Hydroxyethylene isosters of Xaa-Pro dipeptides: synthetic approaches and new HIV-PR inhibitors

The aspartic protease (HIV-Pr) of the human immunodeficiency virus, responsible agent for AIDS, is surely one of the most studied enzymes in terms of structure and activity. HIV-Pr is responsible for cleaving the viral polyprotein precursor into structural proteins and enzymes and plays an essential role in the viral replication and maturation. HIV-Pr has thus become the target of numerous efforts to design antiviral therapeutic agents suitable for the treatment of AIDS. In the field of organic chemistry, the search for effective HIV-Pr inhibitors has boosted the development of new methodologies for the stereoselective synthesis of compounds containing multiple chiral centers, on which reversible inhibitors are generally based. HIV-Pr shows peculiar characteristics as it is able, unlike any other eukaryotic aspartic protease, to hydrolyze amide bonds with proline as the N-terminal residue. Moreover, it is active in a dimeric form, possessing C2 symmetry, in which each monomer contributes a catalytic aspartate. The first part of the present doctoral work described in Chapter 2, has been dedicated to the synthesis of hydroxyethylene Phe-Pro isosters in which the pyrrolidine ring is expanded by a condensed aromatic ring in order to provide a better fit to the enzyme's catalytic site. During the synthesis of the isoster a novel reaction was discovered in which enaminones are directly formed by treatment of ?,?-unsaturated ketones with trimethylsilylazide and fluoride. Phe-Pro isosters based on the enaminone structure showed moderate activity as HIV-Pr inhibitors. The direct amination of ?,?-unsaturated ketones is the subject of Chapter 3. This reaction is demonstrated to be general for enones containing a ?-hydrogen. A mechanism based on azide activation via formation of a pentacoordinated silicon species followed by a 1,3-dipolar cycloaddition is proposed and supported by experimental results and calculations. In Chapter 4 is reported the synthesis of a library of triazole inhibitors by a combinatorial approach based on click chemistry. The library was screened for HIV-Pr inhibition and deconvoluted. A set of promising members from the library was synthesized as single, enantiomerically pure compounds that confirmed to be active HIV-Pr inhibitors. Finally, in Chapter 5 the development of an alternative approach to dipeptide isosters, based on the ring closing metathesis of aminoacid-derived allylamines, is described. Building of the four carbon atom backbone of the isosteres is obtained after mounting the olefins on designed linkers that allow selectivity in the cross metathesis, and easy final cleavage. Carbamate linkers will also allow also protection of the amino groups during the next steps of the synthesis leading to the desired di- and monohydroxyethylene isosters.