Synthesis and evaluation of CC-1065 and duocarmycin analogues incorporating the iso-CI and iso-CBI alkylation subunits: Impact of relocation of the C-4 carbonyl

The synthesis of 2-(tert-Butyloxycarbonyl)-1, 2, 9, 9a-tetrahydrocyclopropa[c]benzo[f]indol-8-one (31, N-BOC-iso-CBI) and 1-(tert-Butyloxycarbonyl)-4-hydroxy-3-[[(methanesulfonyl)oxy]methyl]-2, 3-dihydroindole (19, seco-N-BOC-iso-CI) containing an isomeric structural modification in the CC-1065 and duocarmycin alkylation subunits and their incorporation into analogues of the natural products are detailed. The approach was based on a directed ortho metalation of an appropriately functionalized benzene (13) or naphthalene (24) precursor to regiospecifically install iodine at the C-2 position. Conversion of these respective intermediates to the dihydroindole skeleton utilized an established 5-exo-trig aryl radical cyclization onto an unactivated alkene with subsequent TEMPO trap or the more recent 5-exo-trig aryl radical cyclization onto a vinyl chloride for direct synthesis of the immediate precursors. Closure of the activated cyclopropane to complete the iso-CBI nucleus was accomplished by a selective ortho spirocyclization. The evaluation of the iso-CBI-based agents revealed a significant stability comparable to that of CC-1065 and duocarmycin A, but that it is more reactive than duocarmycin SA (6 - 7x) or the direct comparison CBI-based agents (5x) for which X-ray structure comparisons served to establish the basis for their inherent reaction regioselectivity and reactivity. Resolution and synthesis of a full set of natural product analogues and subsequent evaluation of their DNA alkylation properties revealed that the iso-CBI analogues, even with the relocation of the C-4 carbonyl and the most substantial structural modifications to the alkylation subunit to date, reacted at comparable rates and retain the identical and characteristic sequence selectivity of CC-1065 and the duocarmycins. This observation is inconsistent with the proposal that a sequence-dependent C-4 carbonyl protonation by strategically located DNA backbone phosphates controls the DNA alkylation selectivity but is consistent with the proposal that it is determined by the AT-rich noncovalent binding selectivity of the agents and the steric accessibility of the N3 alkylation site. Confirmation that the DNA alkylation reaction is derived from adenine N3 addition to the least substituted carbon of the activated cyclopropane, and its quantitation (95%) was established by isolation and characterization of the depurination adenine N3 adduct. Consistent with past studies and despite the deep-seated structural change in the alkylation subunit, the agents were found to exhibit potent cytotoxic activity that correlates with their inherent reactivity.