Thioacylsilanes as Synthetic Equivalents of Unstable Thioaldehydes

Thioacylsilanes are potential synthetic equivalents of unstable thioaldehydes, related to their well-known ease of replacing the silyl group with a proton by means of a fluoride ion at the stage of the reaction products. This synthetic equivalence was explored in a large variety of reaction products of thioacylsilanes. 

The protiodesilylation with TBAF/THF or CsF/CH3CN of Diels-Alder cycloadducts 18 with R = Ar is found to occur easily at room temperature giving 36 generally in good yields32 on the contrary, alkyl-substituted derivatives are more reluctant to undergo protiodesilylation. Thus, drastic conditions (TBAF/tolu-ene/110 °C) are required for the tert-butyl derivative (18 R = f-Bu, Si = SiMe3),16 

Protiodesilylation of thioacylsilane 5-oxides 30 (silyl sulfines) offers a unique mild synthetic path to thioaldehyde 5-oxides 40 (monosubstituted sulfines), which 

A detailed study14 of the stereochemistry of fluoro-desilylation (Table 5) showed that the removal of silicon is a stereospecific process occurring with retention of configuration. Accordingly, from the E-isomers of thioacylsilane 5-oxides 30, the Z-thioaldehyde 5-oxides 40 were obtained. The myrtanyl derivative (entry 9) is the first enantiomerically pure thioaldehyde 5-oxide ever prepared. In contrast, loss of stereochemical integrity was observed during the desilylation of tert-butyl trimethylsilyl sulfine (entries 4 and 5) and mesityl-trimethylsilylsulfine (entries 10, 11, and 12). It was demonstrated that the loss of stereospecificity results from a fluoride-induced equilibration of thioaldehyde 5-oxides after the desilylation. 

Thioaldehyde 5-oxides were subjected to 1,4-cycloaddition with a variety of 

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