| Abstract: | Changes in chemical shifts of olefinic protons in a number of α,β- and α,β,γ,δ-unsaturated carboxylic acids caused by ionization of the COOH group were investigated. The ionization shifts of α-H-atoms are ?0.09 to 0.07 ppm, those of β-H-atoms are 0.32?0.47 ppm. The ionization shifts of δ-H-atoms are substantially larger than those of γ-H-atoms. The ionization shifts can be used for immediate determination of the esterification site in monoesters of (2E,4Z)-2,4-hexadienedioic (muconic) acid, which are of interest in connection with synthetic studies on verrucarins. Thus, isomerization by heating in aqueous solution of monoesters of (2Z,4Z)-2,4-hexadienedioic acid yields 1-monoesters rather than 6-monoesters of (2E,4Z)-2,4-hexadienedioic acid, in accordance with the isomerization mechanism involving anchimeric assistance of the free COOH group. Solutions of the ABXY spectra of olefinic protons of monomethyl (2E,4E)- and (2Z,4Z)-2,4-hexadienedioate are reported. |
