Design and synthesis of nitrogen containing heterocyclic fragments via isocyanide chemistry and their biological evaluation as potential inhibitors of HIV-1 replication

Five membered nitrogen-containing heterocycles exhibit a wide range of biological activities. In this study we set out to demonstrate the utility of isocyanides in the synthesis of these heterocycles, and to identify hit scaffolds demonstrating the ability to disrupt important HIV-1host protein-protein interactions. In this project study, isocyanide synthons were used to generate different libraries of five membered nitrogen-containing heterocycles. In the first part of the project (Chapter 2), para-toluenesulfonylmethyl isocyanide (TosMIC) was reacted with N(arylidene)alkylamines (generated from aryl aldehydes and primary amines) under basic conditions using both a conventional and microwave-assisted van Leusen approach to generate a series of novel 5-aryl-1H-imidazole compounds. This family of 25 imidazole-based compounds were assessed for biological activity in an HIV-1 IN-LEDGF/p75 AlphaScreen assay and six compounds were found to be inhibitors of this interaction. In order to improve the potency of the hit fragments, a carboxylic acid functionality was introduced at the imidazole moiety of the hit compound to give novel 1,5-diaryl-1H-imidazole-4-carboxylic acid compounds as second generation of compounds. Biological evaluation of these carboxylic acid scaffolds in the HIV-1 IN-LEDGF/p75 AlphaScreen assay identified compounds exhibiting more than 50% inhibition, with four inhibitors surpassing the percentage inhibition of the hit scaffold. In the second part of the thesis (Chapter 3), TosMIC was reacted with various aryl aldehydes under basic conditions via a microwave-assisted van Leusen reaction to afford a set of 5-aryl-1,3oxazole derivatives. However, attempts to prepare similar scaffolds in acetonitrile afforded the oxazoline intermediates instead, which were easily converted into the corresponding 5-aryl-1,3oxazoles in one additional step. This family of 12 compounds did not significantly inhibit the HIV-1 IN-LEDGF/p75 AlphaScreen assay and the binding of antiVpu to Vpu. To improve the percentage inhibition of the original scaffolds, insertion of an N-aryl-carboxamide at the 4position of the oxazole moiety was carried out. Initial, attempts via formation of 2-isocyano-Naryl-acetamide intermediates and oxidation of N,5-aryl-4,5-dihydrooxazole-4-carboxamide intermediates met with failure. However, this second generation of novel N,5-diaryl-4carboxamide-1,3-oxazole derivatives were then synthesized successfully using benzoyl chloride and ethyl isocyanide as starting reagents. Evaluation of the desired 11 novel N,5-diaryl-4carboxamide-1,3-oxazoles in the HIV-1 IN-LEDGF/p75 assay showed that at least two compounds with the N-aryl-4-carboxamide moiety showed increased inhibition potency at 100 μM relative to the first generation of oxazoles. In the third part of the study (Chapter 4), TosMIC was reacted with ethyl 2(arylideneamino)acylate intermediates (generated from simple amino acids) under basic conditions via a van Leusen reaction to give a family of novel 5-aryl-1H-imidazoyl acylates. However, isolation of adducts derived from D-leucine proved to be impossible. A series of 5aryl-1H-imidazoyl-acylates derived from glycine were further transformed into two novel sets of 5-aryl-1H-imidazoyl-acetic acid and 5-aryl-1H-imidazoyl-acetohydrazide compounds. This family of 25 novel 5-aryl-1H-imidazo-yl-based compounds was screened for biological activity. In direct HIV-1 IN-LEDGF/p75 AlphaScreen assay, one compound was found to be active while seven compounds were identified as inhibitors of HIV-1 Vpu- host BST-2 interactions. To improve the percentage inhibition of these compounds in the HIV-1 IN-LEDGF/p75 AlphaScreen, N'arylidene and carboxamide groups were introduced which resulted in the generation of two sets of novel (E)-N'-arylidene-2-(5-aryl-1H-imidazol-1-yl)acetohydrazides and, N,5-diaryl-2-(1Himidazol-1-yl)acetamide compounds. From this library, at least three compounds with the 4-tertbutylphenyl motif were found to be a disruptor of HIV-1 IN and LEDGF/p75 interactions. In the fourth part of the study (Chapter 5), aryl isocyanide synthons were reacted with 2-haloarylethanone oximes to afford a series of 3-aryl-5-amino-isoxazole derivatives in poor yields. The poor yields were attributed to the low reactivity of the isocyanide and instability of the nitrosoalkene generated in situ from 2-halo-arylethanone oxime intermediates. Better yields of 3aryl-5-amino-isoxazoles were obtained when methyl isocyanoacetate was used instead of aryl isocyanide. Additionally, treatment of 2-halo-arylethanone oxime intermediates with potassium cyanide in MeOH afforded 3-aryl-isoxazol-5-amine derivative in varying yields. From this library, 10 isoxazole-based compounds were biologically evaluated in an HIV-1 IN – LEDGF/p75 interaction assay at 100 μM and, with the exception of two compounds, all the compounds displayed marginal activities. All compounds showing significant inhibitory activity in one of the protein-protein interaction assays were tested for activity in a cell based anti-HIV-1 assay. Unfortunately, none of the compounds was found to be active in this cell based assay. Lastly in Chapter 6, the synthesized libraries of the five membered nitrogen containing heterocyclic compounds were assessed for their antimicrobial activity using MIC methods. At least four compounds exhibited moderate antimicrobial activity against gram positive bacterial strains and a yeast strain.
A thesis submitted to the Faculty of Science University of the Witwatersrand Johannesburg in fulfilment for the requirements of the degree of Doctor of philosophy, 2017