Synthesis and NMR spectroscopic elucidation of four diastereoisomers of oxygenated bisabolane side chain

Article abstract of DOI:10.1002/hlca.201400395

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.