THE MECHANISM OF THE ENANTIOSELECTIVE PHOTOCYCLIZATION OF 2-PIPERIDONE IN THE CLATHRATE CRYSTAL

The achiral molecules of N-(arylcarbonylmethyl)-2-piperidones from 1:1 host-guest complexes with the chiral host molecule derived from tarta ric acid. The guest molecules in the clathrate crystals are converted into the chiral 7-aryl-7-hydroxy-1-azabicyclo[4.2.0]octan-2-one deriva tives with high optical yields on exposure to a high pressure Hg lamp. The crystal structures and absolute configurations of the clathrate c ompound (5a), which consists of s(hydroxydiphenylmethyl)-1,4-dioxaspir o[4,5]decane (4) and N-(p-bromophenylcarbonylmethyl)-2-piperidone (1a) , and of the chiral product of )-(6S,7S)-7-hydroxy-1-azabicyclo[4.2.0] octan-2-one (2a) were determined by X-rays. (5a) C34H34O4.C13H14NO2Br, fw = 802.81, monoclinic C2, a = 24.186(3), b = 9.550(3), c = 19.711(3 ) angstrom, beta = 116.18(1)-degree, V = 4086(2) angstrom3, Z = 4, fin al R became 0.047. (2a) C13H14NO2Br, fw = 296.16, orthorhombic, P2(1)2 (1)2(1), a = 7.047(2), b = 31.151(3), c = 5.696(2) angstrom, V = 1250. 3(5) angstrom3, Z = 4, final R became 0.039. The enantioselectivity of the photocyclization is clearly explained by the two structures if we assume that the reaction proceeds topochemically.