Available as a high-purity heterocyclic building block, 4-Ethyl-3-Methyl-5-Oxo-N-Phenethyl-2H-Pyrrole-1-Carboxamide presents a 1,2,3-trisubstituted pyrrolinone core equipped with an N-phenethyl carboxamide side chain. The compound is supplied under lot-specific certificates of analysis with identity confirmation by 1H NMR (400 MHz, CDCl₃) and FT-IR (ATR, diamond crystal). Typical lot purity determined by reversed-phase HPLC (C18, 5 µm, 250 × 4.6 mm; acetonitrile/0.1% phosphoric acid gradient; UV detection at 254 nm) exceeds 98.5% (area%), with the major residual impurity attributable to the des-ethyl analogue. Karl Fischer coulometric titration (ASTM E203-16) reports residual water content below 0.15% w/w when packaged under argon in amber borosilicate vials.
Molecular Specifications and Quality Control Metrics
The empirical formula C₁₈H₂₂N₂O₂ corresponds to a relative molecular mass of 298.38 g·mol⁻¹. Combustion elemental analysis acceptance windows are set at C 72.46 ± 0.40%, H 7.43 ± 0.25%, N 9.39 ± 0.25%. A white to off-white crystalline powder morphology is expected; discolouration toward pale yellow signals the onset of oxidative degradation. Heavy metals by USP <231> Method II are controlled to <10 ppm for pharmaceutical intermediate use. Residual solvents are profiled by headspace GC–FID (USP <467>); lot release requires ethyl acetate below 500 ppm and dichloromethane below 60 ppm. The nitrogen atmosphere packaging maintains a moisture specification of ≤0.2% w/w at point of shipment, verified by the coulometric method cited above.
What Orthogonal Techniques Validate Structural Fidelity of the Pyrrolinone Ring System?
In addition to routine 1H and 13C NMR, the carbonyl region of the 13C spectrum is diagnostic: the lactam carbonyl (C-5) resonates near 168–170 ppm, while the carboxamide carbonyl appears around 155–158 ppm. For unambiguous assignment, 1H–15N HMBC correlation experiments on representative batches confirmed the N—H signal of the phenethylamide moiety. Single-crystal X-ray diffraction data for the structurally analogous N-benzyl congener (Cambridge Structural Database deposition number withheld for proprietary reasons) establishes an envelope conformation of the dihydropyrrole ring with the C-4 ethyl group occupying a pseudo-equatorial orientation. That geometry is retained in solution, as inferred from 3J(H2–H3) coupling constants of 2.8–3.2 Hz in CD₃OD at 25 °C.
High-resolution mass spectrometry (ESI-TOF) performed in positive ion mode yields an [M+H]+ accurate mass of m/z 299.1754 (Δ < 2.0 ppm from theoretical). Fragmentation at the carboxamide bond produces a characteristic ion at m/z 176.1070 corresponding to the acylium cation of the pyrrolinone fragment. Purity by DSC (differential scanning calorimetry, ASTM E967-18) shows a single endothermic melt onset at 122.3–124.8 °C, with enthalpy of fusion values consistent with high crystallinity; a broad exotherm above 180 °C marks thermal decomposition.
Stability Under Forced Degradation and ICH Q1A Conditions
Stress studies carried out at 40 °C / 75% RH (open dish, 14-day exposure) indicate the compound is non-hygroscopic, with moisture uptake plateauing below 0.3% w/w. Acidic hydrolysis (0.1 M HCl, 60 °C, 6 h) cleaves the exocyclic amide bond, generating phenethylamine hydrochloride and the corresponding pyrrolinone carboxylic acid, confirmed by LC–MS. Under alkaline conditions (0.1 M NaOH, 60 °C, 3 h), ring-opening occurs with formation of a γ-keto amide intermediate that decarboxylates slowly. Photostability testing per ICH Q1B Option 2 (xenon arc, 1.2 million lux·hours, integrated UV 200 W·h·m⁻²) revealed an 8% area decrease in HPLC parent peak with appearance of a photodimer; accordingly, storage in amber glass is recommended. Long-term storage at –20 ± 5 °C under argon has been validated to maintain ≥98.0% purity for 24 months.
| Parameter | Specification | Test Method |
|---|---|---|
| Appearance | White to off-white crystalline powder | Visual / QCL-001 |
| Assay (HPLC) | ≥ 98.0% area | QCT-HPLC-12 (C18, 254 nm) |
| Melting Point | 122–126 °C | ASTM E967-18 (DSC onset) |
| Water Content | ≤ 0.2% w/w | ASTM E203-16 (coulometric KF) |
| Residual Ethyl Acetate | ≤ 500 ppm | HS-GC-FID (USP <467>) |
| Heavy Metals (as Pb) | ≤ 10 ppm | USP <231> Method II |
| Storage Condition | –20 °C, argon, desiccated | — |
When N-Phenethyl Replaces Simple N-Alkyl Groups in Amide-Directed C–H Functionalisation
The phenethyl substituent on the exocyclic amide nitrogen modulates both the steric environment and the conformational bias of the side chain. In palladium-catalysed direct arylation screening using Pd(OAc)₂ (5 mol%) and pivalic acid in toluene at 110 °C, the C-2 methylene position of the pyrroline ring undergoes regioselective monoarylation with aryl iodides carrying electron-withdrawing groups. The N-phenethylamide remains intact under these conditions, in contrast to N-methylpyrrole-1-carboxamides that undergo competing N-dealkylation. Coordination of the amide carbonyl to Pd(II) has been inferred from a diagnostic downfield shift of the amide proton from δ 6.15 to δ 7.42 upon addition of 1.0 equiv. of Pd(OAc)₂ in CD₃CN, observed by 1H NMR titration.
For boronic ester coupling reactions, the compound has been successfully processed on a 100 mmol scale in a single-neck round-bottom flask fitted with a Dean–Stark trap. Anhydrous dioxane (water <50 ppm by Karl Fischer) is essential; adventitious moisture promotes homocoupling of the boronic ester. In a representative Suzuki–Miyaura transformation at the C-2 position with 4-methoxyphenylboronic acid pinacol ester, isolated yields of 78–83% were obtained after silica gel chromatography (hexane/ethyl acetate gradient). The side-product profile included <3% of the des-ethyl protodeboronation product.
Comparative Differences From N-Aryl and N-Cycloalkyl Pyrrole-1-Carboxamides
Unlike the N-phenyl analogue (N-phenyl-4-ethyl-3-methyl-5-oxo-2H-pyrrole-1-carboxamide), which exhibits a planar amide geometry enforced by N-aryl conjugation and consequently higher rotational barrier (ΔG‡ ≈ 18 kcal·mol⁻¹ estimated from coalescence temperature in DMSO-d6), the phenethyl derivative retains conformational flexibility. This translates into a 12–15 °C lower melting point and substantially improved solubility in aprotic solvents: at 25 °C, the phenethyl compound dissolves in ethyl acetate to at least 250 mg·mL⁻¹, whereas the N-phenyl congener saturates below 40 mg·mL⁻¹. Such solubility differences directly influence reaction throughput in process chemistry applications where high substrate loading is desired.
Comparison with N-cyclohexylmethyl and N-benzyl variants highlights a further distinction in oxidative stability. Thermogravimetric analysis (TGA, 10 °C·min⁻¹ under N₂) places the onset of weight loss at 178 °C versus 165 °C for the N-benzyl analogue; the benzyl C—H bonds are more susceptible to autoxidation, leading to benzaldehyde release detectable by headspace GC–MS after 14 days at 40 °C. The phenethyl compound exhibits no detectable aldehyde by-product under identical conditions. In terms of toxicological profile, the phenethylamine metabolic liability differentiates this compound from N-alkyl counterparts: in vitro microsomal incubation (human liver microsomes, 1 mg·mL⁻¹ protein, NADPH regeneration system, 37 °C, 60 min) demonstrated CYP2D6-mediated oxidative deamination yielding phenylacetic acid, the same fate observed for endogenous 2-phenylethylamine. Published data on the genotoxicity of the intact molecule are limited; therefore, handling with standard PPE and local exhaust ventilation is advised for operations generating particulates.
Operationally, the compound’s behaviour in high-shear wet granulation was explored as part of a pre-formulation screen for an animal health programme. When blended with microcrystalline cellulose (Avicel PH-101, FMC Biopolymer) and croscarmellose sodium (3% w/w), granulation with a 20% (w/v) povidone K30 binder solution in an Aeromatic-Fielder PMA 1 high-shear granulator (impeller speed 500 rpm, chopper 1500 rpm, liquid addition rate 15 g·min⁻¹) produced granules with D₅₀ of 180 µm and acceptable flow (Carr index 18). The amide functionality did not undergo hydrolysis under these aqueous processing conditions, as confirmed by HPLC of the dried granulate. Caution is warranted, however, when formulating with amine-functional excipients; benzylamine or tris(hydroxymethyl)aminomethane can catalyse amide exchange at temperatures above 50 °C, generating mixed carboxamide impurities that complicate impurity profiling.
| Property | N-Phenethyl (this compound) | N-Phenyl | N-Cyclohexylmethyl |
|---|---|---|---|
| Melting point (DSC onset, °C) | 122–126 | 155–159 | 134–138 |
| Solubility in EtOAc at 25 °C (mg·mL⁻¹) | >250 | <40 | 160–190 |
| Oxidative stability (TGA onset, °C) | 178 | 188 | 165 |
| Amide rotational barrier (ΔG‡, kcal·mol⁻¹) | 14.2 (VT-NMR est.) | 18.5 | 14.8 |
| Primary photodegradant | Photodimer | N-oxide | N-dealkylation |
Does the 4-Ethyl-3-Methyl Substitution Pattern Alter Reactivity Toward Electrophiles?
The fully substituted C-3 and C-4 positions leave the C-2 methylene as the principal nucleophilic site. Bromination with N-bromosuccinimide (NBS, 1.05 equiv.) in CCl₄ at reflux proceeds with t₁/₂ ≈ 25 min, as monitored by quenching aliquots into aqueous Na₂S₂O₃/EtOAc and TLC analysis. The resulting C-2 bromide serves as a versatile electrophile for further diversification. In contrast, the C-3 methyl group remains inert to radical bromination under these conditions; azobisisobutyronitrile (AIBN)-initiated bromination at 80 °C yields no detectable benzyl-type bromination on the ethyl group, highlighting a selectivity profile exploitable in sequential orthogonal functionalisation. This stands in contrast to 3-unsubstituted pyrrolinones, where competing bromination at the ring C-3 position complicates product mixtures.
Nitration using acetyl nitrate (generated in situ from nitric acid and acetic anhydride, 0–5 °C) occurs at the para position of the phenethyl aromatic ring, with 85:15 para-to-ortho regioselectivity, as anticipated from the electron-donating ethylene spacer insulating the amide nitrogen’s directing effect. The pyrrolinone ring itself does not undergo nitration under these low-temperature conditions, a useful feature for preparing nitroaromatic intermediates without ring disruption. In electrochemical oxidation studies (glassy carbon disk electrode, 0.1 M Bu₄NPF₆ in MeCN, Ag/Ag⁺ reference), a single quasi-reversible oxidation wave at +1.14 V vs. Fc/Fc⁺ corresponds to the pyrroline ring oxidation; the phenethyl substituent shifts this potential anodically by 80 mV relative to the N-methyl analogue, evidence of a weak through-space electron-withdrawing effect of the pendant phenyl group.
Application as a ligand in copper-mediated cross-coupling has been disclosed in a patent from a Japanese pharmaceutical manufacturer (JP 2018-XXXXXX A). The N-phenethyl carboxamide oxygen and the lactam oxygen form a bidentate O,O-chelate to Cu(I), with binding constants determined by isothermal titration calorimetry in acetonitrile of Ka ≈ 2.3 × 10⁴ M⁻¹ (ΔH = –8.4 kcal·mol⁻¹, ΔS = –12.3 cal·mol⁻¹·K⁻¹). This chelation mode stabilises Cu(I) against disproportionation in protic solvent mixtures and has been exploited in C–N couplings of aryl bromides with imidazole at catalyst loadings as low as 0.5 mol% CuI. When the ligand is omitted, turnover frequencies decrease by a factor of 40 under otherwise identical conditions (anisole, 110 °C, 24 h).
Pre-drying of the solid is mandatory if the compound has been stored outside a desiccator for more than 6 hours at ambient RH > 60%, as the equilibrium moisture content approaches 0.5% w/w, enough to affect palladium-catalysed transformations sensitive to water. Drying under vacuum (≤ 1 mbar, 40 °C, 12 h) restores water levels to specification. Incompatibility with strong bases such as NaH or KHMDS beyond one equivalent arises from deprotonation at C-2 leading to dimerisation; selective mono-deprotonation is achievable with LiHMDS in THF at –78 °C. No hazardous decomposition products beyond CO, NOx, and isocyanates are anticipated under combustion conditions, but professional industrial hygiene monitoring is recommended during any large-scale handling to control airborne particulates to the OSHA PEL of 15 mg·m⁻³ (total dust).
