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the major goal of analysis. Single-bead FTIR4 is an effective tool for this type of analysis because most organic reactions involve functional group transformations. This analysis has the advantage of monitoring organic reactions from only one bead without sample preparation, and it can be rapidly performed at any time during synthesis. Additionally color tests for the solution-phase reaction monitoring are transferred to solid-phase chemistry whenever possible in our laboratories.5,6 Some color tests have been developed especially for solid-phase reaction monitoring such as uorescent tagging for various resin-bound functional groups.7,8 Color tests can be conducted in the synthesis laboratories as a quick check for the presence or absence of certain functional groups. They also indicate the extent of reaction completion based on the disappearance of the functional group in the starting material. Therefore a combination of color tests and FTIR analysis can provide a clear picture of progress in solid-phase reactions. Ensuring complete reaction for every step is a more logical way to increase reaction yield than adding excess materials to an inef cient synthesis that still must confront an unavoidable loss in product puri cation. In the following, we show that the monitoring of each reaction during development and subsequent production by FTIR and color test is useful and indispensable. Scaffold loading, the addition of diversity side chains onto the scaffold, and cleavage of the product from resin can all be monitored based on the presence or lack of organic functional groups.
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REACTION MONITORING FOR FEASIBILITY AND SYNTHESIS OPTIMIZATION
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A 90% yield for each reaction step in a four-step synthesis sequence would only give a product with 65% nal yield and a low purity. Therefore, during feasibility and optimization studies, it is important to know that each reaction step is complete. An individual test, FTIR or color test, can reveal whether a reaction is occurring and, in some cases, whether it is complete. The combination of different tests can provide more conclusive information on reaction completion. The coupling of carboxylic acid 2 to Marshall resin 1 to form 3 occurred in the presence of coupling catalysts and base in dichloromethane (Scheme 3.1). To ensure reaction completion, both the phenol test9 and single-bead FTIR are used. Normally, a positive phenol test resulting in dark-purple resin beads indicates the presence of the phenol group. The color intensity is proportional to the phenol concentration (Figure 3.1). FTIR showed the disappearance of the IR band for phenol group at 3278 cm-1 and the appearance of the IR band for ester group at 1710 cm-1 (Figure 3.2). In combining
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reaction monitoring for feasibility and synthesis optimization 55
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O HO OH S Marshall Resin R O S O
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Scheme 3.1. Carbxoylic acid coupling to Marshall resin.
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Figure 3.1. Phenol color test. Colorless beads on left indicate the absence of phenol, dark blue color beads on right indicate presence of phenol.
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the FTIR results and the negative color test for phenol groups, we could prove that a complete carboxylic acid coupling on Marshall resin had occurred. The coupling of Fmoc-protected amino acid 5 to Wang resin 4 to form 6 in the presence of coupling catalysts in DMF (Scheme 3.2) was also monitored. A positive test for hydroxy groups would result in the uorescent resin beads,8 indicating the presence of hydroxyl groups and an incomplete coupling to the Wang resin. Single-bead FTIR showed the disappearance of IR band for hydroxyl stretch at 3437 cm-1 and the appearance of the IR band for the ester carbonyl group at 1710 cm-1 (Figure 3.3). A negative test
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Figure 3.2. Single-bead FTIR spectra of Marshall resin 1 (bottom) and resin 3 (top). Disappearance of the phenol OH group (broad stretch at 3278 cm-1) and the appearance of the carbonyl group (strong stretch at 1710 cm-1) indicate the coupling of the carboxylic acid 2 to the Marshall resin.
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