Abstract
Cyclotheonamides A ( 1 : R = CHO: X, Y = 0) and B (2a: R = Ac; X. Y = 0) have been isolated from a marine sponge of the genus Theonella collected off Hachijo-jima Island, Japan.] These macrocyclic pentapeptides strongly inhibit various proteinases. particularly thrombin. and are the first macrocyclic thrombin inhibitors having two novel amino acid residues: a vinylogous tyrosine and an arginine extended by a carbonyl group in the x-position (a-carbonyl homologue of arginine) (Scheme 1) . Their structural uniqueness as well as intriguing biological activities motivated their total synthesisr2-'] and determination of their mechanism of serine protease inh ib i t i~n . '~ . 61
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