The synthesis of cycloimmoniumsalts of the type , and the corresponding cycloimmonium ylides , are reported. In the reaction with diazonium salts , the pyridinium monosubstituted carbanion ylides , give the pyridinium disubstituted carbanion ylides – . These are new types of isolable stable ylides. The synthesis involves the formation “in situ” of ylides , which are bonded by their nucleophile ylide