Structure Elucidation - Natural Products Chemistry: Sources, Separations, and Structures - page 30

Case Study
Fermentation of a strain of Fusarium larvarum
Isolation of whole broth into acetone, breaks the cells then filtration, retain ace-
tone, concentrate to remove acetone, load aqueous liquid on resin
Reverse phase, CHP20P chromatography, elute aq. acidic-acetonitrile
Elution-extrusion countercurrent chromatography (EECCC), elute mobile phase
1:1:1:1 hexane/ ethyl acetate/ methanol/ water
Parnafungins: A mixture of four closely related compounds—four distinct
stereoisomers and named A1, A2, B1, and B2.
Equilibrium mixture of parnafungins seen by NMR in dimethyl sulphoxide
(DMSO) solution with ratio of 4:1:5:1.
The derivative 12-O-4-chlorobenzyl ether of parnafungin A1 was prepared to
establish the absolute configuration of the C15-hydroxyl group. It was discovered
that parnafungin A1 shows the syn configuration between the 15-hydroxyl and the
neighboring 15a-methyl carboxylate. The complete X-ray structure of the derivative
is displayed in Figure 3.10. Since this was an unusual compound, X-ray analysis
offered (1) absolute proof of structure and (2) resolved the stereo centers—from rela-
tive to absolute stereochemistry. It should be noted that to gain useful information
using X-ray crystallography, it is necessary to obtain good quality crystals from a
pure stable derivative. Usually, the derivative contains a large atom (e.g., chlorine or
bromine atom).
SUMMARY
An introduction to the basic spectroscopic techniques, including nuclear magnetic
resonance (NMR) spectroscopy, H- 13 C-NMR; ultraviolet and infrared spectroscopy;
mass spectrometry; circular dichroism (CD); and X-ray crystallography are pro-
vided. A case study is used on elucidating the structure of a complex molecule using
a combination of techniques.