Synthetic Studies on Sialoglycoconjugates 60: α-Stereocontrolled, Glycoside Synthesis of Trimeric Sialic Acid with Galactose and Lactose Derivatives

One-pot SSA-catalyzed β-elimination: an efficient and inexpensive protocol for easy access to the glycal of sialic acid

Neu5Ac2en1Me per-OAc, the fully protected glycal of sialic acid, is a key intermediate in the discovery of therapeutics and diagnostics, including anti-influenza drugs and proteolysis resistant peptidomimetic foldamers. The synthesis of this sialic acid derivative, however, still relies on standard sugar chemistry that utilizes multi-step methodologies. Herein we report a facile and highly efficient microwave-assisted preparation of Neu5Ac1Me using silica sulfuric acid (SSA) as solid-supported acid catalyst that is one- to two-orders of magnitude faster than standard procedures. We also describe the microwave-assisted and SSA-catalyzed one-pot, rapid, solvent free reaction that combines both peracetylation and β-elimination reactions in one step to generate the glycal from Neu5Ac1Me. We coined the term One-pot SSA-catalyzed Technology for β-Elimination Protocol (OneSTEP) to describe this least laborious, most efficient, and practical preparation to date of Neu5Ac2en1Me per-OAc in terms of yield, time, reagent cost, and waste generation.

Efficient synthesis of 3'-glycosylated LacNAc-based oligosaccharides

LacNAc-based oligosaccharides, including sialyl-(2-->3)-LacNAc, dimeric sialyl-(2-->3)-LacNAc, trimeric sialyl-(2-->3)-LacNAc, beta-glucuronyl-(1-->3)-LacNAc, and 3-sulfo-beta-glucuronyl-(1-->3)-LacNAc, were synthesized efficiently from a single protected LacNAc derivative having both OH-3' and 4' unprotected.

A synthetic approach to the c-series gangliosides containing sialyl-alpha(2-->8)sialyl-alpha(2-->8)sialic acid: synthesis of ganglioside GT4, alpha(2-->6) GT4 and GT3

Trimeric sialic acid [Neu5Ac alpha(2-->8)Neu5Ac alpha(2-->8)Neu5Ac, 1] residue-containing gangliosides, GT4, alpha(2-->6)GT4 and GT3, have been synthesized for the first time. Methyl [phenyl] 5-acetamido-8-O-[5-acetamido-8-O-(5-acetamido-4, 7, 8, 9-tetra-O-acetyl-3, 5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylono-1", 9'-lactone)-4,7-di-O-acetyl-3,5-dideoxy-D- glycero-alpha-D-galacto-2-nonulopyranosylono-1',9-lactone]-4,7-di- O-acetyl -3,5-dideoxy-2-thio-D- glycero-D-galacto-2-nonulopyranosid]onate (3) was prepared from 1, via lactonization, methyl esterification of the carboxyl group at the reducting end, O-acetylation and conversion of the anomeric acetoxy group into a phenylthio group. Iodonium-promoted glycosylation of 3 with 2-(trimethylsilyl)ethyl 2,6-di-O-benzyl-beta-D-galactopyranoside (5), 2-(trimethylsilyl)ethyl 3-O-benzyl-beta-D-galactopyranoside (6), 2-(trimethylsilyl)ethyl 2-O-benzoyl-3-O-benzyl-beta-D-galactopyranoside (9), and 2-(trimethylsilyl)ethyl 2, 3-di-O-benzyl-beta-D-galactopyranoside (11) gave the corresponding tetrasaccharides (13-15, 17) having the (Neu5Ac)3-Gal structure. The peracylated oligosaccharides 18 and 24 derived from 13 and 17, and the previously reported lactose derivative 29 were converted into the alpha-trichloroacetimidates 20, 26 and 31, and coupled with (2S,3R,4E)-2-azido-3-O-benzoyl-4-octadecene-1,3-diol (21) to afford the corresponding beta-glycosides 22, 27 and 32. These protected azidosphingosine derivatives were each transformed into the target gangliosides GT4, alpha(2-->6)GT4 and GT3 via selective reduction of the azido group, subsequent coupling with octadecanoic acid, O-deacylation and saponification of the methyl ester and lactone groups.