Thiohydroxamate esters synthesis

Scheme 23. Barton s thiohydroxamate ester chemistry synthesis of alkyl pyridyl sulfides (127).

The Barton decarboxylation procedure was used in the total synthesis of (-)-verrucarol by K. Tadano et al. The initially formed thiohydroxamic ester was decarboxylated to leave a methylene radical on the cyclopentyl ring, which was then trapped by molecular oxygen. Reductive work-up in the presence of f-BuSH finally provided the hydroxylated product. ... 

A doubie Barton radicai decarboxyiation was utilized during the one-step total synthesis of tyromycin A and its analogs by M. Samadi et al. The b/s-thiohydroxamic ester was irradiated in the presence of citraconic anhydride, and the resulting product was stirred for two days at room temperature to ensure complete elimination. 

The last and the seventh synthesis of chaetomellic acid A was reported in 1997 by the Samadi group from CNRS, France [81]. This one step synthesis involves a Barton radical decarboxylation and gave 77% overall yield (Scheme 8). The method requires preparation of a thiohydroxamic ester of the corresponding alkyl acid followed by in-situ irradiation in the presence of citraconic anhydride and silica gel chromatography. The last intermediate in this synthesis is similar to the synthesis reported by Branchaud and Slate [75,76]. 

In the presence of a hydrogen-atom source, such as tributyltin hydride or a thiol (R SH), the alkyl radical is reduced and the reaction is propagated by the chain carrier (BusSn or R S ). Thus, in a synthesis of a segment of the immunosuppressant FK-506, decarboxylation was effected by heating the thiohydroxamic ester 10 with tert-butyl thiol (4.10). ... 

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