Pseudoerythromycin A enol ether is a degradation product of erythromycin formed by a complex internal rearrangement of erythromycin A on exposure to neutral to weakly alkaline conditions. The C6–OH forms an internal enol ether with the C9 ketone of erythromycin, while the C11-OH attacks the carbonyl of the lactone to reduce the macrocycle from a 14- to an 11-membered macrolide. Synthetically, pseudoerythromycin A enol ether is prepared by reacting erythromycin enol ether with carbonate. Pseudoerythromycin A enol ether is devoid of antibiotic activity but is an important analytical standard for erythromycin A stability studies.