Synthesis of spirotetramates via a Diels-Alder approach.

A study toward the unusual spirotetramate core of the pyrroindomycin antibiotics employing an intermolecular Diels-Alder reaction of an exo-methylene tetramic acid dienophile is described. The exo-methylene tetramate is readily synthesized from S-methylcysteine, and its reactivity as a dienophile is compared with that of related dehydroalanine derivatives. An alternative approach to spirotetramates using a nitroalkene dienophile is also reported.