Recent developments in the chemistry of sydnones and sydnone imines

Abstract The interest of the scientific community in sydnones and sydnone imines, the most known mesoionic compounds, is grounded on their unusual molecular structures, specific physical and biological properties, and chemical behavior. This nonexhaustive review highlights the achievements in sydnone and sydnone imine chemistry reported since 1998 and covers the preparations of these mesoionic compounds, their functionalization both at the 1,3,4-oxadiazoline ring and in side chains (including that one at N6 of sydnone imines), the ring-opening processes, and [3 + 2]-cycloaddition reactions where sydnones and sydnone imines act as the 1,3-dipoles. Special attention is paid to 4-lithio and 4-halogeno derivatives that have manifested themselves as the key intermediate compounds for preparations of a variety of sydnone and sydnone imine derivatives. Biological and some other applications of sydnone and sydnone imines are only slightly mentioned in this review.