Sulfanyl-methylene-5(4H)-oxazolones and b-sulfanyl-a-nitroacrylates as appealing dienophiles for the synthesis of conformationally constrained cysteine analogues

A new class of masked constrained cysteine derivatives containing the norbornen/ane scaffold were prepared by the way of Diels-Alder cycloaddition reaction by exploitation of two different dienophiles, sulfanyl-methylene-5(4H)-oxazolones and beta-sulfanyl-alpha-nitroacrylates. The new norbornen/ane amino acid derivatives can be considered versatile building blocks due to the presence of the alpha,alpha-disubstituted amino acid function, suitable for peptide synthesis, but also by the carbon-carbon norbornene double bond which could be variously functionalized