Synthesis of 4-Amino-4,5-dihydro-1H-1,2,4-triazole-5-ones and their Isatin-3-imine Derivatives

Iminoester hydrochlorides 1 have been synthesized. These compounds were then converted into ester ethoxycarbonyl hydrazones 2, from which in turn a new series of substituted 4-amino-4,5-dihydro-1H-1,2,4-triazole-5-ones, 3, was then prepared. Finally a set of isatin imine derivatives 4 was obtained from the reaction of compounds 3 with isatin. The structures of all the new synthesized compounds were confirmed by elemental analyses, IR, 1H-NMR and 13C-NMR spectra.


Results and Discussion
The new derivatives were prepared following the reaction sequences depicted in Scheme 1. Iminoester starting materials 1a-d were prepared by passing HCl gas through solutions of 2-chlorobenzyl cyanide, 3-chlorobenzyl cyanide, 2-methylbenzyl cyanide or 3-methylbenzyl cyanide and absolute ethanol, followed by precipitation with ether.

Scheme 1
In the IR spectra of compounds 1a-d the characteristic NH 2 + absorption bands appeared at 2980, 2920 and 810 cm -1 and the C=N band at 1640 cm -1 , approximately. Reaction of the iminoesters with ethyl carbazate yielded the ethoxycarbonyl hydrazones 2a-d. The IR spectra of these compounds show the C=O band at 1740 cm -1 and the C=N band at 1600 cm -1 , approximately. The obtained compounds 2a-d were then refluxed with a solution of hydrazine hydrate in water to afford the compounds 3a-d.
The NH 2 , C=O and C=N bands observed in the IR spectra of the latter compounds match the peaks expected for these structures. In addition to this, observation of NH 2 , NH and aromatic protons, as well as the disappearance of CH 2 CH 3 signals in the 1 H-NMR spectra and the triazole C 5 peak at near δ 154 ppm and the triazole C 3 peak at near δ147 ppm in the 13  structures of the isatin-3-imines were verified by the presence in the IR spectra of two different C=O absorbtion bands at around 1700 cm -1 and C=N bands near 1690 cm -1 . In addition to this, the appearance of signals corresponding to two different NH protons and the disappearance of the NH 2 protons in the 1 H-NMR spectra support the proposed structures. The 13 C-NMR spectra also support the expected structures.

Conclusions
New compounds 3a-d, which can be used to prepare many new compounds, have been synthesized. Some of the new derivatives might be important biologically active agents. The compounds 4a-d are also potential biologically active agents and their medical research applications should be investigated.

General
Melting points were determined in open capillary tubes on a Büchi oil heated melting point apparatus and are uncorrected. The IR spectra were recorded for KBr pellets on Perkin-Elmer 1600 FTIR spectrophotometer. 1 H-NMR and 13 C-MR spectra were recorded on a Varian 200A spectrometer (solvent DMSO-d6, TMS as internal standard). Elemental analyses were performed on a Carlo Erba 1106 CHN analyzer. Starting materials were obtained from Fluka or Aldrich.

Synthesis of Iminoester Hydrochlorides 1a-d.
These were synthesized using the reported method [25]. To an ice-cooled solution of the appropriate nitrile (1 mole) in absolute alcohol (1.1 moles), dry hydrogen chloride was added until 1.1 moles had been absorbed. The resulting solution was then allowed to stand at 0 ºC in the refrigerator for 12 hours, after which cold absolute ether was added and the obtained crystals were filtered off immediately, washed with cold absolute ether and dried in a dessicator. The following compounds were thus prepared:

General Procedure for Preparation of Ethyl Substituted Formate Ethoxycarbonyl hydrazones 2a-d.
In a stoppered flask equipped with a magnetic stirrer, the corresponding ethyl imidocarboxylate hydrochloride (1a-d, 0.01 mol) was dissolved in absolute ethanol (50 mL) with ice-bath cooling and ethyl carbazate (0.01 mol) dissolved in absolute ethanol (20 mL) was then added to this solution. After stirring for 6 hr in ice-bath, the mixture was filtered to remove the ammonium chloride which separated from the solution and the filtrate was evaporated at 30-35ºC under reduced pressure. The solid residue, after drying in a dessicator, was recrystallized from petroleum ether to yield compounds 2a-d.

General Procedure for the Preparation of Isatin-3-imines 4a-d.
Equimolar quantities (0.01 mol) of isatin and the corresponding amino compound 3a-d were dissolved in warm ethanol (40 mL) containing glacial acetic acid (0.5 mL). The reaction mixture was refluxed for 4 hr and then kept at room temperature overnight. The resulting solid was washed with ethanol, dried and recrystallized from ethanol-chloroform to afford compounds 4a-d.