I

strong cation exchange weak anion exchange weak anion exchange strong anion exchange

Figure 4 Silica gel-based ion-exchange sorbents.

weak anion exchange weak anion exchange strong anion exchange

Figure 4 Silica gel-based ion-exchange sorbents.

prepacked polypropylene cartridges, filled with various amounts of sorbent. The capacity of the sorbents is given in mequiv. g"' (normally -0.7 mequiv. g_1); thus it is easily determined how much material can be loaded onto the column.

Thus, products able to form ions can be purified by ion-exchange solid-phase extraction in automated solution-phase synthesis. Another possibility to purify a combinatorial solution-phase library is to separate selectively reagents, byproducts and impurities which are able to form ions from the product by ion exchange. Both methods have appeared in the literature, and a few examples will be given here.

For the purification of an amide library [22], a basic ion-exchange resin was used to separate an excess of unreacted acid chloride after addition of water to the reaction mixture (Scheme 9). The authors evaluated nine ion-exchange resins and three solvents, and obtained the best results using the weakly basic AmberliteĀ® IRA-68 in combination with ethyl acetate. Using this method they obtained the desired products 30 in high yields (84-99 % ) and purities (>95 % ), but unfortunately only data of a nine-member library were given. Using this strategy, more than 4500 compounds were synthesized [23], starting from a series of substituted pyrimidine and benzene acid chlorides.

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