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

Abstract

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.

Keywords: alkaloids; photochemistry; reaction mechanisms; rearrangement; spiro compounds.