Total Synthesis of (+)-Calyculin A and (−)-Calyculin B: Cyanotetraene Construction, Asymmetric Synthesis of the C(26−37) Oxazole, Fragment Assembly, and Final Elaboration

The phosphate ester group in secondary metabolites

Covering: 2000 to mid-2021The phosphate ester is a versatile, widespread functional group involved in a plethora of biological activities. Its presence in secondary metabolites, however, is relatively rare compared to other functionalities and thus is part of a rather unexplored chemical space. Herein, the chemistry of secondary metabolites containing the phosphate ester group is discussed. The text emphasizes their structural diversity, biological and pharmacological profiles, and synthetic approaches employed in the phosphorylation step during total synthesis campaigns, covering the literature from 2000 to mid-2021.

Lithium Enolates Derived from Pyroglutaminol: Aggregation, Solvation, and Atropisomerism

Lithium enolates derived from protected pyroglutaminols were characterized by using (6)Li, (13)C, and (19)F NMR spectroscopies in conjunction with the method of continuous variations. Mixtures of tetrasolvated dimers and tetrasolvated tetramers in different proportions depend on the steric demands of the hemiaminal protecting group, tetrahydrofuran concentration, and the presence or absence of an α-fluoro moiety. The high steric demands of the substituted bicyclo[3.3.0] ring system promote dimers to an unusual extent and allow solvents and atropisomers in cubic tetramers to be observed in the slow-exchange limit. Pyridine used as a (6)Li chemical shift reagent proved useful in assigning solvation numbers.

Enantioselective rhodium-catalyzed isomerization of 4-iminocrotonates: asymmetric synthesis of a unique chiral synthon

An enantioselective isomerization of 4-iminocrotonates catalyzed by a rhodium(I)/phosphoramidite complex is described. This reaction uses widely available amines to couple with 4-oxocrotonate to provide a convenient access to a central chiral building block in good yield and high enantioselectivity. Although the mechanism of this new transformation remains unclear, both Rh and the phosphoramidite play a central role.

Alkenenitrile Transmissive Olefination: Synthesis of the Putative Lignan "Morinol I"

Grignard reagents trigger an addition-elimination with α'-hydroxy acrylonitriles to selectively generate Z-alkenenitriles. The modular assembly of Z-alkenenitriles from a Grignard reagent, acrylonitrile, and an aldehyde is ideal for stereoselectively synthesizing alkenes as illustrated in the synthesis of the putative lignan "morinol I."

Total synthesis of enigmazole A from Cinachyrella enigmatica. Bidirectional bond constructions with an ambident 2,4-disubstituted oxazole synthon

The first total synthesis of the cytotoxic marine macrolide enigmazole A has been completed in 22 steps (longest linear sequence). The sensitive, densely functionalized 2,4-disubstituted oxazole fragment was constructed using an efficient Negishi-type coupling of an oxazol-2-ylzinc reagent formed directly from the parent ethyl 2-iodooxazole-4-carboxylate by zinc insertion. Other key steps include a hetero-Diels-Alder cycloaddition to form the central embedded pyran ring, a Wittig reaction to unite Eastern and Western hemispheres, and a ring size-selective Keck macrolactonization.

Calyculins and related marine natural products as serine-threonine protein phosphatase PP1 and PP2A inhibitors and total syntheses of calyculin A, B, and C

Calyculins, highly cytotoxic polyketides, originally isolated from the marine sponge Discodermia calyx by Fusetani and co-workers, belong to the lithistid sponges group. These molecules have become interesting targets for cell biologists and synthetic organic chemists. The serine/threonine protein phosphatases play an essential role in the cellular signalling, metabolism, and cell cycle control. Calyculins express potent protein phosphatase 1 and 2A inhibitory activity, and have therefore become valuable tools for cellular biologists studying intracellular processes and their control by reversible phosphorylation. Calyculins might also play an important role in the development of several diseases such as cancer, neurodegenerative diseases, and type 2-diabetes mellitus. The fascinating structures of calyculins have inspired various groups of synthetic organic chemists to develop total syntheses of the most abundant calyculins A and C. However, with fifteen chiral centres, a cyano-capped tetraene unit, a phosphate-bearing spiroketal, an anti, anti, anti dipropionate segment, an alpha-chiral oxazole, and a trihydroxylated gamma-amino acid, calyculins reach versatility that only few natural products can surpass, and truly challenge modern chemists' asymmetric synthesis skills.

Cobalt(I)-Mediated Preparation of Polyborylated Cyclohexadienes: Scope, Limitations, and Mechanistic Insight

A series of 1,3- and 1,4-diboryl-1,3-cyclohexadienes have been prepared by intermolecular CoCp-mediated [2+2+2] cocyclizations of alkynylboronic pinacolate esters with alkenes, followed by oxidative demetallation with iron(III) chloride. The effect of substitution at the borylated alkyne on chemo- and regioselectivities has been studied, suggesting steric control. The proper choice of substituents allowed the preparation of 1,3-diborylated cyclohexadienes in a highly selective manner. Alternatively, 1,4-diborylated cyclohexadienes could be prepared from diborylated diynes. The scope of this reaction has been examined and found to include electron-poor, electron-rich, linear, and cyclic alkenes. The diborylated cyclohexadienes were submitted to single or double Suzuki-Miyaura cross-coupling reactions with haloarenes to afford polyarylated systems. The mechanism of the title reaction, including the regioselectivity of the cycloaddition steps, has been analyzed by means of DFT computations.

Total Synthesis of Potent Antifungal Marine Bisoxazole Natural Products Bengazoles A and B

The bengazoles are a family of marine natural products that display potent antifungal activity and a unique structure, containing two oxazole rings flanking a single carbon atom. Total syntheses of bengazole A and B are described, which contain a sensitive stereogenic centre at this position between the two oxazoles. Additionally, the synthesis of 10-epi-bengazole A is reported. Two parallel synthetic routes were investigated, relying on construction of the 2,4-disubstituted oxazole under mild conditions and a diastereoselective 1,3-dipolar cycloaddition. Our successful route is high yielding, provides rapid access to single stereoisomers of the complex natural products and allows the synthesis of analogues for biological evaluation.

Apoptolidin: Induction of Apoptosis by a Natural Product

Apoptolidin is a natural product that selectively induces apoptosis in several cancer cell lines. Apoptosis, programmed cell death, is a biological key pathway for regulating homeostasis and morphogenesis. Apoptotic misregulations are connected with several diseases, in particular cancer. The extrinsic way to apoptosis leads through death ligands and death receptors to the activiation of the caspase cascade, which results in proteolytic degradation of the cell architecture. The intrinsic pathway transmits signals of internal cellular damage to the mitochondrion, which loses its structural integrity, and forms an apoptosome that initiates the caspase cascade. Compounds which regulate apoptosis are of high medical significance. Many natural products regulate apoptotic pathways, and apoptolidin is one of them. The known synthetic routes to apoptolidin are described and compared in this Review. Selected further natural products which regulate apoptosis are introduced briefly.

5-Aminooxazole as an Internal Traceless Activator of C-Terminal Carboxylic Acid: Rapid Access to Diversely Functionalized Cyclodepsipeptides

A conceptually novel macrolactonization protocol has been developed. It is a domino process involving a sequence of: 1) protonation of 5-aminooxazole leading to the electrophilic iminium salt; 2) trapping of the iminium species by the neighboring C-terminal carboxylic acid leading to a putative spirolactone; and 3) intramolecular nucleophilic addition of the tethered alcohol to the spirolactone followed by fragmentation. The strategically incorporated 5-aminooxazole serves as an internal traceless activator of the neighboring C-terminal carboxylic acid, since it became an integral part of the peptide backbone after cyclization. No coupling reagent is required and the entire sequence is triggered by just a few equivalents of trifluoroacetic acid under very mild conditions (MeCN as the solvent at room temperature). The spirolactone as an activated form of the carboxylic acid has been evidenced by a sulfur-migration experiment. By combining with a three-component synthesis of 5-aminooxazole, a two-step synthesis of structurally complex cyclodepsipeptides from readily accessible starting materials was developed.

Recent progress in cobalt-mediated [2 + 2 + 2] cycloaddition reactions

For many years, our research group has been interested in the new developments of cobalt-mediated cyclizations. In this article, our recent achievements in the field of inter- and intramolecular [2 + 2 + 2] cyclizations are compiled.

Combinatorial synthesis of benztropine libraries and their evaluation as monoamine transporter inhibitors

A combinatorial synthesis of benztropine analogues is presented. Radical azidonation of 3-benzyloxy-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 3 to 3-(1-azidobenzyloxy)-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 4 was used as a key step in the synthesis. This step was optimized by adding 10% DMF to the reaction. Reaction of 4 with phenyl magnesium bromide followed by Boc removal and N-methylation gave benztropine 1. Reaction of five-component Grignard reagents with 4 was used to create a two-dimensional library of 25 N-normethylbenztropine analogues. Further reaction of this library with five alkyl bromides was carried out to create a three-dimensional library containing 125 compounds. Screening of the libraries towards binding and inhibition of uptake of the human dopamine (hDAT), serotonin (hSERT) and norepinephrine transporters (hNET) was carried out. None of the synthesized compounds were found to be stronger than benztropine, and none were selective for inhibition of binding over monoamine uptake.

Insight into binding of calyculin A to protein phosphatase 1: isolation of hemicalyculin a and chemical transformation of calyculin A

Calyculin A isolated from the marine sponge Discodermia calyx is a potent inhibitor of protein phosphatases 1 and 2A. We attempted to elucidate its mode of binding to the enzymes by examining the activity of natural and chemically transformed derivatives. Ten natural derivatives including a new compound, hemicalyculin A, were provided. The structure of hemicalyculin A, which comprises the southern hemisphere of calyculin A, was firmly established by chemical methods. Six compounds were prepared by selective modifications of functional groups in calyculin A. The enzyme inhibitory activity of these compounds indicated that 17-phosphate, 13-hydroxyl, and the hydrophobic tetraene moieties were all necessary for binding to the enzymes. The derivatives lacking the peptide portion were less cytotoxic even when they possessed full enzyme inhibitory activity.

Solid-phase rhodium carbenoid reactions: an N-H insertion route to a diverse series of oxazoles

[reaction: see text] The solid-phase synthesis of a series of oxazoles is described. The key step in the construction of these molecules involves the rhodium-catalyzed decomposition of polymer-bound alpha-diazo-beta-ketoesters. These reactions are performed in the presence of primary amides and yield the corresponding N-H insertion products. Subsequent cyclodehydration of these alpha-(acylamino)-beta-ketoesters provides the corresponding resin-bound 2,5-disubstituted oxazoles, which are further elaborated during cleavage from the resin.

Chiral action at a distance: remote substituent effects on the optical activity of calyculins A and B

[structure--see text] Calyculins A and B differ only by the (E)- vs (Z)-configuration at C(2). Yet, they show a large difference in optical rotations. We demonstrate a new strategy that provides a physical analysis of this long-range chiro-optical effect by Boltzmann-averaged atomic contribution mapping. The polarizability characteristics of the CN substituent rather than the perturbation of the stereogenic centers or the introduction of asymmetry into the polyene chain give rise to the remarkable difference in rotation angles.