he Rh2(OAc)4-catalyzed reaction of 2H-azirine-2-carbaldehydes with dimethyl diazomalonate proceeds via azirinium ylide formation, isomerization to 2-azabuta-1,3-dienes followed by 1,6-π-electrocyclization to give 2H-1,3-oxazines. According to DFT-calculations ring opening of azirinium ylides should occur stereoselectively to give 2-azadienes with the Cdouble bond; length as m-dashC bond exclusively in the Z configuration. Changing a formyl group for an acetyl group in the azirine leads to a lowering of stereoselectivity and the formation of azadienes with an E configuration of the Cdouble bond; length as m-dashC bond was observed. The reaction of 2-acyl-2-diazoacetates with 2H-azirine-2-carbaldehydes proceeds similarly, but the 2-acetyl-substituted 2H-1,3-oxazines formed are unstable under chromatographic purification and rearrange easily into pyrrole derivatives.