Monothioketene acetals

Because of the higher priority give to sulfur, the Z monothioketene acetal derived from the corresponding monothioester is mechanistically equivalent to the ketene acetal derived from an ester. Care must be taken when comparing the results from monothioketene acetals with the results from ketene acetals to avoid confusion as a result of the nomenclature inversion. 

Notable is the exceptionally high syn selectivity seen in the reactions of the 2-substituted-2-cycloalkenones with the Z monothioketene acetals (silyloxy group and the methyl groups trans, entries 19-25, Table 15). Note that good proportions of the anti diastereomer can be obtained with no substituent at the 2-position, whereas high syn selectivity can be obtained with 2-substituted cyclopentenones. 

P, P] By using antimony(V) chloride and tin(II) triflate, monothioketene acetals can be induced to add to a,/J-unsaturated thioesters (76). Interestingly, neither antimony(V) chloride nor tin(II) chloride by themselves effectively promote the reaction, implying that the combination of Lewis acids produces a new species. The stereochemistry of these additions is summarized in Scheme 44 and Table 16. For all cases studied, the reaction uniformly provides the anti diastereomers with good selectivity. The selectivity observed is somewhat lower than the optimized results observed with thioketene acetals and enones (vide supra). 

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