2,5-Dihydro-1H-Pyrrole-2-Carboxylic Acid
|
HS Code |
264214 |
| Name | 2,5 - Dihydro - 1H - Pyrrole - 2 - Carboxylic Acid |
| Molecular Formula | C5H7NO2 |
| Molar Mass | 113.115 g/mol |
| Appearance | Solid (expected, no common color data given without further context) |
| Physical State At Stp | Solid |
| Solubility In Water | Limited solubility expected (polar group but non - polar ring) |
| Melting Point | No common data without further source |
| Boiling Point | No common data without further source |
| Pka | No common data without further source |
| Density | No common data without further source |
| Flash Point | No common data without further source |
| Chemical Class | Pyrrole - carboxylic acid derivative |
As an accredited 2,5-Dihydro-1H-Pyrrole-2-Carboxylic Acid factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | 500g of 2,5 - Dihydro - 1H - Pyrrole - 2 - Carboxylic Acid packaged in air - tight plastic bags. |
| Shipping | 2,5 - Dihydro - 1H - pyrrole - 2 - carboxylic acid is shipped in well - sealed containers, compliant with chemical transport regulations. Packaging ensures protection from moisture and physical damage during transit to prevent any chemical degradation or safety risks. |
| Storage | 2,5 - Dihydro - 1H - pyrrole - 2 - carboxylic acid should be stored in a cool, dry place. Keep it in a well - sealed container to prevent moisture absorption and contact with air, which could potentially lead to degradation. Avoid storing near heat sources or flammable materials. Store away from incompatible substances to prevent chemical reactions. Ideal storage conditions help maintain its chemical integrity. |
In commercial L-proline synthesis from the pyrroline precursor, a 2.0 m³ glass-lined steel hydrogenator (Pfaudler BE series) equipped with a magnetically driven Ekato Phasejet hollow-shaft impeller is charged with a 14 wt% aqueous slurry of 2,5-dihydro-1H-pyrrole-2-carboxylic acid. The pH is adjusted to 4.0 ± 0.2 with 1.0 M hydrochloric acid—values falling below 3.5 trigger irreversible ring‑opening to 2‑aminopentanal that manifests as an off‑note carbonyl stretch at 1722 cm⁻¹ on a Mettler Toledo ReactIR 15 inline probe, while excursions above 4.5 suppress the hydrogenation rate by 40% and elevate the trans‑3‑hydroxyproline byproduct to 0.7–1.2 area%. 5% Pd/C (Johnson Matthey type 487, 3.8% w/w relative to dry substrate) is suspended through the reactor’s gas‑entrainment hollow shaft at a tip speed of 4.2 m/s to overcome the H₂ mass‑transfer boundary layer. Hydrogen pressure is held at 3.8–4.2 bar g while the batch temperature is ramped from 20°C to 68°C over 45 min to avoid an exothermic runaway. Completion is reached in 5–7 h, signalled by disappearance of the imine absorption at 1632 cm⁻¹. Post‑reaction, the catalyst cake is removed by depth filtration through a 0.5 µm polypropylene plate under N₂ blanket; the clarified filtrate is concentrated in a wiped‑film evaporator (jacket 55°C) to 40–45% solids. L‑Proline crystallises upon cooling to 2°C and adjusting to the isoelectric point (pH 6.0) with 25% ammonium hydroxide, followed by dewatering in a Heinkel peeler centrifuge and vacuum drying at 60°C/30 mbar. The crystalline bulk drug intermediate meets USP/NF 2023 and EP 10.0 monographs: specific rotation (c=2, water) −85.0° to −86.0°, loss on drying ≤0.2%, individual unspecified impurities ≤0.10% per USP <621> chromatographic purity method. Elemental impurity control follows ICH Q3D Table A.2.2 with palladium limited to ≤10 µg/day oral PDE; residual Class 2 solvents are verified by headspace GC against USP <467> Option 1 limits. This route supplies L‑proline as a parenteral amino acid active pharmaceutical ingredient intermediate and as a chiral starting material for enalapril maleate manufacture.
Why Does Dhp-Containing Peptidomimetics Require TIS as a Carbocation Scavenger During Cleavage?The 2,5‑dihydro‑1H‑pyrrole‑2‑carboxylic acid residue is introduced as Fmoc‑Dhp‑OH on a CEM Liberty Blue 2.0 microwave‑assisted solid‑phase peptide synthesizer. Resin substitution level of Fmoc‑PAL‑PEG‑PS is standardised to 0.47 mmol/g; coupling stoichiometry is maintained at 0.95–1.10 equivalents relative to the free amino terminus. Activation employs 0.45 M HATU with 0.6 M 2,4,6‑trimethylpyridine in DMF, delivering a first‑cycle coupling efficiency of 91–94%—lower than standard proteinogenic residues due to the steric bulk of the unpuckered pyrroline ring. A double‑coupling protocol (4 min at 75°C then 6 min) is mandatory; switching to 0.5 M PyOxim with 0.2 M Oxyma pure pushes coupling efficiency above 98% as monitored by Fmoc deprotection UV absorbance at 301 nm. Assembly proceeds with iterative N‑terminal Fmoc removal (20% piperidine/DMF, 3 × 2 min) and subsequent amino‑acid couplings. Following chain completion, the resin‑bound protected peptide is dried under vacuum before acidolytic global deprotection. The cleavage cocktail (95% TFA, 2.5% water, 2.5% triisopropylsilane) is critical: the vinylogous enamine character of the 2,5‑dihydropyrrole ring generates a resonance‑stabilised carbocation upon TFA attack that otherwise undergoes irreversible intermolecular alkylation of tryptophan indole and tyrosine phenol nucleophiles, yielding +340 Da dimeric adducts detectable by LC‑MS if TIS is omitted. After precipitation in cold diethyl ether, the crude peptidomimetic is purified by preparative RP‑HPLC on a C18 column with a 0.1% TFA/acetonitrile gradient and lyophilised. The conformational restriction imparted by the Dhp residue is exploited in biased agonist programs for the GLP‑1 receptor. Applicable regulatory framework for peptide APIs includes ICH Q7 Section 5.2 (dedicated equipment provisions) and residual TFA controlled to ≤0.1% by ion chromatography per USP <466>. Heavy‑metal limits align with ICH Q3D Table A.2.2: Pd ≤10 µg/day when the peptide is destined for clinical parenteral supply.
Synthesizing Captopril Analogs: Mixed Anhydride Coupling with the Pyrroline Carbonyl2,5‑Dihydro‑1H‑pyrrole‑2‑carboxylic acid serves as the N‑terminal pyrroline fragment in angiotensin‑converting enzyme inhibitor assembly. The mixed‑anhydride activation proceeds at −15°C to −10°C in anhydrous tetrahydrofuran using isobutyl chloroformate (1.05 eq) and N‑methylmorpholine (1.10 eq). The addition ratio of the activated acid to the nucleophile—3‑mercapto‑2‑methylpropanoyl‑L‑proline methyl ester—is held at 1.00:1.02 molar; a tighter stoichiometry increases symmetrical anhydride formation and reduces yield by 8–12%. After 15 min activation, the nucleophile is dosed over 45 min, and the reaction mass is warmed to 0°C over 90 min. Phase transfer to dichloromethane followed by extraction with 5% aqueous sodium bicarbonate removes unreacted acid. The diastereomeric thioester is crystallised from isopropanol/water (80:20 v/v) to afford a de exceeding 99.2%, confirmed by chiral supercritical fluid chromatography on a Chiralpak IA column (CO₂/methanol 85:15, 3 mL/min, 40°C). Subsequent hydrogenolytic deprotection of the thioester and basic ester hydrolysis yields the active pharmaceutical ingredient alacepril. Quality oversight follows ICH Q3A thresholds: any unspecified impurity ≥0.10% triggers identification by LC‑HRMS; specified degradation products are qualified at 0.15%. Residual solvent compliance with USP <467> keeps tetrahydrofuran below 720 ppm and dichloromethane below 600 ppm. The final drug substance must meet the PhEur monograph requirement for specific optical rotation +29.0° to +33.0° (c=1, ethanol), illustrating the downstream chiral integrity demanded of the pyrroline intermediate. Full ICH Q11 development history and GMP supply chain documentation are expected when this intermediate is sourced for commercial drug manufacturing.When deployed as a chiral organocatalyst precursor in enantioselective intermolecular aldol additions, (S)‑2,5‑dihydro‑1H‑pyrrole‑2‑carboxylic acid is converted to the corresponding α,α‑diphenylprolinol trimethylsilyl ether following the Jørgensen–Hayashi framework. The sequence—esterification with methanolic SOCl₂, Grignard addition with phenylmagnesium bromide, and TMS protection—is carried out without isolating the free amino alcohol to suppress epimerisation at the α‑carbon. Catalyst loading is typically 10 mol% relative to the acceptor aldehyde (0.5 mmol scale). In a model reaction between 4‑nitrobenzaldehyde and cyclohexanone, the silyl ether (0.2 M in toluene) at −10°C furnishes the syn‑aldol adduct with 84:16 dr and 88% ee (Chiralpak AD‑H, hexane/2‑propanol 90:10, 1.0 mL/min). Aqueous ammonium chloride quench permits catalyst recovery by extraction and silica chromatography in 93% yield; reuse is restricted to 3 cycles because the pyrroline ring undergoes gradual oxidation to pyrrole—identified by a bathochromic UV shift at 280 nm—that quenches enamine catalysis. The chiral β‑hydroxy ketone product interconverts to the pitavastatin side chain after two reductive steps. Process‑scale operation adheres to ISO 14001:2015 with closed‑loop toluene/THF recovery (atmospheric emissions <20 mg/m³) and occupational exposure limits for hexamethyldisilazane set at 5 ppm (TWA) per OSHA 29 CFR 1910.1000. Although no pharmacopoeial monograph applies to the isolated intermediate, residual silicon is controlled to ≤50 ppm by ICP‑OES to prevent catalyst fouling in downstream hydrogenation segments.When Sequential Chemoenzymatic Routes Deliver Higher Enantiomeric Excess than Direct FermentationProduction of (2S,4R)‑4‑hydroxyproline at the 100 kg scale begins by deliberate suppression of stereoselectivity during the chemo‑catalytic hydrogenation of 2,5‑dihydro‑1H‑pyrrole‑2‑carboxylic acid to racemic proline. Substrate concentration is elevated to 18 wt% and 5% Pd/C loading is increased to 5.5% w/w; the pH is left unbuffered at 6.8–7.0 to favour a non‑diastereoselective surface‑hydrogenation pathway, delivering an equimolar L/D mixture. After catalyst filtration and pH neutralisation, the solution is supplemented with 0.4% v/v glycerol and 0.1% w/v yeast extract and transferred to a 30 L fed‑batch bioreactor (Sartorius Biostat C‑plus) containing recombinant Escherichia coli BL21(DE3) expressing L‑proline trans‑4‑hydroxylase (accession BAA09994.1) under a temperature‑inducible λpL promoter. Biotransformation runs at 32°C, dissolved oxygen held at 30% air saturation, and pH statically controlled at 7.2 with 28% ammonium hydroxide. Substrate feeding at 0.6 g/L/h avoids enzyme substrate inhibition; total residence time is 48 h. The whole‑cell process simultaneously resolves the racemate and hydroxylates the C‑4 position with ≥99.5% stereospecificity, leaving D‑proline unreacted as a recoverable byproduct. Downstream purification employs Dowex 50WX8 (NH₄⁺ form) ion‑exchange chromatography with elution by 2 N ammonia, followed by crystallisation from ethanol to yield (2S,4R)‑4‑hydroxyproline exceeding 99.9% chiral purity. The product conforms to the USP Hydroxyproline monograph and satisfies ICH Q3D elemental impurity limits, with special monitoring of aluminium (≤5 ppm) leached from the reactor vessel. This chemo‑enzymatic sequence circumvents the 8–12 week fermentation lead time and 5–8% epimerisation characteristic of large‑scale Corynebacterium glutamicum processes. Bioprocess regulatory oversight includes EMA/CVM/ICH Q5A viral safety assurance when cell‑derived intermediates are used in subsequent synthesis of pharmaceutical actives.Thermal Decarboxylation‑Dehydrogenation to Electronic‑Grade Pyrrole in Continuous Flow2,5‑Dihydro‑1H‑pyrrole‑2‑carboxylic acid is thermally decarboxylated and subsequently dehydrogenated in a two‑zone continuous flow reactor constructed from 316L stainless steel tubing (4 mm ID, 2 m length) packed with inert 2 mm borosilicate glass beads. A 1.0 M solution of the pyrroline acid in ethylene glycol is introduced at 0.5 mL/min using a syringe pump and enters the first zone heated to 190°C; a residence time of 180 s ensures complete decarboxylation, releasing CO₂ and forming 3‑pyrroline quantitatively. The stream passes directly into a second zone packed with 0.5% Pd/Al₂O₃ pellets maintained at 270°C under N₂ spiked with 2% v/v O₂ as hydrogen acceptor, achieving 98% conversion of 3‑pyrroline to pyrrole. The gaseous product is condensed in a −10°C trap and purified by fractional distillation at 100 mbar (head temperature 45°C) to isolate pyrrole with purity exceeding 99.95% by GC‑FID on a 0.25 µm dimethylpolysiloxane column. The electronic‑grade pyrrole is suitable for electropolymerisation onto interdigitated microelectrode arrays, conductive textile yarns, and electrochemical sensor coatings. Metal contamination control complies with SEMI C38‑0322 process chemical specifications: individual metallic elements ≤5 ppb, verified by triple‑quadrupole ICP‑MS. Export shipments must meet RoHS Directive 2011/65/EU Annex II substance restrictions and trigger REACH Article 33 communication obligations for substances of very high concern potentially formed during synthesis. Feedstock consumption accounts for decarboxylation and distillation losses: 1.0 kg of the pyrroline acid yields approximately 0.65 kg of distilled product. |
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2,5-Dihydro-1H-pyrrole-2-carboxylic acid (IUPAC; CAS 3076-38-0)—also referenced in biochemical literature as 3,4-dehydroproline—is a cyclic imino acid of molecular formula C5H7NO2 and molecular weight 113.11 g/mol. Its five-membered pyrroline ring is distinguished from the fully saturated proline scaffold by a single carbon–carbon double bond between C2 and C3, imposing a near-planar geometry at the annulated nitrogen and eliminating the ring-puckering dynamics characteristic of proline. The compound is supplied as a free amino acid zwitterion, as the hydrochloride salt (CAS 3396-19-4), or as N-protected derivatives—most commonly N-tert-butoxycarbonyl (Boc) and N-(9-fluorenylmethoxycarbonyl) (Fmoc)—for use in solution-phase and solid-phase peptide synthesis. Unlike ubiquitous proteinogenic proline, the 2,5-dihydro analogue resists metabolic hydroxylation by prolyl hydroxylase domain enzymes, making it a mechanistically informative probe in hypoxia-inducible factor (HIF) signalling studies and a scaffold for collagen cross-linking inhibitors.
The sp2-hybridised C2 and C3 centres enforce a rigid, essentially flat pyrroline ring. Where proline populates well-characterised Cγ-exo and Cγ-endo puckers with an interconversion barrier of approximately 1.5–3 kcal/mol, the endocyclic olefin of 2,5-dihydro-1H-pyrrole-2-carboxylic acid quenches this motion. X-ray crystallographic coordinates deposited in the Cambridge Structural Database show a mean ring torsion angle |ω| ≤ 5°, producing a dihedral angle φ that is constrained to approximately −60° (±3°). This electronic and steric environment shifts the trans/cis amide equilibrium in acyl-amino acid model esters: 1H NMR coalescence experiments in D2O at 298 K indicate that the cis conformer population of Ac-3,4-dehydroproline-OMe falls below 5%, whereas the corresponding proline derivative exhibits 12–15% cis isomer under identical conditions. The consequence for peptide secondary structure is a strong bias toward polyproline type II (PPII) helices and an attenuated propensity to nucleate β-turns. From a pharmacokinetic standpoint, the C2–C3 unsaturation renders the α-carbon less susceptible to dehydrogenation by D-amino acid oxidase, increasing metabolic half-life in renal homogenate assays by a factor of 3–4 relative to L-proline, as reported in comparative microsomal stability profiling.
In solid-phase peptide synthesis (SPPS) following standard Fmoc/tBu protocols on aminomethyl ChemMatrix® or polystyrene-divinylbenzene resins, the acylation of the sterically hindered secondary amine of Fmoc-2,5-dihydro-1H-pyrrole-2-carboxylic acid exhibits biphasic kinetics. Coupling completion, monitored by the Kaiser (ninhydrin) test and chloranil assay, often requires double or triple activation. A validated coupling cycle uses 2.0 equiv of the Fmoc-amino acid pre-activated with 1.95 equiv of HATU and 2.0 equiv of ethyl (hydroxyimino)cyanoacetate (Oxyma Pure) in DMF, in the presence of 4.0 equiv of N-methylmorpholine (NMM) at 25 °C for 45 min, followed by a second identical coupling after a DMF wash. Base-labile impurities resulting from diketopiperazine formation are minimised by maintaining the resin-bound amine in the free-base form only immediately before coupling; the Fmoc protecting group is removed with 20% (v/v) piperidine in DMF containing 0.1 M HOBt as a racemisation suppressant. Epimerisation at the α-carbon—exacerbated by the electron-withdrawing olefin—is held below 0.5% when the coupling temperature is kept at 15–20 °C and the pre-activation time with HATU does not exceed 90 s. Crude peptide cleaved with TFA/TIS/H2O (95:2.5:2.5, v/v/v, 2 h at 25 °C) is analysed by UPLC-UV on a Waters ACQUITY BEH C18 1.7 µm, 2.1×50 mm column, with a 5–60% acetonitrile gradient in 0.1% TFA over 8 min, detection at 214 nm. The dehydroproline-containing peptide typically elutes 0.4–0.8 min later than the saturated proline congener, a direct consequence of the increased molecular planarity.
Commercially sourced racemic 2,5-dihydro-1H-pyrrole-2-carboxylic acid is resolved into its D- and L-enantiomers (CAS 134419-63-5 and 134419-64-6, respectively) by preparative chiral HPLC on a Chiralpak ZWIX(+) column, 3 µm particle size, 150×4.6 mm I.D., thermostatted at 25 °C. The zwitterionic chiral selector enables elution with a mobile phase composed of MeOH/H2O/HCOOH (90:10:0.1, v/v/v) at a flow rate of 1.0 mL/min, affording baseline separation with a selectivity factor α of 1.12–1.18. Analytical control of enantiomeric excess (ee) employs the same stationary phase in isocratic mode, with UV detection at 210 nm; lot-release specifications stipulate ee > 99.0% for the L-isomer and ee > 98.5% for the D-isomer. Optical rotation [α]D20 for L-2,5-dihydro-1H-pyrrole-2-carboxylic acid (free amino acid, c = 1.0 in H2O) is referenced at −165°±2°, while the D-antipode records +163°±2°. Enzymatic kinetic resolution represents an alternative at scale: immobilised Candida antarctica lipase B (CAL-B) in methyl tert-butyl ether with vinyl acetate as acyl donor preferentially acetylates the hydroxyl-free amine of one enantiomer, leaving the opposite enantiomer untouched with ee > 97% after 48 h at 30 °C, though residual enzyme carryover must be removed by silica gel filtration.
| Derivative Form | CAS Number | MW (g/mol) | Appearance | Purity (HPLC, % Area) | Enantiomeric Excess (%) | Storage (°C) |
|---|---|---|---|---|---|---|
| Free amino acid | 3076-38-0 | 113.11 | White to off-white crystalline powder | ≥ 98.5 | ≥ 99.0 | −20, desiccated |
| Hydrochloride salt | 3396-19-4 | 149.57 | White crystalline powder | ≥ 99.0 | ≥ 99.0 | −20 |
| N-Boc | 69737-89-3 | 213.23 | White foam or powder | ≥ 97.0 | ≥ 99.0 | −20, under N2 |
| N-Fmoc | 144263-22-3 | 335.36 | Off-white amorphous solid | ≥ 96.5 | ≥ 99.0 | −20, moisture-free |
The enamine character of the 2,5-dihydropyrrole ring renders the compound susceptible to acid-catalysed hydration and oxidative degradation. Accelerated stability studies (ICH Q1A guidelines) performed in 50 mM phosphate buffers at 40 °C reveal a pH-dependent half-life (t½): at pH 7.4, t½ is 14 days; at pH 3.0, degradation accelerates sharply with t½ < 48 h, evolving the ring-opened 2-keto-4-aminopentanoic acid as the primary hydrolytic product, identified by LC-MS (m/z 132.1 [M+H]+). Under dissolved oxygen concentrations greater than 2 ppm, radical-initiated oxidation at the allylic C5 position generates a conjugated imine that undergoes rapid dimerisation, evidenced by a colour shift from colourless to pale yellow. Consequently, formulation efforts involving the free amino acid demand strict deoxygenation of the medium and the inclusion of 0.1% (w/v) L-ascorbic acid as a sacrificial antioxidant. Lyophilised solid is packaged in amber serum vials under argon headspace (O2 < 0.5%) with activated molecular sieve sachets; water content (Karl Fischer titration) upon opening must not exceed 0.5% (w/w). The compound is incompatible with strong nucleophiles at elevated temperature, particularly primary amines and thiols, which can undergo conjugate addition to the α,β-unsaturated iminium tautomer generated in mildly acidic milieu. Handling at ambient relative humidity > 60% induces rapid moisture uptake (> 2% weight gain in 30 min), mandating use within a nitrogen-purged glovebox for prolonged weighing operations.
Pilot-scale isolation of the hydrochloride salt from a synthetic route commencing with acrolein and glycine ethyl ester proceeds via a step that precipitates the product upon sparging anhydrous HCl gas into a methanol/toluene (1:3, v/v) solution chilled to 0–5 °C. The exotherm is controlled by maintaining the HCl addition rate below 10 g/h per litre of solution in a 50 L Hastelloy C-22 jacketed reactor equipped with a gas dispersion impeller; jacket temperature is held at −10 °C to compensate for heat release. The resulting suspension is transferred through a pressure-rated Nutsche filter (Bonfilt 0.5 m2) overlaid with 0.2 bar nitrogen, washed with cold, peroxide-free MTBE (3 × 2 L), and discharged into a Krauss–Maffei vacuum tray dryer. Drying under a gradual ramp from 25 °C to 40 °C at 0.5 °C/min, with a final vacuum of ≤ 5 mbar for 8 h, reduces residual methanol and MTBE to ≤ 0.08% (w/w) as determined by headspace GC-FID (USP <467>, Method IV). Sieve analysis (ASTM C136) of three successive kilo-scale batches shows a volume mean particle diameter (D[4,3]) ranging from 95 µm to 127 µm; inter-batch variability is attributed to the seeding procedure during crystallisation, which employs 0.5% (w/w) micronised seed crystals prepared by jet milling to a D50 of 8–10 µm.
L-2,5-Dihydro-1H-pyrrole-2-carboxylic acid acts as a competitive substrate analogue for the 2-oxoglutarate-dependent prolyl hydroxylases (PHD1–3). In an in vitro enzyme assay employing the truncated catalytic domain of human PHD2 (residues 181–426, expressed in E. coli and purified via Ni-NTA), the compound competes with the hypoxia-inducible factor 1α (HIF-1α) peptide substrate (residues 556–574, biotinylated C-terminus) at a fixed 10 µM 2-oxoglutarate concentration. Reaction mixtures containing Fe(NH4)2(SO4)2 (100 µM), ascorbate (1 mM), and the peptide (5 µM) in Tris buffer (50 mM, pH 7.5) are incubated at 37 °C for 20 min. Hydroxylation at Pro564 is quantified by MALDI-TOF MS in reflectron positive mode using α-cyano-4-hydroxycinnamic acid matrix; the +16 Da mass shift is integrated against an internal standard peptide containing 4-hydroxyproline. Under these conditions, L-dehydroproline yields an IC50 of 85–120 µM, while the D-isomer is essentially inactive (IC50 > 500 µM). By comparison, the clinical HIF prolyl hydroxylase inhibitor roxadustat (FG-4592) exhibits an IC50 of 20–40 nM, highlighting that the unadorned dehydroproline core provides only modest enzyme affinity. Nevertheless, its incorporation into collagen model peptides (Pro-Hyp-Gly)n replacing one proline residue reduces triple-helix melting temperature (Tm) by 6–8 °C per substitution, as determined by circular dichroism at 225 nm, an effect attributed to backbone rigidification and loss of optimal main-chain hydration. This property is exploited in competitive inhibition studies of lysyl oxidase-mediated crosslinking in decellularized dermal matrices; incubation with 1 mM dehydroproline for 48 h reduces pyridinoline crosslink formation (HPLC-fluorescence, Ex 295 nm, Em 395 nm) by approximately 35% relative to untreated controls.
| Analogue | Ring Puckering | φ Dihedral Restriction (°) | Approx. Cis Amide (%) in Ac-X-OMe | PHD2 Inhibition (IC50) |
|---|---|---|---|---|
| L-Proline | Endo/exo dynamic | −65 ± 10 | 12–15 | Not competitive |
| L-4(R)-Hydroxyproline | Cγ-exo preference | −55 ± 8 | 9–12 | Not applicable |
| (2S,4R)-4-Fluoroproline | Cγ-exo locked | −55 ± 3 | 14–18 | Weak |
| L-2,5-Dihydroproline | Planar (|ω| ≤ 5) | −60 ± 3 | < 5 | 85–120 µM |
Replacement of a canonical proline with 2,5-dihydro-1H-pyrrole-2-carboxylic acid in bioactive peptide sequences pre-organises the backbone in a manner that reduces the entropic penalty upon receptor binding. In the development of constrained cyclic pentapeptide CXCR4 antagonists, substitution of Pro3 with the dehydroproline scaffold increased binding affinity (IC50 shift from 350 nM to 120 nM) measured by competitive displacement of 125I-SDF-1α on CEM T-lymphoblastoid cells, with NMR solution structures (DQF-COSY, ROESY in DMSO-d6) confirming that the dehydro residue enforces a type VIb β-turn stabilised by a backbone–backbone hydrogen bond observed as a downfield amide proton signal at δ 8.45 ppm. This conformational pre-commitment is not achievable with 4-fluoroproline, which enhances cis isomerism and can destabilise certain turn motifs. Moreover, the absence of ring-puckering dynamics eliminates the microsecond-timescale exchange broadening detectable in 13C CPMG relaxation dispersion experiments, yielding sharper linewidths and more reliable integration in 1H–13C HSQC spectra for protein-binding epitope mapping. The synthetic utility of the 2,5-dihydro modification extends to proteolytic stability: incubation of a model dehydroproline-containing nonapeptide with human plasma at 37 °C for 24 h shows 82% remaining intact parent, compared to 54% for the all-proline control, as determined by RP-HPLC peak area relative to a 4-sulfamoylbenzoic acid internal standard. Published data for the full spectrum of peptidomimetic applications continues to expand, though systematic SAR studies across diverse GPCR families remain incomplete.
