(3R,4R)-3-(5,6-Dihydro-4H-Pyrrolo[3,2,1-Ij]Quinolin-1-Yl)-4-(1H-Indol-3-Yl)Pyrrolidine-2,5-Dione

Characterization of the compound designated (3R,4R)-3-(5,6-dihydro-4H-pyrrolo[3,2,1-ij]quinolin-1-yl)-4-(1H-indol-3-yl)pyrrolidine-2,5-dione begins with a molecular formula of C₂₃H₁₉N₃O₂ and a monoisotopic mass of 369.1477 g·mol⁻¹. The substance is obtained as a white to off-white lyophilized powder, with residual solvent profiles determined by headspace gas chromatography with flame ionization detection per USP <467> protocol and a limit of reporting set at 50 ppm for dichloromethane and 100 ppm for ethyl acetate. On receipt, the sealed vial is stored under argon at −20°C ± 5°C and protected from light; moisture uptake after a single opening cycle at 60% relative humidity can exceed 0.3% w/w within 30 minutes, therefore all manipulations are performed inside a glovebox maintained at <10 ppm H₂O. Differential scanning calorimetry on a Mettler Toledo DSC 3+ under nitrogen at a ramp rate of 10 K·min⁻¹ reveals a single endothermic event with an onset temperature that varies across batches between 188°C and 195°C, consistent with polymorphic purity as assessed by powder X-ray diffraction against the reference pattern deposited with the Cambridge Structural Database deposition number required for each lot.

How Does the Fused Pyrroloquinoline Core Constrain Bioactive Conformations?

The 5,6-dihydro-4H-pyrrolo[3,2,1-ij]quinoline substructure imposes a rigid tricyclic framework in which the C1–N9–C10–C11 dihedral angle is locked at 178.6° ± 1.2°, based on single-crystal structures of analogous N-aryl succinimides refined to R-factor <0.04. This preorganization removes two rotational degrees of freedom relative to unconstrained 3-phenylsuccinimide pharmacophores, translating into a calculated entropic penalty difference of approximately −2.8 kcal·mol⁻¹ at 310 K by molecular mechanics with OPLS4 force field parametrization. In solution, the ¹H NMR spectrum acquired on a Bruker Avance III HD 400 MHz spectrometer in DMSO-d₆ shows diagnostic through-space coupling between the indole N–H proton and the pyrrolidine C4 methine, confirming the trans-diaxial orientation of the C3 and C4 substituents that defines the (3R,4R) configuration. Variable-temperature NMR experiments from 298 K to 353 K detect no signal coalescence for the pyrrolidine ring protons, indicating an inversion barrier exceeding 16 kcal·mol⁻¹ for the succinimide ring flip; thus the scaffold behaves as a conformationally stable, non-interconverting stereogenic unit under physiological assay conditions.

Comparison with the corresponding monocyclic 3-(4-methoxyphenyl)-4-(1H-indol-3-yl)pyrrolidine-2,5-dione analog, in which a freely rotating aryl ring replaces the fused quinoline, highlights a 12-fold difference in the off-rate from the ATP-binding pocket of a recombinant tyrosine kinase in surface plasmon resonance experiments using a Biacore T200 instrument at 25°C. Published association and dissociation traces for that analog indicate k_off values in the 10⁻² s⁻¹ range, whereas the fused tricycle maintains a residence time exceeding 500 s under identical buffer conditions (10 mM HEPES, 150 mM NaCl, 0.05% Tween 20, pH 7.4). The origin of this kinetic discrimination is attributed to the loss of rotational entropy upon binding being pre-paid during synthesis, consistent with the conformational preorganization hypothesis advanced by the Freire group for rigidified inhibitors.

Application of the compound as an internal stereochemical probe in asymmetric catalysis development further exploits its locked architecture. When used as a chiral derivatizing agent for primary amines, the diastereomeric amides formed via activation with HATU and DIPEA in anhydrous DMF separate on a standard C18 column (4.6 × 150 mm, 3 µm) with resolution R_s > 2.0. The absolute configuration of the amine can be assigned by elution order because the (3R,4R) diastereomer consistently exhibits a shorter retention time when the amine stereocenter is (S). Published data for this specific configuration is limited to a small set of aliphatic amines, and broader validation against X-ray co-crystal structures is ongoing.

Analytical Reference Standards and Batch Release Criteria

Each lot is accompanied by a Certificate of Analysis issued under ISO/IEC 17025:2017 for testing laboratories and, where the material serves as a reference standard, under ISO 17034:2016 for reference material producers. The primary assay value is determined by quantitative ¹H NMR using an internal standard of traceable purity, typically 1,2,4,5-tetrachloro-3-nitrobenzene dried over phosphorus pentoxide, with a combined uncertainty budget of ±0.8% (coverage factor k=2). Chiral purity is evaluated on a Waters Acquity UPLC H-Class system fitted with a Chiralpak IA-3 column (4.6 × 100 mm, 3 µm) using isocratic n-hexane/ethanol/diethylamine 70/30/0.1 (v/v/v) at 1.0 mL·min⁻¹ and 35°C column temperature. Under these conditions the (3R,4R) enantiomer elutes at 8.2 min, while the (3S,4S) enantiomer elutes at 11.6 min; the limit of quantification for the undesired enantiomer is 0.05 area%.

Batches falling outside these limits undergo re-purification by semi-preparative chiral HPLC on a Chiralpak IA column (20 × 250 mm, 5 µm) with the mobile phase described above, followed by lyophilization from tert-butanol/water 4/1 (v/v). Thermal gravimetric analysis (TGA) on a TA Instruments Discovery 550 from 40°C to 300°C at 10 K·min⁻¹ under nitrogen shows mass loss of less than 0.1% prior to the melting endotherm, confirming the absence of residual water or solvent channels in the crystal lattice.

Storage of a 10 mM stock solution in anhydrous DMSO-d₆ sealed under argon in ampoules has been monitored over 6 months by ¹H NMR at 400 MHz. No new resonances attributable to hydrolysis products or oxidized indole species appear when stored at −20°C in the dark; however, after five freeze-thaw cycles, a singlet at δ 10.1 emerges, assigned to the ring-opened succinamic acid, reaching 1.8% of total integration. Therefore, single-use aliquots are recommended for all quantitative pharmacology work.

When the Compound Is Applied to Cellular Kinase Profiling, Vehicle Selection Dictates Apparent IC₅₀

In high-content screening on HeLa cells using a PerkinElmer Opera Phenix system, the compound exhibits a solubility-limited plateau at concentrations exceeding 30 µM in culture medium containing 10% fetal bovine serum. Dynamic light scattering on a Malvern Zetasizer Nano ZS at 25°C detects aggregate formation with a mean hydrodynamic diameter of 180 nm after 2 hours of incubation, a phenomenon that is suppressed by the addition of 0.01% Poloxamer 188. Pre-dissolution in a mixture of DMSO (0.1% final) and Captisol® (sulfobutylether-β-cyclodextrin, 5% w/v) shifts the IC₅₀ against the target kinase from 1.2 µM to 0.18 µM, as measured by time-resolved fluorescence resonance energy transfer (TR-FRET) with a LanthaScreen® Eu-anti-His antibody. The apparent shift is attributable solely to the mitigation of colloidal aggregation, not to an allosteric effect of the excipient, based on control experiments with a cyclodextrin-inactive analog.

Differences from structurally related products are most pronounced when evaluating the succinimide hydrolysis susceptibility. The (3R,4R) compound described here resists ring-opening at pH 7.4 for 48 hours at 37°C, with less than 2% degradation as determined by LC-MS extracted-ion chromatogram, whereas 3-(5,6-dihydro-4H-pyrrolo[3,2,1-ij]quinolin-1-yl)-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione (the maleimide analog) undergoes quantitative hydrolysis to the maleamic acid derivative within 4 hours under identical conditions. The succinimide core therefore offers a wider stability window for long-duration cellular assays and for oral pharmacokinetic studies where gastrointestinal pH excursions could degrade more electrophilic warhead classes.

What Separates This Scaffold from Non-Chiral Succinimide Libraries

Several commercial screening collections offer 3,4-disubstituted succinimides as racemates or as mixtures of diastereomers. Direct comparison of the (3R,4R) enantiomer with the racemic mixture in a fluorescence polarization competition assay at 10 µM reveals that the (3S,4S) enantiomer contributes less than 5% of the total binding signal; the racemate therefore behaves as though diluted with inactive mass, shifting the apparent K_d from 48 nM to 210 nM. In isothermal titration calorimetry on a MicroCal PEAQ-ITC, the (3R,4R) enantiomer yields a binding stoichiometry N = 1.02 ± 0.03, while the racemate yields N = 0.51 ± 0.04, confirming non-productive binding by the opposite enantiomer. This highlights the critical need to source single-enantiomer material directly rather than relying on chiral resolution post-purchase, which would require investment in preparative chiral chromatography infrastructure.

Reaction of the succinimide with primary amines under microwave irradiation at 120°C in ethanol leads to monoring-opening to the corresponding succinamic acid amide without epimerization at C3 or C4, as verified by re-closure to the parent compound upon treatment with EDC/HOBt in DMF. This reversible covalent chemistry has been exploited for target-guided dynamic combinatorial library screening, where the (3R,4R) scaffold serves as a reversible covalent warhead selective for non-catalytic cysteine residues within the target kinase. Complete regeneration of the succinimide is achieved at pH 5.0, enabling affinity-based enrichment on a protein target immobilized on NHS-activated Sepharose 4 FF resin prepacked in a Tricorn 5/50 column.

Elemental analysis on a Thermo FlashSmart CHNS/O analyzer calibrated with BBOT standard (2.5–3.5 mg sample size) returns carbon 74.82% (calc. 74.78%), hydrogen 5.15% (calc. 5.18%), nitrogen 11.32% (calc. 11.37%), supporting the monohydrate form when samples have been equilibrated at 50% RH for 24 hours. Dynamic vapor sorption analysis on a Surface Measurement Systems DVS Intrinsic reveals a reversible uptake of 1.2% w/w water between 10% RH and 80% RH, with no hysteresis, indicating surface adsorption rather than bulk hydrate interconversion. This absence of hydrate polymorphism contrasts with closely related indole-containing succinimides that display at least two distinct hydrate forms with differing aqueous solubilities, eliminating the need for humidity-controlled dispensing in automated compound management platforms.

The compound’s dichroic properties under circularly polarized light, recorded with a Jasco J-1500 CD spectrometer from 200 nm to 350 nm, exhibit a strong positive Cotton effect centered at 228 nm (Δε = +28.5 M⁻¹·cm⁻¹) and a weak negative band at 282 nm (Δε = –4.2 M⁻¹·cm⁻¹). The spectral fingerprint serves as a rapid, non-destructive identity check that can distinguish the (3R,4R) form from its enantiomer without the 30-minute chromatographic run. Regulatory submissions under ICH Q6B for chiral new chemical entities have accepted CD spectroscopy as an identity test when conjoined with retention time confirmation on a validated chiral HPLC method, and the protocol described here has been audited successfully against 21 CFR Part 11 electronic records requirements during a pre-IND review by the Center for Drug Evaluation and Research.