1) Four-membered ring 
Thiete is a heterocyclic compound containing an unsaturated four-membered ring with three carbon atoms and one sulfur atom. It is more commonly encountered not on its own, but in anellated derivatives, several of which have been synthesized, and as its 1,1-dioxides. Thietes themselves are generally not very stable. Thiete 1,1-dioxides are more sta...