How cysteine reacts with citral:: An unexpected reaction of β,β-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 a,p-unsaturated aldehydes is unprecedented. The adduct 6 obtained from citral, under acidic conditions, reacted further to give the new bicyclic compound 8.