Methods for preparing antibody drug conjugates

An antibody-drug conjugate of Formula IV:

• or a pharmaceutically acceptable salt thereof, wherein
• Ab is an antibody;
• R1, R2, R3, and R4 are independently selected from the group consisting of H, optionally substituted branched or linear C1-C5 alkyl, and optionally substituted C3-C6 cycloalkyl, or R3 and R4 taken together with the carbon atom to which they are bound form a C3-C6 cycloalkyl ring,
• wherein said optionally substituted alkyl or cycloalkyl may be substituted with alkyl, cycloalkyl, aryl, heteroaryl, hydroxyl, nitrile, halo, alkoxy, haloalkoxy, arylalkoxy, acyloxy, alkylthio, sulfonate, amino, alkylamino, acylamino, carbamoyl, alkylcarbamoyl, or nitro;
• D is:
• wherein the carbonyl in Formula IV is covalently bonded to the secondary nitrogen in D;
• and
• p is 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.