Intramolecular hydrogen bonding and conformational studies of bridged thebaine and oripavine opiate narcotic agonists and antagonists.

A conformational study of a series of oripavine derivatives is reported using the PCILO semiempirical quantum mechanical method. Low-energy conformers of carbinol substituents on C7-C19-R1R2OH are found with and without intramolecular hydrogen bonding to the C6-OCH3 group. The relative energies of these conformers depend on the R1 and R2 groups and the diastereoisomerism of the alcohol. The results are consistent with available NMR and IR studies of intramolecular hydrogen bonding and with crystallographic data. The importance of interaction between specific conformations of C19 carbinols and a lipophilic receptor site is suggested. A hypothesis is formulated to explain observed differences in pharmacological activity between diastereoisomers at C19 in the oripavine series and also to explain how these diastereoisomers alter the established pattern of N-substituent effects on relative agonist/antagonist potency found in other rigid opiates. By contrast, conformational studies of the C19 optical isomers of the C7-C8 etheno form of buprenorphine lead to the prediction of greatly reduced intrinsic potency differences between C19 diastereoisomers for this compound and for buprenorphine itself.

Analgesics. 1. Synthesis and analgesic properties of N-sec-alkyl- and N-tert-alkylnormorphines.

A series of N-sec- and N-tert-alkylnormorphines was synthesized and evaluated for analgesic potency, antagonist activity, and opiate receptor binding. Computer-assisted conformational analysis profiles were utilized to assist in the selection of compounds for synthesis and correlation of receptor events with in vivo observations. N-tert-Alkylnormorphines 5a-c were devoid of agonist activity; however, some sec-alkyl analogues showed interesting mixed agonist-antagnoist actions. N-sec-Butyl- and N-(alpha-methylally)normorphine were separated into R and S isomers, which exhibited quantitative pharmacological differences. The N-sec-butyl S isomer 10a showed analgesia approximating morphine with nalorphine-like antagonist activity. Preliminary testing indicates only slight evidence for physical dependence with this compound.

Quantum chemical studies of N-substituent variation in the oxymorphone series of opiate narcotics.

Quantum chemical calculations were performed on six N-derivatives of oxymorphone including N-methyl- (oxymorphone), N-allyl- (naloxone), N-dimethylallyl- (nalmexone), N-methylcyclopropyl- (naltrexone), N-methylcyclobutyl-(nalbuphone), and N-phenethylnoroxymorphone using the PCILO method. The object of the study was to identify conformational features of the N-substituents which might be responsible for the intrinsic observed pharmacological properties of opiate agonism and antagonism. Both axial and equatorial N-substituent conformers were considered, as well as possible interactions of the C14-OH group with such substituents. Variations of agonist/antagonist potency ratios within this series could not be explained by differing relative energies of equatorial and axial conformations or by varying rates of interconversion between the two. Direct effects of the C14-OH group on conformations of N-substituents also could not account for their relative agonist/antagonist potencies. Consistent with a previous hypothesis, the observed potencies and binding data could be explained most consistently by the availability of several low-energy equatorial conformations of N-substituents and their interactions with the C14-OH group through a common anionic receptor site.

Structure-activity studies of narcotic agonists and antagonists from quantum chemical calculations.

Three classes of flexible opiates have been studied: 4-phenyl piperdines, methadone and enkephalins. Our results show that low energy conformers of the 4-phenyl piperidines have equatorial phenyl rings and cannot completely overlap with rigid opiates at the receptor. A combination of calculated conformational and electronic properties could account for observed potency differences in meperidine, desmethyl, alpha+, alpha-, beta+ and beta- prodines. Our results also indicate that both meperidine and its reverse ester bind to the receptor in a similar mode with the phi ring in approximmately the same position as the phenyl substituent in 5-phenyl benzomorphans. Conformers of methadone which maximally resemble morphine have very high relative energies. The lowest energy conformer has a partial H-bond between the NH and O=C groups. In this conformation methadone resembles meperidine more than morphine. The electronic structure of all three types of opiates indicate a similar cationic charge distribution around the amine nitrogen and imply that their binding to an anionic receptor site could be similar. The determination of peptide opiate conformations present a challenge of a different order of magnitude than the most flexibe exogenous opiates. Because of the extremely large number of possible conformations, search strategies based on energy optimized conformations alone are not adequate to select plausible receptor site candidates. Other criteria such as consistency with known structure activity data and similarities to rigid opiates must be used. With this rationale, we have predicted and characterized a low energy conformer of Met-enkephalin and D-ala2 Met-enkephalin as a likely candidate at the receptor site. With a modest energy input (deltaE approximately 3 kcal/mole) significant overlap of this conformer with the potent opiate PET was obtained. The tyrosine and phenyalanine side chains and the terminal amine and carboxyl groups play a crucial role in this overlap. It is hoped that this calculation with help establish a template for peptide opiate receptor interactions.

Quantum chemical studies of methadone.

Quantum chemical calculations were performed on the flexible methadone molecule to test the hypothesis that it structurally mimics the fused ring structure of morphine. In these calculations using the semiempirical, PCILO method, protonated and nonprotonated conformations were considered representative of different types of intramolecular interaction at the morphine receptor. Calculated energies for these conformations were compared to those calculated for protonated and nonprotonated extended chain and crystal structure conformers. Lowest energy conformations showed intramolecular bonding but the resultant molecular geometries were not very morphine-like. A comparison of the structure of methadone to that of meperidine seemed equally as valid.

Quantum chemical studies of morphine-like opiate narcotics. Effect of N-substituent variations.

Quantum chemical calculations including extensive conformational variations are performed on three morphine-like analgesics with varying N-substituents using the PCILO and INDO methods. The three compounds, morphine, nalorphine, and N-phenethylmorphine, have been shown experimentally to exemplify opiate narcotic agonism, antagonism, and increased agonism, respectively. In this study, these properties are correlated with the electronic and conformational results. The electronic properties of the fused ring skeleton including specifically the cationic region around the nitrogen are relatively unaffected by varying N-substituents. The properties studied include net charges, bond polarities, and the nature and energy of the highest filled and lowest empty molecular orbitals. The conformational behavior appears to be the main cause of differing receptor binding and interaction with the active site and is discussed in these terms.