Nucleic Acid Related Compounds. 94. Remarkably High Stereoselective Reductions of 2′- and 3′-Ketonucleoside Derivatives To Give Arabino, Ribo, and Xylofuranosyl Nucleosides with Hydrogen Isotopes at C2′ and C3′

A Unified Strategy to Fluorinated Nucleoside Analogues Via an Electrophilic Manifold

Herein, we present a strategy for the preparation of 3'-fluorinated nucleoside analogues via the aminocatalytic, electrophilic fluorination of readily accessible and bench-stable 2'-ketonucleosides. Initially developed to facilitate the manufacture of 3'-fluoroguanosine (3'-FG)─a substructure of anticancer therapeutic MK-1454─this strategy has been extended to the synthesis of a variety of 3'-fluoronucleosides. Finally, we demonstrate the utility of the 2'-ketonucleoside synthon as a platform for further diversification and suggest that this methodology should be broadly applicable to the discovery of novel nucleoside analogues.

Diverse Catalytic Reactions for the Stereoselective Synthesis of Cyclic Dinucleotide MK-1454

As practitioners of organic chemistry strive to deliver efficient syntheses of the most complex natural products and drug candidates, further innovations in synthetic strategies are required to facilitate their efficient construction. These aspirational breakthroughs often go hand-in-hand with considerable reductions in cost and environmental impact. Enzyme-catalyzed reactions have become an impressive and necessary tool that offers benefits such as increased selectivity and waste limitation. These benefits are amplified when enzymatic processes are conducted in a cascade in combination with novel bond-forming strategies. In this article, we report a highly diastereoselective synthesis of MK-1454, a potent agonist of the stimulator of interferon gene (STING) signaling pathway. The synthesis begins with the asymmetric construction of two fluoride-bearing deoxynucleotides. The routes were designed for maximum convergency and selectivity, relying on the same benign electrophilic fluorinating reagent. From these complex subunits, four enzymes are used to construct the two bridging thiophosphates in a highly selective, high yielding cascade process. Critical to the success of this reaction was a thorough understanding of the role transition metals play in bond formation.

A practical synthesis of xylo- and arabinofuranoside precursors by diastereoselective reduction using Corey-Bakshi-Shibata catalyst

The Corey-Bakshi-Shibata (CBS) catalyst provides an efficient mechanism to reduce ketones and achieve desired enantiopure alcohols. Herein, the diastereoselective reduction of C-2' and C-3'-keto ribofuranoside derivatives to the corresponding arabino- and xylofuranosides in greater than 95% diastereomeric excess is reported. The stereo-directed substitution with an azido group as well as the synthesis of prodrugs cytarabine and vidarabine are also described. The reported strategy offers superior diastereoselectivity, shorter reaction times, and obviates cooling required with comparable protocols involving achiral reductants.

Essential reactive intermediates in nucleoside chemistry: cyclonucleoside cations

DFT-based modeling as well as experimental examination of model keto nucleosides have revealed that high susceptibility of these compounds to acids is due to formation of intermediate cyclonucleoside cations of low energy. Theoretically established chemical structures of these previously overlooked intermediates explain the reaction courses for a cluster of nucleoside reactions.

Synthesis of D- and L-carbocyclic nucleosides via rhodium-catalyzed asymmetric hydroacylation as the key step

D- and L-carbocyclic nucleosides were obtained by a new procedure involving an enantioselective rhodium/duphos-catalyzed hydroacylation reaction as the key step. The 3-hydroxymethyl-cyclopentanol intermediate was obtained by stereoselective reduction of ketone and by dynamic kinetic resolution (DKR).

Deoxygenative [1,2]-hydride shift rearrangements in nucleoside and sugar chemistry: analogy with the [1,2]-electron shift in the deoxygenation of ribonucleotides by ribonucleotide reductases

A variant of the semipinacol rearrangement that was observed in our laboratory has been applied to the synthesis of several furanose and pyranose derivatives. The process consists of an "orchestrated" [1,2]-hydride shift with departure of a leaving group from the opposite face. Transient formation of a C=O group is followed by rapid transfer of a hydride-equivalent from the same face from which the leaving group departed, which results in double inversion of stereochemistry at the two vicinal carbon atoms. Treatment of 2'-O- and 3'-O-tosyladenosine with lithium triethylborohydride in DMSO/THF gave the respective 2'- and 3'-deoxynucleoside analogues with beta-D-threo configurations. Identical treatment of 5'-O-TPS-2'-O-tosyladenosine gave 9-(5-O-TPS-2-deoxy-beta-D-threo-pentofuranosyl)adenine. The same [1,2]-hydride shift and stereochemistry with the 5'-OH and 5'-O-TPS compounds demonstrated the absence of remote hydroxyl-group participation. Application of this process to other nucleoside 2'-O-tosyl derivatives gave the 2'-deoxy-threo compounds in good yields. The reaction-rate order was OTs approximately Br >> Cl for 2'-O-tosyladenosine, 2'-bromo-2'-deoxyadenosine, and 2'-chloro-2'-deoxyadenosine (all with beta-d-ribo configurations). Analogous results were obtained with mannopyranoside derivatives with either 4,6-O-benzylidene protection or a free OH group at C4. Deuterium labeling clearly defined the stereochemical course as a cis-vicinal [1,2]-hydride shift on the face opposite to the original cis OH and OTs groups followed by hydride transfer from the face opposite to the [1,2]-hydride shift. Synthetic and mechanistic considerations are discussed.

Reduction of ribonucleosides to 2'-deoxyribonucleosides

Ribonucleosides are converted into 2'-deoxyribonucleosides in good yields by a four-step procedure. Selective protection of the 3'- and 5'-hydroxyl groups with 1,3-dichloro-1,1,3,3-tetraisopropyl-1,3-disiloxane is followed by functionalization of the 2'-hydroxyl group with phenoxythiocarbonyl chloride. Free radical-mediated reductive C2'-O2' bond cleavage of these 3',5'-O-TPDS-2'-O-PTC-nucleoside derivatives with tributyltin hydride, followed by removal of the silyl protecting group with tetrabutylammonium fluoride, provides the 2'-deoxyribonucleosides. Adenosine, cytidine, guanosine, and uridine are converted into dA, dC, dG, and dU in overall yields of 60% to 80%. Use of tributyltin deuteride in the reductive cleavage step gives 2'-deuterio-2'-deoxyadenosine in 81% yield from adenosine with >85% retention of configuration at C2'. Application of this four-step protocol with nucleoside analogs is straightforward.

Synthesis of 2'-deuterio and 3'-deuterio cytidine 5'-diphosphate

2'-2H- and 3'-2H-CDP were synthesized from 5'-MMT-3'-O-TBDMS and 2',5'-O-diTBDMS cytidine derivatives, respectively, by oxidation followed by acidic removal of 5'-protection, reduction with [NaBD(OAc)3] and finally displacement of a tosyl group by pyrophosphate.

Stereospecific syntheses of 3'-deuterated pyrimidine nucleosides and their site-specific incorporation into DNA

[reaction: see text] 2'-Deoxy-3'-deutero pyrimidines have been synthesized in high yields and incorporated into deoxyoligonucleotides using standard phosphoramidite chemistry. A key synthetic step is a stereospecific reduction of 3'-keto nucleosides using sodium triacetoxyborodeuteride to give 3'-deuterated thymidine and 2'-deoxy uridine nucleosides. Conversion of the corresponding phorphoramidites 7a and 7b to 4-triazolo derivatives has, for the first time, enabled incorporation of 2'-deoxy-3'-deutero cytidine and 2'-deoxy-3'-deutero-5-methyl cytidine into oligonucleotides.

A practical route to 3'-amino-3'-deoxyadenosine derivatives and puromycin analogues

3'-aminoacylamino-3'-deoxyadenosines, analogues of the antibiotic puromycin, have been synthesized from adenosine. They key 3'-azido derivative 10 was obtained through a 3'-oxidation/reduction/substitution procedure. A modified purification protocol on a larger scale was developed for the oxidation step using the Garegg reagent. The coupling reaction between an Fmoc-l-amino acid and the fully protected form of 3'-amino-3'-deoxyadenosine 11 furnished the aminoacylated compounds 12 in high yields. The puromycin analogues were obtained in 10 steps and up to 23% (14c) overall yield.

The beta-fluorine effect. Electronic versus steric effects in radical deoxygenations of fluorine-containing pentofuranose nucleosides

Stereoselective pyramidalization of free radicals by a vicinal fluorine substituent, the beta-fluorine effect, was invoked to rationalize a 77:23 anti/syn ratio of 2-deuterio-1-fluorocyclopentanes obtained by radical reduction of trans-2-fluoro-1-bromocyclopentane with tributyltin deuteride (Dolbier, W. R., Jr.; Bartberger, M. D. J. Org. Chem. 1995, 60, 4984-4985). We have evaluated analogous reductions of the four possible stereoisomers of some adenine 2'(3')-fluoro-3'(2')-O-phenoxythiocarbonyl nucleoside derivatives. In all cases, the steric effect of adenine on the beta face directs deuterium transfer from the stannane to C2'(C3') on the alpha face of the furanose ring. However, the beta-fluorine effect enhances ratios of deuterium transfer anti to the vicinal fluorine substituent.

Total synthesis of 2',3',4',5',5' '-(2)h(5)-ribonucleosides: the key building blocks for NMR structure elucidation of large RNA

The diastereospecific chemical syntheses of uridine-2',3',4',5',5' '-(2)H(5) (21a), adenosine-2',3',4',5',5' '-(2)H(5) (21b), cytidine-2',3',4',5',5' '-(2)H(5)(2)H(5) (21c), and guanosine-2',3',4',5',5' '-(2)H(5) (21d) (>97 atom % (2)H at C2', C3', C4', and C5'/C5' ') have been achieved for their use in the solution NMR structure determination of oligo-RNA by the Uppsala "NMR-window" concept (refs 4a-c, 5a, 6), in which a small (1)H segment is NMR-visible, while the rest is made NMR-invisible by incorporation of the deuterated blocks 21a-d. The deuterated ribonucleosides 21a-d have been prepared by the condensation of appropriately protected aglycone with 1-O-acetyl-2,3,5-tri-O-(4-toluoyl)-alpha/beta-D-ribofuranose-2,3,4,5,5'-(2)H(5) (19), which has been obtained via diastereospecific deuterium incorporation at the C2 center of appropriate D-ribose-(2)H(4) derivatives either through an oxidation-reduction-inversion sequence or a one-step deuterium-proton exchange in high overall yield (44% and 24%, respectively).

Oxygen-dependent DNA damage amplification involving 5,6-dihydrothymidin-5-yl in a structurally minimal system

5,6-Dihydrothymidin-5-yl (1) was independently generated in a dinucleotide from a phenyl selenide precursor (4). Under free radical chain propagation conditions, the products resulting from hydrogen atom donation and radical-pair reaction are the major observed products in the absence of O(2). The stereoselectivity of the trapping process is dependent on the structure of the hydrogen atom donor. No evidence for internucleotidyl hydrogen atom abstraction by 1 was detected. The tandem lesion (17) resulting from hydrogen atom abstraction from the C1' position of the adjacent 2'-deoxyuridine by the peroxyl radical derived from 1 (3) is observed under aerobic conditions. The structure of this product is confirmed by independent synthesis and its transformation into a second independently synthesized product (24). Internucleotidyl hydrogen atom abstraction is effected selectively by the 5S-diastereomer of the peroxyl radical. The formation of dinucleotide 17 provides further support for the novel O(2)-dependent DNA damage amplification mechanism involving 1 reported previously (Greenberg, M. M.; et al. J. Am. Chem. Soc. 1997, 119, 1828).

Synthesis and cytotoxicity of 4-amino-5-oxopyrido[2,3-d]pyrimidine nucleosides

A number of nucleoside analogues have been either used clinically as anticancer drugs or evaluated in clinical studies, while new nucleoside analogues continue to show promise. In this article, we report synthesis and cytotoxicity of a series of new pyrido[2, 3-d]pyrimidine nucleosides. 2-Amino-3-cyano-4-methoxypyridine was converted, in two steps, to 4-amino-5-oxopyrido[2,3-d]pyrimidine. A variety of 1-O-acetylated pentose sugar derivatives were condensed with silylated 4-amino-5-oxopyrido[2,3-d]pyrimidine, followed by protection, to afford a series of 4-amino-5-oxopyrido[2, 3-d]pyrimidine nucleosides. Further derivatizations provided an additional group of pyrido[2,3-d]pyrimidine nucleosides. These nucleosides were evaluated for in vitro cytotoxicity to human prostate cancer (HTB-81) and mouse melanoma (B16) cells as well as normal human fibroblasts (NHF). A number of compounds (1a,b, 2a-c,f, 3f+4d) showed significant cytotoxicity to cancer cells, with 4-amino-5-oxo-8-(beta-D-ribofuranosyl)pyrido[2,3-d]pyrimidine (1b) being the most potent proliferation inhibitor (EC(50): 0.06-0.08 microM) to all types of cells tested. However, a selective inhibition to the cancer cells was observed for 4-amino-5-oxo-8-(beta-D-xylofuranosyl)pyrido[2,3-d]pyrimidine (2b), which is a potent inhibitor of HTB-81 (EC(50): 0.73 microM) and has a favorable in vitro selectivity index (28).

The synthesis of deuterionucleosides

The synthesis of deuterionucleosides for site-specific incorporation into oligo-DNA or -RAA is herein reviewed for NMR or biological studies. The review covers the following aspects: (i) deuteration of the aglycone; (ii) single-site chemical deuteration of the sugar residues; (iii) multiple-site chemical deuteration of the sugar residues; (iv) enzymatic synthesis of deuterated nucleosides or nucleotides; and (v) synthesis of labelled nucleosides with multiple isotopes

Diastereofacial Selective Addition of Ethynylcerium Reagent and Barton-McCombie Reaction as the Key Steps for the Synthesis of C-3'-Ethynylribonucleosides and of C-3'-Ethynyl-2'-deoxyribonucleosides

We describe the preparation of 3'-alkynyluridine 4a and -adenosine 4b and of 3'-alkynyl-2'-deoxyuridine 16a and -adenosine 16b starting from the corresponding nucleosides. The desired stereochemistry of the C-3' tertiary alcohol was obtained by reaction of an ethynylcerium-lithium reagent on a 3'-ketonucleoside with the hydroxyl group at C-5' unprotected. The 2'-deoxygenation was performed by a Barton-McCombie reaction under appropriate conditions where the addition of tin hydride to the triple bond was suppressed. Evaluation of the anti-HIV activity of the C-3' modified nucleosides is reported.