Add time:08/14/2019         Source:sciencedirect.com

Oxiranes (epoxides) and oxetanes having a carbonyl functional group are chemoselectively isomerized to different heterocyclic compounds via Lewis acid-promoted 1,6- and 1,7-intramolecular nucleophilic attacks of the carbonyl oxygen on the electron-deficient carbon neighboring the oxonium oxygen: for example, cyclic imides to bicyclic acetals, esters to bicyclic orthoesters, sec-amides to 4,5-dihydrooxazole or 5,6-dihydro-4H-1,3-oxazines, and tert-amides to bicyclic acetals or azetidines. The intramolecular attack of a 1,5-positioned carbonyl oxygen predominantly results in a propagating-end isomerization polymerization. On the other hand, cyclic ethers having a 1,8- or farther positioned carbonyl group undergo conventional ring-opening polymerization. A tetrahydrofuran (oxolane) ring does not open, even with a 1,6-positioned carbonyl group.

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