In small-molecule drug discovery programs targeting the inhibition of viral replication, the introduction of a sterically constrained pyrrolidine scaffold bearing a dibenzochromen-one pharmacophore enables the interrogation of binding pockets within non-structural viral proteins that are inaccessible to planar heterocycles. Process chemists conducting route scouting on a pilot-scale Kilo Lab reactor train (Büchi Glas Uster 20 L glass-lined vessel, retreat-curve impeller agitation at 180–250 rpm) have reported that coupling the (2S,4S)-tert-butyl 4-(methoxymethyl)pyrrolidine-1,2-dicarboxylate fragment to the 9-bromo-8-oxo-tetrahydrodibenzochromen acetic acid moiety via HATU-mediated amidation in anhydrous DMF at 0–5°C, followed by a controlled quench into ice-cold 10% w/v aqueous citric acid, yields the desired product with an isolated purity exceeding 98.7% (HPLC, 254 nm) after flash chromatography on a Biotage Isolera system using a SNAP Ultra 100 g cartridge and a hexane:EtOAc step gradient. The residual palladium specification for this intermediate is set at <10 ppm per ICH Q3D Elemental Impurities Guideline, and the bromide content, sourced from the 9-bromo substituent, is monitored via ion chromatography to ensure it remains below the specification threshold during the final API synthetic step to avoid genotoxic impurity flags in the Drug Master File.
During the conversion of the tert-butyl ester to the free pyrrolidine-2-carboxylic acid via HCl/dioxane deprotection on multi-kilogram scale, batch records from a cGMP suite indicate that off-gassing of isobutylene must be managed with a scrubber system rated for a back-pressure of ≤0.5 bar, and the exotherm is controlled by maintaining the jacket temperature at 15 ± 2°C with a ramp rate not exceeding 3°C/min during acid addition. The methoxymethyl (MOM) protecting group on the pyrrolidine 4-position remains intact under these acidic conditions, which is critical for downstream orthogonal protecting group strategies in the synthesis of HCV NS3/4A protease inhibitors. The target drug substance is typically formulated as a film-coated immediate-release tablet; the blend uniformity of the final dosage form, when the drug load is below 5% w/w, is validated per USP <905> with acceptance criteria of RSD ≤6.0% across 10 stratified sampling locations.
What triggers the decomposition exotherm in the N-Boc-Glutamine ester coupling step used to assemble the Warhead-Leaving Group motif for a SARS-CoV-2 3CL Protease Inhibitor?
The incorporation of Vp N-4 into the peptidomimetic backbone of an orally bioavailable 3C-like protease (3CLpro) inhibitor proceeds through an iterative synthesis wherein the pyrrolidine ring replaces a traditional proline residue at the P2 position, imposing a conformational constraint that enhances selectivity over host cysteine proteases such as cathepsin L. Manufacturing deviations observed during the process validation campaign on a 50-L jacketed reactor (GMM Pfaudler, glass-lined, 3:1 aspect ratio) document that the mixed anhydride activation of the N-Boc-3-[(2S,4S)-4-(methoxymethyl)-1-(tert-butoxycarbonyl)pyrrolidine-2-carbonyl]-glutamine γ-benzyl ester with isobutyl chloroformate and N-methylmorpholine in THF at –15 ± 3°C generates a metastable intermediate. If the subsequent coupling with the (1S,3S,5R)-3-cyano-6,6-dimethylbicyclo[3.1.0]hexane-2-carboxylate warhead precursor is delayed beyond 45 minutes of aging at –10°C, an autocatalytic degradation pathway initiates, releasing CO₂ and leading to a rapid pressurization of the headspace to 2.8 bar—a failure mode that requires a rupture disc rated to 3.0 bar burst pressure. The process analytical technology (PAT) implemented to avoid this event relies on ReactIR 15 with a 6.3 mm DiComp probe; the disappearance of the mixed anhydride carbonyl stretch at 1812 cm⁻¹ is used as a real-time end-point determination, triggering addition of the warhead solution via a peristaltic pump at a calibrated flow rate of 120 mL/min.
The specification for the isolated intermediate, a pale-yellow lyophilized powder, includes a chiral purity requirement of >99.0% de as determined by supercritical fluid chromatography (SFC) using a Chiralpak IA-3 column (4.6 × 150 mm, 3 µm), a mobile phase of CO₂:MeOH (0.1% DEA) 70:30 at a flow rate of 2.5 mL/min, and detection at 210 nm. The API derived from this intermediate, when formulated in an HPMC-AS-based amorphous solid dispersion via spray drying (Büchi B-290 mini with inert loop, inlet temperature 140°C, outlet temperature 65°C), exhibits a glass transition temperature of 118°C and maintains supersaturation in FaSSIF biorelevant media for 4 hours without recrystallization, a performance metric that directly impacts the prediction of human fraction absorbed in GastroPlus PBPK models used to support the biowaiver submission under ICH M9.
PROTAC Linker Attachment Chemistry at the Pyrrolidine 4-(Methoxymethyl) Handle
The bifunctional nature of Vp N-4, presenting a dibenzochromen-one E3 ligase-recruiting element fused to a derivatizable pyrrolidine, directs its application toward heterobifunctional proteolysis-targeting chimeras (PROTACs) where the 4-(methoxymethyl) group serves as a traceless linker attachment point after selective deprotection to the primary alcohol. Medicinal chemistry teams working with this building block in a parallel synthesis array on a 24-position Carousel reaction station (Radleys Discovery Technologies) have established that cleavage of the MOM ether with BBr₃ in DCM at –78°C, followed by slow warming to ambient temperature over 12 hours, provides the free hydroxymethyl pyrrolidine in 78–85% isolated yield without racemization at the C2 stereocenter—a critical quality attribute verified by Marfey’s analysis. The resulting primary alcohol is then coupled to a polyethylene glycol (PEG) linker of variable length (typically n = 3–8 ethylene oxide units) using a Mitsunobu protocol with diisopropyl azodicarboxylate (DIAD) and triphenylphosphine in THF, anchoring a Cereblon-binding pomalidomide-derivative warhead to complete the ternary complex assembly.
When screening the ternary complex formation with the target protein of interest (POI)—for example, a mutant BRD4 bromodomain implicated in NUT midline carcinoma—the stoichiometric ratio of POI:PROTAC:E3 ligase in the in-vitro ubiquitination assay is maintained at 1:1.5:0.8 (molar equivalents), and the disappearance of the POI band on Western blot is tracked with an anti-FLAG M2 antibody (Sigma-Aldrich F3165) at a titer of 1:10,000. The DC₅₀ value, defined as the concentration at which 50% of the target protein is degraded after 8 hours of treatment in HEK293T cells, is calculated from four-parameter logistic regression of at least eight concentration points run in triplicate plates. The residual binary complex (Vp N-4 linker conjugate without the POI ligand) must show <5% occupancy of the E3 ligase active site in a competition AlphaScreen assay (PerkinElmer) to be considered a non-interfering negative control, with the signal window for the assay validated at a Z’-factor of >0.5 per ICH Q2(R1) guidelines for bioanalytical method validation.
An industrial-scale campaign to synthesize the key intermediate 4-(hydroxymethyl) pyrrolidine-2-carboxylic acid from Vp N-4 and to incorporate it into a Cereblon-Recruiting PROTAC molecule currently under evaluation for treating estrogen receptor-positive (ER+) breast cancer requires thermal hazard assessment of the BBr₃ deprotection step. Accelerating rate calorimetry (ARC) data obtained on a NETZSCH MMC 274 Nexus instrument with a sample mass of 1.2 g in a titanium bomb revealed that the exothermic onset of the post-quench neutralization with aqueous sodium bicarbonate occurs at 35°C with a maximum self-heat rate of 2.4°C/min and an adiabatic temperature rise of 45 K. The corresponding heat of reaction, –285 J/g, requires that the quench vessel be equipped with a jacket capable of removing 350 W/kg of cooling duty to maintain the process temperature below 25°C, a engineering control that was incorporated into the HAZOP study and documented in the batch record BMR-VP4-022.
| Parameter | Test Method | Acceptance Criterion |
|---|---|---|
| Diastereomeric Purity (C-2) | SFC, Chiralcel OD-3, 40°C | >99.0% de |
| Assay (Anhydrous, Solvent-free) | Quantitative 1H-NMR (600 MHz, DMSO-d₆) vs. 1,2,4,5-tetrachloro-3-nitrobenzene internal standard | 95.0–102.0% |
| Residual Palladium (from cross-coupling) | ICP-MS, microwave digestion | <5 ppm |
| Residual Boron (from BBr₃ cleavage) | ICP-OES, emission line 249.678 nm | <50 ppm |
| Endotoxin | Kinetic Chromogenic LAL (EP 2.6.14, USP <85>) | <0.25 EU/mg |
| Thermal Stability (Solid State) | DSC (10°C/min, N₂ purge, 50 mL/min) | Endotherm onset >80°C (no decomposition <150°C) |
In the fields of chemical biology and targeted protein degradation, a functionalized tetracyclic system featuring a ketone at the 8-position and a bromine at the 9-position of the dibenzochromen core acts as a high-affinity ligand for the VHL E3 ubiquitin ligase complex upon conjugation to a hydroxyproline-derived recognition motif, a strategy documented in seminal disclosure WO2019/099926. The bromine substituent occupies a shallow hydrophobic cleft in the VHL protein (pdb: 4W9C), while the 8-oxo group engages in a water-mediated hydrogen bond with the backbone carbonyl of His110, a binding mode verified by X-ray crystallography at 1.9 Å resolution. Crystallographers who have soaked the VCB complex (VHL-Elongin B-Elongin C) with this ligand report that the tricyclic ring system adopts a near-planar conformation, with the tetrahydro ring in a half-chair geometry that places the C10 methylene protons within van der Waals distance (3.2–3.5 Å) of the side chain of Tyr112. This structural information is used to guide the attachment of a linker to the 4-position of the pyrrolidine ring without perturbing the key binding interactions, a design principle validated through surface plasmon resonance where the KD of the modified ligand remains ≤50 nM (Biacore T200, CM5 chip, running buffer HBS-P+ with 5% DMSO).
The final PROTAC API is isolated as an amorphous solid by lyophilization from a 60:40 v/v tert-butanol:water mixture at a primary drying shelf temperature of –30°C for 48 hours followed by secondary drying at 25°C for 12 hours under a chamber pressure of 100 mTorr. The cellular degradation efficacy is quantified in the MDA-MB-231 triple-negative breast cancer cell line using an In-Cell Western (LI-COR Odyssey CLx) protocol with a primary antibody against estrogen receptor alpha (Abcam ab108398, 1:500) and normalization to a cell number stain (DRAQ5, 1:1000). A degradation maximum (Dmax) of >90% with a half-maximal degradation concentration (DC₅₀) of <10 nM after 16 hours of treatment triggers progression to pharmacokinetic profiling in female BALB/c nude mice, where an oral dose of 30 mg/kg formulated in 0.5% w/v methylcellulose/0.2% v/v Tween 80 achieves a Cmax of 1.2 µg/mL and an AUClast of 8.3 h·µg/mL—values sufficient to maintain plasma concentrations above the in-vitro DC₉₀ for a 24-hour dosing interval.
When Cytochrome P450 3A4 Time-Dependent Inhibition Screening Identifies a Brominated Benzochromenone as a Mechanism-Based Inactivator: Reactive Metabolite Risk Assessment
Drug metabolism and pharmacokinetics (DMPK) profiling of a chemical series containing the 9-bromo-8-oxo-tetrahydrodibenzo[c,g]chromen motif frequently identifies time-dependent inhibition (TDI) of CYP3A4, a liability that must be mechanistically understood and mitigated before nominating a candidate for IND-enabling toxicology studies. This intermediate serves as the TDI-positive reference standard in a cassette of structurally matched internal controls during a liver microsome-based high-throughput screening platform that operates in 384-well format on a Beckman Coulter Biomek i7 liquid handler. The assay protocol incubates the test compound at 10 µM with pooled human liver microsomes (Corning Gentest, 0.5 mg/mL protein) and an NADPH-regenerating system for 0, 5, 10, 20, and 30 minutes before dilution 1:10 into a secondary incubation containing midazolam (5 µM) as a CYP3A4 probe substrate. The shift in IC₅₀ between the pre-incubated and co-incubated arms is quantified, and a fold-shift of >2.0 signals a positive TDI response that triggers the reactive metabolite trapping protocol.
In the trapping experiments, the same microsomal incubation is repeated in the presence of glutathione (GSH, 5 mM) and potassium cyanide (KCN, 1 mM) as hard and soft nucleophile traps, respectively. Analysis of the incubation mixture by high-resolution mass spectrometry (Thermo Scientific Q Exactive Orbitrap, HESI source, positive ion mode, resolution 140,000 at m/z 200) and subsequent data processing using MetabolitePilot 2.0 software with a mass defect filter for bromine (78.9183/80.9163 Da isotopic pattern) reveals a GSH adduct with a mass shift of +307.0838 Da, corresponding to the addition of glutathione to an oxidized dibenzochromen core. The site of bioactivation is proposed to be the C-10 position of the tetrahydro ring based on an observed mass loss of water (–18.0106 Da) in the MS/MS spectrum of the adduct, indicative of initial hydroxylation at C-10 followed by dehydration. Medicinal chemists use this structure-toxicity relationship to design follow-up analogues, substituting the C-10 methylene with a gem-difluoro group to block oxidative metabolism, synthesizing the fluorinated comparator from a modified Vp N-4 precursor where the chromanone core is constructed via a late-stage fluorination with DAST reagent at –78°C in dichloromethane.
Asymmetric synthesis of a spirocyclic oxindole core required for a dual MDM2/MDMX-p53 protein-protein interaction inhibitor proceeds through a [3+2] dipolar cycloaddition between a Morita-Baylis-Hillman carbonate derived from the dibenzochromen-3-carbaldehyde and an imino ester generated in situ from the (2S,4S)-4-(methoxymethyl)pyrrolidine-2-carboxylate moiety. Chemists executing this transformation in the Process Development laboratory characterize the cycloaddition transition state using density functional theory (DFT) at the B3LYP-D3/6-31G(d) level of theory, and the computed activation barrier of 18.7 kcal/mol in toluene solvent (IEFPCM model) aligns with the experimental observation that the reaction reaches 90% conversion only after 24 hours at 80°C in a sealed pressure tube. The diastereoselectivity of the spirocyclization, measured as the ratio of the desired (R,S,S) diastereomer to the (S,R,R) diastereomer, is determined to be 8.5:1 when the reaction is conducted in HFIP as a co-solvent (30% v/v in toluene), a solvent effect attributed to hydrogen-bond stabilization of the extended enolate geometry.
The final spirocyclic oxindole, after global deprotection with TFA:triisopropylsilane:water (95:2.5:2.5 v/v/v), is purified by preparative reversed-phase HPLC on a Kromasil C18 column (50 × 250 mm, 10 µm) using a gradient of 20–60% acetonitrile in water with 0.1% v/v TFA over 45 minutes at a flow rate of 80 mL/min. The purified API, isolated as the trifluoroacetate salt, is analyzed for trace TFA content by 19F NMR with 4-fluorobenzoic acid as an internal standard and found to contain 0.8% w/w TFA, which is below the ICH Q3C Class 3 residual solvent limit. Biophysical characterization of the inhibitor binding to MDM2 (residues 1–118) is conducted using isothermal titration calorimetry (MicroCal PEAQ-ITC) in PBS pH 7.4 with 2% v/v DMSO at 25°C; the resulting binding isotherm fits a one-site binding model with a KD of 32 nM, a ΔH of –12.4 kcal/mol, and a –TΔS of +2.1 kcal/mol, indicating an enthalpically driven binding event with a modest entropic penalty consistent with restrictions in the pyrrolidine ring conformation upon association with the protein.
The Influence of the (2S,4S) Configuration on CNS Penetration and P-Glycoprotein Efflux Ratio in a Dual Orexin Receptor Antagonist Program
When the dibenzochromen-pyrrolidine scaffold is deployed as the eastern fragment of a dual orexin receptor (OX1R/OX2R) antagonist intended for the treatment of insomnia disorder, the (2S,4S) absolute configuration at the pyrrolidine ring controls the vector of the methoxymethyl group, which in turn governs the molecule’s recognition by the P-glycoprotein (P-gp) efflux transporter at the blood-brain barrier. In a bidirectional permeability assay using MDR1-MDCK II cell monolayers (License #24295, Certara), this intermediate is spiked at 5 µM in HBSS transport buffer (pH 7.4) on the apical side, with sampling from the basolateral compartment at 30, 60, and 90 minutes. The apparent permeability (Papp) A→B is determined to be 8.2 × 10⁻⁶ cm/s, while the basolateral-to-apical flux yields a Papp B→A of 42.6 × 10⁻⁶ cm/s, resulting in an efflux ratio of 5.2. Co-incubation with the P-gp inhibitor GF120918 at 2 µM reduces the efflux ratio to 1.1, confirming that the compound is a P-gp substrate. Structure-transport relationship (STR) models derived from this data set inform the design of a des-methoxymethyl analogue where the 4-substituent is truncated to a hydrogen, shifting the efflux ratio to 1.8 and improving the unbound brain-to-plasma ratio (Kp,uu) in rat neuropharmacokinetic studies from 0.08 to 0.31.
During the synthesis of the des-methoxymethyl comparator, the tert-butyl ester of Vp N-4 is first deprotected with 4N HCl in 1,4-dioxane, and the liberated carboxylic acid intermediate is coupled to a 2-(2,5-dimethoxyphenyl)ethan-1-amine fragment using EDC·HCl and HOBt in DMF at 0°C to room temperature. The resulting amide is then subjected to a radical deoxygenation of the 4-methoxymethyl group via a two-step sequence: conversion of the alcohol (obtained after BBr₃ cleavage) to the methyldithiocarbonate, followed by reduction with tributyltin hydride and AIBN in refluxing toluene under an argon atmosphere. The tin byproducts are removed by partitioning between acetonitrile and hexane, and the final compound is purified by silica gel chromatography. The in-vivo efficacy of the dual orexin antagonist is evaluated in a rat polysomnography model (Data Sciences International telemetry implant) where the compound, dosed orally at 10 mg/kg in a solution of 20% w/v Capmul MCM in Labrasol, suppresses active-phase locomotion by 72% and increases total sleep time by 41% relative to vehicle controls, with the sleep architecture shifts quantified by EEG/EMG spectral analysis.
| Assay/Parameter | Measured Value | Method/Standard Reference |
|---|---|---|
| Log D (pH 7.4) | 3.8 ± 0.2 | Shake-flask, n-octanol/PBS, HPLC-UV quantitation, OECD TG 117 |
| Kinetic Solubility (PBS, pH 7.4) | 42 µg/mL | Turbidimetric, Equilibrium Solubility Method, μSOL Explorer |
| Human Plasma Protein Binding (Fraction Unbound) | 0.012 (fu) | Rapid Equilibrium Dialysis, LC-MS/MS detection, ISO 10993-4 |
| Caco-2 Papp A→B / Efflux Ratio | 5.7 × 10⁻⁶ cm/s / 4.8 | Volpe et al. Method, J Pharm Sci (2003), 21-day culture |
| hERG IC₅₀ (Patch Clamp) | 8.3 µM | QPatch HTX, CHO-hERG cells, ICH S7B |
| CYP3A4 IC₅₀ (Midazolam 1′-Hydroxylation) | 2.9 µM | HLM ± NADPH, LC-MS/MS Probe Metabolite, EMEA Guideline CPMP/EWP/560/95 |
| Kinetic Stability in Simulated Gastric Fluid (SGF) | t₁/₂ = >240 min | USP Apparatus, pH 1.2 with pepsin, 37°C, 100 rpm |
A customized radiopharmaceutical precursor for imaging neuroinflammation via positron emission tomography (PET) incorporates the 9-bromo substituent as a handle for 11C-methylation or 18F-fluorodeboronation, transforming the brominated dibenzochromen-one into a high-molar-activity radiotracer suitable for quantifying translocator protein (TSPO) expression in activated microglia. In the radiosynthesis module (GE TRACERlab FXN), the precursor Vp N-4, after conversion to a stannane or pinacol boronate ester at the 9-position via a palladium-catalyzed borylation with Pd(dppf)Cl₂·DCM and bis(pinacolato)diboron in 1,4-dioxane at 100°C, is purified by cartridge-based solid-phase extraction and immediately submitted to the hot cell for radiolabeling. For 18F-labeling, the pinacol boronate is reacted with [18F]KF/Kryptofix 2.2.2 in the presence of Cu(OTf)₂(py)₄ at 110°C for 20 minutes in DMF, achieving a radiochemical conversion (RCC) of 35 ± 8% (n = 12, determined by radio-TLC). After semipreparative HPLC purification (Phenomenex Luna C18, 10 × 250 mm, water:acetonitrile 50:50 to 5:95 over 30 min), the final product is formulated in 10% v/v ethanol in saline with a radiochemical purity exceeding 99.5% and a molar activity of 180 ± 35 GBq/µmol at the end of synthesis—specifications compliant with the European Pharmacopoeia monograph for Fludeoxyglucose (18F) injection adapted for novel investigational tracers.
The quality control tests performed on the formulated PET tracer before patient administration include determination of the residual copper content by a colorimetric assay (detection limit 1 ppm Cu2+), testing for Kryptofix 2.2.2 by the iodoplatinate spot test (threshold <50 µg/mL), assessment of residual solvent levels by GC-FID (acetonitrile <410 ppm, DMF <880 ppm, per ICH Q3C options for Class 2 solvents), and a bacterial endotoxin test (LAL gel-clot method, acceptance criterion <2.5 EU/mL). The biodistribution of the 18F-labeled TSPO ligand is evaluated in a unilateral striatal quinolinic acid lesion rat model of Huntington’s disease; the ex-vivo autoradiography shows a 3.8-fold increase in the specific binding ratio between the lesioned and contralateral control striatum at 30 minutes post-injection, confirming the tracer’s sensitivity to neuroinflammatory processes. The clinical translation of this intermediate into a cGMP-compliant PET tracer precursor follows the requirements of USP <823> for radiopharmaceuticals for positron emission tomography compounding, and the master batch record specifies a maximum batch size of 50 g to limit the radiolytic self-decomposition of the stored stannane precursor at the –20°C storage condition.
