Received December 21, 2000

Abstract:

The synthesis of 5-methoxycarbonyl-1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one (C5-CO₂Me-CBI), a substituted CBI derivative bearing a C5 methoxycarbonyl group, and its corresponding 5-hydroxymethyl derivative are described in efforts to establish substituent electronic effects on the agents' functional reactivity and the resulting effect this has on their rate of DNA alkylation. Resolution of an immediate C5-CO₂Me-CBI precursor and its incorporation into both enantiomers of 16 and 17, analogues of the duocarmycins, are also detailed. A study of the solvolysis reactivity and regioselectivity of N-BOC-C5-CO₂Me-CBI (12) revealed that the introduction of a C5 methyl ester modestly slowed the rate of solvolysis (1.8×, pH 3) without altering the inherent reaction regioselectivity (>20:1). The comparison of the X-ray structures of the N-CO₂Me derivatives of C5-CO₂Me-CBI and CBI revealed correlations with the reaction regioselectivity and the relative reactivity of the compounds. The latter correlated well with the less reactive C5-CO₂Me-CBI exhibiting a shortened N2-C2a bond length (1.386 vs 1.390 Å) and smaller ₁ dihedral angle (8.1 vs 21.2) indicative of greater vinylogous amide conjugation and was accompanied by a diminished (cross-conjugated) cyclopropane conjugation (shorter bond lengths). Establishment of the DNA alkyation properties revealed that C5-CO₂Me-CBI-based agents retained the identical alkylation selectivity of the natural products. More importantly, the C5 methyl ester was found to decrease the rate (0.77×) of DNA alkylation relative to CBI, consistent with its inherent lower reactivity. These results indicate that the previously observed increase in the rate of DNA alkylation for C7-substituted CBI analogues including CCBI (7-cyano-CBI) is contrary to expectations based on their inherent reactivities. Unlike 17, in which the C5 methyl ester does not bind in the minor groove, the C7 substituent lies in the minor groove extending the rigid length of the agents, further enhancing the DNA binding-induced conformational change responsible for activation toward nucleophilic attack and catalysis of the DNA alkylation reaction.

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