Chemoselective reduction of 1,4,6-cholestatrien-3-one and 1,4,6-androstatriene-3,17-dione by various hydride reagents

The chemoselectivity of rigid cyclic α,β-unsaturated carbonyl group on the reducing agents was influenced by the ring size and steric factor. Cholesterol (cholest-5-en-3β-ol) and dehydroepiandrosterone (DHEA) were oxidized with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to form 1,4,6-cholestatrien-3-one and 1,4,6-androstatriene-3,17-dione. They were reduced with NaBH4, lithium tri-sec-butylborohydride (l-Selectride), LiAlH4, 9-borabicyclo[3.3.1]nonane (9-BBN), lithium triethylborohydride (Super-hydride), and BH3·(CH3)2S in various conditions, respectively. Reduction of 1,4,6-cholestatrien-3-one and 1,4,6-androstatriene-3,17-dione by NaBH4 (4 equiv.) produced 4,6-cholestadien-3β-ol and 4,6-androstadiene-3β,17β-diol, respectively. Reduction by l-Selectride (12 equiv.) afforded 4,6-cholestadien-3α-ol and 4,6-androstadiene-3α,17β-diol, chemoselectively. Reaction with Super-hydride (12 equiv.) produced 4,6-cholestadien-3-one and 3-oxo-4,6-androstadien-17β-ol. Reduction of 1,4,6-cholestatrien-3-one by 9-BBN (14 equiv.) produced 1,4,6-cholestatrien-3α-ol, but 1,4,6-androstatriene-3,17-dione was not reacted with 9-BBN in the reaction conditions. Reaction of LiAlH4 (6 equiv.) formed 4,6-cholestadien-3β-ol and 3-oxo-1,4,6-androstatrien-17β-ol. Reduction of 1,4,6-cholestatrien-3-one by BH3·(CH3)2S (11 equiv.) gave cholestane as major compound and unlike reactivity of cholesterol, 1,4,6-androstatriene-3,17-dione by 8 equiv. of BH3·(CH3)2S formed 3-oxo-1,4,6-androstatrien-17β-ol. LiAlH4 and BH3·(CH3)2S showed relatively low chemoselectivity.