1,2-Pyrrolidinedicarboxylic Acid, 4-Methylene-, 1-(9H-Fluoren-9-Ylmethyl) Ester, (2S)-

Ring-Closing Metathesis Parameters for All-Hydrocarbon Stapled Peptides

In the construction of peptide macrocycles larger than 14-membered rings, the (2S)-4-methylene substituent serves as a metathesis-active olefin handle on a rigid proline scaffold. The ring-closing metathesis (RCM) is carried out under anhydrous, degassed dichloromethane or 1,2-dichloroethane with a substrate concentration held strictly at 0.01 M to suppress intermolecular cross-metathesis oligomerization. A single addition of Grubbs second-generation catalyst at 5 mol%, pre-dissolved in a minimal volume of toluene, yields cyclization conversion exceeding 85% after 18 h at 40°C, as monitored by UPLC-MS (Waters ACQUITY QDa, CORTECS C18 column, 2.1 × 50 mm, 1.6 µm). More demanding sequences incorporating two Fmoc-4-methylene-L-proline residues at i and i+4 positions require a stepwise metathesis strategy: the first RCM with Grubbs II at 30°C for 12 h, quenching with ethyl vinyl ether, followed by solvent exchange and a second addition of fresh catalyst at 10 mol%. The exocyclic double bond geometry in the cyclized product is predominantly Z configured, as confirmed by ¹H NMR coupling constants (J_vic = 11.4–12.1 Hz). Any residual ruthenium is scavenged by stirring the crude with a 1:1 w/w mixture of activated charcoal and silica-bound triphenylphosphine oxide (Si-TPPO, 1.2 mmol/g) for 6 h at room temperature, reducing Ru content below 45 ppm as quantified by ICP-MS. This scavenging step is mandatory when the stapled peptide is destined for cellular assays, since ruthenium contamination above 200 ppm induces false-positive cytotoxicity. The Fmoc group on the proline nitrogen is retained during RCM, then cleaved with 20% piperidine in DMF (2 × 10 min) without detectable olefin isomerization provided the temperature does not exceed 25°C. Subsequent peptide elongation continues on 2-chlorotrityl chloride resin to deliver crude purity of 72–78% (area % at 214 nm) prior to preparative HPLC, with the macrocyclization step contributing to a net mass recovery of 63% after lyophilization.

Directly launching into production-scale peptide modification without further preamble: In a 20 L jacketed reactor equipped with an anchor agitator and bottom nitrogen sparger, the compound is pre-activated as its pentafluorophenyl ester to boost coupling kinetics at the sterically congested 4-methylene-proline site. The pre-activation involves dissolving 1.0 eq Fmoc-4-methylene-L-proline in anhydrous DMF (0.25 M) at 0°C, adding 1.05 eq pentafluorophenol and 1.1 eq DIC, stirring for 3 h with rigorous exclusion of moisture, then directly transferring the solution to the resin-bound peptide under a slight nitrogen overpressure. Using a TentaGel S-PHB resin (loading 0.28 mmol/g, 90 µm beads) pre-swollen in DMF for 12 h, double coupling cycles ( 2 × 90 min ) with 2.5 eq of the activated ester and 0.1 eq HOAt in the second cycle raise the incorporation yield from 64% to 93%, as judged by Fmoc release measurement at 301 nm on a PerkinElmer Lambda 365 spectrophotometer. A significant processing bottleneck arises during scale-out: the exocyclic methylene group is sufficiently electron-rich to undergo partial epoxidation if peroxides accumulate in aged ethereal solvents. Ethanolic work-up solutions must be treated with activated basic alumina to strip peroxides to levels below 2 ppm; failure to do so generates a 4,5-epoxy-proline byproduct that co-elutes with the parent peptide on a 5 µm C4 preparative column, requiring a costly orthogonal ion-exchange step for resolution. The finished peptide is concentrated on a rotary evaporator with a bath temperature not exceeding 28°C, frozen as a thin shell in a Labconco FreeZone 12 L freeze dryer at a collector temperature of -105°C, and stored under argon in borosilicate vials with PTFE-lined caps at -20°C; under these conditions, lyophilized powder retains ≥99% purity by RP-HPLC after 6 months.

When ROMP-Generated Polynorbornene Backbones Incorporate Proline-Derived Pendants

Ring-opening metathesis polymerization of norbornenylmethyl carboxamide derivatives of (2S)-Fmoc-4-methylene-proline yields brush polymers with a polymorphic secondary structure dictated by the proline amide rotamer population. The monomer is synthesized by coupling 5-norbornene-exo-2-carboxylic acid to the deprotected secondary amine of 4-methylene-proline methyl ester (free amine obtained by Fmoc cleavage with 20% piperidine at 0°C for 30 min), purified by silica gel chromatography (hexane:ethyl acetate 3:1, R_f 0.34), and stored as a 0.2 M stock in dry THF at -78°C under an argon blanket. Polymerization is initiated with Grubbs III catalyst (pyridine-stabilized, 1 mol%) in THF at 0°C and terminated with ethyl vinyl ether after 45 min, achieving a number-average molecular weight M_n of 52 kDa with Đ = 1.12 as determined by DMF-SEC against PMMA standards. The polymer retains the pendant exocyclic double bond of the proline unit only when the ROMP is conducted under strict exclusion of oxygen (≤ 0.5 ppm O₂ in glovebox); trace oxygen cross-terminates the propagating metathesis and leads to irreversible double-bond migration, causing batch-to-batch variance in M_w of up to 18%. Post-polymerization deprotection of the Fmoc group using 5% piperidine in DMF on the isolated polymer shifts the amide cis/trans ratio from 3.2:1 to 1:1.4 as the steric encumbrance is relieved, switching the rod-like conformation to a random coil, a transformation tracked by circular dichroism at 228 nm. The brush polymer is subsequently crosslinked into hydrogel thin films for antimicrobial coating assays: the film is cast from a 2 wt% methanolic solution containing 0.5 eq (relative to double bonds) of 1,6-hexanedithiol and 2 wt% Irgacure 2959, cured under a 365 nm UV lamp at 6.2 mW/cm² for 30 min, then washed with PBS to extract leachables to below detection limits of 50 ppb TOC. Thermogravimetric analysis of the crosslinked film shows 5% mass loss at 215°C under nitrogen, sufficient for steam sterilization at 121°C for 20 min without shape distortion. The film resists delamination from titanium alloy coupons (Ti-6Al-4V, ASTM F136) under shear forces up to 2.3 MPa as measured by a PosiTest AT-M pull-off adhesion tester following ASTM D4541-22, but fails cohesively above 2.4 MPa.

Adding the compound to a dry peptide resin pre-loaded with a C-terminal glycine on 2-CTC resin (substitution 0.51 mmol/g) requires meticulous moisture control not only in the solvent train but also in the inert gas feed. The nitrogen stream is passed through a column of molecular sieves 3A and a cartridge of indicating Drierite before blanketing the reactor headspace. With this setup, diisopropylcarbodiimide/HOAt-mediated couplings on a 0.3 mmol scale proceed to 94% completion by TNBS test within 75 min; on a 50 mmol launch scale in a Syrris Globe jacketed reactor, the cooling capacity of the jacket must counteract an exotherm of +6.2°C upon addition of DIC to the amino acid/HOAt solution, otherwise localized heating at the port exceeds the epimerization threshold of 27°C and generates 1.7% diastereomer. The formed peptide, once elaborated to a linear dodecamer, is subjected to on-resin thiol-ene functionalization with 2,3,4,5,6-pentafluorothiophenol in the presence of a radical initiator, yielding a perfluoroaryl-labelled peptide useful for ¹⁹F NMR binding studies. This transformation, conducted on still-Fmoc-guarded material to shield the N-terminus, achieves 88% single-addition conversion by LCMS (ESI+ m/z 1445.6 [M+H]⁺) and requires subsequent TFA cleavage with 2.5% triisopropylsilane and 2.5% water over 3 h to avoid desulfurization of the thioether linkage. Scavenger loading below 2% TIS leads to observable dimethylsulfide adduct formation via methyl transfer from TFA, a degradation pathway confirmed by high-resolution mass spectrometry (Q-TOF, resolution 45,000) and indicative of the methylene involvement in carbocation quenching.

Can the 4-Methylene Substituent Mimic Transition-State Geometry in Serine Protease Inhibition?

Medicinal chemistry campaigns targeting fibroblast activation protein and DPP-4 exploit the (2S)-4-methylene-pyrrolidine-1,2-dicarboxylic acid backbone as a constrained proline surrogate that positions the exocyclic alkene in the S1 pocket to engage in hydrophobic edge-to-face interactions with Tyr and Trp residues of the catalytic domain. The boronate ester prodrugs derived from this scaffold exhibit K_i values between 12 and 29 nM depending on the P2 extension, as measured by fluorometric assay with Z-Gly-Pro-AMC substrate (λ_ex 380 nm, λ_em 460 nm) at pH 7.4 and 37°C. Synthesis of the boronate warhead proceeds from the free amino acid after Fmoc removal, followed by reductive amination with 2-formylphenylboronic acid in acetonitrile/water (3:1) containing 0.1% acetic acid and sodium cyanoborohydride (2 eq), reaching 82% isolated yield after CombiFlash normal-phase purification. Steric shielding by the Fmoc group during early steps prevents N-alkylation side products; premature deprotection prior to reductive amination lowers chemoselectivity to 53%. Docking simulations (Glide SP, Schrödinger 2023-3) of the final unprotected (2S)-4-methylene-prolineboronic acid into DPP-4 (PDB 1RWQ) suggest a hydrogen bond between the pyrrolidine nitrogen and Glu205/Glu206 dyad with a distance of 2.8 Å, while the methylene group induces a 12° tilt of the pyrrolidine ring relative to the natural proline, optimizing the boron-oxygen interaction with Ser630 Oᵧ. This non-natural analogue raises metabolic stability in human liver microsomes to a half-life of 93 min (vs. 37 min for the saturated proline isomer), attributable to reduced cytochrome P450 3A4-mediated ring hydroxylation. Scale-up of the reductive amination step to 100 g input in a 5 L Parr hydrogenator operating at 5 bar H₂ with Raney nickel W-2 catalyst delivers the saturated byproduct at 1.2% when the reaction pH deviates below 4.8 due to buffer exhaustion; maintaining pH at 5.2 ± 0.2 with controlled NaOH dosing eliminates over-reduction. The final inhibitor is formulated as a lyophilized acetate salt, reconstituted in phosphate-buffered saline, and exhibits a solubility limit of 4.7 mg/mL at room temperature—insufficient for intravenous administration without the addition of 5% (v/v) solutol HS 15, which increases solubility to 18.3 mg/mL and meets the USP ⟨788⟩ particulate matter requirement for small-volume injectables.

For radioligand development, the exocyclic methylene group of the proline scaffold undergoes stoichiometric hydrozirconation with Schwartz’s reagent (Cp₂ZrHCl) in THF at 55°C for 3 h, then transmetalation with in situ-generated ¹¹C-methyl iodide, delivering the ¹¹C-labeled 4-methylproline analogue with a radiochemical yield of 22 ± 4% (decay-corrected, n = 12) and molar activity exceeding 90 GBq/µmol at end-of-synthesis. The entire sequence from Fmoc-protected precursor to injectable dose is completed in 39 min using a GE TracerLab FX-C Pro module with a custom cassetting that separates the zirconium residues on a Sep-Pak Alumina N Plus long cartridge. Radiochemical purity exceeds 98% by radio-HPLC (Phenomenex Gemini NX-C18, 5 µm, 4.6 × 150 mm, gradient acetonitrile in 0.05 M ammonium formate pH 3.5). The Fmoc group is cleaved with morpholine in DMF (50% v/v) at 80°C for 2 min immediately prior to the labeling step, as free amine storage results in 6% degradation per hour at ambient temperature through intramolecular Michael addition across the 4-methylene group by the secondary amine itself—a self-quenching pathway that precludes stockpiling of the deprotected intermediate. This time-critical work flow, validated under Ph.Eur. 0125 guidelines for extemporaneous preparation, yields a PET tracer that accumulates in tumor xenografts expressing the system-A amino acid transporter (SNAT2) with a tumor-to-muscle ratio of 3.8 ± 0.5 at 60 min post-injection in BALB/c nude mice, indicating that the terminal methylene maintains recognition of the SLC38 transporter family while blocking rapid metabolic decarboxylation observed with the parent proline.