Solvent- and Catalyst-Free Synthesis of Nitrogen-Containing Bicycles through Hemiaminal Formation/Diastereoselective Hetero-Diels–Alder Reaction with Diazenes

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.