Лако-красочные материалы — производство

The ionones and damascones are derived in nature from the degrada­tion of carotenoids. Similarly, the related irones are formed by degradation of other higher terpenes. The ionones are synthesized from citral by aldol condensation with a ketone to form what are known as |/-ionones, which are then cyclized using an acid catalyst, as shown in Scheme 4.39. Some specific syntheses are shown later in Scheme 4.42, along with the syntheses of vitamin A and carotene. The ionones possess odours which are reminiscent of violet, sometimes also with woody notes.

The nomenclature system used for ionones is shown in structure 33. The Greek letters a-, /?- and y — refer to the position of the double bond after ring closure. The prefixes n — and iso — refer to the position of the alkyl group if a higher ketone than acetone is used in the aldol condensation. For example, the ionone which is produced in greatest quantity is known as a-isomethylionone (34). (It should be noted that some older texts erroneously refer to this material as y-ionone.) With

О

a-Isomethylionone

(34)

any given feedstock, the reaction conditions can be manipulated to produce a desired isomer predominantly. It is important to maintain strict control of process and distillation conditions to ensure reprodu­cible odour quality. Quite a range of ionone derivatives are used in perfumery, each having its own particular odour character.

The damascones are a group of materials related to the ionones, but in which the enone unit is transposed. They are components of rose oils and have very intense fruity-floral odours. The damascenones, which contain a second double bond in the ring, are also very much sought after. One synthesis of a-damascone from methyl a-cyclogeranate is shown in Scheme 4.40. A double Grignard addition gives the tertiary alcohol. Heating of this in the presence of base, to prevent elimination

of the alcohol, gives the ketone via an ene reaction. The prefixes а-, /?-, and y — have the same meaning in the damascone series as in the ionones.

The irones are higher analogues of the ionones, in that they contain an additional methyl group in the cyclohexane ring. They are components of orris and have odours reminiscent of that source. One example of a synthesis of the irones, starting from pinene, is given in Scheme 4.41.

Decarbonylation of the ozonolysis product of a-pinene gives a methyl trimethylcyclobutyl ketone. Addition of acetylene to this, followed by the Carroll reaction, gives (trimethylcyclobutyl)heptadienone. The cyclobutane ring is opened by acid to give a methylated |/-ionone which, on cyclization, produces irones. Once again, the Greek prefixes have the same significance as in the ionone series.

It is obvious that the syntheses of the damascones and irones are more difficult than those of the ionones, with the inevitable result that these are much more expensive products.

The economics of ionone production is affected by the fact that ionone is an intermediate for vitamin A and carotene production. However, the volumes of production of ionones are not far from those of the A vitamins and so, while some companies produce both, others can compete effectively even though they are only active in one. Scheme 4.42 shows the routes used to produce vitamin A and carotene from the various monoterpene precursors.