A solvent- and catalyst-free synthesis of nitrogen-containing bicyclic derivatives through a three-bond forming process is reported. Starting from dienals and readily available diazenes, the strategy involving the hemiaminal formation/hetero-Diels-Alder reaction affords the bicyclic products in a highly diastereoselective manner. This simple and green procedure has been applied to a selection of substrates, giving rise to 12 examples of nitrogen-containing bicyclic architectures. These products underwent various synthetic transformations. A sequence involving the cleavage of the hydrazine allowed the preparation of a hydantoin motif bearing an aminopropyl side chain, which is a structure found in natural products. A mechanism has also been suggested to explain the observed selectivities.