Phosphonate bridges

Fig. 9.28 A mono-phosphonate bridged cavitand for alcohol scaising...

Through PXRD and TGA measurements, it was determined that 28 was stable to loss of 80% of the included DMF molecules, which inferred porosity. CO2 and N2 gas sorption analysis gave BET surface areas of 216 m /g and 209 m /g, respectively. Upon complete loss of DMF, a loss of order was observed by PXRD, but order was regained upon resolvation. The addition of two hydroxyl groups ortho to the phosphonate groups provided sufficient steric effects to disrupt the formation of the layered motif and form an open structure in 28. Stability in this structure is sustained by the ID chains of phosphonate-bridged tetrahedral Zn ions, with no solvent molecules coordinating Zn. Stability is evidenced by the permanent porosity of 28. 

An intramolecular cycloaddition also occurred with 3-ylidenepiperazine-2,5-diones such as 124 or 125, obtained by Wittig-Horner-Emmons reaction from phosphonate 121 and aldehydes 122 or 123, respectively. The products of the Diels-Alder reaction are the bridged bicyclo[2.2.2]diazaoctane rings 126 and 127 that have been found in biologically active secondary metabolite such as VM55599 and brevianamide A. The different type of structures employed in this case requires a chemoselective reaction in order to produce the expected products as single diastereoisomers after 20 days (Scheme 18) <2001JOC3984>. 

The general synthetic route for the preparation of bridged phosphorus cavitand is outlined in Scheme 5. From the resorc[4]arene, bearing various substituents at the lower rim, the cyclization step, which leads to the formation of the four fused eight-membered rings, was performed with three-co-ordinated and four-coordinated phosphorus reagent to give respectively tet-ra-phosphite, tetra-phosphonite, and tetra-phosphate or tetra-phosphonate derivatives. 

We will focus now on two series of tetra-bridged phosphorylated cavitands, which are of importance in regards to their potential host-guest properties and therefore as elements for the design of supramolecular systems. These are the phosphate derivatives, and the P-phenyl phosphonate or thio-phosphonate compounds, which have been particularly investigated in our group. 

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