Syntheses of the anti-AIDS drug 2',3'-dideoxycytidine from cytidine

Sustainable Protocol for the Synthesis of 2′,3′-Dideoxynucleoside and 2′,3′-Didehydro-2′,3′-dideoxynucleoside Derivatives

An improved protocol for the transformation of ribonucleosides into 2′,3′-dideoxynucleoside and 2′,3′-didehydro-2′,3′-dideoxynucleoside derivatives, including the anti-HIV drugs stavudine (d4T), zalcitabine (ddC) and didanosine (ddI), was established. The process involves radical deoxygenation of xanthate using environmentally friendly and low-cost reagents. Bromoethane or 3-bromopropanenitrile was the alkylating agent of choice to prepare the ribonucleoside 2′,3′-bisxanthates. In the subsequent radical deoxygenation reaction, tris(trimethylsilyl)silane and 1,1′-azobis(cyclohexanecarbonitrile) were used to replace hazardous Bu₃SnH and AIBN, respectively. In addition, TBAF was substituted for camphorsulfonic acid in the deprotection step of the 5′-O-silyl ether group, and an enzyme (adenosine deaminase) was used to transform 2′,3′-dideoxyadenosine into 2′,3′-dideoxyinosine (ddI) in excellent yield.

Studies toward the oxidative and reductive activation of C-S bonds in 2'-S-aryl-2'-thiouridine derivatives

Studies directed toward the oxidative and reductive desulfurization of readily available 2'-S-aryl-2'-thiouridine derivatives were investigated with the prospect to functionalize the C2'-position of nucleosides. The oxidative desulfurization-difluorination strategy was successful on 2-(arylthio)alkanoate surrogates, while extension of the combination of oxidants and fluoride sources was not an efficient fluorination protocol when applied to 2'-S-aryl-2'-thiouridine derivatives, resulting mainly in C5-halogenation of the pyrimidine ring and C2'-monofluorination without desulfurization. Cyclic voltammetry of 2'-arylsulfonyl-2'-deoxyuridines and their 2'-fluorinated analogues showed that cleavage of the arylsulfone moiety could occur, although at relatively high cathodic potentials. While reductive-desulfonylation of 2'-arylsulfonyl-2'-deoxyuridines with organic electron donors (OEDs) gave predominantly base-induced furan type products, chemical (OED) and electrochemical reductive-desulfonylation of the α-fluorosulfone derivatives yielded the 2'-deoxy-2'-fluorouridine and 2',3'-didehydro-2',3'-dideoxy-2'-fluorouridine derivatives. These results provided good evidence of the generation of a C2'-anion through carbon-sulfur bond cleavage, opening new horizons for the reductive-functionalization approaches in nucleosides.

Molecular structure and vibrational spectra of N4-acetylcytosine

The infrared and FT-Raman spectra of N4-acetylcytosine in the solid phase have been recorded, and the molecular geometrical parameters were optimized using B3LYP and MP2 methods with 6-311++G(d,p) basis set. The theoretical calculations indicate the presence of four stable conformers of N4-acetylcytosine, and the one containing an intramoleuclar six-membered ring is the most stable. The vibrational wavenumbers, infrared intensities, Raman scattering activities and the corresponding assignments of the observed bands based on the most optimized conformer were performed by B3LYP methods using the aug-cc-pvtz basis set. The observed and calculated frequencies are found to be in good agreement.

Synthesis of a fluorescent 2'3'-dideoxycytosine analog, tCdd

The pathway leading to the preparation of a novel tricyclic 2'3'-dideoxycytosine analog, tCdd (1) is reported. A protected 2'3'-dideoxyribose prepared from l-glutamic acid was coupled to a silylated fluorescent base to yield a mixture of the alpha- and beta-anomers of the 2'3'-dideoxyribonucleoside of 1,3-diaza-2-oxophenothiazine, tCdd (1). The fluorescent base analog retains a high fluorescence emission over a large pH range and should be useful in a variety of probe applications.

Efficient lipase-catalyzed enantioselective desymmetrization of prochiral 2,2-disubstituted 1,3-propanediols and meso 1,2-diols using 1-ethoxyvinyl 2-furoate

An efficient lipase-catalyzed desymmetrization of prochiral 2,2-disubstituted 1,3-propanediols was developed using 1-ethoxyvinyl 2-furoate 1b, for which the well-known method using vinyl or isopropenyl acetate has had limited success due to low reactivity and easy racemization of the products through acyl group migration. The reagent 1b is highly reactive and converts various prochiral 1,3-diols to the monoesters having a chiral quaternary carbon center with 82-99% ee. These products were stable against racemization under acidic conditions, and their furoyl groups were compatible with oxidative conditions. Prolonging the reaction time led to the kinetic resolution of the monoesters resulting in an increase of their optical purity. The similar desymmetrization of meso cis-1,2-cycloalkanediols gave the monoesters with 82-97% ee without racemization.

Enantiospecific synthesis of the (9S,18R)-diastereomer of the leukocyte adhesion inhibitor cyclamenol A

Cyclamenol A is one of the very few non-carbohydrate and non-peptide natural products that inhibit leukocyte adhesion to endothelial cells. We report on the first enantioselective total synthesis of the (9S, 18R)-diastereomer of this macrocyclic polyene lactam. Key elements of the synthesis are i) the synthesis of the required chiral building blocks by employing readily accessible building blocks from the chiral pool, that is, (S)-malic acid and (R)-hydroxyisobutyric acid, ii) assembly of a linear polyene precursor by means of Wittig and Horner olefination reactions as key C-C bond-forming transformations, iii) ring closure by means of a vanadium-mediated pinacolisation reaction and iv) conversion of the generated cis-diol into a (Z)-olefin to complete the entire polyene system of the natural product. Attempts to close the macrocyclic ring by a macrolactamisation, a double Stille coupling or direct olefination in a McMurry reaction failed. Crucial to the successful completion of the synthesis was the correct orchestration of the final steps. It was necessary to first deprotect the intermediate formed after macrocycle formation and to generate the sensitive heptaene system in the last step by means of a Corey-Hopkins sequence.

Synthesis of regioselectively protected forms of cytidine based on enzyme-catalyzed deacetylation as the key step

N4-Acetylcytidine (77%) and 2',3'-O, N4-triacetylcytidine (95%) were obtained from the hydrolysis of a common precursor, the peracetylated form of cytidine with Aspergillus niger lipase (Amano A) and Burkholderia cepacia esterase (SC esterase S), respectively, under very mild conditions. The experimental procedure for the conversion of triacetylcytidine to a corresponding phosphoramidite (82%), an intermediate for sugar nucleotide synthesis, is also elaborated.

Synthesis of 2',3'-dideoxyisoguanosine from guanosine

2,3'-Dideoxyisoguanosine was synthesized from guanosine via intermediate 6-[(4-methylphenyl)thio]-2-oxo-9-(2',3',5'-tri-O-acetyl-beta-D-ribofuran osyl)-2,3-dihydropurine (4). The 2-oxo, 6-amino and 5'-hydroxy triprotected isoguanosine derivative was utilized to reduce high polarity and promote poor solubility of intermediates. The protecting groups for oxo and 6-amino were easily removed in reduction of olefin in ribose without additional reaction steps. 2',3'-Vicinal diol in ribose sugar moiety was transformed to olefin with Bu3SnH by radical reaction via bisxanthate. Removing 5'-O-TBDMS protecting group gave final product, 2',3'-dideoxyisoguanosine (12) in a 10% overall yield.

Stereoselective Synthesis of 2'-Amino-2',3'-dideoxynucleosides by Nitrone 1,3-Dipolar Cycloaddition: A New Efficient Entry Toward d4T and Its 2-Methyl Analogue

An efficient access to 2'-(dimethylamino)-2',3'-dideoxynucleosides is reported. The synthetic strategy relies on the 1,3-dipolar cycloaddition of C-alkoxycarbonyl nitrones to allyl acetate, followed by reductive ring opening to substituted lactones, DIBALH reduction to the corresponding 3-(dimethylamino)tetrahydro-2-furanols, and coupling with silylated thymine. The removal of the dimethylamino group by Cope elimination affords a new formal synthesis of d4T and analogous unsaturated 2',3'-dideoxynucleosides.

Synthesis of 2',3'-Didehydro-2',3'-dideoxynucleosides by Reaction of 5'-Protected Nucleoside 2',3'-Dimesylates with Telluride Dianion: A General Route from Cis Vicinal Diols to Olefins

2',3'-Dimesylates of 5'-protected nucleosides are converted into the corresponding 2',3'-didehydro-2',3'-dideoxy compounds by treatment with telluride dianion in the form of the sodium or lithium salt. The method is well-suited to the preparation of unsaturated nucleosides that can be converted into compounds that are believed to be useful in the treatment of AIDS. The deoxygenation is general for vicinal dimesylates that have, or may adopt, a synperiplanar conformation. With straight chain compounds the reaction is stereospecific. In some cases, similar, but slower, deoxygenations can be performed with selenide dianion.

Liquid chromatographic separation of zalcitabine and its stereoisomers

A liquid chromatographic method capable of separating and quantitating the stereoisomers of zalcitabine has been developed and validated. The separation was achieved with an Astec Cyclobond I--RSP column and a mobile phase of 0.25% triethylamine in water adjusted to a pH of 6.5 with glacial acetic acid. All enantiomers were found to exhibit a linear response in the range of 0.1-10% in the presence of 100% zalcitabine. Precision of analysis was found to be less than 1.5% at a level of 1% relative to zalcitabine. The limit of detection for two of the three enantiomeric impurities was determined to be 0.05% relative to zalcitabine. The detection limit for the third was found to be 0.1%. This method was successfully applied to the analysis of reference standards and several production scale batches. All of these materials were found to be stereochemically pure to a level of 99.8% or better.