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, K_TFA/K_{TFA – 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.