Synthesis, Chemical Properties, and Preliminary Evaluation of Substituted CBI Analogs of CC-1065 and the Duocarmycins Incorporating the 7-Cyano-1,2,9,9a-tetrahydrocyclopropa[<i>c</i>]benz[<i>e</i>]indol-4-one Alkylation Subunit:  Hammett Quantitation of the Magnitude of Electronic Effects on Functional Reactivity (1996)

The synthesis of 7-cyano-1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one (CCBI), a substituted CBI derivative bearing a C7 cyano group, is described in efforts that establish the magnitude of potential electronic effects on the functional reactivity of the agents. The CCBI alkylation subunit was prepared by a modified Stobbe condensation/Friedel−Crafts acylation for generation of the appropriately functionalized naphthalene precursors followed by 5-exo-trig aryl radical−alkene cyclization for synthesis of the 1,2-dihydro-3H-benz[e]indole skeleton and final Ar-3‘ alkylation for introduction of the activated cyclopropane. The most concise approach provided the CCBI subunit and its immediate precursor in 14−15 steps in superb overall conversions (15−20%). Resolution of an immediate CCBI precursor and its incorporation into both enantiomers of 34−39, analogs of CC-1065 and the duocarmycins, are detailed. A study of the solvolysis reactivity and regioselectivity of N-BOC-CCBI (25) revealed that introduction of the C7 nitrile slowed the rate of solvolysis but only to a surprisingly small extent. Classical Hammett quantitation of the effect provided a remarkably small ρ (−0.3), indicating an exceptionally small C7 substituent electronic effect on functional reactivity. Additional kinetic studies of acid-catalyzed nucleophilic addition proved inconsistent with C4 carbonyl protonation as the slow and rate-determining step but consistent with a mechanism in which protonation is rapid and reversible followed by slow and rate-determining nucleophilic addition to the cyclopropane requiring both the presence and assistance of a nucleophile (SN2 mechanism). No doubt this contributes to the DNA alkylation selectivity of this class of agents and suggests that the positioning of an accessible nucleophile (adenine N3) and not C4 carbonyl protonation is the rate-determining step controlling the sequence selectivity of the DNA alkylation reaction. This small electronic effect on the solvolysis rate had no impact on the solvolysis regioselectivity, and stereoelectronically-controlled nucleophilic addition to the least substituted carbon of the activated cyclopropane was observed exclusively. Consistent with past studies, a direct relationship between solvolysis stability and cytotoxic potency was observed with the CCBI-derived agents providing the most potent analogs in the CBI series, and these observations were related to the predictable Hammett substituent effects. For the natural enantiomers, this unusually small electronic effect on functional reactivity had no perceptible effect on their DNA alkylation selectivity. Similar effects of the C7 cyano substituent on the unnatural enantiomers were observed, and they proved to be 4−10× more effective than the corresponding CBI-based unnatural enantiomers and 4−70× less potent than the CCBI natural enantiomers.