Acylation of sydnones with acetic anhydride in the presence of montmorillonite K-10

Various 4-acetyl sydnones 2 can be prepared in good yield by reaction of the corresponding 3-arylsydnones (cf. 1) with acetic anhydride at similar to 110 degrees C catalyzed by Montmorillonite K-10. The reaction fails where an ortho-keto moiety is present; therein sydnone ring cleavage occurs to form the corresponding indazole 3.