Copper-mediated synthesis of N-acyl vinylogous carbamic acids and derivatives: synthesis of the antibiotic CJ-15,801

Metal-Free Stereoselective Synthesis of (E)- and (Z)-N-Monosubstituted β-Aminoacrylates via Condensation Reactions of Carbamates

N-monosubstituted β-aminoacrylates are building blocks, which have been used in the preparation of amino acids and pharmaceuticals. Two efficient, stereoselective methods of preparation, via acid- or base-promoted condensation reactions of carbamates, are described. The base-promoted reaction is E-selective, while acid catalysis can, through the choice of solvent, selectively form E or Z. The acid-catalyzed E-selective process proceeds through a crystallization obviating the need for chromatographic purification.

Tertiary Enamides: Versatile and Available Substrates in Synthetic Chemistry

BACKGROUND

Enamines and their variant enamides as powerful and versatile synthons have attracted great attention in synthetic chemistry. Enamides display unique stability and reduce enaminic reactivity in view of the electron-withdrawing effect of N-acyl group. A great deal of satisfactory achievements in the synthesis and application of enamides has been made in recent years. Especially, tertiary enamides without N-H bond regarded as low reactivity of compounds in the past can act as excellent nucleophiles to react with electrophiles for the construction of various nitrous molecules.

OBJECTIVE

This review focuses on recent advances on tertiary enamides in the synthetic strategies and applications including addition, coupling reaction, functionalization and electro- or photo-chemical reaction.

CONCLUSION

Tertiary enamides as electron-deficient nucleophiles display a satisfactory balance between stability and reactivity to offer multiple opportunities for the construction of various functionalized nitrogencontaining compounds. Further exploration of the reactive mechanisms involved tertiary enamides and the development of novel and efficient transformations to generate ever more complex building blocks starting from tertiary enamides are particularly worth pursuing.

Overcoming synthetic challenges in targeting coenzyme A biosynthesis with the antimicrobial natural product CJ-15,801

The biosynthesis of the essential metabolic cofactor coenzyme A (CoA) has been receiving increasing attention as a new target that shows potential to counter the rising resistance to established antimicrobials. In particular, phosphopantothenoylcysteine synthetase (PPCS)-the second CoA biosynthesis enzyme that is found as part of the bifunctional CoaBC protein in bacteria, but is monofunctional in eukaryotes-has been validated as a target through extensive genetic knockdown studies in Mycobacterium tuberculosis. Moreover, it has been identified as the molecular target of the fungal natural product CJ-15,801 that shows selective activity against Staphylococcus aureus and the malaria parasite Plasmodium falciparum. As such, CJ-15,801 and 4'-phospho-CJ-15,801 (its metabolically active form) are excellent tool compounds for use in the development of new antimicrobial PPCS inhibitors. Unfortunately, further study and analysis of CJ-15,801 is currently being hampered by several unique challenges posed by its synthesis. In this study we describe how these challenges were overcome by using a robust palladium-catalyzed coupling to form the key N-acyl vinylogous carbamate moiety with retention of stereochemistry, and by extensive investigation of protecting groups suited to the labile functional group combinations contained in this molecule. We also demonstrate that using TBAF for deprotection causes undesired off-target effects related to the presence of residual tertiary ammonium salts. Finally, we provide a new method for the chemoenzymatic preparation of 4'-phospho-CJ-15,801 on multi-milligram scale, after showing that chemical synthesis of the molecule is not practical. Taken together, the results of this study advances our pursuit to discover new antimicrobials that specifically target CoA biosynthesis and/or utilization.

Benchmark study of benzamide derivatives and four novel theoretically designed (L1, L2, L3, and L4) ligands and evaluation of their biological properties by DFT approaches

Four novel ligands, namely N-benzhydryl benzamide, N, N-diphenethyl benzamide, N, N-dihexyl benzamide, and N, N-dioctyl benzamide (L1, L2, L3, and L4, respectively), based on the benzamide unit were designed and computed for their different properties, such as absorption spectrum, dipole moment, theoretically expected biological properties, and frontier molecular orbitals, by evaluating the HOMO/LUMO energy orbitals strength with DFT approaches and comparing these properties with the R benzamide properties available in literature. All molecules have a suitable frontier molecular orbital diagram and L1 exhibits maximum absorption at 246.8 nm due to the strong electron donating effect of the diphenylmethane ligand group. Moreover, strongly extended conjugated groups caused a redshift in absorption spectra. Newly designed molecules may show strong biological activities against cancer, bacterial diseases, and harmful fungal disorders. Graphical abstract Orbital energy, electron density and frontier molecular orbitals view of four designed novel benzamide derivates.

Synthesis of CPZEN-45: Construction of the 1,4-Diazepin-2-one Core by the Cu-Catalyzed Intramolecular Amidation of a Vinyl Iodide

CPZEN-45 was developed as an antibiotic against Mycobacterium tuberculosis by the chemical modification of caprazamycins. CPZEN-45 has been synthesized in this study by the Cu-catalyzed intramolecular amidation of a complex vinyl iodide precursor bearing uridine and sugar moieties with a secondary amide, allowing for the construction of its 1,4-diazepin-2-one core.

Diversity-Oriented Approaches to Polycyclics and Bioinspired Molecules via the Diels-Alder Strategy: Green Chemistry, Synthetic Economy, and Beyond

We describe diverse approaches to various dienes and their utilization in the Diels-Alder reaction to produce a variety of polycycles. The dienes covered here are prepared by simple alkylation reaction or via the Claisen rearrangement or by enyne metathesis of alkyne or enyne building blocks. Here, we have also included the Diels-Alder chemistry of dendralenes, a higher analog of cross-conjugated dienes. The present article is inclusive of o-xylylene derivatives that are generated in situ starting with benzosultine or benzosulfone derivatives. The Diels-Alder reaction of these dienes with various dienophiles gave diverse polycyclic systems and biologically important targets.

Computational study of the mechanism and selectivity of ruthenium-catalyzed hydroamidations of terminal alkynes

Density functional theory calculations were performed to elucidate the mechanism of the ruthenium-catalyzed hydroamidation of terminal alkynes, a powerful and sustainable method for the stereoselective synthesis of enamides. The results provide an explanation for the puzzling experimental finding that with tri-n-butylphosphine (P(Bu)3) as the ligand, the E-configured enamides are obtained, whereas the stereoselectivity is inverted in favor of the Z-configured enamides with (dicyclohexylphosphino)methane (dcypm) ligands. Using the addition of pyrrolidinone to 1-hexyne as a model reaction, various pathways were investigated, among which a catalytic cycle turned out to be most advantageous for both ligand systems that consists of: (a) oxidative addition, (b) alkyne coordination, (c) alkyne insertion (d) vinyl-vinylidene rearrangement, (e) nucleophilic transfer and finally (f) reductive elimination. The stereoselectivity of the reaction is decided in the nucleophilic transfer step. For the P( n Bu)3 ligand, the butyl moiety is oriented anti to the incoming 2-pyrolidinyl unit during the nucleophilic transfer step, whereas for the dcypm ligand, steric repulsion between the butyl and cyclohexyl groups turns it into a syn orientation. Overall, the formation of E-configured product is favorable by 4.8 kcal mol-1 (Δ‡GSDL) for the catalytic cycle computed with P(Bu)3 as ancillary ligand, whereas for the catalytic cycle computed with dcypm ligands, the Z-product is favored by 7.0 kcal mol-1 (Δ‡GSDL). These calculations are in excellent agreement with experimental findings.

A highly regio- and stereoselective synthesis of α-fluorinated imides via fluorination of chiral enamides

A highly π-facial selective and regioselective fluorination of chiral enamides is described. The reaction involves an enantioselective fluorination exclusively at the electron-rich enamide olefin with N-F reagents such as Selectfluor and N-fluoro-benzenesulfonimide [NFSI] accompanied by trapping of the β-fluoro-iminium cationic intermediate with water. The resulting N,O-hemiacetal could be oxidized using Dess-Martin periodinane, leading to an asymmetric sequence for syntheses of chiral α-fluoro-imides and optically enriched α-fluoro-ketones.

Easy entry into reduced Ar-BIANH2 compounds: a new class of quinone/hydroquinone-type redox-active couples with an easily tunable potential

Ar-BIANH2 bearing different substituents on the aryl rings have been synthesized in high yield by reduction of the corresponding bis(aryl)acenaphthenequinonediimine (Ar-BIAN) compounds. The structure of p-CH3 C6 H4 -BIANH2 in the solid state was determined by X-ray diffraction. An exhaustive voltammetric investigation of the two parallel BIAN and BIANH2 series afforded a first rationalization of the redox properties of these molecules, highlighting their analogies with quinone/hydroquinone systems. Such analogies, in combination with the much more negative reduction potential range of Ar-BIAN compounds with respect to quinones, can afford to extend the range of reduction potentials so far obtainable by the use of quinones.

Copper catalysed Ullmann type chemistry: from mechanistic aspects to modern development

Cu-catalysed arylation reactions devoted to the formation of C-C and C-heteroatom bonds (Ullmann-type couplings) have acquired great importance in the last decade. This review discusses the history and development of coupling reactions between aryl halides and various classes of nucleophiles, focusing mostly on the different mechanisms proposed through the years. Selected mechanistic investigations are treated more in depth than others. For example, evidence in favour or against radical mechanisms is discussed. Cu(I) and Cu(III) complexes involved in the Ullmann reaction and N/O selectivity in aminoalcohol arylation are discussed. A separate section has been dedicated to the synthesis of heterocyclic rings through intramolecular couplings. Finally, recent developments in green chemistry for these reactions, such as reactions in aqueous media and heterogeneous catalysis, have also been reviewed.

Cu-mediated enamide formation in the total synthesis of complex peptide natural products

Cu-mediated C(sp(2))-N bond formation has received intense interest recently, and has been applied to the total synthesis of a wide variety of structurally complex natural products. This review covers the synthetic assembly of peptide natural products in which Cu-mediated enamide formation is the key transformation. The total syntheses of cyclopeptide alkaloids, pacidamycin D, and yaku'amide A exemplify the versatility of the Cu-catalyzed cross-coupling reaction in comparison to other synthetic methods.

Metal free stereoselective synthesis of functionalized enamides

An efficient and expeditious DABCO-mediated synthesis of functionalized enamides from alkenes is delineated. The reaction proceeds through an unprecedented cascade involving an Aza-Michael addition/α-bromination/elimination and a Morita-Baylis-Hillman type reaction to generate functionalized enamides in a regio- & stereoselective fashion.

Direct condensation of functionalized sp3 carbons with formanilides for enamine synthesis using an in situ generated HMDS amide catalyst

Functionalized enamines, including β-enaminoesters were synthesized using in situ generated HMDS amide bases via the combination of aminosilanes and fluoride salts.

Direct C-H functionalization of enamides and enecarbamates by using visible-light photoredox catalysis

Direct C-H functionalization of various enamides and enecarbamates was realized through visible-light photoredox catalyzed reactions. Under the optimized conditions using [Ir(ppy)(2)(dtbbpy)PF(6)] as photocatalyst in combination with Na(2)HPO(4), enamides such as N-vinylpyrrolidinone could be easily functionalized by irradiation of the reaction mixture overnight in acetonitrile with visible light. The scope of the reaction with respect to enamide and enecarbamate substrates by using diethyl 2-bromomalonate for the alkylation reaction was explored, followed by an investigation of the scope of alkylating reagents used to react with the enamides and enecarbamates. The results indicated that reaction takes place with quite broad substrate scope, however, tertiary enamides with an internal C=C double bond in the E configuration could not be alkylated. Alkylation of N-vinyl tertiary enamides and enecarbamates gave monoalkylated products exclusively in the E configuration. Alkylation of N-vinyl secondary enamides gave doubly alkylated products. Double bond migration was observed in the reaction of electron-deficient bromides such as 3-bromoacetyl acetate with N-vinylpyrrolidinone. A mechanism is proposed for the reaction that is different from reported reactions of SOMOphiles with a nonfunctionalized C=C double bond. Further tests on the trifluoromethylation and arylation of enamides and enecarbamates under similar conditions showed that the reactions could serve as a mild, practical, and environmentally friendly approach to various functionalized enamides and enecarbamates.

The antibiotic CJ-15,801 is an antimetabolite that hijacks and then inhibits CoA biosynthesis

The natural product CJ-15,801 is an inhibitor of Staphylococcus aureus, but not other bacteria. Its close structural resemblance to pantothenic acid, the vitamin precursor of coenzyme A (CoA), and its Michael acceptor moiety suggest that it irreversibly inhibits an enzyme involved in CoA biosynthesis or utilization. However, its mode of action and the basis for its specificity have not been elucidated to date. We demonstrate that CJ-15,801 is transformed by the uniquely selective S. aureus pantothenate kinase, the first CoA biosynthetic enzyme, into a substrate for the next enzyme, phosphopantothenoylcysteine synthetase, which is inhibited through formation of a tight-binding structural mimic of its native reaction intermediate. These findings reveal CJ-15,801 as a vitamin biosynthetic pathway antimetabolite with a mechanism similar to that of the sulfonamide antibiotics and highlight CoA biosynthesis as a viable antimicrobial drug target.

Evolution of a protecting-group-free total synthesis: studies en route to the neuroactive marine macrolide (-)-palmyrolide A

A full account of our synthetic work toward the first total synthesis of the neuroactive marine macrolide (-)-palmyrolide A is described. Our first-generation approach aimed to unlock the unknown C(5)-C(7) stereochemical relationship via the synthesis of four diastereomers of palmyrolide A aldehyde, a known degradation product. When these efforts provided inconclusive results, recourse to synthesizing all possible stereocombinations of the 15-membered macrolide was undertaken. These studies were critical in confirming the absolute stereochemistry, yielding the first total synthesis of (+)-ent-palmyrolide A. Subsequent to this work, the first protecting-group-free total synthesis of natural (-)-palmyrolide A is also reported.

Total syntheses of chelidonine and norchelidonine via an enamide-benzyne-[2 + 2] cycloaddition cascade

Total syntheses of chelidonine and norchelidonine featuring an enamide-benzyne-[2 + 2] cycloaddition initiated cascade is described. The cascade includes a pericyclic ring-opening and intramolecular Diels-Alder reaction.

Asymmetric total synthesis and absolute stereochemistry of the neuroactive marine macrolide palmyrolide A

The first asymmetric total synthesis and determination of the absolute configuration for the neuroactive marine macrolide palmyrolide A is described. The highlight of the synthesis is macrocyclization via trans-enamide formation catalyzed by copper(I) iodide and cesium carbonate. Comparison with the authentic spectral data confirms the synthesis of (+)-ent-palmyrolide A.

The structure of glibenclamide in the solid state

The structure of glibenclamide, 5-chloro-N-(2-{4-[(cyclohexylamino)carbonyl] aminosulfonyl}phenyl) ethyl)-2-methoxybenzamide, an important antidiabetic drug, has been studied both in solution and in the solid state by a combination of NMR spectroscopy and theoretical calculations. The possibility that glibenclamide suffers a tautomerization under melting to afford a desmotrope was rejected.

Studies on the Synthesis of Reidispongiolide A: Stereoselective Synthesis of the C(22)-C(36) Fragment

A highly stereoselective synthesis of the C(22)-C(36) fragment 2 of reidispongiolide A is described. This synthesis features the highly stereoselective mismatched double asymmetric crotylboration reaction of the aldehyde derived from 5 and the new chiral reagent (S)-(E)-7 that provides 12 with >15:1 d.r. Subsequent coupling of the derived vinyl iodide 3 with aldehyde 16 provided allylic alcohol 17, that was elaborated by three steps into the targeted reidispongiolide fragment 2.

An efficient and practical entry to 2-amido-dienes and 3-amido-trienes from allenamides through stereoselective 1,3-hydrogen shifts

Preparations of de novo acyclic 2-amido-dienes and 3-amido-trienes through 1,3-hydrogen shifts from allenamides are described. These 1,3-hydrogen shifts could be achieved thermally or they could be promoted by the use of Brønsted acids. Under either condition, these processes are highly regioselective in favour of the α-position, and highly stereoselective in favour of the E-configuration. In addition, 6π-electron electrocyclic ring-closure could be carried out with 3-amido-trienes to afford cyclic 2-amido-dienes, and such electrocyclic ring-closure could be rendered in tandem with the 1,3-hydrogen shift.