Giuseppe Gatti

13 C nuclear magnetic resonance spectra of amaryllidaceae alkaloids

The 13C n.m.r. spectra of the title compounds are reported with complete assignments, based on peak multiplicity, single frequency proton decoupling, the use of lanthanide shift reagents, and empirical calculations of chemical shifts.

Structure determination of two extractives from Aspergillus amstelodami by nuclear magnetic resonance spectroscopy

Spectral evidence supports the assignment of structures (1) and (2) to two substances extracted from the mycelium of Aspergillus amstelodami; they can be obtained by thermal condensation of auroglaucine with neoechinuline B and neoechinuline C, respectively, which are all known metabolites of the fungus.

Stereochemical course of the α,β-desaturation of L- tryptophan in the biosynthesis of cryptoechinuline A in Aspergillus amstelodami

Incorporation of stereospecifically labelled L-tryptophan into cryptoechinuline A (3), which has been assigned the (z)-configuration from examination of its natural and 13C-enriched 13C n.m.r. spectra, proceeds with removal of a pro-S hydrogen atom from the β-methylene position; a new metabolite, cryptoechinuline C (5) has also been isolated.

Ring reversal of cis-cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester

cis-Cyclohexane-1,2,3,4,5,6-hexacarboxylic acid and its hexamethyl ester have been synthesized and shown, by n.m.r., to exist in solution at room temperature as an equilibrium of slowly exchanging chair conformations. From complete line shape analysis of the 13C n.m.r. spectra measured at different temperatures the activation parameters have been determined. A relatively high value (ca. 17 kcal mol–1) of free energy of activation has been found for both molecules, by analogy with the hexamethylcyclohexane case. The energy barrier of the acid has been calculated with the method of molecular mechanics and the computer program MOLBD3: the value obtained (16 kcal mol–1) is a slight overestimate by comparison with the observed value of 13–14.5 kcal mol–1.

Pattern of incorporation of leucine samples asymmetrically labelled with 13C in the isopropyl unit into the C5-isoprenoid units of echinuline and flavoglaucine

The labelling pattern of the C5-isoprenoid moieties of echinuline (18) and flavoglaucine (19) obtained in feeding experiments of Aspergillus amstelodami of 90% enriched (4R)[5-13C]leucine (11) and of the (4S)-isomer (12), determined by 13C-n.m.r. spectroscopy, appears in agreement with a preferential incorporation of the carbon atom of the pro-S methyl group of the stereospecifically labelled amino acid (12) into the C-3 methyl group of the intermediate mevalonic acid and of the carbon atom of the pro-R methyl group of (11) in position 4, respectively.