Configurational assignment of the geometric isomers of α,β-disubstituted vinyl methyl ethers from 13C nmr shift data

¹³C chemical shifts of the C atoms in the geometric isomers of some α,β-disubstituted vinyl methyl ethers have been measured. Configurational assignment is readily accomplished from the relative ¹³C chemical shift values of the β-C atoms, the signal of the Z isomer (with the β substituent in a cis position with respect of the MeO group) lying 11–15 ppm downfield from that of the E isomer. The higher chemical shifts of the β-C of the Z form are ascribed to reduced conjugation in the vinyloxy system, due to the nonplanar gauche configuration of the MeO group about the O-C(sp²) bond. Structural effects on the other ¹³C shift values are also discussed.