Münchnone
Other names: 1,3-oxazol-5-ol
C3H3NO2
34/50 
uncommon
Münchnone is a mesoionic heterocycle first reported by Huisgen and colleagues at the University of Munich in the 1960s. This five-membered oxazolone derivative exhibits unusual electronic properties that make it challenging to represent with conventional Lewis structures. Chemists often depict münchnones using a circle with inscribed "+" and "−" symbols to indicate their mesoionic character—a delocalized system where positive and negative charges exist simultaneously within the ring. This representation acknowledges that no single resonance structure adequately describes the compound's electronic distribution. Münchnones appear as colorful, often yellow to orange crystalline solids that are relatively stable when properly substituted at the nitrogen and carbon positions. Their synthetic utility lies in their behavior as 1,3-dipoles in cycloaddition reactions, particularly with alkynes and alkenes to form pyrroles and other heterocycles. These versatile intermediates serve as valuable building blocks in heterocyclic chemistry and have found applications in pharmaceutical synthesis.