Anethole (1-methoxy-4-(1-propenyl)benzene) is of great commercial interest as a fragrance and flavouring because of its characteristic aniseed odor. More particularly, anethole is used as fragrance in washing and cleaning compositions or cosmetics, and as flavoring in the foods industry.
The preparation of anethole has long been known in the prior art. For instance, one way of obtaining anethole is from natural sources, for example from fennel oil or anise oil—see RU2064920 and CN102491884.
However, the production of fragrances from natural sources is usually costly, and the amounts thus obtainable are limited. Moreover, the purity or production volume of these fragrances often varies because of variable environmental conditions in the production of the raw materials from which they are isolated. There is therefore a great interest in at least partly replacing fragrances from natural sources with substances obtainable by synthetic means.
The prior art discloses various synthetic processes for preparation of anethole. For example, Bauer et al., Common Fragrance and Flavor Materials, 2001, 4th Edition, Wiley-VHC, describes the preparation of anethole by a base-catalyzed rearrangement of 1-methoxy-4-allylbenzene (estragiol).
A further method of preparing anethole, as described, for example, in SU261380 and SU355144, includes a Friedel-Crafts acylation of anisole with propionyl halides or propionic anhydride, followed by the reduction of the carbonyl group and subsequent elimination of water. The acylation of anisole with propionic anhydride can be effected, inter alia, with ZnCl2 and FeCl3 (see Maslozhirovaya Promyshlennost (1974), volume 9, pages 29-30).
CN103058835 describes a process for synthesizing anethole, in which a Friedel-Crafts reaction proceeding from anisole and propionyl chloride is conducted, followed by a reduction of the carbonyl group to the corresponding alcohol with the aid of NaBH4 and subsequent elimination of water with p-TsOH and KHSO4.
DE 2418974 B1 describes a process for preparing anethole, in which, in a first step, anisole is condensed with propionaldehyde in the presence of acidic catalysts to give a mixture of bis(methoxyphenyl)propanes. In a second step, the condensation products are cleaved into p-anethole, or o-anethole and anisole, by using catalytic amounts of acid and by heating to temperatures of 100 to 300° C. in the liquid phase. Specifically, the cleavage of the bis(methoxyphenyl)propanes is conducted in the presence of a catalytic amount of concentrated phosphoric acid at 200° C., with distillative removal of the cleavage products formed at about 5 to 30 Torr. Pure p-anethole is obtained from the distillate by fractional distillation. A disadvantage is that the bis(methoxyphenyl)-propanes are not completely cleaved, i.e. are only partially converted. Moreover, only moderate yields of anetholes are achieved. The long residence time of the bis-(methoxyphenyl)propanes and the cleavage products at elevated temperature additionally leads to increased formation of by-products, unwanted isomers, and oligomers and polymers.