Key words
antibiotics - teixobactin - depsipeptide - unusual amino acid -
l-
allo-enduracididine - Staudinger reaction - Sharpless asymmetric dihydroxylation
According to WHO, ESKAPE pathogens appear as a major public health concern in hospital acquired infections in critically ill or immunocompromised patients.[1] In early 2015, a novel cyclic depsipeptide teixobactin (1) was isolated from screening of an unculturable β-proteobacteria (Eleftheria terrae) by iChip technique.[2] Teixobactin exhibits excellent activities against Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA, MIC 0.25 μg/mL), vancomycin-resistant Enterococcus species (VRE, MIC 0.5 μg/mL) and Mycobacterium tuberculosis (Mtb, MIC 0.125 μg/mL).[2] Teixobactin works as a lipase II inhibitor, like vancomycin, by not allowing pentapeptide incorporation into glycopeptidic cell wall of bacteria, thus rendering it susceptible to rupture.[3] In addition, 1 is also found to inhibit lipase III, another important component of bacterial cell-wall synthesis. Teixobactin is an undecapeptide and encompasses an unusual amino acid, l-allo-enduracididine[4] (L-allo-End) and four d-amino acids (Figure 1). The structure of teixobactin contains a depsipeptide macrolide and peptide side chain.
Figure 1 Structure of teixobactin (1)
The phenomenal biological activity of 1 prompted research groups to take up the total synthesis[5–9] of teixobactin and analogues[10] to elucidate its pharmacophore[11] towards discovering new antibiotics. So far, five total syntheses of 1 are reported, four solid-phase[5–7],[9] and one solution-phase.[8] The bottleneck in the synthesis of teixobactin is the availability of the unnatural amino acid, l-allo-enduracididine. A careful literature survey revealed easy access to l-allo-enduracididine will help in developing faster and affordable steps in synthesis of 1 and analogues on gram scale. The groups which achieved total[5–9] and partial[12] synthesis have relied on the synthesis of enduracididine either from (2S,3R)-4-hydroxy ornithine (which is obtained from l-aspartic acid)[13] developed by Rudolph et al. and Peoples et al. or from protected trans-hydroxyproline[14] developed by Yuan et al. Recently, Rao and co-workers reported l-allo-End precursor on gram scale via intramolecular guanidinylation followed by alcoholysis.[9]
Our own efforts to complete the total synthesis of teixobactin are hinged on the commercial nonavailability of enduracididine. We have already achieved teixobactin peptide side-chain synthesis in solution phase as well as in solid phase.[15] Thus, we desired to develop an alternate synthesis of l-allo-enduracididine which will be scalable and stereoflexible. Herein, we report the synthesis of this unusual amino acid from (S)-glycidol which is commercially available.
Accordingly, the retrosynthetic analysis envisioned the construction of suitably protected l-allo-enduracididine (Boc-End(Cbz)2-OH, 2) through an intramolecular nucleophilic substitution of guanidine compound 3, which in turn could be achieved from diol 4 through guanidinylation. The diol 4 could be obtained from homoallylic alcohol 5 by Staudinger reaction followed by Sharpless asymmetric dihydroxylation (SAD). The homoallylic alcohol 5 could be synthesized by regioselective ring opening of (S)-glycidol (Scheme 1).
Scheme 1 Retrosynthesis of l-allo-enduracididine (Boc-End(Cbz)2-OH, 2)
Based on the retrosynthetic analysis, (S)-glycidol was converted into 2 (Scheme 2). The primary hydroxyl group of commercially available (S)-glycidol was protected as tert-butyldiphenylsilyl ether (in 95% yield)[16] and regioselective ring opening of epoxide was carried out using a reported procedure which gave homoallylic alcohol 5 in 100 g scale.[16,17] The regioselective opening of epoxide was achieved with CuI catalyst and vinylmagnesium bromide to get alcohol 5 in 96% yield. Mesylation of alcohol 5 followed by azide displacement using NaN3 gave azido pentenol 6 with inversion of configuration at C-2 and 90% yield over two steps. The azide 6 was reduced under Staudinger reaction conditions using TPP in THF–H2O (3:1) in the presence of (Boc)2O to provide N-Boc-protected amine 7 in 92% yield. The second chirality was introduced via Sharpless asymmetric dihydroxylation[18] using AD mix-β and methanesulfonamide in t-BuOH–H2O (1:1) at 0 °C for 20 h to realize the diol 4 in 92% yield as a separable diastereomeric mixture (by silica gel column chromatography) in 7:3 ratio with the required diastereomer being the major isomer. Our plan was to convert this diol into amino alcohol to couple with N,N′-Di-Cbz-1H-pyrazole-1-carboxamidine to introduce guanidine moiety. Initially, the diol 4 was monotosylated in situ with Ts2O/2,4,6-collidine/pyridine in CH2Cl2 at ≤ –10 °C, treated with ammonium hydroxide in EtOH at 60 °C to give amino alcohol via epoxide[19] which on further treatment with N,N′-di-Cbz-1H-pyrazole-1-carboxamidine[5,12] gave guanidine derivative 3 in 52% overall yield for four sequential transformations without purification of intermediates. To improve the yield of guanidine derivative 3 further, we thought of an alternative synthetic sequence. Selective mesylation of primary alcohol in compound 4 with MsCl/Et3N in CH2Cl2 at ≤ –30 °C, followed by treatment with NaN3 in DMF at 70 °C gave azido alcohol 8 in 87% yield. Then, the azide 8 was reduced under Staudinger reaction conditions (TPP, THF–H2O) to provide amino alcohol which on further treatment with N,N′-di-Cbz-1H-pyrazole-1-carboxamidine[5,12] gave the guanidine derivative 3 in 85% yield (Scheme 2).[20]
Scheme 2 Synthesis of l-allo-enduracididine 2
The intramolecular cyclization of 3 via triflate[5,12] using triflic anhydride and N,N-diisopropylethylamine at –78 °C allowed us to construct the enduracididine skeleton 9 in 90% yield.[21] This upon deprotection of silyl group with TBAF in THF afforded alcohol 10 in 95% yield. Finally, the oxidation of the obtained primary alcohol 10 using DMP gave aldehyde which upon Pinnick–Lindgren oxidation using a combination of sodium chlorite and NaH2PO4 in t-BuOH–H2O provided the target building block, l-allo-enduracididine (Boc-End(Cbz)2-OH, 2) in 74% yield over two steps, which is being used to complete the total synthesis of teixobactin. A small portion of the carboxylic acid 2 was converted into the corresponding methyl ester 11 using K2CO3/MeI in 76% yield. The present approach allows the synthesis of l-allo-enduracididine in gram scale due to commercial availability of starting material and simple synthetic operations.
In conclusion, a stereoflexible and scalable synthesis of Boc-End(Cbz)2-OH, an unusual amino acid building block of potent depsipeptide antibiotic teixobactin, has been achieved in ten steps with an overall yield of 22.75%. By changing the stereochemistry of starting material, viz., glycidol and dihydroxylating agent, other diastereomers can be synthesized with equal ease.
