Bojarski, Jacek

Tarsa, Monika

Żuchowski, Grzegorz

Acta Poloniae Pharmaceutica - Drug Research

English

Jagiellonian Univeristy Repository

A d a  P o lon iae  P h arm ac eu lica  -- D ru g  R esea rch , V ol. 6 0  N o. 4  pp. 2 4 7 -2 5 2 . 2003 ISSN  000 1 -6 8 3 7  P o lish  P h arm ac eu tica l SocietyHYDROLYSIS OF N - AND S-MONOMETHYL DERIVATIVES OF 2-THIOPHENOBARBITALMONIKA TARSA, GRZEGORZ ZUCHOWSKI and JACEK BOJAR§KIDepartment of Organic Chemistry, Medical College, Jagiellonian University,9 Medyczna Str., 30-688 Krakow, PolandA bstract: K inetics o f hydro lysis  o f  S -m e th y l-2 - th io p h e n o b a rb ita l in aqueous so lu tions w as investigated  using the UV spectro scop ic  m ethod  w ithin the pH  range 1 .5 -12 .9  at 60°C . C h rom atography  w as used to  separate  and iso late the p roducts o f  hydro lysis  o f  this com pound  and its N -m e th y l isom er. T he products w ere iden tified  by spectroscop ic m ethods and the cou rse  o f  hydro lysis  o f  both isom ers w ere com pared .K eyw ords: N -  and  S -m e th y l-2 - th io p h e n o b a rb ita l hydro lysis , kinetic param eters, hydro lysis  p roducts, s tab ilityKinetic investigations on the hydrolysis of 2-thiobarbituric acid derivatives are less frequent than for their 2-oxo analogs although some drugs of this class, such as thiopental (5-ethyl-5-(l-m ethyl- butyl)-2-thiobarbituric acid) and thiamylal (5-al- lyl-5-(l-methylbutyl)-2-thiobarbituric acid), are still in use as intravenous anesthetics. The results of investigations of kinetics of hydrolysis of both these compounds (1, 2), as well as 5,5-diethyl-2-thiobar- bituric acid (3) suggested the similarity of the process with that for 2-oxo analogs with the difference of the magnitude of kinetic parameters. Thus, the log k/pH profiles indicated hydroxyl ion-catalyzed degradation of the non-ionized and mono-ionized species (or kinetically equivalent water attack on the m ono- and di-ionized species), whereas the apparent first-order rate constants were higher for the 2-thiobarbiturates.Since the hydrolysis of thrce-substituted 2-thio- barbiturates was not investigated in detail we were interested in the effect of N-methyl and S-methyl substitution on the hydrolysis of 5-cthyl-5-phe- nyl-2-thiophenobarbital. The kinetics of hydrolysis of N-methyl derivative (I) was already reported (4) and was found to represent the same kinetic model as mentioned above. Here, we present investigations on kinetics of isomeric S-methyl derivative (II) and isolation and identification of several products of hydrolysis of both isomers.CH ,c rh .CpH,EXPERIM ENTALMonomethyl derivatives of 2-thiophenobar- bital were obtained according to (5). Kinetics of hydrolysis of II was investigated in buffer solutions (Table 1) within the pH range 1.5-12.9 at a concen­tration of 4 x 1(T5 mole/dm1. UV spectrophotometric measurements were made with a Gilford-Response II (England) apparatus in thermostated stoppered quarz cuvettes and pH was measured on a mic­rocomputer pH -m eter CP-315M  (Elmetron, Zab- rze) with a combined electrode ESAgP-301 W (Eu­rosensor, Gliwice) calibrated against standard buf­fers (pH=4.0, 7.0 and 10.0 -  POCh, Gliwice).The course of hydrolysis of 1% ethanol-buf- fer solutions of I  (at pH = 10, at 30"C) and I I  (at pH 12.9 and 3.25, at 60"C) was followed by TLC using aluminium sheets 20 x 20 cm with silica gel 60 F254 (Merck) and the following mobile phases:a) n-hexane : ethanol : triethylam ine (7:1:, v/v/v),b) chloroform: 25% am monia (100:0,5, v/v), andc) chloroform : isopropanol (2:1, v/v) for I and d), chloroform : acetone (4:0.5, v/v) and e), chloro­form : toluene : acetone (4.5:3:1, v/v/v) for II. V isualization -  UV light 254 nm (H A -05 lamp -  Haland, W arszawa), iodine vapors and 0.04% ethanol solution of bromocresol green. For iden­tification of hydrolysis products, after 24 h the solutions degraded in flasks heated in a therm os­tated oil bath were evaporated to dryness, the residues were dissolved in methylene chloride and the products of hydrolysis were separated by column chromatography (silica gel .40, Merck, chloroform), circular chromatography (Model 8924 Chromatotron, Harrison Research, USA, si­lica gel 60 PF254, Merck, mobile phases as for247248 MONIKA TARSA et al.T ab le  1. V alues o f  experim en tal rate constan ts  (k) fo r hydro lysis  o f  i l  a t d iffe ren t pH  valuesBuffer [9] pH* k x  10 5 |s  '] log kChloride acc. to 1.50 800 -2.097Clark and Lubs 2.00 300 -2.523Universal acc. to 2.52 86.940 -3.061Britton and Robinson 3.15 26.120 -3 .5833.41 8.217 -4.0853.71 4.630 M .3344.00 3.655 -4.4374.50 1.834 -4 .7374.95 1.370 -4.863Phosphate acc. toMichaelis 5.20 0.854 -5 .069Phosphate acc. to 6.00 0.715 -5.145Clark and Lubs 7.00 0.857 -5.067Borate acc. to 8.00 0.795 -5 .100Bates and Bower 8.65 0.827 -5 .0829.00 0.727 -5.13810.00 0.723 -5.141Phosphate acc. to 11.00 0.897 -5 .047Bates and Bower 11.16 0.841 -5.07511.78 1.568 M .80512.00 2.311 -4 .636Hydroxide acc. to 12.58 3.083 -4.51 !Bates and Bower 12.70 3.987 -4 .39912.80 4.370 -4 .36012.90 6.281 -4 .202* experim en ta l pH m easured  at 60°CTLC) and HPLC/MS [LCQ/MS Finnigan MAT with Ion Trap, USA, column: Purospher RPI8, 125 mm, 3 mm ID, 5 pm, precolumn: Lich- roCART 4 x 4  mm, with Lichrospher 60, 5 pm (Merck), gradient elution with A: 0.1% aqueous trifluoroacetic acid and B: 10% of A + 90% acetonitrile (programmed flow: 5% A + 95% B for 2 min., with 30 min increase to 70% A + 30 B and after 2 min decrease to 55% A + 95% B for 8 min, flow rate 0.4 ml/min, UV detection at 280 nm and MS detection with chemical ionization at atm osp­heric pressure]. The identification of the main products o f hydrolysis was based on their IR (IFS 48 Bruker apparatus, Germany, in KBr discs), 'H -N M R  (Varian M ercury VX 300 MHz appara­tus, USA, CDCL) and MS (Finnigan MAT 95S, USA, ionization energy 70 eV) spectra, rn.p.’s were uncorrected and determined in glass capil­laries on a M el-Tem p apparatus (Laboratory D e­vices Inc., USA). Calculations were performed using the program Quick Statistica PL. (StatSoft, Poland).RESULTSLinear absorbance-concentration relationship was checked in the 1 x 10“* -  5 x 10“5 M range for buffer (pH = 10.0) and ethanol solutions of II  (Figure 1), yielding linear correlation coefficients of 0.998 and 1.000, respectively.Absorbance changes of I I  in tim e (Figure 2) at different pH values were linear (Figure 3) indicating pseudo-first order reaction. The rate constants (k) were calculated from the equation:log (A — 4 ° o )  = log (A,, — A ° ° )  — kt/2.303where A„ is the absorbance at time t = 0; 4°° is the final (residual) absorbance and A is the absorbance at time = t.The results are summarized in Table 1 and the log k/pH profile is shown in Figure 4. The rateF ig u re  1. A b s o rb a n c c -c o n c c n ira t io n  re la tio n sh ip  fo r  c o m ­p o u n d  I I .T ab ic  2. V alues o f  ex perim en ta l rale constan ts  (A) fo r hydro lysis  o f  I I  at d iffe ren t tem pera tu resTemp. [KJ 1/T x  10 ■’ ( 1/K] k x  10* fs '] log k318 3.145 2.291 -5.640323 3.096 2.903 -5.537328 3.049 ■ 4.445 -5.352333 3.003 7.230 -5.141338 2.959 1.134 -4.945H ydrolysis o f  N -  and S -m onom ethy ! d erivatives  o f  2 -th iop h en o b arb ita l 249W avelength [nm] Figure 2. Spectral changes fo r hydro lysis  o f  U .pHFigu re  4. T he log k/pH  profile for hyd ro ly tic  degrada tion  o f  I I  at 60°C.Tlme[min]Figure 3. A pparen t f irs t-o rd e r  plo ts fo r hydro lysis  o f  I I  at d iffe ren t pH  values at 60l’C.1 /T x  1CT3 [1 /K ]Figure 5. T he A rrhen ius p lo t fo r hydro lysis  o f  II in borate buffer pH  = 10.0.constants at different temperatures (45-65”C) are presented in Table 2. The linear relationship log k = f ( I f f )  is shown in Figure 5 and from Arrhenius equation the activation energy for hydrolysis of II at pH=lO.O was calculated yielding Ea = 73.35 kJ/mole.The spectrophotom etric pKa value for II calculated from the H enderson-H asselbalch equ­ation (10) was 5.85 ± 0.29 at 25°C.The results o f TLC m onitoring of hydro­lysis course o f I and II are shown in Figures 6 and 7.The spectroscopic data for identified main products of hydrolysis of I (compounds III and IV -  Figure 8) and II (compounds V and VI -  Figure 8) are presented in Table 3. Additionally, among the products of hydrolysis of I  N-m cthylthiourea was separated and identified by m.p. 119—121 "C, comparison of TLC results (Rf = 0.12 and 0.68 inthe solvent systems b and c, respectively) and m.p. of the mixture (no depression) with an authentic specimen (Fluka).DISCUSSIONFrom the results it follows that the kinetics of hydrolysis of II is a pseudo-first order reaction and in this respect is similar to the hydrolysis of I investigated earlier (4). However, there is a dis­tinct difference in the rate of hydrolysis of both reactions in alkaline medium. For instance, the calculated, from the Arrhenius equation, pseu­do-first order rate constant at pH 10.0 at 25°C for hydrolysis o f II is 3.21 x  10“7 s_l (4). meaning that under these conditions I  is more susceptible to hydrolysis. The opposite is true for acidic region where the unionized form of II is hydrolyzed with evident H+ ion catalysis -  a feature rather uncom-250 MONIKA TARSA et al.Figure 6. T L C  m on ito ring  o f  hydro lysis  o f  I (pH  -  I0, f l )"( ’). A -  m obile  phase: n -h ex an e  : e thanol : trie thy lam ine  ( 7 :1: 1, v/v/v), B -  m obile  phase: ch lo ro fo rm  : 25%  am m on ia  ( 100:0 .5 , v /v), I, I I , IV  -  standards o f  com pounds designed  as in tex t, a  -  N -m ethy lth iou rea . b -  borate  bu ffer pH 10,0.F igure 7. T L C  m on ito ring  hydro lysis  o f  II in acid (A -  un iversa l buffer pH =  3 .25) and a lkaline  (B -  h y d rox ide  bu ffer pH =  12.9) m edium  at 60“C. M obile phase: ch lo ro fo rm  : to luene : acetone (4 .5 :3 :1 , v /v /v ). S tandard  o f  I I  and respective buffers  (a, b) applied  lo r com parisons.F igure 8. S tructu res o f  m ain products o f  hydro lysis  o f  I ( I I I  and IV ) and II  (V and V I).H ydro lysis  o f  N -  and S -m o n o m eth y l d erivatives  o f  2 -th io p h en o b arb ita l 25 1T ab le  3. S pectroscop ic  data fo r degrada tion  p roducts  o f  com pound  I and  IIH1 NMR (CDClj) [ppm] MS (%) IR (KBr) [c m 1]Compound III280 (2) M +235 (100) [C I2H l,N2OS] +162 (20) [C„,H,2NO] +117 (20) [C8H50 ] +91 (3) [C7H7] +Compound IV0.896 (t, 3H, J = 7.5, CH3-C H 2-) 236 (61) M+ 3237N -H31611.774-1.889 (m, 1H, C H ,-C H 2-) 146 ( 100) [C l(,HlüO r2.068-2.211 (m, 1H, C H ,-C H 2-) 118 (13) r c T u . r 3049 C -H  aromat.3.157 (s, 3H, C H j-N H -) 91 (57) [C7H7]+ 2967 C -H  aliph.3.302 (t, 1H, J = 7.5, C2H ,-C H -C (1H5) 1681 C = 07.240-7.384 (m, 5H, - C (,H5) 1567N -H15188.545 (s, 1H, -N H -C H ,)10.457 (s, 1H, -C O -N H -C S -) 1415 C -H  aliph.1359 C -C  12621205 C -N1169 C=SCompound V286 (2.2) M +287 (3.2) [C |2Hi(,N20 4Na]+263 (2) [C ,,H l5N20 4]+221 (5) [C,2H ,7N20 2S]+220 (1.2) [C l2H „N 20 2S]+219 (3.4) [C 12H 15N20 2S r164 (3.3) [C,„Hl,NO]+162 (1.2) [C,„H,2NO]+119 (1.7) [G A ,,]*91 (2.4) [C7Hv]+Compound VI1.004 (t, 3H, J = 7.5, CH.t-C H 2-) 220 (12) M+2.077-2.188 (m, 2H, C H ,-C H 2- ) 219 (5) M +-H3.485 (t, 1H, J = 7.5, C2H5-C H -C 6H5) 164 (3.3) [C,„HL!N O r5.4 (s, 3H, CH3-O - ) 162 (1.2) [C,„HI2N01+7.27-7.457 (m, 5H, -C „H 5) 119 (1.7) [C T InT8.362 (s, 1H, -C O -N H -C = ) 91 (2.4) [C7H7]+10.349 (s, 1H, =C=NH)mon for other barbituric and thiobarbituric acid derivatives.The identified products o f hydrolysis o f I in­dicate the hydrolytic pyrimidine ring opening at 1-6 position and are typical for other barbiturates (especially 2-oxo-analog (6) and thiobarbiturates (3, 7). On the other hand, the products o f hydrolysis of II prove désulfuration of the parent compound and formation of derivatives of isourea. The exact mechanism of such hydrolysis course needs further elucidation.AcknowledgmentThe work was supported by the Grant No. 4P05F02915 from the Polish State Committee forScientific Research (KBN), Warszawa, Poland. The authors are indebted to Dr. hab. M. Kłys, from the Department o f Toxicology, Institute of Forensic Medicine, Medical College of the Jagiellonia Uni­versity for enabling LC/MS measurements.REFERENCES1. Goto E., Furukawa T., Goto S.: Arch. Pract. Pharm. 27, 311 (1967).2. Garrett E.R., Bojarski J.T., Yakatan G.Y.: J. Pharm. Sei. 60, 1145 (1971).3. Bojarski J.: Rocz. Chem. 48, 619 (1974).4. Tarsa M., Bojarski J.: Bull. Acad. Pol. Sei. Chemistry 45, 63 (1997).252 MONIKA TARSA et al.5. Kubaszek M., Paluchowska M., Chmiel E., Bojarski J.: Pol. J. Chem. 68, 117 (1994).6. Fretwurst F.: Arzneim. Forsch. 8, 44 (1958).7. Aspelund H., Eklund B.: Acta Acad. Aboensis, Math. Phys. 21, 3 (1957).8. Bojarski J.T., Mokrosz J.L., Bartoń H.J., Palu­chowska M.H.: Adv. Heterocycl. Chem. 38, 229 (1985).9. Physicochemical manual (Poradnik fizykoche­miczny), p. 640, WNT, Warsaw, 1974.10. Albert A., Serjeant E.P.: in The determination of ionization constants, pp. 44-64, Chapman and Hall Ltd., London, 1971.Received: 10.02.2003

Hydrolysis of N- and S-monomethyl derivatives of 2-thiophenobarbital

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Hydrolysis of N- and S-monomethyl derivatives of 2-thiophenobarbital

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