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A new entry to adenosine analogues via purine nitration - Combinatorial
synthesis of antiprotozoal agents and adenosine receptor ligands
Rodenko, B.
Publication date
2004
Link to publication
Citation for published version (APA):
Rodenko, B. (2004). A new entry to adenosine analogues via purine nitration - Combinatorial
synthesis of antiprotozoal agents and adenosine receptor ligands.
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Summary Summary
AA new entry to adenosine analogues
viaa purine nitration
Combinatoriall synthesis of antiprotozoal agents and
adenosinee receptor ligands
Inn the body adenosine plays an important role as a regulator of many aspects of cellular metabolism.. This endogenous nucleoside is related to bioactive adenine nucleotides such as adenosinee mono-, di- and triphosphate and cyclic adenosine m o n o p h o s p h a t e and it is present ass a structural element in RNA inter alia. Adenosine mediates many of its physiological effects viaa the G-protein coupled adenosine receptors, of which four subtypes have been characterised: thee Ai, A2A, A2B and A3 receptors. In the introductory Chapter 1 the therapeutic implications off drugs acting at these adenosine receptors are indicated. Selective activation of these receptorss can be achieved by modifying the endogenous agonist adenosine. Modified adenosinee analogues also have potential as antiprotozoal therapeutics. Whereas mammalian cellss can synthesise purine nucleosides de novo, protozoan parasites are entirely d e p e n d e n t on purinee salvage from their host. These parasites have evolved intricate uptake mechanisms with aa broad tolerance for purines and purine nucleosides, including unnatural, potentially cytostaticc nucleoside analogues. Parasitic diseases, like malaria and the African sleeping
OH H HO O Adenosine e
sickness,, threatens nearly half the world's population. Drugs currently used for the treatment off these diseases often have serious side effects and are becoming increasingly useless due to thee rapid evolution of drug resistance. Obviously, the development of new antiparasitic drugs iss of cardinal importance. Progress in molecular biology allows for the high-throughput screeningg of drug candidates. Acceleration of the costly drug development process demands t h ee production of a large a m o u n t of compounds in a short period. Combinatorial parallel synthesiss either in solution or on solid phase is generally considered as the answer, which makess the development of new, fast sorting synthetic methodology a highly relevant matter.
Inn C h a p t e r 2 the development of the first reported library of nucleoside monomers entirely preparedd o n a solid support is described. In this case the nucleoside is attached to the solid s u p p o r tt by an ester linkage between the nucleoside 5'-hydroxyl group and a carboxyl functionalisedd polystyrene resin. Nitration of the purine ring in 1 furnished
2-nitro-6-^ ~ - 0 ' ' CI I
6:; ;
NN fo— —
PGO O 1 1 i i J J OPG G nitrationn on solidd support »--CI I 0 2 N ^ N ^ ^ PGO O 2 2 OPG OPG selective e substitution n deprotection n andd cleavage NHR1 1 R2H N ^ N ^ N No— —
H O ^ N f ^ ^
HO O 3 3I I
OH Hchloropurinee nucleoside 2, a highly reactive difunctionalised species. Amines were selectively introducedd by 6-chloro displacement at room temperature without affecting the 2-nitro group. Subsequentt substitution of the 2-nitro group by amines was achieved at 80-90 °C. Removal of thee riboside protective groups u n d e r mildly acidic conditions, followed by cleavage of the nucleosidess from the resin, yielded 2,N6-disubstituted adenosine analogues 3.
Too expand the solid phase methodology to the modification of the ribosyl moiety, a sequencee was developed involving the catch principle, described in C h a p t e r 3. A safety-catchh linker remains inert during the solid supported diversification steps and can be 'switched
NHR R ww H H OH H HO O 4 4 [ox] ] NHR R X ^ N ^ N N R"-NH2 2 NHR R R"HN N OH H XX = Hor NHR'
Summary y
o n '' at will, to allow fot cleavage of the substrate from the resin. A 5'-carboxylate nucleoside scaffoldd is attached to the solid support via an aryl hydrazide linkage. Following diversification reactionss at the purine system and removal of the 2',3'-protective groups, the hydrazide linkage inn 4 is oxidised. The resulting acyl diazene species 5 reacts in situ with amines present, t h u s releasingg 5'-carboxamidoadenosine analogues 6. Two small libraries were synthesised composed off 5',N6-disubstituted and 2,5',N6-trisubstituted carboxamidoadenosine analogues.
Chapterr 4 deals with the construction of conformationally restricted adenosine analogues,
makingg use of macrocyclisations involving the nitro substitution reactions, that were so fruitfullyy applied in the solid supported syntheses described in the preceding chapters. Two typess of conformationally restricted adenosine analogues were synthesised. Type I contains a
NH2 2
v W W
XX
o o
II I XX = COCH2 ~OHH X = NHCO HO O 8 8tetherr between N6 and C2, allowing for the spatial confinement of pharmacophores. Type II containss a chain connecting C 5 ' and C2, thereby covalently restricting the nucleoside in the
synsyn conformation. Binding studies at adenosine receptors revealed A3 selectivity of nucleosides
off type I, while the complete absence of receptor affinity of the syn restricted adenosine analoguess II confirmed that binding to the receptor requires the anti conformation.
T h ee in vitro antiprotozoal evaluation of the adenosine analogues that were synthesised as discussedd in the preceding chapters is described in Chapter 5. Several compounds were identifiedd that displayed significant growth inhibitory activity against Trypanosoma brucei
rhodesiense,rhodesiense, the parasite that causes African sleeping sickness, and against Plasmodium falciparum,
thee parasite that is responsible for the most lethal form of malaria.
Thee versatile purine nitration reaction constitutes the key step in the synthetic strategies describedd in this thesis. T h e mechanism of the purine nitration with a mixture of tetrabutylammoniumm nitrate and trifluoroacetic anhydride was elucidated by using NMR spectroscopy,, as reported in Chapter 6. Extensive monitoring of the nitration of 9 excluded directt nitration of the highly electrophilic C2 position and demonstrated that this reaction occurredd in a three step process. Electrophilic attack by trifluoroacetyl nitrate (TFAN) o n the purinee N7 position results in a n i t r a m m o n i u m species that is trapped by a trifluoroacetate anionn furnishing nitramine intermediate 10. A subsequent nitramine rearrangement generates C2-nitroo species 11 that immediately eliminates TEA to give 2-nitro-6-chloro purine 12. The
I I R R 9 9 TFAN N N 02 2 OTFA A 02NN N Cl l -OTFA A N N I I R R 11 1 Cl l N N Q2N N N N I I R R 12 2 nitraminee rearrangement
involvementt of radicals during the nitramine rearrangement was unequivocally established by
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