Acyl Radical Addition to Activated Olefins: A Stereocontrolled Route to Polysubstituted Tetrahydrofurans and Lactones, and Application to the Total Synthesis of (+)-No. 2106 A: Acyl Radical Addition to Activated Olefins: A Stereocontrolled Route to Polysubstituted Tetrahydrofurans and Lactones, and Application to the Total Sy