Organotitanium-induced stereoselective alkylative endo-cleavage of benzyl pentopyranosides

Abstract

The results presented are the first examples where organotitanium reagents induced alkylative endo-cleavage of carbohydrates. The best conditions for the alkylative transfer of a methyl group to benzyl 2-deoxy-2- C-methyl-4- O-( tert-butyldimethylsilyl)- α- d-arabinopyranoside ( 1) were the application of one equivalent of AlMe 3 followed by four equivalents of MeTiCl 3 generated by mixing TiCl 4 and ZnMe 2 in a ratio 2:1, or, alternatively, treatment of 1 with two equivalents of 1:1 Me 2TiCl 2–ZnMe 2. Both the yields and diastereoselectivities were comparable with those of the reaction with AlMe 3 but the titanium reagents were more reactive and could be applied at much lower temperatures than the aluminium reagent.