Three-membered heterocycles, defined

A y-turn, which is a more rare reverse turn, is defined by a three-residue turn forming a seven-membered hydrogen bonded-ring between the carbonyl of the i residue and the amide NH of the i + 2 residue. Most of the y-turn mimetics described in the literature consist of a six-or seven-membered heterocyclic-or carbocyclic-ring structure (Figure 7). 

Oxygen-containing Heterocycles Three-membered Rings. - The addition of oxygen to double bonds remains the most common method for epoxide synthesis, and several new variations on that theme have appeared this year. Thus, alkenes are converted into epoxides using a solution of elemental fluorine in an acetonitrile/water mixture with complete retention of olefin stereochemistry.1 Prat and Lett have re-examined the use of tungstic acid catalysed epoxidation in an attempt to define the scope and synthetic utility of the process. They have shown that reactivity increases with the nucleophilicity of the double bond and... 

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