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

The olefinic vinyllithium 2, derived from 2-bromo-5-methyl-5-(4-methyl phenyl)-1,6-heptadiene (3) by low-temperature lithium-bromine exchange , undergoes a diastereoselective B-exo cyclization at 0 degrees C in t he 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 appr oximates a chair cyclohexane which preferentially adopts a conformatio n having a pseudoaxial p-tolyl moiety and a pseudoequatorial methyl gr oup at the geminally substituted carbon. In contrast to the behavior o f 2, radical-mediated cyclization of 3 proceeds entirely in a 6-endo f ashion to give 4-methyl-4-(4-methylphenyl)-1-methylenecyclohexan (6) i n 93% yield.