Changing the Odor Properties of Commercial Mixtures of [alpha]-Irones by Simple Chemical Transformations

Journal of Essential Oil Research: JEOR, Jul/Aug 2004 by Brenna, Elisabetta, Fuganti, Claudio, Serra, Stefano

Abstract

The sensory properties of commercial mixtures of [alpha]-irones were modified by means of simple chemical transformations, enriching the original mixtures in their content of [gamma]-isomers. The sequence employed consisted of NaBH^sub 4^ reduction, acetylation with Ac^sub 2^O in pyridine, photoisomerization, saponification and MnO^sub 2^ oxidation. New odor sensations can be elicited by manipulation of commercial odorants.

Key Word Index

Iris germanica, Iris pallida, Iridaceae, irones, isomer distribution, sensory properties, orris root oil, photoisomerization.

Introduction

Orris root oil is a very characteristic fragrance ingredient of high originality and fixativity, having today a reputation of being the most expensive natural perfumery raw material with prices of around 40,000-50,000 euro/kg for the absolute (1). It is still used in numerous fine fragrances, but only in trace quantities.

Orris root oil - or orris butter (beurre d'Iris) as it is also called because of its waxy consistency and yellow appearance - is steam-distilled from the washed, ground and dried rhizomes after some two to three years of maturation. Main constituents are cis-[gamma]-irone [3] and cis-a-irone [1], which are mainly responsible for the typical odor (2). The quality of different orris root oils depends on the distribution of irone isomers [1-3] and enantiomers, and varies with the botanical species of the plant and the region of cultivation (3-6). Orris butter from Iris pallida Lam. contains the dextrorotatory enantiomers while laevorotatory enantiomers are dominant in the orris butter oilris gerinanica L. The average composition (rel.%) of iris butter prepared from I. germanica is: 61.5% (82% ee)oI (-)-cis-[alpha]-irone [(-)-la], 37.6% (38% ee) of (-)-cis-[gamma]-irone [(-)-3a], 0.9% (96% ee) of ( )-imns-[alpha]-irone f( )-lb], and 0.7% of [beta]-irone [2]. When I. pallida rhizomes are used the corresponding iris butter contains 61.3% (96% ee) of ( )-cis-[gamma]-irone [( )-3a], 34.5% (66% ee) of ( )-cis-[alpha]-irone [( )-la], 4.2% (98% ee) of ( )-frons-[alpha]-irone [( )-lb], and 0.16% of [beta]-irone [2] (7). The best quality of orris butter is obtained from I. pallida cultivated around Florence.

With the intention to characterize and explain the sensory differences of orris root oils of different origins, Petrzilka and co-workers (7) separated and evaluated the different enantiomers of irone (7). Racemic irone isomers [1-3] we re resolved by chiral capillary gas chromatography, and the separated enantiopure substances were olfactorily characterized by GCsniffing technique. ( )-eis-[alpha]-Irone [Ia] and ( )-cis-[gamma]-irone [3a] were found to possess the most interesting and prominent organoleptic properties.

In the last four years, we developed chemo-enzymatic approaches to the preparation of the 10 irone isomers (8-10). We thus had in hands enantiopure samples of all irones isomers for sensory evaluation in a chemical purity between 81% and 99%, the results of which are summarized in Table I. We found that (-)-cis-[alpha]-irone [Ia] and (-)-irrtns-[gamma]-irone [3b] showed a delicate orris-butter-like character too. This latter evaluation on isolated samples completed the knowledge of the sensoiy properties of irone isomers. The cis-isomers [Ia] and [3a] proved, however, to be most characteristic for the scent of orris root oils.

During our investigation, we found that it was possible to convert [alpha]-irones into [gamma]-isomers by photoisomerisation of cis-and irans-[alpha]-irol acetates, followed by basic hydrolysis and MnO^sub 2^ oxidation. Despite being a hard task to imitate the complex odor properties of natural fragrant extracts with mixtures of synthetic enantiopure products, it should be possible with the help of the information gathered on the sensoiy properties of irones to improve the quality of commercial irone mixtures. We took advantage of these simple chemical transformations to change the sensory response of commercial [alpha]-irone mixtures by increasing the content of cis-[gamma]-irone.

Experimental

The following [alpha]-irone mixtures were employed in this work: Irone [alpha] (Givaudan) and [alpha]-irone 7OSP (Shin-Etsu). GC/ MS analyses were performed on an HP 6890 gas-chromatograph equipped with a 5973 mass-detector, using an HP-5 M S column (30 m � 0.25 mm, 0.25 �m film thickness). The following temperature program was employed: 60�C (l min), 6�/min, 150�C (l min), 127min, 280�C (5 min). TLC analyses were performed on Merck Kieselgel 60 F^sub 254^ plates. all chromatographic separations were carried out on silica-gel columns.

Reduction and acetylation: The commercial mixtures of [alpha]-irones were reduced with NaBH^sub 4^ in CH^sub 2^CL^sub 2^/methanol (2:1). After the usual work-up, the mixtures of irols were acetylated by treatment of pyridine and acetic anhydride, and then submitted to photoisomerisation.

Photochemical isomerisation: A solution of the mixture of irol acetate derivatives (1.0 g, 4 mmol) in ^sup i^PrOH (90 mL) in the presence of xylene (10 mL) as a photosensitizer was irradiated in quartz vessels in a Rayonet photochemical reactor equipped with 10, 8W high-pressure Hg lamps. The solvent was evaporated and the residue was purified by CC (silica gel; hexane/AcOEt, 9:1).