Orthoformic reaction with esters, cyclic

An efficient pinacol rearrangement mediated by trialkyl orthoformate has been developed [83]. The reactions of various types of 1,2-diol with a catalytic amount of SnCU in the presence of trimethyl orthoformate afford the rearranged product in good yields via a cyclic ortho ester intermediate (Eq. 50). This combined system is applicable not only to cyclic and acyclic tri- and tetrasubstituted diols but also to diols forming acid-sensitive acetals. 

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. 

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