The synthesis of safranal

Abstract

Introduction: Safranal is a cyclic monoterpene which occurs naturally in the saffron plant as a glucoside, picrocrocin. In 1922 Winterstein and TeleCzky obtained safranal by hydrolysis of picrocrocin, but its structure was not determined. Kuhn and Winterstein hydrolyzed picrocrocin in 1933 and obtained safranal and glucose in a one to one ratio. The structures for safranal and picrocrocin were determined. [illustration] Since this time there have been many attempts to devise a practical synthesis of safranal. Among these has been the work of Kuhn and Wendt, who obtained safranal in 1-3% yield by dehydrogenation of [beta]-cyclocitral. Karrer and Ochsner attempted its synthesis by bromination of [alpha]-cyclocitral with N-bromosuccinimide followed by dehydrobromination, but a rearrangement took place. Lunt and Sondheimer obtained a homolog of safranal, 4-methyl safranal, by a Diels-Alder type condensation. Other attempted syntheses have proven unsuccessful. This paper presents the results of attempts to synthesize safranal from the enol acetate of cyclocitral. It is a continuation of the work begun by Vittimberg in 1951. He succeeded in obtaining the enol acetate of cn good yield and determined its structure. Cyclocitral was prepared by first forming a Schiff base from citral (I) and aniline. This was cyclized with concentrated sulfuric acid and steam-distilled to give a mixture of [alpha]- and [beta]-cyclocitral (II and III) in a ratio of 2/3 to 1/3. This mixture was treated with a large excess of isopropenyl acetate to form the enol acetate of cyclocitral. Both [alpha]- and [beta]-cyclocitral give the same enol acetate (IV).