A Brønsted acid catalyzed cyclization of aminodiazoesters with aldehydes is described. This reaction features broad substrate generality and functional group compatibility, affording a wide range of 5−7-membered 3-carboxylate-N-heterocycles containing different functional groups. The title products are able to be further elaborated through simple functional group transformations to produce synthetically useful N-heterocycles.N-Heterocycles are privileged scaffolds in synthetic and medicinal chemistry, 1 which not only frequently exist in a wide range of bioactive molecules but also are valuable synthetic building blocks that can be transformed into a series of valuable structures. Especially, the 3-carboxylate-N-heterocycles are well-known scaffolds for constructing a number of bioactive molecules. 2 For example, the 3-nucleoside derivatives prepared from 3-carboxylate-pyrrole display potent inhibition of the human disease-relevant kinases. 2 (R)-Tiagabine featuring piperidine-3-carboxylic acid serves as an efficient GABA uptake inhibitor, 3,4 and the antiplasmodial exhibits the antimalarial activity (Figure 1). 5 In spite of the substantial