Лако-красочные материалы — производство

The alcohols geraniol-nerol, linalool, citronellol and their esters are the largest tonnage materials of this class. Their syntheses are described in detail above and no further explanation is necessary here. The chemical stability of these materials is limited by the unsaturation in them. To improve stability, particularly in oxidative media such as bleaches and laundry powders, various hydrogenated analogues have been devel­oped. In some, one of the double bonds is reduced, in others both are.

A number of hydrocarbons of this family add oily, green or herbaceous notes to essential oils. Two hydrocarbons, myrcene and dihydromyrcene (also known as citronellene), deserve mention as feedstocks for other fragrance ingredients.

Pyrolysis of /?-pinene gives myrcene as described above. Since it is a 1,3-diene, myrcene readily undergoes the Diels-Alder reaction with a variety of dienophiles. Addition of acrolein gives a mixture of regio — isomers, the major of which is shown in Scheme 4.12. This mixture is known as Myrac Aldehyde® or Empetal®. Hydration of the double bond in the tail gives Lyral®, a widely used muguet ingredient. The reaction with 3-methylpent-3-en-2-one (from the aldol reaction of methyl ethyl ketone with acetaldehyde) is more complex in that a greater number of isomeric products are produced. Acid-catalysed cyclization gives an even more complex mixture, known as Iso E Super®. This mixture has a pleasant, woody, amber odour which is believed to arise predominantly from only a few of its components. The adduct of myrcene with methacrolein is known by the slightly mislead­ing name of Precyclemone B®.

Scheme 4.12

Pyrolysis of pinane gives dihydromyrcene. Hydration of the more reactive, trisubstituted double bond gives dihydromyrcenol, as shown in Scheme 4.13. Direct hydration is difficult, so usually a two-stage process is used. The first involves the acid-catalysed addition of chloride, sulfate or acetate (from the corresponding acids), followed

by hydrolysis to give the alcohol. This alcohol was first introduced as a stable, fresh floral-muguet note for functional products, but its success in the after-shave Drakkar Noir caused a trickle-up to the fine — fragrance market, the reverse of the usual trend with new ingredients.

Hydration of the double bond of citronellal gives the compound known as hydroxycitronellal in a reaction analogous to that used to prepare Lyral®. Like Lyral®, hydroxycitronellal has a muguet odour. In acidic conditions, citronellal cyclizes to isopulegol and so the aldehydic group must be protected during the hydration stage. A typical sequence is shown in Scheme 4.14. The hydration is carried out in concentrated acid, under which conditions the oxazolidine ring is stable. Dilution of the medium allows hydrolysis of the protecting group to occur.

Citral is the key odoriferous principle of lemon oil and is therefore potentially very useful in perfumery. However, it is not stable to oxidation and so cannot be used in functional products containing bleach. Since lemon is associated with cleanliness and freshness, this represents a serious challenge for household products. One of the solutions that has been found is to convert citral into the more stable nitrile, known as geranyl nitrile. Often, nitriles have odours that closely resemble the corresponding aldehyde, this being a case in point. Geranyl nitrile can be prepared from either citral or from methylheptenone, as shown in Scheme 4.15.

There are several cyclic ethers derived from acyclic monoterpenes which are of importance at lower levels in fragrances. Allylic oxidation of citronellol can be used to introduce a leaving group which allows cyclization to form the pyran, rose oxide. Chlorination was one of the first oxidation techniques employed; various others, including electro­chemical methods, have since been developed. An outline of the synthesis is given in Scheme 4.16. Rose oxide occurs in rose and geranium oils, to which it imparts a characteristic dry, green, rosy top-note.

Scheme 4.16

Structurally related to rose oxide is the hydroxypyran shown in Scheme 4.17. This material is known under the tradenames Florosa® and Florol®. It is prepared by the Prins reaction between isoprenol and isovaleraldehyde.

Scheme 4.17

Oxidation of the trisubstituted double bond of linalool leads to the isomeric linalool oxides and their esters, as shown in Scheme 4.18. These materials have odours ranging from floral to woody.