Synthetic strategies stepwise

Scheme 7. The stepwise polycondensation synthetic strategy to polycatenane.

In terms of synthetic strategy, approaches to porphyrins from open-chain tetrapyrroles are the only truly general routes. The principle is to construct, in a stepwise manner, an open-chain tetrapyrrole bearing a pre-determined arrangement of peripheral substituents. Cyclization to produce a porphyrin, or an immediate precursor, under mild conditions which do not cause redistribution of the pyrrole rings, should be accomplished after full characterization of the open-chain intermediate. 

In order to ensue a clear presentation of the results the authors decided to segregate both synthetic principles All synthetic strategies developed from the multifunctional condensations of Stille and Marvel were assigned to this general type of reaction. At the same time the first multistep sequences (polymer-analogous cyclization of poly(methyl vinyl ketone) and polyacrylonitrile) are used as point of reference for the classification of the other type of synthesis (stepwise procedures). 

Having demonstrated the ability to prepare the two extreme cases of surface functionalization, we were confident that our synthetic strategy and the concept of accurately controlling the number and placement of functionality at the chain ends of dendritic macromolecules were viable. Therefore, by a combination of stepwise monomer alkylation and stepwise core alkylation, dendritic macromolecules can be prepared with any number of functionalities at the chain ends. The only requirement governing the choice of the chain-end functionality is that it be compatible with the chemistry employed for growth. 

The nucleophilic, stepwise displacement of chlorine from cyanuric chloride is usually the first method considered for preparation of 1,3,5-triazine derivatives (see Section 6.12.5.1.4). When this approach is not applicable, construction of the 1,3,5-triazine ring from various subunits by formation of carbon-nitrogen bonds is the alternative approach. No new synthetic strategies have emerged over the past decade and synthetic routes are based on the same f undamental principles described in previous reviews. 

This combinatorial approach is applicable to any molecular structure that can be assembled either in a stepwise or concerted fashion from a set of molecular precursors, and where a screen or selection for a desired function exists. This includes oligomeric molecules such as nucleic acids and peptides, nonpolymeric molecules such as natural products, or even solid-state materials such as superconductors or polymers. To illustrate the utility and scope of this new synthetic strategy, as well as the challenges that remain, a number of examples, both from our laboratory and many others, will be described. These applications cover a broad spectrum, ranging from biological catalysis and drug discovery to materials science. 

The entire synthetic strategy relied on the use of Ihioethyl, selenophenyl and glycosyl fluoride donors by stepwise addition and selective activation [120], The synthesis was accomplished by Ley s group in a [2 + 3 + 2] fashion as depicted in Fig. 10. 

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