Relative population of S-form and F-form conformers of bryonolic acid and its derivatives in equilibrium in CDCI3 solutions

Relative populations of S-form (D-E rings: boat-boat form) and F-form (D-E rings: chair-chair form) conformers, in equilibrium in CDCl3, solutions, of 20 derivatives (2-21) of bryonolic acid (D:C-friedoolean-8-en-3beta-ol-29-oic acid) (1) were calculated from NMR spectral data (J-values and chemical shifts), with the aid of molecular mechanic calculation using a MM2/CONFLEX program system. The principal deciding factor of the population ratio was found to be whether the functionality at C-29 is trigonal or tetrahedral; the S-form : F-form was 0 : 100-32 : 68 for the "trigonal" type and 48 : 52-100 : 0 for the "tetrahedral." The reliability of the results is discussed.