Photochemical Synthesis of Both Strained and Macrocyclic (1,7)Naphthalenophanes

Naphthalenophanes are a special type of cyclophanes. While in the past (1,5)-, (1,6)-, and (1,8)­naphthalenophanes were successfully prepared by using the photo-dehydro-Diels–Alder (DDA) reaction, access to (1,7)­naphthalenophanes by this method was hitherto unknown. After numerous unsuccessful attempts to prepare these compounds by thermal DDA, we found that the photoinitiated variant (PDDA) represents a very efficient method to [k]­(1,7)­naphthalenophanes 13. The scope ranged from highly strained (k = 11, 12) to macrocyclic products (k = 22, 24). The extraordinary reactivity could be explained by folded ground-state geometries of diketones 12 used as reactants of the PDDA. Furthermore, we calculated the ring-strain energies with the help of an isodesmic reaction and evaluated structural and spectroscopic (NMR) consequences of ring strain.