Reductive desulfurisation

Ortho-thioquinones are also accessible by this route [238, 239]. They were treated with glycals as dienophiles with success (Table 5, entry 2). These reactions were designed [240] to offer an original glycosyl transfer, by achieving a Diels-Alder reaction with the appropriate a,adioxothione (Table 5, entry 4), and a simple Raney nickel reductive desulfurisation of the cycloadduct. 

The reductive Raney nickel desulfurisation of thioamides is used routinely in the alkaloid series. It proved efficient for the synthesis of Vinca alkaloid related compounds [21], a vinblastine precursor [127], 19-hydroxytubo-taiwine [170], and (-)-monomorine [171]. 

A classic example of this occurred in the USA, when the 1970 Clean Air Act required electric generating companies to install emission abatement technology to limit sulfur emissions. Faced with significant investment costs, some companies switched from higher- to lower-quality coal. The result was an increase in sulfur emissions from dirty coal , offsetting any reductions from the end-of-pipe desulfurisation process [209]. 

A related stereospecific desulfurisation of vinylsulfoxides was observed in the treatment of the vinylsulfoxides (184) and (185) by t rr-butyllithium, in the presence of methanol. When the reduction was performed in the presence of deuteromethanol, the sulfoxide moiety was replaced by a deuterium... 

The proline-catalysed aldol reaction (chapter 26) of the heterocyclic ketone 177 with benzal-dehyde gave the an//-aldol 178. Reduction by the Prasad method (chapter 21) gave the anti,anti-diol 179 and so, by desulfurisation with Raney Ni, the anti,anti-Aio 179. The reduction could be controlled by the neighbouring chiral centre but the Prasad method uses 1,3-control by chelation with BEt2 and external axial delivery of nucleophilic hydride from NaBH4 181. This is 1,3-control via a cyclic intermediate.29... 

Thioacetals are useful because they undergo desulfurisation when treated with Raney nickel in the Mozingo reduction. This is an excellent method for reducing aldehydes or ketones to alkanes. 

Desulfurisation of the resulting adduct (24) gives the required raw -dibenzylbutyrolactone (25). The second approach utilises a Stobbe condensation to generate the unsaturated ester (26), hydrogenation, selective reduction, and alkylation of which leads to the trans-dibenzylbutyrolactone (25) [45-47]. 

Catalytic reductions of the thiophene ring, or of substituents attached to it, are complicated by two factors poisoning of the catalyst, and the possibility of competing hydrogenolysis - reductive removal of sulfur, particularly with Raney nickel - indeed the use of thiophenes as templates on which to elaborate a structure, followed finally by desulfurisation, is an important synthetic strategy... 

Many phosphine sulfides are known but their use in P-stereogenic chemistry has been rather limited until recently. They are easily prepared by reaction of phosphines with elemental sulfur, a step which proceeds with retention of configuration. Hexachlorodisilane and Raney nickel have been formd to be stereoselective reagents for desulfurisation of phosphine sulfides. Some examples of reductions are collected in Table 1.5. 

As an example, indicative costs are provided for the treatment of scrap lead-acid automotive batteries to produce refined lead using battery breaking and paste desulfurisation, followed by melting and reduction in a short rotary furnace, as described in Chapters 10 and 11. A typical small scale unit handling 35 000 tonnes per year of batteries is considered with a lead production of around 19 000 tonnes per year. Relevant production data is as follows ... 

Position 2 of the ergoline skeleton is highly suitable for synthetic modification of EA by both electrophilic and radical substitution. Many modifications have been reviewed by Rutschmann and Stadler (1978) chlorination, bromination and iodination, nitration and reduction of nitro derivatives to amino derivatives and reaction with 2-methoxy-l, 3-dithiolane affording an intermediate which can be desulfurised to a 2-methyl derivative. Troxler and Hofmann (1959) described the oxidation of lysergic acid diethylamide (LSD) to 2-oxo-3-hydroxy-2, 3-dihydrolysergic acid diethylamide. 

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