Grob fragmentation of norbornyl α-diketones: A route to α-ketoenols and aromatic compounds

Khan, Faiz Ahmed and Rao, C N (2011) Grob fragmentation of norbornyl α-diketones: A route to α-ketoenols and aromatic compounds. Journal of Organic Chemistry, 76 (9). pp. 3320-3328. ISSN 0022-3263

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Abstract

An efficient acid-catalyzed Grob fragmentation of symmetrical and asymmetrically substituted norbornyl α-diketones to the corresponding six-membered α-ketoenols is reported. The regio- and stereochemical outcome of the Grob fragmentation of C2 mono- and disubstituted α-diketones was investigated. A single regioisomer resulting from a favorable half-chair intermediate was normally observed. A departure from the normal course was noticed for C2 disubstituted α-diketones possessing an exo-methyl and an endo-methoxycarbonyl derivative, giving the opposite regioisomers due to initial formation of the hemiketal. The bromo analogues of the C2 disubstituted α-diketones furnished an unusual byproduct, which appears to have been formed through highly reactive fused four-membered bicyclo[2.2.0]hexane intermediates. A plausible mechanistic proposal involving the gem-dihalo intermediate, which in one case was actually isolated as its BF2-complex, is outlined. The fragmentation protocol was applied to various norbornyl substrates including bis-α-diketone derivatives. The methodology was successfully utilized for the synthesis of substituted aromatic compounds.

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