1H-Pyrrole-3-Carboxaldehyde, 5-(2-Fluorophenyl)-1-(3-Pyridinylsulfonyl)-

The molecular architecture of 1H-Pyrrole-3-carboxaldehyde, 5-(2-fluorophenyl)-1-(3-pyridinylsulfonyl)- (C₁₆H₁₁FN₂O₃S; formula weight 330.33 g mol⁻¹) integrates three reactive loci within a single compact scaffold: a formyl group at the 3-position for condensation and Knoevenagel chemistry, a 2-fluorophenyl substituent at the 5-position that modulates π-stacking and lipophilicity, and an electron-withdrawing 3-pyridinylsulfonyl moiety on the pyrrole nitrogen that alters the heterocycle’s HOMO–LUMO gap while also serving as a latent directing group for late-stage C–H activation. A CAS registry number for this specific entity is not lodged in public databases as of the current production batch release; the compound is manufactured under an ISO 9001:2015 quality system and released with a proprietary internal material number. The material is supplied exclusively for research use and pilot-scale process development, with a standard pack size of 100 mg, 500 mg, or 2 g in amber borosilicate vials sealed under dry argon (O₂ headspace ≤ 500 ppm post-sealing).

How Does the 3-Pyridinylsulfonyl Group Affect Electrophilicity at the Aldehyde Carbon Compared to a N-Methyl or N-Boc Pyrrole?

Replacements of the N-substituent in pyrrole-3-carboxaldehydes are not cosmetic; they directly re-tune the electron density at the aldehyde carbonyl via inductive and resonance pathways. In the 1-(3-pyridinylsulfonyl) variant, the sulfonyl linker exerts a strong –I effect, pulling electron density from the pyrrole ring and making the aldehyde carbon more electrophilic than in the corresponding N-methyl (σ_p ≈ –0.17 for methyl vs. >0.50 for arylsulfonyl groups) or N-Boc analogues. This is reflected in the ¹³C NMR chemical shift of the formyl carbon: the aldehyde carbon appears at δ 184.1 in DMSO‑d₆ (Bruker Avance NEO 400 MHz), a downfield displacement of 4–5 ppm relative to 1-methyl-5-(2-fluorophenyl)-1H-pyrrole-3-carboxaldehyde. The consequence is accelerated imine formation with anilines: under identical conditions (1.05 eq. 4-toluidine, MeOH, 25 °C, 24 h), the N-pyridinylsulfonyl derivative reaches 92% conversion by HPLC area, versus 68% for the N-methyl congener. The pyridyl nitrogen itself remains basic (pK_aH ≈ 3.1 by potentiometric titration in 50% MeOH/water per OECD Guideline 112) and can coordinate soft metals, a feature absent in N-tosyl or N-mesyl pyrrole aldehydes. Practitioners using palladium-mediated cross-couplings must therefore account for potential catalyst sequestration when adding the substrate before the pre-catalyst activation step; supplementation of the system with an additional 0.5 mol% Pd₂(dba)₃ restores catalytic turnover frequency to > 400 h⁻¹ in Buchwald-Hartwig amination at the 4-position.

In contrast to the N-phenylsulfonyl analogue, the 3-pyridinylsulfonyl group improves solubility in aqueous-organic biphasic systems: log D_7.4 measured by shake-flask (OECD 117) is 1.25, approximately 0.6 log units lower than the benzenesulfonyl variant, facilitating extractive workup without resorting to > 5 vol% of co-solvents. This solubility window becomes critical in scaled Suzuki-Miyaura sequences where boronic acid pinacol ester partitioning into the organic phase can drop product recovery below 75% if the aqueous layer polarity is not precisely maintained. Batch records from 500 g campaigns indicate that maintaining Na₂CO₃ molarity at 1.2 M ± 0.1 M in the aqueous phase keeps product retention in the MTBE layer above 88% per extraction.

Analytical Release Criteria and Residual Element Profiles

Every lot is qualified against a dual-method HPLC protocol using an Agilent Infinity II system equipped with a diode array detector (DAD, 190–400 nm) and a single quadrupole mass spectrometer. Primary purity assessment employs a Waters XBridge C18 column (3.5 µm, 4.6 × 150 mm) thermostatted at 30 °C, with mobile phase A = 0.1% v/v trifluoroacetic acid in LC‑MS‑grade water, B = acetonitrile, gradient 5–95% B over 18 min, flow rate 1.0 mL min⁻¹. Retention time is 12.23 min ± 0.15 min, monitored at 254 nm and 280 nm. The acceptance threshold for main peak purity is ≥ 98.5% area percent; historical process capability (Cpk) for 114 consecutively released batches stands at 1.47. Any individual unspecified impurity detected > 0.10% triggers orthogonal confirmation by UPLC‑QToF (Waters Xevo G2‑XS) in positive ion mode, using collision energy ramping from 20–40 eV.

The 5-(2-fluorophenyl) motif, ortho‑fluorinated on the pendant ring, confers two properties that distinguish this building block from the plain 5‑phenyl or 5‑(4‑fluorophenyl) alternatives. The ortho‑fluorine atom restricts rotation about the C5–aryl bond; variable‑temperature ¹⁹F NMR in DMSO‑d₆ reveals coalescence of the two expected fluorine resonances at 358 K, corresponding to a rotational barrier ΔG^‡ of approximately 68 kJ mol⁻¹. In the solid state, single‑crystal X‑ray diffraction on a representative batch (Rigaku Oxford Diffraction XtaLAB Synergy, Cu Kα, 100 K) shows an interplanar angle of 37.2° between the pyrrole and fluorophenyl rings, which suppresses co‑planar packing and reduces the melting point by roughly 15 °C compared to the 5‑(4‑fluorophenyl) isomer. This conformational bias translates into a 2.1‑fold increase in solubility in DMSO‑d₆ over the 4‑fluoro isomer at 25 °C (52 mg mL⁻¹ vs. 25 mg mL⁻¹), a parameter that directly impacts reagent concentration in parallel medicinal chemistry libraries where DMSO stock solutions at 100 mM are the norm.

When This Scaffold Replaces a 4‑Bromopyrrole‑3‑carboxaldehyde in Fragment‑Based Drug Discovery Pipelines

Halogenated pyrrole aldehydes, particularly 4‑bromo and 4‑iodo variants, have been widely employed as vectors for carbon–carbon bond formation at the 4‑position. However, the presence of a heavy halogen introduces a liability in lead‑optimisation cascades: the C–Br bond can undergo dehalogenation under reductive amination conditions (NaBH(OAc)₃, DCE, pH 5) with rates as high as 12% debromination over 18 h at 40 °C, as verified by GC‑MS spiking studies using 4,4′‑dibromobiphenyl as internal standard. The 5‑(2‑fluorophenyl)‑1‑(3‑pyridinylsulfonyl) compound contains no heavy halogen, eliminating this dehalogenation pathway entirely. Direct C–H arylation at the vacant 4‑position has been demonstrated using Pd(OAc)₂ (5 mol%), P(t‑Bu)₃·HBF₄ (10 mol%), and K₂CO₃ in dimethylacetamide at 110 °C with aryl bromides; conversions typically exceed 85% within 6 h as monitored by inline ReactIR (Mettler‑Toledo), tracking the aldehyde C=O stretch at 1678 cm⁻¹ versus the product band at 1685 cm⁻¹. Regioselectivity is > 20:1 for the 4‑position over the alternative 2‑position, a stark improvement over the N‑tosyl analogue which gives 8:1 under identical conditions due to a less orchestrated directing-group effect from the pyridyl nitrogen of the sulfonamide.

In practice, the absence of a bromine also simplifies the heavy‑metal impurity profile of the downstream active pharmaceutical ingredient (API) intermediate. ICH Q3D elemental impurity limits for palladium (Oral PDE 100 µg day⁻¹) and nickel are more readily met when the input aldehyde does not carry residual copper from halogen-exchange steps or iron from Grignard coupling. Scrub protocols after C–H arylation using SiliaMetS Thiol cartridges (loading 1.2 mmol g⁻¹) reduce Pd levels from a post‑reaction concentration of 320 ppm to < 7 ppm in the isolated crude, verified by microwave‑assisted acid digestion and ICP‑MS. This allows the aldehyde to be carried through three subsequent synthetic steps without intermediate purification, a tangible process advantage documented in a pilot‑plant campaign of 3.4 kg scale where telescoping reduced solvent consumption by 38% relative to a step‑wise approach with the bromo analogue.

Storage stability has been mapped under ICH Q1A(R2) conditions. At 25 °C / 60% RH in closed amber containers, HPLC purity drops by 0.2% over 12 months; at 40 °C / 75% RH, the decline reaches 0.8% over 6 months, with the primary degradant identified as the corresponding carboxylic acid via LC‑MS (m/z 347.1 [M+H]⁺). Vials that have been opened must be purged with dry nitrogen before re‑closure and stored over activated 4 Å molecular sieves if repetitive sampling is intended across > 4 weeks. The compound is incompatible with primary amines in neat storage or in solution without acid buffering; adduct formation between the aldehyde and ethanolamine was observed by NMR within 2 h at 25 °C in DMSO‑d₆.

For solid‑phase peptide synthesis applications exploiting oxime ligation at the formyl group, the compound has been successfully loaded onto aminooxy‑derivatized ChemMatrix resin (loading capacity 0.45 mmol g⁻¹) in 1‑methyl‑2‑pyrrolidinone with 0.5% v/v acetic acid as catalyst. Coupling efficiency, quantified by Fmoc cleavage and absorbance at 301 nm, exceeds 91% after 16 h of gentle agitation at 18 °C. This enables construction of macrocyclic libraries where the 3‑pyridinylsulfonyl group acts as an internal base surrogate, slightly retarding undesired aspartimide formation during subsequent piperidine treatments by maintaining a local pH microenvironment below 9.5 at the resin surface.

Differences from other products in the same supplier portfolio stem primarily from the synergistic effect of the 2‑fluorophenyl and 3‑pyridinylsulfonyl motifs jointly tuning the electronic and conformational properties. The corresponding 5‑(4‑fluorophenyl) isomer shows a higher melting point (183.4 °C) and poorer solubility in THF at −20 °C, limiting its utility in low‑temperature lithiation sequences. The 1‑(phenylsulfonyl) comparative benchmark lacks the pyridyl directing function, reducing C–H activation rates by approximately 60% under the palladium conditions described above. The 5‑(2‑chlorophenyl) analogue has a larger van der Waals footprint at the ortho position, decreasing rotational flexibility but also introducing a slower oxidative‑addition partner in downstream cross‑couplings, which can be detrimental when rapid SAR exploration is prioritised.