Isodityrosine2 (1), isolated in the early 1980s from extensin, a plant cell wall glycoprotein, is the key structural unit which defines a large class of biologically active natural products. Piperazinomycin3 (2a) exhibits antimicrobial and antifungal activity, the cyclic tripeptide K- 134 (2b) is a potent inhibitor of angiotensin I converting enzyme (ACE), and OF4949-I- OF4949-IV5 (2crf) are inhibitors of aminopeptidase B and exhibit antitumor and ...