Abstract
A simple and efficient synthetic route to the novel 3a,4-dihydro-3H,7H- and 4H,7H-pyrazolo[4′,3′:5,6]pyrano[4,3-c][1,2]oxazole ring systems from 3-(prop-2-en-1-yloxy)- or 3-(prop-2-yn-1-yloxy)-1H-pyrazole-4-carbaldehyde oximes has been developed by employing the intramolecular nitrile oxide cycloaddition (INOC) reaction as the key step. The configuration of intermediate aldoximes was unambiguously determined using NOESY experimental data and comparison of the magnitudes of 1JCH coupling constants of the iminyl moiety, which were greater by approximately 13 Hz for the predominant syn isomer. The structures of the obtained heterocyclic products were confirmed by detailed 1H, 13C and 15N NMR spectroscopic experiments and HRMS measurements.
Highlights
The 1,3-dipolar cycloaddition reaction of nitrile oxides as 1,3-dipoles and alkenes/alkynes as dipolarophiles has become an efficient tool in organic synthesis to obtain various substituted isoxazolines/isoxazoles [1,2,3]
Chen et al reported the synthesis of fully substituted isoxazoles from nitrile oxides, which were generated in situ from copper carbene and tert-butyl nitrite [13]
The intramolecular nitrile oxide cycloaddition (INOC) reaction can provide a route for the preparation of isoxazoles or isoxazolines annulated to various carbo- or heterocycles
Summary
The 1,3-dipolar cycloaddition reaction of nitrile oxides as 1,3-dipoles and alkenes/. Nitrile oxides, which are typically generated in situ, undergo subsequent 1,3-dipolar cycloaddition to form appropriate isoxazoles or isoxazolines. The intramolecular nitrile oxide cycloaddition (INOC) reaction can provide a route for the preparation of isoxazoles or isoxazolines annulated to various carbo- or heterocycles. A method for the stereoselective synthesis of novel isoxazoline/isoxazole-fused indolizidine-, pyrrolizidine- and quinolizidinebased iminosugars has been developed, employing N-alkenyl/alkynyl iminosugar Cnitromethyl glycosides as nitrile oxide precursors in 1,3-dipolar cycloaddition reactions [15]. An intramolecular 1,3-dipolar nitrile oxide cycloaddition strategy the regio- and diastereoselective construction of highly functionalized tricyclic tetrahyhas been applied as an efficient synthesis protocol for the regio- and diastereoselective droisoxazoloquinolines [18].
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