Split-mixed synthesis

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 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... 

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. 

FIGURE 2.2 Scheme of the split-mix synthesis. The triangles represent the solid support. The white, gray and black circles are amino acids or other kinds of monomers. 

The split-mix synthesis has several key features that are crucial in the utility of the method in drug discovery or other kinds of applications. 

The combinatorial principle embodied in the synthesis captured the imagination of many researchers all over the world and had a profound effect on the development of the field. The combinatorial nature of the product of the split-mix synthesis is also reflected in its name, combinatorial library. ... 

In the split-mix synthesis, like in other solid phase procedures, the beads behave very much like tiny reaction vessels, which do not interchange their contents with the other ones. Each of the millions of these reaction vessels preserves its content until the end of the synthesis, when they become containers of a single substance. If peptides are produced, their identity can be determined by automatic sequencing [9], It is sufficient to sacrifice a fraction of the total quantity for this purpose. All this means that the split-mix synthesis is, in fact, a parallel procedure, with unprecedented efficiency, however, leading to individual compounds. This feature of the split and mix synthesis allows screening the products in three different ways ... 

A. Split-Mix Synthesis Using Resin Enclosed in Radiofrequency-Encoded Capsules... 

Split-mix synthesis Carbohydrate Small molecule One-bead-one- compound (OBOC) Biosensor-based methods Drug discovery... 

OBOC combinatorial bead-libraries can be considered as chemical microarrays that are spatially separable but non-addressable. The identity of the chemical compound on the positive beads can be determined directly with an automatic sequencer if it is an N-terminally unprotected peptide, by mass spectroscopy, or through chemical encoding. i A synthetic scheme for the OBOC library is shown in Figure 2. Using the highly efficient split-mix synthesis method,literally hundreds of thousands to millions of compounds can be prepared within a week. The recent... 

Figure 1 The "split-mix synthesis" method to generate a one-bead one-compound combinatorial library (a) and a number of permutations for random peptide libraries (b). P, , and T are building blocks (in this case amino acids).

In this method, peptides or small molecules are prepared by the split-mix synthesis method and cleaved from the resin to form an encoded solution-phase library such that each library compound is tethered to a PNA code via a hydrophilic linker (11). The library then is mixed with the target protein and later exposed to planar oligonucleotide microarrays of predetermined sequences. Alternatively, the encoded soluble library can be hybridized to the oligonucleotide microarrays before incubation with the target protein. 

Various sulhir-containing odorants, such as 2-thioalcanes, 3-acetylthio-2-alkyl aikanals, and trialklylated 1,3,5-dithiazines were prepared by conventional, parallel, and split-mix synthesis approaches. 2-Heptanethiol, newly identified in bell peppers, and 3-acetylthio-2-methylpentanal showed relatively low odor thresholds of 10 and 5 pg/kg water, respectively. Several odorants were found to develop interesting notes, which are compatible with both savory and sweet flavors. 

Francis and co-workers [94] prepared a library of linear peptides by split-mix synthesis, and a turn-inducing building block was introduced into the peptide chain. In this way, the peptide chain was able to complex metal ions. In an assay with different transition metals, each metal was able to bind selectively to a number of library components. 

Figure 16.18. FT-IR spectrum of an individual resin bead carrying one defined compound 1, demonstrating the one-bead-one-compound principle of a split-mix synthesis of isoxazolidines.

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