Erythromycin A enol ether is a degradation product of erythromycin formed under acidic conditions by C6–OH internal attack on the C9 ketone to produce a cyclic enol ether. The rearrangement results in a loss of antibiotic activity. This single reaction was the prime driver for the development of second and third generation erythromycins. Erythromycin A enol ether is an important standard for stability studies.