(2S,3S)-1-(Tert-Butoxycarbonyl)-3-Hydroxypyrrolidine-2-Carboxylic Acid

The (2S,3S) isomer of 1-(tert-butoxycarbonyl)-3-hydroxypyrrolidine-2-carboxylic acid, systematically designated as (2S,3S)-N-Boc-3-hydroxyproline (CAS 87691-27-8), serves as a conformationally constrained proline surrogate in solid-phase peptide synthesis, chiral scaffold construction, and peptidomimetic drug design. With a molecular formula of C₁₀H₁₇NO₅ and a molecular weight of 231.25 g·mol⁻¹, the compound presents a pyrrolidine ring bearing a trans-oriented hydroxyl at C-3 and a Boc-protected nitrogen, leaving the carboxylic acid free for uronium- or carbodiimide-mediated activation. The trans relationship between the C-2 carboxyl and C-3 hydroxyl locks the pyrrolidine in a Cγ-endo envelope conformation, as verified by single-crystal X-ray diffraction with a ring puckering amplitude of 0.38 Å, which restricts backbone flexibility and enforces a ψ dihedral angle of approximately +150° when incorporated into a peptide chain. This preorganization contrasts sharply with the (2S,4R)-4-hydroxyproline isomer, where the γ-exo pucker directs the hydroxyl to a pseudo-equatorial position, and directly impacts amide bond geometry in collagen-mimetic peptides and integrin antagonists. Industrial procurement specifications demand an enantiomeric excess exceeding 99.0% as measured by chiral HPLC (Chiralpak IA, hexane:isopropanol 95:5 v/v, 1.0 mL·min⁻¹, UV 210 nm), with residual tert-butanol capped below 0.5% per ICH Q3C guidelines. The orthogonal protecting-group strategy—acid-labile Boc on nitrogen, free hydroxyl available for orthogonal silylation or allylation, and carboxylic acid ready for on-resin coupling—allows iterative assembly of complex non-ribosomal peptide analogs without laborious solution-phase protection adjustments.

What Limits Yield in Boc-3-Hydroxyproline-Mediated Amide Bond Formation?

During the activation of the free carboxylic acid for peptide coupling, two competing side reactions constrain isolated yields: base-catalyzed epimerization at the C-2 stereocenter and intramolecular δ-lactonization between the C-3 hydroxyl and the activated carboxyl group. When the compound is pre-activated with HATU and DIEA in DMF at 0°C, the acyluronium intermediate retains configurational integrity, with the undesired (2R,3S) diastereomer remaining below 0.3% over 30 min as monitored by chiral analytical methods. Raising the activation temperature to 25°C accelerates epimerization; the diastereomeric impurity climbs to 2.1% within the same window, exceeding the typical 1.0% threshold permitted for cGMP intermediate release. Simultaneously, carboxyl activation in the presence of the unprotected C-3 hydroxyl promotes intramolecular nucleophilic attack to form a [3.3.0] bicyclic δ-lactone. Kinetic profiling in DMF with DIC/Oxyma reveals a lactonization rate constant of 0.15 h⁻¹ at 25°C, translating to a 22% loss of active ester within 2 h and a corresponding drop in coupling yield to the target peptide. This dual sensitivity demands strict thermal control and, in most process-scale operations, transient protection of the hydroxyl group prior to the coupling step. In Fmoc/tBu solid-phase synthesis on a CEM Liberty Blue automated microwave synthesizer, the coupling of (2S,3S)-Boc-3-hydroxyproline onto a deprotected peptidyl-resin is executed with 4 eq of amino acid, 4 eq of HATU, and 8 eq of DIPEA in DMF at 75°C for 5 min. Coupling efficiency, quantified by Fmoc deprotection UV absorbance at 301 nm and confirmed by Kaiser test, routinely exceeds 99.5%. The microwave pulse sequence is tuned to avoid hot spots that would trigger lactonization: the initial ramp phase reaches 50°C in 30 s, followed by a stepped increase to the target temperature over 1.5 min. Post-coupling resin shrinkage is mitigated by a wash cycle of 2 × DMF and 2 × DCM, and any trace base carried into the subsequent Fmoc removal step is neutralized by a pre-wash with 0.5 M HOBt in DMF. For scale-up to 2 mmol resin loading, a Biotage Syro I parallel synthesizer with chilled feeding lines (4°C) reproduces these results with a coupling cycle time of 12 min per residue. Under these conditions, the unprotected hydroxyl survives without detectable lactonization, attributing the success to the rapid, high-yielding reaction that outcompetes the slower intramolecular cyclization kinetics.

Orthogonal Chromatographic and Thermal Analysis Defines Release Specifications

Batch conformity is established through a panel of pharmacopoeial and ICH-mandated methods that collectively validate identity, enantiopurity, volatile impurity burden, and elemental safety. The acceptance criteria listed in the following table are applied to every lot released for GMP peptide API synthesis.

Release specifications for (2S,3S)-1-(tert-butoxycarbonyl)-3-hydroxypyrrolidine-2-carboxylic acid

Attribute	Method	Acceptance Criterion
Appearance	Visual inspection (Ph. Eur. 2.2.1)	White to off-white crystalline powder
Identification (IR)	ATR-FTIR (Ph. Eur. 2.2.24)	Matches reference spectrum; characteristic bands at 1745 cm⁻¹ (C=O), 1680 cm⁻¹ (carbamate C=O), 3400 cm⁻¹ (OH)
Assay (HPLC)	RP-HPLC, C18, 210 nm, gradient (Ph. Eur. 2.2.29)	≥ 99.0% area
Enantiomeric excess	Chiral GC (TMS-ether derivative)	≥ 99.5% e.e.
Specific optical rotation	Ph. Eur. 2.2.7	[α]D²⁰ = -35° ± 2° (c 1.0, MeOH)
Water content	Karl Fischer coulometry (Ph. Eur. 2.5.32)	≤ 0.5%
Residual solvents	Headspace GC (Ph. Eur. 2.4.24, ICH Q3C)	Acetone ≤ 500 ppm, DMF ≤ 880 ppm, tert-butanol ≤ 5000 ppm
Elemental impurities	ICP-MS (ICH Q3D)	Pb ≤ 5 ppm, As ≤ 2 ppm, Cd ≤ 1 ppm, Hg ≤ 1 ppm
Residual palladium	ICP-OES	≤ 10 ppm

Each certificate of analysis references these compendial and ICH standards, and the compound is accompanied by a TSE/BSE-free statement. For pharmaceutical development beyond early-phase, the residual Pd limit is tightened to 5 ppm when the final API carries a parenteral route of administration.

When the C-3 Hydroxyl Remains Unprotected During Carboxyl Activation

Process deviations that expose the free hydroxyl to a prolonged activation interval or acidic workup generate the δ-lactone byproduct, which partitions into the organic extract and co-elutes with the desired product on silica gel. In a controlled study, activation with DIC/Oxyma (1.2 eq) in DMF at 25°C followed by glycine methyl ester hydrochloride addition gave a dipeptide yield of 58% after 2 h, whereas the same protocol applied to the TBS-protected analog produced 96% yield. The 38% gap is attributed to lactonization and subsequent ring-opening oligomerization during aqueous workup. Proton-catalyzed lactonization is especially problematic during TFA-mediated global deprotection, where any residual active ester trapped on the resin can cyclize and cleave the peptide chain. To eliminate this variable, pre-silylation is executed with TBSCl (3 eq) and imidazole (6 eq) in anhydrous DMF at 0°C to ambient temperature over 16 h, yielding the TBS ether in 92% after flash chromatography (hexane:EtOAc 8:2). The TBS group is later removed with TBAF·3H₂O (2 eq) in THF at 0°C for 1 h without affecting the Boc carbamate, provided the fluoride solution is pre-dried over molecular sieves. On manufacturing lines equipped with in-line FTIR for real-time process monitoring, the disappearance of the lactone carbonyl stretch at 1780 cm⁻¹ serves as a secondary endpoint criterion for complete hydroxyl reprotection. Long-term storage protocols for multi-kilogram batches dictate double-bagging under nitrogen with silica gel desiccant in HDPE drums, placement in a -20°C walk-in freezer, and semi-annual Karl Fischer verification. Accelerated stability studies at 25°C/60% RH for 6 months show less than 0.2% degradation by HPLC, supporting a retest interval of 24 months at the recommended storage condition. Exposure to ambient humidity above 60% RH for more than 4 h increases water content above the 0.5% limit and necessitates vacuum drying (40°C, 10 mbar, 8 h) before use in moisture-sensitive coupling reactions.

Comparing Diastereomers: (2S,3S) versus (2S,4R) Proline Scaffolds in Peptide Helicity

The stereochemical identity of the hydroxyproline building block dictates backbone torsional preferences, heterocycle solubility, and ultimately the secondary structure of the target peptide. The table below collates experimentally determined and DFT-optimized (B3LYP/6-31G(d)) parameters for the three most commonly encountered N-Boc-hydroxyproline isomers.

Key physicochemical and conformational parameters of N-Boc-hydroxyproline diastereomers

Property	(2S,3S) trans-3-Hyp	(2S,3R) cis-3-Hyp	(2S,4R) trans-4-Hyp
Ring pucker	Cγ-endo	Cγ-exo	Cγ-exo
χ1 torsion (N–Cα–Cβ–Cγ)	+30°	-20°	-25°
Preferred ψ dihedral in tripeptide	+150°	-120°	-140°
Solubility in DCM at 25°C	85 mg/mL	120 mg/mL	95 mg/mL
Half-life of HBTU coupling (DMF, 0°C)	12 min	8 min	15 min
Triple helix Tm shift in (Pro-Xaa-Gly)₁₀	-10°C vs baseline	-18°C	+10°C

In collagen-mimetic peptides, the (2S,3S) configuration induces a polyproline-II helix with a reduced thermal stability relative to the canonical (2S,4R)-4-hydroxyproline, a consequence of altered interstrand hydrogen bonding between the C-3 hydroxyl and the backbone carbonyl of an adjacent chain. Circular dichroism spectra of (Pro-(2S,3S)-Hyp-Gly)₁₀ exhibit a minimum at 225 nm with a molar ellipticity of -28,000 deg·cm²·dmol⁻¹, compared to -38,000 deg·cm²·dmol⁻¹ for the 4-Hyp analog. This difference translates directly into triple helix melting temperatures: 42°C for the 3-Hyp peptide versus 58°C for the 4-Hyp variant, as determined by differential scanning calorimetry (DSC) at a scan rate of 1°C·min⁻¹. Beyond collagen models, the steric and electronic environment sculpted by the (2S,3S) framework proves advantageous in cyclic RGD integrin antagonists. Macrocyclization of a pentapeptide incorporating (2S,3S)-Boc-3-hydroxyproline yields an αvβ3 integrin inhibitor with an IC50 of 0.8 nM in a competition ELISA using vitronectin as the immobilized ligand, while the (2S,4R) diastereomer under identical assay conditions displays an IC50 of 40 nM, highlighting how the trans-3-hydroxyl orientation enhances selectivity for the bent RGD conformation recognized by the integrin headpiece. Published data for this specific configuration in clinical-stage assets remains limited, but the steep SAR gradient underscores the necessity of procuring the defined (2S,3S) isomer with ≥ 99.5% e.e. to avoid confounding pharmacological readouts.