Solid-phase Synthesis of Oligosaccharides Using Novel Alkyne-type Linkers: Selection of Reactive Sites on the Support by Sonogashira Reaction

Abstract

Two new alkyne-type linkers having alkynylmethyloxy moieties were elaborated for solid-phase synthesis of oligosaccharides. A propargyl glycoside-type linker between a sugar residue and a solid support was formed by the Sonogashira reaction of a propargyl glycoside with polymer-supported iodobenzene. A pro­pargyl ester-type linker was also constructed by the same reaction of a 4-(proparygyloxycarbonyl)benzyl glycoside with the latter. Both linkers are stable against acids such as TFA but can be readily cleaved with this acid after conversion to the corresponding alkyne-cobalt complex by treatment with Co₂(CO)₈. The latter ester linker is generally advantageous in that mild cleavage liberates a product as its carboxybenzyl glycoside which is readily purified. The Sonogashira reaction was found to proceed only at spatially reactive sites on the solid support where the reagent accesses readily, so that the subsequent reactions including glycosylation on solid phase proceeded smoothly to result in high total yields of the desired oligosaccharides.

oligosaccharides - solid-phase synthesis - carbohydrates - glycosylations - cross-coupling

References

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12 Commercially available from Argonaut Technologies, San Carlos, California (http://www.argotech.com/resins/index.htm). For investigations of other highly crosslinked macroporous resins, see: Hori M. Gravert DJ. Wentworth P. Janda KD. Bioorganic Med. Chem. Lett.  1998,  8:  2363 

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14 Fukase Y. Fukase K. Kusumoto S. Tetrahedron Lett.  1999,  40:  1169 

15 Izumi M. Fukase K. Kusumoto S. Biosci. Biotechnol. Biochem.  2002,  66:  211 

Sonogashira coupling on solid support:

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17

Commercially available from Mimotopes Pty. Ltd. (http://www.mimotopes.com). Solid-phase oligosaccharide synthesis on SynPhase Crown see ref. [3b] and ref. [7]

18

3: Mp: 98 °C; [α]_D ²² = +13 (c 1.10, CHCl₃); ESI-Mass (positive) m/z 509.3 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 7.48-7.24 (15 H, m, PhCH × 3), 5.54 (1 H, s, PhCH), 5.05 (1 H, d, J = 3.0 Hz, H-1), 4.93-4.73 (4 H, m, PhCH₂ × 2), 4.51 (1 H, t, J = 9.0 Hz, H-3), 4.30 (2 H, d, J = 3.0 Hz, OCH₂-CCH), 4.26 (1 H, dd, J = 10.4, 5.2 Hz, H-6a), 3.88-3.86 (1 H, m, H-5), 3.75-3.72 (2 H, m, H-2 and H-4), 3.62 (1 H, d, J = 5.2 Hz, H-6b), 2.47 (1 H, t, J = 3.0 Hz, OCH₂-CCH). Found: C, 73.79; H, 6.11%. Calcd for C₃₀H₃₀O₆: C, 74.06; H, 6.21%.

19

5: Mp: 162 °C; [α]_D ²² = +37 (c 1.00, CHCl₃); ESI-Mass (positive) m/z 629.3 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 8.00 (2 H, m, CC₆H₄CO₂Me), 7.50-7.21 (17 H, m, PhCH × 3 + CC₆H₄CO₂Me), 5.56 (1 H, s, PhCH), 5.10 (1 H, d, J = 3.6 Hz, H-1), 4.93-4.73 (4 H, m, PhCH₂ × 2), 4.54 (2 , d, J = 3.0 Hz, OCH₂-CCH), 4.26 (1 H, dd, J = 10.3, 4.9 Hz, H-6a), 4.09 (1 H, t, J = 9.1 Hz, H-3), 3.95-3.92 (1 H, m, H-5), 3.92 (3 H, s, CC₆H₄CO₂Me), 3.71 (1 H, d, J = 10.3 Hz, H-6b), 3.65-3.61 (2 H, m, H-2 and H-4). Found: C, 72.92; H, 5.75%. Calcd for C₃₈H₃₆O₈·1/2H₂O: C, 72.48; H, 5.92%.

20

6: ESI-Mass (negative) m/z 605.2 [(M - H)^-]; ¹H NMR (CDCl₃) δ = 8.00 (2 H, m, CC₆H₄CO₂H), 7.50-7.21 (17 H, m, PhCH × 3 + CC₆H₄CO₂H), 5.56 (1 H, s, PhCH), 5.10 (1 H, d, J = 3.6 Hz, H-1), 4.93-4.73 (4 H, m, PhCH₂ × 2), 4.54 (2 H, d, J = 3.0 Hz, OCH₂-CCH), 4.26 (1 H, dd, J = 10.3, 4.9 Hz, H-6a), 4.09 (1 H, t, J = 9.1 Hz, H-3), 3.95-3.92 (1 H, m, H-5), 3.71 (1 H, d, J = 10.3 Hz, H-6b), 3.65-3.61 (2 H, m, H-2 and H-4). Found: C, 72.24; H, 5.78%. Calcd for C₃₇H₃₄O₈·1/2H₂O: C, 72.18; H, 5.73%. A typical procedure for introduction of a monosaccharide 6 on solid support. ArgoPore resin (NH₂-LL: 0.28 mmol/g) (100mg, 28.0 µmol) was placed in a polypropylene tube (Varian) fitted with a filter, and washed with 5% diisopropylamine in CH₂Cl₂ and then CH₂Cl₂. Compound 6 (38.9 mg, 56.0 µmol), HOBt (18.9 mg, 140 µmol), CH₂Cl₂ (3.0 mL), and DIC (8.8 µL, 56.0 µmol) were added to the tube, successively. The reaction mixture was shaken for 3 d with Rotator RT-50 (Taitech) and filtered. The resin was washed with CH₂Cl₂ and the residual amino groups on the resin were then capped with acetic anhydride (1.0 mL) and triethylamine (1.0 mL) in CH₂Cl₂ (2.0 mL) by shaking for 30 min. The resin was washed successively with DMF, MeOH, and CH₂Cl₂.

21

A typical procedure for introduction of 4-iodobenzoic acid 8 on solid support. SynPhase^TM resin (NH₂-HL: 35.0 µmol) was placed in a polypropylene tube (Varian) fitted with a filter, and washed with 5% diisopropylamine in CH₂Cl₂ and then CH₂Cl₂. Compound 8 (17.4 mg, 70.0 µmol), HOBt (23.6 mg, 175 µmol), CH₂Cl₂ (3.0 mL), and DIC (11.0 µL, 70.0 µmol) were added to the tube, successively. The reaction mixture was shaken for 3 d with Rotator RT-50 (Taitech) and filtered. The resin was washed with CH₂Cl₂ and the residual amino groups on the resin were then capped with acetic anhydride (1.0 mL) and triethylamine (1.0 mL) in CH₂Cl₂ (2.0 mL) by shaking for 30 min. The resin was washed successively with DMF, MeOH, and CH₂Cl₂.

22

A typical procedure for Sonogashira coupling of monosaccharide 3 on solid support. SynPhase^TM resin 9 (NH₂: 35.0 µmol) was placed in a polypropylene tube (Varian) fitted with a filter, and washed with THF. CuI (2.6 mg, 14.0 µmol), Pd(PPh₃)₄ (8.1 mg, 7.0 µmol), THF (2.5 mL), TEA (2.5 mL) and 3 (34.1 mg, 70.0 µmol) were added to the tube, successively. The reaction mixture was shaken for 24 h with Rotator RT-50 (Taitech) and filtered. The resin was washed with DMF, MeOH and CH₂Cl₂ to give 7.

23

A typical cleavage reaction of alkynylmethyloxy linker: The ArgoPore^TM resin 7 (100 mg of ArgoPore^TM resin) was shaken with a mixture of Co₂(CO)₈ (14.4 mg, 42.0 µmol) in CH₂Cl₂ (3.0 mL) at room temperature for 1 h. After the reaction mixture was filtered, the resin was washed with DMF and CH₂Cl₂ (3.0 mL). The resin was shaken with TFA (0.5 mL) in CH₂Cl₂ (4.0 mL) and water (0.5 mL) at room temperature for 12 h and then filtered. The resin was then washed with ethyl acetate. The organic layer was combined, washed with saturated NaHCO₃ solution and brine, dried over MgSO₄, and concentrated in vacuo. The residue was purified with preparative silica gel TLC (CHCl₃-MeOH = 5:1) to give colorless solid 10. Yield 5.1 mg (75%). ESI-Mass (positive) m/z 383.1 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 7.35-7.24 (10 H, m, PhCH × 2), 5.24 (1 H, d, J = 3.63 Hz, H-1), 4.93-4.63 (4 H, m, PhCH₂ × 2), 4.26 (1 H, dd, J = 10.4, 5.2 Hz, H-6a), 3.84-3.73 (2 H, m, H-6b), 3.71-3.62 (1 H, m, H-5), 3.57-3.52 (2 H, m, H-2 and H-4), 2.19 (2 H, d, J = 8.3 Hz, OH × 2).

24 Gisin BF. Anal. Chim. Acta  1972,  58:  248 

25

The typical procedure for glycosylation on solid-support: The monosaccharide resin 11 (62% loading) (100 mg of ArgoPore^TM resin) was washed with dry CH₂Cl₂ (3 mL). To the resin were added Molecular Sieves 4A beads, 8-12 mesh (200 mg), a solution of a glycosyl trichloroacetimidate 12 (38.9 mg, 56.0 µmol) in dry CH₂Cl₂ (3.0 mL), and TMSOTf (8.8 µL, 56.0 µmol), successively. The reaction mixture was shaken with Rotator RT-50 (Taitech) at room temperature for 3 h. The solution was removed by filtration and the resin was washed with CH₂Cl₂ and ether. After Molecular Sieves 4Å beads were removed by picking with forceps, the resins were washed with DMF and CH₂Cl₂.

26

14: ESI-Mass (positive) m/z 905.4 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 7.34-7.21 (30 H, m, PhCH × 6), 5.01 (1/2 H, d, J = 3.2 Hz, H-1α), 4.93-4.43 (14 H, m, PhCH₂ × 6, H-1′α, H-1′β, H-1β), 3.84-3.73 (4 H, m, H-2, H-3, H-3′, and H-4′), 3.76-3.62 (4 H, m, H-4, H-5, H-2′, and H-5′), 3.57-3.37 (4 H, m, H-6, and H-6′), 2.50 (2 H, s, OH × 2).

27a DeNinno MP. Etienne JB. Duplantier KC. Tetrahedron Lett.  1995,  36:  669 

27b Debenham SD. Toone EJ. Tetrahedron: Asymmetry  2000,  11:  385 

28

17: ESI-Mass (positive) m/z 541.1 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 7.48-7.24 (15 H, m, PhCO × 2 and PhCH₂), 5.94-5.88 (1 H, m, OCH₂CH=CH₂), 5.75 (1 H, d, J = 9.6 Hz, H-2), 5.37-5.26 (2 H, m, OCH₂CH=CH₂), 5.24 (1 H, t, J = 9.6 Hz, H-3), 5.05 (1 H, d, J = 3.6 Hz, H-1), 4.93-4.73 (4 H, m, PhCH₂ and OCH₂CH=CH₂), 4.26 (1 H, dd, J = 10.4, 5.2 Hz, H-6a), 3.88-3.86 (1 H, m, H-5), 3.75-3.72 (1 H, m, H-4), 3.62 (1 H, d, J = 5.2 Hz, H-6b).

29

18: ESI-Mass (positive) m/z 763.2 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 8.00-7.24 [24 H, m, PhCO × 2, PhCH₂, and C₁₃H₉-CH₂-OCO(Fmoc)], 5.94-5.88 (1 H, m, OCH₂CH=CH₂), 5.75 (1 H, d, J = 9.6 Hz, H-2), 5.56 (1 H, dd, J = 9.6, 7.9 Hz, H-4), 5.37-5.26 (2 H, m, OCH₂CH=CH₂), 5.24 (1 H, t, J = 9.6 Hz, H-3), 5.05 (1 H, d, J = 3.6 Hz, H-1), 4.93-4.73 (4 H, m, PhCH₂ and OCH₂CH=CH₂), 4.26 (1 H, dd, J = 10.4, 5.2 Hz, H-6a), 3.94 [1 H, d, J = 10.3 Hz, C₁₃H₉-CH₂-OCO(Fmoc)], 3.88-3.86 (1 H, m, H-5), 3.75-3.72 (1 H, m, H-4), 3.62 (1 H, d, J = 5.2 Hz, H-6b).

30

20: Mp: 144 °C; [α]_D ²⁴ = +134 (c 1.00, CHCl₃); ESI-Mass (positive) m/z 895.2 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 8.01 (2 H, m, OCH₂C₆H₄CO₂CH₂-CCH), 7.48-7.25 (17 H, m, PhCH₂ × 3, and OCH₂C₆H₄CO₂CH₂-CCH), 4.71 (1 H, d, J = 8.6 Hz, H-1), 4.88-4.45 (8 H, m, PhCH₂ × 3 and OCH₂C₆H₄CO₂H), 4.51 (1 H, t, J = 9.1 Hz, H-3), 4.26 (1 H, dd, J = 10.4, 5.2 Hz, H-6a), 3.91 (2 H, d, J = 3.0 Hz, OCH₂C₆H₄CO₂CH₂-CCH), 3.88-3.86 (1 H, m, H-5), 3.72 (1 H, d, J = 10.3 Hz, H-6b), 3.65-3.57 (2 H, m, H-2 and H-4), 2.47 (1 H, t, J = 3.0 Hz, OCH₂C₆H₄CO₂CH₂-CCH).

31

A typical procedure for solid-phase glycosylation using a glycosyl trichloroacetimidate: The 4-O-Fmoc resin 21 (21% loading) (SynPhase^TM-NH₂; 7.7 µmol) was washed with CH₂Cl₂ (3.0 mL). To the resin was added 25% piperidine in CH₂Cl₂ (3.0 mL). The reaction mixture was shaken with Rotator RT-50 (Taitech) at room temperature for 30 min. The solution was removed by filtration and the resin was washed with CH₂Cl₂, DMF and MeOH. The resin was then washed with 5% TMSOTf in dry CH₂Cl₂ and then dry CH₂Cl₂ (3 mL). Trichloroacetimidate 19 (19.5 mg, 23.1 µmol), dry CH₂Cl₂ (3.0 mL), and TMSOTf (0.3 µL, 1.5 µmol) were added. The reaction mixture was shaken with Rotator RT-50 (Taitech) at room temperature for 30 min to give resin-linked disaccharide 32.
Typical procedure of cleavage from solid support by alkali. SynPhase resin linked with disaccharide via alkynyl ester linker was shaken with a mixture of 28% CH₃ONa in MeOH (1.0 mL), MeOH (1.0 mL) and CH₂Cl₂ (2.0 mL) at room temperature for 12 h. After the reaction mixture was filtered, EtOAc and 0.1 N aqueous HCl were added to the filtrate. The organic layer was combined, washed with saturated NaHCO₃ solution and brine, dried over MgSO₄, and concentrated in vacuo. The residue was purified with preparative silica gel TLC (CHCl₃-MeOH = 5:1) to give the desired compound.
34: Yield 7.2 mg (quant.). ESI-Mass (positive) m/z 945.5 [(M + Na)⁺]; ¹H NMR (CDCl₃) δ = 7.84-7.21 (19 H, m, PhCH₂ × 3 and OCH₂C₆H₄CO₂Me), 4.83-4.43 (10 H, m, PhCH₂ × 3, OCH₂C₆H₄CO₂Me, H-1β, H-1′β, and H-1′′β), 3.93 (3 H, s, OCH₂C₆H₄CO₂Me), 3.84-3.60 (12H, m, H-2, H-3, H-4, H-5, H-2′, H-3′, H-4′, H-5′, H-2′′, H-3′′, H-4′′ and H-5′), 3.57-3.37 (4 H, m, H-6, H-6′, and H-6′′), 2.50 (4 H, s, OH × 7).

32 Fukase Y. Zhang S.-Q. Iseki K. Oikawa M. Fukase K. Kusumoto S. Synlett  2001,  1693