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-1H-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 3200x (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 x 10(-5) s(-1) for the respective acid-catalyzed and uncatalyzed reactions were established, and the uncatalyzed reaction dominates at pH greater than or equal to 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.