The Mechanism of the Enantioselective Photocyclization of 2-Piperidone in the Clathrate Crystal

Abstract

Abstract The achiral molecules of N-(arylcarbonylmethyl)-2-piperidones form 1 : 1 host–guest complexes with the chiral host molecule derived from tartaric 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 derivatives with high optical yields on exposure to a high pressure Hg lamp. The crystal structures and absolute configurations of the clathrate compound (5a), which consists of (−)-(2R,3R)-2,3-bis(hydroxydiphenylmethyl)-1,4-dioxaspiro[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) Å, β=116.18(1)°,V = 4086(2) Å3, Z = 4, final R became 0.047. (2a) C13H14NO2Br, fw = 296.16, orthorhombic, P212121, a = 7.047(2), b = 31.151(3), c = 5.696(2) Å, V = 1250.3(5) Å3, 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.