5-Furan-2yl[1,3,4]oxadiazole-2-thiol, 5-Furan-2yl-4H [1,2,4] triazole-3-thiol and Their Thiol-Thione Tautomerism

5-Furan-2-yl[1,3,4]oxadiazole-2-thiol (Ia) and 5-furan-2-yl-4H-[1,2,4]- triazole-3-thiol (Ib) were synthesized from furan-2-carboxylic acid hydrazide. Mannich bases and methyl derivatives were then prepared. The structures of the synthesized compounds were confirmed by elemental analyses, IR and 1H-NMR spectra. Their thiol-thione tautomeric equilibrium is described.


Results and Discussion
The characterization data of compounds Ia and Ib are given in the Experimental section and that of the other compounds synthesized is summarized in Table 1. All the newly synthesized compounds gave satisfactory analyses for the proposed structures, which were confirmed on the basis of their IR and 1 H-NMR spectral data. The IR spectra of these compounds showed moderately strong bands around 3100-3360 cm -1 , 1600-1650 cm -1 and 1250-1270 cm -1 , characteristic of the NH, C=N and C=S groups, respectively. In the 1 H-NMR spectra, a characteristic signal due to the -N-CH 2 -N-protons appeared at 5.00-6.05. The signal due to the NH protons appeared at 5.50-5.52. The signals due to the aromatic protons appeared as multiplets at 6.50-8.40.
We have observed that extensive thiol-thione tautomerism exists in compounds Ia and Ib. In the 1 H-NMR the signal of the SH protons were recorded, although they were very weak and also the ready synthesis of the Mannich bases IIa, IIIa-g, IIb and Ib [16] from Ia and Ib confirmed the tautomerism. It has been reported that the crystal structures of Ia-and Ib-like compounds correspond to the thione form [17][18][19], but the reaction conditions for the synthesis of IIa prove that Ia can be in the thiol form too. Finally, the crystal structures of Ia and Ib [17,18] corresponded to the thione form, but they showed thiol-thione tautomerism in solution.

General
Melting points were determined in open capillary tubes on a digital Gallenkamp melting point apparatus and are uncorrected. The IR spectra were recorded in KBr with a Mattson 1000 FT-IR spectrometer. 1 H-NMR spectra were recorded on a FX 90 JEOL 90MHz NMR and Varian Gemini 200MHz, spectrometers in CDCl 3 + DMSO-d 6 with TMS as an internal standard. Elemental analyses were done on a LECO-CHNS-938. Starting chemicals were obtained from Merck or Aldrich. [1,3,4]

General Procedure for the Preparation of IIa,b.
A mixture of thione Ia-b (0.005 mole), sodium hydroxide (0.005 mole, 0.2 g), and methyl iodide (0.006 mole, 0.840 g) was stirred in water for 14 h. The resulting thioether solution was removed by vacuum evaporation, and the products collected by filtration, washed with water, dried and recrystallised from a suitable solvent. Spectroscopic and physical data are summarized in Table 1.

General Procedure for the Preparation of IIIa-g.
A mixture of Ia (0.01 mole, 1.56 g) and an alkyl or aryl amine (0.01 mole) was refluxed in ethanol (50 mL) with 36 % formaldehyde (0.02 mole, 1.7 mL) for 3 h. The resulting solid was crystallised from a suitable solvent. Spectroscopic and physical data are summarized in Table 1.