Stereochemical analysis of natural products. Approaches relying on the combination of NMR spectroscopy and computational methods
R. Riccio, G. Bifulco, P. Cimino, C. Bassarello, and L. Gomez-Paloma
Dipartimento di Scienze Farmaceutiche, Università
di Salerno, via Ponte Don Melillo, 8408, Fisciano (SA), Italy
Abstract: The stereochemical study of flexible stereogenic
carbon chains, such as those of many novel natural products, is a particularly
challenging task. Recent applications of our group on the so-called
"J-based approach", a methodology relying on a detailed
analysis of
homonuclear (H-H) and heteronuclear (C-H) 2,3J couplings,
include the study of the sphinxolide family of antitumor macrolides,
a group of molecules characterized by a flexible macrocyclic framework
bearing a number of oxygenated and methylated undetermined stereocenters,
and of ascaulitoxin, a nitrogen-containing phytotoxin with herbicidal
activity produced by a phytopathogenic fungus. An extension of the original
procedure, relying on a Hartree-Fock (HF) ab initio calculation of conformational
equilibrium and an estimate of the Boltzmann averaged 2,3JHH
and 2,3JCH couplings, has been applied to the stereochemical
study of sapinofuranone A, where the conformational equilibrium among
existing rotamers had initially led to controversial results. 13C
NMR chemical shifts are additional useful
parameters in the study of complex organic molecules. Along these lines,
we have lately proposed the use of Hartree–Fock gauge including
atomic orbitals (GIAO) calculated 13C NMR chemical shift
values as a supporting tool for the validation of the structure of new
natural products and the determination of the relative stereochemistry
of diastereomeric flexible compounds that are characterized by multiple
conformer equilibria.
*Pure Appl.Chem. 75,
141�419 (2003). An issue of reviews and research papers based on
lectures presented at the 23rd IUPAC International Symposium on the
Chemistry of Natural Products, Florence, Italy, 28 July � 2 August 2002.
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