Abstract
A series of 1-aryl-substituted 4-penten-1-oxyl radicals 2 were generated from parent N-alkoxypyridinethiones 6. The intermediates 2 undergo 5-exo-trig cyclizations which are sensitive to polar and steric substituent effects. Thus, para groups in the aromatic ring alter the reactivity of radicals 2c–e by one order of magnitude without changing the stereochemical course of the 5-exo-trig reaction (cis:trans = 50:50). Substitution of the ortho methyl groups for hydrogen atoms of the phenyl group affects the cis-ring closures more than the trans reactions which lead to trans-selective alkoxyl radical reactions. Methyl groups at the terminal olefinic double bond cause a significant acceleration of K5-exo in general and of the trans reactions in particular. Thus, trans-2-isopropyl-5-phenyltetrahydrofuran (8i) was the major product in the photoreaction of pyridinethione 6i in the presence of tri-n-butyltin hydride (98% yield, cis:trans, 30:70).
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