Ring oxiranes

Fused-ring Oxiranes, Oxirenes, Thiiranes and Thiirenes... 

Benzene monoxide-oxepin and its sulfur analog are treated elsewhere (Chapter 5.1.7) (67AG(E)385). However, we point out here that electron-withdrawing substituents often favor the benzene oxide tautomer. The first study on oxides of the environmentally hazardous polychloro- and polybromo-biphenyls shows that they exist mainly in the benzene oxide form (81JOC3721). Oxides of polynuclear aromatic hydrocarbons (PAH) also exist mainly in the fused-ring oxirane form. 

Oxetanes are generally much more stable to nucleophilic attack than the more strained three-membered ring oxiranes. However, activation of the oxygen atom by a Lewis acid increases the electronegativity of the adjacent carbon atom and renders oxetanes susceptible to attack from a nucleophile accompanied by subsequent ring opening. 

Due to the high ring strain of 3- and 4-membered rings, oxiranes and oxetanes polymerize practically irreversibly. Thermodynamic polymeri-zability of these groups of monomers is not significantly affected by substi-... 

Unlike for oxirane fusion to three-membered rings, oxiranes fused to four-membered rings (5-oxabicyclo[2.1.0]pen-tanes, cyclobutene oxides) are well known. 

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