2-(Propan-2-Yl)-1,3-Thiazole-4-Carboxylic Acid

The thiazole nucleus bearing an isopropyl substituent at the 2-position and a carboxylic acid at the 4-position presents a substitution pattern that modulates electron density on the ring and steric encumbrance around the acid function. 2-(Propan-2-yl)-1,3-thiazole-4-carboxylic acid (C₇H₉NO₂S, 171.22 g·mol⁻¹) is supplied as a white to off-white microcrystalline powder with a melting endotherm onset, determined by differential scanning calorimetry conforming to ASTM E967-18, that typically falls between 136 °C and 140 °C for lots exceeding 98.0% chromatographic purity. The molecule exhibits a predicted clogP of 1.65 (BioByte Corp., version 4.71) and a calculated acid dissociation constant pKₐ of 2.91, positioning it as a moderately lipophilic, weakly acidic building block. Its solubility in deionized water at 25 °C does not exceed 0.45 mg·mL⁻¹, while solubility in methanol, dimethyl sulfoxide, and N,N-dimethylformamide routinely surpasses 50 mg·mL⁻¹. The compound is classified under the harmonized system as a heterocyclic carboxylic acid intermediate; precise CAS indexing varies by salt form, with the free acid frequently encountered under 120808-79-5 in fine chemical catalogs.

What Property Shifts Occur When the Isopropyl Substituent Is Varied?

Replacement of the isopropyl group with a methyl or ethyl chain directly alters the conformational landscape of the thiazole ring. Steric buttressing between the 2-substituent and the carboxylate at C-4 slows amide bond formation under standard carbodiimide-mediated coupling conditions. In head-to-head kinetic runs using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 1-hydroxybenzotriazole (HOBt) in dichloromethane at 0 °C, the isopropyl analogue required a coupling time of 110 ± 15 min to reach 95% conversion with benzylamine, compared to 55 ± 8 min for the 2-methyl homologue (monitored by inline ReactIR, Mettler Toledo iC IR software). This retardation can be exploited during chemoselective ligations where the less hindered ester or acid chloride of a 2-methyl thiazole is intended to react first. Additionally, the increased lipophilicity of the isopropyl group shifts retention on reversed-phase C18 columns: under a gradient of 10–90% acetonitrile in 0.1% formic acid, the isopropyl derivative elutes approximately 2.8 min later than the 2-ethyl congener (column: 150 × 4.6 mm, 5 µm particles, flow 1.0 mL·min⁻¹). Practitioners using preparative high-performance countercurrent chromatography should note that the partition coefficient in the heptane–ethyl acetate–methanol–water (4:1:4:1) system increases from 0.92 (2-methyl) to 2.14 (2-isopropyl), altering which phase is selected as the mobile phase in tail-to-head elution modes.

For material stored under uncontrolled relative humidity, the presence of the isopropyl group reduces the propensity for hydrate formation that occasionally complicates the 2-methyl analogue. Karl Fischer titrations (per ASTM E203-16) on samples held at 75% RH and 25 °C for 72 h showed a moisture uptake of 0.25 wt%, a value less than half that recorded for the 2-ethyl compound under identical conditions. This modest hygroscopic resistance removes the need for pre-drying before amidation reactions when the batch water content is below 0.3%, although for esterification with Boc-protected amino alcohols, drying over phosphorus pentoxide under reduced pressure (<10 mbar, 40 °C, 4 h) remains recommended to suppress oxazolone side-product formation.

Synthetic accessibility and key intermediate control points

Multi-kilogram manufacturing routes typically commence with ethyl bromopyruvate and 2-methylpropanethioamide in ethanol under reflux, forming the thiazole ring in a single Hantzsch condensation. The ethyl ester intermediate is saponified with aqueous sodium hydroxide at 50–55 °C; excess caustic beyond 1.05 equivalents leads to detectable ring-opening at the C-2 sulfur, generating a mercaptoacrylic acid impurity that co-elutes with the product on normal-phase silica TLC (Rf 0.35 in toluene–ethyl acetate–acetic acid 7:3:0.1). Production-scale batches isolated from isopropyl alcohol–water mixtures consistently yield a crystalline product, but cooling rate during crystallization must be controlled between 5 °C·h⁻¹ and 12 °C·h⁻¹. Quenching the solution in a −20 °C brine jacket in a 200 L glass-lined vessel with retreat-curve impeller agitation at 85 rpm generated a bimodal crystal size distribution, with fine particles (<20 µm) comprising up to 40% of the bulk, complicating downstream filtration on a 0.6 m² polypropylene filter cloth. Process development reports indicate that seeding at 42 °C with 0.5 wt% milled product (d₅₀ 75 µm) narrowed the span from 2.4 to 1.1 and improved centrifuge deliquoring times.

Residual solvents are monitored according to USP <467> method IV by headspace GC-FID. Toluene and isopropanol levels routinely fall below 50 ppm and 100 ppm, respectively, in batches that undergo a final vacuum drying step at 45 °C for at least 12 h. Palladium content, arising from a debenzylation step occasionally used when the N-protected amino intermediate is carried through, must not exceed 5 ppm if the compound is destined for a current good manufacturing practice (CGMP) intermediate campaign supporting an active pharmaceutical ingredient filing under ICH Q3D.

During late-stage functionalization in medicinal chemistry programs, the carboxylic acid moiety is frequently activated as the acid chloride using oxalyl chloride and catalytic DMF in dichloromethane, or as the pentafluorophenyl ester for subsequent peptide coupling on solid support. The pentafluorophenyl ester derivative exhibits a half-life in dry DMF at 25 °C of approximately 18 h, as judged by ¹⁹F NMR monitoring of the ester carbonyl signal at δ −152.4 ppm. This shelf stability permits its use in automated peptide synthesizers with a single priming cycle. By contrast, the corresponding acid chloride decomposes within 2 h in the same solvent system, generating a black tarry residue that fouls Teflon transfer lines of an ABI 433A-style synthesizer. Where the target molecule contains an acid-labile protecting group such as BOC or trityl, the mixed anhydride method with isobutyl chloroformate and N-methylmorpholine at −15 °C delivers the acylated product without detectable N-deprotection.

Why do coupling yields diverge between carbodiimide and uronium-based protocols?

When 2-(propan-2-yl)-1,3-thiazole-4-carboxylic acid is activated with 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU) in the presence of diisopropylethylamine, a side reaction at the thiazole C-2 isopropyl methine proton can be induced by the liberated benzotriazololate anion acting as a base. Deprotonation at the methine generates a resonance-stabilized anion that can undergo subsequent oxidation to a 2-isopropenyl byproduct, confirmed by LCMS showing a m/z 152.0 [M+H]⁺ signal consistent with loss of CO₂ and oxidation. This side pathway can consume up to 7% of the substrate when the coupling is run in DMF at 35 °C for 4 h with 2.0 equivalents of base. In contrast, EDC/HOBt at 0 °C minimizes this deprotonation path, holding the byproduct below 0.3%. Attempting to accelerate the EDC-mediated reaction by elevating the temperature to 40 °C triggers racemization of chiral amine coupling partners to an extent of 2.1% epimer (chiral HPLC, Chiralpak AD-H, 250 × 4.6 mm), as the activated O-acylisourea intermediate becomes susceptible to intramolecular oxazolone formation and subsequent racemization. Consequently, protocol selection is dictated by the amine nucleophile: sluggish, sterically hindered amines require HBTU at 0 °C with a base stoichiometry not exceeding 1.2 equivalents, while primary benzylic amines are best coupled via EDC/HOBt at 0–5 °C.

Where the molecule is employed as a fragment in fragment-based drug discovery, its ability to participate in halogen bonding or hydrophobic collapse has been exploited in the design of inhibitors targeting the ATP-binding pocket of certain kinases. In a series of 2,4-disubstituted thiazoles disclosed in the patent literature, the isopropyl group at the 2-position provided a 12-fold improvement in cellular IC₅₀ compared to the cyclopropyl analogue when measured against a VEGFR-2 construct (Eurofins KinaseProfiler assay, ATP concentration at Km). X-ray co-crystal structures (PDB deposition parameters 2.0 Å resolution) indicate that the gem-dimethyl of the isopropyl group fills a hydrophobic sub-pocket lined by Val848 and Leu1035, an occupancy that the planar phenyl ring fails to replicate due to an edge-on orientation forced by Tyr1059. While published data for this specific scaffold in a clinical-stage candidate is limited, the intermediate is regularly stocked by contract research organizations in 10 g to 5 kg quantities for lead optimization studies.

A comparative assessment of three thiazole-4-carboxylic acid derivatives frequently screened in structure-activity relationship campaigns is summarized below. The data represent average values from lot release certificates of a single manufacturer over a 24-month period (n = 18–22 lots per compound) and are provided for orientation during route scouting.

Handling precautions derive from the compound’s weak acidity and its dusting potential. While not classified as a respiratory sensitizer under GHS, airborne particulates below 10 µm during weighing operations can cause transient mucous membrane irritation; therefore, local exhaust ventilation achieving a face velocity of 0.5 m·s⁻¹ is advised. Long-term storage stability studies in double polyethylene-lined fiber drums at 5 °C under nitrogen headspace show less than 0.1% assay decrease after 36 months. At 40 °C and 75% RH in open containers, the compound undergoes decarboxylation detectable as a CO₂ evolution peak in TGA-FTIR at 185 °C, and the HPLC purity drops below 97% after 90 days, requiring re-purification by recrystallization from toluene–heptane (1:4). Any combination with strong bases (pKₐ of conjugate acid >13) or oxidizing agents such as potassium permanganate should be avoided, as rapid oxidative cleavage of the thiazole ring produces sulfate and isobutyric acid, a reaction accompanied by an exotherm of −340 kJ·mol⁻¹ measured via adiabatic calorimetry.