Abstract
Comparable regio- and stereochemical results were obtained when cyclizations of a series of 2-substituted 3-oxohept-6-enoate (or oct-7-enoate) esters (acetoacetates) were conducted by manganese(III) acetate oxidation or by iodine or bromine atom transfer cyclization. The observed trends support the conclusion that free radicals 6b (rather than Mn(III)-complexed radicals 5b) are involved in the Mn(III)-mediated oxidative cyclization of tertiary malonates and acetoacetates. Most cyclizations proceeded under kinetic control, and several showed large temperature dependences on stereoselectivity. An apparent discrepancy was resolved by demonstrating that ring opening of radical 43 (a reverse 6-exo cyclization) was faster than bromine transfer, but slower than iodine transfer or Cu(II) oxidation. In the process of ring closure/ring opening, a Z-alkene is converted to an E-alkene. Since the E- and Z-alkenes provide distinct stereochemical results on cyclization, the observed stereochemical ratio becomes a very sensitive probe for this radical ring opening. This observation presages the design and use of related probes for radical ring opening.