Synthesis Of4’-Spirouridine Via1,3-Dipolar Cyctoaddition Reaction

Antiviral drugs derived from nucleosides have played an essential role in therapyagainst epidemical diseases caused by virus infection. It has been shown that theconfirmation of the nucleoside is a key factor in its recognition and interaction to thetarget biomolecules. Conformational constrained nucleosides, which restrict theconformation of nucleosides by structural modification, is a useful way to improve theaffinity and selectivity of nucleosides in search for drug candidates.4′-spironucleosides are one type of such important conformationally constrainednucleosides that were much less explored due to the difficulty in synthesis. The fewreported syntheses in general have some of the following disadvantages: long route,low yield and lack of steroselectivities. In this thesis, we committed to studying thesynthesis of4’-spironucleoside, and expanding the application of1,3-dipolarcycloaddition. We synthesized4’-spironucleoside between4’,5’-unsaturated uridineand the higher reactivity dipole via1,3-dipolar cycloaddition reaction.Firstly, nitile oxide was chosen as dipoar reacted with4’,5’-unsaturated uridinevia1,3-dipolar reaction. Nitrile oxide generated in situ under basic condition fromchlorinated benzaldehyde oxime reacted with4’,5’-unsaturated uridine directly. Theexperimental results showed that the raw material was disappeared and the polarity ofproduct was strong. It was difficult to separate the pure product. In order to reduce thepolarity of the product, TBSDMSCl was used to protect2’ and3’-hydroxyl group of4’,5’-unsaturated uridine. When the protected4’,5’-unsaturated uridine reacted withnitrile oxide, we got cycloaddition produt stereospecificly. By de-protecting of theproduct, we obtained the4’-isoxazoline uridine. After optimizing the conditions ofcycloadditon, we improved the yield of the reaction and got high yield. Based on theoptimized condition, we verified the type of substituted nitrile oxide and synthesizedten4’-isoxazoline uridine. We also found that the electronic effects and steric effectsof the nitrile oxide had little effect on the yield and the stereo-selectivity of the reaction. It showed that the1,3-dipolar cycloaddition reaction between nitrile oxideand4’,5’-unsaturated uridine was a highly efficient method of synthesis of4’-spironucleoside. The NOE spectrum of synthesized4′-spirouridine showed that5’-methylene of4′-isoxazoline ring and base of nucleoside are at the opposite side ofthe sugar ring. Finally, we performed a transformation to convert uricil to cytosine,which demonstrated that the isoxazoline in the spironucleoside was stable under thetransformation.Next nitrone was chosen as another1,3-dipolar to react with4’,5’-unsaturateduridine to synthesis the4’-spironucleoside via1,3-dipolar cycloaddition reaction. Itwas different from nitrile oxide that nitrone didn’t react with the unprotected4’,5’-unsaturated uridine under several conditions. when2’ and3’-hydroxyl group of4’,5’-unsaturated uridine protected, it could react with nitrone in high boiling pointsolvent xylene under refluxing, and produced a pair of isomers. After optimization ofcycliaddtion condition, we got two purified isomers via separated by columnchromatography. A pair of desired4’-isoxazolidine uridine was abtained afterdeprotection. According to the optimized condition, we changed the type ofsubstituted nitrone and synthesized eight pairs of4’-isooxazolidine uridine. At thesame time, we found that the stereoselective of the aromatic substituent on the nitronewere better than aliphatic and also found that the yield of electron-withdrawingsubstituents on the nitrone were higher than electron-donating substituents. It provedthat1,3-dipolar cycloaddition reaction between nitrone and4’,5’-unsaturated uridinewas another efficient method of synthesis of4’-spironucleoside. The two isomers wasproved by NOE experiements in NMR that5’-methylene of4′-isoxazolidine ring anduridine nucleotides are at the same side of the sugar ring, but they had differentconfiguration on6’-position. Finally, we successfully performed a transformation ofone of the product to convert uricil to cytosine.