Stannylene acetals oxidation

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. 

Selectivity can be an overriding commodity in cases where reactivity is dictated by logic and accepted concepts. Such is the case with stannylene acetals of diols and trialkyl-stannyl ethers of alcohols. Enhanced nucleophilicity of oxygen attached to tin and well-documented stereoelectrorric effects associated with methine carbon atoms of trialkyhin ethers lead to remarkably selective reactions of (7-substitution and oxidation in polyhydroxy compounds. 

Selective O-Substitution and Oxidation Using Stannylene Acetals and Stannyl Ethers... 

X. Kong and T. B. Grindley, An improved method for the regioselective oxidation of stannylene acetals and dimerization of the a-hydroxyketone products, J. Carbohydr. Chem., 12 (1993) 557-571. 

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... 

Selective O-Substitulion and Oxidation Using Stannylene Acetals and Stan-nyl Ethers. David, S. In Preparative Carbohydrate Chemistry, Hanessian, S., Ed. Dekker New York. 1997 p 69. 

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. 

S. David, Selective O-Substitution and Oxidation Using Stannylene Acetals and Stannyl Ethers, in Preparative Carbohydrate Chemistry, S. Hanessian (Ed.), Marcel Dekker, New York, 1996. 

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]. 

The synthesis of sixteen methyl D-hexopyranosid-2-, 3-, or 4-ulosides by bis(tributyltin)-oxide - bromine oxidation of unprotected methyl glycosides has been described. Axial hydroxy groups in l,2-ci,s-diols are preferentially oxidised by this system. The stmctures of some of the products were confirmed by independent syntheses involving oxidation of partially protected derivatives. In related work some carbohydrate 1,2-diol dibutylstannylene acetals have been regiospecifically oxidized by NBS (rather than bromine) to give a-hydroxyketones. A Sn n.m.r. study on the stannylene acetals has shown some of them to be dimers. ... 

Regioselective Oxidation. Following the original discovery by David and Thieffrey, several stannylenes have been oxidized by bromine to acyloins (eq 8). Hanessian employed this reaction for the synthesis of a densely functionalized intermediate which was converted into (-i-)-spectinomycin. A detailed study of monooxidation of unprotected carbohydrates has also appeared. Stannylenes are oxidized the same way as diols, with Sodium Periodate and Lead(IV) Acetate. ... 

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