Stereoselectivity of the β-lyase-catalyzed cleavage of S-cysteine conjugates of pulegone

Slereoisomers of 8-5-cystemyl-p-menthan-3one synthesized from (R)- and (S)- pulegone by Michael addition of cystcinc were characterized by means of CSC, GC-MS, LC-MS, and 'H-NMR. Conjugates were treated with three sources of cysteine-5-conjugate β-lyase: (i) a crude enzyme extract prepared from Eubacterium limosum, (ii) tryptophanase from E. colt and (iii) yeast cells. The result was liberation of 8-mercapto-p-menthan-3one, a powerful flavoring substance exhibiting a cassistype odor note. The enzyme-catalyzed cleavage of the thio-ether bond proceeded with low enantioselectivity. Discrimination of diastereoisomers was more pronounced with a clear preference of the cis-configured substrates.