Ethyl 2-(4-Hydroxy-3-Nitrophenyl)-4-Methyl-1,3-Thiazole-5-Carboxylate

Introducing a heterocyclic building block with a electron-withdrawing nitro group and a phenolic hydroxyl para to the nitrophenyl-thiazole linkage changes the reactivity profile in cross-coupling and condensation steps. The compound Ethyl 2-(4-Hydroxy-3-Nitrophenyl)-4-Methyl-1,3-Thiazole-5-Carboxylate (CAS RN 885269-84-3, molecular formula C₁₃H₁₂N₂O₅S, molecular weight 308.31 g·mol⁻¹) is supplied as a yellow crystalline powder with purity typically ≥98% by HPLC (area%, detection at 254 nm, C18 column, acetonitrile/water 60:40 v/v). A Product Code TH-4317 designates the research-grade batch; a technical-grade variant (TH-4317T) is produced under ISO 9001:2015 for kilogram-scale campaigns. The substance is soluble in DMF, DMSO, and warm ethanol, sparingly soluble in water (0.2 mg·mL⁻¹ at 25 °C). Differential scanning calorimetry at a heating rate of 10 K·min⁻¹ under nitrogen reveals a sharp melt endotherm with onset at 181 °C and a decomposition exotherm above 270 °C (TGA shows 5% mass loss at 265 °C). These thermal boundaries define safe drying and storage protocols: the compound should be kept in sealed containers under dry nitrogen at 2–8 °C, and exposure to relative humidity above 60% for more than 24 h induces partial ester hydrolysis detectable by FTIR loss of the carbonyl stretch at 1712 cm⁻¹.

Analytical specifications and associated test methods

Parameter	Specification	Method
Appearance	Yellow crystalline powder	Visual, USP 〈785〉
Purity (HPLC)	≥98.0% area	In-house LC-UV, C18, 254 nm
Water content	≤0.5% w/w	Karl Fischer, ISO 760
Melting range	178–182 °C	DSC scan rate 10 K/min, N₂ purge
Residual solvents – ethanol	≤500 ppm	GC-HS, EP 2.4.24
Heavy metals (as Pb)	≤20 ppm	USP 〈231〉 method II
Loss on drying	≤0.3%	60 °C, vacuum, 4 h

In continuous-flow pharmaceutical synthesis, this compound serves as a key intermediate for the construction of 2-aryl-thiazole scaffolds used in selective serine protease inhibitors. A three-step telescoped process executed in a PFA coil reactor (ID 1.0 mm, length 10 m) at 4 mL·min⁻¹ total flow couples the ester with a Boc-protected piperazine via Buchwald-Hartwig amination on the nitro-activated ring. The nitro group facilitates oxidative addition to Pd(dba)₂/XPhos at 85 °C; subsequent nitro reduction to the aniline is performed in a downstream trickle-bed hydrogenation unit with 5% Pt/C at 30 bar H₂. The hydroxyl remains free during this sequence, avoiding a protect-deprotect cycle and reducing step-count by two compared to routes using the analogous methoxy-protected derivative. Batch-mode production in a 100 L glass-lined reactor with pitched-blade agitator at 180 rpm achieved a 72% isolated yield for a 2-aminothiazole analog after recrystallization from ethanol/water 7:3, though scaling the nitro reduction in batch led to an exotherm ΔT of 18 K and required controlled dosing at 0.15 equiv·h⁻¹. Incompatibility with strong bases is critical: treatment with NaOH 2 M at 60 °C results in complete saponification of the ethyl ester within 15 min, generating the corresponding carboxylic acid which precipitates as its sodium salt and complicates extraction.

Why the 4-Hydroxy-3-Nitrophenyl Substituent Delivers Radical-Buffering Performance in Polyolefin Films

The combination of a phenolic hydrogen and an ortho-meta-directing nitro group creates a pendant capable of catalytic hydroperoxide decomposition and radical chain interruption. When the compound is melt-compounded into a linear low-density polyethylene (LLDPE) matrix at 0.5 wt% using a co-rotating twin-screw extruder (L/D = 44, die temperature 210 °C, screw speed 400 rpm), accelerated weathering per ASTM G154 Cycle 1 (UVA-340 lamps, 0.89 W·m⁻² at 340 nm, 8 h light at 60 °C / 4 h condensation at 50 °C) shows retention of tensile strength at break (ISO 527‑2/5A) of 89% after 2000 h, whereas a control film containing Ethyl 2-(4-Methoxyphenyl)-4-Methyl-1,3-Thiazole-5-Carboxylate (nitro-free) retained only 71%. Under identical exposure, the carbonyl index (FTIR, peak area ratio 1715 cm⁻¹ / 1465 cm⁻¹) increased by 0.04 for the nitro-hydroxy analog versus 0.21 for the methoxy analog, indicating suppressed Norrish-type chain scission. The underlying mechanism involves a lower O–H bond dissociation energy calculated by DFT (B3LYP/6-311+G(d,p)) at 337 kJ·mol⁻¹, which facilitates H-atom transfer to peroxyl radicals. Additionally, the nitro group can form a transient quinone-methide tautomer under UV, absorbing in the 300–380 nm range and dissipating energy harmlessly as heat. Processing requires careful temperature control: subjecting the compound to >220 °C for more than 2 min initiates decomposition with gas evolution, observed as a pressure increase of 1.2 bar in a closed DSC pan at 225 °C. Therefore, extrusion zones upstream of the die must not exceed 215 °C.

Hydrolytic Stability Profile of the Ethyl Ester Versus the Methyl Ester at Elevated pH

A systematic comparison of Ethyl 2-(4-Hydroxy-3-Nitrophenyl)-4-Methyl-1,3-Thiazole-5-Carboxylate with its methyl ester analog reveals stark differences in aqueous alkaline hydrolysis relevant to pro-drug design and waste-stream biotransformation. Half-lives were determined in pH 9.0 borate buffer (50 mM) at 37 °C using LC-MS (MRM transition 309→263) and fitted to a pseudo-first-order model. The ethyl ester exhibited t₁/₂ = 42 min, while the methyl ester hydrolyzed more rapidly with t₁/₂ = 17 min. This rate differential originates from the electron-donating effect of the ethyl group, which reduces the electrophilicity of the carbonyl carbon relative to the methyl system; this was corroborated by ¹³C NMR shift of the ester carbonyl at 162.1 ppm for ethyl vs. 160.8 ppm for methyl. Nevertheless, both esters are labile under forced conditions: at pH 12, degradation is instantaneous (< 1 min). The free phenolic group accelerates hydrolysis through intramolecular general-base catalysis when ionized above its pKa (∼7.8), a feature absent in the 4-methoxy-substituted comparative compound. In a manufacturing environment, this mandates a strictly neutral to mildly acidic work-up during aqueous washes, using 5% ammonium chloride solution to quench base. Should the compound be employed as a precursor to water-soluble salts, the ethyl ester is preferred over the methyl because its slower hydrolysis provides a wider processing window during re-crystallization from hot ethanol/water mixtures. Applications in analytical derivatization exploit the strong molar absorptivity of the nitrophenol chromophore. A pre-column derivatization protocol for trace aldehydes in ambient air (DNPH impinger method modified per ISO 16000-3) replaces the standard 2,4-dinitrophenylhydrazine reagent with the hydrazine derivative generated from this thiazole ester. After in-situ condensation on a C18 SPE cartridge, separation on a 150 × 4.6 mm, 3 µm phenyl-hexyl column with acetonitrile/ammonium acetate (pH 4.5) gradient yields detection limits of 0.05 µg·m⁻³ for formaldehyde at 345 nm, with linearity from 0.1–50 µg·m⁻³ (r² >0.999). The nitro-hydroxy phenyl ring delivers a bathochromic shift of approximately 40 nm relative to non-nitrated thiazole hydrazones, moving the λmax away from common co-eluting UV interferences found in diesel exhaust particulates. When compared to the nitro-free analog, selectivity ratios for acetaldehyde over C3 carbonyls improve by a factor of 3.2. A limitation arises with ketones having α-hydrogens: side-reaction aldol condensation under acidic derivatization conditions (pH 2.0, HCl) reduces recovery by up to 30%, necessitating a strictly anhydrous condensation in acetonitrile with 0.1% trichloroacetic acid.

Comparative properties of structurally related 2-aryl-4-methyl-1,3-thiazole-5-carboxylate esters

Compound	Melting point (°C)	Log P (shake-flask, pH 7.4)	λmax (EtOH, nm)	t₁/₂ (pH 9, 37°C, min)
Ethyl 2-(4-Hydroxy-3-nitrophenyl)-4-methyl-1,3-thiazole-5-carboxylate	181	2.95	348	42
Methyl 2-(4-Hydroxy-3-nitrophenyl)-4-methyl-1,3-thiazole-5-carboxylate	197	2.62	347	17
Ethyl 2-(4-Methoxyphenyl)-4-methyl-1,3-thiazole-5-carboxylate	112	3.40	287	110
Ethyl 2-(4-Hydroxyphenyl)-4-methyl-1,3-thiazole-5-carboxylate (nitro-free)	164	2.87	298	95

The presence of the hydroxyl group ortho to the nitro substituent introduces an additional capability not found in the 4-nitrophenyl analog without a neighboring OH: chelation of transition metals. In electroplating wastewater treatment trials, a pilot-scale fixed-bed column packed with silica-immobilized ligand (prepared by coupling the ethyl ester saponified acid precursor to aminopropyl-functionalized silica, particle size 200–400 µm) captured Cu²⁺ from a 50 mg·L⁻¹ synthetic rinse stream with a breakthrough capacity of 0.42 mmol·g⁻¹ at a space velocity of 5 h⁻¹. Regeneration with 0.5 M HNO₃ restored >95% capacity over five cycles. The chelation mode—monitored by XPS showing a shift of the O 1s peak from 531.8 eV to 533.1 eV upon copper binding—relies on the deprotonated phenol and the nitro oxygen, forming a five-membered ring. This contrasts with the 4-nitrophenyl derivative (lacking the hydroxyl) which shows negligible copper retention under identical conditions. However, the ester linkage itself is susceptible to slow hydrolysis in the acidic regenerant (pH ~1.5), causing gradual activity loss after approximately 8–10 cycles; grafting the acid directly via amide bond formation eliminates this decay pathway but reduces copper capacity by 20% due to altered denticity. When evaluated as a photosensitive component in UV-curable offset inks, the compound couples its role as a photoinitiator synergist to a built-in color indicator for cure completeness. Formulations containing 0.3 wt% of the ester with a Type I photoinitiator (BAPO, 1.5 wt%) applied to polypropylene film (corona-treated to 42 mN·m⁻¹) and cured under a medium-pressure mercury lamp (120 W·cm⁻¹, belt speed 25 m·min⁻¹) shifted visually from yellow to pale orange at 0.4 J·cm⁻² cumulative UVA dose, indicating consumption of the nitro-hydroxy chromophore. Real-time FTIR-ATR monitoring (KBF, 4 cm⁻¹ resolution) tracked the acrylate C=C conversion reaching 87% after 3 passes, compared to 82% without the thiazole additive, attributed to the phenolic hydrogen donating to quench peroxyl radicals that otherwise terminate the radical chain. Rub resistance (Sutherland 2000 ink rub tester, 2 lb weight, 100 strokes) resulted in < 10% ink transfer for the cured films. A practical boundary: formulations must avoid amine-based acrylate oligomers, as the nitro group engages in a charge-transfer complex that bathochromically shifts the absorption tail into the visible above 420 nm, imparting an unacceptable brown discoloration and retarding surface cure due to inner filter effects.