Abstract <p>Multi-component reactions (MCRs) play an important role in the one-step synthesis of complex molecular structures owing to their benefits. Herein, one-pot synthesis of thiazolo[3,2-<i>a</i>]pyrimidine derivatives via green and efficient multicomponent reaction of methyl ketones, 2-thioxopyrimidines, and <i>N</i>-bromosuccinimide (NBS), catalyzed by the Brønsted acid ionic liquid, [1-(4-sulfobutyl)-3-methylimidazolium hydrogen sulfate], under ultrasonic irradiation was reported. The protocol combines in situ α-bromination of ketones with subsequent Hantzsch-type annulation, enabling direct construction of the fused heterocyclic scaffold in a single step. A wide range of aromatic and cyclic ketones, as well as substituted 2-thioxopyrimidines, were chosen, affording the desired products in good yields under ultrasound irradiation. Importantly, the Brønsted-acid ionic-liquid catalyst used in this synthesis could be recovered and reused for four cycles without significant loss of efficiency. Moreover, density functional theory (DFT) calculations have been employed to elucidate the regioselectivity of the annulation process, revealing a clear thermodynamic preference for the formation of the 5<i>H</i>-isomer over the 7<i>H</i>-isomer. These computational results were fully consistent with the detailed NMR analyses (¹H, ¹³C, HMBC, and NOESY), confirming the 5<i>H</i>-thiazolo[3,2-<i>a</i>]pyrimidine structure.</p> Graphical abstract <p></p>