(1S,3Ar,6As)-Ethyl Octahydrocyclopenta[C]Pyrrole-1-Carboxylate Hcl

Product Name (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[C]Pyrrole-1-Carboxylate Hcl
Alias Etomethazine
Einecs 'EINECS 695-533-7'
Mininmum Order 1g
Factory Site West Ujimqin Banner, Xilingol League, Inner Mongolia, China
Price Inquiry sales9@bouling-chem.com
Manufacturer Bouling Chemical Co., Limited
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In terms of specification, (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[C]Pyrrole-1-Carboxylate Hcl is supplied with ≥98% purity (HPLC) and ≥99% enantiomeric excess, making it suitable for chiral building block synthesis in pharmaceutical research.

Specifications

HS Code	326274
Chemical Name	(1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole-1-Carboxylate Hcl

As an accredited (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[C]Pyrrole-1-Carboxylate Hcl factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	100g of (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole - 1 - Carboxylate HCl in sealed vial.
Shipping	The chemical (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole - 1 - Carboxylate HCl is shipped in accordance with strict chemical safety regulations. It's packaged securely to prevent leakage, with proper labeling for safe handling during transit.
Storage	Store (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole - 1 - Carboxylate HCl in a cool, dry place away from direct sunlight. Keep it in a tightly - sealed container to prevent moisture absorption and potential degradation. Avoid storing near sources of heat or incompatible substances to maintain its chemical integrity.

Application of (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[C]Pyrrole-1-Carboxylate Hcl

In the registered synthetic route for the HCV NS3/4A protease inhibitor telaprevir (VX-950), the fragment coupling between the P2 bicyclic proline surrogate and the P1-P3 peptidomimetic backbone employs (1S,3aR,6aS)-ethyl octahydrocyclopenta[c]pyrrole-1-carboxylate hydrochloride as the penultimate chiral intermediate prior to saponification and condensation with a cyclopropylamino acid carboxamide. Large-scale execution of this amidation across a campaign of **12–15** batches within a multipurpose cGMP suite at **20–25 kg** input per batch demands meticulous control of the freebasing step because the hydrochloride must be neutralized with a tertiary amine base—typically N-methylmorpholine (**1.05–1.10 eq** relative to the HCl salt) in anhydrous dichloromethane at **−5 °C to 0 °C**—immediately before activation of the carboxylate by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl, **1.03±0.02 eq**) and 1-hydroxybenzotriazole hydrate (HOBt·H₂O, **1.0 eq**), an operation documented to suppress racemisation at the C-1 stereocentre to below **0.3%** of the (1R)-diastereoisomer. The resultant activated ester is held at **−10±2 °C** in a glass-lined 500-litre jacketed reactor under a nitrogen sweep of **0.2–0.5 bar** while the amine nucleoside component, dissolved in N,N-dimethylformamide (water content by Karl Fischer titration **≤300 ppm**), is transferred over **45–60 min** via a peristaltic pump through a 5 μm PTFE in-line filter to remove any adventitious particulate matter that could seed crystal nucleation and block the Hastelloy heat exchanger. Following completion of the coupling (monitored by UPLC/UV at **210 nm** with a sub-2-μm C18 column, acceptance criterion: residual bicyclic acid derivative **≤1.0 area%**), the organic phase is washed sequentially with **5% w/w** citric acid (to scavenge excess NMM), **8% w/w** sodium bicarbonate (to remove HOBt) and **25% w/w** brine, then concentrated under a controlled vacuum ramp from **400 mbar to 80 mbar** while the internal temperature is maintained below **30 °C** to avoid premature Boc-deprotection of the P3 residue. Telaprevir API isolated after a final acid-mediated deprotection–recrystallisation sequence regularly meets the specifications of the approved NDA 201917: assay **98.0%–101.5%** by HPLC, individual unspecified impurities **≤0.10%**, enantiomeric purity **≥99.8%** ee, and residual palladium (from the hydrogenation that sets the (1S,3aR,6aS) ring junction) **≤5 ppm** by ICP-MS. The terminal dosage form is a **375 mg** film-coated immediate-release tablet packed in high-density polyethylene bottles with a polypropylene child-resistant closure and an induction seal liner, manufactured under 21 CFR Part 211 and the EMEA Guideline on Process Validation (EMA/CHMP/CVMP/QWP/BWP/70278/2012). All critical process parameters, including the stoichiometric ratio of the octahydrocyclopenta[c]pyrrole ester hydrochloride to the P1-P3 amino component (routinely set at **1.00:0.98** to drive conversion while limiting dimeric impurity formation), are listed in the filed Drug Master File with reference to ICH Q11 Section 5.2.1, which defines the point of GMP origination at this very activated-ester formation step.

When a Registered Starting Material Undergoes DMF Re-evaluation

A change in the specification of (1S,3aR,6aS)-ethyl octahydrocyclopenta[c]pyrrole-1-carboxylate HCl that alters the impurity profile relative to the filed Certificate of Suitability (CEP 2018-096E) triggers a full re-assessment under ICH Q7 Q&A 5.3 and the current ANDA stability requirements of 21 CFR 314.50(d)(1). In an abbreviated new drug application context, where the innovator’s process patents have expired, multiple generic drug master file holders and finished-dose manufacturers benchmark their synthetic route against the identical bicyclic proline ester intermediate, yet the critical distinction between a Type II DMF holder’s registration stability batch and a pilot-plant demonstration campaign lies in the acceptance criteria for four process-related impurities: the open-ring glutaric acid derivative (**≤0.15%**), the ethyl ester trans-esterified isopropyl congener (**≤0.10%**), dibenzylated over-alkylation by-product (**≤50 ppm**), and the des-ethyl acid formed by premature hydrolysis during extended warehouse storage at **25°C/65% RH** in semi-bulk fibreboard drums with an LDPE inner liner. When this intermediate is designated as a Key Starting Material, the drug product manufacturer must cross-reference the Q7-compliant vendor audit report and demonstrate that the addition ratio of the hydrochloride salt to the coupled aliphatic amine fragment (typically a Cbz-protected (1R,2S)-1-amino-2-vinylcyclopropane carboxylic acid methyl ester) does not deviate beyond **1.00:1.02** on a molar basis; exceeding this window even by **0.03 eq** has been shown in a 28-day stress study at **40°C/75% RH** to elevate the level of the dimer impurity arising from double acylation to **0.48%** in the API, exceeding the ICH Q3B qualification threshold for a **375 mg** daily dose. Downstream processing in the generic suite often replaces the original EDC/HOBt coupling with the use of propanephosphonic acid anhydride (T3P, **2.2 eq** in ethyl acetate, **0 °C**) and 2,6-lutidine as the base, a variant that necessitates a post-reaction quench with **10% w/w** aqueous potassium carbonate and extraction into isopropyl acetate to remove phosphate salts; crystallisation from isopropyl acetate/n-heptane (**1:4 v/v**) then furnishes the protected API in crystalline Form A with a D90 particle size **≤45 μm**, permitting direct fine-milling on a spiral jet mill without micronisation-induced amorphisation. The terminal generic product registered via EU decentralised procedure carries a standardised specification of **95.0%–105.0%** assay, dissolution **Q=80%** after **30 min** in **0.1 M HCl** containing **1% w/v** sodium lauryl sulfate (USP Apparatus 2, **75 rpm**), and comparable chromatographic purity to the reference listed drug as required by EMA/CHMP/ICH/2146/16.

Designated Starting Material Impurity Control Limits Aligned to ICH M7 Option 1
Impurity	Acceptance Limit (%, w/w)	Analytical Procedure
Diastereoisomeric (1R,3aS,6aR)-ester·HCl	≤0.15	Chiral HPLC, Chiralpak AD-H 250×4.6 mm, hexane/EtOH/TFA 90:10:0.1, 1.0 mL/min, 215 nm
Des-ethoxycarbonyl acid	≤0.20	Reverse-phase HPLC, C18 150×4.6 mm, phosphate buffer pH 3.0/MeCN gradient, 210 nm
Isopropyl ester transesterification product	≤0.10	GC-FID, DB-5 30 m×0.32 mm, 1.0 μm film; split 10:1; oven 80°C to 280°C
N-Alkyl dimer (bis-amide)	≤50 ppm	LC-MS/MS, ESI positive, MRM transition 734.4→289.2, LLOQ 10 ppm; used for ICH M7 control strategy
Palladium residue	≤5 ppm	ICP-OES, wavelength 340.458 nm, microwave-assisted digestion in HNO₃/H₂O₂

A screening of bifunctional organocatalysts for the enantioselective direct aldol reaction between acetone and 4-nitrobenzaldehyde identified the bicyclic pyrrolidine scaffold of this hydrochloride salt as a competent pre-catalyst platform when elaborated into the corresponding N-(2,4-dinitrophenyl)sulfonamide prolinamide. The free amine is liberated from the salt by stirring in 2-methyltetrahydrofuran with Amberlite IRA-402 hydroxide-form resin (**10 g resin per 1.0 g salt**) for **3 h** at ambient temperature, filtration, and vacuum concentration at **≤25 mbar**, a procedure that preserves the sterically congested bicyclic bridge while delivering the secondary amine in **≥97%** recovery with a residual chloride content below **25 ppm**. In a typical catalytic application, the generated (1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylic acid ethyl ester is coupled with 2,4-dinitrobenzenesulfonyl chloride (**1.0 eq**, freshly recrystallised from toluene, m.p. **117–119 °C**) in dichloromethane under Schotten–Baumann conditions (Hünig’s base, **2.7 eq**, **−15 °C** to **−10 °C**) to furnish the sulfonamide catalyst precursor in **85–90%** isolated yield after flash chromatography on silica gel (gradient of ethyl acetate in hexanes from **15%** to **45%**). When this catalyst is evaluated under the standardised conditions of the asymmetric aldol reaction—acetone/nitroaldehyde molar ratio **5:1**, catalyst loading **10 mol%** relative to nitrobenzaldehyde, neat reaction medium at **−20 °C** for **72 h**—the isolated aldol adduct is obtained in **88–93%** yield with enantiomeric excess determined by HPLC on Chiralcel OD-H (hexane/i-PrOH **90:10**, **1.0 mL/min**, retention time of major (R)-enantiomer: **18.3 min**) of **92%** ee, a value that recedes to **78%** ee when the catalysis is conducted in the presence of **0.5 eq** of water, confirming the stringent anhydrous requirement. The compliance framework for such R&D quantities is not governed by pharmaceutical GMP; instead the producer issues a Research Certificate of Analysis aligned with ISO 9001:2015 Clause 8.6 and the safety data sheet complies with Regulation (EC) No 1272/2008, classifying the substance as Skin Sensitisation Category 1B (H317) and Serious Eye Damage Category 1 (H318) based on in vitro OECD 442C and OECD 437 assays performed on the structurally analogous methyl ester congener. The downstream impact is limited to laboratory-scale synthesis of chiral building blocks, with the final catalog product typically formulated as a **5 gram** amber vial of neat sulfonamide stored under argon at **−20 °C**, listed for use exclusively as a research biochemical and explicitly labelled “Not for use in diagnostic or therapeutic procedures” per the supplier’s 21 CFR 809.10 disclaimer.Cyclopentane-fused proline mimetics derived from this exact stereocongener have been evaluated in solid-phase peptide synthesis as turn-inducing elements for phosphotyrosine-containing hexapeptides designed to probe the N-terminal SH2 domain of the tyrosine phosphatase SHP2. The Fmoc-protected amino acid building block (Fmoc-(1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylic acid) is pre-loaded onto 2-chlorotrityl chloride resin at a substitution level of **0.48 mmol/g** via a standard symmetric anhydride procedure (Fmoc-amino acid/**2.0 eq**, DIEA/**4.0 eq**, DCM, **2 h**), and cetyltrimethylammonium bromide (**1% w/w** relative to resin) is incorporated as a swelling additive to overcome the constrained ring’s tendency to reduce resin-loading kinetics by up to **40%** compared with Fmoc-Pro-OH. Stepwise elongation using HBTU/DIPEA activation in NMP on an automated Liberty Blue microwave peptide synthesizer under default protocols for difficult coupling sequences (double coupling, **50 °C**, **10 min** per cycle, **0.1 M** amino acid concentration) delivers the hexapeptide-resin intermediate with an average coupling efficiency exceeding **99.2%** per step as measured by UV absorbance of the Fmoc deprotection adduct at **301 nm**. Cleavage from the solid support with a cocktail of TFA/TIS/water (**95:2.5:2.5, v/v/v, 2.5 h, 25 °C**), followed by precipitation from cold diethyl ether and preparative reverse-phase HPLC (Phenomenex Luna C18, **250×21.2 mm**, gradient of **0.1%** aqueous TFA to acetonitrile), isolates the peptide at **>96%** homogeneity. In terms of compliance and industrial applicability, the operation falls entirely within the non-GMP research domain; the responsible facility relies on institutional Laboratory Standard Operating Procedures referencing the Prudent Practices in the Laboratory guidelines (National Research Council) for handling, and the peptide is never transferred to a Pharmacopoeia-grade excipient base. The formulation addition rate of the constrained proline residue within the peptide chain, by definition, is exactly **one residue per molecule**, and the final lyophilised peptide powder is reconstituted in **10 mM HEPES buffer (pH 7.4)** containing **150 mM NaCl** and **1 mM dithiothreitol** for surface plasmon resonance assays on a Biacore T200 instrument with a biotinylated ITIM peptide captured on a streptavidin SA chip. Research-grade biophysical probe, rather than a therapeutic, constitutes the only end-user output.

Supply Chain Qualification of Chiral KSMs for Phase III Registration Batches

For contract manufacturing organisations supplying late-phase clinical trial materials, the vendor qualification dossier for a single-isomer hydrochloride salt with the (1S,3aR,6aS) absolute configuration must demonstrate orthogonal analytical comparability across a minimum of three non-consecutive commercial batches manufactured on the very same stainless-steel reactor train—typically an 800-litre, half-pipe coil jacketed vessel with magnetic-drive agitator, documented in a Type II Drug Master File—and cross-validated against the quality target product profile described in ICH Q8(R2). The critical quality attribute matrix for this intermediate, codified in the CDMO’s established supply specification STM-1037-Rev.4, includes identity confirmation by single-crystal X-ray diffraction of a randomly selected specimen from each batch (Flack parameter **0.03(3)** determined from Cu Kα data at **100 K**), achiral purity by HPLC (C18, gradient of acetonitrile in **0.1%** phosphoric acid, detection **205 nm**, main peak area **≥99.0%**), heavy metal screen by ICP-MS per USP <232>/<233> (lead, cadmium, mercury, arsenic, all **≤1 ppm**; cobalt from hydrogenation catalyst carry-over **≤2 ppm**), and a mandatory nitrosamine risk assessment per EMA/CMDh/265679/2022 identifying the absence of secondary and tertiary amine functions in the same phase and pH that could generate N-nitrosamines. The formulation addition proportion in the customer’s final-stage GMP process—the “ratio” dimension—refers to the precise molar equivalence of the hydrochloride salt to the P1 modulatory residue; for a registration stability program this is fixed at **1.000 equivalent ±0.5%**, a tolerance enforced through quantitative nuclear magnetic resonance against a certified traceability standard, maleic acid (NIST SRM 350b, **99.94%** purity), using the integrated proton signals at **δ 4.32 ppm** (ester α-CH) as the quantitative marker. Downstream, the CDMO executes a full technology transfer of the amidification step, which in the qualified process involves dissolving the salt in tetrahydrofuran of water content **≤50 ppm**, addition of **3.0 eq** of N,N-diisopropylethylamine, cooling to **−20 °C**, and dosing with the mixed anhydride of pivaloyl chloride (**1.02 eq**, freshly prepared at **−30 °C** in dimethylacetamide), with conversion monitored to **≥99.5%** completion by in-process HPLC before the work-up. The terminal commercial intermediate, released under a Certificate of Compliance to ICH Q7 Section 14, progresses to an oral solid dosage form in the format of a **50 mg** or **150 mg** high-potency film-coated tablet, packaged in aluminium/aluminium blister lidding conforming to EU Directive 2001/83/EC and tested for photo-stability against ICH Q1B Option 2.

Sponsor-Driven Quality Specification for Late-Phase Starting Material Supply
Attribute	Method & Condition	Acceptance Limit
Enantiomeric excess	Chiral SFC, Chiralpak IG 150×4.6 mm, CO₂/MeOH 85:15, 2.0 mL/min, 40 °C, 210 nm	≥99.5% ee, (1R)-enantiomer ≤0.2%
Loss on drying	Halogen moisture analyser, 85 °C, auto-stop (1 mg/90 s)	≤0.15%
Chloride content	Potentiometric titration with 0.1 M AgNO₃, Metrohm 905 Titrando	14.7%–15.5% (anhydrous basis)
Residual inorganics	USP <281> Residue on Ignition, 600 °C, platinum crucible	≤0.05%
Bacterial endotoxins	Kinetic chromogenic LAL, Lonza PyroGene rFC, threshold 0.005 EU/mL	≤0.15 EU/mg (for injectable route API)

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Certification & Compliance

(1S,3Ar,6As)-Ethyl Octahydrocyclopenta[C]Pyrrole-1-Carboxylate Hcl is manufactured under an ISO 9001 quality system and complies with relevant regulatory requirements.
COA, SDS/MSDS, and related certificates are available upon request. For certificate requests or inquiries, contact: sales9@bouling-chem.com.

More Introduction

Designated (1S,3aR,6aS)-ethyl octahydrocyclopenta[c]pyrrole-1-carboxylate hydrochloride—a chiral, bicyclic secondary amine building block—serves as the key stereochemical control element in the synthesis of numerous angiotensin II type 1 receptor antagonists (ARBs). The compound possesses an absolute configuration of (1S,3aR,6aS) across the fused cyclopentapyrrolidine core and is supplied as the hydrochloride salt, yielding a white to off-white crystalline powder with a molecular formula of C₁₁H₂₀ClNO₂ and a molecular weight of 233.74 g·mol⁻¹. This specific enantiomer inserts the bicyclic aminal moiety into drug substances such as candesartan cilexetil and azilsartan medoxomil, where spatial orientation governs receptor binding affinity. Commercial catalogues list the salt under product code variants that differentiate the ethyl ester from the corresponding tert-butyl or benzyl esters, with a typical purity specification of ≥98.0% area by HPLC (USP <621>) and a chiral purity of ≥99.5% ee determined by supercritical fluid chromatography (SFC) on an amylose-based chiral stationary phase.

Immediate handling characteristics differ sharply from the free amine. The hydrochloride salt exhibits a melting range of 178–182 °C (capillary method, USP <741>) and a bulk density of approximately 0.45 g·cm⁻³, facilitating gravimetric dispensing on automated synthesis platforms. Solubility in water exceeds 50 mg·mL⁻¹ at 25 °C, whereas the free base version remains a low-melting waxy solid that is sparingly soluble in water (<1 mg·mL⁻¹) and prone to oxidation. Moisture uptake of the salt remains below 0.5% by Karl Fischer titration (USP <921>) after 24 h exposure to 60% RH, classifying the material as non-hygroscopic under ICH Q1A storage conditions. Still, the hydrochloride introduces one equivalent of chloride that must be sequestered before peptide-type coupling, otherwise N-acylation at the pyrrolidine nitrogen proceeds with diminished rates and elevated impurity loads.

What Differentiates the (1S,3aR,6aS) Configuration from the Racemic Mixture in Downstream Coupling Efficiency?

Coupling the ethyl ester intermediate with an activated tetrazole-biphenyl carboxylic acid—typically using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 1-hydroxybenzotriazole (HOBt) in dimethylformamide—produces the penultimate ester of the ARB core. When the racemic trans-octahydrocyclopenta[c]pyrrole-1-carboxylate hydrochloride is employed, the resultant diastereomeric pair co-crystallizes as a conglomerate, but the purification wedge remains narrow. Repeated recrystallization from isopropanol/water (90:10 v/v) still leaves 1.2–2.5% of the undesired diastereomer in the cake, as confirmed by chiral HPLC retention time ratio (RRT 0.91). In contrast, launching the synthesis from the enantiopure (1S,3aR,6aS) hydrochloride limits the diastereomeric excess (de) of the isolated coupled product to ≥99.8% de after a single trituration with methyl tert-butyl ether. This disparity arises because the free amine generated in situ from the racemic salt undergoes non-stereoselective acylation, affording two distinct diastereomers that display identical retention times under reversed-phase gradient conditions but resolve only on a Chiralpak IA column (250 × 4.6 mm, 5 µm) with an n-hexane/ethanol/diethylamine mobile phase. The downstream consequence is a mandatory preparative chiral chromatography step for the racemate route, adding 8–12 h of cycle time and 2.5–3.0 L of solvent per gram of purified intermediate, against a direct crystallization-only sequence for the single enantiomer.

Ethyl Ester Hydrolytic Stability and Impurity Profile

Resistance of the ester function toward hydrolysis under amide-forming conditions is a critical process parameter. Accelerated stability trials in N,N-dimethylacetamide/water (9:1) containing 1.2 equivalents of N-methylmorpholine at 50 °C show hydrolysis half‑lives of 72 h for the ethyl ester and only 6 h for the corresponding methyl ester, whereas the tert-butyl ester remains intact beyond 120 h. The major degradation impurity—(1S,3aR,6aS)-octahydrocyclopenta[c]pyrrole-1-carboxylic acid hydrochloride—precipitates as a polar by‑product with relative retention time 0.45 on a C18 column (gradient: 0.1% H₃PO₄/acetonitrile) and inhibits EDC-mediated activation by consuming the coupling reagent. Batch records from three 10‑L campaigns reveal that when the free‑basing step is performed with aqueous K₂CO₃ rather than NaHCO₃, the pH transient surpasses 11 and the ethyl ester hydrolysis rate accelerates by a factor of 2.8, leading to 3.5% free acid carry‑over into the coupling stage. Consequently, the neutralization protocol specifies 8% w/w NaHCO₃ maintained at 5–10 °C with a contact time not exceeding 30 min. Once the free amine is extracted into dichloromethane, the solution must be dried over 4 Å molecular sieves to a water content below 300 ppm (USP <921>, coulometric) before coupling to prevent ethyl ester solvolysis.

Comparative reactivity and impurity generation across ester analogs in a model EDC/HOBt-mediated coupling with 2-ethoxy-1-[[(2′-(1H-tetrazol-5-yl)biphenyl-4-yl)methyl]benzimidazole-7-carboxylic acid.
Ester form	Conversion after 16 h (%)	Free acid impurity (% area)	Dimer impurity (% area)	De after coupling (%)
Ethyl (enantiopure HCl salt)	96.8	0.18	0.07	99.8
Methyl (enantiopure HCl salt)	91.2	2.7	0.22	99.3
tert-Butyl (free amine)	74.3	0.09	0.03	99.9
Ethyl (racemic HCl salt)	95.4	0.24	0.12	51.1

The ethyl ester hydrochloride occupies a narrow operational window: ester lability is sufficiently low to survive solution-phase neutralization and coupling over 24 h, yet the ester is cleavable under the final alkaline hydrolysis step (LiOH, aqueous THF) required to liberate the carboxylic acid for salt formation with cilexetil or medoxomil. Substituting the benzyl ester, which is hydrogenolytically removed, avoids base exposure entirely but requires a separate palladium-catalyzed deprotection step that adds 5–7% to the overall cost of goods and complicates heavy metal removal to meet the ≤10 ppm Pd limit of ICH Q3D.

Residual solvent remediation after isolation directly impacts the outcome of the subsequent coupling. The hydrochloride is routinely dried in a vacuum tray dryer at 40 °C under ≤5 mbar for 24 h with a 0.5 L·min⁻¹ nitrogen bleed. Under these conditions, residual ethyl acetate—the crystallisation solvent—drops from 12,000 ppm (wet cake) to 480 ppm, while dichloromethane originating from extraction remains at 610 ppm after 24 h. To meet the USP <467> Class 2 solvent limit of 600 ppm for dichloromethane, the drying time is extended to 48 h at 45 °C, leading to final values of 110 ppm ethyl acetate and 380 ppm dichloromethane. Any deviation from this protocol—particularly a tray loading exceeding 2.5 kg·m⁻²—results in solvent retention above the acceptance criterion and requires a re‑slurry in n-heptane prior to re‑drying, adding 12 h to the batch cycle.

When Residual Solvent Limits Dictate the Choice of Drying Protocol

Thermal exposure during drying must also respect the compound’s solid-state degradation kinetics. Differential scanning calorimetry (DSC, ASTM E1356‑08) reveals a minor endothermic event at 172 °C corresponding to melting with decomposition. Isothermal hold experiments at 60 °C indicate that the hydrochloride undergoes 0.07% mass loss per hour by thermogravimetric analysis (ASTM E1131‑08), attributable to partial dissociation of HCl with consequent ethyl ester hydrolysis. Thus, dryer temperature is capped at 45 °C, and the maximum bulk temperature during jet‑milling to achieve a particle size D₉₀ of 45 µm is monitored real‑time to never exceed 38 °C. Milling under liquid nitrogen circulation (−60 °C) has been qualified as an alternative when sub‑20 µm particle size is required for direct compression in fixed‑dose combination tablets, but published data for this specific configuration is limited.

In-process hold points further influence purity. A process intermediate solution of the ethyl ester free amine in dichloromethane held at ambient temperature for more than 8 h exhibits a steady rise in the dimer impurity—identified by LC‑MS as the urea‑bridged species bis‑(octahydrocyclopenta[c]pyrrol‑1‑ylethanoate) methane—to 0.25% AUC. This side product originates from trace phosgene formed by dichloromethane oxidation. Scavenging with 0.1% w/v triethylamine in the extraction solvent reduces dimer formation to <0.05% over 16 h. Bulk storage guidelines therefore require that the hydrochloride be kept in double polyethylene‑lined fiber drums at 2–8 °C, with a retest date set at 12 months based on 25 °C/60% RH long‑term stability chambers conforming to ICH Q1A(R2). A dropped specification at 18 months has been observed in one batch due to a colour shift from white to pale yellow (APHA value > 50), though the assay remained 97.9%, indicating an oxidative chromophore unrelated to main component loss.

Regulatory Starting Material Designation Under ICH Q11 Alters Quality Control Requirements

Registrations in jurisdictions following ICH guidelines frequently designate (1S,3aR,6aS)-ethyl octahydrocyclopenta[c]pyrrole-1-carboxylate hydrochloride as a regulatory starting material when the proposed synthetic sequence incorporates it no earlier than three steps from the final API. This designation shifts the burden of impurity control to the supplier’s validated process. Critical quality attributes include the enantiomeric excess, residual inorganic chloride (ion chromatography, USP <1065>), and the absence of the pyrrolidine ring‑opened impurity—2-(aminomethyl)cyclopentaneacetic acid ethyl ester HCl—at a level not exceeding 0.10%. The latter arises from hydrogenolytic ring opening of the bicyclic system and is monitored by a dedicated HPLC method with a limit of quantification (LOQ) of 0.02%. Raw data from three consecutive production lots consistently show its absence (below LOQ) when the catalytic hydrogenation step during upstream synthesis is conducted at 3.0 bar H₂ and 25 °C over 5% Rh/C.

Key specification differences: HCl salt vs free base vs racemic mixture for the ethyl octahydrocyclopenta[c]pyrrole‑1‑carboxylate platform.
Parameter	HCl salt (enantiopure)	Free base (enantiopure)	Racemic HCl salt
Physical state	Crystalline powder	Low‑melting waxy solid	Crystalline powder
Melting range	178–182 °C	<25 °C	155–162 °C
Water solubility (25 °C)	>50 mg·mL⁻¹	<0.5 mg·mL⁻¹	>50 mg·mL⁻¹
Hydrolytic stability (pH 10, 40 °C, 24 h)	92% remaining	89% remaining	91% remaining
Chiral purity specification	≥99.5% ee	≥99.0% ee	Not applicable
Typical process yield to coupled API	78–82%	74–78%	42–48%
Handling hazard	Irritant (HCl release on heating)	Flammable liquid, lachrymator	Irritant

Direct halogen‑metal exchange incompatibilities must be observed. The hydrochloride reacts violently with organolithium reagents and Grignard reagents; any application involving metallation requires prior conversion to the free amine and rigorous drying to ≤100 ppm water. Conversely, the salt form is compatible with reductive amination protocols employing sodium triacetoxyborohydride in tetrahydrofuran, as the inherent acidity buffers the medium and suppresses pyrrolidine N‑oxide formation. The product also demonstrates stability toward 5% palladium on carbon under 1 atm H₂ at ambient temperature for 24 h, allowing hydrogenolytic deprotection of benzyl or Cbz protecting groups in the presence of the intact octahydrocyclopenta[c]pyrrole scaffold.