Benzyl Octahydrocyclopenta[B]Pyrrole-2-Carboxylate Hydrochloride

In the production of active pharmaceutical intermediates targeting serine protease inhibition within the coagulation cascade, the hydrochloride salt of benzyl octahydrocyclopenta[b]pyrrole-2-carboxylate functions as a pre-activated building block amenable to immediate incorporation into peptidomimetic backbones after neutralization. The ester moiety is preserved during downstream amide bond formation to retain a protecting group strategy compatible with later-stage saponification. A representative coupling procedure liberates the free amine by partitioning the hydrochloride between dichloromethane and saturated aqueous sodium bicarbonate at a controlled pH of 8.0 ± 0.2. The organic phase is dried over anhydrous sodium sulfate and concentrated under reduced pressure at a bath temperature not exceeding 35 °C to minimize thermal lactamization. The resulting amine is coupled to an N-protected amino acid using a dicyclohexylcarbodiimide (1.2 equiv) / 1-hydroxybenzotriazole (1.2 equiv) system in dichloromethane at 0–5 °C for 16 h. The molar ratio of amine to activated carboxylic acid is maintained at 1.0 : 1.05. Solid N,N′-dicyclohexylurea is removed by filtration through a 0.5 µm PTFE membrane, and the filtrate is washed sequentially with 0.1 M hydrochloric acid and brine. The isolated product is purified by flash chromatography on 230–400 mesh silica gel with a cyclohexane/ethyl acetate gradient. Isolated yields in cGMP pilot campaigns (glass-lined reactors, 100 L capacity, impeller tip speed 1.5 m/s) range from 85 % to 92 %, with chemical purity ≥99.0 % by HPLC-UV (210 nm, area normalization). Chiral purity, critical for the downstream drug substance, is verified on a Chiralpak AD-H column (4.6 × 250 mm, hexane/2-propanol/diethylamine 90:10:0.1, 1.0 mL/min) and must exceed 99.5 % enantiomeric excess. Residual solvents are controlled in accordance with ICH Q3C Option 1 limits: dichloromethane ≤600 ppm, cyclohexane ≤3880 ppm. Elemental impurity testing follows USP 〈232〉/〈233〉 with particular attention to palladium (carryover from a prior hydrogenation step) capped at 10 µg/g. The isolated intermediate is stored under argon in double polyethylene-lined fiber drums at −20 °C, with a retest period of 24 months when pre-dried to a loss on drying ≤0.5 % (60 °C vacuum oven, 4 h). Exposure to relative humidity above 30 % leads to hygroscopic clumping and partial ester hydrolysis within 72 h. The terminal dosage form is an oral anticoagulant tablet that must meet EP and USP monographs for assay, content uniformity, and dissolution. This intermediate is supplied with a full certificate of analysis, a transmissible spongiform encephalopathy declaration, and a heavy metal statement prepared under ICH Q7 chapter 11.

What Limits Amide Bond Formation Yield in Fused Pyrrolidine-Derived HCV NS5B Polymerase Inhibitors?

The benzyl ester hydrochloride serves as a direct precursor to the carboxylic acid required for assembling a class of non-nucleoside hepatitis C virus NS5B polymerase inhibitors that occupy the thumb II allosteric pocket. The ester is saponified under carefully buffered conditions to circumvent a competing intramolecular cyclization that forms an inert δ-lactam. In a jacketed stirred-tank reactor with a PTFE-lined Hastelloy shell, the hydrochloride (1.0 kg scale) is suspended in tetrahydrofuran/demineralized water (4:1 v/v, 5 L/kg) and cooled to 0 °C. Lithium hydroxide monohydrate (1.05 equiv) dissolved in demineralized water is added dropwise over 45 min while maintaining the internal temperature between 0 °C and 2 °C. Stirring continues for 2 h, after which the pH is adjusted to 3.5 with 2 M hydrochloric acid at the same temperature. The carboxylic acid is extracted into methyl tert-butyl ether, dried with magnesium sulfate, and vacuum-concentrated to a foam. Due to the acid’s tendency to retro-Michael decomposition at ambient temperature, it must be used within 8 h or stored as a frozen solution in anhydrous dimethylformamide at −80 °C. Coupling to a 4-bromo-2-fluoroaniline fragment proceeds via in situ activation with O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU, 1.1 equiv) and N,N-diisopropylethylamine (3.0 equiv) in anhydrous DMF with a Karl Fischer water specification ≤50 µg/g. The hydrochloride salt of the free base must first be liberated prior to acid activation; failure to do so results in a 30–40 % lower yield due to competing N-acylation. The following table summarizes the outcome of a coupling reagent screen conducted under identical stoichiometry and solvent conditions.

Anhydrous DMF, 25 °C internal temperature, reaction time 10 h. Yield after silica gel chromatography; diastereomeric excess determined by normal-phase chiral HPLC (Chiralpak IA, hexane/ethanol 85:15). The major process risk resides in the slow racemization of the α-carbon to the carboxylate at pH > 4.5 during the saponification work-up. This has been characterized by circular dichroism kinetics: at pH 5.0 and 25 °C, a 1.5 % loss in optical purity was observed over 24 h. Consequently, the work-up pH and temperature are locked into a defined operating window. The final amide intermediate is subjected to a palladium catalyst removal step via silica-bound trimercaptotriazine scavenger resin to reduce residual Pd to ≤5 µg/g, a specification tightened from ICH Q3D for parenteral drug substances. Genotoxic impurity evaluation per ICH M7 addendum on N-nitrosamines confirms the absence of nitrosatable secondary amine functionality in the final product, but N,N-diisopropylethylamine is controlled as a potential mutagenic impurity at ≤75 ppm by LC-MS/MS. The bulk product is shipped as a crystalline solid in amber glass bottles with Teflon-lined caps under argon, meeting the requirements of 21 CFR 210 and 211 current good manufacturing practice for finished pharmaceutical intermediates.

Chiral Ligand Backbone for Asymmetric Hydrogenation Catalyst Manufacturing

The octahydrocyclopenta[b]pyrrole framework embedded in this compound carries three contiguous stereocenters that provide a rigid, concave binding pocket exploited in the design of phosphine and phosphoramidite ligands for rhodium- and iridium-catalyzed asymmetric hydrogenation. The benzyl ester hydrochloride is converted into a diol intermediate through lithium aluminum hydride reduction. In a 50 L glass-lined reactor, lithium aluminum hydride powder (1.5 equiv) is suspended in anhydrous tetrahydrofuran (10 L/kg) at 0 °C. A solution of the hydrochloride (1.0 equiv) in anhydrous THF is added at a rate that keeps the internal temperature below 15 °C. After complete addition, the grey slurry is heated to reflux (66 °C) for 6 h. Quenching follows the Fieser procedure—sequential addition of water (0.57 mL/g LiAlH₄), 15 % aqueous sodium hydroxide (0.57 mL/g), and water (1.7 mL/g)—at 0 °C. The granular inorganic precipitate is filtered, and the filtrate is concentrated. The resulting crude diol (purity ≥95 %, two positional isomers combined) is used without purification in the subsequent mesylation: methanesulfonyl chloride (2.5 equiv) is added dropwise to a solution of the diol and triethylamine (3.3 equiv) in dichloromethane at −10 °C. The bismesylate is isolated by aqueous work-up and subjected to nucleophilic substitution with potassium diphenylphosphide (generated from diphenylphosphine and potassium tert-butoxide) in tetrahydrofuran at −78 °C. The resulting chiral bis(phosphine) ligand coordinates to [Rh(COD)₂]BF₄ in methanol to form the catalyst precursor that is deployed in a 200 L Hastelloy B-2 hydrogenation autoclave for the enantioselective reduction of an α-acetamidocinnamic acid derivative. With a substrate-to-catalyst ratio of 10,000 and a hydrogen pressure of 3.0 MPa, enantiomeric excesses of 99.2 % are routinely recorded for the (S)-N-acetyl phenylalanine methyl ester product. Ligand purity is the primary determinant of catalyst performance: batches yielding ≤99.0 % ee are traced to residual monophosphine oxide impurities formed during work-up. The oxide level is quantified by ³¹P NMR and must be held below 0.8 area%. The ligand is stored under nitrogen in a glovebox (-30 °C) to prevent phosphine oxidation. Heavy metal specifications follow Ph. Eur. 2.4.8 for the finished drug substance: Pd ≤5 µg/g, Rh ≤5 µg/g, Ir ≤2 µg/g. The ligand and its manufacturing process are described in a Type II Drug Master File submitted to the US FDA for reference in generic drug applications.

When Substitution at the Cyclopentane Ring Demands Cryogenic Nitration

This hydrochloride intermediate becomes a scaffold for a developmental insecticide active against piercing-sucking pests in rice and Brassica crops when electrophilic nitration is directed to the cyclopentane ring. The protonated secondary amine in the salt-form deactivates the pyrrolidine nucleus, forcing the incoming nitronium ion to attack the electron-rich cyclopentane ring. A laboratory-scale procedure charges 96 % sulfuric acid (8.0 volumes, w/v) into a double-walled glass reactor cooled to −45 °C by a circulating ethanol bath. The benzyl ester hydrochloride is dissolved portionwise maintaining the temperature below −35 °C. Potassium nitrate (1.2 equiv, pre-dried at 120 °C for 2 h) is added as a finely ground powder over 30 min. The deep orange solution is aged for 1.5 h at −45 °C to −40 °C. The reaction mass is quenched by pouring onto crushed ice ( 20 parts by weight), resulting in the precipitation of a pale yellow solid. The crude product is collected by filtration on a polypropylene Buchner funnel, washed with chilled demineralized water until the washings reach a pH ≥5.0, and dried in a vacuum oven at 35 °C for 24 h. At pilot scale, a 100 L stainless steel stirred reactor with a dipleg recirculation loop and an on-line Fourier Transform Infrared probe (Mettler Toledo ReactIR) monitors the disappearance of the nitrate symmetric stretching band at 1390 cm⁻¹ to determine the reaction endpoint and to minimize over-nitration diastereomer formation. The positional isomer ratio typically fluctuates between 3.2:1 and 5.0:1 depending on the sulfuric acid water content; a water specification of ≤0.5 % in the charging acid tightens the ratio to ≥4.8:1. The following design space parameters, derived from a Central Composite Design, illustrate the sensitivity of isomer selectivity to process variables.

The purified mononitro intermediate undergoes hydrogenation ( 5 % Pd/C, 0.5 wt% loading, methanol, 0.5 MPa H₂, 40 °C) to yield the aromatic amine, which is subsequently coupled to a chloro-substituted heterocyclic acid chloride to assemble the pro-insecticide structure. The standard for acute toxicity classification mandates an Ames test according to OECD 471 using Salmonella typhimurium strains TA98 and TA100; the regenerated free amine of the final active ingredient must return a negative result at 5000 µg/plate. For field trial material shipped under a provisional notification, residual process solvents are aligned with FAO Specification Manual, chapter VI: methanol ≤3000 ppm, dichloromethane ≤600 ppm, toluene ≤890 ppm. The technical grade active ingredient (graded under CIPAC methods) is formulated as a 100 g/L suspension concentrate with a wet sieve residue (75 µm sieve) below 2.0 %. The exothermic nitration step is classified as a critical operation according to OSHA 29 CFR 1910.119 Process Safety Management; the pilot facility is equipped with independent high-temperature interlocks and a dedicated quenching vessel containing 500 L of 5 % aqueous sodium hydroxide to neutralize the entire batch in a runaway scenario.

Latent Amine Functionality Is Integrated into Single-Component Epoxy Matrices

The sterically hindered tertiary amine generated upon thermal dehydrochlorination of benzyl octahydrocyclopenta[b]pyrrole-2-carboxylate hydrochloride functions as a latent nucleophilic initiator for anionic epoxy homopolymerization. This property is harnessed in the formulation of single-pack, storage-stable structural adhesives intended for automotive sensor encapsulation. An epoxy resin base—typically a bisphenol A diglycidyl ether with an epoxy equivalent weight of 184–190 g/eq and a hydrolyzable chloride content ≤300 µg/g—is pre-heated to 70 °C to reduce viscosity. The micronized salt (d₉₀ ≤ 15 µm as measured by laser diffraction on a Malvern Mastersizer) is dispersed at a loading of 7 phr along with 1 phr of fumed silica (BET surface area 200 m²/g) as a rheology modifier. Dispersion is carried out on a three-roll mill (Exakt 80E) with a primary gap of 15 µm and a secondary gap of 5 µm; torque monitoring ensures the dispersion force is insufficient to induce premature deprotonation of the amine salt. The resulting paste undergoes a latent period determination by dynamic viscosity measurement at 40 °C using a Brookfield RVDV-II+ viscometer with a #6 spindle. Per the internal product specification, the viscosity must not increase by more than 20 % over 672 h (four weeks) under 40 °C atmospheric storage, corresponding to a shelf-life extrapolation of 12 months at 25 °C following ASTM D2196 Method A.

Cure kinetics are mapped by differential scanning calorimetry (ASTM E1356) using a 10 mg sample in a sealed aluminum pan. A dynamic temperature ramp from 30 °C to 250 °C at 10 K/min reveals an onset of polymerization at 138 ± 3 °C, a peak exotherm at 162 °C, and an enthalpy of reaction of 320 J/g. Isothermal cure at 150 °C for 30 min raises the glass transition temperature of the network to 135 °C (midpoint, ASTM E1640) as confirmed by thermomechanical analysis. The fully cured adhesive develops a lap shear strength of 12 MPa on grit-blasted aluminum substrates (ASTM D1002). Because the liberated hydrochloride residue remains as a dissociated ionic contaminant, electrical conductivity measurements (IPC-TM-650 2.5.17) highlight a risk in fine-pitch electronic assemblies. Under 85 °C/85 % RH bias for 1000 h, silver migration between adjacent copper traces is detectable at chloride concentrations above 200 µg/g in the cured network. To mitigate this, magnesium oxide powder (3 phr, calcined at 600 °C) is incorporated as an acid acceptor, which reduces extractable chlorides to 45 µg/g and brings the surface insulation resistance above 10⁸ Ω after damp heat exposure. The formulated adhesive complies with UL 94 V-0 at a thickness of 1.5 mm, and the latent hardener component is pre-registered under EU REACH with a tonnage band of 100–1000 t/a for use as an industrial intermediate in polymer production. No RoHS-restricted substances are intentionally introduced, and the cured material passes the IEC 62321 screening test for brominated flame retardants and heavy metals.

Fmoc solid-phase peptide synthesis of macrocyclic peptide drug candidates targeting the glucagon-like peptide-1 receptor incorporates the hydrochloride as a constrained proline isostere to stabilize a type II′ β-turn. The benzyl ester side chain remains intact during chain elongation and serves as a masked carboxylic acid for on-resin cyclization at the conclusion of the linear synthesis. The resin used is a 2-chlorotrityl chloride polystyrene (loading 1.0 mmol/g, 100–200 mesh) functionalized with the first amino acid via C-terminal anchoring. Prior to coupling of the isostere building block, the commercial Fmoc-protected derivative (derived from the hydrochloride after benzyl ester preservation) is dried to constant weight over phosphorus pentoxide under vacuum (<1 mbar, 24 h). Coupling is performed in a microwave peptide synthesizer ( CEM Liberty Blue ) at 60 °C with a four-fold excess of the protected amino acid relative to the resin substitution level. The standard cycle uses 2-(6-chloro-1H-benzotriazole-1-yl)-1,1,3,3-tetramethylaminium hexafluorophosphate (HCTU, 3.95 equiv relative to the amino acid) and N,N-diisopropylethylamine (8.0 equiv) in N,N-dimethylformamide with 5-minute coupling pulses. Because steric crowding from the fused ring reduces the coupling efficiency for the residue immediately following the isostere, a double-couple protocol and subsequent capping with acetic anhydride/pyridine (1:1 v/v) are mandatory to prevent deletion sequences. The completed linear peptide is cleaved from the resin with 20 % hexafluoroisopropanol in dichloromethane while retaining side-chain protecting groups. Macrocyclization between the unmasked C-terminus and the side-chain amine of a lysine residue is achieved with (7-azabenzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyAOP, 1.5 equiv), 1-hydroxy-7-azabenzotriazole (1.75 equiv), and N-methylmorpholine (3.5 equiv) in dimethylformamide at a pseudo-dilution concentration of 2 mM. Global deprotection with trifluoroacetic acid/triisopropylsilane/water (95:2.5:2.5 v/v/v) yields the crude macrocycle, which is purified by preparative RP-HPLC (C18, 250 × 50 mm) with a 0.1 % trifluoroacetic acid / acetonitrile gradient. The purified peptide, isolated as a lyophilized white powder with a purity ≥98.0 % by analytical HPLC, is subjected to endotoxin testing per USP 〈85〉 that must show <0.5 EU/mg for preclinical intravenous pharmacology studies. No additional pharmacopoeial monographs apply at the investigational stage, but the batch record includes the starting material traceable to the hydrochloride lot number with a declared enantiomeric purity of >99.0 %.