The use of dichlorodicyanoquinone DDQ

DDQ has proved to be the most versatile of the high-potential quinones although certain others, notably chloranil (previous section) and tetrachloro-o-benzoquinone have been used on occasion. 

Agnello and Laubach suggested that the dehydrogenation of A -3-ketones to A -3-ketones by chloranil proceeds through the A -enol, which suffers hydride loss from C-7. The failure of 7a-methyl-A -3-ketones to undergo dehydrogenation while the 7/5-isomers do so readily indicates that specific removal of the 7a (axial) hydrogen probably occurs in unsubstituted compounds.  

An explanation for the difference in behavior of chloranil and DDQ towards A -3-ketones was first provided by Ringold and Turner. The A -enol (67) is produced faster than the more stable A -enol (68) but is not attacked appreciably by chloranil, which lacks sufficient oxidizing potential. Instead, the more easily oxidized A -enol (68) is dehydrogenated to (69) as it is produced. With DDQ, the faster formed A -enol (67) can be effectively dehydrogenated and the A -3-ketone (70) is formed  

The reaction of androst-4-ene-3,17-dione with DDQ in refluxing benzene or dioxane leads to the A -3-ketone as the major product, although small amounts of A -3-ketone and A -3-ketone are also produced. The latter arises from the A -3-ketone, since the A -3-ketone is not dehydrogenated further under the usual reaction conditions. A -3-Ketone production is more favored in benzene than it is in dioxane substituents at C-6 can also influence this selectivity. A recent thorough investigation of the mechanism of dehydrogenation of 3-ketones under neutral and acidic condi- 

Similarly, with two equivalents of DDQ, A -3-ketones give A -3-ketones in good yield ( 70%), without isolation of the intermediate A -3-ke-tone/ These trienones are also obtainable directly from A -3-alcohols with three equivalents of DDQ in refluxing dioxane (20 hr), and the overall yield ( 50%) compares favorably with less direct methods. The direct formation of A -3-ketones from A -3-ketones with acid catalysis is not successful. Enol derivatives have proven to be useful for the preparation 

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