Conjugated double bonds, reduction sodium amalgam

The reduction of piperitone to menthone cannot well be brought about by the action of sodium or of sodium-amalgam in alcoholic solution, because, with the latter particularly, a solid bimolecular ketone is formed at once. This is a finely crystallised substance, melts at 148° to 149° C., and has the formula C gHj O. Piperitone thus follows the rule with substances having a conjugated double bond, carvone for instance, also forms a bimolecular ketone on reduction, melting at 148° to 149° C. 

Reductions with sodium amalgam are fairly mild. Only easily reducible groups and conjugated double bonds are affected. With the availability of sodium borohydride the use of sodium amalgam is dwindling even in the field of saccharides, where sodium amalgam has been widely used for reduction of aldonic acids to aldoses. 

Sodium amalgam has also been used in many instances to reduce carbonyl groups, as in the important conversion of strychninonic (XXXVIII) into strychninolic acid (XXXIX) (59). Conjugated double bonds have also been reduced, as in the conversion of Hanssen C19 acid (CXXVII) into its dihydro derivative by reduction of the maleic acid moeity (103), the conversion of 11-benzalstrychnine (LVII) into 11-benzylstrychnine (104), and the reduction of isostrychnine-II (XXXVI R = OH) to its 11,12-dihydro derivative (58). 

The second class of fully conjugated ring systems include the S(vi) oxidation state compounds, such as 85a-d, which react only under forcing conditions. For instance, the 2-alkenylanilines 86a-d have been prepared via the reduction of sulfoximines 85a-d with sodium amalgam (Equation 1) <1995S713>. In the case of disubstituted sulfoximines 85c and 85d, the major products 86c and 86d of this reaction contain a (Z)-double bond. The corresponding ( )-by-products are usually isolated in <10% yield. 

---