Synthesis and Chemical Characterisation of Some New Diheteroaryl Thienothiophene Derivatives

Treatment of 1-(5-acetyl-3,4-dimethythieno[2,3-b]thiophene-2yl)ethanone (1) with dimethylformamide dimethyl acetal afforded enaminone derivative 2, which reacted with amino derivatives to give the corresponding bis-pyrimidine, bis-pyrazole, bis-triazolo-pyrimidine and bis-benzoimidazopyrimidine derivatives.

The structures of the latter products were deduced from their elemental analyses and spectral data. The IR spectrum of compound 5a, for example, showed, the absence of carbonyl bands and revealed the appearance of bands in the 3,417 and 3,100 cm −1 region due to NH 2 groups. The structure of product 5a was confirmed by the 1 H-NMR spectrum, which displayed a new pair of doublet signals at δ 7. 83, 8.37 with J = 12 Hz corresponding to pyrimidine CH protons, as reported for such E-coupled protons [30][31][32]. The 1 H-NMR spectrum also revealed one singlet corresponding to a methyl group at δ 2.21, in addition to the NH 2 protons at δ 4.83 in Scheme 2. The formation of compound 5a would involve an initial addition of the amino group in guanidine to the activated double bond in enaminone derivative 2, followed by deamination to an intermediate which then undergoes cyclization and aromatization via loss of water affording the final isolable product (Scheme 2). The compound 7,7'-(3,4-dimethylthieno[2,3-b]thiophene-2,5-diyl) bis- [1,2,4]triazolo[1,5-a] pyrimidine (6) was formed initially via Michael type addition followed by elimination of dimethylamine and water molecules when treatment of compound 2 with 5-amino-1,2,4-triazole in refluxing ethanol/DMF afforded in (Scheme 3).
In the 1 H-NMR spectra of compound 6 the CH proton appeared as a pair of doublets at 7.66/7.69 ppm (d, 2H, CH, J = 12 Hz), and 9.09/9.12 ppm (d, 2H, CH, J = 12 Hz) due to vicinal coupling with the two magnetically non-equivalent protons of the methylene group at position 5 and 6 of the pyrimidine ring. Also, the 1 H-NMR spectrum showed one singlet corresponding to a methyl group at δ 2.24, in addition to the CH proton of triazole at δ 8.62. The mass spectrum revealed a molecular ion peak at m/z 404, corresponding to C 18 H 12 N 8 S 2 . In a similar manner, when 2 was treated with 2-aminobenzimidazole, the corresponding compound 7 was obtained in high yield.

General
All melting points were measured on a Gallenkamp melting point apparatus. The infrared spectra were recorded in potassium bromide disks on a Pye Unicam SP 3300 or Shimadzu FT IR 8101 PC infrared spectrophotometers. The NMR spectra were recorded on a Varian Mercury VX-400 NMR spectrometer. 1 H spectra were run at 400 MHz and 13 C spectra were run at 75.46 MHz in dimethyl sulphoxide (DMSO-d 6 ). Chemical shifts were related to that of the solvent. Mass spectra were recorded on a Shimadzu GCMS-QP 1000 EX mass spectrometer at 70 e.V. Elemental analyses were carried out at the Microanalytical Center of King Saud University, Riyadh, Saudi Arabia. (2)

General Procedure for the Reaction of Compound 2 with Hydrazine Derivatives
Treatment of compounds 2 (1 mmol) with hydrazine hydrate or phenyl hydrazine (0.1 mL) in dry ethanol (20 mL) under reflux for 7 h afforded the corresponding derivatives 3a and 3b, respectively. The solid products were collected by filteration, washed with ethanol, dried and recrystallized from DMF/EtOH.

General Procedure for the Reaction of Compound 2 with Guanidine, Thiourea and Urea
Treatment of compound 2 (1 mmol) with guanidine, thiourea or urea (2 mmol) after making sure they dissolve in DMF (2 mL) in dry ethanol (20 mL, 99.9%), under reflux for 6-8 h. afforded the corresponding derivatives 5a-c respectively. After the solid products were collected by filtration, washed with ethanol, dried and recrystallized from DMF/EtOH.