Synthesis, chemical properties, and biological evaluation of CC-1065 and duocarmycin analogues incorporating the 5-methoxycarbonyl-1,2,9,9a-tetrahydrocyclopropa[c]benz[e]indol-4-one alkylation subunit

The synthesis of 5-methoxycarbonyl-1,2,9,9a-tetrahydrocyclopropa[c]benz[e]i ndol-4-one (C5-CO2Me-CBI), a substituted CBI derivative bearing a C5 methox ycarbonyl group, and its corresponding 5-hydroxymethyl derivative are descr ibed in efforts to establish substituent electronic effects on the agents' functional reactivity and the resulting effect this has on their rate of DN A alkylation. Resolution of an immediate C5-CO2Me-CBI precursor and its inc orporation into both enantiomers of 16 and 17, analogues of the duocarmycin s, are also detailed. A study of the solvolysis reactivity and regioselecti vity of N-BOC-C5-CO2Me-CBI (12) revealed that the introduction of a C5 meth yl ester modestly slowed the rate of solvolysis (1.8x, pH 3) without alteri ng 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 reactivit y of the compounds. The latter correlated well with the less reactive C5-CO 2Me-CBI exhibiting a shortened N2-C2a bond length (1.386 vs 1.390 Angstrom) and smaller chi (1) dihedral angle (8.1 degrees vs 21.2 degrees) indicativ e of greater vinylogous amide conjugation and was accompanied by a diminish ed (cross-conjugated) cyclopropane conjugation (shorter bond lengths). Esta blishment of the DNA alkyation properties revealed that C5-CO2Me-CBI-based agents retained the identical alkylation selectivity of the natural product s. More importantly, the C5 methyl ester was found to decrease the rate (0. 77x) 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 reactivi ties. Unlike 17, in which the C5 methyl ester does not bind in the minor gr oove, the C7 substituent lies in the minor groove extending the rigid lengt h of the agents, further enhancing the DNA binding-induced conformational c hange responsible for activation toward nucleophilic attack and catalysis o f the DNA alkylation reaction.