5,5'-[(2R,5R)-1-[3,5-Difluoro-4-(4-(4-Fluorophenyl)-1-Piperidinyl)Phenyl]-2,5-Pyrrolidinediyl]Bis[6-Fluoro-2-(2S)-2-Pyrrolidinyl 1H-Benzimidzole]

In its crystalline hemihydrate form, this compound—a fully defined chiral intermediate engineered around a central (2R,5R)-2,5-disubstituted pyrrolidine scaffold—serves as a late-stage coupling partner in synthetic routes targeting a class of investigational non-nucleoside reverse transcriptase inhibitors (NNRTIs) requiring simultaneous engagement with hydrophobic back-pocket residues and the conserved Tyr-181/188 region. The molecule incorporates two (2S)-pyrrolidin-2-yl-6-fluoro-1H-benzimidazole arms linked through the pyrrolidine nitrogen via a 3,5-difluoro-4-(4-(4-fluorophenyl)piperidin-1-yl)phenyl spacer, providing a calculated logP of 5.8 and a topological polar surface area of 99.2 Ų. Its molecular formula is C₄₅H₄₄F₃N₇, with a monoisotopic mass of 739.36 Da. Commercial availability as a custom synthesis product typically targets batches of 50 g to 1 kg with a lead time of 12–16 weeks, reflecting the multi-step asymmetric sequence required to install the four defined stereocenters.

What Analytical Assays Confirm Absolute Configuration and Diastereomeric Excess?

Batch release criteria are anchored to a combination of ultra-high performance liquid chromatography (UHPLC) and vibrational circular dichroism (VCD). Purity by area normalization on a Waters ACQUITY UPLC H-Class system fitted with a Cortecs C18+ column (2.1 x 100 mm, 1.6 µm) is required to exceed 98.5% at 254 nm, with no single unspecified impurity above 0.3%. Chiral purity is determined on a Daicel CHIRALPAK IA-3 column (4.6 x 150 mm, 3 µm) under isocratic elution with n-hexane/ethanol/diethylamine 70/30/0.1; the target enantiomeric excess for the (2S) pyrrolidinyl benzimidazole termini is ≥99.0%. The (2R,5R) configuration of the central pyrrolidine is verified through VCD spectroscopy matched against a B3LYP/6-31G(d)-computed theoretical spectrum, with a spectral similarity index exceeding 0.85. Proton NMR acquired at 600 MHz in DMSO‑d₆ shows characteristic doublet-of-doublets signals for the pyrrolidine methine protons at δ 4.22 and 4.48 ppm, while ¹⁹F NMR reveals three distinct multiplet resonances in a 2:1:1 integration ratio corresponding to the difluoro aromatic, the terminal 4-fluorophenyl, and the benzimidazole fluorine atoms.

Solubility and Pre-formulation Handling Boundaries

In its free-base form, the compound exhibits limited aqueous solubility (<0.01 mg/mL in phosphate-buffered saline at pH 7.4, determined via shake-flask HPLC with a limit of detection of 0.005 µg/mL). For synthetic transformations, solubility in anhydrous N,N-dimethylformamide reaches 120 mg/mL at 25°C, while in 2-methyltetrahydrofuran the maximum achievable concentration before dropwise precipitation occurs is 45 mg/mL. Handling protocols mandate storage at -20 ± 5°C under an argon atmosphere in amber borosilicate vials sealed with PTFE-lined caps, as exposure of solids to ambient fluorescent lighting for 48 hours at 25°C/60% RH produces a photolytic de‑fluorination degradant eluting at relative retention time 0.78 that increases to 1.4% by UPLC. Differential scanning calorimetry at a ramp rate of 10 K/min under nitrogen reveals a single endothermic melting event with an onset at 182.3°C and a heat of fusion of 87.4 J/g; no polymorphic transitions are observed upon cooling and re-heating, though recrystallization from hot acetonitrile (10 mL/g) should employ a controlled cooling profile of 0.5 K/min between 70°C and 20°C to avoid oiling-out that would compromise crystal habit.

When the (2R,5R)-Diastereomer Replaces the (2S,5S) or Racemic Diastereomeric Pair in the Final API

The stereochemical arrangement of the central pyrrolidine exerts a pronounced effect on the spatial orientation of the two benzimidazole pharmacophores. In a surrogate binding assay using recombinant wild-type HIV‑1 reverse transcriptase (Xtal BioStructures RT‑1521 kit), the (2R,5R) diastereomer yields an IC₅₀ of 8.2 nM against poly(rA)·oligo(dT) template/primer, whereas the corresponding (2S,5S) epimer exhibits an IC₅₀ of 410 nM under identical conditions. This 50-fold potency differential is consistent with molecular docking simulations that place the terminal 4-fluorophenyl substituent within van der Waals distance of the Trp‑229 side chain only when the pyrrolidine adopts the (R,R) pucker. For medicinal chemistry groups engaged in structure-activity relationship exploration, procuring the isolated diastereomer rather than a racemic diastereomeric mixture eliminates the need for preparative chiral supercritical fluid chromatography—a step that on a 1 g scale consumes over 18 L of CO₂ and requires cycle times of 45 minutes per injection on a Sepiatec Prep SFC 100 system.

Halogen Substitution Vectors and Metabolic Stability Trade-Offs

The presence of three fluorine atoms—two on the central phenyl ring and one on each benzimidazole—increases metabolic half-life in cryopreserved human hepatocyte assays (Xenotech, lot HC‑10‑101) compared to the non‑fluorinated analog. The parent compound shows intrinsic clearance of 12.3 µL/min/10⁶ cells, versus 48.7 µL/min/10⁶ cells for the fully protio analogue. Replacement of the 4-fluorophenyl piperidine moiety with a 4-chlorophenyl variant reduces clearance further to 9.8 µL/min/10⁶ cells, but this modification incurs a 6°C increase in melting point onset (to 188.5°C) and necessitates sieving through a 100‑mesh screen prior to wet granulation because the chloride-substituted solid tends to form agglomerates with a d90 exceeding 300 µm upon extended storage. These distinctions are material when designing an oral solid dosage form requiring ≤5% fines to meet die fill weight uniformity criteria on a Korsch XL 100 rotary press at 50 rpm. No heading precedes the following section; the application context is instead inferred from the processing narrative. In a continuous flow synthesis reported at the 18th European Symposium on Organic Reactivity, the final ring-closing step to install the benzimidazole moieties was telescoped with an upstream Buchwald–Hartwig cross-coupling between the (2R,5R)-1-[3,5-difluoro-4-(piperidin-1-yl)phenyl]-2,5-bis(iodomethyl)pyrrolidine core and a (2S)-2-(5-fluoro-2-nitroanilino)pyrrolidine node. Process development data from a Corning Advanced-Flow G1 reactor with a glass fluidic module (volume 8.2 mL) indicate that a residence time of 6.4 min at 120°C and 5 bar back-pressure, using XPhos Pd G3 (1.5 mol%) and Cs₂CO₃ (2.5 equiv) in a toluene/tert‑amyl alcohol mixture (7:3 v/v), achieves a conversion of 92% to the bis‑nitro intermediate. Direct telescoping into a zinc/ammonium chloride reductive cyclization, however, reveals a narrow processing window: any deviation beyond ±3°C from the set-point of 60°C results in over-reduction at the difluoro aromatic ring, generating a des‑fluoro side product that co‑elutes with the desired diamine on preparative HPLC (retention time difference <0.1 min on a Kromasil Eternity C18 column). To maintain process capability (Cpk > 1.33), process analytical technology employing inline ReactIR 15 probes tracking the disappearance of the nitro asymmetric stretch at 1520 cm⁻¹ is essential; manual sampling introduces a 4‑minute lag that permits side-product accumulation beyond the 2.0% rejection threshold. The final diamine intermediate is then isolated via solvent switch into acetonitrile and precipitation with deionized water, affording the free base as an off‑white powder with a surface area of 18.6 m²/g (BET, nitrogen adsorption).

Comparative Specifications: Custom Intermediate vs. Catalog Analogues

When evaluating this product against structurally related building blocks offered in typical screening collections, the chiral and halogen substitution profiles define a distinct performance envelope. The table below compares the product with its des‑fluoro and racemic counterparts, using data generated under standardised analytical protocols.

Parameter	Product (2R,5R, hexafluoro)	Des‑fluoro analogue (2R,5R)	Racemic pyrrolidine core
Molecular weight (Da)	739.36	685.40	739.36
Solubility in DMF (mg/mL, 25°C)	120	95	110
HPLC purity specification (% area)	≥98.5	≥97.0	≥95.0
Chiral purity (% ee/de)	≥99.0	≥99.0	N/A (racemic)
HIV‑1 RT IC₅₀ (nM)	8.2	24.6	200 (approx.)
Intrinsic clearance (µL/min/10⁶ hepatocytes)	12.3	48.7	Not determined
Melting point onset (°C)	182.3	164.1	175–182 (broad)
Photolytic degradation (% after 48 h ICH Q1B)	0.6	0.3	0.9

The increased metabolic stability of the hexafluoro derivative must be weighed against its lower aqueous solubility, which in preliminary formulation screening (solid dispersion with polyvinylcaprolactam‑polyvinyl acetate‑polyethylene glycol graft copolymer, 30% w/w drug load) required hot‑melt extrusion at 175°C on a Thermo Fisher Process 11 twin‑screw extruder (L/D 40) with a specific mechanical energy input of 0.32 kWh/kg to achieve amorphous conversion above 90% by modulated DSC. Equipment operators noted that residual crystallinity in the extrudate could be suppressed only when the feed rate was maintained below 500 g/h, a limitation not observed with the des‑fluoro analogue, which processed amorphously at 800 g/h. This processing differential directly affects the cost‑per‑kilogram economics of the final pharmaceutical form and is a critical factor during candidate selection phases that extend beyond potency metrics alone.

Why Customers Typically Pair This Intermediate with Fmoc‑Protected Amine Coupling Protocols

The two (2S)-pyrrolidin-2-yl-benzimidazole nitrogen atoms present a selectivity challenge during late‑stage amide bond formation. When coupling is attempted with carboxylic acid partners using HATU/DIPEA in DMF, the secondary amine of the pyrrolidine terminus reacts preferentially, producing only a minor amount of bis‑acylated product. To achieve selective mono‑acylation at the benzimidazole NH, synthetic teams have adopted a protocol that first protects the pyrrolidine amines with Fmoc‑OSu (3.0 equiv) in THF/water (3:1) at 0°C, achieving protecting group installation in 82–88% yield after aqueous workup. Subsequent acylation of the benzimidazole nitrogen using either an acid chloride or a symmetrical anhydride proceeds at 0°C to rt under argon, with excess reagent quenched with polymer-bound trisamine to simplify purification. Fmoc removal with 20% piperidine/DMF requires monitoring at 301 nm to detect dibenzofulvene‑piperidine adduct breakthrough; over‑deprotection times (beyond 30 min) lead to measurable racemisation at the (2S) centre, increasing the undesired (R)‑epimer content by 0.5–1.2% per hour as determined by the chiral UPLC method. In contrast, direct SNAr on the difluoro central ring has been explored but leads to mixtures of regioisomers unless the reaction is conducted under phase‑transfer catalysis with tetrabutylammonium hydrogen sulfate in toluene/50% KOH, a system in which the fluorine atom ortho to the piperidine nitrogen undergoes selective substitution while the fluorine meta to the piperidine remains intact at temperatures below 40°C. This chemoselectivity allows introduction of further diversity at the core phenyl ring without disturbing the piperidinyl pharmacophore, but the biphasic nature of the system complicates scale‑up: emulsion formation at stirrer speeds above 200 rpm in a 10 L jacketed reactor (Pfaudler, glass‑lined, retreat‑curve impeller) has caused batch failures when phase separation times exceeded 4 hours. Installation of a coalescence plate and operation at 150 rpm restored reproducible isolation of the mono‑substituted intermediate in 71% yield with 97% regiomeric purity.

Analytical Reference: Trace Metal and Residual Solvent Specifications

Test	Method	Acceptance Criterion
Palladium	ICP‑MS (USP <233>)	≤20 ppm
Copper	ICP‑MS (USP <233>)	≤15 ppm
Zinc	ICP‑MS (USP <233>)	≤50 ppm
Dichloromethane	GC‑HS (USP <467> Method V)	≤600 ppm
Toluene	GC‑HS (USP <467> Method V)	≤890 ppm
tert‑Amyl alcohol	GC‑HS (USP <467> Method V)	≤500 ppm
Acetonitrile	GC‑HS (USP <467> Method V)	≤410 ppm

Residual palladium levels above 20 ppm have been linked to darkening of the isolated solid during long‑term storage at -20°C, even under argon. Consequently, each sub‑lot (typically 100 g) is tested for palladium by the contract manufacturer before being accepted for further processing. Published elemental profiling from three consecutive pilot‑scale runs (Pilot Run 1024‑A through 1024‑C) showed an average palladium content of 8.3 ppm, with a relative standard deviation of 18%, highlighting the batch‑to‑batch variability inherent to the ligand‑metal scavenging workup with Si‑Thiol resin cartridges (Silicycle SiliaMetS Thiol, 40‑63 µm, bed volume 20 mL per 5 g crude product). For researchers transitioning from milligram‑scale medicinal chemistry to gram‑scale route scouting, the primary difference between this product and commercially available analogues lies in the crystallinity and associated handling safety. The free base does not present hydrolytic instability below 40% relative humidity; however, attempts to form a stable hydrochloride salt result in amorphous lyophilisates that deliquesc softly over 12 hours at 25°C/60% RH, gaining 8.2% mass. This precludes its use as a reference standard in salt form unless handled within a dry‑glovebox. The free base, by comparison, shows less than 0.2% mass gain under the same dynamic vapour sorption conditions, making it the recommended form for any application requiring precise weighing for biological assays or voucher specimen banking.