Reaction SMILES: [CH3:1][C@H:2]1[C@@H:27]2[O:28][C@@:26]2([CH3:29])[C@@H:25]([O:30][C:31]([C@@H:33]([N:35]([C:37]([CH2:39][CH2:40][SH:41])=[O:38])[CH3:36])[CH3:34])=[O:32])[CH2:24][C:22](=[O:23])[N:21]([CH3:42])[C:14]2=[C:15]([Cl:20])[C:16]([O:18][CH3:19])=[CH:17][C:12](=[CH:13]2)[CH2:11][C:10]([CH3:43])=[CH:9][CH:8]=[CH:7][C@@H:6]([O:44][CH3:45])[C@:5]2([OH:50])[NH:46][C:47]([O:49][C@H:3]1[CH2:4]2)=[O:48].[CH2:51]1[CH:56]([CH2:57][N:58]2[C:63](=[O:64])[CH:62]=[CH:61][C:59]2=[O:60])[CH2:55][CH2:54][CH:53]([C:65]([O:67][N:68]2[C:73](=[O:74])[CH2:72][CH2:71][C:69]2=[O:70])=[O:66])[CH2:52]1.P([O-])([O-])([O-])=O.P([O-])([O-])([O-])=O.[K+].[K+].[K+].C(N(CC(O)=O)CC(O)=O)CN(CC(O)=O)CC(O)=O>C1COCC1>[CH3:1][C@H:2]1[C@@H:27]2[O:28][C@@:26]2([CH3:29])[C@@H:25]([O:30][C:31]([C@@H:33]([N:35]([C:37]([CH2:39][CH2:40][S:41][CH:62]2[C:63](=[O:64])[N:58]([CH2:57][CH:56]3[CH2:55][CH2:54][CH:53]([C:65]([O:67][N:68]4[C:73](=[O:74])[CH2:72][CH2:71][C:69]4=[O:70])=[O:66])[CH2:52][CH2:51]3)[C:59](=[O:60])[CH2:61]2)=[O:38])[CH3:36])[CH3:34])=[O:32])[CH2:24][C:22](=[O:23])[N:21]([CH3:42])[C:14]2=[C:15]([Cl:20])[C:16]([O:18][CH3:19])=[CH:17][C:12](=[CH:13]2)[CH2:11][C:10]([CH3:43])=[CH:9][CH:8]=[CH:7][C@@H:6]([O:44][CH3:45])[C@:5]2([OH:50])[NH:46][C:47]([O:49][C@H:3]1[CH2:4]2)=[O:48]
Reactant: COc1cc2cc(c1Cl)N(C)C(=O)C[C@H](OC(=O)[C@H](C)N(C)C(=O)CCS)[C@]1(C)O[C@H]1[C@H](C)[C@@H]1C[C@@](O)(NC(=O)O1)[C@H](OC)C=CC=C(C)C2
Reactant: O=C(ON1C(=O)CCC1=O)C1CCC(CN2C(=O)C=CC2=O)CC1
Agent: COc1cc2cc(c1Cl)N(C)C(=O)C[C@H](OC(=O)[C@H](C)N(C)C(=O)CCS)[C@]1(C)O[C@H]1[C@H](C)[C@@H]1C[C@@](O)(NC(=O)O1)[C@H](OC)/C=C/C=C(\C)C2
Agent: COc1cc2cc(c1Cl)N(C)C(=O)C[C@H](OC(=O)[C@H](C)N(C)C(=O)CCSC1CC(=O)N(CC3CCC(C(=O)ON4C(=O)CCC4=O)CC3)C1=O)[C@]1(C)O[C@H]1[C@H](C)[C@@H]1C[C@@](O)(NC(=O)O1)[C@H](OC)/C=C/C=C(\C)C2
Agent: O=C(O)CN(CCN(CC(=O)O)CC(=O)O)CC(=O)O
Solvent: C1CCOC1
Product: COc1cc2cc(c1Cl)N(C)C(=O)C[C@H](OC(=O)[C@H](C)N(C)C(=O)CCSC1CC(=O)N(CC3CCC(C(=O)ON4C(=O)CCC4=O)CC3)C1=O)[C@]1(C)O[C@H]1[C@H](C)[C@@H]1C[C@@](O)(NC(=O)O1)[C@H](OC)C=CC=C(C)C2
Reaction time: 0.5 h

Procedure:
A round bottom flask was charged with N2′-Deacetyl-N2′-(3-mercapto-1-oxopropyl)-maytansine (DM1, 67.9 mg, 0.092 mmol), Succinimidyl-4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SMCC, 32.1 mg, 0.096 mmol, 1.05 equivalents) and THF (4 mL). The solution stirred as the reagents were dissolved in the solvent to yield a clear, colorless solution. Phosphate buffer, pH 6, (4 mL, 100 mM Potassium Phosphate, 2 mM EDTA) was then added. The reaction flask was equipped with a septum and stir bar and the reaction proceeded at room temperature with vigorous stirring. The reaction appeared complete within 30 minutes as indicated by reverse-phase HPLC. Following reaction completion, the reaction volume was reduced in vacuo to give a white solid/residue. The product was isolated by silica gel chromatography, eluting with a mixture of 3% methanol in methylene chloride. Product containing fractions were combined and concentrated to dryness in vacuo to give 63 mg (63.9% yield) DM1-SMCC as a white solid. Mass spectral analysis of the isolated product gave the expected m+Na+ (m/z 1094.4) and m+Cl− (m/z 1106.2) major molecular ions (FIG. 43).