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Gas-phase chemistry of acylium ions. Seven-to-five ring contraction of 1,3-dioxepane and 1,3-dioxep-5-ene

✍ Scribed by Moraes, Luiz Alberto B.; Kotiaho, Tapio; Eberlin, Marcos N.

Publisher: John Wiley and Sons
Year: 1999
Tongue: English
Weight: 162 KB
Volume: 34
Category: Article
ISSN: 1076-5174
DOI: 10.1002/(sici)1096-9888(199906)34:6<670::aid-jms819>3.0.co;2-h

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✦ Synopsis

As shown by pentaquadrupole triple-stage mass spectrometric and 18 O-labeling experiments, two sevenmembered cyclic acetals, 1,3-dioxepane and 1,3-dioxep-5-ene, fail to react by transacetalization with the gaseous acylium ions CH 3 C Y O and (CH 3 ) 2 NC Y O. Instead, a novel and less exothermic but more kinetically favored reaction, seven-to-five ring contraction, occurs predominantly, and to great extents with the most reactive acylium ion, (CH 3 ) 2 NC Y O. 1,3-Dioxepane yields O-acylated tetrahydrofurans; 1,3-dioxep-5-ene yields Oacylated 2,5-dihydrofurans.

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1,3,5-Cycloheptatriene (1) and various 7-alkyl-1,3,5-cycloheptatrienes (3, 6, 9, 13, and 16-19) were subjected to gas-phase protonation under and conditions and the MIKE spectra of their [ M + H ] ' ions CI(CH 4 ) CI(iC 4 H 10 ) were measured. Loss of from the parent ion [ 1 + H ] ' and almost exclu