Identification and Total Synthesis of an Unstable Anticancer Macrolide Presaccharothriolide Z Produced by Saccharothrix sp. A1506

Structural Elucidation using Highly Sensitive Labeling Reagents and Total Synthesis of Amoxetamide A, a New Anoikis Inducer

Amoxetamide A is a new anoikis inducer identified in the combined-culture broth of Amycolatopsis sp. 26-4 and Tsukamurella pulmonis TP-B0596. Although its planar structure has been determined through extensive NMR studies, its stereochemical structure remains to be elucidated due to the limited amounts of available samples. Herein, we report its stereochemical determination using our original labeling reagents and chemical synthesis, which was realized by consuming trace amount (ca 10 μg) of natural sample. Moreover, the total synthesis of amoxetamide A is reported. Undesired intramolecular cyclization of an intermediate accelerated by the Thorpe-Ingold effect was completely avoided by using acyl sulfonamide as a protecting group of carboxylic acid. This first total synthesis of amoxetamide A enabled the confirmation of its stereochemistry and anoikis inducing activity.

Twisted Amide-Mediated Synthesis and Rapid Structure-Activity Relationship Study of Medium-Sized Cyclic Peptides

Medium-sized cyclic peptides are expected to be ideal drug leads because these peptides combine the advantages, while compensating for the disadvantages, of small molecules and antibodies. Although medium-sized peptides can be produced by chemical synthesis, two major problems, namely (i) Cα-epimerization during C-terminal modification and (ii) side reactions in the cyclization, remain to be solved. These issues have hampered the synthesis of pure materials for bioassays, making it difficult to accomplish accurate structure-activity relationship (SAR) studies. Herein, we report an efficient synthesis of medium-sized cyclic peptides based on the twisted amide-mediated amidation strategy. First, a variety of linear peptides were synthesized by the "inverse" peptide synthesis and fragment coupling. Second, the C-terminus of the linear peptides were converted to twisted amides, which were then reacted with a variety of α-amino acyl sulfonamides, realizing the rapid C-terminal diversification of peptides. Finally, the resulting linear peptides were cyclized by the intramolecular twisted amide-mediated amidation to afford stereochemically pure cyclic peptides. Using this strategy, total synthesis of acyl-surugamide A, the stereoselective synthesis of 13 non-natural analogs, and the discovery of potent antimicrobial/antifungal peptides beyond the natural product were also achieved.

Twisted Amide-Mediated Peptide Synthesis

A robust, practical, and sustainable isomerization-suppressed peptide bond formation via acyl sulfonamide, a twisted amide, is disclosed. Tosyl isocyanate and pentafluorobenzyl bromide were applied in combination to activate the peptide C-terminus, which then reacted with an amine to yield an elongated peptide with high stereochemical purity. Careful analysis of NMR spectra of the active intermediate revealed the presence of an intramolecular hydrogen bond, suggesting that the hydrogen bond suppressed Cα-epimerization during amidation. The isomerization suppression by the intramolecular hydrogen bond is expected to be effective even under high dilution conditions, making the present method a powerful tool for the synthesis of complex macrocyclic peptides. In addition to peptide synthesis, the developed synthetic entry to twisted amides can be applied to the investigation of transition metal-catalyzed N-C bond activation. Moreover, the application to the N-C bond activation returned insight into peptide synthesis, leading to the use of sulfonamide as a protecting group of carboxylic acid that can be orthogonally removed in the presence of other conventional protecting groups.

Total syntheses of surugamides and thioamycolamides toward understanding their biosynthesis

Peptidic natural products have received much attention as potential drug leads, and biosynthetic studies of peptidic natural products have contributed to the field of natural product chemistry over the past several decades. However, the key biosynthetic intermediates are generally not isolated from natural sources, and this can hamper a detailed analysis of biosynthesis. Furthermore, reported unusual structures, which are targets for biosynthetic studies, are sometimes the results of structural misassignments. Chemical synthesis techniques are imperative in solving these problems. This review focuses on the chemical syntheses of surugamides and thioamycolamides toward understanding their biosynthesis. These studies can provide the key biosynthetic intermediates that can reveal the biosynthetic pathways and/or true structures of these natural products.

Solid-phase total synthesis and structural confirmation of antimicrobial longicatenamide A

Longicatenamides A–D are cyclic hexapeptides isolated from the combined culture of Streptomyces sp. KUSC_F05 and Tsukamurella pulmonis TP-B0596. Because these peptides are not detected in the monoculture broth of the actinomycete, they are key tools for understanding chemical communication in the microbial world. Herein, we report the solid-phase total synthesis and structural confirmation of longicatenamide A. First, commercially unavailable building blocks were chemically synthesized with stereocontrol. Second, the peptide chain was elongated via Fmoc-based solid-phase peptide synthesis. Third, the peptide chain was cyclized in the solution phase, followed by simultaneous cleavage of all protecting groups to afford longicatenamide A. Chromatographic analysis corroborated the chemical structure of longicatenamide A. Furthermore, the antimicrobial activity of synthesized longicatenamide A was confirmed. The developed solid-phase synthesis is expected to facilitate the rapid synthesis of diverse synthetic analogues.

Saccharothrix luteola sp. nov., a novel cellulose-degrading actinobacterium isolated from soil and emended description of the genus Saccharothrix

A novel cellulose-degrading actinobacterium, designated strain NEAU-S10^T, was isolated from soil collected from Chifeng, Inner Mongolia Autonomous Region, PR China, and characterized using a polyphasic approach. Pairwise similarity of the 16S rRNA gene sequence showed that strain NEAU-S10^T was a representative of Saccharothrix and was closely related to Saccharothrix carnea NEAU-yn17^T (99.2 %), Saccharothrix saharensis SA152^T (99.0 %), Saccharothrix texasensis DSM 44231^T (98.5 %) and Saccharothrix xinjiangensis NBRC 101911^T (98.5 %). Physiological and chemotaxonomic characteristics of the strain further supported its affiliation to the genus Saccharothrix. The whole-cell sugars contained galactose, ribose and mannose. The polar lipids contained diphosphatidylglycerol, phosphatidylmonomethylethanolamine, phosphatidylethanolamine, phosphatidylinositol and phosphatidylinositol mannoside. The predominant menaquinones were MK-9(H₀), MK-9(H₂), MK-9(H₄) and MK-10(H₄). The major fatty acids were iso-C_16 : 0, C_16 : 0, anteiso-C_17 : 0, iso-C_15 : 0 and iso-C_17 : 0. The genomic DNA G+C content was 71.8 mol%. The levels of digital DNA-DNA hybridization between isolate and S. carnea NEAU-yn17^T, S. saharensis SA152^T and S. texasensis DSM 44231^T were 40.1 % (37.6-42.6 %), 38.soap8 % (36.3-41.3 %) and 44.8 % (42.2-47.3 %) and the ANI values between them were determined to be 90.2, 89.8 and 91.7 %, the results indicated that strain NEAU-S10^T could be distinguished from its reference strains. The assembled genome sequence of strain NEAU-S10^T was found to be 10 305 394 bp long. The NCBI Prokaryotic Genome Annotation Pipeline (PGAP) revealed 8 994 protein-coding genes. Genomic analysis and Congo red staining test indicated that strain NEAU-S10^T had the potential to degrade cellulose. The genomic and phenotypic results indicate that strain NEAU-S10^T represents a novel species of the genus Saccharothrix, for which the name Saccharothrix luteola sp. nov. is proposed, with NEAU-S10^T (=CCTCC AA 2020037^T=JCM 34800^T) as the type strain.