Opposite Regioselectivity in the Sequential Ring-Opening of 2-(Alkanoyloxymethyl)aziridinium Salts by Bromide and Fluoride in the Synthesis of Functionalized β-Fluoro Amines

Abstract

1-Arylmethyl-2-(bromomethyl)aziridines were converted into the corresponding 2-(alkanoyloxymethyl)aziridines upon treatment with potassium 2-methylpropanoate or potassium 2-­methylbutyrate in DMSO in excellent yields, following regio­selective ring-opening towards N-(2-bromo-3-alkanoyloxy­propyl)amines ­using allyl bromide or an arylmethyl bromide in acetonitrile. Treatment of the latter β-bromo amines with tetrabutyl­ammonium fluoride in acetonitrile afforded 2-amino-1-fluoro­propanes as the major compounds (72-86%) besides the isomeric 1-amino-2-fluoropropanes in minor quantities (14-28%). The ring-opening of the intermediate aziridinium salts by bromide and ­fluoride in acetonitrile resulted in a different regioselectivity with a preferential attack of bromide at the more hindered carbon atom and of fluoride at the less hindered carbon atom of the aziridinium ion.