Hydrogenolysis thioketals

Carbonyl groups can be converted to methylene groups by desulfurization of thioketals. The cyclic thioketal from ethanedithiol is commonly used. Reaction with excess Raney nickel causes hydrogenolysis of both C—S bonds. 

The main interest in dithio-ketals and -acetals has been in connection with their hydrogenolysis by Raney nickel to give the corresponding hydrocarbons, as an alternative to the Clemmensen or Wolff-Kischner reduction on the parent carbonyl compound. This subject has been thoroughly reviewed [90]. Hydrogenolysis of thioketals has also been effected by hydrazine [91, 92, 93] and by alkali metals in liquid ammonia [94]. 

Carbonyl groups can also be reduced to methylene via thioketal intermediates. The preparation of the cyclic thioketals derived from ethanedithiol is common. Reaction of the thioketal with excess Raney nickel causes hydrogenolysis of the C-S bonds. 

A proof of the structure of the adduct (235) and all the intermediates in the synthesis of the D-homosteroid (237) was obtained by carrying out from (238) a stereospecific synthesis of 18-nor-D-homoandrostane-3/3, 17a/5-diol (242) [326]. The addition of h3q)obromous acid totheA )-bond of compound (238) led to the 9o -bromide (240) which was converted into the 11,15-diketone (241). The elimination of both 0x0 groups by the hydrogenolysis of the ethylene thioketals led to the diacetate (242) which, like the diol obtained from it, proved to be identical with the products synthesized by the CD—B— A route [356]. 

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