Cyclization of 6-heptenoyl radicals

6-Heptenyl-type radicals preferentially undergo 6-exo instead of 1-endo cyclizations 85T3925 01OL2217 . 6-Exo cyclizations nevertheless are generally slower than the related 

Satisfactorily, the 1-endo closure became the predominat route when a temporary substituent was introduced at the 6-exo position of the alkene acceptor, a tactic previously used in similar 1-endo cyclizations involving aryl 4TL2335 , vinyl 05JOC519 or alkyl 06OL831 radicals. Hence, cyclization of selenoester 30b, which incorporates a 2-bromo-2-propenyl instead of an allyl moiety, led to the tricyclic substructure of mersicarpine 32 in a synthetically acceptable 62% yield, pyridoindole 31 being now the minor product (19%). The excess hydride ensured the final reductive removal of the bromine atom after the cyclization step 07JOC4562 . 

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Crich and coworkers [37] investigated the influence of oxygen substituents at various carbons on the 1-endo trig cyclization of 6-heptenoyl radicals. Presence of one or two oxygen substituents at C-5 favored the 1-endo trig radical cyclizations along with simple reduction (130 131 -I-132), and the competitive 6-exo cycliza-... 

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