Straightforward Asymmetric Entry to Highly Functionalized Medium-Sized Rings Fused to β-Lactams via Chemo- and Stereocontrolled Divergent Radical Cyclization of Baylis−Hillman Adducts Derived from 4-Oxoazetidine-2-carbaldehydes

DABCO promoted reactions of various activated vinyl systems with optically pure 4-oxoazetidine-2-carbaldehydes 1 gave rise to Baylis-Hillman adducts 3 with excellent syn stereoselectivities, without detectable racemization. Products 3 are used for the asymmetric preparation of unusual 2-azetidinones fused to medium-sized rings via chemo- and stereocontrolled divergent radical cyclization. The formation of bicyclic beta-lactams 4-6 could be rationalized through a tandem radical Michael addition/endo cyclization or a tandem radical addition/Michael addition, depending on the electronic nature of the radical promoter.