Functionalisation of terminal groups

The simplest synthetic approach to the preparation of monofunctional dendritic systems is to functionalise the periphery of existing molecular scaffolds, generally constructed by divergent or convergent growth (Fig. 3.2, Route a) or b), respectively). 

Commercially available POPAM and PAMAM dendrimers (see Chapter 4) bearing terminal primary amino groups are currently the most commonly used divergently grown dendrimer structures, followed by poly(benzyl ether) den-drons or poly(benzyl ether) dendrimers (Frechet type) as representatives of con-vergently synthesised dendritic molecules. 

Follow-on functionalisation with molecular moieties of biological importance, such as anti-tumour agents (e.g. 5-fluorouracil) currently appears particularly attractive. In this context, acetylation is particularly suitable as functionalisation reaction for oligo/polyamine dendrimers. It renders the dendrimers more water soluble, which is essential for biomedical applications [25]. The toxicity of den- 

The principle of dick chemistrf presented by Hawker et al. (see Section 2.3.5) offers an efficient and versatile method of functionalisation. For example, it utilises [3+2]-cycloaddition of azide-functionalised reagents with ethyne end groups of a dendrimer precursor to prepare dendrimers with triazole-functionalised end groups. The mild reaction conditions, almost quantitative reaction, and not least the tolerance towards numerous functional groups permit the use of widely differing molecular frameworks (e.g. poly(benzyl ether), POPAM dendrimer structures or hyperbranched polyesters) and different functionalised azides 

Thus the alkyl ester end groups of convergently grown poly(benzyl ether) dendrimers can be subjected to post-synthetic modification by hydrolysis [28], reduction [29], transesterification/amidation [28] in a variety of ways. Subsequent modification of poly(benzyl ether) dendrons bearing p-bromobenzyl end groups by palladium-catalysed coupling reactions permitted preparation of dendrons with phenyl, pyridinyl, or thiophenyl end groups [30]. 

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