Catalysis, continued ethers

Disubstituted dihydropyrans are produced with high u/iri-selectivity when 2-phenyl-4-(4-tolylsulfonyl)-3,4-dihydro-2H -pyrans ate treated with Al-based Lewis acids <99SL132>. Tetraenes 10, derived from dienes via their epoxides, undergo a double RCM reaction under Ru-catalysis to yield polycyclic ethers 11 in which the dihydropyran units can be joined by a variable number of carbon atoms <99JOC3354>. Continued work on the use of dispiroketals in synthesis has led to an improved route to the enantiomers of bi(dihydropyrans) 12 <99JCS(P1)1639>. 

Ag [39], or Au catalysis [40], most protocols continue to call for significant excess ofthe starting alkane relative to oxidant ((5 100 equiv) and the reactions are often limited in scope to benzylic or allylic positions and cyclic ethers. Readily available iminoiodi nanes of formula Phi = NR (R = Ts, Ns) are generally utilized as the source of nitrogen, but the employment of commercially available chloro or broma mine T [34, 37, 41 44] or arylazides [35, 45] has also been reported. 

This overview about developments in the field of proline-catalysis unfortunately cannot take into full account the vast field of proline-derived catalysts, such as diarylprolinols, 4-silo)yprolines or proline-silyl-ether, to name only a few. These are covered in subsequent chapters of this volume. Furthermore, other great improvements have been made by using immobilised proline catalysts, such as PEG-supported proline or polyelectrolyte-bound pro-line. Going one step further, supported proline catalysts are then applicable in the striving field of continuous-flow reactions. Recent examples include aldol, a-amination reactions and Michael reactions under such conditions. ... 

Among the industrially relevant cyclic ethers tetrahydrojuran (THF) is by far the most important. THF is produced by condensation of 1,4-butandiol that is obtained from several feedstock basis (acetylene, propene, butadiene, and butane, see Scheme 5.3.4). 1,4-Dioxane is obtained by condensation of diethylene glycol that itself is obtained by the addition of water to ethylene oxide. The reaction is catalyzed by sulfuric acid (homogeneous catalysis) or by acidic ion-exchange resins (heterogeneous catalysis) and proceeds at about 160 °C with continuous distillative removal of water. 

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