Ethylidene-4,6-dimethoxycoumaran-3-one: the C2C8 double bond configuration

The configuration about the C2C8 double bond for the major isomer of the natural product ethylidene-4,6-dimethoxycoumaran-3-one isolated from the shrub Pogostemon quadrifolius (Benth.) was consistently determined to be Z by the examination of long-range NOE contacts (ηH,Hs), chemical shifts (δCs), and scalar coupling constants (3JH8,H9, 1JH8,C8, 3JH8,C3, and 2JH8,C2) in conjunction with DFT calculations. The Js was found to be very amenable to the structural analysis and disparate in terms of their magnitude, and in the case of 2JH8,C2, by the sign.