The synthesis of poly-unsaturated aldehydes comprising four double bonds of formula ##SPC1##
Wherein one of the double bonds is represented by the dotted lines, has been thoroughly studied in the past decade. In particular, .alpha.-sinensal of formula Ia, wherein the double bond represented by the dotted lines is located in the main chain, is a particularly useful flavoring ingredient. This compound is in fact a sesquiterpenic aldehyde which has been isolated from the oil of China orange (Citrus sinensis) [cf.: J. Org. Chem. 30, 1690 (1965), Tetrahedron Letters 295 (1966)] and which owing to its specific organoleptic properties has acquired a great value as flavouring agent for the aromatization of foodstuffs in general and beverages.
For the .alpha.-sinensal (Ia) and .beta.-sinensal (Ib) whose structure is derived from cis or trans ocimene and from myrcene, respectively, the denomination .alpha. and .beta. has been adopted by analogy with .alpha.- and .beta.-farnesenes, respectively. In the past, a reverse system had been used (cf. Chem. Commun. 1968, 928), .alpha. in place of .beta. and vice versa.
Although several processes for the preparation of .alpha.-sinensal have been reported in the chemical literature [cf. eg.: Helv. Chim. Acta 50, 2445 (1967); British patent specification No. 1,227,243] none of them has so far found a suitable industrial application. The difficulties encountered by the use of the described prior known processes are of different nature, ranging from poor availability of the starting material to contamination by metal catalyst of the product obtained. It is not surprising therefore that further investigations have been undertaken with the primary goal of synthetizing .alpha.-sinensal in a more economical and industrially convenient way.