Synthesis and antispasmodic effect of aryl substituted N - carbamoyl/thiocarbamoyl isoquinolines

A reaction of tetrahydropapaverine obtained from homoveratryl amine and homoveratric acid by three step procedure with various aromatic isocyanates or isothiocyanates to give the corresponding aryl substituted N-carbamoyl or N-thiocarbamoyl derivatives of isoquinoline has been achieved. All the compounds have been screened for their antispasmodic activities.


Introduction
The chemistry of tetrahydroisoquinoline alkaloids has attracted considerable interest over the years due to their potent biological activities.2][3][4] The natural alkaloids are generally optically active compounds possessing antihypertensive, hemostatic, smooth or skeletal muscle relaxant, antispasmodic, antitussive, antimalarial, narcotic, analgesic or antipyretic activities 5 .The aporphine alkaloids namely (±) thaliporphine, (±) N-methylaurotetanine, (±) isoboldine have been found to be of significant biological and biogenetic interest 6,7 .Papaverine is also a naturally occurring benzylisoquinoline alkaloid isolated from opium.Clinically, papaverine is employed as a vasodilator because of its relaxatory effect on vascular smooth muscle 8,9 .Few N-acyl derivatives of papaverine and related compounds show variety of activities against AIDS, glucoma and fungal infections [10][11][12] .In quest for biologically more potent compounds we envisioned to synthesize aryl substituted N-carbamoyl as well as N-thiocarbamoyl derivatives of tetrahyropapaverine.In this paper, we report a four step procedure that gives aryl substituted Ncarbamoyl or N-thiocarbamoyl derivatives of tetrahydropapaverine and studied their antispasmodic activity.

Antispasmodic activity
The work includes muscle relaxation studies on isolated guinea pig ileum, contracted with acetylcholine [14][15] .Guinea pigs (n = 6) of both sexes (300 -500 g) were used for this study.The animals were killed by a blow to the head, the ileum was removed immediately and placed in aerated Krebs saline at 37 o C.This saline contained (in mM): NaCl, 120.7; KCl, 5.9; CaCl 2 , 2.5; MgCl 2 , 1.2; NaHCO 3 , 15.5; and glucose, 11.5 at pH 7.3.For tension recording 2 cm ileal strips were mounted in a 10 mL organ bath and were connected to physiograph (Polyrite, Recorders and Medicare systems) through force tension transducer.In the concentration range 10µM -150µM papaverine and all its derivatives caused relaxation of spontaneous rhythmic contractions of both guinea pig ileum accompanied by a fall in resting tension.The inhibition contraction was measured simply as percentage reduction in the height of spontaneous contractions.The percentage relaxation of all derivatives is compared in Table 3.The results are expressed as mean ± S.E.The statistical significance was treated with the paired student's t-test.P value < 0.01 were considered to be significant.Increase or decrease in tension was expressed as percent of maximal response to papaverine.

Structure activity relationship
The aromatic proton substitution by electron releasing alkyl groups on the phenyl ring in compound 4i and 4j showed decrease in antispasmodic properties in comparison to papaverine.Compounds with unsubstituted phenyl ring as in compound 4a and 4b showed significant increase in antispasmodic activities in comparison to compound 4i and 4j.Comparable activities corresponding to papaverine were observed by all compounds containing halogen substitution on the phenyl ring 4c, 4d, 4e and 4k.However, dihalogen substitution in the phenyl ring compounds 4f and 4g with ortho and para positions showed significantly enhanced antispasmodic activities in comparison to papaverine.No appreciable change in the antispasmodic activities were observed by altering oxygen to sulphur in the amide moieties of the molecule.

Experimental Section
General Procedures.Melting points are recorded in open capillary tubes on Büchi melting point B-540 instrument and are uncorrected.Solvent system used throughout the experimental work for running TLC plates was ethylacetate -hexane.The 1 H NMR spectra were recorded in CDCl 3 as solvent (using TMS as internal standard) on a Bruker Avance Spectrospin 300 instrument at 300MHz.Mass spectra were run on a MALDI Kratos Analytical Kompact SEQ mass spectrometer using α-cyano-4-hydroxycinnamic acid (4-HCCA) as matrix under positive linear reflectance mode.IR spectra were recorded using KBr discs on a Shimadzu FTIR-8300 spectrophotometer.A solution of p-chlorophenyl isocyanate (1.2 mmole) in dry acetonitrile (5 mL) was added slowly to a solution of tetrahydropapaverine 3 (1 mmole) in dry acetonitrile (5 mL).The mixture was stirred at room temperature for 2 h.After the completion (tlc) of the reaction, evaporated off the solvent, diluted the residue with water (25 mL) and extracted with ethylacetate (2x50 mL).The collective organic portion was washed with brine and dried (Na 2 SO 4 ).It was finally concentrated and chromatographed on silica gel using ethylacetate -hexane as eluent.A final recrystallization from ethylacetatehexane (15:85) affords pure 4e (87%, mp 108-109 o C).The physical and spectral data of compounds are given in Table 1 and Table 2.

Table 1 .
Physical and spectral data of compounds

Table 3 .
Antispasmodic activity of compounds 4a