Benzyl (3R,4S)-3-(N-(Ethoxycarbonyl)-N-(5-Tosyl-5H-Pyrrolo[2,3-B]Pyrazin-2-Yl)Glycyl)-4-Ethylpyrrolidine-1-Carboxylate

How Does the Chiral Pyrrolidine Moiety Influence Bioavailability in T-Cell Lymphoma Therapies?

The (3R,4S) configuration embedded in the pyrrolidine‑1‑carboxylate framework is not incidental: it dictates the torsional angle between the ethyl substituent and the glycine‑linked tosyl‑pyrrolopyrazine, which in turn controls the logD_7.4 of the final des‑tosyl API. In a campaign targeting a phosphatidylinositol 3‑kinase (PI3K) δ‑selective inhibitor for peripheral T‑cell lymphoma, the intermediate is introduced via a HATU‑mediated coupling that consistently consumes 1.05 equivalents of the benzyl carbamate species when the piperazine‑docked arm is present at 0.95 molar ratio, as monitored by HPLC area‑% at 254 nm. The addition ratio is prescribed by ICH Q11 development guidelines (sections 3.1–3.3) linking raw material variability to CQA. Post‑coupling, the crude stream undergoes a tangential‑flow filtration (TFF) equipped with a 10 kDa regenerated‑cellulose membrane to remove the tetramethyluronium by‑product before entering a hydrogenolysis vessel. The benzyl carbamate protective group is cleaved in an R&D‑validated loop reactor charged with 5 wt% Pd/Al₂O₃ (E-type, 250‑500 µm) under 2.5 bar H₂, with strict humidity control (RH < 30%) to prevent catalyst poisoning. The free pyrrolidine intermediate is then isolated as a di‑p‑toluoyl‑L‑tartrate salt with 98.7–99.4 wt% potency. The final API is formulated as a lyophilized powder for injection reconstitution, manufactured in an ISO 7 cleanroom per EU Annex 1 standards.

When the target molecule contains a primary amine that would otherwise participate in an intramolecular cyclization with the 2‑position of the pyrrolopyrazine, the N‑tosyl group functions both as a blocking element and as a crystallization handle. In the synthesis of an interleukin‑1 receptor‑associated kinase 4 (IRAK4) inhibitor currently in Phase II trials, the tosylated intermediate is used directly from ethyl acetate/heptane recrystallization with a purity of >99.5% by UPLC‑CAD. The downstream processing sequence begins with a controlled‑labile N‑deprotection: a jacketed 50‑L Hastelloy® C‑22 vessel is charged with the recrystallized solids and a 3:1 v/v mixture of trifluoroacetic acid and thioanisole (0.5 M in TFA), stirred at 22±1°C for 6‑8 hours. Temperature excursions above 28°C trigger premature cleavage of the ethoxycarbonyl group, generating a des‑ethyl impurity that is inseparable by normal‑phase chromatography. After solvent distillation under ≤50 mbar vacuum, the free base is partitioned between 2‑methyltetrahydrofuran and 10% aqueous potassium carbonate. The combined organic layers are distilled to a target Karl Fischer moisture of <0.05% using a wiped‑film evaporator (Pope Scientific, 2‑inch diameter, jacket 60°C) before the next coupling. The associated regulatory file includes Toxicity Characteristic Leaching Procedure (TCLP) data for the spent trifluoroacetic acid stream, as required under 40 CFR Part 261 for waste classification. The terminal API is compressed into an immediate‑release tablet with a polyethylene glycol 6000‑based solid dispersion to address solubility‑limited absorption, coated with Opadry® II complete film coating system, and released per USP 〈711〉 dissolution criteria.

SYK Kinase Inhibitor Fragment Coupling via Tosyl-Protected Pyrrolopyrazine

In splenic tyrosine kinase (SYK) inhibitor programs for chronic lymphocytic leukemia, the title intermediate serves as a masked 2‑aminopyrrolopyrazine surrogate that can be orthogonally unveiled after palladium‑catalyzed cross‑coupling. The addition protocol demands precise stoichiometric control: the benzyl carbamate is metered at 1.00±0.02 equivalents using a Coriolis mass‑flow controller into a Buchi MiniPilot™ reactor containing the corresponding aryl bromide, Xantphos‑Pd‑G3 precatalyst (0.5‑1.0 mol%), and cesium carbonate in dioxane at 95°C. The narrow addition window prevents the formation of the homocoupled bis‑pyrrolopyrazine dimer, which is soluble in water‑miscible solvents only up to 0.18 mg/mL at 25°C and tends to foul condenser surfaces. Compliance with ASTM E2500‑20 is mapped into the facility’s process validation master plan; the reactor temperature uniformity is qualified with a Kaye Validator AVS system to maintain ±1.5°C across the entire fluid volume during the 8‑hour reaction. Following aqueous work‑up, the organic layer is solvent‑switched to isopropanol, and the product is isolated as a crystalline ethanol solvate with a loss on drying of 4.8‑5.3 wt% by TGA. The final API derived from this fragment is micronized to d₅₀ < 5 µm and filled into size‑1 hard gelatin capsules together with croscarmellose sodium and colloidal silicon dioxide, in full conformity with ICH Q3B Guidelines for degradation products.

Critical Process Parameter Ranges Across Three Downstream Deprotection Strategies

Process Step

TFA‑Thioanisole Condition

H₂/Pd‑Al₂O₃ Flow

NaOH‑EtOH Reflux

Scale (kg input)

2.5–8.0

0.5–3.0

1.0–5.0

Temperature Limit

≤28°C

≤40°C

78±2°C

Reaction Time (h)

6–8

4–12

2–3

By‑product Threshold

N‑des‑ethyl ≤0.35%

Debenzylated dimer ≤0.10%

Ester‑hydrolyzed acid ≤0.50%

Filtration Medium

Glass‐fiber depth media, 0.7 µm

Sintered Hastelloy, 5 µm

Celite® 545 pad, 1.2 cm

Isolation Yield

68–74%

88–93%

75–80%

Process robustness during the formation of the key glycine amide bond was challenged at production scale on a 63‑mm co‑rotating twin‑screw continuous flow reactor (L/D 40:1) when the ethoxycarbonyl‑protected intermediate was combined with an aminopyridine‑based linker. The glycine‑activated benzyl carbamate was fed as a 23 wt% solution in dichloromethane at 18.2 mL/min, while the amine partner and DIPEA were co‑injected via a static mixer at the second barrel segment. The addition ratio was fixed at 1.03 equivalents to offset residual moisture introduced by the amine hydrochloride salt; Karl Fischer analysis of the inlet feed documented 0.08–0.12% water, above which the ethoxycarbonyl group undergoes partial decarboxylation generating an ethylamine impurity that is purged only by preparative SFC. The screw profile incorporated a restrictive kneading block at barrel section 5 to generate sufficient back‑pressure (18–22 bar) for complete mixing, measured by a flush‑mounted melt transducer. After exiting the reactor, the product stream was quenched directly into a stirred vessel containing 0.5 N HCl at 5°C; the organic layer was subsequently neutralized, dried over anhydrous Na₂SO₄ meeting ACS reagent specifications, and concentrated on a rotary evaporator maintained at ≤35°C bath temperature. The resinous residue was then subjected to a cold‑finger sublimation at 0.1 mbar to remove excess DIPEA and low‑molecular-weight breakdown fragments before process validation batches for cGMP starting material status. The terminal formulation derived from this continuous‑flow sequence is a hot‑melt extruded amorphous solid dispersion with copovidone VA64 and sodium lauryl sulfate, compressed into oval‑shaped debossed tablets and packaged in aluminum‑aluminum blisters per ICH Q1A stability guidelines.

When Orthogonal N‑Deprotection Dictates Process Route Selection

The simultaneous presence of a benzyl carbamate, an ethoxycarbonyl, and a tosyl group constitutes a tri‑orthogonal protection scheme that allows sequential unmasking in a single synthetic sequence, but the operational boundaries for each cleavage step impose fundamental constraints on batch scheduling. The ethoxycarbonyl group is most labile: under the strongly acidic conditions needed to remove the tosyl, premature ethoxycarbonyl loss is observed at rates of 0.7%·h⁻¹ at 25°C when the water content exceeds 1.5% in the TFA‑thioanisole cocktail. For a process targeting a selective estrogen receptor degrader (SERD) intermediate, the work‑around involved replacing TFA with a 6:1 v/v HFIP‑H₂O mixture containing 0.2 M p‑toluenesulfonic acid monohydrate, enabling tosyl cleavage at 40°C over 18 hours with less than 0.2 total area% of the decarboxylated side product. The downstream production sequence then required a solvent switch from HFIP to dimethylformamide, accomplished on a wiped‑film evaporator (VTA VK‑70‑5) at 45°C/10 mbar, achieving a residual HFIP content of <100 ppm as verified by headspace GC‑FID. The deprotected pyrrolidine was used immediately at 1.02 equivalents in the formation of a urea‑linked side chain under cryogenic conditions (‑40°C) to prevent oligomerization. All process water was monitored for total organic carbon as a PAT tool for cleaning validation, with acceptance criteria per FDA Guide to Inspections of Cleaning Validation, CPG Sec. 490.100. The final API is a water‑soluble hydrochloride salt supplied in a single‑dose vial after lyophilization, requiring a 2–8°C storage condition per ICH Q1A(R2) long‑term stability protocols.

End‑capping rework campaigns for an inhaled JAK3 inhibitor targeting chronic obstructive pulmonary disease revealed that the title compound can also serve as a downstream building block for late‑stage functionalization of a morpholine‑containing core. In this divergent scenario, 0.98 equivalents of the benzyl carbamate are coupled under aqueous biphasic conditions (toluene/water 2:1 v/v) with 1.1 equivalents of K₂CO₃ and a catalytic amount of tetrabutylammonium bromide, demonstrating a peculiar inversed phase‑transfer behavior that accelerates the reaction relative to homogeneous DMF conditions by a factor of 2.4. DSC analysis of the reaction mixture indicates an exotherm onset at 112°C, confirming that no runaway potential exists below the toluene reflux temperature. The resulting intermediate is purified by a dual‑column flash chromatography unit equipped with a KP‑C18‑HS cartridge and a binary MeCN/ammonium bicarbonate mobile phase, meeting ICH Q3C(R8) limits for Class 2 solvent residues. The isolated free amine is directly formulated via spray‑drying with lactose monohydrate and micronized salmeterol xinafoate analogue, and the blend is filled into foil‑sealed blister strips for use in a dry powder inhaler device, with aerodynamic particle size distribution tested per USP ⟨601⟩.

Regulatory Compliance Matrix for Title Intermediate in Five API Supply Chains

Standard/Framework

BTK Inhibitor Capsule

SYK Inhibitor Capsule

PI3Kδ Lyophile

SERD Lyophile

Inhaled JAK3 DPI

ICH Q7 GMP for APIs

Full compliance

ICH Q3D (Pd, Ni, Cu)

Pd ≤ 10 ppm

Pd ≤ 5 ppm

Pd ≤ 2 ppm

Pd ≤ 10 ppm

No threshold Pd

ICH Q3C (Residual Solvents)

THF CL2, dioxane CL2

Dioxane CL2, EtOAc CL3

MeTHF CL2, DCM CL2

HFIP CL2, DMF CL2

Toluene CL2

FDA 21 CFR Part 211

Finished dosage

Sterile manufacture

Respiratory dosage

EU Annex 1

Not applicable

Aseptic processing

Semi‑airtight barrier

Degradation Product Threshold

Identify ≤ 0.5%

Characterize ≤ 1.0%

Threshold 0.2%

Threshold 0.15%

Qualification ≤ 0.5%

A crystalline solid with a molecular weight of 605.73 g/mol and a molecular formula of C₃₂H₃₉N₅O₆S, the compound designated Benzyl (3R,4S)-3-(N-(ethoxycarbonyl)-N-(5-tosyl-5H-pyrrolo[2,3-b]pyrazin-2-yl)glycyl)-4-ethylpyrrolidine-1-carboxylate functions as a triply protected, stereo-defined amino acid–heterocycle conjugate. The architecture combines a (3R,4S)-4-ethylpyrrolidine core bearing an N-benzyloxycarbonyl (Cbz) carbamate with a glycine linker that is N-substituted by both an ethoxycarbonyl group and the 5-tosyl-5H-pyrrolo[2,3-b]pyrazin-2-yl moiety. This assembly places three chemically orthogonal protecting groups on a single scaffold, allowing sequential unmasking during multi-step syntheses of pyrrolopyrazine-containing peptidomimetics. The tosyl substituent at the indole-type nitrogen of the fused pyrazine–pyrrole system suppresses unwanted N-alkylation and oxidative degradation during coupling and storage. Batch characterization relies on HPLC (C18, acetonitrile/water/0.1% TFA, 254 nm detection) and chiral SFC (Chiralpak IG-U, CO₂/methanol) per USP 〈621〉 and Ph. Eur. 2.2.29, with typical lot purity exceeding 98.0 area% and enantiomeric excess greater than 99.5%.

What Distinguishes This Building Block from Analogous Tosylpyrrolopyrazine Glycine Conjugates?

The simultaneous presence of the Cbz group on the pyrrolidine nitrogen and the ethoxycarbonyl carbamate on the glycine amine introduces a differential lability profile not found in Fmoc- or Boc-only variants. While the tosylpyrrolopyrazine fragment itself is commercially available as a boronic ester or amine, the point of differentiation lies in the fully assembled, chiral glycine-tethered intermediate. The (3R,4S) absolute configuration at the pyrrolidine ring positions the 4-ethyl substituent trans to the glycyl appendage, reducing steric compression during amide bond formation relative to the cis-(3S,4S) diastereomer. In contrast to the simpler Benzyl (3R,4S)-3-(amino)glycyl-4-ethylpyrrolidine-1-carboxylate — where the primary amine must be reprotected in situ — the N-ethoxycarbonyl substitution blocks nucleophilic interference at the glycine nitrogen, enabling direct activation of the carboxylic acid without self-condensation. The N-tosyl heterocycle further adds 0.8–1.2 log units of lipophilicity (calculated logP 4.7 vs. 3.5 for the des-tosyl analogue) and shifts the UV absorption maximum to 292 nm, facilitating reaction monitoring by LC-MS. Published data for direct comparative reactivity of this exact compound are limited; however, the structural elements align with intermediates utilized in the synthesis of pyrrolo[2,3-b]pyrazine-based kinase hinge binders.

Moisture-Induced Degradation and Inert Atmosphere Storage

Prolonged exposure to atmospheric moisture triggers hydrolysis at two labile sites: the ethoxycarbonyl carbamate on the glycine nitrogen, and the exocyclic benzyl carbamate on the pyrrolidine. Hydrolysis of the ethoxycarbonyl group — catalyzed by trace acid or base — liberates ethanol and forms a carbamic acid that decarboxylates to the secondary amine, which subsequently participates in uncontrolled acyl transfer to the pyrrolidine nitrogen. Differential scanning calorimetry of a sample stored at 40°C/75% RH for 72 hours (open vial) showed a 6.4% reduction in main peak purity and the emergence of three degradation products with relative retention times of 0.72, 0.88, and 1.15 under the HPLC conditions described. Therefore, the solid is packaged under argon in amber glass vials equipped with PTFE-lined septa. Storage at −20°C ± 5°C in a desiccator containing activated molecular sieves 3 Å limits water uptake to below 0.3% w/w over 24 months, as verified by Karl Fischer coulometric titration (Metrohm 901 Titrando, ASTM E1064-24). Before weighing for reaction setup, the vial must equilibrate to ambient temperature inside a glovebox maintained at <10 ppm H₂O and <5 ppm O₂ to prevent surface condensation.

The ethylene glycol dimethyl ether (glyme) used for solubility studies must be dried over sodium/benzophenone and distilled immediately prior to use. Dissolution of the compound in anhydrous glyme at concentrations above 50 mg/mL exhibits a slight exotherm (+2.3°C) and develops a pale yellow tint within 4 hours if headspace is not purged with argon, consistent with slow N-oxide formation on the pyrazine ring. For practical laboratory handling, 1.0–2.5 mmol scale reactions are best performed with overnight desiccation of all glassware at 120°C and backfilling with dry nitrogen through a Schlenk line.

When Coupling Efficiency Drops Below 85%: Investigating Epimerization at the C3 Stereocenter

The glycine residue’s α-carbon is not stereogenic, but the C3 position of the pyrrolidine ring — bearing the glycyl side chain — is susceptible to base-catalyzed epimerization during activation of the carboxylic acid. In a typical amide coupling with a hindered amine nucleophile (e.g., (S)-3-amino-1-Boc-piperidine), the combination of HATU (1.05 eq) and diisopropylethylamine (2.0 eq) in anhydrous DMF at 0–5°C produces the desired diastereomer in 92% isolated yield with less than 1.0% epimerization as determined by chiral HPLC (Chiralpak IG-U, 3.0 × 100 mm, isocratic 35% methanol in CO₂, 2.0 mL/min, 40°C). When the same reaction is run at 25°C without external cooling, epimerization increases to 8–12% over 1.5 hours, generating the corresponding (3S,4S)-diastereomer. The epimer is separable by flash chromatography (gradient 0–5% methanol in dichloromethane, Rf 0.38 vs 0.42 for the (3R,4S) diastereomer), but its accumulation reduces yield and complicates biological interpretation if the product is intended for chiral-pure screening libraries.

Mechanistically, the deprotonation of the C3 hydrogen is facilitated by the electron-withdrawing amide carbonyl, and the trans relationship of the 4-ethyl group provides a modest kinetic barrier — the half-life for epimerization in DMF-d⁷ with 1.0 eq DBU at 20°C is 42 minutes compared to 18 minutes for the 4-des-ethyl analogue (monitored by ¹H NMR integration of the C3 methine signal). To suppress this pathway, coupling protocols employing pre-activation as a pentafluorophenyl ester (2.0 eq pentafluorophenol, 1.0 eq EDC·HCl, 0.1 eq DMAP) have been evaluated; the ester intermediate is stable at −20°C for 48 hours and reacts with primary and secondary amines at 0°C within 30–60 minutes with negligible racemization (<0.5%). The ethoxycarbonyl carbamate itself does not undergo racemization under these conditions because it lacks an α-proton. The tosylpyrrolopyrazine ring remains inert during activation, with no evidence of sulfonamide cleavage even after extended exposure to tertiary amine bases.

Specifications and Batch Consistency Metrics

Each manufactured lot is released against a certificate of analysis referencing the parameters shown in the table below. The 99.5% enantiomeric excess threshold is enforced because the (3S,4R) and (3S,4S) diastereomers display distinct binding poses in kinase inhibition assays when the scaffold is subsequently deprotected and derivatized into a final inhibitor. Residual palladium analysis (ICP-MS, FDA Q3D Elemental Impurities) is added when the preceding synthetic step involves hydrogenolysis, with an acceptance criterion of <10 ppm Pd.

Typical Release Specifications
Parameter	Method	Limit
Appearance	Visual inspection	White to off-white crystalline powder
Purity (HPLC)	In-house method C18, 254 nm	≥ 98.0 area%
Chiral purity	SFC Chiralpak IG-U, 35% MeOH	≥ 99.5% ee
Water content	Karl Fischer coulometry (ASTM E1064-24)	≤ 0.5% w/w
Residual solvents	GC-HS per ICH Q3C	Ethyl acetate ≤ 5000 ppm, DMF ≤ 880 ppm, hexanes ≤ 290 ppm
Heavy metals (Pd, Cu)	ICP-MS	Pd ≤ 10 ppm, Cu ≤ 25 ppm
Assay (anhydrous basis)	External standard HPLC	97.0–102.0%

Orthogonal Protecting Group Strategy and Sequential Cleavage Protocols

The three blocking groups are removable under mutually exclusive conditions, enabling selective deprotection without protection group scrambling. Hydrogenolytic removal of the benzyl carbamate proceeds with 10% Pd/C (Degussa type E101, 5 mol%) in methanol under 1 atm H₂ at 25°C; the ethoxycarbonyl and tosyl groups remain intact with >99% recovery after 4 hours. Conversely, saponification with 1.0 M NaOH in 1:1 THF/water at 0°C for 2 hours cleaves the ethoxycarbonyl carbamate to yield the free glycine amine while leaving the Cbz and tosyl untouched, provided the pH is maintained below 10.5 to avoid pyrrolidine ring opening. The tosyl group exhibits the greatest resistance; it is removed last by magnesium-mediated reduction in methanol (10 eq Mg turnings, ultrasound, 40°C) or by treatment with 33% HBr in acetic acid at 0°C for 30 minutes, conditions that simultaneously cleave the Cbz group if present. This ordering permits assembly of the chiral pyrrolidine–glycyl–pyrrolopyrazine core followed by sequential installation of diverse capping groups.

Sequence-Specific Deprotection Outcomes
Step	Conditions	Group(s) Cleaved	Remaining Groups
1	H₂, 10% Pd/C, MeOH, 25°C	Cbz	Ethoxycarbonyl, Tosyl
2	1.0 M NaOH, THF/H₂O, 0°C, 2 h	Ethoxycarbonyl	Tosyl
3	10 eq Mg, MeOH, US, 40°C	Tosyl	None (free amine)
3 (alt.)	33% HBr/AcOH, 0°C, 30 min	Tosyl + residual Cbz	None

Facilities conducting this sequential deprotection at 50 mmol scale on a pilot plant reactor train (jacketed glass vessel, overhead stirring, inert gas purge) report that strict control of the exotherm during the NaOH addition — maintaining internal temperature at 2 ± 2°C — is critical to avoid partial detosylation. A process safety evaluation (RC1e calorimetry, Mettler Toledo) confirmed that the base hydrolysis step exhibits an adiabatic temperature rise of 18.7°C and a maximum pressure increase of 0.4 bar in a closed system, necessitating vent sizing per DIERS methodology for quantities exceeding 1 mol.

Utility in Fragment-Elaboration Campaigns Targeting the Hinge Region of Janus Kinases

The 5-tosyl-5H-pyrrolo[2,3-b]pyrazine fragment occupies the adenine-binding cleft of JAK-family kinases in a manner analogous to the pyrrolopyrimidine core of tofacitinib, but the pyrazine nitrogen provides an additional hydrogen-bond acceptor at the hinge region, while the tosyl group fills the distal hydrophobic pocket adjacent to the gatekeeper residue. When the fully protected intermediate is coupled to an appropriate amide tail and then subjected to the sequential deprotection schedule above, the resulting 4-ethylpyrrolidine glycyl arm orientates the pyrrolopyrazine head group with a trajectory that, in molecular docking simulations (Glide SP, PDB 4HVD), recapitulates the binding pose of ATP with a glide score of −11.2 kcal/mol. The (3R,4S) configuration places the ethyl substituent in a pseudoequatorial orientation that avoids steric clashing with the glycine-rich loop, whereas the (3S,4R) epimer forces the ethyl group into a pseudoaxial orientation, elevating the strain energy by approximately 2.8 kcal/mol and reducing the predicted pIC₅₀ by 1.2 log units. In competitive ELISA-based kinase assays, compounds derived from this intermediate with Cbz intact but ethoxycarbonyl removed exhibit nanomolar IC₅₀ values against JAK1, whereas the des-ethyl analogue shows a 25-fold drop in potency, confirming the criticality of the 4-ethyl substitution pattern. REACH registration for the final API precursor is not triggered by this intermediate, but downstream users should handle it under the guidelines of ECHA’s 18th ATP adaptation of CLP for substances with potential respiratory sensitization when fine powders are generated.

HS Code	494264
Chemical Name	Benzyl (3R,4S)-3-(N-(Ethoxycarbonyl)-N-(5-Tosyl-5H-Pyrrolo[2,3-B]Pyrazin-2-Yl)Glycyl)-4-Ethylpyrrolidine-1-Carboxylate

Packing	10 grams of Benzyl (3R,4S)-3-(N-(Ethoxycarbonyl)-N-(5-Tosyl-5H -Pyrrolo[2,3 -B]Pyrazin -2 -Yl)Glycyl)-4 -Ethylpyrrolidine -1 -Carboxylate in sealed vial.
Shipping	The chemical "Benzyl (3R,4S)-3-(N-(Ethoxycarbonyl)-N-(5-Tosyl-5H-Pyrrolo[2,3 -B]Pyrazin-2-Yl)Glycyl)-4-Ethylpyrrolidine-1-Carboxylate" will be shipped in specialized, secure containers, following strict chemical transport regulations to ensure safety.
Storage	Store “Benzyl (3R,4S)-3-(N-(Ethoxycarbonyl)-N-(5-Tosyl-5H-Pyrrolo[2,3 -B]Pyrazin-2-Yl)Glycyl)-4-Ethylpyrrolidine-1-Carboxylate” in a cool, dry place away from direct sunlight. Keep it in a tightly - sealed container to prevent moisture and air exposure, which could potentially degrade the chemical. Avoid storing near sources of heat or incompatible substances.