Concise Total Syntheses of (±)-Hirsuteine, (±)-Hirsutine, (±)-Corynantheine, and (±)-Dihydrocorynantheine

A concise and protecting-group-free synthesis of the four corynanthe alkaloids has been developed from a readily available indole precursor. The key step features a reagent-controlled cascade Hosomi–Sakurai/Pictet–Spengler cyclization that simultaneously constructs the C and D rings and establishes three stereogenic centers in a single step. Notably, modulation of the Lewis acid enables selective access to either the trans–trans or cis–trans tetracyclic framework, providing divergent entry to both major stereochemical series of Corynanthe alkaloids.