Butane gauche interactions

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... 

Just as conformational behavior in cyclohexane has been explained semi-quantitatively in terms of gauche-butane interactions (cf. 3 4, R = Me), corresponding gauche-propylamine (C—C—C—N) and other interactions can be used to discuss conformational equilibria involving heteroatoms.26... 

Torsional contribution cf. structure with no gauche — butane interactions... 

Actually, nonbonding interactions are already included in the torsional term (as gauche-butane interactions) we might have used an ethane-type torsional function and accounted for CH3/CH3 interactions entirely with nonbonded terms. However, in comparing calculated relative energies the torsional term will cancel out. 

When the chair-boat-chair interconversion occurs with a substituted cyclohexane such as methylcyclohexane, the two cliair conformations are no longer equivalent because the methyl is equatorial in one and axial in the other. The axial methyl group introduces two gauche-butane interactions that are not present in the equatorial-methyl conformation ... 

Includes one skew-pentane interaction, figured as equivalent to four gauche-butane interactions. 

The preference for attack at the Cg-position can clearly be attributed to developing gauche butane interactions with the R-substituent for axial attack at C2, thus necessitating equatorial attack at Cj with consequent increase in the activation energy owing to developing non-bonded twist interactions in 60 (Scheme 41). In contrast to j9-nitrostyrene, 1-nitropropene undergoes both axial and equatorial attack on both 3- and 4-substituted cyclohexanone enamines, except for reaction at C2 of the 3-substituted enamine where only equatorial attack occurs . 

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