Synthesis and photochemical reactivity of new 4-substituted naphtho[1,2-b]pyran derivatives

A new naphtho[1,2-b]pyran possessing an ester substituent in position 4 was prepared from 2,3-dihydro-2,2-diphenyl-4H-naphtho[1,2-b]pyran-4-one and then converted to the carboxylic acid derivative. The photochromic behaviour of these two compounds was studied by UV–vis spectroscopy and the structures of the photoproducts elucidated by NMR. Under UV irradiation the uncoloured ester derivative afforded a single coloured photoisomer having a transoid-trans (TT) configuration while the acid was irreversibly transformed into degradation products. In strong acid medium both compounds were converted to a spiro derivative formed through the opening of the pyran ring followed by an intramolecular lactone ring formation and electrophilic aromatic substitution.