(−)‐β‐Caryophyllene (1) adopts three conformations in solution: αα(48%), βα(28%), and ββ(24%). The conformations were identified by an analysis of the 13C‐ and 1H‐NMR spectra at −87.2 and −153.8° in connection with APT, HETCOR, and COSY spectra, and subsequent NOESY experiments. The activation parameters of the conversion αα → βα were determined from a bandshape analysis of exchange‐broadened 13C‐NMR spectra of 8‐[methylene‐13C]‐1 to give ΔH≠ = 5.9 ± 0.3 kcal/mol, ΔS = −8.1 ± 1.8 cal/mol. · K. and ΔG 298≠ = −8.3 ± 0.8 kcal/mol. The observed population ratio of the different conformers is best described by MM3.
Several new approaches for the enantioselective synthesis of the chromane moiety of vitamin E are described. Sonogashira coupling of 3a with the alkyne 4 and subsequent elimination gave 6, which was bis(hydroxylated) in 93% yield and with 85% ee. Recrystallization gave enantiopure 7a, which was hydrogenated and transformed into the vitamin E precursor 11. The bis(hydroxylation) of 18 and 21 to give 9 and 22, respectively, was less than satisfactory, proceeding with ee values of 28% and 18%. In contrast, stereoselective allylation of ketone 15 followed by removal of the protecting group or ozonolysis of the allyl moiety furnished the allyl alcohol 26 and the aldehyde 27, respectively, in almost enantiopure form, which again could be used as precursors for vitamin E. Partial hydrogenation of 5a gave the alkene 32a and that of 28 the alkene 30b, both of which show interesting atropisomerism.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.