Cyclic Sulfur and Carbonate Derivatives

One attractive feature of the 2,3-dihydroxy groups present in tartaric acid derivatives is the fact that they can be incorporated into cyclic derivatives in which the protecting group also maintains a reactive center, as already discussed in the case of the orthoester protected tartrates. Cylic carbonates and cyclic sulfur compounds of tartaric acids offer additional opportunities for exploitation of the chirality of the tartaric acids in the preparation of very useful chiral intermediates. 

Of the two enantiomeric malic acid methyl esters, the 7 -isomer 539 is considered less accessible and considerably more costly. Several attempts at its synthesis have been made [170,171,172], but a very expedient two-step synthesis from dimethyl (+ )-(i ,i )-tartrate (la) provides (i )-dimethylmalate (539) in 67% yield. Treatment of lb with thiocarbonyldiimidazole affords in 76% yield the thionocarbonate 538 which, when treated with tri-n-butyltin hydride furnishes, after facile chromatographic purification, multigram quantities of pure 539 [173]. 

OH MeOOC. X. COOMe OH MeOOC COOMe TCDI, THF Hi- H n-BuaSnH OH, MeOOC. 

As an aside, it should also be noted that a highly efficient deoxygenation of tartrate diesters via a samarium iodide-induced electron transfer process allows direct conversion to 539. Ethylene glycol, presumably due to its modest acidity and strong coordinating ability with the samarium cation, provides the best results [174]. 

2-Cyclic sulfates 540a-c are conveniently prepared in good yields by a two-step, one-pot transformation of the appropriate diester la or lb with thionyl chloride followed by ruthenium tetroxide oxidation. They behave like epoxides in that they simultaneously activate and protect adjacent functionalized carbon atoms from nucleophilic attack. As a consequence of their cyclic nature, they render competing elimination processes stereochemically unfavorable [175]. 

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