Effect of A-ring modifications on the DNA-binding behavior and cytotoxicity of pyrrolo[2,1-c] [1,4]benzodiazepines

Several A-ring-modified analogues of the DNA-binding antitumor agent DC-81 (5) have been synthesized in order to study structure-reactivity/cytotoxici ty relationships. For two molecules (23 and 30) the modifications required the addition of a fourth ring to give the novel dioxolo-[4,5-h]- and dioxan o[5,6-h]pyrrolo[2,1-c][1,4]benzodiazepin-11-one (PBD) ring systems, respect ively. Another three analogues (34, 38, and 48) have the native benzenoid A -ring replaced with pyridine, diazine, or pyrimidine rings to give the nove l pyrrolo[2,1-c][1,4]pyridodiazepine, pyrrolo[2,1-c][1,4]diazinodiazepine, and pyrrolo[2,1-c][1,4]pyrimidinodiazepine systems, respectively. The other new analogues (16a,b) have extended chains at the C8-position of the DC-81 structure. During the synthesis of these compounds, a novel tin-mediated r egiospecific cleavage reaction of the dioxole intermediate 18 was discovere d, leading to the previously unknown iso-DC-81 (20). In addition, an unusua l simultaneous nitration-oxidation reaction of 4-(3-hydroxypropoxy)-3-metho xybenzoic acid (8) was found to produce 3-(4-carboxy-2-methoxy-5-nitropheno xy)propanoic acid (9), a key intermediate, in high yield. In general, the r esults of cytotoxicity and DNA-binding studies indicated that none of the c hanges made to the A-ring of the PBD system significantly improved either b inding affinity or cytotoxicity in comparison to DC-81. This result suggest s that the superior potency of natural products such as anthramycin (1), to maymycin (2), and sibiromycin (3) is due entirely to differences in C-ring structure, and in particular exo or endo unsaturation at the C2-position an d C2-substituents containing unsaturation. This study also provided informa tion regarding the influence of A-ring substitution pattern on the relative stability of the interconvertible N10-C11 carbinolamine, carbinolamine met hyl ether, and imine forms of PBDs.