The chemical shifts of the CI,-and C,s-hydrogens of 30 14a,l7a-bridged 20-ketopregnanes and of four 14a, 17a-bridged 17-cyanoandrostanes were correlated by means of additive substituent constants. Because the bridged D-ring of these steroids is a substituted bicyclo[2.2.l]Iieptaiie sytem, attempts were made to explain the magnitude of some of these substituent constants by analogy to empirical erects in other such systems as well as by reference to more general theory. The data indicate that the acetyl side chain of the bridged 20-kctopregnanes exists in one rotomer form if the D-ring hears no extra substituents or a I6a-substituent and that it exists in a second form if a 16&suhstituent or a 16substituent on a double hond is present. In the latter case, the carhonyl group deshields the Cls-hydrogens by an extra 0.11 p.p.m. Keyphrases 0 Steroids, hridged [>-ring -effect of D-ring substituents on chemical \hifts of angular methyl protons, NMR spectroscopy C Pregnanes, 13U.17n-bridged-effect of D-ring substituents on chemical shifts of angular methyl protons, NMR spectroscopy 5-Androstenes, 3B-acetoxy-l4a,l7a-ethano-etrect of D-ring substituents on chemical Fhifts of angular methyl protons, NMR spectroscopy G Bridged D-ring steroids-effcct of D-ring substituents on chemical shifts of angular methyl protons 0 NMR spectroscopy-substituent effect on chemical shifts of angular methyl protons in bridged D-ring steroids
During initial investigations of the 14a,l7a-etheno-20-ketopregnane systcm, the use of NMR for the determination of the stereochemistry of the bridged Dsystem was attempted. A comparison of the NMR spectra of some of the first compounds prepared in that series (e.g., X and XI in Table ), if considered according to the theory of that time (I), would have resulted in assignment of wrong stereochemistry for these compounds. Fortunately, internal inconsistencies in the NMR data, together with the fact that other evidence (2) tended to favor the correct structures, recommended the use of other methods. Since a large group of compounds of rigorously proved structure (3-5) is now available for this series, it seemed appropriate to reconsider the spectra of these compounds in the hope of correlating the chemical shifts of their CISand Cls-hydrogens and of explaining the origin of the shifts in terms of currcnt theory. DISCUSSION '1 Measured in parts per million downficld from internal tetramethylsilanc in deuteriochloroform solution. b Re/erence 6. c Re/crence 9 . d Reference 3 .