We have prepared and characterized new examples of 5-substituted-1-aza-4, 6- dioxabicyclo [3.3. 0] octanes (bicyclic amide acetals) and examples of the new heterocyclic system, 5-substituted-1-aza-4-oxa-6-thiabicyclo [3.3. 0] octanes (bicyclic amide monothioacetals). Detailed analysis of NMR coupling constants and X-ray structure determination for an example of each class of compound established the stereochemistry ...