Synthesis and evaluation of 1,2,8,8a-tetrahydrocyclopropa[c]pyrrolo[3,2-e]indol-4(5H)-one, the parent alkylation subunit of CC-1065 and the duocarmycins: Impact of the alkylation subunit substituents and its implications forDNA alkylation catalysis

The synthesis of 1,2,8,8a-tetrahydrocyclopropa[c]pyrrolo[3,2-e]indol-4(5H)- one (CPI), the parent CC-1065 and duocarmycin SA alkylation subunit, is det ailed. The parent CPI alkylation subunit lacks the C7 methyl substituent of the CC-1065 alkylation subunit and the C6 methoxycarbonyl group of duocarm ycin SA, and their examination permitted the establishment of the impact of these natural product substituents. The studies revealed a CPI stability c omparable to the CC-1065 alkylation subunit but which was 6x more reactive than the (+)-duocarmycin SA alkylation subunit, and it displayed the inhere nt reaction regioselectivity (4:1) of the natural products. The single-crys tal X-ray structure of (+)-N-BOC-CPI depicts a near identical stereoelectro nic alignment of the cyclopropane accounting for the identical reaction reg ioselectivity and a slightly diminished vinylogous amide conjugation relati ve to (+)-N-BOC-DSA suggesting that the stability distinctions stem in part from this difference in the vinylogous amide as well as alterations in the electronic nature of the fused pyrrole. Establishment of the DNA binding p roperties revealed that the CPI-based agents retain the identical DNA alkyl ation selectivities of the natural products. More importantly, the C6 metho xycarbonyl group of duocarmycin SA was found to increase the rate (12-13x) and efficiency (10x) of DNA alkylation despite its intrinsic lower reactivi ty while the CC-1065 C7 methyl group was found to slow the DNA alkylation r ate (4x) and lower the alkylation efficiency (ca. 4x). The greater DNA alky lation rate and efficiency for duocarmycin SA and related analogues contain ing the C6 methoxycarbonyl is proposed to be derived from the extended leng th that the rigid C6 methoxycarbonyl provides and the resulting increase in the DNA binding-induced conformational change which serves to deconjugate the vinylogous amide and activate the alkylation subunit for nucleophilic a ttack. The diminished properties resulting from the CC-1065 C7 methyl group may be attributed to the steric impediment this substituent introduces to DNA minor groove binding and alkylation. Consistent with this behavior, the duocarmycin SA C6 methoxycarbonyl group increases biological potency while the CC-1065 C7 methyl group diminishes it.