Alkylation of dibutylstannylene complexes of .3,4,6-tri-g-benzyl-Pmannopyranose, methyl 6-e-trityl-a-E-mannopyranoside and methyl a-pmannopyranoside gives respectively g-g-mannopyranoside and 3-g-alkyl derivatives with high selectivity.
We have been engaged in an investigation of the synthesis of glycopyranosides of high stereoselectivity and yield. The least readily available giycosides are those with a 1 equatorial, 2 axial relationship like the S-_D-mannopyranosides, which have been prepared by Koenigs-Knorr reaction using 4,6-di-c-acetyl-a-~mannopyranosyl bromide 2.,3-carbonate 192 and by stereoselective reduction of glycosides such as methyl S-P arabino-hexopyranosidulose. 3-5 An Important demonstration has been published by Nashed and Anderson6 showing that a cyclic dibutylstannylene derivative of vlclnal equatorial and axial hydroxyl groups selectively activates the equatorial oxygen for e-alkylatlon or acylation. Similar observations have been made by other workers. 7-9
This observation suggested to us an alternative synthesis for S-D_mannopyranosides.
Since mannose derivatives maintain a 4 Cl conformation, the only probable dibutyltin complex with oxygen on C-l and C-2 would involve a 1 eq., 2 ax. relationship. The expected product of 0-alkylation would then be a S-pmannopyranoside.
We now find that dibutyltin oxide forms a complex with 3,4,6-tri-c-benzyl-emannopyranose (l_) as indicated by solution of the tin compound and an +I NMR spectrum with no peaks for OH groups. Reaction of this 1,2-g-dlbutylstannylene complex (2) with methyl iodide (1.5 mole) at 45' in N,IJ-dimethylformamlde followed by chromatography on silica gel with CHC13:MeOH (9:l) yields 94% of methyl 3,4,6-tri-c-benzyl-8-g-mannopyranoside (3); [ali -10.2' (2 0.45, CHC13), 'H NMR data (CDC13): S-anomer,