Conformations of anaesthetic steroids: A H-1 and C-13 NMR study of (2 beta,3 alpha,5 alpha)-2-[(2R)-ethyl-4-morpholinyl]-3-hydroxypregnane-11,20-dione and (2 beta,3 alpha,5 alpha)-2-[(2S)-ethyl-4-morpholinyl]-3-hydroxypregnane-11,20-dione.

The completely assigned H-1 and C-13 NMR spectra of the title diastereoisomers in solutions of CDCI3 and DMSO-d(6) are reported, NOE experiments show that these molecules adopt specific, conformations with no rotation about the C-2-N bond in either solvent, In CDCI,, ring a of the steroid is in a twist-boat conformation and an intramolecular hydrogen bond from the 3 alpha-OH to the morpholine N acts as a conformational lock preventing rotation of the morpholine ring, In DMSO-IZ, solutions, ring A of the steroid is in a chair conformation and van der Waals contacts with the 19-methyl group prevent free rotation of the morpholine ring. Molecular mechanics calculations produced four structures that account far the experimental data. (C) 1997 by John Wiley & Sons, Ltd.