THE PHOTOCHEMICAL REACTION AND THE ABSOLUTE ASYMMETRIC SYNTHESIS IN THE CHIRAL CRYSTALLINE ENVIRONMENT

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The achievement of an asymmetric synthesis starting from an achiral reagent and in the absence of any external chiral agent has long been an intriguing challenge to chemists and is also central to the problem of the origin of optically activity in nature. Stereospecific solid-slate chemical reactions of chiral crystals formed by achiral materials are defined as "absolute" asymmetric synthesis. This asymmetric synthesis involves two aspects : generating chiral crystals and performing topochemically controlled solid-state reactions, which yield chiral products.We demonstrated that the stereo-controlled photochemical phenyl migration could also occur in the solid state. Thioester prepared from o-benzoylbenzoylchloride and o-methylthiophenol crystallized in a chiral space group P2_12_12_1. The circular dichroism (CD) spectra of the two enantiomorphs in KBr were presented. When the enantiomorphic crystals of the thioester was irradiated as white powders, an optically active phthalide was isolated. The formation of phthalide is explained in terms of either arylthio-migration of phenyl-migration process. The phenyl-migration mechanism was confirmed on the basis of the absolute-to-absolute configuration correlation studies of both product and an achiral molecule influenced by chiral crystal lattice.Some of o-benzoylbenzoic acid esters also crystallized in chiral space group, and the photolysis in solid-state gave phthalide derivatives via a cleavage of C(=O)-O bond, which involved the different mechanism from that of thioesters. The absolute asymmetric synthesis was also performed by the photoreaction of N,N-disubstituted o-benzoylbenzamides.The future of this field is intimately connected with progress in the general area of organic solid-state chemistry as well as with deeper understanding of the molecular packing modes of crystals.