Cobalt-Catalyzed Cross-Coupling of 1-Bromo Glycosides and Grignard Reagents

Key words

C-glycosides - cobalt - Grignard reagents

Significance

Numerous metal-catalyzed cross-coupling methods to form anomeric C–C bonds exist, which are important for the synthesis of carbohydrate analogues such as C-glycosides (see Review below). However, β-elimination is a major drawback of these reactions. The authors report a new diastereoselective cobalt-catalyzed cross-coupling between 1-bromo glycosides and aryl and alkenyl Grignard reagents with moderate to good α-selectivity.

Review

L. Somsák Chem. Rev. 2001, 101, 81–136.

Comment

The authors report that there was good α-selectivity for the cross-coupling reaction with mannose and galatose derivatives, but lower α/β ratios for glucose derivatives. Like most cobalt-catalyzed cross-coupling reactions, the stereoselectivity of this reaction supports a radical pathway. Treatment of a δ-olefinic 1-bromoglycoside produced an epimeric mixture of the bicyclic product, which would result from the formation of an anomeric radical that leads to a 5-exo-trig cyclization followed by cross-coupling with PhMgBr.