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(a) Accessibility of the 5-C O and N–C=O groups estimated by the dimension of a sphere (r = 2.5 Å) centered on the carbon atom (red) and on the oxygen atom (blue). Left: (a) right: 3a; (b) accessibility of the 5-C O and N–C=O groups estimated by the dimension of a sphere (r = 2.5 Å) centered on the carbon atom (red) and on the oxygen atom (blue). Left: (b) right: 4a
