Sulfonium ion

A positively charged three-coordinate sulfur centre. Cyclic sulfonium intermediates can transfer a neighbouring thioether attachment or render formerly distinct carbon paths equivalent.

A four-membered cyclic sulfonium ion containing sulfur and three carbons. A bridged version with equivalent methylene paths can allow nucleophilic opening to restore either configuration at an initially stereogenic carbon.

A three-membered ring containing a positively charged sulfur atom and two carbon atoms. Nucleophilic opening at carbon normally involves backside attack and cleavage of one sulfur–carbon bond.