Synthesis and mechanistic studies of a mitomycin dimer containing an eight-membered cyclic disulfide

Dimeric DNA alkylating agents have drawn significant interest because these compounds are expected to provide at least two reactive sites and as a result, generate enhanced levels of DNA interstrand cross-link (DNA ISC) adducts compared to their monomeric agents. We report the synthesis and mechanistic studies of a novel mitomycin dimer, 7-N,7'-N'-(1″,2″-dithiocanyl-3″,8″-dimethylenyl)bismitomycin C (8) connected by an eight-membered cyclic disulfide. Mitomycins require prior activation (i.e., transformation to a good electrophile) for DNA adduction and therefore, 8 was aimed to undergo facile nucleophilic activation and produce enhanced levels of DNA ISC. At the core of this function lies a cyclic disulfide in 8. It was expected that disulfide cleavage by an appropriate nucleophile would successively produce two thiols that may trigger activation of two mitomycin rings in a dimer through intramolecular cyclization to quinine rings. Compound 8 was synthesized from mitomycin A (1) and the key intermediate, cyclic disulfide (11), along with the reference diol mitomycin 7-N,7'-N'-(2″,7″-dihydroxy-1″,8″-octanediyl)bismitomycin C (23) which does not contain the disulfide unit. We found that 8 underwent significantly enhanced nucleophilic activation in the presence of Et(3)P compared with 23, and that the disulfide unit in 8 played a key role for the nucleophilic activation. Based on these findings, we proposed a mechanism for nucleophilic activation of 8. We further demonstrated that 8 generated much higher levels of DNA ISC (94%) compared with 23 (4%) and 2 (3%) in the presence of Et(3)P (and L-DTT) leading to the conclusion that 8 is more efficient for DNA ISC processes than 23 and 2 due to the role of disulfide unit.