Synthesis of 4′-substituted 2′-deoxy-4′-thiocytidines and its evaluation for antineoplastic and antiviral activities

Preparation of a 4'-Thiouridine Building-Block for Solid-Phase Oligonucleotide Synthesis

Starting from a commercially available thioether, we report a nine-step synthesis of a 4'-thiouridine phosphoramidite building-block. We install the uracil nucleobase using Pummerer-type glycosylation of a sulfoxide intermediate followed by a series of protecting group manipulations to deliver the desired phosphite. Notably, we introduce a 3',5'-O-di-tert-butylsilylene protecting group within a 4'-thiosugar framework, harnessing this to ensure regiospecific installation of the 2'-O-silyl protecting group. We envisage this methodology will be generally applicable to other 4'-thionucleosides and duly support the exploration of their inclusion within related nucleic acid syntheses. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: (2R,3S,4R)-2,3-O-Isopopropylidene-5-O-tert-butyldiphenylsilyl-1-(4-sulfinyl)cyclopentane: Sulfoxidation Basic Protocol 2: 2',3'-O-Isopropylidene-5'-O-tert-butyldiphenylsilyl-4'-thiouridine: Pummerer glycosylation Basic Protocol 3: 4'-Thiouridine: Deprotection Basic Protocol 4: 2'-O-tert-Butyldimethylsilyl-3',5'-di-tert-butylsiloxy-4'-thiouridine: 2',3',5'-O-silylation Basic Protocol 5: 2'-O-tert-Butyldimethylsilyl-4'-thiouridine: Selective 3'-5'-desilylation Basic Protocol 6: 2'-O-tert-Butyldimethylsilyl-5'-O-dimethoxytrityl-4'-thiouridine: 5'-O-dimethoxytritylation Basic Protocol 7: 2'-O-tert-butyldimethylsilyl-3'-O-[(2-cyanoethoxy)(N,N-diisopropylamino)phosphino]-5'-O-dimethoxytrityl-4'-thiouridine: 3'-O-phosphitylation.

Substitution of α-Azido Sulfones with Thiolates to Form α-Azido Sulfides

Upon treatment of α-azido sulfones with a thiol in the presence of 1,1,3,3-tetramethylguanidine, substitution of the sulfonyl group with a thiolate occurred, resulting in the formation of α-azido sulfides. Based on experimental results and DFT calculations, a reaction mechanism that involves the addition of a thiolate to the azido group and generation of an alkylidene triazene is proposed.

DAST-Mediated Fluorination of 1-[4-Thio-β-d-arabinofuranosyl]uracil: Investigation of Thiolane vs Thietane Formation and Stereoselective Synthesis of 4′-ThioFAC

The unprecedented DAST-mediated (DAST = diethylaminosulfur trifluoride) deoxygenative fluorination of benzoyl-, TBDPS-, and Bn-protected 1-(β-d-4-thioarabinofuranosyl)uracil at the sugar portion was examined. Three kinds of nucleoside (Ns) products were formed: target thiolane Ns, ring-contracted thietane Ns, and anhydro Ns products. The reaction pathway was determined by the electronic effect of the protecting groups at the sugar and base moieties. The benzoylated uracil starting material gave the 2,2′-anhydronucleoside (anhydro Ns) as a major product, whereas the silylated and benzylated starting materials furnished the corresponding fluorinated products, in which the ring-contracted thietanes predominantly formed. The desired thiolane Ns could be obtained as major product by the addition of a pyridine derivative as an additive. Upon reacting N 3-benzoylated 1-(β-d-4-thioarabinofuranosyl)uracil with DAST in the presence of 2,4,6-collidine, the target 2′-deoxy-2′-β-fluoro-4′-thiouracil nucleoside could be obtained in 72% isolated yield along with the corresponding thietane Ns (7%) and anhydro Ns (3%) (thiolane Ns/thietane Ns/anhydro Ns = 10.3:1.00:0.43), with recovery of the starting material (12%). In this study, the first stereoselective synthesis of the β-anomer of 1-(2-deoxy-2-fluoro-4-thio-β-d-arabino-pentofuranosyl)cytosine (4′-thioFAC) has been developed.

Chemical synthesis of 4'-thio and 4'-sulfinyl pyrimidine nucleoside analogues

Analogues of the canonical nucleosides required for nucleic acid synthesis have a longstanding presence and proven capability within antiviral and anticancer research. 4'-Thionucleosides, that incorporate bioisosteric replacement of furanose oxygen with sulfur, represent an important chemotype within this field. Established herein is synthetic capability towards a common 4-thioribose building block that enables access to thio-ribo and thio-arabino pyrimidine nucleosides, alongside their 4'-sulfinyl derivatives. In addition, this building block methodology is templated to deliver 4'-thio and 4'-sulfinyl analogues of the established anticancer drug gemcitabine. Cytotoxic capability of these new analogues is evaluated against human pancreatic cancer and human primary glioblastoma cell lines, with observed activities ranging from low μM to >200 μM; explanation for this reduced activity, compared to established nucleoside analogues, is yet unclear. Access to these chemotypes, with thiohemiaminal linkages, will enable a wider exploration of purine and triphosphate analogues and the application of such materials for potential resistance towards relevant hydrolytic enzymes within nucleic acid biochemistries.

Recent Advances in the Chemical Synthesis and Evaluation of Anticancer Nucleoside Analogues

Nucleoside analogues have proven to be highly successful chemotherapeutic agents in the treatment of a wide variety of cancers. Several such compounds, including gemcitabine and cytarabine, are the go-to option in first-line treatments. However, these materials do have limitations and the development of next generation compounds remains a topic of significant interest and necessity. Herein, we discuss recent advances in the chemical synthesis and biological evaluation of nucleoside analogues as potential anticancer agents. Focus is paid to 4′-heteroatom substitution of the furanose oxygen, 2′-, 3′-, 4′- and 5′-position ring modifications and the development of new prodrug strategies for these materials.