COBALT CLUSTER-DIRECTED, MN(III)-MEDIATED CHEMOSELECTIVE AND STEREOSELECTIVE RADICAL REACTIONS OF 1-ALKEN-3-YNES

A new strategy has been developed to effect selective Mn(OAc)(3)-media ted oxidative cycloaddition reactions of 1-alken-3-ynes with beta-dica rbonyl compounds. The three-step sequence involves (1) protection of t he triple bond of the substrate with the -Co-2(CO)(6) group, (2) Mn-pr omoted radical addition of the beta-dicarbonyl compounds with the comp lexed enyne, and (3) oxidative demetalation. Mono-, di-, and tricycles , containing 2,3-dihydrofuran and tetrahydrofuran rings, are produced by exclusive attack on the uncomplexed C=C in moderate overall yields; formation of bi- and tricyclic derivatives occurs with excellent cis- stereoselectivity. Molecular mechanics calculations indicate that the cis ring fusion in these systems is thermodynamically favored. Reactio ns of the Co-complexed substrates proceed with Mn(III) promotion alone , whereas the free enynes require combined Mn(III)/Cu(II) mediation to produce furan derivatives, apparently reflecting the relative ease of oxidation of the respective intermediate radicals to carbocations. Fo r the complexed substrates direct experimental proof for the formation of free carbocations along the reaction coordinate has been obtained by methanol trapping.