Recent Advances of Bismuth(III) Salts in Organic Chemistry: Application to the Synthesis of Heterocycles of Pharmaceutical Interest

Semisynthetic Sesquiterpene Lactones Generated by the Sensibility of Glaucolide B to Lewis and Brønsted-Lowry Acids and Bases: Cytotoxicity and Anti-Inflammatory Activities

Sesquiterpene lactone (SL) subtypes including hirsutinolide and cadinanolide have a controversial genesis. Metabolites of these classes are either described as natural products or as artifacts produced via the influence of solvents, chromatographic mobile phases, and adsorbents used in phytochemical studies. Based on this divergence, and to better understand the sensibility of these metabolites, different pH conditions were used to prepare semisynthetic SLs and evaluate the anti-inflammatory and antiproliferative activities. Therefore, glaucolide B (1) was treated with various Brønsted-Lowry and Lewis acids and bases-the same approach was applied to some of its derivatives-allowing us to obtain 14 semisynthetic SL derivatives, 10 of which are hereby reported for the first time. Hirsutinolide derivatives 7a (CC₅₀ = 5.0 µM; SI = 2.5) and 7b (CC₅₀ = 11.2 µM; SI = 2.5) and the germacranolide derivative 8a (CC₅₀ = 3.1 µM; SI = 3.0) revealed significant cytotoxic activity and selectivity against human melanoma SK-MEL-28 cells when compared with that against non-tumoral HUVEC cells. Additionally, compounds 7a and 7c.1 showed strongly reduced interleukin-6 (IL-6) and nitrite (NOx) release in pre-treated M1 macrophages J774A.1 when stimulated with lipopolysaccharide. Despite the fact that hirsutinolide and cadinanolide SLs may be produced via plant metabolism, this study shows that acidic and alkaline extraction and solid-phase purification processes can promote their formation.

Bi-Catalyzed 1,2-Reactivity of Spirocyclopropyl Oxindoles with Dithianediol: Access to Spiroheterocycles

The efficient Bi-catalyzed 1,2-reactivity of spirocyclopropyl oxindoles has been disclosed with dithianediols as the sulfur surrogate to furnish spiroheterocycles at moderate temperature. The procedure provides a potential approach for the construction of spirotetrahydrothiophene scaffolds with functional group diversity. The catalytic 1,2-reactivity of cyclopropanes, mechanistic studies using density functional theory studies, diastereoselectivity, and additive-free mild conditions are the important practical features.

BiCl3-Facilitated removal of methoxymethyl-ether/ester derivatives and DFT study of –O–C–O– bond cleavage

An experimental and DFT study of the cleavage of the MOM group mediated by an eco-friendly reagent, bismuth trichloride.

Bi(OAc)3/chiral phosphoric acid catalyzed enantioselective allylation of seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines

An efficient asymmetric allylation reaction of allylboronates with seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines, is described. The reaction, which is catalyzed by a Bi(OAc)₃/CPA system, gives a range of chiral nitrogen-containing heterocycle structures in high yields and with good enantioselectivities. The conversion of these products to nitrogen-containing heterocycles is also demonstrated.

Metal and Non-Metal Catalysts in the Synthesis of Five-Membered S-Heterocycles

BACKGROUND

A wide variety of biological activities are exhibited by N, O and S containing heterocycles and recently, many reports appeared for the synthesis of these heterocycles. The synthesis of heterocycles with the help of metal and non-metal catalyst has become a highly rewarding and important method in organic synthesis. This review article concentrated on the synthesis of S-heterocylces in the presence of metal and non-metal catalyst. The synthesis of five-membered S-heterocycles is described here.

OBJECTIVE

There is a need for the development of rapid, efficient and versatile strategy for the synthesis of heterocyclic rings. Metal, non-metal and organocatalysis involving methods have gained prominence because traditional conditions have disadvantages such as long reaction times, harsh conditions and limited substrate scope.

CONCLUSION

The metal-, non-metal-, and organocatalyst assisted organic synthesis is a highly dynamic research field. For ßthe chemoselective and efficient synthesis of heterocyclic molecules, this protocol has emerged as a powerful route. Various methodologies in the past few years have been pointed out to pursue more sustainable, efficient and environmentally benign procedures and products. Among these processes, the development of new protocols (catalysis), which avoided the use of toxic reagents, are the focus of intense research.

Organo or Metal Complex Catalyzed Synthesis of Five-membered Oxygen Heterocycles

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The reactions involving the formation of C-O bond using metal as a catalyst have emerged to be one of the most influential reactions for the synthesis of heterocycles in modern organic chemistry. Catalysis by metals offers diverse opportunities to invent new organic reactions with a promising range of selectivities such as chemoselectivity, regioselectivity, diastereoselectivity, and enantioselectivity. The methodologies used earlier for synthesis were less approachable to the organic chemist because of their high cost, highly specified instrumentation and inconvenient methods. For both stereoselective and regioselective formation of five-membered O-containing heterocycles, cyclic reactions that are metal and non-metal-catalyzed have known to be very efficient. The present review article covers the applications of metal and non-metal as a catalyst for the synthesis of five-membered O-containing heterocycles.

Enantioselective and regioselective aza-Friedel–Crafts reaction of electron-rich phenols with isatin-derived ketimines

An efficient asymmetric aza-Friedel–Crafts reaction of phenols with isatin-derived ketimines is described.

α-Alkylidene-γ-butyrolactone Formation via Bi(OTf)3-Catalyzed, Dehydrative, Ring-Opening Cyclizations of Cyclopropyl Carbinols: Understanding Substituent Effects and Predicting E/Z Selectivity

A Bi(OTf)₃-catalyzed ring-opening cyclization of (hetero)aryl cyclopropyl carbinols to form α-alkylidene-γ-butyrolactones (ABLs) is reported. This transformation represents different chemoselectivity from previous reports that demonstrated formation of (hetero)aryl-fused cyclohexa-1,3-dienes upon acid-promoted cyclopropyl carbinol ring opening. ABLs are obtained in up to 89% yield with a general preference for the E-isomers. Mechanistically, Bi(OTf)₃ serves as a stable and easy to handle precursor to TfOH. TfOH then catalyzes the formation of cyclopropyl carbinyl cations, which undergo ring opening, intramolecular trapping by the neighboring ester group, subsequent hydrolysis, and loss of methanol resulting in the formation of the ABLs. The nature and relative positioning of the substituents on both the carbinol and the cyclopropane determine both chemo- and stereoselective outcomes. Carbinol substituents determine the extent of cyclopropyl carbinyl cation formation. The cyclopropane donor substituents determine the overall reaction chemoselectivity. Weakly stabilizing or electron-poor donor groups provide better yields of the ABL products. In contrast, copious amounts of competing products are observed with highly stabilizing cyclopropane donor substituents. Finally, a predictive model for E/Z selectivity was developed using DFT calculations.

Bismuth(III)-catalyzed dehydrative etherification and thioetherification of phenolic hydroxy groups

Use of a bismuth catalyst allowed efficient dehydrative substitution of phenolic hydroxy groups with alcohols and thiols to form C-O and C-S bonds. The reaction required equimolar amounts of two readily available substrates that generated H(2)O as the only byproduct. The relatively mild reaction conditions were compatible with the functional groups selected, and provided excellent chemoselectivity.

A green approach toward quinoxalines and bis-quinoxalines and their biological evaluation against A431, human skin cancer cell lines

Background: The objective of this study was to develop a practical green procedure to synthesize quinoxalines and bis-quinoxalines and evaluate their inhibitory effects on the viability of A431 human epidermoid carcinoma cells. Method: A series of quinoxaline and bis-quinoxaline derivatives have been designed and synthesized following a microwave-assisted and bismuth nitrate-catalyzed eco-friendly route. A detailed comparison has been made between microwave-induced protocol with the reactions occurred at room temperature. The structure of the compounds have been elucidated by various spectroscopic methods and finally confirmed by x-ray crystallographic analyses. Results: Two quinoxaline derivatives, compounds 6 and 12 have demonstrated inhibitory effects on the viability of A431 human epidermoid carcinoma cells when compared with HaCaT nontumorigenic human keratinocyte cells. Conclusion: Notably, compound 6 inhibits Stat3 phosphorylation/activation in A431 skin cancer cells.

Bismuth compounds in medicinal chemistry

In recent years, the chemical potential of bismuth and bismuth compounds has been actively exploited. Bismuth salts are known for their low toxicity, making them potential valuable reagents for large-scale synthesis, which becomes more obvious when dealing with products such as active pharmaceutical ingredients or synthetic intermediates. Conversely, bismuth compounds have been widely used in medicine. After extensive use in the treatments of syphilis and other bacterial infections before the advent of modern antibiotics, bismuth compounds remain important for the treatment of several gastrointestinal disorders and also exhibit antimicrobial properties and cytotoxic activity, among others. This review updates relevant advances in the past few years, concerning the application of bismuth reagents and catalysts in innovative synthetic processes for the preparation of compounds of medicinal interest, as well as the preparation, biological evaluation and potential medicinal uses of bismuth compounds.