NMR proofs of the involvement of an allenyl-naphthol as a key-intermediate in the photochromic process of [3H]-naphthopyrans

The NMR characterization of an intermediate, proved to be involved in the photochromic process of fluoro-substituted-3,3-diphenyl-3H-naphtho[2,1-b]pyrans is reported. An o-allenyl-naphthol structure was deduced and a plausible reaction mechanism was proposed.