SYNTHESES OF THE ENANTIOMERS OF GAMMA-CYCLOGERANIC ACID, GAMMA-CYCLOCITRAL, AND GAMMA-DAMASCONE - ENANTIOSELECTIVE PROTONATION OF ENOLATES

(R)- and (S)-gamma-cyclogeranic acid ((R)- and (S)-9, resp.) were obtained by resolution of the racemate, and their absolute configurations determined by chemical correlation. The gamma-acids (R)- and (S)-9 were converted into (R)and (S)-methyl gamma-cyclogranate ((R)- and (S)-6, resp.), and (R)- and (S)-gamma-damascone ((R)- and (S)-5, resp.). A more direct entry to (R)- and (S)-9 consisted in the enantioselective protonation of a thiol ester enolate with (-)- or (+)-N-isopropylephedrine ((-)- or (+)-20) and subsequent hydrolysis of the (Ii)- and (S)-S-phenyl gamma-thiocyclogeranate ((R)- and (S)-24, resp.; 97 % ee). The esters (R)- and (S)-24 were also used as precursors of(Ii)- and (S)-gamma-damascone ((R)- and (S)-5, resp.). Alternatively, (S)-5 (75% eel was obtained by enantioselective protonation of ketone enolate 29 with (-)-N-isopropylephedrine ((-)-20). Organoleptic evaluation demonstrated that the (S)-enantiomers of methyl gamma-cyclogeranate and gamma-damascone are markedly superior to their (R)-enantiomers.