Synthesis of Standardized Building Blocks as a β-d-Mannosyl Donors with a Temporary Protection to be 3,6-Di-O-glycosyl Acceptors, for Constructing the Inner Core of Glycoproteins and Artificial Antigens

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Abstract
A synthetic scheme has been achieved for the core block of glycoproteins having D-mannose configuration, which is standardized to be a 3,6-di-O-glycosyl acceptor and react stereoselectively as a β-D-mannosyl donor. The synthesis was started by methyl 2,3-O-isopropylidene-α-D-mannopyranoside, which upon benzylation, deisopropylidenation, and selective acetolysis of the 6-O-benzyl group afforded the corresponding tetraacetate, which could be converted to 3,6-di-O-allyl-4-O-benzyl-1,2-O-(1-methoxyethylidene)-β-D-mannopyranose. The orthoester could be separated into the exo and endo isomers. Hydrolysis of the orthoester ring followed by reaction with mesyl chloride gave 3,6-di-O-allyl-4-O-benzyl-2-O-mesytl-α-D-mannopyranosyl chloride, that upon reaction via its 1-O-trifluoroethanesulfonate with 2-[4-(p-toluenesulfonamido)phenyl]ethanol gave the corresponding glycosides in β:α ratio of 3:1. Deallylation of the β-glycosides gave the diol which are suitable for branching and could react selectively at the 6-position allowing its potential for glycosylation by different glycosyl donors.

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