A series of C‐8 mono‐ and disubstituted isatylidenes were prepared in order to examine the influence of the substituent on the polarization of the ethylenic carbons C‐3=C‐8. Donor–acceptor C‐8 substitution resulted in an inversion of the polarization of the ethylenic carbons, as evidenced by the 13C NMR spectra, and because these compounds can exist in solution as equilibrium mixtures of the E‐ and Z‐isomers. The dual donor–acceptor character of the heterocycle is controlled by the cross‐conjugated C‐3=C‐8 double bond, in which the C‐3 carbon atom has the possibility of conjugating with either the donor or the attracting portion of the heterocyclic system, consistent with contributions from isatylidene resonance forms which induce the C‐8 carbon atom to be charged either positively or negatively, thus allowing an opposite sense of the push–pull effect. Copyright © 2000 John Wiley & Sons, Ltd.

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Long-range coupling constants (5) JHortho,OMe were measured in series of methoxyindoles, methoxycoumarins, and methoxyflavones by the modified J doubling in the frequency domain method. The COSY and NOESY spectra revealed the coupling of the -OMe group with a specific proton at the ortho position and its preferred conformation. Homonuclear (1) H-(1) H couplings were confirmed by irradiation of the -OMe signal. Density functional theory calculations of (5) JHortho,OMe using the modified aug-cc-pVTZ basis set evidenced that the Fermi contact term shows good agreement with the experimental J values. Accurate chemical shift and coupling constant values followed after iterative quantum mechanical spectral analysis using the PERCH software.

The leaves of Piscidia carthagenensis provided new 7,2',5'-trimethoxy-3',4'-methylenedioxyisoflavone (1), admixed with known 6,7-dimethoxy-3',4'-methylenedioxyisoflavone (2), and 5,4'-dihydroxy-7,2',5'-trimethoxyisoflavone (3), which were separated by extensive fractional solubillization. Selective irradiation of the H-5 "singlet" of 2 allowed distinction of the two methoxy group signals, whose chemical shift difference is only 0.004 ppm (1.2 Hz at 300 MHz). The (1)H and (13)C NMR data of 3 were assigned with the aid of HETCOR and gHMBC measurements. Although 1 looked inhomogeneous in the solid state, its solution structure followed from (1)H NMR measurements, where it looked homogeneous. To clarify the solid state aspect and confirm the structure of 1, two types of crystals were mechanically separated and subjected to single crystal X-ray diffraction measurements. This study revealed polymorphism because of the concomitant presence of orthorhombic and triclinic crystals, but showed no atropisomerism. The structure of 3 was also verified by X-ray diffraction crystallography.