Strain-Release-Driven Modular Synthesis of Oxetane-Based Amide Bioisosteres: Concise, Robust and Scalable Approach.

Amide bioisoterism is a widely used strategy in drug development to fine-tune physicochemical, pharmacokinetic, and metabolic properties, eliminate toxicity and gain intellectual property rights in uncharted chemical space. Of these, oxetane-amines offer particularly exciting possibilities as bioisosteres, although they are less frequently investigated than warranted due to the lack of simple and widely applicable synthetic methods. Herein, we report a two-step, practical, modular, robust, and scalable method for the construction of oxetane-containing amide bioisosteres that relies on the readily available oxetan-3-one. This operationally simple procedure exploits the enhanced reactivity of the keto group of the commercially available oxetan-3-one to form amine-benzotriazole intermediates, which springloaded adducts are then reacted with various aliphatic and aromatic organometallic reagents under mild conditions to afford various amino-oxetanes in good to high yields. The simplicity and broad applicability of the method greatly facilitates the synthesis of derivatives that were previously difficult or impossible to produce. The usefulness of this method in the field medicinal chemistry was also demonstrated by eliminating the well-known metabolic problem of ketoconazole.

Spiro[3.3]heptane as a Saturated Benzene Bioisostere.

The spiro[3.3]heptane core, with the non-coplanar exit vectors, was shown to be a saturated benzene bioisostere. This scaffold was incorporated into the anticancer drug sonidegib (instead of the meta-benzene), the anticancer drug vorinostat (instead of the phenyl ring), and the anesthetic drug benzocaine (instead of the para-benzene). The patent-free saturated analogs obtained showed a high potency in the corresponding biological assays.

2-Oxabicyclo[2.1.1]hexanes: Synthesis, Properties, and Validation as Bioisosteres of ortho- and meta-Benzenes.

We have developed a general and practical approach towards 2-oxabicyclo[2.1.1]hexanes with two and three exit vectors via an iodocyclization reaction. The obtained compounds have been easily converted into the corresponding building blocks for use in medicinal chemistry. 2-Oxabicyclo[2.1.1]hexanes have been incorporated into the structure of five drugs and three agrochemicals, and validated biologically as bioisosteres of ortho- and meta-benzenes.

General and Scalable Approach to Trifluoromethyl-Substituted Cyclopropanes.

CF3-cyclopropanes with aliphatic, aromatic, and even heteroaromatic substituents were prepared on a multigram scale by deoxyfluorination of cyclopropane carboxylic acids or their salts with sulfur tetrafluoride. For labile α-pyridine acetic acids, only the use of their potassium salts allowed to obtain the needed products. Derivatization of CF3-cyclopropanes into building blocks ready for direct use in medicinal chemistry was performed.

Fluorinated Aliphatic Diazirines: Preparation, Characterization, and Model Photolabeling Studies.

The previously unknown difluoromethyl diazirines and the previously neglected trifluoromethyl-aliphatic diazirines were synthesized and characterized. Model photolabeling experiments and biological studies showed that these compounds could indeed be used as photoaffinity labels.

Fluorinated Pyrazoles: From Synthesis to Applications.

Fluorinated pyrazoles play an important role in medicinal chemistry, drug discovery, agrochemistry, coordination chemistry, and organometallic chemistry. Since the early 1990s, their popularity has grown exponentially. Moreover, more than 50% of all contributions on the topic have been published in the last 5 years. In this review, analysis of novel synthetic approaches to fluorinated pyrazoles that appeared in recent years is performed. A particular emphasis is devoted to a detailed consideration of reaction mechanisms. In addition, the reasons that have led to the ever-increasing popularity of fluorinated pyrazoles in various areas of science are discussed. At the end of the review, several potentially interesting but yet mostly unknown classes of fluorinated pyrazoles are outlined.

Rapid and Scalable Halosulfonylation of Strain-Release Reagents.

Sulfonylated aromatics are commonplace motifs in drugs and agrochemicals. However, methods for the direct synthesis of sulfonylated non-classical arene bioisosteres, which could improve the physicochemical properties of drug and agrochemical candidates, are limited. Here we report a solution to this challenge: a one-pot halosulfonylation of [1.1.1]propellane, [3.1.1]propellane and bicyclo[1.1.0]butanes that proceeds under practical, scalable and mild conditions. The sulfonyl halides used in this chemistry feature aryl, heteroaryl and alkyl substituents, and are conveniently generated in situ from readily available sulfinate salts and halogen atom sources. This methodology enables the synthesis of an array of pharmaceutically and agrochemically relevant halogen/sulfonyl-substituted bioisosteres and cyclobutanes, on up to multidecagram scale.

General Synthesis of 3-Azabicyclo[3.1.1]heptanes and Evaluation of Their Properties as Saturated Isosteres.

A general approach to 3-azabicyclo[3.1.1]heptanes by reduction of spirocyclic oxetanyl nitriles was developed. The mechanism, scope, and scalability of this transformation were studied. The core was incorporated into the structure of the antihistamine drug Rupatidine instead of the pyridine ring, which led to a dramatic improvement in physicochemical properties.

1-Azaspiro[3.3]heptane as a Bioisostere of Piperidine.

1-Azaspiro[3.3]heptanes were synthesized, characterized, and validated biologically as bioisosteres of piperidine. The key synthesis step was thermal [2+2] cycloaddition between endocyclic alkenes and the Graf isocyanate, ClO2 S-NCO, to give spirocyclic ß-lactams. Reduction of the ß-lactam ring with alane produced 1-azaspiro[3.3]heptanes. Incorporation of this core into the anesthetic drug bupivacaine instead of the piperidine fragment resulted in a new patent-free analogue with high activity.

Overcoming Limitations in Decarboxylative Arylation via Ag-Ni Electrocatalysis.

A useful protocol for achieving decarboxylative cross-coupling (DCC) of redox-active esters (RAE, isolated or generated in situ) and halo(hetero)arenes is reported. This pragmatically focused study employs a unique Ag-Ni electrocatalytic platform to overcome numerous limitations that have plagued this strategically powerful transformation. In its optimized form, coupling partners can be combined in a surprisingly simple way: open to the air, using technical-grade solvents, an inexpensive ligand and Ni source, and substoichiometric AgNO3, proceeding at room temperature with a simple commercial potentiostat. Most importantly, all of the results are placed into context by benchmarking with state-of-the-art methods. Applications are presented that simplify synthesis and rapidly enable access to challenging chemical space. Finally, adaptation to multiple scale regimes, ranging from parallel milligram-based synthesis to decagram recirculating flow is presented.

Fluorine-Containing Prolines: Synthetic Strategies, Applications, and Opportunities.

Fluorinated prolines play an important role in peptide studies, protein engineering, medicinal chemistry, drug discovery, and agrochemistry. Since the first synthesis of 4-fluoroprolines by Gottlieb and Witkop in 1965, their popularity started to grow exponentially. For example, during the past two decades, all isomeric trifluoromethyl-substituted prolines have been synthesized. In this Perspective, chemical properties and applications of fluorinated prolines are discussed. Synthetic approaches to all known fluorine-containing prolines are also discussed and analyzed. This analysis unexpectedly revealed an unsolved problem: in strict contrast to fluoro- and trifluoromethyl-substituted prolines, the corresponding analogues with fluoromethyl and difluoromethyl groups are mostly unknown. At the end of the paper, structures of several interesting, yet unknown, fluorinated prolines are disclosed─a good opportunity for chemists to make them.

2,2,2-Trifluorodiazoethane (CF3CHN2): A Long Journey since 1943.

2,2,2-Trifluorodiazoethane (CF3CHN2) has been proven to be a valuable reagent for the rapid synthesis of various classes of trifluoromethyl-substituted organic molecules in both academic and industrial research. The reagent was prepared first in 1943, but only recently it received a great attention from the scientific community. Protocols for in situ generation, in flow generation, and crystalline analogues of CF3CHN2 were developed during the past decade and have led to an ever-increasing number of publications on the topic. The current review summarizes and comprehensively analyzes all manuscripts in a peer-reviewed literature on 2,2,2-trifluorodiazoethane since its discovery in 1943 until today (as of 19 June 2020). These include contributions mostly overseen by scientific community before. A particular emphasis of this review is to classify the existing reactivity modes of CF3CHN2 and to systematically analyze the corresponding reaction mechanisms in details. At the end of this review, several undeveloped areas for CF3CHN2 are outlined, based on the overseen precedents in the literature.

A Practical and Scalable Approach to Fluoro-Substituted Bicyclo[1.1.1]pentanes.

After more than 20 years of trials, a practical scalable approach to fluoro-substituted bicyclo[1.1.1]pentanes (F-BCPs) has been developed. The physicochemical properties of the F-BCPs have been studied, and the core was incorporated into the structure of the anti-inflammatory drug Flurbiprofen in place of the fluorophenyl ring.

Large-Scale Synthesis and Modifications of Bicyclo[1.1.1]pentane-1,3-dicarboxylic Acid (BCP).

In flow photochemical addition of propellane to diacetyl allowed construction of the bicyclo[1.1.1]pentane (BCP) core in a 1 kg scale within 1 day. Haloform reaction of the formed diketone in batch afforded bicyclo[1.1.1]pentane-1,3-dicarboxylic acid in a multigram amount. Representative gram scale transformations of the diacid were also performed to obtain various BCP-containing building blocks-alcohols, acids, amines, trifluoroborates, amino acids, etc.-for medicinal chemistry.

Scalable Approach to Fluorinated Heterocycles with Sulfur Tetrafluoride (SF4).

A general approach to fluorinated (hetero)aromatic derivatives is elaborated. The key reaction is a deoxofluorination of substituted acetophenones with sulfur tetrafluoride (SF4). In contrast to previous deoxofluorination methods, this transformation is fast, scalable (up to 70 g), and high-yielding. More than 100 novel or previously hardly accessible fluorinated heterocycles, interesting for medicinal chemistry and agrochemistry, were synthesized.

Bicyclic Piperidines via [2 + 2] Photocycloaddition.

A synthetic strategy to fused bicyclic piperidines-building blocks for medicinal chemistry-is developed. The key step was an intramolecular [2 + 2]-photocyclization. The photochemical step was performed on a gram scale. Crystallographic analysis of the obtained compounds revealed that they occupy a novel chemical space and can be considered as elongated analogues of 3-substituted piperidines.

Phosphine Oxides (-POMe2) for Medicinal Chemistry: Synthesis, Properties, and Applications.

A general practical approach to hetero(aromatic) and aliphatic P(O)Me2-substituted derivatives is elaborated. The key synthetic step was a [Pd]-mediated C-P coupling of (hetero)aryl bromides/iodides with HP(O)Me2. The P(O)Me2 substituent was shown to dramatically increase solubility and decrease lipophilicity of organic compounds. This tactic was used to improve the solubility of the antihypertensive drug prazosin without affecting its biological profile.

Bicyclic Pyrrolidines for Medicinal Chemistry via [3 + 2]-Cycloaddition.

A general approach to bicyclic fused pyrrolidines via [3 + 2]-cycloaddition between nonstabilized azomethyne ylide and endocyclic electron-deficient alkenes was elaborated. "Push-pull" alkenes and CF3-alkenes did not react with the azomethyne ylide under the previously reported conditions, and we developed a superior protocol (LiF, 140 °C, no solvent). Among obtained products were medchem-relevant bicyclic sulfones, monofluoro-, difluoro-, and trifluoromethyl-substituted pyrrolidines. This approach not only allowed preparation of novel molecules but also significantly simplified synthesis of the existing ones (e.g., sofinicline).

Diazoacetonitrile (N2 CHCN): A Long Forgotten but Valuable Reagent for Organic Synthesis.

Diazoacetonitrile (N2 CHCN) is a small reactive diazoalkane. It has been synthesized for the first time already in 1898 by Theodor Curtius, however, did not gain much recognition in organic synthesis until recently. Only in 2015, after introduction of in situ and flow protocols for the safe generation of diazoacetonitrile, it started gaining popularity. In this minireview, the synthetic properties and applications of this valuable reagent are discussed.

Deoxofluorination of (Hetero)aromatic Acids.

Diverse trifluoromethyl-substituted compounds were synthesized by deoxofluorination of cinnamic and (hetero)aromatic carboxylic acids with sulfur tetrafluoride. The obtained products were used as starting materials in the preparation of novel fluorinated amino acids, anilines, and aliphatic amines - valuable building blocks for medicinal chemistry and agrochemistry.