Selective Inhibition of Collagen‐Induced Platelet Aggregation by a Cyclic Peptide from Drymaria diandra

Four cyclic peptides, diandrine A–D (1–4), were isolated from the MeOH extract of Formosan Drymaria diandra. Their structures were elucidated by chemical and spectroscopic analyses as cyclo(‐Gly1‐Pro2‐Trp3‐Pro4‐Tyr5‐Phe6‐), cyclo(‐Gly1‐Pro2‐Leu3‐Pro4‐Leu5‐Trp6‐Ser7‐Ser8‐), cyclo(Gly1‐Gly2‐Pro3‐Tyr4‐Trp5‐Pro6‐), and cyclo(Gly1‐Gly2‐Pro3‐Tyr4‐Trp5‐Pro6‐), respectively. Compounds 3 and 4 were stable conformational isomers. Cyclopeptide 1 showed a selective inhibitory effect on collagen‐induced platelet aggregation with an IC50 value of 44.2 μM.