Synthetic Studies on η-Pyrromycinone. III. Intramolecular Cyclization of methyl 2-Ethyl-5-hydroxy-6-(2-carboxy-3, 6-dimethoxybenzoyl)-1-naphthoate

Abstract

The Friedel-Crafts condensation of 3, 6-dimethoxyphthalic anhydride (II) with methyl 2-ethyl-5-hydroxy-1-naphthoate (III) gave 2-ethyl-5-hydroxy-6-(2-carboxy-3, 6-dimethoxy-benzoyl)-1-naphthoate (IV), a key intermediate for the synthesis of η-pyrromycinone (I, R=H). An attempt to cyclize IV to η-pyrromycinone dimethyl ether (I, R=CH₃), however, resulted in formation of 9-ethyl-1, 6-dihydroxy-4-methoxynaphthacenequinone (V). The Structure of V was confirmed by alternative synthesis.