An Improved Synthesis of Pyridine−Thiazole Cores of Thiopeptide Antibiotics

Therapies from Thiopeptides

The first part of this contribution describes solutions that were developed to achieve progressively more efficient syntheses of the thiopeptide natural products, micrococcins P1 and P2 (MP1-MP2), with an eye toward exploring their potential as a source of new antibiotics. Such efforts enabled investigations on the medicinal chemistry of those antibiotics, and inspired the development of the kinase inhibitor, Masitinib®, two candidate oncology drugs, and new antibacterial agents. The studies that produced such therapeutic resources are detailed in the second part. True to the theme of this issue, "Organic Synthesis and Medicinal Chemistry: Two Inseparable Partners", an important message is that the above advances would have never materialized without the support of curiosity-driven, academic synthetic organic chemistry: a beleaguered science that nonetheless has been-and continues to be-instrumental to progress in the biomedical field.

Domino synthesis of functionalized pyridine carboxylates under gallium catalysis: Unravelling the reaction pathway and the role of the nitrogen source counter anion

The catalytic potential of various metal Lewis acid catalysts have been assessed to derive a high-yielding, multi-component domino synthesis of functionalized pyridines from (E)-3-(dimethylamino)-1-aryl/heteroaryl-prop-2-en-1-ones, 1,3-dicarbonyl compounds, and an ammonium salt (as the nitrogen precursor). Amongst the various metal halides, tetrafluoroborates, perchlorates, and triflates used as the catalyst GaI3 proved to be the most effective. The mechanistic course of the most plausible pathway has been outlined as the intermediate formation of imine/enamine by the reaction of the 1,3-dicarbonyl compound with ammonia (liberated in situ from the ammonium salt used as the nitrogen source), which participates in the domino nucleophilic Michael reaction to the (2E)-3-(dimethylamino)-1-aryl/hetroarylprop-2-en-1-one by its active methylene carbon through its enamine form followed by intramolecular cyclization and aromatization. The effect of different ammonium salts as the nitrogen source has been investigated and NH4OAc was found to be best. The influence of the acetate counter anion of NH4OAc on the progress of the reaction was studied and its specific role in the cyclization and subsequent aromatization has been revealed. This work offers distinct advantages compared to the literature reported methodologies on the count of several green index parameters.

Regioselective Synthesis of 2,4-Diaryl-6-trifluoromethylated Pyridines through Copper-Catalyzed Cyclization of CF3-Ynones and Vinyl Azides

A novel copper-catalyzed cyclization of readily available vinyl azides with CF₃-ynones is steadily achieved under mild conditions to furnish the versatile 2,4-diaryl-6-trifluoromethylated pyridine products, which are of great interest in medicinal chemistry. The generation of the vinyl iminophosphorane intermediates from vinyl azides through the Staudinger-Meyer reaction ensures the subsequent 1,4-addition process with CF₃-ynones in this transformation.

Synthesis of Colibactin Pyrrolidono[3,4-d]pyridones via Regioselective C(sp3)-H Activation

The synthesis of pyrrolidono[3,4-d]pyridones of relevance to putative genotoxic colibactin structures featuring a doubly conjugated 1,6-Michael acceptor system is reported. We investigated and implemented a highly selective Pd-catalyzed C(sp³)-H activation reaction as a key step and further functionalized the pyridone core. Evaluating the role of this structural unit of relevance to colibactin, we found that this structure displayed a high degree of stability toward both acidic conditions and nucleophiles.

Flavin Nitroalkane Oxidase Mimics Compatibility with NOx/TEMPO Catalysis: Aerobic Oxidization of Alcohols, Diols, and Ethers

Biomimetic flavin organocatalysts oxidize nitromethane to formaldehyde and NO_x-providing a relatively nontoxic, noncaustic, and inexpensive source for catalytic NO₂ for aerobic TEMPO oxidations of alcohols, diols, and ethers. Alcohols were oxidized to aldehydes or ketones, cyclic ethers to esters, and terminal diols to lactones. In situ trapping of NO_x and formaldehyde suggest an oxidative Nef process reminiscent of flavoprotein nitroalkane oxidase reactivity, which is achieved by relatively stable 1,10-bridged flavins. The metal-free flavin/NO_x/TEMPO catalytic cycles are uniquely compatible, especially compared to other Nef and NO_x-generating processes, and reveal selectivity over flavin-catalyzed sulfoxide formation. Aliphatic ethers were oxidized by this method, as demonstrated by the conversion of (-)-ambroxide to (+)-sclareolide.

Synthesis of Polysubstituted Trifluoromethylpyridines from Trifluoromethyl-α,β-ynones

A novel and efficient method for synthesis of polysubstituted trifluoromethylpyridine derivatives by the Bohlmann-Rahtz heteroannulation reaction is described, which use trifluoromethyl-α,β-ynones as trifluoromethyl building blocks to react with β-enamino esters or β-enamino ketones in the presence of ZnBr₂ to form the trifluoromethylpyridine derivatives in good yields. The protocol has the advantages of readily available starting materials, mild reaction conditions, and high atom economy.

Pd-Catalyzed regioselective synthesis of 2,6-disubstituted pyridines through denitrogenation of pyridotriazoles and 3,8-diarylation of imidazo[1,2-a]pyridines

Synthesis of 2,6-disubstituted pyridines from pyridotriazoles through palladium-catalyzed aerobic oxidative denitrogenative reactions has been described.

Phosphine-catalyzed [3 + 2] and [4 + 2] annulation reactions of ynones with barbiturate-derived alkenes

The phosphine-catalyzed [3 + 2] annulation reaction of ynones and barbiturate-derived alkenes has been developed with the assistance of a weak acid, giving functionalized spirobarbiturate-cyclopentanones.

Conjugate Addition of 3-Buytn-2-one to Anilines in Ethanol: Alkene Geometric Insights through In Situ FTIR Monitoring

A convenient, mild and effective conjugate addition of 3-butyn-2-one to a variety of anilines in ethanol is reported. The reaction was monitored and characterized through in situ FTIR, and the dynamics of the facile E/Z alkene geometry interconversion of the resultant aniline-derived enaminones was explored through NMR, FTIR and X-ray crystallography. A straightforward purification protocol that employs direct Kugelrohr distillation was identified, and the method was further extended to other amines and ynones, allowing rapid access to these interesting compounds.

Theoretical investigations toward TMEDA-catalyzed [2 + 4] annulation of allenoate with 1-aza-1,3-diene: mechanism, regioselectivity, and role of the catalyst

A computational study on the reaction between allenoate and 1-aza-1,3-diene catalyzed by TMEDA has been performed using the DFT method.

A copper-catalyzed cascade reaction of o-bromoarylisothiocyanates with isocyanides leading to benzo[d]imidazo[5,1-b]thiazoles under ligand-free conditions

A copper-catalyzed method has been realized for the synthesis of benzo[d]imidazo[5,1-b]thiazoles via intermolecular [3 + 2] cycloaddition and intramolecular C–S bond formation.

Mannich Bases as Enone Precursors for Water-Mediated Efficient Synthesis of 2,3,6-Trisubstituted Pyridines and 5,6,7,8-Tetrahydroquinolines

A highly efficient, regioselective, and environmentally friendly method has been developed for water-mediated synthesis of 2,3,6-trisubstituted pyridines and 5,6,7,8-tetrahydroquinolines. The introduced method allows easy preparation of various polysubstituted pyridines and 5,6,7,8-tetrahydroquinolines via domino reaction of an enolizable ketone, ammonia and enones derived from different Mannich bases in mild reaction conditions. Montmorillonite K-10 promoted this one-pot three-component reaction and gave both new and known 2,3,6-trisubstituted pyridines and 5,6,7,8-tetrahydroquinolines in good yields. The reaction protocol provides a wide array of functionality in construction of polysubstituted pyridines and 5,6,7,8-tetrahydroquinolines from commercially available starting materials in easily applicable and environmentally friendly conditions.

A route to the heterocyclic cluster of the E-series of thiopeptide antibiotics

A concise route to the 3-hydroxypyridine core of thiopeptide antibiotics such as nocathiacin is described. Key phases of the sequence involve a modified Hantzsch pyridine construction and a chemoselective Peng deprotection of a phenolic MOM ether.

Rh(iii)- or Ir(iii)-catalyzed ynone synthesis from aldehydes via chelation-assisted C–H bond activation

A simple and practical synthesis of ynones directly from readily available aldehydes was developed for the first time under mild reaction conditions via a Rh(iii)- or Ir(iii)-catalyzed formyl C–H bond activation.

Synthesis and insecticidal activities of novel pyrazole oxime ether derivatives with different substituted pyridyl rings

BACKGROUND

Pyrazole oxime ether derivatives with different substituted pyridyl rings represent new types of compounds that possess good insecticidal and acarcidal activity against Aphis laburni Kaltenbach and Tetranychus cinnabarinus.

RESULTS

In total, 82 novel pyrazole oxime ether derivatives were synthesized and identified by (1) H NMR, elemental analysis or high-resolution mass spectrometry, and their insecticidal and acarcidal activities were tested against A. laburni Kaltenbach and T. cinnabarinus. Bioassays showed that at a 200 mg L(-1) dosage, one-third of the compounds displayed high insecticidal activity against A. laburni Kaltenbach (> 90%), whereas most of of the compound II series exhibited excellent acarcidal activity against T. cinnabarinus (> 92%). Most compound II series exhibited good activity in both insecticidal and acarcidal tests. In addition, at a low concentration of 10 mg L(-1) , the insecticidal activity of compounds IB9 and IE4 exceeded 90%, and the acarcidal activity of compounds IIB1 and IIB2 was ≥ 95%.

CONCLUSION

Structure-activity relationships were also examined. Results suggested that the tert-butoxycarbonyl group, as well as the position between tert-butoxycarbonyl and the atom N of the pyridyl ring, were essential to obtaining the acarcidal activity of the title compounds.

Synthetic studies on heterocyclic natural products

This paper highlights ongoing efforts toward Erythrina alkaloids, himandrine, tetrodotoxin, and thiopeptide antibiotics such as nosiheptide and describes representative spinoffs in biomedicine that emanated from the author’s research in synthetic organic chemistry.

One-step synthesis of pyridines and dihydropyridines in a continuous flow microwave reactor

The Bohlmann-Rahtz pyridine synthesis and the Hantzsch dihydropyridine synthesis can be carried out in a microwave flow reactor or using a conductive heating flow platform for the continuous processing of material. In the Bohlmann-Rahtz reaction, the use of a Brønsted acid catalyst allows Michael addition and cyclodehydration to be carried out in a single step without isolation of intermediates to give the corresponding trisubstituted pyridine as a single regioisomer in good yield. Furthermore, 3-substituted propargyl aldehydes undergo Hantzsch dihydropyridine synthesis in preference to Bohlmann-Rahtz reaction in a very high yielding process that is readily transferred to continuous flow processing.

Total synthesis and complete structural assignment of thiocillin I

The total synthesis of the thiopeptide antibiotic, thiocillin I, is described. This work unequivocally defines the full structure (constitution and configuration) of the natural product as 1.

A simple reverse genetics approach to elucidating the biosynthetic pathway of nocathiacin

Genomic library screening and genome mining are currently employed to identify biosynthetic gene clusters of thiopeptides. To elucidate the biosynthetic pathway of nocathiacin, we present a new approach with the application of simple reverse genetics. A relationship between structural features of thiopeptides and their biosynthetic pathways is established and is a starting point for speedily elucidating biosynthetic genes of various ribosomally-synthesized bioactive peptides with diverse modifications.

Bohlmann-Rahtz cyclodehydration of aminodienones to pyridines using N-iodosuccinimide

Cyclodehydration of Bohlmann-Rahtz aminodienone intermediates using N-iodosuccinimide as a Lewis acid proceeds at low temperature under very mild conditions to give the corresponding 2,3,6-trisubstituted pyridines in high yield and with total regiocontrol.