Substrate-Controlled Divergent Syntheses of Polycyclic Tetrahydropyrimidin-4-ones and 1,3-Oxazinane-4,6-diones from Dihydroisoquinolines and Meldrum’s Acid

The microwave-assisted reaction of functionalized dihydroisoquinolines with Meldrum's acid unveiled a one-step, divergent access to rare polycyclic pyrimidinones and oxazinanediones. The outcome hinged on the identity of the C1 substituent (R) of the dihydroisoquinolines, where hydrogen favored pyrimidinones, while aryl and alkyl groups afforded oxazinanediones. DFT calculations elucidated the underlying mechanism of this divergence, revealing shared intermediates whose fate is dictated by the stereoelectronic properties of the R group.