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Table 1 Relation between common linkage reactions and most common fragments shown in Figure 2.

From: Estimation of synthetic accessibility score of drug-like molecules based on molecular complexity and fragment contributions

Linkage Reaction

Fragment(s)

Amide bond formation or Urea formation

2, 6, 8, 12 (from primary amine) or 16 (from secondary amine), 28 (only if aniline)

Sulfonamide formation

6, 12 (from primary amine) or 16 (from secondary amine)

Ester formation

2, 6, 8, 10

Reductive amination

4 (the CH2 group from the aldehyde carbon), 12 (from primary amine) or 16 (from secondary amine)