Synthesis of steroid 24,20-lactones by means of phenylselenolactonization

A synthesis of (20 R)- and (20 S)-3β-methoxy-5-cholen-24,20-lactones (7a and 7b) from 3β-methoxy-5-androsten-17-one (2) is described. The 17-ketone 2 was treated with isopropenyllithium to give 3β-methoxy-17α-(prop-l'-en-2'-ylo)-5-androsten-17β-ol (3). Compound 3 on reaction with ethyl orthoacetate and Claisen rearrangement of intermediate 17β-orthoester furnished ethyl esters of (E)- and (Z)-3β-methoxy-chol-5,17(20)-diene-24-acids (4b and 4a). Hydrolysis of ester groups in 4a and 4b and phenylselenolactonization afforded stereospecifically and regioselectively unsaturated (20 R)- and (20 S)-3β-methoxy-chol-5,16-diene-24, 20-lactones(6a and 6b), respectively. Reduction of double bond 16-17 in 6a and 6b gave the final products 7a and 7b. The phenylselenolactonization of(E)- and (Z)-3β-methoxy-chol-5,17(20)-diene-24-acids (5b and 5a) and spontaneous elimination of phenylselenyl moiety was investigated and compared with iodolactonization of the same unsaturated acids.