Octopus dendrimers

These main-chain dendrimers may represent an interesting alternative for the development of original molecular materials having a new architecture. Shibaev et al. foresaw the elaboration of such compounds, but have not yet reported on their synthesis and physical properties [ 151]. Up to now two famines of such systems have been identified, namely the willow-like and the octopus dendrimers. 

Fig. 50 Snapshot of the molecular conformation of the octopus dendrimers with 8 terminal chains and model for their molecular organization in smectic layers...

R = R = OC12H25, R = H) molecular conformations likely predominate to satisfy the geometrical requirements. The supramolecular disks or columns thus result from the molecular association of two or three dendrimers (depending on the aliphatic chain substitution) in the appropriate conformation, as depicted in Fig. 52 and these columns are packed in a hexagonal lattice. Moreover, considering the diblock, alternated chemical nature of these octopus dendrimers, an onion morphology for the columns is most likely probable (Fig. 53) [264]. 

Fig. 55 Self-assembling and self-organization process into columns for the octopus dendrimers with various core connectivity numbers...

Nonetheless, it was a fairly short step from octopus compounds to dendrimers, and the step was taken by Vogtle in the late 1970s when he attempted to use a cascade reaction to prepare a molecule of the dendrimer type that would now be considered a dendron rather than a fully developed dendrimer. It began with the addition of acrylonitrile to an anfine, followed by reduction of the nitrile to amine. This was followed by a further reaction with acrylonitrile, and the process was repeated several times to yield highly branched macromolecules. There were initially problems with the reduction step but these were overcome, and the preparation of these poly(propylene imine) dendrimers was later commercialized. 

M. Dubber and T. K. Lindhorst, Trehalose-based octopus glycosides for the synthesis of carbohydrate-centered PAMAM dendrimers and thiourea-bridged glycoclusters, Org. Lett., 3 (2001) 4019 1022. 

Fig. 49 Schematic representation of homolithic (A) and heterolithic alternated (B), segmented (C) and alternated-segmented (D) octopus-like dendrimers...

Fig. 52 Schematic representation of the self-assembling and self-organization processes of octopus-like dendrimers into the Colh phase...

The choice of the donor is critical for the success of the multi glycosylation step. For instance. Baker and coworkers used a supposed more reactive armed thioglycoside donor (instead of a trichloroacetimidate) in a veiy similar octopus synthesis. Unfortunately, this glycosylation gave such low yield compared to Lindhorst s method that the synthesis of the next dendrimer generation will not be possible (Scheme 2). 

This octopus concept is impressive in that a large number of glycosylated branches are created in one reaction but this strategy was found difficult to continue at the next level due to steric hindrance and moderate yields in per-allylation and per-hydroboration. As a consequence a convergent approach have been preferred in larger constructions. Colonna et al. have described the preparation of a dendrimer with three heptasaccharide elements which was inspired by Polotti s work on the synthesis of the phytoalexin elicitor analogues. ... 

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