Cyclization halogen source

Synthesis.—A new route to alkyl-substituted crown compounds involves" the reaction of polyethylene glycols with alkenes in the presence of a positive halogen source (Scheme 51), followed by base-catalysed cyclization of the halohydrins produced. One approach to aza-crown molecules relies on alkylation of toluene-p-sulphonamide anions, and a study of some methods for N-detosylation of the cyclic products has been published. 

Alkenyl-lithiums, which can be prepared by metal-halogen interchange between an appropriate iodide and t-BuLi at — 78°C, undergo cyclizations on warming to room temperature before quenching with a proton source (equation 117) . This cyclization reaction has been extended for the construction of polycyclic alkanes by sequential polyolefinic cyclizations (tandem cyclizations). A propellane system could be constructed in a single step from his-intramolecular cyclization as shown in equation 118 . 

Cyclization. The base-catalysed cyclization of 1,3-chIorohydrins to oxetans has been the subject of kinetic studies and co-halogenated magnesium alkoxides (38) have been used as a source of various oxygen heterocycles, including oxetans, by heating in hexamethylphosphoramide. ... 

Copper Cyclization via aminohalogenation of (195), affording pyrrolidine derivatives (196) at <93% ee, has been reported to proceed upon catalysis with [(7 ,7 )-Ph-box]Cu(OTf)2 in the presence of Pr X (X = I, Br, Cl) as the source of the halogen atom and Mn02. The reaction is believed to proceed via iyn-aminocupration, followed by atom transfer, to form the C-X bond. ... 

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