13C-NMR chemical shifts of substituted squalenes and model compounds with a heteroatom in d-position to a double bond show that a steric interaction exists between the hetero-atom and the double bond. This points to the presence of "precoiled" conformations in the hetero-substituted squalenes and also in squalene itself. The nature of the interaction remains veiled, but it appears to be a general feature as is shown in a series of 2- and 3-substituted thiophenes with a double bond in the side chain.
van Dommelen, M. E., Ven, van de, L. J. M., Buck, H. M., & Haan, de, J. W. (1977). A 13C-NMR study of squalene, part II : functionalized squalene-like compounds. Recueil des Travaux Chimiques des Pays-Bas, 96(12), 295-301. https://doi.org/10.1002/recl.19770961203