A palladium-catalyzed regio- and diastereoselective diarylation of glycals and pseudoglycals, which is a kind of Heck-Suzuki arylation, is described. A wide range of arylboronic acids reacted under these conditions smoothly. Selectivity was C1-C2(α,α) in the case of glycals but C2-C3(β,β) for pseudoglycals. Quantum chemical analysis has been carried out to establish the reaction mechanism, which may involve Pd(II)/Pd(O). TEMPO plays a key role in the formation of diaryl glycoside due to its radical nature.
A highly stereoselective rapid C-glycosylation reaction has been developed between glycal and unactivated alkynes in the presence of coppertriflate and ascorbic acid at low catalyst loading and at room temperature. A wide variety of glycals and aryl acetylenes participate in the reaction smoothly. TfOH generated during the reduction of Cu(OTf)2 by ascorbic acid may be the active catalyst for the glycosylation.
The palladium‐catalyzed stereoselective synthesis of 2,3‐deoxy‐C‐aryl glycosides was investigated. The strategy is based on the Pd‐catalyzed desulfitative Heck coupling of arylsulfonyl chlorides and glycals with good leaving groups. An attempt was made to establish the reaction mechanism, which may involve PdII/Pd0 interconversion under basic conditions.
Zinc mediated alkynylation reaction was studied for the preparation of C-glycosides from unactivated alkynes. Different glycosyl donors such as glycals and anomeric acetates were tested towards an alkynyl zinc reagent obtained from alkynes using zinc dust and ethyl bromoacetate as an additive. The method provides simple, mild and stereoselective access to alkynyl glycosides both from aromatic and aliphatic acetylenes.
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