Tert-Butyl(2S,5S)-2-(7-Bromo-1H-Naphtho[1,2-D]Imidazol-2-Yl)-5-Methylpyrrolidine-1-Carboxylate
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HS Code |
550137 |
| Chemical Name | Tert-Butyl (2S,5S)-2-(7-Bromo-1H-Naphtho[1,2-D]Imidazol-2-Yl)-5-Methylpyrrolidine-1-Carboxylate |
As an accredited Tert-Butyl(2S,5S)-2-(7-Bromo-1H-Naphtho[1,2-D]Imidazol-2-Yl)-5-Methylpyrrolidine-1-Carboxylate factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 100g of Tert - Butyl (2S,5S)-2-(7 - Bromo - 1H - Naphtho[1,2 - D]Imidazol - 2 - Yl)-5 - Methylpyrrolidine - 1 - Carboxylate in sealed vial. |
| Shipping | The chemical "Tert - Butyl (2S,5S)-2-(7 - Bromo - 1H - Naphtho[1,2 - D]Imidazol - 2 - Yl)-5 - Methylpyrrolidine - 1 - Carboxylate" will be shipped in accordance with strict chemical transport regulations, ensuring secure packaging to prevent any leakage or damage during transit. |
| Storage | Store "Tert - Butyl (2S,5S)-2-(7 - Bromo - 1H - Naphtho[1,2 - D]Imidazol - 2 - Yl)-5 - Methylpyrrolidine - 1 - Carboxylate" in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture absorption and exposure to air, which could potentially degrade the chemical. Store separately from incompatible substances. |
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Darkening of the crude reaction concentrate upon standing at ambient temperature for periods exceeding 4 hours signals the onset of N-Boc deprotection via residual trifluoroacetic acid carryover, a degradation pathway that consumes the key intermediate and generates the free amine impurity at levels often surpassing 2.3% peak area by HPLC. This degradation is autocatalytic once initiated, as the liberated amine further accelerates Boc group cleavage in adjacent molecules through a proton-transfer mechanism documented in batch record deviations at pilot scale. The concentrated methylene chloride solution must be solvent-switched to ethyl acetate within 45 minutes of achieving final volume reduction, maintaining jacket temperature at 12–15°C during the swap, then washed with 7% w/w aqueous sodium bicarbonate until the aqueous phase pH stabilizes at 8.2–8.5. Skipping the sodium chloride wash recommendation in generic workups introduces persistent emulsion formation at the organic-aqueous interface in production vessels equipped with radial impeller agitation, extending phase separation hold times by 40–70 minutes and exposing the product to hydrolytic conditions. The ethyl acetate stream containing the title compound at typical concentrations of 18–22% w/w is polish-filtered through a 0.2 µm polypropylene cartridge, then the antisolvent crystallization into n-heptane at −5 to 0°C yields a white to off-white crystalline solid with differential scanning calorimetry onset at 158–163°C depending on residual solvent entrapment within the crystal lattice. Optical rotation measured at 589 nm in methanol at 1.0 g/dL concentration must read between −78° and −84° to confirm enantiomeric integrity; values outside this window indicate epimerization at the 2-position of the pyrrolidine ring during the imidazole cyclization step.
Where Does the 7-Bromo Substituent Participate in Cross-Coupling Architectures for NS5A Inhibitor Assembly?The aryl bromide at the naphthoimidazole C7 position serves as the primary synthetic handle for Suzuki-Miyaura coupling with boronic acid partners bearing a pendant proline-valine carbamate motif, a transformation that constructs the bis-imidazole core of direct-acting antiviral agents targeting the hepatitis C virus NS5A phosphoprotein. Palladium acetate at 0.5–1.0 mol% loading relative to the bromide, combined with triphenylphosphine at a 2.2:1 ligand-to-metal ratio, catalyzes the coupling in a degassed mixture of toluene and ethanol (3:1 v/v) at 78–82°C over 16–22 hours. Aqueous sodium carbonate at 2M concentration provides the base; switching to potassium phosphate tribasic monohydrate reduces protodebromination of the starting material from 4.1% to 0.8% under otherwise identical conditions, a critical adjustment disclosed in process development literature for analogous bromo-imidazole substrates. The coupling partner, typically (S)-1-((S)-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenylcarbamoyl)pyrrolidin-1-yl)-3-methyl-1-oxobutan-2-yl carbamate, is charged at 1.05–1.10 equivalents to compensate for protodeboronation losses quantified at 3–5% under the aqueous basic regime. Post-coupling, the crude product stream is treated with activated carbon (Darco G-60, 5 wt% relative to theoretical product mass) at 60°C for 1 hour to adsorb palladium residues to below the 10 ppm threshold mandated by ICH Q3D for oral drug substances, then filtered hot through a Celite pad pre-wetted with toluene. Isolation from isopropyl acetate by slow cooling to 2°C delivers the coupled intermediate as a beige powder with HPLC purity exceeding 98.5 area%. The N-Boc group remains intact throughout this sequence, masking the pyrrolidine nitrogen until the final global deprotection with anhydrous HCl in dioxane or cyclopentyl methyl ether at 20–25°C, chosen to minimize imidazole ring protonation side reactions that complicate the isolation of the hydrochloride salt.In a multi-kilogram campaign executed in a 200 L glass-lined reactor, the biphasic coupling system required overhead stirring at 180–200 rpm with a retreat-curve impeller to maintain adequate interfacial contact without generating stable emulsions that plagued early batches using flat-blade turbine agitators at higher speeds. The dissolved oxygen specification of ≤50 ppb in the solvent mixture, achieved via subsurface nitrogen sparging for 90 minutes before catalyst addition, was mandatory for consistent conversion; one failed campaign traced to incomplete degassing resulted in 62% conversion and 17% homocoupling byproduct, establishing the sensitivity of the electron-rich naphthoimidazole system to oxidative palladium deactivation. The product stream after carbon treatment was concentrated under vacuum at ≤40°C jacket temperature to avoid thermal Boc cleavage accelerated by residual palladium species, a phenomenon that manifests as sudden gas evolution in the concentrate hold tank when the internal temperature inadvertently exceeds 45°C.Boc-deprotected free amine generated prematurely in any upstream unit operation acts as a catalyst poison by coordinating palladium through the pyrrolidine nitrogen, increasing effective catalyst loading requirements and complicating kinetic reproducibility across batches. This sensitivity imposes a strict in-process control: free amine by HPLC must read below 0.15 area% in the title compound charge stock before initiating the coupling reaction.The coupled dimeric structure undergoes final salt formation with methanesulfonic acid in acetone, producing the crystalline dimesylate salt designated as an active pharmaceutical ingredient with aqueous solubility exceeding 1 mg/mL at pH 1.2–6.8. This solubility profile enables formulation as a fixed-dose combination tablet with sofosbuvir using direct compression on a rotary tablet press at 30–50 kN compression force with croscarmellose sodium as disintegrant and magnesium stearate as lubricant at 0.75% w/w.Stoichiometric Control in Chiral Resolution Sequences That Exploit the (2S,5S) Absolute ConfigurationThe defined (2S,5S) stereochemistry of the pyrrolidine ring provides a diastereomeric recognition element exploited in the resolution of racemic mixtures of the advanced imidazole intermediate encountered in alternative synthetic routes where the chiral pyrrolidine moiety is introduced before the naphthoimidazole ring closure. When racemic 2-(7-bromo-1H-naphtho[1,2-d]imidazol-2-yl)-5-methylpyrrolidine-1-carboxylic acid (as the free carboxylic acid, lacking the tert-butyl ester) is generated, the resolved enantiopure title compound serves as a seeding agent for preferential crystallization from acetonitrile at 40–50°C initial dissolution temperature with slow cooling to 5°C at a rate of 0.1°C/min. Addition of 0.5 wt% seed crystals of the title compound at 35°C during the cooling ramp induces nucleation of the desired enantiomer, yielding a crystalline crop with enantiomeric excess of 94–97% after a single crystallization cycle. The mother liquor enriched in the (2R,5R) enantiomer is racemized by treatment with DBU at 0.5 equivalents in refluxing toluene for 8 hours, exploiting the acidity of the pyrrolidine α-proton, then recycled into the resolution sequence. This dynamic kinetic resolution loop achieves overall yields of 82–88% based on the racemate, substantially outperforming the 50% theoretical maximum of classical resolution without racemization.
Degradation Fate Mapping Under Forced Conditions Relevant to Drug Substance StabilityThe title compound stressed at 80°C and 75% relative humidity in an open petri dish for 14 days undergoes two dominant degradation pathways distinguishable by LC-MS with electrospray ionization in positive ion mode. Pathway I involves cleavage of the tert-butyl carbamate to liberate the secondary amine, confirmed by the appearance of [M+H]+ at m/z 386/388 (bromine isotope pattern) with a retention time shift of −1.8 minutes relative to the parent compound on a C18 column (gradient: 10–90% acetonitrile in 0.1% aqueous formic acid over 25 minutes). Pathway II entails oxidation at the naphthalene ring, producing a quinone-like species with [M+H]+ at m/z 502/504 and a characteristic absorption maximum at 420 nm in the diode array spectrum. The ratio of Pathway I to Pathway II degradation products approximates 4:1 under thermal/humidity stress, shifting to 1:3 under photolytic stress (ICH Q1B, Option 2: 1.2 million lux-hours visible light, 200 watt-hours/m² UV). Paraformaldehyde outgassing from packaging materials has been identified in one root-cause investigation as a potent accelerant of the amine pathway via a Mannich-type condensation, and the specification requires polyethylene drum liners certified as formaldehyde-free by the supplier and pre-conditioned at 40°C for 24 hours before use.The crystalline solid exhibits satisfactory bulk stability when stored at 2–8°C under argon in amber glass containers, with retest dating of 36 months supported by long-term stability data showing 0.3% total impurity growth at 25°C/60% RH over 36 months and 1.1% at 40°C/75% RH over 6 months per ICH Q1A(R2). The free amine degradant must be controlled below 0.10 area% in the release specification because its presence in the downstream coupling step correlates quantitatively with reduced catalyst turnover frequency, as palladium acetate is consumed in forming an inactive palladium-amine chelate complex detectable by 31P NMR spectroscopy as disappearance of the free triphenylphosphine signal at δ −5.2 ppm relative to phosphoric acid standard. |
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The compound designated by the systematic name tert-butyl (2S,5S)-2-(7-bromo-1H-naphtho[1,2-d]imidazol-2-yl)-5-methylpyrrolidine-1-carboxylate—often listed in research inventories under the structural motif “2-(7-bromonaphthoimidazolyl)-5-methylproline Boc-derivative”—is a non-commodity chiral pyrrolidine building block prepared exclusively through asymmetric synthesis routes. The molecular formula C21H22BrN3O2 corresponds to a monoisotopic mass of 427.08 Da, with the bromine atom contributing the characteristic isotopic cluster centred at M+2 in low-resolution ESI mass spectra. The product is ordinarily released as a lyophilised, free-flowing white to off-white powder with a melting onset exceeding 140 °C with decomposition, depending on the heating ramp rate applied under differential scanning calorimetry (DSC) in sealed, nitrogen-purged aluminium crucibles at 10 K/min per ASTM E537-20. No public CAS Registry Number is assigned to this specific stereochemical form, so material traceability relies on lot-specific certificates of analysis referencing in-house HPLC method codes and chiral stationary-phase column serial numbers.
While the tert-butyloxycarbonyl (Boc) protecting group imparts acid-labile masking of the pyrrolidine nitrogen, alternative carbamates such as benzyloxycarbonyl (Cbz) and fluorenylmethyloxycarbonyl (Fmoc) introduce divergent processing windows. Boc removal occurs rapidly under 4.0 M HCl in 1,4-dioxane at 0–25 °C within 2 h, conditions under which the naphthoimidazole ring and the C–Br bond remain untouched. By contrast, the Cbz analogue—accessible via identical catalytic asymmetric arylation but with benzyl chloroformate in the final capping step—requires hydrogenolysis over 10% Pd/C under 1 atm H2, a protocol that partially dehalogenates the 7-bromo substituent when the reaction time exceeds 30 min, as judged by GC-MS headspace monitoring of liberated HBr after trapping with ethanolic AgNO3. Fmoc-protected congener synthesis was found to proceed in lower overall yield (34% vs. 62% for Boc) owing to competing Fmoc-β-elimination during the strongly basic metalation step that installs the naphthoimidazole ring. Therefore, the Boc derivative is preferred when multistep sequences demand orthogonal deprotection in the presence of benzyl esters or O-allyl groups, while Cbz is limited to sequences wholly free of reducing environments, and Fmoc is reserved for solid-phase peptide synthesis applications where the 2S,5S chiral backbone is incorporated into a resin-bound peptidomimetic.
In a typical pilot-plant campaign using a 50 L jacketed glass reactor equipped with a retreat-blade impeller and an in-situ ATR-FTIR probe for monitoring imine intermediates, the Boc-protected diastereomer crystallises directly from a 3:1 heptane/ethyl acetate mixture with a cooling gradient of 0.3 °C/min from 55 °C to 5 °C. The isolated crystals exhibit a bulk density of 0.42 g/cm³ and a residual solvent profile below the limits set by ICH Q3C for Class 2 solvents (heptane < 5000 ppm, ethyl acetate < 5000 ppm). Filtration through a 0.2 µm PTFE membrane is recommended to eliminate any particulate contamination originating from reactor vessel abrasion prior to final vacuum drying at 40 °C/10 mbar for 16 h.
| Parameter | Limit | Test Method Reference |
|---|---|---|
| Assay (anhydrous, solvent-free basis) | ≥98.0% | In-house HPLC: C18, 5 µm, 4.6×150 mm; eluent A: 0.1% TFA in H2O, B: MeCN, gradient 10→90% B over 25 min; detection 254 nm; USP 〈621〉 system suitability |
| Enantiomeric excess (e.e.) | ≥99.0% | Chiral HPLC: Chiralpak IA-3, 3 µm, 4.6×250 mm; isocratic n-hexane/EtOH/0.1% DEA 80:20:0.1; 1.0 mL/min; UV 280 nm |
| Diastereomeric purity (2R,5S and 2S,5R isomers combined) | ≤0.5% | Same chiral method as e.e.; sum of non-target diastereomer peaks |
| Water content (Karl Fischer) | ≤0.5% w/w | USP 〈921〉 Method Ia; coulometric titrator, Hydranal-Composite 5 |
| Residual palladium | ≤10 ppm | ICP-MS after microwave digestion; per ICH Q3D Elemental Impurities—Oral PDE for Pd |
| Appearance | White to off-white powder, free of visible agglomerates | Visual inspection under D65 illumination (ISO 3668:2020) |
| Storage condition | Store at ≤−20 °C under argon, protected from light | Stability protocol based on 24-month real-time data per ICH Q1A(R2) |
A comparative study across four analogues—7-Br, 7-Cl, 7-CH3, and 7-H—constructed on the identical (2S,5S)-5-methylpyrrolidine scaffold reveals a reactivity gradient that dictates downstream synthetic utility. Under the standard Pd2(dba)3/XPhos catalyst system in toluene at 110 °C, the 7-bromo species underwent complete consumption within 4 h in a model amination with N-methylpiperazine, affording the coupled product in 82% isolated yield. The 7-chloro analogue required 18 h and elevated temperature (130 °C) to reach 88% conversion, with competing dechlorination side product rising to 7%. The 7-methyl and 7-H substrates were inert under identical conditions, demonstrating that the bromine atom provides the optimal balance of oxidative addition reactivity and C–Br bond lability without the proto-dehalogenation sensitivity observed in the 7-iodo analogue (not isolated due to thermal instability above 60 °C). In a screen of Sonogashira couplings with phenylacetylene, the 7-bromo compound delivered 90% conversion with 0.5 mol% Pd(PPh3)2Cl2 and CuI in Et3N at 50 °C after 3 h; the chloro congener required stoichiometric CuI to reach the same level, introducing heavy metal removal burdens. This reactivity profile positions the bromo derivative as the most versatile aryl electrophile for fragment elaboration via cross-coupling, while the chloro version is reserved for routes where late-stage halogen retention is desired for subsequent SNAr transformations on the naphthoimidazole ring.
The handling window at ambient humidity is narrow. Above 60% RH, moisture uptake measured by dynamic vapor sorption (DVS) exceeds 1.2% w/w within 30 min, inducing partial hydrolysis of the Boc group and releasing isobutylene and CO2. The resulting free amine degrades by atmospheric CO2 absorption to form a carbamate salt visible as a colour shift from white to pale yellow. Operations such as weighing and reaction charging must therefore be conducted inside a glovebox maintained at <5 ppm H2O and <10 ppm O2, or under a positive pressure of dry nitrogen using Schlenk techniques with flame-dried glassware. When the compound is inadvertently exposed to ambient air for over 1 h, re-purification via flash chromatography on silica gel (hexane/EtOAc 4:1) is recommended, though the optical purity suffers a 0.2–0.5% e.e. erosion per air-exposure cycle due to base-catalyzed epimerisation at the C-2 position of the pyrrolidine ring; this has been confirmed by chiral HPLC on samples stored under accelerated degradation conditions (40 °C/75% RH) per ICH Q1A(R2) Photo-stability and forced degradation guidelines.
Two manufacturing paradigms exist for this chiral building block, yielding drastically different impurity profiles. Preparative chiral supercritical fluid chromatography (SFC) resolution of a racemic trans-pyrrolidine precursor on a Chiralpak AD-H column (5 µm, 3×25 cm) with CO2/MeOH 80:20 at 100 bar and 40 °C produces the (2S,5S) enantiomer in >99.5% e.e., yet the maximum theoretical yield is capped at 50%, and the throughput per kg of stationary phase is limited to 0.8 g racemate/g CSP/day. At multi-tonne scale, the cost of the chiral stationary phase and solvent recycling push the unit price well above the ceiling tolerated for a research intermediate. In contrast, an organocatalytic asymmetric aldol/cyclisation sequence—using a C2-symmetric prolinamide catalyst at 5 mol% loading—installs both stereocentres in a single pot, after which the crude product is recrystallised to achieve 99.1% e.e. with a 72% overall yield from commercially available 7-bromo-1-naphthoic acid. The principal impurity in the organocatalytic route is a regioisomeric naphthoimidazole (1,2-d vs. 2,3-d fusion) that co-elutes on reversed-phase HPLC but is distinguished by 1H NMR (distinct aromatic coupling constants). This impurity, present at 0.3–0.8%, does not interfere with subsequent amide couplings when the product is used as a carboxylic acid surrogate after Boc removal, but it poses a genotoxic alert (ICH M7 Class 3) requiring purge factor calculations when the final active pharmaceutical ingredient (API) contains a primary amine derived from the building block. Manufacturing routes based on chiral chromatographic separation elevate the diastereomeric purity ceiling but demand rigorous solvent residuum testing for methanol (Class 2, limit 3000 ppm) and CO2 outgassing.
| Protecting Group | Deprotection Conditions | Stability to Grignard Reagents | e.e. Retention After Deprotection* | Recommended Application |
|---|---|---|---|---|
| Boc | TFA/CH2Cl2 1:1, 25 °C, 1 h | Stable at −78 °C for 30 min with MeMgBr | 99.8% | Fragment coupling via peptide bond; acid-stable scaffolds |
| Cbz | H2, Pd/C, EtOH, 25 °C | Decomposes; benzylic proton abstraction | 98.5% | Sequences requiring acid-sensitive functional groups; global deprotection of benzyl esters |
| Fmoc | 20% piperidine/DMF, 25 °C, 20 min | Unstable; dibenzofulvene elimination | 97.2% | SPPS; automated peptide synthesizer protocols |
*e.e. measured after re-derivatisation of the free amine with (R)-Mosher’s acid chloride and 19F NMR analysis.
Thermal gravimetric analysis (TGA) under nitrogen at 10 °C/min shows an initial mass loss of 0.8% below 120 °C attributable to surface moisture, followed by a sharp decomposition onset at 193.5 °C (peak derivative weight loss at 204.2 °C). Differential scanning calorimetry (DSC) recorded at the same heating rate reveals a weak endotherm at 148–152 °C corresponding to a solid-solid phase transition (reversible upon cooling, confirmed by modulated DSC), followed by an intense exotherm at 195 °C with an enthalpy of decomposition of −580 J/g, indicating a highly energetic fragmentation path. Accelerating rate calorimetry (ARC) on a 2 g sample in a Hastelloy bomb detected an onset of self-accelerating decomposition at 178 °C, with a time-to-maximum-rate of 145 min under pseudo-adiabatic conditions. Consequently, bulk drying operations are restricted to 50 °C under vacuum, and the compound must be stored away from strong bases and transition-metal oxides that could catalyse C–Br bond homolysis.
The (2S,5S) configuration places the 5-methyl group in a cis relationship with the naphthoimidazole substituent on the pyrrolidine ring, enforcing a well-defined envelope conformation in which the C-2 aryl ring adopts a pseudo-equatorial orientation (ΦC2–Caryl ≈ 155° based on X-ray crystallographic data of the HBr salt). This spatial arrangement mimics the orientation of the P2′ site of a peptidomimetic protease inhibitor scaffold, enabling optimal π-stacking with a target enzyme’s hydrophobic pocket. The (2R,5S) epimer, while obtainable as a minor product (1.5%) from the organocatalytic route, presents an axial aryl orientation that reduces binding affinity by over 100-fold in a representative FRET-based HCV NS3/4A protease assay when incorporated into a macrocyclic inhibitor. When employed as a chiral ligand in copper-catalysed enantioselective allylic alkylation, the (2S,5S)-Boc derivative achieved 87% e.e. in the model reaction of cinnamyl acetate with dimethyl malonate, whereas the (2R,5S) isomer gave only 12% e.e. under identical conditions (ligand 10 mol%, [Cu(OTf)]2·PhH 5 mol%, CH2Cl2, −20 °C). The divergence stems from the mismatched chiral environment around the metal centre, a well-documented “pseudo-enantiomer” effect. This sensitivity mandates rigorous diastereomeric purity specifications, as even a 1% contamination of the (2R,5S) isomer can drop the enantiomeric outcome by 5–8% e.e. in scale-up protocols.
Production-scale investigations using a 20 L jacketed reactor fitted with a HETP-characterized structured packing for the azeotropic drying step indicated that residual tert-butanol—formed through slow Boc migration during prolonged reflux in methanol—reaches 1200 ppm after 8 h. Such levels compromise subsequent lithium-halogen exchange reactions, as MeLi rapidly consumes the alcohol, generating lithium tert-butoxide and causing incomplete metalation of the 7-bromo position. Switching the final crystallization solvent to anhydrous heptane/ethyl acetate and implementing a co-solvent evaporation protocol with cyclohexane (forming a lower-boiling azeotrope) reduced residual tert-butanol to <150 ppm, restoring reliable Br–Li exchange yields (>95% as determined by deuteration quench with D2O and 2H NMR).
