5-Bromo-2-Phenylthiazole-4-Carboxylic Acid Ethyl Ester

In the manufacture of selective tropomyosin receptor kinase (TRK) inhibitor active pharmaceutical ingredients, 5-bromo-2-phenylthiazole-4-carboxylic acid ethyl ester functions as a heteroaryl electrophile for palladium-catalyzed sp²–sp² carbon bond formation. The compound is charged into a Hastelloy C‑22 jacketed reactor at a molar equivalence of 1.00 ± 0.02 relative to a 2-methylpyridine‑5‑boronic acid pinacol ester (1.15 eq) in a degassed mixture of 1,2‑dimethoxyethane and 2.0 M aqueous tripotassium phosphate (3.5 vol). Reaction initiation with 1.2 mol% [1,1′‑bis(diphenylphosphino)ferrocene]dichloropalladium(II) [Pd(dppf)Cl₂] under a nitrogen blanket at 80 ± 2 °C achieves complete consumption of the bromide within 4–6 h as monitored by in‑line ReactIR analysis tracking the 1045 cm⁻¹ C–Br stretching band disappearance. The downstream manufacturing process includes phase separation, treatment of the organic stream with 5 wt% N‑acetyl‑L‑cysteine immobilized on silica scavenger to reduce residual palladium below the 10 µg g⁻¹ oral permitted daily exposure limit derived from ICH Q3D risk assessment, vacuum distillation to isolate the biaryl ester intermediate, saponification with 1.2 eq lithium hydroxide in THF/water at 25–30 °C to generate the free carboxylic acid, and HATU‑mediated coupling with a chirally pure (R)‑3‑hydroxypyrrolidine to install the amide pharmacophore. The terminal drug substance is a besylate salt formulated into immediate‑release film‑coated tablets at strengths of 40 mg and 80 mg tested per USP <2040> for disintegration. Compliance with ICH Q7A good manufacturing practice for starting materials is documented through a Type II drug master file, and residual solvent limits for 1,2‑dimethoxyethane are validated against ICH Q3C Option 1 concentrations.

When Does the C–Br Activation Window Conflict with Ester Hydrolysis in Sequential Suzuki-Hydrolysis Cascades?

The process intensification imperative to telescope the Suzuki cross-coupling and ester saponification into a single‑pot operation without intermediate isolation introduces a kinetic competition that constrains the operational pH and thermal window uniquely for 5‑bromo‑2‑phenylthiazole‑4‑carboxylic acid ethyl ester. The ethyl ester substituent undergoes base‑promoted hydrolysis with a second‑order rate constant that exhibits a non‑linear dependence on hydroxide ion activity in the aqueous‑diglyme biphasic system. In the presence of 1.5 equivalents of sodium carbonate at 78 °C, Raman spectroscopy quantification of the 1718 cm⁻¹ carbonyl stretch reveals that the ester survives with <3% hydrolysis over the 5 h required for full oxidative addition of the aryl bromide to Pd(dppf)Cl₂; yet raising the carbonate charge to 2.0 equivalents elevates the measured pH of the aqueous phase from 10.2 to 10.9 and accelerates the ester cleavage ten‑fold, converting 15–20% of the substrate to the carboxylate before the coupling achieves 90% completion. The prematurely generated carboxylic acid ligates the palladium center to form a catalytically inert Pd‑carboxylate‑hydroxo dimer that precipitates as a black suspension and arrests turnover. Manufacturing‑scale batches in a 3 000 L glass‑lined reactor have identified a critical process parameter window of pH 10.1–10.3 and internal temperature 76–80 °C; excursions above pH 10.4 for longer than 12 min result in irreversible catalyst deactivation and an increase in the des‑bromo protodehalogenation impurity above the 0.10% ICH Q3A identification threshold, necessitating an additional carbon treatment and recrystallization step to rescue the batch. Equipment specification mandates a pH‑stat controller with a dosing pump capable of delivering 0.5 M sodium carbonate at a rate of 0.3 L min⁻¹ to maintain the pH within the window, along with a retreat‑curve impeller operating at 90–110 rpm tip speed to disperse the organic phase droplet size below 100 µm Sauter mean diameter without shearing the catalyst particles. The terminal drug product, an anaplastic lymphoma kinase inhibitor formulated as a hard gelatin capsule containing 150 mg of the free base, is qualified using a related substances HPLC method with a quantitation limit of 0.03% for the hydrolytically derived carboxylic acid impurity.

C–N Bond Construction via Buchwald-Hartwig Amination for Antibacterial Thiazole Candidates

Incorporation of secondary amine pharmacophores into the thiazole scaffold is achieved by employing 5‑bromo‑2‑phenylthiazole‑4‑carboxylic acid ethyl ester as the oxidative addition partner in a palladium‑catalyzed amine arylation. The substrate is combined with morpholine (1.3 eq) in anhydrous toluene containing 1.5 mol% tris(dibenzylideneacetone)dipalladium(0) and 3.0 mol% 4,5‑bis(diphenylphosphino)-9,9‑dimethylxanthene (Xantphos) together with sodium tert‑butoxide (2.0 eq) as the base. The heterogeneous mixture is heated to 100 °C under an argon atmosphere for 14–18 h until the HPLC area percent of the starting bromide falls below 1.0%. The downstream procedure comprises cooling to 20 °C, filtration through a pad of Celite, acid‑base extractive workup with 1.0 M HCl to protonate the amine and remove palladium residues, concentration, and saponification of the resulting 5‑morpholino‑2‑phenylthiazole‑4‑carboxylic acid ethyl ester with 1.1 eq aqueous sodium hydroxide in ethanol at 40 °C. The liberated carboxylic acid is activated with isobutyl chloroformate and condensed with (S)‑2‑aminopropanamide to furnish a broad‑spectrum β‑lactamase inhibitor intermediate. All operations are conducted under EU GMP Part II guidelines for active substance manufacture, and the final lyophilized sterile powder is filled into Type‑I borosilicate vials at a dose of 500 mg as the sulfate salt. The specification for palladium content is set at <5 µg g⁻¹ in accordance with the parenteral acceptable intake derived from ICH Q3D monograph for elemental contaminants.

If Anhydrous Protocols Are Not Enforced During Negishi Coupling of the Thiazole Core

Preparation of sterically demanding biaryl intermediates through Negishi coupling places exceptionally stringent demands on the exclusion of water at every stage of the process due to the moisture sensitivity of the in‑situ generated arylzinc reagent. 5‑Bromo‑2‑phenylthiazole‑4‑carboxylic acid ethyl ester is pre‑dried in a double‑cone vacuum dryer at 50 °C and 5 mbar until the Karl Fischer titration value remains below 30 µg g⁻¹ H₂O. The anhydrous N,N‑dimethylacetamide solvent is passed through a column of activated 4 Å molecular sieves and sparged with argon to achieve an oxygen content below 5 ppm. Zinc powder (2.2 eq, <10 µm particle size) is activated with chlorotrimethylsilane (0.1 eq) and dibromoethane (0.05 eq) at 65 °C, and the aryl bromide is introduced to form the organozinc species at 50 °C under intensive shearing at 800 rpm in a fixed‑geometry rotor‑stator mixer to maintain the suspension. A failure mode observed in pilot‑scale campaigns (steel reactor, 200 L) occurred when the relative humidity in the glove box rose above 15% during solid transfer; the residual moisture consumed the zinc reagent before oxidative addition, leading to a 30% yield drop and formation of the proto‑dehalogenated impurity that co‑elutes with the product during flash chromatography and demands a multi‑stage fractional distillation for removal. In a validated manufacturing protocol, the arylzinc solution is cannulated into a solution of 2.0 mol% [1,3‑bis(2,6‑diisopropylphenyl)imidazol‑2‑ylidene](3‑chloropyridyl)palladium(II) dichloride (Pd‑PEPPSI‑IPr) in NMP at 0 °C, then warmed to 55 °C over 1 h and maintained for an additional 3 h to complete the cross‑coupling. The terminal product of this sequence, a C‑2 arylated pyrimidine nucleoside analogue, is formulated as a ribavirin co‑tablet for hepatitis C combination therapy in line with ICH M7 mutagenic impurity control, which requires routine GC‑MS monitoring of the bromo intermediate down to a purge factor of 3 000.

Methyl (2‑methyl‑3‑(trifluoromethyl)phenyl)carbamate intermediate formation illustrates the use of this thiazole ester in contemporary succinate dehydrogenase inhibitor fungicide programmes targeting *Rhizoctonia solani* and *Sclerotinia sclerotiorum* in row crops. The thiazole building block is employed in a low‑metal‑burden aqueous Suzuki coupling with 0.25 mol% palladium on carbon (5% Pd/C dry basis) and potassium carbonate in a refluxing ethanol‑water mixture at 78 °C for 8 h, achieving a weight‑based contribution of approximately 32 wt% in the penultimate intermediate of the synthetic route. Post‑coupling, the ethyl ester is hydrolyzed with 1.3 eq of 48 wt% aqueous potassium hydroxide in methanol to give the carboxylic acid, which is treated with thionyl chloride to generate the acyl chloride and immediately condensed with 1‑amino‑2‑methylpropan‑2‑ol to deliver the active ingredient. The technical product is milled via air‑jet micronization to a volume median diameter of 4–6 µm and formulated as a dry flowable (water‑dispersible granule) containing 50 wt% active substance, employing a naphthalenesulfonate‑formaldehyde condensate dispersant. Compliance with FAO specification 599/WG (October 2019) is verified through the CIPAC MT 184 method for suspensibility, and the product label includes an EPA‑mandated hazard statement under 40 CFR § 156.70 respecting the acute oral LD₅₀ in rat of the intermediate.

What Purification Thresholds Enable This Monomer for Stille Polycondensation in Organic Photovoltaics?

Translating 5‑bromo‑2‑phenylthiazole‑4‑carboxylic acid ethyl ester into a functional donor–acceptor conjugated polymer for bulk‑heterojunction photoactive layers demands that the monomer meets electronic‑grade purity specifications that exceed the routine criteria applied in small‑molecule chemical synthesis. The monomer must be processed through sequential liquid‑liquid extraction with 10 wt% sodium metabisulfite solution to remove organic electrophilic impurities, followed by dual recrystallization from ethyl acetate/heptane (1:3 v/v) and a final gradient sublimation under high vacuum (10⁻⁶ mbar) at a source temperature of 105 °C to eliminate non‑volatile inorganic residues. The purified lot is subjected to inductively coupled plasma mass spectrometry (ICP‑MS) per EPA method 6020B with a requirement of total palladium below 0.5 µg g⁻¹, total zinc below 2 µg g⁻¹, and bromide ion concentration below 5 µg g⁻¹ because excess halide quenches charge‑transport mobility by acting as a deep electron trap in the blended morphology. A Stille polycondensation is conducted using 0.98 equivalents of the purified dibrominated thiazole monomer relative to 2,5‑bis(trimethylstannyl)thiophene (1.00 eq) in chlorobenzene at 140 °C with 2.0 mol% Pd₂(dba)₃ and 8.0 mol% tri(o‑tolyl)phosphine under microwave dielectric heating in 15‑minute cycles to achieve a number‑average molecular weight of 28‑35 kg mol⁻¹ and a dispersity 1.6‑2.2 as determined by GPC with polystyrene calibration. End‑capping with 2‑(tributylstannyl)thiophene and 2‑bromothiophene is applied to prevent reactive chain ends from degrading device stability. The precipitated polymer is subjected to Soxhlet extraction with methanol, acetone, and hexane, and the chlorobenzene fraction is collected and spin‑coated from a 12 mg mL⁻¹ solution in o‑dichlorobenzene at 1 000 rpm onto ITO‑coated glass pre‑coated with PEDOT:PSS (Clevios P VP AI 4083). Inverted solar cell stacks with a PFN‑Br electron‑transport interlayer and aluminium cathode yield external quantum efficiency profiles that are recorded according to the spectral response measurement procedure of IEC 60904‑8‑1; long‑term dark storage at 85% RH and 85 °C for 1 000 h demonstrates a stable power conversion efficiency only when the monomer bromide level is confirmed below the 5 µg g⁻¹ threshold prior to polymerization.