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

Abstract

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.

Recent Developments in the Meyer‐Schuster Rearrangement

“…On the other hand, Brønsted acids alone did not show any conversion and the starting material was recovered. This approach has been used subsequently for the preparation of flavor components of iris oils [104] . The benzoate 205 could be prepared in 63 % yield from the sensitive aldehyde 204 .…”

“…This approach has been used subsequently for the preparation of flavor components of iris oils. [104] The benzoate 205 could be prepared in 63 % yield from the sensitive aldehyde 204. Starting from this intermediate, the MSR mediated by the same gold catalyst 28 h afforded in 68 % overall yield 206 as a mixture of the desired (À )-(2S,6R)-cis-αirone 206(E) and its Z-isomer.…”

Transition Metal‐Catalyzed Rearrangement and Cycloisomerization Reactions Toward Hedonic Materials

“…Vidari and Nolan subsequently showed that the Meyer‐Schuster reaction could be performed with configuration retention from an enantiomerically pure substrate (Scheme 9 – C) [23] …”

Synthesis and odour evaluation of double‐bond isomers of DAMASCENOLIDE, 4‐(4‐methylpent‐3‐en‐1‐yl)‐2(5H)‐furanone, which has a citrus‐like odour

“…For example, the odour of geraniol ( 1 , E ‐isomer) is described as sweet, fruity and berry‐like properties, whereas nerol ( 2 , Z ‐isomer) is described as floral, citrus‐like and sweet‐smelling properties (Figure ). In another example, ionones have three double‐bond isomers 3 ‐ 5 , which also have different odours (Figure ). As described above, since many isomers based on a double bond may have different odours, it is very meaningful to study structure‐odour relationships of these isomers.…”