The Nonreductive Cleavage of Oxides

The opening of epoxides with neighboring keto groups, accompanied by vinylogous jS-elimination, has already been mentioned (section V). This is an extension of the jS-elimination reactions which occur when a,p- or jS,y-epoxy ketones are opened with acid or base. a,jS-Epoxy ketones give a-substituted a,jS-unsaturated ketones [(162), for example], and j5,y-epoxy ketones give y-substituted a,jS-unsaturated ketones [(163), for example]  

The immediate product of opening of the a-epoxide (160) is the chlorohydrin (161) which slowly eliminates to give the olefin. In contrast, the epimeric chlorohydrin (165) formed from the jS-epoxide (164) eliminates spontaneously to give the same product (162). This difference is explicable by the known enolization preferences of 5oc- and 5/3-3-ketones. 

The nature of the substituent introduced is determined by the anion present with HX opening, X may be F, Cl, Br, OH, or OMe among others. (See ref. 297, 298, 345, 346, 347, for example). Where X is halogen, the same overall result may sometimes be achieved by addition of halogen followed by elimination.  

Whereas the acid-catalyzed ring opening of 4,5-epoxy-3-ketones in the a-and -series normally gives rise to the type of products discussed above, in some circumstances 4, 5jS-epoxy-3-ketones give abnormal, rearranged products. These products are 2a-hydroxy-A -3-ketones (or esters thereof) (166). 

The acid-catalyzed opening of 16,17-epoxy-20-ketones has also been studied in some detail. In this instance, jS elimination to give a, -unsaturated ketone cannot occur, and in the simplest examples 16 -substituted 17a-alcohols are formed. However, in the presence of a 16jS-methyl substituent, 16-methylene compounds are produced [(167), (168), for example]. Lithium bromide-lithium carbonate in refluxing dimethylformamide gives the A -17a-alcohol.  

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