STEREOSELECTIVE SYNTHESIS OF RID-2-YL)-3-(4-METHYLPHENYLSULFONYL)BUT-3-EN-2-ONE - X-RAY CRYSTAL-STRUCTURE AND CONFORMATIONAL-ANALYSIS

The title compound, gem-ketovinylsulfone 3, was obtained stereoselecti vely (de > 98%) by the action of the oc-anion from p-tolylsulfonylacet one 1 on imidazo[1,2-a]pyridine-2-carbaldehyde 2 in chelation-controll ed conditions in the presence of a Lewis acid (ZnCl2). The X-ray cryst al structure of 3 [C18H16N2O3S: M-I = 340.4, orthorhombic, Pbca, a = 1 2.208(3) Angstrom, b = 18.848(4)Angstrom, c = 14.566(11)Angstrom, V = 3.351(3)Angstrom(3), Z = 8, D-calc = 1.349 g cm(-3), <(lambda)over bar >(CuK alpha) = 1.54178 Angstrom, mu = 1.83 mm(-1), F(000) = 1424, T = 293 K, R = 0.061 for 2.046 observed reflections] was determined, and c onfirmed the (E) configuration. Despite the conjugate position of the vinyl double bond, quasi-coplanar with the imidazopyridine heterocycle , there is no evidence of p-electron delocalization. The crystal cohes ion is ensured by a dense network of van der Waals contacts. The confo rmational analysis of the (E) and (Z) stereoisomers was performed by m olecular dynamics simulation, and showed the (E) isomer to be 9.1 kJ m ol(-1) more stable than the (Z) isomer. (C) 1998 Elsevier Science B.V. All rights reserved.