Compounds of formula (I): in which R1, R2, R3, R3′, R4, R5, R6, R6′, R7, R8, R9, R10, R11, R12, R13, R14, R15, R16, R17, A, D, X, Y, and Z are defined in the specification. Also disclosed is a method of using one of the compounds to lower the blood cholesterol level and treat cancer, atherosclerosis, diabetes, Alzheimer's disease, and corneal arcus.