Radical Cyclization in Heterocycle Synthesis 15. Relationship between a Radical Species and Radical Acceptors of Three Different Types of Double Bond in Radical Addition–Cyclization

Abstract

Relationship between a radical species and radical acceptors of three different types of double bond in radical addition–cyclization was systematically investigated. Substrates carrying α,β-unsaturated amide, isolated olefin, and oxime ether moieties underwent radical addition–cyclization to give differently substituted lactams depending upon the radicals used. The sulfanyl radical addition–cyclization of the substrate proceeded smoothly to give the 5-membered lactam having an alkoxyamino group as a result of preferable addition of an intermediary α-carbonyl radical to the oxime ether. On the other hand, the triethylborane-mediated radical addition-cyclization gave the lactam bearing an iodomethyl group as a result of addition to an intermediary α-carbonyl radical to isolated olefin. The different regioselectivity was explained by the stability of the intermediary radical and the interaction between SOMO and HOMO.