An unprecedented chiral secondary amine-catalyzed formal [4 + 2] annulation of aldehydes and oxidation-generated β-unsubstituted o-QMs is reported. This asymmetric protocol allows direct functionalization of the benzylic C-H bonds and furnishes [4 + 2] cycloadducts, chromanols, with excellent enantioselectivity and in up to 92% yield. The usability of this approach was further demonstrated by the enantioselective synthesis of anticancer Rhinacanthins derivative NKPLS8.
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Zhou et al. (2017) studied this question.
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