Titanium-promoted Intramolecular Photoenolization/Diels–Alder Reaction to Construct Polycyclic Terpenoids: Formal Synthesis of Mycoleptodiscin A

A titanium-promoted intramolecular photoenolization/Diels–Alder (PEDA) reaction was developed to construct the core skeleton of aromatic polycyclic terpenoids bearing an all-carbon quaternary center on the benzylic position. Titanium(IV) isopropoxide [Ti(Oi-Pr)₄] plays a key role during the photo cycloaddition, which may help to accelerate the interaction between dienophile and the stereo-hindered diene species as well as control the diastereoselectivity. This photolysis provides a new solution for the stereospecific formation core structures of aromatic abietane diterpenoids and sesquiterpenoids, which have multiple functional groups for the further transformations. As a synthetic application, it was successfully used in the synthesis of indolosesquiterpenoid mycoleptodiscin A.