Chemistry of Pyrones, Part 5: New Crown Ether and Podand Derivatives of 3,5-Bis(bromomethyl)-2,6-diphenyl-4H-pyran-4-one†

New podand and crown ether derivatives of 3,5-disubstituted 4H-pyran-4-one (8-11) were prepared by the nucleophilic substitution reaction of 3,5-bis(bromomethyl)-2,6-diphenyl-4H-pyran-4-one with o-nitrophenol, 8-hydroxyquinoline, 2-hydroxymethyl pyridine and triethyleneglycol, respectively. The yield of 3,5-bis[(2-formyl-phenoxy)methyl]-2,6-diphenyl-4H-pyran-4-one has been improved by modification of our previous method using NaOH instead of Et3N.


Introduction
Synthetic podands are a family of linear multidentate ligands, which includes acyclic polyethers. They generally form complexes with smaller stability constants than those of corresponding macrocyclic complexes and thus are usually regarded as poor ligands [2]. In contrast, many excellent podands are found in nature. For example, naturally occurring polyether antibiotics, such as monsin and lasalocid, selectively bind several metal cations and effectively transport them across a biomembrane [3]. Synthetic podands have advantages over these biological podands, in terms of facile synthesis and versatility of molecular structure [2].
However, synthesis of 4H-pyran-4-one podands possessing nitrogen atoms, has not been reported as yet. As a continuation of our investigations on the chemistry of pyrones, we now report the synthesis of some new podands and crown ethers derived from 3,5-disubstituted 4H-pyran-4one.

Results and Discussion
In a previous paper we reported the synthesis of podand 7 in 13% yield by means of the reaction between pyrone 6 and salicylaldehyde in DMF and in the presence of Et 3 N at room temperature (Scheme 1) [6]. Recently we have improved the yield of this reaction (77%), using NaOH in EtOH and water under reflux condition .

Scheme1
Recently podand 7 was used as a neutral preferential carrier for cesium over potassium, sodium and other metal ions [11]. In the same manner the podands 8 and 9 were prepared in 47 and 54% yields respectively. As shown in Scheme 2 the reaction of 3,5-bis(bromomethyl)-4Hpyran-4-one [12] with 2-hydroxymethyl pyridine in the presence of ex cess sodium hydride in THF produces podand 10 in 67.5% yield.

Scheme2
Treatment of triethyleneglycol with pyrone 6 in the presence of NaH in THF gave 11 in 6% yield. Unfortunately, the reaction of 6 with tetraethyleneglycol under similar conditions only produced polymeric materials which could not be isolated. The data obtained from mass, IR, 1 Hand 13 C-NMR spectra and elemental analyses of the products are fully consistent with the proposed structures.

Conclusions
In this paper the synthesis of some 3,5-disubstituted podands and crown ether derivatives of 4pyrone (8 -11) is reported. We have also improved the yield of compound 7 by modification of the previous method [6 ]. Complexation properties of these podands are under investigation.

Acknowledgments
Financial support for this work by the Research Council of Tabriz University is gratefully acknowledged.

General
Melting points were determined with an Electrothermal Instrument model 9100 and are uncorrected. Infrared (IR) spectra were run on a Shimadzu IR 435 Spectrophotometer as KBr disks or as smears between salt plates. The 1 H-NMR spectra were recorded on a Varian-EM 390 spectrometer. The 13 C-NMR spectra were recorded on a FT-NMR Brucker 80 MHz spectrometer. Chemical shifts are reported in ppm with TMS as internal standard. Mass spectra were taken with a Varian Mat 711 double focusing mass spectrometer. Elemental analyses were performed on a Heareus, CHN-O-RAPID analyzer.
To a solution of salicylaldehyde (or 2-nitrophenol or 8-hydroxyquinoline, 0.335g , 2.76 mmol) in alcohol (3 mL) was added sodium hydroxide (0.1g, 2.79 mmol) in water (40mL). The mixture was warmed and pyrone 6 (0.6 g, 6.9 mmol) was added. Sufficient ethyl alcohol to produce a homogeneous solution was then added. The solution was refluxed under nitrogen for 12 hr and cooled to 0°C and then filtered. The crystals were washed with water and dried in a desiccator.