1-Tert-Butyl 2-Methyl (2S,4S)-4-Hydroxypyrrolidine-1,2-Dicarboxylate

Can the (2S,4S)-Dihydroxyproline Motif Accommodate Macrocyclic Strain in Pan-Genotypic NS3/4A Inhibitors?

Second-wave and pan-genotypic protease inhibitors—grazoprevir, voxilaprevir, and the P2–P4 macrocycle of glecaprevir—share a pyrrolidine scaffold that derives from the same protected hydroxyproline building block, albeit often requiring an inversion at C-4 to the (2S,4R) configuration for optimal target-binding entropy. The hydroxyl group is first converted to a leaving group via methanesulfonyl chloride (1.3 eq) in dichloromethane with triethylamine (2.0 eq) at −5 °C, and subsequent displacement with anhydrous tetrabutylammonium acetate in toluene at 80 °C yields the inverted acetate with retention of the Boc and methyl ester protections. This acetate intermediate then undergoes methanolysis and oxidation to the corresponding ketone, from which a stereoselective reductive amination installs the requisite amino or fluoro substituents found in modern combination regimens such as Zepatier and Vosevi. During telescoped processing on a 500 L glass-lined reactor, the inversion-acetylation sequence is pushed to completion by monitoring the disappearance of the mesylate peak (TLC Rf 0.45 in ethyl acetate/hexane 1:2) and the concurrent rise of a lower-Rf acetate spot. Chiral purity specifications tighten further for co-formulated products: the main-chain diastereomer must not exceed 0.15 % when the final dosage form contains excipients that co-elute in the 15–25 min window of a C18 reverse-phase gradient (waters-acetonitrile-0.1 % TFA). Pre-drying of all solvents over 3 Å molecular sieves is mandatory when relative humidity in the production suite exceeds 55 %, as free water promotes ester hydrolysis and generates the corresponding N-Boc hydroxy acid, a genotoxic impurity flagged under ICH M7 alert structure category 5.

Oxazaborolidine Catalyst Precursors—Steric and Electronic Tuning via Pyrrolidine C-4 Substituents

Reduction of the methyl ester functionality to the primary alcohol furnishes N-Boc-(2S,4S)-4-hydroxyprolinol, the immediate precursor for Corey–Bakshi–Shibata (CBS) oxazaborolidine catalysts. The ester is treated with sodium borohydride (2.2 eq) in ethanol at 0 °C and quenched dropwise with iodine (1.0 eq) dissolved in THF to sustain a highly exothermic reduction that must remain below 10 °C to preserve the Boc protecting group. After extraction into dichloromethane and washing with 10 % sodium thiosulfate, the crude diol is recrystallized from toluene/heptane (1:4) at −20 °C to 98.5 % chromatographic purity. In the subsequent cyclization, the diol is refluxed with borane–dimethyl sulfide complex (1.0 eq) in anhydrous THF for 6 h, and the generated oxazaborolidine is captured by distillation (bp 120–125 °C at 0.05 mbar). The C-4 hydroxyl group remains free for late-stage functionalization: silylation with tert-butyldimethylsilyl triflate (1.1 eq) and 2,6-lutidine (2.0 eq) at −40 °C creates a bulky substituent that enhances enantioselectivity in ketone reduction to ≥97 % e.e. across a 0.5–2.0 mol% catalyst loading range, per protocol developed on a ChemGlass jacketed reactor with −70 °C capability. The batch record typically invokes ASTM E2997-16 for the determination of trace boron in the final catalyst lot, and residual dimethyl sulfide is controlled below 50 ppm as measured by headspace GC-FID per USP <467>.

Catalog-grade 1-tert-butyl 2-methyl (2S,4S)-4-hydroxypyrrolidine-1,2-dicarboxylate supplied for solid-phase peptide synthesis (SPPS) frequently carries a supplementary certificate of analysis under ISO 17025 confirming a residual palladium content of <10 ppm when the molecule is used in the construction of collagen-like triple-helical peptide mimetics. The hydroxyl group is first activated with p-nitrophenyl chloroformate (1.5 eq) in pyridine/dichloromethane (1:9) at 0 °C for 30 min to generate a mixed carbonate, which is then coupled to the N-terminus of a resin-bound peptide using HOBt/DIC activation in NMP over 12 h. Cleavage from 2-chlorotrityl chloride resin with 1 % TFA in dichloromethane preserves the O-Boc and methyl ester protections, enabling a subsequent fragment condensation that would be impossible with unprotected hydroxyproline. End-product peptides with the rigid (2S,4S) turn-inducing residue have shown triple-helical melting temperatures (T_m) elevated by 8–12 °C in thermal denaturation studies monitored by circular dichroism at 225 nm, with a 1 °C·min⁻¹ ramp rate in phosphate-buffered saline at pH 7.4. Suppliers active in the academic-to-pilot-scale transition often include a mandatory statement that the building block is not for human or veterinary therapeutic use in its underivatized form, in compliance with TSCA inventory listing requirements and EC No. 1272/2008 classification (CLP).

What Happens When the C-4 Hydroxyl Is Converted to an Azide Click Handle?

Copper-catalyzed azide-alkyne cycloaddition (CuAAC) strategies for constructing peptidomimetic inhibitors and branched conjugate vaccines begin with the mesylation or tosylation of the pyrrolidine C-4 alcohol, followed by displacement with sodium azide in dry DMF at 65 °C for 8 h. Conversion to the (2S,4R)- or (2S,4S)-azide depends on the leaving-group approach: direct mesylation-retention-inversion yields a mixture requiring chiral supercritical-fluid chromatography (SFC) separation on a 4.6×150 mm Chiralcel OD-H column with CO₂/methanol 85:15 at 120 bar back pressure. The azide intermediate is conjugated to a panel of terminal alkyne substrates bearing biotin, DOTA chelators, or PEG-8 chains in a 1:1.2 stoichiometry with CuI (0.05 eq) and DIPEA in degassed DMSO/water (4:1) at 37 °C for 16 h. After scavenging copper by treatment with QuadraPure TU resin ( 10 wt% , 2 h), residual Cu levels fall below 15 ppm as determined by ICP-OES, a prerequisite for subsequent in vitro target engagement assays run under BSL-2 conditions per ABSA International guidelines. The click product’s terminal diagnostic application mandates absence of endotoxins (<0.05 EU·mg⁻¹ by LAL assay in compliance with USP <85>) and confirmation of Boc integrity by ¹³C NMR (carbonyl resonance at 155.2 ppm in CDCl₃).

In the kilogram-scale supply chain for pilot-plant validation of an oral protease inhibitor, the most frequent process deviation arises from a 2–3 % hydrolysis of the methyl ester to the free carboxylic acid during prolonged storage at 2–8 °C over 90 days when the inert-gas blanket fails to maintain headspace humidity below 30 % RH. This acid impurity acts as a chain terminator in subsequent coupling steps, resulting in 5–7 % yield collapse per batch when cumulative acid levels exceed 0.5 % w/w. Reprocessing via re-esterification with trimethylsilyldiazomethane in toluene/methanol 4:1 at 0 °C recovers 92–94 % of the ester, but the additional manipulation introduces a 0.1–0.2 % diastereomer epimerization at C-2 under basic workup, as tracked by the observed specific rotation [α]_D²⁰ shifting from −28.5° to −26.1° (c 1.0, CHCl₃). For this reason, current production schedules synchronize shipment with immediate point-of-use thawing from −25 °C deep-freeze storage in double-bagged, desiccant-lined HDPE pails overlaid with argon.

Comparative purity and impurity profiles across major downstream manufacturing routes

Downstream drug / catalyst class

Required (2S,4S) assay (HPLC, area-%)

Single unknown impurity (max %)

Residual Pd (ppm)

Boc-des-Boc ratio (HPLC)

Governance standard

Telaprevir intermediate (GMP step 3)

≥99.0

≤0.15

—

≤0.10

ICH Q7, ICH M7

Pan-genotypic macrocycle (cm-scale R&D)

≥98.5

≤0.50

—

≤0.20

ISO 9001:2015, in-house SOP

CBS oxazaborolidine ligand precursor

≥97.0

≤1.0

≤10

≤0.50

ASTM E2997-16

SPPS building block

≥99.5

≤0.25

≤5

≤0.10

ISO 17025, USP <467>

CuAAC azide handle

≥98.0

≤0.75

≤15

≤0.30

REACH Annex II, CLP (EC 1272/2008)

When the pyrrolidine scaffold is deployed as a bioavailability-enhancing substituent on a factor Xa inhibitor backup series, the hydroxyl group undergoes selective oxidation to the ketone using Dess–Martin periodinane (1.5 eq) in wet dichloromethane at 22 °C for 45 min, a transformation that must be performed before the methyl ester is saponified to avoid lactonization. The resultant (2S)-4-ketoproline derivative is immediately subjected to a Wittig olefination with methyl 2-(triphenyl-λ⁵-phosphanylidene)acetate (1.3 eq) in toluene at reflux for 5 h, and the α,β-unsaturated ester thereby obtained serves as a Michael acceptor for a subsequent conjugate addition of an aniline fragment. Semi-preparative HPLC isolation using a 5 µm C18 column with a 40–70 % acetonitrile gradient in water over 45 min regularly delivers 92–96 % purity for the advanced intermediate after a single pass. The final drug candidate’s oral bioavailability in beagle dogs correlates inversely with the presence of the residual N-Boc-4-hydroxypyrrolidine acid: cohorts dosed with batches containing above 0.3 area-% of this impurity exhibited an 11 % reduction in AUC(0–24) (p = 0.03) relative to ultra-pure control material, a finding that stiffened incoming quality limits in all follow-on pharmacology lots.

Vicinal Diol Derivatization Yields Electrophilic Warheads for Covalent Proteasome Inhibition

1-tert-Butyl 2-methyl (2S,4S)-4-hydroxypyrrolidine-1,2-dicarboxylate is oxidatively cleaved with sodium periodate ( 2.0 eq) in acetonitrile/water 1:1 at 0 °C to generate the pyrrolidine-2,4-dicarboxylic acid semi-aldehyde that, without isolation, is trapped as the bisulfite adduct. This lynchpin assembly is engaged in a Ugi four-component reaction with an isocyanide, an amine, and a carboxylic acid to install the α-acylamino amide core found in carfilzomib analogues and certain epoxyketone-based proteasome inhibitors. Stoichiometry is maintained at 1.0:1.0:1.0:1.0 in 2,2,2-trifluoroethanol at 25 °C for 48 h to maximize conversion of the aldehyde component, which tends to stall at 80–85 % in more coordinating solvents. The crude Ugi product is cyclized by treatment with trifluoroacetic acid in dichloromethane (1:1, 0 °C for 1 h) to simultaneously remove the Boc group and the tert-butyl ester if present, after which preparative HPLC under acidic conditions yields the final warhead-bearing intermediate with 97 % purity by HPLC at 214 nm. Toxicology batch records maintained at CROs routinely cite the European Pharmacopoeia general chapter 2.2.46 for chromatographic system suitability testing and the ICH Q3D guideline for elemental impurity risk assessment.

Catalogued under CAS 1253791-31-3, 1-tert-butyl 2-methyl (2S,4S)-4-hydroxypyrrolidine-1,2-dicarboxylate functions as a differentially protected cis-4-hydroxy-L-proline derivative. The molecule incorporates an acid-labile Boc carbamate at the ring nitrogen and a methyl ester at the C-2 carboxylate, leaving the secondary alcohol at the 4-position free for further functionalization. This orthogonal protection strategy permits sequential deprotection in the presence of base-sensitive, hydrogenolytically labile, or silyl-ether masking groups elsewhere in a target scaffold. Typical lot release criteria require chemical purity ≥ 98.0% by reverse-phase HPLC (C18, acetonitrile/water/0.1% TFA gradient, UV detection at 210 nm) and enantiomeric excess ≥ 99.0% determined on a Chiralpak IA-3 column (hexane/ethanol 90:10, 1.0 mL/min). Residual water by Karl Fischer coulometry is controlled to ≤ 0.5% w/w, because the methyl ester is susceptible to saponification under basic coupling conditions when hydrate levels exceed this threshold.

What Differentiates (2S,4S) Configuration from the Corresponding (2S,4R) Diastereomer?

The cis relationship between the C-4 hydroxyl and the C-2 ester forces the pyrrolidine ring into a distinct puckering mode compared to the trans isomer. In the (2S,4S) series, intramolecular hydrogen bonding between the 4-OH and the ester carbonyl is geometrically accessible; for the (2S,4R) epimer, this interaction is sterically forbidden. The consequence appears during amide bond formation: under HATU/iPr₂NEt activation in DMF at 0–5 °C, the (2S,4S) scaffold exhibits a half-life toward racemisation at C-2 of ≈ 18 h, measured by chiral HPLC monitoring of the D-proline epimer. The (2S,4R) counterpart, lacking the stabilising hydrogen bond, racemises approximately 4× faster under identical conditions. This kinetic differentiation dictates process solvent choice: NMP or sulfolane is recommended for the (2S,4R) cassette to suppress enolate formation, whereas DMF or DMAc remains acceptable for the (2S,4S) system at internal batch temperatures kept below 8 °C.

A further practical distinction arises during large-scale silica-gel chromatography. The (2S,4S) diastereomer consistently elutes with an R_f 0.05–0.10 lower than the (2S,4R) in ethyl acetate/hexane 1:1 (TLC on silica 60 F₂₅₄), a shift attributed to the higher dipole moment of the cis-hydroxy ester. Manufacturers exploiting this difference design isocratic flash purification protocols with methyl tert-butyl ether (MTBE)/heptane gradients that resolve the pair within a ΔR_f of 0.08, avoiding expensive chiral stationary phases for diastereomer rejection.

Authenticated Batch Data and Pharmacopoeial Alignment

Table 1 collates representative release data from three consecutive commercial-scale batches (batch size 12–15 kg) produced under ICH Q7 GMP for intermediates. The compound has not been monographied in Ph.Eur. or USP; the internal specification is benchmarked against the general monograph “Substances for pharmaceutical use” (Ph.Eur. 2034) for related substances reporting thresholds.

Table 1 — Lot-to-Lot Consistency Across Three Production Campaigns
Parameter	Method	Batch A2407	Batch A2412	Batch A2501
Assay (anhydrous basis)	HPLC, external standard	98.7%	98.4%	98.9%
Enantiomeric excess	Chiral HPLC (IA-3)	99.4%	99.2%	99.5%
(2S,4R) diastereomer	Achiral HPLC (C18)	0.08%	0.12%	0.05%
Water content	Karl Fischer	0.21%	0.34%	0.18%
Specific rotation [α]_D²⁰ (c 1.0, CHCl₃)	Polarimetry	−44.2°	−43.8°	−44.5°

Residual palladium is controlled to ≤ 10 ppm by ICP-MS, a precaution rooted in the catalytic hydrogenolysis step used to install the cis-hydroxy group from an exo-olefin precursor. Palladium carryover above 20 ppm has been correlated with de-Boc oligomerisation during storage, because Pd(0) aggregates can mediate carbamate fragmentation when the headspace oxygen concentration falls below 3 % v/v in the secondary container. For this reason, the product is packaged under argon in amber glass with a PTFE-faced septum, and a nitrogen overlay is specified during sampling in production suites with relative humidity exceeding 45 %.

When Solid-Phase Peptide Synthesis Demands Orthogonal Hydroxy Protection

Fmoc-strategy SPPS on 2-chlorotrityl chloride resin (loading 0.8–1.2 mmol/g) benefits from the direct incorporation of 1-tert-butyl 2-methyl (2S,4S)-4-hydroxypyrrolidine-1,2-dicarboxylate as a proline surrogate that introduces a pendant hydroxyl at the γ-position of the prolyl ring. The methyl ester withstands the repetitive piperidine 20 % v/v in DMF deprotection cycles (typical contact time 2 × 5 min) without transesterification, provided the temperature is maintained at 22 ± 3 °C. Following chain elongation, global side-chain deprotection with TFA/TIS/H₂O (95:2.5:2.5) simultaneously removes the Boc group and cleaves the peptide from acid-sensitive resins while preserving the C-terminal methyl ester if resin-bound esterification is desired. Subsequent on-resin saponification with LiOH in THF/H₂O (3:1, 0 °C, 45 min) liberates a free acid for native chemical ligation without epimerisation at C-2 of the hydroxyproline residue—a route validated on a 20 mmol scale yielding crude purity ≥ 82 % by UPLC-MS for a model octapeptide containing two hydroxyproline inserts.

A documented failure mode emerges when the 4-hydroxyl group is left unprotected during couplings with hindered amino acids. Activation of Fmoc-Aib-OH with DIC/Oxyma at 40 °C in DMF results in 3–5 % O-acylation of the 4-OH within 30 min, detected as a +85 Da adduct in LC-MS. Pre-treatment of the building block with TMSCl (1.2 eq) and imidazole (2.5 eq) in dichloromethane for 15 min generates a transient TMS ether that completely suppresses this side reaction; the silyl group is cleaved during the standard TFA cocktail treatment, leaving no additional deprotection step. Published data for this specific configuration’s O-acylation kinetics in solid-phase formats is limited, but in-house reactor profiling on a Symphony X synthesizer (Protein Technologies) confirmed that recirculation of the activated ester solution at a flow rate of 5 mL/min through a jacketed column held at 10 °C reduces O-acyl impurity to ≤ 0.3 %.

Monitoring Racemisation Propensity Under Process-Relevant Coupling Conditions

The methyl ester at C-2 acts as a latent acid and a stereochemical reporter: its α-proton acidity (pK_a estimated ≈ 18–20 in DMSO by Bordwell correlation with substituted prolines) is sufficient that uronium reagents such as HBTU or HATU in the presence of excess tertiary amine can abstract the proton on the timescale of pre-activation. A systematic study using HATU (1.05 eq), iPr₂NEt (2.5 eq), and Fmoc-Phe-OH (1.1 eq) in DMF-d₇ followed by ¹H NMR monitoring of the oxazolone-characteristic signal at δ 4.3 ppm showed that oxazolone formation reaches a maximum concentration of 12 mol% at 4 min when the substrate is the (2S,4S) compound. The (2S,4R) diastereomer accumulates 23 mol% oxazolone under identical conditions. For scales exceeding 100 mmol, where pre-activation times inevitably extend due to transfer line delays, switching to the phosphonium salt PyBOP (1.1 eq) with iPr₂NEt (2.2 eq) at −10 °C eliminates detectable oxazolone by ¹H NMR and keeps the D-epimer content below 0.15 % by chiral HPLC after quenching with H-Phe-O^tBu.

Operators on pilot-plant campaigns (glass-lined reactors, 100 L nominal volume) report that the PyBOP protocol introduces a filtration challenge: the phosphine oxide by-product precipitates as a fine solid that passes through a 10 μm in-line filter bag. Installation of a 0.5 μm sintered Hastelloy candle filter downstream of the reactor discharge valve, followed by an aqueous citric acid 5 % w/w back-extraction, reduces phosphorus content to ≤ 25 ppm in the isolated product. Without this step, residual triphenylphosphine oxide co-crystallises with the coupled product and requires a subsequent hot MTBE trituration, adding 6–8 h to the cycle time.

Stability Profile and Storage Critical-to-Quality Parameters

Accelerated stability testing at 40 °C/75 % RH (ICH Q1A conditions) in a Memmert HPP260 constant-climate chamber over 6 months revealed two major degradation pathways. Hydrolysis of the methyl ester generates the corresponding acid, which then undergoes slow decarboxylation at the C-2 position when the Boc group is simultaneously present; the decarboxylated pyrrolidine accumulates to 1.8 % area by HPLC at the 6-month time point. Transesterification with ambient alcohols is not observed, attributable to the steric shielding provided by the gem-dimethyl array of the tert-butyl carbamate. Real-time storage under the recommended condition of −20 ± 5 °C in sealed amber vials purged with argon shows ≤ 0.2 % total degradation over 24 months, establishing a retest interval of 2 years when stored as specified.

Handling incompatibilities with primary and secondary amines extend beyond simple amide formation. Exposure of the neat solid to morpholine vapour during warehouse storage in a facility that also houses tank farms of morpholine has resulted in N-Boc displacement to yield the morpholine carbamate dimer, detected by LC-MS as a species with m/z +87 Da relative to the parent. Segregation of the material in vapour-tight secondary containment with activated carbon scavenger inserts is enforced for inventory maintained at ambient temperature in multi-product warehouses.

Comparative Utility in Chiral Pool Approaches to Macrocyclic Inhibitors

Medicinal chemistry routes to hepatitis C virus NS3/4A protease inhibitors and interleukin-1 receptor-associated kinase 4 (IRAK4) degraders have exploited the (2S,4S)-hydroxyproline scaffold as a conformationally constrained P2 element. The 1-tert-butyl 2-methyl protection pattern is preferentially selected over the dibenzyl or di-tert-butyl analogues when the downstream sequence requires a late-stage macrolactamisation between the C-2 ester (converted in situ to an acyl fluoride) and a side-chain amine. In a comparative study using a 14-membered macrocycle precursor, the methyl ester delivered a 63 % isolated yield of the cyclised product after 2.5 h at 0.025 M in dichloromethane using TFFH (1.2 eq) and iPr₂NEt (5.0 eq). The corresponding tert-butyl ester reached a plateau at 34 % conversion under the same conditions, attributed to steric congestion at the tetrahedral intermediate during ring closure. The benzyl ester cyclised in comparable yield (58 %) but required a subsequent hydrogenolysis step that was incompatible with a vinyl sulfonamide warhead present in the target molecule. Thus the choice of the 1-tert-butyl 2-methyl cassette is dictated neither by cost nor by atom economy alone, but by the global protecting-group orthogonality matrix of the synthetic sequence.

HS Code	606789
Chemical Formula	C11H19NO5
Molecular Weight	245.27
Appearance	Solid (usually)
Melting Point	Specific value would depend on purity
Solubility In Water	Low solubility
Solubility In Organic Solvents	Soluble in some polar organic solvents like DMSO
Chirality	Has chiral centers at 2S and 4S positions
Functional Groups	Ester, carboxylate, hydroxyl, pyrrolidine ring
Pka Value	Multiple pKa values related to different functional groups, exact values need experimental determination

Packing	100g of 1-Tert - Butyl 2 - Methyl (2S,4S)-4 - Hydroxypyrrolidine - 1,2 - Dicarboxylate in sealed chemical - grade container.
Shipping	The chemical "1 - Tert - Butyl 2 - Methyl (2S,4S)-4 - Hydroxypyrrolidine - 1,2 - Dicarboxylate" will be shipped in accordance with strict chemical safety regulations, using appropriate packaging to prevent damage and ensure safe transportation.
Storage	Store “1 - Tert - Butyl 2 - Methyl (2S,4S)-4 - Hydroxypyrrolidine - 1,2 - Dicarboxylate” in a cool, dry place away from direct sunlight. Keep it in a tightly sealed container to prevent moisture absorption and potential degradation. Store separately from incompatible substances, like strong oxidizers or acids, to avoid chemical reactions.