SERIAL RADICAL REACTIONS OF GLYCALS - READY ROUTES TO HIGHLY FUNCTIONALIZED C-GLYCOSYL DERIVATIVES

The C3-OH of partially protected glycals can be readily converted into mixed acetals of 2-bromoacetaldehyde or into silylmethylene bromides. Reaction of these derivatives with tri-n-butyltin hydride gives a radical that cyclizes efficiently to generate a stabilized radical at C1. The latter can be trapped very efficiently with acrylonitrile, tert-butyl isocyanide, allyltri-n-butyltin, tributyltin acrylates, or a complex pyranoside enone to afford C-glycosides in which a 1,2 disubstitution has taken place. The stereochemistry of the resulting 1,2 disubstituted product is dependent, at C-2, on the stereochemistry of the C3-OH and at the anomeric center (C-1) upon the interplay of (a) steric effects and (b) the electronic bias of the radical anomeric effect.