A streamlined synthetic approach to the truncated linear trisaccharide fragment of QS-21

Abstract

QS-21, a potent immunostimulatory saponin obtained from Quillaja saponaria Molina, a soapbark tree native to Chile, has undergone extensive study for its broad application as a vaccine adjuvant against various infectious diseases and cancers. The structure of QS-21, which features a linear oligosaccharide moiety, provides a critical attachment site for both the labile acyl side chain and the distinctive sugar unit that defines each major saponin variant. In this study, we present an efficient synthetic approach to the truncated linear trisaccharide fragment of QS-21, circumventing the challenges associated with the synthesis of the rare sugar D-fucose. The synthesis of this linear trisaccharide enables streamlined access to a homogeneous QS-21.

Keywords: QS-21; carbohydrate chemistry; glycosylation; linear trisaccharide; vaccine adjuvant.

FIGURE 1

Structure of QS-21 and its saponin conjugate 1.

Scheme 1

Structure of linear trisaccharide moiety 2 from QS-21 and its retrosynthesis. Ac, acetyl; Bn, benzyl; Tol, p-methylphenyl.

Scheme 2

Synthesis of the monosaccharide building blocks 12 (A), 7 (B), and 5 (C). Reagents and conditions: (a) Ac₂O, DMAP, Et₃N, 0 °C, 3 h, 93%; (b) BnNH₂, THF, 0 °C, 16 h, 68%; (c) NaOMe, MeOH, 0 °C, 1 h; (d) 2,2-DMP, CSA, MeCN, RT, 1 h, 72% over 2 steps; (e) BnOH, BF₃•Et₂O, 0 °C, 16 h, 75%; (f) NaOMe, MeOH, 0 °C to RT, 5 h; (g) 2,2-DMP, CSA, MeCN, RT, 30 min, 48% over two steps; (h) Ac₂O, Et₃N, CH₂Cl₂, 0 °C to RT, 3 h, 76%. DMAP, 4-dimethylaminopyridine; BnNH₂, benzylamine; THF, tetrahydrofuran; Me, methyl; 2,2-DMP, 2,2-dimethoxypropane; CSA, 10-camphorsulfonic acid.

Scheme 3

Synthesis of the common disaccharide donor 4. Reagents and conditions: (a) Cl₃CCN, Cs₂CO₃, RT, 0.5 h; (b) TMSOTf, AW-500, CH₂Cl₂, –78 °C to 0 °C, 5 h, 53%; (c) 2% HCl/MeOH, RT, 16 h; (d) Ac₂O, DMAP, Et₃N, EtOAc, 0 °C, 16 h, 81% over two steps. TMSOTf, trimethylsilyl trifluoromethanesulfonate; EtOAc, ethyl acetate.

Scheme 4

First approach in the synthesis of truncated linear trisaccharide domain 2. Reagents and conditions: (a) NIS, TfOH, AW-500, CH₂Cl₂, 0 °C, 2 h, 76%; (b) 7%–8% Mg(OMe)₂ in MeOH, CH₂Cl₂, 0 °C, 1 h; (c) Et₃N, MsCl, CH₂Cl₂, 0 °C, 1 h, 70% (over two steps); (d) TBAI, KI, DMF, 120 °C, 22 h, 58%; (e) H_2(g), Pd(OH)₂/C, THF/MeOH (10/1), RT, 6 h, 95%; (f) H_2(g), Pd(OH)₂/C, THF/MeOH (1/1), RT, 4 h, 89%; (g) H_2(g), Pd(OH)₂/C, THF/MeOH (10/1), RT, 7 h, 89%. NIS, N-iodosuccinimide; TfOH, trifluoromethanesulfonic acid; MsCl, methanesulfonyl chloride; TBAI, tetra-n-butylammonium iodide; DMF, N,N-dimethylformamide.

Scheme 5

Synthesis of acceptor 10 through pre-glycosylation deoxygenation. Reagents and conditions: (a) Et₃N, MsCl, CH₂Cl₂, 0 °C, 2 h, 63%; (b) TBAI, KI, DMF, 120 °C, 22 h, 72%; (c) H_2(g), Pd(OH)₂/C, THF/MeOH (10/1), RT, 7 h, 95%.

Scheme 6

[2 + 1] glycosylation approach for the synthesis of truncated linear trisaccharide 2 using the pre-glycosylation deoxygenation strategy. Reagents and conditions: (a) NIS, TfOH, AW-500, CH₂Cl₂, 0 °C, 2 h, 95%; (b) H_2(g), Pd(OH)₂/C, THF/MeOH (1/1), RT, 7 h, 89%.

FIGURE 2

1D TOCSY analysis of trisaccharide 19 resolving the complex ¹H NMR spectrum into three resolve spectra corresponding to the ring protons of D-xylose (blue), L-rhamnose (red), and D-fucose (green).