1-[(tert-butoxy)carbonyl]azetidine-3-carboxylic acid, identified by CAS 142253-55-2, molecular formula C₉H₁₅NO₄, and relative molecular mass 201.22 g/mol, is supplied as a white to off-white crystalline powder for pharmaceutical synthesis. Material is released under model codes such as AZT-3-BOC-PG or 1-Boc-azetidine-3-carboxylic acid; the model designation refers to the protected azetidine-3-carboxylate series and is not a pharmacopoeial monograph title. The substance is used as a protected building block in API manufacturing rather than as a finished tablet, capsule, granule, or injection component. Release specifications are established on a batch certificate of analysis. Representative acceptance limits include assay by high-performance liquid chromatography NLT 98.0% area per Ph. Eur. 2.2.29 / USP <621>, total impurities NMT 1.0%, single impurity NMT 0.50%, water NMT 0.5% per USP <921> / Ph. Eur. 2.5.12, and residue on ignition NMT 0.1% per USP <281>. Infrared carbonyl bands appear at 1720–1680 cm⁻¹, with aliphatic C–H stretching at 2970–2850 cm⁻¹. Negative-mode electrospray ionization typically shows [M–H]⁻ at m/z 200.1. The product is soluble in dimethyl sulfoxide, methanol, and dichloromethane; aqueous solubility of the free acid is low and increases after pH adjustment above the carboxylate pKa. Published data for the exact ionization constant of this specific protected azetidine is limited.

Does Boc Protection Restrict Direct Compression or Granulation Inputs?

The protected acid is not normally used as an unreactive filler or direct compression excipient in tablet manufacture. Its function in oral solid dosage chains is as a protected intermediate that is coupled and deprotected before final API isolation. Wet granulation with aqueous binder systems is unsuitable for the free acid because low-pH microenvironments and water exposure can initiate slow Boc cleavage; if a granulation study is performed on the protected intermediate, organic solvent-based granulating fluids are used. Fluid-bed granulation and roller compaction data for this specific compound are limited in public literature. For dry processing, the crystalline powder is characterized by laser diffraction according to ISO 13320:2020; a common release target for the protected intermediate is D90 NMT 250 µm. Tapped bulk density of the crystalline powder is generally between 0.35 g/mL and 0.55 g/mL by USP <616> Method I. These values apply to the intermediate, not to a finished dosage form. The material should not be processed under relative humidity above 60% because moisture uptake can cause surface plasticity and partial deprotection in the presence of acidic surface contaminants.

In oral and injectable API routes that require an azetidine-3-carboxylate residue, the tert-butoxycarbonyl group prevents the secondary amine from competing during carboxylic acid activation and amide coupling. The carboxylic acid is activated at controlled temperature using carbodiimide reagents such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride with 1-hydroxybenzotriazole in anhydrous dimethylformamide or dichloromethane; the addition is typically performed at 0–5 °C and then allowed to warm to 20–25 °C. On pilot scale, a glass-lined or stainless-steel jacketed reactor with a retreat-curve impeller operating at 150–200 rpm provides adequate dispersion for similar protected amino acid couplings. The protected intermediate is then deprotected with acid to release the azetidine amine or its salt, which is further processed into the final API. Final presentation as a tablet, capsule, granule, or injectable drug product is determined by downstream API particle size, solubility, and stability, not by this protected building block. The intermediate contributes no pharmacopoeial particle size requirement for finished injectables; that requirement is applied to the final sterilized API or drug product.

When Tert-Butoxycarbonyl Deprotection Is Run Below the Carbon Dioxide Evolution Threshold

Acid-mediated removal of the Boc group proceeds through tert-butyl cation/tert-butanol and carbon dioxide. If deprotection is performed in a closed vessel at temperatures above 20 °C, carbon dioxide accumulates and the headspace pressure increases; therefore, reactors are fitted with a pressure-relief bubbler or gas-outlet line vented to a scrubber. Representative conditions for N-Boc amino acid deprotection employ HCl in dioxane at 2–4 mol/L or trifluoroacetic acid in dichloromethane at 20–50% v/v, with reaction temperatures held between 0 °C and 25 °C and reaction times from 2 h to 24 h. Complete conversion for the azetidine derivative should be confirmed by HPLC rather than by visual gas evolution alone because the four-membered ring can retain acid salts and water. Scale-up equipment includes a jacketed reactor with a PTFE-lined thermowell and a caustic scrubber for acid vapors; the reaction mass is stirred at 150–250 rpm to maintain temperature uniformity. After deprotection, the product is isolated by precipitation with a nonpolar antisolvent such as methyl tert-butyl ether or heptane, filtered under nitrogen, and dried in a vacuum oven at 35–40 °C. Drying above 40 °C is avoided for the deprotected salt because the unprotected azetidine amine can undergo oxidative discoloration. Published kinetic data for this specific compound under production-scale deprotection is limited.

Stability of the pharma-grade protected acid is monitored according to ICH Q1A long-term and accelerated protocols. Long-term storage is set at 25 °C ± 2 °C and 60% RH ± 5% RH; accelerated storage is set at 40 °C ± 2 °C and 75% RH ± 5% RH. The material is filled into double polyethylene bags inside a fiber drum or HDPE container and held under nitrogen. Retain samples are stored in amber glass vials with polyethylene-lined closures at 2–8 °C for comparative testing. A 24-month retest interval is assigned after three consecutive production batches remain within assay and impurity limits at the long-term condition. Open-container stability is not claimed; the container should be re-sealed immediately after dispensing. Exposure to high humidity is more critical for this product than moderate heat because water can hydrate the carboxylic acid and promote slow Boc cleavage at temperatures above 30 °C.

Comparative Process Signatures of Boc-Azetidine Carboxylate Regioisomers and Ester Derivatives

The 3-carboxylate derivative differs from unprotected azetidine-3-carboxylic acid by the presence of the acid-labile tert-butoxycarbonyl group. The unprotected compound exists as a zwitterion with limited solubility in nonaqueous coupling solvents and a free amine that can consume coupling reagents. The Boc derivative removes the nucleophilic amine, improves solubility in dichloromethane, tetrahydrofuran, and ethyl acetate, and allows selective carboxyl activation. In comparison with 1-[(tert-butoxy)carbonyl]azetidine-2-carboxylic acid, the 3-carboxylate derivative places the carboxyl group one carbon farther from the ring nitrogen; the resulting steric and electronic differences alter the rate of carbodiimide-mediated activation and the chromatographic retention time. In comparison with 1-Boc-azetidine-3-carboxylic acid methyl ester, the free acid avoids the need for ester hydrolysis before coupling, which shortens the route and eliminates aqueous extraction losses. However, the free acid may require careful control of water content and may form salts with organic bases such as dicyclohexylamine to improve crystallinity; published data for this specific dicyclohexylamine salt is limited. These differences are relevant when a manufacturer selects between the acid, ester, and hydrochloride salt forms for a given coupling step.

For tablet and capsule presentations, the downstream API derived from the azetidine intermediate is generally milled or sieved to a particle size compatible with direct compression or high-shear granulation. The protected intermediate itself is not a directly compressible powder; its compactibility is low and its crystalline plate-like morphology may cause lamination if compressed as a neat powder. High-shear granulation of the protected intermediate with microcrystalline cellulose and a binder solution is not a registered process; instead, the intermediate is converted to the final API before granulation. Capsule filling of the protected intermediate is likewise not intended. For granule manufacture, dry granulation of the final API with intragranular disintegrants such as crospovidone and fillers such as lactose monohydrate is performed after deprotection and salt formation. The final blend is compressed into tablets using a rotary tablet press with compression force typically between 5 kN and 20 kN, depending on the formulation; this parameter is not influenced by the protected intermediate.

In injectable formulation, the derived API must meet bacterial endotoxin, particulate matter, and sterility requirements. The protected intermediate enters the synthesis before these attributes are fully established; therefore, the final parenteral API is filtered through a 0.22 µm sterilizing-grade membrane and lyophilized or filled aseptically. The protected intermediate should not be introduced after the final bioburden reduction step unless it is depyrogenated and sterile. If the protected intermediate is used in a late-stage step, a pre-treatment with activated carbon or a 0.22 µm filter may reduce bioburden but does not remove endotoxins; depyrogenation by dry heat at 250 °C is not applicable to this heat-sensitive compound. Endotoxin control is therefore achieved through raw material qualification and process design rather than terminal depyrogenation.

Chromatographic purity is determined by reversed-phase HPLC according to USP <621> / Ph. Eur. 2.2.29. The exact column chemistry, mobile phase gradient, flow rate, and detector wavelength are controlled by the manufacturer’s validated method and are reported on the certificate of analysis. Detection in the low UV range at 210 nm is common for the Boc chromophore, but the method may use 205–220 nm depending on baseline noise from the mobile phase. System suitability requires tailing factor NMT 1.5 and theoretical plates NLT 5000 for the main peak. Impurity identification and quantification are supported by liquid chromatography-mass spectrometry; the exact response factors for individual impurities are established during method validation. Elemental impurities are controlled by USP <232> / USP <233> and risk-assessed under ICH Q3D. Residual solvents are tested by headspace gas chromatography per USP <467> / Ph. Eur. 5.4.

Materials are packed in HDPE drums with double polyethylene liners under nitrogen. Each drum carries a tamper-evident seal and a label that includes the model code, lot number, retest date, storage temperature, and net weight. For injectable synthesis chains, the supplier may provide a bacterial endotoxin test result only when the corresponding specification has been agreed; the default pharma-grade release does not include sterility. The product is re-tested at the 24-month interval and may be used after retest if the certificate of analysis remains within specification. Hazard classification and disposal requirements should be confirmed against the safety data sheet and local regulation; fine powder containment should be maintained through engineering controls and closed transfer systems.