Reductive and Brominative Termination of Alkenol Cyclization in Aerobic Cobalt-Catalyzed Reactions

摘要

Aerobic oxidations of substituted pent-4-en-1-ols (bishomoallylic alcohols) occur with notable rates and diastereoselectivity, if catalyzed with appropriate cobalt(II) chelates. The ring closure furnishes substituted tetrahydrofuryl-2-methanols (Scheme 1, Y = OH), which constitute valuable building blocks for natural product synthesis. In an attempt to oxidize (E)- and (Z)-conpgured substrates under such conditions, however, a loss of stereochemical information associated with the olefinic π-bond was observed. This finding was explained with the appearance of configurationaly labile reactive intermediates. In the present report we provide evidence that these intermediates are free carbon radicals that can be converted with a variety of reagents into synthetically useful functional groups. Since oxidative catalytic methods in carbon radical chemistry so far were restricted to hydrocarbon oxyfunc-tionalization, it was our aim in the present study to develop methods for reductive, brominative, and alkylative termination of aerobic cobalt catalyzed alkenol cyclizations.