Oxidation of 5 , 6-Diamino-1 , 3-dimethyl-2 , 4-dioxopyrimidine by Perrhenate . The Crystal Structure of 1 , 3 , 6 , 8-Tetramethylpyrimidopteridine-2 , 4 , 5 , 7-tetrone

A oxidação de 5,6-diamino-1,3-dimetil-2,4-dioxopirimidina (H 2 ddd) por perrenato (ReO 4 ) leva à formação de 1,3-dimetilaloxano, que condensa com H 2 ddd não-oxidado para produzir 1,3,6,8-tetrametilpirimidopteridino-2,4,5,7-tetrona (tppt). A estrutura do tppt consiste de um anel pirazínico central e dois anéis pirimidínicos terminais em posições cis. Os ângulos diedros entre os anéis pirazina e pirimidina são de 1,08° e 1,20°.


Experimental
Ammonium perrhenate and H 2 ddd were obtained commercially from Aldrich.Infrared spectra were obtained using KBr pellets on a Nicolet 20 DXC FTIR spectrophotometer in the 4000-200 cm -1 range, and 1 H NMR spectra were recorded at 300.13 MHz (7.05 T) on a Bruker AMX-300 spectrometer in dimethylsulfoxide-d 6 , (CD 3 ) 2 SO, with tetramethylsilane (tms) as reference.Elemental analysis was carried out by the Department of Chemistry at the University of the Western Cape in Cape Town.

Synthesis of tppt
Equimolar quantities of (NH 4 )[ReO 4 ] (100 mg, 115 µmol) and H 2 ddd (20 mg, 117 µmol) were added to methanol (20 cm 3 ), and the mixture was heated under reflux for 3 h under nitrogen.After cooling to room temperature, the solution was filtered (no precipitate formed) and left to evaporate slowly at room temperature.

X-ray crystallography
Intensity data for tppt were collected at 200(2) K on a Nonius Kappa CCD single-crystal diffractometer, using Mo-K radiation.Unit cell and space group determinations were carried out in the usual manner. 8For the structure factors, corrections for Lorentz and polarization effects and absorption were made.The structure was solved by direct methods and refined by full matrix least-squares procedures using SHELXL-97. 9All non-hydrogen atoms were geometrically constrained.
Crystal data and structure refinement details for tppt are given in Table 1, with selected bond distances and angles shown in Table 2.

Results and Discussion
The reaction of equimolar quantities of (NH 4 )[ReO 4 ] with H 2 ddd in methanol under reflux led to the isolation of tppt as the only product.The heating of H 2 ddd in the absence of perrhenate led to no reaction.The [ReO 4 ] anion is therefore instrumental in the formation of tppt.It has been shown previously that perrhenate reacts with 1,2-diaminobenzene (H 2 dab) to form the rhenium(VII) cationic complex [Re(dab) 3 ] + , where dab is coordinated as the diamide. 10The dab dianion can be readily oxidized by two electrons to the neutral benzoquinonediimine. 11 The product tppt contains the terminal pyrimidine rings in cis positions.This phenomenon has also been observed by the reaction of H 2 ddd with Fe 3+ , which also gives tppt as product. 12However, the metal ions Au 3+ , Hg 2 2+ , Ag + , and Tl 3+ also promote the oxidation of H 2 ddd, but then the product tppt contains a trans disposition of the terminal pyrimidine rings. 13n the infrared spectrum of tppt there are two very strong absorption bands at 1696 and 1653 cm -1 , which correspond to the C=O stretching vibrations.Another strong peak at 1542 cm -1 is assigned to (C=N).The CH 3 deformation vibrations are assigned to the medium-intensity band at   atoms.The angular strain around the nitrogen atoms of the two pyrimidine rings is reflected in the C-N-C bond angles, which vary from 122.1(2)° to 125.4(2)°.In the pyrazine ring, the bond angles C(4)-N(3)-C(5)=115.9(2)° and C(3)-N(4)-C(6)=117.1(2)°indicate sp 2 hybridization of the nitrogens.
The packing of the molecule in the unit cell is complemented by the strong inter-molecular hydrogen bonds C (10)  2), in addition to four intramolecular hydrogen bonds (see Table 3).

Conclusions
The oxidative deamination of H 2 ddd can be achieved by its reaction with perrhenate in methanol, to yield 1,3,6,8-tetramethylpyrimidopteridine-2,4,5,7-tetrone (tppt) as product.This compound is probably formed by condensation of the oxidation product alloxan with unoxidized H 2 ddd.The crystal structure of tppt consists of a central pyrazine ring and two condensed terminal pyrimidine rings in cis positions.I.B. is grateful to the NMMU and NRF for financial support.
1428 cm -1 , as was suggested in the literature. 14Two singlet peaks of equal intensity at 2.89 and 2.73 ppm in the 1 H NMR spectrum are ascribed to the C(9)H 3 /C(11)H 3 and C(10)H 3 /C(12)H 3 hydrogens respectively.The compound is soluble in a wide variety of solvents, including water, chloroform, acetonitrile, dimethylsulfoxide, dimethylformamide and dichloromethane.
The X-ray crystal structure of tppt is shown in Figure 1, together with the atom labelling scheme.The structure consists of tricyclic rings, comprising a central pyrazine ring and two terminal pyrimidine rings.The pyrimidine and pyrazine rings are essentially planar, with maximum deviations from the calculated mean plane of 0.008 Å and 0.004 Å respectively.In the pyrimidine rings there are some steric interactions involving the exocyclic groups, as shown by the short C( 9

Figure 2 .
Figure 2. ORTEP drawing of the intermolecular hydrogen bonds in tppt.

Table 1 .
Crystal data and structure refinement data for tppt