| CPC A61K 31/192 (2013.01) [A61K 47/545 (2017.08)] | 6 Claims |
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1. A compound of (4a) or (4b):
![]() wherein R1 is a carboxylic acid protecting group, R2 is H or a hydroxyl protecting group, R3 is H or a hydroxyl protecting group, and R5 is a polymeric carrier.
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