Acylation, stannylene acetals

Monoalkylation of a vic-glycol.1 Selective monoalkylation or monoacylation of the vic-glycol group of dimethyl L-tartrate is possible by conversion to the O-stannylene acetal (1) by reaction with dibutyltin oxide. The acetal is converted selectively to a mono derivative (3) by reaction with an alkyl halide or acyl chloride (excess) and CsF (about 2 equiv.). KF or Bu4NF are less effective than CsF. 

Hodosi, G. Kovac, P., Manipulation of free carbohydrates via stannylene acetals. Preparation of beta-per-O-acyl derivatives of D-mannose, L-rhamnose, 6-0-trityl-D-talose, andD-lyxose. Carbohydr. Res. 1997,303, 239-243. 

As noted above, the prodncts of acylation reactions rearrange in reactions of stannylene acetals derived from terminal 1,2-diols, bnt do not rearrange at room temperatnre for those derived from trans-diols on pyranose rings. In the absence of added nucleophiles, the dibutylstannylene acetal of methyl 4,6-0-benzylidene-Q -D-mannopyranoside yields an 85 15 ratio of the axial benzoate over the eqnatorial benzoate, only consistent with kinetic acylation on the dicoordinate oxygen atom. °... 

Epimerization at C-2 sometimes occurs during dibutylstannylene complex formation. In particular, 1,2-cis P-per-O-acylation for formation of the stannylene acetals of free mannose, rhamnose, and lyxose is accompanied by epimerization [22]. Scheme 16 shows the epimerization process schematically. 

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