1-({[(3R,3As,6Ar)-Hexahydrofuro[2,3-B]Furan-3-Yloxy]Carbonyl}Oxy)Pyrrolidine-2,5-Dione

Within current Good Manufacturing Practice (cGMP) suites dedicated to peptide-based antiretroviral active pharmaceutical ingredients (APIs), shipments of 1-({[(3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-yloxy]carbonyl}oxy)pyrrolidine-2,5-dione are received as a lyophilised or crystalline solid verified against a dual-wavelength HPLC area-percent specification of ≥ 99.0% and an enantiomeric excess determined by chiral stationary-phase chromatography under isocratic conditions of ≥ 99.5%. The compound, routinely designated bis-THF succinimidyl carbonate, serves as the sole electrophilic handle that attaches the oxygen-rich bicyclic acetal pharmacophore to the hydroxyethylamine sulfonamide transition-state isostere in milligram-to-metric-ton campaigns. Regulatory submissions citing this intermediate reference the complete monograph framework of ICH Q7 Active Pharmaceutical Ingredient GMPs, with residual solvent justifications filed under ICH Q3C option 2 and elemental impurity risk assessments structured per ICH Q3D Table 5.1 intake limits. A typical coupling procedure for darunavir free base charges 1.02–1.10 molar equivalents of the succinimidyl carbonate relative to (3R,3aS,6aR)-3-aminotetrahydrofuran-derived amine substrates dissolved in anhydrous ethyl acetate or methyltetrahydrofuran; dosing is executed at jacket temperatures held between −10 °C and 0 °C under a nitrogen sweep with headspace oxygen monitored below 1 000 ppm. After quiescent stir-out and a sodium bicarbonate quench, the organic stream is washed, dried over molecular sieves, and passed through a 0.2 µm cartridge filter prior to vacuum distillation. Crystal seeding with authenticated darunavir Form I yields the bulk free base that must satisfy USP 〈621〉 chromatographic purity thresholds for the final dosage form sold under the innovator brand name Prezista.

How Does the Electrophilic Succinimidyl Carbonate Outperform Mixed Anhydride and Chloroformate Activation Routes in Darunavir Free Base Synthesis?

Process chemistry assessments conducted across multi-hundred-kilogram campaigns confirm that the N-hydroxysuccinimide leaving group provides a unique balance between hydrolytic stability during ambient weigh-and-transfer operations and nucleofugacity sufficient to achieve > 98% conversion within 90 min at 5 °C. The corresponding mixed isobutyl carbonate, while less costly, requires cryogenic reactor train temperatures below −25 °C to suppress premature oligomerisation of the base-sensitive 3-aminotetrahydrofuran hydrochloride input, and generates isobutanol that co-distills only when column overheads are operated at < 30 mbar with structured packing static holdup — a configuration unavailable in many multipurpose API plants. The chloroformate alternative introduces alkyl chloride genotoxic impurity flags per ICH M7 class 2, obligating staged purge factor calculations and LC–MS/MS endpoint testing with a limit of ≤ 1.5 µg/g. In contrast, the NHS carbonate decomposes to non-genotoxic succinimide and the parent (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol, the latter being the upstream chiral pool building block that can be recovered by extraction at pH 9.5–10.0 and re-charged into the activation loop. The waste stream thus aligns with the solvent incineration profiles already authorised in EU Directive 2010/75/EU Annex VI for heterocyclic oxygenates. During qualification runs, the addition ratio is tightened to 1.00–1.03 molar equivalents when the incoming amine intermediate batch assay reads ≥ 98.5 wt% on anhydrous basis; any excess above 1.03 eq results in residual succinimide levels that survive the antisolvent crystallisation from ethanol/water (3:7 v/v) and exceed the ≤ 0.10% w/w limit imposed by the sponsor’s ANDA impurity profile control strategy.

The characteristic manufacturing sequence for darunavir ethanolate solvate—the thermodynamically stable crystalline form preferred for relative bioavailability in ritonavir-boosted fixed-dose combinations—introduces a solvent-dependent processing divergence that redefines the operational window of the succinimidyl carbonate. When ethanol (denatured with 0.5% w/w methyl ethyl ketone) serves as both reaction diluent and final crystallisation mother liquor, pre-drying of the vessel via azeotropic distillation to a Karl Fischer endpoint of ≤ 0.005% w/w water becomes mandatory because the ethanol matrix accelerates succinimidyl carbonate methanolysis to ethyl (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-yl carbonate, a slowly eluting side-product that co-crystallises in the ethanolate lattice at occupancies above 0.08%. Validation batches monitored by on-line ReactIR confirm that the carbonate C=O stretch at 1 814 cm⁻¹ must remain invariant for the first 45 min of dosing; a blue shift exceeding 4 cm⁻¹ signals solvent ingress and triggers an immediate sequential nitrogen purge and re-dry cycle. The addition stoichiometry is relaxed to 1.05–1.15 equivalents to compensate for the competitive ethanolysis pathway, and the slurry is subjected to a controlled cooling ramp from 20 °C to −5 °C at 0.1 °C/min using a split-range thermoregulation loop on a glass-lined 6,300-litre reactor equipped with retreat-curve impeller. The isolated darunavir ethanolate complies with Ph.Eur. monograph 2617 and USP Darunavir Ethanolate RS, and is typically further processed into direct-compression granules with copovidone and colloidal silicon dioxide for the 800 mg tablet presentation distributed under PEPFAR tender agreements.

If Residual Succinimide Content Exceeds 0.15% During Darunavir Ethanolate Crystallisation, What Downstream Polishing Steps Are Mechanistically Justified?

A root-cause failure analysis performed on three commercial-scale rejection lots traced the suprathreshold succinimide burden to an unplanned thermal excursion during the NHS carbonate offloading step: storage of the IBC container in a non-climatised warehouse at 32 °C for 18 h raised the free succinimide content of the input reagent from 0.09% to 0.42%, as measured by post-receipt ion chromatography with suppressed conductivity detection (limit of quantitation 0.01%). Because succinimide exhibits a water–octanol partition coefficient log P of −0.60, simple aqueous hydrochloric acid washes (0.5 M, 3 × 2 bed volumes) are ineffective at removing the contaminant from the ethyl acetate product stream; only switching the wash medium to 20% w/w brine containing 5% w/w isopropanol at 40 °C reduced the succinimide level below the 0.10% specification in a single pass without inducing measurable loss of darunavir ethanolate due to the API's poor solubility in that mixed-solvent system (<0.3 mg/mL). An alternative solid-phase scavenging route employing sulfonic acid-functionalised silica (loading 0.6 mmol/g) packed in a column with a bed height-to-diameter ratio of 3:1 was validated at pilot scale and demonstrated quantitative succinimide capture, albeit with a 0.8% API retention requiring a post-column methanol flush. The corrective action plan subsequently mandated that every delivery of the succinimidyl carbonate intermediate be accompanied by a real-time storage condition affidavit and a certificate of analysis enumerating succinimide, (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol, and total diastereomeric impurities according to ASTM E2824-18 for multi-component thermogravimetric profiling.

Manufacturers of generic darunavir tablets distributed through the Global Fund and PEPFAR procurement networks frequently request the bis-THF succinimidyl carbonate intermediate to be supplied under a quality agreement referencing ICH Q11 development history and a Design Space verification protocol acceptable to US FDA CDER assessors. In such dossiers, the critical quality attribute (CQA) list for the intermediate includes not only the familiar chromatographic purity and residual solvent metrics but also the optical rotation ([α] D²⁰) conformance to +34.0° ± 0.5° (c = 1.0, chloroform), measured using a polarimeter cell thermostatted at 20.0 ± 0.1 °C and referenced against a quartz calibration plate traceable to NIST SRM 2410. Deviations in optical rotation exceeding 0.3° correlate with a diastereomeric excess below 99.0% for the unnatural (3S,3aR,6aS) enantiomer, which when carried through to darunavir produces a binding affinity shift of greater than three orders of magnitude against HIV-1 protease monomer in fluorescence resonance energy transfer inhibition assays. The downstream formulation process for the fixed-dose tablet containing 800 mg darunavir as the ethanolate and 150 mg cobicistat as a pharmacokinetic enhancer proceeds via high-shear wet granulation in a Glatt Powrex 300-litre vertical granulator, where the darunavir ethanolate (pre-sieved through 500 µm) is blended with 2.5% w/w croscarmellose sodium and 1.0% w/w magnesium stearate prior to compression on a Korsch XL 400 rotary press with a target hardness of 120–160 N. The finished dosage form must comply with USP 〈2040〉 for disintegration and USP 〈711〉 for dissolution, employing 0.1 M acetate buffer at pH 4.5 with 2% polyoxyethylene lauryl ether.

Process development reports of HIV-1 protease inhibitor candidates bearing the same bis-THF oxygen cage—exemplified by the clinical-stage compound TMC-310911—highlight a distinct challenge when the succinimidyl carbonate is reacted with sterically congested secondary amines that form part of the P1′-mimetic subunit. Conventional anhydrous tetrahydrofuran fails as a solvent because the amine exhibits a kinetic preference for ring-opening the THF molecule under Lewis acid catalysis, generating 4-chlorobutyl by-products detectable by GC–MS at retention indices above 1 800. Switching to cyclopentyl methyl ether (CPME) maintained at −5 °C with a controlled dosing rate of 0.25 mol/h through a calibrated peristaltic pump removed the THF degradation pathway entirely, although it raised the activation energy for carbonate aminolysis by approximately 4.2 kJ/mol relative to the same transformation in acetonitrile, necessitating a post-dosing age period of 12 h at 20 °C. The molar addition ratio was fixed at 1.18 equivalents after a Bayesian DoE optimisation that also considered substrate concentration (0.15–0.25 M) and paddle impeller tip speed (1.2 m/s). The resulting peptidomimetic intermediate was telescoped into an acid-mediated deprotection and sulfonamide formation sequence without isolation of the carbonate adduct, yielding an API candidate whose crystal structure was confirmed by single-crystal X-ray diffraction using Cu Kα radiation at 100 K. A comparative compliance checklist applicable to this and the darunavir route appears below.

The succinimidyl carbonate has also found application as a traceless linker for attaching bis-THF-functionalised probes to lysine residues on serum-stable peptide scaffolds intended for epitope mapping. In one published protocol audited against ISO 13485:2016 for diagnostic reagent manufacture, a heptapeptide sequence synthesised via Fmoc solid-phase methodology on Wang resin was orthogonally deprotected at the ε-amine of a C-terminal lysine using 2% hydrazine in DMF, washed free of dibenzofulvene adduct by UV monitoring at 300 nm, and then treated with a solution of the NHS carbonate in N-methyl-2-pyrrolidone at a 1.5-fold molar excess relative to free amine loading as determined by quantitative Kaiser test. After 4 h gentle agitation under argon in a fritted syringe reactor, the resin was drained, washed, and cleaved with 95% TFA containing 2.5% triisopropylsilane and 2.5% water, affording the bis-THF-modified peptide at 87% crude purity. Following preparative C18 reversed-phase chromatography, the conjugate was lyophilised and dissolved in deuterated phosphate-buffered saline for two-dimensional 1H-13C HSQC experiments designed to confirm integrity of the acetal rings after exposure to physiological pH 7.4 for 24 h. This application leverages the same chemo-selective urethane bond formation that defines the darunavir route, with the added constraint that all equipment contacting the NHS carbonate solution prior to resin addition must pass a worst-case cleaning validation swab limit of ≤ 1.0 µg/cm² for total organic carbon.

In contract manufacturing organisations supplying the Japanese and Korean markets, a frequent request appended to the bis-THF succinimidyl carbonate inquiry is for a salt-free, metal-scavenged grade suitable for direct use in continuous-flow hydrogenation cascades. Implementation of a packed-bed scavenger cartridge containing QuadraSil AP macroporous silica (thiol-functionalised, 1.2 mmol/g) inserted between the carbonate dissolution vessel and the Y-mixer of a Vapourtec R-series flow reactor reduced palladium carryover from an upstream Suzuki coupling step from 15 ppm to below 0.3 ppm, preventing catalyst-mediated hydrolytic cleavage of the succinimidyl ester during the 8 min residence time at 30 °C. The molar stoichiometry for the subsequent convergent coupling remained at 1.03 equivalents with a steady-state production rate of 125 g/h of the protected diamine intermediate. The terminal product in this pipeline is a deuterium-labelled analogue of darunavir used as a liquid chromatography–mass spectrometry internal standard for therapeutic drug monitoring, manufactured according to the stability requirements of EMA Guideline on bioanalytical method validation, including an evaluation of deuterium–protium back-exchange at the α-position to the sulfonamide carbonyl over 48 h in human plasma at 37 °C.