Synthesis and Evaluation of a Carbocyclic Analogue of the CC-1065 and Duocarmycin Alkylation Subunits: Role of the Vinylogous Amide and Implications on DNA Alkylation Catalysis

The synthesis and chemical properties of 1,2,9,9a-tetrahydro-1 H -cyclopropa[ c ]benz[ e ]inden-4-one (CBIn, 10 ), a carbocyclic C-ring analogue of the alkylation subunits of CC-1065 and the duocarmycins, are detailed. The core structure of CBIn was prepared with an intramolecular Heck reaction for assembly of the key tricyclic skeleton and a final Winstein Ar-3' spirocyclization to install the reactive cyclopropane. A study of the CBIn solvolysis reactivity, regioselectivity, and mechanism revealed that removal of the nitrogen and resulting vinylogous amide stabilization increased the reactivity 3200× (pH 3) and reversed the inherent regioselectivity, but did not alter the S N 2 reaction mechanism. Thus, the vinylogous amide found in the naturally occurring alkylation subunits is responsible for their unusual stability and significantly impacts the regioselectivity without altering the inherent S N 2 mechanism of nucleophilic addition. More importantly, this solvolysis reactivity proved independent of pH throughout the range of 4−12 including the physiologically relevant range of 5.0−8.0 where CBI is completely stable. Rate constants of 0.093 ± 0.001 M - 1 s - 1 and 4.2 ± 0.4 × 10 - 5 s - 1 for the respective acid-catalyzed and uncatalyzed reactions were established, and the uncatalyzed reaction dominates at pH ≥ 4. These observations have important implications on the source of catalysis for the CC-1065/duocarmycin DNA alkylation reaction supporting the recent proposal that it is not derived from acid catalysis and C4 carbonyl protonation but rather a DNA binding-induced conformational change that disrupts the cross-conjugated vinylogous amide stabilization.