Thioacetals are organic structures consisting of two sulfur atoms connected to the same carbon. Similar to acetals (a carbonyl combined with an alcohol), they can be attached to alkyl chains to form ring structures.

A thioacetal can be used as a protecting group for carbonyl groups on organic molecules. When performing reactions with complex molecules, often a need arises to use a reagent that can attack multiple parts of the molecule. Protecting groups are used to block the reagent from a specific area.

Carbonyl groups (found in aldehydes or ketones) are susceptible to reactions with various chemicals, including organometallics (such as Grignard reagents) or hydrides. A chemist can reversibly change a carbonyl group into a thioacetal, to make it resistant to such reactions. Various reactions can be carried out, and finally the protecting group can be removed, yielding the carbonyl group.

Necessary for this reaction is a dithiol, a molecule containing two thio- groups. A thiol is similar to an alcohol, but containing a sulfur atom in place of oxygen (-SH instead of -OH). A dithiol used for such a reaction preferably is one that will result in a 5- or 6-membered ring for the resulting structure (this reduces ring strain, making the reaction more favorable).

In the following illustrations, R and R2 indicate side chains, the modification of which requires prior protection of the carbonyl.

R	HS   SH 	    R    S
 \	  \_/   , ZnCl2      \ /   \
  C==O ---------------->      C     |
 /			     / \   /
R2			    R2   S

Various reactions can then be performed, without affecting this structure. It is also stable in both acids and bases.

It can be converted back into a carbonyl group via a two step process:

R    S			    R
 \ /   \   1)H2O, HgCl2	     \
  C     | ---------------->   C==O
 / \   /   2) CaCO3, CH3CN   /
R2   S			    R2

Alternatively, you can completely strip the thioacetal structure off the molecule by reduction via Hydrogen gas and a Raney Nickel catalyst, to yield an alkyl chain:

R    S			    R
 \ /   \   Rainey Ni, H2     \
  C     | ---------------->   CH2
 / \   /   		     /
R2   S			    R2

This method of reduction is preferable to applying H2/Pd (Hydrogen gas over a Paladium catalyst) directly to the carbonyl, as the latter is less selective: it will also reduce double bonds in alkenes.

In general, the need for protecting groups is undesirable due to a decreased yield for every additional step in a reaction, but their use is often unavoidable for a specific synthesis.

Source: My Organic Chemistry notes

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