Electroreductive Intermolecular Coupling of Uracils with Aromatic Ketones: Synthesis of 6‑Substituted and cis-5,6-Disubstituted 5,6-Dihydro-1,3-dimethyluracils and Their Transformation to 6‑Substituted 1,3-Dimethyluracils, trans-5,6-Disubstituted 5,6-Dihydro-1,3-dimethyluracils, and 4,5,5-Trisubstituted 3‑Methyloxazolizin-2-ones

The electroreductive coupling of 1,3-dimethyluracil, thymine, and 5-fluorouracil with aromatic ketones in the presence of TMSCl gave 6-substituted and cis-5,6-disubstituted 5,6-dihydro-1,3-dimethyluracils. The dehydro­trimethyl­siloxylation of the adducts afforded 6-substituted and 5,6-fused 1,3-dimethyluracils. The detrimethyl­silylation of the adducts with TBAF or 1 M HCl–MeOH gave 4,5,5-trisubstituted 3-methyl­oxazolizin-2-ones or 3-methyl­oxazolizin-2-imines in addition to simply desilylated alcohols. The cis-5,6-disubstituted 5,6-dihydro-1,3-dimethyl­uracils were isomerized to the corresponding trans-isomers by heating in the presence of cat. DMAP. The cis- and trans-5,6-disubstituted 5,6-dihydro-1,3-dimethyl­uracils were assigned by the coupling constants J5,6 of their 1H NMR spectra.