Unusual stereochemistry in the copper-catalysed ring opening of a carbohydrate oxirane with vinylmagnesium bromide

Abstract

The oxirane ring of 4,6-O-benzylidene-1,5 : 2,3-dianhydrp-D-allitol (1) opened regio- and stereo-selectively at C-2 with Me₂CuLi and allylmagnesium chloride in the presence of catalytic amounts of CuI. With vinylmagnesium bromide, however, the ring opening of (1) was regioselective but a mixture of stereoisomers (10) and (12) was obtained corresponding to retention and inversion at C-2. A mechanism for the formation of the anomolous retention product (12) is proposed.