A simple and expeditious synthesis of 2,3,6-trisubstituted tetrahydropyrans through (3,5)-oxonium-ene cyclization using molecular iodine

Revisiting applications of molecular iodine in organic synthesis

Molecular iodine contributes significantly to organic transformations in synthetic organic chemistry. It works effectively due to its mild Lewis acidic character, ability as an oxidizing agent, good moisture stability, and easy availability.

Ene cyclization reaction in heterocycle synthesis

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.

Regio- and Diastereoselective Synthesis of Dihydropyrans and Pyranopyrans via Oxonium-Ene Reaction of β-Allenols and Aldehydes

Bismuth trifluoromethanesulfonate can be efficiently used for the preparation of dihydropyrans from β-allenols and aldehydes by oxonium-ene reaction in good yields. The reaction is highly regioselective. On the other hand, the same reaction with trimethylsilyl trifluoromethanesulfonate at -45 °C affords the hexahydropyrano[4,3- b]pyran skeleton in moderate yields.

An iodine catalyzed metal free domino process for the stereoselective synthesis of oxygen bridged bicyclic ethers

A domino reaction has been developed for the synthesis of oxygen bridged bicyclic ethers through the coupling of 4-(2-hydroxyethyl)cyclohex-3-enols with aldehydes in the presence of 10 mol% of molecular iodine in dichloromethane at 25 °C. This method is highly diastereoselective affording the corresponding bicyclic ethers, i.e. octahydro-4a,7-epoxyisochromenes in good yields with high selectivity. It is the first report on the synthesis of oxygen bridged bicyclic ethers using a domino Prins strategy.