Evans. breit Conformational Analysis: Bicyclic Ring Systems Chem 206 Conformations of Bicyclic Ring Systems Estimate the energy difference between the two methyl-decalins shown beloy see Elier, p 780 Hydrindane Ring System(6/5) The steroid nucleus provided the stimulation for the conformational analysis, particularly tems. D.H. R. Barton was awarded a nobel prize in 1969 for his contributions in this area flexible igid Decalin Ring System(6/6) AG°=-0.5kcam mobile H I The turnover to favor the cis fusion results from the entropic preference for the less ordered cis isomer The 5-5 Ring System 2 4 kcal/mol 0 Relative△G AG°=+64 kcal/mol Let's identify the destabilizing gauche butane interactions in the cis isomer Gauche-butane interactions c1→c2 c1→c3 c4→c3 AB Trans AB Cis G(est)=3(0.88)=2.64 kcal/mol Rationalize the conformational flexibility of a AB Trans Vs. AB Cis SteroidMe Me H H H O Me H H H H H H H H H H H H H H H Me H H H R C D A B A B Me H H H H H C H H Me H H H R D Evans, Breit Conformational Analysis: Bicyclic Ring Systems Chem 206 Conformations of Bicyclic Ring Systems The steroid nucleus provided the stimulation for the development of conformational analysis, particularly of polycyclic ring systems. D. H. R. Barton was awarded a Nobel prize in 1969 for his contributions in this area. 2.4 kcal/mol 0 Relative DG° rigid Decalin Ring System (6/6) mobile Let's identify the destabilizing gauche butane interactions in the cis isomer 1 2 3 4 Gauche-butane interactions C1 C2 C1 C3 C4 C3 DG°(est) = 3(0.88) = 2.64 kcal/mol Estimate the energy difference between the two methyl-decalins shown below. Hydrindane Ring System (6/5) flexible rigid DG° = –0.5 kcal/mol ■ The turnover to favor the cis fusion results from the entropic preference for the less ordered cis isomer. The 5-5 Ring System favored A/B Trans A/B Cis Rationalize the conformational flexibility of a A/B Trans vs. A/B Cis Steroid! 1 4 7 11 10 13 14 17 DG° = +6.4 kcal/mol see Elier, p 780