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 successful efforts to carry out this transformation enantioselectively through the use of the solid-state ionic chiral auxiliary method. in this method, the reactant, an achiral tropolone ether, is equipped with a carboxylic acid group 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 provide 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 excess depending on the amine employed. Of the amines studied, the best results were obtained with 1-phenylethylamine and 1-amido-2-indanol, which gave enantiomeric excesses in the 60-80% range depending on the extent of conversion. Because the tropolone ring is planar, it is suggested that the stereochemical outcome of the electrocyclization in the solid state is governed by environmental crystal lattice effects rather than by the initial conformation of the reactant. Copyright (C) 2000 John Wiley & Sons, Ltd.