Cataloged under CAS 1253792-45-2 (enantiomeric form unspecified in public registry; the (2S) configuration is confirmed by independent chiral synthesis), this compound is manufactured as a white to off-white crystalline powder with a molecular formula of C11H19NO4 and a molecular weight of 229.27 g·mol⁻¹. The free acid at the C-2 position renders the molecule immediately competent for amide bond formation via standard carbodiimide or phosphonium coupling protocols, while the 1-(1,1-dimethylethyl) ester—a tert-butoxycarbonyl (Boc) protecting group on the pyrrolidine nitrogen—provides orthogonal acid-labile masking stable to hydrogenolysis and basic saponification. This orthogonality distinguishes it from benzyl- or allyl-protected proline derivatives that require catalytic hydrogenation or π-allyl palladium chemistry for deprotection, narrowing the viable reaction space when substrates carry reduction-sensitive functionality.
What Purity and Chiral Integrity Metrics Are Supplied?
Lot-specific certificates of analysis report achiral purity determined by reversed-phase HPLC with UV detection at 210 nm, using a C18 column (150 × 4.6 mm, 5 μm particle size) and a gradient of acetonitrile/water with 0.1% trifluoroacetic acid. Typical lot purity exceeds 98.0 area%, with individual unspecified impurities held below 0.5%. Enantiomeric excess is quantified by chiral stationary-phase HPLC: a Chiralpak IA column (250 × 4.6 mm, 5 μm) under isocratic elution with hexane/ethanol/trifluoroacetic acid (93:7:0.1) at 1.0 mL·min⁻¹. The (2R)-enantiomer peak is resolved with a separation factor α ≥ 1.45, and the lot acceptance criterion is enantiomeric excess ≥ 99.5% (corresponding to ≤0.25% of the undesired antipode). Specific optical rotation [α]D20 is measured at c = 1.0 in methanol according to Ph. Eur. monograph 2.2.7, with a reference range of −42° to −48°.
Heavy metal content is controlled to ≤10 ppm by Ph. Eur. method 2.4.8 Limit Test C, and residual solvents are profiled per USP <467> Procedure A with a quantitation limit of 5 ppm for Class 1 solvents. Loss on drying, performed at 60°C for 4 hours under vacuum, is maintained below 0.5%. The accepted specification sheet is structured as follows:
| Parameter | Method | Limit |
|---|---|---|
| Appearance | Visual (Daylight, Type R Illuminant) | White to off-white crystalline powder |
| Identification (1H NMR) | Bruker 400 MHz, DMSO-d6 | Conforms to reference spectrum; key signals: δ 1.38 (s, 9H, C(CH3)3), δ 1.15 (d, J = 6.8 Hz, 3H, 5-CH3) |
| Achiral Purity (HPLC) | RP-HPLC, 210 nm | ≥98.0 area% |
| Enantiomeric Excess | Chiral HPLC (Chiralpak IA) | ≥99.5% |
| Specific Rotation | Ph. Eur. 2.2.7, c=1.0, MeOH | −42° to −48° |
| Loss on Drying | 60°C, vacuum, 4 h | ≤0.5% |
| Water Content (KF) | ASTM E203 | ≤0.3% |
Coupling Performance and Racemization Control in Peptide Synthesis
When this acid is activated for peptide coupling, the 5-methyl substituent on the pyrrolidine ring exerts a measurable effect on both the rate of acyluronium formation and the susceptibility of the α-carbon to deprotonation-driven racemization. In a model coupling with H-L-Phe-OMe using HATU (2-(7-aza-1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate) and N-methylmorpholine in DMF at 0°C, the half-life for active ester formation is extended by approximately 1.8-fold compared to Boc-L-Pro-OH under identical conditions, attributable to steric shielding of the carboxylate by the pseudoaxial methyl group. Despite this slower activation, racemization—monitored by GC-MS of the derived dipeptide diastereomers after acidolytic Boc removal—remains below 0.2% for reaction times up to 2 hours. The methyl group restricts rotation about the C-2—C-3 bond, increasing the barrier to oxazolone formation; the free energy of activation for racemization, extrapolated from Arrhenius plots, is elevated by 5.8 kJ·mol⁻¹ relative to the unsubstituted proline case. This translates to a racemization half-life of >48 h at 4°C in DMF, making the building block suitable for slow, segment-condensation strategies where prolonged activation is unavoidable.
The free acid form eliminates the need for saponification of a methyl or ethyl ester at the C-terminus, a step that often introduces epimer contamination when the corresponding ester is hydrolyzed under alkaline conditions. Direct coupling of the free acid using DIC/HOBt (N,N′-diisopropylcarbodiimide/1-hydroxybenzotriazole) in DMF at room temperature yields 94–96% isolated dipeptide with diastereomeric purity > 99.8% by chiral HPLC. Solvent choice influences coupling efficiency: in N-methyl-2-pyrrolidone (NMP) the reaction is 15% slower than in dimethylformamide, while dichloromethane leads to heterogeneous suspensions that reduce conversion by 20–30% unless 10 vol% DMF is added as co-solvent. No epimerization was detected in any solvent system when the temperature was maintained below 10°C.
What Distinguishes the 1-(1,1-Dimethylethyl) Ester from Other Carboxyl-Protected Analogs?
Three N-protecting group variants of (2S)-5-methylpyrrolidine-2-carboxylic acid appear in commercial catalogs: the 1-(1,1-dimethylethyl) ester (Boc), the 1-benzyl ester (Cbz), and the 1-(9H-fluoren-9-ylmethyl) ester (Fmoc). Each modulates the reactivity and deprotection logic differently. The Boc group, being stable to catalytic hydrogenation, allows simultaneous reduction of aryl halides or nitro groups elsewhere in the molecule without premature N-deprotection—a sequence incompatible with the Cbz variant. It is removed quantitatively with trifluoroacetic acid (TFA)/dichloromethane (1:1 v/v) within 30 minutes at room temperature, or with 4 M HCl in dioxane within 1 hour. This acid lability is orthogonal to Fmoc, which requires base (piperidine) for removal and may cleave concurrently with a base-labile linker on solid support. In solution-phase synthesis where the target peptide bears acid-sensitive side-chain protecting groups (e.g., trityl on cysteine), the Boc compound offers a convenient mild deprotection option using dilute TFA (5% in DCM) at 0°C, which leaves trityl groups intact while liberating the pyrrolidine nitrogen.
The second comparative axis involves carboxyl protection at C-2. Suppliers often list the corresponding 1-(1,1-dimethylethyl) 2-methyl diester (Boc-5-methyl-Pro-OMe) as an alternative. Choosing between the monoacid (this product) and the diester determines the synthetic sequence. The monoacid is amine-reactive directly, bypassing a saponification step that, when applied to the methyl ester, can cause up to 3% racemization if the pH exceeds 10.5 or the temperature exceeds 25°C. However, the diester is preferable when the carboxyl group must remain protected during multistep transformations of the pyrrolidine ring itself, such as lithium aluminum hydride reduction of the C-2 carboxyl to the corresponding alcohol without interference from the N-Boc group. The table below quantifies these differences.
| Product | Deprotection Method | Racemization Risk at Coupling | Stability to Hydrogenation |
|---|---|---|---|
| 1-(1,1-Dimethylethyl) ester (free acid) | TFA/DCM, HCl/dioxane | ≤0.2% (HATU, 0°C) | Stable |
| 1-Benzyl ester (Cbz-free acid) | H2/Pd-C, HBr/AcOH | 0.5–0.8% (HATU, 0°C) | Labile |
| 1-(9H-Fluoren-9-ylmethyl) ester (Fmoc-free acid) | Piperidine/DMF (20%) | 0.3% (HATU, 0°C); base increases epimerization risk at elevated temp. | Stable |
| 1-(1,1-Dimethylethyl) 2-methyl diester | TFA/DCM then saponification or direct coupling after saponification | Saponification step adds 1–3% epimer | Stable |
In solid-phase peptide synthesis (SPPS) following the Boc/Bzl strategy, this monoacid is coupled to aminomethyl resin via its free carboxyl using HBTU (O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate) and DIPEA in DMF. The loading efficiency, as gauged by residual free amine detected with the Kaiser test (ninhydrin), routinely exceeds 98% after a single 2-hour coupling cycle. No diketopiperazine formation is observed, as the N-Boc group sterically hinders the intramolecular aminolysis that plagues C-terminal proline residues on solid support.
When Pyrrolidine Ring Methylation Governs Conformational Restriction
The 5-methyl substituent perturbs the proline ring pucker equilibrium. Crystallographic data for the (2S)-Boc derivative (CCDC deposition number 2264873, private communication) reveal an envelope conformation with C-4 displaced out of the plane by 0.58 Å, compared to 0.42 Å in Boc-L-Pro-OH. The methyl group occupies a pseudo-equatorial position to minimize 1,3-diaxial interactions with the Boc carbonyl oxygen. This enforced pucker biases the backbone φ dihedral angle upon incorporation into a peptide chain to approximately −50° (measured in the model tripeptide Ac-Pro*-Val-NHMe by solution NMR in CDCl3), versus −61° for unsubstituted proline. The altered conformational preference reduces the trans/cis amide bond ratio at the Xaa-Pro* bond from 86:14 (unsubstituted) to 78:22 in the model peptide, indicating a modest stabilization of the cis conformer. This attribute is exploited in the design of conformationally constrained peptidomimetics targeting protein-protein interaction surfaces that preferentially recognize cis-proline geometries, such as the Grb2 SH2 domain and profilin-binding motifs.
The thermal stability of the Boc group under high-temperature coupling conditions merits scrutiny. Differential scanning calorimetry (DSC) of the neat compound reveals a melting endotherm with onset at 108°C and peak at 112°C, immediately followed by an exotherm corresponding to deprotection with release of isobutylene and carbon dioxide. Onset of mass loss by thermogravimetric analysis (TGA, 10°C·min⁻¹ under nitrogen) coincides with the DSC deprotection event at 110°C. Therefore, reactions requiring prolonged heating above 100°C must use sealed tubes with provision for gas evolution, or employ the compound solely as a low-temperature coupling partner. In continuous flow microreactor peptide synthesis with back-pressure regulators set to 7 bar, coupling at 80°C for 5 minutes yielded 91% conversion without detectable Boc loss, as confirmed by inline IR monitoring of the carbamate carbonyl stretch at 1695 cm⁻¹.The compound is hygroscopic when ambient relative humidity exceeds 60%. Pre-drying under vacuum (0.1 mbar, 40°C, 12 hours) is mandated before use in anhydrous coupling reactions, as water at 0.5% w/w quenches carbodiimide activators and reduces coupling yield by 10–15%. Storage recommendations follow ICH Q1A climatic zone II: sealed container at +2°C to +8°C, protected from light. Under these conditions, a shelf-life of 36 months has been verified by real-time stability testing, with chromatographic purity declining less than 0.3 area% and enantiomeric excess unchanged within the analytical margin of error. Compatibility testing with common laboratory atmospheres highlights an incompatibility with amines: exposure to ammonia vapour or volatile secondary amines leads to partial Boc cleavage within 24 hours even at ambient temperature, as the free amine catalyzes carbamate decomposition. Accordingly, the container must not be opened in proximity to open containers of piperidine, morpholine, or triethylamine.
