Synthesis of Bridged and Nonbridged N‐Heterocycles by Cyclocondensation of Bis(silyl enol ethers) with Iminium Salts

Cyclization reactions of iminium salts with bis(silyl enol ethers) – such as 1,3‐bis(silyloxy)‐1,3‐butadienes and 1,1‐bis(silyloxy)ketene acetals – allow a convenient synthesis of various bridged and nonbridged N‐heterocycles. The iminium salts were generated in situ from pyridines, quinolines, isoquinolines, quinoxalines, pyrazines, quinazolines, and 2,5‐dimethoxypyrrolidines. The products include 4‐oxa‐9‐aza‐7,8‐benzobicyclo[3.3.1]nonan‐3‐ones, 7‐oxa‐1,4‐diaza‐2,3‐benzobicyclo[4.3.0]non‐2‐en‐6‐ones, 7‐oxa‐1,4‐diazabicyclo[4.3.0]non‐2‐en‐6‐ones, 3‐hydroxy‐9‐aza‐7,8‐benzobicyclo[3.3.1]non‐3‐enes, 4‐hydroxy‐1,9‐diaza‐7,8‐benzobicyclo[3.3.1]non‐4‐enes, alkylidene‐7‐oxa‐1,4‐diaza‐2,3‐benzobicyclo[4.3.0]non‐2‐enes, and tropinones.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)