What is "thioacetal"

n. (context organic chemistry English) either a monothioacetal or a dithioacetal

Thioacetals are the sulfur analogues of acetals. There are two classes: monothioacetals and dithioacetals. Monothioacetals are less common, have the functional group RC(OR')(SR")H. Dithioacetals have the formula RC(SR')H (symmetric dithioacetals) and RC(SR')(SR")H (asymmetric dithioacetals).

The symmetric dithioacetals are relatively common. They are prepared by condensation of thiols or dithiols with aldedhydes. These reactions proceed via the intermediacy of hemithioacetals:

thiol addition to give hemithioacetal: RSH + R'CH(O) → R'CH(SR)(OH) thiol addition with loss of water, affording dithioacetal: RSH + R'CH(OH)SR → R'CH(SR) + HO

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

Dithioacetals generated from aldehydes and 1,2-ethanedithiol and 1,3-propanedithiol are useful in organic synthesis:P. Stütz And P. A. Stadler "3-alkylated And 3-acylated Indoles From A Common Precursor: 3-benzylindole And 3-benzoylindole" Org. Synth. 1977, 56, 8.

CH(SH) + R'CHO → R'CHSCH + HO

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

R'CHSCH + RNLi → R'CLiSCH + RNH

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