The reduction of halohydrins, halohydrin esters and epoxides

The reductive elimination of halohydrins provides a means of introduction of double bonds in specific locations, particularly as the halohydrin may be obtained from the corresponding a-halo ketone. This route is one way of converting a ketone into an olefin. (The elimination of alcohols obtainable by reduction has been covered above, and other routes will be discussed in sections IX and X.) An advantage of this method is that it is unnecessary to separate the epimeric alcohols obtained on reduction of the a-bromo ketone, since both cis- and tran -bromohydrins give olefins (ref. 185, p. 251, 271 cf. ref. 272). Many examples of this approach have been recorded. (For recent examples, see ref. 176, 227, 228, 242, 273.) The preparation of an-drost-16-ene (123) is illustrative, although there are better routes to this compound. 

The reductive cleavage of -bromo ethers is a related reaction. If the ether is cyclic, the reverse reaction takes place on treatment with NBS  

The ready reduction of iodohydrins is utilized in the Cornforth reaction for preparing olefins from epoxides. Here the opening and reduction are carried out in one step by treatment of the epoxide, in an acetic acid-sodium acetate buffer, with sodium iodide and zinc. A less common use of iodohy-drin reduction is illustrated in the synthesis of the diene (127)  

The pure olefin is best prepared via the bromohydrin (131) a more direct route from the tosylate (129) yields a mixture of (132) and its A -isomer. The indirect method is most suitable for preparing A -olefins in the 5jS-series. 

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