Synthesis of lactones of ortho-tyrosine, DOPA isomers and tryptophan-ortho-tyrosine hybrid amino acids

A new high-yielding syntheses of lactones of ortho-tyrosine, four isomers of DOPA and a hybrid amino acid incorporating tryptophan and ortho-tyrosine have been achieved in a two step sequence from 2-methylamino-3-nitro-4-methylsulfanyl 4H-chromenes 7. Substitution of the C4 methylsulfanyl group in 7 with hydrogen or with electron rich aromatic compounds furnished corresponding 2-methylamino-3-nitro-4H-chromenes. Reduction of the nitro group with concomitant hydrolysis of the enamine moiety in 2-alkylamino-3-nitro-4H-chromenes or in 4-aryl-2-alkylamino-3-nitro-4H-chromenes with Zn, Ac2O and AcOH led to facile synthesis of lactones of ortho-tyrosine, four isomers of DOPA and the hybrid amino acid incorporating tryptophan and ortho-tyrosine units.