MANGANESE(III)-BASED OXIDATIVE FREE-RADICAL TANDEM AND TRIPLE CYCLIZATIONS

Tandem oxidative free-radical cyclization of 1a with Mn(OAc)3-2H2O and Cu(OAc)2-H2O in acetic acid at 25 °C gives bicyclo[3.2.1]octane 7a in 86% yield. Oxidation of the β-ketoester gives the enol radical 2a which undergoes 6-endo cyclization to give tertiary radical 3a. A second cyclization gives a mixture of primary radicals 5a which is oxidized directly to alkene 7a by Cu(OAc)2-H2O. This reaction has been used to prepare bicyclo[3.2.1 Joctanones 7b-e. Introduction of methyl substituents on the chain leads selectively to 18a-c with an axial methyl group establishing that the (E)-enol radical 15 is formed selectively. Oxidation of 25 leads selectively to the exo-radical 26 which undergoes an oxidative elimination with Cu(OAc)2-H2O with high selectivity for the Hofmann isomer 28. trans-Hydrindanone 52 is formed selectively from Z isomer 47a. cis-Hydrindanones 69a-c are formed selectively from dienes 65a-c. Triple oxidative cyclizations can also be carried out efficiently. Trienes 38 and 58 are converted to tricyclics 41a (39%) and 61 (60%), respectively. © 1990, American Chemical Society. All rights reserved.