Measurement of Atropisomer Racemization Kinetics Using Segmented Flow Technology

Abstract

When stable atropisomers are encountered by drug discovery teams, they can have important implications due to potential differences in their biological activity, pharmacokinetics, and toxicity. Knowledge of an atropisomer's activation parameters for interconversion is required to facilitate informed decisions on how to proceed. Herein, we communicate the development of a new method for the rapid measurement of atropisomer racemization kinetics utilizing segmented flow technology. This method leverages the speed, accuracy, low sample requirement, safety, and semiautomated nature of flow instrumentation to facilitate the acquisition of kinetics data required for experimentally probing atropisomer activation parameters. Measured kinetics data obtained for the atropo isomerization of AMPA antagonist CP-465021 using segmented flow and traditional thermal methods were compared to validate the method. KEYWORDS: Atropisomer, Reaction kinetics, Segmented flow chemistry T he incidence of axial chirality known as atropisomerism 1 in chemical research is steadily increasing due to the advent of new chemical methodologies, which have made sterically encumbered bonds more facile to form. 2 Their existence in a drug discovery setting can have important implications due to possible differences in their biological activity, pharmacokinetics, and toxicity. 3 Knowledge of atropisomer activation parameters facilitates informed decisions about appropriate handling, storage, and reaction conditions. These data can also influence preclinical development strategy. 4 Herein, we report the rapid measurement of atropisomer racemization kinetics utilizing segmented flow technology. This method leverages the speed, accuracy, low sample requirement, safety, and semiautomated nature of flow instrumentation 5 to facilitate convenient acquisition of the kinetics data required for experimentally probing activation parameters.We demonstrate the utility of this method by employing it to generate kinetics data for CP-465021 (Figure 1), a potent and selective noncompetitive α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist whose synthesis and resolution are reported elsewhere. 6−9 This molecule has an axial element of chirality due to hindered rotation of the N-aryl bond. Previously, serial measurements of the chiral stability of this molecule were conducted in a thermally controlled oil bath in 3-methylbutan-1-ol at 120°C. These experiments suggest that interconversion of CP-465021 occurs via a planar, nonionic transition state with a rate constant k rac = 1.07 × 10 −2 min −1 . 10 Segmented flow technology 11 is uniquely suited for measuring thermal interconversion kinetics and has several advantages over traditional methods. First, the reaction temperature can be maintained safely above the boiling point of solvent due to the instrument's ability to contain pressure, thus accelerating reaction times and data acquisition. Second, the thermal mass of the system is much greater than the reaction fluid permitti...

Tautomerism and Atropisomerism in Free‐Base (meso)‐Strapped Porphyrins: Static and Dynamic Aspects

“…The results of the two experiments and definitions of the kinetic and thermodynamic terms referred to in this study are summarised in Figure 14 [Eqs. (1) and (2)]. Assuming reversible and first-order reactions [74] for the interconversion of two atropisomers of different stability, [78][79][80][81][82] the integrated rate law of the atropisomerisation process led to Equation (3) (see Figure 15), adapted from the equations previously reported by Carlier and co-workers for the epimerisation of diastereoisomers with different configurational stability. [76] As seen in Figure 16, the plots of c t (%) versus time for the two experiments are perfectly fitted by first-order exponential curves (decaying or growing) with a non-zero end-point (see the fitting model equation in Figure 15) with the correlation coefficients r = 0.99962 and 0.99968 for experiments 1 and 2, respectively, matching perfectly the integrated rate law.…”

Conformational Studies and Atropisomerism Kinetics of the ALK Clinical Candidate Lorlatinib (PF‐06463922) and Desmethyl Congeners

“…[27] We then envisioned that we would employ the segmented flow technology for the measurement of the racemization kinetics. [28] However,d espite the advantages of this methodology with both the automation and the unprecedented temperature control offered by the Accendo Conjure, [29] there are drawbacks particularly given the introduction of ad isconnect between the emergence/processing of segments from the instrument and the actual chiral analysis.A lthough, for molecules with ahigh barrier to inter-conversion (such as (S)- 1 HNMR comparison of compound 1a/1b and 2 (PF-06463922). MDR BA/AB (ratio) [c] TrkB , the potential errors from this disconnect are practically non-existent and this method is unique in offering the ability to accelerate the kinetics through heating solvents above the boiling point.…”

Conformational Studies and Atropisomerism Kinetics of the ALK Clinical Candidate Lorlatinib (PF‐06463922) and Desmethyl Congeners

“…[26] Va riable temperature NMR was carried out on Compound 1a (see Supporting Information). [28] However,d espite the advantages of this methodology with both the automation and the unprecedented temperature control offered by the Accendo Conjure, [29] there are drawbacks particularly given the introduction of ad isconnect between the emergence/processing of segments from the instrument and the actual chiral analysis.A lthough, for molecules with ahigh barrier to inter-conversion (such as (S)- [27] We then envisioned that we would employ the segmented flow technology for the measurement of the racemization kinetics.…”

“…[27] We then envisioned that we would employ the segmented flow technology for the measurement of the racemization kinetics. [28] However,d espite the advantages of this methodology with both the automation and the unprecedented temperature control offered by the Accendo Conjure, [29] there are drawbacks particularly given the introduction of ad isconnect between the emergence/processing of segments from the instrument and the actual chiral analysis.A lthough, for molecules with ahigh barrier to inter-conversion (such as (S)- BINOL), the potential errors from this disconnect are practically non-existent and this method is unique in offering the ability to accelerate the kinetics through heating solvents above the boiling point. Formolecules though in which t 1/2 is in the range of hours,d elays in sample analyses and background epimerization of stock solutions can introduce ad egree of error.…”