Nitrogen containing heterocycles from aldoximes; a one-pot route to isoxazolobenzodiazepinones, N-substituted and N-unsubstituted isoxazoloquinolinones

Abstract

The aldoximes 1 in the presence of electron poor olefins react to form either the 5,6,7-tricyclic isoxazolobenzodiazepinone 3 or the 5,6,6-tricyclic isoxazoloquinolinone 5 ring skeleton. In each case the ring system formed depends on the relative electrophilicity of the added and internal olefin. The oximes 1a,b,c react with N-methylmaleimide, methyl acrylate or phenyl vinyl sulfone to afford the corresponding regioisomeric isoxazolobenzodiazepinones by a tandem intramolecular dipole formation–intermolecular cycloaddition sequence. With the more electrophilic olefin, methyl vinyl ketone, intermolecular nitrone generation precedes intramolecular cycloaddition and the isoxazoloquinolinone skeleton results and for each oxime reaction proceeds smoothly in a regio- and stereo-specific manner. Steric control of chemoreactivity is observed with the ring or chain substituted aldoximes 1d,e,f. These oximes react in the presence of phenyl vinyl sulfone to give the isoxazolobenzodiazepines 15 and 16 together with varying quantities of the N-unsubstituted isoxazoloquinolines 14. The latter arise via an oxime–nitrone-cycloaddition sequence, in each case the cycloaddition proceeds in a regio- and stereo-specific manner. The oximes 1d,e,f react with methyl vinyl ketone to give single regio- and stereo-isomers of the N-unsubstituted and -substituted isoxazoloquinolinones 14 and 17. The high degree of chemo-, regio- and stereo-selectivity with which the one-pot reaction of the oximes 1 with electron poor olefins proceeds represents a convenient method for the construction of the title tricyclic molecular frameworks.