Synthesis

The synthetic strategies for the preparation of dendrimers comprise divergent and convergent approaches [1, 2]. The convergent procedures often furnish better results concerning the formation of monodisperse dendrimers because the cumulative effects of failures are excluded. This argument is particularly important for higher dendrimer generations. A unimolecular stilbenoid dendrimer is structurally pure, which means constitutionally and configurationally pure. In this chapter, we would like to discuss the general features for the preparation of the compounds 1-12 (Scheme 1) without any details.

Wittig-Horner reactions represent the most applied procedures for the generation of stilbenoid dendrimers. Let us consider here the synthesis of the series 1a-e [19, 20] (see Scheme 2). The diphosphonate 13 with a protected formyl group and the triphosphonate 15 represent the phosphonic ester components and the aldehyde 14 the initial carbonyl component. The Horner reaction 13 + 14 yields the den-dron 9a, whereby the deprotection of the formyl group is a spontaneous part of the work up. The subsequent Horner reaction of 13 and 9a furnishes the next higher dendron generation 9b, which is used for 9c, etc. Thus, the aldehydes 9a-9d can be prepared in a coupled synthesis with the propagation reagent 13. The final step for the generation of the

Scheme 1. Continued.

symmetrical dendrimers 1a-e consists, then, of the Horner reaction of the core component 15 with the corresponding aldehyde.

The purification of the dendrons and the control of the final attachment of three dendrons to the core guarantee structural perfection. One- and two-arm systems can be completely excluded by MALDI-TOF mass spectroscopy and cis configurations to a detection limit of about 3-5% in 1H NMR spectroscopy.

The dendrimers 2a-c and their precursors 10a, b were prepared in an analogous way [23, 39]. Wittig-Horner reactions were also applied for the preparation of 1f, g [21], 3a [40], and in the series 4 [41, 42], 5 [43, 44], 6 [44], 7 [45], 11 [40, 44], and 12 [46-48]. Since the Wittig-Horner reactions are condensation reactions of alkylidenephosphoranes or alkylphosphonates with carbonyl components, it is not possible to keep further present formyl groups intact. Therefore, a protection-deprotection technique, as shown above for 13 ^9a, can be applied [19, 20]. Instead of the protected formyl group, one can introduce a cyano group, which is transformed in a subsequent reduction to the formyl group. The alternating sequence of Horner reactions (i) and reductions (ii) with diisobutylaluminium hydride was established for the following series [47, 48]:

Starting compounds

Generation

A combination of Wittig (i) and Heck (ii) reactions was used for the preparation of the following series [44]:

Starting ii i ii i ii

5 - 11b-- 11c-- 11d-- 11e-- 11g compounds Generation 1

0 0

Post a comment