Diastereoselective heterocyclization of geminal bromo-fluoro arylcyclopropanes by nitrosonium tetrafluoroborate: Access to 4-fluorinated isoxazolines and isoxazoles.

A series of 5-aryl-4-bromo-4-fluoroisoxazolines was synthesized via nitrosation of 2-aryl-1-bromo-1-fluorocyclopropanes with NOBF 4. It was shown that the E -isomers of the cyclopropanes react highly regio- and diastereoselectively leading exclusively to the E -isomers of the isoxazolines. The obtained 5-aryl-4-bromo-4-fluoroisoxazolines were transformed selectively into the corresponding 5-aryl-4-fluoro- or 5-aryl-4-bromoisoxazoles in good yields in the reaction with Lewis acids. Image 1 • Access to 4-fluorinated isoxazolines from gem -bromofluoroarylcyclopropanes. • High regio- and diastereoselectivity for E -4-bromo-4-fluoroisoxazolines. • Highly selective silver nitrate assisted dehydrobromination to 4-fluoroisoxazoles. • Highly selective AlCl 3 assisted dehydrofluorination to 4-bromoisoxazoles. [ABSTRACT FROM AUTHOR]