title: Chameleon Thermal Behavior of Cycloadducts of Nitrones to Methyl 2-Chloro-2-cyclopropylidene- and 2-Chloro-2-spiropentylideneacetates.
abstract: Methyl 2-chloro-2-cyclopropylideneacetate (2) and its spiropentane analogue 3 cycloadd to dihydroisoquinoline N-oxide (9), pyrroline N-oxide (12), and C-phenyl-N-methylnitrone (16) to give 5-spirocyclopropaneisoxazolidines in good yields (58-93%). The thermal behavior of the 5-spirocyclopropaneisoxazolidines is rather differentiated, depending strongly on the constitution of the nitrone and the solvent. As nitrone 9 has the tendency to undergo cycloreversion reactions, the ketoamide rearrangement products 20 and 21 from its cycloadduct derive from the thermodynamically favored 4-spirocyclopropaneisoxazolidine regioisomers formed after the cycloreversion process. In DMSO as solvent different rearrangement processes take place, leading to benzoindolizinones in modest yields (15-21%). The cycloadducts from 12 and 16 undergo a cyclopropyl to cyclobutyl ring enlargement facilitated by the presence of a chlorine substituent on the carbon alpha to the spirocyclopropane ring. Whereas these compounds from nitrone 16 demonstrated an unusual stability, those from nitrone 12 undergo a cascade rearrangement to yield indolizinone derivatives 34, 35 cleanly (73-83% yield). This overall transformation offers a new method for the synthesis of the indolizine skeleton.
authors.affiliation: Dipartimento di Chimica Organica "U. Schiff", and Centro di Studio sulla Chimica e la Struttura dei Composti Eterociclici e loro Applicazioni, C.N.R., Università di Firenze, Via G. Capponi 9, I-50121 Firenze, Italy, Institut für Anorganische Chemie der Georg-August-Universität Göttingen, Tammannstrasse 4, D-37077 Göttingen, Germany, and Institut für Organische Chemie der Georg-August-Universität Göttingen, Tammannstrasse 2, D-37077 Göttingen, Germany.
authors.forename: Chiara
authors.initials: C
authors.lastname: Zorn
authors.forename: Andrea
authors.initials: A
authors.lastname: Goti
authors.forename: Alberto
authors.initials: A
authors.lastname: Brandi
authors.forename: Keyji
authors.initials: K
authors.lastname: Johnsen
authors.forename: Mathias
authors.initials: M
authors.lastname: Noltemeyer
authors.forename: Sergei I.
authors.initials: SI
authors.lastname: Kozhushkov
authors.forename: Armin
authors.initials: A
authors.lastname: de Meijere