Heterocyclic [c]‐Condensed Coumarins from 4‐Azido‐3‐coumarincarbaldehydes 4‐Azido‐3‐coumarincarbaldehydes 2a–d are useful starting materials for syntheses of a variety of 3,4‐disubstituted coumarins as well as heterocyclic [c]‐fused coumarins, e. g. isoxazoles, pyrazoles and 1,5‐diazepines, under mild conditions. In these reactions, 2a–d are – despite of their thermolability — superior to the corresponding 4‐chloro‐3‐coumarincarbaldehydes 1a–d, from which they were prepared. An improved method for the preparation of 1a–d from 4‐hydroxy‐coumarins is described.

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2,7‐Dihydroxy‐2H‐1,4‐benzoxazin‐3(4H)‐one 11 (DHBOA) and 2,4,7‐trihydroxy‐2H‐1,4‐benzoxazin‐3(4H)‐one 14 (TRIBOA) representing aglucones of naturally occurring acetal glucoside type allelo chemicals found in Gramineae have been for the first time synthesized by two pathways both involving selective reductive cyclizations of appropriate 7‐benzyloxy‐2‐nitrophenol derivatives as precursors. TRIBOA 14 and its bioactive naturally occurring 7‐methyl ether DIMBOA have been found to undergo a hitherto unknown transformation to the corresponding 2,6‐dibromo substituted lactam forms 20 and 21 in the presence of hydrogen bromide in acetic acid, which is of value in a better understanding of the possible mode of bioactivity.