Synthesis and properties of bridgehead-substituted bicyclo(n.2.2) bridgehead alkenes.

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The bridgehead-substituted bicyclo[n.2.2] bridgehead alkenes (1b–3b) (1, n=4; 2, n=5; 3, n=6; b, R=OAc) were synthesized based on the oxidative decarboxylation of [n.2.2]propellanecarboxylic acids with lead tetraacetate. The parent alkene (1a–3a) (a, R=H) and other bridgehead-substituted derivatives (1c–3c and 1d–3d) (c, R=OH; d, R=Cl) were prepared from 1b–3b. The examination of the 13C NMR chemical shifts of 1a–c, 2a–c, and 3a–c indicates the presence of electronic interaction between the bridgehead double bond (Cγ) and the opposite bridgehead carbon (Cα), being the homoallylic position of the double bond. From the product study and the kinetic results of the solvolysis of the bridgehead chlorides 1d–3d, it is indicated that the homoallylic participation of the strained bridgehead double bond to the carbonium ion center located at the opposite bridgehead position operates in the solvolysis of 1d and 2d. It may be, therefore, concluded that highly strained bridgehead alkenes, especially, bicyclo[4.2.2]decene system, show remarkable homoallylic type α, γ-interaction both in the ground state and in the transition state (carbonium ion).
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