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

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 (E/Z)-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 (E,E)-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.