Ring–chain tautomerism. Part IV. cis-3-Acylacrylic acids

Abstract

The equilibrium constants for ring-chain tautomerism in a series of cis-3-acylacrylic acids have been determined by using the observed pK, and true pK, (pKST) values in 80% (w/w) 2-methoxyethanol-water. The latter were estimated from the relation between pK, values of a series of cis-and rrans-3-substituted acrylic acids. The results are discussed.EARLIER investigations 2-4 demonstrated the occurrence of ring-chain tautomerism in cis-3-acylacrylic acids, (I). and ( 11), as shown in equation (1). No quantitative studies of these equilibria appear to have been madeRecently, quantitative investigations of this tautornerism in 2-benzoylbenzoic 596 and cis-3-aroylacrylic acids have been described. The qualitative evidence 2-4 regarding the cis-3-acylacrylic acids indicates that these compounds exist predominantly as the ring isomer. The equilibrium constants, Kc, are given by the relation (2). Ke = aRinglaChain(2) The observed pK, can be related to the true pK,T by the relation (3). If a reliable estimate of pKaT can be P K T = pKalog (Ke + 1)(3) made, K , can be found by measuring the observed pK,.In the present study, we have investigated the ionisation of a series of cis-and trans-3-substituted

Thio-arylglycosides with various aglyconpara-substituents: a probe for studying chemical glycosylation reactions

“…With NPhth participation, the positive charge on the corresponding reactive intermediate from GlcN donors becomes more dispersed; hence the electron donating para-substituent on aglycon has lesser stabilizing effect leading to a smaller absolute ρ value (Figure 7). Similar phenomena of smaller absolute ρ value due to neighboring group participation have been observed in several nucleophilic displacement reactions 53, 54…”

Solvent-Dependent Pyranopterin Cyclization in Molybdenum Cofactor Model Complexes

“…25–27 In the current study, equilibrium experiments were performed in seven deuterated solvents to study how the solvent environment affected the equilibrium between the cyclized, pyranopterin and open pterin forms of complexes 1 and 2 . K eq constants were calculated using the integrations of pertinent resonance signals corresponding to the open or pyran forms.…”

Intramolecular Reversible Addition Reactions to the C=O Group

“…Using this method, Bowden and Henry [22] obtained ring-chain equilibrium constants KT of a series of Z-3-acylacrylic acids. They estimated the "true" K~ values comparing Ka values of Z-and E-3-acylacrylic acids.…”

“…Bowden and co-workers [26,27] solved the reverse task: by using the KT constants, determined by NMR method, and the Ka constants, determined by potentiometric method, they have calculated the "true" K~ constants for a series of 2-acylbenzoic and Z-3-acylacrylic acids in 80% methoxyethanol-water solution.…”