3-Nitrocoumarin Amidines: A New Synthetic Strategy for Substituted [1]Benzopyrano[3,4-d]imidazol-4(3H)-ones

A new synthesis of substituted [1]benzopyrano[3,4-d]imidazol-4(3H)-ones, starting from 3-nitrocoumarin N-functionalized amidines 3, has been developed. When the 3-nitro-amidines 3 were treated with NaBH4 in the presence of 10% palladium on charcoal, 2-substituted [1]benzopyrano[3,4-d]imidazol-4(3H)-ones 4 were produced. Structure elucidation of compounds 4 revealed that they exist as one of the three possible tautomeric structures (i.e., 4dα). Alkylation of 4d to give 6, as well as NOE experiments on 6 and energy calculations allowed the most favoured structure of the 4 derivatives to be identified. Alternatively, the reduction of the 3-nitro-amidines 3 by triethyl phosphite, through a nitrene intermediate, cyclization and a [1.5] sigmatropic shift, yielded 2-morpholino-3-alkyl-[1]benzopyranol 3,4-d]imidazol-4(3H)-ones 7.