General Photoinduced Sequential Electrocyclization/[1,9]-Sigmatropic Rearrangement/Ring-Opening Reaction of Diarylethenes

A novel
and efficient photochemical transformation of diarylethenes
comprising a five-membered heterocyclic ring and phenyl moiety is
described. This reaction provides a simple method for the preparation
of functionalized naphthalene derivatives via photorearrangement reaction
of diarylethenes, and the process is characterized by high efficiency
that was determined by NMR monitoring. Some mechanistic aspects of
this process have been also explored. It was found that the reaction
includes tandem transformation of three basic processes: the photocyclization
of the hexatriene system, [1,9]-sigmatropic rearrangement, and heterocyclic
ring opening. Diarylethenes with different heterocycle moieties (thiophene,
benzo­[b]­thiophene, furan, indole, imidazole, thiazole,
oxazole, pyrazole) have been involved into this process, and the target
naphthalenes with good yields have been obtained. The opportunity
for use in the transformation of diarylethenes with different heterocyclic
residues permits synthesis of naphthalenes with desired functional
groups. The general character and high efficiency of the reaction
promise that the transformation can be an effective synthetic route
for the annulation of benzene rings to various aromatic systems, including
heterocycles.