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(O i Pr) 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(8 α )‐ and C(8 β )‐acyloxy isomers was observed in the δ ‐values of HC(8) in CDCl 3 . Within the 8 β ‐acyloxy compounds, the α ‐ and the β ‐epoxides were distinguished by either the J ‐value of HC(8) or the chemical shift of CH 2 (9). Within the 8 α ‐acyloxy compounds, two epoxide isomers were distinguished by the J ‐value of HC(10) in C 6 D 6 or in CD 3 OD.