Dimerization and comments on the reactivity of homophthalic anhydride.

Homophthalic anhydride (HPA) dimerizes under the influence of base to provide, sequentially, the (3-4')-C-acyl dimer, a pair of chiral diastereomeric bis(lactones), 3-(2-carboxybenzyl)isocoumarin-4-carboxylic acid, and finally, 3-(2-carboxybenzyl)isocoumarin. The structures of the bis(lactones) were misassigned in 1970 based on the (presumed) cis thermal decarboxylative elimination reaction of the lower melting one. The preferred pathway should be trans-anti, however, and crystallographic analysis of one of the bis(lactones) reverses the earlier assignment. The formal cycloaddition reaction of HPA with imines occurs in preference to HPA dimerization; the mechanistic implications of this reactivity difference are discussed.