Ynesulfonamide‐Based Silica Gel and Alumina‐Mediated Diastereoselective Cascade Cyclizations to Spiro[indoline‐3,3′‐pyrrolidin]‐2‐ones under Neat Conditions

The spiro[indoline‐3,3′‐pyrrolidin]‐2‐ones were synthesized via a silica gel and alumina‐mediated sequential transformation based on tryptamine‐derived ynesulfonamide substrates under neat conditions. The inherent tendency of C−C bond migration through Wagner‐Meerwein rearrangement in the synthesis of spirooxindole was prevented by water trapping to the spiroindoleninium intermediate. The functional group tolerances of the methodology were investigated using a variety of substrates. The detailed mechanism of the sequential transformation was probed by the isotope‐labeled experiments. This strategy was further applied in the formal syntheses of indole alkaloids coerulescine and horsfiline.