Chlorination of 2,4-, 2,6- and 3,5-di-t-butylphenols gave the chlorocyclohexa-dienones (2a), (10), and (7) and (9), respectively. Base-catalysed methanolysis of the ketones (2a) and (9) produced the rearranged gem-dimethoxycyclohexadienones (5)and (11). Under similar conditions the ketone (7) rearranged to 3,5-di-t-butyl-2,4- dichlorophenol, while (10) was converted into a mixture of 2,6-di-t-butyl-1,4-benzo-quinone and 3,5,3',5'-tetra-t-butyl-4,4'-diphenoquinone. The dibromocyclohexa-dienone (2b) also dimerized in the presence of base, affording a low yield of 2,2'-dihydroxy-3,5,3',5'-tetra-t-butylbiphenyl. The mechanisms of these reactions are discussed.

1,3-Dipolar cycloaddition reactions of nitrones-particularly N-(2'- pyridylmethylene)aniline N-oxide-with arylmethylene cyanoacetates exclusively yield 4,4-disubstituted isoxazolidines, identifiable by 1H n.m.r, spectroscopy and isolable in some cases. Comparative kinetic experiments provide evidence for at least non-synchronous addition via a dipolar intermediate or possibly even two-step addition via a discrete zwitterionic intermediate.

The reactions of C-benzoylnitrones with allylamine afford tetrahydro-5H-pyrrolo[3,4-c]isoxazoles resulting from imine formation and intramolecular 1,3-dipolar cycloadditions. Cycloaddition reactions of the same nitrones with 2-vinylpyridine and N-phenylallylamine afford 5-substituted isoxazolidines in the normal way. Reduction of the imine bond in the tetrahydropyrroloisoxazoles can be achieved by catalytic hydrogenation or sodium borohydride, and, in the case of an N-phenyl derivative, N-O bond cleavage also occurs.

Bei der Chlorierung der Phenole (I), (IV) bzw. (VIII) in Äther bilden sich die Chlorcyclohexadienone (II), (V) und (VI) bzw. (IX).

In diesem Übersichtsartikel werden Cycloadditionen von Nitronen (I), die aus Carbonylverbindungen zugänglich sind , mit ungesättigten Verbindungen wie Alkinen, Olefinen, Allenen, Ketenen, Nitrilen, Isocyanaten, Isothiocyanaten, Carbodiimiden, Aziridinium‐ und Azetidiniumsalzen, Schwefelkohlenstoff, Isothiocyanaten, Thioketenen, Sulfenen, Phosphoranen, N‐Sulfinylverbindungen und Iminophosphoranen zu fünfgliedrigen heterocyclischen Systemen (II) beschrieben.