Stereochemistry and Conformational Anomalies of 1,2,3- and 1,2,3,4-Polycyclohexylcyclohexanes

1,2,3-Tricyclohexylcyclohexane (3) and
1,2,3,4-tetracyclohexylcyclohexane (4) were synthesized by
catalytic
hydrogenation of 1,2,3-triphenylbenzene and 1,2,3,4-tetraphenylbenzene,
respectively. The systems provide examples
of three conformational anomalies:  axial/equatorial,
twist-boat/chair, and eclipsed/staggered stability reversals.
Two
axial substituents are present in the lowest energy conformation of
cis,trans-3. The substituents in
all-cis-3 are
located in an equatorial−axial−equatorial disposition, with the
axial substituent oriented in a nearly eclipsed
arrangement with the central ring. In
all-trans-3 two substituents are gear meshed, which
results in a nonstaggered
arrangement. The central ring in
cis,trans,cis-1,2,3,4-tetracyclohexylcyclohexane
exists in a chair conformation while
in the cis,trans,trans isomer this ring adopts a
twist-boat conformation.