Viicouplings Via Glycosyl Isourea Intermediates

Carbodiimide reagents have long been known to be effective dehydrating agents in coupling reactions [52]. Tsutsumi et al. have shown that simple carbodiimide reagents are also effective in the context of dehydrative couplings with 1-hydroxy glycosyl donors (Scheme 15) [53]. Reactions of alcohols with carbodiimides have been known

Scheme 12 Dehydrative glycosylations via glycosyl oxotitanium intermediates [49].

to occur in the presence of CuCl to form O-alkyl isoureas [54]. This transformation was applied to Cl-glycosyl hemiacetals 1 with dicyclohexyl carbodiimide to generate the corresponding O-glycosyl isourea 68, which proceeds to glycosylate appropriate acceptors via expulsion of dicyclohexyl urea from the anomeric center. This one-pot procedure was found to be effective at elevated temperatures (80-85°C) with a variety of phenolic nucleophiles (e.g., 69-72) in addition to several thiophenolic nu-cleophiles (e.g., 73). The high ^-selectivity of the coupling reactions presumably arises from SN2 displacement of the corresponding a-glycosyl isourea intermediate, which is generated at elevated temperatures in the activation stage.

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