DIASTEREOSELECTIVE SYNTHESIS OF ETHYL THYLIMIDAZO[1,2-A]PYRID-2-YL)-2-PHENYLTHIOACRYLATE - X-RAY CRYSTAL-STRUCTURE AND CONFORMATIONAL-ANALYSIS

The title compound 2, a gem vinyl sulfide ester, has been obtained dia stereoselectively (de > 98%) by action of the ethyl thiophenoxyacetate carbanion on the imidazo[1,2-a]pyridinecarbaldehyde 1 in a basic medi um, at low temperature. The X-ray crystal structure of 2 (C19H18N2O2S: M(r) = 338.43, triclinic, <P(1)over bar>, a = 8.193(3) Angstrom, b = 10.090(2) Angstrom, c = 10.981(4) Angstrom, alpha = 88.12(2)degrees; b eta = 78.66(4)degrees, gamma = 78.53(2)degrees, V = 872.3(6) Angstrom( 3), Z = 2 D-calc = 1.29 g cm(-3), lambda(Mo K alpha) = 0.71069 Angstro m, mu = 0.189 mm(-1), F(000) = 356, T = 293 K, R = 0.043 for 3610 obse rved reflections) has been determined and confirmed the Z configuratio n. The molecule is almost planar except for the phenyl ring situated i n an approximate perpendicular plane. Despite the presence of the conj ugate double bonds of the vinyl ester group (acrylate), coplanar with the imidazopyridine heterocycle, there is no evidence of pi-electron d elocalization over the whole structure. The crystal cohesion is ensure d by a dense network of van der Waals contacts. A conformational analy sis of the Z and E isomers by means of a Monte Carlo search and a stoc hastic dynamics simulation in CHCl3 has shown that according to the me thod the Z isomer is more stable than the E isomer by about 7 to 10 kJ mol(-1).