Convergent synthetic approaches

A convergent synthetic approach developed for 10,10-, 13,13-, and 20,20-difluoroarachidonic acids, based on the reactivity of disymmetric difluor-... 

Figure 5.2 Schematization of the convergent synthetic approach to buildup dendrimers.

Tiam et al. [37] have developed the synthesis of a model compound for the synthesis of camosic type diterpenes by an expedient synthetic route. Instead of following the standard A— B—>C approach as was done by previous authors [28,30], the present authors have developed an efficient convergent synthetic approach (AC—>ABC). This is described in Fig. (16). 

Heathcock and co-woricers recently initiated a convergent synthetic approach to pentacyclic triteipenes, in which preformed AB and DE synthons would be coupled and then the C ring closed. Since in most of the pentacyclic triteipenes the A and B rings are the same, it was proposed that one AB synthon might be used for the synthesis of a variety of triterpenes. The bicyclic enone 153 was chosen as the common AB synthon. 

FIGURE 15.3 Two most common synthetic strategies for dendrimers. (a) Divergent synthetic approach, (b) Convergent synthetic approach. 

An interesting variant on metal polypyridyl-type metallodendrimers is the material 8.34, which was prepared by a convergent synthetic approach (Scheme 8.2) [70]. A further example is the dendrimer 8.35, which possesses 18 ruthenium centers [71]. 

The shimalactones, isolated from the cultured marine fungus Emericdla vane-color, are neuritogenic natural products which contain a bicyclo[4.2.0]octa-2,4-diene core structure in combination with an oxabicyclo[2.2.1]heptane moiety [105]. The complex structure of the shimalactones represents quite a synthetic challenge, which was overcome in 2008 by Trauner and coworkers [106] in a convergent synthetic approach using a domino Stille reaction/electrocyclization. 

Fig. 3 The convergent synthetic approach for second-generation dendrimers the initial modification of a branched oligomer with (c) prospective surface groups, and (b) after a series of activation and (a) branching steps, a last branching reaction with a suitable core yields the final product (bottom).

In 1990, Hawker and Frechet described the convergent (ontside-in) approach, where dendritic polyethereal macromolecnles, based on 3,5-dihydroxybenzyl alcohol as the building block, were synthesized. That same year, Neenan reported the convergent synthetic approach of dendrimers based on 1,3,5-trisubstituted benzenes. " Since then, research on dendrimers has grown to such an extent... 

The dimeric stmcture of C2-symmetric imidodiphosphoric acids enables a highly convergent synthetic approach. The synthesis requires only a single additional synthetic step compared to the synthesis of the corresponding phosphoric acids (Scheme 25) [57]. 

Different synthetic routes to dendrimers have been reported in literature however, the most widely known methods include the classical divergent and convergent synthetic approaches. The development of other synthetic methods for preparation of dendrimer is aimed at decreasing the disadvantages of the classical methods. Most of the newly developed methods have their own merits and drawbacks. Therefore, the S3mthesis method chosen for a particular dendrimer depends on the properties required of the dendrimer. 

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