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.

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.

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.

Scaled up and telescoped synthesis of propofol under continuous-flow conditions

Herein we report a machine-assisted and scaled-up synthesis of propofol, a short-acting drug used in procedural sedation, which is extensively in demand during this COVID-19 pandemic. The continuous-flow protocol proved to be efficient, with great potential for industrial translation, reaching a production up to 71.6 g per day with process intensification (24 h-continuous experiments). We have successfully telescoped a continuous flow approach obtaining 5.74 g of propofol with productivity of 23.0 g/day (6 h-continuous experiment), proving the robustness of the method in both separated and telescoped modes. Substantial progress was also achieved for the in-line workup, which provides greater safety and less waste, also relevant for industrial application. Overall, the synthetic strategy is based on the Friedel-Crafts di-isopropylation of low-cost p-hydroxybenzoic acid, followed by a decarboxylation reaction, giving propofol in up to 84% overall yield and very low by-product formation.

Direct Synthesis of α-Sulfenylated Ketones under Electrochemical Conditions

We investigated the electrochemical sulfenylation reaction in both batch and continuous flow regimes, involving thiophenols/thiols and enol-acetates to yield α-sulfenylated ketones, without using additional oxidants or catalysts. Studies with different electrolytes were also performed, revealing that quaternary ammonium salts are the best mediators for this reaction. Notably, during the study of the reaction scope, a Boc-cysteine proved to be extremely tolerant to our protocol, thus increasing its relevance. The methodology also proved to be scalable in both batch and continuous flow conditions, opening up possibilities for further studies since these relevant functional groups are important moieties in organic synthesis.

Direct C-H photoarylation of diazines using aryldiazonium salts and visible-light

In this study, direct C–H photoarylation of pyrazine with aryldiazonium salts under visible-light irradiation (blue-LEDs) is described, and additional examples including photoarylations of pyrimidine and pyridazine are also covered. The corresponding aryl-diazines were prepared in yields up to 84% only by mixing and irradiating the reaction with no need for an additional photocatalyst. We demonstrate the efficacy of this protocol by the scope with electron-donor, -neutral, and -withdrawing groups attached at the ortho, meta, and para positions of the aryldiazonium salts; the results are better than those reported for ruthenium-complex mediated photoarylations. Additionally, we demonstrate the robustness of this methodology with a 5 mmol scaled-up experiment. Mechanistic studies were carried out giving support to the proposal of a photocatalyzed approach by an electron donor–acceptor (EDA) complex, also highlighting the crucial role that solvents play in the formation of the EDA complex.

An electron donor–acceptor (EDA) approach for the direct C–H photoarylation of diazines using aryldiazonium salts and visible-light is described.

Porphyrins as Photoredox Catalysts in Csp2-H Arylations: Batch and Continuous Flow Approaches

We have investigated both batch and continuous flow photoarylations of enol-acetates to yield different α-arylated aldehyde and ketone building blocks by using diazonium salts as the aryl-radical source. Different porphyrins were used as SET photocatalysts, and photophysical as well as electrochemical studies were performed to rationalize the photoredox properties and suggest mechanistic insights. Notably, the most electron-deficient porphyrin ( meso-tetra(pentafluorophenyl)porphyrin) shows the best photoactivity as an electron donor in the triplet excited state, which was rationalized by the redox potentials of excited states and the turnover of the porphyrins in the photocatalytic cycle. A two-step continuous protocol and multigram-scale reactions are also presented revealing a robust, cost-competitive, and easy methodology, highlighting the significant potential of porphyrins as SET photocatalysts.

Continuous Endoperoxidation of Conjugated Dienes and Subsequent Rearrangements Leading to C-H Oxidized Synthons

We have investigated the continuous flow photooxidation of several conjugated dienes and subsequent rearrangement using a practical and safe continuous-flow homemade engineered setup. End-to-end approaches involving endoperoxidation, Kornblum-DeLaMare rearrangement, and additional rearrangements are comprehensively detailed with optimization, scope, and scale-up to obtain useful hydroxyenones, furans, and 1,4-dicarbonyl building blocks.

Combining batch and continuous flow setups in the end-to-end synthesis of naturally occurring curcuminoids

A successful end-to-end continuous flow synthesis of pure curcumin (1) and two other natural derivatives present in turmeric is described.

Microbial diversification of Diels-Alder cycloadducts by whole cells of Penicillium brasilianum

Functionalizations of cycloadducts are important steps for the use of Diels-Alder reactions in the construction of complex cyclic or polycyclic molecules from relatively simple starting materials. In the present work, we studied the ability of Penicillium brasilianum to perform microbial transformations of racemic Diels-Alder endo-cycloadducts. Thus, Diels-Alder products, obtained from reacting cyclopentadiene or 2,3-dimethylbutadiene with alkylated para-benzoquinones, were transformed by the resting cells of P. brasilianum producing new functionalized polycyclic compounds. These biotransformations yielded novel products of oxidation and ring closure, reduction of the C=C or C=O in [Formula: see text]-unsaturated system, and allylic hydroxylations. The reduction products (conjugated double bond and carbonyl group) were also synthesized, and the enantioselectivity of both in vitro and in vivo processes was evaluated. In all cases, the microbiological transformations were enantioselective. In silico docking studies of the Diels-Alder cycloadducts with P. brasilianum oxidoreductase "old yellow enzymes" shed more light on these transformations.

Porphyrins as Catalysts in Scalable Organic Reactions

Catalysis is a topic of continuous interest since it was discovered in chemistry centuries ago. Aiming at the advance of reactions for efficient processes, a number of approaches have been developed over the last 180 years, and more recently, porphyrins occupy an important role in this field. Porphyrins and metalloporphyrins are fascinating compounds which are involved in a number of synthetic transformations of great interest for industry and academy. The aim of this review is to cover the most recent progress in reactions catalysed by porphyrins in scalable procedures, thus presenting the state of the art in reactions of epoxidation, sulfoxidation, oxidation of alcohols to carbonyl compounds and C–H functionalization. In addition, the use of porphyrins as photocatalysts in continuous flow processes is covered.

NIR bacteriochlorin chromophores accessed by Heck and Sonogashira cross-coupling reactions on a tetrabromobacteriochlorin derivative

The synthesis of a new tetrabromobacteriochlorin BCBr4 is reported. Pd cross-coupling reactions yielded tetra-coupled products with a significant red shift in the UV-Vis bands.

Selective Vilsmeier–Haack aryl-formylations of tetrathienylporphyrin and its Ni(II) complex

Regioselective aryl-formylations of tetrathienylporphyrin and its Ni(II) complex were performed with the Vilsmeier–Haack reaction. We have studied different solvents and temperatures to obtain the mono, di, tri and tetra-formylated porphyrins, all at the α-position of the thienyl groups. No β-formylations were observed and the four different formylated products presented here are suitable intermediates for subsequent extended conjugation reactions.

Exploiting novel process windows for the synthesis of meso-substituted porphyrins under continuous flow conditions

Porphyrin synthesis by classical methods frequently involves harsh conditions yielding products with poor scalability. Herein, a continuous flow approach is described, thus demonstrating a scaled-up procedure in a safe and highly pure manner.

A practical deca-gram scale ring expansion of (R)-(−)-carvone to (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1

A route to enantiopure (R)-(+)-3-methyl-6-isopropenyl-cyclohept-3-enone-1, an intermediate for terpenoids, has been developed and includes a highly chemo- and regioselective Tiffeneau–Demjanov reaction.

Synthesis of (R)-(–)-Carvone Derivatives

(R)-(-)-Carvone, the main constituent of spearmint oil, has several synthetic applications and is used in cosmetics, food, and pharmaceutical preparations. In a recent study we demonstrated that (R)-(-)-carvone shows significant analgesic activity. In the present report, (R)-(-)-carvone derivatives which have pharmacological potential are described. These compounds can be used in studies of structure-activity relationship to develop novel drugs.

Photobiological characteristics of chlorophyll a derivatives as microbial PDT agents

Chlorin-e6 (chl-e6) and a hydrogenated derivative (chl-e6H) were semi-synthesized, and their photophysical properties and photodynamic activity againstEscherichia coli,Staphylococcus aureusandCandida albicansevaluated.

Harvestman phenols and benzoquinones: characterisation and biosynthetic pathway

Benzoquinones are usually present in arthropod defence exudates. Here, we describe the chemical profiles of 12 harvestman species belonging to the neotropical family Gonyleptidae. Nine of the studied species produced benzoquinones, while three produced alkyl phenols. Two benzoquinones and one phenol exhibited biological activity against bacteria and fungi. We also studied the biosynthesis of 2-ethyl-1,4-benzoquinone by feeding Magnispina neptunus individuals with ¹³C-labelled precursors; the benzoquinones were biosynthesised through a polyketide pathway using acetate and propionate building blocks.

(±)-(4aR,5R,8S,8aR)-8-(tert-Butyldimethylsilyloxy)-2,5,8a-trimethyl-4a,5,8,8a-tetrahydronaphthalene-1,4-dione

In the title compound, C₁₉H₃₀O₃Si, both rings adopt a half-boat conformation. Overall, the mol­ecule approximates a U-shape as the cyclo-2-ene-1,4-dione and butyl­dimethyl­sil­yloxy substituents lie to the same side of the central cyclo­hexene ring; the methyl substituent lies to the other side of the mol­ecule. In the crystal, linear supra­molecular chains along the b axis are sustained by C—H⋯O inter­actions.

(4R,4aS,4bS,7R,10aR)-4-Hy-droxy-4a,7-dimethyl-2-(propan-2-yl)-1,4,4a,4b,5,6,7,8,10,10a-deca-hydro-phenanthren-1-one

In the title compound, C(19)H(28)O(2), the A ring adopts a chair conformation, and each of the B and C rings adopts a distorted half-chair conformation with the methine C atom in the CH(2)C(H)C(=O) residue, common to both rings, lying 0.6397 (19) and 0.6328 (18) Å out of the approximate plane defined by the remaining five C atoms (r.m.s. deviations = 0.0791 and 0.0901 Å for rings B and C, respectively). Helical supra-molecular chains along the a axis mediated by hy-droxy-carbonyl O-H⋯O hydrogen bonds feature in the crystal packing.

(4aS,4bR,7R,10aS)-3,7-Dimethyl-10a-(propan-2-yl)-1,4,4a,4b,5,6,7,8,10,10a-decahydrophenanthrene-1,4-dione

In the title compound, C₁₉H₂₆O₂, the A ring adopts a chair conformation, whereas the B and C rings both adopt distorted half-chair conformations with the quaternary C atom common to both rings lying 0.577 (3) and 0.648 (3) Å out of the approximate plane defined by the remaining five C atoms (r.m.s. deviations = 0.1386 and 0.1156 Å) for the B and C rings, respectively. Mol­ecules are assembled in the crystal through C—H⋯O inter­actions involving both carbonyl O atoms, which lead to supra­molecular chains aligned along the b axis.

(4R*,4aS*,4bS*,5R*,10aR*)-4-Hy-droxy-4a,5-dimethyl-2-(propan-2-yl)-1,4,4a,4b,5,6,7,8,10,10a-deca-hydro-phenan-thren-1-one

In the title compound, C₁₉H₂₈O₂, the A ring adopts a chair conformation. Both the B and C rings adopt envelope conformations with the C atoms common to both rings and adjacent to the carbonyl and hydroxyl groups, respectively, lying 0.604 (3) and 0.634 (3) Å out of the mean planes defined by the remaining five C atoms of rings B and C, respectively (r.m.s. deviations = 0.0100 and 0.0157 Å, respectively). The formation of linear supra­molecular C(7) chains along the a axis mediated by hy­droxy-O—H⋯O(carbon­yl) hydrogen bonds is the most prominent feature of the crystal packing.

Larvicidal activity of para-benzoquinones

Plant products may be alternative sources of mosquito larval control agents, since they constitute a rich source of bioactive compounds that are biodegradable into nontoxic products. It has been reported that quinones and derivatives present toxic activity against mosquito larvae Aedes aegypti. Therefore, these facts led us to investigate the larvicidal potential of six structurally related para-benzoquinones against A. aegypti L. (Culicidae) larvae, the vector of dengue fever. All the para-benzoquinones were found to have larvicidal effect. The unsubstituted para-benzoquinone was the compound that exhibited the lowest potency, while 2-isopropyl-para-benzoquinone was the most bioactive. In general, the presence of alkyl groups results in more potent compounds. In addition, the number, position, and size of these groups modulate the potency of the compounds. The experimental results showed that by appropriate structural modification of para-benzoquinones, it may be possible to develop novel insecticidal compounds with potential use to control A. aegypti population.

Synthesis and Analgesic-like Effect of (6R, 4S)-p-Mentha-1,8-dien-6-ylmethylene- p-toluenesulfonamide

The synthesis of a monoterpene-based para-toluenesulfonamide is reported starting from naturally occurring (R)- (−)-carvone (1), by 1,2-addition of HCN followed by reduction with lithium aluminum hydride to afford the amino alcohols 3a and 3b. Tosylation of thismixture with p-toluenesulfonyl chloride furnished sulfonamide 4 in 55% overall yield. Compound 4 was evaluated in behavior animal models to investigate its effects on the central nervous system. It showed low toxicity and sedative action in mice, indicating it to be psychoactive. It also caused a decrease in the spontaneous motor activity of mice. This depressant effect was confirmed in the acetic acid-induced writhing test, which demonstrated a significant antinociceptive response more potent than 1. The present results provide evidence that sulfonamide 4 has analgesic-like psychopharmacological activity.

Core structure of eremophilanes and bakkanes through niobium catalyzed Diels-Alder reaction: synthesis of (+/-)-bakkenolide A

A suitable intermediate for the synthesis of eremophilanes and bakkanes was prepared by a highly regioselective and stereoselective one-step synthesis through a niobium catalyzed Diels-Alder reaction. As a demonstration of the versatility of this intermediate, a total synthesis of (+/-)-bakkenolide A is described.

First comprehensive bakkane approach: stereoselective and efficient dichloroketene-based total syntheses of (+/-)- and (-)-9-acetoxyfukinanolide, (+/-)- and (+)-bakkenolide A, (-)-bakkenolides III, B, C, H, L, V, and X, (+/-)- and (-)-homogynolide A, (+/-)-homogynolide B, and (+/-)-palmosalide C

Cycloaddition of dichloroketene with dimethylcyclohexenes has been used as the key reaction in an efficient, general approach to the bakkanes. New methods and methodologies that have been developed in this work include spiro beta-methylene-gamma-butyrolactonizations, a vicinal dicarboxylation, an angelic ester preparation, a transesterification, an epoxy ketone double reduction, and a retro aldol-aldol approach to low-energy aldol isomers.

An Improved Three Step Synthesis of (-)-3β-Hydroxycarvone from (-)-Carvone

(-)-Carvone (3) has been efficiently transformed into (-)-3β-hydroxycarvone (1), which is expected to be a useful synthon or chiral template in the synthesis of natural molecules. This short and efficient synthesis of compound 1 involves regioselective and stereoselective α-hydroxylation of carvone via the trimethylsilyl-dienyl-ether derivative.