(17R)-17-Ethynyl-13-methyl-7,8,12,13,14,15,16,17-decahydro-6H-cyclopenta[a]phenanthrene-3,17-diol, commonly known as 17α-Ethynylestradiol-17β (or 17α-Ethynylestradiol), is a synthetic steroidal estrogen. It is a derivative of estradiol, featuring an ethynyl group at the C17α position, which significantly enhances its oral bioavailability and metabolic stability compared to the natural hormone. This compound is a key active pharmaceutical ingredient (API) in numerous hormonal contraceptives and hormone replacement therapies. It acts by binding to estrogen receptors, exerting potent estrogenic effects to inhibit ovulation and regulate the menstrual cycle. Its structural modification makes it resistant to first-pass metabolism, allowing for effective oral administration. As a chiral molecule with a defined (17R) configuration, it is also a critical chiral intermediate in the synthesis of more complex steroidal drugs and novel estrogen receptor modulators. Its production and handling require strict controls due to its potent hormonal activity and associated health and environmental concerns.