prov:value
| - yl, nitro, sulfide, disulfide, sulfone, sulfonate, sulfonamide, thiol, and thioalkoxy; provided that X6 is not hydrogen and X6??? is not a bond; [0448] R1 is hydrogen or a hydroxyl protecting group; [0449] R4 is a hydroxyl group or a protected hydroxyl group; [0450] each R5??? and R40 is, independently, hydrogen, C1-C10 alkyl, C2-C10 alkenyl, C2-C20 alkynyl, C6-C14 aryl or an amino-protecting grou
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