Organic Synthesis

Dimethylchlorosulphonium ion is generated in situ from DMSO and oxalyl chloride. The reaction with an alcohol at -78°C leads to an alkoxysulphonium ion.
Deprotonation of this intermediate gives a sulphur ylide, which undergoes intramolecular deprotonation via a five-membered ring transition state and fragmentation to yield the product and DMS (odour!)

If the temperature is not kept near -78°C, mixed thioacetals may result.

One of the alcohol R groups shown can be a hydrogen, leading to an aldehyde.