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Oxabicyclo[3.3.1]nonanone can conveniently be prepared by the reaction of trans-p-menth-6-ene-2,8-diol and aldehyde or epoxide mediated by boron trifluoride etherate in good yields. The reaction proceeds via (3,5)-oxonium-ene-type reaction.

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The ene cyclization has evolved to become an indispensable tool for the synthesis of various ring size heterocyclic compounds. In the past and recent years, many exciting reports have demonstrated the broad scope and synthetic utility of ene cyclization and the versatility of oxonium ion, iminium ion and thionium ion intermediates. Moreover, the ease of regio- and stereoselectivity of ene cyclization has led to the development of new types of heterocyclic compounds. This article aims at reviewing the utilities of ene cyclization reactions for the synthesis of various ring sizes of oxygen, nitrogen and sulfur heterocycles. It also covers some applications in natural product synthesis. The mechanism of the reactions is discussed wherever necessary. The review article covers the time period from 1986 to 2017.

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