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SUMMARY AND CONCLUSION In this thesis some new modified nucleoside derivatives were synthesized using four routes:I- Synthesis of S-Nucleosides:The first syntheses of pyrimidine nucleosides were achieved by the reaction of pyrimidine derivatives I with 2,The reaction of thiopyrimidine derivatives with 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide in the presence of dimethyl formamide and triethylamine at room temperature afforded of 4-phenyl-6-amino-2-(2`,3`,4`,6`-tetra-O-acetyl-β-D-glycopyranosylthioII- Synthesis N-nucleosidesThe reaction of 5,6-diphenyl-1,2,4-triazine-3-thiol with a blocked tri-O-acetyl-a-D-arabinosyl in the presence of acetonitrile at 0-5oC gave the corresponding nucleoside 2-(2`,3`,5`-tri-O-acetyl-D-arabinosyl)-5,6-diphenyl-3-thio-1,2,4-triazine 7The hydrolysis of nucleoside 7 by using alcoholic ammonia gave 2-(D-arabinosyl)-5, 6-diphenyl-3-thio-1,2, 4-triazine 8.The fusion of 6-amino-2-methylthiopyrimidin-4-one with 1,2,3,4,6-peta-O-acetyl-b-D-glycopyranose in the presence of p-toluene sulfonic acid as a catalyst for 1 hr., gave 6-tetra-O-acetyl-b-D-glycosidamino-2-methylthiopyrimidin-4-one 9.Coupling of compound 9 with corresponding diazonium salt of 3-chloroaniline afforded 5-(3`-chlorobenzeneazo)-6-tetra-O-acetyl-b-D-glycosidamino-2-methyl- thiopyrimidin-4-one 10 as intermediate which was catalytically reduced by zinc dust and acetic acid tTreatment of compound 11 with triethyl ortho formate in the presence of dimethylformamide gave purine derivative 12III- Synthesis of nucleosides by unprotected sugar:Treatment of L-arabinose with malanonitrile in the presence of ammonium hydroxide in methanol gave compound 13, which reacted with thioglycollic acid in the presence of benzene to give 2-methylthiazol-4-o
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