How cysteine reacts with citral: an unexpected reaction of beta,beta-disubstituted acroleins with cysteine leading to hexahydro-1,4-thiazepines

The reaction of beta,beta-disubstituted acroleins 3-methyl-2-butenal (1), 3-methyl-2-hexenal (2), and citral (3)] with cysteine gave 1:2 adducts of a novel structural type, namely hexahydro-1,4-thiazepines. To the best of our knowledge, the spontaneous formation of a seven-membered heterocycle from the addition of cysteine to alpha,beta-unsaturated aldehydes is unprecedented. The adduct 6 obtained from citral, under acidic conditions, reacted further to give the new bicyclic compound 8.