Introduced as a chiral heterocyclic building block for structure-activity relationship campaigns targeting phosphoinositide 3-kinase (PI3K) and related lipid kinase isoforms, (S)-benzyl 2-(8-chloroimidazo[1,5-a]pyrazin-3-yl)pyrrolidine-1-carboxylate (CAS registry number applied but not yet publicly indexed) comprises a pyrrolidine core in its enantiopure S-configuration appended to a 8-chloro-substituted imidazo[1,5-a]pyrazine pharmacophore. The carboxybenzyl (Cbz) protecting group at the pyrrolidine nitrogen renders the amine latent during cross-coupling sequences, enabling selective elaboration at the C-3 heteroaryl halide or late-stage deprotection under hydrogenolysis conditions. Typical lot release assays record ≥ 98.0% purity by HPLC at 254 nm (area percent) and chiral purity ≥ 99.0% ee via polysaccharide-based CSP with hexane/2-propanol mobile phase, determined per Ph. Eur. 2.2.29 analogue method.
When the C-3 Chlorine Is Retained as a Late-Stage Diversification Handle
Deliberate preservation of the imidazo[1,5-a]pyrazine C-3 chlorine through multiple synthetic transformations is feasible only if the pyrrolidine N-Cbz group remains intact during palladium-mediated steps. Under standard Buchwald–Hartwig amination with Pd₂(dba)₃ / Xantphos (2 mol% Pd, toluene, 100 °C), the C-3 position undergoes oxidative addition preferentially over any competing dehalogenation of the pyrazine ring, while the Cbz group exhibits < 2% cleavage over 16 h. Fragmentation of the bicyclic core—detected via LCMS as a +16 amu oxidation product—rises sharply above 110 °C in DMF, placing the practical processing window at 90–105 °C for amination chemistry.
In comparison to the corresponding 8-bromo analogue, the chloro derivative demonstrates a 3- to 4-fold slower oxidative addition rate as gauged by reaction calorimetry (omnical SuperCRC, 0.5 M concentration), enabling sequential chemoselective functionalization when both halogens are present in a substrate. The bromide preferentially reacts under mild conditions (Pd-PEPPSI-IPr, K₂CO₃, THF/H₂O, 40 °C), leaving the C-8 chlorine intact for a subsequent nickel-catalyzed Kumada coupling with aryl magnesium bromides in 2-MeTHF at 0–5 °C. This orthogonality has been exploited to construct bis-aryl libraries with > 50 members using parallel synthesis platforms equipped with 24-well reaction blocks (Biotage® Initiator+ microwave reactor, sealed vials).
Physical Form and Processability Constraints in Milligram-to-Kilogram Scale
At ambient conditions the solid is a white-to-off-white microcrystalline powder with an onset melting temperature of 127–129 °C (DSC, 10 °C/min, nitrogen purge), as recorded on a TA Instruments Q2000. The amorphous fraction, which can reach 8–12 wt% after rotary evaporation isolation, reduces oxidative stability: headspace GC-MS of samples stored at 40 °C/75% RH open vial for 30 days detects benzyl alcohol (0.3–0.5 area%) arising from Cbz hydrolysis, whereas crystalline lots show < 0.1 area%. For kilogram-scale campaigns, a slurrying step in n-heptane/ethyl acetate (4:1 v/v) at 50 °C for 2 h is instituted to anneal the solid, followed by vacuum drying at 40 °C until loss on drying falls below 0.5% (Mettler Toledo HX204, 105 °C endpoint).
Solubility in process-relevant solvents, determined by the shake-flask method with HPLC quantitation at 25 ± 1 °C, reveals: THF (62 mg/mL), ethyl acetate (34 mg/mL), acetone (48 mg/mL), DMF (190 mg/mL), and negligible solubility in water (< 0.05 mg/mL) or n-heptane. These values dictate that amidation or Suzuki–Miyaura couplings in aqueous-organic mixtures require a minimum 10 vol% DMF co-solvent to maintain homogeneity at 0.2 M substrate concentration.
| Parameter | Method | Acceptance Criterion |
|---|---|---|
| Assay (anhydrous, solvent-free basis) | HPLC, C18, 0.1% TFA/MeCN gradient | 98.0–102.0% w/w |
| Chiral purity | Chiralpak IA-3, hexane/IPA 80:20 | ≥ 99.0% ee |
| Related substances (total) | HPLC as above, 220 nm | ≤ 1.5 area% |
| Water content | Karl Fischer coulometry (ISO 760:1978) | ≤ 0.5% w/w |
| Residual palladium | ICP-MS (USP <233>) | ≤ 10 ppm |
| Residual nickel | ICP-MS | ≤ 5 ppm |
| Appearance | Visual inspection | White to off-white powder |
What Distinguishes This Scaffold from the 8-Fluoro and 8-Iodo Congeners in Kinase Profiling?
In a panel of 112 wild-type and mutant human kinases (Eurofins KinaseProfiler™, ATP concentration at Km), the Cbz-protected intermediate itself exhibits negligible inhibitory activity (< 15% displacement at 1 µM), confirming its role as a precursor. Once elaborated to the corresponding 3-aryl-8-chloro final analogs, however, selectivity fingerprints diverge markedly from 8-fluoro and 8-iodo series. The chlorine atom’s van der Waals radius (1.75 Å) occupies a volume that is suboptimal for the DFG-out hydrophobic pocket of p110α PI3K (PDB 4L23) but ideally accommodated in the selectivity pocket of DNA-PK (PDB 5LUQ), resulting in 12-fold selectivity shifts measured via IC₅₀ ratios in duplicate. Fluoro analogs (1.47 Å radius) lose this steric interaction, while the iodo substituent (1.98 Å) induces a steric clash that propagates into the hinge-binding region as evidenced by shifted amide 1H-15N HSQC cross-peaks of the protein backbone.
Accelerated stability testing of the final bioactive molecules derived from this building block, formulated as 10 mM DMSO stock solutions stored under argon at −20 °C, revealed no detectable epimerization at the pyrrolidine stereocenter over 24 months when analyzed by chiral SFC (Chiralpak AD-H, CO₂/MeOH with 0.1% diethylamine). This bench stability contrasts with the corresponding unprotected secondary amine, which racemizes to the extent of 2.3% after 6 months at 4 °C in DMSO via an imine-enamine tautomerization pathway.
Processing Boundaries in Continuous Flow Hydrogenolysis
The Cbz deprotection to liberate the pyrrolidine amine is most reliably executed in a continuous flow reactor (ThalesNano H-Cube Pro, 10% Pd/C CatCart, 30 × 4 mm) using methanol/THF (1:1) at 0.5 mL/min, 25 °C, and 1 bar back pressure. Batch hydrogenation in Parr shakers at 50 psi H₂ with 5% Pd/C (10 wt% loading) leads to partial reduction of the imidazo[1,5-a]pyrazine ring (3–5% over-reduction product) when reaction time exceeds 4 h. In flow, residence time is restricted to 3.2 min, eliminating the over-reduction impurity altogether (LCMS detection limit 0.05%). The free amine must be immediately processed or stored as the hydrochloride salt (precipitated from MTBE with 2 M HCl in diethyl ether) because the neat amorphous base darkens upon extended exposure to air, with onset of discoloration at 24 h under ambient fluorescent lighting.
Comparative Binding Affinity and CYP Inhibition Liability vs. the Morpholine Analogues
Replacement of the pyrrolidine with morpholine in otherwise identical C-8-chloro scaffolds reduces logD7.4 by 0.7 log units (shake-flask, octanol/phosphate buffer) and attenuates hERG binding (patch clamp, HEK293, IC₅₀ shifts from 4.8 μM to 28 μM). However, the pyrrolidine-containing series retains 3–5× higher cellular activity in PI3Kδ-dependent anti-IgM-stimulated CD69 upregulation assays in human whole blood (EC₅₀ values of 48 nM vs. 210 nM for the morpholine congener), because the pyrrolidine’s basic nitrogen (calculated pKa 8.9) achieves a higher intracellular concentration due to lysosomal trapping, as confirmed by NanoSIMS imaging of 19F-labeled derivatives. When CYP3A4 time-dependent inhibition was assessed using the testosterone 6β-hydroxylation assay with 20 min NADPH pre-incubation, the pyrrolidine-based compound exhibited an IC₅₀ shift factor of 2.1, compared to 1.1 for the morpholine, indicating mild mechanism-based inhibition that required inclusion of a 30 mg/kg midazolam drug-drug interaction arm in the subsequent rodent pharmacokinetic study (Sprague-Dawley, n = 3 per group).
| Parameter | 8-Cl (This Product) | 8-Br | 8-I |
|---|---|---|---|
| Oxidative addition rate constant (k, s⁻¹, Pd/Xantphos, 100 °C) | 1.2 × 10⁻³ | 4.6 × 10⁻³ | 6.9 × 10⁻³ |
| Typical Pd catalyst loading for C-C bond formation | 1-2 mol% Pd(PPh₃)₄ | 0.5-1 mol% | 0.25-0.5 mol% |
| Melting point (°C) | 127-129 | 138-140 | 152-155 dec |
| Hydrolytic stability (t₉₀, pH 7 buffer, 25 °C) | 180 d | 95 d | 42 d |
| Chiral integrity under Cbz hydrogenolysis | >99% ee retained | >99% ee | 98.2% ee |
Catalyst selection for the pyrrolidine ring construction directly impacts both enantiomeric excess and residual metal profile. Asymmetric transfer hydrogenation of the corresponding N-Boc imine precursor using RuCl[(R,R)-TsDPEN](mesitylene) in HCO₂H/Et₃N at 40 °C delivers the S-enantiomer in 97.3% ee with 85% isolated yield after recrystallization. Subsequent Boc removal with TFA/CH₂Cl₂ and Cbz protection employing benzyl chloroformate in the presence of Na₂CO₃ (aqueous/THF biphasic) furnishes the title compound without erosion of stereochemistry. Quality control of the imidazo[1,5-a]pyrazine fragment formation—a condensation between 3-chloropyrazine-2-amine and chloroacetaldehyde—requires strict control of pH 4.0–4.5 during cyclization to suppress the regioisomeric imidazo[1,2-a]pyrazine by-product, which co-elutes on standard C18 columns and must be resolved by UPLC with a sub-2-µm particle column (Waters ACQUITY BEH C18, 1.7 µm) and column temperature of 50 °C.
In typical medicinal chemistry laboratories, the building block is supplied in 100 mg, 1 g, and 5 g septum-sealed amber vials under argon. For analytical method development, a reference solution at 1.0 mg/mL in acetonitrile is stable for 72 h at autosampler temperature (15 °C), with no growth of impurity peaks exceeding 0.05 area%. The compound meets the requirements for shipment under IATA non-hazardous classification (no UN number assigned) and is accompanied by a certificate of analysis that includes a residual solvent declaration per USP <467> Option 1, typically reporting < 50 ppm each of THF, ethyl acetate, and methanol.
When confronted with high-throughput library production demands, the use of this Cbz-protected chloro analog avoids the chemist’s frequent dilemma between premature amine deprotection and scaffold halogen loss. The absence of a labile benzylic halogen ortho to the pyrazine nitrogen—a feature present in several competing 8-bromoimidazo[1,2-a]pyrazine scaffolds—eliminates the formation of the hydrolysis-derived hydroxypyrazine impurity during automated solid-phase extraction (Biotage® Extrahera, C18 cartridges, basic aqueous/MeCN elution). Stability data from three independent campaign batches processed on a Tecan Freedom EVO® liquid handler with 96-well format confirm < 0.2% of the undesired C-8 OH impurity after 48 h at ambient temperature (22 ± 2 °C) in DMSO stock solution exposed to atmospheric moisture, which is a 40-fold reduction in hydrolysis susceptibility relative to the 8-bromo-1,2-a scaffold.
Storage instructions recommend −20 ± 5 °C in tightly closed containers under desiccant, with a retest period of 24 months from date of manufacture when maintained under these conditions. A forced degradation study per ICH Q1B (Option 2) at 200 Wh/m² UV and 1.2 million lux·h visible light produced 0.8% of a single photodegradant identified as the dechlorinated imidazo[1,5-a]pyrazine congener, underscoring the requirement for amber glassware during synthesis and formulation.
Operational boundaries established through process safety calorimetry (Mettler Toledo RC1e) limit the maximum adiabatic temperature rise for the coupling step to ΔTad = 38 K at 0.25 M concentration, necessitating a jacket temperature ramp not exceeding 1 K/min during scale-up beyond 5 L reactor volume to avoid triggering the undesired exothermic decomposition that initiates at 168 °C (onset by DSC). No incident of thermal runaway has been documented when these limits are observed. Published data for this specific compound’s degradation kinetics at extremes of pH above 12 is limited; users are advised to avoid prolonged exposure to strongly basic conditions unless the stereocenter stability has been independently validated under their specific reaction parameters.
