Desulfurization of mercaptals

Two techniques, electrochemical reduction (section IIl-C) and Clem-mensen reduction (section ITI-D), have previously been recommended for the direct reduction of isolated ketones to hydrocarbons. Since the applicability of these methods is limited to compounds which can withstand strongly acidic reaction conditions or to cases where isotope scrambling is not a problem, it is desirable to provide milder alternative procedures. Two of the methods discussed in this section, desulfurization of mercaptal derivatives with deuterated Raney nickel (section IV-A) and metal deuteride reduction of tosylhydrazone derivatives (section IV-B), permit the replacement of a carbonyl oxygen by deuterium under neutral or alkaline conditions. 

The product composition from these reactions is influenced by the location of the functional group in the substrate. Olefin formation is the most common side reaction and in certain cases, especially with reductions of tosyl-hydrazones (section IV-B), it may become dominant so that the reaction can be used for the preparation of mono-labeled olefins. 

The reduction of oxime derivatives (section IV-C) is useful for the preparation of steroidal amines labeled with a deuterium on the nitrogen-bearing carbon atom 

The preparation of 7,7-d2-cholesterol in 1950 was the first example of deuterium incorporation into steroids via desulfurization of mercaptals with deuterated Raney nickel. A substantially modified version of this reaction subsequently became the first widely used method for site-specific insertion of two deuteriums in place of a carbonyl oxygen. This conversion consists of the preparation of a mercapto derivative (84 85), which usually 

Two serious drawbacks of this method are the extensive deuterium scrambling around the reaction site and the occasional formation of olefinic side products, which are hard to separate by conventional means. The extent of olefin formation may depend on the nature of the Raney nickel since it is known that desulfurization with deactivated Raney nickel can yield olefins. Best results are obtained when the deuterated Raney nickel is prepared very rapidly and used immediately after preparation. 

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Three different methods have been discussed previously (sections III-C,III-D and IV-A) for the replacement of a carbonyl oxygen by two deuteriums. However, in the conversion of a 3-keto steroid into the corresponding 3,3-d2 labeled analog, two of the three methods, electrochemical reduction (section ni-C) and Raney nickel desulfurization of mercaptal derivatives (section IV-A), lead to extensive deuterium scrambling and the third method, Clemmensen reduction (section III-D), yields a 2,2,3,3,4,4-dg derivative. 

Deuteration at C-12 by desulfurization of 12-thioketals with Raney Nickel preparation of the ethylene thioketal, 172 desulfurization of the mercaptal, 173... 

In order to ascertain whether sufficient nickel to complete a given reaction has been used, the liquid phase may be tested for starting material before an attempt is made to isolate a product. In general the sodium fusion test for sulfur is satisfactory for this purpose but in certain individual cases a specific test may be more convenient. Thus, in the desulfurization of thioacetals (mercaptals), unreacted material... 

Other possibilities of converting the aldehyde group to a methyl group are desulfurization of the mercaptal (p. 104) and reduction of azines, hydrazones and tosylhydrazones (p. 106). 

A soln. of tert-dimy alcohol in DME added to a suspension of Ni(II)-acetate, 2,2 -bi-pyridyl, and NaH in the same solvent at 63° under N2, the mixture stirred for 2 h, a soln. of dibenzothiophene in DME added over 2-3 min, and worked up after 5 h - biphenyl. Y 98%. The reagent efficiently desulfurizes S-heteroaromatics, ar. thioethers, sulfoxides, sulfones and mercaptals (although the efficiency of the last depends on the nature of the substituents on the S atom). F.e.s. S. Becker et al.. Tetrahedron Letters 29, 2963-6 (1988). 

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