Enantioselective synthesis of spiro-N,O-ketals via iridium and Brønsted acid co-catalyzed asymmetric formal [4+2] cycloaddition.

We present an iridium and Brønsted acid co-catalyzed enantioselective formal [4+2] cycloaddition reaction of cyclic enamides with 2-(1-hydroxyallyl)phenols. This method yields a wide range of N-unsubstituted spiro-N,O-ketals, with good efficiency (up to 94%) and excellent enantioselectivities (most >95% ee). The protocol features easy scale-up and facile product derivatization.