2-(8-Amino-1-Bromoimidazo[1,5-A]Pyrazin-3-Yl)Pyrrolidine-1-Carboxylate

Coupling Partner in the Synthesis of Fused Heterocyclic Kinase Modulators

Utility of 2-(8-amino-1-bromoimidazo[1,5-a]pyrazin-3-yl)pyrrolidine-1-carboxylate as a late-stage diversification building block in oncology-focused medicinal chemistry is primarily anchored by the C-1 bromine atom, which undergoes oxidative addition with palladium(0) sources under relatively mild thermal activation. In a representative Suzuki-Miyaura protocol conducted at 50 mmol scale, the bromide is combined with 1.2 equivalents of a heteroarylboronic acid pinacol ester, 2 mol% Pd(PPh₃)₄, and 3.0 equivalents of K₃PO₄ in a degassed toluene/water biphasic mixture (4:1 v/v) at 82°C for 16 hours. Conversion monitoring by UPLC-UV (210 nm) typically reveals ≥92% bromide consumption; the major impurity profiled is the dehalogenated byproduct arising from protodebromination, which becomes pronounced when the headspace oxygen concentration in the reactor exceeds 120 ppm. For that reason, process development groups use argon-sparged solvents and jacketed reactors fitted with oxygen probes, targeting dissolved O₂ below 0.5 mg/L before catalyst injection. The pyrrolidine-1-carboxylate ester remains intact under these neutral-to-mildly basic conditions, enabling subsequent elaboration at the C-8 amine. That amine has been acylated with 2-chloro-4-(trifluoromethyl)pyrimidine-5-carbonyl chloride in the presence of DIPEA in dichloromethane at 0–5°C to deliver a biaryl amide that displays potent inhibition of ataxia telangiectasia and Rad3-related (ATR) kinase with an IC₅₀ of 12 nM as measured by a time-resolved fluorescence resonance energy transfer (TR-FRET) assay calibrated against recombinant full-length human ATR/ATRIP complex. Pilot-plant batches destined for IND-enabling toxicology studies are manufactured under ICH Q7 GMP conditions; the active pharmaceutical ingredient intermediate must pass a specification of ≤0.10% palladium via inductively coupled plasma mass spectrometry (ICP-MS) per USP <232/233> and ≤15 ppm residual toluene by headspace GC-FID per USP <467>. A documented failure mode in multi-kilogram campaigns involves agglomeration of the palladium black precipitate causing poor phase separation in the extraction step, which is mitigated by the insertion of a plate-and-frame filter press charged with diatomaceous earth with a nominal pore size of 5 µm and an operating differential pressure maintained below 1.8 bar.

A Buchwald-Hartwig amination pathway leveraging the same C-1 bromide has been successfully executed with morpholine as the nucleophile. The catalytic system employs Pd₂(dba)₃ (0.8 mol%), XPhos (2.4 mol%), and sodium tert-butoxide (1.4 equiv) in 1,4-dioxane at 95°C for 6 hours under a nitrogen atmosphere; isolated yields of the N-arylated product exceed 78% after flash chromatography on silica gel 60 (gradient elution cyclohexane:ethyl acetate 8:1 to 1:1). The primary process safety concern is the exotherm associated with the sodium alkoxide addition: reaction calorimetry (Mettler Toledo RC1) reveals an adiabatic temperature rise of 38 K, mandating controlled dosing over 45 minutes while the jacket temperature is held at 15°C. The resulting amino-substituted intermediate served as a hinge-binding motif in a series of selective TYK2 pseudokinase domain ligands whose equilibrium dissociation constants were determined by surface plasmon resonance on a Biacore T200 instrument, with the immobilization of the target protein performed according to the manufacturer’s amine coupling kit preconditioned with EDC/NHS in HBS-EP+ running buffer at 25°C.

When Electrophilic Bromo-Substitution Enables Agrochemical Lead Optimization

The electron-deficient character of the imidazo[1,5-a]pyrazine core, accentuated by the C-1 bromine and the ester substituent, renders the molecule a competent electrophilic trap for thiols, a feature exploited in the design of contact fungicides targeting ergosterol biosynthesis in Fusarium spp. In one optimization campaign, the bromide was displaced with 4-fluorothiophenol (1.05 equiv) using K₂CO₃ (2 equiv) in DMF at 60°C to afford a diaryl thioether intermediate within 4 hours, with minimal over-alkylation detectable by LC-MS. The pyrrolidine-1-carboxylate ester was subsequently saponified with aqueous LiOH (1 N, 3 equiv) in THF:methanol (1:1) at ambient temperature, and the liberated carboxylic acid was coupled with 2-amino-5-methylbenzonitrile using HATU (1.1 equiv) and N-methylmorpholine (3 equiv) to yield a candidate amide. Greenhouse trials applying the formulated compound as a 200 g/L emulsifiable concentrate revealed 87% control of wheat leaf rust (Puccinia triticina) at a spray rate of 125 g active ingredient per hectare, with no phytotoxicity recorded on Triticum aestivum cv. ‘Chinese Spring’ at the BBCH 31 growth stage. Aquatic toxicity profiling under OECD Test Guideline 203 (96-hour static acute toxicity to Danio rerio) yielded an LC₅₀ of 4.2 mg/L, categorising the material as moderately hazardous and necessitating the inclusion of a buffer zone of 15 meters from surface waters in the label instructions.

On the manufacturing side, the generation of saline wastewater containing lithium and fluoride ions during the ester hydrolysis step requires dedicated effluent treatment infrastructure involving precipitation with calcium chloride and pH adjustment to 9.5–10.0 to achieve Li⁺ levels below 0.2 mg/L as verified by ion chromatography with conductivity detection (IC-CD). The final product is subjected to a 5-step multisolvent recrystallisation sequence — first from acetonitrile:water (7:3) followed by a reslurry in n-heptane at 50°C — delivering a polymorphically pure form (Form A) that exhibits a single endothermic melt at 182.3°C by differential scanning calorimetry (DSC, ramp rate 5 K/min, aluminium pan with pierced lid). Certificate of analysis criteria adhere to the Collaborative International Pesticides Analytical Council (CIPAC) methods: purity by external standard HPLC-UV at 254 nm ≥98.5% area, water content by Karl Fischer coulometry ≤0.35%, and residue on ignition ≤0.10%.

Pilot-scale production documented batch-to-batch variability in the crystallisation yield that was traced to trace iron contamination originating from a 316L stainless steel transfer line; corrosion coupons subsequently installed confirmed a uniform corrosion rate of 0.12 mm/year in the presence of the acidic quench solution. Replacement with PTFE-lined pipework eliminated the issue, improving the overall yield from 68% to 82% over 12 consecutive validation batches.

Thermal Stability Evaluation of Bromoimidazopyrazinyl Esters for Vacuum-Deposited OLEDs

Organic light-emitting diode (OLED) manufacturers evaluating host materials for thermally activated delayed fluorescence (TADF) devices have considered bromoimidazopyrazine derivatives as intermediates for donor-acceptor-donor triads, with the imidazopyrazine acting as the electron-deficient acceptor core. The title compound is submitted to gradient sublimation at a pressure of 5 × 10⁻⁶ Torr using an Anton Paar TGA-sublimation module; the maximum sublimation rate is observed at 218°C, and a recovery of 93% is achieved with a purity uplift from 99.2% to 99.96% as quantified by HPLC-FLD (excitation 340 nm, emission 410 nm). However, thermogravimetric analysis (TGA, N₂ atmosphere, heating rate 10 K/min) reveals that mass loss onset occurs at 227°C with a derivative peak at 239°C, which is uncomfortably close to the sublimation processing window and places a tight constraint of ±4°C on source temperature uniformity. Physical vapour deposition (PVD) operators deploying the intermediate in a Veeco GEN-II cluster tool must ramp the crucible temperature at 1.5 K/min while monitoring the quartz crystal microbalance deposition rate, holding the rate at 0.8 Å/s to prevent resistive heating runaway that triggers premature decomposition visible as a dark residue on the source liner.

The intermediate is subsequently coupled with 9,9-dimethyl-9,10-dihydroacridine via a palladium-catalysed C-N coupling under conditions analogous to those described for kinase inhibitors. The fully conjugated product possesses a glass transition temperature (T_g) of 143°C as determined by DSC (3^rd heating cycle, quench-cooled at 60 K/min) in accordance with ASTM E1356-08. Photophysical characterisation in a toluene solution (10⁻⁵ M, deoxygenated via three freeze-pump-thaw cycles) yields a photoluminescence quantum yield (PLQY) of 0.68 relative to a quinine sulfate standard (0.1 M H₂SO₄, Φ = 0.546), measured with a Hamamatsu C9920-02 integrating sphere system. In a proof-of-concept TADF device with the structure ITO/PEDOT:PSS (40 nm)/TAPC (20 nm)/mCP:emitter (30 nm:8 wt%)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm), the external quantum efficiency (EQE) rolls off from a maximum of 14.2% at 10 cd m⁻² to 8.7% at 1000 cd m⁻², an efficiency droop attributed to singlet-triplet annihilation exacerbated by the relatively long delayed fluorescence lifetime of 18.3 µs determined by transient PL decay using a time-correlated single photon counting (TCSPC) setup. The entire optoelectronic characterisation sequence follows IEC 62321-7-5:2019 guidelines for the determination of regulated substances in electronic products, with specific attention to halogen content verification of finished deposited films by combustion ion chromatography.

A persistent scale-up bottleneck identified on a multi-source cluster tool is the slow desorption of trace isocyanate from the pyrrolidine-1-carboxylate degradation pathway, which poisons the organic layers during subsequent depositions and raises the operating pressure above the 2 × 10⁻⁷ Torr threshold required for contamination-free growth. Mitigation involves a 48-hour bake-out of the source material at 80°C under high vacuum prior to loading.

Conjugation of diagnostic antibodies using this heterobifunctional linker has been vetted through a controlled study in which the C-8 primary amine is first converted to a maleimidohexanoyl moiety via reaction with 6-maleimidohexanoic acid N-hydroxysuccinimide ester (1.02 equiv) in DMF containing 0.5% v/v N,N-diisopropylethylamine. After quenching with glycine and purification by reversed-phase flash chromatography on a Biotage Isolera system (C18, acetonitrile/0.05% trifluoroacetic acid in water gradient), the maleimide-activated intermediate is lyophilised and stored under argon at −20°C until conjugation. Thiolated anti-PD-L1 monoclonal antibody (IgG1 subtype), prepared by treatment with 2-iminothiolane hydrochloride in 50 mM sodium phosphate buffer pH 7.8 containing 1 mM EDTA, is mixed with the maleimide intermediate at a molar ratio of 1:15 (antibody:linker) and incubated in the dark at 22°C for 90 minutes. The resulting immunoconjugate exhibits a drug–antibody ratio (DAR) of 4.6 as determined by hydrophobic interaction chromatography (HIC) on a TSKgel Butyl-NPR column, and retains 92% of its antigen-binding capacity when assayed by cell-based ELISA against recombinant PD-L1-Fc chimera immobilised on MaxiSorp plates. A major limitation documented in the corresponding development report is the gradual hydrolysis of the pyrrolidine-1-carboxylate ester in the conjugation buffer: at pH 7.4 and 37°C, LC-MS quantifies 18% ester cleavage after 24 hours, producing free 8-aminoimidazopyrazine alcohol that no longer reacts with the maleimide handle. This necessitates preparative conjugation runs to be completed within 8 hours of linker reconstitution, with ongoing stability monitored by inline size-exclusion HPLC using a TSKgel G3000SWXL guard column pair. Conjugates intended for histopathology use are validated according to ISO 13485:2016 in vitro diagnostic device quality management system, with particular emphasis on batch records documenting endotoxin levels ≤0.25 EU/mg by Limulus amebocyte lysate (LAL) kinetic chromogenic assay per USP <85>.

The intrinsic Lewis basicity of the imidazopyrazine N-6 and N-4 positions has been harnessed to construct chiral bidentate ligands for iridium-catalysed asymmetric hydrogenation. Condensation of the C-8 amine with (S)-BINAPO-derived phosphinyl chloride in THF with triethylamine as a proton scavenger at −20°C provides a phosphoramidite-imidazopyrazine hybrid. Upon metallation with [Ir(COD)Cl]₂ (0.5 equiv relative to ligand) in dichloromethane followed by anion exchange with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF₂₄), the pre-catalyst is isolated as an air-sensitive orange powder. Catalytic testing in the asymmetric hydrogenation of (E)-2-(4-methoxyphenyl)-1-phenylprop-1-ene was carried out in a 50 mL Premex autoclave charged under argon in a MBraun LabStar glovebox with ≤0.1 ppm O₂ and ≤0.5 ppm H₂O. Operating at 60 bar H₂ and 40°C, the substrate-to-catalyst ratio of 5000 (mol/mol) delivered full conversion in 3 hours; chiral GC analysis on a β-DEX 225 column revealed an enantiomeric excess of 94.2% (R). The ligand’s ester group proved essential for solubility in the fluorinated alcoholic solvent hexafluoroisopropanol (HFIP), which was used at a volume of 3.5 mL per mmol of substrate. Prolonged exposure of the pre-catalyst solution to trace moisture triggered hydrolysis of the BArF₂₄ counterion, generating a non-coordinating boric acid derivative that poisoned the catalytic cycle after 12–15 turnovers; Karl Fischer titrations mandated a moisture content ≤8 ppm in the solvent reservoir. The process was registered under REACH (EC No. 918-552-1) for a total tonnage band of 100–1000 kg/year, requiring a substance classification report addressing the mutagenic potential of the intermediate bromide under OECD 471 (Ames test) with TA98 and TA100 strains.

Integration of the pyrrolidine-1-carboxylate ester function as a reactive plasticiser in polyester-based flame-retardant compounds has been examined at the compounding scale. Pellets of polybutylene terephthalate (PBT, intrinsic viscosity 0.98 dL/g) were pre-dried to a moisture level below 30 ppm and then dry-blended with 12 wt% of the brominated imidazopyrazine, 3 wt% antimony trioxide (Sb₂O₃, median particle size 0.8 µm) as a synergist, and 0.5 wt% pentaerythritol tetrastearate processing aid. The blend was compounded on a Coperion ZSK 26 Mc twin-screw extruder with an L/D ratio of 40 and a screw speed of 300 RPM, applying a barrel temperature profile from 235°C (feed zone) to 260°C (die), followed by strand pelletisation and injection moulding into 3.2 mm-thick test bars on a Demag 80-tonne machine with a mould temperature of 80°C and injection pressure of 900 bar. Under UL 94 vertical burning test conditions, the formulation attained a V-0 classification at 1.6 mm thickness; limiting oxygen index (LOI) as per ASTM D2863-19 measured 31.8%, up from 24.2% for neat PBT. However, notched Izod impact strength (ASTM D256-10, Method A, at 23°C) dropped from 4.8 kJ/m² to 2.1 kJ/m², and scanning electron microscopy of the fracture surface revealed poor dispersion with agglomerates up to 15 µm in diameter, pointing to insufficient shear and a need for the addition of 1–2 wt% maleic anhydride-grafted polyolefin compatibiliser. During extrusion, a gradual buildup of a volatile pyrrolidine degradation byproduct condensed in the vacuum venting system was noted after 45 minutes of continuous operation; vent filter monitoring with a portable Drager tube indicated pyrrolidine vapour at 12 ppm, triggering the activation of a local exhaust ventilation interlock set to 5 ppm threshold limit value (TLV) per ACGIH documentation.

Diagnostic substrate design based on the ester linkage of this compound exploits the ability of certain clinically isolated bacterial esterases to hydrolyse the pyrrolidine-1-carboxylate motif with high specificity. The assay workflow starts with the dissolution of the compound in DMSO at a concentration of 25 mM and subsequent dilution into 50 mM Tris-HCl buffer (pH 8.0, containing 0.15 M NaCl and 0.01% Triton X-100) to a final substrate concentration of 200 µM. Upon addition of the enzyme (e.g., a carboxylesterase from Klebsiella pneumoniae clinical strain KP-714, partially purified by anion-exchange chromatography), the hydrolysis releases a chromophoric imidazopyrazine alcohol whose absorbance at 385 nm is monitored kinetically in a BioTek Epoch 2 microplate reader thermostatted at 37°C. The initial velocity is linear over the first 120 seconds, and following the initial rate method according to ISO 18153:2003, the catalytic activity is reported in katal per litre of specimen. Cross-reactivity testing against 14 other hydrolytic enzymes commonly present in human serum (including butyrylcholinesterase and alkaline phosphatase) yielded activity ratios below 3%, establishing the probe’s selectivity. True clinical applicability is constrained by the substrate’s limited aqueous solubility (0.12 mg/mL in pure buffer); reformulation with 2% (w/v) sulfobutylether-β-cyclodextrin increased solubility to 1.1 mg/mL while preserving 90% of the original hydrolysis rate constant. A formal inter-laboratory validation conducted across three reference centres demonstrated a within-laboratory coefficient of variation (CV) of 5.7% and a between-laboratory CV of 8.3%, meeting the acceptance criterion of ≤10% established for this class of in vitro diagnostic reagents.