Total Synthesis of Two 8‐Oxoprotoberberine Alkaloids: Alangiumkaloids A and B

A new and versatile synthetic route for 8‐oxoprotoberberine 17 through synthesis of isoquinolinone 16 and construction of a B‐ring is described. The key step is the synthesis of isoquinolinone 14 through thermal cyclization of 2‐alkynylbenzaldehyde oxime 12 to afford isoquinoline N‐oxide 13, followed by a Reissert–Henze‐type reaction. The first total synthesis of 8‐oxoprotoberberine alkaloid alangiumkaloids A and B was achieved by using this strategy. We describe the details of the first total synthesis of alangiumkaloids A (1) and B (2) through construction of an isoquinolinone framework formed by thermal cyclization of 2‐alkynylbenzaldehyde oxime to produce isoquinoline N‐oxide, followed by a Reissert–Henze‐type reaction.