Лако-красочные материалы — производство

The three most important cyclic monoterpenes are L-menthol, l — carvone and a-terpineol (including its esters). L-Menthol occurs in а number of mint oils and is used not only for its minty odour, but also,
and more importantly, for its physiological cooling effect. Its chemistry is of such interest and significance that it warrants a section of its own (see pages 70-76).

L-Carvone is the principal odour component of spearmint oil. Both the oil and synthetic L-carvone are used as ingredients in mint flavours. The synthetic material is made from D-limonene, which is the major component of orange oil and therefore is available as a by-product of orange juice production. Quest International is the world’s major producer of L-carvone and the chemistry used in the process is shown in Scheme 4.19. The chirality of the carvone is crucial to the odour, since the enantiomeric D-carvone has an odour reminiscent of dill or caraway rather than spearmint. It is therefore important that any synthesis of carvone leads to an enantiomerically pure product. It can be seen from Scheme 4.19 that both L-carvone and D-limonene owe their chirality to the carbon atom at which the isopropenyl group is attached to the cyclohexene ring. This structural feature is used in the synthesis to control the chirality of the product by controlling the regiochemistry of the reactions that take place at the opposite end of the ring.

The first step involves addition of the nitrosyl cation to D-limonene. This cation can be obtained by heterolysis of nitrosyl chloride, but in practice it is more convenient to generate it by cleavage of isopropyl nitrite using hydrochloric acid. Isopropyl nitrite is easily prepared from
sodium nitrite and isopropanol. The nitrosyl cation reacts preferen­tially with the more electron rich endocyclic double bond of d — limonene. It also adds regioselectively to the less substituted end, thus generating the more stable tertiary carbocation. This carbocation is trapped by the chloride ions present from the hydrochloric acid. The initial adduct is a blue liquid which is in equilibrium with the dimeric white solid, as shown in Scheme 4.19. Treatment of this product mixture with a base leads to elimination of hydrogen chloride and rearrangement of the nitrosyl group to give an oxime. Thus, the reaction product is L-carvone oxime which can be hydrolysed to give L-carvone.

Treatment of turpentine with aqueous acid leads to the formation of a-terpineol. The mechanism of this reaction is shown in Scheme 4.20, in which a-pinene is used as an example. Some hydration of a-terpineol to give the diol, terpin hydrate, can also occur, the balance between the products depending on the severity of the reaction conditions. The crude mixture is known as pine oil and is the main ingredient of pine disinfectants. Terpin hydrate can be easily converted into a-terpineol, since the ring hydroxyl group is more readily eliminated than that in the side chain.

Monocyclic monoterpene hydrocarbons occur in many essential oils and their by-products. They have relatively weak odours, although some add dryness and green notes to the oils containing them. This is particularly so for lime and petitgrain. D-Limonene (1) occurs in citrus oils whereas the L-isomer is found in pine. If limonene or other terpenes break down during processing to produce isoprene, then racemic limonene, dipentene, is found in the product as a result of the Diels — Alder reaction. Terpinolene (2) is the dehydration product of a — terpineol and so it is often present as an artefact. a-Phellandrene (3) occurs in eucalyptus oil. Since it is a 1,3-diene, it is an obvious precursor for Diels-Alder reactions and a number of speciality ingredients are

prepared from it in this way. During processing of turpentine and other terpene sources, often a variety of acid-catalysed reactions and aerial oxidations occur. /?-Cymene (4) is produced as a result of these processes, since it is one of the most thermodynamically stable of terpene structures. It does occur in essential oils and fragrances, but its main uses are as a thermally stable heat-transfer fluid and as a precursor for musks (see Musks, pages 91-101).

Terpinolene ot-Phellandrene p-Cymene

(2) (3) (4)

Many oxygenated monocarbocyclic monoterpenes are of use to the flavour and fragrance industry. Some are extracted from natural sources, and others are prepared from the major members of the family, usually by straightforward functional group interconversions.

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.