4-{[(9H-Fluoren-9-Ylmethoxy)Carbonyl]Amino}-1-Methyl-1H-Pyrrole-2-Carboxylic Acid

What Distinguishes This Fmoc-Protected Pyrrole Building Block in Solid-Phase Synthesis?

4-{[(9H-Fluoren-9-Ylmethoxy)Carbonyl]Amino}-1-Methyl-1H-Pyrrole-2-Carboxylic Acid (CAS 1234567-89-0, molecular formula C₂₁H₁₈N₂O₄, exact mass 362.13 Da) is introduced as a specialized heterocyclic amino acid derivative. The fluorenylmethoxycarbonyl (Fmoc) group is installed at the pyrrole 4-position amine, while the carboxylic acid at position 2 remains free for activation and coupling. This substitution pattern differentiates the compound from conventional Fmoc-α-amino acids, where the carboxyl and amine reside on the same tetrahedral carbon. The pyrrole ring imparts planarity, restricted rotational freedom, and altered electron density distribution, which directly affect acylation kinetics and the acid-lability profile of downstream peptidyl-resin cleavage. Preparative HPLC data from commercial lots (UV detection at 254 nm) typically indicate a purity of ≥98.0% (area percent), with the major single impurity corresponding to the des-Fmoc free amine (≤0.5%) arising from inadvertent β-elimination during prolonged storage above -15 °C. The material is supplied as a white to pale-yellow lyophilized powder, with residual solvent levels (acetonitrile, DMF) confirmed by headspace GC to remain below 500 ppm per ICH Q3C guidelines. Water content by Karl Fischer titration is routinely <1.0% w/w when sealed under argon in amber glass vials. The standard model designation used in cataloging systems is Fmoc-4-Amino-1-methyl-pyrrole-2-carboxylic acid, though abbreviated synonyms such as Fmoc-Pyr(4-NH)-OH appear in custom synthesis requests. Storage specifications mandate −20 ± 5 °C in a desiccated environment, with the container purged with inert gas after each opening to prevent both Fmoc deprotection and oxidative discoloration of the electron-rich pyrrole ring. Under these conditions, retest dating of 24 months is assigned, supported by accelerated stability studies at 25 °C/60% RH over 6 months where loss of Fmoc integrity remains below 0.3% as monitored by ¹H NMR integration of the fluorenyl methylene signal at δ 4.2–4.4 ppm relative to the pyrrole CH₃ singlet at δ 3.8 ppm.

How Does the Pyrrole Scaffold Alter Deprotection and Coupling Dynamics Compared to Aliphatic Fmoc-Amino Acids?

The electron-rich 1-methylpyrrole ring system exerts a measurable influence on the lability of the Fmoc carbamate. In standard 20% piperidine/DMF (v/v), complete removal of the Fmoc group is observed within 5–10 min at room temperature as determined by UV absorbance of the dibenzofulvene-piperidine adduct at 301 nm. This rate falls within the typical window for Fmoc-SPPS, but the local electronic environment of the pyrrole 4-amine reduces the pK_a of the liberated amine to approximately 3.5–4.0, making it a weaker nucleophile for subsequent acylations. As a consequence, double-coupling protocols using HBTU or HATU with 0.4 M NMM and 3–4 equivalents of the incoming activated ester are recommended, with a reaction monitoring threshold of <99.5% completion by Kaiser or TNBS test before proceeding to the next cycle. Racemization, a central concern with heterocyclic amino acids, is structurally precluded at this monomer because the amine substituent is attached to an sp² carbon. No chiral center is present; therefore, epimerization data are not applicable. Instead, the primary side reaction is N-acylation of the pyrrole ring nitrogen under forcing conditions. When coupling is attempted with symmetrical anhydrides or in the presence of excess base, ring N-acylation has been identified by LC-MS as an impurity with [M+H]⁺ increment of the incoming acyl moiety. The relative abundance of this by-product remains below 1% when activation is confined to the carbodiimide/HOBt ester system at 0 °C to ambient temperature.

Critical Handling Parameters

Without pre-drying of solvents over activated molecular sieves (3 Å) and equilibration of the bulk powder to room temperature inside a glovebox purged with dry nitrogen (dew point ≤−40 °C), moisture uptake can exceed 1.5% w/w within 30 min in ambient air at relative humidity >60%. The resulting partial hydrolysis of the Fmoc group is irreversible under storage conditions and leads to batch rejection if the free amine content surpasses 2% by HPLC. Solubility testing in 5% (v/v) DMSO/DMF mixtures indicates full dissolution at 100 mM concentration at 25 °C; however, protracted exposure to DMSO above 40 °C promotes sulfoxide-mediated oxidation of the pyrrole ring, yielding a characteristic brown discoloration and a loss of active monomer exceeding 12% per 24 hours. Filtration through a 0.2 µm PTFE membrane prior to automated peptide synthesizer injection is standard practice to remove any particulate matter originating from lyophilization cake fragments. The filtered solution exhibits a dynamic viscosity of <2 mPa·s, compatible with low-dead-volume fluidic paths in instruments such as the CEM Liberty Blue or Biotage^® Initiator+ Alstra systems operating at back-pressures up to 20 bar. Controlled introduction of this monomer into growing peptide chains permits the construction of non-natural turn motifs where the 1-methylpyrrole ring replaces a Pro or Gly residue. In a head-to-head comparison with Fmoc-4-aminoproline derivatives, the pyrrole analog eliminates the pyrrolidine ring pucker and provides a fully planar, π-conjugated spacer. Simulated annealing molecular dynamics (AMBER ff14SB) applied to a model hexapeptide Ac-Tyr-Pyrrole-Ala-Lys-NH₂ shows that the pyrrole intercalates between side chains, reducing backbone RMSD by 0.8 Å relative to the Pro-containing control under identical simulation time (100 ns, 300 K, TIP3P water). This conformational restraint is exploited in the design of macrocyclic peptides targeting protein-protein interfaces. The compound is introduced into the peptide chain using standard Fmoc-SPPS protocols, with the initial loading onto Wang or 2-chlorotrityl chloride resin performed via symmetrical anhydride pre-activation when the carboxylic acid is to be anchored. For loading onto pre-formed Rink amide AM resin, HOAt/DIC coupling in DMF at 0.5 M concentration for 2 h yields a substitution level of 0.45–0.55 mmol/g as quantified by Fmoc UV absorbance at 301 nm after piperidine treatment.

Structural Stability Under Acidic Cleavage Regimes

The methylpyrrole core remains intact under standard TFA cleavage cocktails (95% TFA, 2.5% TIS, 2.5% water) for up to 3 h at room temperature. Extended exposure beyond 4 h leads to trace amounts of sulfonation at the pyrrole 5-position when thioanisole is used as a scavenger; switching to anisole suppresses this side reaction. Quantitative HPLC-MS analysis of crude cleavage products from a test peptide bearing this residue at the N-terminus confirmed that >99% of the pyrrole remained unmodified when anisole replaced thioanisole in the cleavage mixture. Any residual scavenger-derived adducts were removed by trituration with cold diethyl ether. Following global deprotection and cleavage, reverse-phase purification on a C18 column (linear gradient of 10–60% acetonitrile in 0.1% TFA over 30 min) readily separates the full-length peptide from deletion sequences lacking the pyrrole residue. The characteristic retention time shift of approximately 2.5 min relative to the des-pyrrole analog serves as a diagnostic indicator for successful incorporation. When the monomer is used in fragment condensation approaches on a solid support, the low steric demand of the pyrrole 4-amino group allows efficient coupling of bulky incoming segments. Contrastingly, Fmoc-4-aminobenzoic acid derivatives, despite similar planarity, exhibit slower coupling rates (t_1/2 increased by a factor of 2–3) under identical conditions due to the higher electron-withdrawing effect of the benzene ring. This kinetic advantage is reflected in reduced overall cycle times during automated microwave-assisted synthesis, where the pyrrole building block reaches 99% coupling efficiency within 2 × 5 min couplings at 50 °C, compared to 3 × 5 min for the corresponding phenyl derivative.

Comparative coupling efficiency at 50 °C in DMF with 4 eq. Fmoc-amino acid, HATU, NMM

Monomer	Cycle 1 (%)	Cycle 2 (%)	Cycle 3 (%)	Total time (min)
Fmoc-4-amino-1-methylpyrrole-2-carboxylic acid	92	98.5	99.8	10
Fmoc-4-aminobenzoic acid	81	94		15
Fmoc-4-amino-1-methylimidazole-2-carboxylic acid	88	97	99.5	10

Data acquired on a Liberty Blue automated peptide synthesizer using Rink amide ChemMatrix resin (loading 0.47 mmol/g). Coupling percentages determined by Fmoc UV quantification after each cycle. All values are averages of triplicate runs. The compound’s behavior in high-throughput parallel synthesis libraries has been documented where the 1-methylpyrrole unit acts as a conformationally biased dipeptide mimetic. Library members incorporating this monomer showed an average clogP reduction of 0.3 log units compared to those with 4-aminobenzoic acid, attributable to the additional methyl substituent. This subtle lipophilicity modulation is often leveraged in lead optimization campaigns to fine-tune cellular permeability without incurring excessive molecular weight.

Regulatory and Quality Framework

Batch release documentation adheres to the requirements of ISO 9001:2015 (clause 8.6) and, when destined for GMP peptide APIs, to FDA 21 CFR 211.165(e) for testing of components. A certificate of analysis for a representative lot includes HPLC purity (method TM-427, reverse-phase gradient as above), ¹H and ¹³C NMR conformity (Bruker 400 MHz, DMSO-d₆), mass spectral identity (ESI-TOF, m/z 363.1345 ± 0.0015), and residual solvent analysis by GC-FID per USP <467>. The material is classified under REACH as a substance for product and process-oriented research and development (PPORD) with an annual tonnage band of 0.1–1 metric ton. No harmonized classification for environmental or human health hazards is assigned; however, standard PPE including nitrile gloves and safety goggles is mandated due to potential sensitization common among fluorenyl derivatives. Trace metals analysis by ICP-MS on three consecutive production lots returned values of Pd <10 ppm, Cu <5 ppm, Fe <20 ppm, and Ni <8 ppm, all below the ICH Q3D Option 1 limits for parenteral drug products. The synthetic route employs a palladium-catalyzed Suzuki coupling on the pyrrole scaffold before Fmoc installation; therefore, stringent Pd scavenging with trimercaptotriazine-functionalized silica is integrated into the purification train, yielding end-of-process Pd levels routinely below the limit of quantification (2 ppm).

Trace metal compliance against ICH Q3D Option 1 (oral PDE, μg/day)

Element	Class	PDE (μg/day)	Measured max. (ppm)	Concentration in typical peptide API* (ppm)
Pd	2B	100	8	0.04
Cu	3	1300	4	0.02
Ni	3	600	7	0.035

*Assumes 0.5% w/w of the monomer in the final peptide drug substance and a maximum daily peptide dose of 1 g. When evaluating the compound’s performance in peptide vaccine candidates, immunogenicity risk assessment followed the EMEA/CHMP guideline on assessment of non-clinical safety of oligonucleotide and peptide medicinal products. The 1-methylpyrrole residue, being a non-proteinogenic amino acid, was flagged in silico for potential MHC-II binding using NetMHCIIpan 4.0; however, no T-cell epitope clusters were predicted across common HLA-DR alleles at percentile rank <10. In vivo T-cell proliferation assays in HLA-DR4 transgenic mice confirmed a stimulation index <2, comparable to the vehicle control, supporting a low immunogenic risk profile for peptides incorporating this monomer at a single internal position. Data for this specific configuration in commercial-scale peptide production is limited to pilot-scale batches of 50–100 g; scaling to kilogram quantities maintains purity profiles provided that the Suzuki coupling is conducted in a 20 L jacketed glass reactor with controlled exotherm management (reaction mass temperature maintained at 55 ± 2 °C). Deviation beyond 58 °C results in debenzylation side products rising above 1.5%, which subsequently co-elute with the Fmoc-protected product and necessitate a second silica gel chromatography step, increasing processing cost by approximately 22%.