Enol ethers, silyl sulfenylation

Sulfenylation of lithium enolates with PhS-SPh or of silyl enol ethers with PhS-Cl allows any a-PhS carbonyl compound to be made regioselectively, e.g. 149 and 152 from 147 (see chapter 5). Oxidation with sodium periodate gives the sulfoxide without over-oxidation to the sulfone, but elimination requires reasonably high temperatures (about 120 °C for MeSO but only about 50 °C for PhSO). Together with the unpleasant by-products, the results of disproportionation of unstable PhSOH, this has led to a preference for the selenium version of the reaction, though we must admit that the by-products are even more offensive.22... 

SCHEME 5.8 Asymmetric sulfenylation of silyl enol ethers [9]. 

A catalytic, enantioselective, Lewis base (313)-catalysed a-sulfenylation of silyl enol ethers (311) has been developed by Denmark et al. (Scheme 84). Among a wide variety of chiral Lewis bases investigated in this reaction, selenophosphoramides (313) afforded a-phenylthio ketones (312) in generally high yield and with good enantioselectivity. The selectivity of the reaction did not depend on the size of the silyl group... 

Williams and Rastetter also accomplished an elegant synthesis of ( )-hyalodendrin (83) in 1980 [39]. Beginning with the sarcosine anhydride-derived enolic aldehyde 78, silyl protection of the enal enabled alkylation of the glycine center with benzyl bromide and thiolation using LDA and monoclinic sulfur a la Schmidt. After protection of the thiol with methylsulfenyl chloride and deprotection of the silyl ether, the enol was sulfenylated with triphenylmethyl chlorodisulfide to afford bis(disulfide) 82 as a 2 1 mixture of diastereomers favoring the anti isomer. Reduction of the disulfides with sodium borohydride and oxidation with KI3 in pyridine afforded ( )-hyalodendrin (83) in 29 % yield (Scheme 9.4). 

The sulfenylation of metalloimines is equally applicable to ketones, although using more reactive sulfur electrophiles it is possible to bring about reaction on the unmetallated enamine. ° Sulfenylation of ketone enol silyl ethers also proceeds well with the more reactive sulfur species. Sulfenamides and their derivatives e.g. 11) are particularly suited to the direct sulfenylation of ketones and active methylene compounds such as -diketones, -keto esters and malonates, which undergo facile reaction at room temperature (equation 5). This procedure, however, does not appear to have been exploited for the dehydrogenation of active methylene compounds icf. Section 2.2.4.1). By preparing the dianion (13)... 

As the accompanying equations illustrate, the reactions of enol silyl ethers are regarded as those of either electron-rich olefins or stable metal enolates. The treatments of eno/ silyl ethers with halogens, NBS, sulfenyl halides, and nitronium ions afford the corresponding a-halo, alkylthio, and nitro ketones, respectively (eq (22)) [19]. 

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