Diastereoselective synthesis of ethyl (Z)-3-(8-methylimidazo-[1,2-a]pyrid-2-yl)-2-phenylthioacrylate. X-ray crystal structure and conformational analysis

Abstract The title compound 2 , a gem vinyl sulfide ester, has been obtained diastereoselectively (de > 98%) by action of the ethyl thiophenoxyacetate carbanion on the imidazo[1,2- a ]pyridinecarbaldehyde 1 in a basic medium, at low temperature. The X-ray crystal structure of 2 (C 19 H 19 N 2 O 2 S: M r = 338.43, triclinic, P 1 , a = 8.193(3) A , b = 10.090(2) A , c = 10.981(4) A , α = 88.12(2)°, β = 78.66(4)°, γ = 78.53(2)°, V = 872.3(6) A 3 , Z = 2, D calc = 1.29 g cm −3 , λ( Mo K α) = 0.71069 A , μ = 0.189 mm −1 , F (000) = 356, T = 293 K, R = 0.043 for 3610 observed reflections) has been determined and confirmed the Z configuration. The molecule is almost planar except for the phenyl ring situated in an approximate perpendicular plane. Despite the presence of the conjugate double bonds of the vinyl ester group (acrylate), coplanar with the imidazopyridine heterocycle, there is no evidence of π-electron delocalization over the whole structure. The crystal cohesion is ensured by a dense network of van der Waals contacts. A conformational analysis of the Z and E isomers by means of a Monte Carlo search and a stochastic dynamics simulation in CHCl 3 has shown that according to the method the Z isomer is more stable than the E isomer by about 7 to 10 kJ mol −1 .