Synthesis and organoleptic properties of various 3‐methyl‐2‐oxopentanoates, 2‐hydroxy‐3‐methylpentanoates, and 2‐acyloxy‐3‐methylpentanoates

As part of a research programme concerned with the discovery of new perfumery ingredients, a series of racemic alkyl 3‐methyl‐2‐oxopentanoates 2–8, ethyl and methyl 2‐hydroxy‐3‐methylpentanoates (13[sol ]14 and 15[sol ]16, respectively), together with methyl and ethyl 2‐acyloxy‐3‐methylpentanoates, 17–32, have been prepared and their organoleptic properties evaluated. Ethyl‐3‐methyl‐2‐oxopentanoate (3), exhibiting a strong, fresh walnut, fruity odour, is of special interest, and its enantiomers were thus selectively synthesized from ( l )‐(+)‐isoleucine [(+)‐9] and ( l )‐(+)‐allo‐isoleucine [(+)‐10], respectively. Whilst (+)‐3 and (−)‐3 have a similar odour profile, (+)‐3 is more powerful and less pungent than its antipode. The intermediate hydroxy esters (+)‐13 and (+)‐14 were also found to have a distinct walnut character, as did their respective enantiomers, (−)‐13 and (−)‐14. Apart from 13[sol ]14 (camphoraceous, earthy, fruity), 15[sol ]16 (walnut, cacao, carob bean) and 25[sol ]26 (fruity, camomile, carob bean), the organoleptic properties of the other compounds were found to be less interesting, exhibiting a variety of weak fruity, camomile, bean and medicinal notes. Copyright © 2005 John Wiley & Sons, Ltd.