Dye-sensitized photo-oxygenation of the Aspidosperma alkaloids vincadifformine and tabersonine. A new, convenient approach to vincamine
The dye-sensitized photo-oxygenation of (–)-vincadifformine (1) and (–)-tabersonine (3) is described. Reaction takes place through the intermediate formation of 16-hydroperoxyindolenines, which decompose to give 2,16-seco-products or which can be efficently trapped by reductants to give a new stereoselective synthesis of the 16-hydroxy-[3H]indoles (10) and (14). These compounds are the key precursors to the eburnane alkaloids vincamine (4) and Δ14-vincamine (6). Suitable experimental conditions give compounds (4) and (6) in good yields directly from their Aspidosperma precursor.