Three-Membered Rings with One Heteroatom

A-2-Unsaturated three-membered systems are unknown as stable molecules because they would have a four-electron 7t-system, and thus be antiaromatic. 1//-Azirines occur as reactive intermediates and there is evidence for the existence of 2-thiirene in a low-temperature matrix. Azirines, by contrast, are well-known stable compounds. Thiirene A.A-dioxides are also stable molecules, probably best likened to cyclopropenones. The chemistry of saturated three-membered heterocycles is, however, very extensive - epoxides (oxiranes), and to a lesser extent, aziridines are important intermediates in general synthesis. 

A major advance was the development of an efficient synthesis of epoxides of high optical purity from allylic alcohols and related systems (the Sharpless epoxidation) (below) such epoxides have been used extensively for the synthesis of complex natural products in homochiral form. 

Saturated and Partially Unsaturated Heterocyclic Compounds Reactions and Synthesis 593 

Regioselective opening of Sharpless epoxides, that is those (enantio-enriched) derived from allylic alcohols, has major synthetic significance. The usual outcome is opening at C-3, but the reaction with nucleophiles such as azide, cyanide and thiophenoxide, in the presence of trialkyl borates, can be highly selective for attack at C-2.  

The Payne rearrangement of epoxy-alcohols is a special case of an intramolecular nucleophilic opening of epoxides and is of particular synthetic utility when it is applied to epoxides from the Sharpless procedure.  

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