Synthesis of o-Carboranylmethyl Ethers of Steroids as Potential Target Substrates for Boron Neutron Capture Therapy

o -Carboranylmethyl ethers of steroids were synthesized by insertion of steroidal 2-propynyloxy derivatives into 6,9-bis(acetonitrile)decaborane (12). This reaction afforded compounds with estrane and androstane skeleton, potentionally useful in boron neutron capture therapy of hormone-sensitive forms of cancer: 17β- o -carboranylmethyl ether of estradiol IXb (yield 14%) and 3β- and 17β-carboranylmethyl ethers of androstenediol Vb and VIIb (yield 12% and 13%, respectively). Jones oxidation afforded carboranyl derivative of androsten-17-one VIb in 75% yield. As shown by a study of insertion of 3β-(2-propynyloxy)cholest-5-ene ( IVa ), the low yields of the insertion reaction cannot be increased by change in the reaction conditions. The relative binding affinity of compound IXb to estrogen receptor from rat uterine and human breast tumor cytosol was 3.0 and 0.29% respectively, of that of estradiol.