Continuous flow reactions in the preparation of active pharmaceutical ingredients and fine chemicals.

Herein, we present an overview of continuous flow chemistry, including photoflow and electroflow technologies in the preparation of active pharmaceutical ingredients (APIs) and fine chemical intermediates. Examples highlighting the benefits and challenges associated with continuous flow processes, mainly involving continuous thermal, photo- and electrochemical transformations, are drawn from the relevant literature, especially our experience and collaborations in this area, with emphasis on the synthesis and prospective scale-up.

Selenoneine-inspired selenohydantoins with glutathione peroxidase-like activity.

Chronic diseases such as cystic fibrosis, inflammatory bowel diseases, rheumatoid arthritis, and cardiovascular illness have been linked to a decrease in selenium levels and an increase in oxidative stress. Selenium is an essential trace element that exhibits antioxidant properties, with selenocysteine enzymes like glutathione peroxidase being particularly effective at reducing peroxides. In this study, a series of synthetic organoselenium compounds were synthesized and evaluated for their potential antioxidant activities. The new selenohydantoin molecules were inspired by selenoneine and synthesized using straightforward methods. Their antioxidant potential was evaluated and proven using classical radical scavenging and metal-reducing methods. The selenohydantoin derivatives exhibited glutathione peroxidase-like activity, reducing hydroperoxides. Theoretical calculations using Density Functional Theory (DFT) revealed the selenone isomer to be the only one occurring in solution, with selenolate as a possible tautomeric form in the presence of a basic species. Cytocompatibility assays indicated that the selenohydantoin derivatives were non-toxic to primary human aortic smooth muscle cells, paving the way for further biological evaluations of their antioxidant activity. The results suggest that selenohydantoin derivatives with trifluoro-methyl (-CF3) and chlorine (-Cl) substituents have significant activities and could be potential candidates for further biological trials. These compounds may contribute to the development of effective therapies for chronic diseases such cardiovascular diseases.

Electrochemical reduction of 5-benzylidene thiazolidine-2,4-diones: a greener approach to the preparation of glitazone APIs.

A transition-metal free methodology for the chemoselective reduction of benzylidene thiazolidine-2,4-diones and similar heterocycles is described, allowing the preparation of a broad scope of the corresponding reduced derivatives in up to 90% yield. The protocol has a simple and safe experimental setup, in which water was employed as the hydrogen source. To further demonstrate the synthetic utility of this transformation, the antidiabetic API Pioglitazone was prepared in 81% yield. To the best of our knowledge, this is the first hydride and transition-metal free protocol for the synthesis of Pioglitazone, highlighting its potential utility as a greener alternative in both academic and industrial synthesis.

Recent Advances in the Multistep Continuous Preparation of APIs and Fine Chemicals.

Over the last two decades, with the advent of continuous flow technologies, continuous processes have emerged as a major area in organic synthesis. In this context, continuous flow processes have been increasing in the preparation of Active Pharmaceutical Ingredients (APIs) and fine chemicals, such as complex synthetic intermediates, agrochemicals, and fragrances. Thus, the development of multi-step protocols has attracted special interest from the academic and industrial chemistry communities. In addition to the beneficial aspects intrinsically associated with continuous processes (e.g., waste reduction, optimal heat transfer, improved safety, and the possibility to work under harsh reaction conditions and with more dangerous reagents), these protocols also allow a rapid increase in molecular complexity. Moreover, in telescoped multi-step processes, isolation and purification steps are generally avoided or, if necessary, carried out in-line, presenting an important economy of time, solvents, reagents, and labor. Last, important synthetic strategies such as photochemical and electrochemical reactions are compatible with flow processes and are delivering relevant advances to the synthetic approaches. In this review, a general overview of the fundamentals of continuous flow processes is presented. Recent examples of multi-step continuous processes for the preparation of fine chemicals, including telescoped and end-to-end processes, are discussed, pointing out the possible advantages and/or limitations of each of these methodologies.

Multicomponent Reactions Applied to Total Synthesis of Biologically Active Molecules: A Short Review.

Multicomponent reactions (MCRs) are processes in which three or more starting materials are combined in the same reaction vessel, forming an adduct that contains all or most of the atoms of the starting materials. MCRs are one-pot processes that provide attractive advantages for the total synthesis of target molecules. These reactions allow rapid access to structurally complex adducts from particularly simple starting materials. Moreover, MCRs are generally intrinsically associated with principles of green syntheses, such as atom economy, minimization of isolation, and purification of synthetic intermediates, leading to large solvent economies and avoiding the production of large amounts of reaction waste. Thus, synthetic routes employing multicomponent reactions are generally more convergent, economical and often allow higher overall yields. In total synthesis, the use of MCRs has been mainly applied in the preparation of key advanced intermediates. Progress in the use of MCRs in total synthesis has been described over the last decades, including not only classical MCRs reactions (e.g. isocyanide-based transformations), but also non-traditional multicomponent reactions. Furthermore, reports concerning stereoselective multicomponent transformations are still scarce and present further development opportunities. This review aims to provide a general overview of the application of MCRs as key steps in the rapid preparation of structurally complex derivatives and fine chemicals. In special, some selected examples have been successfully applied for medicinal purposes. Finally, in some representative cases, either key intermediates formed during the reaction vessel or corresponding transition states have been disclosed in order to provide insights into the reaction mechanisms.

Solvent Dependent Competitive Mechanisms for the Ugi Multicomponent Reaction: A Joint Theoretical and Experimental Study in the α-Acyl Aminocarboxamides vs α-Amino Amidines Formation.

A synthetic protocol for the preparation of α-acyl aminocarboxamides and α-amino amidines is proposed. The selectivity toward each of these two possible products was tuned by simple modifications of the reaction conditions. A broad scope is presented, allowing access to the desired products in up to 87% (Ugi adduct) and 93% (α-amino amidine). Theoretical calculations were carried out, and the analysis led to the proposal of a new mechanistic pathway for the Ugi reaction, in which methanol acts not only as the solvent but also as a reagent. High-resolution (tandem) mass spectrometry experiments allowed the detection and characterization of the key intermediate associated with this new and alternative reaction pathway, thus supporting the theoretical proposal.

Diverse 3-Methylthio-4-Substituted Maleimides through a Novel Rearrangement Reaction: Synthesis and Selective Cell Imaging.

A transition metal-free protocol for the preparation of fluorescent and non-fluoresent 3-methylthio-4-arylmaleimides in a single step through a new rearrangement from thiazolidine-2,4-diones is described. By employing the optimized reaction conditions, a broad scope of derivatives was prepared in ≤97% yield. The reaction tolerated several substituted aryl groups, including the challenging preparation of pyridyl-containing derivatives. A series of control experiments strongly suggested that the new rearrangement involves a key isocyanate intermediate and a further reaction with in situ-generated methylthiomethyl acetate. The photophysical properties of some of the synthesized derivatives as well as their use in live cell imaging were also investigated, revealing that some of the substituted maleimides are capable of selectively staining different regions of the cells.

Origin of Enantioselectivity in Chiral Phosphoric-Acid-Catalyzed Azlactone Dynamic Kinetic Resolution.

Theoretical calculations, associated with control experiments, were carried out to gain insights into the mechanism and origin of enantioselectivity in the phosphoric-acid-catalyzed dynamic kinetic resolution of azlactones. The results revealed a Münchnone intermediate as the key species involved in the isomerization of azlactone rings. The developed model was successfully employed in the comprehension and prediction of enantioselectivity under diverse of reaction conditions, including alcoholysis and aminolysis protocols.

Electron-Donor-Acceptor Complex-Enabled Flow Methodology for the Hydrotrifluoromethylation of Unsaturated ß-Keto Esters.

A novel electron-donor-acceptor (EDA) complex-enabled flow photochemical hydrotrifluoromethylation of unsaturated ß-keto esters is described. The developed protocol has an easy experimental procedure and does not require the use of transition-metal-based photocatalysts, allowing the isolation of 14 new compounds in up to 86% yield. Control experiments and computational studies revealed that the reaction proceeds through a Michael-type 1,4-addition of a trifluoromethyl radical, followed by a proton transfer step. Furthermore, the reaction could be scaled up to 1 mmol, and the final product could be employed in the preparation of an isoxazolone and a pyrazolone as trifluoro-substituted heterocycles.

gem-Dichlorocyclopropanation of Dicarbonyl Derivatives.

A novel methodology for the 1,1-dichlorocyclopropanation of dicarbonyl conjugated olefins was described. The developed protocol is simple and uses readily accessible starting materials, allowing the isolation of the desired adducts in moderate to excellent yields (up to 99 %). Furthermore, the reaction tolerated scale up to the gram scale; thus highlighting the synthetic potential of this transformation. Control experiments and DFT studies revealed that the reaction proceeded through a Michael-initiated ring-closure process, in which reaction temperature played a crucial role. Finally, these gem-dichlorocyclopropanes were also employed in the preparation of a trisubstituted naphthyl derivative and a diastereoselective reduction was also demonstrated.

Quantum Chemical-Guided Steglich Rearrangement of Azlactones and Isoxazolones.

The theoretical-guided evaluation of the Steglich rearrangement of azlactones and isoxazolones allowed the determination of the reactivity patterns in these heterocycles, including the factors that drive the regioselectivity toward both possible sites. These results allowed the first experimental report on the regioselective Steglich rearrangement of isoxazolones, affording the nitrogen- or carbon-acyloxy adducts.

Stereoselective Intermolecular [2 + 2] Cycloadditions of Erlenmeyer-Plöchl Azlactones Using Visible Light Photoredox Catalysis.

The first report of the preparation of symmetric and nonsymmetric diaminotruxinic derivatives through the photoredox [2 + 2] cycloadditions of Erlenmeyer azlactones is described, affording the desired compounds in high regio- and diastereocontrol (only head-to-head coupling). Mechanistic studies by DFT suggest that the reaction proceeds through a neutral photocatalytic pathway.

Dual-protected amino acid derivatives as new antitubercular agents.

Tuberculosis is an infectious disease with high incidence and growing drug-resistant rates. In an attempt to develop new antitubercular agents, 35 compounds were synthesized, most of them bearing a carbamate and enantiopure amino acid moiety. These compounds had their activity evaluated toward a Mycobacterium tuberculosis strain (ATCC 27294) and cytotoxicity against fibroblast MRC-5 cells (ATCC CCL-171). Three of the prepared derivatives presented a good antimicrobial inhibition and two of them a moderate cytotoxicity. The lipophilicity seems to play a vital role in the cell growth activity, with best results for the derivatives with a higher logP.

Theoretical Study on the Epimerization of Azlactone Rings: Keto-Enol Tautomerism or Base-Mediated Racemization?

Azlactones are versatile heterocycles employed in a diversity of transformations; the main drawback of these cycles consists in the epimerization of the α-carbonyl stereocenter during its preparation. We hereby present a theoretical study to explain how the racemization occurs. Two hypotheses were investigated: the keto-enol tautomerism and the base-mediated racemization, through an enolate intermediate. The results showed that the latter is consistent with the experimental data and can spontaneously occur at room temperature. The same pathway was evaluated for 2-alcoxy azlactone, showing a slower epimerization ratio, consistent with the literature data.

A Brønsted base-promoted diastereoselective dimerization of azlactones.

A novel Brønsted base system for the diastereoselective dimerization of azlactones using trichloroacetate salts and acetonitrile has been developed. Desired products were obtained in good yields (60-93%) and with up to >19:1 dr after one hour of reaction. Additionally, the relative stereochemistry of the major dimer was assigned as being trans, by X-ray crystallographic analysis. The kinetic reaction profile was determined by using 1H NMR reaction monitoring and revealed a second order overall kinetic profile. Furthermore, by employing this methodology, a diastereoselective total synthesis of a functionalized analogue of streptopyrrolidine was accomplished in 65% overall yield.

Brønsted Acid-Catalyzed Epimerization-Free Preparation of Dual-Protected Amino Acid Derivatives.

An organocatalytic protocol, employing the commercially available EDC as coupling agent, has been developed for the preparation of dual-protected amino acid derivatives without epimerization. This methodology was then applied to different Boc-amino acid and amine derivatives in moderate to excellent isolated yields. In addition, racemization-free Boc deprotection was also demonstrated. Mechanism investigation through electrospray ionization (+)-mass spectrometry/mass spectrometry revealed an acyclic intermediate (no azlactone formation) activated by the camphorsulfonic acid as an organocatalyst as a key step for the sequential attack of the nucleophile.

Azlactone Reaction Developments.

Azlactones (also known as oxazolones) are heterocycles usually employed in the stereoselective synthesis of α,α-amino acids, heterocycles and natural products. The versatility of the azlactone scaffold arises from the numerous reactive sites, allowing its application in a diversity of transformations. This review aims to cover classical and recent applications of oxazolones, especially those involving stereoselective processes. After a short introduction on their structures and intrinsic reactivities, dynamic kinetic resolution (DKR) processes as well as reactions involving stereoselective formation of a new σ C-C bond, such as alkylation/allylation/arylation, aldol, ene, Michael and Mannich reactions will be exposed. Additionally, cycloadditions, Steglich rearrangement and sulfenylation reactions will also be discussed. Recent developments of the well-known Erlenmeyer azlactones will be described. For the most examples, the proposed mechanism, activation modes and/or key reaction intermediates will be exposed to rationalize both the final product and the observed stereochemistry. Finally, this review gives an overview of the synthetic utility of oxazolones.

Synthesis and biological evaluation of a new series of N-acyldiamines as potential antibacterial and antifungal agents.

In continuation of our efforts to find new antimicrobial compounds, series of fatty N-acyldiamines were prepared from fatty methyl esters and 1,2-ethylenediamine, 1,3-propanediamine or 1,4-butanediamine. The synthesized compounds were screened for their antibacterial activity against Gram-positive bacteria (Staphylococcus aureus, Staphylococcus epidermidis), Gram-negative bacteria (Escherichia coli, Pseudomonas aeruginosa) and for their antifungal activity against four species of Candida (C. albicans, C. tropicalis, C. glabrata and C. parapsilosis). Compounds 5a (N-(2-aminoethyl)dodecanamide), 5b (N-(2-aminoethyl)tetracanamide) and 6d (N-(3-aminopropyl)oleamide) were the most active against Gram-positive bacteria, with MIC values ranging from 1 to 16µg/mL and were evaluated for their activity against 21 clinical isolates of methicillin-resistant S. aureus. All the compounds exhibited good to moderate antifungal activity. Compared to chloramphenicol, compound 6b displayed a similar activity (MIC50=16µg/mL). A positive correlation could be established between lipophilicity and biological activity.

Synthesis and antimicrobial activity of novel amphiphilic aromatic amino alcohols.

We report in this work the preparation and in vitro antimicrobial evaluation of novel amphiphilic aromatic amino alcohols synthesized by reductive amination of 4-alkyloxybenzaldehyde with 2-amino-2-hydroxymethyl-propane-1,3-diol. The antibacterial activity was determined against four standard strains (Staphylococcus aureus, Staphylococcus epidermidis, Escherichia coli, Pseudomonas aeruginosa) and 21 clinical isolates of methicillin-resistant Staphylococcus aureus. The antifungal activity was evaluated against four yeast (Candida albicans, Candida tropicalis, Candida glabrata and Candida parapsilosis). The results obtained showed a strong positive correlation between the lipophilicity and the antibiotic activity of the tested compounds. The best activities were obtained against the Gram-positive bacteria (MIC=2-16µgml(-1)) for the five compounds bearing longer alkyl chains (4c-g; 8-14 carbons), which were also the most active against Candida (MIC=2-64µgml(-1)). Compound 4e exhibited the highest levels of inhibitory activity (MIC=2-16µgml(-1)) against clinical isolates of MRSA. A concentration of twice the MIC resulted in bactericidal activity of 4d against 19 of the 21 clinical isolates.