REDUCTIVE ELECTROPHILIC SUBSTITUTION OF PHTHALANS AND RING EXPANSION TO ISOCHROMAN DERIVATIVES

Reductive cleavage of phthalan, 1a, with Li metal in the presence of a catalytic amount of naphthalene leads to the formation of a stable aromatic dililhium compound. The adducts or. the latter with CO2, aldehydes or ketones undergo ring closure to isochroman derivatives, leading to ring expansion of the original heterocycle. The reductive electrophilic substitution procedure was successfully extended to the substituted phthalans 1b and 1c, to afford the corresponding isochroman-3-ones in satisfactory yields.