Unexpected Isomerization of Oxetane-Carboxylic Acids

3-Oxabicyclo[3.1.1]heptanes as Isosteres of meta-Substituted Benzene Rings

Replacement of the aromatic rings in drug candidates with isosteric rigid sp3-rich scaffolds can improve physicochemical properties, increase the chance of progressing the molecule in development, and open new chemical space. Isosteres of meta-substituted benzenes remain challenging due to the difficulty of mimicking the exit vector angles and bond distances. Herein, we report the synthesis of 1,5-disubstituted 3-oxabicyclo[3.1.1]heptanes (oxa-BCHeps), which can serve as saturated isosteres of meta-substituted phenyl rings with a similar geometric arrangement. This structural motif can be obtained under mild reaction conditions via acid-mediated isomerization of (2-oxaspiro[3.3]heptan-6-yl)methanols using catalytic quantities of pyridinium chloride (PyrHCl). We demonstrate the utility of this methodology by preparing various building blocks for use in medicinal chemistry and incorporating 3-oxa-BCHep into the anticancer drug sonidegib, improving its physicochemical properties, such as permeability, metabolic stability, and solubility.

3,3-Difluorooxetane-A Versatile Functional Group for Bioisosteric Replacements in Drug Discovery

Functional group (FG) is one of the cornerstone concepts in organic chemistry and related areas. The wide spread of bioisosterism ideas in medicinal chemistry and beyond caused a striking rise in demand for novel FGs with a defined impact on the developed compound properties. In this work, the evaluation of the 3,3-difluorooxetane unit (3,3-diFox) as a functional group for bioisosteric replacements is disclosed. A comprehensive experimental study (including multigram building block synthesis, quantification of steric and electronic properties, measurements of pKa, LogP, chemical stability, and biological evaluation of the 3,3-diFox-derived bioisostere of a drug candidate) revealed a prominent behavior of the 3,3-diFox fragment as a versatile substituent for early drug discovery programs.

Stereoselective Ring-Opening Reaction of α-Fluorinated Oxetanes: A Practical and Theoretical Investigation

The ring-opening reaction of fluorinated oxetanes by halides, including alkylidene oxetanes and spirocyclic oxetanes, was highly stereoselective and directed by the presence of a fluorine atom. This reaction allowed a stereoselective preparation of tetrasubstituted alkenes and substituted pyrrolidines containing all-carbon quaternary centers. Theoretical calculations were performed to shed light on experimentally observed regioselectivity in the opening of oxetane derivatives. Transition state calculations were carried out to compare the energy of transition states responsible for forming different diastereoisomers. These calculations were performed using several DFT functionals and benchmarked to DLPNO-CCSD(T)/def2-TZVP calculations. Intrinsic reaction coordinated (IRC) calculations were run to confirm if the found transition states connect reactants and products. The IRC paths were then decomposed into the electrostatic, steric hyperconjugative contributions to reaction barriers by using the natural bond orbital (NBO) analysis. The destabilizing Fδ-···Br- electrostatic interaction directs the reaction pathway.

Synthesis of 6-oxa-Spiro[4.5]decane Derivatives by Merging Ring-Opening of Benzo[c]oxepines and Formal 1,2-Oxygen Migration

A facile one-pot synthetic method has been developed for constructing 6-oxa-spiro[4.5]decane skeletons by merging the ring-opening of benzo[c]oxepines and formal 1,2-oxygen migration reactions. More than 30 examples of the 6-oxa-spiro[4.5]decane derivatives have been synthesized under transition-metal-free conditions.

High-throughput optimization of the C-H arylation of oxetanes via Ni/aldehyde photocatalysis

Oxetanes are under-explored in medicinal chemistry, despite their favorable physicochemical properties, in part, because of the challenges associated with their syntheses. High-throughput experimentation (HTE) enables the rapid screening of reaction variables, accelerating the reaction development process. Herein we report the use of HTE in the optimization of a mild C-H arylation reaction of oxetanes, and other ethers, using p-cyanobenzaldehyde as a cheap and effective photoexcited hydrogen-atom transfer catalyst, in conjunction with a Ni catalyst. Our optimized conditions enable the use of a modern, reproducible light source as well as sub-solvent quantity oxetane, while eliminating the need for toxic co-solvents and dangerous sources of UV light.

2,3-Epoxyamide-alcohols in Domino Reactions: En Route to Molecular Diversity

The synthesis of polycyclic γ- and δ-lactams bearing up to four contiguous fully controlled stereocenters is presented. For that purpose, we developed an original approach based on the use of 2,3-epoxyamides in domino reactions by taking advantage of the nucleophilic nitrogen atom and electrophilic epoxide. In reaction with enol ethers bearing gem bis-electrophiles on the double bond as Michael acceptors, four different reaction pathways were observed. They all started with a domino oxa-Michael/aza-Michael/epoxide opening sequence and depending on substrates engaged could be followed either by a lactonization or a hemiketalization/retro-aldol cascade. Thus, four original fully-substituted piperidine- or pyrrolidine-2-one scaffolds were selectively synthesized in good to high yields. Moreover, these polycyclic lactams were obtained in high stereo- and chemo-selectively highlighting the efficiency and molecular diversity offered by this new methodology that should offer various synthetic opportunities in the future.

Three-dimensional saturated C(sp3)-rich bioisosteres for benzene

Benzenes, the most ubiquitous structural moiety in marketed small-molecule drugs, are frequently associated with poor 'drug-like' properties, including metabolic instability, and poor aqueous solubility. In an effort to overcome these limitations, recent developments in medicinal chemistry have demonstrated the improved physicochemical profiles of C(sp3)-rich bioisosteric scaffolds relative to arenes. In the past two decades, we have witnessed an exponential increase in synthetic methods for accessing saturated bioisosteres of monosubstituted and para-substituted benzenes. However, until recent discoveries, analogous three-dimensional ortho-substituted and meta-substituted biososteres have remained underexplored, owing to their ring strain and increased s-character hybridization. This Review summarizes the emerging synthetic methodologies to access such saturated motifs and their impact on the application of bioisosteres for ortho-substituted, meta-substituted and multi-substituted benzene rings. It concludes with a perspective on the development of next-generation bioisosteres, including those within novel chemical space.

Functional group tolerant hydrogen borrowing C-alkylation

Hydrogen borrowing is an attractive and sustainable strategy for carbon-carbon bond formation that enables alcohols to be used as alkylating reagents in place of alkyl halides. However, despite intensive efforts, limited functional group tolerance is observed in this methodology, which we hypothesize is due to the high temperatures and harsh basic conditions often employed. Here we demonstrate that room temperature and functional group tolerant hydrogen borrowing can be achieved with a simple iridium catalyst in the presence of substoichiometric base without an excess of reagents. Achieving high yields necessitates the application of anaerobic conditions to counteract the oxygen sensitivity of the catalytic iridium hydride intermediate, which otherwise leads to catalyst degradation. Substrates containing heteroatoms capable of complexing the catalyst exhibit limited room temperature reactivity, but the application of moderately higher temperatures enables extension to a broad range of medicinally relevant nitrogen rich heterocycles. These newly developed conditions allow alcohols possessing functional groups that were previously incompatible with hydrogen borrowing reactions to be employed.

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.

Synthesis of medium-sized heterocycles from oxetanes based on an allylic amination/ring-opening strategy

The construction of medium-sized ring compounds has been a prominent research area in synthetic chemistry. In this study, we developed a tandem strategy that combines allylic amination and ring-opening of oxetanes to synthesize medium-sized heterocycles. Specifically, N-aryl oxetan-3-amines undergo allylic amination with zwitterionic π-allylpalladium, followed by intramolecular ring-opening, resulting in the formation of medium-sized heterocycles. Notably, we are able to achieve the synthesis of 7-8 membered heterocycles with moderate to good yields by employing different types of zwitterionic π-allylpalladium species.

Oxetanes in Drug Discovery Campaigns

The oxetane ring is an emergent, underexplored motif in drug discovery that shows attractive properties such as low molecular weight, high polarity, and marked three-dimensionality. Oxetanes have garnered further interest as isosteres of carbonyl groups and as molecular tools to fine-tune physicochemical properties of drug compounds such as pKa, LogD, aqueous solubility, and metabolic clearance. This perspective highlights recent applications of oxetane motifs in drug discovery campaigns (2017-2022), with emphasis on the effect of the oxetane on medicinally relevant properties and on the building blocks used to incorporate the oxetane ring. Based on this analysis, we provide an overview of the potential benefits of appending an oxetane to a drug compound, as well as potential pitfalls, challenges, and future directions.

2-Oxabicyclo[2.1.1]hexanes as saturated bioisosteres of the ortho-substituted phenyl ring

The ortho-substituted phenyl ring is a basic structural element in chemistry. It is found in more than three hundred drugs and agrochemicals. During the past decade, scientists have tried to replace the phenyl ring in bioactive compounds with saturated bioisosteres to obtain novel patentable structures. However, most of the research in this area has been devoted to the replacement of the para-substituted phenyl ring. Here we have developed saturated bioisosteres of the ortho-substituted phenyl ring with improved physicochemical properties: 2-oxabicyclo[2.1.1]hexanes. Crystallographic analysis revealed that these structures and the ortho-substituted phenyl ring indeed have similar geometric properties. Replacement of the phenyl ring in marketed agrochemicals fluxapyroxad (BASF) and boscalid (BASF) with 2-oxabicyclo[2.1.1]hexanes dramatically improved their water solubility, reduced lipophilicity and most importantly retained bioactivity. This work suggests an opportunity for chemists to replace the ortho-substituted phenyl ring in bioactive compounds with saturated bioisosteres in medicinal chemistry and agrochemistry.

Visible Light Photoredox-Catalyzed Decarboxylative Alkylation of 3-Aryl-Oxetanes and Azetidines via Benzylic Tertiary Radicals and Implications of Benzylic Radical Stability

Four-membered heterocycles offer exciting potential as small polar motifs in medicinal chemistry but require further methods for incorporation. Photoredox catalysis is a powerful method for the mild generation of alkyl radicals for C-C bond formation. The effect of ring strain on radical reactivity is not well understood, with no studies that address this question systematically. Examples of reactions that involve benzylic radicals are rare, and their reactivity is challenging to harness. This work develops a radical functionalization of benzylic oxetanes and azetidines using visible light photoredox catalysis to prepare 3-aryl-3-alkyl substituted derivatives and assesses the influence of ring strain and heterosubstitution on the reactivity of small-ring radicals. 3-Aryl-3-carboxylic acid oxetanes and azetidines are suitable precursors to tertiary benzylic oxetane/azetidine radicals which undergo conjugate addition into activated alkenes. We compare the reactivity of oxetane radicals to other benzylic systems. Computational studies indicate that Giese additions of unstrained benzylic radicals into acrylates are reversible and result in low yields and radical dimerization. Benzylic radicals as part of a strained ring, however, are less stable and more π-delocalized, decreasing dimer and increasing Giese product formation. Oxetanes show high product yields due to ring strain and Bent's rule rendering the Giese addition irreversible.