Stannylene acetals dibutyltin oxide

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. 

The use of performed dibutyltin dimethoxide is advantageous12-14 reactions of diols are complete in 5-15 min in benzene at reflux. This technique avoids the problems with working in methanol just mentioned, and gives better yields. The preparation of stannylene acetals is also accelerated by microwave irradiation,15 and selective reactions can be performed, albeit in low yields, on some diols and polyols by microwave irradiation in the presence of a catalytic amount of dibutyltin oxide.16,17... 

Hodosi and Kovac observed that, when free sugars are treated with excess dibutyltin oxide in methanol for extended periods of time at temperatures above 60°C, equilibration of the configuration at C-2 occurs.73 This observation led to the efficient formation of 6-O-trityl-D-talose from 6-0-trityl-D-galactose, but also indicates the need for care in the formation of stannylene acetals from free sugars.73... 

Martinelli et al. have discovered that p-toluenesulfonation reactions directly on diols proceed rapidly using catalytic dibutyltin oxide in chloroform at room temperature, only in the presence of a stoichiometric amount of triethylamine or another base. Under conditions where the base is present in less than a stoichiometric amount, the reaction proceeds until the base is consumed and no further. " Sn NMR has been used to show that the tin-containing intermediates are still present as dimers under these conditions. The base must remove by-product HCl, which destroys the stannylene acetal. 

Selective monoprotection of diols is moderately effective. Fluoride ion-promoted monoalkylation of tartrate diesters that have been activated as the stannylene acetal is an extremely effective method for the efficient monoalkylations of diols. An equimolar mixture of dimethyl tartrate (la) and dibutyltin oxide in toluene, heated under reflux to azeotropically remove the formed water, provides a stannylene acetal. The crude acetal in DMF is treated with cesium fluoride and then benzyl bromide to provide in 85% yield the dimethyl (2J ,37 )-2-0-ben-zyltartrate (421). Reaction of la with benzyl bromide and CsF in DMF proceeds to provide 421 in only 5% yield. Benzyl iodide is generally a better alkylating agent, providing 421 in 99% yield [147]. 

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