Use of supported reagents

A complementary approach is afforded by the option to use reagents attached to suitable supports to allow the easy removal of excess or derived products. A useful review of this field was published in 1981 and recent advances in combinatorial chemistry have prompted a revival of interest [24], 

Parlow [25] has demonstrated the use of ammonium exchange resins in the preparation of pooled libraries of aryl and heteroaryl ethers. Two batches of Amberlite IRA-900 were prepared, each of which contained a mixture of 10 aryl or heteroaryl oxides these were then reacted independently with an electrophile to afford a mixture of ethers. 

Kobayashi and Nagayama [26] have reported the preparation of a library of quinoline derivatives using a novel polymer-supported scandium catalyst (Fig. 8) in a three-component coupling reaction. The scandium catalyst has the advantage of being partially soluble in the dichloromethane/acetonitrile mixtures but can be precipitated by the addition of hexanes and thus be removed quantitatively by filtration. 

A series of transformations was also accomplished by the simultaneous use of different polymer-supported reagents (Fig. 10). Parlow [28] published a multistep synthesis from sec-phenethyl alcohol using resin-supported dichromate, perbromide and pyrazol-3-ol. 

15 Bailey, N, Dean, A.W, Judd, D.B, Middlemiss, D, Storer, R. and Watson, S.P, Bioorg. Med. Chem. 

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An alternative to the use of supported reagents is the use of uncatalyzed neat reactions . Under these conditions the radiation is absorbed directly by the reagents and this results in spectacular acceleration, higher yields and purity of the reaction products, a simple workup procedure, and sometimes changes in the selectivity of the cycloaddition. 

New developments in microwave-accelerated solventless organic syntheses are appearing in the literature. This expeditious and solvent-free approach involves the exposure of neat reactants to microwave irradiation in conjunction with the use of supported reagents or catalysts which are primarily of mineral origin (Varma, 1999). 

Possible use of supported reagents-scavenger Simultaneous reaction and filtration Byproducts are retained on the solid phase ... 

Solid-supported reagents which have found utility include Nafion-scandium Lewis acid catalyst (allyl additions to aldehydes) [62], HOBt (medium-ring lactamization) [63], EDC (preparation of active esters) [64], and thiazolium hydrotribromide (brominations) [65], A review has also appeared describing the use of supported reagents in separation science, primarily for the selective sequestration of metal ions [66],... 

The use of supported reagents offers an attractive option for improving the quality of products prepared using solution-phase chemistry. Additionally, liquid-phase synthesis, for example using PEG, provides opportunities to combine some of the benefits of solid-phase approaches with the versatility of solution-phase synthesis. Smart methods such as resin capture for isolating specific compounds from mixtures of products will also help to increase the utility of solution-based approaches. This chapter encompasses developments in each of these areas. 

The use of supported reagents has always been recognized as a powerful tool in classical organic chemistry for a large number of applications, and excellent reviews covered this topic in the past [103-105]. Recently, this technique has been applied to solution-phase combinatorial chemistry, where it looks really promising in terms of simpler work-up procedures, elimination of excess reagents, and isolation of pure reaction products. 

PASP synthesis can be divided into two main approaches these being (a) the use of supported reagents and scavengers, and (b) the adoption of a catch-and-release strategy (Figure 1). Both of these techniques allow the production of clean products, without the need to resort to traditional purification techniques, such as column chromatography. In an ideal case, the incubation of a substrate with a supported reagent causes its complete transformation into a new chemical entity, with any... 

Figure 1 Schematic representation of PASP strategies for the synthesis of carboxamides, (a) Use of supported reagents and scavengers, (b) Catch-and-release strategy...

Oligonucleotide synthesis. - 1.1.1 DNA Synthesis. There have been few publications on improvements to DNA synthesis, and the main area of development has been in the development of modified oligonucleotides (ODNs). There have, however, been some improvements in the use of supports, reagents and protecting groups. Two new thymidine modified solid supports have been prepared suitable for oligonucleotide synthesis in which the support is attached via the thymine N3 position. These modified supports allow for ODN synthesis... 

Three-membered heterocyelie ring systems continue to receive attention from organic chemists. These heterocyclic ring systems provide a useful combination of reactivity, utility and stability. This review is not a comprehensive review but rather covers a selection of interesting and synthetically useful transformations. Some themes that have emerged in the past year include the use of supported reagents, aqueous reactions and solvent free reactions. The organization of this chapter follows that of previous years. 

The use of supported reagents has always been recognized as a powerful tool in classical organic chemistry for a large number of applications, and excellent re-... 

MULTISTEP USE OF SUPPORTED REAGENTS IN NATURAL PRODUCT SYNTHESIS 57... 

MW energy has been shown to accelerate the reduction of many key functional groups in organic chemistry, and Banks and Wagner have recently reviewed the area. In many examples, mild reagents and reaction conditions have been developed, and it has been demonstrated that the technique is widely applicable for the reduction of sensitive substrates the use of supported reagents has reduced the need for solvents and permitted shorter reaction times. The higher yields of products have also simplified the work-up procedures. 

Supported reagents have several advantages in organic chemistry such as the production of resin-bound products, increased stability, and easy purification. Despite these advantages, very few supported reagents have been used in MCRs, in front of the most common approach of using supported catalysts. In this chapter, we will discuss some of the most attractive examples for the use of supported reagents in MCRs. 

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