Abstract
Electrophilic cyclisations of various O-homoallyl
hydroxylamine derivatives are discussed. Non-protected O-homoallyl hydroxylamines undergo oxidative
cyclisations to isoxazolines. The new isoxazoline synthesis comprises
an initial electrophilic cyclisation to form intermediate isoxazolidines,
which are subsequently oxidized to isoxazolines. Cyclisations can
be conducted with various electrophiles (t-BuOCl,
PhSeBr, NBS and NIS). Oxidation can be suppressed if the starting O-homoallyl hydroxylamines are N-sulfonylated. The electrophilic cyclisation
then provides N-sulfonylated isoxazolidines
with moderate cis-selectivity (up to
7:1). Electrophilic cyclisation of N-acylated O-homoallyl hydroxylamines provides
isoxazolines or isoxazolidines depending on the reaction conditions
(reagents). The t-BuOCl-mediated cyclisation
affords isoxazolines via an oxidative cyclisation, whereas the NIS-induced
reaction provides the 5-exo-cyclisation
product with high stereoselectivity (cis:trans = 13:1).
Key words
stereoselective synthesis - electrophilic additions - isoxazolidines - isoxazolines - oxidative cyclisations
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