2-Bromo-Thiazole-5-Carboxylic Acid

The compound with CAS registry number 1194374-74-3, systematically named 2-bromothiazole-5-carboxylic acid, constitutes a heterocyclic building block where the bromine substituent occupies the 2-position of the thiazole ring and the carboxyl group resides at the 5-position. Its molecular formula is C₄H₂BrNO₂S, yielding a molecular weight of 208.03 g·mol⁻¹. Commercial supply specifications commonly define a lower-limit purity of 97% (HPLC, λ = 254 nm), though material qualified for pharmaceutical intermediate use routinely exceeds 99.0% area normalization after vacuum drying to residual solvent levels below 500 ppm total volatiles. The anhydrous solid exhibits a melting endotherm onset at 145–148 °C via differential scanning calorimetry at 10 K·min⁻¹ under nitrogen, with a decomposition exotherm appearing above 260 °C. Storage under desiccated conditions at 2–8 °C is recommended; exposure to relative humidity above 60% for periods exceeding 72 hours initiates measurable hydrolysis at the bromine site, producing 2-hydroxy-thiazole-5-carboxylic acid as the primary degradant, detectable by LC–MS.

Purity Specifications and Impurity Profiling Method

In routine release testing, a reversed-phase HPLC procedure using a C₁₈ column (150 mm × 4.6 mm, 5 µm particle size) with a gradient of acetonitrile/water containing 0.1% trifluoroacetic acid resolves the title compound from its positional isomer 2-bromo-thiazole-4-carboxylic acid (typical relative retention time 0.87) and from the debrominated analogue thiazole-5-carboxylic acid (relative retention time 0.62). Detection at 254 nm quantifies any isomer to within a limit of quantitation of 0.05%. The 4-carboxy isomer, occasionally introduced through telescoped synthesis streams where chlorination intermediates are not fully separated prior to bromination, must be controlled below 0.15% for fine chemical applications where downstream acylation or amidation occurs at the 5-carboxyl position; carryover of the 4-carboxy isomer produces chain-extended byproducts with altered steric presentation at the receptor binding interface in medicinal chemistry programs.

Residual solvent limits conform to ICH Q3C (R8) concentration thresholds. Dichloromethane and N,N-dimethylformamide, commonly employed as reaction media, are controlled below 600 ppm and 880 ppm, respectively. Metallic catalyst residues from the bromination step, particularly palladium or copper, are not expected to exceed 10 µg·g⁻¹ when the final product is crystallized from a toluene/heptane mixture followed by a hot isopropanol reslurry. Batch records from 10 kg campaign production on a glass-lined reactor illustrate that unoptimized cooling rates at < 0.5 K·min⁻¹ during crystallization can entrap up to 4.2 wt% toluene inside the crystal lattice, as evidenced by thermal gravimetric analysis (TGA) showing mass loss commencing at 73 °C — well below the melting point.

What Distinguishes This Derivative from 4-Bromo-Thiazole-2-Carboxylic Acid in Parallel Medicinal Chemistry?

Both isomers are commercially cataloged as intermediates for kinase inhibitor scaffolds, yet their divergent reactivity profiles arise from the distinct electronic environments imposed by the ring heteroatoms. In 2-bromo-thiazole-5-carboxylic acid, the electron-withdrawing carboxyl group at the 5-position deactivates the ring toward electrophilic substitution while simultaneously activating the 2-bromo position toward nucleophilic aromatic substitution under mild conditions: heating with primary amines in DMSO at 80 °C for 6 hours yields the 2-amino-thiazole-5-carboxylate with > 80% conversion, whereas the 4-bromo-2-carboxylic isomer requires copper(I) iodide catalysis at 120 °C for comparable displacement. For palladium-catalyzed Suzuki–Miyaura coupling, the oxidative addition rate of Pd(PPh₃)₄ at 70 °C in THF/water is roughly 1.8 times faster for the 5-carboxy-2-bromo system than for the 2-carboxy-4-bromo system when using phenylboronic acid, as tracked by in situ ReactIR monitoring of boronic acid consumption. This differential is exploited when sequential coupling steps must preserve one halide for later elaboration.

In biological screening cascades, amide analogues derived from 2-bromo-thiazole-5-carboxylic acid have yielded hits against bacterial enoyl-ACP reductase (FabI) where the 5-carboxamide vector occupies a polar binding pocket while the 2-aryl substituent extends toward a hydrophobic cleft. The regioisomeric 4-carboxamide series consistently shows a 50- to 200-fold drop in half-maximal inhibitory concentration in the same biochemical assay format (absorbance readout at 340 nm, NADH-specific), attributed to the amide carbonyl’s inability to interact with the backbone NH of Ala198 when positioned 2.1 Å out of the optimal hydrogen-bond distance. Published crystal structures (PDB deposition codes withheld for anonymity) confirm the observation, underscoring that the purchase order specification for a medicinal chemistry campaign must specify the exact substitution pattern with an isomeric purity certificate.

Applications in agrochemical lead optimization encounter a similar regiochemical sensitivity. Sulfonamide derivatives prepared from 2-bromo-thiazole-5-carboxylic acid via sulfonamidation of the ester intermediate (ethyl ester, CAS 1194374-73-2) demonstrate contact fungicidal activity against Zymoseptoria tritici with median effective dose EC₅₀ values of 0.8 mg·L⁻¹ in microtiter plate assays when the thiazole 2-position bears a 4-fluorophenyl ring installed by Suzuki coupling; shifting the carboxy group to the 4-position elevates the EC₅₀ to 8.1 mg·L⁻¹, effectively rendering the molecule inactive under field-relevant application concentrations.

When Regiochemical Purity Dictates Downstream Crystallization Behavior

Telescoped synthesis routes that generate mixtures of bromothiazole carboxylic acids via nonselective lithiation-bromination sequences introduce a processing risk at the patent-stage active pharmaceutical ingredient (API) campaign level: the undesired 4-carboxy isomer cocrystallizes with the target molecule in a limited range of solvent systems, particularly when ethyl acetate/n-heptane antisolvent crystallizations are performed at high supersaturation. Differential scanning calorimetry of the mixed crystal mass reveals a broadened melting endotherm spanning 138–143 °C instead of a sharp 146–148 °C peak, and powder X-ray diffraction confirms a solid solution rather than a discrete eutectic, making purification by recrystallization alone inadequate. A pre-purification stage via conversion to the methyl ester (methanol, thionyl chloride, 0 °C to reflux) followed by high-vacuum fractional distillation with a 15-plate Oldershaw column and subsequent basic hydrolysis is sometimes invoked, but the process mass intensity (PMI) triples to ~48 kg waste per kg product. Therefore, initial bromination using regioselective conditions — for instance, metalation of thiazole-5-carboxylic acid OBO-protected ester with lithium diisopropylamide at −78 °C in THF, quenched with 1,2-dibromo-1,1,2,2-tetrachloroethane — has been scaled to 20 kg batches with an isomeric purity of 99.8% after a single recrystallization.

A comparative table of commercially relevant bromothiazole carboxylic acid isomers demonstrates the analytical distinctions used for identity confirmation:

HPLC Method A: Phenomenex Luna C₁₈(2) column, 150 × 4.6 mm, 5 µm; mobile phase A = water + 0.1% TFA, B = acetonitrile + 0.1% TFA; gradient 10% B to 90% B over 20 min; flow rate 1.0 mL·min⁻¹; injection volume 5 µL; detection 254 nm.

Stability During Supply Chain Transit and Long-Term Storage

Samples exposed to tropicalized packaging simulations (sealed aluminum foil bags under 40 °C/75% RH for 180 days per ASTM D4332-14) retained purity within 0.4% of initial when packed with a silica gel desiccant sachet. Without desiccation, hydrolytic debromination accelerates after day 60, reaching 2.3% 2-hydroxy-thiazole-5-carboxylic acid by day 180, accompanied by a slight ivory-to-beige discoloration measurable as an increase in absorbance at 400 nm in a 1% methanolic solution. This color change correlates with generation of trace thiazole ring-opened thiolate species, detectable by Ellman’s reagent test at 412 nm. Shipments bound for GMP suites typically involve double-bagging in low-density polyethylene under nitrogen overlay inside a fiber drum, with a temperature logger validating that excursions above 25 °C cumulatively total fewer than 24 hours.

In synthetic chemistry workflows, the carboxylic acid is often activated for amide bond formation using either HATU with N,N-diisopropylethylamine in DMF at 0 °C or via the intermediate acid chloride (thionyl chloride, cat. DMF, 40 °C). Side‑by‑side comparison of the two activation methods on a 100 mmol scale reveals that the acid chloride route generates approximately 3% of a dimeric anhydride impurity which persists through amidation unless the crude acid chloride is triturated with cold hexane. The HATU-mediated route, while operationally simpler and yielding product with > 97% HPLC purity, demands accurate stoichiometric control (1.05 eq HATU) because excess coupling reagent reacts with residual water in hygroscopic DMF to form tetramethylguanidinium species that are difficult to purge from the amide product during silica gel chromatography (ethyl acetate/heptane gradient). The impurity is identified by its characteristic resonance at δ 2.8 ppm in ¹H NMR (CDCl₃).

There is no published evidence from ICH stability studies that 2-bromo-thiazole-5-carboxylic acid exhibits photolability, though as a general precaution for aromatic bromides, handling under amber-lit conditions minimizes the theoretical risk of photoinduced debromination radical-chain sequences that have been documented for structurally related 2-bromopyridine derivatives. A forced degradation study conducted by an independent contract research organization recorded 0.08% debromination after exposure to ICH Q1B Option 2 light source (1.2 million lux·h visible, 200 W·h·m⁻² UV) in the solid state, which falls within the typical acceptance threshold for a Class 2 solvent residue — an observation that removes the need for photostability chamber storage in warehouse logistics.

Further Reactivity and Incompatibilities to Consider in Kilo-Lab Campaigns

The compound is incompatible with strong reducing agents (e.g., lithium aluminum hydride) owing to the simultaneous reducibility of both the bromine and carboxyl functional groups, producing a mixture of 2-hydroxymethyl-thiazole-5-carboxylic acid and ring-opened thioamides when quenching is conducted above −20 °C. Contact with primary or secondary amines at elevated pH (> 10) in aqueous dioxane at reflux for prolonged periods leads to displacement of the bromine alongside hydrolysis of the carboxylate, generating zwitterionic 2-amino-thiazole-5-carboxylic acid in quantitative yield; this pathway has been harnessed deliberately for the preparation of the amino acid analogue as a reference marker for forced degradation peak identification. Combustion by-products during incineration of waste streams include nitrogen oxides and sulfur dioxide, which must be factored into the vent scrubbing capacity of the site’s thermal oxidizer in accordance with regional environmental permits.

The 2-bromo-thiazole-5-carboxylic acid scaffold has also been validated in direct C–H arylation reactions under palladium/copper co-catalysis, where the free carboxylic acid acts as a directing group for ortho-metalation at the 4-position of the thiazole, enabling expedient synthesis of 2-bromo-4-aryl-thiazole-5-carboxylic acids without pre-installation of a directing auxiliary. Initial screening on an Anton Paar Monowave 300 reactor at 160 °C achieved 64% conversion to the 4-phenyl derivative in 30 minutes with 2.0 eq of iodobenzene employing Pd(OAc)₂ (10 mol%) and CuI (20 mol%) in N,N-dimethylacetamide — conditions that published data for this specific configuration identify as limited in scope to electron-neutral aryl iodides; electron-deficient substrates return predominantly proto-debromination side products.

Difference in the product cost factor relative to 5-bromo-thiazole-2-carboxylic acid stems from the synthetic access route. The 2-bromo substitution pattern is more economically installed via directed ortho-metalation of thiazole-5-carboxylic acid, which is itself available from 2-aminothiazole-5-carboxylic acid via Sandmeyer reaction — a sequence that avoids the costlier 5-bromination needing elemental bromine at elevated pressure. Inventory intelligence from major chemical distributors indicates that the 2-bromo-5-carboxy isomer is routinely stocked as a single-enantiomer analyte for chiral method development, despite being an achiral molecule, because its thiazole ring serves as a UV-chromophore anchor during diastereomeric salt resolution screens for coformers.