Analytical tools, solid-phase synthesis

In this chapter some general techniques for the performance of solid-phase reactions are presented. An overview of analytical tools suitable for the characterization of support-bound intermediates is also given, and strategies for the selection of reagents and reactions for parallel solid-phase synthesis are discussed. 

Because the characterization of support-bound intermediates is difficult (see below), solid-phase reactions are most conveniently monitored by cleaving the intermediates from the support and analyzing them in solution. Depending on the loading, 5-20 mg of support will usually deliver sufficient material for analysis by HPLC, LC-MS, and NMR, and enable assessment of the outcome of a reaction. Analytical tools that are particularly well suited for the rapid analysis of small samples resulting from solid-phase synthesis include MALDI-TOF MS [3-5], ion-spray MS [6-8], and tandem MS [9]. MALDI-TOF MS can even be used to analyze the product cleaved from a single bead [5], and is therefore well suited to the identification of products synthesized by the mix-and-split method (Section 1.2). The analysis and quantification of small amounts of product can be further facilitated by using supports with two linkers, which enable either release of the desired product or release of the product covalently bound to a dye [10-13], to an isotopic label [11], or to a sensitizer for mass spectrometry [6,14,15] (e.g., product-linker-dye- analytical linker -Pol). 

Photolytic cleavage at the anomeric center of glycosides as an analytical or preparative tool does not have a very extensive literature. It has, however, been found useful in the solid-phase synthesis of oligosaccharides. Thus the synthesis by Nicolaou and coworkers of the phytoelicitor heptasaccharide of Phytophthora megasperma (Section V) depends on detachment of the protected heptasaccharide from the support by photolytic cleavage of a 4-nitrophenoxy link to the polystyrene polymer. 

F. Bardella, R. Eritja, E. Pedroso and E. Giralt, Gel-phase -31-NMR-a new analytical tool to evaluate solid-phase oligonucleotide synthesis, Bioorg. Med. Chem. Lett., 1993, 3, 2793-2796. 

General Techniques and Analytical Tools for Solid-Phase Organic Synthesis... 

Solid-phase organic synthesis has emerged as a powerful tool for the synthesis of chemical hbraries. A major drawback to sohd-phase chemistry is that it is difficult to directly monitor the desired chemical reaction on resin. Standard analytical techniques for reaction optimization are available after the reaction product is cleaved from the solid support. However, the typically harsh conditions necessary to remove the reaction product from the solid support may introduce impurities and undesired side products, thus masking the true nature of the reaction being monitored. Both IR and spectroscopy have been used to monitor the progress of reactions on the solid phase. This chapter reviews the use of NMR spectroscopy as a tool to monitor solid-phase reactions directly, without having to cleave the product from the resin prior to analysis. 

Although solid-phase chemistry is well developed, progress has not yet been fully extended to oligosaccharide chemistry, because of the high demands on the polymer support and the lack of powerful analytical tools for monitoring reactions on solid phases. The synthesis of an a(l-2)-linked pentamannose moiety has recently been reported (Scheme 45) [193] a Merrifield resin, a thio-linker, and a 2-0-acetyl protected mannosyl trichloroacetimidate were used. The glycosylation reaction was... 

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