Photo-induced transformations. Part 69. The formation of bridged oxabicyclic compounds by intramolecular radical addition of oxyl radicals generated from B-homocholest-5-en-7a-ol hypoiodites

Abstract

Reduction of B-homocholest-5-en-7a-one, synthesized by a Lewis acid-catalysed ring enlargement of cholest-5-en-7-one, with lithium aluminium hydride, gave B-homocholest-5-en-7aα-ol while reduction with sodium in ethanol afforded a more stable B-homocholest-5-en-7aβ-ol exclusively. The photo-induced reaction of the 7aα-ol in the presence of mercury(II) oxide and iodine gave 5,7aβ-epoxy-B-homo-5α-cholestane as the major product along with 5,7aα-epoxy-6β-iodo-B-homo-5α-cholestane and 5,7aβ-epoxy-6α-iodo-B-homo-5β-cholestane as the minor products. However, the photo-induced reaction of the 7aβ-ol in the presence of mercury(II) oxide and iodine gave 5,7aβ-epoxy-B-homo-5β-cholestane as the major product, with 5,7aβ-epoxy-6α-iodo-B-homo-5β-cholestane as the minor product. All these 5,7-epoxy-B-homocholestanes are formed by intramolecular addition of the oxyl radicals to the 5,6-double bond. The formation of 5,7aβ-epoxy-6α-iodo-B-homo-5β-chplestane from the 7aα-ol occurs via cyclization of a tetrahydropyranyl radical (D) which is formed by intramolecular combination of the allyl radical with carbonyl oxygen. This β-scission of the oxyl radical followed by a new type of radical cyclization was also found in the reaction of the oxyl radical generated from A-homo-cholest-4a-en-3β-ol.