Synthesis of the Guaianolide Ring System via Cycloaddition of a Bicyclic Carbonyl Ylide with Allyl Propiolate

A cyclic carbonyl ylide with a trans-annulated cyclopentane ring was generated by a Rh2(OAc)4-catalyzed reaction from a diazoketone precursor and trapped with allyl propiolate. The 1,3-dipolar cycloaddition led to the stereoselective formation of an oxygen-bridged polycycle. Via Curtius degradation, the cycloadduct was transformed to the ring skeleton typical of the sesquiterpene family of guaianolides.