Enantioselective synthesis and absolute stereochemistry of both the enantiomers of trans-magnolione, a fragrance structurally related to trans-methyl jasmonate

Both enantiomers of the trans stereoisomer of magnolione 4 have been prepared by asymmetric Michael addition of ethyl acetoacetate to 2-pentylcyclopentenone, followed by hydrolysis/decarboxylation. The Michael reaction occurs under solid/liquid phase transfer catalysis in the presence of N-methylanthracenylquininium (or quinidinium) chloride. Enantiomeric excesses up to 76% are obtained. The trans structure of the compounds has been fully established by a careful NMR analysis, while the absolute configuration has been assigned as (2S,3S) for (+)-4 by the analysis of the CD spectrum.