Cycloadditions of 6H-1,3,4-oxadiazin-6-ones (4,5-diaza-α-pyrones). Part 16.1 4-Substituted and 4,5-disubstituted methyl 3-phenyl-α-pyrone-6-carboxylates from γ-oxoketenes via α,δ-dibromo-δ-lactones
Abstract
The addition of bromine to the γ-oxoketenes 4, 10 and 22 and the elimination of two equivalents of hydrogen bromide from the resulting α,δ-dibromo-δ-lactones 5, 12 and 23 to give the α-pyrones 7, 19 and 20, respectively, are described. Since the isolation of neither 4, 10 and 22 nor 5, 12 and 23 proved necessary, the α-pyrones have been prepared in one-pot procedures from methyl 6-oxo-5-phenyl-6H-1,3,4-oxadiazine-2-carboxylate 6.