Photocyclisation of 1-o-alkylphenylpropane-1,2-diones: stereochemistry

Abstract

The photoreactions of 1-[o-(s-butyl)- and 1-[o-(α-methylbenzyl)-phenyl]propane-1,2-diones are reported. In hydroxylic solvents the major products are 2-hydroxyindan-1-ones, whereas in aprotic solvents substantial amounts of substituted isochroman-4-ones are also formed. It is shown that the formation of the hydroxyindinanones may proceed with a high degree of stereospecificity, both epimers possessing the same chirality at C-3, and that the photocyclisation of 1-(o-s-butylphenyl)propane-1,2-diones results in predominant inversion of configuration at the alkyl group of the hydroxyindanones. Photoracemisation of the starting diketones is also found to occur, and mechanisms for these processes are discussed.