SYNTHESIS AND EVALUATION OF A CARBOCYCLIC ANALOG 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 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. Th e core structure of CBIn was prepared with an intramolecular Heck reac tion for assembly of the key tricyclic skeleton and a final Winstein A r-3' spirocyclization to install the reactive cyclopropane. A study of the CBIn solvolysis reactivity, regioselectivity, and mechanism revea led that removal of the nitrogen and resulting vinylogous amide stabil ization increased the reactivity 3200x (pH 3) and reversed the inheren t regioselectivity, but did not alter the S(N)2 reaction mechanism. Th us, the vinylogous amide found in the naturally occurring alkylation s ubunits is responsible for their unusual stability and significantly i mpacts the regioselectivity without altering the inherent S(N)2 mechan ism of nucleophilic addition. More importantly, this solvolysis reacti vity proved independent of pH throughout the range of 4-12 including t he physiologically relevant range of 5.0-8.0 where CBI is completely s table. Rate constants of 0.093 +/- 0.001 M-1 s(-1) and 4.2 +/- 0.4 x 1 0(-5) s(-1) for the respective acid-catalyzed and uncatalyzed reaction s were established, and the uncatalyzed reaction dominates at pH great er 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 ac id catalysis and C4 carbonyl protonation but rather a DNA binding-indu ced conformational change that disrupts the cross-conjugated vinylogou s amide stabilization.