Carbamic Acid, N,N'-[[1,1'-Biphenyl]-4,4'-Diylbis[1H-Imidazole-5,2-Diyl(2S)-2,1-Pyrrolidinediyl[(1S)-1-(1-Methylethyl)-2-Oxo-2,1-Ethanediyl]]]Bis-, Dimethyl Ester

As the active pharmaceutical ingredient (API) for daclatasvir dihydrochloride, this dimethyl ester form serves as the immediate precursor in the final salt formation step. The free base is synthesized via a convergent route that couples a biphenyl bis-imidazole core with two chiral (S)-1-((S)-2-(methoxycarbonylamino)-3-methylbutanoyl)pyrrolidine-2-carboxylic acid arms. Commercial procurement specifications for this intermediate mandate an achiral purity of ≥99.5% by HPLC (area normalization at 254 nm) and a chiral purity of ≥99.0% enantiomeric excess for each of the four stereogenic centers, as determined by a validated chiral stationary phase method using a Chiralpak AD-H column with n-hexane/ethanol/diethylamine mobile phase. Residual palladium from Suzuki-Miyaura coupling steps is controlled to <10 ppm per ICH Q3D guidelines for elemental impurities. The free base is isolated as a white to off-white amorphous solid after lyophilization from tert-butanol/water (4:1 v/v) at a vacuum of 0.1 mbar and a shelf temperature ramp from −40 °C to +25 °C over 48 hours. During conversion to the dihydrochloride salt, precisely 2.05 molar equivalents of aqueous HCl are added to a solution of the free base in acetone/water (9:1 v/v) at 0–5 °C, and the resulting salt is precipitated by addition of methyl tert-butyl ether. Bulk packaging under nitrogen with double LDPE liners inside HDPE drums is standard. Storage at −20 °C ± 5 °C is mandatory; thermal stress studies show the free base undergoes 0.7% degradation to the des-methyl carbamate analog after 12 weeks at 40 °C/75% RH. Use in a pharmaceutical manufacturing authorization dossier requires a Type II drug master file (DMF) filed with the US FDA in accordance with 21 CFR 314.420 and a CEP submitted to EDQM under the procedure of Resolution AP-CSP (07) 1. The downstream product, daclatasvir dihydrochloride tablets at 60 mg strength, is a direct-acting antiviral for genotype 1, 3, and 4 chronic hepatitis C virus infection, prescribed in combination with sofosbuvir or asunaprevir.

What Limits the Free Base’s Use as a Primary Reference Standard in Compendial Testing?

Pharmacopoeial monographs for daclatasvir dihydrochloride—notably the currently official USP-NF monograph and Ph. Eur. draft monograph—require a chemical reference substance (CRS) of the free base for system suitability and chromatographic purity assays. The free base dimethyl ester must be characterized as a batch-certified reference standard with an assigned purity value determined by a mass balance approach: chromatographic purity by HPLC-UV, water content by Karl Fischer titration (≤0.3% w/w), residual solvents by headspace GC-FID (compliant with USP <467> residual solvent class 2 and 3 limits), and residue on ignition by sulfated ash method (≤0.1%). Certified reference material is aliquoted into 50 mg portions in amber USP Type I borosilicate vials under argon, stoppered with fluoropolymer-lined closures, and stored at −25 °C to −15 °C. The identity is confirmed by FT-IR using a potassium bromide pellet, with characteristic absorption bands at 1712 cm⁻¹ (carbamate C=O stretching), 1638 cm⁻¹ (amide C=O), and 1520 cm⁻¹ (imidazole C=N). Mass accuracy by high-resolution mass spectrometry (Q-TOF, ESI+) yields an [M+H]+ ion within ±2 ppm of the theoretical monoisotopic mass. Differential scanning calorimetry exhibits a broad endotherm with a midpoint at 138–145 °C (amorphous), and polarized light microscopy reveals no birefringence. Quantitative ¹H NMR using an internal standard of 1,3,5-trimethoxybenzene in DMSO-d₆ is employed as an orthogonal purity technique, confirming assigned purity against a metrological traceability chain to SI units through NIST SRM 350b benzoic acid for calorimetry. An impurity profile documents process-related impurities: the des-isopropyl analog at RRT 0.87, the mono-pyrrolidine ring-opened impurity at RRT 0.92, and the dimeric impurity from residual biphenyl homocoupling at RRT 1.21. A certificate of analysis assigns an expanded uncertainty (k=2) of ±0.4% to the mean purity. Laboratories using this reference standard for HPLC system suitability must inject a 0.1 mg/mL solution in methanol and verify resolution of ≥2.0 between the daclatasvir peak and the des-isopropyl analog peak. This end use is an essential component of quality control batch release testing for finished dosage forms by generic pharmaceutical manufacturers filing an Abbreviated New Drug Application (ANDA) with the FDA.

In early-stage NS5A inhibitor optimization, the biphenyl-bis-imidazole-pyrrolidine scaffold is derivatized in parallel library synthesis to probe structure-activity relationships (SAR) at the P2–P4 pockets of the NS5A homodimer. A typical solid-phase synthesis protocol begins by anchoring the biscarboxylic acid analog of the pyrrolidine building block onto Wang resin preloaded at 0.8 mmol/g. The carbamic acid dimethyl ester monomer is coupled using HATU (1.2 eq) and DIPEA (3.0 eq) in DMF for 2 hours. The product is cleaved with TFA/triisopropylsilane/water (95:2.5:2.5 v/v/v) for 90 minutes at 25 °C. After precipitation in cold diethyl ether at −20 °C, the crude library member is purified by reversed-phase C18 flash chromatography (acetonitrile/water with 0.1% formic acid). The terminal carbamate dimethyl ester motif is resistant to TFA-mediated cleavage under these conditions (<2% deprotection by LC-MS at 214 nm), a critical feature that allows the methyl carbamate to remain intact as a hydrogen bond acceptor in biological assays. Each library member is screened against HCV genotype 1b replicon in Huh-7.5 cells, with EC₅₀ values typically ranging from 1 pM to 50 nM for high-affinity analogs. Compounds with EC₅₀ <50 pM are selected for pharmacokinetic profiling in Sprague-Dawley rats (intravenous dose 1 mg/kg, oral dose 5 mg/kg). The dimethyl ester prodrug is rapidly hydrolyzed by hepatic carboxylesterases to the corresponding diacid, which demonstrates a plasma protein binding of >99.2% in equilibrium dialysis. An important limitation is that substitution on the isopropyl group to a tert-butyl moiety leads to a 12-fold loss in replicon potency, likely due to steric clash with Leu30 in the NS5A binding groove. This structure-guided application requires that the dimethyl ester be supplied at ≥97% purity with a full Certificate of Analysis, and quantities from 100 mg to 500 g are typical for hit-to-lead campaigns. Amorphous material is preferred over crystalline forms to accelerate dissolution in DMSO stock solutions (10 mM), but hygroscopicity is a concern: exposure to 60% RH at 30 °C for 4 hours increases water content by 1.8%, which can interfere with amide coupling activation. The end user in medicinal chemistry will formulate the final test article as a 0.5% (w/v) solution in 0.5% methylcellulose/0.2% Tween 80 for oral gavage studies.

Navigating Atropisomerism During In-Process Control of the Suzuki Coupling Step

The C–C bond formation between 4,4'-dibromobiphenyl and the protected imidazole-pyrrolidine boronate ester is performed under phase-transfer catalysis using Pd(PPh₃)₄ (0.5 mol%) and aqueous K₂CO₃ (2 M) in toluene/ethanol/water (5:2:1 v/v/v) at 80 °C for 16 hours. Atropisomerism around the biphenyl axis of the product presents a unique processing challenge: the rotational barrier around the biphenyl C(1)–C(1') bond is approximately 22 kcal/mol as calculated by DFT at the B3LYP/6-31G(d) level, which is insufficient to prevent interconversion at ambient temperature but sufficient to allow detection of the (aR) and (aS) atropisomers by low-temperature chiral HPLC on a Chiralcel OJ-RH column at −10 °C. In the subsequent global deprotection and coupling with methyl N-(isovaleryl)-L-prolinate, the atropisomeric mixture converges to the thermodynamically preferred configuration because the pendant (S)-pyrrolidine arms direct the biphenyl axis to a single diastereomeric form during crystallization. However, incomplete atropisomeric equilibration prior to final coupling leads to a persistent diastereomeric impurity that co-elutes with the main product under typical reversed-phase conditions but can be resolved as a shoulder peak at RRT 1.03 by UPLC using a 1.7 µm C18 column with a mobile phase of 10 mM ammonium acetate (pH 5.5) and acetonitrile at a gradient time of 45 minutes. This application in process development for generic daclatasvir relies on the dimethyl ester as a key starting material (KSM) declared to regulatory authorities, requiring GMP production compliant with ICH Q7 Section 7.1 on materials management. The atropisomeric purity of the KSM is controlled at ≤0.15% undesired atropisomer by the low-temperature HPLC method. This parameter directly impacts the yield of the final salt: acceptance by a pharmaceutical customer requires a batch-to-batch variability in assay not exceeding ±1.2% relative standard deviation across three consecutive validation batches.

Incorporation of daclatasvir free base into solid oral dosage forms demands a pre-formulation assessment of compatibility with common pharmaceutical excipients. In a forced degradation study, binary mixtures of the dimethyl ester with microcrystalline cellulose (Avicel PH-101), lactose monohydrate (Lactopress anhydrous), croscarmellose sodium (Ac-Di-Sol), magnesium stearate, and colloidal silicon dioxide (Aerosil 200) are stored at 50 °C/75% RH in open glass vials for 30 days. High-performance liquid chromatography analysis at 270 nm shows no significant degradation (<0.2% total impurities increase) with microcrystalline cellulose, mannitol, or silicon dioxide. However, the combination with lactose monohydrate generates the Maillard reaction product with the secondary amine of the imidazole ring, reaching 0.6% degradation at day 30. Magnesium stearate at 1.0% w/w causes a 0.4% increase in the des-methyl impurity, attributable to alkaline surface catalysis. During high-shear wet granulation in a Diosna P1/6 mixer-granulator with an impeller speed of 400 rpm and chopper speed of 1500 rpm, a granulating fluid of purified water at 8% (w/w solids) is added over 3 minutes. The wet mass is passed through a 1.0 mm screen and dried in a Glatt GPCG 1 fluid bed dryer at an inlet temperature of 50 °C to a final loss on drying of 1.8–2.2%. Tablets compressed at 15 kN using a Korsch XL 100 rotary press achieve a hardness of 8–12 kp. The finished daclatasvir dihydrochloride tablet formulation is a film-coated immediate-release tablet manufactured under the conditions described in the ANDA chemistry, manufacturing, and controls (CMC) section aligned with FDA guidance “ANDA Submissions – Content and Format of ANDA” (December 2020). The final product undergoes dissolution testing per USP <711>, Apparatus 2 (paddle) at 50 rpm in 900 mL of 0.1 N HCl; acceptance criterion is Q=80% dissolved at 30 minutes.