Steroid organoborane reactions Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/vq27zr24f

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  • Steroid organoboranes have been found to be useful synthetic intermediates. Attempted pyrolyses, based on simple trialkylborane cyclizations, have not yielded C-19 or C-18 functionalized steroids, but have produced isomerized systems not otherwise available. Thus, hydroboration of cholesterol followed by pyrolysis at 215° gives 31% of 2-cholestene-6α-ol and 49% of a mixture of cholestenes, predominantly Δ⁴ and Δ⁵. The alcohol arises by a facile borate pyrolysis, which can also be invoked to explain the presence of the cholestenes. 6-Methylcholesterol on hydroboration gives the previously unknown 6β-methylcholestane-3β, 6α-diol, with no 5α-alcohol detected. The organoborane isomerizes readily to the 6α-methylene system which on oxidation produces the 6α-hydroxymethylene compound. The latter is formed directly from 6-methylenecholestanol, indicating a facile organoborane isomerization. Cyclization from a 21-organoborane also does not occur at temperatures to 215°, sufficient to cause borate eliminations. An interesting intermolecular oxidation-reduction reaction of testosterone borate has been discovered. Routes to the various cholestenes, primarily involving organoboranes, have been studied and the properties of the products recorded. Purification of the olefins has been accomplished using chromatography on silver nitrate impregnated silica gel. Hydroboration-elimination of 4-cholestene-3, 6-dione furnishes a facile route to the homoallylic Δ³-6α-alcohol system. The solvolysis reactions of this system have been investigated and the stereochemistry of diborane reduction at C-6 has been examined. A review of organoborane chemistry, particularly that of steroids, is given.
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