Organotitanium-induced stereoselective alkylative endo-cleavage of benzyl pentopyranosides

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 AlMe3 followed by four equivalents of MeTiCl3 generated by mixing TiCl4 and ZnMe2 in a ratio 2:1, or, alternatively, treatment of 1 with two equivalents of 1:1 Me2TiCl2–ZnMe2. Both the yields and diastereoselectivities were comparable with those of the reaction with AlMe3 but the titanium reagents were more reactive and could be applied at much lower temperatures than the aluminium reagent.