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Water-Assisted Nitrile Oxide Cycloadditions: Synthesis of Isoxazoles and Stereoselective Syntheses of Isoxazolines and 1,2,4-Oxadiazoles
Ist Teil von
Angewandte Chemie (International ed.), 2016-03, Vol.55 (12), p.3997-4001
Auflage
International ed. in English
Ort / Verlag
Germany: Blackwell Publishing Ltd
Erscheinungsjahr
2016
Link zum Volltext
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
Conventional methods generate nitrile oxides from oxime halides in organic solvents under basic conditions. However, the present work revealed that water‐assisted generation of nitrile oxides proceeds under mild acidic conditions (pH 4–5). Cycloadditions of nitrile oxides with alkynes and alkenes easily occurred in water without using catalysts, thus yielding isoxazoles and isoxazolines, respectively, with excellent stereoselectivity toward five‐ and six‐membered cyclic alkenes. A double stereoselective cycloaddition of two units of a nitrile oxide with cyclohexene was also achieved, thus yielding 1,2,4‐oxadiazole derivatives having a unique hybrid isoxazoline‐oxadiazole skeleton. Enantiomerically pure isoxazolines were prepared from monoterpenes with a ring strain. In one case, the isoxazoline with a butterfly‐like structure was simply prepared, and it might be used as a ligand in asymmetric catalysis.
Water changes things: In water, nitrile oxides can be generated from oxime halides without using catalysts. Unexpectedly, the nitrile oxide generation proceeds under mild acidic conditions. The catalyst‐free cycloaddition of the formed nitrile oxides with certain alkenes in water had excellent stereoselectivity, and it was applied to the synthesis of isoxazoles, novel oxadiazoles, and enantiomerically pure isoxazolines.