Nucleophilic substitution of thiamin (11, vitamin B1, by sulfite ion gives pyrimidinium sulfonate 2 and the corresponding free thiazole.'Although this reaction was first reported in 1935 it was not until 1977 that a mechanism was advanced: one sulfite ion adds to the pyrimidinium ring, made electrophilic by protonation, to give intermediate 3. A second sulfite ion, the one appearing in the observed product, then reacts either with 3 or with another ...
