Menthenes Derivatives

With acidic potassium dichromate adsorbed on silica-zirconia, regioselective allylic oxidation is achieved. This reagent is better than CrO -S.S-dimethylpyrazole for oxidation of 1-menthene derivatives because the 3-keto products are largely absent. 

Chemical Properties. Hydrogenation of menthols yields / -menthane oxidation with chromic acid or catalytic dehydrogenation yields menthones. Dehydration under mild conditions yields 3-/ -menthene as the main product. Reaction with carboxylic acids or their derivatives yields menthyl esters, which are used mainly as aroma substances and in pharmaceutical preparations and formulations. The esterification of menthols with benzoic acid is used on an industrial scale in the resolution of racemic menthol. 

For the Type A monocyclic terpene hydrocarbons (para, ortho, and meta derivatives of menthane) use the menthane, menthene, and menthadiene names and the well-established fixed numbering of menthane. 

Rule 2 Form the names of univalent radicals derived from the unsaturated menthane-type monocyclic terpene hydrocarbons containing one double bond, which may be either in the ring or outside the ring, by replacing the final e" of the hydrocarbon name (menthene) with "yV That is menthenyl. 

Derivatives of Menthene.— The most important alcohols and ketones derived from the menthene unsaturated group of terpenes are terpineol, di-hydro carveol, di-hydro carvone and pulegone. The first one, the alcohol terpineol, occurs in its dextro form in cardamon oil and marjoram oil, in its leoo form in neroU oil and in its inactive form in cajeput oil. The constitution is proven by Perkin s synthesis from As-tetra-hydro para-toluic acid by means of the Grignard reaction. 

Monoterpenes derive fi om fra 5-/ -menthane (4-isopropyl-1-methyl-cyclohexane), respectively from the unsaturated menthene, CioHig, or menthadiene, CioHie a-terpinene and S-limonene are isomeric forms of menthadiene see H. Beyer, W. Walter, Lehrbuch fur Organische Chemie, Hirzel Verlag, Stuttgart, 1984. 

Essential oil analyses of note this year are of Buchu leaf (some p-menthane sulphur derivatives), Hyssopus officinalis (methyl myrtenate, 2-hydroxyisopinocamphone, pinic acid, and pinonic acid), Laggera aurita (m-menth-6-en-8-ol), some Cymbopogon species (up to 89% of unusual p-menthadienols), and Trichostema lanceolatum (55% p-menthen-4-ol). There... 

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-... 

Derivatives of m-menthenes and m-menthadienes were also almost exclusively produced on treatment of car-4-ene oxide664 or 5-acetylcar-2-ene oxide665 with HC1. Reaction of car-3-ene oxide with ZnBr2 gave m-menth-5-en-2-one.666... 

The only derivative of 1,4-cineole (554) we feel constrained to mention is not even a natural product. As a herbicide, however, Cynmethylin (622) will doubtless soon be present in the environment. It was synthesized by epoxidation of l-menthen-4-ol (547), using vanadium-catalyzed rerf-butyl hydroperoxide. 

Neomenthyl derivatives eliminate to give a mixture of 2- and 3-menthenes but menthyl derivatives only yield 2-menthene under E2 conditions as anti stereospecificity is only possible in this case (88) . ... 

It is important to note that when El elimination is effected on the epimeric chloro-derivative [neomenthyl chloride, (l/ 35,45)-3-chloro-4-(l-methylethyl) methylcyclohexane]. Scheme 7.33, which has the chlorine axial and the two alkyl substituents equatorial, and where, as a consequence of that arrangement, there are now two anti (or antarafacial) and axial protons situated P to the chlorine (Cl), the same product mixture is obtained, but now, with anri-elimination possible, the ratio of 3-menthene 2-menthene is 98 2. 

Together with (H-)-pulegone (78%) [derived from alcohol (49)], two new /7-menthene ketones, viz. (+)-4-hydroxypiperitone (92) 410) and ( —)-8-hydroxy-4-/7-menthen-3-one (93) 411), were isolated from Seto-mint grown in the Tokai area. Both hydroxyketones (92) and (93) develop a strong minty odor impression 443). A detailed report on the natural occurrence, the methods of preparation and the chemistry of piperitone [carbonyl derivative of (48)] and related compounds was published recently 422). 

Mercaptoketones (83) and (84) can be prepared in a simple manner by adding H2S to pulegone [carbonyl derivative of (49)] which was also found to be a component of Buchu leaf oil (311). Hydroxyketones (91) and (93), which belong to different steric series, are both formed by photooxygenation of the corresponding pulegones (552). Furthermore, 3-hydroxy-6-menthen-2-one (94) in the form of its angelate was initially detected in the root oil of a Pluchea species (48). 

Acyclic terpenes and tetramethylcyclohexane-type representatives are named systematically. Unsaturated derivatives of the above trivially named systems are characterized in the usual way, e.g. 3-p-menthene, 2,5-norbornadiene etc. 

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