Stereo- and Regioselective Introduction of 1- or 2-Hydroxyethyl Group via Intramolecular Radical Cyclization Reaction with a Novel Silicon-Containing Tether. An Efficient Synthesis of 4'alpha-Branched 2'-Deoxyadenosines(1)

Generation of 4'-Carbon Radicals via 1,5-Hydrogen Atom Transfer for the Synthesis of Bridged Nucleosides

The rapid and facile generation of 4'-carbon radicals from oxime imidates of nucleosides via 1,5-hydrogen atom transfer induced by iminyl radicals was developed. The cyclization of 4'-carbon radicals with olefins, followed by the hydrolysis of imidate residues, provided various 2'-O,4'-C- and 3'-O,4'-C-bridged nucleosides. This operationally simple approach can be applied to the few-step syntheses of 6'S-methyl-2'-O,4'-C-ethylene-bridged 5-methyluridine (6'S-Me-ENA-T) and S-constrained ethyl-bridged 5-methyluridine (S-cEt-T).

[Medicinal Chemical Studies Based on the Theoretical Design of Bioactive Compounds]

I have engaged in medicinal chemical studies based on the theoretical design of bioactive compounds. First, I present a three-dimensional structural diversity-oriented conformational restriction strategy for developing bioactive compounds based on the characteristic steric and stereoelectronic features of cyclopropane. Using this strategy, various biologically active small molecule compounds, such as receptor agonists/antagonists and enzyme inhibitors, were effectively developed. The strategy was also applied to develop versatile peptidomimetics and membrane-permeable cyclic peptides. Next, studies on Ca²⁺-mobilizing second messengers, cyclic ADP-ribose (cADPR) and myo-inositol trisphosphates (IP₃), are described. In these studies, stable equivalents of cADPR were developed, since biological studies of cADPR have been limited due to its instability. Various potent IP₃ receptor ligands, which were designed using the d-glucose structure as a bioisostere of the myo-inositol moiety of IP₃, have been identified. Organic chemistry studies have also been extensively performed, because excellent organic chemistry is essential for promoting high-level medicinal chemical studies. For examples, new methods for the synthesis of chiral cyclopropanes, new radical reactions with silicon tethers, and kinetic anomeric effect-dependent stereoselective glycosidations have been developed.

1,2-Boron Shifts of β-Boryl Radicals Generated from Bis-boronic Esters Using Photoredox Catalysis

1,2-Bis-boronic esters are versatile intermediates that enable the rapid elaboration of simple alkene precursors. Previous reports on their selective mono-functionalization have targeted the most accessible position, retaining the more hindered secondary boronic ester. In contrast, we have found that photoredox-catalyzed mono-deboronation generates primary β-boryl radicals that undergo rapid 1,2-boron shift to form thermodynamically favored secondary radicals, allowing for selective transformation of the more hindered boronic ester. The pivotal 1,2-boron shift, which has been demonstrated to be stereoretentive, enables access to a wide range of functionalized boronic esters and has been applied to highly diastereoselective fragmentation and transannular cyclization reactions. Furthermore, its generality has been shown in a radical cascade reaction with an allylboronic ester.

Tethered 1,2-Si-Group Migrations in Radical-Mediated Ring Enlargements of Cyclic Alkoxysilanes: An EPR Spectroscopic and Computational Investigation

5- to 6-member ring enlargements of 3-oxa-2-silacyclopentylmethyl to 4-oxa-3-silacyclohexyl radicals were investigated by EPR spectroscopy and QM computations of model indano-oxasilacyclopentane and oxasilinanyl compounds. Both experimental and computational evidence favored a mechanism via a concerted 1,2-migration of the "tethered" Si-group. Thus, the "forbidden" 1,2-Si-group migration from carbon to carbon becomes allowed when the Si-group is "tethered". The EPR data from 3-oxa-2-silacyclopentylmethyl radicals disclosed ground state conformations having semioccupied p-orbitals close to antiperiplanar with respect to their β-Si-C bonds, but indicated Si-hyperconjugation (β-silicon effect) was insignificant in radicals. Kinetic data was obtained by the steady state EPR method for ring enlargement of indano-3-oxa-2-silacyclopentylmethyl radicals. The scope of the novel rearrangement in terms of other ring types and sizes, as well as the analogous 1,2-migration of "tethered" C-centered groups, was explored computationally.

A review of methods to synthesise 4′-substituted nucleosides

Modified nucleosides have received a great deal of attention from the scientific community, either for use as therapeutic agents, diagnostic tools, or as molecular probes.

[Development of highly nuclease-resistant chemically-modified oligonucleotides]

Chemical modification of therapeutic oligodeoxyribonucleotides (ODNs) is necessary to avoid not only degradation by endo- and exo-nucleases but also recognition by sensors such as an innate immune system. We have been developing modified nucleosides having an aminoalky linker at the pyrimidine nucleobase or sugar moiety. ODNs containing 5-N-(6-aminohexyl)carbamoyl-2'-deoxyuridine (7) were thermally stabilized about 3°C per modification and were about 160 times more stable to hydrolysis by snake venom phosphodiesterase (a 3'-exonuclease) than unmodified ODNs, but not by endonucleases. On the other hand, ODNs containing 4'-C-(aminoethyl)thymidine (14b), which was synthesized by a newly developed radical cyclization-ring-enlargement reaction by us, were 87 times more stable to hydrolysis by DNase I (an endonuclease) and 133 times more stable in 50% human serum than unmodified ODNs. The highly stereoselective synthesis of 4'-thioribonuclesides ((S)Ns) was also developed using a Pummerer reaction. Human thrombin RNA aptamer (CII-1-37) containing 4'-thiouridine and 4'-thiocytidine was obtained by SELEX with a K(d) value of 4.7 nM, while a previously known RNA aptamer (RNA-24) has a K(d) value of 85 nM. Studies of the modification pattern-RNAi activity relationships by using (S)Ns have been carried out against luciferase genes. We found that siRNAs, which have 4 residues of (S)Ns on both ends of the sense strand and 4 residues on the 3'-end of the antisense strand, were the most effective. 4'-ThioRNA is about 1100 times more stable in 50% human plasma than unmodified RNA. However, oligoribonucleotides ((SM)ONs) containing 2'-O-methyl-4'-thioribonucleosides were 9800 times more stable in 50% human plasma than unmodified RNA. Since (SM)ON duplexes were thermally more stable than unmodified ON duplexes, therefore they would be quite suitable to use for oligonucleotide therapeutics.

[Medicinal chemistry targeting nucleosides and nucleic acids based on fine synthetic chemistry]

Nucleosides and nucleotides are one of the most important elements for cells by the fact that they are components of DNAs and RNAs. In addition, they play important roles in most fundamental cellular metabolic pathways such as energy donors, second messengers, and cofactors for various enzymes. Therefore, there exists a rich source in drug discovery targeting nucleosides and nucleotides. In order to utilize nucleosides and nucleic acids on the drug development, it is very important to develop reactions and methods, by which the highly coordinating and labile nucleoside intermediates can be used. With these in mind, we have been working on synthetic nucleoside and nucleic acid chemistry. First, branched sugar nucleoside derivatives, which are potential antitumor agents, have been synthesized utilizing samarium diiodide (SmI(2)) mediated Reformatsky reaction or aldol reaction. 3'-beta-Carbamoylmethylcytidine (CAMC) was found to exhibit potent cytotoxicity against various human tumor cell lines. Synthetic methodology of the caprazamycins, which are promising antibacterial nucleoside natural products, was also developed by the strategy including beta-selective ribosylation without using a neighboring group participation. Our synthetic route provided a range of key analogues with partial structures to define the pharmacophore. Simplification of the caprazamycins was further pursued to develop diketopiperazine analogs. Medicinal chemistry of oligodeoxynucleotides has been conducted. Thus, novel triazole-linked dumbbell oligodeoxynucleotides and modular bent oligodeoxynucleotides were synthesized. They exhibit excellent binding affinity to NF-kappaB or HMGB1 A-box protein, which are important therapeutic targets. Therefore, the results obtained conclusively demonstrated these oligodeoxynucleotides could be proposed as powerful decoy molecules.

Synthesis of 3′-β-carbamoylmethylcytidine (CAMC) and its derivatives as potential antitumor agents

3'-beta-Carbamoylmethylcytidine (CAMC) and its derivatives were synthesized using an intramolecular Reformatsky-type reaction promoted by SmI2 as the key step. In vitro tumor cell growth inhibitory activity was evaluated and CAMC was found to exhibit potent cytotoxicity against various human tumor cell lines. From a structure-activity relationship study it was postulated that the cytotoxic mechanism of action of CAMC did not require phosphorylation at the 5'-hydroxyl group. This study provides a novel strategy for the development of a new type of antitumor nucleoside.

Sulfanyl radical promoted C4′–C5′ bond scission of 5′-oxo-3′,4′-didehydro-2′,3′-dideoxynucleosides

Treatment of C5'-aldehydes, under mildly basic conditions leads to the formation of 3',4'-didehydroaldehydes, and furfural. Sulfanyl radical addition eventually gives rise to the lactones, through C4'-C5' bond scission of the 1,2-dioxetane intermediates.

4'-Ethynylstavudine (4'-Ed4T) has potent anti-HIV-1 activity with reduced toxicity and shows a unique activity profile against drug-resistant mutants

A nucleoside analogue 4'-ethynylstavudine (4'-Ed4T) was recently synthesized during chemical studies directed towards the development of a new route to 4'-carbon-substituted nucleosides. This compound was found to be more anti-HIV-1 active than the parent compound stavudine (d4T) and much less toxic to various cells and also to mitochondrial DNA synthesis. It became apparent that 4'-Ed4T is a better substrate for human thymidine kinase than d4T, and very much more resistant to catabolism by thymidine phosphorylase. The study of 4'-Ed4T against various drug-resistant HIV-1 mutants has disclosed its unique activity profile.

Spirocyclic restriction of nucleosides. Synthesis of the first exemplary syn-1-oxaspiro[4.4]nonanyl member

[reaction: see text] The potential benefits associated with the spirocyclic restriction of nucleosides are summarized. Following exploration of a pi-allylpalladium route to 5'-alpha- or syn-dideoxy examples, we evaluated MOM protection of the 5'-hydroxyl as being suited to the synthesis of the first member of this new class of nucleoside mimic.

Synthesis and biological activity of 4'-C-hydroxymethyl-2'-fluro-D-arabinofuranosylpurine nucleosides

A series of 4'-C-hydroxymethyl-2'-fluoro-D-arabinofuranosylpurine nucleosides was prepared and evaluated for cytotoxicity. The details of a convenient synthesis of the carbohydrate precursor 4-C-hydroxymethyl-3,5-di-O-benzoyl-2-fluoro-alpha-D-arabinofuranosyl bromide (13) are presented. Proof of the structure and configuration at all chiral centers of the sugars and the nucleosides were obtained by proton NMR. All five target nucleosides were evaluated for cytotoxicity in human tumor cell lines. The 4'-C-hydroxymethyl clofarabine analogue (16beta) showed slight cytotoxicity in CCRF-CEM leukemia cells.

An efficient method for the preparation of 1'alpha-branched-chain sugar pyrimidine ribonucleosides from uridine: the first conversion of a natural nucleoside into 1'-substituted ribonucleosides

The 1'alpha-phenylselenouridine derivative 13 was successfully synthesized by enolization of the 3',5'-O-TIPDS-2'-ketouridine 8, and was subjected to a radical reaction with a vinylsilyl tether--an efficient procedure for preparing 1'alpha-branched-chain sugar pyrimidine nucleosides. Successive treatment of 8 with LiHMDS and PhSeCl in THF at < -70 degrees C gave the desired 1'-phenylseleno products in 85% yield as an anomeric mixture of the 1'alpha-product 11 and the 1'beta-product 12 (11/12= 2.5:1). Highly stereoselective reduction at the 2'-carbonyl of the 1'alpha-product 11 occurred from the beta-face by using NaBH4/CeCl3 in MeOH, and subsequent introduction of a dimethylvinylsilyl tether at the 2'-hydroxyl gave the radical reaction substrate 14. The photochemical radical atom-transfer reaction of 14 by using a high-pressure mercury lamp proceeded effectively in benzene to give the exo-cyclized PhSe-transferred product 18, in which (PhSe)2 proved to be essential as an additive for radical atom-transfer cyclization reactions. Subsequent phenylseleno-group elimination of 18 gave the sugar-protected 1'alpha-vinyluridine. With this procedure, 1'alpha-vinyluridine (22) and -cytidine (25), designed to be potential antitumor agents, were successfully synthesized. This study is the first example of functionalization at the anomeric 1'-position of a nucleoside by starting from a natural nucleoside to produce a ribo-type 1'-modified nucleoside.

Synthesis and antiproliferative activity of some 4'-C-hydroxymethyl-alpha- and -beta-D-arabino-pentofuranosyl pyrimidine nucleosides

A suitably protected 4-C-hydroxymethyl-arabino-pentofuranose was prepared and condensed with the following nucleobases: uracil, 5-fluorouracil and thymine. The corresponding cytosine and 5-fluorocytosine derivatives have also been obtained respectively from the uracil and 5-fluorouracil nucleosides. Separation of the anomeric mixtures followed by deprotection afforded the target compounds that were found to be non-cytotoxic to CCRF-CEM leukemia cells.

Nucleosides and nucleotides. 183. Synthesis of 4'alpha-branched thymidines as a new type of antiviral agent

A series of 4'alpha-branched thymidines was synthesized and evaluated as potential antiviral agents. 4'-Ethylthymidine (3), 4'-ethenylthymidine (5), and 4'-ethynylthymidine (6) exhibited potent anti-HSV-1 and anti-HIV-1 activities with no significant cytotoxicity.