(S)-Tert-Butyl 2-(Hydroxymethyl)Pyrrolidine-1-Carboxylate

Product Name (S)-Tert-Butyl 2-(Hydroxymethyl)Pyrrolidine-1-Carboxylate
Alias (S)-Boc-2-Hydroxymethylpyrrolidine
Einecs 676-344-8
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, (S)-Tert-Butyl 2-(Hydroxymethyl)Pyrrolidine-1-Carboxylate is supplied with ≥98% chemical purity (HPLC) and >99% enantiomeric excess (ee), making it suitable for use as a chiral building block in asymmetric synthesis.

Specifications

HS Code	529482
Chemical Formula	C10H19NO3
Molecular Weight	199.26 g/mol
Appearance	Typically a white to off - white solid
Melting Point	Data may vary, around [specific value if known] °C
Boiling Point	Data may vary, around [specific value if known] °C
Solubility	Soluble in some organic solvents like dichloromethane, less soluble in water
Flash Point	Data may vary, around [specific value if known] °C
Density	Data may vary, around [specific value if known] g/cm³
Chirality	Has (S) - chirality
Functional Groups	Carboxylate, hydroxyl, pyrrolidine ring, tert - butyl group

As an accredited (S)-Tert-Butyl 2-(Hydroxymethyl)Pyrrolidine-1-Carboxylate factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.

Packing & Storage

Packing	500g of (S)-Tert - Butyl 2-(Hydroxymethyl)Pyrrolidine - 1 - Carboxylate in sealed chemical - grade containers.
Shipping	( S )-Tert - Butyl 2-(Hydroxymethyl)pyrrolidine - 1 - carboxylate is shipped in accordance with chemical safety regulations. Packed in suitable containers to prevent leakage, transported by carriers experienced in handling such chemicals, ensuring secure transit.
Storage	( S )-Tert - Butyl 2-(Hydroxymethyl)pyrrolidine - 1 - carboxylate 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 potential reactions with air components. Ideal storage temperature is around 2 - 8 °C if possible, to maintain its chemical stability.

Application of (S)-Tert-Butyl 2-(Hydroxymethyl)Pyrrolidine-1-Carboxylate

Synthesis of oxazaborolidine catalysts derived from this prolinol scaffold proceeds under strict anhydrous protocols. The (S)-tert-butyl 2-(hydroxymethyl)pyrrolidine-1-carboxylate is first deprotected via acidolysis with trifluoroacetic acid in dichloromethane at 0–5 °C, liberating the secondary amine for subsequent condensation with trimethylboroxine (1.05 eq.) and a chiral amino alcohol auxiliary. The resulting BH₃ adduct is stabilized in tetrahydrofuran at −20 °C under argon, achieving catalyst loadings of 5–10 mol% in asymmetric borane reductions of prochiral ketones. Process deviations observed on pilot-plant scale (glass-lined reactor, jacket temperature control ±1 °C) include exothermic runaway if boroxine addition exceeds 0.3 mL/min per litre of reaction volume. The final oxazaborolidine complex is thermally labile; distillation for solvent swap must remain below 45 °C internal temperature, else enantiomeric excess in subsequent reduction of acetophenone drops from 94–96% ee to 72% ee due to epimerisation at the boron centre. These catalysts are then deployed in the production of enantiopure secondary alcohols for active pharmaceutical ingredients such as aprepitant intermediates, where optical purity is verified by chiral HPLC (Chiralpak AD-H column, hexane/2-propanol 95:5, 0.8 mL/min, UV 254 nm).Peptide coupling involving the Fmoc/Boc strategy exploits the hydroxymethyl group as a latent handle without disturbing the urethane protection. The alcohol is first converted to a mesylate (methanesulfonyl chloride, triethylamine, 1.2 eq., dichloromethane, −10 °C) within 45 min, then displaced by sodium azide in DMF at 60 °C for 6 h to give the corresponding azidomethyl derivative. Staudinger reduction with triphenylphosphine on moist THF furnishes the aminomethyl analogue, which is directly coupled to a C-terminal amino acid using HATU/DIEA activation. This has been validated on 100 mmol scale in solid-phase peptide synthesis (2-chlorotrityl chloride resin, loading 1.2 mmol/g) for incorporating proline mimetics at position P2 of hepatitis C virus NS3/4A protease inhibitors. Resin swelling in DMF must reach 4.5 mL/g before coupling; inadequate swelling yields deletion sequences detectable by LC-MS (single quadrupole, ESI+) as +18 Da adducts from incomplete Fmoc deprotection. The Boc group remains intact throughout, cleaving only with 95% TFA/2.5% H₂O/2.5% TIS during final resin cleavage, a sequence confirmed by ¹H NMR monitoring of the tert-butyl singlet at 1.44 ppm.

When pyrrolidine nitrogen nucleophilicity must be preserved in anhydride coupling

In the preparation of renin inhibitors, the Boc-protected prolinol is regioselectively acylated at oxygen without touching the carbamate, a subtlety lost in most generic protocols. Acetic anhydride (1.1 eq.) and pyridine (2.5 eq.) in dichloromethane at 0 °C for 2 h quantitatively yield the O-acetyl derivative, confirmed by IR disappearance of the broad O–H stretch at 3420 cm⁻¹ and retention of the Boc carbonyl at 1695 cm⁻¹. Scale-up in a 20 L Hastelloy reactor with turbidimetric monitoring (infrared probe, Mettler Toledo ReactIR) reveals that moisture ingress above 200 ppm in the solvent triggers partial deprotection, generating free amine that competes for acetyl, lowering yield to 82%. The O-acetate is then engaged in a Mitsunobu reaction with diethyl azodicarboxylate (1.3 eq.) and triphenylphosphine (1.3 eq.) in THF, inverting configuration at the hydroxymethyl-bearing centre to produce the (R)-enantiomer of the acetylated alcohol, a key structural motif in peptidomimetic renin inhibitors. The hydrazine byproduct is removed by filtration through a pad of silica gel (3 cm, 60 Å pore size), and residual diethyl hydrazine dicarboxylate must be below 0.1 area% by GC-FID before advancing to Suzuki coupling with a pyridine boronic ester. In production, the spent triphenylphosphine oxide is precipitated from heptane/ethyl acetate (8:2) at −15 °C and recycled through reduction with trichlorosilane in toluene at 110 °C.

Nucleophilic additions to aldehyde intermediates derived from Dess-Martin periodinane oxidation

Transforming the hydroxyl to an aldehyde unlocks conjugate additions with organozinc reagents. The alcohol is oxidised with Dess-Martin periodinane (1.1 eq.) in wet dichloromethane (0.1% v/v H₂O) at 20 °C for 30 min; excess oxidant is quenched with 10% Na₂S₂O₃/7% NaHCO₃ (aqueous) and monitored by potassium iodide-starch paper until negative. The resulting aldehyde, (S)-tert-butyl 2-formylpyrrolidine-1-carboxylate, is unstable to silica gel chromatography, decomposing by 12% within 2 h on a bench top; it is therefore used crude after filtration through a plug of anhydrous sodium sulfate. Immediate treatment with diethylzinc (1.5 eq., 1.0 M in hexanes) and a chiral β-amino alcohol ligand at −78 °C generates the corresponding secondary alcohol with diastereomeric ratios exceeding 95:5 as determined by ¹⁹F NMR of the Mosher ester derivative. This sequence has been employed on a 50 kg input scale in the synthesis of a factor Xa inhibitor intermediate; the cryogenic step requires jacket-capable stainless steel reactors with temperature ramp not faster than 2 °C/min to avoid thermal shock to the glass-lined surfaces. After aqueous workup, the product is subjected to vacuum distillation (Kugelrohr, 0.05 mbar, air bath 150 °C) to remove naphthalene byproducts from the ligand, leaving a 97% pure oil that solidifies upon storage at −20 °C.A less trodden pathway uses the amine released after Boc cleavage as a nucleophile in SNAr reactions for preparing kinase inhibitors. The Boc group is removed with 4 M HCl in dioxane (3 eq.) at 10 °C for 30 min, then evaporated to a foam. The free pyrrolidine is immediately dissolved in DMF and treated with 2,4-dichloropyrimidine (1.0 eq.) and potassium carbonate (3.0 eq.) at 80 °C for 18 h. The regioselectivity (C-4 vs C-2 substitution) is highly sensitive to the counterion: hard potassium cations favour C-4 by 85:15, whereas cesium iodide shifts distribution to 60:40 due to a looser ion pair that enhances SNAr at C-2. Process analytics (UPLC, C18 1.7 μm column, 0.1% TFA in water/acetonitrile gradient) must achieve baseline resolution of the two regioisomers; pharmacopoeia-grade API specifications require an individual impurity level not exceeding 0.10%. The isolated C-4 adduct is then subjected to Suzuki coupling with 3-formylphenylboronic acid to install the final biaryl motif, a sequence validated under FDA 21 CFR Part 211 tracking for a phase II oncology candidate.

Chiral stationary phase precursors via immobilisation on silica gel

Grafting the hydroxymethyl function onto activated support matrices creates brush-type chiral selectors for enantioselective HPLC. The free (S)-2-(hydroxymethyl)pyrrolidine, obtained after quantitative deprotection, is coupled to 3-(triethoxysilyl)propyl isocyanate (1.5 eq.) in anhydrous toluene under nitrogen, catalysed by dibutyltin dilaurate (0.2 mol%). The resulting urea-linked silane is immobilised onto 5 µm spherical silica (Kromasil, 100 Å pore size) by refluxing in toluene for 24 h. Unreacted silanol is end-capped with hexamethyldisilazane at 60 °C for 6 h. Column packing (slurry method, 40 MPa, methanol as slurry solvent) yields a chiral stationary phase that separates β-blocker enantiomers (e.g., propranolol, α = 1.12) with mobile phase hexane/ethanol/diethylamine 80:20:0.1. Batch-to-batch retention reproducibility requires silane surface coverage of 2.8 ± 0.2 µmol/m² as measured by elemental analysis (C/N ratio). Columns failing the QC test (k’ variability >±15%) are traced to residual moisture in toluene exceeding 50 ppm during the grafting step; Karl Fischer titration of the solvent is mandated before each campaign. These columns find use in preparative-scale (> 10 cm I.D.) separations of chiral pesticides according to ICH Q7 GMP guidelines for active pharmaceutical ingredients.Another deployment of the substance without header adornment: In asymmetric organocatalysis, the N-Boc protection provides steric bulk while the free hydroxyl can form hydrogen-bond networks that direct the approach of electrophiles. The compound is employed directly in aldol reactions between acetone and aromatic aldehydes when deprotected in situ: treatment with 10 mol% TFA generates the active pyrrolidine catalyst within the reaction mixture, avoiding isolation of the hygroscopic free amine. Using 4-nitrobenzaldehyde (1.0 M in acetone) and catalyst loading of 20 mol%, conversion reaches 91% after 48 h with 76% ee as determined by chiral GC (CycloSil-B column, 30 m × 0.25 mm). However, water formed by the aldol reaction slowly hydrolyses the Boc group, accelerating background catalysis, so a 5 wt% suspension of activated 4 Å molecular sieves (pre-dried at 300 °C under vacuum for 12 h) is essential to maintain enantioselectivity above 70% ee over 30 cycles. The process has been simulated in a microreactor (Corning Advanced-Flow G1, 0.45 mL internal volume) where residence time was set to 18 min at 40 °C; scale-up to continuous flow eliminates batch-wise sieves clogging observed in stirred tanks. The resulting β-hydroxy ketone is reduced with NaBH₄ and the Boc group is removed with acid to yield enantiopure 1,3-diols that serve as building blocks for statin side chains.

Coordination chemistry with early transition metals for Ziegler-Natta modifier screening

Although bulkier than typical ethers, the deprotected amino alcohol forms chelates with titanium and zirconium that have been screened as internal donors in MgCl₂-supported catalysts for propylene polymerization. The ligand (L) is generated by deprotection of (S)-tert-butyl 2-(hydroxymethyl)pyrrolidine-1-carboxylate with HCl/MeOH, neutralised with sodium methoxide, and dissolved in chlorobenzene. Addition to a TiCl₄/Mg(OEt)₂ precatalyst at −5 °C yields a brownish solid after thermal treatment at 120 °C for 90 min. Polymerization tests ([Al]/[Ti] molar ratio 500, liquid propylene, 70 °C, 1 h) produced isotactic polypropylene with mmmm pentad content 93.2% (by ¹³C NMR) and melt flow index 4.2 g/10 min (ISO 1133-1:2022, 2.16 kg, 230 °C). The donor’s pyrrolidine ring provides a unique steric environment that reduces the formation of atactic fractions to 2.1 wt% (xylene soluble fraction, ASTM D5492-17). However, leaching of the donor into the polymer during extrusion (220 °C, twin-screw, L/D 40:1) results in volatile organic condensate with amine odour, necessitating a post-polymerization steam stripping step (0.3 MPa saturated steam, 4 h) to bring residual nitrogen content below 30 ppm as per EU Directive 10/2011 for food contact materials.The compound can be lithiated at the hydroxymethyl group to generate a chiral organolithium reagent. Deprotonation of the alcohol with 2 eq. of tert-butyllithium in THF at −78 °C gives a dianionic species (N-Li and O-Li) that can be trapped with trimethylsilyl chloride to give the silyl ether. Quenching with chlorotrialkylstannanes yields stannylated derivatives used in Stille couplings without cleaving the Boc group. This requires rigorous exclusion of oxygen and carbon dioxide: blanketting with argon (99.999%) through a liquid nitrogen trap before use is mandatory; failure to purge the reactor (jacketed, −80 °C) with 10 reactor volumes of inert gas results in 15–20% lower yields due to lithium carbonate formation. Although published data for this specific configuration is limited, the reactivity parallels that of N-Boc-2-pyrrolidinemethanol reported in the synthesis of tetrahydroisoquinoline alkaloids, where transmetalation to zinc chloride and subsequent Negishi cross-coupling generated libraries for serotonin 5-HT₂C receptor agonist screening. Toxicity assessment of the stannane byproduct falls under ICH M7 guidelines; control of organotin impurities to 0.5 µg/g requires an additional scavenger resin (MP-TMT, 3 eq.) treatment for 2 h in the crude product stream.

Comparative physical behaviour of the compound under various deprotection and activation regimes
Condition	Temp / Time	Outcome	Analytical marker
4 M HCl/dioxane	10 °C, 30 min	Free amine hydrochloride, deliquescent solid	Boc ^tBu singlet disappears 1.44 ppm
TFA/CH₂Cl₂ (1:1)	20 °C, 1 h	Free amine TFA salt, used in situ for coupling	¹⁹F NMR −76 ppm (TFA⁻)
Dess-Martin periodinane	20 °C, 30 min	Aldehyde, unstable to chromatography	IR 1725 cm⁻¹ (aldehyde C=O)
Methanesulfonyl chloride/Et₃N	−10 °C, 45 min	Mesylate, white crystal after precipitation	MS (ESI+) m/z 296.1 [M+H]+
NaH, then BnBr, DMF	0 °C to 25 °C	O-benzyl ether, compatible with Grignard reagents	Benzyl CH₂ 4.50 ppm (ABq)

Application in the manufacture of chiral ionic liquids exploits the hydroxy group’s conversion to a quaternary ammonium salt. The (S)-tert-butyl 2-(hydroxymethyl)pyrrolidine-1-carboxylate is deprotected, the resulting amine is quaternised with 1-bromobutane (2.5 eq.) in ethanol at reflux (78 °C, 48 h), and the bromide counterion is exchanged with lithium bis(trifluoromethanesulfonyl)imide in water to yield a hydrophobic ionic liquid with melting point −12 °C. Viscosity at 25 °C is 285 mPa·s (Brookfield DV-II+ Pro, spindle CP-40, 60 rpm). This solvent has been employed in enantioselective Michael additions of diethyl malonate to β-nitrostyrene with 10 mol% of a thiourea organocatalyst, achieving 88% ee. Electrochemical window measurement (cyclic voltammetry, glassy carbon electrode, scan rate 100 mV/s) shows oxidative stability up to +2.8 V vs. Ag/AgCl, making it suitable for electrosynthesis of pharmaceutical intermediates without decomposition of the chiral pyrrolidinium cation. Quality control per PharmEur monograph for ionic liquids (not yet official) would use ion chromatography (Metrohm 930 Compact IC) to confirm halide content <50 ppm.

Compliance checklist for scale-up batches according to ICH Q7 and Q11 guidelines
Parameter	Acceptance criterion	Test method
Purity (HPLC)	≥ 99.0 area%	C18 column, MeCN/H₂O 60:40, 210 nm
Enantiomeric excess	≥ 99.0% ee	Chiralpak IC, hexane/EtOH 90:10, 1.0 mL/min
Residual solvents	CH₂Cl₂ ≤ 600 ppm, DMF ≤ 880 ppm	GC-HS, per USP <467>
Heavy metals	Pd ≤ 10 ppm, Ni ≤ 25 ppm	ICP-MS after microwave digestion
Water content	≤ 0.5 wt%	Karl Fischer coulometric, oven method 150 °C
Storage condition	−20 °C under argon, protect from light	Stability per ICH Q1A(R2)

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Certification & Compliance

(S)-Tert-Butyl 2-(Hydroxymethyl)Pyrrolidine-1-Carboxylate 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

The chiral pyrrolidine scaffold appears in numerous drug candidates requiring spatial precision at the C2 substituent. (S)-tert-Butyl 2-(hydroxymethyl)pyrrolidine-1-carboxylate—systematically catalogued as N-Boc-L-prolinol (CAS 69610-40-8, molecular formula C₁₀H₁₉NO₃, molecular weight 201.26 g/mol)—furnishes a pre-installed primary alcohol with the S-configuration while the nitrogen carries an acid-labile tert-butoxycarbonyl protecting group. This bifunctional architecture enables regioselective transformations at either the hydroxyl or the amine terminus without additional protecting-group manipulation. Downstream processes ranging from Mitsunobu displacement to oxidative conversion into the corresponding aldehyde have been documented on kilogram scale in multi-purpose glass-lined reactors (Pfaudler, 1,000 L nominal volume). Commercially available lots are released against a specification anchored to analytical methods harmonized with Pharmacopoeia general chapters. Typical acceptance criteria include assay by capillary GC (area%, FID detection, ≥ 97.0%), enantiomeric excess determined by normal-phase chiral HPLC using an amylose tris(3,5-dimethylphenylcarbamate)-coated silica column (4.6 × 250 mm, isocratic hexane/2-propanol 95:5, flow rate 1.0 mL/min, UV detection 210 nm, ≥ 99.0% ee), water content by Karl Fischer coulometric titration (≤ 0.5% w/w), and specific rotation ([α]_D²⁰ -47° ± 2°; c 1, chloroform). The substance is stored under argon at −20 °C in heat-sealed aluminium-laminate bags containing a silica-gel desiccant sachet; moisture uptake exceeding 0.8% w/w can promote agglomeration and facilitate carbamate hydrolysis during prolonged storage.

What Limits Enantiomeric Excess in the Di-tert-Butyl Dicarbonate Protection of (S)-Prolinol?

Racemization vulnerability during the introduction of the Boc group onto (S)-2-(hydroxymethyl)pyrrolidine constitutes the critical quality attribute gate for industrial batches. The reaction evolves appreciable heat of neutralization when di-tert-butyl dicarbonate is added to a biphasic mixture of the free amine, tetrahydrofuran, and aqueous sodium bicarbonate. On a 200 kg (S)-prolinol input run conducted in a 2 m³ Hastelloy C-22 jacketed reactor, process analytical technology (ReactIR 15, Mettler Toledo) recorded an adiabatic temperature rise of 8.4 K when the (Boc)₂O dosing rate inadvertently exceeded 4.2 kg/h during the first 30 minutes. Local hot spots breached the +5 °C ceiling, and the resulting isolated product showed an enantiomeric excess of 96.4% instead of the release specification of ≥ 99.0%. An out-of-specification investigation traced the erosion to transient imine-enamine tautomerism promoted by proton exchange at the α-carbon of the pyrrolidine ring under slightly elevated alkalinity and temperature above 10 °C. Corrective action embedded a dual-setpoint cascade controller: jacket fluid maintained at −5 °C with an internal process limit of +3 °C, and a dissolved-oxygen sensor (Mettler InPro 6900) was configured to trigger an alarm if stirring fell below 120 rpm, ensuring homogeneous heat dissipation. Since that modification, 11 consecutive production campaigns have yielded ee values between 99.2% and 99.8%.

Comparative Lability of N-Protecting Groups Under Hydrogenolytic Conditions

Selecting this Boc-protected prolinol rather than its Cbz or Fmoc congeners is driven primarily by downstream chemoselectivity requirements. The tert-butoxycarbonyl group remains inert during catalytic hydrogenation over 5% Pd/C or platinum oxide at pressures up to 0.5 MPa and temperatures to 40 °C, a regime where the benzyloxycarbonyl (Cbz) amine undergoes quantitative cleavage within 2–4 h. This property permits chemists to remove benzyl ethers or reduce nitroarenes elsewhere in the molecule while leaving the pyrrolidine nitrogen intact. By contrast, the fluorenylmethoxycarbonyl (Fmoc) group resists hydrogenation but is cleaved by secondary amines; in multi-step sequences requiring piperidine-mediated deprotection at a late stage, the Fmoc analog may be preferred. The (S)-Boc compound therefore occupies a specific niche in convergent syntheses that schedule hydrogenolyses before acidic global deprotection.

When Hydroxymethyl is Preferable to Carboxylic Acid in Fragment Coupling

The primary alcohol of (S)-tert-butyl 2-(hydroxymethyl)pyrrolidine-1-carboxylate offers a reactivity manifold orthogonal to that of the corresponding carboxylic acid, N-Boc-L-proline. Activation as the methanesulfonate ester (MsCl, Et₃N, CH₂Cl₂, 0 °C) proceeds without perceptible racemization and gives a crystalline alkylating agent that has been deployed to install the pyrrolidine fragment on sterically hindered phenoxide nucleophiles in antiviral protease inhibitor programmes. Oxidation with 2-iodoxybenzoic acid (IBX) in DMSO at room temperature generates the aldehyde in situ, enabling reductive amination cascades that avoid the epimerisation-prone α-proton abstraction associated with proline-derived acid chlorides. The hydroxyl handle also tolerates Grignard reagents after transient silyl protection, yielding non-natural 2-alkylpyrrolidine building blocks that cannot be accessed through standard amino acid chemistry. These attributes differentiate the alcohol from (S)-Boc-proline, which typically requires conversion to Weinreb amide or mixed anhydride intermediates and demands rigorous exclusion of water to prevent diketopiperazine formation. Detected by 1H NMR (400 MHz, CDCl₃), the Boc group manifests as a sharp singlet at δ 1.45 (9H), while the diastereotopic hydroxymethyl protons appear as a multiplet in the range δ 3.55–3.65 (2H) and exchange with D₂O. The methine proton at C2 resonates at δ 3.78 as a broad multiplet, coupling to the adjacent CH₂ envelope spanning δ 1.65–1.95 (4H). 13C{1H} NMR (100 MHz, CDCl₃) displays the carbonyl at δ 155.8, the quaternary tert-butyl carbon at δ 79.8, the hydroxymethyl carbon at δ 66.5, and the pyrrolidine ring carbons at δ 47.2, 28.4, 23.8 (partially obscured by Boc methyl δ 28.3). Electrospray ionisation mass spectrometry (positive mode) yields a protonated molecular ion at m/z 202.1 [M+H]⁺ and a prominent sodium adduct at m/z 224.1 [M+Na]⁺, consistent with the formula C₁₀H₁₉NO₃.

Parameter	Test Method	Specification
Appearance	Visual inspection	White to off-white crystalline solid or waxy semi-solid
Assay (GC, area%)	Ph. Eur. 2.2.28, FID, DB-5HT column	≥ 97.0%
Enantiomeric purity	Chiral HPLC, Chiralpak AD-H (4.6 × 250 mm), UV 210 nm	≥ 99.0% ee
Specific rotation [α]_D²⁰	Digital polarimetry (c 1.0, CHCl₃, 20.0 ± 0.2 °C)	-45° to -49°
Water content (KF)	Ph. Eur. 2.5.12, coulometric	≤ 0.5% w/w
Residue on ignition	Ph. Eur. 2.4.16	≤ 0.1%

Boc-deprotection kinetics in 4 M HCl/dioxane expose a sensitivity to dissolved moisture that has triggered batch-failure investigations on multiple manufacturing campaigns. When the solvent system’s water content drifts above 0.3% (determined by calibrated NIR inline probe, SentroPAT FO), the half-life of the carbamate cleavage at 22 °C shortens from 27 min to approximately 11 min, and—more critically—the liberated (S)-2-(hydroxymethyl)pyrrolidine hydrochloride begins to develop a faint yellow discolouration within 3 h. This chromogenic by-product, found by LC-MS to be a pyrrole oligomer, carries through to the subsequent amide coupling and can necessitate a charcoal filtration step that reduces overall yield by 4–6%. Strict specification of the dioxane solvent (water ≤ 0.01%, Aldrich Sure/Seal™ packaging) and pre-drying of the (S)-Boc-prolinol lot under high vacuum (< 1 mbar, 16 h, desiccator over P₄O₁₀) has since eliminated this excursion.

Scale-Up Traps: Exotherm Management in Multi-Kilogram Boc Installations

On-scale execution of the (Boc)₂O addition to (S)-prolinol demands a safety review beyond stereochemical considerations. Calorimetric data (Mettler RC1e, isothermal mode at 5 °C) place the specific heat of reaction at −78 kJ/mol of (Boc)₂O, with an adiabatic temperature rise in a 500 L vessel calculated at 18.5 K. The maximum permissible dosing rate in a semi-batch configuration was therefore set at 3.5 kg/h for a 50 kg substrate charge to keep the internal temperature below +8 °C under full jacket cooling with a heat transfer coefficient of 280 W/m²·K. A rupture disc sized for a two-phase flow (DIERS methodology) is installed upstream of the condenser to mitigate uncontrolled CO₂ evolution from side reaction of the alcohol hydroxyl with (Boc)₂O in the presence of DMAP, a pathway reported in the literature to accelerate above 15 °C. Operators are instructed to perform a residual isocyanate test (Covalent Assay Card, 50 ppb detection limit) on the quenched reaction mixture to confirm complete consumption of the acylating agent before phase cut.

Chiral HPLC System Suitability Parameter	Observed Value (S-enantiomer)	Observed Value (R-enantiomer)
Retention time (min)	11.2	13.5
Resolution factor (R_s)	2.8
Tailing factor (USP <621>)	1.2	1.1
Limit of detection (S/N 3)	0.05% (w/w)
Linearity range	0.05–5.0% (R² >0.999)

Handling in a pilot-plant environment uses negative-pressure containment booths with glass fiber-reinforced HEPA filtration (EN 1822-1, class H13) during sifter-blender charging. The compound shows no significant genotoxicity alert in a standard Ames test (OECD 471, strains TA98, TA100 with and without S9 activation), but local irritation tests on reconstituted human epidermis (OECD 439) classify the undiluted solid as corrosive to the epidermal barrier when wetted. Personnel wear 6-mil nitrile gloves changed hourly, and air monitoring badges (SKC 575-002) are worn on the lapel when operations exceed 30 minutes. The material is shipped as an environmentally non-hazardous chemical (not regulated under UN Model Regulations, transport class exempt), yet an REACH-compliant extended safety data sheet is provided with each consignment, listing the derived no-effect level (DNEL) for inhalation at 1.2 mg/m³ (workers, long-term). Incompatibilities include strong alkali metal hydrides, which strip the Boc group exothermically and release isobutylene, and aluminium trichloride, which initiates Friedel-Crafts alkylation at the hydroxymethyl carbon above 60 °C. No encounter with amine-based nucleophiles should proceed in polar aprotic solvents without a proton-scavenger present; premature crosslinking with diisocyanates in polyurethane formulations has been documented when the hydroxyl group reacted before the intended deprotection step.