Rearrangement ketene thio-Claisen

Z)-.S -Allylic ketene aminothioacetals underwent thio-Claisen rearrangement at room temperature to give iVjV-dimethyl /1-hydroxy a-allylic thioamides96. /3-I Iydroxy-/V,/V,-dimethylthioamides were deprotonated with LDA to afford a chelated dianion with Z-configuration. Alkylation of this dianion gave the corresponding Z a-hydroxy S-allylic... 

Base-induced S-allylation of dithioesters [17,18], thionoselenoesters [19] as well as tertiary thioamides [20-23] with allylic halides and alcohols [24] have been commonly used to generate S-allyl ketene-S,W-acetals (W = S,Se, N), which are prone for a thio-Claisen rearrangement. In fact, the marked trend of the starting thiocarbonyl compounds to render the cis-... 

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... 

The efFicienqr of sulfoxides as chiral inductors for the thio-Claisen rearrangement has been demonstrated with N,S-ketene acetals derived from racemic 2-sulfinyl dithioacetates [72], Moreover, these reactions were successfully extended to the enantiopure series using chiral thioamides as substrates [125]. The source of chirality is the easily available and cheap D-glucose [126]. Remarkably, both enantiomers of the starting 2-sulfinyl thioacetamides can be selectively obtained from the same D-glucose. 

Similarly, but in the cyclic series, the thio-Claisen rearrangement of substrates derived from chiral thiolactams was reported to be facile but poorly stereoselective [129]. The low stereocontrol observed in both cases may be explained by the lack of facial selectivity resulting from the free rotation around the C-N bond of the N,S-ketene acetals. This critical issue has been solved either by constructing a rigid bicyclic framework or by using C2-symmetric amines as chiral inductors. The former strategy, developed by Meyers et al. [45], involved bicycUc thiolactams, which were transformed into N,S-ketene acetals by deprotonation with LDA, followed by S-allylation with various allyl halides (Scheme 9.27). 

Applications of the thio-Claisen rearrangement towards the synthesis of natural products are also well documented. Takano utilized a thio-Claisen rearrangement during the course of studies toward the synthesis of velbanamine. " Alkylation of 435 was followed by treatment with base to generate ketene A ,5 -acetal 437 which smoothly rearranged to thioamide 438. 

Rainer has studied the sulfonium ylide-mediated thio-Claisen rearrangement. Rhodium carbenoid 443, generated from rhodium acetate and alkenyl diazoacetate 441, reacted with the 2-ethylthioindole 440 to reveal the sulphur ylide 445. Proton shift generated the ketene aminethio acetal 446 which underwent thio-Claisen rearrangement to yield indoline 442 in excellent yield. 

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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