Department of Chemistry, Indian Institute of Technology, Powai, Mumbai-400 076, India Manuscript received 18 August 1999 The specificity of progesterone action largely depends on the conformation of its ring A in solution. Selectivity and specificity of haptens for immunoassay is dependent on their conformational similarity to that of the parent molecule. Conformational distortions due to bulky substituents have often led to a loss in activity. Therefore, determination of conformation of the

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... roidal rings has always been an area of high intrinsic interest. Conformational analysis of the A/B rings of the synthesized steroid hormone derivatives, viz. 16α-methylprogesterone, 6β-hydroxy-16α-methylprogesterone, 16α-methylpregn-4-en-3,6,20-trione, 16α-ethylprogesterone, 6β-hydroxy-16 α-ethyl progesterone, 16α-ethylpregn-4-en-3,6,20-trione, 16α-prop-2'- enylprogesterone, 6β-hydroxy-16α-prop-2' -enylprogcsterone, 16α-prop-2' -enylpregn-4-en-3,6,20-trione has been carried out using high resolution 1D proton NMR spectra. Calculations are carried out by employing three of the latest extensions to the Karplus equations, viz. those suggested by Colucci, Osawa and Barfield. The A ring conformations of these steroids are determined by calculating dihedral angles of the ethane fragments consisting of C1-C2 and C6-C7 from the vicinal coupling constants derived from high field 1H NMR spectra of the hormone derivatives. The dihedral angle θ could not be calculated directly since the nature of the equations is not truly quadratic. A program has been devised wherein the calculated J value has to be fitted to the observed J value for each of the Colucci, Osawa and Barfield equations. Molecular modeling of the 4-en-3-one steroid hormone derivatives has also been carried out using MM2 calculations.