The essential oil of Mentha rotundi folic cultivated in Japan is highly dextro-rotatory. It has a high refractive index and a high specific gravity as compared with those of European oils. By polarographic method it was found that this oil contained a substance which was more easily reduced at the dropping mercury cathode than any other oils of Mentha plants were. The principal component was isolated as a crystal by vacuum fractional distillation. Its physical properties are as follows: m. p. 27.5° [α] ¹⁰D + 199.6⊃ n¹⁰D 1.5045 d³⁰4 1.053 This substance gives negative test with ferric chloride solution and Schiff's reagent. It reduces Fehling solution and ammoniacal silver nitrate. Its Malaprade reaction is positive after being stood for several hours. It shows an intense absorption maximum at 260 mμ (logε = 3.95 in EtOH). It gives easily monosemicarbazone which melts at 180° (with decomp.) and has λ max at 273 mμ. From the results of elementary analyses of the original ketone and its semicarbazone, this substance is found to have the molecular formula C₁₀H₁₄O₂ and to be an α,β-unsaturated ketone. The structure of rotundifolone has been established to be 1-methyl-4-isopropylidene-1, 2-epoxy-cyclohexanone-3 from the following facts: 1) The ultraviolet spectra of rotundifolone and its semicarbazone show the absorption bands which are attributable to the presence of α,β-unsaturated ketone. 2) The infrared spectra of rotundifolone and its epoxide do not show the absorption band characteristic of hydroxyl group near 3μ. 3) Catalytic reduction of rotundifolone over palladium barium sulfate gives dihydro-rotundifolone, which rearranges to diosphenol in acid media. This indicates that rotundifolone has p-menthane carbon skeleton, in which each of the carbon atoms 2 and 3 has an oxygen atom. 4) The formations of triol C₁₀H₂₀O₃ and ketol C₁₀H₁₈O₃ from dihydroro-tundifolone suggest the presence of an epoxide linkage in the original ketone. 5) By the periodic acid oxidation of diol (XI), acetone is obtained. This fact shows that rotundifolone has an isopropylidene group. The reactions of rotundifolone above mentioned are shown in Fig. 10. By lithium aluminum hydride reduction, various diols are obtained from rotundifolone and dihydro-rotundifolone, as shown in Fig. 14. Rotundifolone which has two oxygen atoms in p-menthone ring, is an interesting substance from the biochemical point of view. It has been shown from the cytogenetical studies that Mentha rotundi folia has the least number of chromosomes and supposed to be one of the prototypes or the types connected to the prototype of Mentha. This interpretation seems to support that rotundifolone will be a "turn-table" between the menthone and carvone series, instead of diosphenol which was suggested by Walker and Read. Diosphenol is well known to be one of the components of the leaves of Barosma, but has never been isolated from the essential oils of the genius Mentha, as a natural product. By mixed melting point of semicarbazones, dihydrorotundifolone has been found to be identical with l-piperitone oxide which was recently shown to be the principal component of the essential oil of American Mentha sylvestris by Reitsema et al. In 1955 K.K. Chakravarti et al isolated a new α,β-unsaturated ketone C₁₀H₁₄O₂ from Indian Mentha viridis and named it as "mintglyoxal". The infrared spectra of their ketone, its epoxide and diol from epoxide coincided completely with those of our rotundifolone series respectively. However mintglyoxal formula presented by them can not be accepted for rotundifolone from our experimental results.
新テルペンケトンRotundifoloneの研究
