NMR study of alkaloids. V.13C NMR spectra and reconsideration of the structures of 11- and 10-hydroxypleiocarpamines and the structures of new alkaloids fromVinca erecta
The structures of phenolic alkaloids isolated previously fromVinca erecta Rgl. et Schmalh. have been reconsidered on the basis of the results of a study of their13C NMR spectra, and it has been shown that a base with mp 228–229° has the structure of 11-hydroxystrictamine and an amorphous base of the same composition isomeric with it is 10-hydroxystrictamine. The structures of two new indolenine alkaloids — ercinamine and ercinaminine — have been determined from an analysis of13C NMR spectra, and also on the basis of PMR and IR spectra and chemical transformations.
Nachweis von Desacetylakuammilin und 10-Methoxy-desacetylakuammilin in Vinca minor L. 147. Mitteilung über Alkaloide†‡
The two indole alkaloids desacetylakuammiline (2) and 10-methoxy-desacetylakuammiline (4) were shown to occure in the upper parts of the small periwinkel Vinca minor L. The structure of the hitherto unknown 10-methoxy-desacetylakuammiline was established on the bases of chemical, spectroscopic and o.r.d. evidence.
Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules
The NICSzz-scan curves of aromatic organic, inorganic and “all-metal” molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the σ-, π- and/or double (σ + π)-aromaticity. The NICSzz-scan curves of σ-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 A with the induced diatropic ring current arising from Tx,y-allowed transitions involving exclusively σ-type molecular orbitals. Broad NICSzz-scan curves (half-band width approximately higher than 3 A) characterize double (σ + π)-aromaticity, the chief contribution to the induced diatropic ring current arising from Tx,y-allowed transitions involving both σ- and π-type molecular orbitals. NICSzz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (σ + π)-aromatics where the π-diatropic ring current overwhelms the σ-type one. In the absence of any contribution from the σ-diatropic ring current, the NICSzz(0) value is close to zero and the molecule exhibits pure π-aromaticity.