Rapid Enantiomeric Excess Measurements of Enantioisotopomers by Molecular Rotational Resonance Spectroscopy

Abstract

Recent work in drug discovery has shown that selectively deuterated small molecules can improve the safety and efficacy for active pharmaceutical ingredients. The advantages derive from changes in metabolism resulting from the kinetic isotope effect when deuterium is substituted for a hydrogen atom at a structural position where rate limiting C–H bond breaking occurs. This application has pushed the development of precision deuteration strategies in synthetic chemistry that can install deuterium atoms with high regioselectivity and with stereocontrol. Copper-catalyzed alkene transfer hydrodeuteration chemistry has recently been shown to have high stereoselectivity for deuteration at the metabolically important benzyl C–H position. In this case, stereocontrol results in the creation of enantioisotopomersmolecules that are chiral solely by virtue of the deuterium substitutionand chiral analysis techniques are needed to assess the reaction selectivity. It was recently shown that chiral tag molecular rotational resonance (MRR) spectroscopy provides a routine way to measure the enantiomeric excess and establish the absolute configuration of enantioisotopomers. High-throughput implementations of chiral tag MRR spectroscopy are needed to support optimization of the chemical synthesis. A measurement methodology for high-throughput chiral analysis is demonstrated in this work. The high-throughput ee measurements are performed using cavity-enhanced MRR spectroscopy, which reduces measurement times and sample consumption by more than an order-of-magnitude compared to the previous enantioisotopomer analysis using a broadband MRR spectrometer. It is also shown that transitions for monitoring the enantiomers can be selected from a broadband rotational spectrum without the need for spectroscopic analysis. The general applicability of chiral tag MRR spectroscopy is illustrated by performing chiral analysis on six enantioisotopomer reaction products using a single molecule as the tag for chiral discrimination.

Assignment of the absolute configuration of molecules that are chiral by virtue of deuterium substitution using chiral tag molecular rotational resonance spectroscopy

“…Similarly low ee has been found for three reaction products containing nitrogen heterocycles (ethylpyridine- d1 , 8-ethylquinoline- d1 , and 2-(4-ethylphenyl)pyridine- d1 ). 25 There is an interesting correlation that all low ee substrates have strong hydrogen bond acceptor positions where the chiral tag complex with TFIP prefers to attach via formation of a classical hydrogen bond ( Figure 7 ) instead of through interaction with the aromatic ring ( Figures 1 and 11 ). Future synthetic methodology development will explore ways to achieve high enantioselectivity in these substrates with hydrogen bond acceptor sites.…”

“…The rms rotational constant error for the scaled rotational constants used to assign the homochiral and heterochiral tag complexes to establish the absolute configuration is about thirty times smaller, 0.030%. Previous work demonstrated that chiral tag rotational spectroscopy provides high-speed ee measurement capabilities 25 and that MRR spectroscopy is a quantitative technique for measuring both the isotopolog and isotopomer distributions in deuterated molecules. 10 MRR spectroscopy delivers the full analysis capabilities required to support the development of synthetic chemistry approaches to precision deuteration chemistry.…”

“…The development of synthetic chemistry methods requires a routine, rapid method for measuring the enantiomeric excess (ee) of the reaction products. 25 Fast measurement times are needed so that a wide range of reaction conditions can be explored to optimize the enantioselectivity of the chemistry. The small chemical difference in isotopically chiral molecules limits the application of chiral chromatography and only a few cases of partial chiral resolution have been reported—requiring long separation times.…”

“… 54 – 56 As we recently discussed, ee measurements in chiral tag rotational spectroscopy can be performed without the need of any spectroscopic analysis. 25 This feature makes the technique a routine approach for ee measurements. However, establishing the absolute configuration requires a detailed analysis of the structures of the chiral tag complexes formed in the pulsed-jet expansion that introduces the sample into the spectrometer.…”

Cage-based sensors for circular dichroism analysis

“…9 The ee% also indicates how much of a drug is available in a therapeutic dose relative to the theoretical optimal dose. 10 Because living creatures are so sensitive to introduced molecules’ chirality, recognizing and quantitatively sensing chiral compounds is of great medical and biological significance.…”