Synthesis and oxidative cleavage of oxazinocarbazoles: atropselective access to medium-sized rings

Polycyclic systems can be converted into medium-sized-ring-containing compounds through the controlled oxidative cleavage of internal double bonds. This approach is particularly accessible in systems that contain a suitably substituted indole ring. Here, a robust approach to the synthesis of the understudied oxazinocarbazole system is reported. After regioselective incorporation of a carbonyl functional group, m-chloroperoxybenzoic acid (MCPBA) is used to cleave the indole 2,3-double bond that this system contains. This results in a competition between two processes, oxidative cleavage of the double bond and a pinacol-type rearrangement, both of which occur with very high diastereoselectivity. The balance between the two processes is studied as a function of the substrate structure. Extensive use of X-ray crystallographic analysis of the products enables detailed mechanistic conclusions to be drawn.