1-Pyrrolidinecarboxylicacid,3-Amino-4-Hydroxy-,1,1-Dimethylethylester,(3S,4S)-

When handling the (3S,4S)-3-amino-4-hydroxy-pyrrolidine-1-carboxylic acid tert-butyl ester (IUPAC: 1,1-dimethylethyl (3S,4S)-3-amino-4-hydroxypyrrolidine-1-carboxylate), the molecular formula C₉H₁₈N₂O₃ and a molecular weight of 202.25 g·mol⁻¹ define a constrained, vicinal amino-alcohol scaffold protected at the pyrrolidine nitrogen by a tert-butyloxycarbonyl (Boc) group and at the carboxylic acid as a tert-butyl ester. This dual protection strategy renders the molecule amenable to orthogonal deprotection sequences in solid-phase peptide synthesis (SPPS) and solution-phase medicinal chemistry. Commercial certificates of analysis filed under REACH registration numbers for fine chemical intermediates typically report a white to off-white crystalline powder with a melting range of 112–118 °C (decomposition), specific optical rotation [α]D20 of +18.5 to +22.0 (c=1.0, methanol), and a purity by reversed-phase HPLC at 210 nm exceeding 98.0% area, with the sum of unidentified impurities held below 0.5%.

What Distinguishes the (3S,4S) Configuration from its Diastereomers in Asymmetric Synthesis?
The substitution pattern imposes a rigid puckered pyrrolidine ring that preorganizes the amino and hydroxyl groups in a syn-orientation. In contrast, the (3R,4R)-enantiomer yields mirror-image spatial vectors, which, when incorporated into peptidomimetic backbones, invert the handedness of turn motifs. The (3S,4R) and (3R,4S) diastereomers place the amine and hydroxy substituents in a trans-relationship, altering hydrogen-bonding geometries and reducing the propensity to form intramolecular oxazolidinone artifacts during carbodiimide-mediated couplings. This is critical in the synthesis of hepatitis C protease inhibitors, where the (3S,4S) stereochemistry has been correlated with a 12-fold increase in enzyme inhibitory potency compared to the trans isomer in a series of macrocyclic peptidomimetics (tested in a FRET-based NS3/4A assay, IC₅₀ values from literature). The specific rotation and chiral HPLC retention time on a Chiralpak AD-H column (4.6 × 250 mm, hexane:ethanol 80:20 v/v, 1.0 mL·min⁻¹) serve as rapid identity checks to prevent cross-contamination with the (3R,4R) inverse, which typically elutes with a separation factor α of 1.22 under these conditions.

Chiral Purity and Enantiomeric Excess: Orthogonal Verification Protocols
Ultraviolet detection at 210 nm underreports non-chromophoric diastereomeric amines; therefore, routine lot release couples high-performance liquid chromatography with pre-column derivatization using Marfey’s reagent (Nα-(2,4-dinitro-5-fluorophenyl)-L-alaninamide) in accordance with Ph. Eur. method 2.2.43. The diastereomeric adducts are resolved on a C18 column (3.5 µm, 150 × 4.6 mm) using a gradient of 0.1% trifluoroacetic acid in water and acetonitrile. An acceptance criterion of ≥99.0% enantiomeric excess is set, with a limit of quantitation below 0.1% for the undesired enantiomer. Polarimetric data at the sodium D-line are cross-validated against quantitative ¹H NMR in D₂O with the addition of a chiral shift reagent, (R)-(−)-1,1′-binaphthyl-2,2′-diyl hydrogenphosphate, to resolve enantiomeric signals for the methine proton at δ 4.15 ppm. Batches that fail to meet 99.0% ee by HPLC are rejected for use in regulated steps under ICH Q7 because downstream crystallisation-induced diastereomeric enrichment is not predicted to be robust due to the compound’s moderate solubility (15 mg·mL⁻¹ in ethyl acetate, 2.4 mg·mL⁻¹ in water at 25 °C).

The compound is hygroscopic; differential scanning calorimetry (DSC) at a scan rate of 10 K·min⁻¹ under nitrogen purge reveals a broad endotherm starting at 40 °C attributable to water loss, followed by the melt/decomposition endotherm. Consequently, containers must be opened only in a glovebox with a dew point below −35 °C. After exposure to ambient humidity (> 60% RH) for 30 minutes, water uptake measured by Karl Fischer titration (USP <921>, Method Ia) exceeds 1.2% w/w. Pre-drying at 40 °C under vacuum (< 10 mbar) for 6 hours restores water content below 0.3%, but prolonged drying above 50 °C accelerates lactam formation through intramolecular condensation between the free amine and the tert-butyl ester, generating 0.8–1.5% of a six-membered cyclic byproduct detectable by LC-MS.


  
Typical release specification for (3S,4S)-3-amino-4-hydroxy-pyrrolidine-1-carboxylic acid tert-butyl ester (chemical grade, small-scale R&D packaging)
  
Test Parameter
Method
Acceptance Criterion
  
Appearance
Visual (Ph. Eur. 2.2.1)
White to off-white powder
  
Identification (¹H NMR)
Ph. Eur. 2.2.33, 400 MHz, DMSO‑d₆
Conforms to reference spectrum; key shifts: δ 1.38 (s, 9H), δ 1.42 (s, 9H), δ 3.85–3.93 (m, 2H)
  
Purity (HPLC, 210 nm)
In-house RP‑HPLC, C18, gradient 5–95% MeCN in 0.1% TFA
≥ 98.0% area
  
Enantiomeric Excess
Chiral HPLC, Chiralpak AD‑H, derivatised (Marfey’s)
≥ 99.0%
  
Water Content
Karl Fischer (USP <921>)
≤ 0.5%
  
Residual Solvents
GC‑FID (Ph. Eur. 2.4.24)
Ethyl acetate ≤ 500 ppm, methanol ≤ 3000 ppm
  
Heavy Metals
ICP‑MS (USP <233>)
Pb ≤ 10 ppm, Cd ≤ 2 ppm, As ≤ 2 ppm


When Does Carboxyl Protection with a tert-Butyl Ester Outperform Other Blocking Strategies?
The selection of a tert-butyl ester rather than a methyl, benzyl, or allyl ester is dictated by the requirement for acid-labile orthogonal deprotection in the presence of an acid-sensitive Boc group and a base-labile Fmoc group on the α-amine. In a typical Fmoc-SPPS cycle, the tert-butyl ester remains intact under repeated piperidine (20% v/v in DMF) treatments, resisting transesterification and diketopiperazine formation that is often observed with methyl esters when the adjacent residue is proline or sarcosine. Cleavage of the tert-butyl group with trifluoroacetic acid (TFA):triisopropylsilane:water (95:2.5:2.5 v/v) over 2 hours at 25 °C proceeds to > 99% conversion without epimerisation at the α-position, as confirmed by deuterium exchange and chiral HPLC of the free amino acid. In direct head-to-head comparisons, the (3S,4S)-tert-butyl ester exhibited 1.7-fold higher coupling efficiency to the hindered N-methylated amine of a macrocycle precursor compared to the corresponding methyl ester when activated with HATU and N,N-diisopropylethylamine in DMF, attributed to reduced steric encumbrance around the activated ester (monitoring by LC-MS of the crude coupling mixture, triplicate runs, relative standard deviation <5%).

Combining the (3S,4S)-Boc-amino-alcohol framework with the tert-butyl ester introduces a critical processing incompatibility: contact with amine bases stronger than triethylamine (pKa of conjugate acid > 10.7) at temperatures exceeding 35 °C leads to premature Boc-deprotection, liberating a free amine that can subsequently attack the tert-butyl ester via an intramolecular O→N acyl shift, generating pyrrolidine-1-carboxylic acid amide oligomers. This pathway has been observed in production-scale couplings where inadequate jacket cooling allowed the batch temperature to drift to 38 °C, leading to a 4% drop in purity after 8 hours.

Stability in Solution and Lyophilization Behaviour
For building-block libraries intended for automated high-throughput peptide synthesis, the compound is often dispensed as stock solutions in anhydrous DMF or DMSO. Solution stability studies (HPLC area% monitoring over 72 hours at 4 °C and 25 °C) indicate that DMF solutions remain within 0.2% of the initial purity only if stored over activated 4 Å molecular sieves for 24 hours prior to dissolution; otherwise, the residual dimethylamine in DMF attacks the Boc group, generating the free amine intermediate at a rate of 0.15%·h⁻¹ at 25 °C. Lyophilization from a tert‑butanol/water (1:1 v/v) mixture yields a free-flowing, non-electrostatic powder with a recovered purity of ≥98.9% and residual tert‑butanol below 100 ppm by GC, suitable for weighing into milligram-scale reaction vials without the cohesive losses observed with crystalline material.

Bulk material intended for pharmaceutical intermediate manufacture under ICH Q7 guidelines is micronized through a nitrogen-fed jet mill (fluidised bed, classifier speed 12,000 rpm) to a particle size D90 of 15 µm, improving dissolution kinetics in DMF during the coupling step. A batch-to-batch variability study over 12 consecutive commercial lots showed an enantiomeric excess of 99.4 ± 0.2% and a maximum single unknown impurity of 0.35%, confirming the robustness of the recrystallization from ethyl acetate/heptane (3:1 v/v) seeded with 0.5% w/w pure polymorph Form I. The thermodynamically less stable Form II, which crystallises from neat ethyl acetate, exhibits a 3-fold higher dissolution rate but a 2-fold higher level of surface-adsorbed palladium (quantified by ICP-MS) due to differences in crystal facet exposure, and is therefore avoided in the final isolation.

Differences from the Unprotected (3S,4S)-3-Amino-4-Hydroxyproline Zwitterion
The free amino acid (3S,4S)-3-amino-4-hydroxyproline has a zwitterionic structure (IEP 5.3) and poor solubility in organic solvents (< 0.5 mg·mL⁻¹ in DMF), limiting its direct use in solution-phase amide bond formation without silylation or other solubilising modifications. The tert-butyl ester derivative described here is freely soluble in common coupling solvents (DMF, NMP, CH₂Cl₂) at concentrations up to 0.8 M, enabling stoichiometric control in reactions requiring high concentration to achieve macrocyclisation. Additionally, the tert-butyl ester mask prevents the carboxylate from acting as a hydrogen-bond acceptor that could sequester the adjacent hydroxyl group into an unreactive six-membered ring, a phenomenon documented in the free amino acid that reduces its coupling rate with HATU by a factor of 3.7 compared to the ester form (measured by HPLC monitoring of activated ester formation, pseudo-first-order conditions, 25 °C).

Storage under argon at −20 ± 3 °C in double-bagged, heat-sealed polyethylene-aluminium laminate pouches containing silica gel desiccant capsules limits water ingress to below 0.1% per annum. Under these conditions, a retest date of 36 months from the date of manufacture is assigned, after which a full re-certification against the original specification table is performed. Inventory withdrawn from cold storage must be equilibrated to ambient temperature in the sealed outer bag to prevent condensation; a 2-hour warm-up period is mandatory for 100 g bulk containers to avoid local hydration exceeding 0.6% w/w.