Synthesis and Pharmacological Evaluation of (6-Substituted 4-Oxo-4 H -chromene-3 yl) methyl N-substituted Aminoacetates

: Synthesis Pharmacological Evaluation of N-substituted Aminoacetates A series of the title compounds were synthesized and characterized by spectral data. All the compounds were evaluated for in vitro antihistaminic activity by inhibition of isotonic contractions induced by histamine on isolated guinea pig ileum and the compound 6-k showed signiﬁ cant activity. A few compounds have also been screened for in vivo bronchodilatory activity. These compounds exhibited signiﬁ cant protection against histamine-induced convulsions in guinea pig at the dose of 50 µmol.

Chromone moiety is a component of a number of biologically active substances of both synthetic and natural origin having medical signifi cance 1 . Thus it is of great interest to medicinal chemist for molecular manipulation and pharmacological evaluation. Chromone is reported to have coronary spasmolytic 2 , bronchodilatory 3 , antiallergic 4 , antianaphylactic 4,5 , platelet antiaggregatory 6 and anti-asthmatic 7 activities. 3-(Hydroxymethyl)-4H-chromen-4-one is a key intermediate for the synthesis of many drugs 8,9 . It can be prepared by the two reported methods. The fi rst method involves reduction of 4-oxo-4H-chromen-3-carbaldehyde using sodium borohydride in the presence of aluminum chloride and borane in THF 10 , and the second method uses condensation of 1-(2hydroxyphenyl)-2-(methylsulfi nyl)ethanone/2′-hydroxy-2-(methylsulfinyl)acetophenone with formaldehyde, followed by thermal elimination of methylsulfinyl group 11 . The fi rst method resulted in poor yields and formation of complex mixture which required tedious purifi cation process. Hence we followed the second method. Disodium chromoglycate, which contains chromone nucleus, is well known for prophylaxis and treatment of asthma, and on the other hand several antihistamines posses basic nitrogen. Hence it was proposed to synthesize title compounds, taking care to preserve the chromone skeleton along with the substituent in the benzo group and also to introduce a basic N-substituted amino group at the acetate group, separating the chromone ring and basic nitrogen with four atoms. Such a molecular framework is expected to retain the mast cell stabilizing potency and H 1 -receptor blockade and can be useful as both prophylactic as well as therapeutic agent in allergic patients.
Melting points were determined in open capillary tubes and are uncorrected. IR spectra were recorded on Perkin-Elmer spectrum, Bx-I IR spectrometer, 1 H NMR on Jeol-300D (300 MHz) using TMS as internal standard and mass spectra on VG Micromass 7070H instrument. The title compounds were prepared from 3-hydroxymethyl-4-oxo-chromene 11 as depicted in Scheme 1.

Scheme 1: Synthesis of the title compounds -NR1R2 in which R1 is H and R2 is 2-pyridyl or morpholino or 4-methylpiperazino or 4-ethylpiperazino or piperidino
As shown in Scheme 1, the key intermediates were prepared and identifi ed based on the reported data [10][11][12][13] . Compounds 2 were synthesized by refl uxing appropriate 3-(hydroxymethyl)-4H-chromen-4-one (1; 0.01 mol) with chloroacetyl chloride (0.01 mol) in dry benzene (20 ml) under anhydrous conditions, using calcium chloride guard tube, for 2 h. The product thus formed was fi ltered, washed with small portions of benzene to remove unreacted chloroacetyl chloride and dried.  Table 1. The compounds were screened for in vivo bronchodilatory activity and were subjected for toxicity studies, and found to be nontoxic and safe in experimental animals (guinea pigs) up to a dose of 300 mg/kgbw (i.p.).
The test compounds were found to be free from CNS depression and did not cause any changes in the behavioural pattern of animals tested viz., impairment in awareness, mood and motor activity. Fifteen compounds of series 3a-o containing chromone ring attached to a basic moiety through four atom unit (-CH 2 -O-CO-CH 2 -) have been tested for their ability to inhibit histamine induced isotonic contractions in isolated guinea pig ileum (in vitro model). Pheniramine maleate was used as standard (50 μg; 96.6% inhibition). The results are included in Table 1.
All of them exhibited signifi cant antihistaminic activity at the concentration of 60 μg. Of all the compounds in the series, 3k (R = Cl; R 1 = H, R 2 = 2-pyridyl) was found to be the most potent exhibiting 100% inhibition at the selected dose. Compound 3e with piperidine as basic moiety exhibited 49.3% inhibition and the activity increased to 94.8% with chlorine substitution at 6 position (3o; R = Cl, -NR 1 R 2 = piperidino). Also a methyl substituent at the same position (3j; R = CH 3, -NR 1 R 2 = piperidino) could cause 73.4% inhibition. However compound 3a with 2-pyridylamino group showed only 44.6% inhibition in the absence of a chlorine substituent (R = H). Thus both chlorine and methyl substituents being at 6 th position is contributing positively to enhance the activity. Compounds 3d and 3c with 4-alkyl piperazine as basic nucleus have been found to be more active than the compound with morpholino group (3b). As seen in case of piperidine containing compounds, chlorine substitution at 6 th position in compounds with peperazine also enhanced the activity.
Compounds 3k, 3n and 3o which showed greater in vitro potency, have been screened for in vivo bronchodilatory activity as well. All the three compounds exhibited significant protection against Among these three compounds, compound 3k (R = Cl; R 1 = H, R 2 = 2-pyridyl) showed relatively higher activity with 48.34% protection against 43.34% protection exhibited by standard drug (aminophylline), whereas 3o, 3n with piperidine and 4-ethylpiperazine groups showed only 38.34% and 35.99% protection respectively ( Table 2).
Selected compounds from the series (having greater in vitro potency) have been found to be devoid of toxicity and no mortality was observed upto a dose of 300 mg/kgbw (i.p.) in the experimental animals. Compounds 3k and 3o, which have comparable in vitro antihistaminic activity, differed in their in vivo bronchodilatory activity. Compound 3k with pyridyl moiety exhibited 48.34% protection against 38.34% protection shown by compound 3o (pyridine moiety) as compared to that of the standard aminophylline, which showed 43.34% protection.