Stabilization of γ-lactam and lactone ring-fused norcaradienes by protonation: DFT calculations of norcaradiene and the corresponding cycloheptatriene structures

Stabilization of norcaradiene structures of γ-lactone and lactam ring-fused 7-vinylnorcaradienes 1a–1e in their norcaradiene – cycloheptatriene valence isomerization was achieved by protonation. Induced 13C NMR chemical shifts caused by protonation were monitored for the cyclopropane ring carbons (C1, C6 and C7) of the norcaradiene structures as indices of their stabilities. DFT calculations (B3LYP/6-311+G(d) level) of norcaradienes, the corresponding cycloheptatrienes, and their protonated structures supported the experimental results.