Stereocontrolled formation of spiro enones by radical cyclization of bromo acetals

Aldol condensation of ketones with 2-[[(1,1-(dimethylethyl)diphenylsilyl]oxy]propanal (7), dehydration, NaBH4 reduction and treatment of the resulting alcohols with ethyl vinyl ether in the presence of NBS gives bromo acetals (e.g. 11) that undergo 5-exo-trigonal radical cyclization, affording compounds that are easily converted into spiro enones (e.g. 16). The stereochemistry at the spiro center is controlled by the stereochemistry at the hydroxyl-bearing carbon of the intermediate alcohol.