Design, synthesis and biological evaluation of 8- oxoguanine derivatives as DNA glycosylases inhibitors and efficient functionalization of 2-amino-6- chloropurines at C-8 via lithiated species

Abstract

Resistance to chemotherapy and/or radiotherapy limits the effectiveness of cancer treatment. One factor that can contribute to resistance is the process of DNA repair. Among the different DNA repair pathways, the base excision repair (BER) is a multi-step, multi-enzyme pathway that is able to recognize and correct small changes in native nucleobases of the DNA. 8-Oxoguanine DNA glycosylase (OGG1) is one of the enzymes in this pathway which removes the oxidized guanine lesion from DNA. Inhibitors of OGG1 might improve the outcome of certain cancer treatments by temporarily inhibiting the BER pathway in tumour cells and may act as adjuvants in cancer treatment. It was envisaged that 8-oxoguanine derivatives may act as OGG1 inhibitors as they contain the signature scaffold of oxidized native guanine. The present thesis is focused on the design and synthesis of 8-oxoguanine derivatives and their biological evaluation as DNA glycosylase inhibitors. Suitable synthetic strategies were developed to obtain 8-oxoguanine derivatives with various N-9 substituents. The 8-oxoguanines were efficiently synthesized using a threestep strategy: N-alkylation of guanine precursors at N-9, C-8 bromination and hydrolytic cleavage of bromide. Purines are known to give varying ratio of N-9/ N-7 regioisomers depending on the nature of substituents and methods of N-alkylation. Two guanine precursors were alkylated by different N-alkylation strategies viz. base induced alkylation, Mitsunobu coupling, and palladium catalyzed allylation. The regioisomeric outcome of these strategies was studied. In the next step, N-9-alkylated purines were brominated either by direct bromination or by a lithiation/halogenation protocol depending on the nature of the substituent and its compatibility with brominating conditions. Finally, the brominated derivatives were hydrolyzed to the target compounds, 8-oxoguanine derivatives, during which the partially hydrolyzed 6-chloro-8-oxoguanine derivatives were also isolated. The 8-oxoguanine derivatives and 6-chloro-8-oxoguanine derivatives were evaluated for their ability to inhibit OGG1 using an OGG1 assay. The synthesized compounds showed a moderate inhibitory effect on OGG1. During the course of the study, 9-alkylated 2-amino-6-chloropurine in the presence of a strong base such as LDA, gave the ring-opened products. 9-Alkylated 2-amino-6- chloropurines were functionalized at C-8 via lithiation/halogenation protocol using appropriately protected 2-amino-6-chloropurine. The scope of the C-8 lithiation was evaluated using various electrophiles. The findings of the work will be useful in choosing efficient synthetic strategies for new derivatives of 8-oxoguanines and 6-chloro-8-oxoguanines for future development of DNA glycosylase inhibitors.