Ring Opening Reactions of Epoxides. A Review

Allylic Epoxides Increase the Strain Energy of Cyclic Olefin Monomers for Ring-Opening Metathesis Polymerization

Ring-opening metathesis polymerization (ROMP) is an effective method for synthesizing functional polymers, but since the technique typically relies on high ring strain cyclic olefins, the most common monomers are norbornene derivatives. The reliance on one class of monomer limits the obtainable properties of ROMP polymers. In this work, we investigate new bicyclic monomers synthesized via epoxidation of commercial dienes. DFT estimates of these monomers' ring strains suggests a significant increase in strain for cyclic olefins containing allylic epoxides. We found that the eight-membered (3,4-COO) and five-membered (CPO) cyclic olefins were particularly effective for ROMP. CPO was of especially intriguing due to its excellent polymerizability when compared to the limited reactivity of other five-membered rings. Unlike polynorbornenes, the resulting polymers of both monomers displayed glass transition temperatures well below room temperature. Interestingly, poly(3,4-COO) showed both high stereo- and regioregularity while poly(CPO) showed little regularity. Both polymers could be readily modified via post-polymerization ring-opening of the reactive allylic epoxides. With a high epoxide density in poly(CPO), CPO is an exciting new ROMP monomer that is easily synthesized, can be polymerized to high conversion at room temperature, and may be facilely modified to yield a wide range of functional materials.

Electrically Conductive Injectable Silk/PEDOT: PSS Hydrogel for Enhanced Neural Network Formation

With no effective treatments for functional recovery after injury, spinal cord injury (SCI) remains one of the unresolved healthcare challenges. Human induced pluripotent stem cell (hiPSC) transplantation is a versatile patient-specific regenerative approach for functional recovery after SCI. Injectable electroconductive hydrogel (ECH) can further enhance the cell transplantation efficacy through a minimally invasive manner as well as recapitulate the native bioelectrical microenvironment of neural tissue. Given these considerations, we report a novel ECH prepared through self-assembly facilitated in situ gelation of natural silk fibroin (SF) derived from mulberry Bombyx mori silk and electrically conductive PEDOT:PSS. PEDOT:PSS was pre-stabilized to prevent the potential delamination of its hydrophilic PSS chain under aqueous environment using 3% (v/v) (3-glycidyloxypropyl)trimethoxysilane (GoPS) and 3% (w/v) poly(ethylene glycol)diglycidyl ether (PeGDE). The resultant ECH formulations are easily injectable with standard hand force with flow point below 100 Pa and good shear-thinning properties. The ECH formulations with unmodified and GoPS-modified PEDOT:PSS, that is, SF/PEDOT and SF/PEDOTGoP maintain comparable elastic modulus to spinal cord (~10-60 kPa) under physiological condition, indicating their flexibility. The GoPS-modified ECHs also display improved structural recoverability (~70%-90%) as compared to the unmodified versions of the ECHs (~30%-80%), as indicated by the three interval time thixotropy (3ITT) test. Additionally, these ECHs possess electrical conductivity in the range of ~0.2-1.2 S/m comparable to spinal cord (1-10 S/m), indicating their ability to mimic native bioelectrical environment. Approximately 80% or more cell survival was observed when hiPSC-derived cortical neurons and astrocytes were encapsulated within these ECHs. These ECHs support the maturation of cortical neurons when embedded for 7 days, fostering the development of a complex, interconnected network of long axonal processes and promoting synaptogenesis. These results underline the potential of silk ECHs in cell transplantation therapy for spinal cord regeneration.

Modifications of Prenyl Side Chains in Natural Product Biosynthesis

In recent years, there has been a growing interest in understanding the enzymatic machinery responsible for the modifications of prenyl side chains and elucidating their roles in natural product biosynthesis. This interest stems from the pivotal role such modifications play in shaping the structural and functional diversity of natural products, as well as from their potential applications to synthetic biology and drug discovery. In addition to contributing to the diversity and complexity of natural products, unique modifications of prenyl side chains are represented by several novel biosynthetic mechanisms. Representative unique examples of epoxidation, dehydrogenation, oxidation of methyl groups to carboxyl groups, unusual C-C bond cleavage and oxidative cyclization are summarized and discussed. By revealing the intriguing chemistry and enzymology behind these transformations, this comprehensive and comparative review will guide future efforts in the discovery, characterization and application of modifications of prenyl side chains in natural product biosynthesis.

Solid-Acid Catalyzed Continuous-Flow Aminolysis of Epoxides

We report a solid-acid catalyzed aminolysis of epoxides under continuous-flow conditions. A titania-zirconia supported molybdenum oxide catalyst demonstrated exceptional substrate compatibility, enabling the synthesis of β-amino alcohols in excellent yields with high catalyst durability. Characterization of the catalyst revealed the crucial role of the titania-zirconia ratio in optimizing its performance. Furthermore, this method was applied to the efficient, sequential-flow synthesis of a rivaroxaban intermediate (an oral anticoagulant and the first direct factor Xa inhibitor), combining a hydrogenation step with the aminolysis reaction without the need for intermediate isolation.

Accessing the synthesis of natural products and their analogues enabled by the Barbier reaction: a review

The Barbier reaction is significantly referred to as one of the efficient carbon-carbon bond forming reactions which involves the treatment of haloalkanes and carbonyl compounds by utilizing the catalytic role of a diverse range of metals and metalloids. The Barbier reaction is tolerant to a variety of functional groups, allowing a broad substrate scope with the employment of lanthanides, transition metals, amphoteric elements or alkaline earth metals. This reaction is also water-resistant, thereby overcoming the challenges posed by moisture sensitive organometallic species involving C-C bond formation reactions. The Barbier reaction has significantly found its applicability towards the synthesis of intricate and naturally occurring organic compounds. Our review provides an outlook on the synthetic applications of the Barbier reaction and its variants to accomplish the preparation of several natural products, reported since 2020.

C3 Selective Hydroxylation of Pyridines via Photochemical Valence Isomerization of Pyridine N-Oxides

The C-H hydroxylation of the pyridine C3 position is a highly desirable transformation but remains a great challenge due to the inherent electronic properties of this heterocycle core which bring difficulties in chemical reactivity and regioselectivity. Herein we present an efficient method for formal C3 selective hydroxylation of pyridines via photochemical valence isomerization of pyridine N-oxides. This metal-free transformation features operational simplicity and compatibility with a diverse array of functional groups, and the resulting hydroxylated products are amenable to further elaboration to synthetically useful building blocks. The synthetic utility of this strategy is further demonstrated in the effective late-stage functionalization of pyridine-containing medicinally relevant molecules and versatile derivatizations of 3-pyridinols.

Engineering poly(dehydroalanine)-based gels via droplet-based microfluidics: from bulk to microspheres

Biomedical applications such as drug delivery, tissue engineering, and functional surface coating rely on switchable adsorption and desorption of specialized guest molecules. Poly(dehydroalanine), a polyzwitterion containing pH-dependent positive and negative charges, shows promise for such reversible loading, especially when integrated into a gel network. Herein, we present the fabrication of poly(dehydroalanine)-derived gels of different size scales and evaluate them with respect to their practical use in biomedicine. Already existing protocols for bulk gelation were remodeled to derive suitable reaction conditions for droplet-based microfluidic synthesis. Depending on the layout of the microfluidic chip, microgels with a size of approximately 30 μm or 200 μm were obtained, whose crosslinking density can be increased by implementing a multi-arm crosslinker. We analyzed the effects of the crosslinker species on composition, permeability, and softness and show that the microgels exhibit advantageous properties inherent to zwitterionic polymer systems, including high hydrophilicity as well as pH- and ionic strength-sensitivity. We demonstrate pH-regulated uptake and release of fluorescent model dyes before testing the adsorption of a small antimicrobial peptide, LL-37. Quantification of the peptide accommodated within the microgels reveals the impact of size and crosslinking density of the microgels. Biocompatibility of the microgels was validated by cell tests.

Superior and efficient performance of cost-effective MIP-202 catalyst over UiO-66-(CO2H)2 in epoxide ring opening reactions

This study explored the catalytic performance of two robust zirconium-based metal-organic frameworks (MOFs), MIP-202(Zr) and UiO-66-(CO2H)2 in the ring-opening of epoxides using alcohols and amines as nucleophilic reagents. The MOFs were characterized by techniques such as FT-IR, PXRD, FE-SEM, and EDX. Through systematic optimization of key parameters (catalyst amount, time, temperature, solvent), MIP-202(Zr) achieved 99% styrene oxide conversion in 25 min with methanol at room temperature using 5 mg catalyst. In contrast, UiO-66-(CO2H)2 required drastically harsher conditions of 120 min, 60 °C, and four times the catalyst loading to reach 98% conversion. A similar trend was observed for ring-opening with aniline -MIP-202(Zr) gave 93% conversion in one hour at room temperature, while UiO-66-(CO2H)2 needed two hours at 60 °C for 95% conversion. The superior performance of MIP-202(Zr) likely stems from cooperative Brønsted/Lewis acid sites and higher proton conductivity enabling more efficient epoxide activation. Remarkably, MIP-202(Zr) maintained consistent activity over five recycles in the ring-opening of styrene oxide by methanol and over three recycles in the ring-opening of styrene oxide by aniline. Testing various epoxide substrates and nucleophiles revealed trends in reactivity governed by electronic and steric effects. The results provide useful insights into tuning Zr-MOF-based catalysts and highlight the promise of the cost-effective and sustainable MIP-202(Zr) for diverse epoxide ring-opening reactions on an industrial scale.

Emin: A First-Principles Thermochemical Descriptor for Predicting Molecular Synthesizability

Predicting the synthesizability of a new molecule remains an unsolved challenge that chemists have long tackled with heuristic approaches. Here, we report a new method for predicting synthesizability using a simple yet accurate thermochemical descriptor. We introduce Emin, the energy difference between a molecule and its lowest energy constitutional isomer, as a synthesizability predictor that is accurate, physically meaningful, and first-principles based. We apply Emin to 134,000 molecules in the QM9 data set and find that Emin is accurate when used alone and reduces incorrect predictions of "synthesizable" by up to 52% when used to augment commonly used prediction methods. Our work illustrates how first-principles thermochemistry and heuristic approximations for molecular stability are complementary, opening a new direction for synthesizability prediction methods.

Three-Membered Aluminacycles

Three-membered-ring scaffolds of carbocycles, namely, cyclopropanes and cyclopropenes, are ubiquitous in natural products and pharmaceutical molecules. These molecules exhibit a peculiar reactivity, and their applications as synthetic intermediates and versatile building blocks in organic synthesis have been extensively studied over the past century. The incorporation of heteroatoms into three-membered cyclic structures has attracted significant attention, reflecting fundamental differences in their electronic/geometric structures and reactivities compared to their carbon congeners and their associated potential for exploitation in applications. Recently, the chemistry of low-valent aluminum species, alumylenes, dialumenes, and aluminyl anions, has dramatically developed, which has allowed access to hitherto unprecedented aluminacycles. This Perspective focuses upon advances in the chemistry of three-membered aluminacycles, including their synthetic protocols, spectroscopic and structural properties, and reactivity toward various substrates and small molecules.

Merging Two Strained Carbocycles: Lewis Acid Catalyzed Remote Site-Selective Friedel-Crafts Alkylation of in Situ Generated β-Naphthol

Lewis acid catalyzed tandem activation of the two smallest carbocycles, 3-ethoxy cyclobutanones, and donor-acceptor cyclopropanes has been demonstrated. The diphenyl-substituted 3-ethoxy cyclobutanone rearranges itself by intramolecular cyclization for the in situ generation of 1-phenyl 2-naphthol, which further undergoes remote site-selective Friedel-Crafts alkylation with donor-acceptor cyclopropane to synthesize a series of γ-naphthyl butyric acid derivatives. Further control experiments for mechanistic investigations and synthetic applications have also been carried out.

Continuous Flow Epoxidation of Alkenes Using a Homogeneous Manganese Catalyst with Peracetic Acid

Epoxidation of alkenes is a valuable transformation in the synthesis of fine chemicals. Described herein are the design and development of a continuous flow process for carrying out the epoxidation of alkenes with a homogeneous manganese catalyst at metal loadings as low as 0.05 mol%. In this process, peracetic acid is generated in situ and telescoped directly into the epoxidation reaction, thus reducing the risks associated with its handling and storage, which often limit its use at scale. This flow process lessens the safety hazards associated with both the exothermicity of this epoxidation reaction and the use of the highly reactive peracetic acid. Controlling the speciation of manganese/2-picolinic acid mixtures by varying the ligand:manganese ratio was key to the success of the reaction. This continuous flow process offers an inexpensive, sustainable, and scalable route to epoxides.

Recent Advances in the Biomedical Applications of Functionalized Nanogels

Nanomaterials have been extensively used in several applications in the past few decades related to biomedicine and healthcare. Among them, nanogels (NGs) have emerged as an important nanoplatform with the properties of both hydrogels and nanoparticles for the controlled/sustained delivery of chemo drugs, nucleic acids, or other bioactive molecules for therapeutic or diagnostic purposes. In the recent past, significant research efforts have been invested in synthesizing NGs through various synthetic methodologies such as free radical polymerization, reversible addition-fragmentation chain-transfer method (RAFT) and atom transfer radical polymerization (ATRP), as well as emulsion techniques. With further polymeric functionalizations using activated esters, thiol-ene/yne processes, imines/oximes formation, cycloadditions, nucleophilic addition reactions of isocyanates, ring-opening, and multicomponent reactions were used to obtain functionalized NGs for targeted delivery of drug and other compounds. NGs are particularly intriguing for use in the areas of diagnosis, analytics, and biomedicine due to their nanodimensionality, material characteristics, physiological stability, tunable multi-functionality, and biocompatibility. Numerous NGs with a wide range of functionalities and various external/internal stimuli-responsive modalities have been possible with novel synthetic reliable methodologies. Such continuous development of innovative, intelligent materials with novel characteristics is crucial for nanomedicine for next-generation biomedical applications. This paper reviews the synthesis and various functionalization strategies of NGs with a focus on the recent advances in different biomedical applications of these surface modified/functionalized single-/dual-/multi-responsive NGs, with various active targeting moieties, in the fields of cancer theranostics, immunotherapy, antimicrobial/antiviral, antigen presentation for the vaccine, sensing, wound healing, thrombolysis, tissue engineering, and regenerative medicine.

Bis(oxiranes) Containing Cyclooctane Core: Synthesis and Reactivity towards NaN3

Reactions of oxirane ring opening provide a powerful tool for regio- and stereoselective synthesis of polyfunctional and heterocyclic compounds, widely used in organic chemistry and drug design. Cyclooctane, alongside other medium-sized rings, is of interest as a novel molecular platform for the construction of target-oriented leads. Additionally, cyclooctane derivatives are well known to be prone to transannular reactions, which makes them a promising object in the search for novel approaches to polycyclic structures. In the present work, a series of cyclooctanediones was studied in Corey-Chaykovsky reactions, and novel spirocyclic bis(oxiranes) containing cyclooctane core, namely, 1,5-dioxadispiro[2.0.2.6]dodecane and 1,8-dioxadispiro[2.3.2.3]dodecane, were synthesized. Ring opening of the obtained bis(oxiranes) upon treatment with sodium azide was investigated, and it was found that the reaction path is determined by the reciprocal orientation of oxygen atoms in the oxirane moieties. Diastereomers of the bis(oxiranes) with cis-orientation underwent independent ring opening, supplying corresponding diazidodiols, while in the case of stereoisomers with trans-orientation, domino-like reactions occurred, including intramolecular nucleophilic attack and the formation of a novel three- or six-membered O-containing ring. Summarily, a straightforward approach to polyfunctional compounds containing cyclooctane or oxabicyclo[3.3.1]nonane cores, employing bis(oxiranes), was elaborated.