Iodohydrin deoxygenation

Deoxygenation of epoxidesEpoxides are converted to alkenes with retention of configuration by treatment with this combination in hexane at 0 -> 25°. The reaction involves anti-opening of the epoxide to an iodohydrin, which undergoes trans-elimination to an alkcnc. 

Deoxygenation of epoxides. The reagent effects deoxygenation of epoxides via an intermediate iodohydrin (11,30), which can be isolated as the trans-isomer in some cases. This reagent can effect selective reactions in the case of some diepoxides. 

Organophosphorus reagents based on triphenylphosphine, or trimethylsilyl iodide, may be used to deoxygenate epoxides to re-form the parent alkene. Reactions based on this, or on a related scheme using the reduction of an iodohydrin, have been used in the synthesis and protection of alkenes as their epoxides (Scheme 2.23). 

Deoxygenation via iodohydrin intermediates using chlorosilane/sodium iodideand p-toluenesul-fonic acid/sodium iodidehas also been reported. In a natural product synthesis, an epoxide has been deoxygenated with a large excess of triethylsilane at 300 C for 30 h (equation 51). 

The stereoselective reductive deoxygenation of aliphatic cw-oxiranes to the corresponding c/5-alkenes via iodohydrin intermediates has been accomplished by convenient one-pot procedures (equations 6 and... 

Elimination. The highly stereoselective conversion of iodohydrin esters to alkenes [erythro ( ), threo — (Z)] by the allyltrimethylsilane-TiCl mixture enables stereoretentive deoxygenation of epoxides via the iodohydrins. 

A freshly-prepared, pale yellow, soln. of AII3 in dry benzene added with stirring to a soln. of 5,6a-oxido-5a-cholestane in acetonitrile, and reaction continued for 5 min - cholest-5-ene. Y 95%. Longer reaction times were required for less reactive epoxides which initially afforded rra/i5-1,2-iodohydrins. F.e. and preferential deoxygenation s. P. Sarmah, N.C. Barua, Tetrahedron Letters 29, 5815-6 (1988). 

On treatment with PhaPL, u/c-iodohydrins give high yields of olefins via stereospecific trani-elimination from the phosphorylated iodohydrin. A more useful extension of this process is the one-flask deoxygenation of epoxides to olefins via uic-halohydrins using 1.1 molar equivalents each of Ph3P.HI and PhaPL. The reduction takes place at or below room temperature, in high yield (91—98%), and with better than 98% retention of configuration. 

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