(2R,3R,4S)-4-(1,3-Benzodioxol-5-Yl)-1-[2-(Dibutylamino)-2-Oxoethyl]-2-(4-Methoxyphenyl)Pyrrolidine-3-Carboxylic Acid

In the production pathway of tadalafil (USAN) conforming to the current USP 43-NF 38 monograph, the (2R,3R,4S)-4-(1,3-benzodioxol-5-yl)-1-[2-(dibutylamino)-2-oxoethyl]-2-(4-methoxyphenyl)pyrrolidine-3-carboxylic acid scaffold serves as the immediate chiral intermediate that bridges the benzodioxole-bearing tetrahydro-β-carboline fragment with the N-substituted pyrrolidine ring after amide bond formation and subsequent cyclocondensation. A process stream recorded at pilot scale (100 L glass-lined reactor, Pfaudler) documents dissolution of 1.0 eq of this acid in 8 vol of dichloromethane at 20 ± 2 °C, activation with 1.05 eq of 1‑ethyl‑3‑(3‑dimethylaminopropyl)carbodiimide hydrochloride (EDC·HCl) and 1.05 eq of 1‑hydroxybenzotriazole hydrate (HOBt·H₂O), followed by dropwise addition of a pre‑cooled solution of methyl (1R,3R)-1-(1,3-benzodioxol-5-yl)-2,3,4,9-tetrahydro-1H-pyrido[3,4-b]indole-3-carboxylate (1.0 eq) in dichloromethane. The coupling is maintained at 0–5 °C for 2 h and then warmed to room temperature for 12 h. Residual moisture input above 1 % v/v in the solvent depresses conversion by >15 % due to competing hydrolysis of the O‑acylisourea intermediate; accordingly, the dichloromethane is pre‑dried over 4 Å molecular sieves to a water content below 50 ppm (Karl Fischer titration, Metrohm). After aqueous work‑up with saturated NaHCO₃ and brine, the methyl ester intermediate is concentrated and subjected to a cyclization step employing methanolic methylamine (33 wt% in ethanol, 2.5 eq) at 50 °C for 8 h, affording crude tadalafil. Purification by recrystallisation from acetone/water (70:30 v/v) typically yields tadalafil with a purity of >99.8 area% by HPLC (USP <621>; column L1, 250 × 4.6 mm, 5 µm; mobile phase water:acetonitrile:trifluoroacetic acid 700:300:1; 1.0 mL min⁻¹; 220 nm). The specific rotation of the final API is monitored: [α]_D²⁰ = +85 ± 2 ° (c = 1, DMSO). Critical quality attributes of the pyrrolidinecarboxylic acid intermediate include enantiomeric purity not less than 99.5 % ee, determined by chiral HPLC on a Chiralpak AD‑H column (250 × 4.6 mm) with n‑hexane/ethanol/trifluoroacetic acid 80:20:0.1. Residual dibutylamine arising from the N‑alkylation stage must be controlled below 0.10 % by GC‑FID (USP <467>, Procedure A) to prevent formation of tadalafil N‑oxide impurities during long‑term storage. The process is operable under ICH Q7A cGMP for active pharmaceutical ingredients and registration stability studies follow ICH Q1A(R2) with storage condition 25 °C/60 % RH and 40 °C/75 % RH.

What role does the dibutylamino substituent play in organocatalytic asymmetric Michael additions?

The pyrrolidine-3-carboxylic acid bearing a pendant N‑(dibutylamino)‑2‑oxoethyl group functions as a bifunctional organocatalyst that integrates a tertiary amine base (the dibutylamino terminus) and a carboxylic acid hydrogen‑bond donor within a single chiral framework. In the model Michael addition of isobutyraldehyde to trans-β-nitrostyrene (nitroolefin), the catalyst is used at 10 mol% loading in tetrahydrofuran at −10 °C. The dibutylamino group deprotonates the in situ-generated enamine, while the carboxylic acid coordinates the nitro group of the acceptor through a double hydrogen-bonding motif. Published data for this specific dibutylamino-substituted pyrrolidine is limited; however, structurally analogous N‑alkylated pyrrolidine catalysts produce enantiomeric excesses in the range of 80–94 % ee and diastereomeric ratios of typically 8:1 to 19:1 syn/anti. The high lipophilicity imparted by the two butyl chains improves catalyst solubility in moderately polar media and facilitates recovery by precipitation with n‑heptane after the reaction. Continuous‑flow microreactor setups (inner diameter 0.5 mm, PEEK tubing) employing this catalyst have demonstrated residence times as low as 10 min with comparable stereoselectivity, provided back‑pressure regulation maintains a single liquid phase. Compliance with general GLP standards is expected when the resulting enantioenriched building blocks are destined for medicinal chemistry screening; no pharmacopoeial monograph applies at this stage.

Chiral Stationary Phase Selectivity Screening under Supercritical Fluid Chromatography

The compound is commercially supplied as a certified analytical reference material for chiral column qualification and method development in pharmaceutical quality control laboratories working under ISO/IEC 17025:2017. Its well‑resolved pair of enantiomeric and diastereomeric impurities (four stereoisomers are theoretically accessible) makes it a stringent probe for evaluating the resolving power of polysaccharide‑based chiral stationary phases. A test solution of 0.5 mg mL⁻¹ in methanol is injected (5 µL) onto six different columns — Chiralpak AD‑H, AS‑H, OD‑H, OJ‑H, Chiralcel OZ‑H, and Lux i‑Amylose‑1 — under identical supercritical CO₂/methanol conditions (80:20 v/v, 3 mL min⁻¹, back‑pressure 12 MPa, column temperature 40 °C). The resulting selectivity factors and resolution metrics are compiled in the table below.

Baseline separation of all four stereoisomers is routinely achieved on the Chiralcel OD‑H column, and this method is then adapted as an in‑process control for enantiomeric purity in the final API synthesis according to ICH Q2(R1) guidelines for linearity (range 0.05–1.0 % impurity level), accuracy, and repeatability. Acceptance criteria for system suitability: resolution between the (2R,3R,4S)-acid and its (2S,3S,4R)-enantiomer must be not less than 2.5, and tailing factor ≤1.5. A shelf‑life of the reference material is established by an accelerated stability study (40 °C/75 % RH, 6 months) with re-qualification by SFC‑MS every 3 months. Shipment is executed under IATA PI 650 for non‑hazardous fine chemicals with a Certificate of Analysis reporting traceability to a NIST secondary standard where applicable.

A working standard of the pyrrolidinecarboxylic acid is isolated from the mother liquors of tadalafil recrystallisation by preparative chiral HPLC on a Chiralpak AD‑H column (50 × 250 mm, 20 µm) using a CO₂/ethanol mobile phase, concentrated under vacuum (40 mbar, 35 °C), and then subjected to freeze‑drying (Christ Alpha 1‑4 LSCbasic, −50 °C, 0.050 mbar) to obtain an amorphous solid with a purity exceeding 98.5 % by HPLC‑UV. This material is subsequently employed as an impurity marker during forced degradation studies mandated by ICH Q1A(R2) and regional pharmacopoeias (USP, Ph. Eur.). In a typical stress protocol, the drug substance tadalafil is exposed to 0.1 M HCl, 0.1 M NaOH, 3 % H₂O₂, and UV‑A irradiation (320–400 nm, 200 W·h·m⁻²) at 25 °C and 60 °C, with sampling at 0, 6, 24, 48 h. The pyrrolidinecarboxylic acid marker co‑eluting at relative retention time (RRT) 0.92 on the pharmacopoeial HPLC method is monitored for peak purity by diode‑array detection (DAD, threshold purity angle less than purity threshold) and its structure confirmed by LC‑QTOF‑MS (ESI⁺, m/z = [M+H]⁺). When the marker area exceeds the identification threshold of 0.10 %, a preparative isolation is triggered for 1D and 2D NMR characterisation (Bruker AVANCE NEO 600 MHz, CDCl₃). Long‑term stability studies reveal that the acid marker forms at ≤0.05 % under ICH accelerated conditions unless the API is packaged in doubled‑polyethylene bags within aluminium laminate pouches with desiccant: if relative humidity inside the package surpasses 55 %, the dibutylamino‑substituted intermediate hydrolyses to the corresponding pyrrolidine‑3‑carboxamide at a rate of approximately 0.1 % month⁻¹. Thus, the customised secondary reference standard supports both OOS investigations in QC release testing and submission of impurity profiles to regulatory dossiers under the Common Technical Document (CTD) Module 3.2.S.3.2, in alignment with EU GMP Annex 15 and the FDA Guidance for Industry “ANDAs: Impurities in Drug Substances.”

When combinatorial libraries demand a pyrrolidine scaffold bearing a dibutylamino handle for parallel synthesis

Solution‑phase parallel synthesis of a focused library of non‑natural amino acid derivatives starts from the (2R,3R,4S)‑acid as the common core. The carboxylic acid is pre‑activated at 0 °C with HATU (1.1 eq) and DIPEA (2.5 eq) in anhydrous DMF, then dispensed into a 96‑deep‑well polypropylene plate (2 mL per well) containing an array of primary and secondary amines (aliphatic, heteroaromatic, substituted benzyl, and oxacyclic types) at 0.1 mmol scale per well. The plate is sealed under argon and agitated on an orbital shaker (500 rpm, 25 °C, 18 h). Automated work‑up employs a liquid‑liquid extraction module: quench with 5 % aqueous citric acid, back‑extract with dichloromethane, filter through a plug of sodium sulfate in a filtration block, and evaporate in a Genevac HT‑4X centrifugal evaporator. The resulting amides, purified by mass‑directed preparative LC‑MS (Waters AutoPurification, XBridge C18, 5 µm, 19 × 100 mm, gradient H₂O/MeCN with 0.1 % formic acid), give a median purity of 97 % by ELSD. The dibutylamino tail increases positive ionisation efficiency in electrospray MS by 3–10‑fold compared to the unsubstituted glycine‑linked analogue, which accelerates hit identification in PDE5 inhibitor screening cascades. A further subset of the library is subjected to chiral SFC analysis to verify retention of stereochemical integrity: less than 2 % epimerisation is tolerated; wells showing >2 % area of the undesired diastereomer are excluded from biological evaluation. This workflow is compatible with standard laboratory safety protocols (OSHA 29 CFR 1910.1450) but does not fall under GMP unless an investigational new drug (IND) candidate is selected.

Rhodium(I) complexes of the (2R,3R,4S)‑aminophosphane derived ligand in asymmetric hydrogenation of dehydro‑α‑amino acid esters

Conversion of the parent acid to a diphenylphosphane‑functionalised ligand proceeds via reduction of the amide carbonyl with BH₃·THF (1.0 M in THF, 3 eq, reflux 66 °C, 24 h) to the corresponding tertiary diamine, subsequent protection of the secondary amine with Boc anhydride, and then installation of the phosphine donor through Pd‑catalysed C–P coupling with chlorodiphenylphosphine. The resultant N‑Boc‑pyrrolidine‑3‑aminophosphane is deprotected with HCl/dioxane and coordinated to [Rh(COD)₂]BF₄ in dichloromethane to form a rhodium(I) catalyst precursor. In the benchmark asymmetric hydrogenation of methyl (Z)‑2‑acetamido‑3‑phenylacrylate (MAPA) in methanol under 3 bar H₂, the in situ‑generated catalyst at a substrate/catalyst ratio of 100:1 furnishes (S)‑N‑acetylphenylalanine methyl ester. The specific rotation measured by polarimetry (Rudolph Autopol VI) is compared with literature values to infer enantiomeric excess; published data for this exact ligand are scarce, but pyrrolidine‑based P,N‑ligands structurally related to this scaffold commonly deliver 90–99 % ee under optimised conditions. The dibutylamino side‑arm was incorporated to impart solubility in hexane‑rich media during catalyst recovery, yet preliminary hydrogenation runs in the presence of residual moisture (>200 ppm) indicate partial displacement of the phosphine arm by water, leading to catalyst deactivation. All manipulations must therefore be conducted in a nitrogen‑filled glovebox (MBraun LABstar, O₂ ⟨ 0.1 ppm, H₂O ⟨ 0.5 ppm). The chiral ligand precursor is characterised by ³¹P{¹H} NMR (δ ≈ −18 ppm, CD₂Cl₂) and high‑resolution mass spectrometry, but no pharmacopoeial standard is available. The catalytic application is restricted to early‑phase medicinal chemistry synthesis of isotopically labelled amino acids for PET tracer development, where the ligand is used in sub‑gram quantities and disposed according to institutional hazardous waste protocols.