Synthesis of 3D-Rich Heterocycles: Hexahydropyrazolo[1,5‑a]pyridin-2(1H)‑ones and Octahydro‑2H‑2a,2a1‑diazacyclopenta[cd]inden-2-ones

Two cyclic azomethine imines, 7-methyl- and 7-phenyl-2-oxo-Δ7-hexahydropyrazolo­[1,5-a]­pyridin-8-ium-1-ide, were prepared in seven steps from the respective commercially available δ-keto acids. The addition of Grignard reagents followed by N-alkylation at position 1 afforded the 1,7,7-trisubstituted hexahydropyrazolo­[1,5-a]­pyridin-2­(1H)-ones, whereas 1,3-dipolar cycloadditions of these dipoles to typical acetylenic and olefinic dipolarophiles gave 4a-substituted 2a,2a1-diazacyclopenta­[cd]­indene derivatives as the first representatives of a novel heterocyclic system. Regio- and stereoselectivity as well as the mechanism of these [3 + 2]-cycloadditions were evaluated using computational and experimental methods. The data obtained were in agreement with the polar concerted cycloaddition mechanism via the energetically favorable syn/endo-transition states.