Ketones, tosyl hydrazones, reduction

Some advantages of this reaction are high yield if the tosylate is in a sterically accessible position excellent isotopic purity of the product (usually higher than-95%) and perhaps most important, access to stereospecifically labeled methylene derivatives. For example, deuteride displacement of 3j -tosylates (183) yields the corresponding Sa-d derivative (185) in 96-98% isotopic purity. Application of this method to the labeled sulfonate (184), obtained. by lithium aluminum deuteride reduction of a 3-ketone precursor (see section HI-A) followed by tosylation, provides an excellent synthesis of 3,3-d2 labeled steroids (186) without isotopic scrambling at the adjacent positions. The only other method which provides products of comparable isotopic purity at this position is the reduction of the tosyl-hydrazone derivative of 3-keto steroids (section IV-B). 

In general, the reduction of tosyl hydrazones with boron hydrides is a convenient alternative to the classical procedures of Wolff-Kishner, Clemmen-sen, and Raney nickel reductions when one wants to delete the carbonyl of ketones and aldehydes after using it for the very many synthetic operations it allows. 

A final method of alkene formation from a-derivatized ketones entails formation of unsaturated tosyl-hydrazones from a,P-unsaturated ketones followed by conjugate reduction with sodium cyanoborohy-diide in acid media. This transformation, which proceeds with regioselective migration of the original double bond, has been applied to the synthesis of quassinoids by Ganem et al. (Scheme 22). A similar... 

A method for the conversion of carbonyl compounds into the corresponding hydrocarbons involves reduction of the derived toluene-p-sulfonyl (tosyl) hydra-zones with sodium cyanoborohydride in acidic dimethylformamide (DMF). The reaction is specific for aliphatic carhonyl compounds aromatic compounds are normally unaffected. The tosyl hydrazone need not be isolated but can be prepared and reduced in situ. For example, the ketone 97 was reduced to the alkane 98 (7.89). 

With a, -unsaturated carbonyl compounds, reduction of the tosyl hydrazone is accompanied by migration of the double bond. Thus, cinnamaldehyde tosylhydra-zone gives 3-phenyl-1-propene in 98% yield and the a, -unsaturated ketone 99 gives the alkene 100 (7.90). The mechanism for this reaction involves reduction of the iminium ion to the tosylhydrazine 101, elimination of p-toluenesulfinic acid... 

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