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Publication Title | NEW TOOLS AND CONCEPTS FOR MODERN ORGANIC SYNTHESIS

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REVIEWS

NEW TOOLS AND CONCEPTS FOR MODERN ORGANIC SYNTHESIS

Steven V. Ley and Ian R. Baxendale

The increasing need to efficiently assemble small molecules as potential modulators of therapeutic targets that are emerging from genomics and proteomics is driving the development of novel technologies for small-molecule synthesis. Here, we describe some of the general applications and approaches to synthesis using one such technology — solid-supported reagents — that has been shown to significantly improve productivity in the generation of combinatorial libraries and complex target molecules.

The unprecedented increase in the number of new drug targets arising from genomics and proteomics translates directly into a need for new methods to rapidly assemble highly pure small molecules (Mr ~250–800) that possess an ever-increasing level of structural complexity. These strategically important processes are also required to be environmentally cleaner, more efficient and lead to greater structural variation in as short a period of time as possible. Such demands have driven the development of novel technologies, which have begun to produce compounds at a greater rate than previously thought possible. One such molecule-assembly technology is that of solid-supported reagents. The use of solid- supported reagents in chemical synthesis in a multistep mode has been shown to markedly improve productiv- ity in crucial aspects of the generation of fine chemical entities and complex target molecules. Further studies are under way to show the full range of advantages that these reagents offer — not only because of their signifi- cant importance in the field of COMBINATORIAL CHEMISTRY and PARALLEL SYNTHESIS, but also because of the possibili- ties that they present for a much wider impact generally on all synthetic chemistry.

Solid-supported reagents — summary

Chemists have generally addressed the question of improving their throughput by applying substrate- supported chemistry. In such a strategy, the substrate is temporarily immobilized on a polymeric resin, taken through a synthetic sequence and then cleaved back into solution (FIG. 1a). This makes it possible to drive

reactions to completion with excess reagents and to create a high level of molecular diversity, often by the use of a robotic synthesizer. Although this strategy has many advantages, it also has several drawbacks: reac- tions can be slow and are difficult to monitor in real time compared with solution-phase chemistry; extra steps are required to attach and release the substrate from the resin; part of the resin attachment is often found in the final product; CONVERGENT SYNTHESES are not possible; resin loading is often poor; optimization of reactions can be very time-consuming and long linear sequences are difficult to achieve.

Many of these issues have been addressed by the application of solid-supported reagents. Supported reagents are reactive species that are associated with a support material. They transform a substrate (or sub- strates) into a new chemical product (or products), and the excess or spent reagent can then be easily removed by filtration. In a similar fashion, impurities can be removed from solution using a ‘scavenger’ immobilized on a support. A schematic representation of how these concepts work in practice to give clean products is shown in FIG. 1b.

This concept of immobilizing reagents on a solid support provides many advantages over both conven- tional solution-phase and solid-phase preparative routes (BOX 1). Moreover, it could be argued that this approach actually combines the best attributes from both of these synthetic approaches, which results in a more efficient and powerful methodology. It is certainly conceivable that, with the appropriate choice of support

COMBINATORIAL CHEMISTRY The generation of large collections, or ‘libraries’, of compounds by synthesizing all possible combinations of a set of smaller chemical structures.

PARALLEL SYNTHESIS

Creation of a series of individual compounds through reactions performed simultaneously, rather than one at a time.

Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK. Correspondence to S.V.L. e-mail: svl1000@cam.ac.uk doi:10.1038/nrd871

NATURE REVIEWS | DRUG DISCOVERY

VOLUME 1 | AUGUST 2002 | 573

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