Dendrimers, divergent synthesis

Scheme 13. Divergent synthesis of platinum-acetylide dendrimers...

Keywords Macromolecules m Dendrimers m Fractal Structure m Hyperbranched Polymers m Nanostructure m Divergent Synthesis m Convergent Synthesis m Dendrylation... 

At the center of the dendrimer is the core that represents the first synthetic step in divergent synthesis, or the last step in convergent synthesis. Each concentric shell of branched units is a generation (X), designated (GX), of identical repeat units. Hundreds of chemical repeat units have been reported [3, 4], but our characterization has been focused primarily on poly(amidoamine) (PAMAM), -(CH2CH2CONHCH2CH2N) < [1], or poly(propyleneimine)... 

Tomalia, D. A. and Dvornic, P. R. Dendritic polymers, divergent synthesis (starburst polyamidoamine dendrimers) , in Salamone, J. C. (ed.), Polymeric Materials Encyclopedia, Vol. 3, CRC Press, Boca Raton, FL, 1996, pp. 1814-1830. 

DIVERGENT SYNTHESIS OF PAMAM DENDRIMERS VIA EXCESS REAGENT METHOD PREPARATION OF ESTER TERMINATED PAMAM STAR-BRANCHED PRECURSOR [NH2-(CH2)2 6-NH21 (G = -0.5) STAR-PAMAMICO E) [1]... 

Scheme 3. Divergent synthesis of the polyphenylene dendrimers based on the Diels-Alder cycloaddition... 

Fig. 2-22 Divergent synthesis of PAMAM dendrimer. Generation is 1 for LXIV and 5 for LXV.

Goodman and co-workers177-3 have approached the divergent synthesis differently. In their syntheses of collagen models consisting of three copies of homomeric peptide chains, solid-phase schemes begin with monomers 44 and then the template, the Kemp tricarboxylic acid (KTA) 10,[80 is used to link three monomers, also on resin, to form the peptide dendrimers 46 via 45 (Scheme 12). 

Difficulties that have been encountered in divergent synthesis can be attributed to the clustered nature and sequences of peptide dendrimers that form interchain hydrogen bonds and occlude coupling reactions. Solvent combinations such as DMSO and NMP, as well as coupling at elevated temperatures (50-60 °C),[94] have been used to overcome these side reactions and should be used if a single coupling fails to complete the reaction. It is best not to dry the peptide resin at any stage of the synthesis because re-solvation of dried peptide dendrimer resin is difficult. 

Scheme 19 Divergent Synthesis of a Peptide Dendrimer through Intramolecular Cyclization via a Thiazoli-...

Dendrimer chemistry began to emerge in the early 1980s, and like many discoveries it was a gradual evolution of the chemistry already available [174-178], The first identifiable example of a true dendrimer was the divergent synthesis by Vogtle in... 

Fig. 22 Divergent synthesis of ammonium-terminated polyaminophosphine dendrimers [213]...

In the present context, solid-phase synthesis has been used primarily for the preparation of peptide and glycopeptide dendrimers [23]. For example, a second-generation dendrimer could be prepared by successive addition of branched polyproline building blocks to a solid phase [24]. The divergent synthesis of polyamide dendrimers on polystyrenes was accomplished [25] by Frechet el al. 1991 (cf. Fig. 2.8) [26]. PAMAM dendrons could be grown up to the fourth-... 

Fig. 3.1 a) Convergent, b) supramolecular, and c) divergent synthesis of dendrimers with functional core (schematic)... 

Divergent synthesis of POPAM dendrimers is based on the concept developed by Vogtle et al. in 1978 (see Section 1.1) [4]. Acrylonitrile is added to a primary mono- or oligo-diamine in a Michael reaction (Fig. 4.2). Suitable reducing agents are Co(II)-borohydride complexes or diisobutylaluminium hydride [5]. Subsequent iterative reaction cycles permit repeated addition of acrylonitrile followed by reduction until the limiting generation is reached. 

Grinstaff succeeded in growing bio-dendrimers via divergent synthesis with branched AB2 monomers, which could be of future use in the clinical sector. Adducts of glycolic acid and lactic acid or derivatives of adipic acid are examples of the monomers used [72 a]. 

Scheme 14.2 Divergent synthesis of polypropylene imine) dendrimers (A and B) and possible side reactions resulting in defects and chain termination (C missed Michael additions D cyclisation).

Wiesler, U.-M., Berresheim, A.J., Morgenroth, F., Lieser, G. and Miillen, K. (2001) Divergent synthesis of polyphenylene dendrimers the role of core and branching reagents upon size and shape. Macromolecules, 34,187-99. Weil, T., Wiesler, U.M., Herrmann, A., Bauer, R., Hofkens,)., De Schryver, F.C. and Mullen, K. (2001) Polyphenylene dendrimers with different fluorescent chromophores asymmetrically distributed at the periphery. Journal of the American Chemical Society, 123, 8101-8. 

Dendrimers are hyperbranched polymers that emanate from a single core and ramify outward with each subsequent branching unit [1,2]. In the commonly employed divergent synthesis, dendrimers can be prepared through sequential, alternating reactions of two smaller units, one of which has a point of bifurcation. As is described elsewhere in this volume, several classes of dendrimers are known, including polypropyleneimine (PPI), polyamidoamine (PAMAM), and Frechet-type polyether dendrimers [1,2]. 

Majoral et al.1114 reported the facile divergent synthesis of a novel P-dendrimer series (Scheme 4.27). Treatment of the sodium salt of 4-hydroxybenzaldehyde (100) with trich-lorothiophosphorus(V) gave the trialdehyde 101, which, when treated with three equivalents of the hydrazine derivative 102, quantitatively afforded the first generation dendrimer 103 possessing six P-Cl bonds juxtaposed for repetition of the sequence. Construction of the second, third, and fourth (104) generations followed this iterative sequence. Key features of this sequence included no protection-deprotection procedures and the... 

Diederich et al.11921 reported the divergent synthesis of dendrimers possessing porphyrin cores with the aim of modeling redox potentials of electroactive chromophores via environmental polarity modification. The dendrimers thus can be considered as electron-transfer protein mimics for such proteins as cytochrome c oxidation potentials for cytochrome c in aqueous solution are known to be 300-400 mV more positive than those reported for similarly ligated heme mimics lacking hydrophobic peptide encapsulation J193a ... 

Consider the divergent synthesis of PAMAM dendrimers, 76 whereby undesired intra-or inter-molecular events lead to increased polydispersity and lost ideality. Although these events can be minimized or in selected cases eliminated, interestingly these processes can give rise to randomly positioned polydendritic systems. Thus, amidation of polyesters (e.g., 1) with a diamine can lead to intramolecular bisamidation which can give rise to topological dendrimer connection (Scheme 9.1) as depicted by catenated bisdendri-... 

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