Issue 8, 1989

Ring closure to ynone systems: 5- and 6-endo- and -exo-dig modes

Abstract

The rates of cyclisation of 2-hydroxy-2-methyl-5-arylpent-4-yn-3-ones in trifluoroacetic acid are reported. An approximate ρ value of –4.0 (using σ+) and a kinetic isotope effect, KTFA/KTFA – d, of 4.2 are observed, which suggest that the reaction proceeds via rate-limiting triple-bond protonation and does not entail carbonyl protonation along the reaction co-ordinate. The base-catalysed ring closure occurs via the vinylic carbanion. A similar mechanistic picture appears to be involved in the cyclisation of 1-(2-hydroxyphenyl)-3-arylprop-2-yn-1-ones which yield a mixture of flavone and aurone in base, whilst in acid only the flavone is observed. In contrast with recent theoretical studies, the base-catalysed results appear to conform to Baldwin's original proposal of acute-angle approach of a nucleophile to a triple bond.

Article information

Article type
Paper

J. Chem. Soc., Perkin Trans. 2, 1989, 957-961

Ring closure to ynone systems: 5- and 6-endo- and -exo-dig modes

C. M. Brennan, C. D. Johnson and P. D. McDonnell, J. Chem. Soc., Perkin Trans. 2, 1989, 957 DOI: 10.1039/P29890000957

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