New neplanocin analogues. 10. The conversion of adenosine to neplanocin A, a carbocyclic nucleoside antibiotic with potent antiviral activity

Chemical Approaches to Carbocyclic Nucleosides

Nucleoside analogues are at the forefront of antiviral therapy for last decades. To circumvent some of their limitations, based on their metabolism, and in order to improve their anti-viral potency and selectivity, several families of nucleoside analogues have been described through structural modifications at the sugar and heterocycles. The replacement of the oxygen of the nucleoside by a methylene has led to the family of carbocyclic (or cyclopentane) nucleoside analogues. Various potent anti-HIV and anti-HBV drugs belong to this family. Main syntheses of carbocyclic analogues of nucleosides used Diels-Alder reactions (in racemic or asymmetric series), but also started from carbohydrates (ribose, glucose), as a source of optically active compounds, which then had to be transformed into carbacycles under various conditions. The growing interest in carbocyclic nucleosides has led several groups, including ours, to develop new analogues and to explore novel routes. This article will review some of the recent chemistry developed on selected five-membered ring carbocyclic nucleosides.

A novel route to a chiral building block for the preparation of cyclopentenyl carbocyclic nucleosides. Synthesis and anticancer activity of enantiomeric neplanocins A

The synthesis of both enantiomers of 3-[(tert-butyldimethylsilyl)oxy]methyl-4,5-O-isopropylidenecyclopent-2-en-1-ol was accomplished in six steps based on optically inactive dimethyl meso-tartrate. This key intermediate in the synthesis of cyclopentenyl carbocyclic nucleosides was subsequently applied in the preparation of enantiomeric neplanocins A. The toxic effect of these compounds was investigated for a series of suspension and adherent cancer cell lines and normal human fibroblasts. (-)-Neplanocin A ((-)-NPA) was more toxic against all tested cancer cell lines than its dextrorotary counterpart. The highest toxicity with IC50 values of 7 and 10 μM was observed for the MOLT-4 and A431 cells, respectively. Moreover, (-)-NPA also induced apoptosis in A431 cell while this effect was not observed for (+)-NPA.

Inhibitors of Measles Virus

Measles virus (MV) infections have been almost eradicated in some industrialized nations. However, MV continues to cause severe disease and mortality in the world and is responsible for clusters of exogenous-borne disease in essentially disease-free countries. Because of the ebb and flow of immunization campaigns, especially in the poverty-stricken and war-torn Third World, and the ominous potential for severe disease and mortality, it is vital that research for discovery of therapeutic countermeasures should continue. To that end, a number of compounds have been evaluated for efficacy in vitro and in animal models, and several therapeutic modalities have been tested in the clinic. The only current therapies used in the clinic include ribavirin administered orally or intravenously, alone or in combination with immune serum globulin; these therapies have demonstrated variable efficacy. Therefore, drug discovery efforts have been launched to supplement the existing treatments for MV infections. Antisense molecules, adenosine and guanosine nucleosides, including ring-expanded ‘fat’ nucleoside analogues, brassinosteroids, coumarins, peptide inhibitors, modulators of cholesterol synthesis and a variety of natural products have been screened for efficacy and toxicity both in vitro and in animals. However, none of these agents has gone into human clinical trials and most will not merit further development due to toxicity concerns and/or low potency. Thus, further research is needed to develop more potent and less toxic drugs that could be used for treating MV infections to supplement the existing MV vaccine campaigns.

Synthesis of cyclopentenyl carbocyclic nucleosides as potential antiviral agents against orthopoxviruses and SARS

A practical and convenient methodology for the synthesis of chiral cyclopentenol derivative (+)-12a has been developed as the key intermediate that was utilized for the synthesis of biologically active carbocyclic nucleosides. The selective protection of allylic hydroxyl group followed by the ring-closing metathesis (RCM) reaction with Grubbs catalysts provided (+)-12a on a 10 g scale with 52% overall yield from D-ribose (4). The key intermediate (+)-12a was utilized for the synthesis of unnatural five-membered ring heterocyclic carbocyclic nucleosides. The newly synthesized 1,2,3-triazole analogue (17c) exhibited potent antiviral activity (EC(50) 0.4 microM) against vaccinia virus and moderate activities (EC(50) 39 microM) against cowpox virus and severe acute respiratory syndrome coronavirus (SARSCoV) (EC(50) 47 microM). The 1,2,4-triazole analogue (17a) also exhibited moderate antiviral activity (EC(50) 21 microM) against SARSCoV.

Total synthesis of (-)-neplanocin a by using lithium thiolate-initiated Michael-aldol tandem cyclization reaction

(-)-Neplanocin A (1), S-adenosylhomocystein hydrolase inhibitor, was synthesized. The characteristic of this synthesis is a stereoselective construction of five-membered ring of neplanocin A by intramolecular aldol reaction of the lithium enolate that was generated by conjugate addition of lithium thiolate. TBS-protected chiral omega-oxo-alpha,beta-unsaturated ester 16, which was prepared from D-mannitol, was treated with 1.2 equiv of lithium benzylthiolate in THF at -20 degrees C to give three separable cyclization products in good yields and stereoselectivity. After conversions of protective groups, the benzylsulfanyl part of 21 was removed by oxidation to sulfoxide and subsequent thermal elimination to give the requisite double bond. Through the functional group transformations of 30, total synthesis of (-)-neplanocin A (1) was accomplished.

The latest progress in the synthesis of carbocyclic nucleosides

This review presents the latest developments in the field of carba-nucleosides (1994-1998). Special attention is paid to the synthesis of key precursors to those carba-nucleosides that possess significant biological activities or have novel structures.

9-[(2'S,3'S)-3'-formyl-2',3'-dihydroxypropyl]adenine: a facile affinity-labeling probe of human S-adenosyl-L-homocysteine hydrolase

Treatment of human recombinant S-adenosyl-L-homocysteine (SAH) hydrolase with 9-[(2'S,3'S)-3'-formyl-2',3'-dihydroxypropyl]adenine (FDHPA) caused irreversible inactivation in a time- and concentration-dependent manner (Ki = 8.8 microM, k(inact) = 0.09 min(-1)). FDHPA behaved as a facile affinity-labeling probe of SAH hydrolase.

Chemotherapy of respiratory viruses: prospects and challenges

Billions of people are infected with respiratory viruses annually. Infants and young children, the elderly, immunocompromised individuals and those debilitated by other diseases or nutritional deficiencies are most at risk for serious disease. There are few vaccines available for use against these viruses, and even where there are (influenza, measles and adenovirus), infections remain common. The continued prevalence of respiratory virus infections has lead to renewed efforts to find safe agents effective against the most medically important respiratory viruses: influenza, respiratory syncytial, parainfluenza, measles, rhino- and adenovirus. Copyright 1999 Harcourt Publishers Ltd.