Enantioselective Total Synthesis of Octalactin A Using Asymmetric Aldol Reactions and a Rapid Lactonization To Form a Medium-Sized Ring

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.

β-Hydroxy Esters as Malonic Semialdehyde Proelectrophiles in Enantioselective Butadiene-Mediated Crotylation: Total Synthesis of Octalactins A and B

Tractable and commercially available esters (and amides) of β-hydroxypropionic acid serve as malonic semialdehyde proelectrophiles in enantioselective ruthenium-catalyzed hydrogen autotransfer crotylations mediated by butadiene. Through iterative asymmetric butadiene-mediated crotylations of ethyl 3-hydroxypropanoate, total syntheses of the polyketide natural products octalactin A and B were achieved in fewer steps than previously possible.

Total Synthesis of the Sesquiterpene (-)-Merrillianin

The first total synthesis of (-)-merrillianin (1), which is a natural sesquiterpene with a tricyclic structure having a cyclopentane ring and five- and seven-membered lactone parts, is demonstrated. This asymmetric total synthesis enabled the absolute stereostructure determination of naturally occurring (-)-1.

Late-Stage Functionalization: Total Synthesis of Beauveamide A and Its Congeners and Their Anticancer Activities

Asymmetric total synthesis of cyclotetradepsipeptide beauveamide A has been achieved for the first time. A macrolactamization strategy involving two possible sites has been explored to find the most effective route for cyclization. A late-stage functionalization approach has been adopted for easy access of non-natural analogues of beauveamide A for further biological evaluation. Interestingly, the anticancer activity of one of the synthesized analogues was better than that of the parent natural product.

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 Syntheses of Scabrolide A and Nominal Scabrolide B

The marine natural product scabrolide A was obtained by isomerization of the vinylogous 1,4-diketone entity of nominal scabrolide B as the purported pivot point of the biosynthesis of these polycyclic norcembranoids. Despite the success of this maneuver, the latter compound itself turned out not to be identical with the natural product of that name. The key steps en route to the carbocyclic core of these targets were a [2,3]-sigmatropic rearrangement of an allylic sulfur ylide to forge the overcrowded C12–C13 bond, an RCM reaction to close the congested central six-membered ring, and a hydroxy-directed epoxidation/epoxide opening/isomerization sequence to set the “umpoled” 1,4-dicarbonyl motif and the correct angular configuration at C12.

Design of C2-symmetric alkaloidal chiral amphiphiles and configurational effects on self-assembly

Alkaloids are a cornerstone in the development of medicinal and synthetic compounds due to their capability of specific recognition of targeted biomacromolecules, and uses in optical resolution and asymmetric reactions. To explore the untapped potential of the rigid and densely functionalized structures of alkaloids with precisely regulated configurations as optically active core scaffolds of self-assembling molecules, here we report the design, syntheses, chiroptical properties and self-assemblies of C2-symmetric alkaloidal amphiphiles with anti/syn stereochemical variations. Bispyrrolidinoindoline (BPI) was chosen as the optically active core scaffold. It was synthetically modified with hydrophobic alkyl chains and hydrophilic tetraethylene glycol tails to provide amphiphilicity. The anti/syn configurational differences in the amphiphiles significantly influenced the chiroptical, dynamic and supramolecular properties. Amphiphiles with anti-configurations responded to a solvent polarity change by altering their conformations, while the conformational changes of the syn-type amphiphiles were largely restricted. Furthermore, the anti-type amphiphile having the highest structural flexibility showed a characteristic split Cotton effect in an organic medium and formed the largest aggregates upon addition of water with a significant change in the circular dichroism (CD) profile, while amphiphiles having conformational restriction by the syn-configuration or a macrocyclic structure showed monomodal CD signals and afforded significantly smaller aggregates upon addition of water. Hence, the C2-symmetric alkaloidal BPI structure is demonstrated to be a useful core scaffold for supramolecular chemistry to design amphiphiles with controllable configurational diversity, which allows for the customization of chiroptical properties, conformational flexibility and self-assembly.

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.

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.

Scalable solution-phase synthesis of the biologically active cyclodepsipeptide destruxin E, a potent negative regulator of osteoclast morphology

The scalable solution-phase synthesis of the cyclodepsipeptide destruxin E (1) has been achieved. Diastereoselective dihydroxylation of the terminal alkene in a 2-alkoxy-4-pentenoic amide, 7, was successfully accomplished utilizing (DHQD)2PHAL as the chiral ligand, and it was found that the use of the l-proline moiety in the substrate as a chiral auxiliary was essential for the induction of high diastereoselectivity to afford the key compound 4 on a gram scale. MNBA-mediated macrolactonization of 3 was also performed without formation of the dimerized product even under higher-dilution conditions, and it is noteworthy that the internal hydrogen bonds and s-cis configuration of the amide bond between N-methylalanine and N-methylvaline in the cyclization precursor 3 would assist in the macrolactonization to provide the macrolactone 2 without forming a dimerized product. Finally, epoxide formation in the side chain afforded destruxin E (1) on a gram scale in high purity (>98%).

Synthesis of a pladienolide B analogue with the fully functionalized core structure

Starting from (R)-(-)-linalool (6), terminus differentiation and chain extension via aldol type reactions led to ketophosphonate 16 (C1-C8 building block). In a Horner-Wadsworth-Emmons reaction, 16 reacted with aldehyde 22, which contained the vicinal anti-Me-OH pattern and a vinyl iodide function, to provide the C1-C13 part of pladienolide B. After Shiina macrolactonization, reduction of the enone 26 gave the core structure 27. A Stille cross-coupling of vinyl iodide 27 with tributylphenylstannane eventually furnished analogue 30.

Efficient synthesis of the C(7)-C(20) subunit of amphidinolides C and F

Synthesis of the C(7)-C(20) subunit of amphidinolides C and F has been accomplished utilizing a Me(3)Al-mediated ring opening of a vinyl iodide/allylic epoxide to establish the C(12,13)anti stereochemistry, an organolithium coupling/olefination sequence to construct the C(9)-C(11) diene moiety and a sulfone alkylation/hydroxylation strategy to join the C(7)-C(14) and C(15)-C(20) fragments.