Regioselective Palladium-Catalyzed Alkylation of Allylic Halides with Benzylic Grignard Reagents. Two-Step Synthesis of Abietane Terpenes and Tetracyclic Polyprenoid Compounds

A highly regioselective palladium-catalyzed alpha-alkylation of allylic bromides 1a,c-e and chloride 1b with substituted and unsubstituted benzylic Grignard reagents is reported. The resulting all-trans polyenehomobenzene derivatives were obtained in excellent yields and regioselectivity. These products were easily converted to abietane-type diterpenes (10-12) and tetracyclic polyprenoid compounds (13, 14) through a Lewis acid-promoted cascade polyene cyclization reaction.