Enantioselective Desymmetrization of <i>para</i>-Quinamines through an Aminocatalyzed Aza-Michael/Cyclization Cascade
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Abstract
An unprecedented organocatalytic
asymmetric desymmetrization of <i>para</i>-quinamines leading
to functionalized hydroindoles,
a common motif in many alkaloids, has been reported. The ability of
diarylprolinol silyl ethers to promote iminium and enamine activation
of α,β-unsaturated aldehydes in one catalytic cycle is
the centerpiece of the strategy involving a challenging aza-Michael/intramolecular
cyclization cascade reaction. A range of prochiral <i>para</i>-quinamines and α,β-unsaturated aldehydes were investigated
to afford 16 examples of hydroindoles possessing four contiguous stereocenters
including one quaternary carbon. The hydroindole structures include
multiple orthogonal functionalities, which underwent various transformations