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

A four-step procedure for homologation of methyl α-d-mannofuranoside and α-d-mannopyranoside 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 CH2OR (or oxidation of the CH2SiMe2Ph) group. From methyl α-d-mannosides, stereoisomeric dαd and lαd methyl heptosides and from them, methyl octosides of d-threo- and l-erythro-α-d-manno configuration were obtained.