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The final combinatorial step involved manipulation of the ketone group of most of the dihydrobenzopyrans. Resin pools 4-6 and ll-41 were combined to give resin batch C and then re-divided into three equal portions (52-54). Batch 52 was retained, while the ketones in 53 were reduced with NaBH4, and those in 54 transformed into the cyclic thioacetal with ethane dithiol (Figure 20). Batches 52-54 were then uniquely encoded with two distinct molecular tags.