ELUCIDATION OF REACTION PATHWAYS IN HOST-GUEST COMPLEXES BY CRYSTAL ENGINEERING. PHOTOADDITION OF CARBONYL GROUP OF GUEST ACETOPHENONES AND PROPIOPHENONES TO HOST DEOXYCHOLIC ACID

Article abstract of DOI:10.1016/S0040-4020(01)90262-5

The two inclusion complexes of guest acetophenone and m-chloro-acetophenone with host deoxycholic acid (DCA) each yielded on UV irradiation a single diastereomeric photoaddition product at steroid atom C5, of configuration S at the generated stereogenic centre (DCAOHCH3C6H4).Comparison of the host-guest arrangement at the sites of reaction with the S configuration at the new chiral atom C of the photoproducts indicates a net rotation of 180 degree by the guest acetyl group prior to C-C bond formation since the guest molecules expose their Re faces to the C5-H5 steroid bonds.To elucidate this rotation, photochemical and crystallographic studies were performed on four DCA complexes with engieered guest ketone arrangements.The guests were p-fluoroacetophenone and (p-substituted) propiophenones (XC6H4CO2C2H5, X=H, F, Cl).The crystal structures of two isolated photoproducts are also reported.The channels of DCA-fluoroacetophenone contain two independent guest molecules G' and G, respectively exposing their Re and Si faces to the steroid C5-H5 bond vectors at the potential sites of reaction.Only one diastereomeric photoproduct was obtained with addition at atom C5, with absolute configuration S at the generated stereogenic centre, akin to that of the DCA-acetophenone complexes.The reaction is interpreted in terms of a 180 degree rotation of the acetyl group of the G' molecule; the absence of the R diastereomer is explained by a preclusion of photoaddition of guest G to DCA because of guest packing along the channel.The guest arrangements in the two complexes DCA-propiophenone and DCA-p-flouropropiophenone are almost isomophous.There are two independent guest molecules G and G<*> related by a pseudo centre of inversion.Each of the two guest molecules exposes its Re face to a steroid C5-H5 bond at a site of reaction.Thus photoaddition takes place with and without 180 degree rotation of the propionyl group since the two complexes each yield the two diastereomeric photoproducts at C5.DCA-p-chloropropiophenone yields only one diastereomeric photoproduct at C5, with absolute configuration R about the new stereogenic centre.The crystal structure analysis suggests three independent guest molecules G, G', and G<*>; G and G' are related by pseudo two fold screw symmetry along the channel axis; G<*> is related to G and G' by pseudo centre of inversion; once again the guests G, G', and G<*> appear to expose their Re faces to the C5-H5 bonds at the sites of reaction.