AN NMR AND MOLECULAR MECHANICS STUDY OF SQUALENE AND SQUALENE DERIVATIVES

Various squalene derivatives, including squalene, squalene 2,3-epoxide (monoepoxide, SQME), squalene 2,3;22,23-diepoxide (SQDE), 2-aza-2,3-d ihydrosqualene (SQN) and 2-aza-2,3-dihydrosqualene N-oxide (SQNO), wer e studied in chloroform solutions using 1D high-resolution H-1 spectra and C-13 longitudinal relaxation studies, 2D proton NOESY and COSY an d 2D proton-carbon HETCOR spectroscopy. A full interpretation of the H -1 and C-13-NMR spectra is presented. Staggered conformations along th e C11-C12 bond are favoured and a relatively rigid structure of the ce ntral part of the chain is indicated in relaxation and coupling data, while further away from the central part the molecular mobility grows. A detected NOE dipolar interaction between terminal and central parts of the molecule indicates the presence of dynamically folded structur es in solution. The proposed model also explains the selective reactiv ity of the mobile chain endings with respect to the central part which is protected by these moving ends. Different solvents at different co ncentrations induce some variations in this molecular model with a sho rtening or a lengthening of the mean path covered by the tail endings. Molecular mechanics and molecular dynamics calculations on the free s qualene molecule indicate that the mobility of the chain is almost equ ivalent in all its isoprenic moieties, and the greater mobility of the chain ends may be ascribed to co-operative movements from the center to the tails. The solvent probably plays an important role in hinderin g the motion of the central part of the molecule.