A chemoenzymatic total synthesis of the structure assigned to the alkaloid (+)-montabuphine

Design, synthesis and evaluation of bioactivity of peptidomimetics based on chloroalkene dipeptide isosteres

Ample biologically active peptides have been found, identified and modified for use in drug discovery to date. However, several factors, such as low metabolic stability due to proteolysis and non-specific interactions with multiple off-target molecules, might limit the therapeutic use of peptides. To enhance the stability and/or bioactivity of peptides, the development of "peptidomimetics," which mimick peptide molecules, is considered to be idealistic. Hence, chloroalkene dipeptide isosteres (CADIs) was designed, and their synthetic methods have been developed by us. Briefly, in a CADI an amide bond in peptides is replaced with a chloroalkene structure. CADIs might be superior mimetics of amide bonds because the Van der Waals radii (VDR) and the electronegativity value of a chlorine atom are close to those of the replaced oxygen atom. By a developed method of the "liner synthesis", N-tert-butylsulfonyl protected CADIs can be synthesized via a key reaction involving diastereoselective allylic alkylation using organocopper reagents. On the other hand, by a developed method of the "convergent synthesis", N-fluorenylmethoxycarbonyl (Fmoc)-protected carboxylic acids can be also constructed based on N- and C-terminal analogues from corresponding amino acid starting materials via an Evans syn aldol reaction and the Ichikawa allylcyanate rearrangement reaction involving a [3.3] sigmatropic rearrangement. Notably, CADIs can also be applied for Fmoc-based solid-phase peptide synthesis and therefore introduced into bioactive peptides including as the Arg-Gly-Asp (RGD) peptide and the amyloid β fragment Lys-Leu-Val-Phe-Phe (KLVFF) peptide, which are correlated with cell attachment and Alzheimer's disease (AD), respectively. These CADI-containing peptidomimetics stabilized the conformation and enhanced the potency of the cyclic RGD peptide and the cyclic KLVFF peptide.

Diastereoselective Construction of Fused Carbocyclic Pyrrolidines via a Copper-Catalyzed [3 + 2] Cycloaddition: Total Syntheses of Pancratinines B-C

A Cu-catalyzed diastereoselective [3 + 2] cycloaddition of 2-arylaziridines and cyclic silyl dienol ethers was developed to efficiently construct fused-[5,n] carbocyclic pyrrolidines, which are widespread in bioactive natural products. Mechanistic studies revealed that the high diastereoselectivity of this transformation arose from a sequential retro aza-Michael/epimerization/aza-Michael process. Taking advantage of this newly developed method, the first total syntheses of pancratinines B and C were concisely realized.

Pancracine, a Montanine-Type Amaryllidaceae Alkaloid, Inhibits Proliferation of A549 Lung Adenocarcinoma Cells and Induces Apoptotic Cell Death in MOLT-4 Leukemic Cells

Pancracine, a montanine-type Amaryllidaceae alkaloid (AA), is one of the most potent compounds among natural isoquinolines. In previous studies, pancracine exhibited cytotoxic activity against diverse human cancer cell lines in vitro. However, further insight into the molecular mechanisms that underlie the cytotoxic effect of pancracine have not been reported and remain unknown. To fill this void, the cell proliferation and viability of cancer cells was explored using the Trypan Blue assay or by using the xCELLigence system. The impact on the cell cycle was determined by flow cytometry. Apoptosis was evaluated by Annexin V/PI and by quantifying the activity of caspases (-3/7, -8, and -9). Proteins triggering growth arrest or apoptosis were detected by Western blotting. Pancracine has strong antiproliferative activity on A549 cells, lasting up to 96 h, and antiproliferative and cytotoxic effects on MOLT-4 cells. The apoptosis-inducing activity of pancracine in MOLT-4 cells was evidenced by the significantly higher activity of caspases. This was transmitted through the upregulation of p53 phosphorylated on Ser392, p38 MAPK phosphorylated on Thr180/Tyr182, and upregulation of p27. The pancracine treatment negatively altered the proliferation of A549 cells as a consequence of an increase in G1-phase accumulation, associated with the downregulation of Rb phosphorylated on Ser807/811 and with the concomitant upregulation of p27 and downregulation of Akt phosphorylated on Thr308. This was the first study to glean a deeper mechanistic understanding of pancracine activity in vitro. Perturbation of the cell cycle and induction of apoptotic cell death were considered key mechanisms of pancracine action.

Chloroalkene dipeptide isosteres as peptidomimetics

To date various biologically active peptides have been discovered, characterized and modified for drug discovery. However, the utilization of peptides as therapeutics involves some limitation due to several factors, including low metabolic stability owing to proteolysis and non-specific interactions with multiple off-target molecules. Hence, the development of "peptidomimetics," in which a part or whole of a molecule is modified, is a desirable strategy to enhance the stability or bioactivity of peptide-based drugs. In this situation, we have designed and developed a synthetic method for chloroalkene dipeptide isosteres (CADIs), which involves replacement of an amide bond in peptides with a chloroalkene structure and are classified as peptidomimetics. By a developed synthetic method, an N-tert-butylsulfonyl protected CADI can be obtained utilizing diastereoselective allylic alkylation with organocopper reagents as a key reaction. This CADI can be transformed into an N-fluorenylmethoxycarbonyl protected CADI in short steps. In addition, CADIs are used in Fmoc-based solid-phase peptide synthesis and introduced into a bioactive peptide. Protocols for practical preparation of some CADIs and peptide mimetics containing a CADI are described as detailed recipes.

Chemical and Biological Aspects of Montanine-Type Alkaloids Isolated from Plants of the Amaryllidaceae Family

Plants of the Amaryllidaceae family are promising therapeutic tools for human diseases and have been used as alternative medicines. The specific secondary metabolites of this plant family, called Amaryllidaceae alkaloids (AA), have attracted considerable attention due to their interesting pharmacological activities. One of them, galantamine, is already used in the therapy of Alzheimer’s disease as a long acting, selective, reversible inhibitor of acetylcholinesterase. One group of AA is the montanine-type, such as montanine, pancracine and others, which share a 5,11-methanomorphanthridine core. So far, only 14 montanine-type alkaloids have been isolated. Compared with other structural-types of AA, montanine-type alkaloids are predominantly present in plants in low concentrations, but some of them display promising biological properties, especially in vitro cytotoxic activity against different cancerous cell lines. The present review aims to summarize comprehensively the research that has been published on the Amaryllidaceae alkaloids of montanine-type.

Synthesis of Transition-State Inhibitors of Chorismate Utilizing Enzymes from Bromobenzene cis-1,2-Dihydrodiol

In order to survive in a mammalian host, Mycobacterium tuberculosis (Mtb) produces aryl-capped siderophores known as the mycobactins for iron acquisition. Salicylic acid is a key building block of the mycobactin core and is synthesized by the bifunctional enzyme MbtI, which converts chorismate into isochorismate via a S_N2″ reaction followed by further transformation into salicylate through a [3,3]-sigmatropic rearrangement. MbtI belongs to a family of chorismate-utilizing enzymes (CUEs) that have conserved topology and active site residues. The transition-state inhibitor 1 described by Bartlett, Kozlowski, and co-workers is the most potent reported inhibitor to date of CUEs. Herein, we disclose a concise asymmetric synthesis and the accompanying biochemical characterization of 1 along with three closely related analogues beginning from bromobenzene cis-1S,2S-dihydrodiol produced through microbial oxidation that features a series of regio- and stereoselective transformations for introduction of the C-4 hydroxy and C-6 amino substituents. The flexible synthesis enables late-stage introduction of the carboxy group and other bioisosteres at the C-1 position as well as installation of the enol-pyruvate side chain at the C-5 position.

Asymmetric Synthesis of Isoquinoline Alkaloids: 2004-2015

In the past decade, the asymmetric synthesis of chiral nonracemic isoquinoline alkaloids, a family of natural products showing a wide range of structural diversity and biological and pharmaceutical activity, has been based either on continuation or improvement of known traditional methods or on new, recently developed, strategies. Both diastereoselective and enantioselective catalytic methods have been applied. This review describes the stereochemically modified traditional syntheses (the Pictet-Spengler, the Bischler-Napieralski, and the Pomeranz-Fritsch-Bobbitt) along with strategies based on closing of the nitrogen-containing ring B of the isoquinoline core by the formation of bonds between C₁-N₂, N₂-C₃, C₁-N₂/N₂-C₃, and C₁-N₂/C₄-C_4a atoms. Methods involving introduction of substituents at the C1 carbon of isoquinoline core along with syntheses applying various biocatalytic techniques have also been reviewed.

Simple and efficient synthesis of tetrahydro-β-carbolines via the Pictet–Spengler reaction in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)

1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) can act as both the solvent and the catalyst to effectively promote the Pictet–Spengler reaction.

The role of biocatalysis in the asymmetric synthesis of alkaloids

Alkaloids are not only one of the most intensively studied classes of natural products, their wide spectrum of pharmacological activities also makes them indispensable drug ingredients in both traditional and modern medicine. Among the methods for their production, biotechnological approaches are gaining importance, and biocatalysis has emerged as an essential tool in this context. A number of chemo-enzymatic strategies for alkaloid synthesis have been developed over the years, in which the biotransformations nowadays take an increasingly 'central' role. This review summarises different applications of biocatalysis in the asymmetric synthesis of alkaloids and discusses how recent developments and novel enzymes render innovative and efficient chemo-enzymatic production routes possible.

Stereoselective construction of 5,11-methanomorphanthridine and 5,10b-phenanthridine structural frameworks: Total syntheses of (±)-pancracine, (±)-brunsvigine, (±)-maritidine, and (±)-crinine

The core structure of the complex pentacyclic 5,11-methanomorphanthridine skeleton and the vicinal quaternary and tertiary stereocenters of the 5,10b-phenanthridine skeleton are constructed stereospecifically in one step employing intramolecular 1,3-dipolar cycloaddition of a nonstabilized azomethine ylide (AMY) generated by the sequential double desilylation of appropriate bis-trimethylsilylmethyl amines using Ag(I)F as a single-electron oxidant. The strategy is successfully applied for the total synthesis of biologically active alkaloids such as (±)-pancracine, (±)-brunsvigine, (±)-maritidine, and (±)-crinine.

Flexible synthesis of montanine-like alkaloids: revisiting the structure of montabuphine

An efficient and stereocontrolled synthetic strategy towards the synthesis of montanine-like alkaloids was developed. Our results suggest that the structure elucidation for natural montabuphine needs further elaboration.

Making mixtures to solve structures: structural elucidation via combinatorial synthesis

A domino Horner-Wadsworth-Emmons olefination strategy has been used to prepare homologous series of (polyen)ones, and through combinatorial elaboration, corresponding families of highly branched hydrocarbons. Gas chromatography-mass spectrometry of the mixtures has enabled the rapid and unambiguous identification of several highly branched alkanes of geochemical importance. This is the first example of the use of combinatorial synthesis for the elucidation of structural connectivity.

The Regio- and Stereo-Selective Formation of Allylic Chlorides During the Overman Rearrangement of Trichloroacetimidates Derived from Certain Brominated Conduritols

Microwave irradiation of imidate 2 at 165°C affords a ~3:1 mixture of the Overman rearrangement product 3 and the allylic chloride 4 (83% combined yield). Under the same conditions the deoxy-conduritol 6 gives a comparable mixture of compounds 7 and 8. The single-crystal X-ray structure of a derivative of chloride 8 is reported together with a mechanism that accounts for the selective formation of this compound and congener 4.