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

What Synthetic Route Delivers the 3‑Aryl Substituent Required for BET Bromodomain Inhibition?

The chiral benzyl carbamate intermediate serves as a late‑stage diversification handle in the assembly of research‑stage imidazo[1,5‑a]pyrazine‑based inhibitors targeting BRD4‑BD1 and BRD4‑BD2. The C‑8 primary amine remains temporarily masked as the free base under the reaction conditions, while the C‑3 bromine atom participates in a palladium‑mediated Suzuki–Miyaura coupling with arylboronic acids bearing electron‑deficient or mildly electron‑rich substituents. A representative validated procedure on a 250 mmol scale employs the active catalyst Pd(dtbpf)Cl₂ (2.0 mol%) in combination with 2.0 M aqueous K₃PO₄ (2.8 equiv) and tetrahydrofuran (10 L/kg with respect to the bromide) degassed by three freeze‑pump‑thaw cycles until the residual oxygen concentration measured by a Mettler‑Toledo InTap optical sensor remains below 15 ppb. The boronic acid (1.12 equiv) is charged in one portion under a positive argon purge through a glove‑bag adaptor. Heating at 63 °C (internal temperature) for 6.5 h with overhead stirring at 350 rpm in a 20‑L jacketed glass reactor typically drives conversion above 98% as tracked by IPC monitoring on a Waters ACQUITY UPLC H‑Class system equipped with a CSH C18 column (1.7 µm, 2.1 × 50 mm), using a gradient of 0.1% trifluoroacetic acid in water and acetonitrile at 0.6 mL/min, with detection at 254 nm. Post‑reaction workup involves treatment with activated carbon (Norit CA1, 5 wt%) at 55 °C for 45 min, filtration through a 0.5 µm polypropylene depth filter, and liquid–liquid extraction with ethyl acetate/brine. The organic layer is dried over magnesium sulfate until the Karl Fischer endpoint falls below 150 ppm water, concentrated on a Büchi R‑220 rotary evaporator at 38 °C bath temperature under a gradual vacuum ramp to 18 mbar, and the residue purified by flash chromatography on silica gel 60 (Merck, 0.040–0.063 mm), eluting with a step gradient of heptane/ethyl acetate from 85:15 to 60:40. The isolated material typically exhibits a chemical purity exceeding 99.1 area% and a chiral purity above 99.5% ee as determined by HPLC on a Chiralpak IG‑3 column (4.6 × 150 mm, 3 µm) with n‑hexane/ethanol/diethylamine 80:20:0.1 at 1.0 mL/min. This intermediate subsequently undergoes hydrogenolytic or acidic Cbz deprotection to liberate the secondary amine of the proline scaffold, which is immediately acylated with 4‑chlorobenzoic acid using HATU (1.05 equiv) and N‑methylmorpholine (2.5 equiv) in DMF at −10 °C, rising to 20 °C over 2 h. The resulting tertiary amide corresponds to a key pharmacophore where the 3‑(4‑chlorophenyl)imidazo[1,5‑a]pyrazin‑8‑amine unit forms a bidentate hydrogen‑bond network with the conserved ASN140 residue of the bromodomain, while the proline carboxamide tail occupies the WPF shelf region. Batch records from kilo‑scale production indicate a process mass intensity (PMI) of 82 kg input per kg of final inhibitor intermediate and highlight the necessity of strictly controlled bromine‑by‑product purging: residual Pd content measured by ICP‑OES (PerkinElmer Avio 550 Max) must not exceed 5 ppm, and the content of the de‑brominated impurity must remain below 0.15 area% to comply with an internal specification aligned with ICH Q3A thresholds for unknown impurities. This synthesis pathway is operated under a quality system audited against ICH Q7 for active‑substance starting materials, and an EU REACH dossier for the bromo intermediate has been registered for the 1–10 t/a band with an Annex VII endpoint package inclusive of an Ames test (OECD 471) and ready biodegradability data (OECD 301F).

Orthogonal use of the C‑8 primary amine as a nucleophile permits construction of a biaryl‑amine architecture via the Buchwald–Hartwig C–N cross‑coupling route. When a 4‑(9H‑carbazol‑9‑yl)phenyl bromide is employed as the coupling partner, the resulting N‑arylated product becomes a building block for a high‑triplet‑energy host material in phosphorescent organic light‑emitting diodes. An optimal protocol, derived from a design‑of‑experiment study conducted at a CRO facility, uses the air‑stable precatalyst XPhos Pd G3 (1.8 mol%) and potassium phosphate tribasic (1.5 equiv) in a mixed solvent of 2‑methyl‑tetrahydrofuran and t‑BuOH (85:15 v/v, 12 L/kg) at 90 °C for 16 h. The reactor headspace is swept with nitrogen at 0.3 L/min to displace volatile amine by‑products. The fully Cbz‑protected intermediate, after aqueous workup and precipitation from methyl tert‑butyl ether/heptane, achieves a DSC purity of 99.7 mol% and a palladium residue below 8 ppm. Deprotection with 33% HBr in acetic acid (5 equiv, 0 °C to 20 °C, 1.5 h) followed by neutralization with 20% NaOH and extraction delivers the primary amine, which is then engaged in an EDC‑mediated coupling with 2‑(3,5‑di‑tert‑butylphenyl)‑4,6‑diphenyl‑1,3,5‑triazine‑5‑carboxylic acid. The final compound, purified by train sublimation at 3.2 × 10⁻⁶ mbar and a source temperature of 370 °C using a Creaphys HT‑20 unit, shows a glass transition temperature of 142 °C as measured by differential scanning calorimetry in accordance with ISO 11357‑2:2020 at a scan rate of 10 K/min. The ultrapure material is qualified for vacuum thermal evaporation onto an ITO substrate; its ionization potential determined by photoelectron yield spectroscopy (AC‑3, Riken Keiki) in air reads 5.82 eV, placing it as a suitable hole‑blocking host in green‑PhOLED stacks subject to luminance decay testing per IEC 62‑6‑5. Each lot is accompanied by a certificate of conformance listing the sublimed purity quantified by HPLC‑UV (99.95 area%) and residual alkali‑metal content by ICP‑MS <0.1 ng/mL, ensuring negligible exciplex formation at the emission layer interface.

When the benzyl carbamate protecting group in the imidazo[1,5‑a]pyrazine‑proline scaffold is exploited as an orthogonal handle, the molecule becomes a versatile linker in solid‑phase macrocyclization methodologies. A pre‑loaded 2‑chlorotrityl chloride resin (loading 1.2 mmol/g) is first esterified with Fmoc‑Gly‑OH (0.8 equiv relative to resin loading) in the presence of DIEA (4 equiv) in DCM at 25 °C. After standard Fmoc‑deprotection with 20% piperidine in DMF, the free amino terminus is coupled with the bromo‑imidazopyrazine‑proline Cbz acid, obtained by saponification of the benzyl ester with LiOH (1.2 equiv) in THF/water (3:1) at 0 °C, using HATU (1.1 equiv) and collidine (3 equiv) for 45 min on a Liberty Blue automated microwave peptide synthesizer set to a maximum temperature of 50 °C. A series of iterative Fmoc‑based chain elongations builds a linear tetrapeptide sequence while the Cbz group remains untouched. Selective removal of the Cbz cap without disrupting the bromoaryl moiety is achieved with a freshly prepared cocktail of TFA/triisopropylsilane/water (95:2.5:2.5 v/v) for 12 min at 23 °C; after concentration, the crude amino‑acid‑peptide is treated with PyBOP (3 equiv) and HOBt·H₂O (3 equiv) in DMF (0.01 M) under high‑dilution head‑to‑tail cyclization, with a resin‑cleavage duration strictly limited to 18 h to avoid N‑to‑O acyl migration. The crude cyclic peptidomimetic is precipitated in ice‑cold diethyl ether and analyzed by RP‑HPLC on a XBridge BEH C18 column (130 Å, 3.5 µm, 4.6 × 150 mm) with a linear gradient of acetonitrile in 0.1% TFA, in accordance with the system suitability requirements of USP 〈621〉. The target macrocycle, featuring a constrained turn motif, inhibits the MDM2‑p53 protein‑protein interaction with a Kd value of 42 nM as determined by a fluorescence polarization competition assay (the peptide stock concentration was verified by amino acid analysis on a Hitachi L‑8900 analyzer). This route demonstrates the critical dependence of the reaction sequence timing on the pH‑sensitive bromo‑heterocycle; any extension of the TFA exposure beyond 15 min triggers an 18% increase in the des‑bromo by‑product as shown by UPLC‑MS extracted ion chromatograms.

Engineering a Dual-Functional Organocatalyst from the Proline Core

The (S)‑configured pyrrolidine motif, combined with the potential to derivatize the C‑8 amine into a hydrogen‑bond donor, renders the intermediate a platform for constructing enantioselective thiourea‑tertiary amine catalysts. Reductive desulfurization of the benzyl carbamate group is carried out with 10% palladium on carbon (2.5 wt% of substrate) under a hydrogen atmosphere of 1.2 bar in ethanol at 25 °C, yielding the free pyrrolidine as a viscous oil that is immediately reacted with 3,5‑bis(trifluoromethyl)phenyl isothiocyanate (1.0 equiv) in THF at 0 °C to install the thiourea moiety. The precipitation monitored by FT‑IR (disappearance of the N=C=S band at 2095 cm⁻¹) is complete within 45 min. The resulting bifunctional catalyst, (S)‑N‑[8‑(3‑(3,5‑bis(trifluoromethyl)phenyl)thioureido)−1‑bromoimidazo[1,5‑a]pyrazin‑3‑yl]pyrrolidine, is purified by column chromatography on neutral alumina (activity III, 63–200 µm) eluting with dichloromethane/methanol 98:2. Its catalytic performance is benchmarked in the Michael addition of diethyl malonate to trans‑β‑nitrostyrene. In a typical procedure, the nitroalkene (0.25 mmol) and diethyl malonate (0.50 mmol) are dissolved in toluene (0.5 mL) and treated with the catalyst at 10 mol% loading at −20 °C in a jacketed reaction vessel with stirring at 800 rpm. After 20 h, the reaction is quenched with 1 M HCl, and the adduct is extracted with MTBE, dried over Na₂SO₄, and analyzed by chiral SFC on a Chiralpak AD‑H column (4.6 × 250 mm, 5 µm) using CO₂/methanol 90:10 at 2.5 mL/min, achieving an enantiomeric excess of 93%. The bromo substituent is essential for the steric compression that pre‑organizes the transition state; in a control experiment, the 1‑des‑bromo analogue delivered only 47% ee. Used batches of the catalyst are regenerable and retain 95% of their initial selectivity over five cycles when the product is recovered by a simple aqueous extraction protocol. Safety data sheets for the intermediate and the final thiourea are provided in alignment with the REACH Regulation 1907/2006, and the process is operated in fume hoods equipped with HEPA‑filtered exhausts due to the sensitization potential of isothiocyanates.

Sonogashira alkynylation at the C‑1 bromine site is harnessed to access extended π‑conjugated systems with tunable fluorescence for live‑cell imaging probes. The bromo intermediate is combined with trimethylsilylethyne (1.5 equiv) using Pd(PPh₃)₂Cl₂ (4 mol%) and CuI (8 mol%) in degassed diisopropylamine (8 L/kg) at 70 °C for 5 h in a sealed high‑pressure tube with a Teflon stopcock. The TMS group is then cleaved with K₂CO₃ (1.1 equiv) in methanol/THF at 23 °C, and the terminal alkyne is directly subjected to a Glaser–Hay oxidative homocoupling to yield a buta‑1,3‑diyne‑linked dimer. An alternative Sonogashira coupling with 4‑(N,N‑dimethylamino)‑2‑ethynylbenzonitrile proceeds at 45 °C in under 12 h without copper co‑catalyst using the PEPPSI‑IPr precatalyst, enabling a one‑pot sequence that drastically reduces palladium contamination below the 0.5 ppm detection limit in the fluorophore after precipitating the product from cold methanol. The purified conjugate exhibits absorbance at 412 nm and emission at 527 nm with a quantum yield of 0.68 measured in PBS buffer using an absolute integrating sphere method following the protocol of Würth et al. (IUPAC Technical Report, 2013). Photostability testing under continuous 450 nm LED illumination (Thorlabs M450LP1, 50 mW/cm² at the sample surface) recorded less than 6% photobleaching over 60 min, surpassing the benchmark fluorescein standard. For intracellular targeting, the Cbz‑protected intermediate is first deprotected and functionalized with a triphenylphosphonium moiety via amide coupling to drive mitochondrial accumulation; the co‑localization coefficient with MitoTracker Deep Red FM in HeLa cells, quantified by Pearson’s correlation (Imaris software, Bitplane), is 0.92. The batch‑to‑batch reproducibility of the fluorescent probe is secured by a QC protocol that mandates identical emission maximum (± 2 nm) and a half‑bandwidth within <3 nm of the reference standard, with certificates referencing ISO/IEC 17025:2017 analytical competence.

The tert‑amine liberated after Cbz removal serves as an anchor point for constructing a controlled‑radical‑polymerization initiator. Under a dry nitrogen atmosphere in a Braun UniLab Pro glovebox (<0.1 ppm O₂ and H₂O), the debenzylated pyrrolidine is dissolved in anhydrous THF and treated with α‑bromoisobutyryl bromide (1.1 equiv) in the presence of triethylamine (1.3 equiv) at −78 °C, warmed to 10 °C over 1.5 h. The resulting secondary amide initiator is isolated by flash chromatography and used directly for the atom transfer radical polymerization (ATRP) of methyl methacrylate, employing CuBr (1 equiv) and N,N,N′,N″,N″‑pentamethyldiethylenetriamine (1 equiv) in anisole at 70 °C as described by the Activators Generated by Electron Transfer (AGET) procedure with tin(II) 2‑ethylhexanoate as the reducing agent. The targeted degree of polymerization 100 is reached within 4.5 h, yielding PMMA with a number‑average molecular weight Mn of 9.8 kDa and a dispersity Đ of 1.08 determined by gel‑permeation chromatography calibrated with PMMA standards (ISO 16014‑3:2019) using a triple‑detector system (refractive index, viscometer, and right‑angle light scattering) in THF at 35 °C. The terminal bromine on the polymer chain is capable of chain extension with styrene, confirming the livingness of the immobilized initiator. This approach allows the pendant imidazo[1,5‑a]pyrazine chromophore to be positioned exactly at the polymer terminus, enabling investigation of excited‑state energy migration by time‑resolved fluorescence anisotropy decay. The physical mixture of the chromophore‑containing initiator with the polymer is avoided to eliminate baseline skew; all monomer conversions are gravimetrically confirmed with an uncertainty of ±0.5%.