Allyl ketene dithioacetals

Chirahty transmission can also be induced in thio-Claisen reactions by a chiral center adjacent to the allyl vinyl sulfide core. The group of Metz-ner examined the diastereoselectivity of the rearrangement of acyclic S-allyl ketene dithioacetals bearing a chiral center adjacent to carbon 6. hi these substrates, the sigmatropic shift proceeded smoothly and with modest syn anti diastereoselectivity [74]. In contrast, higher selectivities are foimd for thio-Claisen precursors bearing a chiral center adjacent to carbon 1 mainly due to steric effects and allylic strain [75]. In 1991, the group of Beslin examined the effect of a hydroxyl substituted chiral center attached to C-1 of Z and E S-allyl ketene dithioacetals [76,77] (Scheme 7). Under the reaction conditions, these... 

Methyl 3-hydroxybutanedithioate (R = Me) (0.6 g 4 mmol) was added dropwise to a solution of LDA (2 eq) in THF cooled at -78°C. The mixture was stirred for 20 min. The yellow dithioester colour disappeared. Allyl bromide (1 eq) was added. Stirring was maintained for 45 min. The solution was quenched by an aqueous solution of ammonium chloride. The mixture was partitioned between ether and brine. The organic layer was washed with brine, dried with magnesium sulfate and concentrated. The crude ketene dithioacetal was left at ambient temperature for X d to achieve the rearrangement. A yellow oil was isolated by flash chromatography (elution with a 90 10 mixture of cyclohexane/ethyl acetate), yield 74%, as a 90 10 syn/anti mixture of diastereoisomers. 

Ally I ketene dithioacetals. In the presence of these two reagents, ketene dithioacetals (1) react with allyl methyl ethers to provide allyl-substituted ketene dithioacetals (equation I). The combination of ClSi(CH3)3 and SnCl2 is an equally... 

Lithiosilanes derived from cyclopropane dithiocetals add to aldehydes to give precursors for Peterson olefinations - one of the best ways of making alkylidene cyclopropanes. In the example below, the lithiated allyl sulfide 72 adds cleanly to a ketene dithioacetal to give cyclopropane 73. Successive reductive lithiations give silane 74 and then a mixture of... 

Ketene dithioacetals are deprotonated with LDA-HMPA and complexed with copper(l) iodide (Scheme 36). This reagent reacts with allylic halides exclusively at the y-position with allylic rearrangement (5n20. The reaction of the lithium reagent with simple alkylating reagents gives mostly a-attack. Ketene dithioacetals can be converted to esters by aqueous mercury(II) chloride. 

The substituted ketene S,S-dithioacetal (76) has been developed as a P-lithioacrylate equivalent (Scheme 22). Deprotonation of (76) leads to an allylic anion (77) that reacts with a wide range of electophiles to give the T-substituted adduct (78), as the only observed product. Hydrolysis of the ketene thioacetal then releases the carboxyl residue, and this activates the system towards elimination of thiophenol, thereby unmasking the acrylate unit. Dianion (79) may also be regarded as a modified B-lithio-acrylate equivalent, but the chemistry of this species is diverse. Not only have B-substituted a, B-unsaturated amides been synthesized, but the same reagent reacts, for example, with epoxides to give ultimately dihydropyrans the species (79) is therefore perhaps more accurately represented as equivalent to the dipolar... 

Thioacylketen dithioacetals participate with alkenes and sulphenes in 1,4-cycloaddition reactions. Thio-Claisen rearrangement of 1-allylthio-l-aminoalkenes to thioamides or isothiocyanates, and of 5 -allyl-5 -methyl-keten mercaptals to dithioesters, has been described. 

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