Solution to “little western flower” challenge

Abstract

The ionones are a series of closely related compounds that are part of the so-called rose ketones, which also include the damascones and damascenones. These important fragrance compounds are found in a variety of essential oils, including rose oil and violet oil. Ionones are the key odour-contributing compounds not only in flowers, but also in aged tobacco, grapes and many fruits. The ionones originate from enzymatic degradation of carotenoids—the pigment compounds that are responsible for the bright-orange colour of some fruits and vegetables. From several ionone isomers the three double bond position isomers α-ionone, β-ionone, and γ-ionone are most familiar and have been well reviewed with respect to their organoleptic properties. The odour of all ionone isomers is characterised by a woody flavour. α-Ionone has a warm, resin-like smell with sweet and floral tones. β-Ionone is a colourless to yellowish liquid with a floral, sweet, fruity, berry-like odour of medium strength with a green berry background [1]. Two steroisomeric γ isomers with different olfactory properties exist. The odour of (R)-(-)-γ-ionone is described as weak green, fruity and pineapple-like with metallic aspects, quite different from the typical ionone odour, whereas the odour of (S)-(+)-γ-ionone is very pleasant, floral, green and woody with a very natural violet tonality [2]. The latter is arguably one of the most powerful and pleasant isomers; however, the correct answer to the challenge is β-ionone. This compound is a significant contributor to the aroma of roses, and numerous other plants also contain β-ionone. Between 1995 and 2004, some 125 publications reported the occurrence of this substance in plants [3]. Viola odorata (Fig. 2) is used as a source for the perfume industry; however, 1 t of fresh flowers yields only 28–43 g of violet essential oil [4]. As described in the challenge, Josephine de Beauharnais, the first wife of Napoleon Bonaparte, adored violets, not only CH3 O