REACTIVITY AND GEOMETRY IN ALLYLIC SYSTEMS: III. PHOTOSENSITIZED OXYGENATION OF CHOLEST-4-ENE, A STEROID OLEFIN WITH A CONFORMATIONALLY FLEXIBLE RING

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Abstract
Photosensitized oxygenation of cholest-4-ene with hematoporphyrin in pyridine followed by reduction of the derived hydroperoxides gave five allylic alcohols in these relative amounts: cholest-4-en-3α-ol (Va, 37%); cholest-4-en-3β-ol (IVa, 7.5%); 5α-cholest-3-en-5-ol (VIIIa, 29%); 5β-cholest-3-en-5-ol (VIa, 2%); and cholest-5-en-4β-cl (Xa, 8.5%). A by-product from breakdown of the hydroperoxides was cholest-4-en-3-one (III, 6.5%). The Δ4-stenols are not primary products from the photosensitized process but arose by gradual isomerization of the initially derived 5-hydroperoxycholest-3-enes. When the original hydroperoxide mixture was allowed to stand in chloroform, the rearrangement went to completion and no isolable amounts of the Δ3-stenols remained. After rearrangement, Va and IVa were present in a ratio of about 8:1, and this sequence affords a convenient new route to cholest-4-en-3α-ol. The photooxidation products and their relative proportions are readily understood if oxygen can attack both half-chair conformations of ring A in cholest-4-ene and can cyclically abstract a quasi-axial (or an axial) allylic hydrogen. Conclusions are drawn on the importance of steric hindrance and of ring inversion on the photosensitized oxygenation reaction.

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