H calculations of opiates

Journal of Molecular Structure (Theochem), 207 (1990) 1-14 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SEMIEMPmJCAL

MNDO/H CALCULATIONS

1

OF OPIATES

Part 1. Building blocks: conformations of piperidine derivatives and the effect of hydrogen bonding

AMIRAM GOLDBLUM*

and OMAR DEEB

Department of Pharmaceutical Chemistry, School of Pharmacy, Hebrew University of Jerusalem, Jerusalem 91120 (Israel) GILDA H. LOEW Molecular Research Institute, 845 Page Mill Rd., Palo Alto, CA 94304 (U.S.A.) (Received 22 June 1989; in final form 10 October 1989)

ABSTRACT Conformations of piperidine, N-methylpiperidine, N-methyl-3-piperidinol and N-methyl-4-piperidinol were studied by MNDO/H in the gas phase. A few representative conformers were considered for each compound, and both the enthalpy and entropy were calculated and compared with experimental values, where possible. The results obtained indicate that, for each compound, several conformers are expected to coexist at room temperature in various amounts determined by the free energy Boltzmann distribution. Hydrogen bonding contributes to the stabilization of some piperidinol conformations with axial substituents as well as some boat or flexible forms. In general, protonation increases the stabilization of the chair compared with the boat or flexible conformers, except for 4-hydroxy-N-methylpiperidine-H+, where the boat form predominates. However, the maximum stabilization by internal hydrogen bonding is 4-5 kcal mol-‘, so that hydration can break these bonds. In N-methylpiperidine stabilization of the N-Me, conformer with respect to the axial methyl group increases to 1.4 kcal mol-’ from the value of 0.4 kcal mol-’ for the same equilibrium of N-H in piperidine. The theoretical distribution of conformers in Nmethyl-3-piperidinol is consistent with the known experimental results.

INTRODUCTION

Piperidine rings form part of many drugs, of which the best known are the opiate analgesics. In the rigid opiates, piperidine rings have very limited flexibility due to their participation in fused-ring systems such as morphine (Fig. l), levorphanol, morphinans and benzomorphans [ 11. Despite this rigidity, nitrogen inversion might still take place and the piperidine rings can assume a boat conformation, but with carbon and not nitrogen at the “flagpole” posi*Author to whom correspondence

0166-1280/90/$03.50

should be addressed.

0 1990 Elsevier Science Publishers

B.V.

Fig. 1. Stereoscopic plot of morphine. The piperidine ring in this figure is in the chair conformation, and the N-methyl group is in the equatorial position.

tion. The effects of N-substituent variation on the affinity and activity profiles of opiates have been studied [ 21 and attributed in some cases to conformational preference of equatorial to axial positions of the N-substituents [ 31. Even if such a preference is established, the drug could bind to the receptor in its less abundant conformation, thus generating more of this conformer from the equilibrium mixture. Thus it is important to establish whether such minor conformers could potentially be present under experimental conditions. A further complication in deciding on which bioactive form binds to the receptor is the existence of protonated species. This is highly probable for most opiates under biological pH [ 41. In addition to the rigid opiates, other, more flexible analogs are known such as the 4_phenylpiperidines, exemplified by meperidine (pethidine) and prodine [ 5 1. A third complication is the possible role that internal hydrogen-bonding can play in determining preferred conformations and bioactive forms of compounds. Both flexible and rigid opiates can achieve such internal hydrogen-bonds in neutral or protonated states. For example, in naloxone, a potent “rigid” opiate antagonist, such a bond can be formed in the piperidine ring, which has a 3-OH substituent. Thus it is important to include both neutral and protonated forms in this study and to examine the contribution of hydrogen-bonding in order to characterize the energetically accessible conformations available to the common piperidine moiety in such molecules. Such a characterization might shed more light upon the mechanisms of interaction with the opiate receptor. Very little experimental information is available on opiate conformations in solution [ 61. Known X-ray structures indicate that the piperidine rings of such opiates are in the “chair” form, with equatorial alkyl substitution. More experimental information is available on piperidines themselves and the influence of substituents on their conformational stabilities [ 71. Internal hydrogen-bonding can play a role in the conformational equilibria of some 3-hydroxy as well as in 4-hydroxy substituted N-methylpiperidines [&lo]. After a long controversy, experiments which were designed to determine axial vs. equato-

3

rial preferences of the N-H bond in piperidines have now been definitely interpreted [ 111 in favour of the equatorial N-H. Interpretations of NMR experiments on 3-hydroxy-N-methylpiperidine [91 are still questionable since unproven assumptions about the accessibility of certain conformations were made. In a theoretical approach to the study of conformational equilibria, a compromise between methodology and model size must be achieved. Extended ab initio basis sets are of limited practical use for the study of many conformers of rather large molecules due to time limitations. Small ab initio basis sets do not seem to be suitable for piperidine studies. One such study over-estimated the energy differences between equatorial and axial N-H and N-methyl conformers [ 12 1. Among the semiempirical quantum-mechanical methods, we have recently modified MNDO [ 131 so that it can better describe hydrogen bonding geometries and energies. We have demonstrated the ability of this modified version (MNDO/H) to reproduce gas phase experimental results for a variety of small molecules [ 141. MNDO/H was also useful for calculating acidities in the active site of aspartic proteinases [ 151 and demonstrated the modification of their pK, values by hydrogen-bonding to protein residues. We report here the first step in applying this method to the study of opiates, a test of its ability to predict the conformational behaviour of piperidine and its derivatives which can be relevant to such studies. In the present study we have not included the role of the solvent in conformational equilibria of piperidines. However, this “gas phase” approach is appropriate for comparison to experiments in apolar solvents such as heptane [ 71 and could also be relevant to the bioactive forms of piperidine rings in opiates which bind to receptor sites that are not in contact with bulk water. METHODS

MNDO/H [ 141 was employed for all calculations. Enthalpies of formation, A&, and entropies of each species were calculated at room temperature (T= 298.15 K). A few initial conformers of each molecule were chosen as starting geometries for full optimization. To eliminate maximally any bias in the choice of conformers, geometries included both local minima as well as some strained geometries which are expected to have higher energies. By this careful selection of initial conformers, it is likely that all stable species representing both “global” and “local” minima are characterized for each of the molecules. Entropies were calculated by mass-weighting of the diagonalized Hessian matrix for vibrational frequencies and adding the vibrational component to the rotational and translational ones. In some cases we report the equilibrium value for two conformers, A and B, of a molecule. Such values were calculated using the standard relationship

4

K ~=e~[-(~~B-~As~)/~~l

(1)

where A&n = AFIt - AH: (enth~pies of formation of the two conformers) and ASAB= 8: - SL. Entropies are reported in entropy units ( 1 e.u, = 1 cal mol-l K-“). The fraction of a conformer A in a mixture of conformers was determined as %A=lOO/Cl+~CBfA[exp-(~AB-TASAB)/RTJ)

(2)

where B denotes all the other conformations which have been cahlated,

Scheme 1 presents some of the basic ~onfo~at~ons that must be considered for piperidine ( 1) and its protonated forms, and Table I lists the energies and percentage abundance of each species in this scheme.

ICX ~~~

‘~~~_“I‘;

f_

ICP

13P

Scheme 1. Conformations

of neutral and protonated

piperidine.

The conformations of N-methy$iperidine (2) are depicted in Scheme 2 and the corresponding energies are listed in Table 2.

2CXP

ZCQP

Scheme 2. Conformations

2BXP

of neutral and protonated

N-methylpiperidine.

The addition of a second substituent increases the number of conformers to be considered. The 3-hydroxy-~-methylpip~ridines (3 ) can undergo both ring and nitrogen inversion to achieve internal hydrogen-boning. The relevant

5

3cc1

3TC

3CT2

,,

‘HZ

Scheme 3. Conformations of 3-hydroxy IV-methylpiperidine.

conformers are shown in Scheme 3, and the corresponding energies are listed in Table 3. The internally hydrogen bonded molecules 3CTl and 3TC in Scheme 3 undergo -OH rotation upon protonation so that the donor and acceptor roles are then reversed. TABLE 1 Enthalpies and entropies for the neutral and protonated forms of piperidine Compound Neutral 1cx

1CQ 1BQ 1BX Protonated 1CP IBP

AH;

S”

% in mixture

- 18.49 - 19.00 - 16.95 - 16.46

74.79 74.90 77.85 77.75

25.1 62.4 8.8 3.7

146.32 148.81

75.08 77.49

95.1 4.9

TABLE 2 Enthalpies and entropies of the neutral and protonated conformers of N-methylpiperidine Compound Neutral 2cx

2CQ 2Tl Protonated PCXP PCQP 2BXP

AH;

S”

% in mixture

- 13.37 14.67 - 12.70

82.49 82.73 84.98

8.3 82.4 9.3

153.61 152.15 154.80

81.49 81.75 83.88

6.8 90.1 3.1

-

TABLE 3 ~utha~pies and entropies for the neutral and protonated conformers of 3-hydroxy-Nmethylpiperidine Compound

Neutral

Protonated

w

S”

%

AK

S”

%

3CTl 3CT2 3CT3

-56.39 - 53.76 -54.15

87.40 88.10 88.44

73.1 1.3 2.9

108.40 115.11 _a

87.09 87.37 _‘J

97.5 0.002 -_a

3TC 3CCl 3cc2

- 54.06 - 54.06 -55.14

90.41 87.64 88.36

6.6 1.7 14.5

111.46 112.24 113.53

89.91 86.50 87.61

2.4 0.1 0.02

“rhe protonated product of 3CT3 is similar to the one from 3CTl. TABLE 4 Enthalpies and entropies for the neutral and protonated conformers of 4-hydroxy-Nmethylpiperidine Compound

4CCl 4CC2 4CTl 4CT2 4BC

Neutral

Protonated

‘w

S”

%

m;

S”

%

-55.01 - 53.63 -53.36 - 54.84 -54.20

88.06 88.10 87.90 88.26 89.48

39.8 4.0 2.3 33.1 20.8

110.83 114.79 112.86 113.35 109.35

87.91 88.13 87.73 88.39 86.54

14.1 0.02 0.4 0.4 85.14

Finally, it is possible that the boat conformers of 4-hydroxy-N-methylpiperidine (4) are also stabilized by internal hydrogen bonding. These and other important conformations are drawn in Scheme 4, and their associated energies are listed in Table 4. Upon protonation, the hydroxyl in 4BC rotates in order to accommodate a proton on nitrogen. Figure 2 presents stereo drawings of all

4cc2

4CT2

Scheme 4. Conformations of 4-hy~oxy-N-methylpiperidine.

7

a

st~~tureswhich were found to have s~biii~atj~n bonding in their ~onformat~ons~ for both neutral and ~ro~nat~

the relevant

fmmhalogen mofecules.

Comparison with experiments

The comparison of the theoretical results with the experimental ones are limited mostly due to the lack of direct correspondence of the computations to the complicated experimental setting, Most of the structural information on piperidine and its derivatives was gained indirectly from physical measurements such as NNfR, II& and spoke-rnorn~~t me~u~ments [7 f sOnly gs\s phase rneasu~rn~~~ alfow a strict comparison with the ~ornpu~t~o~ done in this study and enabb a reasonable assessment of their quality. The most recent microwave structure of piperidine in the gas phase fl6] shows that the canformer with N-H,, predominates with respect to N-H,, by a ratio of 3.5: 1 corresponding to a difference of ca. 0.75 kcal mol-” in free energy. A relatively recent electron diffraction study of piperidine also concluded that it exists mainly as the N-H equatorial conformer in the gas phase [ 171, with the possibility of minor amounts of N-H,,. A similar conclusion about the preference of equatorial N-W was finally obtained from interprotations of a solution ex~riment~ after many years of dispute f I1 ] _The presently accepted value for the free energy of the equilibrium N-E&S N-Ha is AG 2 0.4 kcal mai-‘, This is close to the L%.FI value fount_3from IR overtone measurements o~~~~~i~ne in the gas phase f I$]. Chr calculated difference for A.&+ S 0 of these two species (Table 1 and Scheme 1,1CX and 1CQ) is in good agreement with that difference. However, the MNDO structure of piperidine differs from the gas phase TABLE 5

N-C

c-c C-N-C N-C-C c-c4-c c-cs-c N-C-C-C C-C-C-C C-N-C-C

1.466

1.542,1.537 116.6 111.7 113.6 113.9 47 43.5 53.3

9

structure as found by electron diffraction [ 171. A comparison of the structural parameters is given in Table 5. The calculated chair form is too “flat”, and this is responsible for the concomitant increase of bond angles, as compared with the experimental ones. Thus, although we did obtain a reasonable value for the free energy difference between axial and equatorial N-H conformations of piperidine, the “flattening” of the calculated ring should increase the 1,3-diaxial distance and reduce interactions for both the proton and the lone pair on the nitrogen atom. According to our calculations, both the chair and the boat forms are expected to contribute to the gas phase conformations of piperidine in a &M/12%ratio, and each should have a minor concentration of ca. 25% of the N-H,, conformer. To the best of our knowledge, no boat conformations have been inferred for piperidine in any of the experimental conditions reported. It is difficult to determine whether this apparent discrepancy is due to deficiencies in the calculations or the insensitivity of experiments to such small concentrations, or both. Conclusions from gas phase experiments were based on some assumptions, as were our calculations. For example, we implicitly did not allow a role for activation energy between the various conformers, thus assuming equilibrium conditions. Another limitation in the results of this study is the use of static geometries to define a conformer and to calculate its concentration in an equilibrium mixture. The flexibility in such a conformer, i.e. the ability to change its geometry with only small additional free energy, should lead to an abundance of closely related structures. These other “conformers” are found by experiments to contribute values similar to their more stable counterpart, but in theoretical studies they are usually ignored. The distribution of conformers for N-methyl-3-piperidinol was studied by IR spectroscopy, in order to differentiate between structures with a “free” and “hydrogen-bonded” hydroxyl group [ 81. From these studies it was concluded that 57% of the hydroxyl groups are internally hydrogen bonded. This value should be compared with the calculated sum of 80% shown in Table 3 for the conformers with such internal hydrogen bonding (Scheme 3) 3CTl+ 3TC. In an NMR experiment, designed to limit nitrogen inversion by quaternization, the relative fraction of (3CTl+ 3CT2) was compared with (3CCl+ 3CC2) [9]. It was estimated from the NMR study that 3CT1+3CT2 is about 66% of the mixture by ‘H NMR, or 71% by 13C NMR, close to the calculated value of 74%, given in Table 3. N-Methyl-4-piperidinol is believed to prefer an -OH equatorial [ 71 position by about 0.8 kcal mol-‘. This amounts to nearly 20% of the conformers with an axial -OH. Our results for this molecule show a nearly equal distribution of axial (4CCl+ 4CTl) vs. equatorial (4CT2 +4CC2 ) as well as some 20% of “boat”, the internally hydrogen-bonded conformer 4BC. This “boat” conformer turns out to have a much greater abundance following protonation, and can be in equilibrium with both protonated 4CC 1 and 4CT2, but not 4CTl or 4CC2.

10

Axial vs. equatorial N-substituents The experimental values reported for the equilibrium between N-Me,, and N-Me,,,conformers in N-methylpiperidine are spread over a large range, from low values of 0.4-0.65 kcal mol-’ [ 19,201 obtained from dipole-moment studies, to higher values of ca. 2.5 kcal mol-l obtained from 13C NMR of model systems [ 211, always in favour of an equatorial methyl group. Those higher values were based upon relations to model systems, while the results for iV-methylpiperidine from 13C NMR gave a lower range of 1.35-1.77 kcal mol-‘. The authors of this 13C NMR study [ 211 questioned their own results because of the lack of a suitable model system and because of potential solvent stabilization by CHCl, on equatorial lone-pairs in the case of N-Me,, conformation. The presently “accepted” value of 2.7 kcal mol-’ [ 71 is large compared with the equatorial/axial distribution for a methyl group in cyclohexane [lo]. Our computed free energy for the N-Me (equatorial to axial) in the chair form is AG= 1.37 kcal mol-‘. The calculated structures give some insight into the observed differences in the experimental values and indicate why the lower value might be correct. We find that C-N-C-C angles in the N-Me,, conformer are slightly increased with respect to piperidine with N-H,,. It is possible that the axial methyl group somewhat deforms the chair conformation and thus the model compounds from which the larger AG difference was obtained are not directly applicable, since they are more rigid and have stronger diaxial repulsions [21]. The addition of a 3-hydroxy group to N-methylpiperidine increases the number of possible N-Me, to N-Me,, equilibria which have to be considered. As expected, if OH is equatorial, this equilibrium (3CC2 =3CT2) was unaffected relative to N-methylpiperidine (2CQS2CX), with I&N 10. With an axial hydroxyl, this iv-inversion (3CTl e3CC 1) equilibrium energy is only slightly increased, to 2.3kcal mol-’ (compared with 1.4kcal mol-‘), despite the favourable hydrogen bonding in 3CTl. The expected syn-diaxial repulsion in 3CCl which should have made this difference greater is reduced due to the electrostatic attraction of positively charged methyl hydrogens to the negative oxygen (Fig. 3 ). The effect of this attraction is also evident in comparing 3CC 1

Fig. 3. The optimized structure of 3CC1, which is stabilized by interaction of the N-methyl protons with the axial 3-OH group (stereoscopic plot).

11

with 3GT3. In 3GT3, the N-Me group is equatorial, while 3GGl with an axial N-Me is nearly as stable. In the 4-OH series, the N-Me axial and equatorial conformers are separated by 1.26 kcal mol-’ if 4-OH is equatorial (4GT2=4GG2 ), while the energy difference is 1.70 kcal mol-’ when it is axial (4GGlS4GTl). Both these differences are proportionately increased upon protonation (to 1.52 and 2.08 kcal mol- ’ , respectively).

Boat vs. chair conformations and the hydrogen bonding effect None of the boat conformers shown in the schemes had been considered to play a role in the equilibria of piperidines, except for some special cases where 1,2_interactions of large substituen~ together with hydrogen bonding can force a distortion of the chair of the more flexible conformation. In neutral piperidine, a boat (b) conformation is some 2.0 kcal mol-l higher than a chair (c) form for both N-H,, and N-H,,. Even less of the b form is expected for protonated piperidines (Table 1) . In addition, we find that the calculated b forms are closer in geometry to a “twist boat” conformation. For N-methylpiperidine, the distorted ring (2Tl) is only 1.3 kcal mol-’ above the lowest c conformer so that, if only three conformers (2GX, 2GQ and 2Tl) are present, it represents 9% of the total conformations. For both neutral and protonated ~-methylpiperi~nes, except 4BG, initial b forms derived from the optimized boat structure of non-substituted piperidine were greatly transformed during geometry optimization. This result indicates that the piperidine b forms are not local or global minima for most of the substituted molecules included in this study. We could not obtain a “boat” conformer of N-methylpiperidine with an axial methyl group. All attempts to reach such a “local minimum” ended in a flexible “boat” conformer similar to the one obtained for the N-Me,, counterpart. This flexible form is not symmetric. Addition of a 3-OH group stabilizes a conformer (3TG) which is a “twist” boat. It has a weak internal hydrogen bond (Table 6) with a large H* **N distance and a consistently small charge variation of the atoms involved directly in hydrogen bonding, compared with the non hydrogen bonded conformers. Charge variation, i.e. the polarization of the atoms involved in hydrogen bonding, is a well-known effect associated with the proximity and proper alignment of systems of the X-H* - *Y type, with both X and Y being electronegative atoms [ 141. In this case, the 0-H. lN atoms have charges of - 0.388,0.233 and -0.518 in 3TG, and -0.393, 0.227 and -0.506, respectively, in 3CC2, with no hydrogen bonding. The AG value for 3GG2+3TG (0.5 kcal mol-‘) should be compared with that of 2GQ*2Tl (1.3 kcal mol-‘), so we expect this hydrogen bond to stabilize 3TG by less and 1 kcal mol-‘. Protonation has a different effect on those partial equilibria. It stabilizes the l

12 TABLE 6 Geometries of hydrogen bonded species Molecule

X-Y (A,

H-Y (A,

X-H.. I”)

3CTl 3CT1, 3TC 3TC, 4BC 4BC,,

3.073 2.880 3.097 2.800 3.161 2.532

2.761 2.445 2.814 2.337 2.681 1.704

100.2 104.2 98.3 105.7 112.0 130.6

*Y

equilibrium of 3TC,- “3CC.2, (2.1 kcal mol-‘) in favour of the flexible form. By contrast, without the 3-OH group, the chair conformation is favoured and 2CQPsBBXP has AG= 2.0 kcal mol-‘. Here the hydrogen bond contributes some 4.0 kcal mol-’ of stabilization: this is shorter than the bond in 3TC (RN-O =2.80 A and RH_O=2.34 A, vs. 3.10 and 2.81 of 3TC), the hydrogenbond angle (N-H-O) is 105.7” (98.3” in 3TC) and the charges involved are larger (q. = -0.421, q,=o.273, qN+ = -0.245). Another partial equilibrium which demonstrates the stabilizing effect of intramolecular hydrogen bonding is found in the comparison of 3CTl ti3CC2 (Scheme 3,l.O kcal mol-l) with the equilibrium 4CCl+QCT2 {Scheme 4, 0.1 kcal mol-I). Both those equilibria are for OH, to OH, with the N-methyl fixed in the equatorial position. Upon protonation, this equilibrium for the 4-OH compounds is characterized by AGE 2.4 kcal mol-’ with no hydrogen bonding, while the corresponding value for the 3-OH isomer is nearly 5.0 kcal mol-‘. Thus we may estimate the hydrogen bonding stabilization in this case at about 2.5 kcal mol-‘. Rotation of the hydroxyl group (3CT3) from its hydrogen bonding position towards the nitrogen leads to a loss of 1.9 kcal mol-‘, which is a sum of the hydrogen bond loss and additional repulsion among oxygen and the lone pair on nitrogen. This value indicates that the hydroxyl group has some rotational freedom despite the hydrogen bonding. The conformers of neutral and protonated 4-hy~oxy-~-methylpiperi~ne can also be stabilized by internal hydrogen bonding. The equilibrium 4CT2+4BC (0.3 kcal mol-‘) should be compared with 2CQti2Tl (1.3 kcal mol- ‘) as a lower limit since 2Tl is more flexible than the boat 4BG. Protonation strengthens this hydrogen bond by an additional 3.0 kcal mol-“, or an overall value of 4.0 kcal mol-l. The hydrogen bonding parameters of protonated 4BC have the shortest interatomic distances (Table 6) and the largest atomic charges. The greater flexibility of the boat forms, either classical or not, is reflected in their larger entropies as compared with the chair forms. Analysis of the

relative contributions to the entropy from rotational and v~ratio~al partition functions demonstrates that this larger entropy is an outcome of more low energy vibrations of the flexible forms. CONCLUSIONS

The theoretical study of conformations of piperidine analogs by MNDO/H described here has led not only to the characterization of their lowest energy conformers but also to an assessment of the contributions of some higher energy forms. Comparisons of the MNDO/H results with the experimental ones indicate that it can reasonably represent the conformations and energetics of such piperidine rings. In the neutral form of the four analogs, a chair nonformation with N-equatorial dominates in each case. The greatest contribution from other forms is found in the unsubstituted piperidine itself (25% N-axiah 11% boat), An M-Me, 3-OH,, hydrogen bonded form dominates in the 3-OH compound, while in the 4-OH compound there are two important N-Me,, conformers, one with axial 4-OH (40%), the other with equatorial 4-OH (33%), neither of which are hydrogen bonded. In none of the derivatives can the presence of conformers other than predominant chair conformations with N-RW (R=H or Me) be ruled out. The presence of minor amounts of such conformers should affect both the physical properties and the interactions of these molecules in the gas phase or in a nonpolar environment, such as a membrane bound receptor binding site. Our calculatians are directly applicable to such an en~ronment_ If the neutral form of piperidine containing opiates binds to such a receptor site, two types of 4-OH conformers as well as a hydrogen bonded 3-OH conformer are candidates for the bioactive form. Protonation shifts the equilibria of the neutral compounds and, for all but the 4-OH compound, the same lowest energy conformer which was found in the neutral state increases in abundance to 90-97%. The most dramatic effect of protonation is seen in the 4-OH piperidine. Strong internal hydrogen bonding makes a boat form the dominant one (85% ). If, as generally presumed, piperidine containing opiates bind in a protonated form to receptors, the possibility that this new type of conformer could be involved should be considered, at least in the flexible analogs‘ The extension of these studies to piperidine containing opiates and to the effect of water on the equilibria is now under way. ACKNOWLEDGEMENTS

One of us (G.L.) gratefully acknowledges support of this work by the National Institute on Drug Abuse grant DA02622. A.G. and O.D. thank the He-

14

brew University Computer Centre for providing computer time.

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