When erythromycin A is heated in diethylamine-acetic acid, an erythromycin hemiketal is obtained, which can be further transformed into a new enol ether and spiroketal. The new enol ether is also obtained in equilibrium with the normal one on heating erythromycin A or B in pyridine-acetic acid. The novel compounds, which will be called pseudoerythromycin derivatives, are characterized by a translactonization between the Cll-hydroxyl and the ...