From methyl mannosides to methyl octosides by a stepwise homologation with Grignard C1 reagents

A four-step procedure for homologation of methyl α- d - manno furanoside and α- d - manno pyranoside was examined. The reactions consisted in (i) oxidation of the terminal hydroxymethyl group in a protected sugar derivative to an aldehyde; (ii) reaction with allyloxymethylmagnesium chloride (or (phenyldimethyl)silylmethyl–magnesium chloride); (iii) protection of the newly formed secondary alcohol group; (iv) deprotection of the terminal CH 2 OR (or oxidation of the CH 2 SiMe 2 Ph) group. From methyl α- d -mannosides, stereoisomeric d α d and l α d methyl heptosides and from them, methyl octosides of d - threo - and l - erythro -α- d - manno configuration were obtained.