Assignment of stereochemistry to cyclohexenylidenecyanoacetates by 1H NMR spectroscopy

Abstract

AbstractThe stereochemistry of various pairs of isomeric 2‐cyclohexen‐1‐ylidenecyanoacetates was assigned using 1H NMR spectroscopy. The isomers with the γ‐methylene or the γ‐vinyl protons cis to the carbalkoxy group were found to have the signals of these protons at approximately 0.3 ppm and 1 ppm, respectively, downfield relative to their geometrical isomers or the corresponding 2‐cyclohexen‐1‐ylidenemalononitriles. The observation regarding the γ‐vinyl proton proved useful for the assignment of configuration to cyclohex‐enylidenecyanoacetates derived form cholest‐4‐en‐3‐one. The large and constant downfield shift (c. 1 ppm) of the γ‐vinyl proton when cis to the ester group results from the rigid cyclohexenylidenecyanoacetate system, in which the vinylic proton can approach more closely to the magnetically anisotropic ester carbonyl group.