Total synthesis of mesembrine

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Structure of Mesembrine

Shamma’s route for total synthesis of (±)-Mesembrine

Yamada’s asymmetric total synthesis of (±)-Mesembrine.

Mesembrine is an azizoaceae alkaloid isolated from Mesembryanthemum tortuosum. It was first isolated and characterized by Bodendorf, et al in 1957. Mesembrine is a bicyclic molecule and has two bridge-head chiral carbons between the five-membered ring and the six-membered ring (Figure "Structure of Mesembrine"). Because of its special structure and bioactivity, Mesembrine has been one of the most interesting targets in synthetic organic chemistry over the past 40 years. Over 40 total syntheses have been reported for Mesembrine, most of which focused on different approaches and strategies for the construction of the bicyclic ring system and the quaternary carbon.

The first total synthesis of Mesembrine was reported by Shamma, et al in 1965 (Figure "Shamma’s route for total synthesis of (±)-Mesembrine"). This route has 21 steps, which was among the longest synthetic routes for Mesembrine. Key steps involve the construction of the six-membered ketone ring by Diels-Alder reaction, α-allylation for synthesis of the quaternary carbon, and conjugate addition reaction for the final five-membered ring closure. The final product from this route is a racemic mixture of (+) and (-) Mesembrine.

In 1971, Yamada, et al reported the first asymmetric total synthesis of (+)-Mesembrine. The quaternary carbon was introduced by asymmetric Robinson annulation reaction mediated by an L-proline derivative (Figure "Yamada’s asymmetric total synthesis of (±)-Mesembrine").