The result of synthetic studies on sesquiterpene alkaloids (dendrobine 1 and nobiline 2) obtained from Dendrobium nobile L. is presented. Starting from a tetralone 3, a conjugated cyclohexenone 12 was prepared, which was cyclized to give a tricyclic keto acid 13. A common intermediate for the synthesis of dendrobine 1 and nobiline 2, a bicyclic keto acid 18 was prepared by ozonization of the enol acetate 15 of the acid 13. In the carboxyl group of the compound 18 nitrogen function was introduced to afford derivatives such as 21, 24, and 26. Attempts were made to form the pyrrolidine ring (or pyrrolidone ring) of dendrobine 1 via tricyclic compounds (21, and 26) containing a piperidone ring.
