Synthesis and NMR Spectroscopic Elucidation of Four Diastereoisomers of Oxygenated Bisabolane Side Chain

Four possible stereoisomers of a model compound of highly O‐bearing bisabolane sesquiterpenes were synthesized and their NMR spectra were compared. Starting from isopulegol, allylic oxidation and Grignard reaction afforded a mixture of alcohols at C(8), which was separated. After metathesis reaction, both α‐ and β‐epoxides were obtained via non‐stereoselective epoxidation, while VO(OⁱPr)₃‐catalyzed epoxidation afforded a single diastereoisomer selectively. NMR Spectra of twelve synthesized compounds, four stereoisomers of acetates, isobutyrates, and tiglates, were measured. A difference between C(8α)‐ and C(8β)‐acyloxy isomers was observed in the δ‐values of H? C(8) in CDCl₃. Within the 8β‐acyloxy compounds, the α‐ and the β‐epoxides were distinguished by either the J‐value of H? C(8) or the chemical shift of CH₂(9). Within the 8α‐acyloxy compounds, two epoxide isomers were distinguished by the J‐value of H? C(10) in C₆D₆ or in CD₃OD.