Synthesis of some biologically active 5-benzylidene-2-aryl-5,6-dihydro-4 H -[1,3]oxazin-6-ones and 4-benzylidene-1-aryl-3,4-dihydropyrimidobenzimidazoles with bridged nitrogen

Facile synthesis of differently substituted 5-benzylidene-2-aryl-5,6-dihydro-4 H -[1,3]oxazin-6-ones 2a-2h has been achieved by heating a mixture of substituted N-benzoyl-β -aminopropanoic acids 1a-1d with several aromatic aldehydes in the presence of sodium acetate and acetic anhydride. These compounds 2a-2h, on refluxing with o-phenylenediamine in dry benzene resulted in the formation of differently substituted 4-benzylidene-1-aryl-3,4-dihydropyrimidobenzimidazoles 3a-3h. The synthesized compounds 2a-2h and 3a-3h were characterized by elemental analysis and spectral studies (IR, 1 H NMR and 13 CNMR).

Pyrimidines and their derivatives are known for their pharmacological properties including antiviral, antibacterial [16][17][18][19][20] , antitumor 21 and antihypertensive 20 effects.As a result of their pharmacological, biological, physiological, and medical significance, substituted and condensed pyrimidines form class of compounds of importance and still growing interests.Keeping this in view, it was thought of interest to synthesize certain pyrimidobenzimidazoles with the expectation of their activities supplemented or atleast comparable to those of benzimidazole and pyrimidine derivatives 22 .These compounds have been prepared as per Scheme 1.The mechanisms of formation of 2 and 3 have been shown in Scheme 2 and 3 respectively.
Condensation 24-25 of 3-benzoylaminopropanoic acid 1a and 4-methylbenzaldehyde in 1:1 molar ratio in the presence of anhydrous sodium acetate and acetic anhydride yielded a crystalline compound 2a.Based on spectral data and elemental analysis, the compound obtained by the condensation of 1a and 4-methyl benzaldehyde was assigned the structure as 5-(4methylbenzylidene)-2-phenyl-5,6-dihydro-4H- [1,3]oxazin-6-one 2a (R 1 = R 2 = R 3 = H and R 4 = CH 3 ).In IR spectrum of 2a, the peaks (υ) at 1685cm -1 and 1785cm -1 speaking of (-C=N-) and a characterstic δ-azlactone carbonyl respectively; and presence of a sharp singlet at δ 5.85 for the ethylenic proton and a singlet due to two protons at δ 2.95 for the =N-CH 2 protons and a singlet at δ 2.2 due to three benzylic methyl protons in 1 HNMR spectrum confirm unambiguously the assigned structure for compound 2a.The tautomeric shift of proton may generate the corresponding isomeric benzyl analogues 2 / from benzylidene analogues.But in this latter case, the presence of =N-CH 2 gp.chemical shift (δ 2.95) and the shift due to benzylidene proton (-CH= at δ 5.85) would have been lacking.Infact, there ought to have been the presence of a chemical shift of a proton underlined in structure 2 / in Scheme 2 in the down aromatic region and the presence of benzylic protons around (δ 2.3) favouring structures 2 / but this is not the case as both are wanting and hence ruling out the formation of isomeric benzyl analogues 2 / and conforming unambigousoly the structure 2. Condensation of 1a with veretraldehyde and other substituted 3-benzoylaminopropanoic acids 1b-1d with four different aldehydes, two for each of 1b-1d in 1:1 molar ratio yielded differently substituted 5-benzylidene-2-aryl-5,6-dihydro-4H- [1,3]oxazin-6-ones 2a-2h respectively in good yields Table 1.The structures of all these compounds were established on the basis of spectral data and elemental analysis.Further, condensation [26][27][28] of 5-(4-methylbenzylidene)-2-phenyl-5,6-dihydro-4H- [1,3]oxazin-6-one 2a (R 1 = R 2 = R 3 = H and R 4 = CH 3 ) with o-phenylenediamine in 1:1 molar ratio, in the presence of dry benzene yielded a crystalline compound 3a.Based on spectral data and elemental analysis, the compound obtained was assigned the structure as 4-( 4 in the IR spectrum of which the band due to δ-lactone carbonyl group was wanting and the presence of characterstic proton signals in 1 HNMR corresponding to those of the 2a confirmed unequivocally the assigned structure of compound 3a.Condensation of other differently substituted 5-benzylidene-2-aryl-5,6-dihydro-4H- [1,3]oxazin-6-one 2b-2h with ophenylenediamine in dry benzene yielded differently substituted 4-benzylidene-1-phenyl-3,4dihydropyrimido [1,6-a] benzimidazoles 3b-3h respectively in good yields Table 2.The structures of all these compounds were established by elemental analysis and spectral studies (IR, 1 HNMR and 13 CNMR).
The mechanism for the reactions is outlined in Scheme 2 and Scheme 3. (3)

Pharmacology
On preliminary pharmacological screening all the 1,3-oxazines 2a-2h and pyrimido [1,6a]benzimidazole derivatives 3a-3h have been found to be promising bronchodilatory and oxytocic agents having activities comparable to those of alkaloid vasicine and its natural and synthetic analogues.The detailed study of the evaluation of these biological activities is under active exploration from our research laboratory.The drugs employed in this study are 7,8,9,10tetrahydroazepino[2,1-b]quinazolin-12(6H)-one; Aminophyllin injection I.P(Burroughs Wellcome & Co.); Histamine diphosphate (Sigma); Adrenaline tartarate (IP); Propanolol HCl (ICI); 5-hydroxytryptamine; and Egg albumin (BDH).The comparative SAR of various compounds 29 and the results of other details regarding these activities are being currently determined.All the pyrimido[1,6-a]benzimidazole derivatives 3a-3h have been found to be weakly to moderately active antimicrobial agents.Compounds 3b-3h have been found to be highly promising, as regards as 'Tracheal smooth muscle activity' and 'Antitussive activity'.
Antimicrobial activity.The compounds 3a-3h have been screened for their antifungal activity against Aspergillus, Penicillium and Cladosporium species.For antibacterial activity, these compounds have been screened against E.coli, Bacillus subtilis and Bacillus cereus.Both the activities were evaluated at the same concentration of 1000µg and through well diffusion technique.The standard antifungal agent fluconazole and the antibacterial agent norfloxacin were also screened under similar conditions for a comparative study.The inhibition zones formed were measured in mm and are listed in Table 3.  24.9 and (+)-glucose 11.1.The responses were recorded isotonically on a kymograph.The tissue was adjusted to an initial tension of 1.5 g and allowed to equilibrate (60-90) minutes.Relaxation effect of the drug was studied on tracheal chain precontracted with histamine diphosphate (1× 10 -6 g/mL) or acetylcholine chloride (1× 10 -6 g/mL).The test drugs were added 8 minutes after the tonic contraction reached plateau.The responses were calculated as per cent to relaxing of precontracted muscle back to base line tension (10% relaxation).If there was relaxation to muscle slightly below the base line it was also taken as 100% relaxation.
Antitussive activity.Kobayshi's 31 method was used in this study.Guineapigs (300-400 g) were aneasthetised by I/P urethane (6.5 mL/kg; 25%) and fixed in dorsal position.The trachea was exposed and a small incision made at a distance of 1.5 cm from the clavicle.A fine and very thin polythene tube was inserted into the incision as deep as 3 cm to give the stimulus.The stimulus was applied two times before and 15, 30, 45, 60, 90 and 120 minutes after the drug dministration by oral route.If no coughing occurred in 2 or more out of 5 trails after drug administration, the drug was estimated as effective.Results are shown in Table 4.

Experimental Section
General Procedures.The melting points were determined in open capillary tubes in melting point apparatus and are uncorrected.The purity of the products was checked on TLC plates coated with silica gel-G and detected by iodine vapours.The IR spectra were recorded on Perkin Elmer Infrared model S99-B and on Shimdzu IR-435 spectrophotometer (υ max in cm -1 ).

hydrochloride 100 4
Benzoylation of β-alanine and 3-benzoyl aminopropanoic acids 1a-1d.Dissolved (0.33 mol) β-alanine (CDH, Laboratory reagents Pvt.Ltd.) in 250 mL of 10% NaOH solution contained in a conical flask.Added 45 mL (0.385 mol) of corresponding benzoyl chloride in five portions to the solution.Stoppered the vessel and shook vigorously after each addition till all the acid chloride had reacted.Transfered the solution to a beaker and added few gms of crushed ice to it.Added concentrated HCl slowly with stirring till the mixture was acidic to congo red paper.Collected the resulting crystalline precipitates of substituted 3-benzoylaminopropanoic acids 1a-1d by filtration, recrystallised the dried product in each case from boiling water (about 500 mL) and dried them in an oven.Synthesis of differently substituted-5,6-dihydro-4H-[1,3]oxazin-6-ones 2a-2h.A mixture of 3-benzoylaminopropanoic acid (0.25 mol), aromatic aldehyde (CDH, Laboratory reagents Pvt.Ltd.) (0.25 mol), acetic anhydride (0.75 mol) and anhydrous sodium acetate (0.25 mol) was taken in a 250 mL conical flask and heated on an electric hot plate with constant shaking.As soon as the mixture liquified completely, transfered the flask to a water-bath and heated for 2 hour.Then added 100 mL of ethanol slowly to the contents of the flask and allowed the mixture to stand overnight.Filtered the crystalline product in each case, washed with ice-cold alcohol, washed with 25 mL of boiling water and dried at 100 °C when 2a-2h were obtained.Synthesis of differently substituted-3,4-dihydropyrimido [1,6-a]benzimidazole 3a-3h.Added (0.25 mol) of substituted 5,6-dihydro-4H-[1,3]oxazin-6-ones 2a-2h and (0.25 mol) of ophenylenediamine (Thomas Baker Chm. Ltd.Mumbai, India) in dry benzene in a 250 mL round bottom flask and refluxed on a water-bath initially for two hours.Then refluxing was continued for a further period of 10 minutes by the addition of few drops of acetic acid.Within few minutes the colourless compound that separated out in each reaction was filtered and recrystallised from acetic acid to give 3a-3h.one (2a).IR (KBr, υ, cm -1 ): 3065 (C-H, aromatic); 2825 (C-H, aliphatic); 1785 (C=O); 1685 (C=N); 1520 (C ….. C of aromatic ring); 1454 (C=H bending of methylene); 1072 (C-O-C).