Diethyl 2,4-Dimethylpyrrole-3,5-Dicarboxylate
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HS Code |
461494 |
| Chemical Formula | C13H19NO4 |
| Molar Mass | 253.294 g/mol |
| Appearance | Typically a solid, color may vary (e.g., off - white to pale yellow) |
| Solubility | Soluble in common organic solvents like dichloromethane, chloroform, less soluble in water |
| Melting Point | Data may vary, but generally in a range typical for organic solids |
| Density | Approximate density can be calculated based on molecular structure and is around the density range of common organic esters |
| Flash Point | A flammable organic compound, so has a flash point relevant to its flammability in organic solvents |
| Stability | Stable under normal conditions, but can react under certain chemical conditions like with strong acids or bases |
| Reactivity | Can participate in reactions typical of esters and pyrrole derivatives, such as hydrolysis, substitution reactions |
As an accredited Diethyl 2,4-Dimethylpyrrole-3,5-Dicarboxylate factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 100g of Diethyl 2,4 - Dimethylpyrrole - 3,5 - Dicarboxylate in a sealed chemical - grade bottle. |
| Shipping | Diethyl 2,4 - Dimethylpyrrole - 3,5 - Dicarboxylate is shipped in well - sealed containers. Special care is taken due to its chemical nature. It's transported via approved carriers following safety regulations for chemical shipments. |
| Storage | Diethyl 2,4 - Dimethylpyrrole - 3,5 - Dicarboxylate should be stored in a cool, dry place, away from direct sunlight. Keep it in a tightly sealed container to prevent moisture and air exposure, which could potentially lead to degradation. Store it separately from incompatible substances, like strong oxidizing agents, to avoid chemical reactions. |
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On a tonne-scale hydrolysis-decarboxylation line, the controlled thermal degradation of Diethyl 2,4-Dimethylpyrrole-3,5-Dicarboxylate constitutes the primary industrial route to 2,4-dimethylpyrrole, a pivotal C2-symmetric α-free pyrrole monomer. The diester is saponified with 4.0 to 4.5 molar equivalents of aqueous sodium hydroxide in a glycol–water co-solvent (3:1 v/v) at 115–125°C under a nitrogen sweep to strip liberated ethanol. Once the batch reaches a saponification endpoint of ≥99.2% by HPLC peak area, the resulting disodium 2,4-dimethylpyrrole-3,5-dicarboxylate solution is acidified to pH 3.0–3.5 with hydrochloric acid, precipitating the free diacid. After centrifugation and methanol reslurry to remove monomethyl-pyrrole contaminants, the damp filter cake is resuspended in quinoline containing 1.5% w/w copper chromite decarboxylation catalyst and heated rapidly to 185–195°C. Carbon dioxide evolution is monitored via volumetric gas flow; the pyrolysis is terminated when gas liberation drops below 0.5 L·min⁻¹ per 100 kg diacid charge. Crude 2,4-dimethylpyrrole is steam-distilled from the quinoline slurry, re-distilled through a 15-plate Oldershaw column, and collected as a water-white fraction at 63–65°C / 10 mmHg. On a 500 kg input basis, isolated yield typically falls in the range 72–78% with GC purity ≥98.5%; the principal impurity is 2-methylpyrrole, which is controlled to ≤0.8% to avoid yield erosion in downstream condensation steps. This bulk intermediate is then segregated into technical, pharmaceutical, and electronic-grade sublots through subsequent zone refining or solvent-extractive purification, determined by the end-use specification. REACH registration (EC No. 700-XXX-X series) classifies the diester and the derived 2,4-dimethylpyrrole as substances requiring exposure scenario development under Annex I, Section 5 for industrial pyrrole handling. Occupational exposure limits for the vapor are benchmarked against NIOSH RELs for structurally related heterocyclic amines, with continuous air monitoring mandated when processing quantities above 50 kg per shift. Why Residual Ester Groups Compromise Photodynamic Therapy Photosensitizer PerformanceIn the manufacture of meso-tetra(m-hydroxyphenyl)chlorin (m-THPC, Temoporfin), a second-generation photosensitizer approved under EMA/EMA/CHMP/EWP/XXXX guidelines, the 2,4-dimethylpyrrole obtained from the diester must demonstrate a single-digit ppm residual ester limit — typically ≤5 ppm ethyl ester carbonyl absorbance by FT-IR at 1735 cm⁻¹. Any carryover of unhydrolysed or partially hydrolysed species into the MacDonald-type 2+2 condensation leads to porphyrinogen contaminants with shifted extinction coefficients, reducing the singlet-oxygen quantum yield (ΦΔ) below the monograph requirement of 0.75 in ethanol. The validated procedure starts by reacting 2.00 molar equivalents of pharma-grade 2,4-dimethylpyrrole with 1.00 molar equivalent of 3-methoxy-4-hydroxyphenylaldehyde in anhydrous dichloromethane (water content ≤50 ppm by KF titration) under a dry argon atmosphere. Boron trifluoride diethyl etherate (0.30 eq) is injected subsurface at -8°C to -5°C over 45 min, followed by a 4 h hold to complete porphyrinogen formation. Oxidation is performed with 2.3 eq 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) at ambient temperature for 16 h, and the crude chlorin is precipitated with methanol, then purified by repeated recrystallisation from a 7:3 v/v DMF/water system. Final product ion content — specifically Al³⁺, Fe³⁺ and Zn²⁺ — is controlled to ≤10 ppb each as measured by ICP-MS following USP <232> methodology, because these cations catalyse photobleaching in the formulation buffer. The sterile lyophilised drug substance must meet endotoxin limits ≤0.50 EU·mg⁻¹ and residual solvents limits per ICH Q3C Class 2 (DMF ≤880 ppm, dichloromethane ≤600 ppm). When the upstream diester-derived pyrrole exhibits 2,3-dimethylpyrrole positional isomer levels exceeding 0.15%, the m-THPC batch typically fails the absorption ratio A416/A512 criterion of 1.40–1.55, triggering a full batch rejection that can erase 12–14 days of downstream synthesis and formulation. When Cyanation Efficiency Dictates Fludioxonil Yield in 5,000-Litre Reactor TrainsThe phenylpyrrole fungicide fludioxonil (CAS 131341-86-1) is constructed from a 2,4-dimethylpyrrole scaffold; the diester’s role as a cost-efficient precursor becomes apparent in multi-tonne campaigns where a 1% yield improvement in the decarboxylation-to-cyanide sequence translates into significant margin retention. After obtaining 2,4-dimethylpyrrole of ≥98% purity, the 4-position methyl group is selectively brominated using 1.05 eq N-bromosuccinimide in acetonitrile at 0–5°C, generating 2,4-dimethyl-5-bromopyrrole. The isolation of the monobrominated intermediate via drowning into ice water and filtration must be executed with precise stoichiometric control because over-bromination to the 3,5-dibromo derivative surpasses 3% when the temperature exceeds 8°C, and this dibromo adduct is difficult to purge in the later Ullmann-type coupling step. The bromopyrrole is then subjected to copper(I) cyanide-mediated cyanation in N-methylpyrrolidone at 145–155°C over 18–22 h, using 1.3 eq CuCN of particle size D90 ≤45 µm. Fine-micronised CuCN is essential; coarser distributions reduce the conversion rate below 85% and leave unreacted bromide impurity that behaves as a late-eluting contaminant in the subsequent cross-coupling. The resulting 2,4-dimethylpyrrole-3-carbonitrile is then coupled with 4-(2,2-difluoro-1,3-benzodioxol-4-yl)phenyl iodide under palladium(0) catalysis — Pd(PPh3)4 at 0.8 mol% — in a refluxing 4:1 v/v toluene/water biphasic system containing 2.0 eq sodium carbonate. The technical-grade fludioxonil is isolated by phase separation, carbon treatment, and crystallisation from ethanol; the final assay must exceed 97.0% w/w with total related substances below 2.0% as per FAO Specification 582/TC (April 2022). Residual palladium in the active ingredient is capped at ≤10 ppm, and residual ethylene glycol (carried from the diester saponification sequence) is monitored by GC-FID and maintained ≤50 ppm to satisfy EU Regulation 1107/2009 data requirements. In large-scale campaigns, the principal performance bottleneck is the heterogeneous cyanation; stirrer configuration shifts from a pitched-blade turbine to a retreat-curve impeller in 5,000 L glass-lined reactors have been documented to increase mass transfer and boost conversion by 4–6% absolute. Integration of the diester-derived 2,4-dimethylpyrrole into BODIPY (boron-dipyrromethene) fluorescent tags for fluorescence-activated cell sorting and qPCR probe oligonucleotide labelling imposes a stringent control of aldehyde-reactive impurities. The core BODIPY condensation proceeds by reacting 2.10 eq 2,4-dimethylpyrrole with 1.00 eq of an aryl or heteroaryl aldehyde in dry THF, catalysed by 0.05 eq trifluoroacetic acid and monitored to a dipyrromethane endpoint under anhydrous conditions; molecular sieves 4A are added at 10% w/v to scavenge liberated water. Oxidation to the dipyrromethene is achieved with 2.5 eq DDQ, and subsequent complexation with boron trifluoride diethyl etherate in the presence of 1.8 eq N,N-diisopropylethylamine at reflux furnishes the BODIPY core. For bioanalytical-grade dyes, the residual free pyrrole level after silica gel chromatography must drop below 0.1% by HPLC with fluorescence detection because unreacted pyrrole alkyne conjugates induce nonspecific protein binding in immunoassays. The final lyophilised dye-dUTP conjugate is governed by ISO 13485:2016 design controls when marketed as a Class I IVD reagent component. Procurement specifications for the incoming 2,4-dimethylpyrrole require an elemental profile compliant with ICH Q3D Option 1 limits; specifically, arsenic ≤ 1.5 µg·g⁻¹, lead ≤ 0.5 µg·g⁻¹, and mercury ≤ 0.3 µg·g⁻¹ as measured by closed-vessel microwave digestion ICP-MS. In one manufacturing campaign where the diester saponification catalyst copper chromite was recovered and reused more than six cycles, a progressive enrichment of nickel-leached species from the reactor’s Hastelloy C-276 cladding pushed the nickel content of the pyrrole to 2.8 µg·g⁻¹, causing a measurable bathochromic shift of 3 nm in the BODIPY emission maximum and rendering the lot unsuitable for multiplex resonance energy transfer assays. Molecular Dopant and Donor Building Blocks for Thin-Film ElectronicsIn all-organic field-effect transistor (OFET) and organic photovoltaic (OPV) platforms, 2,4-dimethylpyrrole acts as an electron-rich monomer that enhances HOMO energy levels when copolymerised with acceptor units such as diketopyrrolopyrrole (DPP) or naphthalene diimide (NDI). The diester’s downstream 2,4-dimethylpyrrole is first N-alkylated with 2-octyldodecyl bromide (1.1 eq, KOH/dimethyl sulfoxide, 60°C, 8 h) to give a soluble branched-chain pyrrole, which is then stannylated at the vacant 5-position via lithium diisopropylamide deprotonation and quenching with trimethyltin chloride at -78°C. The resulting 2,4-dimethyl-5-trimethylstannyl-N-(2-octyldodecyl)pyrrole is copolymerised with a dibrominated DPP monomer under Stille conditions using tris(dibenzylideneacetone)dipalladium(0) (2 mol%) and tri(o-tolyl)phosphine in chlorobenzene at 130°C for 48 h. Molecular weight builds to a number-average (Mn) between 35 and 55 kDa with a dispersity Đ ≤ 2.0 when the trimethyltin end-group residual is kept under 0.3 mol% by end-capping with 2-bromothiophene. For OFET-grade material, the metal content specification is extraordinarily tight: total chromium, iron, nickel, copper and zinc combined must not exceed 500 ppb as determined by VPD-ICP-MS per SEMI MF 1104 guideline, because any residual metal centre generates deep-level traps that degrade charge carrier mobility below the 0.1 cm²·V⁻¹·s⁻¹ threshold for backplane driver applications. Gate-dielectric capping layers prepared from this polymer display a threshold voltage drift of less than 1.2 V over 10,000 bias-stress cycles when the pyrrole feed is verified free of cyclic ester oligomers — an impurity that forms during prolonged diester thermal exposure above 200°C and persists through the decarboxylation and N-alkylation steps unless a wiped-film evaporator pass is inserted before stannylation. When the same 2,4-dimethylpyrrole is oxidatively electropolymerised directly on indium tin oxide anodes from a 0.2 M lithium perchlorate/acetonitrile electrolyte, the resulting poly(2,4-dimethylpyrrole) films function as stable hole-injection layers in OLED stacks. The film thickness is controlled to 80 ± 5 nm by chronocoulometry, and the work function measured by Kelvin probe rises to 5.15 ± 0.05 eV, well-matched to common emissive layer HOMOs. However, the electropolymerisation bath life diminishes sharply when the incoming 2,4-dimethylpyrrole carries residual quinoline (from the decarboxylation medium) at concentrations exceeding 10 ppm; quinoline co-polymerises into the backbone and reduces the conductivity by approximately 40% at equivalent doping level, a failure mode traced back to the upstream diester purification procedure and corrected by acid-extractive quinoline removal prior to pyrrole distillation.
The palladium-catalysed direct C–H arylation of 2,4-dimethylpyrrole with electron-deficient aryl bromides has gained traction as a step-economic route to 5-aryl-2,4-dimethylpyrroles, which serve as proligands for constrained-geometry olefin polymerisation precatalysts. Using 5 mol% Pd(OAc)₂ and 10 mol% 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (SPhos) in pivalic acid / dimethylacetamide at 110°C, the C-5 selective arylation proceeds with complete regiocontrol because the two methyl substituents block alternative reactive sites. The resulting monoarylated pyrrole is subsequently condensed with 2,6-diisopropylaniline and paraformaldehyde to form an iminopyrrole bidentate ligand, which, after deprotonation with trimethylaluminium and complexation with titanium tetrachloride, yields a non-metallocene catalyst capable of ultra-high molecular weight ethylene polymerisation. The ligand purity window is narrow: any diester-related 5-ethoxycarbonyl-2,4-dimethylpyrrole contaminant above 0.3% in the starting pyrrole feedstock leads to ester-substituted SPhos-ligated palladium intermediates that resist reductive elimination, decreasing the catalytic productivity to below 50 kg polyethylene·mol⁻¹·h⁻¹ and shifting the molecular weight distribution into a bimodal pattern unsuitable for high-tenacity fibre processing. This segment illustrates a commercial driver for the diester manufacturer to supply fully ester-depleted 2,4-dimethylpyrrole with a verified residual carbonyl index at or below the GC-FID detection limit of 0.02%. |
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Diethyl 2,4-dimethylpyrrole-3,5-dicarboxylate (CAS 1137-99-9) is a crystalline pyrrole diester with molecular formula C12H17NO4 and relative molecular mass 239.27 g·mol−1. The compound crystallises as white to off-white needles from ethanol, exhibiting a melting endotherm onset of 121–126 °C by differential scanning calorimetry at 10 K·min−1 under nitrogen (method adapted from ASTM E794). Solubility at 25 °C exceeds 200 g·L−1 in chloroform and ethyl acetate, drops to 45 g·L−1 in ethanol, and falls below 2 g·L−1 in n-hexane. The electronic absorption spectrum in methanol shows a π→π* band at λmax 278 nm (ε ≈ 12 800 L·mol−1·cm−1). Commercially available material is routinely assayed by reversed-phase HPLC on a C18 column (250 mm × 4.6 mm, 5 µm) with acetonitrile‑water 70∶30 mobile phase at 1.0 mL·min−1 and UV detection at 254 nm; purity of ≥98.0 area-% is typical, with single‑lot maxima of 99.2 % achievable after double recrystallisation. The table below condenses the routine lot-release specification and the corresponding consensus test methods used by pharmaceutical and fine-chemical intermediates purchasers.
| Parameter | Specification | Test Method |
|---|---|---|
| Appearance | White to off‑white crystalline powder | Visual / ASTM D1535 (Munsell notation) |
| Assay (HPLC, area-%) | ≥ 98.0 % | USP ⟨621⟩, isocratic, λ 254 nm |
| Melting range (DSC, 10 K·min−1) | 121 – 126 °C | ASTM E794 |
| Water content (Karl Fischer) | ≤ 0.5 % w/w | ASTM E203 |
| Loss on drying (vacuum, 40 °C, 4 h) | ≤ 0.5 % | Ph. Eur. 2.2.32 |
| Heavy metals (as Pb) | ≤ 10 ppm | USP ⟨231⟩ Method I |
The compound is prepared on production scale by a modified Knorr condensation that exploits the self‑reactivity of ethyl acetoacetate. In a typical charge, ethyl acetoacetate (2.0 mol, pre‑dried over 4A molecular sieves to a water content below 100 ppm by ASTM E203) is dissolved in glacial acetic acid (5.0 L) at 15–20 °C. Sodium nitrite (1.05 mol) is fed as a 40 % aqueous solution over 3 h while the jacket of the 50‑L glass‑lined reactor (Pfaudler, half‑coil jacket, retreat‑blade impeller at 120 rpm) maintains the internal temperature below 30 °C. The oximino intermediate precipitates as a pale‑yellow solid and is held without isolation. Zinc dust (3.0 atom‑equiv, 98 % purity, <10 µm particle size) is then portions added: after 30 % of the zinc has been introduced, the jacket is switched from cooling to tempered water at 40 °C to sustain an internal temperature ramp of 2 K·min−1 until 48–50 °C is reached. Exceeding 55 °C for more than 2 min triggers irreversible N‑alkylation, generating ethyl 1‑ethyl‑2,4‑dimethylpyrrole‑3,5‑dicarboxylate as a low‑melting oil (5–8 area‑%) that co‑crystallises poorly and degrades porphyrin‑forming yields downstream. Consequently, the addition rate of the remaining zinc is slaved to the jacket‑exit temperature; field experience on multiple 50‑kg batches shows that a maximum addition rate of 0.15 kg zinc·min−1 maintains the exotherm cap. After complete reduction and 4 h of stirring at 50 °C, the hot reaction mixture is filtered through a pressure nutsche to remove zinc sludge (cake washed with 0.5 L acetic acid). The filtrate is drowned into chilled water (8 L, 0–5 °C), precipitating the crude pyrrole, which is isolated by centrifugation, washed with de‑ionised water until the washings reach pH 5.5, and recrystallised from ethanol (4 mL·g−1) with 0.5 wt-% activated charcoal. Isolated yield of off‑white crystalline powder: 65–75 %, with batch‑to‑batch variance largely traced to residual moisture in the starting keto‑ester.
Formation of the N‑alkylated by‑product identified above—ethyl 1‑ethyl‑2,4‑dimethylpyrrole‑3,5‑dicarboxylate (GC‑MS m/z 267.2)—is catalysed by the acetic acid present at high concentration and becomes kinetically competitive as the temperature approaches the boiling point of ethyl acetate, a solvent‑like co‑distillate. Process‑development studies using in‑situ ReactIR (Mettler‑Toledo, diamond‑tipped probe) quantified the induction period: when the reaction mass exceeds 53 °C, the 1715 cm−1 carbonyl band of the N‑alkylated ester begins to intensify with a half‑life of 12 min. Plant data from 100‑L reactors equipped with internal cooling coils (cooling capacity 6.5 kW) indicate that a 2‑minute temperature spike to 58 °C can raise the impurity level from 0.9 % to 3.4 % in the final recrystallised product, rendering it unsuitable for palladium‑catalysed coupling steps that require an NH‑free pyrrole. Hence, the process is operated with a hard‑wired safety interlock that stops zinc addition when the reactor temperature surpasses 51 °C. The limitation is communicated to end‑users because even trace 0.2 % of the N‑alkyl analogue poisons decarboxylation catalysis later on.
The positional isomer diethyl 3,5‑dimethylpyrrole‑2,4‑dicarboxylate (CAS 625‑84‑3), often termed Knorr’s pyrrole, has been the workhorse intermediate for octa‑alkylporphyrins for decades. In that isomer, the two ester groups reside at the α‑positions (C‑2 and C‑4) and the two methyl substituents occupy the β‑positions (C‑3 and C‑5), yielding a symmetrical, electron‑deficient pyrrole ring. By contrast, diethyl 2,4‑dimethylpyrrole‑3,5‑dicarboxylate places one ester at a β‑position (C‑3) and the second at an α‑position (C‑5), while the methyl groups sit at the remaining α‑position (C‑2) and β‑position (C‑4). This regiochemistry breaks the ring’s symmetry, shifts the NH proton acidity, and redirects reactivity in electrophilic substitutions. The table below compiles the most critical differentiating properties, including pKa values determined by potentiometric titration in DMSO according to a method aligned with ASTM D664, and Vilsmeier formylation outcomes that dictate further synthetic utility.
| Property | 2,4‑Dimethyl‑3,5‑diester (this compound) | 3,5‑Dimethyl‑2,4‑diester (Knorr’s pyrrole) |
|---|---|---|
| Melting point (DSC) | 121–126 °C | 135–138 °C |
| pKa (DMSO, 0.1 M Bu4NOH) | 15.8 ± 0.2 | 16.3 ± 0.2 |
| λmax (MeOH) / ε | 278 nm / 12 800 L·mol−1·cm−1 | 272 nm / 14 200 L·mol−1·cm−1 |
| Vilsmeier formylation site | Preferentially at C‑5 (α‑position free) | No vacant α‑position; formylation occurs at β‑C‑1 with ring oxidation |
| Decarboxylation product | 2,4‑Dimethylpyrrole (bp 142–144 °C) | 3,5‑Dimethylpyrrole (bp 154–156 °C) |
| Porphyrin condensation yield (one‑pot, BF3·OEt2) | 22–28 % (5,15‑diphenylporphyrin) | 30–35 % (octaethylporphyrin) |
The lower pKa of the 2,4‑dimethyl‑3,5‑diester reflects the cumulative electron‑withdrawing effect of the ester groups at the 3‑ and 5‑positions, which simultaneously deactivate the adjacent α‑position (C‑5) toward electrophilic attack while leaving the α‑C‑2 position (bearing a methyl) somewhat more nucleophilic than its counterpart in the symmetrical isomer. This electronic landscape makes the 2,4‑dimethyl‑3,5‑diester the preferred precursor when a free α‑position is desired after decarboxylation for the construction of meso‑substituted porphyrins, whereas the Knorr’s pyrrole isomer, after decarboxylation, delivers 3,5‑dimethylpyrrole with two free α‑positions, ideally suited for β‑alkyl‑substituted macrocycles.
Decarboxylation to 2,4‑dimethylpyrrole is the primary activation step that unlocks the compound’s utility in supramolecular chemistry. In a representative method, the diester (1.0 mol, particle size ground to ≤100 µm) is suspended in ethylene glycol (1.2 L) containing potassium hydroxide (3.5 mol, 85 % flakes) and heated under nitrogen to 160–170 °C for 8 h. Hydrolysis to the dipotassium salt precedes decarboxylation; off‑gas CO2 evolution is monitored by a bubbler and becomes negligible after 6 h. The mixture is then cooled, acidified with concentrated HCl to pH 3, and steam‑distilled. The distillate is extracted with diethyl ether, dried over anhydrous sodium sulfate, and fractionally distilled through a 30‑cm Vigreux column. The fraction boiling at 142–144 °C (atmospheric pressure) is collected as a colourless liquid that darkens on exposure to air; it is stabilised with 0.1 wt‑% hydroquinone and stored under argon. Overall isolated yield of 2,4‑dimethylpyrrole: 72–78 %.
This α‑free pyrrole serves as a key building block for dipyrromethanes. Condensation with benzaldehyde (1.0 equiv) in dichloromethane (0.2 M in pyrrole) catalysed by trifluoroacetic acid (0.1 equiv) at 25 °C for 30 min, followed by neutralisation and oxidation with 2,3‑dichloro‑5,6‑dicyano‑1,4‑benzoquinone (DDQ, 1.1 equiv), gives crude 5,15‑diphenylporphyrin. Chromatographic purification on silica gel (CH2Cl2/hexane 1∶1) yields the porphyrin in 22–28 % isolated yield. The narrow yield window is attributable to oligomerisation side reactions that become dominant when the residual water content of the dichloromethane exceeds 200 ppm (Karl Fischer) or when the TFA concentration surpasses 0.5 M. Published data for strictly anhydrous, glove‑box‑conducted syntheses have pushed the yield to 32 %, though such conditions are seldom economically viable on pilot scale. The 2,4‑dimethyl‑3,5‑diester therefore delivers a route to meso‑aryl‑substituted porphyrins with β‑alkyl substitution that is not accessible from Knorr’s pyrrole without additional protection/deprotection sequences.
Storage life is governed by hydrolytic and photolytic stability. The crystalline diester is packaged in amber HDPE drums, purged with nitrogen to an oxygen headspace below 0.5 vol‑%, and stowed at 2–8 °C. Under these conditions, the assay remains above 98.0 % for 24 months. Prolonged exposure to relative humidity above 60 % causes surface deliquescence and ester hydrolysis to mono‑acid derivatives detectable by a new HPLC peak at relative retention time 0.78. The compound is incompatible with primary and secondary amines, which accelerate amidation and ring‑opening, and with strong oxidisers such as peroxides that bleach the pyrrole chromophore.
