Methodology for the Absolute Configuration Determination of Epoxythymols Using the Constituents of Piptothrix...

Since epoxythymols occur in Nature either as scalemic mixtures or as pure enantiomers, the knowledge of their chiral composition and of the absolute configuration (AC) of the dominant enantiomer turns out to be mandatory. This task has already been faced using 1,1-bis-2-naphthol (BINOL), as a chiral solvating agent in accurate ¹H NMR quantifications to determine the enantiomeric ratio, and vibrational circular dichroism (VCD) to evidence the AC of the dominant enantiomer. We now explore the use of electronic circular dichroism (ECD) to determine the AC of an epoxythymol for which time-expensive DFT calculations would be required unless the AC of a related molecule is already known, from either VCD studies or single-crystal X-ray diffraction analysis, since one could correlate the ECD Cotton effect with the AC because in ECD only chromophores and their neighborhoods are evidenced. This method is now applied by using the epoxythymols from Piptothrix areolare. Known areolal (1) and 10-cinnamoyloxy-8,9-epoxythymol isobutyrate (2) were isolated from the roots, while known 7-acetoxy-10-cinnamoyloxy-8,9-epoxythymol isobutyrate (3) and 10-cinnamoyloxy-7-hydroxy-8,9-epoxythymol isobutyrate (4), as well as the new enantiopure 7-acetoxy-10-cinnamoyloxy-6-hydroxy-8,9-epoxythymol isobutyrate (5) and 10-cinnamoyloxy-8,9-epoxy-6-hydroxy-7-northymol isobutyrate (6), were obtained from the extract of the flowers. Chemical correlation of epoxythymols 1 and 3 was achieved. Compounds 1-4 were obtained as scalemic mixtures, and 5 and 6 as the pure (8S) enantiomers. In addition, the new 10-cinnamoyloxy-7-oxo-8,9-dehydrothymol isobutyrate (7) was isolated from the roots. The structures of 5-7 followed from NMR and HRMS data, while enantiomeric compositions of 1-6 were determined by ¹H NMR-BINOL measurements. The AC determination for 2-6 was done by ECD using a sample of 1 to reference the ECD Cotton effect. In turn, the AC of 1 was determined by VCD and extensive DFT calculations. The ECD-BINOL methodology turned out to be some 500 times more sensitive than that combining VCD and ¹H NMR-BINOL.