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

Abstract The configuration about the C 2 C 8 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 ( η H,H s), chemical shifts ( δ C s), and scalar coupling constants ( 3 J H8,H9 , 1 J H8,C8 , 3 J H8,C3 , and 2 J H8,C2 ) in conjunction with DFT calculations. The J s was found to be very amenable to the structural analysis and disparate in terms of their magnitude, and in the case of 2 J H8,C2 , by the sign.