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. Racemic total syntheses of the four title spirocyclic oxindole alkaloids are reported starting from 4‐hydroxytryptamine. The key steps were a base‐mediated Mannich spirocyclisation to form the C‐ring, and a Pd‐catalysed cyclisation of a keto ester enolate onto an allylic carbonate to form the D‐ring. Thus, convenient syntheses of the alkaloids were achieved, for three of them for the first time.