SOLVENT EFFECTS ON MANGANESE(III)-BASED OXIDATIVE FREE-RADICAL CYCLIZATIONS - ETHANOL AND ACETIC-ACID

Ethanol complements the typical solvent, acetic acid, for Mn(III)-based oxidative free-radical cyclizations. Cyclization of enol ether 1c to give gibberellic acid intermediate 6c is successful in ethanol, but not in acetic acid. Ethanol acts as a reducing agent for primary radicals, converting 13 and 33 to alkanes 17 and 32. Acetylenes can be used as substrates since the vinyl radicals 24 and 25 are reduced by ethanol to alkenes 26-28. The 1-hydroxyethyl radical obtained from ethanol is oxidized to acetaldehyde by Mn(III). The effect of solvent on the oxidative cyclization of unsaturated beta-keto esters 35 and 48 was examined. A higher percentage of 5-exo product is obtained in ethanol. The primary cyclopentanemethyl radicals 40 and 53 are oxidized mainly to alkenes 43 and 57 in ethanol and mainly to the alcohols 42 and 55 and lactones 41 and 58 in acetic acid.