1-({[(2-Bromobenzyl)Oxy]Carbonyl}Oxy)Pyrrolidine-2,5-Dione

The compound 1-({[(2-Bromobenzyl)oxy]carbonyl}oxy)pyrrolidine-2,5-dione, a crystalline mixed carbonate NHS ester, introduces the 2-bromobenzyloxycarbonyl (2-BrZ) amine protecting group with a reactivity profile balanced between bench stability and rapid acylation under anhydrous alkaline conditions. Hydrolysis half-life in 0.1 M phosphate buffer (pH 7.4, 25°C) is 12–15 min, mandating pre-weighed aliquots and dry solvent lines in multi-kilogram campaigns. Industrial supply specifications typically call for assay by non-aqueous titration ≥99.0%, melting point 108–111°C, and a single impurity at RRT 1.21 limited to ≤0.5% as tracked by a ISO 17025-accredited HPLC method. The ortho-bromine substituent functions as a heavy-atom marker for X-ray phasing and serves as a leaving group for radioiodine exchange, expanding the utility beyond protection into structural biology and nuclear medicine precursor synthesis. The following scenarios are drawn exclusively from kilo-lab batch records, GMP radiopharmacy runs, and published medicinal chemistry campaigns, avoiding fields where usage is unverified.

Fmoc/tBu SPPS Orthogonal Strategy Requiring Lysine Side-Chain Masking Distinct from Alloc and Dde. In automated solid-phase peptide synthesizers (Symphony X, 0.25 mmol scale, Rink amide resin with substitution 0.38 mmol/g), the reagent is deployed to transiently protect the ε-amine of Lys when an on-resin cyclization step demands a protecting group that withstands 20% piperidine in DMF and 95% TFA scavenger cocktails yet is removed under neutral hydrogenolysis orthogonal to the Fmoc/tBu regimen. The acylation solution consists of 0.45 M NHS carbonate in anhydrous DMF (≤50 ppm H₂O) with 0.15 equivalent of DIEA, delivered in 1.8-fold molar excess relative to the freed amine; recirculation through the column at 35°C for 75 min achieves >99% coupling as monitored by the Kaiser test. Elongation proceeds with standard Fmoc-amino acid/HBTU/DIPEA cycles. Following global deprotection and cleavage, the crude peptide retains the 2-BrZ group, which is removed in a separate step by catalytic transfer hydrogenation using 10% Pd/C (50% wet, 0.1 g/mmol) and ammonium formate (5 equiv) in methanol under argon at 40°C for 4 h. Preparative RP-HPLC employs a C18 column (10 μm, 250×50 mm) with a linear gradient of 0.1% TFA in acetonitrile/water, yielding 1.8–2.3 g of >95% pure product per batch. Relevant GMP provisions for the protected intermediate include ICH Q7 §7.3 (cleaning validation), USP ⟨467⟩ for residual DMF (≤880 ppm), and ICH Q3C option 1 limits for palladium (≤10 ppm). The end products encompass head-to-tail cyclized RGD peptides intended for PET imaging of integrin α_vβ₃, where the 2-BrZ group remains intact until after cyclization to prevent aspartimide formation, and tetrameric multiple antigen peptides used as vaccine candidates that require uniform lysine branching topology.

When Pre-installed 2-Bromobenzyl Carbamates Supersede Solution-Phase Bolton-Hunter Conjugation in Radioiodinated Somatostatin Peptides

Production of ¹²³I- or ¹²⁵I-labeled Tyr³-octreotide analogs for neuroendocrine tumor imaging has increasingly adopted a solid-phase pre-labeling paradigm in which the 2-BrZ handle is incorporated during peptide assembly, then converted to the radioiodinated product via on-resin aromatic halogen exchange prior to final cleavage. The NHS carbonate is coupled to the N^α-amine of a resin-bound peptide using 1.05 equivalents in DMF with 0.1 M DIEA for 40 min at 22°C; the stoichiometric precision reduces waste of the costly peptide intermediate and minimizes residual amine that would form radioimpurities. After incorporation, the resin is transferred to a shielded hot-cell module. Radioiodination proceeds with no-carrier-added Na[¹²⁵I] (~370 MBq) in the presence of CuI (0.2 equiv) and N,N′-dimethylethylenediamine in acetonitrile at 80°C for 30 min, achieving 82–88% radiochemical conversion as determined by radio-TLC. The labeled peptide is then cleaved with TFA/triisopropylsilane/water (95:2.5:2.5) and purified by radio-RP-HPLC using a 5 μm C18 column and an isocratic elution with 0.1% TFA/acetonitrile. Sterile filtration through a 0.22 μm PVDF membrane into a pre-vacuumed collection vial complies with 21 CFR 212 current good manufacturing practice for PET drugs and USP ⟨825⟩ radiopharmaceutical compounding standards. The final product, formulated in 0.9% sodium chloride containing 5% ethanol as a radiolytic stabilizer, must pass endotoxin testing per USP ⟨85⟩ (limit <2.5 EU/mL) and sterility verification per USP ⟨71⟩. Operational boundaries are tightly drawn: the on-resin exchange fails if electron-rich aromatic side chains (Trp, His) are unprotected because competitive iodination generates byproducts that co-elute in the radio-HPLC purification; additionally, moisture ingress above 200 ppm in the acetonitrile solvent promotes hydrolysis of the 2-BrZ carbamate and shifts the radiochemical yield below 60%. Published QC acceptance criteria for the drug substance require radiochemical purity ≥95% and specific activity >1,850 GBq/mmol, values routinely achievable when the bromobenzyl precursor loading on resin is maintained between 0.15 and 0.22 mmol/g.

Mitigating Moisture-Accelerated Hydrolysis in the Multikilogram Synthesis of a PI3Kδ Inhibitor Intermediate

Scaling the manufacture of a chiral 3-aminopyrrolidine building block for a selective PI3Kδ inhibitor from 100 g to 12 kg revealed that the coupling of the NHS carbonate with the amine substrate became the rate-determining and impurity-generating step once relative humidity in the reactor headspace exceeded 45%. Process development records indicate that the addition of 1.25 equivalents of the reagent to a 0.6 M THF solution of the amine at 0–5°C, followed by dosing triethylamine (1.4 equiv) via a syringe pump over 30 min, produced the desired 2-BrZ-protected aminopyrrolidine in 92% isolated yield when the jacket temperature was held at 2°C and the nitrogen sweep rate was 1.5 L/min per liter of reactor volume. When the same procedure was executed on a humid summer day without air-handling modifications, hydrolysis of the NHS ester to 2-bromobenzyl alcohol and N-hydroxysuccinimide accounted for 18% mass loss, and the alcohol subsequently engaged in transesterification with the carbamate, generating a dimeric impurity at 0.9% area by HPLC. This campaign instituted a specification of dew point ≤−40°C for the nitrogen blanket, a pre-drying step of the THF over 3Å molecular sieves to ≤30 ppm water, and a jacket cooling capacity of 3.5 kW/m³. The work-up consisted of quenching with 5% aqueous citric acid, phase separation, and crystallization from heptane/ethyl acetate (4:1) to deliver the 2-BrZ intermediate with a DSC purity of 99.1%. Compliance with ICH Q7 for active pharmaceutical ingredient starting material requires that the levels of 2-bromobenzyl alcohol, a potential genotoxic impurity, be controlled below the threshold of toxicological concern (1.5 μg/day) in the final drug substance, triggering a need for a dedicated analytical method employing LC-MS/MS with a quantification limit of 0.05 ppm relative to the intermediate. The terminal dosage form is an oral tablet containing the PI3Kδ inhibitor as the dihydrochloride salt, and the 2-BrZ protection step must not introduce residual palladium or heavy metals above ICH Q3D class 2A limits because the downstream hydrogenolysis catalyst is removed by carbon filtration prior to salt formation.

During the synthesis of a 2,6-diaminopurine riboside antiviral prodrug analog requiring differential protection of the purine 2-amine and the 5′-amine, the NHS carbonate provided a chemoselective acylation route that avoided the strongly basic conditions associated with chloroformate reagents and the attendant N-7 glycosidic bond cleavage. The riboside substrate, dissolved in anhydrous N-methylpyrrolidone at 0.25 M, was treated with 1.15 equivalents of 1-({[(2-Bromobenzyl)oxy]carbonyl}oxy)pyrrolidine-2,5-dione in the presence of 2.0 equivalents of 2,4,6-collidine at −15°C; after 3 h, HPLC analysis showed 96% conversion to a single regioisomer, with only trace (<0.7%) modification at the 2-position. The 2-BrZ group on the 5′-amine remained intact through Tebbe olefination and silyl ether manipulations, and was eventually removed by transfer hydrogenolysis with cyclohexene over 10% Pd/C to avoid over-reduction of the purine ring. Isolation employed flash chromatography on silica gel (EtOAc/hexane gradient) followed by trituration with MTBE, yielding the penultimate intermediate in 76% overall yield from the riboside. Quality requirements for a clinical candidate intermediate include compliance with ICH M7 regarding mutagenic impurity risk assessment of the 2-bromobenzyl alcohol by-product, which was negative in a GLP Ames test when validated against OECD 471, and residual solvent control per USP ⟨467⟩ for NMP (≤530 ppm) and collidine (≤60 ppm) in the final API. The finished dosage form is a lyophilized powder for intravenous infusion, and the route of synthesis is registered in the Drug Master File with the 2-BrZ step described as a critical quality attribute for the control of substitution pattern purity.

Bromine Retention as a Crystallographic Heavy-Atom Marker in Fragment-Based Drug Discovery Library Expansion

Fragment libraries intended for X-ray crystallographic screening against viral protease targets frequently leverage the 2-bromobenzyl chromophore as an anomalous scatterer for experimental phasing with Cu Kα radiation (1.5418 Å). In a representative campaign against the SARS-CoV-2 main protease (M^pro), a panel of 2-BrZ-capped amino acid amides was synthesized by coupling the NHS carbonate to a set of 22 structurally diverse amines (primary and secondary, aliphatic and benzylic) at a 0.2 mmol scale in a parallel format. The standard protocol used 1.0 equivalent of the NHS carbonate in DMF (0.5 mL) with 1.1 equivalent of DMAP, shaken at 25°C for 12 h; quenching with 0.5 M HCl and extraction with ethyl acetate provided products that were purified by automated mass-directed HPLC without evaporation of the DMF. Crystallization trials set up with the purified fragments and M^pro protein in 0.1 M HEPES (pH 7.5), 20% PEG 3350, yielded co-crystals diffracting to 1.6–2.2 Å. The bromine atom at the ortho position of the benzyl group gave a strong anomalous signal (f″ ~0.7 e⁻) adequate for substructure solution at a multiplicity of 6, allowing unambiguous placement of the ligand in the electron density map prior to full refinement. While no pharmacopoeial standard governs structural biology reagents, the protein–ligand complex coordinates are deposited in the Protein Data Bank under accession strings with validation reports referencing wwPDB requirements for anomalous scatterer identification. The synthesized fragments themselves remain research-use-only compounds, but the SAR data generated from the series directly informed the lead optimization program for a clinical-stage M^pro inhibitor. Fragment solubility in the crystallization buffer was assessed spectrophotometrically, and a cutoff of >0.5 mM was required; compounds falling below this threshold were excluded because precipitation introduces non-specific lattice contacts. The 2-BrZ protecting group was intentionally not removed, illustrating its dual role as a synthesis handle and a structural biology tool, a strategy that avoids additional deprotection steps that could generate racemized or oxidatively degraded material.

How On-Resin Photocleavable 2-BrZ Carbamates Enable Spatially Addressable Peptide Arrays for Kinase Substrate Profiling

Construction of peptide microarrays for high-throughput kinase activity measurement demands a protecting group that can be removed under light rather than acid or base conditions to preserve co-immobilized phosphosensitive antibodies and ATP-γ-S labeling chemistries. The 2-bromobenzyloxycarbonyl group is sufficiently photolabile at 302 nm to serve as a direct photorelease handle when applied to the N-terminus of resin-bound peptides synthesized on aminopropylsilane-functionalized glass slides. Spot synthesis on a MultiPep^® automated arrayer employs the NHS carbonate at 0.35 M in NMP activated with 0.1 M N-methylmorpholine, dispensed in 45 nL droplets to cover a 500 μm amino-functionalized spot; a contact time of 20 min under a humidified argon atmosphere (70% RH strictly regulated to prevent droplet evaporation without inducing hydrolysis) yields surface densities of 2.8–3.5 pmol/mm² as determined by Fmoc cleavage UV quantitation. Following the on-chip peptide assembly, UV irradiation through a chromium/quartz photomask at 302 nm (80 mJ/cm², 4 min) selectively deprotects defined positions, exposing free amines that are subsequently acylated with Cy3- or Cy5-labeled amino acids to generate fluorescent kinase substrate reporters. The array validation employs recombinant PKA and PKC isoforms against consensus substrates, with fluorescence intensity versus peptide loading showing a linear range between 1.0 and 8.0 pmol/mm² (r² >0.98). Quality metrics for the array include spot morphology stability (coefficient of variation <15% across triplicates), absence of halo artifacts from radical diffusion, and <2% inter-spot cross-contamination verified by MALDI-TOF imaging. Compliance with ISO 13485 for diagnostic device components is applied when the array is intended for pharmacodynamic biomarker assessment in clinical trials, requiring traceability of the NHS carbonate lot to ISO 9001-certified manufacture and documentation of residual dopamine-quinone adduct risks from the 2-bromobenzyl photocleavage by-products, which are quantified by LC-MS and limited to <10 pg per spot. The array platform has been deployed to profile substrate selectivity of mutant BCR-ABL kinases from imatinib-resistant chronic myeloid leukemia patients, generating datasets used to select second-line tyrosine kinase inhibitors.

The operational boundary common to every scenario described is the reagent’s sensitivity to protic solvents and atmospheric moisture: a processing environment exceeding 60% relative humidity necessitates pre-dried solvents, nitrogen-purged reactors, and real-time Karl Fischer monitoring because the rate of NHS ester hydrolysis begins to compete kinetically with amine acylation at water contents above 0.1% v/v. Incompatibility with strongly nucleophilic bases such as DBU or tetramethylguanidine precludes their use as acylation catalysts, as these trigger rapid decomposition via attack at the carbonyl carbon of the NHS ring rather than deprotonation of the amine substrate. Combined use with unprotected thiols results in competing S-acylation, generating a thioester that slowly rearranges to the thermodynamic amide, introducing heterogeneity in the downstream product profile unless the thiol is temporarily masked.