The Synthetic Access to Fused 6,7,8,9-Tetrahydro- 5H-pyridoazepines: Evaluation of Ring-Closure Strategies

The synthetic accessibility of fused pyridoazepane frameworks was investigated through a series of strategies designed to construct differently fused azepane systems. Several precursor designs enabling alternative ring-closure topologies were explored. A “lactam” pathway proved synthetically inaccessible under various conditions due to chemoselectivity issues and competing intermolecular processes. In contrast, an efficient route to the 6,7,8,9-tetrahydro-5H-pyrido[3,2-c]azepine framework was achieved via an intramolecular cyclization strategy, in which the amine functionality was introduced prior to ring assembly. The developed route proceeds under practical laboratory conditions using inexpensive reagents and was demonstrated on a gram scale. These findings provide insight into the structural factors governing ring-closure efficiency in pyridoazepine systems and establish a practical entry to a previously underexplored fused heterocyclic scaffold.