Enantioselective photoelectrocyclization of a tropolone derivative in the crystalline state

Achiral tropolone ethers are well known to undergo photochemically induced disrotatory electrocyclic ring closure in solution to form racemic bicyclo[ 3.2.0]hepta-3,6-dien-2-one derivatives. The present paper reports successfu l efforts to carry out this transformation enantioselectively through the u se of the solid-state ionic chiral auxiliary method. in this method, the re actant, an achiral tropolone ether, is equipped with a carboxylic acid grou p to which an optically pure amine can be attached by salt formation. Salts such as these are required to crystallize in chiral space groups, which pr ovide asymmetric reaction cavities capable of differentiating enantiomeric transition states. Irradiation of these materials in the solid state leads to enantiomerically enriched products in moderate to high enantiomeric exce ss depending on the amine employed. Of the amines studied, the best results were obtained with 1-phenylethylamine and 1-amido-2-indanol, which gave en antiomeric excesses in the 60-80% range depending on the extent of conversi on. Because the tropolone ring is planar, it is suggested that the stereoch emical outcome of the electrocyclization in the solid state is governed by environmental crystal lattice effects rather than by the initial conformati on of the reactant. Copyright (C) 2000 John Wiley & Sons, Ltd.