The stereochemistry of the conjugate addition of cuprates to the title compounds is mainly governed by the ?-methyl group, but the alkoxy substituent also plays a role. A ?-methyl group exerts the dominant effect in the conjugate addition of a cuprate on a ?-methyl-d-oxy-a,ß-enone, leading to pure anti-addition to the syn substrate, whereas for the anti substrate, a chelating group on oxygen (MEM) definitely assists anti addition. The syn enone reacts exclusively according to the Morokuma model, whereas the anti enone exhibits a chelate effect and fits better with the Isobe model. In both cases, capture of the enolate by iodomethane is stereoselective, so that four contiguous substituted carbons can be created from the initial two, with good stereoselectivity.