PHENOTHIAZINE AKTHELMTSTICS
11:Lrch 1969
20.5
T.mr,r: I Electrode potential,"
Coiripd
Lett&
In\'
.\ntlielniintic art., c/rh zt rtd error
IIanmett siilmtitilent
Iiarariieter
3,Yf 2 ft J . 3 - 1.%O" hlethyleiie blue 378C 3 1 , .i - 1,:32e €3 Thionine 43lC 2 f0 . . i -0.66 C 3-ilminophenot hiazirie 473 0+0 - 0 , .54e I) 3,i-Dimethoxyphenothiazine 548 1 & O..i E :3,4,6,7-L)ibenzophenothia~iiie 380 29 + 9 -0.2.; F 3-Ethoxyphenothiazine .;!I0 72 + 6..i - 0.27 c; 3-1Iethoxyphenothiazine .iB0 1 i.o..; -0 ,3 4 e I3 35,:- Ilimethylphenot hiazine I 3,4-Benzophenothiazine 628 7 =k 3 . . i 65 1 40 f 11 -0.17 J 3-3Iethylphenothiazine K 2-Chloro-i-met,hoxyphenothiazine 662 100 f 0 . 2 +o. 10e L 4-Chloro-7-met hoxyphenot hiazine 668 100 f 0 . 2 + 0 . 10e 679 l i l $0.01 RI 3-Phenylphenothiazine 696 70 f 7 0.00 N Phenothiazine 722 23 i 1 1 . 5 $0.06 0 3-Fluorophenothiazine 763 70 rt ;i.3 +0.2:3 P 3-Chlorophenothiazine 766 24 & 6.-5 +0.232 Q %Bromophenothiazine +0.28 I: 3-Iodophenot hiazine 738 6 i%.*i S 3-Kitrophenothiazine 900d 1 &0.5 +0. 7 s b Reference 1, and J. Cymerman-Craig, unpublished data. For the Reference 1, measured a t 20" in 80% v,'v A4cOH(pH -2). method of determination of biological activity, see ref Id. L. llichaelis in "The Enzymes," 1-01. 11, Part, I, J. B. Sumiier and Ii. Myrbiick, Ed., Academic Press, New York, N. Y., 1951. d Approximate value only; potential became unstable between 25 and 50% of the uriivalent titration step, and value was obtained by extrapolation. e For derivatives having more t,han one substituent the constants are added. f Pee Figures 1, 4,and 6.
A
+
ambient electromotive force of the physiological system; let [R], [S+], and [T+] represent the thermodynamic activities of the reduced, semiquinone, and totally oxidixed forms of phenothiazine, respectively. The rela-
-0.9.-
2 ~0.8-
2z 0.7 w
-e-
-e-
0.6
+e4
i)
semiquinone ion
phenothiazine
[o:n]' w
+ H +
phenazothionium ion
tionships between E and the standard potent,ialsEl" and E," for t,he tivo urlivalent steps 1 and 2 Rre sholvn in eq 3 and 4. S+ + e- = R (1) T-
E E = E?"
=
El0
+ H + + e - = s+
+ ( R T / F ) In ([S+]/[R])
+ ( R T / F )In ([T+]/[S+])+
(2)
(3)
( R T / F ) In [H+l (4) Adding (3) and (4) to eliminate [S+], one obtains E
=
',/z(E1O + EZ0)+ (RT/2F) In ([T+]/[R]) + ( R T / 2 F ) In [H+] (5)
When T + and R have the same activity, E equals the bivalent midpoint electrode potential E,, so that
+
+
Em = '/z(E1° Ez') (RT/2F) In [ H + ] ( 6 ) The difference AE between E and E , at 30" then becomes AI!' = I3 - E,,, = (RT12F) In ([T+]/[R])=
0.030 log ( [ T + ] / [ R ] ) (7)
-12
-08
-0.4 0 0.4 SUBSTITUENT PARAMETER
0.6
-
Figure 1.-Reduction aotential of the fir.t iuiivalent oxidation step of the substituted phenothiazines as a function of the Hammett suhstitlient parameter: A, iinsubstituted; 0, moilosubstituted; 0,disubstituted. For identifying letters, see Table I.
The stability of the semiquinone in the rapidly reverqible chemical reaction R T + H + = 2S+ (reverse of the disproportionation reaction) is expressed by the semiquinone formation equilibrium constant K . This
+
+
equilibrium constant is related to the standard reduction potentials EI" and E2* as follows.
EsQ- El0
=
( R T / F )In K
The subsequent algebra is simplified by letting K' K [H+],from which also
E z - El
=
( R T / F ) In K'
(9) =
(9')
The electrode potentials of the first oxidation step (El) for the phenothiazine derivatives are given in Table I. It is of considerable interest and of predictive value that the reduction potentials yield a linear Hammett plot (Figure 1) when the para substituent con-
3000L
I
-06
l
l
I
I
-0,4 -0.2 0 SUBSTITUENT PARAMETER
l
0.2
l
K L
I
k
:loo
80
70
$ z W
In
a@
“I
)=
50
4
40
30
20 10
0
-.io
-io
-.io
0
-.io
-.io
-.‘30
Figure 4.-Katio of semiquinone coiiceiitration to its masimiim value (left scale) as a furictioii of AE for various valiies [ ~ fthe semiquiiioiie formation eqriilibrium coiistant K‘. The atit helmiiitic activities (right scale) of the pheiiothiaziiie derivative. are superimposed as a fuiictioii of E, so that the LE at maximum activity is zero: A, utisribstituted; 0, moiiosiibstitutetl; 0, disubstituted. For identifying letter?:, see Table I. J‘ertical bars represent standard errors where these are > + 2 .
CIE
Figure 3.-Per w i l t seriiiquinotie a.3 a fuiictioii of the difference A B betweeii the poised potential of the solutioii E and the bivaleiit midpoiiit electrode potential E,,, for various values of the semiquiiioiie formation equilibrium cotistaut.
The correlation coefficient is -OS34 ( P < 0.001), and the slope and its standard error are -2.3s f 0.44. Since this analysis is based on only a small fraction of the compounds studied, and taking into account the sources of error involved in the methods of determination of K’, a high precision cannot be expected for the above numerical values. For example, the equilibrium involving protonation of 3-amino, B,T-diamino, or 3,7-bis(dimethylamino) derivativesi3is undoubtedly responsible for their not possessing the values of log K’ which would be predicted from Figure 2 . Assignment of a substituent constant for these compounds is complicated because, in contrast to the amino group, the ammonium ion has a large positive substituent constant. Combination of eq 9’, 10, and 11yields
E2
=
0.791
+ 0.127~~
Concentration of Semiquinone.-The
(12)
concentration
\ \
\ \LOG
K’(pH7)
\.
0.4
\
1
1-4 \
-0.8
-0.4
0
0.4
0.0
SUBSTITUENT PARAMETER
Figure 5.-Experirnental values of E1 and log Iloreover, such a correction would result only in a bulk shift of the data as well as the curves as they
299
appear in Figures 4 and 6, and mould not affect any conclusions derived from these figures. I t should be emphasized that in the foregoing discussion the site and mechanism of action of the phenothiazine anthelmintics are hypothetical, and little is therefore known about the possible effects of other factors such as distributive and metabolic parameters. However, the observed correlation appears interesting and significant enough to encourage further investigation of systems in which semiquinone free radicals are suspected to be the biologically active species.
cr,a,a-Trifluorotoluamides as Anticocciclial Agents DEANE. WELCH,ROBERT R. BARON, A N D BRUCE A. BURTON Research Division, Salshury Laboratories, Charles City, Iowa 50616 Received September 3, 1968 The preparation and anticoccidial activity of a number of a,a,a-trifluorotoluamides and related compounds are reported. Several active compounds were obtained, but the most active were the amide, dimethylamide, ethylamide, and diethylamide in which the trifluoromethyl and a nitro group are in a 3,s relationship. One other amide with 2-chloro-5-trifluoromethyl showed similar activity.
The use of nitrated and halogenated benzamides (1-3) as feed additives for the control of poultry coccidiosis has been known for several In these compounds the nitro group is known to be essential for significant anticoccidial a ~ t i v i t y . ~Certain aminobenzoic acids and related compounds are also known to have anticoccidial activity.'j These compounds are believed to act as p-aminobenzoic acid (PABA) antagonists because simultaneous administration of PABA is reported to reduce their efficacy. Also, it has long been recognized that certain coccidia are sensitive to known PABA antagonists such as the sulfonamides and 4,4'-diaminophenyl sulfone^.^-^^ I n contrast there is no direct evidence that compounds such as 1-3 act as PABA antagonists. During the past 20 years a substantial effort has been devoted to the replacement of hydrogen, nitro, halogen, or methyl by fluorine or trifluoromethyl in prototype molecules which are known to have chemotherapeutic This work has led to some (1) N. F. Morehouse a n d W.C. ZIcGuire, Poultry Sei., 36, 1143 (1857); 37, 1228 (1958); 98, 410 (1959).
H y m a s a n d G. T. Stevenson, ibid., 89, 1261 (1960). (3) R. R . Baron, hI. IT. hloeller, a n d 4.F. RIorehouse, ibid., 46, 411 (1966). (4) S. J. Ball a n d E. TI-. Parnell, Suture, 199, 612 (1863). ( 5 ) Salshury Laboratories, unpublished results.
(6) E. F. Rogers, R. L. Clark, H. .J. Becker, A. A. Pessolano, IT. .J. Leanza, E. C . XlcManus, F. J. Andriuli, and A. C. Cukler, Proc. SOC.Exp.B i d . M e d . , 117, 488 (1964). ( i )C. Horton-Smith and E. Boyland, Brit. J. Pharmacol., 1, 139 (1946). ( 8 ) L.P. Joyner and S.B. Kendall, ibid., 11, 454 (1956). (9) E. Waletzky and C. 0. Hughes, Amer. J . Vet. Res., 7 , 365 (1946). (10) E. H. Peterson, ibid., 9 , 77 (1958). (11) L. C . Grumhles, .J. P. Delaplane, a n d T. C. Higgins. Poultry Sci., 27, ti05 (1Y48). (12) For a relien. of t h e trifluoromethyl group in medicinal chemistry a n d reEerences to its inductive a n d liygerconjugati\.e comparison to other groups, see H . L. \-ale, J . .lied. Phorm. Chem., 1, 121 (1858). f 1 3 ) .t. Burger. "hledicinal Cliemistry." Interscience Publisliers, Inc.. 1960, p 82. S e u Y u r k , S . I-.,
compounds with interesting and often more powerful and varied biological activity. As part of a continuing search for new and improved anticoccidial agents and prompted by previous work on organofluorine drugs, we became interested in trifluoromethylbenzamides similar to 1-3. The object of the study was to determine if replacement of a nitro group by a trifluoromethyl group would give a compound with anticoccidial activity, and, if so, what structural requirements were necessary for this activity. Chemistry.-The compounds initially prepared for testing are listed in Table I. AIost of the amides were prepared from the acid chloride using commercially available a,a,a-trifluoro-nz-toluic acid (50) as a starting point. However, several attempts to prepare the Saminoethyl- and S-hydroxyethylamides by this rout e always gave the disubstituted derivatives 22 and 23. *kmides 31, 34, and 35 were obtained from hydrolysis of the appropriate nitriles. The amino derivative 26 and the o-hydroxyamide (27) were prepared from the esters 46 and 48 and concentrated IYHdOH under pressure. A cursory attempt to prepare 26 from the o-amino ester 45 was not successful. The preparation of 24 was best accomplished by catalytic reduction of 7 rather than ammonolysis of the ester 49. The other amides were prepared by the acid chloride-SH3 route. During the course of this investigation it was of interest to determine if a change in the amide portion of the molecule would give compounds with anticoccidial activity. Consequently the thioamide 53, sulfonamide 55, nitriles 51 and 56, arid amidine derivatives 52 and 54 mere prepared as described in the Experimental Section.
