Acyl radicals, generation, tris silane

Generation of 3-indolylacyl radicals from the selenoesters 149, using either /j-Bu3SnH or tris(trimethylsilyl)silane (TTMSS) followed by reaction with various alkenes, offers a route to 3-acylindoles 150. On the other hand, the use of n-Bu Sn2 under irradiation gave cyclopent[6]indole derivatives such as 151 via a cascade involving initial addition of the acyl radical to the alkene, and a subsequent oxidative cyclization at the indole C-2 <02JOC6268>. 

In contrast to the oxidative generation of radicals described above, reductions of alkyl iodides using tris(trimethylsilyl)silane also produces alkyl radicals under conditions suitable for Minisci-type substitution." Carboxylic acids (a-keto acids) are also useful precursors for alkyl (acyl ) radicals via silver-catalysed peroxide oxidation, or from their l-hydroxypyridine-2-thione derivatives using Barton s method,the latter in non-aqueous conditions. 

Alkyl, alkenyl, aryl and acyl radicals can all be used in cyclization reactions. Acyl radicals can be generated by addition of alkyl radicals to carbon monoxide, or more conveniently from acyl selenides, and undergo a variety of radical reactions. A synthesis of the sesquiterpene (—)-kamausallene made use of the radical cyclization from the acyl selenide 69 (4.61). Tris(trimethylsilyl)silane and triethylborane in air were used to promote the reaction, which is highly selective (32 1) in favour of the cis stereoisomer 70, as expected from a chair-tike transition state. Best yields in the cyclization reactions of acyl radicals are found with electron-deficient alkenes, indicating the nucleophilic character of acyl radicals. 

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