A Cyclic Side-Chain-Linked Biphenyl Ether Tripeptide: H₃N⁺-cyclo-[Phe^(4-O)-Phe-Phe^(3-O)]-OMe.Cl^-

The crystal structure of the chloride salt of H₃N⁺-cyclo-(Phe^(4-O)-Phe-Phe^(3-O))-OMe, cyclo-phenylalanyl-phenylalanyl-phenylalaninium chloride methyl esther, C₂₈H₃₀N₃O₅⁺.Cl^-, is described. It is oxidatively linked through a biaryl ether linkage formed from the hydroxyl of 4-hydroxyphenylalanine and the meta position of the distal phenylalanine residue. This is the first reported crystal-structure determination of a cyclic 17-membered biphenyl ether tripeptide, a class which includes the natural products K-13 and OF4949 I-IV. An unusual C-H...O hydrogen bond is formed between the methine H atom of the N-terminal C[alpha] and a carbonyl O atom of a neighboring molecule [C...O = 2.995 (4) Å].

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