A Facile Method for the Synthesis of Oxoketene-N , Sand-N , N-acetals from Reactions of Amino Compounds

2-Aroyl-3,3-bis(alkylsulfanyl)acrylaldehydes reacted with various primary amines, namely, o-phenylenediamine, ethylenediamine, and anilines to produce functionalized oxoketene-N,S-acetals and N,N-acetals in good yields. Imidazolo derivatives synthesized with o-phenylenediamine and ethylenediamine containing a formyl group could act as valuable starting materials for a variety of substituted heterocyclic compounds.


Introduction
Oxoketene-N,S-and -N,N-acetals are highly versatile synthons for heterocyclic synthesis [1][2][3].They are quite stable and can be stored indefinitely without any decomposition.They exhibit the nucleophilic displacement reactions by various binucleophiles followed by intramolecular cyclisation leading to the formation of cyclic compounds, and such reactions are characteristics of enamines [1][2][3].Junjappa and coworkers prepared a series of functionalized heterocycles by treating them with binucleophiles like hydrazines, hydroxylamines, guanidines, cyanoacetamides, and so forth, [4][5][6].They have also prepared functionalized quinolines from α-oxoketene-N,S-acetals employing Vilsmeier-Haack reaction [7].A similar strategy was reported for the synthesis of quinoxalines from nitroketene-N,S-acetals, important benzoheterocycles displaying a broad spectrum of biological activities [8].Our research group has also made attempts to explore the synthetic utility of oxoketene-N,S-acetals derived from thioamides [9].We have shown that their reactions with α-halo ketones or ethyl bromoacetates afford amino thiophenes [10].
The α-oxoketene-N,S-acetals are generally prepared by the direct amination reactions of oxoketene-S,S-acetals by appropriate amines which resulted in a mixture of oxoketene-N,S-acetals and -N,N-acetals in most of the cases.They have also been synthesized directly from active methylene ketones by treating them with phenyl isothiocyanate followed by alkylation.We envisioned that the 2aroyl-3,3-bis(alkylsulfanyl)acrylaldehydes 2 which we had reported recently could be transformed into aroyl formyl ketene-N,S-acetals or aroyl formyl ketene-N,N-acetals by direct amination reaction.The literature survey shows that such formyl ketene-N,S-acetals and formyl ketene-N,Nacetals have not been reported so far, and such compounds will be established in near future as valuable pivot for a variety of amino substituted heterocyclic compounds.Therefore, it appeared worthwhile to utilize 2-aroyl-3,3bis(alkylsulfanyl)acrylaldehydes for the synthesis of those synthons.
In 1 H NMR spectrum of 5, a doublet at δ 8.11-8.07 with a coupling constant J = 12.9 Hz, corresponding to a vinylic proton was present, and the doublet revealed the presence of a neighbouring proton.The NH proton was also appeared as a doublet at δ 11.92-11.49(J = 12.9 Hz) showing a non hydrogen bonded NH group with a neighboring proton.Due to the presence of doublet for NH proton and vinylic proton in the 1 H NMR spectrum of 5, we expected the structure of the compound as 5B.In the IR spectrum, there was no characteristic NH peaks indicating that there was a chance for rapid exchange of the NH protons between the two structures 5A and 5B.
The literature survey shows that such α-oxoketene-N,S acetals can be transformed to functionalized quinolines by Vilsmeier-Haack reaction [7].So we expected 5 would be easily cyclized under Vilsmeier condition.Therefore, 2-benzoyl-3,3-bis(methylsulfanyl)acrylaldehyde 2a dissolved in DMF was heated with aniline for 10 hrs at 100 • C after 5 h Vilsmeier-Haack reagent was added to the reaction mixture, and it was further stirred for 5 hrs.The reaction on workup gave a mixture of products which we could not isolate in pure forms.Then we expected 3-anilino-1-aryl-3-(methylsulfanyl)-2-[(phenylimino)methyl]-2-propene-1-ones 5 may be cyclized in the presence of glacial acetic acid to get substituted quinolines.Therefore, the acrylaldehydes were treated with two equivalents of aniline in acetonitrile in the presence of glacial acetic acid.The reaction afforded 1-aryl-3,3dianilino-2-propen-1-ones in moderate yields.As these compounds have been effectively prepared from α-oxoketene dithioacetals in good yields [1-3], we have not made further attempts to establish their synthetic utility.
In conclusion, we have developed facile methods for the synthesis of various structurally diverse α-formylketene-N,S-acetals and α-formylketene-N,N-acetals which can serve as starting materials for functionalized heterocyclic compounds.

Experimental
Melting points were determined on Buchi 530 melting point apparatus and were uncorrected.The IR spectra were on KBr pellets on a Schimadzu IR-470 spectrometer, and the frequencies are reported in cm −1 .The 1 H NMR & 13 C NMR spectra were recorded on a Brucker WM (300 & 75.47 MHz) spectrometer using TMS as internal standard and CDCl 3 or acetone as solvents.The CHN analyses were done on an The FAB mass spectra were recorded on a JOEL SX 102/DA-6000 Mass Spectrometer/Data System using Argon as the FAB gas.The EIMS spectra were recorded on a MICROMASS QUATTRO 11 triple quadrupole mass spectrometer All reagents were commercially available and were purified before use.The previously reported aroylketene dithioacetals and formylketene dithioacetals were prepared by the known procedure [1][2][3]11].Anhydrous sodium sulphate was used as drying agent.All purified compounds gave a single spot upon TLC analyses on silicagel 7GF using an ethyl acetate/hexane mixture as eluent.Iodine vapors or KMnO 4 solution in water was used as developing agent for TLC.