3-Hydroxy-4-pyranones

Fig. 1. General formulae for hydroxypyranones and hydroxypyridinones 1, 3-hydroxy-2-pyranone 2, 3-hydroxy-4-pyranones 3, 3-hydroxy-2-pyridi-nones 4, 3-hydroxy-4-pyridinones 5, l-hydroxy-2-pyridinone. In each case the ring atoms are numbered anticlockwise, starting with the ring-oxygen or ring-nitrogen atom.

Stereochemistry AND Degree of Hydration of 3-Hydroxy-4-pyranonate Complexes of 3+... 

Stepwise stability constants decrease regularly in the normal manner 178), log > log K2 > log K, as may be exemplified by the values for the Ni -maltolate system, viz. 5.5, 4.3, and 2.7, respectively 179). Stability constants (log Kf) for a selection of 3-hydroxy-4-pyranonate and 3-hydroxy-4-pyridinonate complexes of some first-row transition metal 2+ cations are listed in Table IV 10,128,180—184). The values for the 3d transition metal cations conform to the long-established Irving-Williams order... 

Solubilities (mM) of Tris(3-hydroxy-4-pyranonate) and Tris(3-hydroxy-4-pyridinona-te)metal(III) Complexes in Water at 298 K... 

A selection of distribution (partition) coefficients for 3-hydroxy-4-pyranones and their complexes is set out in Table XIX,... 

Distribution Coefficients for Selected 3-Hydroxy-4-pyranones and their Complexes... 

Zr L = a bidentate hydroxypyranonate or hydroxypyridinonate ligand and X is a halide or alkoxide leaving group (264,265). Replacement of chloride by thiocyanate, pyrazine, or water (in acetonitrile solution) in 3-hydroxy-4-pyranonate or 3-hydroxy-4-pyr-idinonate tin(IV) complexes in all cases follows a two-term rate law of the type... 

In recent years hydroxypyranones and hydroxypyridinones have been increasingly investigated for the control of metal ion levels in the body (324-327). The 3-hydroxy-4-pyranones maltol and ethylmaltol are of relatively low toxicity 328,329), and indeed have the pharmacological advantage of being permitted food additives. Hydroxypyridinones are particularly attractive for pharmaceutical purposes since their structure allows tailoring of many of their properties, as outlined in Section II.A.3.b earlier. They have been used in, or tested or proposed for, chelation therapy to remove excess of several toxic elements. This will be illustrated later in the sections... 

Figure 1 Transfer chemical potentials for selected iron complexes from water into aqueous methanol (on the molar scale, at 298 K). Ligand abbreviations not appearing in the list at the end of this chapter are acac = acetylacetonate (2,4-pentanedionate) dmpp = l,2-dimethyl-3-hydroxy-4-pyridinonate, the anion from (24) malt = maltolate (2-methyl-3-hydroxy-4-pyranonate, the anion from (233)).

Studies on the tautomerism of heterocycles, including pyrans and derivatives, have been extensively reviewed <2006AHC(91)1>. Computational studies have been used to examine a number of 3-hydroxy-4-pyranones such as maltol 68, ethyl maltol 69, and pyromeconic acid 70, and it has been determined that the 3-hydroxy-4-oxo tautomer is the most stable <2003JMT(639)87>. 

The structures of several 3-hydroxy-4-pyranones, such as pyromeconic acid 152 (R1 = R2 = H), maltol 152 (R1 = Me R2 = H), ethylmaltol 152 (R1 = Et R2 = H) and kojic acid 152 (R1 = H R2 = CH2OH) were calculated using 6-311 + +G(d,p) basis set (03JST(639)87). The 3-hydroxy-4-oxo forms shown were found to be predominant in all cases with the second most stable tautomer being 2/7-3,4-dioxo form. 

The crystal structures of three polymorphs of ethyl maltol (2-ethyl-3-hydroxy-4-pyranone) have been reported, with differing modes of hydrogen bonding giving rise to the three crystal forms [23], Form 1 contains nearly planar chains of molecules, Form 2 contains three-dimensional or spiral chains, and Form 3 contains hydrogen bonded dimers. The crystallographic data reported for these three forms are summarized in Table 4. 

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