2-(1-(Tert-Butoxycarbonyl)Pyrrolidin-2-Yl)Thiazole-4-Carboxylic Acid

The fate of the Boc-pyrrolidine-thiazole-4-carboxylate fragment under superacid-catalyzed electrophilic fluorination conditions and its application to ¹⁸F-radiotracer synthesis

Positron emission tomography (PET) radiolabeling of heterobifunctional degrader molecules for pharmacokinetic imaging studies exploits the carboxy-thiazole moiety as a synthetic handle for late-stage ¹⁸F-fluorination via the corresponding p-nitrophenyl ester or acyl fluoride. The Boc-protected intermediate is dissolved in anhydrous acetonitrile containing 4 Å molecular sieves and treated with 1.1 equivalents of p-nitrophenyl chloroformate and pyridine (1.2 equiv) at 0°C for 3 hours to generate the p-nitrophenyl ester in 85–92% isolated yield after precipitation from cold diethyl ether; this activated ester is stable to storage at −20°C under argon for at least 6 months. Reaction of the p-nitrophenyl ester with ¹⁸F-fluoride in the presence of Kryptofix 2.2.2 and K₂CO₃ in DMSO at 120°C for 10 minutes produces the [[¹⁸F]fluoromethyl]thiazole radiotracer intermediate, which after Boc-deprotection with TFA (1 minute at 80°C) and semi-preparative HPLC purification (40% EtOH/60% 10 mM NaH₂PO₄, pH 4.5) yields the injectable PET tracer with molar activity >37 GBq/μmol and radiochemical purity >99%. The free pyrrolidine generated post-deprotection is directly conjugated to a bifunctional VHL E3 ligase ligand bearing an activated PEG₃-COOH linker under HATU/DIPEA conditions in DMF (30 minutes, 25°C), producing the ternary degrader molecule. Quality control per ICH Q3C (Residual Solvents) quantifies acetonitrile (Class 2, limit 410 ppm), DMF (Class 2, limit 880 ppm), and TFA (Class 4, reporting threshold 0.1%) by headspace GC-FID; sterility and bacterial endotoxin tests per USP <71> and USP <85> respectively are required for any lot intended for clinical PET imaging under an Exploratory IND (eCFR 21 Part 312). The specific radioactivity decay-corrected to end-of-synthesis is a critical release parameter, with failure modes including insufficient drying of the azeotropically dried ¹⁸F-fluoride/K2.2.2/K⁺ complex, which results in residual water competing for the activated ester and reducing radiochemical yield below 5% (non-decay corrected).

Proteolysis-targeting chimeras (PROTACs) designed to recruit Cereblon (CRBN) or von Hippel-Lindau (VHL) E3 ubiquitin ligases to oncogenic drivers (e.g., BTK, AR-V7, BRD4) utilize the (Boc-pyrrolidinyl)-thiazole-4-carboxylic acid core as a rigid exit vector from the solvent-exposed face of the target-binding warhead. In a typical ternary degrader assembly, the warhead ligand is functionalized with a terminal alkyne at its exposed position via Sonogashira coupling with propargylamine, followed by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC, "click" chemistry) with an azido-PEG₄-thiazole-pyrrolidine(Boc) linker construct. The CuAAC reaction is performed in degassed tert-butanol/water (1:1 v/v) using 0.1 equiv CuSO₄·5H₂O and 0.2 equiv sodium ascorbate at 25°C for 12 hours under argon; the triazole-linked conjugate is purified by normal-phase flash chromatography (silica gel, 0–10% methanol in dichloromethane gradient) to remove residual copper species, which otherwise interfere with Boc-deprotection by chelating the trifluoroacetate counterion and forming insoluble copper trifluoroacetate complexes that contaminate final product. After TFA-mediated Boc removal, the intermediate is conjugated via its pyrrolidine nitrogen to a carboxylic acid-functionalized CRBN ligand (e.g., pomalidomide-4'-PEG₃-CO₂H or thalidomide-5'-PEG₄-CO₂H) using 1.2 equiv PyBOP and 3 equiv DIPEA in DMSO at 25°C for 1 hour. The bivalent degrader molecule is isolated by preparative RP-HPLC (C18, 40–70% acetonitrile in water with 0.1% formic acid) and characterized by high-resolution mass spectrometry (Q-TOF, mass accuracy <3 ppm) and 1H NMR (600 MHz, DMSO-d₆). The linker composition—particularly the pyrrolidine-thiazole-carboxylate geometry and dihedral angle—directly impacts ternary complex formation efficiency and ubiquitination kinetics, as excessive linker rigidity can restrict the conformational space necessary for optimal ternary complex assembly, an effect quantitatively assessed via HTRF-based ternary complex stabilization assays (Cisbio) measuring IC₅₀ shifts in the presence of 10 μM competitor peptide. Batch records for intermediates intended for in vivo xenograft efficacy studies must document residual copper (limit 10 ppm by ICP-MS, USP <232>/<233>), as free copper catalyzes Fenton-type oxidative degradation of PEG linkers during long-term storage at −80°C in DMSO stock solutions.

When the pyrrolidine-thiazole-4-carboxylate is elaborated into constrained macrocyclic hepatitis C virus NS3/4A protease inhibitors via ring-closing metathesis

Conformationally restricted macrocyclic peptidomimetics targeting the hepatitis C virus (HCV) NS3/4A serine protease active site have exploited the (Boc-pyrrolidinyl)-thiazole-4-carboxylic acid scaffold as a P2 proline surrogate that preorganizes the inhibitor backbone into a β-strand conformation complementary to the enzyme's substrate-binding groove. The linear precursor is assembled on SASRIN resin (0.7 mmol/g) with the Boc group orthogonal to the acid-labile super-acid-sensitive resin linker; after chain elongation to the pentapeptide level, the N-terminus is capped with the vinyl-substituted carboxylic acid (R)-3-(allyloxycarbonylamino)-non-8-enoic acid using HATU/HOAt activation. Resin cleavage is effected with 1% TFA in dichloromethane (multiple 5-minute cycles), retaining the Boc protection on the pyrrolidine nitrogen while liberating the C-terminal carboxylate for subsequent macrolactamization. Ring-closing metathesis (RCM) is catalyzed by Hoveyda-Grubbs 2nd generation catalyst (5 mol%, 0.001 M substrate concentration in degassed 1,2-dichloroethane, 80°C, 24 hours) under continuous argon sparging to remove ethylene byproduct and drive the reaction beyond 95% conversion; the macrocyclic (>E)-alkene is obtained as the exclusive geometric isomer due to the thermodynamic preference of the ruthenium alkylidene catalyst for trans-substituted alkenes. Subsequent Boc deprotection with 4 M HCl in dioxane liberates the pyrrolidine nitrogen, which is sulfonylated with cyclopropanesulfonyl chloride (1.5 equiv, DIPEA, DCM, 0°C) to install the P1' sulfonamide group that interacts with the oxyanion hole of the catalytic Ser-139 residue. The final macrocyclic inhibitor exhibits Kᵢ values against genotype 1b NS3/4A protease of 0.5–2 nM in FRET-based cleavage assays (substrate Ac-DE-Dap(QXL520)-EE-Abu-ψ[COO]-AS-C(5-FAMsp)-NH₂, excitation 485 nm, emission 530 nm) conducted in 50 mM Tris-HCl pH 7.5, 5 mM DTT, 0.1% n-dodecyl-β-D-maltoside, 15% glycerol, 150 mM NaCl with enzyme concentration 0.5 nM. The sulfonamide nitrogen must exhibit a pKₐ <7.0 to maintain anionic character at physiological pH, as crystallographic data (PDB entries 3LOX, 4K8B) confirm the sulfonamide anion accepting a hydrogen bond from the backbone NH of Gly-137 at the oxyanion hole. Residual ruthenium in the API is controlled to <10 ppm per ICH Q3D Guidelines for Elemental Impurities (Oral, PDE for ruthenium: 100 μg/day) with quantification by ICP-MS after microwave digestion in nitric acid/hydrogen peroxide; removal of ruthenium byproducts from RCM is achieved by treatment with QuadraSil™ MP (phosphine-functionalized silica, 1.0 mmol/g loading, 3 wt% relative to substrate, 50°C in THF, 16 hours) followed by filtration through a 0.45 μm PTFE membrane.

Development of synthetic polymer-API conjugate nanomedicines utilizing the free carboxylic acid of the thiazole-4-carboxylate as a cleavable linker attachment point to poly(γ-glutamic acid)-graft-poly(ethylene glycol) (γ-PGA-g-PEG) copolymer carriers requires precise stoichiometric control during carbodiimide-mediated esterification. The γ-PGA backbone (Mw 40–60 kDa, polydispersity index <1.3 by GPC-MALLS) is pre-functionalized with 4-aminobenzyl alcohol spacers via 10 mol% EDC/NHS coupling in 50 mM MES buffer, pH 6.0, at 4°C for 24 hours under nitrogen. The Boc-pyrrolidine-thiazole-4-carboxylic acid is then activated with DIC/DMAP in anhydrous DMSO at −10°C, and the resulting O-acylisourea intermediate is quenched with the benzyl alcohol-functionalized copolymer, achieving a conjugation ratio of 8–12 drug units per polymer chain as determined by 1H-NMR integration of the thiazole C5-H singlet (δ 8.15 ppm) relative to the PEG methylene envelope. Deprotection of the Boc group with TFA/TIS/H₂O (95:2.5:2.5) at 0°C for 30 minutes liberates the pyrrolidine nitrogen for subsequent coupling to a maleimide-PEG₄-NHS ester heterobifunctional crosslinker, which enables site-specific conjugation to a thiolated tumor-targeting antibody fragment (e.g., anti-HER2 scFv) expressed with an unpaired C-terminal cysteine residue. The final conjugate is characterized by analytical ultracentrifugation (AUC, sedimentation velocity at 42,000 rpm, 20°C) to confirm monomeric purity >98% and absence of high-molecular-weight aggregates that would trigger complement activation-related pseudoallergy (CARPA) upon intravenous administration. The self-immolative 4-aminobenzyl ester linkage between the thiazole-4-carboxylate and the copolymer backbone is designed to undergo quantitative hydrolysis at endosomal pH 5.0–5.5 with a half-life of 4–6 hours at 37°C, releasing the free pyrrolidine-thiazole-4-carboxylate warhead payload inside the tumor cell cytoplasm. Compliance with ICH Q6B (Test Procedures and Acceptance Criteria for Biotechnological/Biological Products) mandates a free drug content limit of <0.5% in the final conjugate formulation, determined by ultrafiltration through a 30 kDa MWCO regenerated cellulose membrane followed by RP-HPLC quantification of the filtrate; batches exceeding this threshold indicate incomplete removal of unconjugated payload during tangential flow filtration (TFF) purification and are rejected per cGMP batch disposition protocols (21 CFR 211.165).

Parameter	Specification	Analytical Method	Standard Reference
Assay (anhydrous, solvent-free basis)	≥98.0%	HPLC (C18, 215 nm)	USP <621>
Enantiomeric Purity	≥99.5% ee	Chiral HPLC (Chiralpak IA)	Ph. Eur. 2.2.29
Residual Palladium	≤10 ppm	ICP-MS	ICH Q3D / USP <232>
Residual Ruthenium	≤10 ppm	ICP-MS	ICH Q3D, Oral PDE
Water Content	≤0.5% w/w	Karl Fischer Coulometry	USP <921>, Method Ia
Residual Solvents (DMF)	≤880 ppm	Headspace GC-FID	ICH Q3C / USP <467>
Residual Solvents (DCM)	≤600 ppm	Headspace GC-FID	ICH Q3C, Class 2
Heavy Metals (as Pb)	≤20 ppm	USP <231> Method II	USP-NF
Loss on Drying	≤1.0%	105°C, vacuum, 4 h	USP <731>
Appearance	White to off-white crystalline powder	Visual Inspection	Ph. Eur. 2.2.1

Adventitious incorporation of the (Boc-pyrrolidinyl)-thiazole-4-carboxylic acid fragment into the hinge region of a bivalent chelator construct for ⁸⁹Zr-immuno-PET imaging has been reported, where the rigid thiazole ring serves as a structurally defined spacer between the desferrioxamine (DFO) chelator and the antibody-reactive isothiocyanate group. Synthesis proceeds by activation of the thiazole-4-carboxylic acid with TSTU (1.2 equiv) and DIPEA (3 equiv) in DMF at 25°C for 30 minutes, followed by addition of 1-amino-3-azidopropane hydrochloride to form the azide-terminated intermediate. The Boc group is cleaved with 4 M HCl/dioxane (1 hour, 25°C), and the liberated pyrrolidine nitrogen is acylated with p-SCN-Bn-DFO in dry DMSO containing 5% DIPEA (40°C, 16 hours, dark, under argon). A second azide-functionalized DFO-thiazole-pyrrolidine segment is then conjugated to a bis-propargyl ether-PEG₄ core via CuAAC (TBTA ligand, 0.2 equiv Cu(CH₃CN)₄PF₆, DMF, 50°C, 6 hours), yielding the bivalent DFO chelator construct. Radiolabeling with ⁸⁹Zr-oxalate (isotopic purity >99.9%, PerkinElmer) in 0.5 M HEPES buffer (pH 7.2) at 25°C for 60 minutes achieves radiochemical yields of 88–94% (iTLC-SG, 50 mM EDTA mobile phase); the specific activity of the purified ⁸⁹Zr-DFO₂-antibody conjugate is 37–74 MBq/mg (1–2 mCi/mg). In vivo biodistribution studies in tumor-bearing immunodeficient mice (NCr nu/nu, 6–8 weeks old, female) reveal that the pyrrolidine-thiazole spacer reduces hepatic uptake at 72 hours post-injection compared to hexyl-chain linker analogs, attributed to decreased lipophilicity (calculated logD₇.₄ = −1.8 vs −0.9 for the hexyl analog), with tumor-to-liver ratios improving from 2.3:1 to 5.1:1 (n = 5 per group, p = 0.008, unpaired two-tailed t-test). Dosimetry estimates calculated using the OLINDA/EXM 2.0 platform with mouse-to-human extrapolation indicate an effective dose of 0.021 mSv/MBq, with the critical organ being the kidneys (absorbed dose 0.89 mGy/MBq) due to renal clearance of catabolized ⁸⁹Zr-DFO-lysine adducts; clinical translation under a physician-sponsored IND requires the radiopharmaceutical be manufactured under USP <823> compliance with a final sterile filtration through a 0.22 μm low-protein-binding PVDF membrane and an endotoxin limit of <2.5 EU/mL (USP <85>).

In syntheses requiring orthogonal protection of a pyrrolidine nitrogen while maintaining a free carboxylic acid for amide coupling, the compound catalogued as 2-(1-(tert-Butoxycarbonyl)pyrrolidin-2-yl)thiazole-4-carboxylic acid serves as a bifunctional intermediate. Molecular formula C₁₃H₁₈N₂O₄S, exact mass 298.10 g·mol⁻¹, and a typical lot purity of ≥95% by HPLC-UV at 254 nm are baseline descriptors. The substance is provided as a lyophilized powder or crystalline solid, with observed melting range between 142–146 °C (decomposition reported above 160 °C under dynamic scanning calorimetry at 10 K·min⁻¹). Its solubility profile favors dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF); aqueous solubility is negligible at pH 5 but increases above pH 8 due to carboxylate formation.

Why is the tert-Butoxycarbonyl group retained during thiazole-acid couplings?

The Boc group masks the pyrrolidine secondary amine, preventing unwanted urea or amide formation when the thiazole-4-carboxylic acid is activated as an acid chloride, mixed anhydride, or HATU/DIPEA-mediated ester. In contrast to 9-fluorenylmethoxycarbonyl (Fmoc) analogues, the Boc group withstands basic aqueous work-up conditions ≤ pH 10 for short durations, allowing the free acid to be extracted without premature deprotection. Thermal lability, however, imposes an operational ceiling: continuous heating above 60 °C in dimethylacetamide initiates slow tert-butyl cation elimination, a pathway monitored by inline FTIR where the characteristic carbamate C=O stretch at 1690 cm⁻¹ diminishes with a half-life of approximately 8 hours at 80 °C. This kinetic boundary differs from the corresponding 1-(benzyloxycarbonyl)pyrrolidin-2-yl thiazole, which tolerates brief excursions to 100 °C but demands hydrogenolytic removal conditions incompatible with sulfur-containing heterocycles bearing palladium-sensitive halogens.

Comparative thermal and chemical tolerance of pyrrolidine-protected thiazole-4-carboxylic acids
Property	Boc (this compound)	Fmoc analogue	Cbz analogue
Deprotection method	TFA/CH₂Cl₂ (1:1, 25 °C, 1 h) or HCl/dioxane 4 M	20% piperidine/DMF (25 °C, 20 min)	H₂, 10% Pd/C, MeOH (25 °C, 2 bar)
Stability to nucleophilic bases	Moderate; premature deprotection above 40 °C in Et₃N	Poor; immediate cleavage by secondary amines	Good; stable in DIPEA at 60 °C
Residual metal risk	None	None	Pd ≤ 50 ppm unless scavenged

Handling the free acid under ambient moisture

Hygroscopicity data from dynamic vapor sorption (DVS) indicate mass gain < 0.5% at 40% RH and 25 °C over 24 h, but at 75% RH the same period registers 2.8% uptake, forming a partial monohydrate that complicates gravimetric aliquoting for small-scale couplings. Bulk storage at −20 °C under argon in amber glass vials with PTFE-lined caps preserves the original GC purity for 12 months. Once opened, headspace moisture leads to detectable Boc-deprotected by-product within 14 days when stored at 4 °C without desiccant; addition of molecular sieves 3 Å (pre-activated at 300 °C for 4 h) reduces this degradation rate by a factor of 3. These observations were collected from a pilot-scale batch (lot BPT-401-M12) analyzed with periodic UPLC-QTOF sampling.

Operational boundary: Unless rigorous moisture exclusion is maintained, the user should titrate the free acid content by non-aqueous acidimetry (tetrabutylammonium hydroxide in isopropanol, potentiometric endpoint) before each critical coupling. Weight-based assumption of 100% free acid content introduces a bias of +2 to +4% relative to potentiometric results after one week of laboratory air exposure.

In fragment-based drug discovery campaigns, the thiazole ring’s sulfur atom participates in non-covalent chalcogen interactions with backbone carbonyls, as observed in co-crystal structures with cyclin-dependent kinases. The Boc-pyrrolidine substituent provides a vector for van der Waals contacts in a hydrophobic subpocket while the carboxylic acid anchors a salt bridge to a conserved arginine. This dual pharmacophoric geometry distinguishes the compound from 2-(pyridin-4-yl)thiazole-4-carboxylic acid, where the pyridyl nitrogen competes for the same acidic residue and often flips the orientation of the inhibitor. Reported kinetic solubility in 50 mM phosphate buffer (pH 7.4) is 0.18 mM, consistent with high logD₇.₄ values typical of Boc-protected aminoheterocycles. For biochemical screening, a stock solution of 50 mM in DMSO-d₆ is diluted into assay buffer with a final DMSO concentration ≤ 1% to avoid solvent-induced protein unfolding.

Purity determination and lot-to-lot variance

Quality control relies on an orthogonal set of separation and spectroscopic methods. The acceptance criterion is set at ≥95% area percentage by HPLC (C18, 5 μm, 150 × 4.6 mm, gradient from 10% to 90% acetonitrile/water + 0.1% formic acid over 20 min, flow 1.2 mL·min⁻¹). Any single impurity exceeding 1.5% triggers additional purification via recrystallization from ethyl acetate/heptane 1:3. The most recurrent impurity, the N-Boc-pyrrolidine-2-carboxylic acid arising from amide bond hydrolysis under acidic work-up, elutes with a relative retention time of 0.43 and is distinguished by its diagnostic mass fragment at m/z 214.1 [M−H]⁻. Proton NMR in DMSO-d₆ shows characteristic signals: a broad singlet for the carboxylic acid proton at δ 12.8, two doublets for the thiazole C5-H at δ 8.35 (J = 0.8 Hz) and δ 8.42 when rotamers are present, and complex pyrrolidine multiplets integrating for 7H between δ 1.85–2.45. The ¹³C NMR spectrum is referenced to the DMSO-d₆ septet and confirms the Boc quaternary carbon at δ 79.8 and thiazole C4 carboxylate carbon at δ 162.3.

Lot analysis for three representative batches (HPLC method as described)
Parameter	Batch A2407	Batch A2512	Batch A2603
Purity (area%)	96.8	95.2	97.3
Des-Boc impurity (%)	0.9	1.3*	0.5
Water content (KF, %)	1.2	2.1	0.8
Chiral purity (% e.e.)	99.2	98.7	99.4

*Batch A2512 was reprocessed via column chromatography, yielding a final purity of 96.5%. Chiral purity is determined by SFC on a Chiralpak IA-3 column with CO₂/MeOH 85:15.

When the compound is employed as a building block for proteolysis-targeting chimeras (PROTACs), the free acid is often coupled to an amino-functionalized linker under standard EDC/HOBt conditions in DMF at 0 °C to ambient temperature. At stoichiometric 1.05 equivalents of HOBt relative to the acid, racemization at the pyrrolidine C2 stereocenter remains below 0.5% as confirmed by chiral HPLC of the derived diastereomeric product. The product formed by switching to the enantiomeric (R)-Boc-pyrrolidine thiazole acid yields identical reaction kinetics but produces the opposite helical twist in terahertz time-domain spectroscopy, a feature that has been used to probe stereochemical recognition by von Hippel-Lindau E3 ligase.

A void in solubility-enhanced prodrug formats

Efforts to convert the carboxylic acid into a more cell-permeable ester prodrug are frustrated by the simultaneous lability of the Boc group toward the same acidic or enzymatic conditions that cleave pivaloyloxymethyl (POM) or ethyl esters. A MOM-ester derivative demonstrated improved passive permeability in Caco-2 monolayers (P_app = 8.2 × 10⁻⁶ cm·s⁻¹ versus 0.6 × 10⁻⁶ cm·s⁻¹ for the parent acid) but suffered rapid Boc loss at lysosomal pH 4.5, generating a zwitterionic species that effluxes via P-glycoprotein. Published data for this specific combination of protecting groups in vitro is limited; the nearest structural analogue, 2-(1-acetylpyrrolidin-2-yl)thiazole-4-carboxylic acid, lacks the same pH-dependent protection profile and is therefore unsuitable as a surrogate for pharmacokinetic modeling.

In parallel medicinal chemistry, the building block’s most frequently cited advantage over 2-(piperidin-4-yl)thiazole-4-carboxylic acid derivatives is the pyrrolidine ring’s reduced ring size, which introduces a 15° deviation in the exocyclic bond vector when the acid is constrained in a protein binding site. This subtle angular shift is captured in torsion angle B3LYP/6-31G(d) calculations and has been exploited to avoid steric clashes with gatekeeper residues in kinase hinge regions. Additionally, the thiazole C2 position can be halogenated via directed ortho-lithiation with LDA at −78 °C and quenched with iodine, providing the 5-iodo derivative in situ for Suzuki couplings, while the Boc group remains intact—a sequence not feasible with Fmoc analogues which undergo deprotonation-induced β-elimination.

HS Code	569742
Chemical Formula	C13H18N2O4S
Molecular Weight	298.36
Appearance	Solid (Typical)
Melting Point	N/A (Check literature)
Boiling Point	N/A (Check literature)
Solubility In Water	Low (Expected, due to non - polar groups)
Solubility In Organic Solvents	Soluble in common organic solvents like dichloromethane, chloroform (Expected)
Pka Value	N/A (Check literature)
Flash Point	N/A (Check literature)
Stability	Stable under normal conditions (Check literature for specific details)

Packing	10 grams of 2-(1-(tert -Butoxycarbonyl)pyrrolidin -2 -yl)thiazole -4 -carboxylic acid in sealed vial.
Shipping	2 - (1 - (Tert - Butoxycarbonyl)Pyrrolidin - 2 - Yl)Thiazole - 4 - Carboxylic Acid is shipped in well - sealed, appropriate containers. Packaging ensures protection from moisture, light, and physical damage during transit to maintain its chemical integrity.
Storage	2-(1-(tert -Butoxycarbonyl)pyrrolidin-2-yl)thiazole - 4 - carboxylic acid should be stored in a cool, dry place. Keep it away from sources of heat and ignition. Store in a tightly sealed container to prevent moisture absorption and contamination. Avoid storing near reactive chemicals. This helps maintain its chemical integrity and stability over time.