Total Synthesis of the Spirocyclic Oxindole Alkaloids Corynoxine, Corynoxine B, Corynoxeine, and Rhynchophylline

Racemic total syntheses of four spirocyclic oxindole alkaloids are reported. The general starting material was an N‐2‐butenylated 2‐hydroxytryptamine, which underwent a base‐mediated Mannich spirocyclisation with a functionalised aldehyde to generate the C‐ring. The second key step was a Pd‐catalysed cyclisation of an α‐keto ester enolate (in the original aldehyde) onto an allylic carbonate (in the N‐substituent) to form the D‐ring. The stereoselectivities of the key steps were moderate, but the isomers were readily purified, so that the natural products could be conveniently prepared, three of them for the first time.