Chirality: a key parameter in chemical probes.

Many small molecule bioactive and marketed drugs are chiral. They are often synthesised from commercially available chiral building blocks. However, chirality is sometimes incorrectly assigned by manufacturers with consequences for the end user ranging from: experimental irreproducibility, wasted time on synthesising the wrong product and reanalysis, to the added cost of purchasing the precursor and resynthesis of the correct stereoisomer. Further on, this could lead to loss of reputation, loss of funding, to safety and ethical concerns due to potential in vivo administration of the wrong form of a drug. It is our firm belief that more stringent control of chirality be provided by the supplier and, if needed, requested by the end user, to minimise the potential issues mentioned above. Certification of chirality would bring much needed confidence in chemical structure assignment and could be provided by a variety of techniques, from polarimetry, chiral HPLC, using known chiral standards, vibrational circular dichroism, and x-ray crystallography. A few case studies of our brushes with wrong chirality assignment are shown as well as some examples of what we believe to be good practice.

Synthesis of Stereoenriched Piperidines via Chemo-Enzymatic Dearomatization of Activated Pyridines.

The development of efficient and sustainable methods for the synthesis of nitrogen heterocycles is an important goal for the chemical industry. In particular, substituted chiral piperidines are prominent targets due to their prevalence in medicinally relevant compounds and their precursors. A potential biocatalytic approach to the synthesis of this privileged scaffold would be the asymmetric dearomatization of readily assembled activated pyridines. However, nature is yet to yield a suitable biocatalyst specifically for this reaction. Here, by combining chemical synthesis and biocatalysis, we present a general chemo-enzymatic approach for the asymmetric dearomatization of activated pyridines for the preparation of substituted piperidines with precise stereochemistry. The key step involves a stereoselective one-pot amine oxidase/ene imine reductase cascade to convert N-substituted tetrahydropyridines to stereo-defined 3- and 3,4-substituted piperidines. This chemo-enzymatic approach has proved useful for key transformations in the syntheses of antipsychotic drugs Preclamol and OSU-6162, as well as for the preparation of two important intermediates in synthetic routes of the ovarian cancer monotherapeutic Niraparib.

Focusing on C-4 position of Hantzsch 1,4-dihydropyridines: Molecular modifications, enantioseparation, and binding mechanism to L- and T-type calcium channels.

1,4-Dihydropyridines (DHPs) represent the blockbuster class of L-type calcium channel blockers that have tremendous therapeutic value against cardiovascular conditions. Due to their abilities to additionally target other subtypes of calcium channels, DHPs are also considered promising molecules for the treatment of neurological and psychiatric disorders. Having been in the market for more than forty years, DHP is one of the most modified scaffolds for the development of novel molecules acting on calcium channels. Taking the chemical structures of approved DHPs into account, it is noteworthy that C-4 position is the least modified part of the ring system. Therefore, in the present study, we focused on this location and carried out various molecular modifications to obtain twelve potential calcium channel blockers with a DHP-based hexahydroquinoline scaffold (DA1-DA12). The whole-cell patch clamp technique applied to analyze the blocking ability of the synthesized compounds on both L- (Cav1.2) and T- (Cav3.2) type calcium channels revealed five blockers with different selectivity profiles. Introducing naphthyl moiety onto the C-4 position of the main scaffold led to the identification of a selective blocker of Cav1.2 (DA8). The benzodioxole-substituted derivative (DA1) was the most potent and selective Cav3.2 inhibitor, therefore, its enantiomers were separated using HPLC on a chiral stationary phase. Retesting single isomers on Cav3.2 revealed that S-enantiomer was mainly responsible for the block. Finally, DA compounds were docked into two generated homology models of L- and T-type calcium channels. Molecular dynamics (MD) simulations and 3D pharmacophore modeling provided further insights into the detailed binding mechanism of DHPs to Cav1.2 as well as to Cav3.2.

Design, Synthesis, and Physicochemical Studies of Configurationally Stable ß-Carboline Atropisomers.

Axially chiral atropisomers have energetic barriers to rotation, ΔGrot, that prevent racemization of the respective enantiomers. We used computational modeling to develop a suite of 10 bio-inspired 1-aryl-ß-carbolines with varying ΔGrot, from which a strong structure-activity relationship was observed for 2-substituted-1-naphthyl substituents. We then synthesized two of these atropisomers, 1d and 1f, by a four-step racemic synthesis and resolved the enantiomers via chiral chromatography. Racemization studies revealed experimental ΔGrot values of 39.5 and 33.0 kcal/mol for 1d and 1f, respectively, which were consistent with our computational results. These atropisomers exhibited long half-lives, which allowed for their physicochemical characterization and stereochemical assignment via UV-vis spectroscopy, fluorescence spectroscopy, electronic circular dichroism, and vibrational circular dichroism.

Tuaimenal A, a Meroterpene from the Irish Deep-Sea Soft Coral Duva florida, Displays Inhibition of the SARS-CoV-2 3CLpro Enzyme.

Cold water benthic environments are a prolific source of structurally diverse molecules with a range of bioactivities against human disease. Specimens of a previously chemically unexplored soft coral, Duva florida, were collected during a deep-sea cruise that sampled marine invertebrates along the Irish continental margin in 2018. Tuaimenal A (1), a cyclized merosesquiterpenoid representing a new carbon scaffold with a highly substituted chromene core, was discovered through exploration of the soft coral secondary metabolome via NMR-guided fractionation. The absolute configuration was determined through vibrational circular dichroism. Functional biochemical assays and in silico docking experiments found tuaimenal A selectively inhibits the viral main protease (3CLpro) of SARS-CoV-2.

Generation of Axially Chiral Fluoroallenes through a Copper-Catalyzed Enantioselective ß-Fluoride Elimination.

Herein we report the copper-catalyzed silylation of propargylic difluorides to generate axially chiral, tetrasubstituted monofluoroallenes in both good yields (27 examples >80%) and enantioselectivities (82-98% ee). Compared to previously reported synthetic routes to axially chiral allenes (ACAs) from prochiral substrates, a mechanistically distinct reaction has been developed: the enantiodiscrimination between enantiotopic fluorides to set an axial stereocenter. DFT calculations and vibrational circular dichroism (VCD) suggest that ß-fluoride elimination from an alkenyl copper intermediate likely proceeds through a syn-ß-fluoride elimination pathway rather than an anti-elimination pathway. The effects of the C1-symmetric Josiphos-derived ligand on reactivity and enantioselectivity were investigated. Not only does this report showcase that alkenyl copper species (like their alkyl counterparts) can undergo ß-fluoride elimination, but this elimination can be achieved in an enantioselective fashion.

Chiral resolution and absolute configuration determination of new metal-based photodynamic therapy antitumor agents.

The advent of cisplatin as a cancer drug in the late 1960s generated considerable interest in the use of transition metal complexes as cancer therapy agents. Despite enhanced research in this area, there has yet to be any non-platinum-based transition metal complex cancer drugs approved by the Food and Drug Administration (FDA). Recently a Ru(II) metal-organic dyad (TLD1433) has provided promising results as a photodynamic therapy (PDT) agent for some types of cancer. This particularly effective PDT compound has an oligothiophene chain appended to an imidazophenanthroline ligand which chelates Ru(II). The entire complex is chiral and is synthesized as a racemate. Five such chiral Ru(II) and Os(II) PDT agents were synthesized and their enantiomers separated for the first time. The enantiomers of these compounds are not easily crystalized. However, preparative LC provided sufficient amounts of these novel PDT agents to determine their absolute configurations by vibrational circular dichroism (VCD). The synthesis, separation and absolute configuration determinations are described and discussed in detail.

Multigram Preparation of BRD4780 Enantiomers and Assignment of Absolute Stereochemistry.

The development of a multigram synthesis of 3-exo-isopropylbicyclo[2.2.1]heptan-2-endo-amine hydrochloride (1) (also known as BRD4780 and AGN-192403) is described. The process involves protection of the amine as 4-nitrobenzyl carbamate, pNZ, which enables chiral SFC chromatography. The absolute configuration (AC) of the individual enantiomers has been determined by Mosher's amide method, VCD spectroscopy, and X-ray crystallography. We highlight the VCD approach as a rapid and effective means of AC determination that can be deployed directly on the target compounds.

Triple Helical Ir(ppy)3 Phenylene Cage Prepared by Diol-Mediated Benzannulation: Synthesis, Resolution, Absolute Stereochemistry and Photophysical Properties.

Cyclometalation of a triple helical N-doped phenylene cage prepared by ruthenium(0)-catalyzed diol-diene benzannulation delivers a chiral, conformationally constrained Ir(ppy)3 analogue. Like the parent complex, fac-Ir(ppy)3 , the iridium-containing PAH-cage is phosphorescent, but displays enhanced resistance to oxygen quenching.

A Vibrational Circular Dichroism Microsampling Accessory: Mapping Enhanced Vibrational Circular Dichroism in Amyloid Fibril Films.

We report the first vibrational circular dichroism (VCD) measurement of spatial heterogeneity in a sample using infrared (IR) microsampling. Vibrational circular dichroism spectra are typically measured using a standard IR cell with an IR beam diameter of 10 mm or greater making it impossible to investigate the spatial heterogeneity of a solid film sample. We have constructed a VCD sampling assembly with either 3 mm or 1 mm spatial resolution. An XY-translation stage was used to measure spectra at different spatial locations producing IR and VCD maps of the sample. In addition, a rotating sample stage was employed using a dual photoelastic modulator (PEM) setup to suppress artifacts due to linear birefringence in solid-phase or film samples. Infrared and VCD mapping of an insulin fibril film has been carried out at both 3 and 1 mm spatial resolution, and lysozyme films were mapped at 1 mm resolution. The IR spectra of different spots vary in intensity due primarily to sample thickness. The changes in the VCD intensity across the map largely correlate to corresponding changes in the IR map. Closer inspection of the insulin map revealed changes in the relative intensities of the VCD spectra not present in the parent IR spectra, which indicated differences in the degree of supramolecular chirality of the fibrils in the various spatial regions. For lysozyme films, in addition to different degrees of supramolecular chirality, reversal of the net fibril chirality was observed. The large signal-to-noise ratio observed at 1 mm resolution implies the feasibility of further increasing the spatial resolution by one or two orders of magnitude for protein fibril film samples.

DNA repair in response to anthracycline-DNA adducts: a role for both homologous recombination and nucleotide excision repair.

Doxorubicin, a widely used anthracycline anticancer agent, acts as a topoisomerase II poison but can also form formaldehyde-mediated DNA adducts. This has led to the development of doxorubicin derivatives such as doxoform, which can readily form adducts with DNA. This work aimed to determine which DNA repair pathways are involved in the recognition and possible repair of anthracycline-DNA adducts. Cell lines lacking functional proteins involved in each of the five main repair pathways, mismatch repair (MMR), base excision repair (BER), nucleotide excision repair (NER), homologous recombination (HR) and non-homologous end-joining (NHEJ) were examined for sensitivity to various anthracycline adduct-forming treatments. The treatments used were doxorubicin, barminomycin (a model adduct-forming anthracycline) and doxoform (a doxorubicin-formaldehyde conjugate). Cells with deficiencies in MMR, BER and NHEJ were equally sensitive to adduct-forming treatments compared to wild type cells and therefore these pathways are unlikely to play a role in the repair of these adducts. Some cells with deficiencies in the NER pathway (specifically, those lacking functional XPB, XPD and XPG), displayed tolerance to adducts induced by both barminomycin and doxoform and also exhibited a decreased level of apoptosis in response to adduct-forming treatments. Conversely, two HR deficient cell lines were shown to be more sensitive to barminomycin and doxoform than HR proficient cells, indicating that this pathway is also involved in the repair response to anthracycline-DNA adducts. These results suggest an unusual damage response pathway to anthracycline adducts involving both NER and HR that could be used to optimise cancer therapy for tumours with either high levels of NER or defective HR. Tumours with either of these characteristics would be predicted to respond particularly well to anthracycline-DNA adduct-forming treatments.

Design, synthesis, and preliminary evaluation of doxazolidine carbamates as prodrugs activated by carboxylesterases.

The synthesis and tumor cell growth inhibition by doxazolidine carbamate prodrugs are reported. The carbamates were designed for selective hydrolysis by one or more human carboxylesterases to release doxazolidine (Doxaz), the formaldehyde-oxazolidine of doxorubicin that cross-links DNA to trigger cell death. Simple butyl and pentyl, but not ethyl, carbamate prodrugs inhibited the growth of cancer cells that overexpress carboxylesterase CES1 (hCE1) and CES2 (hiCE). Relative CES1 and CES2 expression levels were determined by reverse transcription of the respective mRNAs, followed by polymerase chain reaction amplification. More complex structures with a p-aminobenzyl alcohol (PABA) self-eliminating spacer showed better growth inhibition (IC50=50 nM for Hep G2 liver cancer cells) while exhibiting reduced toxicity toward rat cardiomyocytes, relative to the parent drug doxorubicin. Pentyl 4-(N-doxazolidinylcarbonyloxymethyl)phenylcarbamate, the lead compound for further investigation, appears to be activated in Hep G2 cells that express both CES1 and CES2.