Decarboxylation via O-Acyl Thiohydroxamates

See also in (a) Motherwell, W. B. and Imboden, C., Decarboxylation via O-acyl thiohydroxamates in Radicals in Organic Synthesis Renaud, P. and Sibi, M., Eds., VCH, Weinheim, 2001 (b) Barton, D. H. R. and Parekh, S. 1., Half a Century of Free-Radical Chemistry, Cambridge University Press, Cambridge, U.K., 1993 (c) Crich, D. and Motherwell, W. B., Free Radical Chain Reactions in Organic Synthesis, Academic Press, London, 1992. 

The decarboxylation of peifluoroalkanoic acids ntay also be achieved via the O-acyl thiohydroxamates. When carried out in the presence of electron rich alkenes such as ethyl vinyl ether the perfluoroal-... 

Barton Esterification Reductive Decarboxylation. O-Acyl thiohydroxamates or Barton esters are useful precursors of carbon-centered radicals via thermolysis or photolysis. Several different methods are available for converting carboxylic acids into Barton esters (eq 1). These reactions generally proceed via the attack of a 2-mercaptopyridine-N-oxide salt on an activated carboxylic acid that has either been preformed (acid chloride, mixed anhydride) or generated in situ (with 1,3-dicyclohexylcarbodiimide or tri-n-butylphosphine + 2,2 -dithiodipyridine-l,r-dioxide). However, HOTT has the distinct advantages of (1) being easy to prepare and handle without the need for any special precautions, (2) facilitates efficient Barton esterification of carboxylic acids, and (3) simplifies subsequent work-up and purifications by avoiding the need to remove by-products like 1,3-dicyclohexylurea. 

The photolysis or thermolysis of O-acyl-thiohydroxamates in the presence of appropriate H-donors gives rise to reductive decarboxylation via a free-radical mechanism. Hydrogen donors can be tin hydrides such as BujSnH or PhjSnH, silicon hydrides such as (TMSljSiH, or more generally thiols such as McjCSH or PhSH. The general usefiilness of the reaction has been validated in a number of syntheses.The scope of the reaction is illustrated in Scheme 9, which shows the reduction of a key intermediate in the synthesis of the ( )-kopsidasine. ... 

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