CONFORMATIONAL-ANALYSIS OF RING-A AND TOTAL ASSIGNMENT OF 19-FUNCTIONALIZED 4-EN-3-ONE STEROIDS - APPLICABILITY OF 2D NOE AS A CRUCIAL TECHNIQUE

The conformational analysis of ring A of 4-en-3-one steroids with three kinds of functionalizations viz. the 19-hydroxy, 19-oxo, and the 6-beta,19-epoxy groups, is discussed using a simple protocol-COSY 90 + NOESY + HETCOR. The 2D NOE technique is found to be of immense importance in deducing the conformational preferences of these steroids. The conformations are derived by connectivity pattern analysis and independently by torsional angle calculations employing the Karplus equation. While steroids 1 and 2 show ''normal'' 1-alpha,2-beta half-chair conformation, steroid 3 exhibits ring A in the 1-beta,2-alpha ''inverted'' conformation. The conformational analysis highlights aspects like interactions of the 1,3-diaxial type and the effect of tilting the 19-carbon toward ring A or ring B. A comment is made to that effect.