Stereoselective preparation of (RP)-8-hetaryladenosine-3′,5′-cyclic phosphorothioic acids

Bond Formation at C8 in the Nucleoside and Nucleotide Purine Scaffold: An Informative Selection

This paper presents methods for the introduction and exchange of substituents in a nucleobase and its nucleosides and nucleotides with emphasis on the C8-position in the purine skeleton. The nucleobase is open for electrophilic and nucleophilic chemistry. The nucleophilic chemistry consists mainly of displacement reactions when the C8-substituent is a good leaving group such as a halogen atom. The heteroatom in amines, sulfides, or oxides is a good nucleophile. Halides are good reaction partners. Metal-promoted cross-coupling reactions are important for carbylations. Direct oxidative metalation reactions using sterically hindered metal amides offer chemo- and regio-selectivity besides functional tolerance and simplicity. The carbon site is highly nucleophilic after metalation and adds electrophiles resulting in chemical bond formation. Conditions for metal-assisted reactions are described for nucleobases and their glycosides.

Azolo substitution into the purine scaffold in nucleoside cyclic 3',5'-phosphorothioates

Azolation in the 8-position in the purine scaffold of cAMP (adenosine 3′,5′-cyclic monophosphate) and cAMPS (adenosine 3′,5′-cyclic monophosphorothioate) provided derivatives with an azole ring directly attached to the purine via an annular azole nitrogen. Electrophilic bromination in the 8-position was followed by nucleophilic substitution with metalated azoles to afford 8-imidazo and 8-triazolo derivatives. The substrates were appropriately protected (Sp)-3′,5′-cyclic N-benzylphosphoramidate. A subsequent carbon disulfide promoted thiation reaction afforded corresponding (Rp)-8-azolo-3′.5′-cAMPS products. The reactions were stereoselective. The products as tri-n-butylammonium salts were soluble in organic solvents and were purified by chromatography. The ammonium salts were converted to sodium salts.

Graphical abstract

Preparative Synthesis of an R P-Guanosine-3',5'-Cyclic Phosphorothioate Analogue, a Drug Candidate for the Treatment of Retinal Degenerations

Cyclic guanosine monophosphorothioate analogue 1a is currently showing potential as a drug for the treatment of inherited retinal neurodegenerations. To support ongoing preclinical and clinical work, we have developed a diastereoselective synthesis via cyclization and sulfurization of the nucleoside 5′-H-phosphonate monoester, which affords the desired R_P-3′,5′-cyclic phosphorothioate in 9:1 ratio to the undesired S_P-diastereomer. This route was made viable as a result of the silyl protection sequence used, which achieved >80% selectivity for 2′,5′-hydroxyls over 3′,5′-hydroxyls. Finally, the chromatography-free process allowed for a scale-up, as intermediates and the final product were isolated by crystallization to give 125 g of 1a (13.8% total yield) with over 99.9% HPLC purity.

Evaluation and Development of Methodologies for the Synthesis of Thiophosphinic Acids

Thiophosphorus acids R¹R²P(S)OH constitute an important class of organophosphorus compounds, in which the phosphorus atom is intrinsically chiral if R¹ ≠ R². In connection with a project aimed at the preparation of chiral thiophosphorus acids, various available literature methods were considered, but few fit the requirement of odorless reagents. Herein, the results of our studies on the synthesis of thiophosphinic acids are reported. Ultimately, two major approaches were selected: (1) the Stec reaction of phosphorus amides with carbon disulfide; and (2) the one-pot synthesis of thiophosphorus acids from H-phosphinates, an organometallic nucleophile, and quenching with elemental sulfur. An application to the preparation of a potential chiral phosphorus organocatalyst is also reported.

cAMPS derivatives. A minireview over synthetic medicinal chemistry

Cyclic nucleosides belonging to the cAMPS system can exert antagonistic or agonistic effects on the cAMP/PKA type 1 inhibitory pathway affecting T-lymphocyte replications. The stereochemistry at the phosphorus atom in the phosphate group is important for the expressed selectivity. The two stereoisomers at the phosphorus atom in the phosphate arise by selective replacements of one of the oxygens pendant from the phosphorus atom by a sulfur atom. Methods for the preparation of cAMPS derivatives as stereochemical mixtures at the phosphorus atom and separation of stereoisomers have been developed into highly stereoselective syntheses. Methods for halogenation in the purine 8-position afford corresponding halides. Heteronucleophilic substitution of the halides afford corresponding amines, ethers or sulfides. Transition metal catalysis for carbylation of the 8-halides affords simple and efficient routes for the preparation of 8-aryl, 8-hetaryl or 8-alkyl cAMPS derivatives. Preparations of prodrugs for improved cell membrane penetration are described. The prodrugs are S-alkylated derivatives which are constructed for selective cleavage of the SP bond by an esterase to regenerate the bioactive cAMPS species at the site of the desired action.

Two single-enantiomer amidophosphoesters: a database study on the chirality of (O)2P(O)(N)-based structures

The crystal structures of two single-enantiomer amidophosphoesters with an (O)₂P(O)(N) skeleton, i.e. diphenyl [(R)-(+)-α-methylbenzylamido]phosphate, (I), and diphenyl [(S)-(-)-α-methylbenzylamido]phosphate, (II), both C₂₀H₂₀NO₃P, are reported. In both structures, chiral one-dimensional hydrogen-bonded architectures, along [010], are mediated by N-H...OP interactions. The statistically identical assemblies include the noncentrosymmetric graph-set motif C(4) and the compounds crystallize in the chiral space group P2₁. As a result of synergistic co-operation from C-H...O interactions, a two-dimensional superstructure is built including a noncentrosymmetric R₄⁴(22) hydrogen-bonded motif. A Cambridge Structural Database survey was performed on (O)₂P(O)(N)-based structures in order to review the frequency of space groups observed in this family of compounds; the hydrogen-bond motifs in structures with chiral space groups and the types of groups inducing chirality are discussed. The ^2,3J_X-P (X = H or C) coupling constants from the NMR spectra of (I) and (II) have been studied. In each compound, the two diastereotopic C₆H₅O groups are different, which is reflected in the different chemical shifts and some coupling constants.

Protein kinase A antagonist inhibits β-catenin nuclear translocation, c-Myc and COX-2 expression and tumor promotion in Apc(Min/+) mice

Background

The adenomatous polyposis coli (APC) protein is part of the destruction complex controlling proteosomal degradation of β-catenin and limiting its nuclear translocation, which is thought to play a gate-keeping role in colorectal cancer. The destruction complex is inhibited by Wnt-Frz and prostaglandin E₂ (PGE₂) - PI-3 kinase pathways. Recent reports show that PGE₂-induced phosphorylation of β-catenin by protein kinase A (PKA) increases nuclear translocation indicating two mechanisms of action of PGE₂ on β-catenin homeostasis.

Findings

Treatment of Apc^Min/+ mice that spontaneously develop intestinal adenomas with a PKA antagonist (Rp-8-Br-cAMPS) selectively targeting only the latter pathway reduced tumor load, but not the number of adenomas. Immunohistochemical characterization of intestines from treated and control animals revealed that expression of β-catenin, β-catenin nuclear translocation and expression of the β-catenin target genes c-Myc and COX-2 were significantly down-regulated upon Rp-8-Br-cAMPS treatment. Parallel experiments in a human colon cancer cell line (HCT116) revealed that Rp-8-Br-cAMPS blocked PGE₂-induced β-catenin phosphorylation and c-Myc upregulation.

Conclusion

Based on our findings we suggest that PGE₂ act through PKA to promote β-catenin nuclear translocation and tumor development in Apc^Min/+ mice in vivo, indicating that the direct regulatory effect of PKA on β-catenin nuclear translocation is operative in intestinal cancer.

Stereoselective synthesis of (RP)-8-substituted-N⁶-acylated and N⁶-alkylated adenosine-3',5'-cyclic phosphorothioic acids as cAMP antagonists

N(6)-Monoalkylated, N(6)-dialkylated and N(6)-acylated (R(P))-adenosine 3',5'-cyclic phosphorothioic acids have been prepared by stereoselective syntheses from cAMP for a study of protein kinase A antagonist activity. The antagonist activity of the parent primary 6-amino cAMP derivative was reduced after N-monoalkylation. No significant activity was detected in the N,N-dialkylated derivative. Mono N-acylation had little effect on the activity. Hydrogen bonding involving the 6-amino group in cAMPS seems necessary for activity.

The synthesis of the 8-C-substituted 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP) derivatives by diverse cross-coupling reactions

Diisopropyl 8-bromo-2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine was used as a starting material for the synthesis of the 8-C-substituted 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP) analogues. A systematic screening of diverse cross-coupling reactions was carried out. Stille, Suzuki–Miyaura, Negishi, and Sonogashira cross-couplings, as well as Pd-catalysed reactions with trialkylaluminiums, were employed for the introduction of various alkyl, alkenyl, alkynyl, aryl, and hetaryl substituents to the C-8 position of the 2,6-diaminopurine moiety. In contrast to the potent parent compound PMEDAP, which exhibits potent antiretroviral and antitumor activity, none of the sixteen newly synthesized 8-C-substituted analogues of PMEDAP showed any specific antiviral activity.