Abstract
An efficient and convenient strategy for the enantioselective synthesis of enantiomerically enriched 10-ethyl-7,8-dihydro-γ-ionone isomers (R)-(+)-7, and (S)-(−)-7 are described utilizing a lipase mediated resolution protocol, and reductive elimination of the secondary allylic alcohol as the key step. The enantioselective and diastereoselective lipase kinetic acetylation of 4-hydroxy-γ-ionone derivatives 6a afforded the 4-acetyl-γ-ionone derivatives (−)-8, and the 4-hydrox-γ-ionone derivatives (+)-6a, which are suitable precursors of the desired products. Stereospecific palladium-mediated elimination of allylic acetate provides the target compounds with an excellent enantiomeric excess and yield. Additionally, the novel 4,5-didehydro-α-ionone 13 is obtained from readily prepared (2,6,6-trimethylcyclohexa-2,4-dien-1-yl) methanol 9. The structures of all newly synthesized compounds have been elucidated by 1H, 13C NMR, GC-MS, and IR spectrometry. These compounds represent a new class of odorants that may be of pivotal relevance in industrial perfumery.
Highlights
Timberol® and Ionones are among the most important fragrance constituents due to their distinctive fine woody, amber, violet and rose scents [1,2]
Taking advantage of processes based on the enzyme-mediated resolution, Fuganti et al [6] have reported a synthetic approach to the olfactory active components of the woody odorant timberol® whereas Serra et al [7,8] described the enantioselective synthesis of a number of natural odorants with the ionone skeleton
We have focused our attention on the odorants in a combination of timberol and ionone framework (Figure 1) that are of pivotal relevance in industrial perfumery
Summary
Timberol® and Ionones are among the most important fragrance constituents due to their distinctive fine woody, amber, violet and rose scents [1,2]. Many fragrances are sold as a mixture of isomers whose specific contribution to the perceived odor may be very different. Olfactory evaluation shows that the regioisomeric purity and the absolute stereochemistry of these compounds dramatically determines the fragrance properties, sometimes with amazingly pronounced differences between the notes and the odor thresholds of the isomers. Taking advantage of processes based on the enzyme-mediated resolution, Fuganti et al [6] have reported a synthetic approach to the olfactory active components of the woody odorant timberol® whereas Serra et al [7,8] described the enantioselective synthesis of a number of natural odorants with the ionone skeleton. The exocyclic double bond confers characteristic nuances to the fragrance, which can favorably complement other widely used compound from the same family [9,10,11,12,13,14,15]
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