Intramolecular Polar [4^⊕+2] Cycloadditions of Aryl‐1‐aza‐2‐azoniaallene Salts: Unprecedented Reactivity Leading to Polycyclic Protonated Azomethine Imines

Two-Step Sequence of Cycloadditions Gives Structurally Complex Tetracyclic 1,2,3,4-Tetrahydrocinnoline Products

“…6,17 We recently discovered that 1-aza-2-azoniaallene salts, generated by the reaction of SbCl 5 with an α-chloroazo species, can participate in intramolecular [4 + 2]-cycloaddition reactions to give tetracyclic 1,2,3,4-tetrahydrocinnoline products containing an N-aminoiminium motif (Scheme 1). 18 Limitations on the preparation of α-chloroazo compounds and the environmental implications of using stoichiometric quantities of SbCl 5 limited the full potential of this transformation and prompted us to look for alternative methods to effect this cycloaddition. Ultimately, we discovered that treating α-trifluoroacetoxyazo compounds with TMS-triflate was a milder and more environmentally friendly way to effect this transformation.…”