Scheme 3
Scheme 4
The chiral BLAs having counteranion triflimide (Tf2N− ) provides remarkable catalytic stability compared to that bearing trilfate (CF3SO3−). In addition, BLAs with triflimide are found to be versatile catalysts for wide range of Diels-Alder reactions. For examples, the reactions of the challenging unsymmetrical benzoquinones with 2-triisopropyloxy-1,3-butadiene has been shown with excellent enantio- and regioselectivities (Scheme 4).
The observed results suggest that the BLA coordination to the oxygen of unsymmetrical quinones takes place as shown in Scheme 5. The coordination predominately takes place with the more basic oxygen of the quinones.
Scheme 5
The catalytic system is also effective for intramolecular reactions to afford trans -fuzed bicyclic structures with excellent enantioselectivity (Scheme 6).
Scheme 6