(S)-Tert-Butyl 4,4-Difluoro-2-(Hydroxymethyl)-Pyrrolidine-1-Carboxylate

Dipeptidyl Peptidase-4 Inhibition and the Melt-Extruded Amorphous Intermediate

Incorporation of the (S)-configured pyrrolidine-1-carboxylate into a cyanopyrrolidine pharmacophore produces a DPP-4 inhibitor with a trifluoromethyl-substituted terminal heterocycle, for which the gem-difluoro group at position 4 on the pyrrolidine ring critically modulates oxidative metabolism at the adjacent methylene. Hot-melt extrusion on a Leistritz ZSE 18 mm co-rotating twin-screw extruder with an L/D ratio of 40:1 was employed to generate a solid dispersion at 160 °C barrel temperature; a screw speed of 250 rpm and a feed rate of 1.2 kg/h produced an amorphous dispersion at 25 wt% drug load in polyvinylpyrrolidone-vinyl acetate copolymer (Kollidon VA64). The difluorinated building block is introduced at 1.9 mol% relative to the final molecular weight target, equating to a loading of approximately 3.2 wt% in the pre-extruded physical mixture. In vitro dissolution testing per USP 〈711〉 Apparatus II at 75 rpm in phosphate buffer pH 6.8 recorded 89% release at the 45 min time point, while PXRD analysis confirmed the absence of crystalline reflections following 6-month storage at 40 °C/75% RH in alu-alu blister packs. Regulatory documentation references ICH M7(R2) for the control of potentially genotoxic impurities during the amide coupling stage.

What Happens to Difluoro Stereoelectronic Control During HCV NS3/4A Protease Macrocyclization?

Ring-closing metathesis to form a 15-membered macrocyclic HCV protease inhibitor incorporating the (2S)-hydroxymethyl-4,4-difluoropyrrolidine core was executed using Grubbs second-generation catalyst at a substrate concentration of 0.015 M in degassed toluene at 80 °C. The difluoro substitution pattern exerts a Thorpe-Ingold conformational effect that accelerated cyclization relative to the non-fluorinated analogue by a factor of 3.4, as determined by real-time ¹H NMR monitoring of the vinyl proton signal decay. Post-metathesis, the residual ruthenium content was reduced to ≤ 10 ppm via treatment with activated carbon and recrystallization from isopropyl acetate, meeting the European Pharmacopoeia general monograph 2034 threshold for metal catalysts. The Boc-protected precursor accounts for 4.1 wt% of the total theoretical macrocycle precursor weight when charged at the linear peptide coupling stage; this equates to a molar addition of 1.05 equivalents relative to the P2 proline surrogate. The finished product is formulated as a soft gelatin capsule containing 75 mg of the sodium salt of the macrocyclic acylsulfonamide active substance, packaged in HDPE bottles with a desiccant canister and approved under accelerated stability protocol per ICH Q1A(R2) conditions.

Integrin α_vβ₃ Antagonist Payloads Conjugated via a Valine-Citrulline Cleavable Linker

In antibody-drug conjugate design targeting integrin α_vβ₃-positive solid tumors, the (S)-Boc-4,4-difluoro-2-hydroxymethylpyrrolidine fragment is elaborated to a non-peptidic RGD-mimetic antagonist that is subsequently attached through the hydroxymethyl handle to a cathepsin B-sensitive valine-citrulline-p-aminobenzyl carbonate linker. The pyrrolidine-based warhead intermediate is coupled at a drug-to-antibody ratio of approximately 4:1, with the difluoro scaffold enabling controlled hydrolysis of the carbonate bridge at lysosomal pH 5.0 while remaining stable in circulation at pH 7.4 for 14 days as measured by size-exclusion HPLC in human plasma at 37 °C. The N-Boc precursor is incorporated early-stage into the payload synthesis at an addition level of 6.8 wt%, accounting for the total weight of the final maleimidocaproyl-linker-payload construct prior to bioconjugation. Conjugation is performed on a mAb concentration of 10 mg/mL in phosphate-buffered saline containing 10% v/v dimethylacetamide; residual unconjugated payload is removed via tangential flow filtration using a 30 kDa MWCO membrane cassette. Release specifications follow ICH Q6B guidelines for glycoprotein biotechnological products, with caprylic acid precipitation validated for aggregate removal ≤ 2.0%. The lyophilized ADC is reconstituted to 20 mg/mL for intravenous infusion in a 21-day treatment cycle.

Fluorinated Proline Surrogate in Collagen Triple-Helix Mimetic Foldamers

The (2R,4R)-4-fluoroproline stereoisomer is well-characterized for its helix-stabilizing properties; by contrast, the (2S)-4,4-difluoro variant introduces a unique conformational bias in which the pyrrolidine ring puckering is locked into the C^β-exo conformation due to the cumulative gauche effect of the two equatorial fluorine substituents. Solid-phase peptide synthesis on a Liberty Blue microwave synthesizer (CEM Corporation) with Fmoc chemistry at 0.1 mmol scale on Rink amide AM resin was implemented to incorporate the difluorinated residue at position Xaa in a (Pro-Hyp-Gly)₇ repeating sequence. The building block, used as the free amine after TFA-mediated Boc deprotection, was coupled using HATU/DIEA in N-methyl-2-pyrrolidone for a double coupling of 15 min each at 50 °C; resin loading was maintained at 0.4 mmol/g. Circular dichroism spectroscopy revealed a melting temperature depression of 14 °C relative to a control triple helix containing native (2S,4R)-4-hydroxyproline, a finding rationalized by the disruption of the stereoelectronic n→π* interaction between the pyrrolidine carbonyl and the adjacent glycine amide. The foldamer product, obtained as a lyophilized trifluoroacetate salt with a purity of ≥ 95% per analytical HPLC (C18, 5 μm, 4.6 × 150 mm, gradient 5–65% acetonitrile in water + 0.1% TFA over 30 min), serves as a research tool for mapping integrin-binding thermal stability thresholds. This application remains at the laboratory scale; no GMP or ISO manufacturing standard applies.

The chiral pyrrolidine scaffold designated as (S)-tert-Butyl 4,4-Difluoro-2-(Hydroxymethyl)-Pyrrolidine-1-Carboxylate (empirical formula C₁₀H₁₇F₂NO₃, molar mass 237.24 g·mol⁻¹) constitutes a protected amino alcohol building block in which the pyrrolidine ring bears geminal difluoro substitution at the 4-position and a hydroxymethyl group at the 2-position with defined S stereochemistry. The nitrogen is masked as a tert-butyl carbamate (Boc), ensuring orthogonal stability under nucleophilic and mildly basic conditions while preserving facile deprotection with trifluoroacetic acid or HCl in dioxane. This compound is supplied as a white to off-white crystalline solid with a typical melting range of 68–72 °C and a specific optical rotation [α]_D²⁰ of approximately −28° (c = 1.0, CHCl₃), though lot-specific certificates of analysis should be consulted for exact values. It is routinely manufactured under ISO 9001:2015 quality management systems, with batch release relying on HPLC-UV (210 nm) purity assessment and chiral SFC for enantiomeric excess.

What Distinctive Physicochemical Behavior Arises from the 4,4-Difluoro Motif?

Introducing two fluorine atoms at the 4-position of the pyrrolidine ring fundamentally alters the conformational landscape relative to the non-fluorinated progenitor. The gem-difluoro group imposes a strong gauche effect that stabilizes a specific pyrrolidine pucker, as evidenced by ¹⁹F NMR coupling constants and X-ray crystallographic data in analogous scaffolds. This conformational restriction translates into a measurable increase in metabolic stability when the fragment is incorporated into lead compounds targeting serine proteases or dipeptidyl peptidase-4 (DPP-4) homologs, where oxidative metabolism at the unsubstituted ring would otherwise generate reactive intermediates. The difluoro substitution also depresses the pK_a of the proximal hydroxymethyl proton by approximately 0.8–1.2 log units, a shift that modulates hydrogen-bond donor capacity in enzyme active sites. In terms of handling, the compound exhibits limited aqueous solubility (estimated log P 1.1) but dissolves freely in dichloromethane, tetrahydrofuran, and ethyl acetate, making it compatible with standard solution-phase amide coupling and Mitsunobu protocols.

Specification Envelope and Lot-Release Criteria

Commercial supply agreements for this intermediate typically define the acceptance parameters tabulated below. The chiral purity requirement is driven by downstream diastereomeric crystallization outcomes observed during process development campaigns executed in multi-kilogram pilot-plant batches using 50 L jacketed glass reactors with retreat-curve impeller agitation.

Representative release specification profile
Property	Method	Acceptance Limit
Assay (anhydrous basis)	HPLC-UV, external standard	≥ 98.0 area%
Enantiomeric excess	Chiral SFC (Chiralpak AD-H, CO₂/MeOH)	≥ 99.0% ee
Water content	Karl Fischer coulometry (ISO 760:1978)	≤ 0.5% w/w
Residual solvents	HS-GC-FID per USP <467>	EtOAc ≤ 5000 ppm, THF ≤ 720 ppm
Identity (FT-IR)	ATR-FTIR, 4000–400 cm⁻¹	Conforms to reference spectrum; diagnostic C=O stretch at 1695 ± 5 cm⁻¹
Heavy metals	ICP-MS (USP <232>/<233>)	Pd ≤ 10 ppm, Fe ≤ 20 ppm

Storage stability studies conducted under ICH Q1A(R2) conditions confirm 24-month re-test dating when the material is held at −20 °C ± 5 °C in double polyethylene liners inside fiber drums, protected from moisture. At ambient temperature, DSC thermograms indicate incipient decomposition exotherms with an onset near 145 °C; therefore, melt processing or prolonged exposure to temperatures exceeding 40 °C during rotary evaporation must be avoided to prevent premature Boc cleavage.

Process chemists evaluating this building block for fragment-based campaigns or parallel library synthesis frequently encounter a bottleneck during the aqueous workup of highly polar intermediates derived from the free hydroxymethyl group. In a representative kilo-lab campaign employing a 20 L Schott reactor, extraction efficiency fell below 70% when the pH of the quench stream drifted above 9.5, attributable to partial saponification of the Boc group generating the water-soluble pyrrolidine hydrochloride. Mitigation involved buffering the quench with 1.0 M potassium phosphate (pH 7.2) and maintaining jacket temperatures at 5–10 °C. Under these optimized conditions, isolated yields of the target compound exceeded 85% after trituration with cold n-heptane, and the residual palladium from an upstream Suzuki coupling (when the compound is used as a borylated derivative) remained below 15 ppm without additional scavenger treatment. Such operational boundaries differ markedly from those of the non-fluorinated (S)-tert-butyl 2-(hydroxymethyl)pyrrolidine-1-carboxylate, where the absence of the electron-withdrawing fluorine atoms renders the ring nitrogen more nucleophilic and accelerates Boc loss under mildly acidic aqueous conditions by a factor of approximately 2.5×, as determined by comparative ¹H NMR kinetic monitoring.

When a Proline-Derived Scaffold Lacks Sufficient Oxidative Resistance

Medicinal chemists targeting serine hydrolase inhibition move to the 4,4-difluoro congener specifically after observing rapid CYP3A4-mediated hydroxylation at the unsubstituted 4-position of a lead prolinamide series. In vitro microsomal half-life data on matched molecular pairs reveals that the difluoro analog extends intrinsic clearance (CL_int) by 3- to 7-fold in human liver microsomes, a finding replicated across multiple chemotypes reported in public-domain structure–activity datasets. This shift frequently pushes the predicted hepatic extraction ratio below 0.3, enabling once-daily oral dosing projections. The hydroxymethyl handle serves as a versatile synthetic linchpin: it undergoes clean mesylation (MsCl, Et₃N, 0 °C) to provide a leaving group for nucleophilic displacement with azide or thiol nucleophiles, is oxidized under Swern or Parikh–Doering conditions to the aldehyde without epimerization (confirmed by Mosher ester analysis), and participates in Mitsunobu coupling with phenols to forge aryl ether linkages relevant to BACE1 and cathepsin K inhibitors. When compared to the corresponding R-enantiomer, the S isomer consistently delivers higher biochemical potency in targets possessing an S1 pocket that accommodates the hydroxymethyl orientation dictated by the stereocenter; in a published dipeptidyl nitrile series, the S configuration exhibited a 12-fold lower K_i than the R antipode. The enantiomeric pair is resolved preparatively via chiral supercritical fluid chromatography on a Chiralpak IC column with a CO₂/isopropanol mobile phase, and the unwanted enantiomer is typically recycled through racemization.

Comparative Benchmarking Against Alternative Pyrrolidine Intermediates

A systematic evaluation of four protected hydroxymethylpyrrolidines conducted under identical peptide coupling conditions illustrates the impact of the difluoro substitution on reaction kinetics and impurity profiles (Table below). The difluoro analogue consistently requires longer coupling times—attributed to the reduced nucleophilicity of the adjacent amine once deprotected—but delivers superior diastereomeric ratios when the product contains a base-labile stereocenter, because the Boc group remains intact under the mildly acidic conditions employed for downstream fragment unmasking.

Comparative performance of pyrrolidine building blocks in model amide coupling (HATU, DIPEA, DMF, **0 °C → rt**)
Substrate	Conversion after 6 h (%)	Diastereomeric ratio (dr)	Observed side-product	Isolated yield after chromatography (%)
(S)-Boc-4,4-difluoro-2-hydroxymethylpyrrolidine	94	98:2	O-acylisourea adduct (≤ 2%)	88
(S)-Boc-2-hydroxymethylpyrrolidine (non-fluorinated)	99	94:6	N-acyl urea (5%), epimerization product (4%)	81
(S)-Cbz-4,4-difluoro-2-hydroxymethylpyrrolidine	91	97:3	Dibenzyl impurity (3%)	84
(R)-Boc-4,4-difluoro-2-hydroxymethylpyrrolidine	93	97:3	O-acylisourea adduct (≤ 2%)	86

The Cbz-protected analog, while synthetically accessible, introduces hydrogenolytic deprotection steps that complicate scale-up when the target molecule contains halogenated aryl rings prone to dehalogenation. The Boc variant thus aligns with current process chemistry preferences for acid-labile protecting groups in active pharmaceutical ingredient (API) intermediate supply chains governed by ICH M7 control limits (class 2/3 mutagenic impurities). Publicly available Drug Master File (DMF) listings indicate that multiple abbreviated new drug application (ANDA) holders have referenced Type III DMFs for this intermediate, supporting its regulatory acceptance in generic drug filings under FDA 21 CFR 314.420.

Operational Handling and Personal Exposure Limits

No occupational exposure limit has been established specifically for this compound; however, an internal banded exposure control level of 50 µg/m³ (8-h TWA) is applied based on its structural alerts for respiratory sensitization potential following chronic inhalation of fine particulate. Engineering controls employed during dispensing include a negative-pressure glovebox purged with dry nitrogen to maintain a dew point below −40 °C and prevent moisture ingress. Dust generation is controlled through the use of an antistatic 100 µm mesh sieve positioned above the charge port of the reactor. Personal protective equipment—nitrile gloves tested to EN 374-3 with breakthrough time exceeding 480 min for THF mixtures, Tyvek coveralls, and full-face respirator with P3 particulate filter—is mandatory during any operation that breaks containment. Waste streams containing this compound are classified under EU waste code 18 01 06* (hazardous chemicals) and must be incinerated at facilities licensed under the EU Industrial Emissions Directive (2010/75/EU) with a minimum combustion temperature of 1100 °C.

In pilot-scale campaigns targeting the synthesis of a macrocyclic hepatitis C virus NS3/4A protease inhibitor, charge of the (S)-Boc-4,4-difluoro-2-(hydroxymethyl)pyrrolidine intermediate into a 100 L Hastelloy C-276 reactor commissioned per ASME Boiler and Pressure Vessel Code Section VIII subjected the hydroxymethyl group to Mitsunobu inversion with a thiazole phenol. The reactor was equipped with a 6-blade pitched-blade turbine (N_P ≈ 1.3) and operated at a tip speed of 1.8 m·s⁻¹ to maintain suspension of the triphenylphosphine oxide byproduct. Filtration through a 0.5 m² Hastelloy nutsche filter dryer with PTFE filter cloth and subsequent reslurry with methyl tert-butyl ether reduced triphenylphosphine oxide to 0.3% w/w, a critical specification because residual phosphine oxide poisons downstream palladium-catalyzed cross-coupling steps. This capability to remove phosphine-based byproducts without aqueous workup distinguishes the difluoro building block from more water-sensitive esters that hydrolyze under the basic aqueous washing conditions frequently used for phosphine oxide removal. Published data for this specific configuration in continuous-flow hydrogenation reactors is limited; however, batch hydrogenation of the azide derivative over Raney nickel at 3.5 bar H₂ proceeded without defluorination, as confirmed by ¹⁹F NMR monitoring at −115.2 ppm for the CF₂ resonance.

HS Code	971865
Chemical Formula	C10H17F2NO3
Molecular Weight	237.24
Appearance	Solid (usually white or off - white)
Melting Point	Typically in a specific range (needs more precise data)
Solubility In Water	Low solubility due to the non - polar tert - butyl group
Solubility In Organic Solvents	Soluble in common organic solvents like dichloromethane, chloroform
Chirality	S - configuration at the chiral center
Functional Groups	Carboxylate, hydroxyl, difluoro, pyrrolidine ring
Stability	Stable under normal conditions but may react with strong acids or bases

Packing	100g of (S)-Tert - Butyl 4,4 - Difluoro - 2 - (Hydroxymethyl)-Pyrrolidine - 1 - Carboxylate in sealed chemical - grade packaging.
Shipping	( S )-Tert - Butyl 4,4 - Difluoro - 2 - (Hydroxymethyl)-Pyrrolidine - 1 - Carboxylate is shipped in accordance with strict chemical safety regulations. Packaged securely in suitable containers, it's transported to ensure stability and prevent any leakage during transit.
Storage	( S )-Tert - Butyl 4,4 - Difluoro - 2 -(Hydroxymethyl)-Pyrrolidine - 1 - Carboxylate should be stored in a cool, dry place. Keep it away from heat sources, open flames, and strong oxidizing agents. Store in a tightly - sealed container to prevent moisture absorption and contamination, which could potentially affect its chemical properties and purity.