Free radical reaction of vinyl epoxides

Abstract

Radical Mediated Ring Expansion Reactions The radical ring expansion reactions of vinyl epoxides were initiated by n-$Bu_3Sn$ radical addition to the vinyl epoxide, followed by epoxide, followed by epoxide fragmantion to the alkoxy radical, β-cleavage to produce the carbon-centered radical, and cyclization. Several noteworthy findings are as follows. First, ring expansion of vinyl oxaspiroheptanes with n-$Bu_3SnH$ and AIBN proceeded cleanly, yielding 2-vinylcyclohexanones in high yields due to the strong preference of 6-exo ring closure over 7-endo closure. Second, β-cleavage of alkoxy radicals occurred at the more highy substituted carbon. Finally, ring expansion of vinyl oxaspirooctane failed due to unfavorable β-cleavage of the alkoxy radical and 7-exo closure. Also, three-carbon ring expansion reactions of silylated vinyl endo epoxides with PhS radical proceeded cleanly, allowing 8-,9-,10-membered rings in high yields. Radical cyclization via Radical Translocations by a Novel 1,5n-$Bu_3Sn$ Group or a 1,5-Hydrogen Atom Transfer. The reaction of the vinyl exo epoxide with n-$Bu_3SnH$ occurred smoothly, yielding initially the alkoxy radical bearing allyltin moiety. 1,5-n-$Bu_3Sn$ transfer proceeded rapidly and cleanly, yielding the allylic radical which underwent reduction, cyclization, and allylation. The vinyl epoxide with a cinnamyl substituent afforded exclusively the 5-exo product whereas allyl substituent afforded a mixture of 5-exo, 6-endo, and ayclized product. The exclusive 6-endo cyclization was realized with methallyl substituent and ayclic compound was not detected. The radical allylation of allyl radical, obtained from 1,5-Sn translocation, with allyltri-n-butylstannane with activated allylstannanes such as 2-carbethoxyallyltri-n-butylstannane and 2-cyanoallyltri-n-butylstannane afforded allylated products.