The reaction of nitrile oxides with Ai6-steroids substituted at C-17 by COCH3, OCOCH,, CN, NHCOCH3 and H afforded 1601, 17~-[3,1-(2-isoxazolino)]-steroids. From the reaction of C ,N-diphenylnitrone with pregna-5,16-diene-3p-ol-2O-one acetate two isomeric products were isolated, 1601, 1701 -(2,3-diphenyl-3 ,l-isoxazolidino)-pregn-5-en-3p-ol-20one acetate and 1601,17a -(2,3 -diphenyl -1 , 3isoxazolidino)-pregn-5-en-3p-01 -20 -one acetate.Certain steroids containing a heterocyclic ring fused at positions C -1 6 and C-17, such a s the 1601,17adioxolane derivatives of 901 -fluoro-l601hydroxyprednisolone (1) and of 1601,17a -dihydroxyprogesterone (2) have been reported by other investigators. The present investigation was undertaken in order to prepare steroids having other heterocyclic ring systems fused at these positions.The cycloaddition of 1 , 3 -dipolar reagents to unsaturated compounds, which has been reviewed by Huisgen (3), affords a simple method for the synthesis of five-membered nitrogen-containing heterocycles. The 1,3-cycloaddition of nitrile oxides to olefins affords A'-isoxazolines (4), and i t was of interest to use this reaction a s a direct method for synthesizing isoxazolino-steroids. While this paper was being written Fritsch, Seidl and Ruschig (5) reported that benzonitrile oxide and acetonitrile oxide added to 30acetoxypregna-5,16-dien-2O-one (Ia) to give isoxazolines with structure VIa (Ri =
