Benchtop 19F nuclear magnetic resonance (NMR) spectroscopy provides mechanistic insight into the Biginelli condensation towards the chemical synthesis of novel trifluorinated dihydro- and tetrahydropyrimidines as antiproliferative agents

Abstract

Benchtop nuclear magnetic resonance (NMR) spectroscopy has enabled the monitoring and optimization of chemical transformations while simultaneously providing kinetic, mechanistic, and structural insight into reaction pathways with quantitative precision. Moreover, benchtop NMR proton lock capabilities further allow for rapid and convenient monitoring of various organic reactions in real-time, as the use of deuterated solvents is not required. The complementary role of 19F NMR-based kinetic monitoring in the fluorination of bioactive compounds serves many benefits in the drug discovery process, since fluorinated motifs additionally improve drug pharmacology. In this study, 19F NMR spectroscopy was utilized to monitor the synthesis of novel trifluorinated analogs of monastrol, a small molecule dihydropyrimidine kinesin-Eg5 inhibitor, and to probe the mechanism of the Biginelli cyclocondensation, a multicomponent reaction used to synthesize dihydropyrimidine and tetrahydropyrimidines through a Bronsted- or Lewis-acid catalyzed cyclocondensation between ethyl acetoacetate, thiourea, and an aryl aldehyde. In the present study, a trifluorinated ketoester serves a dual purpose as being the source of the trifluoromethyl group in our fluorinated dihydropyrimidines and as a spectroscopic handle for real time reaction monitoring and tracking of reactive intermediates by 19F NMR. Further, upon extending this workflow to a diverse array 3- and 4-substituted aryl aldehydes, we were able to derive Hammett linear free energy relationships (LFER) to determine stereoelectronic effects of para- and meta- substituted aryl aldehydes to corresponding reaction rates and mechanistic routes. In addition, we used density functional theory (DFT) calculations to corroborate our experimental results through the thermodynamic values of key intermediates in each mechanism. Finally, these studies cumulate in the synthesis of a novel trifluorinated analog of monastrol and its subsequent biological evaluation in vitro. More broadly, we show an application of benchtop 19F NMR spectroscopy as an analytical tool in the real-time investigation of a mechanistically and chemically complex multicomponent reaction mixture.