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Synthesis and Evaluation of Duocarmycin and CC-1065 Analogues Containing Modifications in the Subunit Linking Amide
journal contribution
posted on 1999-06-24, 00:00 authored by Dale L. Boger, Alejandro Santillán, Mark Searcey, Qing JinThe preparation and evaluation of 6 and 7, analogues of the duocarmycins and CC-1065 in which
the subunit linking amide has been replaced with an amidine and thioamide, are described.
Consistent with the increased electron-withdrawing properties and conjugation of thioamides
relative to amides, 7 showed increased solvolysis reactivity (t1/2, 160 h versus 230 h) at pH 3,
attributable to a diminished vinylogous amide stabilization of the reacting alkylation subunit.
Amidine 6 proved to be even more unstable (t1/2, 12 h) despite the diminished electron-withdrawing
properties, but underwent preferential N2 amidine linkage hydrolysis rather than solvolysis of the
alkylation subunit, attributable to preferential N2 vinylogous amide versus amidine conjugation.
The natural isomers (+)-6 and (+)-7 exhibited an identical DNA alkylation selectivity as (+)-CBI-TMI and (+)-duocarmycin SA but were less efficient (10−100×). Biological studies of (+)-6 and
(+)-7 (0.75 and 1.1 nM, respectively) indicated the analogues retained good cytotoxic activities
(L1210), but were less potent than (+)-duocarmycin SA (0.01 nM, 100×) and (+)-CBI-TMI (0.02
nM, 50×). The enhanced properties of the linking amide versus amidine or thioamide established
the N2 amide as the optimal linking unit examined to date and revealed that it provides a beautiful
balance between competing amide (reactivity) and vinylogous amide (stability) conjugation.