Mimicking the pineapple scent: Synthesis and properties of (semi)conjugated triene carbonyl derivatives

<p>The unexpected formation of previously nondescribed (semi)conjugated ethyl trienoate revealed its powerful aroma of fresh pineapple. Thus, we have designed, prepared and evaluated a set of its carbonyl analogues as mixtures of (<i>E</i>/<i>Z</i>)-isomers. Although their synthesis from natural ocimene led to target compounds in low yields, optimized preparation from geranyl acetate furnished an aldehyde as high-yielding common intermediate on multigram scale. A series of its Wittig olefinations provided corresponding (<i>E,E</i>)-configured carbonyl dienes. Final acid-catalyzed elimination of allylic acetates provided the desired “pineapple” target in moderate yield. Sensory analysis revealed that only the parent compound possesses the typical pineapple aroma. Although analogous ^tBu- and/or Bn-esters feature an additional green note, the most similar Me-ester differs by its fresh woody aroma analogously to methyl ketone.</p