An efficient three component one-pot synthesis of some new octahydroquinazolinone derivatives and investigation of their antimicrobial activities

An efficient and convenient procedure has been developed for the synthesis of some new octahydroquinazolinones 5a-p in good yields. They have been achieved by the reaction between corresponding tetrazolo[1,5-a ]quinoline-4-carbaldehyde 2a-d, dimedone or cyclohexane-1, 3-dione 3a-b and (thio) urea 4a-b in the presence of concentrated HCl in ethanol. The structures of new compounds have been evaluated on the basis of elemental analysis, FT-IR, 1 H NMR and 13 C NMR spectral data. They have also been screened for their antimicrobial activities.


Introduction
In recent years, dihydropyrimidinones (DHPMs) and their derivatives have occupied an important position in natural and synthetic organic chemistry, due mainly to their wide range of biological activities 1 , such as antibacterial, antiviral, antihypertensive, antitumor effects and calcium channel blockers.Scaffold decoration of DHPMs is highly important for creating structural diversity to produce "drug-like" molecules for biological screening.The synthesis of DHPMs was first reported by Biginelli in 1893 2 and has been reviewed recently 3 .Improved procedures and new Biginelli-like scaffolds have been reported over the past decade and a variant of the Beginelli condensation has been described for its application to the total synthesis of bioactive guanidine alkaloids 4 .Basically, these methods are all similar in the use of different Lewis acid catalyst as well as protic acid under classical reflux 5 .Other studies have focused on the use of ionic liquids 6 , microwave irradiation 7 and combinatorial chemistry 8 .The use of boron compounds 9 , TMSCl 10 and heterogeneous catalysts, such as tangstophosphoric acid 11 , Zeolite 12 , montmorillonite 13 and ion-exchange resins 14 have been reported.However, to the best of our knowledge, there have been relatively few reports of the synthesis of fused DHPMs from cyclic β-diketones.More recently, the Biginelli reaction has been employed for the synthesis of octahydroquinazolinones, which used cyclic β-diketones instead of open-chain dicarbonyl compounds using concentrated HCl 15 and H 2 SO 4 16 as the catalyst.Octahydroquinazolinone derivatives have attracted considerable attention since they exhibit potent antibacterial activity against Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa 17 and calcium antagonist activity [18][19] .Literature survey reveals that number of octahydroquinazolinone 20 derivatives have been synthesized by Biginelli reaction conditions using various aldehyde but not a single reference have been found where tetrazolo[1,5-a]quinoline-4-carbaldehyde is used.In view of the above observation and in continuation of our work on biologically active heterocyclic compounds [21][22][23] , We now wish to report herein this heterocyclic aldehyde which is biologically active [24][25] with a view to obtain more active heterocyclic systems containing two biologically active moieties tetrazoloquinolines 26 and octahydroquinazolinone [27][28] .

Evaluation of antimicrobial activity
The in vitro antimicrobial activity of compounds 5a-p were carried out against 24 hr old cultures of three bacteria Escherichia coli as Gram-negative bacteria and Bacillus substilis and Staphylococcus aureus as Gram-positive bacteria and two fungi Aspergillus niger and Rhizopus oryzae by disc-diffusion method [31][32] .Nutrient agar and potato dextrose agars were used to culture the bacteria and fungus respectively.The compounds were tested at a concentration of 1000 µg/mL in DMF solution.
Ciprofloxacin, Ampicillin and Griseofulvin were used as standards for comparison of antibacterial and antifungal activities respectively.Inhibition was recorded by measuring the diameter of the inhibition zone at the end of 24 hr.for bacteria at 35 °C and 48 hr. for fungus at 28 °C.The protocols are summarized in (Table 1).The antifungal evaluation of the synthesized compounds revealed that among all the compounds 5b, 5f, 5g, 5h, 5k, 5l, 5m, 5o and 5p showed good antifungal activity against fungal strain namely Rhizopus oryzae.Similarly compound 5a, 5c, 5d, 5e, 5g, 5i, 5j and 5n against Rhizopus oryzae and compound 5c against Aspergillus niger showed moderate antifungal activity.
While remaining compounds of the series are slightly active against the Gram-positive, Gram-negative bacterial species and fungal species.

Conclusion
A convenient and efficient method for the direct synthesis of octahydroquinazolinone derivatives over concentrated HCl as the catalyst with good yield has been developed.This successful reaction expands the synthetic scope of the multicomponent Biginelli reaction.Further, the present procedure may be readily amenable to large-scale synthesis and the generation of combinatorial octahydroquinazolinones derivatives.It can be concluded from Table I that among all the compounds 5b, 5f, 5g, 5h, 5k, 5l, 5m, 5o and 5p showed good antifungal activity against Rhizopus oryzae while compound 5a, 5c, 5d, 5e, 5i, 5j and 5n against Rhizopus oryzae and 5c against Aspergillus niger showed moderate antifungal activity.Compound 5m showed good antibacterial activity against Staphylococcus aureus.Compounds 5a, 5d, 5e, 5f, 5h, 5l, 5o and 5p against Escherichia coli, 5a, 5b, 5f and 5g against Bacillus substilis, 5b, 5c, 5d, 5i, 5l, 5n and 5o against Staphylococcus aureus showed moderate antibacterial activity while remaining compounds of the series are slightly active against the Gram-positive, Gram-negative bacterial species and fungal species.

Experimental Section
General Procedures.All the reagents were obtained commercially and used with further purification.All melting points were taken in open capillaries and are uncorrected.The monitoring of the progress of all reactions and homogeneity of the synthesized compounds was carried out by TLC.TLC was runned using TLC aluminum sheets silica gel 60 F 254 (Merck).
Elemental analysis (% C, H, N) was carried out by Perkins Elmer 2400 CHN elemental analyzer.IR spectra were recorded on a shimadzu FTIR 8401 spectrophotometer in KBR. 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer using solvent peak as internal standard.Mass spectra were scanned on a shimadzu LCMS 2010 spectrometer.