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492 Organic Chemistry, Compound Detection

FIGURE 15 COSY NMR spectrum of stevioside (stacked plot).

generated in which the F 2 axis corresponds to 13 C chem- cynarin (isolated from artichokes), allowing the distinc-
ical shifts (δ c ) or other nonproton nuclei and the F 1 , axis tion between two possible structures to be made. The
1
which corresponds to the proton H shifts (δ H ). The di- complex structures of griseochelin and grisorixin metabo-
agonal relationship of the signals in such 2D-NMR spec- lites, which are antibiotics obtained from strains of Strep-
tra is a direct consequence of the parallel between car- tomyces, were solved by COSY techniques. HETCOR
bon chemical shifts and attached hydrogen shifts. The experiments appear for the most part to have been used
closely analogous Homonuclear shift correlation (HOM- as a simple tool for assignment in the area of natural
COR) or COSY (COrrelated Spectroscop Y) maps out all products. In the studies of bile acids, isomugineic acid
homonuclear couplings in a single spectrum. In a typical (a metabolite derived from barley), the heteronuclear cor-
relation was used to transfer proton assignments that had
COSY spectrum of stevioside (Fig. 15), both the F 1 and F 2
1
axes correspond to H chemical shift scales. The normal been made by other means onto the 13 C spectrum. HET-
1 COR techniques have also been applied in long-range
H NMR spectrum appears along the diagonal, while all
1
1
off-diagonal signals corresponds to H- H homonuclear experiments to obtain structural information; for exam-
couplings. ple, gracilin B (a bisnor-terpene isolated from the sponge
The HOMCOR technique was originally used by Spongionellagracilis),ekeberginine(analkaloidextracted
Jenner in 1971, when he obtained a COSY spectrum of β- from Ekebergia senegalensis), and didehydroryanodine
chloroacrylic acid. It was only a decade ago that 2D-NMR (an insecticide obtained from Ryania speciosia) were all
became a routine technique. determined with the aid of long-range proton-carbon cor-
relations using the HETCOR method.
a. Applications of 2D-NMR spectroscopy. Ade- Perhaps one of the recent exciting and fruitful areas
quate COSY spectra were obtained from only 90 µgof of FTNMR research is in biochemistry. It is only been