Diosphenols rearrangements

In practice, the scope of the benzilic rearrangement as a tool for the ring contraction of steroids is defined by the availability of a-diketones, or the corresponding diosphenols. 

Benzilic rearrangement of diosphenol (44) gives the A-norsteroid (45) in 90 % yield. 

Kubota and co-workers describe a novel oxidative rearrangement of the diosphenol (58) of 17iS-hydroxyandrost-4-ene-2,3-dione to the A-nor-A -1,2-diketone (59) in 33 % yield by the action of specially prep d manganese dioxide in boiling acetone. The rate of ring contraction is very sensitive to the source of the oxidant, and a trace of dilute sulfuric acid in the reaction mixture causes oxidative fission of ring A. 

Ring A diosterols.3 The ring A diosterols (3 and 4) of triterpenes can be prepared from the A2-alkene (1) by osmylation to form the two possible cis-diols (2), which on Swern oxidation give the a-diketone (3). The same diketone is also obtained by Swern oxidation of the 2(3,3a-diol, the product of peracid oxidation followed by acid cleavage. The diketone 3 rearranges to the more stable diosphenol (4) in the presence of base. 

Considerable interest has been shown in the rearrangement of the a-diketones of steroids and of di-and tri-teipenoids. Of particular interest is the abutment of the reaction center to a chiral system of rigid stereochemistry. This allows an examination of the regiochemistry and the stereospecificity and stereoselectivity of the reaction with alkali. The isomeric diosphenols (64) and (65), respectively 3-hydroxy-5a-cholest-3-en-2-one and 2-hydroxy-5a-cholest-l-en-3-one, are interconvertible in the presence of acid or base. On reaction of either with potassium hydroxide in propanol a single benzilic acid rearrangement... 

Claisen rearrangement, diosphenol-Claisen rearrangement, enzymatic Claisen... 

The thermal Claisen rearrangement of so-called diosphenol allyl ether has been investigated by Ponaras [18]. In 1983, he reported half-life times for the rearrangement in different solvent systems. Selected data are given in Schemes 11.9 to 11.11. Unfortunately, the half-life times have been determined at varying temperatures, which limits the quantitative comparability of many of the available data. 

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