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Viral mRNA cap methyltransferases (MTases) are emerging targets for the development of broad-spectrum antiviral agents. In this work, we designed potential SARS-CoV-2 MTase Nsp14 and Nsp16 inhibitors by using bioisosteric substitution of the sulfonium and amino acid substructures of the cosubstrate S-adenosylmethionine (SAM), which serves as the methyl donor in the enzymatic reaction. The synthetically accessible target structures were prioritized using molecular docking. Testing of the inhibitory activity of the synthesized compounds showed nanomolar to submicromolar IC50 values for five compounds. To evaluate selectivity, enzymatic inhibition of the human glycine N-methyltransferase involved in cellular SAM/SAH ratio regulation was also determined, which indicated that the discovered compounds are nonselective inhibitors of the studied MTases with slight selectivity for Nsp16. No cytotoxic effects were observed; however, this is most likely a result of the poor cell permeability of all evaluated compounds.

The reaction of methyl 2-isothiocyanatobenzoate and 1-azido-3-(4-substituted phenyl)propan-2-ones in the presence of triphenyl phosphine in dioxane by heating produced tricyclic (Z)-2-(4-substituted benzylidene)-2,3-dihydro-5H-oxazolo[2,3-b]quinazolin-5-ones instead of the expected methyl 2-{[5-(4-substituted benzyl)oxazol-2-yl]amino}benzoates. A one-pot procedure for the synthesis of 2-substituted 5H-oxazolo[2,3-b]quinazolin-5-ones from appropriate azidomethyl ketones and 2-isothiocyanatobenzoate was developed.