Ethyl 2,4-Dimethyl-5-Propanoyloxy-1H-Pyrrole-3-Carboxylate

Designated product code EP-7592, the compound Ethyl 2,4-Dimethyl-5-propanoyloxy-1H-pyrrole-3-carboxylate (empirical formula C₁₃H₁₇NO₅, molecular weight 267.28 g·mol⁻¹) is supplied as a white to off-white crystalline powder with a minimum purity of 98.0% (HPLC area%, detection at 254 nm). The product is obtained via acylation of the corresponding 5-hydroxy pyrrole ester using propanoyl chloride in the presence of triethylamine, followed by recrystallization from ethyl acetate/hexane. This derivative distinguishes itself from the parent 5-hydroxy compound and the structurally related 5-acetoxy ester through its elevated thermal stability under inert atmosphere and its distinct reactivity profile in palladium-catalyzed transformations, where the propanoyloxy moiety participates as a directing group and a latent leaving group. In medicinal chemistry workflows, the propanoyloxy substituent imparts increased lipophilicity (calculated LogP 3.1) compared to the hydroxy analog (LogP 2.4), which can alter pharmacokinetic properties in early-stage screening. When incorporated into a pyrrole nucleus destined for cytochrome P450 inhibition assays, the ethyl ester at the 3-position undergoes hydrolysis by esterases more slowly than the corresponding methyl ester, as determined in human liver microsomal stability studies (t₁/₂ > 60 min). This characteristic makes EP-7592 a more suitable intermediate for prodrug design where controlled release is desired. The propanoyl group at C5 can be enzymatically cleaved to reveal a hydrogen-bond donor, enabling a two-stage activation sequence not feasible with the corresponding 5-methyl or 5-bromo analogs. Furthermore, metabolite identification studies using quadrupole time-of-flight mass spectrometry confirmed that oxidative metabolism primarily occurs at the 2- and 4-methyl groups rather than at the ester functionalities, a regioselectivity that simplifies toxicological profiling relative to 5-alkoxycarbonyl pyrroles that generate reactive acyl glucuronides.

Physicochemical Specifications and Storage Stability

The compound exhibits a sharp melting endotherm when analyzed by differential scanning calorimetry at a heating rate of 10 °C·min⁻¹ under a nitrogen purge of 50 mL·min⁻¹; the observed onset temperature is 82 °C with a peak maximum at 84 °C, meeting the acceptance range of 82–85 °C. A representative lot analysis release sheet is reproduced below.

Parameter	Specification	Method
Appearance	White to off-white crystalline powder	Visual inspection
Assay (HPLC area%)	≥ 98.0%	USP <621>; C18, 254 nm
Water content	≤ 0.5% w/w	ASTM E203 (coulometric KF)
Residual ethanol	≤ 5000 ppm	GC-FID per ICH Q3C
Residual hexane	≤ 290 ppm	GC-FID per ICH Q3C
Heavy metals (as Pb)	≤ 10 ppm	USP <231>
Melting range	82–85 °C	USP <741> Class I

Long-term stability studies conducted at −20 °C under argon atmosphere demonstrate less than 0.2% degradation over 12 months. At ambient humidity (relative humidity > 60%), hydrolytic ring-opening of the propanoyl ester becomes kinetically significant after 48 h; therefore, handling in a dry nitrogen glovebox (<1 ppm H₂O) is mandatory for moisture-sensitive downstream chemistry. Exposure to primary or secondary amines in concentrated solutions leads to aminolysis of the 5-propanoyl ester, liberating the corresponding amide and the 5-hydroxypyrrole species. Consequently, formulation development for amine-containing media must employ aprotic conditions or protect the ester. The compound is classified as a non-regulated substance under REACH for imported volumes below 1 tonne·annum⁻¹, and it contains no substances of very high concern (SVHC) at concentrations exceeding 0.1% w/w.

How Does the Propanoyloxy Substituent Influence Reactivity in Cross-Coupling Reactions?

The electron-withdrawing 3-ethoxycarbonyl group deactivates the pyrrole ring toward electrophilic aromatic substitution, but the 5-propanoyloxy center engages in oxidative addition with Pd(0) complexes under forcing conditions. In a Negishi coupling protocol evaluated on a 1.0 mmol scale using a Carousel 12 reaction station, EP-7592 (1.0 equiv) was treated with 1.2 equiv of 4-methoxyphenylzinc bromide in anhydrous THF, catalyzed by 2 mol% Pd₂(dba)₃ and 4 mol% XPhos at 80 °C for 18 h. The reaction furnished the 5-aryl product in 78% isolated yield after flash chromatography (SiO₂, hexane:EtOAc 8:2). Under identical conditions, the 5-bromo analog (EP-7580) gives 92% yield, yet its synthesis involves corrosive brominating agents and generates halogenated waste streams. The propanoyloxy variant thus offers a halogen-free route to 5-arylpyrroles, albeit at the cost of higher catalyst loading. The ethyl ester at C3 remains inert throughout the coupling, eliminating the need for orthogonal protecting groups that would be required for 3-carboxylic acid derivatives. Transesterification with nucleophilic organometallics (e.g., Grignard reagents) is not observed at temperatures below −20 °C, as demonstrated by stopped-flow IR monitoring of the carbonyl stretching frequency while titrating PhMgCl into an EP-7592 solution in THF at −40 °C. In comparison, the 5-acetoxy ester (EP-7591) suffers 12% transesterification at −20 °C under the same protocol, attributed to the lower steric bulk of the acetyl group. When the propanoyloxy group is leveraged solely as a directing group in C–H activation, competition with C2-methyl activation is minimal; a kinetic isotope effect study (k_H/k_D = 2.3) using deuterated 2-CD₃ analog confirmed that palladation occurs preferentially at the C5 position. This selectivity is reversed in the 5-methoxy derivative, where C2-arylation dominates. Thus, EP-7592 occupies a unique niche among 2,4-dimethylpyrrole-3-carboxylates as a substrate that undergoes C5-selective functionalization with predictable regiochemical outcome.

When Considering this Ester as a Monomer for Conducting Polymers

Electropolymerization of EP-7592 on indium tin oxide (ITO) electrodes was performed in a three-electrode cell with a Pt wire counter electrode and a Ag/AgCl reference, using 0.1 M tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile and a monomer concentration of 10 mM. Cyclic voltammetry revealed an irreversible oxidation peak at +1.24 V (vs Ag/AgCl), which is cathodically shifted by 180 mV relative to 2,4-dimethylpyrrole under identical conditions, attributable to the electron-withdrawing ester groups. The resulting poly(pyrrole) film exhibited a conductivity of 1.5 × 10⁻² S·cm⁻¹ after doping, measured by four-point probe following 48 h of solvent evaporation. While the conductivity is an order of magnitude lower than that of poly(3-methylpyrrole) prepared under comparable conditions, the film’s post-modification versatility offers compensatory advantages. Immersion of the coated electrode in 0.1 M aqueous NaOH at 40 °C for 2 h hydrolyzed the propanoyl ester, generating surface-confined hydroxyl groups quantifiable by toluidine blue O staining (4.3 nmol·cm⁻²). These sites were subsequently coupled with NHS-biotin, enabling streptavidin binding for biosensor architectures. The 5-acetoxy analog delivers a similar surface hydroxyl density but undergoes a 20% decrease in film adhesion during hydrolysis, as assessed by cross-hatch tape test (ASTM D3359-17), likely due to faster ester cleavage creating microporosity. In contrast, 5-alkoxycarbonyl pyrroles (e.g., 5-ethoxycarbonyl) resist hydrolysis under these mild basic conditions, preventing surface activation altogether. Therefore, EP-7592 serves as a processable monomer for functional electrode coatings where a balance between electroactivity and post-deposition derivatization is required. When pilot-scale synthesis of EP-7592 was conducted in a 50 L glass-lined reactor with anchor stirrer and jacket temperature control, the exothermic propionylation step (ΔH = −168 kJ·mol⁻¹) required a controlled addition rate of propanoyl chloride over 90 min to maintain an internal temperature below 5 °C. A single recrystallization from 1:3 ethyl acetate/hexane yielded 8.2 kg (87% yield) of material conforming to the above specifications. This batch-to-batch consistency has been verified across 15 consecutive production lots, with the HPLC purity coefficient of variation remaining below 0.8%. In agricultural intermediate synthesis, the compound’s 2,4-dimethyl substitution provides steric shielding that directs alkylating or acylating reagents toward the reactive C5 position after propanoyl removal. This contrasts sharply with 2,4-dimethyl-5-ethoxycarbonylpyrrole, where deprotection requires refluxing with strong base and generates ethanol that can compete for electrophiles. To illustrate, in the preparation of a herbicidal lead compound, the sequence propanoyl hydrolysis (0.5 M LiOH, THF/water, 23 °C, 2 h) followed by in situ S-alkylation with ethyl 2-bromoacetate gave a 74% overall yield, whereas the parallel sequence starting from the 5-ethoxycarbonyl pyrrole required 16 h at 60 °C with 2.0 equivalents of LiOH and afforded only 41% yield due to competitive ring degradation. These results underscore the operational advantage of the propanoyloxy group in sequential one-pot transformations.

Comparative Data Across 2,4-Dimethyl-5-O-Acyl-1H-Pyrrole-3-Carboxylate Esters

The table below aggregates key differentiating metrics for three structurally related products within the EP series, enabling direct selection based on synthetic strategy.

Property	EP-7590 (5-OH)	EP-7591 (5-OAc)	EP-7592 (5-OPr)
Melting range (°C)	138–142	96–99	82–85
Calculated LogP	2.4	2.8	3.1
Hydrolytic half-life t₁/₂ (pH 7.4 buffer, 37 °C)	Not applicable	4.2 h	8.7 h
Negishi coupling yield (5-(4-MeO-Ph) product)	No reaction	62%	78%
Electropolymerization onset potential (V vs Ag/AgCl)	+1.08	+1.15	+1.24
Film adhesion after base hydrolysis (cross-hatch rating)	5B	3B	4B

These differences arise from the interplay of steric demand, electronic withdrawal, and leaving-group ability of the 5-acyl moiety. Selection of EP-7592 is advantageous when a hydrolytically more robust protecting group is required during multi-step sequences, when higher coupling yields in Pd-catalyzed transformations are prioritized over absolute reaction rate, or when a wider electrochemical window is needed to suppress premature polymerization during functional monomer storage. The product is not recommended for applications where the 5-acyl group must be removed under strongly acidic conditions (e.g., concentrated H₂SO₄ at ambient temperature), as decarboxylation at the 3-position becomes a competitive side pathway.