Atom-Transfer Tandem Radical Cyclization Reactions Promoted by Lewis Acids This work was supported by The University of Hong Kong and the Hong Kong Research Grants Council. D.Y. acknowledges the Bristol-Myers Squibb Foundation for an Unrestricted Grant in Synthetic Organic Chemistry and the Croucher Foundation for a Croucher Senior Research Fellowship.

Various polycyclic ring skeletons (e.g. 3) are formed from unsaturated α-bromo β-keto esters (e.g. 1) in a Lewis acid catalyzed atom-transfer tandem radical-cyclization reaction in moderate to good yields and with excellent stereoselectivities. Furthermore, in the presence of chiral complexes such as [Yb(Ph-pybox)(OTf)3] (2), the enantioselective cyclization gave up to 84 % ee. OTf = trifluoromethanesulfonate, pybox = 2,6-bis(2-oxazolin-2-yl)pyridine.