Destruxin E backbone modification effects on osteoclast Morphology: Synthesis and SAR study of N-Desmethyl and N-Methyl analogs

The design and synthesis of N-desmethyl and N-methyl destruxin E analogs have been demonstrated. The X-ray single crystal structure of destruxin E (1a) revealed a stable three-dimensional (3D) structure, including a s-cis amide bond at the MeVal-MeAla moiety and two intramolecular hydrogen bonds between NH(β-Ala) and OC(Ile) and between NH(Ile) and OC(β-Ala). N-Desmethyl analogs 2a (MeAla → Ala) and 2b (MeVal → Val) were synthesized through macrolactonization similar to our previously reported synthesis of 1a. Conversely, for the synthesis of N-methyl analogs 2c (Ile → MeIle) and 2d (β-Ala → Meβ-Ala), macrolactonization did not proceed; therefore, cyclization precursors 10c and 10d were designed to maintain the intramolecular hydrogen bonds described above during their cyclization. The macrolactamization proceeded despite the presence of a less reactive N-methylamino group at the N-terminus in both cases. Analog 2a, which exhibits multiple conformers in solutions, was inactive at 50 μM, whereas analog 2b, which exhibits a conformation similar to that of 1a in solutions, exhibited morphological changes against osteoclast-like multinuclear cells at 1.6 μM. The activity of the MeIle analog 2c, which cannot take the intramolecular hydrogen bond (Ile)NH•••OC(β-Ala) in 1a, was markedly diminished compared with that of 1a, and that of the Meβ-Ala analog 2d, which cannot take the intramolecular hydrogen bond (β-Ala)NH•••OC(Ile) in 1a, was further reduced to one-fourth of that of 2c. The overall results indicate that both the s-cis amide bond at the MeVal-MeAla moiety and two intramolecular hydrogen bonds (β-Ala)NH•••OC(Ile) and (Ile)NH•••OC(β-Ala) are important for constraining the conformation of the macrocyclic peptide backbone in destruxin E, thereby exhibiting its potent biological activity.

Synthetic studies for destruxins and biological evaluation for osteoclast-like multinucleated cells: a review

Synthesis of various destruxin analogs was accomplished using Shiina's macrolactonization as a key reaction. Combinatorial synthesis of cyclization precursors using solid-phase peptide synthesis and macrolactonization in solution were successful. In the synthesis of destruxin E and its analogs, the hydroxyacid-proline (HA¹-Pro²) dipeptide with an acetonide-protected diol moiety was synthesized in an asymmetric manner, and the protected diol was converted to an epoxide after macrocyclization. Destruxin E was synthesized on a gram scale using solution-phase synthesis. The structure-activity relationships of destruxins were elucidated through biological evaluation of synthetic destruxins A, B, and E and their analogs for morphological changes in osteoclast-like multinucleated cells.

Structure Revision of Trichomide D by Total Synthesis

A structure revision of trichomide D has been achieved by its total synthesis. The sterically hindered peptide sequence was successfully prepared using not only a conventional amidation with EDCI but also coupling with an Fmoc-protected amino acid chloride derivative. The cyclization precursor was synthesized by coupling of a tetrapeptide with an acylproline derivative and subsequent removal of silyl groups at the N- and C-termini. Macrolactonization using MNBA/DMAPO followed by preparation of a chlorohydrin moiety furnished the proposed structure of trichomide D, whose spectra were not identical to those of the natural product. Finally, we succeeded in the elucidation of the true structure of trichomide D by its total synthesis, and the absolute configuration of the chlorohydrin moiety was revised to be S. The cytotoxicities of the natural product and its synthetic derivatives against MCF-7 and HeLa S3 cells were evaluated by the MTT method, revealing that the configuration of the chlorohydrin moiety is a pivotal factor for exhibiting potent cytotoxicity against cancer cells.

Total Synthesis and Structural Determination of Cyclodepsipeptide Decatransin

The structure determination of the 30-membered cyclodepsipeptide decatransin was demonstrated on the basis of total synthesis. Both (R)- and (S)-2-hydroxy-5-methylhexanoic acid derivatives were prepared via the Evans asymmetric alkylation. N-Alkyl-enriched peptide fragments were synthesized by the Cbz strategy in the solution phase without formation of diketopiperazine and epimerization. The synthesis of putative candidates was achieved by convergent peptide coupling of three peptide fragments, followed by macrocyclization under the Mitsunobu conditions.

Rapid mild macrocyclization of depsipeptides under continuous flow: total syntheses of five cyclodepsipeptides

A systematic investigation of the flow macrocyclization approaches for five destruxin analogues natural products at three different cyclization point has been reported.

4-(Dimethylamino)pyridine N-Oxide-Catalyzed Macrolactamization Using 2-Methyl-6-nitrobenzoic Anhydride in the Synthesis of the Depsipeptidic Analogue of FE399

A depsipeptidic analogue of FE399 was efficiently synthesized mainly through macrolactamization using 2-methyl-6-nitrobenzoic anhydride (MNBA), and a detailed investigation of the desired 16-membered macrolactam core of FE399 was performed. It was determined that the combination of MNBA and a catalytic amount of 4-(dimethylamino)pyridine N-oxide exhibits much higher activity than that of conventionally used coupling reagents such as hexafluorophosphate azabenzotriazole tetramethyl uronium and benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate.

Lactones in the Synthesis of Prostaglandins and Prostaglandin Analogs

In the total stereo-controlled synthesis of natural prostaglandins (PGs) and their structural analogs, a vast class of compounds and drugs, known as the lactones, are encountered in a few key steps to build the final molecule, as: δ-lactones, γ-lactones, and 1,9-, 1,11-, and 1,15-macrolactones. After the synthesis of 1,9-PGF2α and 1,15-PGF2α lactones, many 1,15-lactones of E2, E3, F2, F3, A2, and A3 were found in the marine mollusc Tethys fimbria and the quest for understanding their biological role stimulated the research on their synthesis. Then 1,9-, 1,11-, and 1,15-PG lactones of the drugs were synthesized as an alternative to the corresponding esters, and the first part of the paper describes the methods used for their synthesis. The efficient Corey procedure for the synthesis of prostaglandins uses the key δ-lactone and γ-lactone intermediates with three or four stereocenters on the cyclopentane fragment to link the PG side chains. The paper describes the most used procedures for the synthesis of the milestone δ-Corey-lactones and γ-Corey-lactones, their improvements, and some new promising methods, such as interesting, new stereo-controlled and catalyzed enantioselective reactions, and methods based on the chemical/enzymatic resolution of the compounds in different steps of the sequences. The many uses of δ-lactones not only for the synthesis of γ-lactones, but also for obtaining 9β-halogen-PGs and halogen-substituted cyclopentane intermediates, as synthons for new 9β-PG analogs and future applications, are also discussed.

Studies toward the Synthesis of Chartelline C

Chartelline C, a marine alkaloid, possesses a unique core scaffold including indolenine β-lactam and imidazole moieties linked by an unsaturated 10-membered ring. A new synthetic approach for the construction of the indolenine β-lactam was planned, based on the inherent reactivity of chartelline A with NaOMe, triggered by bromination of bromoenamide, and proceeding through an N-acyl imine. However, the N-acyl imine intermediate was not observed. Instead, the corresponding bromoindolenine was obtained, which led to the desired indolenine β-lactam in 92% yield.

Total Synthesis and Antimicrobial Activity of Tumescenamide C and Its Derivatives

Tumescenamide C (1) is an antimicrobial compound produced by Streptomyces sp. KUSC_F05 and consists of a cyclic depsipeptide core and a polyketide side chain with branched methyl groups. Here, we report the total synthesis of tumescenamide C and two derivatives, mainly using Fmoc solid-phase peptide synthesis (SPPS). In addition, a biological evaluation of these compounds revealed the critical partial structure in 1 for antimicrobial activity.

Total Synthesis and Antimicrobial Evaluation of 23-Demethyleushearilide and Extensive Antimicrobial Evaluation of All Synthetic Stereoisomers of (16Z,20E)-Eushearilide and (16E,20E)-Eushearilide

A novel stereoisomer of eushearilide, 23-demethyleushearilide, was synthesized, and the structure–activity relationships of this compound along with known eushearilide stereoisomers were investigated in order to design novel lead compounds for the treatment of fungal infections. It was discovered that all of these congeners, together with the natural product, exhibited a wide range of antimicrobial activity against not only fungi but also against bacteria, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE).

Total synthesis and biological activity of dolastatin 16

The total synthesis of dolastatin 16, a macrocyclic depsipeptide first isolated from the sea hare Dolabella auricularia as a potential antineoplastic metabolite by Pettit et al., was achieved in a convergent manner. Dolastatin 16 was reported by Tan to exhibit strong antifouling activity, and thus shows promise for inhibiting the attachment of marine benthic organisms such as Amphibalanus amphitrite to ships and submerged artificial structures. Therefore, dolastatin 16 is a potential compound for a new, environmentally friendly antifouling material to replace banned tributyltin-based antifouling paints. The synthesis of dolastatin 16 involved the use of prolinol to prevent formation of a diketopiperazine composed of l-proline and N-methyl-d-valine during peptide coupling. This strategy for the elongation of peptide chains allowed the efficient and scalable synthesis of one segment, which was subsequently coupled with a second segment and cyclized to form the macrocyclic framework of dolastatin 16. The synthetic dolastatin 16 exhibited potent antifouling activity similar to that of natural dolastatin 16 toward cypris larvae of Amphibalanus amphitrite.

Enantioselective Total Synthesis of Antibiotic CJ-16,264, Synthesis and Biological Evaluation of Designed Analogues, and Discovery of Highly Potent and Simpler Antibacterial Agents

An improved and enantioselective total synthesis of antibiotic CJ-16,264 through a practical kinetic resolution and an iodolactonization reaction to form the iodo pyrrolizidinone fragment of the molecule is described. A series of racemic and enantiopure analogues of CJ-16,264 was designed and synthesized through the developed synthetic technologies and tested against drug-resistant bacterial strains. These studies led to interesting structure-activity relationships and the identification of a number of simpler, and yet equipotent, or even more potent, antibacterial agents than the natural product, thereby setting the foundation for further investigations in the quest for new anti-infective drugs.

Asymmetric Total Synthesis of (-)-Astakolactin and Confirmation of Its Stereostructure

The originally proposed structure of astakolactin was revised, and an asymmetric total synthesis of the newly proposed structure was achieved. The key transformations in the synthesis were a Johnson-Claisen rearrangement, an asymmetric Mukaiyama aldol reaction, and a Mitsunobu-type cyclodehydration. The spectroscopic data and specific rotation of the compound obtained matched well with those reported for naturally occurring astakolactin.

Combinatorial Solid-Phase Synthesis and Biological Evaluation of Cyclodepsipeptide Destruxin B as a Negative Regulator for Osteoclast Morphology

Combinatorial synthesis and biological evaluation of cyclodepsipeptide destruxin B have been achieved. The cyclization precursors were prepared by solid-phase peptide synthesis via a split and pool method utilizing SynPhase lanterns with colored tags and cogs, followed by cleavage from the polymer-support. Macrolactonization utilizing MNBA-DMAPO in solution-phase was successfully performed in parallel to afford the desired 64-member destruxin analogues in moderate to good yields. Biological evaluation of the synthesized analogues indicated that a MeAla residue for the building block A is required to induce the desired morphological changes in osteoclast-like multinuclear cells (OCLs), and introduction of the substituent at the R(4) position of a proline moiety is tolerated by the morphology and may enable the preparation of a molecular probe for the target identification in the osteoclasts.

Design and synthesis of a series of serine derivatives as small molecule inhibitors of the SARS coronavirus 3CL protease

Synthesis of serine derivatives having the essential functional groups for the inhibitor of SARS 3CL protease and evaluation of their inhibitory activities using SARS 3CL R188I mutant protease are described. The lead compounds, functionalized serine derivatives, were designed based on the tetrapeptide aldehyde and Bai's cinnamoly inhibitor, and additionally performed with simulation on GOLD softwear. Structure activity relationship studies of the candidate compounds were given reasonable inhibitors ent-3 and ent-7k against SARS 3CL R188I mutant protease. These inhibitors showed protease selectivity and no cytotoxicity.

Enantioselective total synthesis of naturally occurring eushearilide and evaluation of its antifungal activity

The asymmetric total synthesis of a newly proposed structure of (3S,16E,20E,23S)-(+)-eushearilide was achieved primarily through an asymmetric Mukaiyama aldol reaction, Schlosser-modified Wittig reaction and 2-methyl-6-nitrobenzoic anhydride-mediated macrolactonization. Based on detailed spectroscopic analyses, the obtained synthetic compound was found to be identical to natural eushearilide. Therefore, we were able to determine the true structure of eushearilide. Moreover, the synthetic compound was found to exhibit significant in vitro antifungal activity against various fungi and bacteria.

Total synthesis of the proposed structure of astakolactin

The first total synthesis of the proposed structure of astakolactin, a sesterterpene metabolite isolated from the marine sponge Cacospongia scalaris, has been achieved, mainly featuring Johnson-Claisen rearrangement, asymmetric Mukaiyama aldol reaction and MNBA-mediated lactonization.