Modular Synthesis of Multi‐Substituted Cyclopenta[ <i>b</i> ]Indoles via Binary‐acid Complex Catalyzed In Situ Generated Carbocations

ABSTRACT The binary‐acid complexes HfCl 4 or ZrCl 4 /diphenyl phosphate PA could promote in situ generation of stabilized carbocations from γ‐indolyl‐β,γ‐unsaturated α‐keto esters, which were further trapped by different nucleophiles (e.g., thiols and indoles). Under mild conditions, the reaction afforded multi‐substituted cyclopenta[ b ]indoles in up to 91% yield with low to good stereoselectivity.