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.