Synthesis of Solid-Phase Pool Libraries

The use of heterogeneous supports in organic synthesis has been extensively covered, and its relevant implications on the reaction outcome were discussed in Chapter 1. A major advantage of heterogeneous versus homogeneous reactions for the generation of pool libraries is related to the handling and the purification of intermediate or final pools. 

The second approach, named many compounds per bead (Fig. 7.3), starts by coupling the solid support in a single reaction vessel with an equimolar mixture of the 100 amines (step a) then the mixture is reduced (step b) and the resin-bound amines are reacted with an equimolar mixture of the 100 acylating agents (step c). The 10,000-member library is obtained as a single 50-g pool of resin, and each bead contains similar quantities of each library individual. A bead has typically 10 -10 reaction sites, so that each bead will contain an average of lO -lO copies of each library individual. The library synthesis could technically be considered successful if all the monomers react properly and the 10,000 compounds are acmally present, but the identification of positives from this library for any specific application is not feasible. In fact, the cleavage of resin-bound materials produces an equimolar mixture of all the components, whose activity, if any, is the activity of a 10,000-member unresolved mixmre. As a consequence of this major limitation, this SPS approach is not used for library synthesis. 

The third approach can be considered as a compromise between parallel synthesis and mixtures on SP. It is named one compound per bead (13) and allows the synthesis 

The synthesis of our hypothetical SP pool hbrary of 10,000 amides by mix and split is reported in Fig. 7.4. Preparation of the 100 resin-bound amines follows an identical course to the one seen for parallel synthesis (steps a, b, and c), but then the 100 resin aliquots are mixed to give a single 50 g portion (step d). This portion is then split into 100 aliquots each containing all the 100 amines (step a), but each bead is loaded with 

500 mg-batches 100 resin-bound amines 1 compound per bead 

---

Analytical Methods for the Synthesis, Quality Control, and Purification of Solid-Phase Pool Libraries... 

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 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. 

The primary library synthesis is then carried out using any of the techniques that will be illustrated in detail in the next three chapters for solution- or solid-phase, pool or discrete libraries. Many choices in terms of instrumentation or automation are available, and each of them may afford high-quality results. The same is true for analytical quality control, which determines the synthesis outcome and decides if the hbrary is suitable for screening against desired targets. 

Among the other published works, Parlow et al. [60] and Xu et al. [61] reported the use of anion-exchange resins for the synthesis of solution-phase libraries of aryl and heteroaryl ethers via formation of a resin-bound phenol salt and its subsequent alkylation [60] (two pools of 10 compounds each) or with a one-pot procedure [61] (13 discretes). Polyme supported reagents being established tools in classical organic chemistry, their use for the synthesis of high-quality solution-phase libraries, possibly in combination with other solid-support-... 

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]. 

---