Diketopiperazines as Advanced Intermediates in the Biosynthesis of Ecteinascidins

The diketopiperazines of phenylalanine, tyrosine and DOPA have been synthesized from 14C-labeled amino acids and tested as intermediates in the biosynthesis of the ecteinascidins. Biosynthetic experiments were performed using an enzyme preparation of the tunicate Ecteinascidia turbinata, the source of the ecteinascidins. Tyrosine and DOPA diketopiperazines were both transformed to the ecteinascidins whereas the diketopiperazine of phenylalanine was not modified. It is now apparent that the biosynthesis of the ecteinascidins is initiated by the condensation of tyrosine to its cyclic dipeptide followed by a subsequent oxidation to the diketopiperazine of DOPA.