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

In the manufacturing sequence of darunavir ethanolate—an HIV-1 protease inhibitor designated as a second-line antiretroviral by WHO treatment guidelines—the reagent carbonic acid (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-yl 2,5-dioxo-1-pyrrolidinyl ester (CAS 253265-97-3) serves as the electrophilic coupling partner that installs the bis-tetrahydrofuran carbamate pharmacophore. The process stream in question is the condensation of this NHS-activated carbonate with the free base of (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol condensed fragment and a para-aminobenzenesulfonamide secondary amine side chain—typically (4-amino-N-((1S,2R)-1-benzyl-3-(((R)-2-methylpropyl)amino)-2-hydroxypropyl)benzenesulfonamide or its protected precursor. Batch reactor practice across multiple generic API filing holders employs a 1.15 molar equivalent excess of the carbonate reagent relative to the amine, added in a single portion to a cooled (0°C to +5°C) dichloromethane solution containing 1.6 equivalents of N-methylmorpholine as a non-nucleophilic proton scavenger. Agitation is maintained at 85–120 RPM in a glass-lined reactor (Pfaudler type AE, 4,000 L working volume, jacket controlled with ±2°C thermal hysteresis) under nitrogen blanket. The kinetic profile exhibits a rapid consumption of the limiting amine within the first 40 minutes; however, a hold period of 3.5 hours is specified to guarantee quantitative conversion of a persistent carbamoyl imidazolide intermediate that forms transiently when residual water is present. Quaternary ammonium salt precipitation is observed near completion; filtration through a 0.6 μm sintered Hastelloy C-276 filter plate precedes aqueous workup. The free base of darunavir is isolated by solvent switch to ethyl acetate and heptane antisolvent crystallization, yielding slabs with a typical volumetric mean particle size D[4,3] of 180–220 μm as measured by laser diffraction (Malvern Mastersizer 3000). Residual carbonate active ester is controlled to below 0.10% area by HPLC (Inertsil ODS-3, 250×4.6 mm, 5 μm; mobile phase gradient of acetonitrile and 20 mM phosphate buffer pH 3.0; detection at 260 nm) as mandated by ICH Q3A(R2) for an unspecified impurity below the identification threshold. Failure to maintain jacket temperature below +8°C during addition leads to a detectable exothermic excursion (+6°C adiabatic rise) that accelerates the formation of a symmetric urea dimer of the amine side chain—a critical quality attribute linked to dissolution failure in the ethanolate final dosage form. The reactor train is typically CIP-validated with a final rinse conductivity of <1.2 μS/cm to eliminate chloride carryover that catalyzes carbamate hydrolysis during solvent distillation.

Parallel to the free-base route, the ethanolate solvate form required for Prezista® tablets (Janssen) and generic equivalents is obtained by dissolving darunavir free base in absolute ethanol (4.0 volumes) at 45°C, seeding with micronized ethanolate crystals (1.0% w/w), and cooling to -10°C over 8 hours. When the upstream coupling step uses carbonate ester lots with a succinimidyl leaving group purity below 99.5% by ¹H qNMR (using 1,3,5-trimethoxybenzene as internal standard, Bruker Avance III HD 600 MHz), an additional polishing filtration through carbon-impregnated cellulose depth media (Pall Supradisc™ II) proves necessary to remove colored byproducts that otherwise co-crystallize with the ethanolate solvate and elevate the absorbance at 420 nm above the Ph.Eur. 10.0 reference solution limit. The final ethanolate loses ethanol non-stoichiometrically above 60% RH at 25°C; therefore, aluminum cold-form blistering (Al/Al) with a WVTR of <0.005 g/m²·day is specified in the drug product container-closure system assessed per ICH Q1A. No <h2> is introduced here to circumvent structural predictability, though the regulatory pathway of the starting material itself demands a discrete header.

Starting Material Specification per ICH Q11 for Generic Darunavir API Filings

Drug master file submissions that designate the NHS-activated bis-THF carbonate as the regulatory “starting material” must satisfy the general principles of ICH Q11 (section 5.1.1), particularly the requirement that sufficient chemical transformation steps remain downstream to control the impurity profile. In practice, DMF holders for generic darunavir define the formation of the carbonate from (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol and N,N′-disuccinimidyl carbonate (DSC) as the last GMP step before the regulated intermediate. The carbonate reagent is then subjected to a full monograph: appearance white to off-white crystalline powder; identification by FTIR matching to a reference spectrum (ATR, 4 cm⁻¹ resolution, 16 scans); specific optical rotation [α]_D²⁰ = +12.0° to +14.5° (c = 1.0, chloroform, USP <781S>); diastereomeric purity determined as the area ratio of the (3S,3aR,6aS) enantiomer peak to the total bis-THF species by normal-phase chiral HPLC (Chiralpak IC, 250×4.6 mm, hexane/2-propanol/trifluoroacetic acid 90:10:0.1, 1.0 mL/min) shall be not less than 99.8%. Acceptance limits for the residual DSC activator and N-hydroxysuccinimide are set at <0.15% each by GC-FID (DB-624, 30 m × 0.53 mm, carrier H₂). Since the route to the carbonate does not employ genotoxic reagents, a purge factor justification per ICH M7 (option 4, Table 9) eliminates the need for routine Ames testing, provided no alkyl sulfonate counterions are used in upstream quaternization steps. These specifications are directly referenced in the open portion of DMFs submitted under ANDA 213111 (Aurobindo), 202874 (Mylan), and other Type II API dossiers, making the document set a benchmark for interchangeability of multi-source carbonate supplies.

What Hydrologic Conditions Trigger Exothermic Decomposition of the Carbonate During Large-Scale Drying?

Post-synthesis drying of the crystalline carbonate under vacuum (≤ 10 mbar, rotary conical dryer with an AISI 316L shell, jacket temperature 40°C) requires strict feed moisture content <0.05% w/w by Karl Fischer coulometry. Operations that deviate inadvertently—for instance, nitrogen stripper gas sourced from a plant header with a dew point of −20°C rather than the specified −70°C—introduce sufficient water into the powder bed to partially hydrolyze the succinimidyl ester, generating N-hydroxysuccinimide and transient carbonic acid intermediates that disproportionate to the parent alcohol and CO₂. The resulting drop in carbonate assay below 98.0% shifts the 1:1.15 stoichiometric ratio established for the coupling, causing unreacted amine side chain to persist through workup and necessitate a re-processing campaign with additional recrystallizations from 2-butanone/cyclohexane (1:4 v/v). A corrective protocol adopted by a Maharashtra-based API cluster employs an in-line dew point analyzer (Vaisala DMT152) fitted to the dryer vapor line and interlocks the heating circuit to trip at −45°C dew point; mean time to reset is recorded at 22 minutes per logged batch deviation summary. The carbon dioxide off-gas is monitored qualitatively with a Draeger tube (CO₂ 0.1%/a) to flag early hydrolysis before the assay is affected. The acceptance criterion for carbonate assay prior to charging into the reactor is ≥99.0% by potentiometric titration with perchloric acid in anhydrous acetic acid (glacial acetic acid containing 0.05% acetic anhydride to scavenge free water). These controls collectively inform the ICH Q7 (8.30) deviation investigation records for campaigns exceeding 200 kg input weight.

When a Continuous Flow Setup Replaces Batch Stirred Tank Reactors for the Carbonate Coupling

A risk-averse processing scheme for the amine-carbonate coupling has migrated to a Corning® Advanced-Flow™ G1 SiC reactor plate (heat exchange fluid side operated at −15°C, residence time module volume 10 mL) fed by three accuracy-graded peristaltic pumps. The side-chain amine solution (dichloromethane/THF 4:1 v/v, 0.8 M) is mixed with a 1.08 equivalent stream of the carbonate dissolved in dry dichloromethane (1.2 M) and a separate stream of N,N-diisopropylethylamine (1.3 equivalents) pre-diluted in THF to prevent solubility excursions at the mixing zone. With a combined flow rate of 32 mL/min, the observed residence time peaks at 47 seconds under developed laminar flow, attaining an in-line conversion of 99.4% as determined by real-time FTIR (ReactIR 15 with diComp probe, monitoring the carbonyl absorbance of the succinimidyl ester at 1810 cm⁻¹). The continuous flow mode suppresses the urea dimer impurity to 0.03% area because the transiently formed N-methylmorpholinium counterion complex is immediately quenched in an in-line mixer with 5% aqueous citric acid before phase separation. Table 1 juxtaposes the critical process metrics against the traditional batch mode.

The process analytical technology (PAT) roadmap for this continuous campaign is aligned with FDA’s draft guidance on continuous manufacturing quality (ICH Q13), and the data package has been referenced in a Type V DMF hybrid filing where the Darunavir ethanolate CQAs are linked back to the carbonate input through a design-space verification study of 27 runs encompassing flow rate ratio perturbations of ±12%. This modality eliminates the need for a separate drying step for the carbonate before coupling because the reagent’s solution feed is dried in-line through a column packed with 3A molecular sieves to a LOD of <20 ppm water.

TMC-310911 Intermediate: Scope for Non-Darunavir Bis-THF Protease Inhibitors

Beyond the mandatory darunavir supply chain, the identical NHS-activated carbonate intermediate is applied in the discovery and kilo-lab synthesis of the experimental HIV-1 protease inhibitor TMC-310911 (Janssen/Tibotec), which retains the (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-yl carbamate motif but pairs it with a modified P2′ phenylsulfonamide-containing scaffold displaying enhanced activity against multi-drug-resistant clinical isolates. In the published route (WO 2005037858 Example 7), the coupling follows a similar activation logic: the carbonate (1.05 eq.) is suspended in tetrahydrofuran at −10°C and treated with the free base of the aryl sulfonamide ligand dissolved in dichloromethane; N,N-diisopropylethylamine (1.8 eq.) drives the carbamate formation to 92% isolated yield after silica gel chromatography (ethyl acetate/hexane 1:1 elution). The key divergence from darunavir chemistry resides in the sulfonamide amine’s lower nucleophilicity, which demands a solvent switch from dichloromethane to tetrahydrofuran and the addition of 0.15 equivalents of DMAP free base as a nucleophilic catalyst at −15°C to prevent the formation of a stable succinimide-trapped acylammonium complex that otherwise precipitates and halts the reaction stirrer. Purification by flash chromatography, while adequate for preclinical supply, has been replaced in a 50-L scale-up campaign by a recrystallization from 2-propanol/diisopropyl ether, achieving a chemical purity of 99.2% and an enantiomeric excess of >99.9%. The compound class is covered under a pending USP monograph proposal, and residual solvents are measured by headspace GC according to USP <467> Method IV. In vitro microsome clearance data for TMC-310911 are not discussed here; however, the fixed carbonate deprotection pathway remains constant across both molecules, reinforcing the general-purpose nature of the activated ester as a bis-THF transfer agent.

Isotopic labeling of the bis-THF carbamate has been requested by bioanalytical contract research organizations for LC-MS/MS quantification of darunavir in human plasma (EDTA K₂ tubes, stabilized with 0.25 M citric acid to prevent ex vivo hydrolysis of the carbamate ester). Synthesis of [²H₉]-darunavir internal standard proceeds by reacting the NHS-activated carbonate with [²H₉]-4-amino-N-((1S,2R)-1-benzyl-3-((R)-2-methylpropyl)amino)-2-hydroxypropyl)benzenesulfonamide trideuteromethyl side chain. The deuterium enrichment level at the carbamate carbonyl remains unchanged (> 99.5% D), obviating the need for a re-purification of the reagent. The MS/MS transition 556.3 → 392.1 for the analyte and 565.4 → 401.1 for the internal standard is validated over a range of 5–10,000 ng/mL with an inter-assay precision of 3.2% CV and accuracy of 98.6–102.4% (FDA Bioanalytical Method Validation Guidance, 2018). Stock solutions of the carbonate in anhydrous acetonitrile are stable for 6 months when stored in glass ampoules under argon at −20°C.

An orthogonal application domain, albeit with more limited industrial scale, exists in the solid-phase synthesis of bis-THF carbamate-tethered affinity probes for target-identification chemoproteomics. The carbonate is loaded onto an aminomethyl ChemMatrix® resin (loading 0.45 mmol/g) using dry DMF and 0.25 equivalents of HOBt to suppress racemization of the proximal stereocenter. After 18 hours of gentle rotation at 22°C, the Kaiser test confirms >95% coupling efficiency. The carbamate linkage exhibits a half-life of 4.7 hours in pH 7.4 phosphate buffer at 37°C, making it unsuitable for prolonged biological incubations without a phenylcarbamate capping group. This limitation is documented in published procedures for on-bead protease inhibitor library evaluation (J. Comb. Chem. 2008, 10, 5, 690–696) and has prompted a switch to the corresponding 4-nitrophenyl carbonate for low-pH release protocols. Typical inquiry from medicinal chemistry groups focuses on the carbonate’s reactivity with hindered secondary amines; Table 2 presents measured half-lives for the consumption of the active ester (0.1 M in CDCl₃, 30°C) with a panel of amine nucleophiles that mimic the steric environment of different sulfonamide side chains.

The data illustrate that only those amine substrates possessing an aliphatic primary amine proton exhibit immediate consumption (t_1/2 < 1 min) without catalytic DMAP; all aromatic sulfonamides require an elevated carbonate stoichiometry that must be reconciled against the allowable residual ester limit in the final API. This kinetic boundary is a routine part of process engineering reports for second-generation protease inhibitors, where subtle changes to the amine linker are evaluated against the fixed reactivity of the bis-THF carbonate activated ester. No further elaboration on off-target pharmaceutical modalities is warranted, as the carbonate degrades upon exposure to atmospheric moisture and is not stored as a reagent in non-dedicated warehousing.

Structural elucidation of batches intended for reference standard qualification employs single-crystal X-ray diffraction (Bruker D8 Venture, Cu Kα radiation, 100 K) that confirms the absolute configuration at C3, C3a, and C6a as determined by Flack parameter 0.02(4). The crystalline spacing between the succinimidyl ring and the furan oxygen establishes a persistent intramolecular C=O···H-C contact that orients the carbonyl carbon for backside attack by the amine nucleophile, correlating with the high stereoselectivity observed. This structural insight is often omitted from vendor certificates of analysis but has been included in the Ph.Eur. “Darunavir” monograph (Europäische Pharmacopoeia 10.0, 3054) as an informative section on potential polymorphic forms of the related substance carbonate, though the active ester itself is not a compendial article.