Divergent Procedures

The growth step procedures for the cycloaddition reaction are very simple. Combination of an ethynyl-substituted dendrimer and an excess of the cyclo-pentadienone in a refluxing solvent such as o-xylene, diphenylether, or methyl-naphthalene (with b.p. higher than 130 °C) typically results in quantitative conversion within 24 h. The refluxing of the solvent is necessary to accelerate the elimination of the carbon monoxide in the cycloaddition. The purity of the resulting compounds was checked by MALDI-TOF mass spectrometry which showed quantitative reaction, facilitating work-up. By repeated precipitation in methanol, the pure product can be isolated as white amorphous powders in yields higher than 90%. 

The deprotection step and subsequent purification are equally facile. Exposure of a TiPS-ethynyl-protected dendrimer to tetrabutylammonium fluoride in THE results in a quantitative reaction within 2 h. The tetrabutylammonium salts can be removed by siHca plug filtration rendering pure product, and the obtained yields are higher than 95%. If deprotection is effected instead with ammonium fluoride and catalytic amounts of tetrabutylammonium fluoride, even silica plug filtration is unnecessary. 

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Newkome, G.R. and C.D. Shreiner, Poty(amidoamine), polypropylenimine, and related dendrimers and dendrons possessing different 1 —> 2 branching motifs An overview of the divergent procedures. Polymer, 2008. 49(1) p. 1-173. 

The two dendritic structures, 5 and 6, incorporating tetra- and deca-macrocyclic sites, respectively, were both synthesised by a divergent procedure. This involved the use of (mono)tosylated N-protected tacn as the precursor for generation of the branching synthons and tacn itself for the core [13], Spectrophotometric titrations in acetonitrile and DMF demonstrated that these ligands take up the expected four and ten Cu(II) ions, respectively. 

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