Enantioselective diethylzinc addition to the exocyclic C [dbnd]N double bond of some 4-arylideneamino-3-mercapto-6-methyl-4 H-1,2,4-triazin-5-one derivatives

Abstract

4-Arylideneamino-3-mercapto-6-methyl-4 H-1,2,4-triazin-5-ones 2a– f have been evaluated as substrates in the enantioselective diethylzinc addition reaction in the presence of (1 S,2 R)- N-alkyl- N-benzylnorephedrines 3a– d as chiral ligands. The utility of using a dual catalytic system (amino alcohol/halosilane) for the diethylzinc addition reaction has been also examined. The addition products 4-(1-arylpropyl)amino-3-mercapto-6-methyl-4 H-1,2,4-triazin-5-ones 4a– f were obtained in high yields and with enantiomeric excesses of up to 92%. The treatment of arylimines 2a– f with a diethylzinc reagent did not affect the hetero-ring opening although the C N double bond of the lateral chain did undergo an addition reaction to yield the C-ethylated products 4a– f. The reductive cleavage of the 1,2,4-triazinyl heterocyclic ring from addition products 4a– f led smoothly to the corresponding free primary amines 5a– f without a significant loss of enantiomeric purity.