1.3- Dimethyl-6- uracil

Treating glycosyl isothiocyanates 415 with 5,6-diamino-1,3-dimethyl-uracil (416) gave thioureas 417, which on oxidative cyclization with N-bromosuccinimide afforded 5,7-dioxopyrimido[5,4- ][1,2,4]triazine nucleosides 418 (80MI1 82MI2). 

The only information about the variation of the hydrate formation rate with light intensity is that the photolysis rate of 1,3-dimethyl uracil is independent of intensity over a 50-fold range.7 Since dimer formation must be bimolecular,106 and since it is a photoreversible phenomenon, the rates of dimer formation are complicated functions of the... 

Six-membered azaenones [e.g., 2,3-dihydropyridin-4(l//)-one,106 uracil,107,1,3-dimethyl-uracil l08] afford [2 + 2] photocycloadducts 34 with 2,3-dimethylbut-2-ene. 

It is interesting to look at the effect of substituents on the dipole moment of uracil (or thymine). An examination of the data collected in Table XX shows that among the four positions (N-l, N-3, C-5, C-6) available for substitution, the one at C-6 is the most sensitive. An alkyl substituent at C-6 of uracil (or thymine) increases the dipole moment by about 0.4 0.5 D, while the same substituent at N-l, N-3, or C-5 has only a small effect. Similarly, a halogen does not greatly alter the dipole moment of the molecule when at C-5 whereas 6-halogen decreases the value of the dipole moment considerably (cf. the data for 1,3- dimethyl-uracil and for its 5- and 6-chloro derivatives, Table XX). An attempt has been made by Kufakowska and Wierzchowski402 to interpret these... 

Reaction of various aromatic or heteroaromatic diamines with iV-acyl isothiocyanates gave the corresponding benzo- or heterocycle-fused l,3,5-triazepine-2-thiones, for example, 16 (Figure 3) from 5,6-diamino-1,3-dimethyl-uracil and D-gluconyl isothiocyanate <1981CPB1832>. Cycloaddition of iVjiV -disubstituted ethylenediamines with bis(isocyanato)dimethylsilane or bis(isothiocyanato)dimethylsilane followed by treatment with 1,1 -carbonyldi-imidazole or 1,1 -thiocarbonyldiimidazole afforded a series of 1,5-disubstituted l,3,5-triazepine-2,4-diones, -2,4-dithiones, and 4-oxo-l,3,5-triazepine-2-thiones <1980AGE327>. 

Theruvathu JA, AravindakumarCT, Flyunt R, von Sonntag C (2001) Fenton chemistry of 1,3-dimethyl-uracil. J Am Chem Soc 123 9007-9014... 

A mixture of 5,6-diamino-1,3-dimethyl-uracil 1 (0.2 g, 1.2 mmol), chalcone 2 (0.25 g, 1,2 mmol) and 0.4 ml of glacial acetic acid in 20 ml of methanol was refluxed for 3 h (Scheme A.33). After cooling, the colorless crystals of 3 precipitated and were filtered. Yield 82%. Melting point 229-231°C. 

The proton affinity and acidity of uracil itself (2a) has also been the subject of computational investigation, primarily by the groups of Zeegers-Huyskens and Lee.52-57 Lee and coworkers have also conducted a series of experimental investigations that have established that uracil has four sites that are more acidic than water (Nl, N3, C5, and C6) and that 04 is 8 kcal mol-1 more basic than 0254-56 Gronert and coworkers also conducted clever mass spectrometric experiments that effected decarboxylation of orotate to form the C6-deprotonated uracil (3a), which was then used to measure the acidity of the C6-H.57,58a Gronert s calculations and experiments, later confirmed by Lee and Kurinovich using different uracil derivatives,55 established that the C6 site of uracil is quite acidic with a gas phase acidity (A//acid) of —369 kcal mol-1, this site is as acidic as acetone. Recent experiments in water, however, indicate that the C6-H of 1,3-dimethyl uracil has a pK of 34, considerably less acidic than that of acetone (pA a — 19).58b... 

Our study of the photostability issue of nucleic acid bases began with isolated bases, and later extended to their water complexes [15, 32], Our observation has thus revealed not only intrinsic properties of the bases, but also the effect of the environment on the decay mechanism of the excited state. Most of our results have been obtained from 1,3-dimethyl uracil (DMU) due to the ease of vaporization. To generalize our conclusion, however, we have also investigated 1-methyl uracil (MT), thymine (T), and 1,3-dimethyl thymine (DMT). In particular, we believe that 1-methyl uracil is an excellent mimic of uradine because of the similar substitution position on uracil. 

Figure 11-6. Lifetimes of 1-methyl uracil, 1,3-dimethyl uracil, 1,3-dimethyl thymine, and thymine at different excitation wavelengths. (Reproduced with permission from J. Phys. Chem. 2004, 108, 943-949. Copyright 2004 American Chemical Society.)...

More recently the term bifurcated has also been applied to the configuration 5, proposed for interactions between water and dimethyl phosphate or formate ions [68]. An unusual bifurcated configuration 6 is reported in 1,3-dimethyl uracil [69]. 

This is equivalent to the corresponding CC12 configuration in cytosine (Fig. 16.2), which is least commonly observed with this base. It is also interesting to note that the same type of molecular association UU12 occurs in 1,3-dimethyl uracil [DMURACJ (Fig. 16.4), where the NH -0=0 bonds are replaced by an unusual H... 

Cycloaddition between electron-rich 6-[(dimethylamino)methylene]amino-1,3-dimethyl uracil 242 and various electron-deficient substrates such as quinones 243 and coumarins 244 was considered to take place by elimination of dimethyla-mine from the respective cycloadducts, followed by oxidative aromatization in a highly regiospecific manner to give pyrido[2,3-solvent-free conditions, the times were reduced to 6-7 min and the yields were increased from 70-80% to 85-94% (04SL1179). 

The Paternd-Buchi reaction of 1,3-dimethylthymine and 1,3-dimethyl-uracil with benzophenones gives rise to two regioisomeric oxetanes (31). Substituent, temperature and heavy atom effects on the reaction are discussed in terms of entropy v.s. enthalpy control. Irradiation of uracil in frozen aqueous solution produces two diastereomeric (6-4) products. 

Acaianuno (bzw. 5)>[4.aeataminO phai l].imidazol 26 1 682. 4 Amino -benq deDaiiiiiio-1.3 dimethyl-uracil 25, 495. 

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