Anolytico Chimico Acto. 133 (1981) 667-676 Computer Techniques and Optimization Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THE ORTNO EFFECTJN QUANTITATIVE STRUCTURE-ACTIVITY CORRELATIONS

TOSHIO FUJITA

Department of Agricultural Chemistry, Kyoto University, Kyoto 606 (Japan)

(F&eived 23rd January 1981)

SUMMARY

A procedure for assessment of the effects of o-substituents in physical-organic systems by means of linear combination of substituent parameters is described and its applicability to various data sets on reactivity and equilibrium is shown. The procedure seems to provide a physicochemical background to the quantitative analysis of structureactivity relationships for biologically active aromatic ortho compounds. The ortho effect in bio- logical activity is analyzed in terms of a linear combination of the electronic parameter op and field-inductive F, hydrophobic n and steric E, parameters. Depending on the type of sterlc effect, other parameters may be used. The model is shown to be applicable toseveral biological activity data sets of aromatic pesticide congenen including osubstituted derivatives.

Numerous quantitative structureactivity studies based on free energy- related physicochemical substituent parameters and regression analysis [l--6] have shown that the substituent effect on a certain biological activity of a series of congeneric compounds should be separable into electronic, hydrophobic, steric and other components. For the m and p substituted congeners, the Hammett u constant can be used for the electronic effect of substituents [ 7]_

The structure-activity relationship is analyzed quantitatively by means of

log (l/C) = PO + XeiEi + constant (1)

where C is the concentration (or dose) of the congener giving a standard response (e.g., Et&,,, LB,,) on a molar basis; E, corresponds to the free energy-related substituent parameters (e.g., hydrophobic and steric) other than those related to electronic effects; p and ei are the regression coefficients of respective terms whose level of significance is examined statistically.

Ortho-substituted compounds, however, cannot be dealt with directly by eqn. (1) because the Hammett u constant is usually not available for o-sub stituents. Generally, o-substituents exert various proximity effects such as

steric and direct field electric effects which are otherwise insignificant, but these effects contribute to varying degrees in different systems, and the effect of o-substituents cannot be expressed by a single generally applicable set of “Oo” parameters in physical-organic systems. Thus, in order to develop

0378-4304/81/0000-0000/so2.50 0 1981 Elsevier Scientific Publishing Company

a model similar to eqn. (1) for structureactivity studies of o-substituted compounds, as well as to provide a physicochemical background for such a model, a suitable procedure for establishing o-substituent effects was required. The procedure developed is based on linear combination of sub- stituent parameters and can be incorporated into a model for quantitative structure--activity analysis of ortho compounds [Cl] _ The procedure is out- lined below, and csamplcs of this application to sets of aromatic pesticide congcners including o-substituted derivatives arc given.

THE ORTHO EFFECT IN PHYSICAL-ORGANIC SYSTEMS

To establish the model: the following five assumptions were made [8]. First, the total effect of o-substituents, except for those capable of internal hydrogen bonding, is composed of the “ordinary” polar effect that can be considered in common with m- and p-substituents, and proximity effects (polar and stcric) inherently associated with o-substituents. Second, the “ordinary” polar effects of o-substituents and p-substituents are the same, i.e., a, = up, for any reaction type (91. Third, the o-substituents are closer to the reaction center than the p-substituents. Thus, the definition u0 = u,, may underestimate the electronic effect for ortho compounds. This undcr- estimation, or the difference from the “ordinary” effect, is taken into account by the “proximity” polar effect which is represented by the Swain- Lupton F constants [lo] as improved and extended by Hansch et al. [ 111. Fourth, the steric effect of o-substitucnts can be espressed by the Taft E, constants [12] as modified by Kutter and Hansch [ 131. Fifth, when the secondary steric effect of the o-substituent (e.g., steric inhibition of resonance of the side-chain functional group) is significant as, for instance, in reactions where phenoxide formation is critical, the “ordinary” electronic effect of o-substituents is given by up (= u,) while the effects from m- and p-sub stitucnts arc expressed by urneta and upom, respectively. (The composite of CJ, (= up), urn and up is denoted as u=.)

With these assumptions, the reactivity data of a series of o-substituted derivatives can be expressed by

logk,=po,+6E,0’thO+ fF,+c (2)

whem 6 and f are susceptibility constants and c is the intercept. If the p value of eqn. (2) does not differ from that of the correlation for the same reactivity of the corresponding m- and p-derivatives, the data for the series including o-, m- and p-substituted derivatives can be combined into eqn. (3).

log k am. P =po ,-,. m,p + S EFtho + fF, + c (3)

Then, the proximity effects of o-substituents are considered to be well separated from the ordinary polar effect. The susceptibility constants and the intercept are determined by regression analysis.

The F values used in eqn. (3) are the improved values [ll] (see above). The scale of the original set is not correct. The E, values in eqn. (3) are not

the Taft E,O values defined for the aromatic o-substituents f 123. Charton [ 141 advanced evidence that the E,O value is linked rather with the polar sub- stituent effect despite the original definition, whiIe the E, value determined for the ahphatic system is a reaI steric parameter [ 14]. Kutter and Hansch [13] found a relationship such as

E, = -1.839 r, (av) + 3.484 (n = 6, r = 0.996, s LL 0.132) (4)

for symmetrical substituents such as 1-I and CX3 (X = H, Me or halogen) where r, (av) is the average van der WaaIs radius [ 13]_ The steric constants of heteroatom substituents were estimated by using eqn. (4) not only for other symmetrical substituents such as halogens but also for unsymmetrical substituents such as NR2, OR, NO1 and phenyl [ 131. Here the extended E, values were used along with the original values for the alkyl substituents [ 123. The reference substituent was shiftid to H for simplicity so that E,(H) = 0.

Numerous data sets were found [ 81 to obey eqn. (3): o-, m- and p-substi- tuted derivatives for diverse reactions and equilibria including acid dissociation, hydrolysis and formation of esters and amides, electrophiiic and nucleophiiic substitution reactions, addition reactions and physicochemicaf properties. Some examples are shown in Table 1 along with the corresponding correla- tions for the m- and p-substituted compounds.

Depending on the conditions and type of reaction, not all the terms in eqn. (3) are statisticdlly significant. For example, for dissociation (sets 1 and 2) or ion-pair formation (set 3) and Ph,CN,-esterifieation (sets 4 and 5) of substituted benzoic acids, one or other of the proximity effects becomes ~si~ific~t when the reaction medium is changed from hydroxyiic solvents (water or ethanol) to aprotic solvents (DMSO, DMF and benzene). For the dissociation of phenoxyacetic acids and the Ph,CN,csterification of phenyl- acetic acids, the proximity effects are totally unimportant (sets 6 and ‘7) which is attributable to the reaction site being distant from the benzene ring. When the polar effect is not significant in critical steps associated with the reaction mechanism, neither the a nor the F term is required as observed for the acid-catalyzed hydrolysis of benzamides (set 8).

These examples indicate that the effect of o-substituents can be established with the use of crp for the ordinary electronic effectz and the significance of the proximity effects can be examined by adding the E, and F parameters. Thus, a model of the same form as eqn. (1) should apply to quantitative analysis of the biological activity of o-substituted compounds.

ANALYSIS OF THE BIOLOGICAL ACTIVITY OF O~~~O~SUBSTITUTED COMPOUNDS

In general, the o-substituent effect in biological activity can be analyzed in terms of a linear combination of substituent parameters as in the equation

log (l/C) = aa + po + 6E, f fF f c (6)

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TABLE I

Correlation of reactivity data’ with eqn. (3): log k = PO + 6 E, + fF + c

P 6 f C n “0 S rb - __-- 1. Dissociation of ArCOOH in H,O at 25°C; log K,, (M) [ 151

1.000 -4.197 24 0 0.021 0.998 (x0.029) (zO.011)

0.950 -0.392 1.469 -4.179 36 12 0.087 0.987 (2 0.099) (-.O.OSO) (TO.179) (zO.039)

2. Dissociation of ArCOOH in 50% DMSO at 25°C; log K,, (XI) [ 16 ] 1.345 -5.838 6 0 0.033 0.998

(*O.lOS) (~0.038) 1.302 0.832 -5.800 11 5 0.113 0.987

(~0.248) (I 0.4 11) (x0.089)

3. Ion-pair formation of _kCOOII with (PhNH),C=NH in benzene at 25”C;Iog K (MM’) [ 17 ] l.i89 -0.613 22 0 0.141 0.979

(:O.lilr) (- 0.068) 1.678 -0.470 -0.598 30 8 0.190 0.964

(tO.194) (~0.161) ( z 0.085)

4. Esterification of ArCOOH with Ph,CN, in EtO!i at 30°C; log fz (M” min ‘) [ 18, 191 0.877 0.032 14 0 0.032 0.994

(~0.061) ( t 0.020) 0.878 -0.193 0.718 0.024d 24 10 0.053 0.989

(-O.Oi9) (TO.033) (-0.143) (TO.029)

5. Esterification of ArCOOH with Ph ,CN, in DMF at 3O’C; log k (MM’ min.‘) [ 191 1.557 -1.294 5 0 0.068 0.993

(10.346) (10.129) 1.7i4 0.359= -1.388 13 8 0.081 0.989

(~0.206) (r0.252) (~0.058)

6. Esterification of ArCH$OOII: with Ph $!N, in EtOH at 30°C; log h (M-’ min.‘) [ 201 0.368~1~ 0.067 10 0 0.021 0.985

(10.053) (~0.021) 0.3820” 0.068 19 9 0.023 0.983

(2 0.036) (-0.013)

7. Dissociation of ArOCH,COOH in H,O at 25°C; log K, (IV) [ 211 0.3130” -3.188 17 0 0.031 0.950

(? 0.056) (~0.023) 0.3170” -3.186 25 8 0.034 0.947

(tO.047) (? 0.019)

8. Acid hydrolysis of ArCONH, in HI0 (HCI) at 100°C; log h (MM’ min-‘) [22] 0.085” -1.735 14 0 0.058 0.502

(eO.092) (tO.041) 0.707 -1.733 23 9 0.129 0.965

(-0.087) (20.066)

‘For each set, the correlation on the first line is for m- and p-substituted derivatives, whereas the correlation on the second line includes o-substituted derivatives. Unless othenvise noted, the values of p, 6 and fare justified by t-tests at better than the 99.5% level of significance. The figures in parentheses are the 95% confidence intervals. “I’he number of data used in the correlation is denoted by n; n, denotes the number of data of osubstituted derivatives included in the correlation; s is the standard deviation and r is the multiple correlation coefficient. The unit of equilibrium or rate constant is shown in parentheses after KA or k. =Justified at a level between 97.5 and 99.0%. dJustified at a level between 90 and 95%.

671

although each of the terms is not always statistically significant. In eqn- (5), CI is taken as up; n is the hydrophobic substituent parameter defined from l-octanol/water partition coefficients, P, as n = log Px - log P, where X denotes substituted compounds [23]. Depending on the type of stericeffect, E, can be replaced by other parameters. The correlation for compounds substituted at other positions can be combined with eqn. (5) if the corre- sponding substituent effects are equivalent for substituents at the o-, m- and p-positions. Some examples of the application of eqn. (5) and it-s extension from quantitative structureactivity studies of aromatic pesticides are presented in the following paragraphs.

Neuroexcitatory activity of substituted-benzyl (+)-trans.chrysanthemates 1241

Pyre~rin-tie compounds have little effect on mammals and are widely used as domestic and garden insecticides. Numerous classical as well as recently developed synthetic analogs having high potency against insects are esters of chrysanthemic acid with substituted benzyl alcohols. In the study described here, the activity of substituted benzyl (+)-trans.chrysanthemates (I) (in terms of the minimum effective concentration, MEC in M) needed to induce hyperexcitatory symptoms (i.e., a repetitive train of nerve impulses in response to a single stimulus) was determined against the excised nerve cords of American cockroaches. Quantitative analysis for the o-substituted derivatives yielded the equation log(l/MEC) = -1.010(+0.673)a - 0.301. (+0.332)n* t- 0.406(%0.332)AV,” + 5.502(*0.479), with R = 14, s = 0.372 and r = 0.878. In this equation, AV, is the group van der Waals volume relative to that of H defined as A V,,, = V,(X) - V,(H) [25] and scaled by 0.1 to make the size comparable to that of other parameters- It is suggested that the larger the volume, the more favorable are the o-substituents to excitatory activity on the nerve cords as far as the compounds used for the correlation are concerned. The 7r2 term indicates that the optimum hydro- phobicity for substituents is located around n = 0. Similar estimates for the m- and p-substituted derivatives for substituents at each of the stituents from the central part value; this shows that the steric becomes stricter.

showed that there is an optimum A VW value other positions. The more distant the sub- of the molecule, the smaller the optimum requirement for interaction with the target

(11 (II)

Insecticidal and antiacetylcholinesterase activities of substituted phenyl-N- methytcarbamates (261

Among this series of compounds, the o-secbutyl, o-isobutyl, 2,3-dimethyl and 3,4dimethyl derivatives have been used practically in Japan against

planthoppers and leafhoppers, which are major pests in rice production. The insecticidal activity of this class of compound (II) has been determined. The smaller brown planthoppers were liberated on rice seedlings which had been immersed in solutions containing various concentrations of the car- bamates along with enough piperonyl butoxide to inhibit oxidative metabolic degradation. The concentration which caused 50% mortality after 24 h, LCSO, was estimated for each compound. For the o-substituted compounds, the equation established waslog(l/LCsO) = 0.953(?0.428)n - 0.436(+0.526)~” i 0_723(?0_375)E, + 6.944(+0.226), for n = 13 with s = 0.147 and r = 0.871. In this equation, the u” term is significant only at the 90% level. The steric and hydrophobic effects are well separated.

Carbamate insecticides arc believed to deactivate acetylcholinesterase in the nervous system of the insect [ 271. The inhibitory activity against acetyl- cholinesterase prepared from planthoppers was therefore determined. The variation in the enzyme-inhibitor binding equilibrium constant (Kd’) reflects the variation in the inhibitory activity_ For the log K-,’ values of o-substituted derivatives, the equation derived was log Z&’ = 1_442(?0_292)n + 0.645- (+0.398)o” -f- 1.098(?0.312)HL3 + 4.898(?0.244) (with n = 14, s = 0.194 and r = 0.962). Here HR is an indicator variable that takes a value of 1 for hydrogen-accepting substituents such as nitro, cyano and alkoxy groups, but is otherwise zero. The steric effect does not seem to be significant. The difference in the relative hydrogen-bonding effects of substituents between phases involved in binding with acetylcholinesterase and the 1-octanol/water partitioning phases used as the reference system to estimate 7 values is accounted for by the HB term 1281.

The differences between the above equations for log (l/LC,,) and log Kd’ may reflect the factors other than oxidative degradation involved in biotrans- formation before the target is reached through the plant and/or insect body.

Antiacetylcholinesterase activity of O,O-dimethyl o-substituted phenyl phosphates [29J

This class of compounds (III) is generated in vivo by oxidative desulfura- tion of the corresponding phosphorothioates [30]. Among the phosphoro- thioates, the 3-methyl-4-nitro-, 3-chloro-4-nitro- and 4-cyano- derivatives have been widely used as selective insecticides with low mammalian toxicity. The insecticidal activity of the phosphorothioates is due to the inhibitory activity of the corresponding phosphates on acetylcholinesterase [ 271. The kinetic constants were determined [29] for the inhibition reaction of this class of compound on an acetylcholinesterase preparation from house-fly heads. Similarly to the inhibition of acetylcholinesterase by phenyl N- methylcarbamaies, the inhibitory potency is determined primarily by the binding step with the enzyme (l/K& Because the number of o-substituted compounds showing appreciable enzyme inhibition is small, the log (l/Kd) value of the o-compounds was analyzed together with that of the m-deriva- tives; up0 and o”, were used as the electronic parameters of the o- and m- substituents, respectively. The equation fount, was

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(III) (lx)

log(l/K,) = 0.138n,S(+0.053) + 2.246u”(k0.205) + 2_884(?O_llS)

(n = 12, s = 0.046, r = 0.992) (6)

In eqn. (6), =23 is the hydrophobic parameter for the o- and m-substituents. The correlation shown by eqn. (6) is very good and the standard deviation is low, indicating that no effect overlaps the ordinary electronic and hydro- phobic effects which are equivalent for the o- and m-substituents. The p- substituents exert no hydrophobic effect on the binding; their electronic effect is correlated with d by

log(l/&) = 2.2016(+0.904) + 2.951(?0.786) (n = 7, s = 0.266, r = 0.942)

(7)

The p value of eqn. (7) is practically equal to that of eqn. (6). For o-. m- and p-substituted congeners, u1 can be used as a common electronic param- eter to derive the equation

log(l/K,) = 0.176n,,(+0.160) + 2.253~‘;(?0_304) + 2.892(+0.215)

(n = 19, s = 0.153, r = 0.970) (8)

which suggests that there is a through-resonance effect of p- but not o- substituents in the binding process.

Herbicidal activity of N-chloroacetyl-N-phenylglycine esters (311 This class of compounds (IV) exhibits various degrees of herbicidal

activity against annual grasses. To examine the selectivity in favor of the rice plant, the molar ISo concentrations which inhibit shoot elongation of a bam- yard grass (Echinochloa Cruss-galli var. frumentaceus) and of rice plants to half the length of the control after 6 days were determined. The equations derived for the activity of o- and m-substituted derivatives, and some 2,6- disubstituted compounds, were as follows:

against the rice plant,

PISO = -0.33n(+0.18) - 0.95E,““h0 (iO.14) - 0_620(?0.46) + 4.10(+0.19)

(n = 28, s = 0.261, r = 0.959) (9)

and against the barnyard grass,

PIZO = -0.77E,0tih0(*0.16) - 0.22E,mc’“(~0.20) + 3.99(?0.24)

(n = 28, s = 0.295, r = 0.905) (10)

The activity data of the p-derivatives, which were very low and did not vary significantly, were not included in the correlations. For the effects of 2,6- disubstitution, Cn, C u and ZE, were used in the correlation. It is reasonable

to consider that the steric effect of o-substituents represented by the E,Ortho term involves both intra- and inter-molecular types, while that of m-sub stituents expressed by the ESmcta term is only for the intermolecular type. The negative sign of the E, terms in eqns. (9) and (10) indicates that bulk

is favorable to activity. Substituent effects providing selectivity for different plant species are evident from these two correlations, and information on selectivity between two plants can be obtained by comparing the equations.

Cytohinin-active substituted diphenylureas 1321 Bruce and Zwar 1331 prepared a large number of substituted N,N’-

diphenylureas exhibiting the cytokinin activity in varying degrees. Their activity data in terms of the minimum concentration (C) to show detectable cell division on cultured tobacco pith were analyzed for compounds where

one of the benzene rings is unsubstituted (V) to give the equation

log(l/C) = 0.9Ou(+O.33) - 0.85L,(fO.25) - 0.27LJkO.22) -t- 1.04r;,(“O.58)

+ 2.00 (20.32) (n = 39, s = 0.38, r = 0.91) (11)

Not only monosubstituted but also polysubstituted derivatives were included in the correlation. For polysubstituted derivatives, the electronic effect is represented as Xu. The hydrophobic and steric effects of substituents at various positions in polysubstituted molecules are not additive but are pri- marily governed by substituents at particular positions. The L, and I,, para-

meters are the length of the substituents at the c- and p-positions, respectively, along the “principal” bond axis as defined by Verloop et al. [ 341. Thep value, 0.90, suggests that an interaction with electron donors or nucleophiles at either the imino hydrogen or the carbonyl carbon is important for enhancing the activity. At the o- and p-positions, substituent length is unfavorable.

At the m-position, very hydrophobic substituents enhance activity.

DISCUSSION

The above examples show that structure-activity correlations of bio- logically active o-substituted compounds can be obtained in essentially the

same way as for the m- and p-substituted compounds. The “ordinary” as we11 as the position-specific and/or j>roximity effects of o-substituents can be analyzed by means of linear combination of physicochemical parameters. One of the most serious problems in dealing with this type of multiparameter correlation is collinearity among substituent parameters. This point was examined and it was confirmed that collinearity was not significant for most of the examples presented above.

Although further such examples can be expected, the procedure is not

675

applicable to all problems_ For instance, there are several examples where two o-substituents together exert specific effects on biological activity. In the phenoxyacetic acid plant growth regulators, a 2-chloro substitucnt greatly enhances the activity of the 4-chloro compound (leading to 24-D) whereas the 2,6-dichloro analog shows very little activity [35]. In a study of the activity of N-benzoyl-N’-(4chlorophenyl)urea derivatives against the larvae of the rice stem borer, a 2-fluoro substituent at the benzoyl moiety significantly lowers the activity of the unsubstituted benzoyl compound whereas the 2,bdifluoro substitution considerably increases the activity [36]. The effect of two o-substituents is apparently not additive in either example. The present procedure, which is primarily intended for assessment of the effect of single o-substituents and assumes the additivity of component effects, should be extended and modified to cover even those complex problems.

Finally, it should be emphasized that the analysis is just the first step in understanding the physicochemical mode of action of biologically active compounds. Numerous efforts will be needed from various directions to c!arify the physicochemical significance of substitucnt effects at the (sub)- molecular level for biologically active compounds and for the receptors_

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