Prasoon Kumar Kaushik , Research Scholar, Chemistry Division, Forest Research Institute, Dehradun
“Schiff bases and their use in fragrance compositions”
A fragrance is not a single material of clearly defined properties, but rather a mixture of individual chemicals, each behaving according to its own unique attributes. The discovery and utilization of fragrant materials began with an elite few and had religious connotations. The great world religions of Hinduism, Christianity, Islam, Buddhism, and Shintoism use fragrance in pursuance of their belief. Thus religious and pleasurable pursuits have been the main drives in the phenomenal growth of fragrance uses throughout the centuries. In the fragrance industry there is a lasting interest in the development of novel fragrances in order to create new odorant molecules in augmenting the aroma of fragrance compositions for perfumery or for new extended applications. Schiff bases (or azomethines / anils / imines) are well known in the art of flavoring and perfumery. Schiff bases are the products of a condensation reaction between a carbonyl (an aldehyde or ketone) compound with a primary amine. Structurally these compounds are distinguished by the >C=N linkage. A great majority of the Schiff bases are reported to possess floral, fruity odors, namely of the citrus type and, more particularly, reminiscent of the odor of the orange-flower. The Schiff bases can be used in wide limits which can range in compositions, for example, from about 0.1 (detergents)-about 20% (-OH solutions). The Schiff bases are also known to be useful as intermediates in producing other fragrance materials.
A large number of odorants have been produced since the dawn of organic chemistry 200 years ago, and a vast database of odorants and their corresponding odor profiles has built up. The field of fragrance synthesis, though still small in comparison to, say, pharmaceuticals, is a 8-billion dollar industry. Odorant chemicals that are inexpensive, highly substantive, long lasting and non toxic are highly desirable in the art of perfumery and flavorings. This has been a global need to create perfumes that present much less hazard to human health in view of the recent concerns over the safety of the perfumes. Many of the natural materials which contribute desired nuances to perfumery compositions are high in cost, vary in quality from one batch to another and/or are generally subject to the usual variations of natural products. Aromachemicals or fragrance compounds containing a carbonyl (aldehyde / ketone) functional group constitute an important group of raw materials for fragrance and flavor industry. However, most of such chemicals containing an aldehydic functional group are inherently unstable at the aldehyde moiety to oxidation to the corresponding carboxylic acid, thereby losing their fragrance characteristics and hence limiting their use. The ketone compounds of natural origin, except few cyclic terpene ketones and aromatic ketones are of minor importance as fragrance chemicals. The commercially used fragrant ketones are generally of synthetic origin. Recently the use of musk ketones has been stringently regulated due to their stability and consequently deleterious effect on the environment. The chemical transformation of carbonyl compounds into their novel derivatives, Schiff bases, therefore, presents an innovative approach to overcome their above mentioned undesired attributes. Thus, it will be of great scientific and technical interest to develop novel odorant Schiff bases of the reactant compounds from natural, nature identical or synthetic origin which have not been derivatized in the past.
Brief review of the status of research on the subject
Organic chemists have carried on romance with fragrant chemicals from the early days of organic chemistry. In the beginning the challenge was to isolate and identify the fragrant chemicals form natural products. Next, effort was directed toward the synthesis and commercial production of nature identical chemicals for the growing perfume industry. Finally, new chemicals were designed and synthesized for their valuable odor properties. Progress in this field has been amply recorded in scientific journals, patents and several books. Researches and reviews reported in the literature usually treat the chemistry of each material individually or group the materials according to structure or functionality. There have been numerous researches on novel odorant Schiff bases disclosed in patents and research papers. Being a classical fragrance raw material having an orange flower note, methyl 3,7-dimethyl-7-hydroxyoctylideneanthranilate, a Schiff base derived from hydroxycitronellal and methylanthranilate, has been recognized as an indispensable aroma chemical that is compounded for manufacturing perfumes within the class of floral family (1969). A number of Schiff bases including their organoleptic properties have been published (1969). The commercial availability of a number of Schiff bases has been reported (1987). Parliament, 1986 has reported flavoring of foodstuffs with α, β-keto-imine having a nutty corn, cereal aroma while the use of aldimines as chocolate-like flavors has been by Rizzi, 1971. The Schiff bases have also been reported to be useful as intermediates in producing other fragrance materials by Schreiber et al., 1975. Review of literature reveals that interest in development of Schiff bases as novel perfuming ingredient is high and global efforts are going on in this direction.