9- hypoxanthine preparation

Since chemistry of pterines and purines has been already reviewed (42,48,491), only recent studies will be described here. Guanine (93) was prepared by reaction of 4-hydroxy-2,5,6-triaminopyrimidine sulfate (588 see Scheme 73) with HCONH2 with removal of H2O from the reaction system in an excellent yield (492). Also, irradiation of oxygenated aqueous solutions of 6-mercaptopurine with near-UV light gave hypoxanthine (92) as a minor product (< 10%) together with purine-6-sulfinate (589). It also arises from degradation of purine-6-sulfonate obtained from photooxidation of the sulfinate (589) (493). 

Some phosphoric acid derivatives of 2-desoxy-D-ribose have been obtained by enzymic methods of preparation. A reaction analogous to the phosphorolysis of glycogen to D-glucose 1-phosphate241 has been effected with either hypoxanthine- or guanine-D-riboside, both of which could be split by enzymic phosphorolysis with the formation of D-ribose 1-phosphate.242 The successful conclusion of these experiments prompted similar investigations with desoxyribonucleosides. 

Manson and Lampen243 reported that they obtained the phosphorolysis and arsenolysis of hypoxanthine desoxyriboside by enzyme preparations from calf-thymus gland and rat liver. An acid-stable phosphate ester was isolated as a product of phosphorolysis. Results to be outlined suggested that this ester was 2-desoxy-D-ribose 5-phosphate and evidence was obtained for its formation by a mutase type reaction from 2-desoxy-D-ribose 1-phosphate. This evidence was extended and reinforced when Manson and Lampen244 obtained indications for the formation of desoxy-D-ribose 1-phosphate during the phosphorolysis of thymidine. Consequently the conversions outlined may be depicted as shown. 

Preparation of AIC from Hypoxanthine. A search of the chemical literature revealed a paper by Friedman and Gots,21 who found that hypoxanthine was stable to heating in 1 N sulfuric acid. However, the same authors showed that when hypoxanthine was heated in 1.5 N sulfuric acid, with zinc dust added, extensive degradation occurred. The imidazole moiety of hypoxanthine was shown to be stable to the reducing conditions and the product of degradation was found to be a mixture of AIC and a structurally related compound. However, the mixture could not be separated, nor was the unknown compound identified. 

None of the compounds in the sequence was isolated or synthesized, but then-work validated the idea that the best approach to preparing AIC from hypoxanthine was likely to result from initially reducing hypoxanthine to its dihydro derivative and then hydrolyzing the XXIX produced to AIC. 

The ion formation theory also provides an explanation for the fact that strong alkali fails to hydrolyze 8-chloropurine,63 whereas even weak alkali rapidly converts the 6-chloro analog to hypoxanthine. Similarly, as expected, both 2,6- and 6,8-dichloropurine are hydrolyzed only at the 6-position. By using an easily removable blocking group such as tetrahydropyranyl at the 9-position of trichloropurine, it is possible to prepare either 6- or 8-substituted derivatives as required.61 (Scheme 1 )... 

Poly(vinyl alcohols) bonded with nucleic acid bases through phosphate linkages were prepared. Contents of uracil, thymine and hypoxanthine in the polymers were about 50 to 60 mol-%, and that of adenine was about 10%. Interactions of these polymers with DNA in aqueous solution were studied. The apparent hypochromidty was 6.5% for adenine substituted poly(vinyl alcohol) - DNA and 3% for the corresponding uracil substituted derivative83. ... 

The enzyme 5 -nucleotidase dephosphorylates IMP to inosine and P. Thus, since this reaction represents a possible fate for the IMP formed by the transferase (Fig. 10.7), reconstitution studies were undertaken with the nucleotidase. These studies were carried out using the HPLC assay method developed for the HGPRTase activity. A reaction mixture was prepared that contained hypoxanthine and PRibPP as substrates. The reaction was started by the addition of purified HGPRTase enzyme. Samples were removed and were analyzed by HPLC. The chromatographic profiles obtained at 0,10, 20, and... 

While the formation of AMP from hypoxanthine is surely a good start, the salvage will be truly successful only if the AMP is converted to ATP. However, prior to continued phosphorylation, the AMP formed by the two-enzyme reaction sequence described above can undergo another fate—deamination to form IMP and ammonia (see Fig. 10.7). Since the HPLC method is able to separate AMP and IMP, reconstitution experiments were again undertaken to determine whether the HPLC could follow this reaction as well. A reaction mixture was prepared, AMP formed, and an AMP deaminase was added to the reaction mixture. Samples were again removed and, as shown in Figure 10.10, the addition of the AMP deaminase resulted in the conversion of AMP to IMP. Thus, this reaction sequence also can be followed. 

In some cases the combination of two biocatalysts in a multistep process allows, in an indirect manner, the preparation of some compounds or increases in their yields. Thus, Scheme 10.21 shows that the synthesis of a nucleoside analog containing a hypoxanthine base takes place in a two-step fashion using a combination of both nucleoside phosphorylase and adenine deaminase, obtaining the final analog in high yield. However, the direct preparation of this compound with hypoxanthine takes place with a very low yield due to the low solubility of this base [48]. 

Bromopurines have also -been prepared from oxopurines and phosphoryl bromide. In this way 6-bromopurine was produced in 40% yield from hypoxanthine (56JA3508) but xanthine only furnished a low (10%) yield of 2,6-dibromopurine. 

Ethyl acetate and ethyl propionate in ethanolic sodium ethoxide with 5-aminoimidazole-4-carboxamide and its riboside have provided a route to 2-methyl- and 2-ethyl-hypoxanthine arid corresponding 2-alkylinosines, respectively (67JOC3258,68JA2661). 2-Methylhypoxan-thine has also been prepared from the same imidazole and ethyl orthoacetate via an intermediate ethoxymethylene derivative which cyclized to the purine when heated (60JA3144). The same method has been applied to the synthesis of 2,8-dimethyl- and XS-diphenyl-hypoxanthine from appropriate imidazole precursors. 

The purines are exceptional when compared to the majority of heterocyclic compounds in that the most useful starting materials for their preparation are often of natural occurrence. This reflects of course the widespread occurrence of adenine and guanine and their nucleosides as components of nucleic acids. Thus adenine and guanine may be obtained by acid hydrolysis of nucleic acid (38CB718) and hypoxanthine is readily produced by deamination of adenine with nitrous acid (38CB718) or by adenosine deaminase. 

Examples of all the four purine iV-oxides are known. Purine 1-oxide may be prepared from purine and perbenzoic acid over two weeks (62JOC567), and adenine 1-oxide is similarly prepared from adenine over 2 days (58JA2759). On the other hand hypoxanthine 1-oxide is best obtained by deamination of adenine 1-oxide (66JOC966). Reaction of 5-amino-4-hydroxyformamidinoimidazole with carbon disulfide may be used to make 2-thioadenine 1-oxide which with alkaline peroxide provides a useful route to isoguanine 1-oxide (67JOC1151). 

A major route to thioxopurines involves thiation of the corresponding oxopurine. Thus a good yield of 6-thioxo-l,6-dihydropurine is obtained from hypoxanthine and phosphorus pentasulfide (61JA4038). Thiation of an appropriate diaminopyrimidine and concomitant cyclization to a thioxopurine is sometimes possible as in the preparation of 2-amino-l-methyl-6-thioxo-l,6-dihydropurine from 2,6-diamino-5-formamido-3-methyl-4-oxo-3,4-dihydropyrimidine and phosphorus pentasulfide in refluxing pyridine over 30 hours... 

A rational synthesis of purine from 2-carbamimidoyl-2-(formylamino)acetamide with formamide gives hypoxanthine (8). Hypoxanthine (8) can also be prepared from 2-amino-2-carbamimi-doylacetamide and ethyl orthoformate. 

All of the N-monomethyl derivatives of adenine, hypoxanthine, guanine, xanthine and uric acid have been synthesized. Several A -methylpurine derivatives were prepared by Traube himself, e.g. 1-methylguanine (1), or by the protocol of the Traube synthesis. ... 

To prepare the 2-trifluoromethyl derivative of hypoxanthine 17, 5-aminoimidazole-4-carb-oxamide hydrochloride is refluxed with trifluoroacetamide. ... 

Useful precursors for the preparation of substituted hypoxanthine derivatives are alkyl 4(5)-aminoimidazole-5(4)-carboxylates. Thus, reaction of ethyl 4-amino-l-arylimida7ole-5-carb-oxylates with thioamides in the presence of a catalytic amount of formic acid gives 2-substituted 7-arylhypoxanthines, e.g. formation of l. °... 

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