Stereocontrolled Synthesis of Complicated Oxacyclic Compounds via Platinum-Catalyzed [4+2]-Cycloadditions and Annulations of Enynals with Allylic Alcohols

We report stereoselective synthesis of tricyclic ketals through Pt(II)-catalyzed [4 + 2]-cycloaddition of benzopyrilium/1-en-3-ol functionalities. For nonaromatic enynals, we obtained distinct tricyclic oxacyctic ketones due to occurrence of a new annulation reaction. This work reports the first example that [4 + 2]-cycloadducts are successfully intercepted from such functionalities in diversified approaches. The value of such cycloadditions and annulations are shown by their high diastereoselectivities and chemoselectivities.