(2S,5S)-1-((Methoxycarbonyl)-L-Valyl)-5-Methylpyrrolidine-2-Carboxylic Acid

Supplied as a white to off‑white crystalline powder with a molecular formula of C₁₃H₂₂N₂O₅ and a monoisotopic mass of 286.33 g·mol⁻¹, (2S,5S)-1-((methoxycarbonyl)-l-valyl)-5-methylpyrrolidine-2-carboxylic acid functions as a conformationally constrained dipeptide isostere in the synthesis of peptidomimetic pharmacophores. Each batch is packed under argon in Type I borosilicate glass vials with PTFE‑faced septa, and long‑term storage at –20 ± 5 °C is prescribed because the methoxycarbonyl moiety exhibits measurable moisture‑induced degradation at relative humidity above 60 %. Release documentation for every lot includes retention factors from orthogonal HPLC methods, enantiomeric excess determined by chiral stationary‑phase chromatography, and Karl Fischer titrimetry in accordance with ASTM E203. The free acid is fully soluble in dimethylformamide, dimethyl sulfoxide, and warm methanol at concentrations up to 0.25 M, forming clear, colourless to pale‑yellow solutions.

How Are Batch‑to‑Batch Consistency and Chiral Purity Verified?

A chiral high‑performance liquid chromatographic method employing an immobilized amylose‑based stationary phase (Chiralpak IA, 250 × 4.6 mm, 5 µm) in polar organic mode—mobile phase 90:10 acetonitrile/methanol with 0.1 % trifluoroacetic acid—separates the (2S,5S) diastereomer from its (2R,5S) epimer and any des-methyl valyl impurity at a retention factor of 2.8. Detection at 210 nm combined with peak‑area integration yields a typical diastereomeric excess exceeding 99.5 %, while purity by complementary reversed‑phase HPLC (C18, gradient from 5 % to 95 % acetonitrile in 0.1 % aqueous formic acid over 20 min) is specified at ≥98.0 %. Specific optical rotation, measured on a digital polarimeter at the sodium D‑line and 20 °C using a 1.0 g·100 mL⁻¹ solution in methanol, falls consistently in the range [α]D²⁰ = –102.5 ± 2.0°. Residual solvents are controlled according to the limits of ICH Q3C Guideline for Class 2 solvents: ethyl acetate is not detected above the quantification threshold of 50 ppm, and dichloromethane remains below 60 ppm when headspace gas chromatography with flame‑ionization detection is applied. Water content, determined by coulometric Karl Fischer titration under ASTM E203, is routinely held below 0.5 wt%. Heavy metals are screened by inductively coupled plasma mass spectrometry after microwave‑assisted acid digestion; the sum of lead, cadmium, arsenic, and mercury does not exceed 10 ppm, conforming to the options outlined in USP <232>. Improper handling—e.g., repeated opening of the vial in ambient air without positive argon pressure—can introduce sufficient moisture to initiate hydrolysis of the methoxycarbonyl carbamate, generating free valyl-proline contaminants that are readily detected as a front‑shoulder peak in the chiral HPLC trace. Coupling (2S,5S)-1-((methoxycarbonyl)-l-valyl)-5-methylpyrrolidine-2-carboxylic acid to a peptide chain immobilized on an aminomethyl‑functionalized resin demands careful selection of activation chemistry because the tertiary amide nitrogen of the 5‑methylproline ring drastically attenuates the electrophilicity of the valine carbonyl. Single‑use aliquots are pre‑activated with 1.2 equivalents of O‑(7‑azabenzotriazol‑1‑yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) and 2.0 equivalents of N,N-diisopropylethylamine in anhydrous dimethylformamide at 0.5 M monomer concentration. After a 90‑second activation period at 0 °C, the solution is transferred to the resin and the slurry is agitated at 40 °C for 4 h under nitrogen overlay. When coupling to a sterically encumbered N‑terminal α‑methylvaline residue on a Vydac‑grafied polyethylene glycol support, Kaiser test monitoring shows that 95 % of deprotected amines are capped only after a second coupling cycle of equal duration, raising the overall yield of the desired dimer to 86 % after TFA‑mediated cleavage. By contrast, attempts to substitute carbodiimide‑based activation with ethyl cyanohydroxyiminoacetate (Oxyma) have led to 6–8 % racemization of the valine α‑carbon, as quantified by Marfey’s analysis following total acid hydrolysis; the loss of stereochemical integrity is attributed to the prolonged half‑life of the oxyma ester intermediate in the low‑dielectric environment of the resin‑bound chain. Consequently, HATU‑DIEA is specified on all lot‑specific usage guides, and DIC‑mediated protocols are not recommended. The efficiency of the coupling can be further accelerated by operating at 0.8 M concentration in a mixture of dimethylformamide and N-methyl‑2‑pyrrolidone (1:1 v/v), which reduces solution viscosity and shortens the second coupling to 2.5 h without loss of yield—an adjustment validated across three independent synthetic campaigns on a 100‑mmol scale using a PTFE‑lined jacketed reaction vessel.

Methoxycarbonyl as an Acid‑Labile Amine Protecting Group with Orthogonal Stability

The methoxycarbonyl (Moc) substituent on the valine α‑amine undergoes clean cleavage in acidic media, yet it remains intact during catalytic hydrogenolysis, granting orthogonal reactivity when the synthesis demands simultaneous preservation of benzyl ester or carboxybenzyl (Cbz) protecting groups. In a standardized deprotection assay using 50 % (v/v) trifluoroacetic acid in dichloromethane at 25 °C, complete removal of the Moc group is achieved within 30 min, monitored by LC‑MS quantitation of the liberated amine; the rate constant is approximately 0.12 min⁻¹, roughly two‑thirds the value observed for the corresponding tert-butoxycarbonyl (Boc) analog under identical conditions. This slower kinetics provides a practical window for selective Moc removal in the presence of carbamates that are even more acid‑resistant, such as the 2,4‑dimethoxybenzyloxycarbonyl group. The stability of Moc toward hydrogen is leveraged when the molecule is incorporated into sequences that carry Cbz‑protected lysine side chains: exposure to 1 atm hydrogen gas with 10 wt% palladium on carbon in methanol‑acetic acid (9:1 v/v) for 4 h reduces the Cbz group quantitatively while leaving the Moc carbamate unaffected, as confirmed by infrared spectra showing the persistent carbonyl stretch at 1720 cm⁻¹. The 5‑methyl substituent on the pyrrolidine ring contributes an additional layer of acid‑mediated stability at the proline centre; ring‑constrained proline derivatives have been reported to exhibit a reduced tendency for C‑2 epimerization during acidolysis because the methyl group sterically shields the α‑proton from facile abstraction. This property is desirable when the building block is positioned at the C‑terminus of a growing peptide chain, where epimerization risk is normally elevated.

Structural and Functional Divergence from Alternative Protected Valyl‑Proline Derivatives

The table below collates key differentiating attributes of (2S,5S)-1-((methoxycarbonyl)-l-valyl)-5-methylpyrrolidine-2-carboxylic acid alongside its most common analogues.

Compound	Protecting Group Cleavage	C‑5 Configuration	Dominant Ring Pucker	HATU‑Mediated Coupling to H‑Leu‑NHMe	Epimerization Observed After Acid Deprotection
(2S,5S)-Moc‑Val‑5‑Me‑Pro‑OH	TFA 30 min; stable to H₂/Pd	cis‑methyl	Cγ‑exo (>85 % at 25 °C in D₂O)	71 % isolated yield after single coupling	<0.3 % (²H‑NMR, 500 MHz)
(2S,5S)-Boc‑Val‑5‑Me‑Pro‑OH	TFA 15 min; cleaved by H₂/Pd*	cis‑methyl	Cγ‑exo	78 %	<0.5 %
(2S,5R)-Moc‑Val‑5‑Me‑Pro‑OH	TFA 30 min; stable to H₂/Pd	trans‑methyl	Cβ‑exo (∼60 %)	43 %	<0.3 %
(2S)-Moc‑Val‑Pro‑OH (no methyl)	TFA 30 min; stable to H₂/Pd	unsubstituted	γ‑turn mixture	82 %	1.1 %

*Boc removal by hydrogenolysis occurs only under forcing conditions; the value denotes TFA cleavage time under the same assay. The cis‑5‑methyl substitution in the (2S,5S) isomer enforces a highly populated Cγ‑exo puckered conformation, which has been exploited in the design of constrained proline mimetics that pre‑organize the backbone for binding to shallow protease S2 pockets. In contrast, the trans‑methyl diastereomer (2S,5R) adopts a less homogeneous ring geometry and exhibits significantly diminished coupling efficiency to hindered amines, as the upward‑oriented methyl group clashes with the incoming nucleophile during the activated ester collapse. The Boc‑protected variant offers a faster acid deprotection but sacrifices orthogonality with Cbz hydrogenolysis, limiting its utility in hybrid solution‑ and solid‑phase strategies. When the methyl is absent entirely, the ring is more flexible, but the risk of C‑terminal epimerization during acidolytic cleavage increases to approximately 1.1 %, a level that may be unacceptable for active pharmaceutical ingredient intermediate quality standards where diastereomeric purity must exceed 99.5 %. For these reasons, the Moc‑protected (2S,5S) congener is frequently selected in early‑stage structure–activity campaigns where stereochemical fidelity and the ability to carry an orthogonal Cbz‑protected lysine are both mandatory.

When Stereochemistry at C5 Inverts: Consequences for Macrocyclization Outcome

A practical distinction emerges when the dipeptide building block is deployed in macrocyclization protocols. Ring‑closing metathesis of an allylglycine‑containing linear heptapeptide that incorporates (2S,5S)-Moc‑Val‑5‑Me‑Pro at the i→i+3 position occurs with a cyclization yield of 52 % under Grubbs second‑generation catalyst (5 mol%) in refluxing dichloromethane, whereas the identical sequence prepared with the (2S,5R) fragment delivers the macrocycle in only 18 % yield. Conformational analysis by ROESY spectroscopy indicates that the cis‑methyl isomer locks the proline residue in a turn‑stabilising geometry that brings the olefinic side chains into a proximity of 3.2 Å, while the trans‑methyl counterpart populates an extended conformation that forces the tethers apart. Given that the syntheses of strained 14‑membered lactams are highly sensitive to pre‑ring closure distance distributions, this conformational pre‑organisation is a decisive factor in selecting the (2S,5S) stereochemistry. Batch records from a kilo‑laboratory campaign employing a cyclic RCM strategy underscore that resin‑bound intermediates built with the (2S,5S) building block exhibited ≤2 % dimeric by‑products, a stark reduction from the 12 % dimer content observed with the trans isomer, thereby obviating a costly preparative HPLC step downstream.