Abstract
The applicability of homonuclear gradient enhanced NMR experiments is demonstrated in the structure determination of steroid derivatives using stigmasterol as a model compound. High resolution 1 H NMR spectra of steroids very often display well resolved multiplets usually in the low-field region, and these signals can be used as starting points in several selective NMR experiments to study scalar ( J coupling), and dipolar (NOE) interactions. Selective excitation was carried out using a double pulsed-field gradient spin-echo sequence (DPFGSE) in which 180° Gaussian pulses are sandwiched between sine shaped z-gradients. Scalar interactions were studied by homonuclear DPFGSE-COSY, DPFGSE-relay-COSY and DPFGSE-TOCSY experiments, while DPFGSE-NOESY was used to monitor spatial environment of the selectively excited proton. These methods provided unambiguous assignments for signals of the main skeleton and the side-chain of the steroid molecule. In addition, they allowed determination of the conformationally important homonuclear proton–proton coupling constants ( J).
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