| Abstract
| - Cysteine conjugates, resulting from the addition of cysteine to α,β-unsaturated carbonyl compounds,are important precursors of odorant sulfur compounds in food flavors. The aim of this work was tobetter understand this chemistry in the light of the unexpected double addition of cysteine to twounsaturated aldehydes. These reactions were studied as a function of pH. When (E)-2-methyl-2-butenal (tiglic aldehyde, 4) was treated with cysteine in water at pH 8, the major product formed wasthe new compound (4R)-2-(2-{[(2R)-2-amino-2-carboxyethyl]thio}methylpropyl)-1,3-thiazolidine-4-carboxylic acid (6). Under acidic conditions (pH 1), we also observed a double addition, but the secondcysteine was linked by a vinylic sulfide bond to form the previously unreported major product,(2R,2‘R,E)-S,S‘-(2,3-dimethyl-1-propene-1,3-diyl)bis-cysteine (7). When (E)-2-hexenal (12) was treatedwith cysteine under acidic conditions, the major product was the novel (4R,2‘ ‘R)-2-[2‘-(2‘ ‘-amino-2‘ ‘-carboxyethylthio)pentyl]-1,3-thiazolidine-4-carboxylic acid (13), and the formation of an vinylic sulfidecompound analogous to 7 was not observed. Reduction of the acidic crude reaction mixture withNaBH4 afforded 13 and the cysteine derivative (R)-S-[1-(2-hydroxyethyl)butyl]cysteine (14) in 14%yield. Treating (E)-2-hexenal with cysteine at pH 8 followed by NaBH4 reduction yielded the newproduct (3R)-7-propylhexahydro-1,4-thiazepine-3-carboxylic acid (15). Addition of cysteine to mesityloxide (16), at pH 8, followed by reduction with NaBH4 furnished (R)-S-(3-hydroxy-1,1-dimethylbutyl)cysteine (3) and the new compound (3R)-hexahydro-5,7,7-trimethyl-1,4-thiazepine-3-carboxylic acid(18). Keywords: Cysteine conjugate; (E)-2-methyl-2-butenal; (E)-2-hexenal; mesityl oxide; thiazepane-3-carboxylic acid; hexahydro-1,4-thiazepine; S-(1,2-dimethyl-3-oxopropyl)-l-cysteine; S-[1-(2-oxoethyl)butyl]-l-cysteine; 4-methyl-4-mercapto-2-pentanol; 3-mercaptohexanal
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