Split-and-pool technique

The principal differences between those two methods are reflected in their applications. The random spht-and-pool method is suited for the synthesis of smaller quantities of large sizable libraries (i.e., million compounds), whereas the directed split-and-pool technique is suited for the synthesis of larger quantities of smaller compound collections (i.e., several hundred to several thousand compounds). In this chapter, a simple technique for directed spht-and-pool technique is described. 

Carbonylative macrolactonization is also possible. Takahashi and coworkers successfully prepared a library of macrosphelide targets using solid phase synthesis under Pd(0) catalysis (Scheme 2.5). By using the split-and-pool technique, 122 compounds were synthesized [15]. 

Solid-phase combinatorial synthesis can be performed using the split-and-pool technique based on the combination of variously substituted compounds together for the same reaction in an appropriate reaction step, as well as by parallel synthesis, in which all compounds are segregated during all the reaction steps (see next chapters). Although parallel synthesis is an efficient way to prepare arrays of structurally unrelated compounds, it is not necessarily a combinatorial approach conventionally based on substituent modifications of one structural motif. Thus, combinatorial chemistry is not parallel synthesis, albeit combinatorial chemistry can be performed in parallel fashion. 

Solid-phase organic syntheses typically use large excesses of reagents to drive reactions to completion so that, ideally, products liberated from resins should not require purification. Optimization of conditions is a critical part of solid-phase syntheses. Transfer of organic reactions in solution to a solid matrix is not a trivial undertaking, and lack of analytical methods accentuates this problem. Libraries prepared without adequate refinement of conditions tend to be of poor quality. For libraries so large that all the constituents cannot be fully characterized, well-optimized reaction conditions are absolutely essential. Techniques like split and pool, 2 for instance, can only be applied successfully after thorough optimization. 

The Encore Technique A Novel Approach to Directed Split-and-Pool Combinatorial Synthesis... 

Researchers of Praecis Pharmaceuticals, which was later acquired by GlaxoSmithKline (GSK), invented another encoding method. The key encoding technique involved enzymatic ligation of double-stranded DNA codons. Scheme 10.2 shows the DNA ligation and chemical synthesis on a sym-chlorotriazine scaffold, which were cycled in a split-and-pool manner to generate different encoded libraries with up to 4 biUion compoimds. 

Combinatorial syntheses can be performed in two ways (i) split-and-pool (also referred to as the split-and-mix or the split-and-recombine method)and its variant directed sorting" and (ii) parallel synthesis. A hybrid approach is the split-and-split technique. " ... 

V. Krchnak, V. Padera, The encore technique a novel approach to directed split-and-pool combinatorial synthesis, in B. A. Bunin, G. A. Morales, (Eds), Methods in Enzymology, Vol. 369, Elsevier, 2003, pp. 112-124. 

Synthesis of combinatorial arrays of compounds can, in principle, be performed three different ways. The Split and Mix (split and pool/recombine) concept introduced by Furka (4-6) is the most efficient method for preparation of sizable libraries (tens of thousands of compounds). However, the technique requires tracking of the chemical history of the resin beads and limits the quantity of synthesized material to the loading per solid phase unit (one unit can be represented by one particle, bead, lantern, etc., or one container, T-bag, Kan, etc.). Parallel synthesis, on the other hand, can supply any quantity per compound, but it requires handling large numbers of reaction vessels at one time. 

Today s parallel methods remain limited in terms of their ability to produce the large numbers of compounds that many believe will be required by the large number of novel targets predicted to evolve from the Human Genome Project. These novel bioassays coupled with HTS will place an enormous burden on compound production and informatics capabilities. The techniques prepared to handle this need are the split synthesis and the direct divide solid phase synthesis methods. These methods involve the division and pooling of resin particles such that large libraries can be easily produced, usually with a minimum investment in automation. 

Iterative deconvolution, like many other methods presented in the next paragraphs and chapters, requires the use of the mix and split [36,37] technique, which is briefly described in Figure 2. A hypothetical three-step library of 3 x 3 x 3 = 27 members is prepared by first splitting the resin into three equivalent portions, then adding a different monomer of the subset A to each portion. The three portions are then mixed and split again into three portions, now containing similar amounts of the three A monomers but with a unique monomer on each resin bead. Repeating this procedure with monomers of the subset B, three pools where B is determined and A is randomized are obtained. Another mix and split ... 

With the advent of combinatorial chemistry in the 1990s, their came the use of split-pool techniques that, when combined with advances in automation, made possible the efficient synthesis of literally millions of compounds. These libraries, however, were generally made up of broadly similar structures, with any variation between molecules resulting from appendage alteration of building blocks assembled around a common scaffold. Libraries of this sort have had limited success in the discovery of novel biologically active agents, a fact that is mainly attributed to the relative lack of diversity within the libraries. Therefore, it is believed that the quality, in terms of structural complexity and diversity, of these libraries is as important, if not more so, than the number of the compounds synthesized." ... 

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