Synthesis, Photochemical Properties, and Cytotoxicities of 2<i>H</i>‑Naphtho[1,2‑<i>b</i>]pyran and Its Photodimers

A 2H-naphtho­[1,2-b]­pyran, prepared by dimerization of 2-bromo-3-methyl-1,4-naphthoquinone and O-methylation, readily undergoes solid-state [2 + 2] photodimerization to give a photodimer in excellent yield and with excellent selectivity. Retro [2 + 2] cycloaddition can be achieved by irradiation of a solution of the photodimer in chloroform. Interestingly, the 2H-naphtho­[1,2-b]­pyran dimerizes with a skeletal rearrangement to afford 2,5-dihydro-1-benzoxepin dimers upon irradiation in methanol or via irradiation with hexamethylditin. Furthermore, treatment of the resulting dimers with triethylamine regenerates the 2H-naphtho­[1,2-b]­pyran monomer. Significant differences in the color, fluorescence, and cytotoxic properties of the monomer and dimers were observed.