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
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 , 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 Amberlite® 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 , starting from a series of substituted pyrimidine and benzene acid chlorides.
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