Studies On The Synthesis Of Pyrazofurin Triphosphate And Ketalization Catalyzed By Hf?OTf?₄

Pyrazofurin,a special C-nucleoside isolated from Streptomyces,has antiviral and antitumor activity.It can effectively inhibit 5'-monophosphate orotic acid decarboxylase by interfering with the de novo synthesis of uridine nucleotides,leading to cytotoxicity.Therefore,the research on the biological and pharmaceutical aspects of pyrazofurin is of great significance.Pyrazofurin triphosphate is an important metabolic product.It plays an important role in the study of in vitro antiviral activity and mechanism,and the differences in the effects of viral and human polymerases.However,due to the presence of reactive enol-type hydroxyl group on the base of pyrazofurin,the chemical synthesis of pyrazofurin triphosphateuse via conventional methods has never been achieved since the discovery of pyrazofurin nearly 40 years ago.Thus,the chemical synthesis of pyrazofurin triphosphate is still a huge challenge to organic chemists.On the basis of previous reports on the synthesis of pyrazofurin moiety,the current research project is focused on the modified preparation of the nucleoside part,and selective monophosphorylation at 5'-Oh.Finally,P?V?-N activation strategy was employed to obtain the target product,pyrazofurin 5'-triphosphate.The detailed research work are summarized as follows:1)Optimized synthesis of pyrazofurin moiety.According to the synthetic route reported in the literature,D-ribose was used as the starting material and the 2?and3?-hydroxyl groups are subjected to isopropylidene protection,and then the5'-hydroxyl group is subjected to trityl protection.The protected ribose intermediate was treated with a triphenylphosphonate to give a carbohydrin precursor by Wittig reaction.In this step,the amount of benzoic acid catalyst was systematically optimized and greatly improved the reaction rate.Subsequently,the reactive methylene group was treated with azodation,and then the pyrazole ring was cyclized under alkaline conditions.After column chromatographic separation of the target product precursor with?-configuration,the ester was amminolyzed with ammonium hydroxide instead of ammonia in methanol.Finally,trityl was removed with TFA to afford the pyrazofurin moiety.2)Synthesis of pyrazofurin 5'-monophosphate intermediate.In a previous report,benzyl ether was used to protect the enol of pyrazofurin.However,this protective group is hard to remove in later stage.Therefore,my research focused on the selective monophosphorylation of the 5'-OH of ribose in the presence of active enol.In this part of work,we tested a series of phosphorylating/phosphitylating reagents including phosphorus oxychloride,chlorophosphoramidite,etc.Finally,the selective monophosphorylation of the 5'-OH was achieved by using dibenzylphosphoramidite.3)Synthesis of pyrazofurin 5'-triphosphate.The triphenylphosphine/disulfide oxidation-reduction condensation system was employed to prepare the pyrazofurin5'-phosphoropiperidate precursor efficiently.Subsequently,we used the P?V?-N activation strategy and 4,5-dicyanoimidazole as the activating reagent to achieve efficient coupling of the phosphoropiperidate with pyrophosphate.As expected,³¹P NMR tracing experiment showed that the P?V?-N activation method has very high base-tolerance,and the reaction rate and yield are essentially unaffected by the acidic enol.For the first time,chemical synthesis of pyrazofurin triphosphate has been achieved.In addition,the present study also found that the isopropylidene protection of D-ribose with conventional p-toluene sulfonic acid was problematic.The reaction rate was slow and hard to go completion,and the yield was relatively low.Therefore,on the basis of the previous research of our group on transition metal Lewis acid catalysts,my research explored the possibility of using Group IV B transition metal Lewis acid for the isopropylidene protection of D-ribose and achieved excellent results.Subsequently,we further optimized this method and extended this novel method to a variety of substrates and established a highly efficient Hf?IV?-catalyzed catalyst for the use of ketalization reaction.The detailed research work are summarized as follows:On the basis of literature reports and the precedent research of our group,we noticed that the tetravalent cations of the Group IV B transition metal had strong activation effect on the carbonyl group.Therefore,my research focused on the screening of the catalytic effect of a series of Group IV B metal Lewis acids in the model ketalization reaction.The experimental results showed that Hf?OTf?₄ exhibited distinctively high catalytic activity.Subsequent optimizations of the reaction conditions,such as the amount of activating reagent,solvent,were performed.The established method was successfully applied to a series of 1,2-diol and 1,3-diol substrates and even more complicated sugars.27 different cyclic ketal compounds were successfully synthesized by this method.Due to the reversibility of this reaction,the deprotection of cyclic ketal compounds using Hf?OTf?₄ was systematically studied,and an efficient catalytic method was also established for the deketalization reaction.