Intramolecular Substitution Reaction of Lithium Alkylidene Carbenoids. Regioselective Synthesis of Indenes

When 4,4-dibromo-3-alkenols are treated with butyllithium, an intramolecular substitution reaction with alkoxide moiety occurs at the lithium alkylidene carbenoid center to give dihydrofurans. The reaction mechanism of this intramolecular substitution reaction is studied by B3LYP density functional calculations with the 6-31+G (d) basis set, and the substitution is found to proceed in a concerted manner. This substitution reaction is applied to the regioselective preparation of indene derivatives. That is, treatment of 3-(o-bromophenyl)-1,1-dibromopropene derivatives 23 with butyllithium results in the formation of intramolecular substitution intermediates, 3-indenyllithiums D, which are trapped with electrophiles to afford substituted indenes regioselectively.