Fischer indolization of terpenes and steroids

Abstract

Here we report a new synthetic approach to several intricate indoles and hydrazones starting with commercially available terpenes and steroids via Fischer indolization (FI) sequence under deep eutectic solvents (DES) conditions or by employing acetic acid reflux conditions. During the course of the reaction with pulegone 22 and pipertone 25 we observed Fischer indolization along with aromatization is occurring. In another instance, with thujone 32 we found cyclopropane ring is opening under thermal conditions leading to aromatized Fischer indole product. We also studied the FI sequence of some diketo derivatives to synthesize the corresponding bis indole derivatives, but instead we obtained mono hydrazone derivative which can be explained on the basis of steric reasons. In addition, we also modified other terpenes to study the FI sequence where we realized the formation of both hydrazone and indole derivatives. All the compounds synthesized here are well characterized by NMR and HRMS data.