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Six T-O-, 3'-O-glycosylated diosgenyl p-D-glucopyranosides (4-9), which have a typical structural pattern of diosgenyl saponins, were synthesized; their synthetic routes are discussed.

KeywordsOne-step reaction, protected D-glucopyranosides with free 2-hydroxyl group An important field in carbohydrate chemistry is to look for regioselective reactions of saccharides to simplify the chemical synthesis of oligosaccharides. One of such efforts nowadays is directed to the convenient preparation of protected Dglucopyranosides having only one free hydroxyl group either on 2-C or on 3-C, because their preparation is usually much more difficult than that of D-glucopyranosides having free 4-C or 6-C hydroxyl group.' Several reported methods' for the preparation of glucosides having free hydroxyl group on 2-C are all multistep procedures involving several steps of regioselective protections and deprotections of the monosaccharide, which makes the synthesis of these intermediates very tedious and low yielding. We wish to report herewith a new, facile method for the syntheses of various protected D-glucopyranosides having free 2-C hydroxyl group in one step starting from 0-peracetylated P-D-glucopyranose (1). The glucosides obtained are important intermediates for the chemical synthesis of numerous natural oligosaccharides containing 1-2 Glc fragment,2 especially in saponins.Reaction of peracetylated monosaccharides with one equivalent of alcohol in the presence of Lewis acid, such as BF3-Et20, TMSOTf and AgC104, generally affords their glyc~sides.~ However, we recently found that when 0-peracetylated ,f3-D-glucopyranose was treated with excess amount of allylic alcohol in the presence of boron trifluoride, the glycosylation reaction was accompanied by deacetylation on 2 -0 , and then afforded a substantial amount of the ally1 glucosides with 2-C hydroxyl group free (2a, Scheme 1). Systematic experimentation on the reaction revealed that optimized yield (63%) of 2a was obtained with a molecular ratio of 1,

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