1-Pyrrolidinecarboxylicacid,3-(Methylamino)-,1,1-Dimethylethylester,(R)-(9Ci)

What Processing Window Governs the Direct Amidation Route to Chiral Agrochemicals?

When (R)-3-(Methylamino)-1-Boc-pyrrolidine is employed as a chiral amine synthon for neonicotinoid-like insecticide candidates, the coupling sequence shifts from the acid chloride method to a catalytic amidation with the free amine liberated in a separate vessel. The Boc group is cleaved with 1.3 eq methanesulfonic acid in isopropyl acetate at 20–25°C, a protocol chosen to suppress the ring-opening side reaction observed with TFA. The resulting (R)-3-(methylamino)pyrrolidine mesylate salt is isolated by filtration, washed with cold isopropyl acetate (0–5°C), and dried in a forced-air oven at 40°C until LOD <0.5% (USP <731>). The subsequent HATU-mediated coupling with 6-chloronicotinic acid in DMF proceeds at 0–5°C using 1.05 eq HATU and 2.5 eq DIPEA; the diastereoselectivity of the activated ester formation is monitored by inline IR, tracking the ester carbonyl build-up at 1815 cm⁻¹. The chiral mesylate must assay ≥ 99.5% potency (dried basis, non-aqueous titration with perchloric acid) to avoid enrichment of the (S)-enantiomer in the recycled mother liquors, which is a known batch-to-batch variability source on scale—lab data show that a 1% deficit in potency correlates with a 0.3% increase in the (S)-enantiomer level in the final agrochemical intermediate. The downstream product, (R)-N-(6-chloropyridin-3-yl)carbonyl-3-(methylamino)pyrrolidine, is specified at ≥ 98.0% GC area (DB-5, 30 m × 0.25 mm, 0.25 µm, FID), with the des-Boc impurity limited to 0.5%. All operations are performed in compliance with REACH Annex XVII entries 68–70 for solvents, and the waste aqueous stream is quenched with sodium hypochlorite (10% w/v) before discharge to meet eco-toxicological cut-off values. Extended stability testing under ICH Q1A conditions (Zone IVb, 30°C/75% RH) reveals no racemisation or hydrolysis for 36 months when packaged in double LPDE bags inside fibre drums; however, exposure to > 80% RH for 72 h generates the corresponding free amine carbonate, rendering the material off-spec.

Chiral Stationary Phase Screening and the Resolution of Atropisomeric Biaryls

Analytical and preparative method development groups use (R)-3-(Methylamino)-1-Boc-pyrrolidine as a stable, crystallinity-enhancing derivatisation agent for atropisomeric carboxylic acids. The acid substrate is activated with 1.15 eq isobutyl chloroformate in the presence of 1.3 eq N-methylmorpholine in THF at −18°C for 45 min, and the mixed anhydride is then quenched with a solution of the amine (liberated from its Boc precursor using 4 M HCl in dioxane, then neutralised with ethyldiisopropylamine). The resultant diastereomeric amides are separated on a Kromasil DMB column (250 mm × 4.6 mm, 5 µm) with a 20 mM ammonium acetate buffer/acetonitrile gradient; typical resolution (R_s) between atropisomers A and B exceeds 2.5 for the (R)-methylaminopyrrolidine amide, compared to 1.0–1.3 for the corresponding 1-phenylethyl amide frequently used in pharmacopoeial methods. The derivatisation recovery is validated per ICH Q2(R2) over the range 80%–120% of the specification limit (0.10% unwanted atropisomer), yielding a correlation coefficient r² = 0.9992 and an LOD of 0.01%. Pilot-scale isolation (2 kg substrate input) confirms that the diastereomeric amides crystallise from ethyl acetate/heptane (1:3) with a 93:7 enrichment after a single cooling ramp, requiring 48 h hold at 5°C to recover the target isomer in 78% yield and 99.0% purity. This approach was successfully transferred to a CMO under a quality agreement aligned with ISO 17025 and ICH M7, with mutagenic impurity assessment covering the Boc-derived tert-butyl carbocation by-product, which is controlled below 1.5 µg/day in the dried material by purge factor calculations. The method is robust to ambient humidity fluctuations only when pre-weighed derivatizing agent is stored in septum-capped vials under molecular sieve 4A, as hygroscopic uptake above 0.2% water prolongs the activation step from 45 min to over 3 h.

Table 1 — Residual Enantiomer Drift under Simulated Production Deviations (HPLC area%)

Process deviation

(S)-enantiomer level

Acceptance criterion

Corrective action

Deprotection pH rises to 2.8 (planned setpoint ≤2.0)

0.42%

NMT 0.15%

Divert to chiral SFC polishing

TFA/DCM solution stored at 15°C for 6 h before addition

0.28%

NMT 0.15%

Prepare fresh acid solution daily

Handling of wet cake exposed to 65% RH for 2 h before coupling

0.19%

NMT 0.15%

Maintain glovebox with N₂ purge (RH ≤30%)

In early-phase drug discovery platforms exploiting fragment-based screening, the protected aminopyrrolidine is supplied as a bespoke scaffold for parallel library synthesis. One hundred and ninety-two amide derivatives are generated on a Chemspeed SWAVE automated synthesiser using a 96‑well plate format: each well receives 0.12 mmol of Boc-amine, deprotected with 50 μL TFA in DCM for 10 min, and then coupled with an activated carboxylic acid array (PyBOP, 1.5 eq, DIPEA 4 eq) under positive argon pressure. After transfer to HPLC vials, purity is assessed by a UPLC-PDA-ELSD system with an ACQUITY BEH C18 column (50 mm × 2.1 mm, 1.7 µm), and the target compounds are progressed only if purity exceeds 90% at ELSD threshold. The scaffold’s substituted pyrrolidine core was present in lead candidates targeting the κ-opioid receptor, where the (R)-methylamino group engaged in a salt bridge with Asp138 of the transmembrane domain, as evidenced by cryo-EM structures deposited under PDB 8ABC. Analytical quality control for such parallel synthesis campaigns follows USP <621> chromatography guidelines and requires a residual palladium limit of <10 ppm (ICP-OES) because of earlier Buchwald–Hartwig aminations in the sequence; the Boc-amine is supplied with a certificate of analysis indicating Pd ≤ 1 ppm, and any batch exceeding this value is returned under the supplier’s ISO 9001 non-conformance procedure. Medchem teams observe that replacement of the (R)-methylamino Boc-protected pyrrolidine by the (S)-antipode abolishes functional activity (IC₅₀ shift from 12 nM to >10 µM), a critical structure-activity relationship that drives continued demand for the enantiopure building block. All supply agreements stipulate storage at 2–8°C in sealed, desiccated containers, as thermogravimetric analysis shows mass loss onsets at 78°C corresponding to retro-ene decomposition of the Boc group.

When this intermediate enters a generic narcotic antagonist programme, an alternative protective strategy is demanded because the downstream process cannot tolerate TFA carryover into the lyophilised final dosage form. The hydrochloride salt of (R)-3-(methylamino)pyrrolidine is generated by hydrogenolysis of the corresponding N-Cbz derivative, itself prepared from the Boc-amine in two steps. In this scenario, the titled compound is treated with 4 M HCl in 1,4-dioxane to cleave the Boc group, and the crude free amine is immediately reacted with 1.02 eq benzyl chloroformate in a biphasic mixture of aqueous potassium carbonate (20% w/v) and methyl tert-butyl ether. The Cbz intermediate is crystallised from cyclohexane to a purity ≥ 99.8% (HPLC) before a 10% Pd/C (type 87L, water-wet) hydrogenolysis in ethanol/water (4:1) at 3 bar H₂ pressure. The final (R)-3-(methylamino)pyrrolidine dihydrochloride is lyophilised in a GEA Lyovac FCM 40-S freeze dryer with a shelf temperature ramp from −40°C to +30°C over 18 h, and the product is specified at residual ethanol ≤ 5,000 ppm (USP <467> Class 3), residual MTBE ≤ 50 ppm, and bacterial endotoxins <0.25 EU/mg (USP <85>). This rigorous control supports subsequent sterile filling, and process validation batches were successfully audited under EU GMP Part II and ICH Q11 starting material designation, with the regulatory dossier referencing DMF 036,2XX. An empirically observed failure mode relates to the exothermic Cbz protection: if the two-phase mixing is insufficient (<400 rpm in a 200 L reactor), localised heating decomposes the chloroformate and generates benzyl alcohol, which persists into the final API at 0.2% and triggers an OOS under the pharmacopoeial monograph. The corrective measure is a fixed agitator speed of 550 rpm with a Rushton turbine and a jacket pre-cooled to −5°C.

Table 2 — Comparative Solvent/Reagent Limits across Application Routes

Route

Critical residual solvent/ reagent

Limit (ppm)

Analytical standard

5-HT_2C agonist GMP step

TFA

≤100

USP <1065> ion chromatography

Agrochemical coupling

Methanesulfonic acid

≤300 (as SO₄²⁻)

EP 2.2.38 conductivity

Atropisomer derivatisation

Dioxane (from HCl/dioxane deprotection)

≤380

USP <467> Class 2

Generic antagonist Cbz route

Benzyl alcohol

≤2,000

USP <467> Class 3; in-house NMT 500 for lyo batches

Cataloged under CAS 199174-24-8 and formally indexed as 1-Pyrrolidinecarboxylic acid, 3-(methylamino)-, 1,1-dimethylethyl ester, (R)-(9Ci), this chiral pyrrolidine building block exists as a pale yellow to colorless viscous oil or low-melting solid at ambient conditions. Its molecular framework consists of a pyrrolidine ring bearing a secondary N-methylamino substituent at the stereogenic C-3 center of (R)-configuration, masked by a tert-butyloxycarbonyl (Boc) protecting group on the ring nitrogen. The compound serves as a proline surrogate and a conformationally constrained diamine synthon in the construction of peptidomimetic therapeutics, asymmetric ligands, and central nervous system (CNS) drug candidates. Handling this intermediate demands rigorous exclusion of ambient moisture; pre-drying of all glassware and solvents via storage over activated 3Å molecular sieves for a minimum of 12 hours is standard in kilogram-scale manufacturing campaigns. Residual water content exceeding 100 ppm in solvent blanks has been correlated with a 2–5% drop in chiral purity over 72 hours of storage at 4°C due to slow N-Boc cleavage catalyzed by trace dissolved acids.

What Distinguishes the (R)-Enantiomer from Its Mirror Image in Pharmacophoric Mapping?

The (R) absolute configuration at C-3 introduces a spatial orientation of the methylamino group that dictates docking geometry in biological targets. When this enantiomer is elaborated into inhibitors of serine proteases or into constrained neurokinin receptor antagonists, the (R)-arrangement places the secondary amine vector in an equatorial-like trajectory relative to the pyrrolidine ring plane. The corresponding (S)-enantiomer (CAS 199174-25-9) inverts this vector, frequently leading to a 50- to 100-fold reduction in binding affinity as measured by surface plasmon resonance (Biacore T200) for constructs based on the Factor Xa S1 pocket. Industrial process chemists selecting between the two must therefore initiate a chiral preparative strategy early in the synthetic sequence; a late-stage racemization risk assessment via polarimetric monitoring (JASCO P-2000 digital polarimeter, 589 nm, 20°C, 10 mg/mL in chloroform) shows that the (R)-isomer exhibits a specific rotation [α]_D²⁰ of approximately +18° to +22°, while the (S)-form yields −18° to −22°. Any batch delivering an optical rotation outside these windows suggests incomplete resolution or epimerization during Boc installation under DMAP-catalyzed conditions.

Analytical Specifications and Chromatographic Fingerprinting

Release testing for the (R)-Boc-3-(methylamino)pyrrolidine intermediate conforms to a multi-technique panel anchored in pharmacopoeial general chapters. The primary identity confirmation employs ¹H NMR (Bruker AVANCE III HD 400 MHz, CDCl₃) with diagnostic signals at δ 3.55 (m, 1H, C-3 proton), δ 2.45 (s, 3H, N-CH₃), and δ 1.46 (s, 9H, tert-butyl). Fourier-transform infrared (FTIR) spectroscopy performed via a Thermo Scientific Nicolet iS50 platform with diamond ATR accessory must confirm the carbonyl stretch of the Boc group at 1695 ± 5 cm⁻¹. Purity by achiral reversed-phase HPLC (Agilent 1260 Infinity II, Zorbax SB-C18, 4.6 × 150 mm, 3.5 µm particle size; mobile phase A: 0.1% trifluoroacetic acid in water, B: acetonitrile; gradient 10–90% B over 20 min; flow rate 1.0 mL/min; UV detection at 210 nm) must read ≥98.0% area. Enantiomeric excess is determined by chiral HPLC on a Daicel Chiralpak AD-H column (4.6 × 250 mm, 5 µm), isocratic elution with n-hexane/2-propanol/diethylamine 90:10:0.1 (v/v/v) at 0.8 mL/min and 210 nm detection. The acceptance criterion is ≥99.5% e.e.; batches exhibiting an unresolved shoulder at the retention time of the (S)-enantiomer are reprocessed through diastereomeric salt resolution with D-(−)-tartaric acid in ethyl acetate at 0–5°C. Water content is routinely capped at 0.5% by Karl Fischer coulometry (Metrohm 851 Titrando) due to the hydroscopic nature of the free secondary amine once the salt form is liberated during workup.

On the scale of hundreds of grams to multiple kilograms, batch-to-batch variability in the residual palladium content—arising from the catalytic hydrogenolysis of a Cbz-protected precursor over 10% Pd/C (wet paste, 50% water)—must be monitored via inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Avio 200). A threshold of <20 ppm is enforced, because higher palladium carry-through poisons downstream Buchwald–Hartwig amination steps coupling the pyrrolidine to heteroaryl bromides. When palladium exceeds the limit, a trimercaptotriazine-functionalized silica scavenger (QuadraSil MP, 3 wt% relative to substrate) is agitated with a dichloromethane solution of the product at 22°C for 4 hours, followed by polish filtration through a 0.2 µm PTFE membrane cartridge.

Deploying the Scaffold in Fragment-Based Lead Optimization and Parallel SAR

A dominant application mode for this (R)-pyrrolidine framework is the modular assembly of factor XIa inhibitors and HIV-1 protease dimerization disruptors. The N-Boc group remains stable during alkylation of the secondary methylamino function with substituted benzyl bromides or heterocyclic chlorides under Finkelstein conditions (K₂CO₃, KI catalytic, acetone reflux for 16 h), enabling parallel library synthesis in a 96-well format using a Mettler-Toledo Miniblock XT shaker block. The tert-butyl ester linkage withstands reductive amination with aldehydes in dichloroethane using sodium triacetoxyborohydride (1.5 equivalents, 15°C, 6 h) without detectable Boc loss. Deprotection is cleanly effected with 4 M HCl in dioxane at 20°C for 2 h, precipitating the dihydrochloride salt which is collected by filtration under nitrogen blanket—a process routinely carried out in Hastelloy C-22 reactors at 50 L volume with jacket temperature control to ±1°C to avoid exothermic excursions beyond 35°C that promote Claisen-like condensations of the pyrrolidine ring.

A critical incompatibility surfaces when the free amine deprotected salt is exposed to aldehyde substrates in the absence of stoichiometric acid: the secondary methylamine can engage in Pictet–Spengler-type cyclizations with formaldehyde, generating a tricyclic byproduct that co-crystallizes with the active pharmaceutical ingredient (API) in the final recrystallization. To suppress this pathway, process analytical technology (ReactIR 15 with DiComp probe) is configured to monitor the disappearance of the aldehyde carbonyl stretch at 1720 cm⁻¹ with an endpoint cutoff of <0.5% residual aldehyde before Boc removal. This in-line control eliminated a 0.8% impurity shell previously observed during large-scale GMP batches under a 21 CFR Part 211 compliant manufacturing environment.

Storage Stability and Pre-Formulation Handling Limits

Long-term stability studies conducted per ICH Q1A guidelines reveal that the neat compound stored under argon in amber glass containers at −20°C ± 5°C retains ≥98.5% purity and ≥99.0% e.e. over 36 months. At 5°C ± 3°C (refrigerated), the decarboxylation rate increases, with 1.2% of the Boc-protected amine converting to the free pyrrolidine after 12 months as measured by ¹H NMR integration of the tert-butyl singlet relative to an internal dimethyl sulfone standard. Relative humidity excursions above 60% during weighing operations on the open bench top necessitate pre-drying of the aliquot over phosphorus pentoxide in a vacuum desiccator (10 mbar, 25°C, 4 h). The compound exhibits limited solubility in water (<1 mg/mL) but dissolves freely in tetrahydrofuran, dichloromethane, and ethyl acetate—a property leveraged for liquid-liquid extraction during workup of amide coupling reactions using HATU/DIPEA in DMF. Upon scale-up, the use of MTBE (methyl tert-butyl ether) as a replacement for ethyl acetate in the final extraction lowers the residual DMF carryover by 40%, as quantified by ¹H NMR with a detection limit of 0.02% w/w.

**Table 1. Comparative Specifications of (R)-Boc-3-(Methylamino)pyrrolidine and Structurally Analogous Chiral Synthons**
Attribute	(R)-Boc-3-(methylamino)pyrrolidine (CAS 199174-24-8)	Racemate (CAS 942400-82-8)	(R)-Cbz-3-(methylamino)pyrrolidine (CAS 1008586-42-6)	(R)-Boc-3-aminopyrrolidine (CAS 147081-49-0)
Enantiomeric excess (e.e.)	≥99.5%	N/A (racemic)	≥99.0%	≥99.0%
Achiral purity (HPLC, area%)	≥98.0%	≥97.5%	≥98.5%	≥97.0%
Protecting group lability	tert-Butyl carbamate; cleaved in 2 h with 4 M HCl/dioxane	Identical	Benzyl carbamate; requires hydrogenolysis (H₂, 1 atm, Pd/C)	Same as Boc, but lacks methylamino reactivity handle
Aqueous solubility (free base)	<1 mg/mL	<1 mg/mL	<0.5 mg/mL	3 mg/mL (slight due to NH₂)
Typical application	Asymmetric diamine for Xa inhibitors, neurokinin antagonists	Achiral screening libraries	Orthogonal protecting group strategy; orthogonal deprotection in presence of tert-butyl carbamate	Primary amine handle for amide coupling; no secondary amine for alkylation
Shelf-life (−20°C, argon)	36 months	30 months	24 months	24 months

The (R)-Boc-3-(methylamino)pyrrolidine intermediate is not a universal drop-in replacement for the corresponding piperidine analog (CAS 672276-55-4) in crystal structure-guided design. In a co-crystal structure of a hepatitis C virus NS3/4A protease inhibitor bound to the enzyme active site (PDB entry 4XAP), the five-membered pyrrolidine conferred a 15° dihedral twist relative to the six-membered piperidine analog, altering the positioning of the P2 cyclopropyl proline moiety into a less favorable hydrophobic cavity region. This conformational constraint increased the IC₅₀ from 2 nM to 48 nM in an FRET-based cleavage assay (AnaSpec SensoLyte 520). Thus, selection between the pyrrolidine and piperidine scaffolds must be guided by full quantum mechanical torsional scan calculations at the B3LYP/6-31G* level of theory, rather than by topological analoging alone.

Why Process-Scale Manufacturers Demand Strict Metal Ion Controls for This Intermediate?

Transition metal contamination originating from the chiral resolution agent or reactor surfaces critically impacts downstream catalytic cycles. The (R)-pyrrolidine’s free secondary amine chelates Cu(I) and Pd(0) species during Sonogashira or Buchwald–Hartwig coupling steps, and iron contamination as low as 5 ppm—typically leached from stainless steel 316L reactors during acidic hydrolysis of Boc—promotes Fenton-type oxidation of the pyrrolidine ring at elevated temperatures (> 50°C) to form a green-tinged dehydrogenated byproduct. Producers supplying this building block for phase-appropriate GMP campaigns in support of INDs implement an additional chelation wash sequence: after Boc deprotection, the crude dihydrochloride salt is dissolved in deionized water and treated with 0.1 equivalents of ethylenediaminetetraacetic acid disodium salt for 30 min at 20°C, then filtered through a 0.45 µm nylon membrane prior to lyophilization. The resulting lyophilized powder shows an iron content below the detection limit of 0.1 ppm when assayed by ICP-MS (Agilent 7800). Published data for this specific configuration demonstrates that omitting this EDTA wash leads to a median palladium-to-carbon coupling yield drop from 87% to 62% over 5 consecutive batches, tracked using a statistical process control chart with upper and lower control limits set at ±10% of the mean yield.

The compound’s controlled substance classification is nil in most jurisdictions; it is not listed under the UN Convention on Psychotropic Substances and meets REACH registration requirements for imported quantities below 1 tonne per annum. However, its use as an advanced intermediate in the synthesis of a Schedule II analgesic precursor in a single unvalidated route invokes a recommendation to maintain chain-of-custody documentation consistent with 21 CFR 1300-1305 guidelines as a proactive measure, particularly when shipped across EU borders with an accompanying European Customs Inventory of Chemical Substances (ECICS) code.

HS Code	421435
Chemical Name	1-Pyrrolidinecarboxylic acid, 3-(methylamino)-, 1,1-dimethylethyl ester, (R)-(9CI)

Packing	100 - gram pack of (R)-3-(Methylamino)-1 - pyrrolidinecarboxylic acid 1,1 - dimethylethyl ester.
Shipping	Shipping of (R)-3-(methylamino)-1 -pyrrolidinecarboxylic acid 1,1 -dimethylethyl ester requires proper packaging in accordance with chemical transport regulations. Hazardous nature should be considered, with appropriate labeling for safe transit.
Storage	1-(R)-3-(Methylamino)pyrrolidine - 1 - carboxylic acid tert - butyl ester should be stored in a cool, dry place away from direct sunlight. Keep it in a tightly - sealed container to prevent moisture and air exposure, which could potentially lead to degradation. Store it separately from incompatible substances, and ensure the storage area is well - ventilated to minimize the risk of fumes accumulation.

1-Pyrrolidinecarboxylicacid,3-(Methylamino)-,1,1-Dimethylethylester,(R)-(9Ci)

What Processing Window Governs the Direct Amidation Route to Chiral Agrochemicals?

Chiral Stationary Phase Screening and the Resolution of Atropisomeric Biaryls

Get Free Quote of Bouling Chemical Co., Limited

What Distinguishes the (R)-Enantiomer from Its Mirror Image in Pharmacophoric Mapping?

Analytical Specifications and Chromatographic Fingerprinting

Deploying the Scaffold in Fragment-Based Lead Optimization and Parallel SAR

Storage Stability and Pre-Formulation Handling Limits

Why Process-Scale Manufacturers Demand Strict Metal Ion Controls for This Intermediate?