Enantioselective Synthesis and Olfactory Evaluation of 13‐Alkyl‐Substitutedα‐Ionones

Stereoselective Synthesis of Terpenoids through Lipase-Mediated Resolution Approaches

This review article focuses on the scientific developments concerning the lipase-mediated synthesis of terpenoids that have been reported in the literature during the last twenty years. More specifically, this review describes in depth the resolution approaches that allow the preparation of the chiral building blocks used for the stereoselective synthesis of bioactive terpenoids. The synthetic methods that have given new and innovative perspectives from a scientific standpoint, and the preparative approaches that possess industrial importance, are described thoroughly.

A General Strategy for the Stereoselective Synthesis of the Furanosesquiterpenes Structurally Related to Pallescensins 1⁻2

Here, we describe a general stereoselective synthesis of the marine furanosesquiterpenes structurally related to pallescensins 1–2. The stereoisomeric forms of the pallescensin 1, pallescensin 2, and dihydropallescensin 2 were obtained in high chemical and isomeric purity, whereas isomicrocionin-3 was synthesized for the first time. The sesquiterpene framework was built up by means of the coupling of the C₁₀ cyclogeranyl moiety with the C₅ 3-(methylene)furan moiety. The key steps of our synthetic procedure are the stereoselective synthesis of four cyclogeraniol isomers, their conversion into the corresponding cyclogeranylsulfonylbenzene derivatives, their alkylation with 3-(chloromethyl)furan, and the final reductive cleavage of the phenylsulfonyl functional group to afford the whole sesquiterpene framework. The enantioselective synthesis of the α-, 3,4-dehydro-γ- and γ-cyclogeraniol isomers was performed using both a lipase-mediated resolution procedure and different regioselective chemical transformations.

Competitive Gold-Promoted Meyer-Schuster and oxy-Cope Rearrangements of 3-Acyloxy-1,5-enynes: Selective Catalysis for the Synthesis of (+)-(S)-γ-Ionone and (-)-(2S,6 R)-cis-γ-Irone

We report a simple, highly stereoselective synthesis of (+)-(S)-γ-ionone and (-)-(2S,6R)-cis-γ-irone, two characteristic and precious odorants; the latter compound is a constituent of the essential oil obtained from iris rhizomes. Of general interest in this approach are the photoisomerization of an endo trisubstituted cyclohexene double bond to an exo vinyl group and the installation of the enone side chain through a [(NHC)Au(I)]-catalyzed Meyer-Schuster-like rearrangement. This required a careful investigation of the mechanism of the gold-catalyzed reaction and a judicious selection of reaction conditions. In fact, it was found that the Meyer-Schuster reaction may compete with the oxy-Cope rearrangement. Gold-based catalytic systems can promote either reaction selectively. In the present system, the mononuclear gold complex [Au(IPr)Cl], in combination with the silver salt AgSbF6 in 100:1 butan-2-one/H2O, proved to efficiently promote the Meyer-Schuster rearrangement of propargylic benzoates, whereas the digold catalyst [{Au(IPr)}2(μ-OH)][BF4] in anhydrous dichloromethane selectively promoted the oxy-Cope rearrangement of propargylic alcohols.

The Importance of the 5-Alkyl Substituent for the Violet Smell of Ionones: Synthesis of Racemic 5-Demethyl-α-ionone

The synthesis and the odor tonalities of racemic 5-demethyl-α-ionone are described. This synthetic ionone derivative did not show the typical floral-woody violet smell of α-ionone, definitely proving the importance of a suitably sized and spatially oriented alkyl substituent at C(5) for stimulating olfactory receptors of ionones.

Biomimetic Cyclization of Geraniol Derivatives, a Useful Tool in the Total Synthesis of Bioactive Monocyclic Terpenoids

This review is a detailed account of authors’ work in the field of biomimetic cyclization of geraniol-like dienes. The very high regio- and enantioselectivity achieved made these elegant reactions a viable tool for the synthesis of monocyclic building blocks used in the synthesis of valued terpenoids, like the precious aroma and perfume constituents’ ionones and irones.

Synthesis of the spirochroman core of dihypoestoxide and stereochemical proposal for the natural product

The tricyclic spirochroman core of dihypoestoxide has been synthesized from geranoic acid in seven steps using a hetero-Diels-Alder cycloaddition as a key step, thus providing support for the proposed biosynthesis of the natural product. Furthermore, analysis of the (13)C NMR data obtained for all four diastereoisomers of the synthetic spirochroman core has allowed us to propose a full stereochemical assignment for dihypoestoxide.

A simple and versatile re-catalyzed Meyer-Schuster rearrangement of propargylic alcohols to alpha,beta-unsaturated carbonyl compounds

Rhenium does the job! A readily available rhenium complex efficiently catalyzed the direct Meyer-Schuster-like rearrangement of different alkyl- and aryl-substituted propargylic secondary and tertiary alcohols to the corresponding alpha,beta-unsaturated compounds, which were produced with virtually complete E stereoselectivity. The reaction proceeded under neutral conditions and no racemization of potentially enolizable stereocenters was observed.