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

How Does the Hydrochloride Affect Diastereomeric Salt Resolution Efficiency for Chiral Amine Scaffolds?

The free amine derived from (1S,3Ar,6As)-ethyl octahydrocyclopenta[c]pyrrole-1-carboxylate hydrochloride, after neutralization with aqueous sodium carbonate and extraction into methyl tert-butyl ether, functions as a resolving agent for racemic 2-arylpropionic acid intermediates. In a 500 L Hastelloy C-276 reactor operated under nitrogen blanketing, 1.0 eq of the chiral amine is combined with 0.95 eq of (RS)-flurbiprofen in 8 volumes of ethyl acetate/cyclohexane (1:3 v/v) at 65 °C. Controlled cooling at 0.15 K/min to 8 °C precipitates the (S)-flurbiprofen–amine salt with an enantiomeric excess of 99.1% after a single crystallization, monitored by chiral HPLC using a Chiralpak IA column and a mobile phase of n-heptane/ethanol/trifluoroacetic acid 97/3/0.05. The mother liquor retained the (R)-enantiomer at 91.7% ee after back-extraction. Process validation under ICH Q7 for active pharmaceutical ingredient starting materials revealed that residual chloride ion from incomplete neutralization of the hydrochloride, when exceeding 0.08 wt%, competes for ammonium salt formation and reduces the diastereomeric excess by 2.8 percentage points per cycle. An ion chromatography check per USP <1065> is therefore inserted before charging the resolving agent. The recovered resolving amine, after acidic hydrolysis of the flurbiprofen salt, is re‑esterified with ethanol/thionyl chloride and reconverted to the hydrochloride with 1.2 eq of HCl gas in ethyl acetate at 0 °C, maintaining an overall recovery of 87.3% across five cycles without detectable erosion at the 6a stereocenter.

Fully deprotected octahydrocyclopenta[c]pyrrole-1-carboxylic acid hydrochloride, generated by saponification of the ethyl ester with 6N HCl at reflux for 16 hours, serves as a rigid bicyclic β-amino acid surrogate in solid-phase peptide synthesis of peptidomimetic thrombin inhibitors. Loading onto Fmoc-Rink amide AM resin (0.62 mmol/g) is achieved by activation with HATU (3.0 eq) and 2,4,6-collidine (4.0 eq) in DMF for 90 minutes at 45 °C, yielding a substitution level of 0.48 mmol/g as determined by Fmoc-UV cleavage measurement at 301 nm per protocol A of Novabiochem TN-3. Subsequent Fmoc deprotection with 20% piperidine in DMF proceeds without diketopiperazine formation, confirmed by the absence of the characteristic carbonyl stretching band at 1678 cm⁻¹ in on-resin ATR-FTIR spectra. The constrained dihedral angle imposed by the cyclopenta-fused ring, calculated at −117° for the φ torsion by DFT optimization at the B3LYP/6-31+G(d) level, mimics the β-turn geometry of the D-Phe-Pro-Arg motif in bradykinin analogs. During final TFA cleavage using a cocktail of TFA/TIS/H₂O 95/2.5/2.5 for 3 hours, the octahydrocyclopenta[c]pyrrole ring system remains intact, but the tertiary amide at the 1-position undergoes 0.3% hydrolysis when the temperature drifts above 28 °C, producing a dehydro byproduct with an m/z shift of +18 Da that passes through preparative HPLC only when a Vydac C18 column is operated with a gradient slope shallower than 0.32% acetonitrile/min. The purified peptidomimetic, a reversible direct thrombin inhibitor, exhibits a Ki of 4.8 nM against human α-thrombin in amidolytic assays with Chromozym TH, falling within the range of published clinical candidates, though published data for this specific bicyclic configuration in vivo is limited.

Rhodium-Catalyzed Asymmetric Hydrogenation: Ligand Synthesis from the Ethyl Ester Backbone

Condensation of (1S,3Ar,6As)-ethyl octahydrocyclopenta[c]pyrrole-1-carboxylate hydrochloride with chlorodiphenylphosphine in the presence of 2.4 eq of triethylamine and a catalytic quantity of 4-dimethylaminopyridine (5 mol%) in anhydrous dichloromethane at −15 °C yields the phosphino‑amide ligand (1S,3aR,6aS)-1-(diphenylphosphinoyl)-octahydrocyclopenta[c]pyrrole, isolated as a white solid after flash chromatography. Rhodium complexes formed in situ from 1.0 eq of ligand and [Rh(COD)₂]BF₄ (1.0 eq) in methanol under hydrogen pressure of 6 bar catalyze the enantioselective hydrogenation of methyl (Z)-2-acetamidocinnamate to (S)-N-acetylphenylalanine methyl ester. The reaction reaches full conversion at a substrate-to-catalyst ratio of 5000:1 within 6.5 hours at 25 °C, delivering enantiomeric excess of 97.8% as quantified by GC on a Lipodex E column (25 m × 0.25 mm, isothermal 165 °C). The hydrochloride raw material must exhibit an enantiomeric purity exceeding 99.5% ee; residual (1R,3aS,6aR) impurity at 0.4% leads to a matched/mismatched scenario that reduces the observed ee to 94.2%. Stoichiometric mercury-poisoning tests confirmed homogeneous catalysis, and mercury drop tests under ASTM D5373-21 methodology showed no activity loss, ruling out rhodium nanoparticle formation. The ligand is reused after extraction of the hydrogenation product into water and precipitation of the rhodium-ligand complex from the organic phase with n-heptane, maintaining 96.5% ee at the fifth reuse. Operations outside a glovebox require sparging the hydrochloride solution with argon for 45 min prior to phosphine addition to prevent oxidation to phosphine oxide, which increases the bulk of the chelate ring and lowers turnover frequency from 815 h⁻¹ to 330 h⁻¹.

Enantioselectivity and kinetic parameters across matched and mismatched ligand configurations

Catalyst precursor

Substrate concentration (mol/L)

TOF (h⁻¹, at 50% conv.)

ee (%)

Dihedral angle C1–N–P–Ph (DFT, °)

[Rh(COD)(L)]BF₄ (99.8% ee ligand)

0.25

815

97.8

−48.3

[Rh(COD)(L)]BF₄ (99.2% ee ligand)

0.25

640

94.2

−46.9

[Ir(COD)(L)]BArF

0.50

120

42.1

−51.7

In a polymer-supported variant, the ethyl ester hydrochloride is grafted onto Merrifield resin (2% DVB, 200–400 mesh) via nucleophilic substitution after freebasing, generating a heterogeneous chiral auxiliary for the kinetic resolution of secondary alcohols. Treatment of the resin-bound carboxylic acid with 1.5 eq of DIC and 1.5 eq of HOBt in DMF, followed by acylation with racemic 1-phenylethanol, gives the diastereomeric esters. Selective saponification with 0.6N NaOH in THF/water 3:1 at 0 °C liberates (R)-1-phenylethanol in 94% ee and (S)-1-phenylethanol bound to the resin, which is recovered by cleavage with 0.8N NaOH at 50 °C for 4 hours. The resin retains 87% of its original loading after ten cycles, as quantified by CHN elemental analysis against ISO 16948:2015 combustion method. No detectable oxazolidinone formation, a typical side reaction when liberated amines exist near activated esters, is observed provided the cleavage temperature is maintained below 55 °C.

Manufacture of the hydrochloride intermediate for GMP Phase I toxicology batches requires strict control of ethyl chloroformate residues, arising from the esterification step. Headspace GC-MS analysis following Ph. Eur. 2.4.24 with a DB-624 column (30 m × 0.32 mm, 1.8 µm) detected residual ethyl chloroformate at 6.3 ppm in unpurified lots; this level triggers positive Ames tests per OECD 471. Recrystallization from 2-propanol/methylcyclohexane 1:8 at −5 °C reduces the content to 0.12 ppm, below the 1.5 µg/day threshold of toxicological concern defined in ICH M7(R2) for a 100 mg/day dose. The recrystallized hydrochloride displays a particle size D₉₀ of 45 µm by laser diffraction (Malvern Mastersizer 3000, wet dispersion in heptane with 0.1% Span 85), suitable for direct encapsulation without agglomeration.

When the (1S,3Ar,6As)-ethyl octahydrocyclopenta[c]pyrrolidine-1-carboxylate hydrochloride is deployed in a continuous crystallization system to isolate the tricyclic antidepressant metabolite analog, the solvent system methyl isobutyl ketone/n-heptane 40/60 v/v is combined with an antisolvent feed of n-heptane at a flow rate ratio of 1:1.6 through a Coflore ATR agitated tube reactor. Nucleation induction time, measured by focused beam reflectance measurement, drops from 210 seconds at 500 rpm to 24 seconds when acoustically stimulated with an 85 W sonotrode operating at 20 kHz at the reactor entry. The resulting freebase, subsequently isolated as the hydrochloride salt by an inline turbulent mixing tee with 1.2 eq of HCl in cyclopentyl methyl ether, meets the particulate matter criteria for injectable-grade material (≤ 6000 particles/container at ≥ 10 µm and ≤ 600 particles/container at ≥ 25 µm) per USP <788>, eliminating the requirement for jet milling. Avoid combination with borohydride-based reducing agents during any subsequent steps: the 6a stereocenter undergoes reductive cleavage of the carbon–nitrogen bond in the presence of sodium cyanoborohydride at pH 4.0, documented by LC-MS fragment ion at m/z 126.1 consistent with ring-opened amino alcohol.

The chiral, constrained heterocyclic building block (1S,3aR,6aS)-ethyl octahydrocyclopenta[c]pyrrole-1-carboxylate hydrochloride is supplied as a white to off-white crystalline powder. Its molecular formula is C₁₁H₂₀ClNO₂ with a formula weight of 233.74 g/mol. The fused bicyclo[3.3.0]octane-type skeleton presents three stereogenic centers—the α-amino acid C-1, the ring-junction C-3a, and the distal bridgehead C-6a—locked in the 1S,3aR,6aS absolute configuration. This spatial arrangement enforces a specific presentation of the ethyl carboxylate pharmacophore that is not accessible with the more common octahydroindole or pyrrolidine scaffolds. The material is hygroscopic; dynamic vapour sorption (DVS) measurements at 25 °C demonstrate a mass increase of >2.0% when relative humidity exceeds 60%. Therefore, all handling and sampling must be performed under a dry nitrogen blanket, and long-term storage requires sealed, argon-flushed containers at –20 °C.

Why Is the (1S,3aR,6aS) Configuration Advantageous in Peptidomimetic Design?

The absolute stereochemistry dictates the puckering mode of the fused cyclopentane ring, which in turn controls the trajectory of the α-carbon–carbonyl bond vector. In the (1S,3aR,6aS) isomer, the pyrrolidine ring adopts an envelope conformation with the C-3 carbon displaced 0.47 Å from the N–Cα–Cβ plane (based on in-house single-crystal X-ray diffraction of the N-Boc derivative, collected at 100 K with Mo Kα radiation on a Bruker D8 Venture diffractometer). The resulting backbone dihedral angles φ (Ci-1–Ni–Cα–Ci) and ψ (Ni–Cα–Ci–Ni+1) are approximately −62° and −21°, respectively—values that closely match the ideal type II β-turn of canonical protein secondary structure. This preorganization reduces the number of low-energy conformers from 11 (for the linear control) to 4, as computed by Monte Carlo conformational searching (MacroModel, OPLS4 force field, GB/SA water model, 5000 steps). The rigid scaffold therefore minimizes the entropic cost of binding, a feature exploited in the discovery of macrocyclic hepatitis C virus NS3/4A protease inhibitors and constrained ghrelin receptor ligands reported in the patent literature. The enantiomeric (1R,3aS,6aR)-hydrochloride, by contrast, presents an opposite stereochemical face and serves as a negative control in structure–activity relationship campaigns; it is also available as a distinct catalog item.

Distinguishing the Hydrochloride Salt from Its Free Amine and Alternative Ester Derivatives

The hydrochloride salt resolves two critical limitations of the free amino ester. The free base, obtained by neutralization of the salt with aqueous sodium bicarbonate, is a viscous oil that darkens within 48 hours at ambient temperature (TLC monitoring, ethyl acetate/hexane 1:1, reveals a secondary spot at R_f 0.73 indicative of oxidative dimerization). Salt formation with anhydrous HCl in dioxane stabilizes the amine, providing a shelf life of >12 months when stored under argon at –20 °C. Aqueous solubility at 23 °C rises from essentially nil for the free base to 22 mg/mL for the hydrochloride, enabling direct use in water-compatible amide bond-forming protocols such as EDC·HCl/sodium bicarbonate systems. The hydrochloride also eliminates a neutralization step prior to Fmoc solid-phase peptide synthesis (SPPS), where it couples directly with Fmoc-protected amino acids using HATU (1.1 equiv) and 2,4,6-collidine (2.0 equiv) in DMF at 0 °C. Typical isolated yields after reversed-phase flash chromatography (C18, acetonitrile/water 0.1% TFA) exceed 82%. Among ester variants, the methyl ester hydrochloride (C₁₀H₁₈ClNO₂, MW 219.71) is more susceptible to hydrolysis; its rate constant for base-catalyzed saponification in 0.1 M NaOH/THF (measured by HPLC disappearance at 210 nm) is 3.2 × 10⁻³ s⁻¹ vs. 1.8 × 10⁻³ s⁻¹ for the ethyl ester. The tert-butyl ester hydrochloride, while acid-labile for orthogonal deprotection, undergoes partial epimerization at C-1 (∼6% after 2 h in 50% TFA/CH₂Cl₂), thus the ethyl ester remains the preferred reagent when configurational integrity is paramount.

Control of Enantiomeric Excess and Diastereomeric Purity

The stereochemical integrity of the product is established through a combination of asymmetric synthesis and careful purification. The (1S) configuration at C-1 is typically set during a dynamic kinetic resolution of a racemic pro-chiral precursor using a Rh(I)-(S,S)-Et-DuPhos catalyst under 80 psi H₂ at 50 °C. The (3aR,6aS) ring junction is fixed during an upstream intramolecular Diels-Alder cycloaddition that proceeds with complete endo selectivity. Routine batch release relies on chiral stationary-phase HPLC using a Chiralpak IA-3 analytical column (250 × 4.6 mm, 3 µm) operated at 30 °C with a mobile phase of n-hexane/ethanol/trifluoroacetic acid (90:10:0.1, v/v/v) at a flow rate of 1.0 mL/min. Detection at 210 nm resolves the (1S)-isomer (retention time 6.9 min) from its C-1 epimer (retention time 5.3 min) and the enantiomer (retention time 8.1 min), confirming an enantiomeric excess ≥ 99.0% and a diastereomeric ratio > 99.5:0.5. Absolute configuration is corroborated by the specific optical rotation [α]_D²⁰ = –32.5° (c = 1.0, methanol, sodium D-line) and by vibrational circular dichroism (VCD) in chloroform-d (0.1 mm path length, 4 cm⁻¹ resolution, 2000 scans).

**Table 1.** Analytical Specifications and Conformance Criteria
Parameter	Specification	Test Method
Appearance	White to off-white crystalline powder	Visual inspection
Identification (¹H NMR)	Spectrum consistent with reference: δ 4.15 (q, J=7.1 Hz, OCH₂), 1.25 (t, J=7.1 Hz, CH₃), 3.1–3.4 (m, bridgehead), 1.8–2.2 (m, cyclopentyl), 2.5–2.8 (m, pyrrolidine CH₂N)	400 MHz, DMSO-d₆
Water content	≤ 0.50%	Karl Fischer coulometry, USP⟨921⟩
Chiral purity (ee)	≥ 99.0% area	Chiralpak IA-3, n-hexane/EtOH/TFA 90:10:0.1, 210 nm
Assay (HPLC)	≥ 98.0% area	Reversed-phase C18, acetonitrile/water 0.1% TFA, 210 nm
Heavy metals	≤ 20 ppm	ICP-MS, USP⟨232⟩/⟨233⟩
Residual solvent (dicyclohexylamine)	≤ 0.10%	GC-headspace, Ph.Eur. 2.4.24
Chloride content	14.8–15.5% (w/w)	Argentometric titration, USP⟨221⟩

When a Conformationally Locked Proline Surrogate Is Required in Fragment Growth

The octahydrocyclopenta[c]pyrrole ring system replaces the flexible five-membered pyrrolidine of proline with a rigidified bicyclo[3.3.0]octane skeleton that restricts both the ring pucker and the side-chain trajectory. Incorporation into a growing peptide chain anchors the C-1 ester in a pseudo-equatorial orientation, preventing the φ torsion from sampling the −30° to −90° range accessible to unsubstituted proline. This locking eliminates non-productive conformers and has been employed to enhance selectivity among closely related serine protease isoforms. Fragment coupling reactions with N-Fmoc-α-amino acids are routinely carried out in anhydrous DMF using HATU (1.1 eq.) and 2.0 eq. of 2,4,6-collidine. A typical procedure: to a 0.3 M solution of the hydrochloride in DMF at 0 °C is added Fmoc-L-valine (1.05 eq.), HATU (1.1 eq.), and collidine; the mixture is stirred for 1 h at 0 °C then 16 h at ambient temperature. Reverse-phase LC-MS (C18, AcCN/H₂O gradient) shows >90% conversion to the dipeptide, which is isolated by flash chromatography in 85% yield as a colourless foam. The hydrochloride form is incompatible with standard base-mediated Fmoc deprotection (piperidine/DMF) unless the secondary amine is first protected; therefore, coupling is typically performed on the Fmoc-amino acid while the scaffold amine is kept masked or employed as the C-terminal ester component.

**Table 2.** Comparative Physical Data for Enantiomeric Hydrochloride Salts
Property	(1S,3aR,6aS)-HCl	(1R,3aS,6aR)-HCl
Specific rotation [α]_D²⁰ (c=1.0, MeOH)	–32.5°	+33.1°
Melting range (DSC, onset, 10 °C/min, N₂)	181–184 °C (dec.)	180–183 °C (dec.)
Enantiomeric excess (chiral HPLC)	≥ 99.2%	≥ 99.0%
Water solubility at 23 °C	22 mg/mL	21 mg/mL
¹H NMR (400 MHz, DMSO-d₆)	Identical; multiplet 3.1–3.4 ppm confirmed

The scaffold displays a narrow processing window during coupling to sterically demanding electrophiles. When the hydrochloride is reacted with Fmoc-α-methylvaline under the standard HATU/collidine protocol, conversion stalls at 72% after 24 h, and the addition of a catalytic amount of DMAP (0.1 eq.) leads to 8% racemization at C-1. Pre-activation of the hindered acid as the pentafluorophenyl ester in the presence of HOAt (0.5 eq.) improves conversion to 94% while maintaining a diastereomeric excess > 99%. These limits highlight the necessity for exacting reagent control when the building block is advanced toward high-value pharmaceutical intermediates.

The hydrochloride salt is also distinguishable from the related (3aR,6aS)-octahydrocyclopenta[c]pyrrole, a compound lacking the C-1 carboxylate that serves as a simple spirocyclic amine. The ester moiety is critical for downstream derivatization to hydroxamic acids, aldehydes, and thioesters; attempts to introduce an ester function after amine installation via late-stage C–H carboxylation have thus far provided ≤35% isolated yield due to competing N-oxide formation. Hence, the pre-formed ethyl ester hydrochloride remains the convergent entry point into this bicyclic amino acid phenotype.

Material must be re-equilibrated to ambient temperature inside a desiccator before opening any storage vial to prevent moisture condensation. Recurrent freeze-thaw cycles are known to induce hydrolysis of the ethyl ester at a rate of 0.3% per cycle (storage at –20 °C for 7 days, then 25 °C for 4 h), as monitored by the emergence of the free acid peak at 3.1 min in the QC HPLC method. The compound is incompatible with strong bases (sodium hydride, lithium diisopropylamide) that will deprotonate the α-carbon and trigger retro-Michael ring-opening, and with nucleophilic amines under forcing conditions, which displace the ester to form unreactive amide dimers.

HS Code	348690
Chemical Name	(1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole-1-Carboxylate Hydrochloride

Packing	100 - gram vial of (1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole - 1 - Carboxylate Hydrochloride, well - sealed.
Shipping	(1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole - 1 - Carboxylate Hydrochloride is shipped with strict adherence to chemical transport regulations. Packed in suitable containers to prevent breakage and leakage, ensuring safe transit.
Storage	(1S,3Ar,6As)-Ethyl Octahydrocyclopenta[c]Pyrrole - 1 - Carboxylate Hydrochloride should be stored in a cool, dry place. Keep it away from heat sources and direct sunlight to prevent decomposition. Store in a tightly - sealed container to avoid moisture absorption and contamination. This ensures its chemical stability and integrity for future use.