Fluxional nature in cyclohexane and cyclopentane: spherical conformational landscape model revisited

Abstract

Spherical conformational landscape model was revisited to include yet another class of cyclic compounds; the derivatives of cyclohexane. The updated model is not only capable of explaining Raman spectral features in fluxional cyclopentane but is also capable of revealing similarities between cyclopentane and cyclohexane derivatives for the first time. At the heart of the model lies the aspect of B/T ring coordinates (B/T conformational platform) that represents different levels of puckering (q). DFT-ωb97xd/6-311 + G* computations confirmed by MP2/aug cc-pVTZ computations were used to fully investigate 16 different derivatives of both cyclohexane and cyclopentane. Intrinsic reaction coordinate, IRC, computations were performed to gain insight into patterned inter-platform pathways connecting ring coordinates. These pathways revealed the coupling strength between bent/boat, B, and twist, T, ring coordinates. The coupling is found to be stronger for cyclopentane compared to cyclohexane. Some spectral features in the overlap region near 1400 cm−1 show promising signs on spin–spin relaxation, T2, mechanism. The work opens up an avenue for conformational studies of medium-sized rings. Also, ongoing studies to unravel potential relationships between conformational flexibility and bioactivity of cyclic compounds are underway. Conformationally mobile cyclic carbonic skeletons like cis-Octahydropentalenes are occurring in a number of natural products. A complete conformational analysis of cis-Octahydropentalenes reveals how some natural products populate their bioactive form. The pattern successfully explains the bioactive conformation in (+)-epi-goniofufrone and Hirsutic acid, both of which are natural products with potent bioactivity. Analysis like these are illustrative of how nature can use some special scaffolds to achieve maximal bioactivity, which in turn would fuel research in drug design.