Split-pool synthesis solid-phase

The isorniinchnone cyclization/isocyanate cycloreversion process for substituted furan synthesis has been well studied, as exemplified by the conversion of 104 to 106 (Scheme 19.19). In a solid-phase adaptation of this transformation, two groups independently utilized this reaction to estabhsh a traceless self-cleaving method for the synthesis of substituted furans [176, 177]. Further investigation of the thermal requirements of this cycloreversion led to its application in the split-pool synthesis of a small library of amides [178]. 

Solid-phase synthesis techniques applied either in a parallel fashion in automated synthesis or in split-pool-synthesis enable the rapid production of compound libraries containing thousands of members. But the original expectation that new hits will be discovered solely by the creation of a large quantity of library members was not fulfilled. Some of the libraries contained hardly any hit, because the underlying structures therein were not biologically relevant. Thus an old question returned Where in the almost indefinite space of thinkable chemical compounds are the structures which are of biological relevance [2] ... 

Developments in synthetic methodology like solid-phase synthesis, split-pool synthesis, etc. have helped considerably in streamlining the initial stages of the discovery... 

A number of different solid supports and uniquely designed reaction vessels are adopted for the parallel synthesis of organic compound libraries. The yields of the individual compounds synthesized vary widely from nanomoles to millimoles. Unlike split-pool synthesis, which requires a solid support, parallel synthesis can be done either on solid phase or in solution. 

Schreiber s early efforts in this area were focused on libraries of compounds having structural features reminiscent of rigid, complex, stereochemically rich natural products. In a key early example, solid-phase split-pool synthesis was used to generate a combinatorial library of over two million complex, polycyclic compounds derived from shikimic acid [17]. A stereoselective tandem acylation-nitrone cycloaddition was used to generate 18 tetracyclic scaffolds, to which 30 alkynes were coupled using a Sonogashira reaction, 62 amines were coupled via y -lactone aminolysis, and 62 carboxylic acids were coupled by alcohol esterification (Fig. 9.1-3(c)). In addition, a portion of the solid supports were left unreacted at each of the last three steps to generate a skip codon that further increased the diversity of the library. 

The concept of reducing the number of reaction vessels and exponentially increasing the number of synthesized compounds was brought to a next level of simplicity by the split-and-pool method of Furka et al.5 The split-and-pool method was independently applied by Lam et al.6 in a one-bead-one-compound concept for the combinatorial synthesis of large compound arrays (libraries) and by Houghten et al.7 for the iterative libraries. Now several millions peptides could be synthesized in a few days. In Furka s method the resin beads receiving the same amino acid were contained in one reaction vessel—identical to Frank s method—however, the beads were pooled and then split randomly before each combinatorial step. Thus the method is referred to as the random split-and-pool method to differentiate it from Frank s method in which each solid-phase particle was directed into a particular reaction vessel (the directed split-and-pool method). 

The algorithm described above is for a three-step combinatorial synthesis. However, the method is not limited to only three-step combinatorial libraries the solid-phase support can be derivatized before the directed split-and-pool synthesis on the Encore synthesizer. The necklace coding can also be a very useful tool during the chemistry development process. 

Combinatorial chemistry has moved from specially centralized laboratories, often equipped with multimillion-dollar robots, onto the bench of individual medicinal chemists. This change in direction requires the availability of personal chemistry tools that are simple to operate, easy to arrange in the laboratory, and reasonably priced. Such instruments are now available for the effective synthesis of combinatorial libraries. The Encore synthesizer represents a simple and efficient personal chemistry tool that allows the execution of directed split-and-pool combinatorial synthesis. The current version of the Encore synthesizer is designed for solid-phase synthesis on SynPhase Lanterns however, it can be modified for synthesis on alternative solid supports such as resin plugs from Polymer Laboratories (e.g., StratoSpheres Plugs). 

The split-and-pool synthesis not only simplifies the complexity of the combinatorial synthetic process, but also offers additional important benefits. To undertake a full range of solid-phase chemical reactions, elaborate reaction conditions are needed for some chemical transformations. These include, but are not limited to, low temperature and inert atmosphere conditions. Parallel synthesis of a thousand compounds requires handling of a thousand reaction vessels. The timely addition of sensitive reagents (e.g., butyl lithium) at low temperature (—78°) under inert atmosphere during parallel synthesis is not a trivial task. It can be done if sophisticated automated synthesizer equipment is designed to handle and tolerate such reaction conditions. Such a synthesis can alternatively be performed easily in a manual fashion using a split-and-pool method that requires only a limited number of reaction vessels. Examples from Nicolaou s17 and Schrei-ber s18,19 laboratories have shown that the split-and-pool method is the methodology of choice for the synthesis of complex and diversity-oriented combinatorial libraries. 

Generally, solution-based approaches for the generation of inorganic split and pool libraries have substantial advantages over approaches where solid phases are introduced as chemical sources during the different synthetic steps. Solution chemistry offers, potentially, a wide range of synthetic opportunities that can be exploited not only for the purpose of parallel synthesis but also for synthetic steps for Split Pool library creation. 

Fig. 7.1. The strategy used to develop GGTI. A pilot library consisting of 171 compounds were screened for the ability to inhibit GGTase-I using RhoA as a substrate. This led to the identification of two groups of compounds, one with tetrahydropyridine scaffold and the other with dihydropyrrole scaffold. Solid-phase split-and-pool combinatorial synthesis of a large number of analogs of these initial hits led to the identification of P3-E5 and P5-H6. More than 700-fold increase in IC50 value for the inhibition of GGTase-I was obtained in the case of P3-E5 compared with the initial compound. Further derivatization of P5-H6 and P3-E5 led to cell active compounds P61-A6 and P61-E7, respectively.

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