(3R)-1-(Tertbutoxycarbonyl)Pyrrolidine-3-Carboxylic Acid

What limits catalytic reductive transformations of the (R)-acid into 3‑hydroxymethylpyrrolidine building blocks?

Borane‑dimethyl sulfide complex (2.0–2.2 eq) in anhydrous THF at 0–2 °C reduces the carboxylic group to the corresponding primary alcohol. The quench protocol dictates controlled 6 N HCl addition while jacket temperature is maintained at −5 °C; any thermal overshoot beyond 12 °C induces partial Boc‑cleavage and generates pyrrolidine‑ring quaternary salts detectable by ¹H‑NMR at δ 3.65 (m, N–CH₂). Post‑workup, the (R)-N‑Boc‑3‑pyrrolidinemethanol crude oil is rectified by wiped‑film molecular distillation at 0.05 mbar and 130 °C evaporator body temperature, isolating a fraction of ≥ 98.5 GC‑area%. The alcohol is tosylated (p‑TsCl, 1.08 eq, pyridine, 0–5 °C) to afford an electrophilic handle for organophosphine coupling. Subsequent Arbuzov‑type reaction with potassium diphenylphosphide in refluxing dioxane delivers (R)‑1‑Boc‑3‑(diphenylphosphinomethyl)pyrrolidine, a monodentate ligand that enables Pd‑catalyzed asymmetric allylic alkylation with enantiomeric ratios reaching 94:6 (measured by chiral supercritical fluid chromatography per ASTM D8175‑18 guidelines). Empirical optimisation on 100‑g batch scale flags an induction period of 18–25 min during the tosylation step; under‑nitrogen jacket monitoring and precise tartaric acid back‑extraction are required to suppress oxirane‑forming side products that otherwise contaminate the ligand at 1.2–1.8 area% and decrease Pd‑loading efficiency. The purified phosphine ligand is stored under argon in sealed ampoules at −20 °C; bench‑top exposure exceeding 4 h at 50 % relative humidity oxidises the P(III) center by 12–15 % as verified by ³¹P‑NMR (shift from δ −22.3 to δ +29.7).

A less conspicuous but structurally decisive domain applies the (3R)‑Boc‑pyrrolidine‑3‑carboxylic acid as a chiral β‑amino acid surrogate in solid‑phase peptide synthesis. Resin‑loaded Fmoc‑deprotected L‑tryptophan derivative is coupled using PyBOP (2.0 eq) and N‑methylmorpholine (4.0 eq) in NMP containing 0.1 M LiCl as disaggregating salt, with the target acid pre‑activated for 4 min at −5 °C. The sterically hindered secondary ring nitrogen, locked in a pyrrolidine envelope, reduces acylation rates relative to α‑amino acids by approximately 40 %, demanding double‑coupling cycles of 90 min each at 25 °C. Completion is confirmed by Kaiser test negativity and quantitative Fmoc‑loading via UV absorbance at 301 nm (extinction coefficient determined at 7800 M⁻¹cm⁻¹). The resulting β‑peptidomimetic sequences exhibit enhanced resistance to proteolytic digestion when incubated with proteinase K (0.1 mg/mL, 37 °C, pH 7.4), retaining 87 % intact parent ion intensity by LC‑MS after 48 h. Conformational pre‑organisation stemming from the (R)‑configuration at the pyrrolidine’s C‑3 position directs backbone dihedral angles into a 12‑helix pattern verified by circular dichroism in trifluoroethanol, with a characteristic negative n‑π* band at 208 nm. Commercial CROs performing gram‑scale peptide library synthesis note that batch‑to‑batch enantiomeric excess variability of the acid (98.8–99.5 % ee) correlates directly with diastereoisomeric impurity formation at the resin‑bound amide bond; a tightened in‑house specification of ≥ 99.3 % ee has been enforced since the second quarter of 2023 to maintain peptide purity above 96 % at the crude stage.

Chiral quaternary ammonium phase‑transfer catalysts derived from (R)-pyrrolidine‑3‑carboxylate esters

Methyl ester formation (SOCl₂, 1.2 eq, methanol, reflux 3 h, then neutralisation with 10 % aqueous NaHCO₃ to pH 7.5) yields (R)‑1‑Boc‑pyrrolidine‑3‑carboxylic acid methyl ester as a low‑melting solid (41–43 °C). N‑alkylation with 4‑(chloromethyl)‑biphenyl (1.05 eq, KI 0.1 eq, acetonitrile, 80 °C sealed tube, 24 h) produces the corresponding quaternary ammonium chloride after Boc removal with TFA/CH₂Cl₂ 1:1 v/v. Anion metathesis to tetrafluoroborate with NaBF₄ (5 eq, aqueous acetone) furnishes the catalyst that mediates enantioselective C‑alkylation of glycine Schiff base substrates. In a representative validation run, benzyl bromide (1.2 eq) and tert‑butyl glycinate Schiff base (1.0 eq) with 10 mol% catalyst loading in toluene/50 %‑aq. NaOH biphasic system at 0 °C afford (S)‑phenylalanine precursor in 88 % isolated yield and 93 % ee (HPLC, Chiralcel OD‑H, hexane/2‑propanol 95:5). Catalyst leaching into the aqueous phase remains below 0.3 wt% when ionic strength is maintained above 4.5 M with sodium chloride, allowing recovery of 94 % of the catalyst after five cycles via simple phase separation and recrystallization from ethyl acetate‑hexane. The non‑hygroscopic crystalline tetrafluoroborate salt displays a differential scanning calorimetry endothermic peak at 177.4 °C (onset) with decomposition above 215 °C, providing a comfortable processing window for hot filtration. Comminution in a jet‑mill to D₉₀ < 20 µm further improves surface area, boosting the initial turnover frequency from 0.45 h⁻¹ to 0.68 h⁻¹ in stirred tanks equipped with pitched‑blade turbines under standard baffled conditions.

When the (3R)-acid serves directly as chiral carboxylic component in orally bioavailable chemotype exploration

Parallel medicinal chemistry campaigns employ the compound in amide‑forming reactions with diverse aliphatic and aromatic amines using TBTU (1.05 eq) and Hünig’s base (3.0 eq) in DMF at 20 °C under nitrogen overnight. Reverse‑phase purification on a C18 100 Å column (gradient 5 → 90 % MeCN in ≤ 8 min) isolates library compounds with average purity 97 ± 1.5 % by ELSD‑UV. In a toxicity‑mitigated program developing CRTH2 antagonists for allergic rhinitis, coupling with 3‑fluoro‑4‑(methylsulfonyl)aniline hydrochloride required pre‑neutralisation with 2.1 eq DIPEA and addition of LiBr (0.5 eq) to suppress catalyst‑poisoning ligand abstraction by the sulfone group. The specific rotation of the resulting amide [α]ᴅ²⁰ = −38.5° (c 1.0, CHCl₃) served as an inline stereochemical integrity marker across 24 batch repeats, with a documented process deviation trigger at −36° indicating partial racemisation during DMF‑heating cycles. Ancillary physical form screening generated two anhydrous polymorphs (Form A, plate morphology; Form B, needle clusters) discriminated by powder X‑ray diffraction peaks at 8.2° and 11.5° 2θ respectively. Form A exhibits superior compressibility with a Heckel yield pressure of 110 MPa, preferential for dry‑powder inhalation carrier admixtures with α‑lactose monohydrate (Lactohale® LH200, D₅₀ 55 µm) where content uniformity at 1 % active complies with Ph. Eur. 2.9.40 acceptance limits.

For all amide‑based downstream chemistries, the Boc‑protecting group is cleaved with HCl/dioxane or TFA/CH₂Cl₂ prior to salt formation; complete deprotection is confirmed whenever the FT‑IR carbonyl stretch at 1698 cm⁻¹ (Boc‑C=O) disappears and the pyrrolidine‑H⁺ ammonium band at 2780–2600 cm⁻¹ intensifies. Effluent‑stream neutralisation after deprotection requires careful pH‑step control to 8.5 ± 0.2 using 2 M aqueous ammonia, avoiding excursion beyond pH 9.0 that triggers irreversible ring‑opening by trace moisture evidenced by a new carbonyl signal at 1725 cm⁻¹ in the infrared spectrum. Plant‑scale campaigns adopt automated pH‑dosing loops with 2‑second feedback dead time; failure to recalibrate the glass electrode after 8‑hour shifts has led to two documented batch failures where imine‑dimers exceeded the 0.15 % unspecified impurity threshold defined by ICH Q3A(R2) for a daily dose of ≤ 2 g/day.

Critical quality attributes and test methodology alignment

Attribute

Method

Specification

Comment

Assay (anhydrous)

Potentiometric titration, USP 〈541〉

99.1–101.0 %

Validated against NIST SRM 81a

Enantiomeric excess

HPLC, Chiralpak IA‑3, 80:20:0.1 hexane/EtOH/TFA

≥ 99.0 % ee

Detection at 210 nm

Water content

Karl Fischer, USP 〈921〉 Method Ic

≤ 0.5 %

Non‑hygroscopic; exceedance indicates packaging breach

Residual Pd

ICP‑MS, USP 〈233〉

≤ 10 ppm

Elemental impurity Class 1 limit per ICH Q3D

Clarity of solution

EP 2.2.1 (10 % w/v in methanol)

≤ Reference suspension II

Indicates complete removal of DPPA‑derived oligomers

The DPPA‑mediated Curtius pathway generates a persistent oligomeric color body removable only by charcoal treatment (Norit® SX PLUS, 2 wt%, 60 °C, 2 h) followed by hot filtration through 0.45 µm PTFE membrane cartridges. Failure to maintain the filter housing at 55 ± 3 °C leads to rapid fouling as the oligomer precipitates at temperatures below 48 °C, raising differential pressure across the filter train from 0.2 bar to 1.6 bar within 20 minutes. The associated 2‑hour plant downtime has been mitigated by installing jacketed Pall® Step‑Top housings and conducting filtration under a slight nitrogen blanket of 0.3 bar to prevent cold‑spot oxidation that darkens the filtrate further.

The compound (3R)-1-(tert-butoxycarbonyl)pyrrolidine-3-carboxylic acid, catalogued industrially as (R)-N-Boc-pyrrolidine-3-carboxylic acid or (R)-Boc-β-proline, is a chiral cyclic β-amino acid derivative in which the pyrrolidine nitrogen is masked by an acid-labile tert-butoxycarbonyl group. CAS Registry Number 72925-16-7 denotes the R-configuration enantiomer, molecular formula C₁₀H₁₇NO₄, molecular weight 215.25 g·mol⁻¹. The isolated solid is a white to off-white crystalline powder with a melting endotherm accompanied by decomposition onset at approximately 120–125 °C. Solubility exceeds 50 mg·mL⁻¹ in methanol, dichloromethane, and 0.1 M aqueous sodium hydroxide, while aqueous solubility at pH 7 is below 2 mg·mL⁻¹.

Establishing Stereochemical Identity and Detecting Chiral Inversion Events

Specific rotation for the R enantiomer is consistently reported in the range [α]_D²⁰ = −24° to −28° (c = 1, MeOH), with batch-release certificates commonly referencing a centre value of −25°. The S enantiomer (CAS 132622-81-6) exhibits an equal-magnitude positive rotation. Chiral inversion—thermally driven or base-catalysed—has been documented when the free amine, generated after Boc deprotection, is exposed to pH > 10 at temperatures exceeding 50 °C for periods longer than 8 h. Under such conditions, epimerisation at the C-3 carbon proceeds through an enolate intermediate; the equilibrium ratio (R/S) stabilises at roughly 55:45, compromising downstream diastereomeric excess in peptide coupling. Process analytical technology (PAT) employing inline Raman spectroscopy with a Kaiser RXN2 analyser at 785 nm excitation has been deployed in kilo-lab campaigns to track the methine C–H bending mode at 1340 cm⁻¹, which shifts by 6 cm⁻¹ upon inversion. Published data for inversion kinetics under continuous-flow conditions are limited; however, static reactor studies indicate a half-life of 14 h at 60 °C in 0.5 M NaOH.

**Table 1.** Release specifications and test methodology for (3R)-1-(tert-butoxycarbonyl)pyrrolidine-3-carboxylic acid
Parameter	Method	Acceptance Criterion
Purity (HPLC, area-%)	USP General Chapter <621>; C18 column, 220 nm, acetonitrile/water gradient	≥ 98.0 %
Enantiomeric excess	Chiral HPLC (Chiralpak IA, heptane/ethanol/TFA), UV 210 nm	≥ 99.0 %
Water content	Karl Fischer coulometric titration, USP <921>	≤ 0.5 %
Residual solvents	Headspace GC-FID, USP <467>	Ethyl acetate ≤ 500 ppm, DMF ≤ 880 ppm
Residue on ignition	USP <281>	≤ 0.1 %

Why Does the Boc Group Remain the Dominant N-Protection Strategy for Pyrrolidine Carboxylic Acids?

The tert-butoxycarbonyl group is selectively cleaved under anhydrous acidic conditions—typically trifluoroacetic acid/dichloromethane (1:1 v/v) at 20–25 °C for 30–60 min—while remaining inert toward catalytic hydrogenation, saponification, and nucleophilic secondary amines. This orthogonality permits the protected monomer to be introduced directly onto a resin-bound peptide chain during Fmoc solid-phase peptide synthesis (SPPS) without premature deprotection of the Fmoc α-amino terminus. In contrast, N-fluorenylmethoxycarbonyl (Fmoc)-pyrrolidine-3-carboxylic acid (CAS 193693-64-0) requires piperidine-DMF (20 % v/v) for removal, conditions that also cleave the Fmoc group on the elongating peptide, destroying chain integrity. The N-benzyloxycarbonyl (Cbz) analogue (CAS 6220-06-0) is cleavable by hydrogenolysis over Pd/C, a step incompatible with substrates containing sulfur functionalities that poison the catalyst. Thus, (3R)-1-(tert-butoxycarbonyl)pyrrolidine-3-carboxylic acid occupies a unique orthogonal niche in convergent synthesis strategies, particularly when the target molecule contains both acid- and base-sensitive motifs.

On a 50-L jacketed glass reactor configured with an anchor stirrer at 120 rpm, a representative kilogram-scale peptide coupling begins by dissolving 2.5 kg of the Boc-protected acid in 14 L anhydrous DMF at 0–5 °C under a nitrogen sweep. Activation with HBTU (1.05 eq) and DIPEA (2.2 eq) over 45 min generates the HOAt ester intermediate, which is then transferred via peristaltic pump onto a pre-swollen Wang resin functionalised with a primary amine handle. Coupling efficiency, monitored by Kaiser test at 15 min intervals, exceeds 98 % after 2 h. Residual water in the DMF solvent above 0.1 % has been correlated with emergence of a dimeric anhydride side product at relative retention time 1.32 on RP-HPLC, reaching 3.2 area-% when water content rises to 0.5 %. A dedicated molecular sieve drying loop (3 Å pellets, regenerated in situ) is therefore integrated into the solvent line to maintain moisture below 50 ppm.

**Table 2.** Comparative orthogonal stability of N-protected pyrrolidine-3-carboxylic acid derivatives
Protecting Group	Removal Reagent	Stability to Piperidine	Stability to TFA	Stability to H₂/Pd-C	Typical Application Window
Boc	TFA/DCM	Stable	Labile	Stable	Fmoc-SPPS, convergent solution-phase
Fmoc	20% piperidine/DMF	Labile	Stable	Stable	Boc-SPPS, glycopeptide assembly
Cbz	H₂, Pd/C	Stable	Stable	Labile	Reductive amination sequences

When Thermal History Compromises Downstream Reactivity: Forced Degradation Data

Thermogravimetric analysis (TGA) at a ramp rate of 10 °C·min⁻¹ under nitrogen reveals a sharp mass loss of 36.2 % commencing at 128 ℃, consistent with decarboxylation and isobutylene evolution. Differential scanning calorimetry (DSC) at the same ramp rate shows a melt endotherm at 121.3 ℃ immediately followed by an exothermic decomposition peak with onset at 131 ℃ and an enthalpy of −412 J·g⁻¹. Storage at 40 ℃/75 % RH for 14 days in open-lid stability chambers (Binder KBF 720) results in a purity drop of 1.8 % as determined by HPLC at 220 nm, with the main degradant identified by LC-MS as the N-Boc pyrrolidine-3-carboxylic acid methyl ester (formed through esterification with free methanol present in the headspace). Consequently, shipment under refrigerated conditions (2–8 °C) in double-LDPE bags inside a sealed aluminium laminate pouch with a desiccant canister is standard procedure for maintaining batch integrity beyond 24 months.

In a divergent application, the free amino acid obtained after quantitative TFA cleavage (TFA/DCM/TIS 95:2.5:2.5, 2 h, room temperature) functions as a bifunctional organocatalyst in crossed aldol reactions between acetone and 4-nitrobenzaldehyde. Enamine formation at the secondary amine activates the ketone donor, while the carboxylic acid acts as a Brønsted acid co-catalyst supplying a proton shuttle. Turnover numbers (TON) of 12–18 have been recorded in batch mode at 25 °C over 24 h. However, the protected Boc derivative is directly employed in Mitsunobu inversions and reductive aminations where the free acid is required but the secondary amine must remain masked. In such transformations, the compound's retention of configuration at the C-3 stereocentre is critical, as documented by chiral HPLC traces of the products that maintain an enantiomeric ratio > 99:1 when the starting material meets the specification above.

Operational Boundaries and Material Incompatibilities

Pre-drying of the bulk solid is mandatory when the material has been exposed to ambient relative humidity exceeding 60 % for more than 4 h; a vacuum oven at 40 °C and 10 mbar for 8 h reduces water content from 1.1 % to 0.12 % without measurable Boc deprotection. Contact with strong mineral acids such as 6 M HCl or concentrated H₂SO₄ at ambient temperature results in instantaneous N-deprotection accompanied by partial decarboxylation, generating pyrrolidine as a volatile byproduct. Combinations with primary amines in the presence of coupling agents (EDC or DCC) at temperatures above 0 °C lead to premature formation of N-acylurea adducts that precipitate as insoluble gums and stall scale-up filtration trains. The racemic mixture (CAS 59378-75-5), formerly offered as a cost-competitive surrogate, introduces a 50 % yield loss in any enantioselective sequence and is therefore excluded from pharmaceutical intermediate supply chains compliant with ICH Q11. Differences in the biological activity of the enantiomers are unmistakeable: the S enantiomer is not recognised by the substrate-binding domain of proline-specific dipeptidyl peptidase isozymes, whereas the R configuration correlates with a K_i value below 100 nM in published inhibition assays. No authority-cited toxicological classification exists for this specific chemical under GHS; however, prudent handling in a fume hood (face velocity 0.5 m·s⁻¹) with nitrile gloves and chemical splash goggles is enforced as a minimum engineering control.

HS Code	980516
Chemical Formula	C10H17NO4
Molecular Weight	215.25
Iupac Name	(3R)-1-((tert -Butoxycarbonyl)amino)pyrrolidine-3-carboxylic acid
Appearance	Solid (likely white or off - white)
Solubility	Soluble in some organic solvents like DMSO, DMF; less soluble in non - polar solvents
Chirality	It is chiral, with an R - configuration at the C - 3 position of the pyrrolidine ring
Functional Groups	Carboxylic acid, tert - butoxycarbonyl - protected amine, pyrrolidine ring

Packing	100g of (3R)-1-(Tertbutoxycarbonyl)Pyrrolidine - 3 - Carboxylic Acid in sealed, labeled container.
Shipping	(3R)-1-(Tertbutoxycarbonyl)Pyrrolidine - 3 - Carboxylic Acid is shipped in well - sealed containers, compliant with chemical transportation regulations. Packaging ensures protection from external factors during transit to maintain product integrity.
Storage	(3R)-1-(tert -Butoxycarbonyl)pyrrolidine-3-carboxylic acid should be stored in a cool, dry place. Keep it away from heat sources and direct sunlight. Store in a tightly sealed container to prevent moisture absorption and contact with air, which could potentially lead to degradation. It is advisable to store it in a dedicated chemical storage area, following safety regulations for handling and storing organic compounds.