The geminal coupling and paramagnetic shift of exomethylene protons in the α,β′-unsaturated γ-lactone group of sesquiterpene lactones containing C 8-α-hydroxyl groups

Abstract

Exomethylene protons (H 13a and H 13b) in the α,β′-unsaturated lactone groups in sesquiterpene lactones which contain C 8-αOH groups exhibit NMR geminal coupling. In addition, H 13a undergoes a paramagnetic shift relative to the corresponding compound which differs only in that it does not contain the C 8-α-hydroxyl group. Data are presented which indicate that the geminal coupling and concurrent paramagnetic shift mainly result from the van der Waal's proximity effect of the C 8-αOH group upon H 13a. The application of the phenomenon for making certain stereochemical and conformational assignments for sesquiterpene lactones is discussed.