Synthesis and conformational analysis of 6-C-methyl-substituted 2-acetamido-2-deoxy-beta-D-glucopyranosyl mono- and disaccharides

Abstract

Several 6-C-substituted 2-acetamido-2-deoxy-beta-D-glucopyranosides (beta-D-GlcNAc monosaccharides 1a-3a and 1,4-linked disaccharides 1b-3b) were studied by solution NMR spectroscopy. Conformational analysis of the (6S)- and (6R)-C-methyl-substituted beta-d-GlcNAc monosaccharides indicates that the stereodefined methyl groups impose predictable conformational biases on the exocyclic C-5-C-6 bond, as determined by (1)H-(1)H and (13)C-(1)H coupling constants. Variable-temperature NMR experiments in methanol-d(4) were performed to determine DeltaDeltaH and DeltaDeltaS values derived from the two lowest energy conformers. These indicate that while the influence of 6-C-methyl substitution on conformational enthalpy is in accord with the classic principles of steric interactions, conformational preference in solution can also be strongly affected by other factors such as solvent-solute interactions and solvent reorganization.

SCHEME 1

Synthesis of 6-C-Substituted Monosaccharides^{a a} Reagents and conditions: (a) (i) AcOH, THF:H₂O, 45 °C, (ii) TBS-Cl, Et₃N, imidazole, CH₂Cl₂:THF (93%, two steps); (b) (i) dihydropyran, PPTS, CH₂Cl₂, (ii) n-Bu₄NF, THF (76%, two steps); (c) 7: (i) (COCl)₂, DMSO, CH₂Cl₂, −78 °C, Et₃N, 0 °C (62%), (ii) NaBD₄, CH₂Cl₂:MeOH, −10 °C (6S:6R 2:1), (iii) p-TsOH, MeOH (46%, two steps); (d) 8: (i) (COCl)₂, DMSO, CH₂Cl₂, −78 °C, Et₃N, 0 °C, (ii) AlMe₃, CuCN, THF, −55 °C to rt (6S:6R 6:1), (iii) p-TsOH, MeOH (25%, three steps); (e) 9: (i) and (ii) same as (d), (iii) (COCl)₂, DMSO, CH₂Cl₂, −78 °C; Et₃N, 0 °C (81%), (iv) ZnCl₂, i-Bu₂AlH, THF, −78 °C (6S:6R 1:6), (v) p-TsOH, MeOH (44%, two steps); (f) (i) (CH₂NH₂)₂, n-BuOH, 100 °C, (ii) Ac₂O, pyridine (10: 74%, 11: 87%, 12: 93%, two steps); (g) NaOMe, MeOH:CH₂Cl₂ (1:1, v/v) (1a: 71%, 1b: 98%, 1c: 100%). Selected acronyms: All = allyl, Phth = phthalimido, PMP = p-methoxyphenyl, TBS = tert- butyldimethylsilyl, THP = tetrahydropyranyl.

SCHEME 2

Synthesis of 6-C-Substituted Disaccharides^{a a} Reagents and conditions: (a) 13: TBS–Cl, Et₃N, imidazole, CH₂Cl₂:THF (89%); (b) 14, 15: (i) p-MeOC₆H₄CH(OMe)₂, camphorsulfonic acid, 4A mol sieves, toluene, 90 °C, (ii) NaBH₃CN, HCl, THF:Et₂O, 4A mol sieves, −30 °C (14: 43%, 15: 27%, two steps); (c) 16 (2 equiv), TMSOTf, 4A mol sieves, CH₂Cl₂, −30 °C (17: 51%, 18: 98%, 19: 78%); (d) 1b: n-Bu₄NF, THF; (e) 2b, 3b: DDQ, t-BuOH, pH 7 buffer, CH₂Cl₂; (f) (i) (CH₂NH₂)₂, n-BuOH, 100 °C, (ii) Ac₂O, pyridine, (iii) NaOMe, MeOH:CH₂Cl₂ (1b: 99%, 2b: 79%, 3b: 71%, four steps). Selected acronyms: All = allyl, Phth = phthalimido, PMB = p-methoxybenzyl, TBS=tert-butyldimethylsilyl.

FIGURE 1

Staggered gt, tg, and gg conformations of 6-C-substituted β-d-GlcNAc derivatives 1–3.

FIGURE 2

(6R)-C-Methyl-substituted glucosamine in Geneticin (G418).

FIGURE 3

Parametrized Karplus relationships describing C-5-C-6 conformations as a function of ³J_5,6, based on the empirical formulations of Haasnoot et al. C-5 hydroxymethyl and C-5 hydroxyethyl conformations were evaluated by using Karplus curves parametrized for 1,2-dialkoxypropanes (left) and 1,2-dialkoxybutanes (right), respectively.

FIGURE4

Potential energy curves for 6-C-substituted β-d-GlcNAc derivatives as a function of dihedral angle. Gas-phase conformational energies were calculated by using molecular mechanics (Amber) with ε = 1 (a–c); conformations in methanol were simulated by using ε = 32.6 (d–f).

FIGURE 5

Van't Hoff plots describing conformational preferences of the C-5–C-6 bond for 6-C-substituted β-d-GlcNAc derivatives. (a) gg vs gt for 6-C-d-substituted β-d-GlcNAc monosaccharide 1a (filled circles) and disaccharide 1b (open squares). (b) gg vs tg for (6S)-C-methyl-substituted β-d-GlcNAc monosaccharide 2a, with tg = gt (filled circles) or tg > gt (open squares). (c) gg vs tg for (6S)-C-methyl-substituted β-d-GlcNAc disaccharide 2b, with tg = gt (filled circles) or tg > gt (open squares). (d) gt vs tg for (6R)-C-methyl-substituted β-d-GlcNAc 3a, with tg = gg (filled circles) or tg > gg (open squares).