E Conformations in Polycyclic Molecules

Hydrindanes have the five-membered ring in the envelope conformation with the six-membered ring in a chair. The c/s-indane (241) shows a 1,3-diaxial Rl—H interaction and one of the bridgehead hydrogens interacts with the two R groups, giving another gauche- i]/.e. interaction (similar to Corey s G values) Another 

3- diaxial-like interaction is apparent between R and R on the six- and five-membered rings. In the transisomer (242), the R-R1 interaction is lessened, but there is now a Rl—bridgehead hydrogen interaction. The 

3- diaxial R-H interaction remains, but the extended conformation in 242 is slightly lower in energy (about 1 kcal moRl, or 4.19 kJ mol l) than 243. 

In all four cases (239 - 242), there is little conformational mobility and steric interactions are more important than in a conformationally mobile system. Similarly, cis-decalin (243) and trans-decalin (244) show the same bent and extended forms with both six-membered rings in a chair conformation, There is a 

3-diaxial R-R interaction in 243 and an axial interaction of R with the bridgehead hydrogens. The R-R1 groups on adjacent rings are also in close proximity. Examination of 244 shows 1,3-diaxial R-H and R -H interactions but the overall energy of 244 is lower due to decreased nonbonded interactions. Eliel describes the conformations of several other bicyclic and polycyclic compounds. 

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