Theoretical Studies of Some New Anti-Malarial Drugs

COLIN THOMSON NFCR Project, Department of chemistry, University of St. Andrews, St. Andrews, KY16 9ST, Scotland

MARSHALL CORY AND MICHAEL ZERNER Quantum Theory Project, University of Florida, Gainesville, Florida, USA

Abstract

Qinghaosu (Artemisinin), is a sequiterpene lactone containing a peroxide group, which has been used from ancient times as an anti-malarial agent in Chinese medicine [ I ] . Unlike chloroquine and related compounds used as anti-malarial agents, resistant strains of malaria have not developed so far to qinghaosu. In this article; we present the results of a theoretical study of the structure of this and related molecules, using both semi-empirical and ab-initio quantum chemical methods, and an investigation of the known structure/activity (S/A) relationships of Qlnghaosu and related molecules as anti-malarials using Molecular Electrostatic Potential (MEP) maps. The unusual 1,2,4-trioxane ring system, and other ring systems con- taining the peroxide linkage occumng in qinghaosu have also been studied in some detail. We speculate that the reactivity of this system in this context is related to a rather dramatic region of negative potential that surrounds the molecule and that includes the -0-0- linkage as well as the ring oxygen atom.

Introduction

The search for new anti-malarial drugs is of very great importance in view of the large increase in incidence of the disease in the Third World during the last few years [ 1,2]. This is partly a consequence of the development of resistance to the current anti-malarial drugs, such as chloroquine (Fig. 1).

A very interesting new drug which may provide a lead compound for better drugs is the compound Qinghaosu (also known as Artemisinin, Fig. l), which has been used in Chinese medicine for centuries [ 1,3]. The structure of this compound has been determined by X-ray methods [4], and the chemistry of it reviewed [ 11. It is rather unique among natural products in that it contains the very unusual 1,2,4- trioxane ring system (Fig. 1). This system has only recently received concentrated attention [5], but so far only a variety of derivatives have been synthesized [5-141.

There is little information about this ring system, and we believe that theoretical studies might be able to shed light on the structure and mechanism of action of the drug and help in the prediction of new and more potent anti-malarials. We have therefore investigated in some detail the structure of the basic 1,2,4-trioxane ring system, and that of a large variety of other compounds containing this ring.

International Journal of Quantum Chemistry: Quantum Biology Symposium 18, 23 1-245 (1991) 0 1991 John Wiley & Sons, Inc. CCC 0360-8832/9 1/01023 1-1 5$04.00

232 THOMSON, CORY, AND ZERNER

Chloroquine

Qinghaosu

Figure 1 . Formulae of Chloroquine, Qinghaosu, and BC.

We report in this article a preliminary account of these theoretical investigations. We have used a variety of theoretical methods in order to compute the structure and properties of these molecules, ranging from Molecular Mechanics to ab-initio methods. In so doing, we are not only examining the properties of these systems, but also the reliabilities of the methodologies used for such investigations.

The results of this work are conveniently divided into the studies of the basic ring system, and studies of the anti-malarials themselves.

Methods and Computing Requirements

In order to obtain theoretical information on the three-dimensional structures and the electronic properties of the molecules of the size mentioned above (con- taining up to 50 atoms), we need to be able to calculate the three-dimensional shape of these molecules reliably. This is now feasible, because we know from extensive work on small molecules that ab-initio computations at the SCF level of theory can give such structures, providing flexible basis sets are employed [ 151.

However, geometry optimizations at this level of theory are quite time consuming for large molecules, since the number of internal coordinates rapidly becomes very large, and the number of soft modes and minima increases rather dramatically with increasing size. Nevertheless, the energy gradient methods which are used in ob- taining structure have been refined considerably during the last few years, and such calculations are certainly feasible.

ANTI-MALARIAL DRUGS

H

233

0-

3 1 CH3 1,2.4-Trioxane

Figure 2. Numbering of 1,2,4-trioxane and structure of Compound 31.

In addition, the improvements in semi-empirical SCF methods, such as MNDO [ 161, and more recently AMI [17] and PM3 [ 18,191, has been of great importance, since these methods have been shown in several recent studies to yield reliable molecular geometries for quite large systems [20,2 11. Bond lengths, interbond-angles, and torsion angles can be computed which are in excellent agreement with exper- iment, where these values are known. This observation is extremely useful when studying the properties of large molecules, because it means that the expense of geometry optimization at the ab-initio level of theory can often be avoided, but a reasonably accurate ab-initio wave function computed at the AM^ or PM3 geometry can be used to compute molecular properties for the many large molecules of interest in biology.

We have optimized the geometry of the molecules shown in Figures 1-4, and at the optimized geometry computed a variety of molecular properties, especially the Molecular Electrostatic Potential (MEP), which has been shown to be a very useful quantity for comparing the behavior of molecules in biology [22,23]. The underlying assumption here is that the MEP is related to the recognition of the drug for its receptor. This quantity can be computed rigorously, but we have found that there

6 8

Figure 3. Structure of two ring compounds 6 and 8.

234 THOMSON, CORY, AND ZERNER 9 H 3 C T C H 3

0

ABCD 0

1 1-Deoxyqinghaosu

OR BCD

R = H 2-Hydroxy-2-deoxoqinghaosu

R = Et Arte-ether Figure 4. Formulae of BCD, ABCD, 1 1-Deoxyqinghaosu, and the more effective deriv-

atives of Qinghaosu.

is good agreement between the ab-initio MEP and those computed using the simple charge approximation, with the charges computed by the AM^ method [24]. The MEP maps in the present article have been computed in a variety of ways, but the most recent method we have used involves a multipole expansion method as im- plemented in the CAChe modelling system [25], which gives MEP in excellent agree- ment with more rigorous calculations at a fraction of the cost. The MEP are usually displayed in color on a graphics device either using our own programs [24,26], or that implemented on the CAChe system [25] monitor in 3-D.

Our strategy has been, first, to optimize the geometry of the molecule using the MM2 Molecular Mechanics Force field [26], followed by optimization with the AMI or PM3 methods [ 17-19]. This was followed by the calculation of the molecular properties using MOPAC [28], such as the electrostatic potential, as well as the vi- brational spectra. We have also investigated some of these molecules using ab-initio SCF calculations, and limited geometry optimization. These calculations are referred to in the text below. Some calculations have also been carried out using the ZINDO

ANTI-MALARIAL DRUGS 235

program to study the electronic spectra [29], and to compare the MEP computed with different methods.

The calculations were initiated at QTP, and carried out on the CAChe system or on the IBM RISC/6000 work station there, and subsequent calculations were carried out on the MICROVAX-2/GPX workstation, and the two processor FPS- 500 EA minisupercomputer at St. Andrews, together with the CAChe system at St. Andrews.

Results

The structures of the molecules studied in this work are all given in Fig- ures 1-4.

A. 1,2,4-Trioxane and Derivatives

A.1. 1,2,4-Trioxane. This molecule has not yet been synthesized, and it is of great interest to investigate the conformational properties of this molecule and related systems.

Geometry optimization with the AM 1 Hamiltonian gives the same qualitative results as MM2 optimizations: namely that the Chair form is the most stable form, to the extent of about -3 kcal/mol compared to the alternative boat/twist confor- mation. This is confirmed by the ab-initio SCF calculations, using both a 3-21 G and a 6-3 1 G* basis set, and in these cases the energy difference is 4.1 and 4.8 kcal/ mol, respectively. The calculated values of the geometrical parameters are given in Table I.

Of particular interest are the values of R(0-0), which is computed to be 1.29 A using the AM^ method, but considerably longer, 1.58 A, when the PM3 Hamil- tonian is used. Both of the ab-initio calculations give intermediate values, namely 1.47 A and 1.40 A for the 3-2 1G and 6-3 lG* basis sets, respectively.

As we shall see below, the experimental values in several compounds have been measured, and the values vary from 1.45 A to 1.48 A [30,3 1,4]. It should be noted that SCF calculations have difficulty giving the correct structure for H202 [31], producing too short a bond length. On the basis of these results, it seems as if the AM^ method gives a much poorer value of the 0-0 bond length than the PM3 method.

We have also computed the vibrational spectrum of 1,2,4-trioxane, using the CAChe system, using the AMI model implemented in the CAChe system [25]. This is of interest in view of the measurements of the spectra of a variety of derivatives containing this ring system [32].

However, because the different methods give different values of the 0-0 bond length, the calculated vibrational frequencies for the -0-0- vibration are not very reliable, especially as the calculated values for dimethyl peroxide, which are known [28], vary widely. It does seem likely, however, that there is a band at a calculated frequency of between 700 and 800 cm-' which is due to a combination

236 THOMSON, CORY, AND ZERNER

TABLE I. Ring geometrical parameters of 1,2,4-trioxane.

Chair form Boast/twist form

Parameter PM3 AM1 3-21G 6-31G' PM3 AM1 3-21G 6-31G*

0 1 0 2 02C3 c 3 0 4 04C5 C5C6

0102C3 02C304 C304C5 04C5C6

0 102C304 02C304C5 C304C5C6

1.577 1.294 1.381 1.436 1.411 1.412 1.416 1.427 1.392 1.488

107.3 111.7 107.6 105.8 113.4 113.0 107.3 111.7

-67.8 -66.7 63.8 59.2

-51.5 -52.4

1.466 1.438 1.415 1.466 1.524

104.9 110.6 112.8 108.9

-66.6 57.3

-51.1

1.396 1.39 I 1.382 1.405 1.518

107.3 111.1 112.1 109.3

-66.0 57.6

-50.3

1.569 1.290 1.382 1.438 1.414 1.411 1.41 I 1.425 1.398 1.450

107.6 112.6 112.8 109.2 116.2 114.7 106.2 111.4

-44.4 -34.7 -16.1 -30.9

5 1 . 1 58.5

1.466 1.438 1.422 1.411 1.538

103.7 109.0 112.1 108.4

-46.8 -24.2

65.5

1.391 1.393 1.388 1.400 1.533

106.1 110.8 112.8 108.7

-48.2 -2 1.2

59.7 ~~ ~

AHr -65.97 -67.05 -62.20 -64.5 I Energy' -339.6769 -341.5593 - -339.6730 -341.5607 Dipole momentb I .SO 1.97 2.48 2.12 1.54 1.93 2.21 I .95

a I Hartree =H=627.5 kcal/mol. Values in Debyes (D).

of 0-0 stretching and bending modes. Ab-initio calculations of the frequencies using the 6-3 lG* basis set results are in progress.

A.2 Tetramethylhydroxymethyl-1,2,4-trioxane (Compound 31). This derivative (Fig. 2) is of particular interest, as it is one of the simplest of the derivatives for which there is an X-ray crystal structure [ 5 ] . Once again the optimized structures are in the chair form, and in excellent agreement with the X-ray structure. A com- parison of the calculated ring parameters at the AM^ and PM3, 3-21G and 6-3 1G levels with the X-ray is given in Table 11. Apart from parameters involving the peroxide linkage, there are only small differences between the results with diffirent methods.

B. Compounds Containing Two Rings

There are more examples of this type of compound, and several of these have been described by Jefford's group [ 5 ] . In Ref. 5, the structures of several of these are described, and the computed structures we obtain are in every case in excellent agreement with the X-ray data (details of these computational results are not given here), but the structures of these are illustrated in Figure 3.

It is an intriguing possibility that the activity of Qinghaosu is determined by the two-ring system, labelled BC (Fig. l), and for this reason we have made a particularly detailed study of this (as yet unknown) compound.

ANTI-MALARIAL DRUGS 237

TABLE 11. Ring geometrical parameters of compound3 1.

Chair form

Parameter AM 1 PM3 3-21G 6-31G Experimenta

0 1 0 2 02C3 C304 04C5 C5C6

0102C3 02C304 C304C5 04C5C6

0102C304 02C304C5 C304C5C6

1.287 1.449 1.417 1.437 1.553

111.8 104.7 114.3 108.1

-65.2 60.1

-56.0

1.545 1.404 1.418 1.434 1.585

107.3 105.6 114.9 108.6

-67.8 66.0

-55.3

1.465 1.446 1.413 1.451 1.543

106.0 109.2 116.0 106.5

-61.7 58.2

-55.4

1.449 1.437 1.410 1.449 1.557

107.1 108.6 118.4 106.3

-59.9 56.8

-53.4

1.459 1.464 1.419 1.440 1.562

107.9 108.6 114.5 108.9

-66.7 59.8

-52.0

AHf - 122.4729 -120.8990 Energy -608.23 -61 1.33 Dipole moment 2.41 2.46 3.00 3.1 1

a Ref. 5.

Table I11 list the results of a series of studies on the BC ring conformation found in Qinghaosu, although we have also investigated other conformers. In the case of BC, we have carried out a more extensive series of calculations with several different basis sets, at the ab-initio level, which will be reported at a later date. The present results, which are shown in Table 111, are for the 3-21G basis set and involved a partial optimization of the trioxane ring in BC.

In addition, we have investigated in some detail those molecules described in Ref. 32, in which Jefford's group attempted, using isotopic substitution, to assign the vibrational spectra to specific modes.

Our own studies of these molecules, whose structures are given in Figures 2-4, are described in detail elsewhere: at this stage we report the vibrational spectra of compounds 6 and 8 (Fig. 3) which are shown in Figure 5.

It can be seen that the -0-0- stretching vibration is predicted to occur at a wavelength of around 800 cm-'. The general appearance of the computed spectra are in good agreement with the experimental data.

C. Three Ring Compounds

The rings in Qinghaosu are designated A, B, C, and D [4], and we have called the relevant three ring compounds, BCD, ABC, and ACD, one of which is shown in Figure 4. They have not yet, so far as we know, been synthesized, but they all

238 THOMSON, CORY, AND ZERNER

TABLE 111. Ring geometrical parameters for two ring compound BC.

Chair form

Parameter AM I PM3 3-2 IG

0 1 0 2 1.294 I .568 1.460 02C3 1.447 I .403 1.443 C304 1.424 1.427 1.428 04C5 1.428 1.415 1.449 C506 1.523 1.537 1.526

0102C3 1 14.0 112.5 109.9 02C304 104.0 107.2 106.4 C304C5 114.6 115.8 115.0 04C5C6 112.5 113.6 113.2

01 02C304 -75.0 -63.0 -75.1 02C304C5 41.4 52.4 43.1 C304C5C6 14.0 3.0 15.1

AHf (kcal/mol) 70.490 -70.937 Energy (HI -493.7960 Dipole moment 1.85 2.35 2.89

-

contain the peroxide link which seems to be necessary for activity. Table IV lists the computed heats of formation of these compounds.

1. Qinghaosu and Its Derivatives (Figure 1)

Qinghaosu. Qinghaosu is the only known example of a naturally occumng com- pound containing the 1,2,4-trioxane ring system, and is a powerful anti-malarial having significant activity against strains of the disease which are resistant to chlo- roquine. The activity seems to reside in the -0--0-- linkage, since replacement of it by -0- in the derivative deoxyartemisinin destroys the activity completely.

On the other hand, replacement of the carbonyl group in ring D with the -CH-OEt group gives a derivative which is more water soluble and more effective.

The crystal structure of the compound was determined several years ago [4], and we have computed the structure of the molecule at the .4M 1, PM3, and ZINDO levels. The agreement with X-ray structure is excellent, the only exception being the value of R(0-0) which is too short by 0.19 A at the AM^ level, but 0.06 A too long at the PM3 level. The calculated values of AHf are -161.75 kcal/mol AM^) and - 164.32 kcal/mol (PM3).

A limited number of ab-initio calculations were also carried out, using GAUS- SIAN-88 on the FPS-500, and the DIRECT-SCF method. In these cases, the value of 0-0 is - 1.44 A which is quite close to the experimental value of 1.478 A. A comparison of the theoretical results with the crystal structure data for the trioxane

ANTI-MALARIAL DRUGS

1 .Compound6

239

Compound 9.

80-

m 60-

& * , h i Iwr mrlurlw (cn-lf bo75 1(Ms 97s as a75 625 7751

Figure 5. Computed vibrational spectra for compounds 6 and 8 (Fig. 3) using PM3 geometries.

240 THOMSON, CORY, AND ZERNER

TABLE IV. Computed values of AHf for Qinghaosu and related molecules using AMI and P M ~ methods.

Molecule AM I PM3

Qinghaosu Qin-oh" Arte-ether Qoco Qoch2n Qoch2 Desml Iqb Qch2

BC BCD ABCD ACD ABC

- 161.7491 -182.6643 - 193.5933 -266.6913 -165.1571 - 195.22 15 -158.7115 -160.5442

-70.52170 - 150.0239 - 153.3024 -169.6256

-88.27341

-164.3203 -174.6288 - 191.0977 -220.17392 -146,0963 -183.05 18 - 149.5455 -149.9340

-70.9375 -155.3480 - 153.2010 -158.6715 -80.4508

a 2-Hydroxy-2-deoxoqinghaosu (Fig. 4). 1 I-Deoxyqinghaosu (Fig. 4).

ring part of Qinghaosu is given in Table V. The general agreement with the X-ray data is very good, and the conformation of the molecule as a whole is well reproduced by all the calculations.

Calculations of the vibrational spectra are of interest in view of a disagreement between the original Chinese observations [31] and the work of Jefford and co-

TABLE V. Comparison of calculated and experimental values of the 1,2,4-trioxane ring parameters in Artemisinin.

Parameter AM 1 PM3 ZlNDO Experiment

0 1 0 2 02C3 C304 04C5 C5C6

1.289 1.544 1.240 1.478 1.427 1.402 1.404 1.403 1.427 1.428 1.402 1.437 1.416 1.403 1394 1.390 1.537 1.555 1.499 1.529

0 102C3 112.5 110.3 112.4 107.5 02C304 103.6 104.8 106.7 107.3 C304C5 115.5 116.0 1 1 1.8 114.1 04C5C6 113.5 115.2 114.1 113.3

0 102C3C4 -77.7 -73.3 -76.9 02C304C5 41.9 52.7 34.6 C304C5C6 11.5 2.8 21.1

ANTI-MALARIAL DRUGS 24 1

workers [32]. Our results predict the 0-0 stretching band should be at -750 cm-' at the AM^ level and 760 cm-' at the PM3 level. These values are reasonably close to the experimental values assumed by Jefford for this vibrational mode.

Deoxyartemisinin. The replacement of the 0-0 linkage by -0- completely destroys the anti-malarial activity of Qinghaosu, and we have investigated the struc- ture of this molecule with the structure being optimized in the same way. The MEP are compared with that of Qinghaosu in the next section.

Other Derivatives of Qinghaosu. Several other compounds have been synthesized in which the -CO-OC- linkage has been replaced by -0-CO-, -CO-CO--, -0-CH2--, and finally with -CH2-CH2- [33]. The heats of formation of these molecules are shown in Table IV.

Finally, recently some different derivatives have been prepared which are as active or more active as anti-malarials than Qinghaosu. In particular, replacement of the carbonyl group by 0-CH2-CH3 results in a more active compound, and we have studied the derivative with -OH and OEt in this position, and also those with -NHAr [34].

Comparison of the MEP Maps. The MEP have been calculated in two different ways: in the first case, we have computed the MEP from the charges computed with the semi-empirical wave functions, as described in our earlier work [23]. These were then displayed on the GPX work station, and Figure 6 shows the MEP of Qinghasosu and Deoxyartemisinin computed in this manner from AM 1 charges.

In the second case, we have computed the MEP using the multipole expansion implemented in the CAChe system. These results are in excellent agreement with more rigorous calculations. Figure 7 shows the MEP for Qinghasou and three de- rivatives. However, it is not a trivial matter to obtain more quantitative correlations between features of these maps and biological structure activity (S/A) data, but the following conclusions can be drawn of the basis of this data.

There seems to be a distinct difference between the MEP of the active molecules and those which are definitely inactive, such as deoxyartemisinin. The active mol- ecules have a much wider band of negative potential around the part of the molecule containing the -0-0- linkage. We are currently investigating the similarity indices of these maps [35] for a more quantitative analysis.

Discussion

The study of the conformations of the 1,2,4-trioxane ring system and the deriv- atives of this system which have anti-malarial activity are reported.

In this initial report, we concentrate on the conformational aspects of the mol- ecules, since at the moment the mechanism of action of these molecules in vivo is still under discussion. It is clear that the molecules must contain the 0-0 linkage, and this must be in the trioxane ring. The recent synthesis of smaller ring systems such as ABC [36] should clarify the structural features necessary for activity.

There has been a suggestion that the activity is a consequence of the metabolism of these compounds: in particular, that the ring may be opened, either by addition

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ANTI-MALARIAL DRUGS 243

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244 THOMSON, CORY, AND ZERNER

of an electron or by protonation. In either case, we have found in preliminary calculations that both these processes at the SCF level lead to a ring opened structure for the parent ring compound. More detailed studies ofthese processes are underway.

Acknowledgment

The authors would like to thank Dr. A. R. Butler for drawing their attention to this problem. This work was begun at the Quantum Theory Project at the University of Florida in September 1990 and CT would like to acknowledge this hospitality. CT is also indebted to DEC for the donation of the MICROVAX/GPX work station, and the National Foundation for Cancer Research (USA) and the Association for International Cancer Research (UK) for continued financial support, and for funding the purchase of the Tektronix CAChe system which has been an invaluable tool in this investigation. MCZ acknowledges the donation of a CAChe workstation system on which much of initial part of this work was done. The authors would also like to thank Drs. George Purvis and Sam Cole (Tektronix) for much useful advice about the use of this system.

Bibliography

[I] D. L. Klayman, Science 228, 1049 (1985). [2] Tropical Disease Research, 7th Program Report, WHO, Geneva (1985). [3] China Cooperative Research Group on Qinghaosu and its derivatives as anti-malarials, J. Trad.

(41 Qinghaosu Research Group, Scientifica Sinica 23, 380 (1980). [5] C. W. Jefford et al., Stud. Org. Chem. 31, 113 (1986). [6] C. W. Jefford, J. Velrade, and G. Bemardinelli, Tetrahedron Lett. 30, 4485 (1989). [7] A. J. Lin, D. L. Klayman, and W. K. Milhous, J. Med. Chem. 30,2147 (1988). [8] C. W. Jefford, Y. Wang, and G. Bernardinelli, Helv. Chim. Acta 71, 2042 (1988). [9] C. W. Jefford, E. C. McGoran, J. Boukouvalas, G. Richardson, B. L. Robinson, and W. Peters,

[lo] H. R. Chang, C. W. Jefford, and J-C. Pechere, Antimicrobial Agents and Chemotherapy 33, 1748

[ I I ] C. W. Jefford, J. Velarde, and G. Bemardinelli, Tetrahedron Lett. 30, 4485 (1989). [I21 B. Ye and Y-L Wu, Tetrahedron 23, 7287 (1989). [I31 A. J. Lin, L-Q. Li, D. L. Klayman, C. F. George, and J. Flippen-Anderson, J. Med. Chem. 33,

[14] M. Jung, X. Li, D. A. Bustos, H. N. EISohly, J. D. McChesney, and W. K. Milhous, J. Med. Chem.

[ 151 W. J. Hehre, L. Radom, P. von R. Schleyer, and J. A. Pople, Ab-Initio Molecular Orbital Theory,

[I61 M. J. S. Dewar, J. Am. Chem. Soc. 99,4899 (1977). [I71 M. J. S. Dewar, E. G. Zoebisch, E. F. Healey, and J. J. P. Stewart, J. Am. Chem. Soc. 107, 3902

[I81 J. J. P. Stewart, J. Comput. Chem. 10, 209 (1989). [I91 J. J . P. Stewart, J. Comput. Chem. 10, 221 (1989). [20] C. Thomson, Carcinogenesis 10, 317 (1989). [21] C. Thomson and P. Scano, J. Comput. Chem. 12, 172 (1991).

Chin. Med. 2, 3 (1982).

Helv. Chim. Acta 71, 1805 (1988).

( 1 989).

2610 (1990).

33, 1516 (1990).

(Wiley, New York, 1986).

(1985).

ANTI-MALARIAL DRUGS 245

[22] P. Politzer and D. G. Truhlar, Eds., Chemical Applications of Atomic and Molecular Electrostatic

[23] C. Thomson, D. Higins, and C. Edge, J. Mol. Graphics 6, 171 (1988). [24] D. Higins, Ph.D. thesis, University of St. Andrews, St. Andrews, Scotland (1988). [25] Tektronix CAChe system, Tektronix (1989). [26] C. Edge, Ph.D. thesis, University of St. Andrews, St. Andrews, Scotland (1987). [27] U. Burkhart and N. L. Allinger, Molecular Mechanics, ACS Monograph 177 (American Chemical

[28] J. J . P. Stewart, MOPAC-V5.0, USAFA (1989). [29] A. D. Bacon and M. Zerner, Theor. Chim. Acta. 53, 21 (1979). [30] K. Christe, Spectrochim. Acta, Part A27, 463 (1971). [31] R. S. Mulliken and W. C. Ermler, Polyatomic Molecules (Academic, New York, 198 I ) . [32] M. Mohnhaupt, H. Hagemann, J-P Perler, H. Bill, J. Boukouvalas, J-C Rossier, and C. W. Jefford,

[33] B. Ye and Y-L. Wu, Tetrahedron Lett. 45, 7287 (1989). [34] A. Brossi, B. Vemgopalan, L. D. Gerpe, H. J. C. Yeh, J. L. Flippen-Anderson, P. Buchs, X. D.

[35] C. Thomson and M. Zerner, to be published. [36] C. W. Jefford, private communication.

Potentials Plenum, New York, (Plenum, 198 1) .

Society, 1982).

Helv. Chim. Acta 71,992 (1988).

Luo, W. Milhous, and W. Peters, J. Med. Chem. 31, 645 (1988).

Received June 2 1, 199 1