Thiocarbonates, reductive elimination

The di-O-tosylates (prepared by action of tosyl chloride in pyridine) are reduced with zinc (Nal/Zn route e Tipson-Cohen reaction) [13]. Cyclic ortho-esters (prepared by reaction of the diol with ethyl orthoformate) are transformed into olefins by simple heating in the presence of acids (Eastwood reaction, route b) [14]. Cyclic thiocarbonates (obtained by reaction of a diol with thiophosgene or (V,(V -thiocarbonyl-di-imidazole) are reduced to olefin with trimethyl phosphite (Corey-Winter method, route c) [15]. Finally, reduction of vicinal di-xanthates with tri- -butyltin hydride according to the Barton procedure [16] affords olefins via a reductive elimination process route a). The Corey-Winter, Garegg, and Tipson-Cohen methods are most commonly applied for deoxygenation of sugar diols. 

Eliminations. Thionocarbonates derived from 1,2-diols are transformed into iodo thiocarbonates with RI, which decompose into alkenes on treatment with RLi. The three-step process represents a reductive deoxygenation. 

The isomerization can occurr in situ, while accomplishing other reactions. Johnson and Poulos [21] in synthesising (Z)-methyl triacont-21-enate (4a), via the reduction of the thiocarbonate 4b or iodo 4c derivatives with (TMS)3SiH, have obtained (JE)-methyl triacont-21-enate. That is, during the reduction process a (Z)-( ) isomerization occurred, by the addition-elimination of (TMS)3Si radical. 

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