(1R,2R)-2-(4-Methylenecyclohex-2-enyl)propyl (1R,4S)-camphanate

Abstract

Esterification of a single diastereomer of 2‐(4‐methylene­cyclohex‐2‐enyl)propanol, (II), with (1R,4S)‐(+)‐camphanic acid [(1R,4S)‐4,7,7‐trimethyl‐3‐oxo‐2‐oxabicyclo[2.2.1]heptane‐1‐carboxylic acid] leads to the crystalline title compound, C20H28O4. The relative configuration of the camphanate was determined by X‐ray diffraction analysis. The outcome clarifies the relative and absolute stereochemistry of the naturally occurring bisabolane sesquiterpenes β‐turmerone and β‐sesquiphellandrene, since we have converted (II) into both natural products via a stereospecific route.

Stereospecific Synthesis of (−)-β-Turmerone and (−)-Bisacurol

“…104.58, [a] 22 D À 19.6 (c 2.67, CHCl 3 )) is undoubtedly the one depicted in Scheme 1 as was established by an X-ray diffraction analysis of 2b [8]. 2 ) As an additional experimental feature, it is notable that pure (À)-isopulegon was provided in at least 90% isolated yield by application of Swern oxidation on 5 as described in the case of compounds 7a/7b (see below, Scheme 2 ) from pentane at À 208 is suitable to achieve maximum stereoisomeric purity of 2a.…”

“…Thus, 20a was found to be a 80 : 20 mixture of enantiomers (60% ee), while 20b was found to be totally enantiomerically pure with respect to 1 H and 13 C-NMR analysis of the camphanoates. 10 ) An X-ray diffraction analysis of the ()-(1R,4S)-camphanic acid ester of 20b was performed [22], establishing unambiguously not only the relative configuration of the ester, but also the absolute configuration of 20b (and indirectly that of 20a). cyanide ion in DMSO at room temperature resulted in clean formation of the unsaturated nitriles 22a,b.…”

“…7 ) Assignment of the relative configuration was performed by comparison of 1 H-and 13 C-NMR data with those of the well-known monoterpenes cis-and trans-piperitol [18] [19]. 8 ) Traces of concomitantly formed 17b were removed by selective formation of its Diels-Alder adduct with maleic anhydride, followed by alkaline workup and column chromatography. 9 ) At this stage, we also carried out esterifications of 20a,b with both (À)-(1S,4R) and ()-(1R,4S)-camphanic acid according to Gerlach and coworkers [21] to determine the enantiomer purity of these compounds.…”