Sulfenyl reagents ketones

The asymmetric a-sulfenylation of ketones is a particularly challenging reaction, as demonstrated by the poor success reported in the stereoselective variants via classical enolate/azaenolate reaction with an electrophilic sulfur reagent [71]. An umpolung approach has been devised by Coltart and co-workers [72] to effect the first asymmetric a-sulfenylation of ketones with arene thiols. Nitroso alkene derivatives, in i/tM-generated under basic conditions from a-chloro oximes, reacted with arene thiols in the presence of cinchona thiourea 27, which promoted the conjugate addition of thiophenol (Scheme 14.25). The chiral nonracemic a-sulfenylated oximes were directly hydrolyzed by IBX to ketones in high yield and good enantioselectivity. 

From intermediate 12, the path to periplanone B (1) is short but interesting. Enolization of 12 with lithium bis(trimethylsilyl)amide at -78 °C, followed by sulfenylation using Trost s reagent,12 affords a 16 1 mixture of regioisomeric monosulfenylated ketones favoring intermediate 17. The regioselectivity displayed in this reaction is... 

Acyl anion equivalents. Benzotriazol-l-ylmethyl methyl sulfide can undergo consecutive C-alkylations. The products are readily hydrolyzed with acidic aqueous methanol to give ketones. A related process involving alkylation and sulfenylation followed by treatment with a Grignard reagent to eliminate benzotriazole completes the synthesis of ketene dithioacetals. ... 

Sulfenylation. Ketones and amines are sulfenylated with the reagent 1 under very mild conditions. 

Diketones. The reagent is somewhat more reactive than diphenyl disulfide for sulfenylation of lithium enolates of ketones. The products can be converted into protected fojms of 1,2-diketones or into the products of cleavage. 

Bis-sulfenylation of y-butyrolactone, as well as of cyclic ketones, has been achieved with this reagent via the corresponding lithium enolates. This reaction has been utilized in an efficient synthesis of 3-substituted 2-buten-4-olides (eq 4). 

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