Synthesis of 22‐Cupriosteroids, a New Route to Steroids Possessing Oxygenated Side Chains

A new method for the synthesis of steroids with oxygenated side chains starting from C 22 steroids is described. Rieke copper, obtained by reduction of lithium 2‐thienylcyanocuprate 4 with lithium naphthalenide at −78°C or −100°C, reacts with the 22‐bromosteroids 1a, 2, 14 to afford the corresponding cupriosteroids. In addition to the metalation a very smooth retro Diels‐Alder reaction takes place in the case of 14 at the B ring. The steroid‐copper compounds react with (±)‐1,2‐epoxypropane to furnish epimeric mixtures of the 24‐hydroxysteroids 6, 8 and 15 in good yields. The procedure described is useful for a short synthesis of (24 R )‐cholesta‐5,7‐diene‐3β,24,25‐triol ( 17 , as 3‐silyl ether), the provitamin of the main metabolite (24 R )‐24,25‐dihydroxyvitamin D 3 , using 14 , Rieke copper and the highly functionalized chiral building block 10 / n ‐butyllithium.