Visible‐Light Catalytic Photooxygenation of Monoterpene Indole Alkaloids: Access to Spirooxindole‐1,3‐oxazines

• Published in: Chemistry : a European journal 2018-07, Vol.24 (40), p.10253-10259
• Main Authors: von Drathen, Thorsten, Hoffmann, Frank, Brasholz, Malte
• Format: Article
• Language: English
• Subjects: Alkaloids; Anthraquinone; Catalysis; Chemistry; Indoles; Oxazine; Oxindole alkaloids; photochemistry; reaction mechanisms; rearrangement; Reserpine; Ring opening; spiro compounds
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201801882

Few natural oxindole alkaloids possess an exceptional spiro‐[(1,3)oxazinan‐3,6′‐oxindole] core structure, which results from an unusual oxidative indole rearrangement. The Rauvolfia alkaloid reserpine can be converted into the spirooxindole‐1,3‐oxazines dioxyreserpine and trioxyreserpine through efficient visible‐light catalytic photooxygenation with anthraquinone photocatalysts. A mechanistic investigation sheds new light on the photooxidative rearrangement of reserpine and related monoterpene indole alkaloids, and the spirooxindole‐1,3‐oxazine products can be valorized by reductive ring opening, to obtain cis‐decahydroisoquinolines as new enantiopure synthetic building blocks, as demonstrated for dioxyreserpine. Switching the core: Few natural oxindole alkaloids possess the exceptional spirooxindole‐1,3‐oxazine core structure, which results from an unusual oxidative indole rearrangement. The Rauvolfia alkaloid reserpine can be converted into the photoproducts dioxyreserpine and trioxyreserpine through efficient visible‐light catalytic photooxygenation with anthraquinone photocatalysts (see scheme).