Carveol reduction

Di-hydro Carveol, Di-hydro Carvone.—Di-hydro carveol, the other important menthen-ol, is present in kummel oil, together with the corresponding ketone, di-hydro carvone, from which it may be obtained by reduction. This ketone is the di-hydrogen addition product of a mentha-di-ene ketone known as carvone which we shall presently consider. 

Alkylation of isovaleramide with 1,3-dichlorobut-2-ene yields (139) after methyl-ation acid-catalysed hydrolysis and internal aldol condensation gives ( )-piperitone. The value of piperitenone and isopiperitenone formation, probably via electrocyclic reaction of the pyrolytic acetic acid-elimination product from A - and A -isomers of (49), cannot be assessed in the absence of reaction yields. (S)-(-)-Pulegone is obtained in good yield from (- )-citronellol by oxidation with pyridinium chlorochromate followed by double-bond isomerization. Low-temperature reduction of ( —)-carvone to ( —)-cz5-carveol (140) and... 

Carveol (5) is one of the minor components responsible for the odour of spearmint, and is easily prepared by reduction of carvone. Isopulegol (6) is prepared from citronellal, as discussed in the section on menthol below, and is a precursor to other materials in the group. The phenols carvacrol (7) and thymol (8) are important in some herbal odour types, but the major use for thymol is as a precursor for menthol q.v. Piperitone (9) and pulegone (10) are strong minty odorants, the latter being the major component of pennyroyal oil. 1,8-Cineole (11) is the major component of such eucalyptus oils as Eucalyptus globulus. These oils are inexpensive and so there is no need to prepare cineole synthetically. Menthofuran (12) is an important minor component of mint oils and can be prepared from pulegone. 

Complex (17) is also known as an excellent catalyst for Oppenauer oxidation. For example, in the presence of (17) (5 mol%) carveol was oxidized by pivalaldehyde to provide the corresponding ketone in 91% yield (Scheme 6.59) [76b]. As a further synthetic application, MPV reduction/Oppenauer oxidation sequence of... 

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