A Rapid Aza-Bicycle Synthesis from Dendralenes and Imines.

The diene-transmissive 2-fold Diels-Alder sequence between carbon-based dienophiles and [3]-dendralenes is becoming an established method for polycarbocycle synthesis. Here, we demonstrate for the first time that imines are competent participants in intermolecular formal [4 + 2] cycloadditions with dendralenes. After a second Diels-Alder process with a carbadienophile, hexahydro- and octahydro-isoquinoline structures are formed. The formal aza-Diels-Alder reaction, which requires Lewis acid promotion, proceeds in high regio- and stereoselectivity under optimized conditions. omega B97XD/Def2-TZVP//M06-2X/6-31+G-(d,p) calculations reveal a stepwise ionic mechanism for the formal aza-dienophile cycloadditions and also explain an unexpected Z -> E olefin isomerization of a non-reacting C=C bond in the first formal cycloaddition.