Photochemical reactions of barbituric acids

The [2+2] Photocycloaddition of Uracil Derivatives with Ethylene as a General Route to cis-Cyclobutane β-Amino Acids

“…In several cases, the desired cyclobutane 2 was accompanied by uracil photodimers (entries 1, 3, 14, [16][17][18]. With uracil (1a, entry 1) the photodimer was formed in small amounts and was easily eliminated by the acetone wash or, if necessary, by passage through a short column of silica gel.…”

“…15 2-Hydroxy compounds 2 (R 1 = H or Et, R 2 = OH) have been described but these compounds were obtained by photolysis of barbital via a Norrish type II cyclization. 16 For our work, we selected a range of uracils 1, which were substituted by alkyl, aryl or functional groups, and were readily available (Table 1; the prototype uracil substrate 1a is included for comparison). All compounds were obtained commercially, with the exception of the following, which were prepared in one step using straightforward procedures: 5-phenyluracil (1c), 17 5-(benzylamino)uracil (1h), 18 5-(benzyloxycarbonylamino)uracil (1i), 19 6-phenyluracil (1n) 20 and hexyl orotate (1r).…”