Photolysis hydroxypyridine-2-thiones

The N-hydroxypyridine-2-thione esters of Boc-protected amino acids undergo decarboxylation on photolysis in the presence of a hydrogen-atom-transfer reagent such as t-butyl mercaptan in 80-95% yield. Decarboxylation of the side-chain carboxy groups of suitably protected aspartic and glutamic acids can also be effected, but yields are lower (50-80%). ... 

The generation and synthetic uses of carbon radicals obtained by photochemically initiated elimination of carbon dioxide from thiohydroxamic esters, and in particular from esters of W-hydroxy-2-thiopyridone, have again been widely reported. Exploratory studies of these W-hydroxypyridine-2-thione esters using laser flash photolysis techniques have been described,and improved... 

In addition to dehydroxylation, a useful protocol for decarboxylation has been developed. The procedure was introduced by Barton, using thiohydroxamic esters 9, prepared from activated carboxylic acids (RCOX) and the sodium salt of N-hydroxypyridine-2-thione. Simple thermolysis or photolysis of the esters (homolysis of the N—O bond) results in the production of alkyl radicals R, which can attack the sulfur atom of the thiocarbonyl group to propagate the fragmentation (4.9). 

Similar to esters derived from N -hydroxypyridine-2(lH)-thione 1, the O-esters of 4-methyl and 4-phenyl N-hydroxythiazoline-2-thiones (7a and 7b) undergo smooth thermal decarboxylative rearrangement in refluxing toluene but require UV photolysis at -320 nm (medium mercury pressure) for the photochemical reaction. 

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