Total Synthesis of WS9326A, a Potent Tachykinin Antagonist from Streptomyces violaceoniger

Total synthesis of the cyclic peptide lactone WS9326A, a potent tachykinin antagonist isolated from Streptomyces violaceoniger strain 9326, has been achieved via Cbz-Thr(Boc-allo-Thr-Asn-Ser(Bzl))-(E)ΔMeTyr-Leu-D-Phe-OTce, which was cyclized (Phe and allo-Thr) using an active ester method with N-hydroxysuccinimide. Finally the unique N-acyl group, the 2-(1(Z)-pentenyl)cinnamoyl moiety, was introduced onto the amino group in the Thr unit. The key step of the synthesis involves the preparation of the E-isomer of the dehydro-N-mehyltyrosine (ΔMeTyr) unit. The debenzoxylation reaction of the threo- and erythro-isomers of β-benzoxy-N-methyltyrosine drivatives gave exclusively the Z-isomer of Cbz-Thr-ΔMeTyr(MOM)-OMe, which was then converted to the desired E-isomer by photochemical isomerization of Cbz-Thr(TBDMS)-(Z)ΔMeTyr(MOM)-Leu-D-Phe-OTce at a later step.