Enantioselective Divergent Synthesis of (−)-cis-α- and (−)-cis-γ-Irone by Using Wilkinson’s Catalyst

A simple, efficient synthesis is reported for (-)-cis- a- and (-)-cis-g-irone, two precious constituents of iris oils, in > 99 % diastereomeric and enantioselective ratios. The two routes diverge from a common intermediate prepared from (-)-epoxygeraniol. Of general interest in this approach is the installation of the enone moiety of irones through a NHC-AuI-catalyzed Meyer–Schuster-like rearrangement of a propargylic benzoate and the use of Wilkinson’s catalyst for the stereoselective hydrogenation of a prostereogenic exocyclic double bond to secure the critical cis stereochemistry of the alkyl groups at C2 and C6 of the irones. The ste- reochemical aspects of this reaction are rationally supported by DFT calculation of the conformers of the substrates un- dergoing the hydrogenation and by a modeling study of the geometry of the rhodium h2 complexes involved in the dia- stereodifferentiation of the double bond faces. Thus, compu- tational investigation of the h2 intermediates formed in the catalytic cycle of prostereogenic alkene hydrogenation by using Wilkinson’s catalyst could be highly predictive of the stereochemistry of the products.