Quinestrol, also known as ethinylestradiol 3-cyclopentyl ether (EE2CPE), is a synthetic estrane steroid and a derivative of estradiol.[1][2] It is an estrogen ether, specifically the C3 cyclopentyl ether of ethinylestradiol (17α-ethynylestradiol).[16][17] Closely related estrogens include mestranol (ethinylestradiol 3-methyl ether) and ethinylestradiol sulfonate (EES; Turisteron; ethinylestradiol 3-isopropylsulfonate).[16][17]