Ecteinascidins 729, 743,745,759A, 759B, and 770, tris(tetrahydroisoquino1ines) with potent in vivo antitumor activity, have been isolated from the colonial tunicate Ecteinascidia turbinata, and their structures have been assigned. A new acyl anion equivalent, the protected hydroxymalonitrile 2, has been developed as a masked activated ester equivalent. Alkylation or allylation of 2a proceeded in high yields under mild basic or neutral conditions, respectively. Treatment of the tosylimine 18 with 2a gave the dipeptide 20 via the a-amino acid having a masked activated functionality 19a.