First Application of Biopolymers for Chiral Reaction Field of Asymmetric Photoreactions

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In this study, the author has proposed a new strategy of asymmetric photochemistry or a new concept of supramolecular photochirogenesis, in which the chiral information of biomolecules is transferred to or multiplied in photoproducts through the solution-phase supramolecular interaction of chiral biomolecular hosts with prochiral guest substrates and the subsequent Uni, and bimolecular photochemical reactions of the resulting ,host-guest complexes. All of the examined supramolecular photochirogenic approaches, employing modified cyclodextrins, (oligo) nucleotides, DNA, RNA, and protein as chiral biomolecular hosts, were successful in creating optically active photoproducts in good optical yields. Thus, the original idea of using the supramolecular interactions of chiral biomolecules in photochemical asymmetric synthesis has been demonstrated to be useful as a novel photochirogenic methodology, and is potentially extended to the other supramolecular systems. The major findings obtained … More in this thesis may be summarized as follows.The enantiodifferentiating Z-to-E photoisomerization of cyclooctene included and sensitized by chromophoreappended cyclodextrins was investigated in aqueous solution at varying temperatures. Indeed, the original poor ee of <1 % obtained upon direct irradiation at 185 nm of (Z)-cyclooctene accommodated in native b-cyclodextrin was dramatically enhanced up to 24 % ee.The double stranded DNA, as well as nucleosides and (oligo)nucleotides, have been shown for the first time to function as effective chiral photosensitizers and/or chiral photoreaction field in aqueous solution to effect the enantiodifferentiating photoisomerization of (Z)-cyclooctene. The best ee of the chiral (E)-isomer produced upon, photosensitization with DNA was 15 % in favor of (S)-(+)-(E)-cyclooctene. (R)-(E)-cyclooctene was obtained in an ee as high as 26 % by using d(T)_<15> d(A) _<15> as a photosensitizer, whereas the photosensitizations by the other duplexes gave nearly racemic (E)-isomer. It is concluded therefore that the composition and the sequence of nucleobases are important factors in determinins the enantioselectivity, and the consecutive oligothymidine sequence is essential for realizing the highly enantioselectm photoisomerization of (Z)-cyclooctene. Less