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In a first [3+2] cycloaddition between the alkyne and ozone, a first molozonide-type intermediate (Need a refresher? Check the Ozonolysis of alkene sheet) is formed. This latter is unstable and undergoes retrocycloaddition to yield a zwitterionic intermediate bearing both an oxide carbonyl and a carbonyl function. A final rearrangement leads to an ozonide-type intermediate. At this stage, depending on the workup conditions, different products can be obtained.
Neutral workup: The ozonide intermediate undergoes rearrangement to generate an anhydride. In the presence of water, the anhydride undergoes hydrolysis releasing after equilibrium a pair of carboxylic acids.
Reductive workup: In the presence of dimethyl sulfide (or other reductants such as Zn or phosphine), the ozonide-type intermediate is attacked by one lone pair on the sulfur atom producing a zwitterionic compound. After elimination of DMSO, a 1,2-diketone is finally obtained. Case of terminal alkyne: under neutral workup, carboxylic acid is formed with formic acid which subsequently decomposes in carbon dioxide. Under reductive workup, a α-ketoaldehyde is produced.