1-Tert-Butyl 2-Methyl 2H-Pyrrole-1,2(5H)-Dicarboxylateboc-3,4-Dehydro-L-Proline Methyl Ester

The compound (S)-1‑tert‑butyl 2‑methyl 2,5‑dihydro‑1H‑pyrrole‑1,2‑dicarboxylate (CAS 130930‑49‑3; molecular formula C₁₁H₁₇NO₄, 227.26 g mol⁻¹) is the N‑Boc‑protected methyl ester of 3,4‑dehydro‑L‑proline—a conformationally restricted proline surrogate wherein the C3–C4 single bond is replaced by an endocyclic double bond. The product is supplied as a colourless to pale‑yellow oil and serves as a chiral building block for the introduction of a pseudo‑proline turn constraint into peptide sequences. Its utility in medicinal chemistry programmes and solid‑phase peptide synthesis (SPPS) derives from the ability to enforce a specific backbone dihedral geometry while retaining the reactivity of the secondary amine after Boc removal.

Purity (HPLC, 210 nm)	≥ 98.0 area%
Enantiomeric excess (chiral HPLC)	≥ 99.0 % ee
Specific rotation [α]D20	−118 ± 2° (c 1.0, MeOH, NIST‑traceable quartz control plate)
Appearance	Colourless to pale‑yellow oil
Density (20 °C)	1.12 g mL⁻¹
Refractive index nD20	1.483–1.486

Maintaining Enantiomeric Integrity Under Standard Handling Conditions

Routine chiral stationary‑phase HPLC analysis employs a Chiralpak IA column (250 × 4.6 mm, 5 µm) with an isocratic mobile phase of n‑hexane/2‑propanol (90:10 v/v) at a flow rate of 1.0 mL min⁻¹ and UV detection at 215 nm; the (R)‑enantiomer elutes with a resolution factor R_s ≥ 2.5 from the main peak. Optical rotation is verified on a polarimeter calibrated against NIST SRM 928 quartz control plates in accordance with USP 〈781〉. Batches exhibiting [α]_D²⁰ outside the specified window are rejected regardless of chromatographic purity, as even 1–2 % of the opposite enantiomer can propagate into diastereomeric peptide sequences during fragment condensation. Storage at –20 ± 2 °C in tightly sealed amber borosilicate vials under dry argon is mandatory to preserve N‑Boc integrity. Exposure to ambient relative humidity > 60 % for periods exceeding 30 minutes initiates autocatalytic tert‑butyloxycarbonyl cleavage, with a measured water uptake of 2.8 mg g⁻¹ h⁻¹ at 25 °C and 75 % RH (gravimetric data on Mettler Toledo HX204). Containers should be allowed to equilibrate to room temperature before opening to prevent condensation, and any withdrawn aliquot not consumed within 4 hours must be discarded—redistillation or re‑chromatography of partially deprotected material introduces epimerisation products that co‑elute with the target compound on standard silica gel.

How Does the 3,4‑Dehydro Modification Alter Ring Conformation and Reactivity?

The endocyclic double bond flattens the pyrroline ring, shifting the preferred backbone torsion angle φ from approximately −60° in saturated proline to −120° in the dehydro analogue, as determined by solution NMR coupling constants and X‑ray crystallography of derived amides. This conformational restriction amplifies the propensity for a type‑II′ β‑turn when incorporated at the i+1 position, making the derivative especially valuable for stapled peptide libraries. Reactivity diverges markedly from Boc‑L‑proline methyl ester: the enamine character of the double bond renders the ring susceptible to electrophilic attack. Reaction with N‑bromosuccinimide in dichloromethane at 0 °C yields the 3,4‑dibromo adduct quantitatively within 15 min, whereas the saturated scaffold remains inert under the same conditions. This susceptibility restricts the choice of coupling additives: HOAt‑based reagents are preferred over HOBt, as traces of bromine from HOBt synthesis can degrade the product.

If N‑Boc Removal Proceeds in the Presence of Triisopropylsilane as Cation Scavenger

Standard global deprotection with trifluoroacetic acid (TFA)/triisopropylsilane (TIS)/water (95:2.5:2.5 v/v/v) at 25 °C for 60 min achieves > 99 % Boc removal, as monitored by LC‑MS (ESI⁺) for the free amine [M+H]⁺ m/z 128.1. Under these conditions methyl ester hydrolysis remains below 0.5 %, verified by the absence of the corresponding carboxylic acid ion (m/z 114.1). Omission of TIS results in a 4–7 % increase in tert‑butyl carbocation‑derived alkylation by‑products that are difficult to remove. Extended exposure beyond 90 min at 25 °C or temperature elevation to 40 °C raises methyl ester saponification to 1.8 %, and isolation of the amine must be performed immediately via precipitation with cold diethyl ether; lyophilisation from dilute acetic acid yields the acetate salt with 97 % recovery of optical purity. Coupling of the free amine after neutralisation in situ is the preferred route. Activation of Boc‑3,4‑dehydroproline as its HATU ester in DMF at 0 °C for 3 min prior to addition of the amino component—typically 1.2 equiv of H‑AA‑OMe·HCl with 4.0 equiv DIEA—consistently delivers dipeptides with isolated yields of 82–88 % on a 10 mmol scale. Sterically demanding nucleophiles such as H‑Aib‑OMe (α‑aminoisobutyric acid methyl ester) reduce single‑coupling efficiency to 65–70 % under identical conditions. Double coupling with PyAOP (2.5 equiv) and extended reaction time (12 h) raises incorporation to ≥ 85 %, and residual unreacted amine is capping with acetic anhydride to simplify purification. Racemisation during activation is suppressed by the N‑Boc urethane; derivatisation with (R)‑α‑methoxy‑α‑trifluoromethylphenylacetic acid followed by GC‑MS reveals epimerisation < 0.2 %. Unlike Boc‑L‑proline methyl ester (CAS 59936‑29‑7), which is a fully saturated, puckered pyrrolidine, the dehydro derivative cannot undergo cis/trans amide bond isomerisation, eliminating the kinetic complexity often seen in prolyl peptide coupling. This simplifies reaction monitoring by HPLC, as a single diastereomer predominates. However, the double bond introduces a sensitivity to hydrogenation conditions that saturated proline esters tolerate: attempted catalytic hydrogenation over 10 % Pd/C at atmospheric pressure quantitatively reduces the pyrroline to the proline scaffold, which is an intentional transformation in some synthetic routes but must be avoided where the unsaturated constraint is desired.

	Boc‑3,4‑dehydro‑L‑proline‑OMe	Boc‑L‑proline‑OMe
Ring conformation	Half‑chair, φ ≈ −120°	Puckered, φ ≈ −60°
Amide bond rotamers	Essentially single (>95 % trans)	Mixture of cis/trans (15–20 % cis)
Coupling efficiency with Aib‑OMe (single coupling)	65–70 %	55–60 %
Saponification half‑life (pH 10.5, 25 °C, H2O/dioxane 1:1)	4.2 min (epimerisation 2.5 % at 50 % conversion)	3.8 min (epimerisation 8 % at 50 % conversion)
Stability to electrophilic halogenation	Rapid addition across double bond	Inert

Direct saponification of the methyl ester for C‑terminal elongation is discouraged. Attempted hydrolysis with lithium hydroxide (1.0 equiv) in THF/water at 0 °C leads to epimerisation at Cα within 5 min, as observed by a drift in [α]_D²⁰ of +12° within the first 10 % conversion. This susceptibility originates from the enhanced acidity of the α‑proton in the planarised pyrroline ring, where the pK_a is estimated to be 0.8 log units lower than that of saturated proline methyl ester. When free‑acid intermediates are required, alternative protective‑group strategies that bypass ester hydrolysis—such as initial assembly on a 2‑chlorotrityl chloride resin followed by acidic cleavage—are recommended.