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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