A novel transamination process in 3-arylamino-5 , 5-dimethylcyclohex-2-en-1-one with nucleophiles and antimicrobial activity of selected products

Reaction of dimedone with arylamines afforded the respective enaminones. Some N-acetyl derivatives were prepared. The reaction of enaminones with hydroxylamine was found to be dependent on the nature of the arylamine moiety and the molar ratio of hydroxylamine to give the mono-, dior tri-oximes. Cyclization of the trioxime by acetic anhydride gave dihydrobenzo[c][1,2,5]oxadiazol-4(5H)-one-5-O-acetyloxime. Coupling of the synthesized enaminones with benzene diazonium chloride gave 2,4-bis-phenylhydrazones or a mixture of 2-mono hydrazones and bis-hydrazones depending on the nature of the arylamine moiety. Trans-amination of the arylamine of the bis hydrazones with hydroxylamine gave the same1,3-bis oxime derivative. The structures of the synthesized compounds were confirmed by IR, HNMR, C NMR spectra. The antimicrobial activity of some selected products showed promising results.

Herein, we report a mild and efficient method for the synthesis of 3-arylamino-5,5-dimethylcyclohex-2-en-1-one 8-13 (Scheme 1) in 88-96% yield, by boiling dimedone with the arylamines 1-6 in acetic acid for two hours.Under similar conditions, the antipyrinyl derivative 5,5-dimethyl-3-[N-(2,3dimethyl-1-phenylpyrazolin-5-on-4-yl)]aminocyclohex-2-en-1-one 14 was obtained in low yield.A better yield (50%) of 14 was obtained when dimedone reacted with 7in formic acid either under reflux for 30 minutes or at room temperature for 24 hours.The structure of 14 was deduced from its spectral data.Thus, the FAB mass spectrum showed a molecular ion peak at m/z 326 [M +• +1], and its elemental analysis agreed with the molecular formula C19H23N3O2.The 1 H-NMR spectrum of 14 showed the vinylic C-2 proton as a singlet atδ 5.04 ppm and the NH-proton as a broad singlet at δ 7.98 ppm.The other signals agreed with the assigned structure (experimental section).Scheme 1. Synthesis of enamine 8-14 and their N-acetyl derivatives Acetylation of 9-11 with acetic anhydride under reflux afforded the N-acetyl derivatives 15-17, respectively as that deduced from their spectral analyses.The mass spectrum of 16 showed a molecular ion peak at m/z 271 which in agreement with the introduction of only one acetyl group.The 1 H-NMR spectra of 15-17 showed the absence of the signal corresponding to the NH-proton and the appearance of a singlet at δ 1.82-2.11ppm corresponding to only one acetyl group.Their vinylic C-2 proton resonated at δ 5.12-5.51ppm, and one of the two methylene protons were assigned to the two singlets at δ 2.12 -2.30 and 2.46-2.73ppm.Whereas, the second methylene of 16 appeared as ABq at δ 2.95 ppm with JABq 17.2 Hz.The 13 CNMR spectra of 16 agreed with the assigned structure.
The reaction of 9 and 11 with 1.2 equivalent of hydroxylamine hydrochloride in the presence of sodium acetate for two hours afforded the mono oxime derivatives 18 and 19, respectively.Their 1 H-NMR spectra showed a signal for the N-OH proton at the downfield region (δ 10.58 ppm) in addition to the NH-proton assigned at δ 11.07-11.20 ppm.When compounds 10, 12 and 13 were similarly treated, only slight conversion to their mono oximes was detected by using TLC.On the other hand, increasing the molar equivalents of hydroxylamine hydrochloride to 2.4 was required to yield the dioxime 20 within 3-4 hours 31 .Thus, the mass spectrum of 20 showed a molecular ion peak at m/z 170 which indicated a trans-amination process of the aryl amine moiety had taken place with an oxime group.Also, the 1 H-NMR spectrum of 20 showed the disappearance of the aryl protons and the presence of D2O exchangeable singlet of two protons intensity at δ 10.42 ppm due to the two N-OH protons.
The possible existence of the enol form 21 was excluded since C-4, and C-6 methylene protons were assigned for a singlet at δ 2.13 ppm of four protons intensity whereas those of C-2 were assigned at a higher frequency (δ 3.40 ppm) in addition to the absence of the vinyl H-2 proton.The 13 C-NMR of 20 showed signals corresponding to C-1 and C-3 at higher frequency region at δ 153.04 ppm.The two methylene carbons C-4 and C-6 were assigned at δc 32.18 ppm where the flanked C-2 was assigned to the higher frequency signal at δc 44.35 ppm because of the two oxime groups at C-1 and C-3.
When the hydroxylamine hydrochloride was increased to five equivalents in its reaction with enaminones 9-13, an unexpected product was obtained within two hours from each, in 50-76 % yield which could be assigned the trioxime structure 22 32 .The 1 H-NMR spectrum of 22 showed the presence of two singlet signals at δ 2.42 and 2.45 ppm which were assigned to the two methylene protons.The spectrum showed the absence of signals corresponding to the aryl moiety and the presence of three D2O exchangeable signals at the downfield region at δ 11.33, 11.89 and 12.77 ppm corresponding to the three N-OH protons.The presence of last three signals at a low magnetic field indicated their involvement in hydrogen bonding; two within the chelated structure and the third one can be hydrogen bonded with DMSO-d6that was used as a solvent (Scheme 2).The mass spectrum of 22 confirmed the assigned structure which showed a molecular ion peak at m/z 198 [M +• +1].Loss of NOH fragment gave a bis-oxime 20 as the base peak at m/z 170.The 13 C-NMR spectrum of 22 showed a signal for C-1, C-2 and C-3 at δc 153.79 ppm whereas C-4, C-5 and C-6 were assigned at lower frequency region at δc 37.45, 32.18 and 44.32 ppm.4][35][36] The synthesis of 1,2,5oxadiazole can be readily achieved by the dehydration of vicinal dioximes in basic or acidic media [37][38][39][40] .Thus, dehydrative cyclization of 22 with acetic anhydride under reflux for two hours gave the respective 1,2,5-oxadiazole 24.The 1 H-NMR spectrum of 24 showed the absence of N-OH protons at the downfield region of its precursor 22 and the appearance of a new signal at δ2.27 attributed to an N-O-acetyl proton that confirmed the cyclization of the vicinal dioxime to form the 1,2,5-oxadiazole 23 which subsequently acetylated under the reaction conditions to give 24 (Scheme 2).The cyclization process and O-acetylation of the oxime group were confirmed from its 13 C-NMR (CDCl3) spectrum that showed three signals at δc134.2, 144.51 and 161.3 ppm corresponding to C-7a, C-4a, and C-4, respectively.The O-acetyl group carbons were confirmed by the presence of signals at δ 19.85 (Me) and 171.93 ppm (CO).
On the other hand, coupling the enaminones 9-11 with benzene diazonium chloride (5 molar equivalent) give compounds 29-31, their structures were deduced from their 1 H NMR spectral data which showed the absence of signals corresponding to both vinylic H-2 and the presence of only one of the methylene singlets of dimedone moiety.The disappearance of one of the methylene groups indicated its involvement in the reaction.Moreover, extra ten protons at δ 6.61-7.38 ppm were assigned to the aromatic protons, attributed to two phenyl groups in addition to those of the pre-existing aryl group.Furthermore, the mass spectrum of 30 showed a molecular ion peak at m/z 437.0 [M +• -1] which in agreement with the bis-hydrazone 30 rather than their monohydrazone 26.Accordingly, a double azo coupling reaction took place by successive electrophilic attacks of two diazonium ions on C-2 to give 26 which subsequently attacked one of the methylene group at C-6 to give 28 or on the methylene group at C-4to give 29-31.The formation of 29-31 rather than 28 was confirmed from 13 C-NMR spectrum of 30 which showed three signals at the lower frequency region at δc 28.41, 37.61 and 52.67 ppm characteristic for two methyl groups and methylene group at C-5 and C-6, respectively whereas signals corresponding to C-4, C-2 and C-3 were shifted to the downfield to resonate at δ 144.54, 150.31 and 154.47 ppm, respectively.The introduction of the second phenylhydrazone group at C-4 rather than C-6 could be as a result of activation of this methylene group by the neighboring enamine group.The formation of the hydrazones 29-31 rather than azo tautomer was established from the assignment of only two exchangeable N-H protons δ 14.00-14.59ppm whereas the second N-H was overlapped with the aromatic protons for compounds 29 and 31 whereas compound 30 showed two NH signals at δ at 14.60 ppm and 14.88 ppm.The presence of N-H proton at the higher frequency region (14.00-14.60 and 14.54-14.88)suggested its involvement in a strong intramolecular hydrogen bonding 41 .Moreover, the assignment of the location of the second molecule from diazonium salt at C-4 as hydrazone rather than at C-6 as in 28 has been based on spectral studies of related compounds reported by Simunek et al. [42][43] .
On the other hand, the enaminones12 and 13 under the same reaction conditions afforded a mixture of the monohydrazone (26a, 26b) and the bishydrazones (32 and 33), respectively which were fractionally separated by crystallization from ethanol.Consequently, decreasing the equivalent of benzene diazonium chloride to 0.6 molar equivalent on coupling reaction with 12 and 13 for two hours resulted in the formation of the monohydrazones 26a and 26b with a minor amount of 32 and 33, respectively.The 1 H-NMR and 13 C-NMR spectra of 32 and 33 showed a similar pattern to those of compounds 29-31.On the other hand, the 1 H-NMR spectra of 26a and 26b showed only one exchangeable N-H proton at δ 14.55-14.59ppm.Moreover, two methylene protons of C-4 and C-6 were assigned to the two singlets at δ 1.56-1.58ppm and 2.73-2.75ppm, respectively which confirmed the formation of the monohydrazone derivatives.Furthermore, only nine protons were assigned in the aromatic region which agrees with the assigned structure.The 13 C-NMR spectra of 26a and 26b also confirmed the introduction of the monohydrazone moiety since three signals corresponding to C-5, C-4 and C-6 were assigned at δc 37.72-37.91,37.92-37.99and 52.52-52.64ppm, respectively whereas the other three carbons C-2, C-3 and C-1 were resonated at higher frequency region at δc 145.41

Antimicrobial study for some selected compounds
Compounds 11, 14, 17, 18, 19, 29, 31 and 35 were screened for their antimicrobial activity against S. aureus and C. albicans using agar diffusion method 44 and ampicillin and clotrimazole as references.The results (Table 1) showed that compounds 14, 17, 18, and 19 have moderate inhibition against S. aureus (IZ 21-28 mm).On the other hand, the presence of the phenylhydrazone moieties in compounds 29 and 31 resulted in the loss of activity against S. aureus.However, the chloro compound 19 showed a higher antimicrobial activity (IZ 30-31 mm) against C. albicans and p-tolyl analogue 18 showed the same activity (IZ 23 mm) compared to that of clotrimazole.The other tested compounds showed lower inhibition activity (IZ 14-20).

Conclusion
The enaminones derived from dimedone have proved its utility as reactive intermediates upon reaction with nucleophilic and electrophilic reagents.The formation of the oxime derivatives can be rationalized by nucleophilic attack of hydroxylamine at C-1 carbonyl group to give the respective C-1 mono oximes that underwent a trans-amination process of the arylamine moiety with hydroxylamine to form the bis-oximes.Further reaction of the bis oxime with NH2OH, probably at the more reactive methylene carbon C-2 rather than C-6 gave the tri-oxime.The preference for the first attack of hydroxylamine at C-1 rather than C-3 can be deduced from the formation of the mon-oximes without affecting the arylamine residue.The trans-amination process has supported by a displacement of such type, but with hydrazine, of 3-benzylamino group in 3-benzylamino-2-carboxamido derivative of dimedone with hydrazine 45 , and reactions of enamines with nitrogen nucleophiles 3 .Moreover, good antimicrobial activity against S. aureus and C. albican was shown.

Experimental
Melting points were determined with a Mel-Temp apparatus and are uncorrected.TLC was performed on Baker-Flex silica gel 1B-F (1.5-5.0 cm) plates, and the spots were detected by UV light absorption.IR spectra were recorded in a matrix of KBr with Perkin-Elmer 1430 spectrometer. 1H NMR spectra were recorded on Jeol spectrometer (500 MHz) and Bruker AC (300 MHz) spectrometer.Chemical shifts () are given in ppm relative to the signal for TMS as an internal standard. 13CNMR were recorded on the Bruker Avance AV spectrometer at 75 MHzChemical shifts () are given in ppm.Mass spectra were recorded using electron ionization (EI) on a Finnigan MAT 312 spectrometer orJeol (JMS.600H)instrument and fast-atom-bombardment (FAB) on a Kratos MS50 spectrometer.

3-N-(Aryl)amino-5,5-dimethylcyclohex-2-en-1ones (8-14). Method a.
A mixture of dimedone (0.01mol) and arylamines1-6 or 4-aminoantipyrine7 (0.01mol) in acetic acid (5 mL) was heated under reflux for 2 hours.The reaction mixture was left to cool, then poured onto cold water (50 mL).The product was filtered off and crystallized from ethanol.Method b.A mixture of dimedone(0.01 mol) and 4aminoantipyrine (0.01 mol) was heated under reflux in formic acid (98 %, 20 mL) for 30 min, or left at room temperature for 24 hours then evaporated under reduced pressure.The residue was washed with warm water and then re-crystallized from ethanol to give 14 (50 % yield) identical with that obtained from method a.

3-(N-Acetyl-N-Aryl)amino-5,5-dimethylcyclohex-2-en-1-ones (15-17). General method.
A suspension of 9 -11 (5 mmol) in acetic anhydride (15 mL) was heated under reflux for 1hour.The reaction mixture was cooled then poured onto crushed ice.The separated syrup was dissolved in chloroform, and the solution was washed with a dilute solution of NaHCO3in water.The chloroform layer was dried over anhydrous sodium sulfate, then evaporated and the residue was recrystallized from ethanol to give 15-17, respectively.
-152.54, 155.49-155.51 and 196.59-197.15ppm,respectively.Treatment of compounds 29-33 with excess hydroxylamine hydrochloride in the presence of sodium acetate in ethanol under reflux did not afford the respective oxime derivative 34, but the same crystalline product 35 resulted from each of them.Its elemental analysis as well as its mass spectrum which showed a molecular ion peak at m/z 378, were inconsistent with 34, but in agreement with the molecular formula C20H22N6O2 indicating the displacement of the arylamine residue by an oxime group.The 1 HNMR spectrum of the isolated product, measured in deuterated chloroform, showed the absence of signals corresponding to the aryl moiety, and the presence of signals corresponding to ten protons of two phenyl groups.Moreover, four exchangeable singlets at δ 8.22, 12.35, 12.93 and 15.06 ppm can be assigned to the 2 OH, and 2 NH and at least two of them are involved in hydrogen bonding.