CONFORMATIONAL CONTROL IN THE CYCLIZATION OF AN UNSATURATED VINYLLITHIUM - SYNTHESIS OF (+/-)-LAURENE

The olefinic vinyllithium 2, derived from 2-bromo-5-methyl-5-(4-methylphenyl)-1,6-heptadiene (3) by low-temperature lithium-bromine exchange, undergoes a diastereoselective B-exo cyclization at 0 degrees C in the presence of TMEDA to afford the naturally occurring sesquiterpene (+/-)-laurene (1) in 60% isolated yield along with 17% of the isomeric (+/-)-epilaurene (4). The diastereoselectivity of the cyclization of 2 is discussed in terms of a transition state for the process that approximates a chair cyclohexane which preferentially adopts a conformation having a pseudoaxial p-tolyl moiety and a pseudoequatorial methyl group at the geminally substituted carbon. In contrast to the behavior of 2, radical-mediated cyclization of 3 proceeds entirely in a 6-endo fashion to give 4-methyl-4-(4-methylphenyl)-1-methylenecyclohexan (6) in 93% yield.