3-Hydroxymethyl-Pyrrolidine-1-Carboxylic Acid Ter-Butyl-Ester

Table 1 — Comparative Activation Performance of N-Boc-3-hydroxymethylpyrrolidine in Pilot-Plant Batches

Activation Method

Reagent (eq)

Solvent System

Isolated Yield (%)

HPLC Purity (area% at 210 nm)

Main Impurity

Sulfonate — MsCl

1.15 eq MsCl, 1.40 eq NMM

DCM, -5°C

92–96 (solution assay)

98.5–99.1

elimination β-pyrroline (0.15%)

Sulfonate — TsCl

1.25 eq TsCl, 2.10 eq pyridine

DCM, 0°C

88–93 (isolated solid)

97.0–98.4

3-chloromethyl byproduct (0.3–0.8%)

Succinimidyl Carbonate — DSC

1.50 eq DSC, 0.05 eq DMAP

MeCN, 25°C

78–85 (chromatographed)

98.8–99.6

N-hydroxysuccinimide (0.2%)

What Limits Batch Reproducibility in Succinimidyl Carbonate Activation?

Preparation of a heterobifunctional linker for an antibody-drug conjugate platform frequently employs N,N′-disuccinimidyl carbonate (DSC) to convert the sterically congested hydroxymethyl group of N-Boc-3-hydroxymethylpyrrolidine into a reactive succinimidyl carbonate ester. The reaction is performed in anhydrous acetonitrile (water content <300 ppm by Karl Fischer) under a positive pressure of argon within a 200 L glass-lined reactor. DSC (1.50 eq) and 4-dimethylaminopyridine (0.05 eq) are charged at 22±2°C, and the progression of carbonate formation is monitored by ¹³C NMR — the carbonyl resonance shifts from 155.7 ppm (Boc C=O) to a new peak at 151.2 ppm (carbonate C=O). After 6–8 h, the mixture is filtered through a bed of Celite ^® 545 to remove precipitated N-hydroxysuccinimide, and the filtrate is concentrated on a rotary evaporator with bath temperature not exceeding 30°C to prevent thermal reversion to the symmetric urea.

Critical batch-to-batch variance originates from residual water in the starting material. When the water level surpasses 800 ppm, DSC is hydrolytically consumed, creating a deficit that leaves 3–8% of the alcohol unreacted. This necessitates a second addition of 0.30 eq DSC and extends cycle time by 4 h, which in turn increases the formation of the ring-opened N-(hydroxycarbonyloxy)succinimide dimer impurity (M ⁺ +256 Da). Process robustness is further challenged by residual DMAP, a genotoxic impurity that must be reduced to <500 ppm in the isolated linker intermediate. A tandem wash protocol employing 0.5 N HCl (pre-cooled to 5°C) followed by 20% w/v sodium chloride limits DMAP carryover without inducing noticeable Boc loss — exposure time to the aqueous acid is restricted to 90 seconds per wash cycle. The resulting carbonate ester is stored under argon in amber glass bottles at -20°C, and its identity is confirmed by IR (carbonate C=O stretch at 1758 cm⁻¹) and ESI-MS ([M+Na] ⁺ calc. 323.12). The compound serves as an amine-reactive module in a cathepsin B-cleavable linker system regulated under FDA’s 21 CFR Part 211 for injectable biologic conjugates, and the genotoxic impurity control strategy follows the staged TTC approach described in ICH M7(R1).

Conversion of the hydroxyl group to a mesylate, followed by substitution with potassium diphenylphosphide, furnishes a chiral phosphine intermediate that, after Boc deprotection and condensation with 2-hydroxybenzaldehyde, yields a P,N-ligand applied in palladium-catalyzed asymmetric allylic alkylation. The mesylation is run identically to the previously described protocol but on a 50–100 mmol laboratory scale within a nitrogen-filled glovebox (O₂ < 10 ppm, H₂O < 5 ppm). The mesylate solution in anhydrous tetrahydrofuran is transferred via cannula to a solution of KPPh₂ (1.30 eq, 0.5 M in THF) containing 18-crown-6 (0.15 eq) at -20°C. The temperature must be rigorously maintained below -15°C throughout the addition; excursion to 0°C causes β-elimination to the 3-methylene pyrrolidine carbamate, detected by GC-MS as m/z 213.1. After 6 h, the mixture is quenched with degassed methanol at -10°C and filtered through neutral alumina (activity grade II) to remove potassium salts. The crude phosphine oxide is purified by flash chromatography on silica gel (EtOAc/hexane 1:4) to deliver the N-Boc-protected phosphine in 82–89% yield and 99.5% ee as determined by chiral HPLC (Chiralpak ^® AD-H, hexane/IPA 90:10, 1.0 mL/min).

Removal of the Boc group with 4.0 M HCl in 1,4-dioxane at 0°C for 2 h, followed by neutralization and immediate reaction with 1.05 eq of 2-hydroxybenzaldehyde in ethanol at reflux, installs the imine framework. The final ligand is typically stored as a toluene solution under argon to inhibit oxidation of the phosphine to phosphine oxide. ³¹P NMR (CDCl₃) shows a single resonance at -15.4 ppm, and complexation with [Pd(C₃H₅)Cl]₂ in degassed THF yields the pre-catalyst with a diagnostic allyl proton signal at 5.36 ppm (dd, ³J = 12.5 Hz, 6.8 Hz). The ligand system has been evaluated in the enantioselective alkylation of racemic 1,3-diphenyl-2-propenyl acetate with dimethyl malonate, giving isolated yields of 94–96% and enantiomeric excess consistently above 95% when reactions are performed in methylene chloride at -20°C. The process falls outside the scope of mandatory GMP because the ligand is not incorporated into a drug substance, but all syntheses are documented following ISO 9001:2015 quality management requirements. Exposure of the phosphine to laboratory atmosphere for longer than 30 min causes more than 10% oxidation and renders the batch unusable — this operational boundary mandates strict inert-atmosphere handling.

Polyurethane Side-Chain Functionalization via N-Boc Protected Pyrrolidine Monools

Thermoplastic polyurethane elastomers designed for post-extrusion functionalization are prepared by end-capping a mid-block isocyanate prepolymer with N-Boc-3-hydroxymethylpyrrolidine as a chain terminator bearing a latent secondary amine. A poly(tetramethylene ether) glycol-based prepolymer (Mn 2000 g/mol, NCO content 5.22% by ASTM D2572-19) is dissolved in anhydrous 2-butanone at 35% w/v and reacted with the monool at an NCO:OH equivalent ratio of 1.00:1.00 in the presence of dibutyltin dilaurate (0.10 wt% of solids). The batch is held at 70°C in a sealed 20 L stainless steel reactor fitted with an anchor stirrer until the isocyanate band at 2270 cm⁻¹ vanishes in ATR-FTIR spectra, typically within 90–110 min. The resultant polymer is precipitated into excess hexane, dried at 50°C under vacuum (10 mbar) for 24 h, and compression-moulded into 1.5 mm sheets at 155°C.

Post-processing deprotection of the pendant pyrrolidine nitrogen is accomplished by exposing the moulded films to anhydrous HCl gas in a sealed chamber at 25°C for 4 h. Quantitative Boc removal is verified by solid-state ¹³C CP/MAS NMR (disappearance of the tert-butyl quaternary carbon at 28.5 ppm). The resulting free amine can participate in Schiff-base crosslinking with terephthalaldehyde or in amide bond formation with diacid chlorides. Mechanical properties before and after deprotection were characterized according to ASTM D412-16 (Die C): the tensile strength drops from 32.4 MPa to 27.9 MPa, while elongation at break increases from 450% to 510%, attributed to disruption of hard-segment ordering. Yellowing upon acid exposure remains a persistent limitation; incorporation of a hindered phenol antioxidant (Irganox ^® 1010, 0.50 phr) reduces the yellowness index (ASTM E313-20) from 12.8 to 5.6. For applications in indirect food-contact materials, migration testing under EU Regulation 10/2011 (simulant B, 3% acetic acid, 40°C for 10 d) must show total polyurethane constituents < 10 mg/dm². The presence of residual tin catalyst above 50 ppm in the extract is a known threshold that triggers non-compliance; therefore a post-polymerization scrubbing step with 0.1 N HCl at 50°C for 2 h is incorporated before precipitation.

When Hydroxymethyl Esters Enable Controlled-Release Agrochemical Conjugates

Conjugation of phenoxy acid herbicides to N-Boc-3-hydroxymethylpyrrolidine via ester linkage creates a pro-herbicide with altered lipophilicity and soil mobility. 2,4-dichlorophenoxyacetic acid (1.00 eq) is dissolved in ethyl acetate (12 L/kg) together with the pyrrolidine alcohol (1.05 eq), and the mixture is cooled to 0°C. N,N′-dicyclohexylcarbodiimide (DCC, 1.20 eq) and 4-dimethylaminopyridine (0.10 eq) are introduced under nitrogen. The slurry is warmed to 22°C over 2 h and stirred for an additional 16 h. Precipitated dicyclohexylurea (DCU) is removed by filtration through a 10 μm polyethylene frit, and the filtrate is washed with 5% w/v sodium bicarbonate to remove unreacted acid. After drying over anhydrous magnesium sulfate, the crude ester is purified by isocratic silica gel chromatography (toluene/ethyl acetate 9:1) to deliver the product as a viscous oil. The ¹H NMR spectrum exhibits a diagnostic downfield shift of the hydroxymethyl methylene protons from 3.47 ppm (t, J=5.8 Hz) in the starting alcohol to 4.12 ppm (d, J=7.2 Hz) in the ester.

Hydrolytic stability is the governing parameter that determines release rate; the conjugate has a half-life of 14.2 d in phosphate-buffered saline (PBS) at pH 7.4 and 37°C, but this accelerates to 2.8 h at pH 9.0. Soil column leaching studies under OECD 312 guidelines indicate that the esterified form retards downward mobility by a factor of 2.3 relative to the free acid in a sandy loam matrix. The technical material is formulated as a 200 g/L emulsifiable concentrate containing 10% v/v N-methylpyrrolidone and an ethoxylated tristyrylphenol phosphate surfactant (8% w/v). For registration purposes, five-batch analysis data must comply with FAO specification 1.2/4 for acid equivalent content, and relevant CIPAC methods (CIPAC 1, MT 15, MT 46) are deployed for emulsion stability and wet sieve testing. Storage stability at 54°C for 14 d (accelerated storage, based on CIPAC MT 46.3) shows <3% active ingredient degradation, provided the filled polyethylene containers are sealed with aluminum/PET composite liners to exclude moisture ingress.

A late-stage fluorination approach for neuroreceptor imaging relies on the tosylate precursor derived from N-Boc-3-hydroxymethylpyrrolidine. The crystalline tosylate is prepared in advance by reacting the alcohol with p-toluenesulfonyl chloride (1.25 eq) in pyridine/DCM (1:3 v/v) at 0°C for 18 h, yielding a stable white powder after recrystallization from ethanol/water (1:2). In the hot-cell automated synthesis module (GE TRACERlab FX_N or similar), anhydrous [¹⁸F]fluoride (15–25 GBq) trapped on a QMA cartridge is eluted with a solution of Kryptofix 2.2.2 (5.0 mg) and potassium carbonate (0.50 mg) in acetonitrile/water (800 μL/200 μL). The eluate is azeotropically dried by repeated evaporation with acetonitrile (0.5 mL × 3) at 85°C under a gentle helium stream. The tosylate precursor (2.0 mg) in anhydrous acetonitrile (300 μL) is added, and the sealed reaction vessel is heated at 100°C for 10 min. Radiochemical incorporation yield, determined by radio-TLC (silica gel, ethyl acetate/hexane 1:1), typically ranges between 45% and 62% (decay-corrected). The crude product is purified by semi-preparative HPLC (Waters XBridge C18, 10×250 mm, acetonitrile/0.1% aqueous H₃PO₄ 55:45) with simultaneous UV (254 nm) and radioactivity detection.

The isolated ¹⁸F-labeled compound is formulated as an isotonic saline solution for intravenous injection, and quality control tests follow Ph. Eur. monograph 2468 on radiopharmaceutical preparations. Residual Kryptofix 2.2.2 must not exceed 50 μg/mL (spot test with iodoplatinate), and residual acetonitrile is maintained below 0.04% as per ICH Q3C. The Boc protecting group remains intact during the rapid fluorination step because exposure to the weakly basic carbonate-Kryptofix complex is brief; however, any deviation in drying time that leaves residual water leads to formation of the free [¹⁸F]fluoride spot at the TLC origin and requires re-purification. The shelf life of the final product is 8 h from the end of synthesis, governed by radiochemical purity remaining ≥ 95% as assessed by radio-HPLC. Published pharmacokinetic data for this specific pyrrolidine tracer are limited, but the sulfonate displacement route consistently delivers higher specific activity (80–120 GBq/μmol) compared to direct aliphatic labeling methods, making it the preferred path for receptor occupancy studies requiring low-mass-dose tracer.

Supplied as a white to off-white crystalline powder with a purity specification of ≥ 98.0% (HPLC, area%), this heterocyclic building block carries the empirical formula C₁₀H₁₉NO₃ and a molecular weight of 201.26 g·mol⁻¹. The substance is catalogued under CAS 170491-63-1 and typically ships in amber glass vials purged with argon to maintain a water content below 0.5% (KF). Storage recommendations derive from accelerated stability studies: the compound remains within specification for 24 months at −20 ± 5 °C in a desiccated environment, while exposure to ambient humidity (> 60% RH) for periods exceeding 8 hours initiates detectable carbamate hydrolysis and formation of the free pyrrolidine, detectable by TLC (silica gel 60 F₂₅₄, cyclohexane/EtOAc 7:3, ninhydrin stain).

What Distinguishes the tert-Butyl Carbamate from Methyl or Benzyl Analogues in Multi-Step Sequences?

In contrast to the corresponding methyl carbamate, which requires strongly basic saponification conditions or trimethylsilyl iodide for cleavage—conditions incompatible with acid-sensitive tertiary alcohols and certain heterocycles—this N-Boc derivative undergoes rapid deprotection with neat trifluoroacetic acid (TFA) or 4 M HCl in dioxane at 0–25 °C. Kinetic profiling by ¹H NMR (DMSO‑d₆, 400 MHz) indicates that the deprotection half-life in TFA/CH₂Cl₂ 1:1 v/v at 20 °C is 12 ± 2 min, whereas the benzyl carbamate (Cbz) analogue requires hydrogenolysis over Pd/C (10 wt%, 50 psi H₂) in a dedicated Parr shaker vessel, a step that introduces catalyst cost and residual metal compliance concerns under ICH Q3D. The benzyl ester nonetheless finds preference in sequences where global acid lability must be avoided; the Cbz-protected 3-hydroxymethylpyrrolidine retains the hydroxymethyl group unchanged during Boc deprotection elsewhere in the molecule. The methyl ester, with a deprotection endpoint requiring pH > 12, is largely restricted to early-stage intermediate elaboration where epimerization risk is absent.

A further operational difference emerges in scalability. The tert-butyl carbamate’s deprotection off-gases isobutylene and carbon dioxide, which can be safely vented through a bleach scrubber in pilot-plant settings; benzyl deprotection consumes hydrogen and demands explosion-proof infrastructure compliant with ATEX Directive 2014/34/EU. Published data for direct calorimetric comparison of deprotection exotherms is limited, but adiabatic reaction calorimetry (ARC) conducted on structurally analogous N-Boc-pyrrolidines places the onset temperature of autocatalytic decomposition above 120 °C, imparting a comfortable safety margin during ambient-temperature cleavage.

Specifications are verified against a certificate of analysis that includes retention time relative to an authenticated reference standard. Identity is confirmed by ¹H and ¹³C NMR, IR (C=O stretch at 1690 ± 5 cm⁻¹, broad O–H absorption centred at 3420 cm⁻¹), and high-resolution mass spectrometry. Enantiomeric excess for the (R)- and (S)-configured forms is controlled to ≥ 99.0% (chiral HPLC, Chiralpak AD‑H column, n-hexane/isopropanol 90:10, 1.0 mL·min⁻¹, 210 nm).

Application of 3-Hydroxymethyl-Pyrrolidine-1-Carboxylic Acid Ter-Butyl-Ester in Medicinal Chemistry Campaigns

The primary utility lies in its function as a rigid, sp³-rich fragment for introducing a basic amine anchor into lead compounds targeting CNS aminergic GPCRs and ion channels. Its hydroxymethyl handle undergoes clean Mitsunobu inversion with phenols or phthalimide, Jones oxidation to the carboxylic acid (CrO₃/H₂SO₄, acetone, 0 °C), or conversion to the mesylate leaving group, which is sufficiently reactive toward NaN₃ in DMF at 60 °C to install an azide for click chemistry. In a published dopamine D3 receptor antagonist programme, (S)-3-hydroxymethyl-pyrrolidine-1-carboxylic acid tert-butyl ester was alkylated with 4-(4-chlorophenyl)piperazine under standard Williamson conditions (NaH, THF, 0 °C to rt, 18 h, 82% isolated yield after flash chromatography) to deliver the protected intermediate en route to the hydrochloride salt. The crystalline nature of the Boc derivative permits trituration in cold heptane to upgrade purity to 99.5% prior to amide bond formation, a significant advantage over oily analogues (e.g., the corresponding N-formyl compound) that demand chromatography at every stage.

Simulated process mass intensity (PMI) modelling on a 1 kg batch of the Boc-pyrrolidine alcohol in a multi-purpose 50 L jacketed glass reactor (Büchi Glas Uster) indicates that the exotherm upon NaH addition can be controlled within ΔT < 5 °C by dosing the reagent in 0.25 eq portions at 15-minute intervals while maintaining jacket temperature at −5 °C. No pressure accumulation above 0.1 bar was observed, consistent with the low headspace hydrogen evolution rate.

Comparative physicochemical profile of pyrrolidine N-protecting groups
Parameter	tert-Butyl carbamate	Benzyl carbamate	Methyl carbamate
Deprotection reagent (lab scale)	TFA/DCM, 4 M HCl/dioxane	H₂, Pd/C, EtOH	NaOH aq./MeOH, Δ
Typical deprotection time (25 °C)	1–4 h	12–48 h	6–24 h
Residual metal risk (ICH Q3D)	Low	Pd ≤ 10 ppm	Low
Free amine storage form	TFA or HCl salt	Free base or HCl salt	Free base
Solid-state stability at 25 °C/60% RH	6 months	12 months	24 months
Cost index (€/mol, lab grade)	1.0 (reference)	0.7–0.9	0.4–0.6

The tert-butyl ester moiety is stable towards nucleophilic acyl substitution and Grignard reagents at low temperature, permitting functionalization of the hydroxymethyl group without premature unmasking. However, exposure to Lewis acids such as BF₃·OEt₂ or TMSOTf at concentrations above 0.1 M leads to partial Boc cleavage even at −78 °C; coordination of the carbonyl oxygen to the Lewis acid centre is presumed responsible, a limitation not observed with the more robust tosyl-protected analogue. Thus, transformations requiring stoichiometric BF₃·OEt₂ are better served by the benzyl carbamate, accepting the consequent deprotection overhead.

When Does Batch-to-Batch Colour Variation Signal a Process Impurity Rather Than Degradation?

Off-white to pale yellow colouration detected upon reception is occasionally traced to residual palladium from an upstream Suzuki coupling step if the pyrrolidine ring is constructed via an intramolecular Heck reaction, rather than to oxidative degradation of the hydroxymethyl group. Elemental analysis by ICP-MS (Agilent 7800) quantifies Pd below the ICH Q3D oral permitted daily exposure of 100 μg/day in batches cleared for GMP campaigns. Where colour is accompanied by a melting point depression greater than 3 °C (pure compound melts at 67–70 °C with decomposition onset at 185 °C by DSC at 10 K·min⁻¹), silica gel plug filtration in ethyl acetate followed by crystallisation from n-heptane/EtOAc (5:1 v/v) restores the specification appearance. For supply chain security, an incoming QC protocol per ISO 9001:2015 clause 8.4.1 specifies rejection criteria: any lot exhibiting HPLC purity below 97.0%, single unknown impurity exceeding 0.5%, or residual solvent above ICH Q3C limits (ethyl acetate ≤ 5000 ppm, heptane ≤ 5000 ppm, dichloromethane ≤ 600 ppm).

In medicinal chemistry laboratories operating under the NIH Chemical Genomics Center guidelines, the compound is routinely dissolved in anhydrous DMSO to a stock concentration of 20 mM and dispensed into 384-well plates, where free-thaw cycles (up to 10) demonstrated < 2% decomposition by LCMS, provided the DMSO is dried over 4 Å molecular sieves (20% w/v, activated at 300 °C for 12 h). Aqueous solubility of the neutral carbamate is low (0.8 mg·mL⁻¹ in phosphate buffer pH 7.4); the hydrochloride salt generated in situ increases solubility to > 25 mg·mL⁻¹.

Packing for intercontinental shipment follows UN 4G fibreboard box specifications with vermiculite cushioning. The compound is classified as non-hazardous under DOT 49 CFR and ADR, but local regulations concerning dust explosion (K_st value 0 bar·m·s⁻¹, St 0) should be consulted when handling quantities exceeding 5 kg. Customs harmonised system code 2933.99 applies.

The free hydroxymethyl group exhibits the anticipated reactivity profile toward acyl chlorides and isocyanates. In a typical preparative procedure documented across multiple pharma process development reports, the alcohol (1.0 eq) in dichloromethane (10 vol) is treated with triethylamine (1.5 eq) and acetyl chloride (1.1 eq) at 0 °C, furnishing the acetate ester in quantitative conversion after 30 min. The acetate, being a liquid at room temperature, is rarely isolated; instead it is telescoped directly into the subsequent Boc deprotection and HATU-mediated amide coupling with Fmoc-protected amino acids. This telescoped process reduces solvent consumption by 40% relative to stepwise isolation, an important factor under the ACS Green Chemistry Institute’s Pharmaceutical Roundtable metrics.

HS Code	948408
Name	3-Hydroxymethyl-Pyrrolidine-1-Carboxylic Acid Ter-Butyl-Ester

Packing	100g of 3 - Hydroxymethyl - Pyrrolidine - 1 - Carboxylic Acid Ter - Butyl Ester in a sealed chemical - grade container.
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