Pyrimidines base-catalyzed

Hydrolyses of aminopyridopyrimidines to the corresponding pyridopyrimidones by means of acid, base, and nitrous acid have been reported. 4-Amino compounds are stable to nitrous acid, but are much more labile than the 2-amino derivatives toward acid- or base-catalyzed hydrolysis. The aminochloro-pyrido[2,3-d]pyrimidine (160) has been converted into the 2,4-dianilino analog (161) by reaction with aniline." ... 

E. C. Taylor and his co-workers have demonstrated an important principle in the ring-opening of pyridopyrimidines and other fused pyrimidine systems to o-aminonitriles. They have demonstrated that based-catalyzed cleavage of a 4-substituted pyrimidine will occur provided that (a) the anion formed by the attack by the base at the 2-position can be stabilized by appropriate structural features in the remainder of the molecule and (b) that the substituent attached to the 4-position is capable of departure with its bonding pair of electrons in... 

Assignment of the l,2,4-triazolo[l,5-c]pyrimidine structures to the products obtained from the previously described cyclizations and not the alternative [4,3-c] structures has been rationalized and corroborated on the basis of (a) preference of cyclization at the more nucleophilic triazole ring N2 rather than at its less nucleophilic N4 (65JOC3601 88JMC1014), (b) inability of the obtained products to undergo acid- or base-catalyzed Dimroth rearrangement, a property characteristic of the thermodynamically less stable [4,3-c] isomers (91JMC281), (c) comparison with unequivocally prepared... 

The nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are biological polymers that act as chemical carriers of an organism s genetic information. Enzyme-catalyzed hydrolysis of nucleic acids yields nucleotides, the monomer units from which RNA and DNA are constructed. Further enzyme-catalyzed hydrolysis of the nucleotides yields nucleosides plus phosphate. Nucleosides, in turn, consist of a purine or pyrimidine base linked to Cl of an aldopentose sugar—ribose in RNA and 2-deoxyribose in DNA. The nucleotides are joined by phosphate links between the 5 phosphate of one nucleotide and the 3 hydroxyl on the sugar of another nucleotide. 

Chiral 4,7-disubstituted perhydropyrazino[l,2-f]pyrimidine-l,6,8-triones 170 were synthesized from [4+2] atom fragments by reacting separately, the four diastereomers of 169 with isocyanates in a base-catalyzed cyclization (Scheme 23) <2002TL4899>. 

Figure 11.3 shows spectra from a cubane pair in an enzyme called DPD, dihydropyrimidine dehydrogenase (Hagen et al. 2000). It catalyzes the first step in the breakdown of pyrimidine bases. The two cubanes have unusually low reduction potentials, and so they are reduced to the [4Fe-4S]1+ form with S = 1/2 ground state by means of... 

Dioxane solutions of 4(5)-aminoimidazole (64) have been treated with several reagents [92JCS(P1)2789], Imidazo[l,5-a]-l,3,5-triazines (65 R = Ph, Me SMe) were obtained by reaction with methyl N-cyanobenzimidate (57 R = Ph), ethyl N-cyanoacetimidate (58 R = Me), and dimethyl yV-cyanodithioiminocarbonate (59 R = SMe). The uncy-clized product (68 R = H) (73%), obtained by condensation with ethyl N-cyanoformimidate (58 R = H), underwent facile cyclization to give the imidazo[l,5-a]-l,3,5-triazine (65 R = H). Likewise, the condensation adduct (69 R = H) (78%) was obtained using ethoxy methyleneurethane (60 R = H) and this underwent base-catalyzed cyclization to give the imidazo[ 1,5-a]-1,3,5-triazinone (67). Imidazo[l, 5-a]pyrimidines (66 R = H, Me) were obtained from the condensation of 4(5)-aminoimidazole... 

Base-catalyzed addition of glycosyl oxides for anomeric O-activation has been extended meanwhile to trifluoroacetonitrile (see Scheme 9), to dichlor-oacetonitrile, to l-aryl-l,l-dichloroacetonitriles, and to ketene imines (46,51,52). Also 2-(glycosyloxy)-pyridine and -pyrimidine derivatives were readily prepared from the corresponding 2-halo precursors (78). However,... 

Addition of nucleophiles to the cyano group of cyanothiadiazole under basic conditions takes place with unusual ease <88AG(E)434,94ACS372). Hydrolysis to the amide, for example, can be effected at 0°C in the presence of a catalytic amount of sodium hydroxide or basic ion-exchange resin. At reflux temperature, hydrazine and monosubstituted hydrazines convert 3,4-dicyano-l,2,5-thia-diazole into the l,2,5-thiadiazole[3,4-. The base-catalyzed addition of acetone to cyanothiadiazole forms an enamino ketone, used as a key intermediate for the synthesis of a number of heterocyclic ring systems, e.g. isothiazole, isoxazole, pyrazole, pyrimidine, and thiazole <77H(6)1985>. 

Free purine and pyrimidine bases are constantly released in cells during the metabolic degradation of nucleotides. Free purines are in large part salvaged and reused to make nucleotides, in a pathway much simpler than the de novo synthesis of purine nucleotides described earlier. One of the primary salvage pathways consists of a single reaction catalyzed by adenosine phosphoribosyltransferase, in which free adenine reacts with PRPP to yield the corresponding adenine nucleotide ... 

Few pyrimidine bases are salvaged in human cells. However, the pyrimidine nucleosides uridine and cytidine can be salvaged by uri-dine-cytidine kinase, deoxycytidine can be salvaged by deoxycytidine kinase, and thymidine can be salvaged by the enzyme thymidine kinase. Each of these enzymes catalyzes the phosphorylation of a nucleoside(s) utilizing ATP, and forming UMP, CMP, dCMP, and TMP. 

Just as orotic acid is converted to a ribonucleotide in step e of Fig. 25-14, other free pyrimidine and purine bases can react with PRPP to give monoribonucleotides plus PP . The reversible reactions, which are catalyzed by phosphoribosyltransferases (ribonucleotide pyrophosphorylases), are important components of the salvage pathways by which purine and pyrimidine bases freed by the degradation of nucleic acids are recycled.273 However, thymine is usually not reused. Thymine will react with deoxribose 1-P to form thymidine plus inorganic phosphate (thymidine phosphorylase), and thymidine is rapidly... 

This approach was also employed for pyridolT id thienoP -Z pyrimidine derivative 139, which can be prepared according to the following two procedures by the one-pot base-catalyzed reaction of pyrimidinethione 140 with ethyl 4-chloro-acetoacetate or by cascade heterocyclization of pyrimidine 141 (2003IZV2069). 

Chloroperoxidase derived from the microorganism Caldariomyces fumago combined with sodium chloride and hydrogen peroxide in buffered aqueous solution converts olefins to chlorohydrines374"376. Chloroperoxidase immobilized on a polymeric support catalyzes the bromination of barbituric acid in a continuous flow system377. Detailed mechanistic study on the mono- and dibromination of barbituric acid catalyzed by bromoperoxidase from Ascophyllum nodosum has been reported by Franssen and coworkers378. Pyrimidine bases are chlorinated and brominated by chloroperoxidase379. 

Itahara T, Ide N (1987) Chloroperoxidase catalyzed halogenation of pyrimidine bases. Chem Lett 12 2311-2312... 

Second-Order Rate Coefficients for Base-Catalyzed Deuterium-Hydrogen Exchange in 5-Substituted Pyrimidine A/-Oxides"... 

In pyrimidine, the most likely metallation site for lithiation is C-5 (cf. base-catalyzed hydrogen exchange, Section 2), but even two methoxy groups in the 4- and 6-positions failed to allow lithium-hydrogen exchange at -40°C in this system (65JCS6695). At lower temperature (<-70°C), however, 5-pyrimidinyllithium derivatives can be prepared by... 

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