Split-mix

Combinatorial chemistry has significantly increased the nurnjjers of molecules that can be synthesised in a modern chemical laboratory. The classic approach to combinatorial synthesis involves the use of a solid support (e.g. polystyrene beads) together with a scheme called split-mix. Solid-phase chemistry is particularly appealing because it permits excess reagent to be used, so ensuring that the reaction proceeds to completion. The excess... 

Illustration of the split-mix approach to combinatorial s jnthesis, using sets containing three monomers. 

The major impetus for the development of solid phase synthesis centers around applications in combinatorial chemistry. The notion that new drug leads and catalysts can be discovered in a high tiuoughput fashion has been demonstrated many times over as is evidenced from the number of publications that have arisen (see references at the end of this chapter). A number of )proaches to combinatorial chemistry exist. These include the split-mix method, serial techniques and parallel methods to generate libraries of compounds. The advances in combinatorial chemistry are also accompani by sophisticated methods in deconvolution and identification of compounds from libraries. In a number of cases, innovative hardware and software has been developed tor these purposes. 

An important breakthrough in that respect was the use of soHd-phase organic synthesis (SPOS) where the attachment of the substrate to an insoluble support allowed for easy workup (filtration) and for rapid generation of products via split-mix procedures [1,2]. An important subsequent development consisted of the immobihzation of reagents, scavengers and catalysts. This technique, coined polymer-assisted solution phase chemistry (PASP), allowed solution phase synthesis of compoimds, yet still enjoying the bene-... 

The synthesis of libraries of structurally defined compounds can potentially be achieved either by split-mix synthesis or by parallel synthesis of individual compounds. The synthesis requires a reliable methodology of oligosaccharide synthesis, where stereochemistry and regioselectivity have to be achieved unlike other library approaches. Development of synthetic methodologies that can provide access to any oligosaccharide structure is underway. 

The development glycopeptide libraries obtained by the split-mix method is severely hampered by the lack of concurrent development of a general, facile separation and characterization technology. Some headway has been made with chemical coding of the libraries, but very few direct methods of analysis exist. One promising method that could be applied to the direct characterization of both types of libraries is mass spectrometry. More specifically, post-source-decay matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (PSD-MALDI-TOF-MS) and CID-FAB/MS/MS have been used to characterize glycopeptides.53-55... 

While parallel synthesis of arrays of glycopeptides is readily achieved by implementation of the building-block approach (Scheme 14.1, Strategy 2),101 glycopeptide library synthesis in a combinatorial manner via the split-mix method has yet to prove routine. The difficulty lies in the structural analysis of the vast number of compounds generated in picomolar quantities on a single bead. Whereas peptides on... 

The split-mix synthesis technique can allow geometric numbers of compounds to be prepared using an arithmetic number of reaction chambers.5... 

Split-Mix Synthesis Using Macroscopic Solid Support Units... 

Split-mix synthesis1 3 made it possible to prepare new compounds in practically unlimited numbers. That procedure was based on the solid-phase method4 in which each coupling cycle was replaced by the following operations ... 

We first reported the one-bead one-compound (OBOC) combinatorial library method in 1991.1 In this method, compound beads are prepared by a split-mix synthesis approach1 3 (Fig. 1) that results in the display of many copies of the same compound on one single bead.1,4 Tens of thousands to millions of these compound beads can easily be prepared. The bead library is then mixed with a target molecule, such as a protein, an... 

Fig. 1. Split/mix synthesis approach to generate an OBOC combinatorial library.

The expense of screening depends very much on the number of samples tested. Consequently, the density format of titer-plates has increased in recent years from 96-well up to the 9600-well format. The next big step towards miniaturization would be the complete avoidance of any container, which then results in the smallest well possible and a well-less, so-called lawn-format assay develops. This is exactly what is proposed by a number of authors (see review [47]). Screening in a lawn format does not mean avoiding any structure or arrangements. Samples are still prepared on beads, which are produced by split-mix synthesis, but the beads are arrayed directly on the well-less assay. A typical matrix applied for such biological screening is the agarose lawn. Active beads are then picked from the assay matrix and decoded for compound identification. 

One of these methods is the split-mix procedure, originally called portioning-mixing method, invented by Furka et al. [1-3]. This method was originally developed to enable the user to prepare millions of new peptides, but, later, was successfully used to synthesize organic libraries, too. The method is an embodiment of the combinatorial principle and the combinatorial thinking , which constitutes the theoretical background of the method and proved to be so fruitful in other areas, too. 

The split-mix method realizes the combinatorial distribution rule by mixing the products after each reaction step, then distributing the mixture into equal portions. 

The split-mix method was developed for preparing peptide libraries. The method is based on Merrifield s solid phase procedure, published in 1963 [8], Each coupling cycle of the solid phase synthesis is replaced by the following simple operations ... 

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