Stereochemical course of Knoevenagel condensation of furan compounds

(2E,4E)-2-Cyano-4-methyl-5-thiocyanato-2,4-hexadienoic acid ethyl ester

“…The condensation of aromatic aldehydes with a cyanoacetic acid ester gives stereospecific E isomers (Baker & Howes, 1953). However, when unsymmetrical ketones are condensed with a cyanoacetic acid ester, a mixture of E and Z isomers is obtained (Hayashi, 1966;Hayashi, Igarashi, Hayashi & Midorikava, 1965;Cernayova, Kovac, Dandarova & Rajniakova, 1977) in which the isomer with a trans relation between the more bulky substituent and the alkoxycarbonyl group predominates. The stereochemistry of the condensation products of aliphatic aldehydes with a cyanoacetic acid ester under Knoevenagel conditions has not yet been thoroughly investigated (Popp & Catala, 1961;Alexander & Cope, 1944;Astle & Zaslowsky, 1952;Mastagli, Lambert & Andric, 1956).…”