Nucleosides and Nucleotides. 163. Synthesis of 3‘-β-Branched Uridine Derivatives via Intramolecular Reformatsky-Type Reaction Promoted by Samarium Diiodide1

A convenient synthesis of branched-chain nucleoside isothiocyanates via aza-Claisen rearrangement

Stereocontrolled introduction of a nitrogen atom at either C-2' or C-3' positions of nucleosides derived from uridine, 4-N-benzoylcytidine and adenosine was investigated. An efficient and rapid procedure was employed for creating new chiral centers at C-2' and C-3' positions using [3,3]-sigmatropic aza-Claisen rearrangement of allyl thiocyanates under conventional and microwave conditions. Structure of isothiocyanate products was confirmed by 1-D and 2-D NMR spectral analyses including selective ¹H 1-D-NOE experiments.

A reliable Pd-mediated hydrogenolytic deprotection of BOM group of uridine ureido nitrogen

The benzyloxymethyl (BOM) group has been utilized widely in syntheses of a variety of natural and non-natural products. The BOM group is also one of few choices to protect uridine ureido nitrongen. However, hydrogenolytic cleavage of the BOM group of uridine derivatives has been unrealizably performed via heterogeneous conditions using Pd catalysts. One of the undesirable by-products formed by Pd-mediated hydrogenation conditions is the over-reduced product of which the C5-C6 double bond of the uracil moiety was saturated. To date, we have generated a wide range of uridine-containing antibacterial agents, where the BOM group has been utilized in their syntheses. In screening of deprotection conditions of the BOM group of uridine ureido nitrogen under Pd-mediated hydrogenation conditions, we realized that the addition of water to the (i)PrOH-based hydrogenation conditions can suppress the formation of over-reduced uridine derivatives and the addition of HCO(2)H (0.5%) dramatically improve the reaction rate. An optimized hydrogenation condition described here can be applicable to the BOM-deprotections of a wide range of uridine derivatives.

[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.

Design, synthesis, and antiviral evaluation of some 3'-carboxymethyl-3'-deoxyadenosine derivatives

3'-Carboxymethyl-3'-deoxyadenosine derivatives were prepared from 2'-O-TBDMS-3'-[(ethoxycarbonyl)methyl]-3'-deoxyadenosine (1) via simple and efficient procedures. Conversion of 1 to its 5'-azido-5'-deoxy derivative 5 was accomplished via a novel one-pot method employing 5'-activation (TosCl) followed by efficient nucleophilic displacement with tetramethylguanidinium azide. Compound 5 was converted to 5'-[(N-methylcarbamoyl)amino] derivative 8 via one-pot reduction/acylation employing H(2)/Pd-C followed by treatment with p-nitrophenyl N-methylcarbamate. N(6)-phenylcarbamoyl groups were introduced by treatment with phenylisocyanate, and an efficient new method for lactonization of 2'-O-TBDMS-3'-[(ethoxycarbonyl)methyl]-3'-deoxyadenosines to give corresponding 2',3'-lactones was also developed. Target compounds were evaluated for anti-HIV and anti-HIV integrase activities, but were not active at the concentrations tested.

[Synthetic study of nucleoside antibiotics]

Some of nucleoside antibiotics include complex structures as well as sensitive functionality, which are challenging targets for organic chemists. Among complex nucleoside antibiotics, there are also good drug candidates because they possess a variety of interesting biological properties. Herbicidin B and fully protected tunicaminyluracil, which were undecose nucleoside antibiotics, were synthesized using a samarium diiodide (SmI(2)) mediated aldol reaction with the use of alpha-phenylthioketones as enolate sources. The characteristics of the SmI(2)-mediated aldol reaction are that the enolate can be regioselectively generated and the aldol reaction proceeds under near neutral condition. This reaction is proved to be a powerful reaction for the synthesis of complex nucleoside antibiotics. The synthesis of caprazol, the core structure of caprazamycins, was conducted by the strategy including beta-selective ribosylation without using a neighboring group participation and the construction of a diazepanone by a modified reductive amination. Our synthetic route would provide a range of key analogues with partial structures to define the pharmacophore, which can be a lead for the development of more effective anti-bacterial agents.

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.

Synthesis of tunicaminyluracil derivatives

A tunicaminyluracil derivative, which is a key component of the tunicamycin nucleoside antibiotics, was synthesized using a samarium diiodide (SmI2) mediated aldol reaction and intramolecular Pummerer reaction as the key steps. The alpha-phenylthio ketone 11, the precursor of the samarium enolate, was prepared from D-galactose. Treatment of 11 with SmI2 at -40 degrees C resulted in complete conversion to the corresponding samarium enolate, and subsequent addition of uridine 5'-aldehyde 12 afforded the desired aldol products 13a,b. Compound 13a was converted to the sulfoxide 15 by a sequential diastereoselective reduction of the ketone and an oxidation with mCPBA. Activation of 15 with Tf2O provided the desired cyclized compound 17. In this reaction, the aldol product 13a was also obtained as a consequence of a competitive intramolecular version of DMSO-oxidation via a 7-membered ring intermediate. Compound 18 or 19 are ready for use as a glycosyl donor in glycosylations to provide a range of analogues as potential glycosyltransferase inhibitors as well as related natural products.

A highly stereoselective samarium diiodide-promoted aldol reaction with 1'-phenylseleno-2'-keto nucleosides. Synthesis of 1'alpha-branched uridine derivatives

Since 1'-branched nucleosides are biologically important targets in medicinal chemistry, more efficient methods for preparing them are required. The 1'alpha-branched uridine derivatives were successfully synthesized via a samarium diiodide (SmI(2))-promoted aldol reaction. Treatment of the 1'alpha-phenylseleno-2'-ketouridine derivative 6, readily prepared from uridine, with SmI(2) at -78 degrees C in THF reductively cleaved the anomeric Se-C bond to generate the corresponding samarium enolate, which was highly stereoselectively condensed with aldehydes, such as PhCHO, MeCHO, i-PrCHO, or (CH(2)O)(n)(), to give the corresponding 1'alpha-1' 'S-branched products 12a-d. This is the first time an enolate has been generated by reductively cleaving a C-Se bond. The highly selective stereochemical results suggest that the aldol reaction proceeds via a chelation-controlled transition state. When an excess of aldehyde was used and the reaction mixture was gradually warmed, the tandem aldol-Tishchenko reaction proceeded to give the "arabino-type" nucleosides 14a-c, having a 2'-"up" hydroxyl and 1'alpha-1' 'S-branched chain. 1'alpha-Hydroxymethyluridine (21), which is the uracil version of the antitumor antibiotic angustmycin C, was synthesized from the aldol reaction product 10.

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.

Glucose-derived 3'-(carboxymethyl)-3'-deoxyribonucleosides and 2', 3'-lactones as synthetic precursors for amide-linked oligonucleotide analogues

Treatment of a 1,2-O-isopropylidene-3-ketopentofuranose derivative (obtained from D-glucose) with [(ethoxycarbonyl)methylene]triphenylphosphorane and catalytic hydrogenation of the resulting alkene gave stereodefined access to 3-(carboxymethyl)-3-deoxy-D-ribofuranose derivatives. Esters of 5-O-acetyl- or 5-azido-5-deoxy-3-(carboxymethyl)-D-ribofuranose were coupled with nucleobases to give branched-chain nucleoside derivatives. Ester saponification and protecting group manipulation provided 2'-O-(tert-butyldimethylsilyl) ethers of 5'-azido-5'-deoxy- or 5'-O-(dimethoxytrityl) derivatives of 3'-(carboxymethyl)-3'-deoxyribonucleosides that are effective precursors for synthesis of amide-linked oligoribonucleosides.

C-3'-Branched Thymidines as Precursors for the Selective Generation of C-3'-Nucleoside Radicals

C-3'-nucleoside radicals can be generated via Norrish type I photocleavage of C-3'-acyl nucleoside derivatives. In monomer experiments employing C-3'-acylthymidine derivatives 2 and 3, a 1:1 mixture of isomers of the H-abstraction products was obtained when the photolysis was carried out in the presence of a hydrogen donor. Derivatives 2 were synthesized by an approach which involves the formation of a silyl-protected cyanohydrin, which is subsequently alkylated with organolithium reagents, followed by hydrolysis. Derivatives 3 could be obtained via a multistep synthesis starting from diol 7. Several different methods were attempted to oxidize the unprotected diol to the alpha-hydroxy aldehyde. Finally, a route was chosen which involves a protection-deprotection sequence followed by oxidation of the free primary alcohol. The resulting modified nucleosides should facilitate the study of C-3'-DNA radicals.