Лако-красочные материалы — производство

A large number of aliphatic fragrance ingredients are used, but few in significant tonnage. This is largely because the materials of use are mostly aldehydes, nitriles and lactones, the majority of which have very intense odours that limit the amount that can be incorporated into a fragrance. A number of volatile esters are also used to give fruity top — notes but, again, these are not used at high levels. The majority of ingredients of this class can be prepared by straightforward synthetic reactions and functional group interconversions, starting from both natural and petrochemical precursors. Some examples are shown below (45-50).

5-Decalactone Hexyl acetate

(peach, coconut) (fruity, pear)

(47)

OMe

Beauvertate® (vegetable, earthy)

(50)

The aliphatic fragrance materials of natural origin are mostly derived from fatty acids and related materials. As a result of their biosynthetic pathway (see Chapter 3), those formed directly all have an even number of carbon atoms in the chain. Any with an odd number of carbons in the chain are likely to be breakdown products. Fatty acids are useful precursors for aliphatic fragrance materials, but the same observations about the number of carbon atoms in the chain, of course, also apply. Thus, for example, octanal can be prepared by oxidation of octanol, which can be obtained from coconut oil. Similarly, materials with 10-, 12- or 14-carbon chains can be obtained from other fats. One example of a fatty acid being used in fragrance is myristic acid, the isopropyl ester of which is employed as a solvent. Once a fatty acid or alcohol is available, appropriate oxidation and reduction reactions open routes to

the other members of the series. To take octanol as an example again, it can be dehydrogenated to give octanal, which is used both as an ingredient in its own right and as a precursor for other materials such as HCA (see page 107).

Two major reactions are used to break longer chains and give odd- numbered fragments as fragrance building blocks. The first is oxidative cleavage of a double bond in an unsaturated fatty acid. Ozonolysis is a convenient method for doing this and is used to provide pelargonic acid (heptanoic acid) and azelaic acid (nonanedioic acid) from oleic acid, the major constituent of olive oil, and brassylic acid (tridecanedioic acid) from the erucic acid (13-docosenoic acid) of rapeseed oil.

The other reaction is pyrolytic cleavage (via a retro-zne reaction) of a fatty acid containing a homoallylic alcohol in the chain. This is used to produce two important feedstocks, heptanal and undecylenic acid, from ricinoleic acid, the major fatty acid component of castor oil. The mechanism of this reaction is shown in Scheme 4.61.

co2h

C02H

Scheme 4.61

Undecylenic acid is important as a starting material for a number of fragrance materials, some of which are shown in Scheme 4.62. The two aldehydes are obtained by simple functional group manipulation. Both are known by trivial, and somewhat misleading, names in the industry; 10-undecenal as Aldehyde Cll undecylenic or simply Aldehyde Cl 1, and undecanal as Aldehyde Cll undecylic. Treatment of undecylenic acid with a strong acid causes the double bond to migrate along the chain by repeated protonation and deprotonation. Once a carbocation has formed on the fourth carbon of the chain, it can be trapped by the carboxylic acid group to form a stable y-lactone, undecalactone, which has a very powerful coconut-like odour. Esterification of undecylenic acid followed by anti-Markovnikov addition of hydrogen bromide

Scheme 4.62

gives the co-bromoester. Addition of butan-l,4-diol to this with sub­sequent polymerization and depolymerization (see page 99) gives the musk Cervolide®.

Synthetic precursors for aliphatic materials mirror the pattern of their naturally derived counterparts in that the commonest units are even in carbon chain length, because they are usually derived from ethylene through oligomerization. Thus, coupling of two ethylene molecules produces a four-carbon chain, three produces six and so on. To obtain an odd number of carbon atoms in the chain, one of the simplest techniques is to add a single carbon to an even chain; this can be achieved, for example, by hydroformylation. Hydroformylation also introduces an alcohol function and opens the way for oxidation to aldehydes and acids. Three carbon units are available from propylene as well as by reaction of ethylene with a 1 carbon unit. cw-Hex-3-enol occurs in a number of natural sources, such as freshly cut grass and strawberries. It possesses a very intense odour, which falls into the odour class known to perfumery as green. Green odours are those that resemble foliage, and stems of plants. m-Hex-3-enol is very character­istic of cut grass and is used to add a fresh green topnote to fragrances. A number of its esters, such as the acetate and salicylate, are also of use.

Scheme 4.63

The synthesis of these materials is shown in Scheme 4.63. But-2-yne is obtained as a by-product stream from a petrochemical process. The hydrogen atoms adjacent to the acetylene bond are acidic enough to be removed by a very strong base, which allows the triple bond to migrate to the end of the chain. Once there, the terminal hydrogen atom is lost to give the relatively stable acetylide anion, and eventually all of the material is present in this form. The anion reacts with ethylene oxide to give m-hex-3-ynol, which can be converted into cw-hex-3-enol by hydrogenation over a Lindlar catalyst. The same acetylide anion can also be produced from 1,2-butadiene which is another petrochemical by-product.

The Prins reaction, the acid-catalysed addition of an aldehyde or ketone to a double bond, is a useful reaction in perfumery chemistry and a number of aliphatic fragrance ingredients are prepared in this way. One example is shown in Scheme 4.64. When oct-l-ene is reacted with formaldehyde, the result is a complex mixture of products of which the pyran shown in Scheme 4.64 is the major component. In the case of this pyran, the octene molecule reacts with two molecules of formaldehyde to give an intermediate cation, which is trapped by the acetic acid present in the reaction medium. The product mixture is widely used for its fatty jasminic character and is sold under a variety of trade names, such as Jasmopyrane®.

Two other Prins reactions are shown in Scheme 4.65. The precursor for both of these is 2-methylpent-l-en-4-ol, which is produced from acetone. Acetone readily undergoes a self-aldol reaction in the presence of base to give diacetone alcohol, 2-methylpentan-4-on-2-ol, which can be reduced to hexylene glycol, 2-methylpentan-2,4-diol. Careful dehy­dration under mild conditions gives the unsaturated Prins precursor. Dehydration under stronger conditions gives 2-methylpenta-1,3-diene, which is the more thermodynamically stable of the two possible diene products. Prins reaction of 2-methylpent-l-en-4-ol with pentanal gives Gyrane® and with benzaldehyde gives Pelargene®. In both cases, the cationic intermediate is trapped by the alcohol of the starting material to form a pyran ring. Both products have odours which are rosy and green: Gyrane® is fresh and radiant and Pelargene® resembles crushed leaves. Methylpentadiene is also a useful precursor for fragrance ingredients; it undergoes a Diels-Alder reaction to give Ligustral® (Scheme 4.65), a very intense green ingredient in its own right and also a precursor for Karanal®, a powerful ambergris material.