Cascade C-H Annulation of Aldoximes with Alkynes Using O₂ as the Sole Oxidant: One-Pot Access to Multisubstituted Protoberberine Skeletons

A cascade double C-H annulation of aldoximes with alkynes to produce benz[a]acridizinium salts is developed by using a simple catalytic system of [CpRh(OAc)₂]₂ in the presence of Zn(OTf)₂ with oxygen as the sole oxidant. In addition, the challenging C-H annulation of aldoximes with alkynes, especially arylalkynes, to synthesize 1H-isoquinolines is also achieved under slightly modified conditions. This protocol provides an efficient one-pot access to multisubstituted dehydroberberinium skeletons from simple starting materials, which can be easily transformed into berberinium and tetrahydroberberine skeletons by controlled hydrogenation.