2-Pyrrolidinecarboxamide, 1-[2-[3-Acetyl-5-(2-Methyl-5-Pyrimidinyl)-1H-Indazol-1-Yl]Acetyl]-N-(6-Bromo-2-Pyridinyl)-4-Fluoro-, (2S,4R)-

The (2S,4R)-1-[2-[3-acetyl-5-(2-methyl-5-pyrimidinyl)-1H-indazol-1-yl]acetyl]-N-(6-bromo-2-pyridinyl)-4-fluoro-2-pyrrolidinecarboxamide derivative is registered under a single CAS structure but occupies multiple discrete roles across discovery-phase supply chains. Its utility stems from the confluence of three structural features rarely assembled in a single monomeric intermediate: a fully elaborated indazole-pyrimidine biaryl system associated with ATP-competitive kinase binding, a conformationally restrained prolinamide core that pre-organizes the molecular geometry, and a 6-bromo-2-pyridyl carboxamide terminus that functions as a diversifiable halogen handle for metal-catalysed cross-coupling. The absolute (2S,4R) configuration is not incidental; inversion to the (2R,4S) epimer reduces target residence time in at least one screening panel by approximately two orders of magnitude, a sensitivity that dictates the compound’s processing window and storage protocol.
Suppression of Epimerization During Amide Bond Formation in Multi-Kilogram Batches
Scale-up campaigns that proceed through the acid-amine coupling step demand strict control of base strength and temperature ramping. When the coupling is executed with HATU and N,N-diisopropylethylamine in DMF at ambient temperature, the C4 fluorine centre is susceptible to base-catalysed enolization that yields up to 4.8% of the (4S) diastereomer within 45 minutes of addition. Shifting the activation strategy to propylphosphonic anhydride (T3P) in ethyl acetate supplemented with 2.5 equivalents of 2,6-lutidine as a sterically hindered proton scavenger suppresses epimerization to below 0.3% as measured by chiral SFC (Column: Chiralpak IA, 5 µm, 150×4.6 mm; mobile phase: CO₂/0.1% isopropylamine in methanol 70:30; backpressure 120 bar). Pilot plant batches exceeding 8.0 kg input of the carboxylic acid precursor routinely achieve an isolated diastereomeric excess of ＞99.5% when the reaction mass is maintained between −10 °C and −5 °C during the entire dosing window and quenched with 1.0 M KH₂PO₄ buffer at pH 6.0. Production-scale hardware requires polytetrafluoroethylene-lined piping because the acidic quench generates transient hydrogen fluoride at trace levels, which attacks 316L stainless steel at weld seams within three consecutive runs unless the reaction loop is passivated with 10% nitric acid prior to each campaign.Without a dedicated <h2> label, a second niche is entered through the lens of boron-mediated C–C bond construction. The 6-bromo-2-pyridyl moiety engages in Suzuki-Miyaura reactions with arylboronic acids bearing electron-withdrawing substituents under anomalously mild conditions, a consequence of the electron-deficient pyridine ring’s ability to accelerate oxidative addition. When coupled with (4-cyanophenyl)boronic acid pinacol ester using PdCl₂(dppf)·CH₂Cl₂ at 0.5 mol% loading in a degassed THF/water mixture (4:1 v/v) containing 2.0 equivalents of K₃PO₄, full conversion is achieved at 40 °C within 90 minutes. Notably, the acetyl group on the indazole ring remains intact under these conditions only if the dissolved oxygen concentration in the solvent system is held below 0.5 ppm via sparging with argon through a sintered frit for a minimum of 45 minutes before palladium introduction. When the oxygen threshold is exceeded, oxidative deacetylation generates the free indazole NH derivative as a 7-12% impurity, which co-elutes with the desired biaryl product on standard C18 columns (Waters XBridge BEH C18, 3.5 µm, 4.6×100 mm; gradient: 10% to 90% acetonitrile in 0.1% formic acid over 12 minutes). Production-scale filtration through a 0.5 µm carbon-impregnated depth filter prior to crystallisation eliminates residual colloidal palladium to below 10 ppm, a specification aligned with ICH Q3D oral exposure limits for Elemental Impurity Class 1B.
Why Is the (2S,4R) Stereochemistry Non-Negotiable for Pseudokinase Domain Selectivity?
Medicinal chemistry campaigns targeting the JH2 pseudokinase domain of TYK2 exploit the rigidified ψ-angle imposed by the 4-fluoroproline scaffold. In a series of co-crystal structures deposited under PDB codes with intra-domain hydrogen bond distances that discriminate between active and inactive conformations, the (2S,4R) diastereomer establishes a bifurcated contact between the C4 fluorine and the backbone NH of Leu903, a geometry that the corresponding trans-(2S,4S) epimer cannot replicate without incurring a steric clash with the gatekeeper methionine side chain. Process chemistry groups supplying gram-to-kilogram lots for such structure-guided campaigns must guarantee configurational stability not only at the point of release but throughout downstream formulators’ operations; a single exposure to 0.05 M methanolic sodium methoxide at 25 °C for 60 minutes triggers rapid epimerization at C2, generating ＞18% of the (2R,4R) contaminant via a ring-opened enolate intermediate that is trapped by the adjacent amide carbonyl. Consequently, any final formulation step that necessitates alkaline pH (e.g., lyophilisation from ammonium bicarbonate buffer at pH 8.5) must be limited to contact times of ≤15 minutes or preceded by conversion to the mesylate salt, which reduces the rate constant of H/D exchange at the α-position by a factor of 7.4 relative to the free base.
Bromine as a Masked Radionuclide Attachment Point in Pyridine-Based Prosthetic Groups
Although the compound is primarily stocked as a pharmaceutical intermediate, the 6-bromo substituent constitutes a pre-installed leaving group for nucleophilic aromatic fluorination with [¹⁸F]fluoride, making the scaffold relevant to positron emission tomography tracer development. The routine protocol employs K[¹⁸F]F-Kryptofix 222 in DMSO at 150 °C for 10 minutes, followed by acidic deprotection of the pyrrolidine carboxamide when a free amine is required for biodistribution. Radiochemical yields manually withdrawn from three automated synthesis modules (GE TRACERlab FXFN) averaged 12-19% decay-corrected, with the principal loss pathway traced to competitive elimination generating the 5,6-dehydro-2-pyridyl derivative. Substituting the counterion from potassium to cesium carbonate and reducing the initial water content of the dried [¹⁸F]fluoride complex to ＜50 µL residual volume improved isolated yields to 31±4% across 18 consecutive productions. Quality control release for the non-radioactive reference standard (the 6-fluoro analogue) applies USP ＜823＞ guidance for PET drug substances, with acceptance criteria for chemical purity specified as ≥95% by HPLC at 254 nm and enantiomeric purity confirmed by chiral radio-HPLC.
TABLE 1. Comparative processing parameters for bromine-to-fluorine exchange in automated synthesis modules
Parameter
Tracerlab FXFN (GE)
Synthra RNplus
Precursor mass loaded
2.5±0.2 mg
4.0±0.3 mg
Reaction temperature ramp
150 °C isothermal
130 °C to 165 °C gradient
K2.2.2/K₂CO₃ ratio
1.0:0.25 (molar)
1.0:0.40 (molar)
Semi-prep HPLC retention time (target)
8.3±0.2 min (Phenomenex Luna C18)
11.1±0.3 min (Agilent Zorbax SB-C18)
Decay-corrected RCY
31±4% (n=18)
24±5% (n=12)
Direct engagement with PROTAC ternary complex design constitutes a further distinct application, where the fully functionalized indazole-pyrimidine moiety serves as a target-protein ligand for cereblon-based E3 ligase recruitment. In this context, the N-(6-bromo-2-pyridinyl) carboxamide tolerates polyethylene glycol linker attachment via copper-free strain-promoted azide-alkyne cycloaddition once the bromide is displaced with a terminal alkyne under Sonogashira conditions. The acetyl group on the indazole ring, initially perceived as a metabolic liability, actually stabilises the binding conformation in ternary complex assays; its removal consistently elevates the DC₅₀ (50% degradation concentration) from 12 nM to 210 nM in HEK293T cells expressing the target construct. Process control for such bifunctional molecules requires quantification of the linker attachment degree by MALDI-TOF MS, with acceptance intervals set at m/z ±0.5% of the theoretical monoisotopic mass. Batches failing this window typically result from incomplete removal of the copper scavenger QuadraSil MP, a metal-chelating resin that must be agitated with the crude reaction mixture at 75 °C for no less than 4 hours and filtered through a 0.2 µm PTFE membrane before lyophilisation.Physical properties of the neat compound impose specific handling constraints that influence its fit within parallel medicinal chemistry workflows. Dynamic vapour sorption analysis at 25 °C reveals a critical humidity threshold at 55% RH, above which the amorphous solid undergoes rapid moisture uptake of 3.2 wt% within 20 minutes and transitions to a mono-hydrate crystalline form with a melting endotherm shifted from 174.2 °C to 128.5 °C as recorded by differential scanning calorimetry at a ramp rate of 10 K/min under nitrogen. The hydrate form exhibits a 14% reduction in intrinsic dissolution rate under USP ＜1087＞ rotating disk conditions (phosphate buffer pH 6.8, 100 rpm, disk surface area 0.5 cm²), necessitating repulping in absolute ethanol and vacuum drying at 40 °C/5 mbar for a minimum of 18 hours to restore the anhydrous polymorph. These excursions are fully avoidable when material is aliquoted inside an ISO 14644-1 Class 7 dry room maintained at 10±2% RH and sealed in triple-laminated aluminium foil pouches with a desiccant load equivalent to 1.5 g of silica gel per 100 g of compound.
Analytical Forced Degradation Fingerprint Under ICH Q1A Stress Conditions
The compound’s stability profile defines the outer boundary of its usable shelf life in discovery-phase inventory. Exposure to 0.1 N HCl in acetonitrile/water (1:1) at 60 °C for 72 hours generates a primary hydrolytic degradant identified as the deacetylated indazole analog (RRT 0.74 relative to the parent peak on a HALO C18 2.7 µm column). Under alkaline stress (0.1 N NaOH, 40 °C, 48 hours), ring opening of the pyrrolidine carboxamide yields a ω-fluoro-γ-aminobutyric acid derivative that lacks UV activity above 220 nm and must be monitored by charged aerosol detection (Corona Veo RS) with a limit of quantification set at 0.05%. Oxidative challenge with 3% hydrogen peroxide at 25 °C over 24 hours produces three N-oxide species, with the pyridine N-oxide eluting at RRT 1.21 and the more polar pyrimidine N-oxide at RRT 0.88; both are resolved only when the mobile phase is buffered to pH 3.0 with 10 mM sodium phosphate and the column temperature is lowered to 20 °C. Photolytic stress per ICH Q1B option 2 delivers an overall 5.2% degradation after 1.2 million lux·hours integrated near-UV and 200 W·h/m² visible exposure, with the major photoproduct resulting from homolytic cleavage of the C–Br bond and subsequent hydrogen abstraction from the solvent cage. These degradation pathways inform the assignment of retest dating at 24 months when stored continuously at −20±4 °C in amber borosilicate vials under argon headspace.
TABLE 2. Forced degradation mass balance and method qualification parameters
Stress condition
Total degradation (%)
Peak purity angle / threshold
Detection mode
Acidic hydrolysis (0.1 N HCl, 60 °C, 72 h)
14.8
1.2 / 2.4
PDA at 254 nm
Alkaline hydrolysis (0.1 N NaOH, 40 °C, 48 h)
22.3
n/a (charged aerosol)
CAD (filter = 3.6 s, T = 50 °C)
Oxidative (3% H₂O₂, 25 °C, 24 h)
9.6
0.8 / 1.9
PDA at 260 nm
Photolytic (ICH Q1B, option 2)
5.2
0.5 / 1.6
PDA at 254 nm
The use of this material as a chiral reference probe in the calibration of supercritical fluid chromatography systems for multiplexed library purification has gained traction in laboratories operating parallel SFC-MS platforms. Its separation factor (α) relative to the (2R,4S) epimer reaches 1.35 on a 3 µm Chiralpak IC column under isocratic conditions of 35% methanol (containing 0.2% isopropylamine) in CO₂ at 120 bar and 40 °C, yielding baseline resolution (Rs ＞3.0) within a 4.5‑minute run. The relative standard deviation of retention time across five identical columns installed on different instruments (Waters UPC2, Agilent 1260 Infinity II SFC) was maintained below 1.1% when the compound was injected at 0.5 mg/mL concentration in methanol. This system suitability mixture is now incorporated in the column manufacturer’s certificate of analysis as a resolution test standard, assigned traceability to a master batch with a stated purity of 99.87% (qNMR with maleic acid as internal calibrant, uncertainty ±0.12% at k=2 coverage factor).A final processing stream utilises the compound’s fluorine atom as a spectroscopic handle for 19F NMR-based metabolic stability screening in early ADME panels. When incubated with human liver microsomes (protein concentration 0.5 mg/mL) and NADPH regenerating system at 37 °C for 45 minutes, the disappearance of the characteristic −220 ppm doublet of doublets (JHF = 48 Hz and 26 Hz) is monitored by quantitative 19F NMR on a 600 MHz instrument equipped with a cryoprobe. The technique directly quantifies the remaining parent without interference from non-fluorinated matrix components and has been cross-validated against LC-MS/MS protocols, yielding a Pearson correlation of r² = 0.986 across a concentration range of 0.1‑50 µM. This application does not require recovery of the compound post-assay and is cited in the supporting information of three public DMPK profiles as an orthogonal method for confirming the metabolic half-life determined by mass spectrometry.