ChemInform Abstract: STABLE CARBO CATIONS FROM TERPENOIDS. I. GENERATION OF CARBO CATIONS FROM NATURAL SESQUITERPENES ‐ HEXAHYDRONAPHTHALENE DERIVATIVES

Abstract

Hexahydronaphthalin‐Derivaten.

Cationic molecular rearrangements of natural terpene compounds in superacidic media: reality and predictions

“…A temperature-dependent change in the ratio of protonation rates of two endocyclic double bonds has been described. 23 Thus ions 66 and 49 are generated upon protonation of the diene 48 at 7120 8C in HSO 3 F ± SO 2 FCl, whereas heating to 750 8C yields ions 67 and 49 in a * 2.5 : 1 ratio. If the ions are generated at 770 8C, the ion 49 is predominantly formed (see Section II).…”

“…The pathways of molecular rearrangements of mono-and dications often differ, which largely increases the diversity of terpene transformations. Thus it has been shown 23 that molecular rearrangements of mono- (65) and dications (66) generated from the same natural diene 48 give rise to diastereomeric allylic ions 49 and 67.…”

“…This selectivity is provided by a combination of regio-and stereocontrol. Thus studies of diene 48 conversions in HSO 3 F ± SbF 5 ± -SO 2 FCl have shown 23 that at temperatures from 7120 to 750 8C all ionic isomerisations occur exclusively as 1,2-hydride shifts and deprotonation-protonation, but not as C7C shifts. Although the theoretically possible protonation ± deprotonation reactions and 1,2-hydride shifts in the diene 48 can lead to an extremely complex mixture, this gives rise to a limited number of stable ions that are generated by several relatively short pathways.…”

“…This suggestion seems unlikely, since diastereomeric ions 49 ± 51 generated separately are stable and are not interconverted under these conditions. 23…”

“…Thus stable diastereomeric allylic ions 49 ± 51 generated from (7)-a-murolene (48) differ only in the configuration of the asymmetric centre at C (6) or C (10). 23 This, in turn, affects the conformation of the corresponding ion and, ultimately, the dihedral angle between the vacant p-orbital of the cationic centre and the C(10) ± C(9) s-bond. This angle determines the probability of intramolecular rearrangements in these ions with a `turnover' of a six-membered ring relative to another.…”