Thioacetal

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General structure of a thioacetal.
General structure of a dithioacetal.

Thioacetals are the sulfur analogue of acetals. They are prepared in a similar way to acetals: by reacting a thiol with an aldehyde:

RSH + R'CHO → R'CH(OH)SR

Dithioacetals are prepared similarly to thioacetals, which are intermediates:

RSH + R'CH(OH)SR → R'CH(SR)2 + H2O

Such reactions typically employ either a Lewis or Brønsted acid catalyst.

Dithioacetals generated from aldehydes and 1,2-ethanedithiol are useful in organic synthesis:

C2H4(SH)2 + R'CHO → R'CHS2C2H4 + H2O

Whereas the carbonyl carbon of an aldehyde is electrophilic, the deprotonated derivatives of dithioacetals feature nucleophilic carbon centers:

R'CHS2C2H4 + R2NLi → R'CLiS2C2H4 + R2NH

Again, this reactivity is most commonly exploited in the 1,3-dithiolanes. The inversion of polarity between R'(H)Cδ+Oδ- and R'CLi(SR)2 is referred to as umpolung.

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