IICT Communication no. IICT/Pubs./2022/207
Key words
macrolides - melearoride A - PF1163B - Julia–Kocienski olefination - ring-closing metathesis
Marine-derived fungi receive significant attention as natural sources of drugs because of their impressive biological activities.[1] During recent decades, the pharmacology of antimycotics has advanced significantly, although common invasive fungal infections are still believed to have a high mortality rate.[2]
[3] Melearoride-A (1), a novel 13-membered macrolide isolated from marine-derived fungus Penicillium meleagrinum var. viridiflavum by Koyama and co-workers in 2016 has been demonstrated to show synergic effects with fluconazole against azole-resistant Candida albicans.[4] The structure of 1 was elucidated from spectroscopic data (NMR, MS, IR). PF1163B (2), another 13-membered macrolide (Figure [1]) was isolated along with PF1163A as new antifungal antibiotics from the Penicillium sp., by Sasaki and co-workers.[5]
[6] The structure of PF1163B has been deduced by chemical and X-ray crystallographic analyses, and was observed to be the first known inhibitor of ERG25p, a C-4 methyl oxidase.[7] The antifungal activity of PF1163A was found to be four times higher than that of PF1163B, despite possessing a near identical skeleton except for the presence of an additional hydroxyl group in the side chain of PF1163A. The two macrolides, melearoride-A and PF1163B, are structurally and stereochemically similar and differ in alkyl chain appendage; wherein the phenolic group of the amino acid fragment l-tyrosine is coupled with a different alkyl chain.
Figure 1 Structures of 13-membered macrolides melearoride-A, B, and members of the PF1163 family
The impressive biological properties and structural architecture of these macrolides have stimulated several synthetic groups and culminated in the synthesis of individual members of this class. To our knowledge, there is only one synthesis on melearoride A[8] and three reported synthetic routes for PF1163B.[9]
[10]
[11]
In a continuation of our interest in the total synthesis of biologically active natural products,[12] we report herein the stereoselective synthesis of two 13-membered macrolides, melearoride-A and PF1163B, by following a similar strategy and by two different approaches.
Our retrosynthetic analysis (Scheme [1]) revealed a common macrocyclic core 8, which can be alkylated with 1-bromo-3-methylbut-2-ene or a 2-haloethan-1-ol derivative to furnish the corresponding alkylated ethers melearoride A and PF1163B, respectively. The macrocyclic framework 8 could be obtained from two key fragments, 9 and 10. While the latter can be synthesized from readily available amino acid l-tyrosine, 9 can be synthesized from commercially available n-hexanal through a sequence of synthetic transformations involving Keck allylation, cross-metathesis and auxiliary-based chiral alkylation reaction.
Scheme 1 Retrosynthetic analysis for 1 and 4
Our synthetic efforts began with the synthesis of precursor building block 9 from commercially available n-hexanal. Initially, n-hexanal was subjected to Keck asymmetric allylation to afford homallylic alcohol 11 (Scheme [2]).[13] The TBS protected homo allylic alcohol 12,[14] on ozonolysis followed by C-2 Wittig reaction, provided the unsaturated ester 13 in 85% yield over two steps. The unsaturated ester 13, on reduction with NaBH4 and NiCl2·6H2O in methanol, gave saturated ester 14.[15] Ester 14, on hydrolysis with LiOH·H2O in THF/H2O (3:1), afforded acid 15, which was used for N-acylation of (S)-4-benzyl-2-oxazolidinone in the presence of Et3N, pivaloyl chloride and LiCl in THF to give the corresponding N-acyl derivative 16 in 85% yield.[16] Compound 16, on diastereoselective methylation with CH3I in the presence of LiHMDS as base, afforded 17 with a 20:1 diastereomeric ratio. Exposure of 17 to LiBH4 provided the primary alcohol 18,[17] which, upon oxidation with Dess–Martin periodinane in CH2Cl2 followed by homologation using the C1 Wittig salt in the presence of n-BuLi, provided alkene 19 in 70% yield over two steps. Finally, deprotection of the TBS protected alcohol under TBAF, THF conditions afforded secondary alcohol fragment 9 in 86% yield.
Scheme 2 Synthesis of 9. Reagents and conditions: (a) (S)-BINOL, TiCl4, allyl tributylstannane, Ag2O, CH2Cl2, –15 to 0 °C, 16 h, 85%, 98% ee; (b) TBSCl, imidazole, CH2Cl2, 6 h, 96%; (c) O3, DMS, CH2Cl2, 2 h; (d) PPh3CHCO2Et, CH2Cl2, 4 h, 85% (over two steps) 9:1 (E/Z); (e) NiCl2·6H2O, NaBH4, CH3OH, 1 h, 90%; (f) LiOH·H2O, THF/H2O (3:1), 2 h, 80%; (g) PivCl, Et3N, LiCl; (S)-4-benzyl-2-oxazolidinone, THF, –20 °C, 4 h, 85%; (h) LiHMDS, CH3I, THF, 1 h, 80%, 20:1 dr; (i) LiBH4, CH3CH2OH, THF, 0 °C to r.t., 86%; (j) DMP, CH2Cl2, r.t., 2 h; (k) Ph3PCH2Br, n-BuLi, THF, 0 °C to r.t., 70% (over two steps) (l) TBAF, THF, 0 °C to r.t., 86%.
The amino acid fragment 10 was synthesized from commercially available l-tyrosine. Accordingly,l-tyrosine was treated with SOCl2 in methanol under reflux to provide the corresponding methyl ester 20. Boc protection followed by benzylation was achieved with Boc2O in the presence of triethylamine and then BnBr in the presence of KI and K2CO3 in acetone to provide 21, followed by 22, respectively.[18] Ester 22, upon hydrolysis with LiOH, provided acid 23, which was subjected to N-methylation with MeI and NaH to produce the enantiomerically pure aromatic fragment 10 (Scheme [3]).[19]
Scheme 3 Synthesis of 10. Reagents and conditions: (a) SOCl2, CH3OH, reflux, 3 h, 100%; (b) (Boc)2O, Et3N, CH2Cl2, r.t., overnight, 95%; (c) BnBr, K2CO3, KI, acetone, reflux, 6 h, 96%; (d) LiOH·H2O, THF/H2O (3:1), 2 h, 86%; (e) NaH, CH3I, THF, 6 h, 80%.
With the two key fragments in hand, we proceeded to the esterification of acid 10 with alcohol 9 under Yamaguchi conditions to afford ester 24 (Scheme [4]).[20] Boc deprotection with TFA afforded secondary amine, which was then acylated with pent-4-enoic acid using DIPEA / Pybop to give the corresponding acylated α,ω-diene 25 in 80% yield over two steps. Ring-closing metathesis of diene 25 was achieved in toluene under reflux to provide the requisite macrocycle containing a mixture of (E)- and (Z)-diastereomers in 70% yield. Since the geometry of the olefin was not of concern, we proceeded to the one-pot reduction of the alkene and debenzylation by hydrogenation with Pd-C in EtOAc to give the desired macrocycle 8 in 85% yield. In this synthetic route, the macrocyclic intermediate 8 was obtained from n-hexanal in 17 steps with 2.7% overall yield.
Scheme 4 Synthesis of 8. Reagents and conditions: (a) 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, toluene, 0 °C to r.t., 8 h, 90%; (b) TFA, CH2Cl2, 3 h; (c) Pent-4-enoic acid, Pybop, DIPEA, CH2Cl2, 6 h, 80% (over two steps); (d) Grubbs’ second-generation catalyst (10 mol%), toluene, reflux, 12 h, 70%; (e) H2, Pd/C, EtOAc, r.t., 85%.
Scheme 5 Alternate approach for the synthesis of 8. Reagents and conditions: (a) DMP, CH2Cl2, r.t., 2 h, 76%; (b) 26, KHMDS, THF, 1 h, –78 °C, 80% (14:1 E:Z ratio); (c) TBAF, THF, 0 °C to r.t., 85%; (d) 10, 2,4,6-trichlorobenzoyl chloride, Et3N, DMAP, toluene, 0 °C to r.t., 8 h, 90%; (e) H2, Pd/ C, EtOAC, r.t., 80%; (f) BAIB, TEMPO, CH3CN, pH 7, r.t., 4 h, 81% (g) TFA, CH2Cl2, r.t., 3 h; (h) Pybop, DIPEA, CH2Cl2, 6 h, 80% (over two steps).
An alternate strategy to ring-closing metathesis to obtain 8 was also adopted (Scheme [5]). Thus, alcohol 18 was oxidized under Dess–Martin periodinane conditions to yield the corresponding aldehyde, which, on Julia–Kocienski olefination with sulfone 26 using KHMDS as base, afforded the alkene 27 as a diastereomeric mixture (E/Z 14:1) in 80% yield.[21] Sulfone 26 was synthesized starting with 1,4-butanediol, which was monoprotected as the corresponding benzyl ether 28 and the alcohol was converted into sulfide 29 under Mitsunobu conditions on treating with 1-phenyl-1H-tetrazole-5-thiol (Scheme [6]). m-CPBA oxidation of sulfide 29 afforded the required sulfone fragment 26 in 90% yield.[22] Deprotection of the TBS group in 27 was achieved with TBAF in THF to provide secondary alcohol 30 in 85% yield (Scheme [5]). Esterification of acid 10 with alcohol 30 under Yamaguchi conditions afforded ester 31. One-pot reduction of the double bond and debenzylation was achieved with Pd-C (10%) in EtOAc, under hydrogen to afford primary alcohol 32 in 80% yield. Then, alcohol 32 was oxidized to acid 33 using BAIB, TEMPO oxidation conditions,[23] in 81% yield. Boc deprotection with TFA followed by intramolecular coupling of acid with secondary amine with DIPEA, PyBOP afforded 8 in 80% yield over two steps.
Scheme 6 Synthesis of 26. Reagents and conditions: (a) NaH, BnBr, THF, 4 h, 85% (b) PPh3, DIAD, 1-phenyl-1H-tetrazole-5-thiol, THF, 8 h, 90%; (c) m-CPBA, CH2Cl2, 16 h, 90%.
In this synthetic route we produced fragment 8 from n-hexanal in 16 steps in 3.4% overall yield. Finally, O-alkylation of phenol 8 using prenyl bromide with Cs2CO3 as base and a catalytic amount of KI in DMF afforded the target molecule (–)-melearoride-A in 90% yield.[24] The broad signals in the 1H NMR spectrum are attributed to the presence of conformers.[6] Using similar etherification conditions, O-alkylation of 8 with 2-bromoethoxy-tert-butyldimethylsilane, followed by subsequent TBS deprotection afforded PF1163B in 87% yield over two steps (Scheme [7]). The 1H NMR spectroscopic data of the resulting product were found to be in accordance with previously reported data.[11]
Scheme 7 Synthesis of compound melearoride A (1) and PF1163B (4). Reagents and conditions: (a) K2CO3, KI, prenyl bromide, DMF, 4 h, reflux, 90%; (b) (2-bromoethoxy)(tert-butyl)dimethylsilane, K2CO3, KI, DMF, 4 h, reflux; (c) TBAF, THF, 6 h, 83% (over two steps).
In conclusion, we have accomplished the stereoselective total synthesis of macrolides melearoride A and PF1163B in good overall yields. For the first time, Julia–Kocienski olefination has been applied to extend the C-4 carbon chain in the synthesis of members of this family as an alternative to conventional Grubbs ring-closing metathesis. This strategy allows easy access to various analogues by varying the side chain on the aromatic amino acid fragment for further screening of antifungal and antibiotic properties.
All reagents were used as received from commercial sources unless otherwise noted. All air- and moisture-sensitive reactions were conducted under nitrogen or argon in flame-dried or oven-dried glassware with magnetic stirring. CH2Cl2 was stirred over CaH2 and distilled prior to use. THF was dried with Na/benzophenone and distilled prior to use. Toluene was freshly distilled from CaH2 before use. Reactions were monitored by thin-layer chromatography, using Merck silica gel 60 F254 and UV light, iodine or p-anisaldehyde for visualization. Column chromatography was carried out on silica gel (60–120 mesh or 100–200 mesh). Technical grade ethyl acetate and petroleum ether were used for column chromatography and were distilled prior to use. 1H and 13C NMR spectra were recorded in CDCl3 on 300 MHz, 400 MHz, 500 MHz or 600 MHz spectrometers. Coupling constants (J) are given in Hz. Chemical shifts (δ) are reported in ppm downfield from TMS with use of the residual solvent peak in CDCl3 (H: δ = 7.26 and C: δ = 77.0 ppm) or TMS (δ = 0.0 ppm) as internal standards. Signal patterns are indicated as follows: s = singlet, d = doublet, dd = doublet of doublets, dt = doublet of triplets, t = triplet, q = quartet, m = multiplet, br = broad. IR spectra were recorded with a Bruker infrared spectrophotometer and are reported in cm–1. High-resolution mass spectra (HRMS) were recorded with a Waters-TOF. Specific rotations were measured using a 1 mL cell with a 1 dm path length.
(R)-Non-1-ene-4-ol (11)
To a solution of TiCl4 (0.54 mL, 5 mmol) in CH2Cl2 (100 mL) was added anhydrous Ti(OiPr)4 (4.48 mL, 15 mmol) at 0 °C under argon, and the solution was warmed to r.t. After 1 h, silver(I) oxide (2.3 g, 10 mmol) was added at r.t., and the mixture was stirred for 5 h with the exclusion of direct light. The mixture was diluted with CH2Cl2 (160 mL) and then treated with (S)-binol (5.72 g, 20 mmol) at r.t. for 2 h to furnish chiral bis-(S)-Ti(IV) oxide. The in situ generated bis-(S)-Ti(IV) oxide was cooled to –15 °C and treated sequentially with hexanal (10 g, 100 mmol) and allyltributylstannane (34 mL, 110 mmol) at –15 °C. The mixture was warmed to 0 °C, stirred for 8 h, and then quenched with saturated NaHCO3 (100 mL). The resulting mixture was extracted with diethyl ether (2 × 500 mL), the combined organic extracts were washed with brine (2 × 200 mL), filtered and dried over anhydrous Na2SO4, and concentrated in vacuo. Purification by flash column chromatography on silica gel (5% EtOAc/hexanes to 6% EtOAc/hexanes) afforded compound 11 (12.07 g, 85% yield, 98.74% ee by analytical HPLC analysis).
HPLC [Chiral Pak-IC (2504.6 mm, 5 μm); 5% isopropanol (IPA) in hexane]: tR
= 14.571 (major isomer 99.4%) and 13.196 min (minor isomer 0.6%).
Colorless oil; Rf
= 0.7 (20% EtOAc/ hexanes); [α]D
25 9.0 (c = 1.0, CHCl3).
IR (neat): 3405, 3359, 3077, 2925, 1453, 1129 cm–1.
1H NMR (500 MHz, CDCl3): δ = 5.88–5.79 (m, 1 H), 5.16–5.11 (m, 2 H), 3.65 (s, 1 H), 2.33–2.28 (m, 1 H), 2.17–2.11 (m, 1 H), 1.66–1.57 (m, 1 H), 1.49–1.25 (m, 8 H), 0.89 (t, J = 6.9 Hz, 3 H).
13C NMR (101 MHz, CDCl3): δ = 134.9, 118.1, 70.7, 42.0, 36.8, 31.9, 25.4, 22.7, 14.1.
HRMS (ESI): m/z [M + NH4]+ calcd. for C8H22NO: 160.1701; found: 160.1697.
(R)-tert-Butyldimethyl Non-1-en-4-yloxysilane (12)
(R)-tert-Butyldimethyl Non-1-en-4-yloxysilane (12)
To a stirred solution of alcohol 11 (5.0 g, 35.21 mmol) in CH2Cl2 (20 mL) was added imidazole (4.78 g, 70.42 mmol), followed by TBDMS-Cl (7.92 g, 52.81 mmol) at 0 °C. The ice bath was removed, and the reaction mixture was allowed to warm to r.t. with continuous stirring over 12 h. The reaction mixture was quenched with saturated aqueous NH4Cl and the mixture was extracted with CH2Cl2. The combined organic layers were dried over Na2SO4, filtered, and evaporated, and the residue was purified by silica gel column chromatography (petroleum ether/EtOAc, 99:1) to afford 12 (8.65 g, 96%) as a colourless oil.
Rf
= 0.7 (5% EtOAc/hexanes); [α]D
25 9.9 (c = 1.0, CHCl3).
IR (neat): 2954, 2858, 1466, 1253, 1056, 912 cm–1.
1H NMR (500 MHz, CDCl3): δ = 5.84–5.76 (m, 1 H), 5.03–4.98 (m, 2 H), 3.69–3.64 (m, 1 H), 2.24–2.14 (m, 2 H), 1.45–1.19 (m, 8 H), 0.88–0.85 (m, 12 H), 0.04 (s, 6 H).
13C NMR (101 MHz, CDCl3): δ = 135.6, 116.5, 72.1, 42.0, 36.8, 32.0, 25.9, 25.7, 25.0, 22.7, 18.2, 14.1, –2.9, –4.3, –4.5.
HRMS (ESI): m/z [M + Na]+ calcd. for C15H32OSiNa: 279.1591; found: 279.1588.
Ethyl (R,E)-5-tert-Butyldimethylsilyloxydec-2-enoate (13)
Ethyl (R,E)-5-tert-Butyldimethylsilyloxydec-2-enoate (13)
To a stirred solution of 12 (6.4 g, 26.5 mmol) in CH2Cl2 (60 mL) and MeOH (60 mL), O3 was passed through a gas dispersion tube. When the color of the solution turned blue, dimethyl sulfide (16.4 mL) and triethylamine (2.4 mL) were added. The solution was stirred for 2 h and was concentrated under reduced pressure to afford the crude aldehyde as a colorless oil that was used directly in next step. To the aldehyde (6 g, 23.25 mmol) in CH2Cl2 (60 mL), was added (ethoxycarbonlymethylene)triphenylphosphorane (12.1 g, 34.88 mmol) at r.t. After 4 h, the solvent was removed under reduced pressure and the crude product was purified by silica gel column chromatography (petroleum ether/EtOAc, 95:5) to afford 13 (6.97 g, 85%) over two steps as a colorless oil.
Rf
= 0.2 (5% EtOAc/hexane); [α]D
25 6.6 (c = 1.0, CHCl3).
IR (neat): 2931, 1722, 1655, 1465, 1257, 1046 cm–1.
1HNMR (500 MHz, CDCl3): δ = 6.99–6.93 (m, 1 H), 5.83 (d, J = 15.0 Hz, 1 H), 4.21–4.14 (m, 2 H), 3.82–3.74 (m, 1 H), 2.39–2.28 (m, 2 H), 1.46–1.20 (m, 2 H), 1.37–1.21 (m, 9 H), 0.90–0.87 (m, 12 H), 0.04 (s, 6 H).
13C NMR (101 MHz, CDCl3): δ = 166.5, 146.2, 123.2, 71.4, 60.1, 40.2, 37.3, 31.9, 25.9, 25.0, 22.6, 18.1, 14.3, 14.0, –4.5.
HRMS (ESI): m/z [M + H]+ calcd. for C18H37O3Si: 329.2512; found: 329.2509.
Ethyl (R)-5-tert-Butyldimethylsilyloxydecanoate (14)
Ethyl (R)-5-tert-Butyldimethylsilyloxydecanoate (14)
To a solution of 13 (18 g, 57.32 mmol) in MeOH (180 mL) was added NiCl2·6H2O (2.7 g 11.46 mmol), then NaBH4 (57.32 mmol) in portions over 5 min at 0 °C. After stirring of 30 min, with completion of reaction being indicated by TLC, the reaction mixture was then filtered through a pad of Celite and solvent was removed under reduced pressure. To the residue was added aqueous NH4Cl and the mixture extracted with EtOAc (2 × 60 mL). The organic layer was washed with brine (2 × 30 mL), dried over anhydrous Na2SO4, filtered, and concentrated in vacuo. The resulting crude product was purified by column chromatography on silica gel (4% EtOAc/hexanes) to afford 14 (16.29 g, 90%).
[α]D
25 –1.3 (c = 1.0, CHCl3).
IR (neat): 2933, 2860, 1737, 1463, 1374, 1250, 1165 cm–1.
1H NMR (500 MHz, CDCl3): δ = 4.12 (q, J = 7 Hz, 2 H), 3.68–3.60 (m, 1 H), 2.29 (t, J = 5 Hz, 2 H), 1.73–1.58 (m, 2 H), 1.50–1.38 (m, 4 H), 1.34–1.21 (m, 9 H), 0.90–0.87 (m, 12 H), 0.04 (s, 6 H).
13C NMR (101 MHz, CDCl3): δ = 173.7, 72.0, 60.2, 37.0, 36.4, 34.6, 32.1, 25.9, 25.0, 22.7, 20.9, 18.2, 14.3, 14.1, –4.4.
HRMS (ESI): m/z [M + H]+ calcd. for C19H38OSi: 321.2663; found 321.2653.
(R)-5-tert-Butyldimethylsilyloxydecanoic Acid (15)
(R)-5-tert-Butyldimethylsilyloxydecanoic Acid (15)
To 14 (10 g, 30.30 mmol) in THF/H2O (3:1) was added LiOH (1.4 g, 60.6 mmol) at 0 °C and the reaction was then allowed to stir at 23 °C for 2 h. The reaction was then quenched with 1 M HCl to raise the pH to 4. The aqueous layer was extracted with EtOAc (3 × 50 mL), the combined organic layers were washed with water, brine and dried over anhydrous Na2SO4. After filtration, the solvent was removed under reduced pressure and the crude acid was purified by silica gel column chromatography (90:10, petroleum ether/EtOAc) to afford acid 15 (7.32 g, 80%) as a colorless oil.
Rf
= 0.5 (20% EtOAc/hexane); [α]D
25 –1.0 (c = 0.4, CHCl3).
IR (neat): 2934, 2860, 1712, 1463, 1375, 1255, 1075 cm–1.
1H NMR (500 MHz, CDCl3): δ = 3.67–3.63 (m, 1 H), 2.35 (t, J = 7.4 Hz, 2 H), 1.74–1.59 (m, 2 H), 1.52–1.40 (m, 4 H), 1.35–1.20 (m, 7 H), 0.89–0.86 (m, 12 H), 0.04 (s, 6 H).
13C NMR (101 MHz, CDCl3): δ = 179.8, 71.9, 37.0, 36.3, 34.2, 32.1, 25.9, 25.0, 22.7, 20.5, 18.1, 14.1, –4.5.
HRMS (ESI): m/z [M + H]+ calcd. for C16H35O3Si: 303.2356; found: 303.2355.
(S)-4-Benzyl-3-(R)-5-tert-Butyldimethylsilyloxydecanoyloxazolidin-2-one (16)
(S)-4-Benzyl-3-(R)-5-tert-Butyldimethylsilyloxydecanoyloxazolidin-2-one (16)
To a stirred solution of acid 15 (6 g, 19.86 mmol) in THF (60 mL) at –20 °C was added Et3N (5.67 mL, 39.72 mmol) followed by PivCl (2.4 mL, 19.86 mmol). After stirring for 1 h at –20 °C, LiCl (1.2 g, 29.79 mmol) followed by (S)-oxazolidinone (3.5 g, 19.86 mmol) were added. Stirring was continued for 1 h at –20 °C and then 2 h at 0 °C. The mixture was then quenched with saturated aqueous NH4Cl (30 mL) and extracted with EtOAc (2 × 80 mL). The combined organic layers were washed with brine (60 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography (petroleum ether/EtOAc 90:10) to afford 16 (7.78 g, 85%) as a viscous liquid.
Rf
= 0.5 (20% EtOAc/hexane); [α]D
25 29.15 (c = 1.3, CHCl3).
IR (neat): 2937, 2860, 1787, 1704, 1464, 1387, 1255, 1083 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.35–7.31 (m, 2 H), 7.29–7.27 (m, 1 H), 7.22–7.19 (m, 2 H), 4.70–4.64 (m, 1 H), 4.22–4.14 (m, 2 H), 3.70–3.65 (m, 1 H), 3.31 (dd, J = 11.36 Hz, 3.32 Hz, 1 H), 3.01–2.86 (m, 2 H), 2.76 (dd, J = 13.36 Hz, 9.68 Hz, 1 H), 1.54–1.40 (m, 4 H), 1.33–1.22 (m, 8 H), 0.90–0.88 (m, 6 H), 0.06 (s, 3 H).
13C NMR (151 MHz, CDCl3): δ = 173.2, 153.5, 135.3, 129.4, 129.0, 127.3, 72.0, 66.2, 55.2, 37.9, 37.0, 36.4, 35.7, 32.1, 25.9, 25.0, 22.7, 20.1, 18.2, 14.1, –4.4.
HRMS (ESI): m/z [M + H]+ calcd. for C26H44NO4Si: 462.3039; found: 462.3033.
(S)-4-Benzyl-3-(2S,5R)-5-tert-butyldimethylsilyloxy-2-methyldecanoyloxazolidin-2-one (17)
(S)-4-Benzyl-3-(2S,5R)-5-tert-butyldimethylsilyloxy-2-methyldecanoyloxazolidin-2-one (17)
To a solution of compound 16 (3.0 g, 6.5 mmol) in anhydrous THF (20 mL) at –78 °C, LiHMDS (1 M in THF, 9.75 mL, 9.75 mmol) was added dropwise with stirring under nitrogen. After stirring at –78 °C for 30 min, MeI (0.6 mL, 9.75 mmol) was added and the reaction mixture stirred for an additional 3 h at –78 °C. Then the reaction was quenched with saturated aqueous NH4Cl (30 mL), warmed to r.t. and extracted with EtOAc (2 × 60 mL). The combined organic extracts were washed with brine (50 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography (petroleum ether/ hexane, 95:5) to yield 17 (2.47 g, 80%) as a colorless viscous liquid.
Rf
= 0.5 using (10% EtOAc/ hexane); [α]D
25 41.59 (c = 0.5, CHCl3).
IR (neat): 2934, 2859, 1781, 1699, 1459, 1382, 1245 cm–1.
1H NMR (500 MHz, CDCl3): δ = 7.34–7.31 (m, 2 H), 7.28–7.26 (m, 1 H), 7.22–7.20 (m, 2 H), 4.69–4.65 (m, 1 H), 4.20–4.15 (m, 2 H), 3.70–3.59 (m, 2 H), 3.27 (dd, J = 13.35, 3.25 Hz, 1 H), 2.77 (dd, J = 12.9, 9.5 Hz, 1 H), 1.74–1.67 (m, 1 H), 1.55–1.39 (m, 5 H), 1.32–1.21 (m, 9 H), 0.89–0.86 (m, 6 H), 0.04 (s, 6 H).
13C NMR (101 MHz, CDCl3): δ = 177.1, 153.1, 135.4, 129.5, 129.0, 127.4, 72.3, 66.0, 55.4, 37.9, 37.2, 34.5, 32.1, 29.3, 26.0, 24.9, 22.7, 18.2, 17.4, 14.1, –4.4.
HRMS (ESI): m/z [M + H]+ calcd. for C27H46NO4Si: 476.3190; found: 476.3174.
(2S,5R)-5-tert-Butyldimethylsilyloxy-2-methyldecan-1-ol (18)
(2S,5R)-5-tert-Butyldimethylsilyloxy-2-methyldecan-1-ol (18)
To a solution of compound 17 (2 g, 4.22 mmol) in THF (20 mL) at 0 °C was added EtOH (0.99 mL 16.91 mmol) followed by LiBH4 (110 mg, 5.07 mmol) portion-wise and the mixture was stirred at the same temperature for 10 min. The reaction was quenched with saturated aqueous NH4Cl (15 mL) and the mixture was extracted with EtOAc (2 × 40 mL). The organic extracts were washed with brine (30 mL) and dried over anhydrous Na2SO4. The solvent was filtered, evaporated under reduced pressure and the crude product was purified by silica gel column chromatography (petroleum ether/EtOAc 90:10) to afford alcohol 18 (1.09 g, 86%) as a colorless viscous liquid.
Rf
= 0.5 (20% EtOAc/hexane); [α]D
25 –5.2 (c = 0.5, CHCl3).
IR (neat): 2931, 2859, 1463, 1375, 1252, 1125, 1045 cm–1.
1H NMR (400 MHz, CDCl3): δ = 3.64–3.59 (m, 1 H), 3.52 (dd, J = 10.4, 5.16 Hz, 1 H), 3.42 (dd, J = 10.4 Hz, 6.8 Hz, 1 H), 1.47–1.11 (m, 14 H), 0.92–0.86 (m, 15 H), 0.04 (s, 6 H).
13C NMR (101 MHz, CDCl3): δ = 72.6, 68.4, 37.1, 36.0, 34.3, 32.1, 28.6, 26.0, 25.1, 22.7, 18.2, 16.7, 14.1, –4.4.
HRMS (ESI): m/z [M + H]+ calcd. for C17H39O2Si: 303.2718; found: 303.2715
tert-Butyldimethyl (3S,6R)-3-Methylundec-1-en-6-yloxysilane (19)
tert-Butyldimethyl (3S,6R)-3-Methylundec-1-en-6-yloxysilane (19)
The aldehyde obtained from oxidation of alcohol 18 was subjected to C-1 Wittig olefination without purification. To methyl triphenyl phosphonium bromide (1.78 g, 5 mmol) in anhydrous THF was added LiHMDS (1 M in THF, 3.3 mL, 3.32 mmol) at 0 °C. The mixture was stirred for 30 min and to this was added dropwise a solution of aldehyde (500 mg, 1.66 mmol) in THF (5 mL). The reaction mixture was then stirred for 1 h, quenched with saturated aqueous NH4Cl solution (20 mL) and extracted with EtOAc (20 mL). The organic extracts were washed with brine (10 mL) and dried over anhydrous Na2SO4. The solvent was filtered and evaporated under reduced pressure, and the resulting crude product was purified by silica gel column chromatography using hexane as eluent to give product 19 (0.34 g, 70%).
[α]D
25 3.75 (c = 0.4, CHCl3).
IR (neat): 2928, 2858, 1463, 1373, 1252, 1128, 1058 cm–1.
1H NMR (500 MHz, CDCl3): δ = 5.72–5.65 (m, 1 H), 4.96–4.89 (m, 2 H), 3.63–3.58 (m, 1 H), 2.09–2.03 (m, 1 H), 1.47–1.26 (m, 12 H), 0.98 (d, J = 6.7 Hz, 3 H), 0.89–0.87 (m, 12 H), 0.03 (s, 6 H).
13C NMR (101 MHz, CDCl3): δ = 144.9, 112.4, 72.5, 37.9, 37.1, 34.6, 32.2, 26.0, 25.0, 22.7, 20.3, 18.2, 14.1, –4.4.
HRMS (ESI): m/z [M + H]+ calcd. for C18H39OSi: 299.1265; found: 299.1263.
(3S,6R)-3-Methylundec-1-en-6-ol (9)
(3S,6R)-3-Methylundec-1-en-6-ol (9)
To a stirred solution of O-TBS protected alkene 19 (200 mg, 0.67 mmol) at 0 °C in THF (5 mL), tetrabutylammonium fluoride (1.34 mL 1.34 mmol, 1.0 M in THF) was added at 0 °C. Stirring was continued from 0 °C to r.t. for 6 h, then the reaction was quenched with ice-cold water (5 mL), and the mixture was extracted with EtOAc (5 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography over silica gel (petroleum ether/EtOAc, 95:5) to afford alcohol 9 as a colorless liquid (106 mg, 86%).
Rf
= 0.5 (10% EtOAc/hexane); [α]D
25 5.2 (c = 0.5, CHCl3).
IR (neat): 3545, 3418, 3160, 2930, 2861, 1459 cm–1.
1H NMR (400 MHz, CDCl3): δ = 5.73–5.65 (m, 1 H), 4.98–4.90 (m, 2 H), 3.57 (s, 1 H), 2.17–2.05 (m, 1 H), 1.49–1.28 (m, 13 H), 1.00 (d, J = 6.7 Hz, 3 H), 0.89 (t, J = 6.8 Hz, 3 H).
13C NMR (101 MHz, CDCl3): δ = 144.6, 112.7, 72.3, 38.0, 37.5, 35.2, 32.6, 31.9, 25.3, 22.7, 20.2, 14.1.
HRMS (ESI): m/z [M + NH4]+ calcd. for C12H28NO: 202.1786; found: 202.1794.
Methyl l-Tyrosinate (20)
A solution of l-tyrosine (5.0 g, 27.6 mmol) in MeOH (30 mL) was stirred at 0 °C and thionyl chloride (3.0 mL, 41.4 mmol) was added dropwise. The reaction was then allowed to warm to r.t. before being heated to reflux for 3 h. The solvent and volatiles were evaporated under reduced pressure and the product was triturated with EtOAc to give the methyl ester hydrochloride salt 20 as a colorless solid (5.4 g, quant.).
[α]D
25 8.80 (c = 1.0, CHCl3).
IR (neat): 3595, 3423, 2938, 2601, 2141, 1874, 1338 cm–1.
1H NMR (400 MHz, DMSO): δ = 9.51 (s, 1 H), 8.60 (s, 3 H), 7.01 (d, J = 8.2 Hz, 2 H), 6.72 (d, J = 8.0 Hz, 2 H), 4.16 (s, 1 H), 3.67 (s, 3 H), 3.08–2.95 (m, 2 H).
13C NMR (101 MHz, CDCl3): δ = 174.7, 161.9, 135.6, 129.5, 120.7, 58.7, 57.8, 40.3.
HRMS (ESI): m/z [M + H]+ calcd. for C10H14NO3: 196.0973; found: 196.0971.
Methyl N-tert-Butoxycarbonyl-l-tyrosinate (21)
Methyl N-tert-Butoxycarbonyl-l-tyrosinate (21)
To a mixture of methyl l-tyrosinate 20 (5.0 g, 27.6 mmol) in CH2Cl2, was added triethylamine (11.6 mL, 82.9 mmol), and di-tert-butyl dicarbonate (7.6 mL, 33.1 mmol) at 0 °C, and the resulting mixture was stirred at r.t. overnight. The resulting mixture was partitioned between CH2Cl2 (100 mL) and water (50 mL). The aqueous phase was removed and the organic phase was dried over Na2SO4, filtered, and concentrated. The residue was purified by silica gel column chromatography (petroleum ether/EtOAc, 90:10) to afford methyl (tert-butoxycarbonyl) tyrosinate 21 as a white solid (7.14 g, 95%).
Rf
= 0.5 (20% EtOAc/ hexane); [α]D
25 52.6 (c = 1.0, CHCl3).
IR (neat): 3381, 2982, 1686, 1512, 1241, 1159, 1054 cm–1.
1H NMR (400 MHz, CDCl3): δ = 6.96 (d, J = 7.28 Hz, 2 H), 6.73 (d, J = 7.68 Hz, 2 H), 5.01 (s, 1 H), 4.54 (dd, J = 13.2, 6.08 Hz, 1 H), 3.71 (s, 3 H), 3.05–2.93 (m, 2 H), 1.42 (s, 9 H).
13C NMR (101 MHz, CDCl3): δ = 172.7, 155.3, 155.1, 130.4, 127.6, 115.5, 80.2, 54.6, 52.3, 37.6, 28.3.
HRMS (ESI): m/z [M + Na]+ calcd. for C15H21NO5Na: 318.2107; found: 318.2108.
Methyl (S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylamino)propanoate (22)
Methyl (S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylamino)propanoate (22)
To a mixture of Boc-l-Tyr-OMe 21 (2.0 g, 6.77 mmol), K2CO3 (1.4 g, 10.15 mmol), and KI (112 mg, 0.67 mmol), in acetone (20 mL), was added BnBr (0.9 mL, 8.13 mmol), slowly. The mixture was then heated to reflux overnight and then quenched with water (20 mL). The reaction mixture was extracted with EtOAc (30 mL), dried over anhydrous Na2SO4, filtered, concentrated and purified by silica gel column chromatography (petroleum ether/EtOAc, 93:7) to afford 22 (2.5 g, 96%).
Rf
= 0.3 (10% EtOAc/ hexane); [α]D
25 23.3 (c = 0.3, CHCl3).
IR (neat): 3368, 2978, 1691, 1514, 1364, 1229, 1060 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.43–7.30 (m, 5 H), 7.04 (d, J = 8.52 Hz, 2 H), 6.90 (d, J = 8.64 Hz, 2 H), 5.04 (s, 2 H), 4.96 (s, 1 H), 4.54 (dd, J = 12.8, 5.8 Hz, 1 H), 3.71 (s, 3 H), 3.07–2.96 (m, 2 H), 1.42 (s, 9 H).
13C NMR (101 MHz, CDCl3): δ = 172.5, 157.9, 155.1, 137.0, 130.3, 128.6, 128.3, 128.0, 127.5, 115.0, 79.9, 70.1, 54.6, 52.2, 37.5, 28.3.
HRMS (ESI): m/z [M + Na]+ calcd. for C22H27NO5Na: 408.1779; found: 408.1778.
(S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylmethylamino)propanoic Acid (23)
(S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylmethylamino)propanoic Acid (23)
To compound 22 (2.0 g, 5.2 mmol) in THF/H2O (3:1) was added LiOH (435 mg, 10.38 mmol) at 0 °C. The reaction was allowed to stir at r.t. for 2 h and was then quenched with 1 M HCl to pH 4. The aqueous layer was extracted with EtOAc (3 × 50 mL), the combined organic layers were washed with water, brine and dried over anhydrous Na2SO4. After filtration, the volatiles were removed under reduced pressure, and the crude acid was purified by silica gel column chromatography (petroleum ether/EtOAc, 60:40) to yield 23 as a white solid (1.65 g, 86%).
Rf
= 0.2 (60% EtOAc/hexane); [α]D
25 19.8 (c = 0.9, CHCl3).
IR (neat): 3424, 2978, 1711, 1509, 1399, 1168, 1025 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.42–7.29 (m, 5 H), 7.10 (d, J = 8.56 Hz, 2 H), 6.91 (d, J = 8.56 Hz, 2 H), 5.03 (s, 2 H), 4.94 (d, J = 7.68 Hz, 1 H), 4.56 (d, J = 5.88 Hz, 1 H), 3.15–3.00 (m, 2 H), 1.42 (s, 9 H).
13C NMR (101 MHz, CDCl3): δ = 175.7, 158.0, 155.5, 137.0, 130.4, 128.6, 128.0, 127.5, 115.0, 80.3, 70.1, 54.4, 36.9, 28.3.
HRMS (ESI): m/z [M + Na]+ calcd. for C21H25NO5Na: 394.1742; found: 394.1612.
(S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylmethylamino)propanoic Acid (10)
(S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylmethylamino)propanoic Acid (10)
A suspension of sodium hydride (96 mg, 4.03 mmol) in THF (10 mL) was cooled in an ice-water bath under nitrogen. To the mixture was added a solution of (S)-3-(4-(benzyloxy)phenyl)-2-((tert-butoxycarbonyl)(methyl)amino)propanoic acid 23 (1.0 g, 2.69 mmol) in THF (5 mL) slowly, the mixture was stirred for 30 min and then methyl iodide (0.51 mL, 8.07 mmol) was added dropwise. The reaction mixture was stirred at r.t. for 2 h and then quenched with ice-cold water and diluted with EtOAc (30 mL). The aqueous layer was separated and extracted with EtOAc (2 × 50 mL), the combined organic layers were washed with brine (2 × 10 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure to give the crude product 10 as a white solid (0.85 g, 82% yield).
[α]D
25 –15.10 (c = 1.0, CHCl3).
IR (neat): 3202, 2977, 1695, 1451, 1327, 1241, 767 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.43–7.29 (m, 5 H), 7.13–7.08 (m, 2 H), 6.92–6.88 (m, 2 H), 5.04 (s, 2 H), 4.69–4.54 (m, 1 H), 3.24 (dd, J = 14, 4.8 Hz, 1 H), 3.12–2.94 (m, 1 H), 2.71 (s, 3 H), 1.41 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 176.1, 175.5, 157.7, 156.5, 155.1, 137.0, 130.0, 129.7, 129.4, 128.6, 128.0, 127.5, 115.0, 115.0, 80.8, 80.7, 70.1, 61.6, 61.1, 34.5, 33.9, 33.2, 32.6, 28.3.
HRMS (ESI): m/z [M + H]+ calcd. for C22H28NO5: 386.1967; found: 386.1963.
(3S,6S)-3-Methylundec-1-en-6-yl (S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylmethylamino)propanoate (24)
(3S,6S)-3-Methylundec-1-en-6-yl (S)-3-[4-(Benzyloxy)phenyl]-2-(tert-butoxycarbonylmethylamino)propanoate (24)
To a solution of 10 (400 mg, 1.03 mmol) in toluene (5 mL) was added Et3N (0.22 mL, 1.54 mmol) and 2,4,6-trichlorobenzoyl chloride (0.20 mL, 1.23 mmol) at 0 °C and the mixture was stirred at r.t. for 30 min. After the formation of the mixed anhydride, the solution was cooled to 0 °C and a solution of DMAP (628 mg, 5.15 mmol) and alcohol 7 (229 mg, 1.24 mmol) was introduced dropwise to the reaction mixture. The mixture was then warmed to r.t. and was stirred for an additional 5 h. After completion of the reaction (TLC), the mixture was quenched with saturated aqueous NaHCO3 (5 mL) and the aqueous layer was washed with EtOAc (10 mL). The combined organic layers were washed with brine (5 mL), dried over Na2SO4, filtered and the solvent was evaporated to give a pale-yellow oil. Purification of the residue by silica gel column chromatography (petroleum ether/EtOAc, 95:5) afforded ester 24 (0.49 g, 86%) as a colorless oil.
Rf
= 0.5 (10% EtOAc/hexane); [α]D
25 2.86 (c = 0.7, CHCl3).
IR (neat): 3091, 2965, 2867, 1889, 1737, 1622, 1384 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.43–7.30 (m, 5 H), 7.12 (dd, J = 16, 8 Hz, 2 H), 6.89 (d, J = 7.92 Hz, 2 H), 5.69–5.59 (m, 1 H), 5.03 (s, 2 H), 4.97–4.87 (m, 3 H), 3.26–3.18 (m, 1 H), 2.95–2.86 (m, 1 H), 2.76 (s, 3 H), 2.11–2.03 (m, 1 H), 1.52–1.17 (m, 22 H), 1.01–0.86 (m, 6 H).
13C NMR (101 MHz, CDCl3): δ = 174.5, 172.7, 157.6, 144.2, 137.1, 129.9, 128.6, 127.9, 127.5, 115.0, 114.8, 112.9, 80.1, 79.8, 75.8, 75.5, 70.1, 60.9, 37.7, 34.4, 34.2, 34.0, 32.0, 31.7, 31.1, 29.7, 28.3, 24.9, 22.5, 20.2, 14.0.
HRMS (ESI): m/z [M + H]+ calcd. for C34H50NO5: 552.3689; found: 552.3675.
(3S,6S)-3-Methylundec-1-en-6-yl (S)-3-[4-(Benzyloxy)phenyl]-2-(N-methylpent-4-enamido)propanoate (25)
(3S,6S)-3-Methylundec-1-en-6-yl (S)-3-[4-(Benzyloxy)phenyl]-2-(N-methylpent-4-enamido)propanoate (25)
To a solution of 24 (200 mg, 0.362 mmol) in CH2Cl2 (5 mL) cooled to 0 °C was added trifluoroacetic acid (0.55 mL, 7.24 mmol) dropwise. Upon completion of addition, the reaction was warmed to r.t. and stirred for 1 h; at that time, TLC analysis showed complete consumption of starting material. The reaction was concentrated in vacuo to afford a red oil that was subsequently dissolved in CH2Cl2 (5 mL). This solution was cooled to 0 °C before the sequential addition of 4-pentenoic acid (0.046 mL, 0.66 mmol), PyBoP (343 mg, 0.66 mmol), and N,N-di-isopropylethylamine (0.3 mL, 1.76 mmol). The reaction was allowed to stir at r.t. for 6 h; whereupon, TLC analysis indicated complete consumption of starting material. The reaction was diluted with CH2Cl2 and quenched with saturated aqueous NH4Cl. The layers were separated, and the aqueous layer was washed with CH2Cl2 (5 mL). The combined organic layers were washed with saturated aqueous NaHCO3, brine, dried over Na2SO4, filtered, concentrated in vacuo and purified by silica gel column chromatography using petroleum ether/ EtOAc (90:10) to afford the desired product 25 as a colorless oil (212 mg, 90% yield).
Rf
= 0.5 (20% EtOAc/ hexane); [α]D
25 2.36 (c = 01.4, CHCl3).
IR (neat): 2941, 2864, 1731, 1510, 1392, 1113 cm–1.
1H NMR (500 MHz, CDCl3): δ = 7.44–7.30 (m, 2 H), 7.09 (dd, J = 23.5, 8.5 Hz, 1 H), 6.92–6.86 (m, 1 H), 5.81–5.59 (m, 1 H), 5.38 (m, 1 H), 5.02 (s, 1 H), 5.00–4.84 (m, 2 H), 4.52 (dd, J = 9.8, 5.0 Hz, 1 H), 3.27 (m, 1 H), 2.87 (d, J = 31.4 Hz, 1 H), 2.34–1.90 (m, 2 H), 1.57–1.46 (m, 1 H), 1.32–1.19 (m, 3 H), 0.97 (dd, J = 6.7, 3.4 Hz, 1 H), 0.90–0.85 (m, 1 H).
13C NMR (101 MHz, CDCl3): δ = 172.7, 170.9, 170.0, 157.9, 157.6, 144.3, 137.5, 137.1, 129.8, 128.58, 128.0, 127.5, 115.2, 115.0, 114.8, 113.0, 75.7, 70.0, 62.0, 57.9, 37.7, 34.5, 34.1, 33.9, 32.7, 32.5, 32.0, 31.7, 28.9, 24.9, 22.5, 20.2, 14.0.
HRMS (ESI): m/z [M + H]+ calcd. for C34H48NO4: 534.3583; found: 534.3580.
(3S,10R,13R)-3-[4-(Hydroxy)benzyl]-4,10-dimethyl-13-pentyl-1-oxa-4-azacyclotridecane-2,5-dione (8)
(3S,10R,13R)-3-[4-(Hydroxy)benzyl]-4,10-dimethyl-13-pentyl-1-oxa-4-azacyclotridecane-2,5-dione (8)
A solution of compound 25 (180 mg, 0.337 mmol) in toluene (100 mL) was purged with argon, treated with Grubbs’ second-generation catalyst (14 mg, 0.016 mmol) and allowed to stir at 90 °C for 6 h. The reaction mixture was filtered through a short pad of silica gel, washed with EtOAc and concentrated to afford a colorless oil (122 mg, 72% yield), which was taken forward to the next step without further purification.
A solution of RCM product (122 mg, 0.240 mmol) in EtOAC (15 mL) was passed through an H-cube R flow reactor® (40 °C, at 6 bar with a 10 mol-% Pd/C cartridge at 1 mLmin–1. Additional EtOAc (20 mL) was passed through the apparatus, and the solvent was removed in vacuo to give compound 8 (85 mg, 85% yield) as a colorless oil.
[α]D
25 –53.8 (c = 0.5, CHCl3).
IR (neat): 3316, 3268, 3202, 2935, 2861, 1733, 1456, 1231 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.10–6.98 (m, 2 H), 6.78–6.67 (m, 2 H), 5.88–5.84 (m, 1 H), 4.92–4.81 (m, 1 H), 4.55 (m, 0.2 H), 3.17–3.12 (m, 1 H), 3.03–2.87 (m, 3 H), 2.72–2.58 (m, 1 H), 2.21–2.09 (m, 1 H), 1.49–1.25 (m, 20 H), 0.92–0.80 (m, 7 H).
13C NMR (151 MHz, CDCl3): δ = 174.8, 174.4, 171.0, 170.3, 155.4, 129.9, 129.6, 127.6, 127.3, 115.8, 115.4, 75.8, 74.7, 61.9, 55.4, 35.0, 34.9, 33.9, 33.5, 32.9, 32.6, 31.6, 30.7, 29.7, 29.5, 29.4, 28.5, 28.3, 28.1, 26.5, 25.3, 25.1, 24.8, 23.7, 23.4, 23.0, 22.7, 22.5, 20.6, 14.0.
HRMS (ESI): m/z [M + H]+ calcd. for C25H40NO4: 418.2965; found: 418.2957.
4-Benzyloxybutan-1-ol (28)
4-Benzyloxybutan-1-ol (28)
To a vigorously stirred suspension of NaH (640 mg, 26.6 mmol, 1.2 equiv, 60% dispersion in mineral oil) in THF (20 mL) at 0 °C was quickly added a solution of 1,4-butanediol (2 g, 22.2 mmol) in THF (10 mL) via an addition funnel. After stirring for 20 min at r.t., the reaction mixture was again cooled to 0 °C and BnBr (0.2 mL, 22.2 mmol, 1.0 equiv) was added dropwise via syringe. After stirring for 14 h at r.t., the reaction was quenched with sat. aqueous NH4Cl (10 mL) and water (10 mL). The layers were separated and the aqueous phase was extracted with Et2O (2 × 20 mL). The combined organic extracts were dried over MgSO4, filtered, and concentrated under reduced pressure. The residue was purified by column chromatography (hexane/EtOAc, 2:1 → 1:1) to afford 28 (3.4 g, 85%) as a colorless oil.[13]
Rf
= 0.28 (hexane/EtOAc, 2:1).
1H NMR (400 MHz, CDCl3): δ = 7.36–7.28 (m, 5 H), 4.51 (s, 2 H), 3.62 (t, J = 5.9 Hz, 2 H), 3.51 (t, J = 5.8 Hz, 2 H,), 2.4 (s, 1 H), 1.74–1.62 (m, 4 H).
13C NMR (101 MHz, CDCl3): δ = 138.1, 129.6, 128.5, 127.8, 73.1, 70.4, 62.6, 30.0, 26.6.
5-(4-(Benzyloxy)butylthio)-1-phenyl-1H-tetrazole (29)
5-(4-(Benzyloxy)butylthio)-1-phenyl-1H-tetrazole (29)
To a cooled, stirred solution of 4-(benzyloxy)butan-1-ol 28 (2.0 g, 11.1 mmol), in anhydrous THF (20 mL) was added PPh3 (6.45 g, 16.64 mmol), 1-phenyl-1H-tetrazole-5-thiol (4.4 g, 22.18 mmol) and DIAD (2.17 mL, 11.1 mmol) dropwise. The reaction mixture was then vigorously stirred for 8 h at r.t. and then the solvent was removed using a rotary evaporator. The crude material was purified by column chromatography (petroleum ether/EtOAc 95:5) to afford sulfide 29 (3.4 g, 90%) as a colorless liquid.
Rf
= 0.5 (10% EtOAc/hexane).
IR: 2928, 2859, 1500, 1397, 1258, 1100 cm–1.
1H NMR (500 MHz, CDCl3): δ = 7.59–7.51 (m, 1 H), 7.35–7.26 (m, 1 H), 4.50 (s, 2 H), 3.51 (t, J = 6.25 Hz, 2 H), 3.45–3.40 (t, J = 7.3 Hz, 2 H), 1.97- 1.92 (m, 2 H), 1.79–1.75 (m, 2 H).
13C NMR (101 MHz, CDCl3): δ = 154.4, 138.4, 133.7, 130.1, 129.8, 128.4, 127.7, 123.9, 73.0, 69.5, 33.2, 28.7, 26.1.
HRMS (ESI): m/z [M + H]+ calcd. for C18H21N4OS: 341.1431; found: 341.1432.
5-(4-(Benzyloxybutyl)sulfonyl)-1-phenyl-1H-tetrazole (26)
5-(4-(Benzyloxybutyl)sulfonyl)-1-phenyl-1H-tetrazole (26)
To a solution of 29 (1.00 g, 2.94 mmol) in CH2Cl2 (20 mL) at 0 °C was added m-CPBA (1.51 g, 8.82 mmol, 70% wt% suspension in water) in portions. The reaction mixture was stirred at ambient temperature for 16 h, quenched with saturated aqueous NaHCO3, dried over Na2SO4, filtered, and evaporated under reduced pressure. The crude material was purified by silica gel column chromatography (petroleum ether /EtOAc 9:1) to afford the sulfone 26 (0.98 g, 90%) as a colorless oil.
Rf
= 0.5 (10% EtOAc/hexane).
IR: 3069, 2949, 2868, 1715, 1495, 1341, 1102 cm–1.
1H NMR (500 MHz, CDCl3): δ = 8.08–7.97 (m, 1 H), 7.70–7.57 (m, 5 H), 7.36–7.26 (m, 4 H), 4.50 (s, 2 H), 3.79 (t, J = 7.9 Hz, 2 H), 3.53 (t, J = 5.9 Hz, 2 H), 2.12–2.06 (m, 2 H), 1.84–1.75 (m, 2 H).
13C NMR (101 MHz, CDCl3): δ = 169.7, 153.5, 138.1, 134.7, 133.8, 133.1, 131.5, 131.0, 130.3, 129.7, 128.5, 127.7, 125.1, 73.1, 69.1, 55.9, 28.1, 19.6.
HRMS (ESI): m/z [M + H]+ calcd. for C18H21N4O3S: 372.1256; found: 373.1254.
(6R,9S,E)-14-Benzyloxy-9-methyltetradec-10-en-6-yloxy tert-Butyldimethylsilane (27)
(6R,9S,E)-14-Benzyloxy-9-methyltetradec-10-en-6-yloxy tert-Butyldimethylsilane (27)
To a solution of alcohol 18 (0.25 g, 0.82 mmol) in CH2Cl2 (5 mL) at 0 °C was added Dess–Martin periodinane (4.77 g, 1.64 mmol). After 4 h at r.t., the reaction mixture was diluted with CH2Cl2 (5 mL) and then poured into a mixture of saturated aqueous NaHCO3 and 25% aqueous Na2S2O3 (1:1, 5 mL). The separated aqueous layer was extracted with CH2Cl2 (5 mL), and the combined organic extracts were washed with brine (2 mL), dried with anhydrous Na2SO4, and filtered. The filtrate was concentrated under reduced pressure to obtain the aldehyde as a colorless liquid, which was taken into the next step without purification.
To a solution of sulfone 26 (500 mg, 1.34 mmol) in anhydrous THF (10 mL) at –78 °C, was added KHMDS (1.34 mL, 1 M solution in toluene, 1.34 mmol) dropwise. The resulting yellow solution was stirred for 30 min, followed by the dropwise addition of the crude aldehyde (201 mg, 0.67 mmol) in THF (5 mL). The reaction mixture was stirred at –78 °C for 1 h and then quenched at this temperature after the reaction had been demonstrated to be completed by TLC. Saturated aqueous NH4Cl was added, the mixture was allowed to warm to ambient temperature and extracted with Et2O (3 × 30 mL). The combined organic layers were dried with Na2SO4, filtered, and evaporated under reduced pressure. The crude product was purified by silica gel column chromatography (petroleum ether/EtOAc, 98:2) to afford a mixture of (E,Z)-diastereomers of 27 (14:1) (0.29 g, 80%, over two steps) as a colorless liquid.
Rf
= 0.5 using (5% EtOAc/hexane); [α]D
25 3.8 (c = 1.0, CHCl3).
IR (neat): 2941, 2860, 1461, 1368, 1255, 1107, 973 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.33–7.25 (m, 5 H), 5.40–5.22 (m, 2 H), 4.49 (s, 2 H), 3.62–3.56 (m, 1 H), 3.48–3.44 (m, 2 H), 2.09–1.94 (m, 3 H), 1.70–1.63 (m, 2 H), 1.43–1.36 (m, 4 H), 1.34–1.19 (m, 9 H), 0.94–0.88 (m, 2 H), 0.89–0.85 (m, 12 H), 0.02–0.01 (m, 6 H).
13C NMR (101 MHz, CDCl3): δ = 138.7, 137.0, 130.90, 129.6, 128.3, 127.7, 127.6, 127.5, 72.9, 72.5, 72.4, 72.3, 69.8, 37.2, 37.1, 36.9, 36.6, 34.8, 32.7, 32.7, 32.1, 29.7, 29.7, 29.1, 29.1, 26.0, 25.3, 25.1, 21.0, 14.1, –4.4.
HRMS (ESI): m/z [M + H]+ calcd. for C28H51O2Si: 447.7919; found: 447.7910.
(6R,9S,E)-14-Benzyloxy-9-methyltetradec-10-en-6-ol (30)
(6R,9S,E)-14-Benzyloxy-9-methyltetradec-10-en-6-ol (30)
To a cold stirred solution of diastereomeric O-TBS protected alkene 27 (200 mg, 2.35 mmol) in THF (5 mL), tetrabutylammonium fluoride (4.71 mL 4.71 mmol, 1.0 M in THF) was added at 0 °C. Stirring was continued at 0 °C to r.t. for 6 h, then the reaction was quenched with ice-cold water (10 mL), extracted with EtOAc (10 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The crude product was purified by column chromatography over silica gel (petroleum ether/EtOAc, 95:5) to yield an inseparable mixture of (E,Z)-alcohols 30 (126 mg, 85%) as a colorless liquid.
Rf
= 0.5 (10% EtOAc/hexane); [α]D
25 1.8 (c = 1.0, CHCl3).
IR (neat): 3405, 3070, 2925, 1733, 1225, 1175 cm–1.
1H NMR (500 MHz, CDCl3): δ = 7.34–7.26 (m, 5 H), 5.41–5.23 (m, 2 H), 4.50 (s, 2 H), 3.56 (br s, 1 H), 3.49–3.45 (m, 2 H), 2.09–1.97 (m, 3 H), 1.70–1.64 (m, 2 H), 1.47–1.37 (m, 6 H), 1.33–1.25 (m, 8 H), 0.98–0.93 (m, 2 H), 0.89 (t, J = 6.9 Hz, 3 H).
13C NMR (101 MHz, CDCl3): δ = 138.7, 136.7, 130.6, 129.9, 128.4, 128.0, 127.6, 127.5, 72.90, 72.3, 71.9, 69.8, 37.5, 36.9, 35.3, 33.1, 32.6, 31.9, 29.7, 29.6, 29.1, 29.1, 25.6, 25.4, 22.7, 20.9, 14.1.
HRMS (ESI): m/z [M + H]+ calcd. for C22H37O2: 333.2793; found: 333.2785.
(6R,9R,E)-14-Benzyloxy-9-methyltetradec-10-en-6-yl (S)-3-(4-(Benzyloxy)phenyl)-2-(tert-butoxycarbonylmethylamino)propanoate (31)
(6R,9R,E)-14-Benzyloxy-9-methyltetradec-10-en-6-yl (S)-3-(4-(Benzyloxy)phenyl)-2-(tert-butoxycarbonylmethylamino)propanoate (31)
To a solution of 10 (0.5 g, 1.29 mmol) in toluene (5 mL) was added Et3N (0.27 mL, 1.93 mmol) and 2,4,6-trichlorobenzoyl chloride (0.24 mL, 1.55 mmol) at 0 °C and the resultant mixture was stirred at r.t. for 30 min. After formation of the mixed anhydride, the solution was cooled to 0 °C and a solution of DMAP (0.78 g, 6.45 mmol) and alcohol 30 (0.51 g, 1.55 mmol) was introduced dropwise. The reaction mixture was warmed to r.t. and was stirred for an additional 5 h. After completion of reaction as indicated by TLC, it was quenched by addition of saturated aqueous NaHCO3 (10 mL) and the aqueous layer was washed with EtOAc (3 × 10 mL). The combined organic layers were washed with brine (5 mL), dried over Na2SO4, filtered and the solvent was evaporated to give a pale-yellow oil. Purification of the residue by silica gel column chromatography (petroleum ether/EtOAc, 96:4) afforded ester 31 (0.90 g, 86%) as a colorless liquid.
Rf
= 0.2 (6% EtOAc/Hexane); [α]D
25 –12.89 (c = 0.9, CHCl3).
IR (neat): 3028, 2865, 1735, 1513, 1458, 1328, 1176 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.43–7.24 (m, 10 H), 7.11 (dd, J = 15.8, 7.6 Hz, 2 H), 6.89 (d, J = 8.2 Hz, 2 H), 5.39–5.19 (m, 2 H), 5.03 (s, 2 H), 4.95–4.67 (m, 2 H), 4.50 (s, 2 H), 3.47 (t, J = 6.48 Hz, 2 H), 3.26–3.11 (m, 1 H), 2.95–2.86 (m, 1 H), 2.76 (s, 3 H), 2.17–1.93 (m, 3 H), 1.70–1.63 (m, 2 H), 1.51–1.19 (m, 22 H), 0.99–0.85 (m, 5 H).
13C NMR (101 MHz, CDCl3): δ = 163.2, 157.5, 138.7, 129.9, 128.6, 128.4, 127.9, 127.6, 127.5, 114.9, 79.8, 72.9, 72.1, 70.1, 69.8, 36.9, 35.0, 34.2, 33.9, 33.5, 32.4, 32.0, 31.7, 29.7, 29.1, 28.3, 24.9, 22.5, 20.9, 14.0.
HRMS (ESI): m/z [M + H]+ calcd. for C44H62NO6: 700.4577; found: 700.4556.
(6R,9R)-14-Hydroxy-9-methyltetradecan-6-yl N-tert-Butoxycarbonyl-N-methyl-l-tyrosinate (32)
(6R,9R)-14-Hydroxy-9-methyltetradecan-6-yl N-tert-Butoxycarbonyl-N-methyl-l-tyrosinate (32)
A solution of 31 (0.2 g, 0.286 mmol) in EtOAc (20 mL) was passed through an H-cube R flow reactor® (40 °C, at 6 bar with a 10 mol% Pd/C cartridge, 1 mL min–1). Additional EtOAc (20 mL) was passed through the apparatus, and the solvent was removed in vacuo to obtain a colorless oil, which was purified by silica gel column chromatography (petroleum ether/ EtOAc, 80:20) to afford alcohol 32 (119 mg, 80%) as a colorless liquid.
Rf
= 0.5 (40% EtOAc/hexane); [α]D
25 –23.5 (c = 1.6, CHCl3).
IR: 3316, 3268, 2935, 2861, 1733, 1513, 1456, 1090 cm–1.
1H NMR (500 MHz, CDCl3): δ = 7.05–7.02 (m, 2 H), 6.75–6.68 (m, 2 H), 6.2 (s, 1 H), 4.97–4.87 (m, 1 H), 4.70 (s, 1 H), 3.65 (t, J = 7.3 Hz, 1 H), 3.24–3.17 (m, 1 H), 2.89 (t, J = 12.6 Hz, 1 H), 2.78, 2.73 (s, 3 H), 2.38–2.01 (m, 1 H), 1.56–1.46 (m, 8 H), 1.38–1.27 (m, 22 H), 0.88–0.84 (m, 6 H).
13C NMR (101 MHz, CDCl3): δ = 171.2, 170.9, 156.1, 155.5, 155.1, 154.8, 129.0, 128.8, 115.4, 115.3, 80.5, 80.1, 75.9, 75.8, 75.7, 75.5, 63.0, 63.0, 61.2, 59.5, 36.8, 34.4, 34.0, 33.8, 32.7, 32.5, 32.3, 32.1, 31.7, 31.5, 31.4, 29.7, 29.3, 29.2, 28.3, 28.2, 26.7, 25.9, 25.6, 25.2, 25.1, 24.9, 22.5, 19.7, 19.6, 14.0.
HRMS (ESI): m/z [M + Na]+ calcd. for C30H51NO6Na: 544.3614; found: 544.3605.
(6R,9R)-9-(N-tert-Butoxycarbonyl-N-methyl-l-tyrosyloxy)-6-methyltetradecanoic Acid (33)
(6R,9R)-9-(N-tert-Butoxycarbonyl-N-methyl-l-tyrosyloxy)-6-methyltetradecanoic Acid (33)
BAIB (0.86 g, 0.27 mmol) and TEMPO (0.21 mg, 0.13 mmol) were added sequentially to a stirred solution of alcohol 32 (50 mg, 0.09 mmol) in acetonitrile phosphate buffer solution (pH 7) (1:1, 2 mL) at r.t. and the mixture was stirred for 2 h. After completion of reaction, saturated aqueous 1 M Na2S2O3 (5 mL) and Et2O (10 mL) were added and the organic layer was separated. The organic layer was washed with saturated aqueous NaHCO3 (5 mL), brine (5 mL), dried over anhydrous Na2SO4, filtered, and evaporated under reduced pressure. The crude residue was purified by silica gel column chromatography using petroleum ether/EtOAc (6:4) to give acid 33 (41 mg, 81%) as a colorless liquid.
Rf
= 0.5 (80% EtOAc/hexane); [α]
d
25 2.5 (c = 0.6, CHCl3).
IR: 3539, 3423, 1712, 1463, 1255, 1054 cm–1.
1H NMR (500 MHz, CDCl3): δ = 6.9–6.78 (m, 2 H), 6.23–6.14 (m, 2 H), 5.06–4.83 (m, 2 H), 2.76, 2.73 (s, 3 H), 2.59–2.50 (m, 1 H), 2.38–2.30 (m, 3 H), 2.20–2.03 (m, 2 H), 1.48–1.40 (m, 12 H), 1.33–1.25 (m, 15 H), 0.89–0.84 (m, 7 H).
13C NMR (126 MHz, CDCl3): δ = 185.1, 176.9, 170.8, 156.3, 150.7, 149.6, 149.5, 130.0, 129.0, 128.3, 127.7, 81.3, 81.2, 81.1, 68.5, 55.1, 40.1, 37.2, 36.0, 33.9, 31.9, 31.6, 29.7, 28.3, 28.2, 26.1, 24.9, 22.5, 19.8, 14.0.
HRMS (ESI): m/z [M + H]+ calcd. for C30H50NO7: 536.1669; found: 536.1655.
(3S,10R,13R)-3-(4-Hydroxybenzyl)-4,10-dimethyl-13-pentyl-1-oxa-4-azacyclotridecane-2,5-dione (8)
(3S,10R,13R)-3-(4-Hydroxybenzyl)-4,10-dimethyl-13-pentyl-1-oxa-4-azacyclotridecane-2,5-dione (8)
To a solution of compound 33 (50 mg, 0.093 mmol) in CH2Cl2 (4.0 mL) at 0 °C was added TFA (0.14 mL, 1.86 mmol). The reaction mixture was stirred for 1 h at r.t.; at that time, TLC analysis showed complete consumption of starting material. The reaction was concentrated in vacuo to afford a red oil that was subsequently dissolved in CH2Cl2 (4 mL) and cooled to 0 °C before the sequential addition of N,N-di-isopropylethylamine (0.09 mL, 0.55 mmol) and BOP-Cl (72 mg, 0.13 mmol). The reaction mixture was stirred at r.t. for 16 h, then it was concentrated and saturated aqueous NaHCO3 (2 mL) was added. The aqueous layer was extracted with CH2Cl2 (2 × 4 mL) and the combined organic extracts were dried over Na2SO4, filtered, evaporated, and the product was purified by silica gel chromatography (petroleum ether/EtOAc, 80:20) to afford 8 (31 mg, 80% over 2 steps) as a colorless oil.
Rf
= 0.5 (40% n-hexane/EtOAc).
Preparation of (–)-Melearoride A (1)
Preparation of (–)-Melearoride A (1)
A mixture of compound 8 (20 mg, 0.047 mmol), prenyl bromide (0.027 mL, 0.235 mmol), cesium carbonate (0.75 g, 5.4 mmol) and DMF (4 mL) was heated under reflux for 4 h. After completion of the reaction, the solid was filtered off and the solvent was evaporated. The residue obtained was purified by silica gel column chromatography (petroleum ether/EtOAc, 70:30 as eluent) to give melearoride-A (1) (19 mg, 85%) as a colorless oil.
Rf
= 0.2 (60% EtOAc/hexane); [α]D
20 –92.5 (c = 0.5, CH3OH); [α]D
25 –95.5 (c = 0.48, CH3OH).
IR (neat): 2928, 2862, 1734, 1647, 1510, 1388, 1232, 1078, 823 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.15–7.07 (m, 2 H), 6.84–6.80 (m, 2 H), 5.79 (t, J = 7.4 Hz, 1 H), 5.48 (t, J = 6.4 Hz, 1 H), 4.87–4.83 (m, 1 H), 4.57 (dd, J = 9.5, 5.7 Hz, 0.2 H), 4.46 (m, 2 H), 3.48–3.16 (m, 2 H), 3.02–2.70 (m, 3 H), 2.42–2.12 (m, 2 H), 1.79 (s, 3 H), 1.73 (s, 3 H), 1.58–1.08 (m, 19 H), 0.90–0.80 (m, 6 H).
13C NMR (101 MHz, CDCl3): δ = 174.0, 173.4, 171.2, 170.3, 158.0, 157.6, 138.3, 138.0, 130.1, 129.6, 128.5, 128.2, 119.8, 119.6, 115.0, 114.6, 75.9, 75.3, 74.6, 67.5, 64.8, 61.7, 55.5, 40.4, 35.2, 35.0, 34.7, 34.1, 33.8, 33.7, 33.0, 32.0, 31.7, 30.7, 30.2, 29.7, 29.5, 29.4, 29.1, 29.0, 28.8, 28.4, 28.3, 27.9, 26.6, 25.9, 25.3, 25.1, 24.8, 24.1, 23.8, 23.5, 23.0, 22.7, 22.5, 20.6, 20.1, 18.2, 14.0.
HRMS (ESI): m/z [M + H]+ calcd. for C30H48NO4: 486.3583; found: 486.3576.
Synthesis of PF1163 B (4)
Synthesis of PF1163 B (4)
A mixture of 8 (15 mg, 0.035 mmol), cesium carbonate (0.75 g, 0.071 mmol), 2-bromoethoxy tert-butyldimethylsilane (33 mg, 0.14 mmol), potassium iodide (0.58 mg, 0.003 mmol) and DMF (3 mL) was heated under reflux for 4 h. After completion of reaction, the solid was filtered off and the solvent was evaporated. The residue was filtered through a short pad of silica gel, washing with EtOAc, and concentrated to afford a colorless oil (15 mg, 75%) that was taken forward to the next step without further purification.
A magnetically stirred solution of the crude silyl ether (15 mg, 0.026 mmol) in THF (3 mL) at 0 °C was treated with TBAF (1.0 M solution in THF, 0.104 mL, 0.104 mmol). Stirring was continued for 20 min and then the reaction mixture was warmed to r.t. and stirring was continued for a further 8 h. The reaction was quenched with ice cold water (5 mL), and the mixture was extracted with EtOAc (5 mL), dried over Na2SO4, filtered and concentrated under reduced pressure. The residue thus obtained was purified by silica gel column chromatography (petroleum ether/EtOAc, 70:30) to afford PF1163B 4 (10 mg, 86%) as a colorless oil.
Rf
= 0.5 (60% EtOAc/ hexane); [α]D
20 –85.24 (c = 0.4, CH3OH).
IR (neat): 3431, 3368, 2939, 2865, 1734, 1634, 1513, 1395, 1248, 1181 cm–1.
1H NMR (400 MHz, CDCl3): δ = 7.18–7.09 (m, 2 H), 6.86–6.81 (m, 2 H), 5.81–5.77 (m, 0.5 H), 4.92–4.79 (m, 1 H), 4.58–4.55 (m, 0.2 H), 4.06–4.04 (m, 2 H), 3.95 (m, 2 H), 3.74–3.16 (m, 2 H), 3.04–2.71 (m, 3 H), 2.41–1.96 (m, 3 H), 1.47–1.05 (m, 20 H), 0.90–0.80 (m, 6 H).
13C NMR (126 MHz, CDCl3): δ = 173.9, 173.5, 170.3, 157.3, 130.2, 129.7, 129.2, 114.9, 114.5, 76.0, 75.4, 74.7, 69.1, 61.5, 55.5, 34.1, 33.0, 31.7, 30.7, 29.7, 29.0, 26.5, 25.1, 23.5, 23.0, 22.5, 21.9, 20.6, 14.0.
HRMS (ESI): m/z [M + H]+ calcd. for C27H45NO5: 462.3297; found: 462.3216.
