Δ2-1 2-Diazetines:: Regioselective acylation reactions and rearrangement into 4H-1,3,4-oxadiazines

Reaction of 1-substituted Delta(2)-1,2-diazetine with acid chlorides or anhydrides resulted in the derivatives 3a,b,f that have been acylated at a ring position. On the other hand, trifluoroacetic anhydride yields a new 1,3,4-oxadiazine 4e under the same conditions. The structure of 4e could be confirmed by X-ray structural analysis. Acylated 1,2-diazetidines of type 3a,b can be nearly quantitatively transformed into 1,3,4-oxadiazines 4a,b by thermally induced ring-expansion reactions. Differential scanning calorimetry measurements provide evidence for a monomolecular mechanism in which a strong dependence of the acyl group on the rate of reaction is observed.