Piperitol

An optically active, secondary terpene alcohol. ( —)-Piperilol is found in various eucalyptus oils and (-l-) piperitol in the oil from a species of Andropogon. A somewhat viscous oil of pleasant smell. It yields piperitone on oxidation with chromic acid. 

Fig. 5. Alkaline hydrolysis of piperityl chloride to cis- and /n j -2-menthen-l-ol and acid-catalyzed rearrangement to piperitols (78) and (79).

Piperitol is a secondary alcohol, corresponding with the ketone, piperitone, occurring in several of the so-called peppermint group of eucalyptus oils. ... 

Mentholfrom (—)-Fiperitone or Piperitol. (—)-Menthol can also be prepared from (-)-piperitone, the main component of Eucalyptus dives Typus oils. Hydrogenation in the presence of Raney nickel yields a mixture of menthols, from which (—)-menthol can be separated by crystallization and saponification of its chloroacetate. 

Analogously, (+)-/ra i-piperitol (obtained from a.- or /3-phellandrene via piperityl chloride [80]) can be hydrogenated to give a mixture of 97% (+)-isomenthol and 3% (+)-menthol. Pure (+)-isomenthol is obtained by crystallization and undergoes rearrangement to give an equilibrium mixture of (+)-neomenthol and (-)-menthol the latter is separated by distillation. 

Mixed arylation of bifunctional substrates, such as those containing two /3-ketoester groups in their structure can be made. This possibility was used to synthesize unsymmetrically substituted compounds containing two different aryl groups (Scheme 2). One-pot treatment of the dibenzyl 3,7-dioxobicyclo [3.3.0]-octane-2,6-dicarboxylate 6 with a mixture of two aryllead triacetates 7 and 8 afforded a mixture of the unsymmetrical diaryl derivative 10 with the two symmetrical diaryl compounds 9 and 11. When the arylation was performed with 1.1 equiv. of each aryllead reagents 7 and 8, a mixture of 9-11 in a ratio 0.46 1 0.72 was obtained. This ratio became 0.43 1 0.49, when 0.86 equiv. of 7 and 1.33 equiv. of 8 were used. This compound 10, isolated in a 33% yield, was eventually elaborated to complete a total synthesis of ( ) methyl piperitol.25... 

Benzyl acetate, geraniol, a-ionone, phenylethyl alcohol, terpineol fert-butylcarbinol, cyclohexanone, menthone, piperitol, 1,1,3-trimethyl-cyclo-5-hexanone... 

Cp2TiCl-mediated 5-exo-trig cyclizations have been intensively exploited by Roy et al. for the total synthesis of ( )-dihydroprotolichesterinic and ( )-roccellaric acids, ( )-sesamin, ( )-dihydrosesamin, ( )-acuminatin, ( )-eudesmin, ( )-lariciresinol, ( )-pinoresinol, ( )-piperitol, ( )-acumi-natin methyl ether, (zh)-sanshodiol methyl ether, ( )-piperitol methyl ether, ( )-pinoresinol monomethyl ether, ( )-lariciresinol monomethyl ether, and ( )-lariciresinol dimethyl ether [ 100-103]. Moreover, this group has very recently reported the enantioselective synthesis of (-)-sesamin (Scheme 19), (-)-dihydrosesamin, (-)-acuminatin, and (-)-methyl piperitol by radical cyclization of chiral epoxides initiated by Cp2TiCl [104]. 

Other useful p-menthane syntheses of no great novelty are of cis- and trans-piperitol from 2a,3o -epoxycarane (silica-catalysed rearrangement to ds-p-menth-2-en-l,8-diol is also reported), of ( )-dihydrocarvone, isopulegone, and p-menthofuran via /S-keto-sulphoxides, of p-mentha-l,4(8)-diene via a bromination-dehydrobromination sequence, and of trans-carveol by benzoyl peroxide-CuCl oxidation of a-pinene. Further details for the conversion of (-)-(142) into (+)-(142), via its epoxide, are reported (Vol. 5, p. 25 cf. Vol. 3, p. 44). " ... 

Menthone was elegantly transposed into carvomenthone by the sequence (196)->(197).439 Piperitol was isomerized into isomenthone over cobalt catalysts.440 Piperitone, p-menth-3-en-2-one, and 8,9-dihydrocarvone can by pyrolysed to aromatics that have lost the Pr group in fair yields (30—50%),441 and menthone can be converted into piperitone and other p-menthen-3-ones via bromination and Zn treatment.442 Pulegone on reaction with HOC1 formed 4-chloro-/>-menth-8-en-3-one,448 and O-acetylated dienolates of pulegone have been prepared.444 Carvo-menthene oxide was isomerized to p-menth-1 (7)-en-2-ol, carvotanacetol, and cyclopentane derivatives over solid acids and bases,445 and 6-thiophenoxy-8,9-... 

Reduction of the more unsaturated 3-oxygenated menthanes is a well-known route to menthones thus treatment of pulegone (46) with cyclooctadiene and an iridium catalyst yielded 61% menthone (521) and 39% isomenthone (583). cw-Piperitol (584) was also converted to Isomenthone (583) by treatment with low-valence cobalt complexes coordinated with phosphines. ... 

The essential oil of Eucalyptus dives contains L-piperitone and this provides a starting material for L-menthol using the process shown in Scheme 4.25. The L-piperitone is reduced to a mixture of piperitols, which are separated, and the major isomer, D-/ra j-piperitol, hydrogenated to give D-isomenthol containing a small amount of D-menthol. After purification, the former can be isomerized into L-menthol using aluminium isopropoxide as catalyst. About 30 tonnes per annum of l-menthol are produced by Keith Harris Co. in Australia using this route. 

Some unexpected rearrangements occur when palladium chloride (PdCl2) is allowed to react with allyl alcohols of the p-menthane series in the presence of carbon monoxide. 7t-Allylpalladium compounds are formed, but after 10 days that from piperitol (128) has rearranged to the 3,4,8-tn7iapto-complex (162) which is obtained from pulegol (163) after 1 h. Menth-3-en-2-ol (127) leads to the same complex after 9 days." ... 

---