Supports solid, dendronized

Frechet and coworkers explored the synthesis of polyester dendrons on solid support. These dendrons, based on 2,2-bis(hydroxymethyl)propanoic acid monomers, were assembled up to the fourth generation on poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate) resin and decorated with chiral proline derivatives (Scheme 15.15). T vo methods for preparation of these materials were reported. The solid-phase divergent approach yielded polymer with higher loading but a less precise architecture than the second approach. The second approach, the divergent synthesis of the dendrons in solution, followed by the focal point deprotection and then attachment to the solid support, led to well-defined structures on the polymer, but relatively low loading. 

Dendronized Solid-phase Supports for Organic Synthesis... 

Scheme 7.9 Synthesis of a library of amidine-based CP llb-llla antagonists on a dendronized solid support.

In another example of the dendronization of solid supports, Rhee et al. described the design of silica-supported chiral dendritic catalysts for the en-antioselective addition of diethylzinc to benzaldehyde (Fig. 28) [60-62], The immobilized dendritic systems were formed in two different ways one by stepwise propagation of dendrimers and the other by direct immobilization... 

Alternatively, initiator cores have been tethered to solid support (Merrifield resins [83]). Using protein synthesis procedures that avoid the workup difficulties involved in handling large excesses of reagents, one can readily control dendrimer growth. Unfortunately, only noncleavable linkers have been examined to date with these PAMAM dendrimers. The use of initiator cores possessing cleavable linkers should make dendrimer synthesis and isolation very facile. This was demonstrated for mono-dendrons derived from poly(lysines) as reported by Tam et al. [105-107] (Fig. 26). 

Another application of the rearrangement in dendrimers focuses on the development of novel polythioether dendrons on a solid support for supported catalysis applications [94]. 

Two series of triazine-based dendrons were efficiently prepared by a convergent method and their properties studied <06OL1541>. A practical syntheis of [l,3,5]triazine dendritic molecules on solid supports has been described <06MI2248>. 

Despite the advantage of their easy separation, the use of conventional insoluble polymer-supported catalysts often suffered from a reduced catalytic activity and stereoselectivity, due either to diffusion problems or to a change of the preferred conformations within the chiral pocket created by the ligand around the metal center. In order to circumvent these problems, a new class of crosslinked macromolecule-namely dendronized polymers-has been developed and employed as catalyst supports. In general, two types of such solid-supported dendrimer have been reported (i) with the dendrimer as a hnker of the polymer support and (ii) with dendrons attached to the polymer support [12, 113]. 

Scheme 5 Remarkable dendritic effect in the Heck reaction using aryl ether dendrons on PS solid support...

Scheme 15.1 The divergent approach to dendron assennbly on solid support.

Scheme 15.2 The convergent approach to dendron assembly on solid support.

In addition to the aforementioned postsynthetic immobilization, the materials of the chosen architecture can be assembled via stepwise growth of the dendron on the support, applying the technique of solid-phase synthesis. This approach is used in the majority of the cases and is the leading route to most dendritic motifs used for the dendronization of insoluble supports. 

Scheme 15.1. The divergent approach to dendron assembly on solid support. (See the color version of this figure in Color Plates section.)...

Scheme 15.4. Synthesis of polylysine dendrons on solid support.

Figure 15.2. Various types of conjugates generated on solid support with polylysine dendrons.

Figure 15.3. Peptide dendrons prepared by Reymond et al. on solid support.

---