The diastereoselectivity in the alkylation and Michael addition of "naked" 6-substituted delta-lactolates has been studied by density functional (B3LYP) calculations with ab initio (MP2) energy refinements. The resulting proposed model for the origins of stereocontrol in this reaction has been tested by experiment. The reactions lead to a high cis diastereoselectivity across the THP ring due to the preference for both the alkoxide and the 6-substituent to sit equatorial in the alkylation transition structure. In the oxy-Michael addition of these lactolates to beta-substituted nitroolefins, we propose that the high diastereoselectivity beta- to the nitro group is a result of a combination of steric, stereoelectronic and solvation factors.