The 6-(but-2'-enylidene)-1,5,5-trimethylcyclohex-1-enes: Important volatile constituents of the juice of the purple passionfruit Passiflora edulis

Two important flavour components of the juice of the purple passionfruit Passflora edulis (Sims) have been shown by g.l.c., mirco-reaction g.l.c., u.v., i.r., N.M.R., mass spectrometry, and by synthesis to be (6E,2'E)- and (6Z,2'E)-6-(but-2'-enylidene)-1,5,5-trimethylcyclohex-1-ene (2a) and (2b). Evidence is also reported for the presence in this juice of trace amounts of the stereoisomers (2c,d). The 6-(but-2'-enylidene)-1,5,5-trimethylcyclohex-1-enes are very susceptible to oxidation and, in dilute acetic acid solution, are rapidly converted into 'ionene', 1,1,6-trimethyl-1,2,3,4-tetrahydro- naphthalene (10). The significance of this reaction in the loss of flavour during pasteurization of passionfruit juice is discussed.