13dithianes

13-dithianes are typically referred to in the literature as 1,3-dithianes. They are five-membered cyclic thioacetals formed by the condensation of a carbonyl compound (aldehyde or ketone) with 1,3-propanedithiol. The ring contains two sulfur atoms at the 1 and 3 positions and three methylene carbon units. This structure makes 1,3-dithianes robust protecting groups for carbonyls, while also enabling distinctive reactivity at the methylene bridge.

Preparation of 1,3-dithianes is commonly accomplished by reacting a carbonyl compound with 1,3-propanedithiol under acid catalysis, typically with an acid such as p-toluenesulfonic acid or catalytic HCl. The resulting dithiane protects the carbonyl from nucleophiles and bases, allowing synthetic sequences that would otherwise be incompatible with carbonyl functionality. Dithianes are relatively stable to a variety of reagents, a feature that underpins their widespread use in complex molecule synthesis.

A key aspect of 1,3-dithianes is their utility in carbonyl umpolung, notably via the Corey-Seebach approach. Deprotonation at the methylene position between the two sulfur atoms generates a lithio-dithiane or similar carbanion, which acts as an acyl anion equivalent. This carbanion can react with electrophiles to form new carbon–carbon bonds, enabling strategic C–C bond construction while the carbonyl is masked as a dithiane. After the desired transformations, desulfurization or oxidation removes the dithiane and regenerates the carbonyl, completing the sequence.

Desulfurization methods vary and may involve reductive or oxidative conditions, including reagents such as Raney nickel or mercuric salts in appropriate media. The choice of conditions depends on the substrate and the desired outcome.