Chiral dendrimer scaffold

Chiral dendrimer scaffold, made up of chiral branching building blocks or chiral spacers... 

Chiroptical studies on dendrimers with chiral dendrimer scaffold... 

Fig. 4.72 Dendrimer with completely chiral dendrimer scaffold based on chiral branching units (according to Sharpless et al.)...

Does the chiral core unit permit enantioselective (dendrimer) host/(substrate) guest complexation in the interior of the dendrimer scaffold ... 

Overall, studies on poly(benzyl ether) dendrimers with various chiral core units show that the influence of the achiral dendrimer scaffold on the chiropti-cal properties of the core unit depend primarily upon the origin of their chirality. 

Studies performed on chiral core dendrimers have provided valuable information about the influence of the achiral dendrimer scaffold on the chiroptical properties of the core unit. Yet they also show that prediction of the chiroptical properties of the dendrimer is difficult, since the chiral relationship between the local chirality of the core unit and the nanoscopic conformation of the overall dendrimer structure is influenced by numerous structural factors. Further studies will be required to attain a fuller understanding of how an individual chiral building block can induce chirality in the entire dendrimer architecture (see Section 4.2.7). 

Fig. 4.70 Dendrocleft (according to Diederich et al.) for stereoselective host/guest interaction depending upon the size of the dendrimer scaffold surrounding the chiral core (marked with a red asterisk)...

In contrast, the diastereoselectivity of the dendritic host increases. This indicates that on shielding of the chiral core unit with sterically more demanding dendrons in higher-generation dendroclefts the monosaccharide guests are no longer bound in the immediate vicinity of the chiral core unit instead, a less specific host/guest interaction takes place with the dendrimer scaffold. 

Chow and Mak came to a similar conclusion on investigating the chiroptical properties of dendrimers containing enantiomerically pure threitol building blocks obtained from tartaric acid as spacers between the achiral phloroglucin branching units (see Fig. 4.73) [27]. They found that the chiral spacers in the dendrimer scaffold do not influence one another and contribute additively to the overall rotation. Moreover, they also observed that on introduction of both enantiomers one (R,R)-threitol unit precisely compensated the rotational contribution of one (S,S)-threitol unit, provided that the enantiomeric building blocks were located at equivalent positions within the dendrimer scaffold. However, CD-spectroscopic data revealed that the contribution of the exterior threitol units to the total rotation must be slightly different from that of the interior units. 

Possible applications of dendrimers with chiral branching scaffold... 

Owing to their size and the feasibility of controlling the dendrimer structure and globular morphology via their synthesis, dendrimers with chiral branching scaffold are of interest as potential protein mimics. In addition, the introduction of chiral branching units or spacers into the molecular scaffold should lead to the development of non-symmetrical macromolecular conformations and provide chiral cavities for asymmetrical catalysis or chiral recognition processes. 

Soai et al. studied the influence of the flexibility of the dendritic support scaffold on catalytic properties for the chiral dendrimers 4 and 5 (Fig. 6.45). In the... 

This indicates that the measured molar optical rotation is caused by the chiral building blocks and not by a stable conformational array of the dendrimer scaffold [83]. 

Specific attractive interactions between the dendrimer building blocks (e.g. hydrogen bonds between amino acid building blocks) or complexation of metal ions [86] by suitable functional groups in the dendrimer scaffold can favour the formation of chiral substructures and effect distortion or folding of the overall structure. Interactions with solvent molecules can also have a pronounced effect on the overall structure. 

A) Dendrimers with chiral core and achiral branching scaffold (see Section 4.2.3)... 

Compound 43 was found to exhibit a chiral nematic phase (see the texture in Fig. 48), hexagonal disordered columnar (see Fig. 49) and rectangular disordered columnar phases, in the sequence g 5.4 Col d 30 Col 102.3 N 107.7 °C Iso Liq [94]. Thus the increase in the number density of the meso-gens bound to the central scaffold transforms the situation from the octamers 41 and 42, which exhibit calamitic phases, to one where the hexadecamer 43 exhibits columnar phases. However, the formation of columnar phases when the dendrimer possesses rod-like mesogenic groups is not easy to vi-... 

In this paper we will focus more specifically on the third one, in which the scaffold unit is originated from monosaccharides. Compare to classical Pamam, Boltorn cores, carbohydrate units add much more 3D information coded in the chirality in almost each carbon. The diversity of such stmcture has been calculated for oligosaccharides and is veiy impressive, Laine found over 1.05 x 10 possible branched hexasaccharides starting from a basis set of 6 different sugars. Compared with a set of 6 different amino acids, there is nearly 10 times less hexapeptides Thereby the possible structures of a carbohydrate-based dendrimers are tremendous and open a large possible choice of scaffolds and thus properties. In addition, various characteristics of such dendrimer can be easily controlled such as their sizes, shapes, topologies, flexibilities and their surface properties. 

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