Accelerated convergent synthesis

While the convergent strategy, as described above, has been exceptionally successful, a considerable amount of effort has been devoted to improving its speed and synthetic efficiency. The remainder of this chapter reviews the major methodological developments and improvements in the accelerated, covalent, convergent synthesis of dendrimers. Dendrimers constructed via self-assembly and noncovalent interactions, as well as inorganic dendrimers [25], are beyond the scope of this brief chapter. 

Frechet et al. 8fi reported a branched-monomer approach to the convergent synthesis of dendritic macromolecules this approach permits an accelerated growth by the replacement of the simplest repeat unit with a larger repeat unit of the next generation. In essence, the traditional AB2 monomer is replaced with an AB4 unit. Scheme 5.28 depicts the transformation of the AB4 unit (115) to the trimethylsilyl protected tetraester 116... 

Moore et al.[104 106a reported a double exponential dendrimer growth , which is an accelerated convergent process for the synthesis of monodendrons by means of a bidirectional procedure, where the degree of polymerization follows Equation (5.1). 

Other related systems, which contain the 5-ylidenepyrrol-2-(5//)-one structural unit are (-)-ampullicin (91a) and (+)-isoampullicin (91b), sesquiterpene amides isolated from a culture filtrate of an Ampulliferine-like fungus sp. no. 27 [161], obtained from a dead pine tree (Pinus thunbergii). The latter compounds accelerate the root growth of lettuce, and their structures have been determined by X-ray diffraction and NMR data [162]. The total synthesis of both compounds (91) by means of a convergent synthesis from a tricyclic aldehyde and the phosphonate derived from a substituted maleimide has been reported [163,164]. (Scheme 42). 

Though even higher oligomers of 9 could undoubtedly be prepared by further repetition of this procedure, a somewhat more streamlined, convergent synthesis proved more practical, even for the preparation of pentayne 11 (Fig. 9-4). Noteworthy here is the utility of acetyl chloride for the mild conversion of a methyl ether to an alkyl chloride (and methyl acetate), presumably via an S l process that is facilitated by the adjacent triple bond Lewis acids accelerate the reaction but are not essential. 

The other accelerated approach for dendrimer synthesis, the orthogonal approach, involves convergent growth with two different monomers. The monomers, an AB2 and a CD2 must be carefully selected such that the focal functionalities of each individual monomer will only react with the periphery of the other monomer (B couples only with C and D only with A) thus removing the need for activation reactions. As... 

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