Configurational isomers decalins

When we look at the structures of trans- and cis-decalin, it is apparent that a further plane of symmetry, through the ring fusion, is present in both structures. This means that each isomer is superimposable on its mirror image consequently, there are only two configurational isomers of decalin, one trans and one cis. 

The situation in trans- and ciG-decalin is complicated by the symmetry elements. If this symmetry is destroyed, e.g. by introducing dimethyl substituents, we get back to reassuringly familiar territory in which two chiral centres lead to four configurational isomers. The same is true in the trans- and cw-1,2-dimethylcyclohexane series. 

The fusion of rings of different sizes reduces symmetry in the structures instead of the rather unusual situation with the decalins, where there are only two configurational isomers, the hydrindanes exist in the... 

Figure 9.18 Configurations of decalin-bridged donor-acceptor complexes, (a) 2,7 equatorial-equatorial and (b) 2,7 axial-axial complexes. The donor acceptor centre-centre separation distance is 12.5 A for the equatorial-equatorial isomer and 6.5 A for the axial-axial isomer.

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

Probtem 9.48 Decalin, C,oH,g, has cis and trans isomers that differ in the configurations about the two shared C s as shown below. Draw their conformational structural formulas. 

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