Benzyl3-(Tert-Butoxycarbonylamino)-4-(Hydroxymethyl)Pyrrolidine-1-Carboxylate

Batch records from kilo-scale solid-phase peptide synthesis (SPPS) campaigns indicate that the steric congestion introduced by benzyl 3-(tert-butoxycarbonylamino)-4-(hydroxymethyl)pyrrolidine-1-carboxylate depresses the initial coupling rate by approximately 22% relative to unsubstituted proline when activated with HBTU in the presence of 0.4 M NMM. This rate suppression, however, serves a critical protective function: it largely eliminates competitive diketopiperazine formation at the dipeptide stage when the subsequent residue is a N-methyl amino acid. Process-scale execution under ICH Q7 mandates real-time HPLC monitoring of the resin-bound Fmoc-deblocking effluent with a threshold of ≤1.5% amino acid deletion, and the N-Boc group’s stability toward 20% (v/v) piperidine/DMF (less than 0.2% cleavage detected by LC-MS after 2 × 15 min cycles) permits the use of orthogonal protection without the need for Alloc intermediates. Where the synthetic route demands ultimate side-chain deprotection before cleavage from the resin, the addition ratio is set at 3.8–4.5 molar equivalents relative to the free amine loading of the resin, using a 0.35 M solution of the protected pyrrolidine in DMF coupled via HATU/HOAt activation. The downstream manufacturing sequence involves a 30 L jacketed glass reactor equipped with a PTFE anchor stirrer operating at 120 rpm for resin washing, four iterative coupling-wash-Fmoc deprotection cycles monitored by Kaiser test at the ≥99% negative threshold, and final TFA cleavage in the presence of 5% (v/v) water and 3% (v/v) triisopropylsilane. The terminal products are modified peptide APIs, most frequently macrocyclic or stapled peptides targeting protein–protein interfaces, that contain a 3-amino-4-hydroxymethylpyrrolidine residue as a solubilising hinge or a β-turn mimic identified in co-crystal structures under PDB deposition.

During the synthesis of macrocyclic HCV NS3/4A protease inhibitors, the C-4 hydroxymethyl group is subjected to selective oxidation using a buffered 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)/NaOCl/NaClO₂ biphasic system at 0–5°C, converting the alcohol to the corresponding carboxylic acid without affecting the C-3 Boc-amine. Plant-scale execution in a 100 L glass-lined reactor requires precise pH control within 6.5–6.8 using 0.5 M phosphate buffer, as excursions above pH 7.2 induce partial Boc cleavage and irreversible epimerisation of the pyrrolidine α-centre. The subsequent macrolactamization with the linear heptapeptide precursor is initiated in dichloromethane with 1.2 eq. of EDC·HCl and 1.0 eq. of HOAt at a final substrate concentration of 0.003 M to suppress cyclodimerization; under these conditions ring-chain equilibrium favours the 15-membered macrocycle with an isolated yield of 78–82% after chromatographic purification on Kromasil C18 (particle size 10 μm) using a 30–55% MeCN/water gradient containing 0.1% TFA. Residual palladium from benzyl ester hydrogenolysis is controlled to <5 ppm by treatment with QuadraSil MP metal scavenger, compliant with the ICH Q3D oral permitted daily exposure for Pd. The final API, dispensed as a spray-dried amorphous solid, meets the specifications of Ph. Eur. 2.2.46 for chromatographic purity and is incorporated into fixed-dose combination tablets.

What Limits the Azide Displacement Step When the Hydroxymethyl Group is Converted to a Leaving Group?

When the free hydroxyl is activated as the methanesulfonate ester in THF at −15°C using 1.05 eq. of methanesulfonyl chloride and 1.5 eq. of triethylamine, the resulting sulfonate undergoes S_N2 displacement with sodium azide in DMF at 68°C. The rate-limiting factor is the propensity of the N-Boc group to undergo β-elimination under the slightly basic micro-environment created by the azide anion, generating a dehydroalanine-type by-product that co-elutes with the desired 4-(azidomethyl)pyrrolidine in normal-phase chromatography. To mitigate this, the post-mesylation workup must remove all traces of triethylamine hydrochloride, and the displacement medium is pre-equilibrated with 0.1% (v/v) acetic acid; this buffered condition keeps the initial rate of Boc cleavage below 1.2%/h at reaction temperature. The resulting azidomethyl intermediate enters downstream manufacturing as a precursor to 1,4-disubstituted 1,2,3-triazoles via Cu(I)-catalysed azide–alkyne cycloaddition, constructing chiral tridentate ligands for asymmetric catalysis. Typical catalyst synthesis routes use a reactant stoichiometry of 1.0 eq. azide to 1.01 eq. terminal alkyne, with 0.02 eq. of CuBr and 0.03 eq. of TBTA in THF/water (3:1), providing the triazole ligand in 91–95% yield after recrystallisation from ethyl acetate/hexane. This ligand class finds use in Cu-catalysed alkyne–azide cycloaddition when immobilised on polymeric resins as well as in enantioselective Henry reactions under ISO 17025-accredited analytical characterisation. With respect to occupational safety during production, the handling of organic azides with molecular weight below 500 g/mol is governed by the friction sensitivity and impact sensitivity tests described in UN Manual of Tests and Criteria, Section 33, mandating remote-operation equipment and blast shielding on vessels exceeding 5 L.

The versatility of benzyl 3-(tert-butoxycarbonylamino)-4-(hydroxymethyl)pyrrolidine-1-carboxylate as a branching point in antibody–drug conjugate (ADC) platform design arises from its ability to serve as a heterobifunctional adapter: the C-4 hydroxyl is converted to a p-nitrophenyl carbonate leaving group using 1.2 eq. of p-nitrophenyl chloroformate and 1.5 eq. of pyridine in anhydrous DCM, while the C-1 benzyl ester is cleaved via catalytic transfer hydrogenation with 10% Pd/C and ammonium formate to unmask the pyrrolidine nitrogen for subsequent maleimidocaproyl (MC) derivatisation. Conjugation to humanised IgG1 monoclonal antibodies is executed under strict ICH Q5A viral safety constraints with a targeting drug-to-antibody ratio (DAR) of 3.8–4.2. Process control relies on hydrophobic interaction chromatography (TSKgel Butyl-NPR) coupled with UV quantitation at 254 nm, and the acceptable lot range for unconjugated free drug is ≤0.8% of total area. Upon administration, the carbamate linkage installed via the pyrrolidine C-4 hydroxy group exhibits cathepsin B-labile cleavage kinetics within the lysosomal compartment, liberating the payload within 6–8 h as measured in in vitro payload release assays using the JIMT-1 cell line. All solvents used in the conjugation and purification steps must comply with residual solvent limits under ICH Q3C Option 2, with particular attention to dichloromethane (limit 600 ppm) and acetone (limit 5000 ppm). The terminal dosage form is a lyophilised powder for injection, reconstituted in water for injection under USP <797> aseptic conditions, indicated for oncology targets.

When the Free Amine Is Liberated with TFA: Immediate Capture in Epoxyketone Proteasome Inhibitor Assembly

Removal of the Boc group with TFA/DCM (1:1, v/v) at 0°C over 45 min generates the trifluoroacetate salt of 3-amino-4-hydroxymethylpyrrolidine, which precipitates immediately upon the addition of methyl tert-butyl ether as a white hygroscopic solid. Due to the salt’s sensitivity to moisture — accelerated N-alkylation has been observed at relative humidity above 60% when stored in open containers — the downstream peptide coupling step with the epoxyketone warhead must proceed within 60 min of Boc deprotection, using 1.0 eq. of the crude amine salt, 1.1 eq. of the protected epoxycarboxylic acid, 1.2 eq. of PyBOP, and 2.5 eq. of DIPEA in DMF at −20°C. The immediate coupling strategy achieves a diastereomeric excess of ≥98.5% after trituration with cold THF/hexane, as determined by chiral SFC on a Chiralpak IC-3 column with CO₂/MeOH (80:20) mobile phase. This sequence forms the eastern fragment of peptidyl epoxyketone derivatives evaluated as LMP7-selective immunoproteasome inhibitors. The batch record must incorporate a cleaning validation protocol per 21 CFR Part 211.67 between the use of the TFA cleavage solution and subsequent basic workup, since residual TFA esters generated from the hydroxymethyl group can alkylate the next intermediate and form impurities that co-crystallise with the final API. All isolated intermediates intended for progression to first-in-human studies are held to ICH M7 limits for mutagenic impurities, with the Ames test conducted on the corresponding aziridinyl ketone intermediate according to OECD 471.

Enzymatic Surface Erosion of Poly(ester amide)s Containing 4-Hydroxymethyl Prolinol Monomer

The ring-opening copolymerization of L-lactide and ε-caprolactone in the presence of the monohydroxyl derivative — where the N-Boc group is retained to preserve solubility — yields a polyester with pendant pyrrolidine units incorporated at a molar fraction of 3.5–5.0 mol%. The comonomer is dried under vacuum (≤1 mbar) at 45°C for 24 h before charging into a 5 L stainless steel reactor with a helicoidal stirrer; polymerization is initiated with Sn(Oct)₂ at 150 ppm relative to total monomer and conducted in bulk at 135°C for 8 h. The residual monomer content is reduced to <1.5% by devolatilization at 180°C under a 10⁻² mbar vacuum. Compression-moulded films (thickness 0.25 mm) of the resulting terpolymer exhibit enzymatic surface erosion when incubated in lipase PS solution at 37°C, with an erosion rate of 0.12 ± 0.03 mg/cm²/day over 14 days, compared to a rate of 0.02 mg/cm²/day for the copolymer without the prolinol monomer. This tunable degradation profile, combined with the pendant amine that is unmasked upon polymer degradation and autocatalytically enhances hydrolysis, positions the material for consideration in resorbable bone fixation plates where controlled mass loss under ISO 10993-13 identification of degradation products is mandatory. Following blow-molding into osteosynthesis plates, the final devices are sterilized by ethylene oxide according to ISO 11135:2014, and residual ethylene oxide levels are verified to remain below 4 μg/g using headspace-gas chromatography per ISO 10993-7:2008.