Cycloaddition of bifunctional nitrones 1 containing a nitrone function as part of the 1,4diazabutadiene system to electron-rich monosubstituted alkenes has been investigated. The nitrone group manifested predominant 1,3-dipolar reactivity compared with that of the 1,3heterodiene system leading to isoxazolidine derivatives 2 and 3. An atypical assistance of the 1,4-diazabutadiene system was demonstrated by a reverse regioselectivity of cycloadditions to allyl alcohol yielding bicyclic heterocycles 2 via 3,4-disubstituted isoxazolidines. Cycloadditions with allyl halides gave 3,5-disubstituted isoxazolidines 3 showing common regioselectivity for the nitrone function. The mechanism and configuration of the products are discussed.
Cycloaddition von bifunktionellen Nitronen an monosubstituierte Alkene, I
Die Cycloaddition der bifunktionellen Nitrone 1, die die Nitronfunktion als Teil eines 1,4-Diazabutadien-Systems enthalten, an elektronenreiche monosubstituierte Alkene wurde untersucht. Gegenuber dem 1,3-Heterodiensystem wies die Nitrongruppe eine bevorzugtc 1,3-dipolare Reaktivitit auf, wobei die Isoxazolidinderivate 2 und 3 entstanden. Eine atypische Beteiligung des 1,4-Diazabutadiensystems zeigte sich durch eine umgekehrte Regioselektivitat der Cycloadditionen an Allylalkohol, die bicyclische Heterocyclen 2 uber 3,4disubstituierte Isoxazolidine ergaben. Cycloadditionen mit Allylhalogeniden fuhrten gemal3 der iiblichen Regioselektivitat der Nitrofunktion zu 3,5-disubstituierten Isoxazolidinen 3. Der Mechanismus sowie die Konfiguration der Produkte werden diskutiert.
Although 1,3-dipolar cycloadditions of simple nitrones with alkenes have widely been studied'), well recognized'), and utilized for organic syntheses", such reactions of bifunctional nitrones possessing a nitrone function incorporated into a 1,3-heterodiene skeleton have hardly been investigated. Those which were r e p ~r t e d ~. ~) , concerned exclusively on bifunctional nitrones containing the I-oxa-4-azabutadiene system. Our attention has been directed towards cycloaddition of such bifunctional nitrones in which a nitrone group is a part of the 1,4-diazabutadiene system. This system can be considered as an efficient "masked 1,3dipol regarding a number of reactions with aryl isothio-and isocyanates6). The bifunctional 1,4-diazabutadiene-nitrones 1 were readily available via condensation of acetophenone anils with nitrosobcnz~ncs~~~'.