Diastereomeric (19S)- and (19R)-19-ethynyl-19-acetoxy derivatives of androst-4-ene-3,6,17-trione (AT) (9 and 10) and 19,19-difluoro AT (12) were synthesized. The 19,19-difluoro compound (12) was an effective competitive inhibitor of humanplacentalaromatase with an inhibition constant (ki) of 1.8 microM but the acetylenic 9 and 10 were poor inhibitors of the enzyme with k(is) of 75 and 67 microM,