Activation of Enediynes by Photochemical Ring Contraction: Design, Synthesis and Reactivity of Cyclic Enediynes Containing Diazodicarbonyl Moiety

Of all recently discovered antibiotics, the natural enediyne antibiotics possibly show the most promise as prospective drugs for the treatment of various forms of cancer. Unfortunately, low selectivity and high general toxicity of natural enediynes, as well as designed compounds based on essential structural features of natural enediyne antibiotics, prevented their use in clinical practice thus far. In an attempt to address this issue we employed an approach to pro-drug design that makes it possible not only to utilise stable and thus relatively non-toxic enediynes, but also to achieve a certain degree of control over their action. We have suggested to explore the feasibility of photochemical or thermal ring contraction reactions as tools to activate cyclic enediynes toward Bergman cycloaromatisation. A number of cyclic enediynes have been prepared in this work. While oxacyclic enediyne 3.29 could not be prepared in a satisfactory yield and cyclic enediyne precursors 6.14 and 6.26 were found to be thermally unstable, carbocyclic diazodicarbonyl enediynes 4.18 and 5.15 were prepared in excellent synthetic yields. We have shown that eleven-membered and bigger homologous cyclic diazoenediynediones are thermally stable at room temperature, however they undergo an efficient photochemical ring contraction reaction transforming them into smaller cyclic systems prone to Bergman cyclisation. Thermal or photochemical Wolff rearrangement of stable diazoenediynediones results in formation of unstable ring contraction products, which undergo facile Bergman cyclisation. We have also found that Wolff rearrangement of a diazodicarbonyl moiety conjugated with the extended π-system of the enediyne fragment results in formation of highly enolised oxocarboxylic acid derivatives. Addition of a functionalised double bond was found to provide enough activation to the enediyne fragment to facilitate Bergman cyclisation of eleven-membered enediynes such as 5.16a, which was found to have a lifetime of 3 hours at 28° C. A series of DNA cleavage experiments showed that diazoenediynediones 4.18 and 5.15 exhibit moderate DNA cleavage activity upon irradiation at 350 nm. In most experiments single strand cleavage was observed. Higher concentrations of the cleavage agents or longer irradiation times resulted in the appearance of double strand cleavage products