STEREOSELECTIVE CYCLIZATIONS AND REARRANGEMENTS IN VINYL RADICALS PROMOTED BY REGIOSELECTIVE SULFANYL RADICAL-ADDITION TO ENYNES

Regioselective radical addition of 4-cyanotoluenethiol (1a) to enynes 3-6 leads to vinyl radicals 7-10 that can undergo five- or six-membere d cyclization onto styrene or terminal double bonds in competition wit h 5-exo cyclization onto the aryl ring, The latter affords spiro-cyclo hexadienyl radical intermediates which can either be trapped by 2-cyan oisopropyl radicals or give 1,4-aryl migration products. Regioselectiv e radical addition of phenethanethiol (1b) to enynes 3-6 gives radical s 11-14 which undergo five- or six-membered cyclization onto the alken e double bond; the stereoelectronically disfavored 6-exo cyclization c an compete with intermolecular hydrogen abstraction and (to a small ex tent) with 1,5-hydrogen migration. The 5- and 6-(pi-exo)exo cyclizatio n of vinyl radicals 7, 9, 11-13 is highly stereoselective and exclusiv ely or predominately affords products deriving from the (Z)-isomers. S tereochemical evidence indicates that the six-membered endo-cyclizatio n products 30 and 50 could derive from a direct 6-endo cyclization of (E)-radicals (E)-10 and (E)-14 rather than from a 5-exo cyclization/ri ng expansion process. Sulfanyl radical addition to enynes 3-5 is highl y regioselective to the terminal triple bond. In contrast, reaction el f thiols 1a,b with enyne 6 leads to products deriving from sulfanyl ra dical addition to both the CC triple and double bond. This behavior is accounted for by assuming that sulfanyl radical addition to the alkyn e triple bond is not reversible, while the addition to the alkene doub le bond is. Vinyl radical 10 affords product 33 by a rare 1,4-hydrogen migration/fragmentation process.