Desulphonylation

Solanesol and other prenyl alcohols are important as metabolites in mulberry and tobacco leaves and in the synthesis of isoprenoid quinones. Hence, Sato and collaborators107 have developed a stereoselective synthesis of all-trans-polyprenol alcohols up to C50. Construction of the requisite skeletons was accomplished by the alkylation of a p-toluenesulphonyl-stabilized carbanion, followed by reductive desulphonylation of the resulting allylic sulphonyl group. This was achieved most efficiently by the use of a large excess of lithium metal in ethylamine (equation (43)), although all reaction conditions led to mixtures. The minor product results from double bond rearrangement. 

The yields of the desulphonylation step are generally in the range of 70 to 80%. A related alkene synthesis is described in Section III.C.l. 

Trost published a desulphonylation procedure for aryl alkyl sulphones using an excess of sodium amalgam in buffered ethanol126 (equation 52). Trost claimed that this is superior to earlier reactions using sodium amalgam in ethanol because of a couple of factors the use of the acid phosphate buffer to prevent formation of significant amounts of sodium methoxide is particularly important, since this can cause isomerizations in base-sensitive substrates, and the temperature should be kept low, but optimized for each substrate. 

Trost found126 that aryl sulphones containing a range of other functionalities could be successfully desulphonylated without attack at alkenes, esters or nitriles, and without any detectable degree of alkene migration. 

The reagent has also been used to desulphonylate vinyl sulphones to give alkenes141-143. The solvent was refluxing 10% aqueous THF and the yields reported were over 80%. 

An unusual desulphonylation process has been developed by Julia s group170 in which a masked a-hydroxysulphone spontaneously loses the elements of phenylsulphinic acid... 

Eisch, Behrooz and Galle196 give compelling evidence for the intervention of radical species in the desulphonylation of certain acetylenic or aryl sulphones with metal alkyls having a lower oxidation potential at the anionic carbon. The primary evidence presented by these workers is that the reaction of 5-hexenylmagnesium chloride outlined in equation (85) gives a mixture of desulphonylation products, in accord with the known behaviour of the 5-hexenyl radical, in which the cyclopentylmethyl radical is also formed. 

This evidence is undoubtedly directly relevant to all of the reactions described in this section and should be valuable in guiding further studies into an interesting and potentially useful area of desulphonylation reactions. 

Examples of a desulphonylation procedure on complex molecules are provided by Fuchs and coworkers199 who reported on a triply convergent synthesis of L-( —)-prostaglandin E2. The molecules can be classified as homoallylic sulphones and reductive desulphonylation was best achieved with a mixture of sodium methoxide and sodium borohydride in methanol, with yields being better than 90%, despite the complexity of the molecules involved (equation 87). 

Finally, an ingenious synthetic sequence by Trost, Cossy and Burks201 includes a unique desulphonylation reaction that involves an electron-transfer process. The synthetic sequence uses 1, l-bis(phenylsulphonyl)cyclopropane as a source of three carbon atoms, since this species is readily alkylated even by weakly nucleophilic species. Given an appropriate structure for the nucleophile, Trost found that desulphonylation with lithium phenanthrenide in an aprotic solvent allowed for an efficient intramolecular trapping of the resultant carbanion (equation 88). This desulphonylation process occurs under very mild conditions and in high yields it will undoubtedly attract further interest. 

The desulphonylation of /J-ketosulphones with the Hantzsch ester is described in Reference 226. 

In all the desulphonylation reactions discussed in Sections III.B and III.C the sulphur is lost from the starting sulphone and is reduced in the process simultaneously, the former carbon-sulphur bond is either reduced to a C—H bond or is converted into a C=C bond. The reactions described in this section have the common thread that the sulphur atom is lost with reduction at sulphur, but the carbon atom is converted directly into a carbonyl group. Formally, these reactions offer a route from alkyl halides to aldehydes or ketones. 

Another approach to an oxidative desulphonylation reaction is to oxidize an a-sulphonyl carbanion with an oxidizing agent that is also nucleofugal. An example of this was presented by Little and Sun Ok Myong203 who used a molybdenum peroxide complex (M0O5.Pyridine.HMPA) as the oxidant. However, this reagent is expensive and... 

Perhaps of more significance is a detailed study132 into the reductive desulphonylation of 7-methyl-7-phenylsulphonylestratrienes. The goal was stereoselective removal of the sulphonyl group, and hydride reductions, alkali-metal-amalgam reductions and electrochemical reductions were explored. The latter proved to be the most effective and the best results are illustrated in Scheme 3. 

Details of the conditions used in these reductions are discussed, especially the fact that this is apparently the first example in which the stereochemical aspects of an electrochemical desulphonylation reaction on a complex molecule have been examined. It is likely that further work will be profitable, given suitable substituents on a molecule, since sulphones (especially vinyl and aryl sulphones) should be good candidates for this type of reduction. 

Absorption spectroscopy of sulphonyl radicals 1093, 1094 of sulphoxides 895, 897, 902-905 Acetoxysulphones 953 electrochemical reactions of 1036, 1037 / -Acyloxysulphones, desulphonylation of 948 Alcohols... 

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