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n‐Octanol Partition Coefficients of 1,2‐Dithiole‐3‐thiones
WaterhOctanol Partition Coefficients of 1,2-Dithiole-3-thiones M. BONA*,P. BOUDEVILLE*, 0. ZEKRI*, M. 0. CHRISTEN*,
AND
J. L. BUR GOT*^
Received December 29, 1994, from the *UFR des Sciences Pharmaceutiques et Biologiques, Deparfement &Etudes Physicochimiques et ~iocin~iiques des Phafmacosysiemes, Labofafoife de Chimie Ana/yfique, 2 Av. du Pr leon-l3efnard, 35043 Flennes cedex, France, and GOLVAY-PHARMA (L.T.M), 42 Rue Rouget-de-Lisle, 92151 Suresnes-Cedex, France. Accepted for publication May 11, 1995@. Abstract 0 Waterlnoctanolpartition coefficients (log p) for 33 1,Pdithiole3-thiones and for 18 1,Pdithiol-3-ones were determined by RP-HPLC measurement of the concentration of the solute in aqueous solution after equilibrium. Depending on the nature of the substituents (alkyl or aryl) and their position(s) (4, 5, or both) on the dithiole nucleus, some peculiar behaviors were revealed. Therefore, different fragmental constants containing the 1,2-dithioIe-3-thione nucleus were inferred in order to calculate in a complementary work, a prjori, the log P values of new dithiolethiones and dithiolones
Table 1-Alkyl- and Dialkyl-l,2-dithiole-3-thiones and -1,2-dithiol-3-ones Studied
4-Alkyl
5-Alkyl
Introduction 172-dithiole-3-thiones(X = S ) are compounds of pharmaceutical interest. One of them, Anetholtrithione (R5=p-CH30-
4,5-Dialkyl
Cycloalkyl
CsH4; R4 = H ) has been marketed since 1947l and is prescribed for its cholereticz and syalagogue proper tie^.^ Another one, Oltipraz 35972 R.P. (R4 = CH3; R5 = 2-pyrazi-
R4
R5
a
H H H CH3 H C2H5 H C3H7 H C4H9 H C5Hll CH3 H C2HS H C3H7 H H CH(CH3)z CH3 CH3 C2H5 CH3 CH3 CZH5 C2H5 C2H5 CH3 C3H7 C3H7 CH3 C3H7 C2Hs -(CHz)r -( CH2)4-CH2)5-( CH2)6-
X=S
Ref
X=O
Ref
1 14a 14b 1 4 ~ 14d 14e 15a 15b 15c 15f 14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5C 149 1459 145h 145i
28 29 10 32 a a 29 10 6 34 35
2 24a 24b 24c
28 30 31 a
25a 25b 25c
33 a
24a5a
33
24b5b 24c5a
a a
2459
33
a
10
10 10 36 a 37 37 37 37 37
Original compound.
Table 2-1,2-Dithiole-3-thiones and 1,2-DithioISones with an Aryl Fragment Studied R5 Olripraz
Ancrholuithionc
n ~ l is) in ~ advanced de~elopment.~ Interest in this class of compounds has grown for about 10 years6 because of their chemoprotective properties against some cancer^.^^^ We report, here, the determination of waterln-octanol partition coefficients (log P) of some 1,2-dithiole-3-thiones(X = S ) and of some l72-dithio1-3-ones(X= 0). This work was undertaken essentially to determine which fundamental fragments, including the whole 1,2-dithiole-3-thionenucleus, must be considered to calculate accurately log P values of the most simple 1,2-dithiole-3-thiones and l72-dithio1-3-ones. In a subsequent work, we shall use these fundamental fragments to calculate a prwri the log P values of more complex derivatives according to a constructionist approach, owing to the great importance of this parameter in pharmacochemistry.9 In this first work, we focused our attention on the determination of log P values by direct measurement of the concentration of solute in water once equilibrium between the two phases was reached. It was necessary to develop this methodology since no log P data for these compounds have yet been published. We limited our investigations to dithiolethiones and dithiolones, the substituents of which R5 and R4 were only hydrocarbon groups. Owing to the fact that we were primarily concerned with log P values, we give, here, @Abstractpublished in Advance ACS Abstracts, July 1, 1995.
0 1995, American Chemical Society and American Pharmaceutical Association
R4
X=S
Ref
X=O
Ref
14j 14j5a 14j5b 14m5a 141 15j 14a5j 15k 14a5k 15m 151 145j
38 35 6 40 38 41 42 41 43 44 45 32
24j 24j5a 24j5b
30 39
25)
33
25k
33
245j
33
a
only a preliminary interpretation of the results. Moreover, this first family grounded set of values will be, of course, used later for indirect determination of log P through measures of capacity factors.
Experimental Section Nomenclature Used-To mark mono- or disubstituted dithiolethiones and dithiolones, a simplified nomenclature is used: (i) 1 = dithiolethione, 2 = dithiolone, (ii) 4 and 5 denote the 4 and 5 substituent positions, respectively, (iii) a = methyl, b = ethyl, etc. (see Tables 1 and 2), (iv) 45, followed by a letter, signifies that the substituent forms a cycle between Cq and C g (see Tables 1 and 2).
0022-3549/95/3184-1 107$09.00/0
Journal of Pharmaceutical Sciences / 1107 Vol. 84, No. 9, September 1995
Table 3-Some Characteristics of the New Dithiolethiones and Dithiolones Studied in This work" NMR 'H (CDC13, b ppmTTMS)
14d 14a5c 14e 25b 25c 24c 24b5b 24c5a 24j5b a
Physical State
Red liquid mp 49 "C Red liquid Yellow liquid Yellow liquid Yellow liquid Yellow liquid Yellow liquid mp 82 "C
0.85 (t, ZH), 1.3 (m, 4H), 2.6 (t, 3H), 8.2 (s, 1H) 1.0 (t, 3H), 1.8 (m, ZH), 2.2 (s, 3H), 2.8 (t, 2H) 0.8 (t, ZH), 1.4 (2m, 6H), 2.6 (t, 3H), 8.2 (s, 1H) 1.2 (t, 3H), 2.7 (9, 2H), 6.4 (s, 1H) 1.0 (t, 3H), 2.7 (t, 2H), 1.65 (m, 2H), 6.45 s, 1H) 0.9 (t, 3H), 1.5 (m,2H), 2.4 (t, ZH), 8.0 (s, 1H) 1.0 (t, 3H), 1.25 (t, 3H), 2.75 (q, ZH), 2.4 (q, 2H) 0.85 (t, 3H), 1.35 (m,ZH), 2.4 (s, 3H), 2.4 (t, 3H) 1.2 (t, 3H), 2.75 (9, ZH), 7.4 (m, 5H)
1,
(C=O)
(cm-I, KBr)
1636 1658 1660 1652 1652 1652
Satisfactory microanalysis.
Chemistry-Thirty-three 1,2-dithiole-3-thiones and 18 1,2-dithiol3-ones were investigated (tables 1 and 2 ) . Some of these compounds were newly synthesized for this work. Already described dithiolethiones and dithiolones (tables 1 and 2) were prepared according to the given references. The novel dithiolethiones 14d, 14e, and 14a5c (respectively: 4-butyl; 4-pentyl, and 4-methyl-5-propyl-l,2dithiole-3-thiones) were all synthesized by action of phosphorus pentasulfide with corresponding P-keto esters in dry xylene according to the method of Legrand and Lozac'h.lo The starting esters for 14d and 14e were ethyl a-formylhexanoate and ethyl a-formylheptanoate prepared by allowing ethyl formiate to react with corresponding esters according to ref 11. The starting ester used in the synthesis of 4-methyl-5-propyl-1,2-dithiole-3-thione (14a5c) was ethyl(2-methylbutyry1)acetate prepared by alkylation of commercial ethyl butyrylacetate.12 The novel dithiolones were prepared by action of benzonitrile oxide generated in situ according to ref 13. Some of the characteristics of these new compounds are given in Table 3. The purity of solutes was checked before each determination by TLC with two pairs of solvents. log P Determination-The main difficulty of this determination was the great insolubility of dithiolethiones and dithiolones in water. The water solubility of the most soluble dithiolethiones was only 5 x 10-3 mol L-' and for most of them was below 5 x 10 mol L-I. As a result, the transfer of solute from n-octanol into water could not be evidenced by appreciable variation of its concentration in the organic phase (in the few cases where the amount of solute in the aqueous phase at the equilibrium was superior to 1% of the amount remaining in the organic phase, corrections were made). Moreover, the studied solutes were not functionalized on the substituents R5 and R4 and, in particular, did not exhibit a n acidic-basic property which would have made determination easier by solubility measurements at different pH values. Therefore, we had to determine the concentration of solutes in water once the partitioning equilibrium was reached. We resorted to RP-HPLC with spectrophotometric detection. The partition coefficient P was calculated as the ratio of concentrations in octanol and in water. It was leotimate to equate concentrations to activities, as the concentrations of the solutes in water were very low and the solutes were not ionic.'* Our methodology was close to that followed by Camirelli et ~ 1 . and ' ~ Hansch et ~ 1 . ' ~ Ten milliliters of a n octanolic solution of solute were introduced with 50 mL of water into a 250 mL separatory funnel and mechanically shaken for 30 min a t ambient temperature, 20 f 1 "C. (Equilibrium was very quickly reached: we verified that shaking for more than 30 min did not significantly change the log P values.) The solutions were then left to stand for 2 h until the two layers were separated. The aqueous phase was kept and centrifuged in stoppered tubes a t 20 i 1 "C a t 6000 rpm for 30 min, and the residual octanol on the water layer was aspirated. The water layer was directly analyzed by RP-HPLC through a 100 pL sample loop. The sample concentration was determined by comparison to a calibration curve constructed with five points chosen in the same range of concentrations as the measured one (from 0.5 to 1.5 times the sample concentration). Owing to the low water solubility of solutes, the solutions used to build up the calibration curve were prepared by dissolving a known amount of solute in methanol; this solution was firstly diluted with methanouwater (50/50 v/v). Then the latter solution was diluted with pure water. Each calibration solution was chromatographed three times. For each compound, four replicates were performed according to the above procedure and the four
1108 / Journal of Pharmaceutical Sciences Vol. 84, No. 9, September 1995
resultant aqueous phases were analyzed three times. So the aqueous concentration used for computation was the mean value of the 12 data points obtained. From the standard deviation of the 12 experimental results and from the calibration curve, the uncertainty of the aqueous concentration of solute was estimated, and the resulting precision of the log P determination was estimated to be f 0 . 1 log P unit. For example, if we consider 5-phenyl-1,2-dithiole-3-thione (15j),its water solubility was determined as 3 x mol L-I; in the shake-flask experiment the 15j concentration in the octanolic phase was 5 x mol L-l and after partition the 15j concentration found in the aqueous mol L-l. phase was 1.07 x To validate our methodology, we applied it to some compounds for which the log P was already known: p-nitroacetophenone (found (fd), 1.49; literature (lit.), 1.5317),p-nitroanisole (fd, 2.02; lit., 2.0016 and 2.0318),p-nitrotoluene (fd, 2.43; lit., 2.429 and 2.3716), anisole (fd, 2.15; lit., 2.10: 2.1l,lfi 2.0819), thioanisole (fd, 2.81; lit., 2.749), nitronaphthalene (fd, 3.21.; lit., 3.1g9). Moreover, the study of some of the least lipophilic derivatives mentioned above was carried out by direct analysis of the aqueous phase by direct and pulse polarography on the one hand and by UV-visible spectrophotometry on the other hand. The values obtained by polarography (1.58 and 2.03 for p-nitroacetophenone and p-nitroanisole) and those obtained by spectrophotometry (1.49, 2.02, and 2.44 for p-nitroacetophenone, p nitroanisole, andp-nitrotoluene, respectively) were in good agreement with those obtained by RP-HPLC and with those reported in the literature. The same comparison of the log P values obtained by polarography, spectrophotometry, and RP-HPLC was carried out for the least lipophilic dithiolethiones derivatives 1, 15a, and 2 (Tables 4 and 7). It is worth noting that we were not able to determine the log P values of all the dithiolethiones we would have liked to study because their solubility in water was too low mol L-l), and therefore our methodology could not be used. Materials-The HPLC chromatograph was a n LDC-Milton Roy system (constametric 111) isocratic pump with a (spectromonitor 11) UV-visible detector. Solutes were detected a t their analytical wavelength (ranging from 400 t o 430 nm for the dithiolethiones and 280 to 330 nm for the dithiolones). The RP-HPLC column was a stainless steel tubing (4.5 mm in diameter and 15 cm long) filled with 5 pm ODs2 stationary phase (Hichrom). The mobile phase was water/methanol mixtures (20/80or 30/70 v/v). The flow rate was 1 mL min-' a t 20 "C. Water was doubly deionized on an ion-exchange resin. Water and n-octanol were mutually saturated before use. The UV-visible spectrophotometer was a double beam Uvikon 930 (Kontron) with baseline correction. Differential pulse polarography was performed with a Tacussel EPL 3 Ti-Puls polarograph. All the solutions used for calibration and supporting electrolyte (m030.1 mol L-') were prepared with water saturated by n-octanol (n-octanol influenced the peak shape).
Results and Discussion Experimental measurements of log P allowed us to calculate simultaneously the values of the dithiole nucleus fragments and those of CH3, CHe, and H, flCH3), flCHz), and AH), respectively. If ACH3), flCHz), AH) values found were in agreement with those reported in the literature, we considered that the values obtained for the dithiole nucleus fragments were accurate. Alkyl- and Dialkyl-,2-dithiole-3-thiones-The log P valare ues we found for alkyl and dialkyl-1,2-dithiole-3-thiones mentioned in Table IV. Consideringthe very small differences in the log P values found for dithiolethione parent (1) and for 5-methyl-l,Z-dithiole-3-thione (15a),independently obtained from HPLC, polarography, and UV-visible spectrophotometry experiments (Table 4),their respectively log P values 1.58 f 0.02 and 1.86 f 0.02 cannot be questioned. The fact that the log P of the dithiolethione parent (1) was so firmly grounded was a very important point (see below). In order to detect some possible inaccuracies in the experimental log P values of some other dithiolethiones or, alternatively, to detect their unexpected behavior, we decided to calculate log P starting from the well-established value for 1. We assumed, therefore,
Table 4-Experimental and Calculated log PI Values (See the text) of Alkyl-, Dialkyl-, and Aryldithiolethiones [log PI = 40) 1(R4) 4R5)]
+
Fragment used
Dithiolethionesc
log P(expl) 1.58 1.59a 1.566 1.85 1.85a 1.8gb 2.31 2.83 2.18 2.67 3.18 4.06 2.45 2.94 2.95 3.51 3.39 3.42 3.76 2.53 3.10 3.27 3.75 3.20 3.17 3.52 3.78 3.73 3.67 3.95 3.82 4.10 3.43 4.26 3.57
1
15a
0
f(0) = 1.22
~~
15b 15c 14a 14b 14c 14e 14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5c 145f 1459 145h 145i 14j 14j5a 14j5b 14m5a 141 15j 14a5j 15k 14a5k 15m 151 145j
Table 5-Experimental and Calculated log P2 Values of Alkyl- and Dialkyldithiolethionesa
+
log PI
-
Fragments Used
A
Dithiolethiones
-
1 2.09
-0.16
2.60 3.13 2.09 2.60 3.13 4.17 2.60 3.12 3.12 3.65 3.65 3.65 4.16 2.78 3.30 3.82 4.34 3.23 3.75 4.27 4.27 3.74 3.23 3.75 3.32 3.84 3.46 3.74 2.62
-0.29 -0.30 +0.09 +0.07 +0.05 -0.1 1 -0.15 -0.18 -0.17 -0.14 -0.26 -0.23 -0.40 -0.25 -0.20 -0.55 -0.59 -0.03 -0.58 -0.75 -0.49 -0.01 +0.44 +0.20 +0.50 +0.26 -0.03 +0.52 +0.95
~
UV-visible spectrophotornetry. Polarography. See tables 1 and 2. A = log P(expl) - log PI.
15a f(1) = 1.67
r2
f(II) = 2.00
f(ll1) = 2.69
15b 1% 14a 14b 14c 14d 14ed 14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5cd 145f 1459 145hd 145id
log P2
log P(expl) 1.58 1.596 1.56c 1.85 1.85b 1.8gC 2.31 2.83 2.18 2.67 3.18 3.70 4.06 2.45 2.94 2.95 3.51 3.39 3.42 3.76 2.53 3.10 3.27 3.75
Aa
-
-
-
-
2.36 2.88 -
2.69 3.21 3.73 4.25 -
2.96 2.96 3.47 3.48 3.48 3.99 2.61 3.13 3.65 4.17
-0.05 -0.05 -0.02 -0.03 -0.03 -0.19 -0.02 -0.01 4.04 -0.09 -0.06 -0.23 -0.08 -0.03 -0.38 -0.42
a Calculations from log P values of 5-methyl; 4-methyl- and 4,5-dimethyldithiolethiones (see the text). UV-visible spectrophotornetry. Polarography. d A = log P(expl) - log P2.
and 15a, which should have the same log P I , exhibited important discrepancies. It was clear that the same alkyl fragment substituted in position 4 or 5 did not give the same log P. Therefore, a new calculation of log P taking into account the three basic fragments I, 11, and 111was performed.
a
that it was possible to attribute to the correspondingfragment 0 the fragmental constant value fC0) = log P(exp1 dithiolethione 1) - 2flH).
qs 0
According to this hypothesis, the log P of any dithiolethione could be calculated by relation 1 (see Table 4):
+
logP, = f l0) + ftR5) AR4)
fcI), AID, AIII) were calculated after log P(exp1) of the derivatives 14a, 15a,and 14a5a by substraction of AH), AH), and 2f(H), respectively, that is to say: fcI) = 1.85 - 0.18 = 1.67, AII) = 2.18 - 0.18 = 2.00, AIII) = 2.45 - (2 x 0.18) = 2.09. As a result, log P values were calculated differenciating For example, for 5-alkyl-, 4-alkyl-, 4,5-dialkyldithiolethiones.
(1)
The fragmental constants RR5) and AR4) that we used were those of Rekker et aLZ0[f(CH3)= 0.69, flCH2) = 0.52, AH) = 0.181 because, for our purpose, this “reductionist” statistical approach was sufficient, since only short and unbranched ~ a result, f l 0 ) = 1.58 - 2 x alkyl groups were p r e ~ e n t .As 0.18 = 1.22. A study of the log P(exp1) and log PI values revealed that practially all dithiolethiones were deviant (see Table 4). Simultaneous resolution by a nonlinear least squares process (in a matricial way) of relation 1applied to all the alkyldithiolethiones in Table 4 gave f ( 0 ) = 0.98 (a = 0.15),ACHs) = 0.73 (a = 0.071, flCH2) = 0.48 (a= 0.02), AH) = 0.33 (a= 0.06) (a is the square root of the variance of each fragment obtained from the varianceskovariances matrix). The flH) value was quite unsatisfactory. Moreover, methyldithiolethiones 14a
/,y
Cn,
I
R4’
+
5-alkyldithiolethiones,log Pa = AI) f(R5‘), and so on for other alkyldithiolethiones. The values obtained in this way are given in Table 5. However, some deviant dithiolethiones still remained, as shown by the values of A (last column). This was confirmed by the corresponding matricial calculation which gave fcI) = 1.69 (a = 0.06), PII) = 2.06 (0= 0.06), AIII) = 2.14 (0= 0.08), fcH) = 0.18 (a = 0.051, flCH2) = 0.43 (a = 0.02),flCH3) = 0.68 (a = 0.03). If the literature value flCH2) = 0.52 is not questionable, our value ACHz) = 0.43 is unsatisfactory. Journal of Pharmaceutical Sciences / 1109 Vol. 84, No. 9, September 1995
Table 6-Experimental and Calculated log P3 Values of Alkyl- and Dialkyldithiolethionesa Fragments Used
Dithiolethiones
log Ffexpl)
15b 15c 14a 14b 14c 14d 14ed
1.58 1.5gb 1.56c 1.85 i,856 i,890 2.31 2.83 2.18 2.67 3.1 8 3.70 4.06
14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5cd 145f 1459 145hd 145id
2.45 2.94 2.95 3.51 3.39 3.42 3.76 2.53 3.10 3.27 3.75
1
15a f(1) = 1.64
f(I1) = 1.99
f(111) = 2.09
log f
3
-
Ae -
1.82
+0.03
2.34 2.86 2.16 2.67 3.1 9 3.71 4.23
-0.03 -0.03 4.02
2.45 2.97 2.97 3.49 3.48 3.48 3.99 2.61 3.13 3.65 4.17
0.00 -0.03 -0.02 4.02 -0.09 -0.06 -0.23 -0.08 -0.03 -0.38 -0.42
+o.oo -0.01 -0.01 -0.17
dCalculations from ql), @I), and qlll) values obtained in a matricial way (see the text). UV-visible spectrophotometry. Polarography. Dithiolethiones discarded (see the text). e A = log qexpl) - log 4.
Since flCHz) was still unsatisfactory, the same matricial strategy was finally adopted, but dithiolethiones 14e,14b5c, 145h,and 145i were discarded because the difference (A) (see Table 5)between their log PZand log P(exp1)values was more than twice the experimental uncertainty. From the matricial calculation performed with remaining dithiolethiones, we adopted the definitive values for the searched for fragmental constants as fl1)= 1.64 (a = 0.03),f(I1) = 1.99 (a = 0.03),flII1) = 2.09 (a = 0.041, and those for other fragments a s AH) = 0.18 (0 = 0.02), flCHz) = 0.50 (a= 0.01), flCH3) = 0.69 (a = 0.02). The log P values of dithiolethiones calculated with these last fragmental values AI), flII), and f(II1) and noted P3 are listed in Table VI. The four discarded dithiolethiones remain deviant, but all other calculated values are in good agreement with experimental log P (A < 0, 1). The flCH3), flCHz),and f ( H )values obtained simultaneously with flI), flII), and flIII) (after dithiolethiones 14e, 14b5c,145h,and 145i were discarded) compared nicely with those found in the literature. This is a good argument in favor of the accuracy of all our log P(exp1) values. As a result, taking into account the fragmental constants flI),f(II), and flIII) provides a good means to calculate the log P values of alkyldithiolethiones. We performed another analysis of the log P values of alkyldithiolethiones. Remembering that the fragment 5-(1,2dithiole-3-thione)-ylis an electron-withdrawing group (ap-= 1.14),21,2z we singled out the hydrogen fragment in position 5, the value of which would be f(H*) = 0.46 according to Rekker.zo This allowed us to introduce the fragments values ffO*) and f(II*).
qsqs '0
tTz I!'
Satisfactory results were obtained only when we took into 1110 / Journal of Pharmaceutical Sciences Vol. 84, No. 9, September 1995
account simultaneously these two fragments and when we discarded the same four dithiolethiones 14e,14b5c,145h,and 145i as we did previously. The values obtained were AH) = 0.18 (a = 0.02), AH*) = 0.43 ( U = 0.02),flCH2) = 0.50 (a= 0.01), flCH3) == 0.69 (a= 0.02),f l O * ) = 0.95 (a = 0.04), flII*) = 1.56 (a = 0.04). Therefore, again, dthiolethiones 14e, 14b5c,145h,and 145 exhibit abnormal behavior. According to log P(exp1) data, it is quite clear that 4-alkyl1,2-dithiole-3-thiones are more lipophilic than 5-alkyldithiolethiones [log P(4-alkyldithiolethiones)- log P(5-alkyldithiolethiones) sz 0.351. Moreover, a comparison of log P (expl) and log P I (Table 4) shows that 4-alkyl-l,2-dithiole-3-thiones are more lipophilic and 5-alkyl-l,2-dithiole-3-thiones are less lipophilic than expected. The reinforced lipophilic property of 4-alkyl-1,2-dithiole-3-thiones can be explained by the proximity of the substituent in position 4 and of the thiocarbony1 group with generation of steric hindrance.z3 Since dithiolethiones are endowed with significant dipolar mom e n t ~it, is ~ ~reasonable to rationalize the lowered lipophilic property of 5-methyl-l,Z-dithiole-3-thione in terms of some polar effect. 4,5-Dialkyl-1,2-dithiole-3-thiones exhibit simultaneously the behaviors of 5-alkyl- and 4-alkyl-1,Z-dithiole3-thiones. Otherwise, another trend is perceptible: the longer the chain substituted in the 4 position is, the more log P deviates from the expected value. This is not totally surprising. Such a phenomenon has been taken into account by Leo et al. in their system of fragmental constants to calculate the log P values of chain^.^ Unfortunately, we could not further verify this trend because 4-hexyl-l,Z-dithiole-3-thione was, a priori, too poorly soluble in water to be studied. It is likely that the same phenomenon could probably be evidenced for 5-alkyl-1,2-dithiole-3-thiones if they were easily synthesized. Another finding is the lower than expected lipophilicity of the cyclic dithiolethiones 145h and 145i (see A, Table 6). This phenomenon seems to increase with the length of the cycle. It has been noticed and taken into account by Leo et aL9 The dithiolethione derived from cyclohexane 145g can be considered as normal, and that derived from cyclopentane 145f as quasinormal. To conclude as far a s alkyl-1,2-dithiolethiones are concerned, it is interesting t o note that the branched 5-isopropyl-1,2-dithiole-3-thione 15f [log P(exp1)= 2.751 seems t o be less lipophilic than 5-propyl-l,Z-dithiole-3-thione 15c [log P(exp1) = 2.831. This is in agreement with literature data.g Aryl-1,2-dithiole-3-thiones-In Table 6 are mentioned the log P values of dithiolethiones with an aromatic fragment included in their R4 or R5 substituents. A comparison of log P(exp1) and log PIvalues (obtained with f(0) = 1.22, flCsH5) = 1.83,20f(p-CHsOCsH4)= 1.92: and fcpCH3CsH4) = 2.3420)shows the failure of the additivity of fragmental constants without supplementary corrections for 5-aryl-l,2-dithiole-3-thiones. The failure is particularly striking for 5-phenyl; 5-(ptolyl)- and 5-@-methoxyphenyl)-l,2dithiole-3-thiones 15j (A = 0.44), 151 (A = 0.50),and 15k (A = 0.52),which are more lipophilic than expected (see log P I , Table 4). This finding must undoubtedly be related to the conjugation of the 5-aryl substituent with the dithiole nucleus, which results in the coplanarity of the whole molecule.25 Such a conjugation is encountered with biphenyl, for which experimental log P values 3.95,4.09, and 4.179 are higher than the log P calculated from the simple addition of two flC6H5): log P(ca1c) = 3.66 (A = 0.44 f 0.12). A corrective factor has been proposed to take this effect into account.z0 This conjugation effect is less marked in 5-phenyl-4-methyl-l,2-dithiole-3-thione (14a5j)and in 5-@-methoxyphenyl)-4-methyl-1,2-dithiole-3thione (14a5k). Their lipophilicity increase is about half that of the 4-demethylated dithiolethiones 15j and 15k. This effect is likely to be significant, because it is grounded on the values of two 5-aryl-4-methyl-dithiolethiones and three 5-aryldithiolethiones and because the difference A = 0.20is most certainly
+
the very upper limit of the log P uncertainty. This is quite understandable as the 4-methyl substituent lowers the conjugation of the 5-aryl group by steric hindrance. Our semiempirical calculations indicate that the aryl and the dithiole nucleus are twisted at an angle of 120°,whereas in 5-aryldithiolethiones, the two aromatic fragments are coplnar. A comparison of the log P(exp1)and log PI values of 4-phenyl (14and 141)shows that and 4-(p-tolyl)-l,2-dithiole-3-thiones they exhibit a normal behavior. This can be related to the hypothesis formulated by some authors according to whom the aryl fragment is not conjugated with the dithiole nucleus as simple LCAO-MO calculain 4-aryl-1,2-dithiole-3-thiones, tions suggest.26 Our own findings by molecular modelingz7 indicate that the aromatic substituent is twisted at an angle of 111" t o the dithiole nucleus. With such a geometry, solvation of the thiocarbonyl group by water is not hindered. Very puzzling are the behaviors of 5-methyl- and 5-ethyl4-phenyl-1,2-dithiole-3-thiones (14j5a)(logP(exp1)= 3.17) and (14j5b)(log P(exp1) = 3.52) which are less lipophilic than expected (A = -0.30 or -0.55) as the following calculations for 14j5a indicate: fcC6H5) fcI) = 3.47(A = -0.30) or fcCH2) 3.20 = 3.72 (A = -0.55) (3.20 is the logP(exp1)of 14).14j5b exhibits the same behavior: f(C6H.5) fcI) f(CH2)= 3.99 (A = -0.47) or 2fcCHz) 3.20 = 4.24 (A = 0.72). Introduction of fragment values AIv) and fcV) should take into account these peculiar behaviors.
+
+
+
+
+
Table 7-Experimental and Calculated log P, Values (see the Text) of the Dithiolones Studied
log ffexpl) log A 2 24a 24b 24c 25a 25b 25c 24a5a 24b5b 24c5a 2459
0.82 0.82a 0.84b 1.33 1.90 2.38 1.26 1.69 2.24 1.73 2.78 2.96 2.40
1.33 1.85 2.38 1.33 1.85 2.38 1.84 2.88 2.88 2.54
log Qexpl) log 9
Ac
A
0.00 +0.05 0.00 -0.07 -0.16 -0.14 -0.11 -0.10 +0.08 -0.10
24j 24j5a 24j5b 25j
2.64 2.93 3.15 3.01
2.47 2.99 3.51 2.47
+0.17 -0.05 -0.36 +0.54
25k
3.19
2.56
+0.63
24j5j
4.07
4.12
-0.05
2451
2.73
1.94
+0.79
a UV-visible spectrophotometry. Polarography. A = difference between log flexpl) and log 4.
than the corresponding dithiolethione 14j5b. 4-phenyl-1,2dithiol-3-one(24)is somewhat more lipophilic than expected. 4,5-diphenyl-l,2-dithiol-3-one (24j5j)(its corresponding dithiolethione 14j5j was too lipophilic to be studied) seems to give a normal value (log P(exp1) = 4.07 for log PI = 4.12).
Conclusion
Even, 4-benzyl-5-methyl-l,2-dithiole-3-thione (14m5a)exhibits the same behavior. So far, we have no satisfactory explanation for these phenomena. On the other hand the log (15m)is nearly P(exp1f(3.43) of 5-benzyl-l,Z-dithiole-3-thione identical t o the sum fcCsH5) fcI) = 3.47 (expected log P). Unfortunately, we were not able to study other 4- or 5-aryl1,2-dithiole-3-thiones because of their lack of solubiiity in water. Alkyl- and Aryl-1,2-dithiole-3-ones-logP values of dithiolones are mentioned in Table 7. As above, we performed calculations of log PI of dithiolones by introducing the fragment 0'
+
qo 0'
+
+
log PI =fro') f(R4) AR5) [f(0') = log P(exp1)dithiolone2) ZfcH)]. The dithiolones are considerably less lipophilic than the corresponding dithiolethiones (A = 0.60 f 0.16). At first glance, the log P(exp1) - log P I differences of the former seem to be less marked than those of the corresponding dithiolethiones. This is particularly noticeable for 4-methyl- and 5-methyl-l,2-dithiole-3-ones (24aand 25a),which exhibited nearly the same value. We were able to distinguish three types of dithiolethiolones by considering the differences of log P(exp1) between dithiolethiones and correspondingdithiolones. They are 5-alkyldithiolones: A log P(exp1) x 0.60; 4-alkyl1,2-dithiole-3-ones: A log P(exp1) x 0.81; 4,5-dialkyl-1,2dithiol-3-ones: A logP(exp1)x 0.73. As far as 5-aryl- or 4-aryl1,2-dithiolonesare concerned, the interesting findings are that 5-methyl-4-phenyl-l,2-dithiol-3-one (24j5a)is not deviant a t all (contrary to the corresponding dithiolethione 14j5a) and that 5-ethyl-4-phenyl-1,2-dithiol-3-one(24j5b)is less deviant
To sum up, it is possible to calculate, a priori, the log P values of most alkyl- and aryldithiolethiones. It is necessary, however, to take into account the fragmental constant fcI) for 5-alkyl-1,2-dithiole-3-thiones, AII) for 4-alkyl-1,2-dithiole-3Using thiones, and f(II1) for 4,5-alkyl-1,2-dithiole-3-thiones. and the same strategy for 4- or 5-aryl-l,2-dithiole-3-thiones the fragment constants fcIv)and fcV),respectively, would also probably give reasonable estimations of their log P values Unfortunately, we were not able t o verify this method of calculation since higher homologues of dithiolethiones were too poorly soluble in aqueous phase to be studied. We are currently working on the determination of functionalized dithiolethiones log P values.
References and Notes 1. Laboratoire de Therapeutique Moderne, Suresnes, France. 2. Halpern, B. N.; Gaudin, 0.Arch. Int. Pharmacodyn. Thr. 1950, 83, 49. 3. Leland, G.; Mercat, C.; Fuseiller, C. Gaz. Med. Fr. 1969, 76, 2257. 4. Bieder, A,; Decouvelaere, B.; Gaillard, C.; Depaire, A,; Heusse, D.; Ledouse, C.; Lemar, M.; Le Roy, J. P.; Raynaud, L.; Snazzi, C.; Gregaire, J . Arzeim-Forsh./Drug. Res. 1983, 33 (II), 1989. 5. RhBne-Poulenc-Rorer Industries, Recherche, Vitry sur Seine, France. 6. Abazid, M.; Bertrand, H. 0.; Christen, M. 0.; Burgot, J. L. Phosphorus Sulfur Silicon Relat. Elem. 1994, 88, 195. 7. Egner, P. A.; Kensler, T. W.; Prestera, T.; Talalay, P.; Libby, A. H.; Joyner, H. H.; Curphey, T. J. Carcinogenesis 1994, 15 (21, 177. 8. Li. Y.; Lafuente, A.; Trush, M. A. Life Sci. 1994, 54 (131, 901. 9. Hansch, C.; Leo, J. A. In Substituent Constants for Correlation Analysis in Chemistry and Biology; John-Wiley and Sons: New York, 1979; p 13. 10. Legrand, L.; Lozac'h, N. Bull. Soc. Chim. Fr. 1955, 79. 11. Wislicenus, W. Ber. Dtsch. Chem. Ges. 1887, 20, 2930. 1959,81,2907. 12. Ireland, R. E.; Marshall, J. A. J . Am. Chem. SOC. 13. Boberg, F.; Knoop, J. Liebigs Ann. 1967, 148. 14. Butler, J. N. In Zonic Equilibrium: a Mathematical Approach; Addison-Wesley, Publishing Co. Inc.: New York, 1964; p 439. 15. Camilleri, P.; Munro, D.; Weaver, K.; Williams, D.; Rzepa, A. J . Chem. SOC.Perkin Trans. 2 1989, 1935. 16. Fujita, T.; Iwasa, J . E.; Hansch, C. J . Am. Chem. Soc. 1964,80, 5175.
Journal of Pharmaceutical Sciences / 1111 Vol. 84, No. 9, September 1995
17. Chapman, J . D.; Raleigh, J . A,; Borsa, J.; Webb, R. G.; Witehouse, R. Znt. J . Radiat. Biol. Relat. Stud. Phys. Chem. 1972, 21. 475. 18. AlIGalany, K. A.; Bridges, J . W.; Netter, K. J . Biochem. Pharmacol. 1975, 24 (81, 867. 19. Lu, P. Y.; Metcolf, F. L. Enuiron. Health Pers ect 1975,10, 269. 20. Rekker, R. F.; de Kort, H. M. Eur. J . Med. & e i . 1979,14 (6), 479. 21. Saidi, M., Ph.D. Thesis; University of Rennes I, France, 1988. 22. Burgot, J.-L.; Darchen, A.; Jehan, P.; Saidi, M. Proceedings of Journ6es d’Electrochimie Brest (France), 1991, poster no. 4-49. 23. Mayer, J . M.; Van de Waterbeernd, H.; Testa, B. Eur. J . Med. Chen. 1982, 17 (l), 17. 24. Luttringhaus, A,; Grahman, J . 2. Naturforsh. 1955, l o b , 365. 25. Wang, Y.; Lin, H. C.; Wei, C. H. Acta Crystallogr. 1985, (241, 1242. 26. Vorankov, M. G.; Minhin, V. I.; Osipav, 0. A.; Kogan, M. G.; Lapina, T. V. Khin. Geterosikl. Soedin. 1967, 3, 758. 27. Bertrand, H. 0.;Burgot, J. L.; unpublished results. 28. Meinetsberger, E.; Schofer, A,; Behringer, H. Synthesis 1977, 802. 29. Legrand, L.; Mollier, Y.; Lozac’h, N. Bull. SOC.Chim. Fr. 1953, 327. 30. Brown, R. F. C.; Rae, I. D.; Sternhell, S. Austr. J . Chem. 1965, 18, 1211.
1112 1Journal of Pharmaceutical Sciences Vol. 84, No. 9, September 1995
31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.
Fabian, J.; Melhorn, A,; Mayer, R. 2.Chem. 1965,5 (11, 22. Mayer, R.; Wittig, F. 2. Chem. 1972, 12(31, 91. Raoul. P.: Vialle. J. Bull. SOC.C h i n . Fr. 1959. 1670 Mouchel, P ; Thuillier, A. C. R . Sci. 19671264?18), 1552. Thuillier, A,; Vialle, J . Bull. SOC.Chim. Fr. 1962, 2187. Wander, A. G. Germ. Offen. 909.097, apr 12, 1954. Thuillier, A,; Vialle, J . Bull. SOC.Chim. Fr. 1962, 2194. Fields, F. K. J . Am. Chem. SOC.1955, 77, 4255. Yoshitomi. Pharmaceutical Industrie, Ltd. JP 51125737, 1975. Wander, A. G. Germ. Offen. 1958, 909.097 (C.A.; 1958, 52, 10205). Bottcher, B.; Liittringhaus, A. Ann. 1948, 557, 89. Hoffman, F. W. Chem. Abstr. 1953,47, 2168h. Elkaschef, M. A. F.; Abdel-Megeid, F. M. E.; Elbarbary, A. A. Acta. Chim. Acad. Sci. Hung. 1977, 93 (21, 167. Beer, R. J . S.; Carr, R. P.; Cartwright, D.; Harris, D.; Slater, R. A. J . Chem. SOC.C 1968,20, 2490. Block, B. P. Chem. Abstr. 1953, 47, 11877h.
Acknowledgments Authors thank G. Bouer for his technical assistance. JS940701S
AND
J. L. BUR GOT*^
Received December 29, 1994, from the *UFR des Sciences Pharmaceutiques et Biologiques, Deparfement &Etudes Physicochimiques et ~iocin~iiques des Phafmacosysiemes, Labofafoife de Chimie Ana/yfique, 2 Av. du Pr leon-l3efnard, 35043 Flennes cedex, France, and GOLVAY-PHARMA (L.T.M), 42 Rue Rouget-de-Lisle, 92151 Suresnes-Cedex, France. Accepted for publication May 11, 1995@. Abstract 0 Waterlnoctanolpartition coefficients (log p) for 33 1,Pdithiole3-thiones and for 18 1,Pdithiol-3-ones were determined by RP-HPLC measurement of the concentration of the solute in aqueous solution after equilibrium. Depending on the nature of the substituents (alkyl or aryl) and their position(s) (4, 5, or both) on the dithiole nucleus, some peculiar behaviors were revealed. Therefore, different fragmental constants containing the 1,2-dithioIe-3-thione nucleus were inferred in order to calculate in a complementary work, a prjori, the log P values of new dithiolethiones and dithiolones
Table 1-Alkyl- and Dialkyl-l,2-dithiole-3-thiones and -1,2-dithiol-3-ones Studied
4-Alkyl
5-Alkyl
Introduction 172-dithiole-3-thiones(X = S ) are compounds of pharmaceutical interest. One of them, Anetholtrithione (R5=p-CH30-
4,5-Dialkyl
Cycloalkyl
CsH4; R4 = H ) has been marketed since 1947l and is prescribed for its cholereticz and syalagogue proper tie^.^ Another one, Oltipraz 35972 R.P. (R4 = CH3; R5 = 2-pyrazi-
R4
R5
a
H H H CH3 H C2H5 H C3H7 H C4H9 H C5Hll CH3 H C2HS H C3H7 H H CH(CH3)z CH3 CH3 C2H5 CH3 CH3 CZH5 C2H5 C2H5 CH3 C3H7 C3H7 CH3 C3H7 C2Hs -(CHz)r -( CH2)4-CH2)5-( CH2)6-
X=S
Ref
X=O
Ref
1 14a 14b 1 4 ~ 14d 14e 15a 15b 15c 15f 14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5C 149 1459 145h 145i
28 29 10 32 a a 29 10 6 34 35
2 24a 24b 24c
28 30 31 a
25a 25b 25c
33 a
24a5a
33
24b5b 24c5a
a a
2459
33
a
10
10 10 36 a 37 37 37 37 37
Original compound.
Table 2-1,2-Dithiole-3-thiones and 1,2-DithioISones with an Aryl Fragment Studied R5 Olripraz
Ancrholuithionc
n ~ l is) in ~ advanced de~elopment.~ Interest in this class of compounds has grown for about 10 years6 because of their chemoprotective properties against some cancer^.^^^ We report, here, the determination of waterln-octanol partition coefficients (log P) of some 1,2-dithiole-3-thiones(X = S ) and of some l72-dithio1-3-ones(X= 0). This work was undertaken essentially to determine which fundamental fragments, including the whole 1,2-dithiole-3-thionenucleus, must be considered to calculate accurately log P values of the most simple 1,2-dithiole-3-thiones and l72-dithio1-3-ones. In a subsequent work, we shall use these fundamental fragments to calculate a prwri the log P values of more complex derivatives according to a constructionist approach, owing to the great importance of this parameter in pharmacochemistry.9 In this first work, we focused our attention on the determination of log P values by direct measurement of the concentration of solute in water once equilibrium between the two phases was reached. It was necessary to develop this methodology since no log P data for these compounds have yet been published. We limited our investigations to dithiolethiones and dithiolones, the substituents of which R5 and R4 were only hydrocarbon groups. Owing to the fact that we were primarily concerned with log P values, we give, here, @Abstractpublished in Advance ACS Abstracts, July 1, 1995.
0 1995, American Chemical Society and American Pharmaceutical Association
R4
X=S
Ref
X=O
Ref
14j 14j5a 14j5b 14m5a 141 15j 14a5j 15k 14a5k 15m 151 145j
38 35 6 40 38 41 42 41 43 44 45 32
24j 24j5a 24j5b
30 39
25)
33
25k
33
245j
33
a
only a preliminary interpretation of the results. Moreover, this first family grounded set of values will be, of course, used later for indirect determination of log P through measures of capacity factors.
Experimental Section Nomenclature Used-To mark mono- or disubstituted dithiolethiones and dithiolones, a simplified nomenclature is used: (i) 1 = dithiolethione, 2 = dithiolone, (ii) 4 and 5 denote the 4 and 5 substituent positions, respectively, (iii) a = methyl, b = ethyl, etc. (see Tables 1 and 2), (iv) 45, followed by a letter, signifies that the substituent forms a cycle between Cq and C g (see Tables 1 and 2).
0022-3549/95/3184-1 107$09.00/0
Journal of Pharmaceutical Sciences / 1107 Vol. 84, No. 9, September 1995
Table 3-Some Characteristics of the New Dithiolethiones and Dithiolones Studied in This work" NMR 'H (CDC13, b ppmTTMS)
14d 14a5c 14e 25b 25c 24c 24b5b 24c5a 24j5b a
Physical State
Red liquid mp 49 "C Red liquid Yellow liquid Yellow liquid Yellow liquid Yellow liquid Yellow liquid mp 82 "C
0.85 (t, ZH), 1.3 (m, 4H), 2.6 (t, 3H), 8.2 (s, 1H) 1.0 (t, 3H), 1.8 (m, ZH), 2.2 (s, 3H), 2.8 (t, 2H) 0.8 (t, ZH), 1.4 (2m, 6H), 2.6 (t, 3H), 8.2 (s, 1H) 1.2 (t, 3H), 2.7 (9, 2H), 6.4 (s, 1H) 1.0 (t, 3H), 2.7 (t, 2H), 1.65 (m, 2H), 6.45 s, 1H) 0.9 (t, 3H), 1.5 (m,2H), 2.4 (t, ZH), 8.0 (s, 1H) 1.0 (t, 3H), 1.25 (t, 3H), 2.75 (q, ZH), 2.4 (q, 2H) 0.85 (t, 3H), 1.35 (m,ZH), 2.4 (s, 3H), 2.4 (t, 3H) 1.2 (t, 3H), 2.75 (9, ZH), 7.4 (m, 5H)
1,
(C=O)
(cm-I, KBr)
1636 1658 1660 1652 1652 1652
Satisfactory microanalysis.
Chemistry-Thirty-three 1,2-dithiole-3-thiones and 18 1,2-dithiol3-ones were investigated (tables 1 and 2 ) . Some of these compounds were newly synthesized for this work. Already described dithiolethiones and dithiolones (tables 1 and 2) were prepared according to the given references. The novel dithiolethiones 14d, 14e, and 14a5c (respectively: 4-butyl; 4-pentyl, and 4-methyl-5-propyl-l,2dithiole-3-thiones) were all synthesized by action of phosphorus pentasulfide with corresponding P-keto esters in dry xylene according to the method of Legrand and Lozac'h.lo The starting esters for 14d and 14e were ethyl a-formylhexanoate and ethyl a-formylheptanoate prepared by allowing ethyl formiate to react with corresponding esters according to ref 11. The starting ester used in the synthesis of 4-methyl-5-propyl-1,2-dithiole-3-thione (14a5c) was ethyl(2-methylbutyry1)acetate prepared by alkylation of commercial ethyl butyrylacetate.12 The novel dithiolones were prepared by action of benzonitrile oxide generated in situ according to ref 13. Some of the characteristics of these new compounds are given in Table 3. The purity of solutes was checked before each determination by TLC with two pairs of solvents. log P Determination-The main difficulty of this determination was the great insolubility of dithiolethiones and dithiolones in water. The water solubility of the most soluble dithiolethiones was only 5 x 10-3 mol L-' and for most of them was below 5 x 10 mol L-I. As a result, the transfer of solute from n-octanol into water could not be evidenced by appreciable variation of its concentration in the organic phase (in the few cases where the amount of solute in the aqueous phase at the equilibrium was superior to 1% of the amount remaining in the organic phase, corrections were made). Moreover, the studied solutes were not functionalized on the substituents R5 and R4 and, in particular, did not exhibit a n acidic-basic property which would have made determination easier by solubility measurements at different pH values. Therefore, we had to determine the concentration of solutes in water once the partitioning equilibrium was reached. We resorted to RP-HPLC with spectrophotometric detection. The partition coefficient P was calculated as the ratio of concentrations in octanol and in water. It was leotimate to equate concentrations to activities, as the concentrations of the solutes in water were very low and the solutes were not ionic.'* Our methodology was close to that followed by Camirelli et ~ 1 . and ' ~ Hansch et ~ 1 . ' ~ Ten milliliters of a n octanolic solution of solute were introduced with 50 mL of water into a 250 mL separatory funnel and mechanically shaken for 30 min a t ambient temperature, 20 f 1 "C. (Equilibrium was very quickly reached: we verified that shaking for more than 30 min did not significantly change the log P values.) The solutions were then left to stand for 2 h until the two layers were separated. The aqueous phase was kept and centrifuged in stoppered tubes a t 20 i 1 "C a t 6000 rpm for 30 min, and the residual octanol on the water layer was aspirated. The water layer was directly analyzed by RP-HPLC through a 100 pL sample loop. The sample concentration was determined by comparison to a calibration curve constructed with five points chosen in the same range of concentrations as the measured one (from 0.5 to 1.5 times the sample concentration). Owing to the low water solubility of solutes, the solutions used to build up the calibration curve were prepared by dissolving a known amount of solute in methanol; this solution was firstly diluted with methanouwater (50/50 v/v). Then the latter solution was diluted with pure water. Each calibration solution was chromatographed three times. For each compound, four replicates were performed according to the above procedure and the four
1108 / Journal of Pharmaceutical Sciences Vol. 84, No. 9, September 1995
resultant aqueous phases were analyzed three times. So the aqueous concentration used for computation was the mean value of the 12 data points obtained. From the standard deviation of the 12 experimental results and from the calibration curve, the uncertainty of the aqueous concentration of solute was estimated, and the resulting precision of the log P determination was estimated to be f 0 . 1 log P unit. For example, if we consider 5-phenyl-1,2-dithiole-3-thione (15j),its water solubility was determined as 3 x mol L-I; in the shake-flask experiment the 15j concentration in the octanolic phase was 5 x mol L-l and after partition the 15j concentration found in the aqueous mol L-l. phase was 1.07 x To validate our methodology, we applied it to some compounds for which the log P was already known: p-nitroacetophenone (found (fd), 1.49; literature (lit.), 1.5317),p-nitroanisole (fd, 2.02; lit., 2.0016 and 2.0318),p-nitrotoluene (fd, 2.43; lit., 2.429 and 2.3716), anisole (fd, 2.15; lit., 2.10: 2.1l,lfi 2.0819), thioanisole (fd, 2.81; lit., 2.749), nitronaphthalene (fd, 3.21.; lit., 3.1g9). Moreover, the study of some of the least lipophilic derivatives mentioned above was carried out by direct analysis of the aqueous phase by direct and pulse polarography on the one hand and by UV-visible spectrophotometry on the other hand. The values obtained by polarography (1.58 and 2.03 for p-nitroacetophenone and p-nitroanisole) and those obtained by spectrophotometry (1.49, 2.02, and 2.44 for p-nitroacetophenone, p nitroanisole, andp-nitrotoluene, respectively) were in good agreement with those obtained by RP-HPLC and with those reported in the literature. The same comparison of the log P values obtained by polarography, spectrophotometry, and RP-HPLC was carried out for the least lipophilic dithiolethiones derivatives 1, 15a, and 2 (Tables 4 and 7). It is worth noting that we were not able to determine the log P values of all the dithiolethiones we would have liked to study because their solubility in water was too low mol L-l), and therefore our methodology could not be used. Materials-The HPLC chromatograph was a n LDC-Milton Roy system (constametric 111) isocratic pump with a (spectromonitor 11) UV-visible detector. Solutes were detected a t their analytical wavelength (ranging from 400 t o 430 nm for the dithiolethiones and 280 to 330 nm for the dithiolones). The RP-HPLC column was a stainless steel tubing (4.5 mm in diameter and 15 cm long) filled with 5 pm ODs2 stationary phase (Hichrom). The mobile phase was water/methanol mixtures (20/80or 30/70 v/v). The flow rate was 1 mL min-' a t 20 "C. Water was doubly deionized on an ion-exchange resin. Water and n-octanol were mutually saturated before use. The UV-visible spectrophotometer was a double beam Uvikon 930 (Kontron) with baseline correction. Differential pulse polarography was performed with a Tacussel EPL 3 Ti-Puls polarograph. All the solutions used for calibration and supporting electrolyte (m030.1 mol L-') were prepared with water saturated by n-octanol (n-octanol influenced the peak shape).
Results and Discussion Experimental measurements of log P allowed us to calculate simultaneously the values of the dithiole nucleus fragments and those of CH3, CHe, and H, flCH3), flCHz), and AH), respectively. If ACH3), flCHz), AH) values found were in agreement with those reported in the literature, we considered that the values obtained for the dithiole nucleus fragments were accurate. Alkyl- and Dialkyl-,2-dithiole-3-thiones-The log P valare ues we found for alkyl and dialkyl-1,2-dithiole-3-thiones mentioned in Table IV. Consideringthe very small differences in the log P values found for dithiolethione parent (1) and for 5-methyl-l,Z-dithiole-3-thione (15a),independently obtained from HPLC, polarography, and UV-visible spectrophotometry experiments (Table 4),their respectively log P values 1.58 f 0.02 and 1.86 f 0.02 cannot be questioned. The fact that the log P of the dithiolethione parent (1) was so firmly grounded was a very important point (see below). In order to detect some possible inaccuracies in the experimental log P values of some other dithiolethiones or, alternatively, to detect their unexpected behavior, we decided to calculate log P starting from the well-established value for 1. We assumed, therefore,
Table 4-Experimental and Calculated log PI Values (See the text) of Alkyl-, Dialkyl-, and Aryldithiolethiones [log PI = 40) 1(R4) 4R5)]
+
Fragment used
Dithiolethionesc
log P(expl) 1.58 1.59a 1.566 1.85 1.85a 1.8gb 2.31 2.83 2.18 2.67 3.18 4.06 2.45 2.94 2.95 3.51 3.39 3.42 3.76 2.53 3.10 3.27 3.75 3.20 3.17 3.52 3.78 3.73 3.67 3.95 3.82 4.10 3.43 4.26 3.57
1
15a
0
f(0) = 1.22
~~
15b 15c 14a 14b 14c 14e 14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5c 145f 1459 145h 145i 14j 14j5a 14j5b 14m5a 141 15j 14a5j 15k 14a5k 15m 151 145j
Table 5-Experimental and Calculated log P2 Values of Alkyl- and Dialkyldithiolethionesa
+
log PI
-
Fragments Used
A
Dithiolethiones
-
1 2.09
-0.16
2.60 3.13 2.09 2.60 3.13 4.17 2.60 3.12 3.12 3.65 3.65 3.65 4.16 2.78 3.30 3.82 4.34 3.23 3.75 4.27 4.27 3.74 3.23 3.75 3.32 3.84 3.46 3.74 2.62
-0.29 -0.30 +0.09 +0.07 +0.05 -0.1 1 -0.15 -0.18 -0.17 -0.14 -0.26 -0.23 -0.40 -0.25 -0.20 -0.55 -0.59 -0.03 -0.58 -0.75 -0.49 -0.01 +0.44 +0.20 +0.50 +0.26 -0.03 +0.52 +0.95
~
UV-visible spectrophotornetry. Polarography. See tables 1 and 2. A = log P(expl) - log PI.
15a f(1) = 1.67
r2
f(II) = 2.00
f(ll1) = 2.69
15b 1% 14a 14b 14c 14d 14ed 14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5cd 145f 1459 145hd 145id
log P2
log P(expl) 1.58 1.596 1.56c 1.85 1.85b 1.8gC 2.31 2.83 2.18 2.67 3.18 3.70 4.06 2.45 2.94 2.95 3.51 3.39 3.42 3.76 2.53 3.10 3.27 3.75
Aa
-
-
-
-
2.36 2.88 -
2.69 3.21 3.73 4.25 -
2.96 2.96 3.47 3.48 3.48 3.99 2.61 3.13 3.65 4.17
-0.05 -0.05 -0.02 -0.03 -0.03 -0.19 -0.02 -0.01 4.04 -0.09 -0.06 -0.23 -0.08 -0.03 -0.38 -0.42
a Calculations from log P values of 5-methyl; 4-methyl- and 4,5-dimethyldithiolethiones (see the text). UV-visible spectrophotornetry. Polarography. d A = log P(expl) - log P2.
and 15a, which should have the same log P I , exhibited important discrepancies. It was clear that the same alkyl fragment substituted in position 4 or 5 did not give the same log P. Therefore, a new calculation of log P taking into account the three basic fragments I, 11, and 111was performed.
a
that it was possible to attribute to the correspondingfragment 0 the fragmental constant value fC0) = log P(exp1 dithiolethione 1) - 2flH).
qs 0
According to this hypothesis, the log P of any dithiolethione could be calculated by relation 1 (see Table 4):
+
logP, = f l0) + ftR5) AR4)
fcI), AID, AIII) were calculated after log P(exp1) of the derivatives 14a, 15a,and 14a5a by substraction of AH), AH), and 2f(H), respectively, that is to say: fcI) = 1.85 - 0.18 = 1.67, AII) = 2.18 - 0.18 = 2.00, AIII) = 2.45 - (2 x 0.18) = 2.09. As a result, log P values were calculated differenciating For example, for 5-alkyl-, 4-alkyl-, 4,5-dialkyldithiolethiones.
(1)
The fragmental constants RR5) and AR4) that we used were those of Rekker et aLZ0[f(CH3)= 0.69, flCH2) = 0.52, AH) = 0.181 because, for our purpose, this “reductionist” statistical approach was sufficient, since only short and unbranched ~ a result, f l 0 ) = 1.58 - 2 x alkyl groups were p r e ~ e n t .As 0.18 = 1.22. A study of the log P(exp1) and log PI values revealed that practially all dithiolethiones were deviant (see Table 4). Simultaneous resolution by a nonlinear least squares process (in a matricial way) of relation 1applied to all the alkyldithiolethiones in Table 4 gave f ( 0 ) = 0.98 (a = 0.15),ACHs) = 0.73 (a = 0.071, flCH2) = 0.48 (a= 0.02), AH) = 0.33 (a= 0.06) (a is the square root of the variance of each fragment obtained from the varianceskovariances matrix). The flH) value was quite unsatisfactory. Moreover, methyldithiolethiones 14a
/,y
Cn,
I
R4’
+
5-alkyldithiolethiones,log Pa = AI) f(R5‘), and so on for other alkyldithiolethiones. The values obtained in this way are given in Table 5. However, some deviant dithiolethiones still remained, as shown by the values of A (last column). This was confirmed by the corresponding matricial calculation which gave fcI) = 1.69 (a = 0.06), PII) = 2.06 (0= 0.06), AIII) = 2.14 (0= 0.08), fcH) = 0.18 (a = 0.051, flCH2) = 0.43 (a = 0.02),flCH3) = 0.68 (a = 0.03). If the literature value flCH2) = 0.52 is not questionable, our value ACHz) = 0.43 is unsatisfactory. Journal of Pharmaceutical Sciences / 1109 Vol. 84, No. 9, September 1995
Table 6-Experimental and Calculated log P3 Values of Alkyl- and Dialkyldithiolethionesa Fragments Used
Dithiolethiones
log Ffexpl)
15b 15c 14a 14b 14c 14d 14ed
1.58 1.5gb 1.56c 1.85 i,856 i,890 2.31 2.83 2.18 2.67 3.1 8 3.70 4.06
14a5a 14b5a 14a5b 14b5b 14c5a 14a5c 14b5cd 145f 1459 145hd 145id
2.45 2.94 2.95 3.51 3.39 3.42 3.76 2.53 3.10 3.27 3.75
1
15a f(1) = 1.64
f(I1) = 1.99
f(111) = 2.09
log f
3
-
Ae -
1.82
+0.03
2.34 2.86 2.16 2.67 3.1 9 3.71 4.23
-0.03 -0.03 4.02
2.45 2.97 2.97 3.49 3.48 3.48 3.99 2.61 3.13 3.65 4.17
0.00 -0.03 -0.02 4.02 -0.09 -0.06 -0.23 -0.08 -0.03 -0.38 -0.42
+o.oo -0.01 -0.01 -0.17
dCalculations from ql), @I), and qlll) values obtained in a matricial way (see the text). UV-visible spectrophotometry. Polarography. Dithiolethiones discarded (see the text). e A = log qexpl) - log 4.
Since flCHz) was still unsatisfactory, the same matricial strategy was finally adopted, but dithiolethiones 14e,14b5c, 145h,and 145i were discarded because the difference (A) (see Table 5)between their log PZand log P(exp1)values was more than twice the experimental uncertainty. From the matricial calculation performed with remaining dithiolethiones, we adopted the definitive values for the searched for fragmental constants as fl1)= 1.64 (a = 0.03),f(I1) = 1.99 (a = 0.03),flII1) = 2.09 (a = 0.041, and those for other fragments a s AH) = 0.18 (0 = 0.02), flCHz) = 0.50 (a= 0.01), flCH3) = 0.69 (a = 0.02). The log P values of dithiolethiones calculated with these last fragmental values AI), flII), and f(II1) and noted P3 are listed in Table VI. The four discarded dithiolethiones remain deviant, but all other calculated values are in good agreement with experimental log P (A < 0, 1). The flCH3), flCHz),and f ( H )values obtained simultaneously with flI), flII), and flIII) (after dithiolethiones 14e, 14b5c,145h,and 145i were discarded) compared nicely with those found in the literature. This is a good argument in favor of the accuracy of all our log P(exp1) values. As a result, taking into account the fragmental constants flI),f(II), and flIII) provides a good means to calculate the log P values of alkyldithiolethiones. We performed another analysis of the log P values of alkyldithiolethiones. Remembering that the fragment 5-(1,2dithiole-3-thione)-ylis an electron-withdrawing group (ap-= 1.14),21,2z we singled out the hydrogen fragment in position 5, the value of which would be f(H*) = 0.46 according to Rekker.zo This allowed us to introduce the fragments values ffO*) and f(II*).
qsqs '0
tTz I!'
Satisfactory results were obtained only when we took into 1110 / Journal of Pharmaceutical Sciences Vol. 84, No. 9, September 1995
account simultaneously these two fragments and when we discarded the same four dithiolethiones 14e,14b5c,145h,and 145i as we did previously. The values obtained were AH) = 0.18 (a = 0.02), AH*) = 0.43 ( U = 0.02),flCH2) = 0.50 (a= 0.01), flCH3) == 0.69 (a= 0.02),f l O * ) = 0.95 (a = 0.04), flII*) = 1.56 (a = 0.04). Therefore, again, dthiolethiones 14e, 14b5c,145h,and 145 exhibit abnormal behavior. According to log P(exp1) data, it is quite clear that 4-alkyl1,2-dithiole-3-thiones are more lipophilic than 5-alkyldithiolethiones [log P(4-alkyldithiolethiones)- log P(5-alkyldithiolethiones) sz 0.351. Moreover, a comparison of log P (expl) and log P I (Table 4) shows that 4-alkyl-l,2-dithiole-3-thiones are more lipophilic and 5-alkyl-l,2-dithiole-3-thiones are less lipophilic than expected. The reinforced lipophilic property of 4-alkyl-1,2-dithiole-3-thiones can be explained by the proximity of the substituent in position 4 and of the thiocarbony1 group with generation of steric hindrance.z3 Since dithiolethiones are endowed with significant dipolar mom e n t ~it, is ~ ~reasonable to rationalize the lowered lipophilic property of 5-methyl-l,Z-dithiole-3-thione in terms of some polar effect. 4,5-Dialkyl-1,2-dithiole-3-thiones exhibit simultaneously the behaviors of 5-alkyl- and 4-alkyl-1,Z-dithiole3-thiones. Otherwise, another trend is perceptible: the longer the chain substituted in the 4 position is, the more log P deviates from the expected value. This is not totally surprising. Such a phenomenon has been taken into account by Leo et al. in their system of fragmental constants to calculate the log P values of chain^.^ Unfortunately, we could not further verify this trend because 4-hexyl-l,Z-dithiole-3-thione was, a priori, too poorly soluble in water to be studied. It is likely that the same phenomenon could probably be evidenced for 5-alkyl-1,2-dithiole-3-thiones if they were easily synthesized. Another finding is the lower than expected lipophilicity of the cyclic dithiolethiones 145h and 145i (see A, Table 6). This phenomenon seems to increase with the length of the cycle. It has been noticed and taken into account by Leo et aL9 The dithiolethione derived from cyclohexane 145g can be considered as normal, and that derived from cyclopentane 145f as quasinormal. To conclude as far a s alkyl-1,2-dithiolethiones are concerned, it is interesting t o note that the branched 5-isopropyl-1,2-dithiole-3-thione 15f [log P(exp1)= 2.751 seems t o be less lipophilic than 5-propyl-l,Z-dithiole-3-thione 15c [log P(exp1) = 2.831. This is in agreement with literature data.g Aryl-1,2-dithiole-3-thiones-In Table 6 are mentioned the log P values of dithiolethiones with an aromatic fragment included in their R4 or R5 substituents. A comparison of log P(exp1) and log PIvalues (obtained with f(0) = 1.22, flCsH5) = 1.83,20f(p-CHsOCsH4)= 1.92: and fcpCH3CsH4) = 2.3420)shows the failure of the additivity of fragmental constants without supplementary corrections for 5-aryl-l,2-dithiole-3-thiones. The failure is particularly striking for 5-phenyl; 5-(ptolyl)- and 5-@-methoxyphenyl)-l,2dithiole-3-thiones 15j (A = 0.44), 151 (A = 0.50),and 15k (A = 0.52),which are more lipophilic than expected (see log P I , Table 4). This finding must undoubtedly be related to the conjugation of the 5-aryl substituent with the dithiole nucleus, which results in the coplanarity of the whole molecule.25 Such a conjugation is encountered with biphenyl, for which experimental log P values 3.95,4.09, and 4.179 are higher than the log P calculated from the simple addition of two flC6H5): log P(ca1c) = 3.66 (A = 0.44 f 0.12). A corrective factor has been proposed to take this effect into account.z0 This conjugation effect is less marked in 5-phenyl-4-methyl-l,2-dithiole-3-thione (14a5j)and in 5-@-methoxyphenyl)-4-methyl-1,2-dithiole-3thione (14a5k). Their lipophilicity increase is about half that of the 4-demethylated dithiolethiones 15j and 15k. This effect is likely to be significant, because it is grounded on the values of two 5-aryl-4-methyl-dithiolethiones and three 5-aryldithiolethiones and because the difference A = 0.20is most certainly
+
the very upper limit of the log P uncertainty. This is quite understandable as the 4-methyl substituent lowers the conjugation of the 5-aryl group by steric hindrance. Our semiempirical calculations indicate that the aryl and the dithiole nucleus are twisted at an angle of 120°,whereas in 5-aryldithiolethiones, the two aromatic fragments are coplnar. A comparison of the log P(exp1)and log PI values of 4-phenyl (14and 141)shows that and 4-(p-tolyl)-l,2-dithiole-3-thiones they exhibit a normal behavior. This can be related to the hypothesis formulated by some authors according to whom the aryl fragment is not conjugated with the dithiole nucleus as simple LCAO-MO calculain 4-aryl-1,2-dithiole-3-thiones, tions suggest.26 Our own findings by molecular modelingz7 indicate that the aromatic substituent is twisted at an angle of 111" t o the dithiole nucleus. With such a geometry, solvation of the thiocarbonyl group by water is not hindered. Very puzzling are the behaviors of 5-methyl- and 5-ethyl4-phenyl-1,2-dithiole-3-thiones (14j5a)(logP(exp1)= 3.17) and (14j5b)(log P(exp1) = 3.52) which are less lipophilic than expected (A = -0.30 or -0.55) as the following calculations for 14j5a indicate: fcC6H5) fcI) = 3.47(A = -0.30) or fcCH2) 3.20 = 3.72 (A = -0.55) (3.20 is the logP(exp1)of 14).14j5b exhibits the same behavior: f(C6H.5) fcI) f(CH2)= 3.99 (A = -0.47) or 2fcCHz) 3.20 = 4.24 (A = 0.72). Introduction of fragment values AIv) and fcV) should take into account these peculiar behaviors.
+
+
+
+
+
Table 7-Experimental and Calculated log P, Values (see the Text) of the Dithiolones Studied
log ffexpl) log A 2 24a 24b 24c 25a 25b 25c 24a5a 24b5b 24c5a 2459
0.82 0.82a 0.84b 1.33 1.90 2.38 1.26 1.69 2.24 1.73 2.78 2.96 2.40
1.33 1.85 2.38 1.33 1.85 2.38 1.84 2.88 2.88 2.54
log Qexpl) log 9
Ac
A
0.00 +0.05 0.00 -0.07 -0.16 -0.14 -0.11 -0.10 +0.08 -0.10
24j 24j5a 24j5b 25j
2.64 2.93 3.15 3.01
2.47 2.99 3.51 2.47
+0.17 -0.05 -0.36 +0.54
25k
3.19
2.56
+0.63
24j5j
4.07
4.12
-0.05
2451
2.73
1.94
+0.79
a UV-visible spectrophotometry. Polarography. A = difference between log flexpl) and log 4.
than the corresponding dithiolethione 14j5b. 4-phenyl-1,2dithiol-3-one(24)is somewhat more lipophilic than expected. 4,5-diphenyl-l,2-dithiol-3-one (24j5j)(its corresponding dithiolethione 14j5j was too lipophilic to be studied) seems to give a normal value (log P(exp1) = 4.07 for log PI = 4.12).
Conclusion
Even, 4-benzyl-5-methyl-l,2-dithiole-3-thione (14m5a)exhibits the same behavior. So far, we have no satisfactory explanation for these phenomena. On the other hand the log (15m)is nearly P(exp1f(3.43) of 5-benzyl-l,Z-dithiole-3-thione identical t o the sum fcCsH5) fcI) = 3.47 (expected log P). Unfortunately, we were not able to study other 4- or 5-aryl1,2-dithiole-3-thiones because of their lack of solubiiity in water. Alkyl- and Aryl-1,2-dithiole-3-ones-logP values of dithiolones are mentioned in Table 7. As above, we performed calculations of log PI of dithiolones by introducing the fragment 0'
+
qo 0'
+
+
log PI =fro') f(R4) AR5) [f(0') = log P(exp1)dithiolone2) ZfcH)]. The dithiolones are considerably less lipophilic than the corresponding dithiolethiones (A = 0.60 f 0.16). At first glance, the log P(exp1) - log P I differences of the former seem to be less marked than those of the corresponding dithiolethiones. This is particularly noticeable for 4-methyl- and 5-methyl-l,2-dithiole-3-ones (24aand 25a),which exhibited nearly the same value. We were able to distinguish three types of dithiolethiolones by considering the differences of log P(exp1) between dithiolethiones and correspondingdithiolones. They are 5-alkyldithiolones: A log P(exp1) x 0.60; 4-alkyl1,2-dithiole-3-ones: A log P(exp1) x 0.81; 4,5-dialkyl-1,2dithiol-3-ones: A logP(exp1)x 0.73. As far as 5-aryl- or 4-aryl1,2-dithiolonesare concerned, the interesting findings are that 5-methyl-4-phenyl-l,2-dithiol-3-one (24j5a)is not deviant a t all (contrary to the corresponding dithiolethione 14j5a) and that 5-ethyl-4-phenyl-1,2-dithiol-3-one(24j5b)is less deviant
To sum up, it is possible to calculate, a priori, the log P values of most alkyl- and aryldithiolethiones. It is necessary, however, to take into account the fragmental constant fcI) for 5-alkyl-1,2-dithiole-3-thiones, AII) for 4-alkyl-1,2-dithiole-3Using thiones, and f(II1) for 4,5-alkyl-1,2-dithiole-3-thiones. and the same strategy for 4- or 5-aryl-l,2-dithiole-3-thiones the fragment constants fcIv)and fcV),respectively, would also probably give reasonable estimations of their log P values Unfortunately, we were not able t o verify this method of calculation since higher homologues of dithiolethiones were too poorly soluble in aqueous phase to be studied. We are currently working on the determination of functionalized dithiolethiones log P values.
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Acknowledgments Authors thank G. Bouer for his technical assistance. JS940701S