Facile and convenient synthesis of 2,6‐disubstituted 4 H ‐1,3‐oxazin‐4‐one derivatives

Facile and convenient methods for the preparation of a variety of 2,6‐disubstituted 4 H ‐1,3‐oxazin‐4‐ones 3 by three complementary methods are described. Treatment of the branched aliphatic imidate 2c,d with diketene 1 in the presence of a catalytic amount of acetic acid affords 2‐substituted 6‐methyl‐1,3‐oxazin‐4‐ones 3c,d , whereas the unbranched imidate 2b,e gave oxazines 3b,e and pyrimidines 4b,e (Method A). The reaction of acyl Meldrum's acid 5 with imidate 2 afford 2,6‐disubstituted oxazine 3, though the alkylimidate with acetyl Meldrum's acid yielded 3 and 5‐acetyl‐1,3‐oxazine‐4,6‐dione 8 (Method B). The cylodehydration of acylacetylcarboxamide 13 with acid, such as 70% perchloric acid or fluorosulfonic acid, afforded 1,3‐oxazines 3 (Method C).