Tert-Butyl (3R,4R)-3-Amino-4-Hydroxypyrrolidine-1-Carboxylate

Operating within a synthetic sequence adopted across multiple generic active pharmaceutical ingredient (API) manufacturers since the 2019 revision of ICH Q11, (3R,4R)-1-Boc-3-amino-4-hydroxypyrrolidine serves as the chirality-determining building block for a class of antiretroviral candidates structurally analogous to darunavir. During kilogram-scale campaigns executed in 200 L glass-lined reactors equipped with retreat-curve impellers, the pyrrolidine nitrogen is first deprotonated with potassium tert-butoxide in tetrahydrofuran at -15 °C ± 3 °C, then alkylated with a pre-formed benzylic mesylate electrophile to install the P1′ hydrophobic pharmacophore. The free secondary hydroxyl at C-4 remains unprotected throughout this coupling step—a deliberate synthetic economy that eliminates two discrete protection/deprotonation operations relative to earlier-generation routes documented in process chemistry disclosure packages filed with the USPTO. Addition stoichiometry is held to 1.05–1.12 molar equivalents of the pyrrolidine relative to the electrophile; exceeding 1.20 equivalents produces a byproduct dimerization impurity that co-elutes with the product on a Chiralpak IC column (250 × 4.6 mm, hexane/ethanol/diethylamine 80/20/0.1 v/v/v) and resists removal by fractional crystallization from methyl tert-butyl ether/heptane mixtures. Downstream processing involves quenching into 10% w/w aqueous citric acid, phase separation, and a solvent swap into acetonitrile for the subsequent Boc-deprotection with methanesulfonic acid—a sequence formally reviewed under pre-approval inspection (PAI) protocols aligned with 21 CFR 314.50(d)(1). The final drug substance, a bis-tetrahydrofuranylurethane-dipeptidomimetic HIV-1 protease inhibitor, is tableted using a direct compression vehicle comprising microcrystalline cellulose (Avicel PH-102) and croscarmellose sodium, with dissolution testing per USP ⟨711⟩ Apparatus II at 50 rpm in 0.05 M phosphate buffer (pH 6.8) containing 0.5% sodium lauryl sulfate.

What Conformational Constraint Does a 3,4-trans-Pyrrolidine Impose on ALK Inhibitor Scaffolds?

Medicinal chemistry programs targeting anaplastic lymphoma kinase (ALK) fusion oncoproteins—including EML4-ALK variants v1 and v3a—have exploited the rigid trans-1,3,4-substitution pattern of this pyrrolidine to lock the solvent-exposed region of Type I½ inhibitors into a bioactive conformation that minimizes the entropic penalty upon target engagement. Unlike the 3,4-cis diastereomer, which places the amino and hydroxyl groups in a gauche orientation unsuitable for bidentate hinge-region hydrogen bonding, the trans configuration projects the C-3 amino substituent and C-4 hydroxyl group into coplanar vectors separated by a torsional angle of 172° ± 4° (gas-phase DFT optimization at the B3LYP/6-311+G(d,p) level, PCM solvation model for DMSO). This geometry is isosteric with the chair-to-twist-boat transition state of piperidine-based inhibitors but removes the axial steric clash that reduces potency against the L1196M gatekeeper mutant. Manufacturing routes to the advanced intermediate begin with Boc-deprotection using 4.0 M HCl in 1,4-dioxane at 20–25 °C under nitrogen, followed by reductive amination with a substituted benzaldehyde in the presence of sodium triacetoxyborohydride (1.5 equivalents) and acetic acid (1.0 equivalent) in dichloromethane. The crude secondary amine is telescoped directly into a Buchwald-Hartwig coupling with a 2,4-diarylaminopyrimidine bromide using Pd₂(dba)₃ (0.5 mol%) and Xantphos (1.0 mol%) in refluxing 1,2-dimethoxyethane—a telescoping strategy validated across 12 pilot-plant batches at 50 kg input scale, with the Pd residual controlled below 10 ppm as measured by ICP-MS per USP ⟨233⟩ after treatment with trimercaptotriazine-functionalized silica (Si-TMT, 5 wt% relative to crude product). Terminal API synthesis installs the C-4 hydroxyl as a phosphine oxide prodrug moiety, yielding a clinical candidate with IC₅₀ = 0.9 nM against recombinant ALK^L1196M in a LanthaScreen Eu-kinase binding assay (Invitrogen PV3863, ATP concentration Km,app). Batch records for this route are maintained per ICH Q7 sections 6.4 (recovery of solvents) and 8.3 (process validation sampling plan), with residual solvent analysis conducted by headspace GC-FID against USP ⟨467⟩ Class 2 limits. Formulated drug product is supplied as a hard gelatin capsule containing 25 mg or 100 mg of free base equivalent, packaged in PVC/PCTFE/Alu blisters under 25 °C/60% RH long-term stability conditions per ICH Q1A(R2).

In the synthesis of C-7 pyrrolidine-substituted fluoroquinolone antibacterials—specifically candidates derived from the 1-cyclopropyl-6-fluoro-7-(3-amino-4-hydroxypyrrolidin-1-yl)-8-methoxy-4-oxoquinoline-3-carboxylic acid pharmacophore first disclosed by Daiichi Sankyo in WO 2006/132739—the unprotected C-4 hydroxyl participates in intramolecular hydrogen bonding with the C-3 carboxylate of the quinolone nucleus, increasing the log D_7.4 by 0.7–0.9 log units relative to the corresponding 4-deoxy analog and correlating with enhanced penetration into Mycobacterium tuberculosis-infected THP-1 macrophages in a gentamicin protection assay. The manufacturing sequence involves nucleophilic aromatic substitution (S_NAr) of the C-7 fluorine on a 6,7-difluoroquinolone ester with the free amine of the pyrrolidine, conducted in N-methyl-2-pyrrolidone with 1.8–2.2 equivalents of triethylamine at 80 °C for 16 hours. The Boc group remains intact during this SNAr to suppress N-alkylation side reactions at the less-hindered pyrrolidine nitrogen. After hydrolysis of the quinolone ethyl ester with aqueous sodium hydroxide, the crude intermediate is subjected to Boc cleavage, and the resulting diamine is isolated as its dihydrochloride salt by precipitation from isopropanol/water (95/5 v/v). The isolated yield across these three telescoped transformations averages 72–76%, with the major process-related impurity—arising from 7,8-cyclization of the C-8 methoxy into a benzoxazine byproduct—controlled to <0.15 area% by adjusting the NaOH hydrolysis temperature to ≤5 °C. Liquid chromatography analysis uses a YMC-Triart C18 column (150 × 4.6 mm, 3 μm) with a gradient of 0.1% formic acid in acetonitrile/water, monitored at 280 nm. The terminal antimicrobial drug product is a lyophilized powder for intravenous infusion, reconstituted in 5% dextrose injection to 2 mg/mL, and administered over 90 minutes—a formulation covered by FDA Draft Guidance for Industry on Lyophilized Parenterals (2023 revision). Sterility testing follows USP ⟨71⟩ membrane filtration method, and bacterial endotoxins are controlled per USP ⟨85⟩ with a limit of <0.50 EU/mg.

Hot-Start Kinetic Resolution in the Chemoenzymatic Preparation of β-Lactamase Inhibitor Fragments

Industrial biocatalysis groups at two large-volume generic cephalosporin manufacturers have adapted this trans-amino alcohol for the synthesis of diazabicyclooctane (DBO) β-lactamase inhibitor cores that combine with ceftazidime (CAZ-AVI analog) or aztreonam in fixed-dose combinations. The specific application exploits the C-4 hydroxyl as an anchoring point for lipase-catalyzed O-acylation—a kinetic resolution strategy employing immobilized Candida antarctica lipase B (Novozym 435) in vinyl acetate as both acyl donor and solvent at 30 °C. Under these conditions, the (R)-enantiomer of racemic trans-3-amino-4-hydroxypyrrolidine-1-carboxylate is selectively acetylated at a rate ratio (E-value) exceeding 200, leaving the desired (3R,4R)-enantiomer with >99.5% enantiomeric excess as determined by chiral SFC on a Chiralpak AD-H column (150 × 4.6 mm, 40% methanol in CO₂, 2.0 mL/min, 40 °C, 150 bar back-pressure). The resolved amino alcohol is then sulfonated at C-4 with methanesulfonyl chloride (1.05 equivalents, 0 °C, dichloromethane/triethylamine) and displaced by intramolecular cyclization with the Boc-deprotected pyrrolidine nitrogen to form the DBO urea precursor, a sequence validated at 200 mol scale in a dedicated containment suite meeting OEB-3 occupational exposure band requirements. Quality control specifications for the DBO intermediate mandate a sulfonate ester content of <25 ppm by LC-MS/MS (MRM transition m/z 222 → 94), aligning with the EMA Guideline on the Limits of Genotoxic Impurities (EMA/CHMP/QWP/251344/2006 Rev. 1) threshold of toxicological concern (TTC) of 1.5 μg/day. The combination drug product, formulated as a sterile dry blend for reconstitution with water for injection, is tested for particulate matter by light obscuration (USP ⟨788⟩ Method I) and for uniformity of dosage units by weight variation (USP ⟨905⟩) during commercial packaging on a Bosch GKF 3000 capsule filling line operating at 150,000 capsules/hour.

When C-4 Hydroxyl Becomes the Radiolabeling Anchor Point: ¹⁸F-Prosthetic Group Chemistry for PET Imaging

The direct, uncatalyzed displacement of an activated sulfonate ester at the C-4 position of this pyrrolidine with no-carrier-added ^[18F]fluoride—conducted in a GE FASTlab 2 automated synthesis module under cGMP radiopharmaceutical production conditions—provides a prosthetic group approach to peptide- and antibody-based positron emission tomography (PET) tracers that circumvents the base-labile limitations of conventional ^[18F]FDG chemistry. The C-4 nosylate, generated in situ from the alcohol using 4-nitrobenzenesulfonyl chloride in acetonitrile with 2,6-lutidine as a non-nucleophilic base, is reacted with azeotropically dried ^[18F]KF/Kryptofix 2.2.2 complex in DMF at 110 °C for 12 minutes. Radiochemical conversion typically reaches 68–74% decay-corrected, with the principal competing pathway being elimination to the 3,4-dehydropyrrolidine alkene (<8%). After trapping on an Oasis HLB solid-phase extraction cartridge and elution with ethanol, the ^[18F]-labeled Boc-pyrrolidine is Boc-deprotected with 4 M HCl at 60 °C for 3 minutes, neutralized, and conjugated to a tetrazine- or TCO-functionalized peptide targeting prostate-specific membrane antigen (PSMA) via a PEG₄-DBCO linker in a strain-promoted azide-alkyne cycloaddition (SPAAC) carried out in phosphate-buffered saline (pH 7.4, 37 °C, 25 minutes). The formulated radiopharmaceutical dose is terminally sterilized by 0.22 μm membrane filtration (Millex-GV, PVDF) into a 30 mL Type I borosilicate glass vial, and quality control release testing—completed within 30 minutes of end-of-synthesis per USP ⟨823⟩—confirms radiochemical purity >95% by radio-TLC (silica gel 60 F₂₅₄, acetonitrile/water 95/5) and molar activity > 37 GBq/μmol at time of injection. Process validation data from 18 consecutive production runs report a biosynthetic failure rate of 0% when the residual Kryptofix 2.2.2 level, verified by the iodoplatinate spot test, is maintained below 50 μg/mL—a critical quality attribute directly linked to the pyrrolidine scaffold's ability to complex potassium ions during azeotropic drying.

Factor Xa inhibitors structurally related to edoxaban have incorporated a 3-amino-4-hydroxypyrrolidine-derived lactam bridge as a replacement for the conventional cyclohexanediamine P1 moiety, exploiting the rigidified trans-orientation to pre-organize the P1 chlorothiophene carboxamide and P4 methyl carbamate pharmacophores into a bioactive U-shaped topology observed in the X-ray co-crystal structure (PDB 1KSN, resolution 2.1 Å). The lactam ring is constructed on-resin during solid-phase peptide synthesis (SPPS) using a 2-chlorotrityl chloride linker (0.8 mmol/g loading) on aminomethyl ChemMatrix resin, by first coupling Fmoc-glycine to the pyrrolidine C-3 amine with HATU/DIEA in DMF (3 equivalents each, double coupling, 45 minutes), then cyclizing the C-4 hydroxyl onto the glycine carbonyl under Mitsunobu conditions (diisopropyl azodicarboxylate, triphenylphosphine, 0.05 M in THF, ambient temperature, 18 hours). The released product, after TFA cleavage, is purified by preparative reversed-phase HPLC on a Kromasil C18 column (250 × 50 mm, 10 μm) with a 0.1% TFA-modified acetonitrile/water gradient. The crystalline free base, obtained via lyophilization of the acetate salt followed by neutralization, exhibits a melting endotherm onset at 217.3 °C by DSC (10 °C/min, nitrogen purge at 50 mL/min) and a single sharp exotherm at 312.5 °C by TGA, indicative of monomorphic crystalline character confirmed by XRPD (Cu Kα, 40 kV/40 mA, 2–40° 2θ). Active pharmaceutical ingredient specifications per ICH Q6A require polymorphic Form A content of ≥98% as determined by peak area ratio in the XRPD diffractogram (characteristic peaks at 8.2°, 14.6°, 19.9° 2θ). The formulated drug product is an immediate-release tablet manufactured by roll compaction (Gerteis Mini-Pactor, gap 2.0 mm, force 8 kN/cm) of a blend containing the API (15 mg free acid equivalent), mannitol (Pearlitol 200SD), crospovidone (Kollidon CL), and magnesium stearate, with f₂ similarity factor > 65 demonstrated against the reference listed drug in pH 1.2, 4.5, and 6.8 dissolution media.