Stereoselective functionalization of the 1'-position of 4'-thionucleosides

S-Ribosylhomocysteine analogs containing a [4-thio]ribose ring

The [4-thio]-S-ribosylhomocysteine (SRH) analogs containing substitution of a sulfur atom for the endocyclic oxygen were synthesized by coupling of the 4-thioribose substrates with a thiolate generated from the protected homocysteine. Coupling of the protected 1-deoxy-5-O-mesyl-S-oxo-4-thio-D-ribofuranose with homocysteinate salt gave the C4 epimers of [4-thio]-SRH at the sulfoxide oxidation level lacking a hydroxyl group at anomeric carbon. Treatment of these sulfoxides with BF3⋅Et2O/NaI affected simultaneous reduction to sulfide and global deprotection affording 1-deoxy-4-thio-SRH analog. Treatment of the protected 1-deoxy-S-oxo-4-thio-D-ribofuranose sulfoxide with DAST/SbCl3 resulted in the fluoro-Pummerer rearrangement to give 4-thio-β-D-ribofuranosyl fluoride. Mesylation of the latter at 5-hydroxyl position followed by coupling with homocysteinate salt and subsequent global deprotection with trifluoroacetic acid afforded [4-thio]-SRH thiohemiacetal.

Lead tetraacetate mediated one pot oxidative cleavage and acetylation reaction: an approach to apio and homologated apio pyrimidine nucleosides and their anticancer activity

An efficient and versatile strategy towards apio and homologated apio pyrimidines has been described via one pot oxidative cleavage and acetylation using Pb(OAc)₄.

Synthesis of 4'-Ethynyl-2'-deoxy-4'-thioribonucleosides and Discovery of a Highly Potent and Less Toxic NRTI

The synthesis of 4'-ethynyl-2'-deoxy-4'-thioribonucleosides was carried out utilizing an electrophilic glycosidation in which 4-ethynyl-4-thiofuranoid glycal 16 served as a glycosyl donor. Electrophilic glycosidation between 16 and the silylated nucleobases (N⁴-acetylcytosine, N⁶-benzoyladenine and N²-acetyl-O⁶-diphenylcarbamoylguanine) was carried out in the presence of N-iodosuccinimide (NIS) leading to the exclusive formation of the desired β-anomers 29, 33 and 36. Anti-HIV studies demonstrated that these 4'-thio nucleosides were less cytotoxic to T-lymphocyte (i.e. MT-4 cells) than the corresponding 4'-ethynyl derivatives of 2'-deoxycytidine (44), 2'-deoxyadenosine (45) and 2'-deoxyguanosine (46). Comparison of the selectivity indices (SI) was made between 4'-thionucleosides (32, 41 and 43) and the corresponding 4'-oxygen analogues 44-46 by using the reported CC₅₀ and EC₅₀ values. In the case of cytosine and adenine nucleosides, comparable SI values were obtained: 32 (545) and 45 (458); 41 (>230) and 45 (1,630). In contrast, 4'-ethynyl-2'-deoxy-4'-thioguanosine 43 was found to possess a SI value of >18,200, which is twenty times better than that of 46 (933).

Structure-activity relationships of truncated D- and l-4'-thioadenosine derivatives as species-independent A3 adenosine receptor antagonists

Novel D- and l-4'-thioadenosine derivatives lacking the 4'-hydroxymethyl moiety were synthesized, starting from d-mannose and d-gulonic gamma-lactone, respectively, as potent and selective species-independent A 3 adenosine receptor (AR) antagonists. Among the novel 4'-truncated 2-H nucleosides tested, a N(6)-(3-chlorobenzyl) derivative 7c was the most potent at the human A 3 AR (K i = 1.5 nM), but a N(6)-(3-bromobenzyl) derivative 7d showed the optimal species-independent binding affinity.

Stereoselective synthesis of 4'-selenonucleosides using the Pummerer glycosylation reaction

The syntheses of four selenonucleosides, namely 4'-beta-selenoadenosine, -cytidine, -thymidine, and -uridine are described. Commercially available D-ribonolactone was converted to the key intermediate 1,4-anhydro-4-seleno-D-ribitol in seven steps in overall excellent yield. Oxidation of the seleno-d-ribitol with MCPBA gave a single diastereomeric selenoxide in excellent yield, which upon Pummerer reaction in the presence of silylated purine or pyrimidine bases gave stereoselectively the corresponding 4'-beta-selenonucleosides. The stereochemistry at the anomeric center was determined by means of 1D-NOE experiments.

Design and synthesis of truncated 4'-thioadenosine derivatives as potent and selective A3 adenosine receptor antagonists

We have established structure-activity relationships of novel truncated D-4'-thioadenosine derivatives from D-mannose as potent and selective A(3) adenosine receptor (AR) antagonists. At the human A(3) AR, most of N(6)-substituted analogues showed high potency and selectivity and acted as pure antagonists in a cyclic AMP functional assay. Among compounds tested, 2-chloro-N(6)-3-chlorobenzyl and N(6)-3-chlorobenzyl analogues displayed very high binding affinities (K(i) = 1.66 nM and 1.5 nM, respectively) at the human A(3) AR. Truncated 4'-thioadenosine derivatives studied here are regarded as an excellent template for the design of novel A(3) AR antagonists to act at both human and murine species.

Discovery of a new nucleoside template for human A3 adenosine receptor ligands: D-4'-thioadenosine derivatives without 4'-hydroxymethyl group as highly potent and selective antagonists

Truncated D-4'-thioadenosine derivatives lacking the 4'-hydroxymethylene moiety were synthesized starting from D-mannose, using cyclization to the 4-thiosugar and one-step conversion of the diol to the acetate as key steps. At the human A3 adenosine receptor (AR), N6-substituted purine analogues bound potently and selectively and acted as antagonists in a cyclic AMP functional assay. An N6-(3-chlorobenzyl)purine analogue 9b displayed a Ki value of 1.66 nM at the human A3 AR. Thus, truncated D-4'-thioadenosine is an excellent template for the design of novel A3 AR antagonists to act at both human and murine species.