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)-mediated oxidative cycloaddition reactions of 1-alken-3-ynes with beta-dicarbonyl compounds. The three-step sequence involves (1) protection of the triple bond of the substrate with the -Co-2(CO)(6) group, (2) Mn-promoted radical addition of the beta-dicarbonyl compounds with the complexed 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. Reactions 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. For the complexed substrates direct experimental proof for the formation of free carbocations along the reaction coordinate has been obtained by methanol trapping.