Studies on Hopf Cyclization in Cyclic-Aza and Oxa-Dieneyne Systems and Exploring their DNA Cleavage Properties

This paper describes, for the first time, the synthesis of cyclic aza- and oxa-dieneynes through intramolecular aza-Wittig, Wittig, and lactonization reactions as key steps. The synthesized 9- and 10-membered cyclic aza-compounds (75 and 81) undergo Hopf Cyclization (HC) under normal conditions, resulting in hydroxy dihydroisoquinoline derivatives. The 9- membered aza-analogue 75 exhibits DNA cleavage properties. We have also attempted the synthesis of cyclic dieneynes that incorporate an oxygen atom within their ring structure and have explored their thermal reactivity as well as the DNA cleavage properties.

Our objective was to incorporate heteroatoms, specifically nitrogen and oxygen, into the carbocyclic dieneyne structure in different arrangements. In one configuration, a nitrogen and an oxygen atom substitute for one of the carbon atoms in the saturated moiety of the chain. The purpose was to explore the impact of introducing nitrogen and oxygen heteroatoms on the Hopf Cyclization temperature within the carbocyclic dieneyne framework. Furthermore, we aimed to determine whether intermediates from these oxa- and aza-Hopf cyclizations could cleave DNA, given that some intermediates might have sufficiently long half-lives to interact with external entities, such as biomacromolecules.

The synthesis of target molecules involved palladium-catalyzed cross-coupling, followed by a series of group transfer reactions, and concluded with ring closure reactions using a high dilution technique. Subsequently, these molecules were exposed to thermal conditions to achieve the desired ring closure. We evaluated their DNA-cleavage activity by incubating the compounds with double-stranded DNA (plasmid) in millimolar concentrations.

After completing the synthesis and examining the thermochemical reactivity of azadieneynes in relation to Hopf Cyclization, we evaluated their ability to cleave DNA. Consequently, when the 9-membered aza-compound 75 was incubated, it resulted in moderate DNA cleavage at millimolar concentrations after 48 and 72 hours. In the case of oxadieneyne synthesis, the final ring closure reaction, even under conditions of extreme dilution, led to the formation of dimeric oxa-dieneynes rather than the intended monomers and did not undergo cyclization under normal conditions. The 11-membered cyclic dieneyne lactone was successfully synthesized, but instead of the anticipated radical cyclization, decomposition occurred when subjected to cyclization conditions.

These results underscore the interplay among ring size, heteroatom type, and substitution pattern in Hopf Cyclization reactions. Aza-dieneynes are promising because of their reactivity and ability to cleave DNA, whereas oxa-dieneynes are less feasible. Future studies should investigate modified substitution patterns or photochemical activation to overcome the challenges associated with oxygen incorporation. The DNA-cleaving activity of compound 75 illustrates that dieneynes with heteroatom substitutions can connect Hopf intermediates to biological reactivity. These systems can serve as templates for molecular tools in chemical biology, particularly in applications involving DNA cleavage.

We have successfully synthesized the aza analogue of the 9- and 10-membered dieneyne (75 and 81) for the first time and conducted DNA cleavage experiments. The ninemembered imine 75 exhibited moderate DNA-cleaving activity. Our approach involved initially closing the ring, followed by forming a double bond within the cyclic molecule, which was effectively applied to create the 10- and 11-membered oxa-dieneynes. However, neither of these compounds underwent HC under normal conditions. When heated to 110ºC in toluene, they decomposed.