Thioacetals reductive desulfurization

Chrzanowska and Rozwadowska (60) performed a total synthesis of ( )-43, using amine 49 and the thioacetal of methoxycarbonylpiperonal (26) as substrates (Scheme 13). These two synthons were joined together under the influence of LDA, and the resulting addition product 50 was subjected to reductive desulfurization with Raney nickel to give racemic peshawarine (43). 

A somewhat similar configurational correlation between (leva)-glyceraldehyde and (dexfro)-lactic acid has been made by Wolfrom, Lemieux, Olin and Weisblat.36 Reductive desulfurization of tetra-acetyl-2-methyl-D-glucose diethyl thioacetal (XXVII) and hydrolysis of the product gave 2-methyl-l-desoxy-D-glucitol (XXVIII) oxidation... 

In- the preceding section the reductive desulfurization of some substances which are both thioglycosides and thioacetals has been mentioned here the hydrogenolysis of the simple 1-thioglycosides will be discussed. 

Alkyl fluorides.1 a-Fluoro sulfides, available by reaction of mercury(II) fluoride with thioacetals, are convertible into alkyl fluorides by reductive desulfurization with sodium in ethanol. 

Thioacetals are valuable intermediates for organic synthesis in their own right. For example, thioacetals are desulfurized by Raney nickel to give the corresponding hydrocarbon. This provides another path from an aldehyde or ketone to a methylene group that does not require either strong acid or strong base. Therefore, this method complements the Wolff—Kishner and Clemmensen reductions. 

An unusual reductive elimination can ensue from titanacyclobutanes possessing an alkenyl group at the carbon a to the titanium atom. Thus, alkenylcarbene complexes 48, prepared by the desulfurization of (fy-unsaturated thioacetals 49 or l,3-bis(phe-nylthio)propene derivatives 50 with a titanocene(II) reagent, react with terminal olefins to produce alkenylcyclopropanes 51 (Scheme 14.22, Table 14.4) [37]. This facile reductive... 

Ra-NL A mild procedure for deoxygenation of aldehydes or ketones is via desulfurization of their thioacetal derivatives. For example, reduction of thioacetals with Ra-Ni, derived by treatment of an Ni-Al alloy with NaOH, produces the corresponding alkane moieties. The hydrogen atoms in the deoxygenated product come from the hydrogen gas adsorbed on the Ra-Ni surface during its preparation. 

Deoxygenation of the carbonyl group of aldehydes and ketones via the intermediacy of their hydrazone derivatives, known as the Wolff-Kishner reduction,offers an alternative to the thioacetal desulfurization method. The Wolff-Kishner reduction in the presence of hydrazine and NaOH (or KOH) has been replaced largely by the Huang-Minlon method,where the deoxygenation is carried out with hydrazine in refluxing ethylene glycol. 

The reduction of 55 with LiAlHa and subsequent acetylation afforded the diphenylmethyl thioacetate 150, while heating with copper powder in diglycol diethyl ether caused desulfurization to tetraphenylethylene 151 (Scheme 19) <1997H(45)507>. Treatment of 55 with a fourfold excess of (Ph3P)2Pt(77 -C2H4) in either THF or toluene solution yielded [Pt2(Ph3P)4(/i-S2)] and the platinum(O) compound 152 (Scheme 19) <2000ZNB453>. 

Conventional methods for the reduction of a carbonyl to a methylene group that do not require the conjugative assistance of a heteroatom are the well-known Clemmensen (Zn(Hg), HQ), and Wolff-Kishner (H2NNH2/KOH) reductions, and the desulfurization of the corresponding thioacetal with Raney nickel. 

This method involves formation of the thioacetal followed by desulfurization with Raney nickel. It is the third method we have encountered for achieving this type of transformation. The other two methods are the Clemmensen reduction (Section 19.6) and the WoUF-Kishner reduction (Section 20.6). 

In Section 17-5 we reviewed methods by which carbonyl compounds can be reduced to alcohols. Reduction of the C=0 group to CH2 (deoxygenation) also is possible. Two ways in which this may be achieved are Clemmensen reduction (Section 16-5) and thioacetal formation followed by desulfurization (Section 17-8). This section presents a third method for deoxygenation—the Wolff-Kishner reduction. 

Wolff-Kishner reduction complements the Cletnmensen and thioacetal desulfurization methods of deoxygenating aldehydes and ketones. Thus, the Clemmensen reduction is unsuitable for compounds containing acid-sensitive groups, and hydrogenation of multiple bonds can accompany desulfurization with hydrogen and Raney nickel. Such functional groups are generally not affected by Wolff-Kishner conditions. 

In Summary The Wolff-Kishner reduction is the decomposition of a hydrazone by base, the second part of a method of deoxygenating aldehydes and ketones. It complements Clemmensen and thioacetal desulfurization procedures. 

Raney nickel is a highly active, finely divided form of the metal prepared by reaction of a nickel/aluminum alloy with concentrated sodium hydroxide, which removes most of the aluminum as Na[Al(OH)4], Although active for a very wide range of reductions, it has been particularly widely used for the reduction of nitriles, and the desulfurization of thioacetals, thioethers, and dithianes (see Section 19.3.3) (Figure 23.22). The careful disposal of the catalyst after use is very important once it has dried out, the metal is highly pyrophoric. 

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