Reactive oxygen species such as singlet oxygen have been a major focus of research in medicine. The effect of singlet oxygen on sterols within biological membranes is becoming increasingly more important. Ergosterol, a vitamin D precursor, is one such sterol. The benzoylation of ergosterol and subsequent reaction with singlet oxygen to form an endoperoxide provide two important reactions for the undergraduate laboratory. The benzoylation of ergosterol coordinates well with carboxylic acid chemistry generally covered in introductory organic chemistry. The photooxidation of ergosterol benzoate provides a unique dienophile for a [4 + 2] cycloaddition reaction. Singlet oxygen, generated by photosensitization, adds to the diene of the B ring of the sterol. Two explanations for the photosensitization are provided. The first explanation is for introductory organic chemistry students. The second explanation is for upper-level students and delves into electronic states and energy transfer. An optional microscale photosensitization is given using a laser pointer as a light source. This two-step synthesis augments lecture material while providing students the opportunity to develop pertinent laboratory skills including UV-vis and NMR spectroscopy, photochemistry, and TLC and column chromatography. (Contains 2 schemes.)