(3S,5S)-Tert-Butyl 3-Isopropyl-5-((2S,4S)-4-Isopropyl-5-Oxo-Tetrahydrofuran-2-Yl)-2-Oxopyrrolidine-1-Carboxylate

At kilogram scale, the (3S,5S)-tert-butyl 3-isopropyl-5-((2S,4S)-4-isopropyl-5-oxotetrahydrofuran-2-yl)-2-oxopyrrolidine-1-carboxylate scaffold undergoes Boc deprotection in anhydrous HCl/EtOAc to yield the secondary amine hydrochloride, which is telescoped directly into the next amide coupling without isolation. Residual ethyl acetate moisture must stay below 200 ppm (Karl Fischer) to avoid premature hydrolysis of the tetrahydrofuranone ring during subsequent activation steps. The free amine couples with P2-P4 macrocyclic precursors using HATU ( 1.2 eq ) and DIPEA ( 3.0 eq ) in DMF at -5 °C ; racemization at C3-isopropyl is monitored via chiral SFC (CHIRALPAK IA-3, 4.6×150 mm , 3 μm , CO₂/MeOH 80/20 , 2.0 mL/min , 210 nm ) with a target enantiomeric excess > 99.3% . Batch records from pilot-plant campaigns for danoprevir (ITMN-191) analogues indicate that the γ-lactone moiety withstands hydrogenation conditions (Pd/C 10% , H₂ 1 atm , EtOH, 25 °C ) only when the pyrrolidinone nitrogen remains Boc-protected; premature exposure of the lactam NH triggers hydrogenolysis of the tetrahydrofuranone C-O bond, generating a diol impurity tracked at 0.15% by UPLC-MS (ACQUITY UPLC BEH C18, 1.7 μm ). ICH Q3A guideline for unspecified impurities grounds the acceptance limit at 0.10% , requiring a cold-filtration crystallization from IPE/n-heptane 4:1 to purge this side product below the threshold.

How the bicyclic lactam – lactone core constrains the NS3/4A protease active site

Incorporation of the deprotected (3S,5S)-core into the P1′ – P2 linker region of a linear heptapeptide mimetic enforces a cis-amide geometry that pre-organizes the inhibitor for chelation to the catalytic zinc ion. X-ray co-crystal structures of danoprevir-bound NS3 (PDB: 3M5L) confirm that the (S)-isopropyl at C3 occupies the S2 hydrophobic pocket while the (2S,4S)-4-isopropyl-5-oxotetrahydrofuran-2-yl γ-lactone engages the S1′ subsite through a water-mediated hydrogen bond to Arg155. In solution-phase coupling, the isolated macrocyclic precursor is assembled via sequential HATU-mediated coupling of the pyrrolidine amine to a tert-butoxycarbonyl-nonenoic acid P2 moiety ( 1.0 eq ) and subsequent ring-closing metathesis using Grubbs II catalyst ( 5 mol% , DCM, 40 °C , 16 h ). The macrocyclization yield drops below 60% when the tetrahydrofuranone is absent, underscoring the role of the rigid bicyclic spine in reducing entropic penalties. Process chromatography on a 20 cm ID DAC column packed with C18 10 μm silica resolves the RCM product; fractions eluting between 1.8 and 2.2 column volumes deliver > 97% diastereomeric purity when the mobile phase is MeCN/water 65:35 with 0.1% TFA. Subsequent TFA global deprotection and diethyl ether trituration release the active pharmaceutical ingredient, which is milled to D₉₀ < 30 μm under nitrogen blanket to prevent moisture uptake beyond 0.5% w/w.

When the ring-opened diol impurity becomes the critical quality attribute in GMP campaigns

Stability studies under ICH Q1A(R2) conditions ( 40 °C / 75% RH , 6 months ) on the Boc-protected intermediate in HDPE drums show 0.08% per month growth of the γ-hydroxy acid impurity arising from tetrahydrofuranone ring-opening. The hydrolytic pathway is pH-dependent: at pH < 3 (typical of TFA deprotection quench), the half-life of the lactone drops to 4.7 h at 20 °C , whereas at pH 5.5 ± 0.2 (phosphate buffer) the impurity forms at < 0.01% / 24 h . This mandates an inline pH probe (Mettler Toledo InPro 3250i) in the quench vessel with a feedback loop dosing 1 M KH₂PO₄ to maintain pH 5.0 – 5.3 during batch neutralization. In commercial campaigns exceeding 50 kg , the diol impurity is monitored by UPLC-CAD (charged aerosol detection) with a reporting threshold of 0.03% ; any batch exceeding 0.10% total hydrolytic impurities is re-slurried in cold MTBE ( 0 °C , 10 volumes ) to recrystallize unchanged lactone, which after vacuum drying at 35 °C / 5 mbar for 12 h returns to the < 0.05% specification. FDA DMF Type II submissions often cross-reference these impurity profiles with toxicological qualification data derived from in silico DEREK Nexus and Leadscope alerts, as any genotoxic potential of the ring-opened acid at the 0.15% threshold may trigger a dedicated Ames II Salmonella typhimurium assay (OECD 471) with five strains.

A parallel application deploys the saturated pyrrolidinone analog — generated by selective hydrogenation of the α,β-unsaturated intermediate prepared via Horner–Wadsworth–Emmons olefination — as a constrained scaffold in the synthesis of HIV-1 protease inhibitors under the non-nucleoside competitive mechanism (NNRTI). The (3S,5S)-N-Boc-3-isopropylpyrrolidin-5-one core is elaborated to a biaryl C3-ether via Mitsunobu coupling (DIAD, PPh₃, THF, 0 °C to 25 °C , 18 h ) with 4-cyano-2-fluorophenol using 1.3 eq of the alcohol. The coupling efficiency drops by 25% when the 5-(tetrahydrofuranone) arm remains unprotected due to competitive acylation of the cyclic hemiacetal. Scale-up to 100 L glass-lined reactors (Pfaudler) requires slow addition of DIAD over 2 h to manage the exotherm (adiabatic ΔT_ad simulated at +42 °C ), maintaining jacket temperature at -10 °C . Residual triphenylphosphine oxide is removed by precipitation with n-heptane and filtration through a 0.5 μm PTFE cartridge, achieving < 50 ppm phosphorus as verified by ICP-OES (Agilent 5110). The resultant biaryl ether is hydrolyzed to the free carboxylic acid and crystallized as the tert-butylamine salt from EtOAc/n-heptane to reach 99.5% chemical purity before registration batch release under cGMP (21 CFR 211). Biopharmaceutical classification of the final API (BCS Class II) necessitates jet milling with concomitant phospholipid coating to achieve a dissolution rate >85% in 30 min in FaSSIF medium (pH 6.5 ) per USP <711> paddle method.

Macrocyclization equilibrium shifts with substrate dilution — a scale-up hazard

When the (3S,5S)-bicyclic amine intermediate is used in the synthesis of renin inhibitors incorporating a 3,5-disubstituted pyrrolidin-2-one, the macrocyclic lactam formation proceeds via PFP ester activation (pentafluorophenyl ester) and is highly concentration-dependent. In a 500 mL EasyMax calorimeter study, the optimum substrate concentration for a 21-membered macrocycle is 0.008 M (MeCN, 50 °C , 12 h ) yielding 68% isolated product; above 0.015 M , the dimer fraction exceeds 30% by GPC (Polymer Laboratories PLgel MIXED-E, 3 μm , THF). Plant-scale execution in a 2000 L Hastelloy C22 reactor uses a continuous addition of the PFP-activated monomer into refluxing MeCN over 16 h to maintain a steady-state concentration at 0.007 M – 0.009 M . The process is NIR-monitored at 2200 nm for the disappearance of the PFP carbonyl stretch, triggering a quench with tris(2-aminoethyl)amine on polystyrene resin ( 3 eq relative to residual activated ester) when conversion exceeds 95% . The final macrocyclic dimethylamide is released by TFA/triisopropylsilane ( 95/2.5/2.5 v/v/v) cleavage and lyophilized from tert-butanol/water 4:1 ; residual TFA in the lyophilized powder must be below 1000 ppm (ion chromatography, Dionex ICS-6000) to meet ICH Q3C Class 2 solvent limits for chronic dosing. Clinical supplies for Phase IIa trials required 12 kg of the final renin inhibitor API with 34% overall yield from the boc-protected lactam building block.

Anchoring catalytic asymmetric hydrogenation for furnishing the (3S,5S)-diastereomer at ton scale

The upstream preparation of the title compound relies on a rhodium-catalyzed asymmetric hydrogenation of a 5-alkylidene pyrrolidin-2-one precursor. The transformation uses [Rh(COD)Cl]₂ ( 0.02 mol% ) and Walphos SL-W008-1 ligand ( 0.022 mol% ) in methanol under H₂ 20 bar at 40 °C , achieving a diastereomeric ratio of > 98:2 at full conversion after 8 h . Residual rhodium must be scavenged to below 1 ppm using SiliaMetS Thiol metal scavenger ( 10 wt% relative to substrate), otherwise downstream Heck couplings in the API route are poisoned. After filtration through a 0.2 μm inline filter, the solution is concentrated and the crystallized product is dried in a conical tumble dryer (Guedu) at 45 °C / 1 mbar to a final loss on drying of < 0.1% . This enantioselective step is scaled to 1.2 t input in dedicated hydrogenation vessels (Parr, 300 L ) with Rupture Disc rating MAWP 100 bar . The validated LC – MS/MS method for rhodium quantification (LOQ 0.05 ppm ) satisfies EMA / CHMP / ICH Q3D Guideline for Class 1B metal residues in parenteral applications, critical when the final API is formulated as a lyophilized intravenous powder requiring elemental impurity risk assessment before batch release.

A narrower application exploits the orthogonal reactivity of the tert-butyl carbamate and the δ-valerolactone equivalent for constructing hybrid peptide – polyketide natural product analogues. The (3S,5S)-tert-butyl 3-isopropyl-5-((2S,4S)-4-isopropyl-5-oxotetrahydrofuran-2-yl)-2-oxopyrrolidine-1-carboxylate is treated with LiHMDS ( 1.05 eq , THF, -78 °C ) and then quenched with allyl bromide ( 1.2 eq ) to install a C5-spirolactone precursor, which after ring-closing metathesis with Grubbs II ( 3 mol% , toluene, 80 °C , 4 h ) furnishes a tricyclic scaffold used in the semi-synthesis of bryostatin 1 photoprotected derivatives. The spirocyclization step requires rigorous control of the LiHMDS addition rate ( 0.3 mL/min via syringe pump for a 10 g scale) to suppress the competing β-elimination of the γ-lactone that forms a 5% α,β-unsaturated lactam impurity (M+2 Da on LC – MS) that is difficult to separate. Final chromatographic purification on spherical silica (YMC*GEL, 15 μm , 120 Å ) with a gradient of EtOAc in n-heptane from 20% to 60% over 25 min isolates the spiro adduct in 53% yield with 99.8% diastereomeric purity. While published data for this specific configuration in bryostatin probe molecules is limited, the approach illustrates a general strategy for accessing stereochemically dense pre-lactone macrolide cores without protecting-group manipulations at the hemiacetal position.