The total synthesis and functional evaluation of fourteen stereoisomers of yaku'amide B. The importance of stereochemistry for hydrophobicity and cytotoxicity.

Yaku'amide B is a highly unsaturated linear tridecapeptide and an extremely potent cytotoxin. Herein, we describe the synthesis of fourteen new stereoisomers of yaku'amide B using a unified assembly strategy. The hydrophobicities and cytotoxicities of these analogues were analyzed, along with those of four previously prepared isomers. Although all of the analogues share a common planar structure, their log D values varied significantly (3.39-5.32), presumably reflecting their distinct three-dimensional shapes. Subnanomolar-level cytotoxicity was observed for the natural yaku'amide B and its epimer of the N-terminal acyl group, whereas the other sixteen isomers exhibited 13- to 1200-fold weaker activities than that of the natural isomer. These data indicated the importance of the overall stereostructure of the 13-mer sequence of yaku'amide B for exerting its potent toxicity.

Elucidation and Total Synthesis of the Correct Structures of Tridecapeptides Yaku'amides A and B. Synthesis-Driven Stereochemical Reassignment of Four Amino Acid Residues.

Yaku'amides A (1) and B (2) possess four α,ß-dehydroamino acid residues in their linear tridecapeptide sequence and differ in their residue-3 (Gly for 1 and Ala for 2). The highly unsaturated peptide structure, characteristic cytotoxicity profile, and extreme scarcity from natural sources motivated us to launch synthetic studies of 1 and 2. Here, we report the total synthesis of the originally proposed structure of yaku'amide B (2a) by applying the route to 1a, which was previously established in our group. However, this accomplishment only proved that 2a and natural 2 were structurally different and prompted investigations directed toward determining the true structure of 2. Extensive Marfey's analyses of minute amounts of natural 2 and its degradation products presented us the possible stereoisomers, all of which were synthetically prepared for chromatographic comparison with the authentic fragments of 2. Based on this detective work, we proposed a corrected structure for yaku'amide B (2c), in which the orders of residues-7 and -8 and residues-11 and -12 are reversed. Finally, the total synthesis of 2c led to confirmation of its structural identity. Moreover, the revised structure of yaku'amide A (1c) was constructed by switching Ala-3 to Gly-3 and was found to be chromatographically matched with the re-isolated natural 1. The present work demonstrated the high reliability and sensitivity of the MS- and LC-based structural analyses and the indispensable role of chemical synthesis in structural elucidation of scarce natural products.

Cu-mediated enamide formation in the total synthesis of complex peptide natural products.

Cu-mediated C(sp(2))-N bond formation has received intense interest recently, and has been applied to the total synthesis of a wide variety of structurally complex natural products. This review covers the synthetic assembly of peptide natural products in which Cu-mediated enamide formation is the key transformation. The total syntheses of cyclopeptide alkaloids, pacidamycin D, and yaku'amide A exemplify the versatility of the Cu-catalyzed cross-coupling reaction in comparison to other synthetic methods.

Total synthesis and complete structural assignment of yaku'amide A.

Here we report the first total synthesis and the complete stereochemical assignment of yaku'amide A. Yaku'amide A (1) was isolated from a sponge Ceratopsion sp. as an extremely potent cytotoxin. Its structure was determined except for the C4-stereochemistry in the N-terminal acyl group (NTA). This tridecapeptide consists of 2 proteinogenic and 11 nonproteinogenic amino acid residues and is capped with NTA and a C-terminal amine (CTA). α,ß-Dehydrovaline, E- and Z-α,ß-dehydroisoleucines are the most unusual nonproteinogenic residues of 1 and necessitated development of new methodologies for their assembly. Consequently, Cu-mediated cross-coupling reactions were efficiently employed for E/Z-selective syntheses of the three dipeptides with the dehydroisoleucines and for construction of the tetrapeptide with the dehydrovaline. The peptide was then elongated from the tetrapeptide in a stepwise fashion to deliver the two possible C4-epimers of 1. Extensive NMR studies revealed that the natural 1 possessed the C4S-stereochemistry, and biological assays using P388 mouse leukemia cells demonstrated that both C4-epimers possessed comparable toxicities. The present synthetic methodologies for construction of the highly unsaturated peptide sequence of 1 will allow studies of the relationships between the conformational properties of dehydro amino acid residues and cytotoxicity.