The [4 + 2] cycloaddition reactions of 2H-1,4-oxazin-2-ones 5 to olefins and the generation of new 2-oxa-5-azabicyclo[2.2.2]oct-5-en-3-ones of type 6 is studied. The cycloaddition is promoted by the 5-Cl substituent in the oxazinone and can be performed with electron-poor as well as with electron-rich alkenes. The NMR data for the isolated compounds show exclusive formation of endo adducts for symmetrically disubstituted olefins; this is interpreted in terms of secondary orbital interactions and repulsion between the lactone function and both substituents in the exo-transition state. The lower endo selectivity observed for some monosubstituted alkenes is in agreement with the expected decrease of this repulsion factor. The addition of the electron poor methyl acrylate shows rather low regioselectivity but addition of other dienophiles is always highly regioselective, the ortho regioisomer being favoured.