NEW TANDEM RADICAL CYCLIZATIONS DIRECTED TOWARD THE SYNTHESIS OF CRINIPELLIN-A

We describe a new tandem radical cyclization strategy for the construction of the congested angular triquinane portion of the naturally occurring tetraquinane crinipellin A. The preparation and cyclization of three 5,5-disubstituted-1.4-dimethyl-1,3-cyclopentadienes are detailed. This cyclization strategy results in a 1,4-functionalization of the cyclopentadiene nucleus, mediated by an allylic radical cyclization. Each tandem cyclization produces two diastereomeric triquinanes in a ratio of 5:1. The minor diastereomer possesses the correct relative stereochemistry for the D-ring isopropyl group of crinipellin A. A tandem cationic cylization, paralleling the radical cyclizations, is also described. The generation and subsequent addition or cyclization reactions of acyl radicals has been accomplished by the reduction of acyl methyl selenides with tin hydride. This new method is the most direct route for conversion of a methyl ester to an acyl radical. © 1990, American Chemical Society. All rights reserved.