Electroreductive Intermolecular Coupling of Uracils with Aromatic Ketones: Synthesis of 6‑Substituted and <i>cis</i>-5,6-Disubstituted 5,6-Dihydro-1,3-dimethyluracils and Their Transformation to 6‑Substituted 1,3-Dimethyluracils, <i>trans</i>-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.