Spiro 1,3-indandiones: intramolecular photochemical reactions of carbonyl groups with carbon-carbon double bonds

2-Substituted 1,3-indandione derivatives are known to undergo a photochemical rearrangement into phthalides. We find that in the presence of a substituent involving a double carbon-carbon bond in the vicinity of the carbonyl group in spiro-1,3-indandiones, a variety of other types of intramolecular photochemical reactions readily occur: 1,3-acyl shift, intramolecular reversible Paternò-Büchi reaction, and formal oxa-[4+2]-cycloaddition. A specific feature of these reactions of spiro-1,3-indandiones is their photo-reversibility. The structures of polycyclic products of these reactions were determined spectroscopically and, in most cases, by single crystal X-ray structure determinations. A number of the products, especially those involving benzazulene or oxatricyclo[4.2.1.0^3,8]nonane moieties are synthetic analogues of natural compounds of interest owing to a variety of their biological activities.