Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis and Structural Elucidation of a Pyranomorphinan Opioid and in Vitro Studies Mohd. Imran Ansari,† Jason R. Healy,‡ Kellie Hom,† Jeffrey R. Deschamps,§ Rae R. Matsumoto,‡,∥ and Andrew Coop*,† †
Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, 20 North Pine Street, Baltimore, Maryland 21201, United States ‡ West Virginia University, One Medical Center Drive, Morgantown, West Virginia 26506, United States § Naval Research Laboratory, Code 6910, 4555 Overlook Avenue, SW, Washington, DC 20375, United States ∥ Touro University California, College of Pharmacy 1310 Club Drive, Vallejo, California 94592, United States S Supporting Information *
ABSTRACT: During optimization of the synthesis of the mixed μ opioid agonist/δ opioid antagonist 5-(hydroxymethyl)oxymorphone (UMB425) for scale-up, it was unexpectedly discovered that the 4,5-epoxy bridge underwent rearrangement on treatment with boron tribromide (BBr3) to yield a novel opioid with a little-studied pyranomorphinan skeleton. This finding opens the pyranomorphinans for further investigations of their pharmacological profiles and represents a novel drug class with the dual profile (μ vs δ) predicted to yield lower tolerance and dependence. The structure was assigned with the help of 1D, 2D NMR and the X-ray crystal structure.
T
he current opioid crisis1 necessitates the development of efficacious analgesic agents lacking the dependence associated with traditional, clinically used opioids. Opioids are the gold standard for the management of severe pain; however, the development of dependence leads to continued use outside of the clinical setting.2 Opioids act as agonists on μ opioid receptors, and dependence and tolerance occurs due to the down-regulation of μ receptors in response to the presence of exogenous opioid agonists.3 Numerous approaches have been investigated toward identifying μ opioids that do not cause dependence, as recently reviewed in The Scientist. Such efforts have included targeting receptor dimers, the development of biased agonists, and the development of opioids with a polypharmacological profile.4 In addition to μ opioid receptors, there are also κ, δ, and ORL-1 receptor subtypes. Our approach to the design of μ opioid analgesics lacking dependence has focused on the findings that δ opioid antagonists attenuate or eliminate the dependence and tolerance to μ agonists.5 There is significant evidence from coadministration, knockout studies, and dualprofile peptides that a polypharmacological profile of μ opioid agonism and δ opioid antagonism will lead to lessened dependence and tolerance.6 To this end, we recently reported 5-(hydroxymethyl)oxymorphone (UMB425) as a novel opioid with a dual profile of μ agonism/δ antagonism, which showed reduced antinociceptive tolerance in mice compared to morphine.7 © XXXX American Chemical Society
As part of our studies into optimization of the synthesis of UMB425, we uncovered a unique rearrangement of (4ahydroxy-7a-(hydroxymethyl)-9-methoxy-3-methyl-2,3,4,4a,5, 6hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7(7aH)-one) (1) during 3-O-demethylation with BBr3 which gave a 7-bromo-substituted pyranomorphinan. The ease of synthesis, coupled with the installation of functionality at the 7-position, opens up the chemistry of pyranomorphinans for further investigation (Scheme 1). With a 10 °C temperature difference, an unknown product was obtained in a 30% yield, and structural elucidation was therefore undertaken to determine the mechanism of the formation of UMB426 (later determined to be (6S,7aS)-6-bromo-4a,10-dihydroxy-3methyl-2,3,4,4a,5,6,7a,8-octahydro-4,13-methanochromeno[4,3-e]isoquinolin-7(1H)-one through a change in conditions. The structure of UMB426 was solved by means of mass spectrometry and 1D and 2D NMR spectroscopy and confirmed by X-ray crystallography. The current studies allow a facile method for its synthesis, and in vitro assays demonstrate a profile of μ agonism/δ antagonism. Mass spectral analysis showed the loss of −OH and the addition of bromine and suggested that the unknown compound might be compound 2 (Figure 1) in which the −OH group was directly replaced by a bromine. However, 1H NMR analysis in CDCl3 was not consistent with 2, as the Received: March 30, 2018
A
DOI: 10.1021/acs.orglett.8b01025 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Scheme 1. Synthesis of UMB425 and Rearranged UMB426
Scheme 3. Synthesis of UMB 453
Figure 1. Structure of rearranged product.
Scheme 2. Previous Reports of Some Unusual Pyranomorphinans Figure 4. X-ray crystal structure of UMB426.
Scheme 4. Proposed Mechanism for the Formation of UMB426
Figure 2. Suggested structures of rearranged product.
Table 1. Optimization Table for UMB426 Conversion from UMB425 T (°C) −40 to −20 −20 to −10 −10 to 0
conversion of UMB425 to UMB426 (%) no reaction conversion conversion
0 50 100
group, which suggested a possible rearrangement having taken place. Significant efforts over the years have been extended to modify the 4,5-epoxymorphinan skeleton of opioids to uncover the ideal opioid analgesic, and efforts at the 4,5epoxy bridge have focused mainly on removing the oxygen bridge to give the morphinans, a removal that leads to changes to the conformation of the remainder of the skeleton.8 Ring expansion from the dihydrobenzofuran to
Figure 3. 2D NMR correlation diagram of UMB425 and UMB426.
proton spectrum of UMB426 showed no peak of methylene protons corresponding to the proposed terminal 5-CH2Br in the proton spectrum. In addition, the characteristic peaks corresponding to 5-CH2OH in UMB425 (two doublets of one proton at δ 4.23 and δ 4.0, respectively) were absent from the spectrum of UMB426. This showed a loss of the 5-CH2X B
DOI: 10.1021/acs.orglett.8b01025 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 2. In Vitro Pharmacological Profiles of Morphine and UMB 426* Ki ± SEM (nM) morphine UMB426
EC50 ± SEM (nM)
% Emax ± SEM
pA2
μ
δ
κ
μ
δ
κ
μ
δ
κ
δ
1.7 ± 0.34 33 ± 3.1
87 ± 6.6 121 ± 7.2
69 ± 1.3 920
38 ± 4.9 342 ± 94
316.5 ± 4.9 n/e
484 ± 213 n/e
81 ± 2.3 79 ± 7.3
103 ± 7 nd
62 ± 7 nd
nd 6.31 (−1.08)
*
Receptor binding and [35S] GTPγS functional activity for morphine and UMB 426 are performed in CHO cell membranes stably transfected and overexpressing the human μ, δ, and κ opioid receptors. Competition binding for compounds ([3H]DAMGO for μ, [3H]DPDPE for δ, and [3H]U69,593 for κ) were performed in triplicate of duplicates and reported as mean Ki values ± SEM. Mean EC50 and % Emax values ± SEM for the [35S]GTPγS functional assays were performed in triplicate of duplicates. pA2 is defined as the negative logarithm of antagonist concentration needed to shift the dose response curve by a factor of 2. A slope of at or near −1 is indicative of competitive antagonism for the drug at the receptor B. n/e = no effect, nd = not determined.
alcohol. The lone pair of dihydrofuran oxygen directs the expulsion of HOBBr2 (D) and facilitates the formation of an epoxide ring (E). Then, the bromide ion attacks the double bond, which enables the opening of the epoxide ring with ring expansion. Although these steps are likely concerted, they are depicted in sequential steps for the sake of simplicity. Intermediate (G) undergoes keto−enol tautomerism to change into a more stable configuration: α of UMB426. The X-ray crystal structure of the product UMB426 also confirmed that it is in the α-configuration (Figure 4). We screened several other Lewis acids, InCl3, AlCl3, FeCl3, FeBr3, ZnBr2, and BF3·EtO2, for possible conversion. All of these acids were ineffective, and the starting material was recovered. To further evaluate the reaction, we also showed that the 3-phenolic UMB425 also undergoes the rearrangement reaction with boron tribromide. We found that this reaction was also temperature specific, and there was a complete conversion of starting material to the product (Table 1). We obtained the desired product UMB426 exclusively, in more than 50% yield, at a temperature of −10 °C, and up to 0 °C. For the conversion of UMB425 to UMB426, the screened Lewis acids InCl3, AlCl3, FeCl3, FeBr3, and BF3·EtO2 were also ineffective for any rearrangement. All pharmacological assays were performed as reported in our previous publication.7 The binding affinities of UMB426 and morphine at human μ, δ, and κ opioid receptors are summarized in Table 2. As reported previously, morphine had higher binding affinity for μ receptors, compared to δ and κ receptors (δ/μ = 51; κ/μ = 41). Compared to morphine, the UMB426 receptor affinity ratios were quite different (δ/μ ≈ 4 ; κ/μ ≈ 28), as predicted due to expansion of the ring. Although UMB426 demonstrated greater selectivity for μ receptors relative to δ or κ receptors, compared to morphine it had about 20 times less binding affinity toward μ receptors (Table 2). Agonist and antagonist activity data obtained from [35S]GTPγS functional assays for morphine and UMB426 are summarized in Table 2. As shown in Table 2, UMB426 displayed 10 times less agonistic potency at μ receptors compared to morphine (EC 50 column). As reported previously, morphine was highly efficacious at the δ receptor,12 but UMB426 displayed no significant agonist activity through δ or κ receptors (% Emax column), yet it did demonstrate antagonism at the δ receptor indicated by a corresponding pA2 value (Table 2). In conclusion, we have studied and identified a novel rearranged pyranomorphinan opioid, UMB426. The rearrangement was temperature dependent where a slight variation in temperature affected the product formation, as
the corresponding dihydrobenzopyran is a more subtle change: it modifies the conformation while retaining the bridge with the hydrogen bond accepting oxygen intact (Scheme 2). Such pyranomorphinans have received little attention due to their synthesis involving light-induced rearrangement of 5-methyl-4,5-epoxymorphinans of the metopon class,9 but limited studies showed antinociceptive activity in rodent models (suggesting μ agonism).10 We envisioned a similar ring expansion in the unusual product. Previously, Gates et al.11 and others achieved the cyclization of the furan ring through bromination of dihydrothebainone. Based upon the earlier reports, we looked into the various possible cyclized products. Mass spectrometry analyses, however, ruled out the possibility of structure 3, whereas the NMR analysis, as it showed chemical shifts of the two protons exclusively in the aromatic region, removed the likelihood of structure 4 (Figure 2). We assessed the NMR spectra (2D HMQC and DEPT) based on this proposed structure (Figure 3). The 1H NMR spectra of UMB426 in CDCl3 showed two double doublet peaks at δ 5.24 and δ 4.25, which may be the result of coupling between C−CH and CH2O (H5−H19, UMB426, Figure 3) or may be due to CH−Br and CH2−C− OH (H7−H8, UMB426, Figure 3). Though both protons are equally distant from the carbonyl group which shifts the proton downfield, due to the attachment of the proton at position 7 with electronegative bromine, the double doublets may be shifted further downfield than the proton at position 5 (Figure 4). The structure was further established by 2D HMQC, which correlated a proton attached to the geminal carbon. The most useful information obtained from HMQC was that the carbon at δ 52.4 showed a clear correlation with a double doublet of one proton at δ 5.24. This correlation was further confirmed by a DEPT 90 experiment in which the carbon at δ 52.4 was assigned as CH (see the Supporting Information). To support this assumption, we performed the dehalogenation reaction of UMB426 using Pd/C/H2 and found that in the 1H NMR spectrum of the new compound UMB453 in CDCl3, the double doublet of the proton at δ 5.24 and the carbon peak at δ 52.4 were absent from the 1H NMR and 13C NMR results respectively (Scheme 3). These findings confirmed that the double doublet of one proton at δ 5.24 was the proton on the carbon at position 7 attached to a Br. X-ray crystal structure analysis was consistent with the structure as determined by NMR, and demonstrated the stereochemistry of the 7-bromo substituent as being 7αbromo (see the SI). The proposed mechanism of formation of the rearranged product is as discussed here (Scheme 4). Boron tribromide forms a complex (C) with the methyl C
DOI: 10.1021/acs.orglett.8b01025 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
1136. (c) Costantino, C. M.; Gomes, I.; Stockton, D.; Lim, M. P.; Devi, L. A. Expert Rev. Mol. Med. 2012, 14, e9. (d) Stockton, S. D., Jr.; Devi, L. A. Drug Alcohol Depend. 2012, 121, 167−172. (e) DiMaio, J.; Schiller, P. W. Proc. Natl. Acad. Sci. U. S. A. 1980, 77, 7162−7166. (f) Porreca, F.; Mosberg, H. I.; Hurst, R.; Hruby, V. J.; Burks, T. F. J. Pharmacol. Exp. Ther. 1984, 230, 341−348. (g) Kieffer, B. L.; Evans, C. J. Cell 2002, 108, 587−590. (7) Healy, J. R.; Bezawada, P.; Shim, J.; Jones, J. W.; Kane, M. A.; MacKerell, A. D.; Coop, A.; Matsumoto, R. R. ACS Chem. Neurosci. 2013, 4, 1256−1266. (8) (a) Smith, T. A.; Thatcher, L. N.; Coop, A. Bioorg. Med. Chem. Lett. 2007, 17, 5175−5176. (b) Coop, A.; Rothman, R. B.; Dersch, C.; Partilla, J.; Porreca, F.; Davis, P.; Jacobson, A. E.; Rice, K. C. J. Med. Chem. 1999, 42, 1673−1679. (c) Li, F.; Yin, C.; Chen, J.; Liu, J.; Xie, X.; Zhang, A. Chem. Biol. Drug Des. 2009, 74, 335−342. (d) Spetea, M.; Greiner, E.; Aceto, M. D.; Harris, L. S.; Coop, A.; Schmidhammer, H. J. Med. Chem. 2005, 48, 5052−5055. (9) (a) Schultz, A. G.; Graves, D. M.; Green, N. J.; Jacobson, R. R.; Nowak, D. M. J. Am. Chem. Soc. 1994, 116, 10450−10462. (b) Schultz, A. G.; Napier, J. J.; Sundararaman, P. J. Am. Chem. Soc. 1984, 106, 3590−3600. (10) Mclaughlin, J. P.; Nowak, D.; Sebastian, A.; Schultz, A. G.; Archer, S.; Bidlack, J. M. Eur. J. Pharmacol. 1995, 294, 201−206. (11) (a) Gates, M.; Shepard, S. A. J. Am. Chem. Soc. 1962, 84, 4125−4130. (b) Jones, R. N.; Ramsay, D. A.; Herling, F.; Dobriner, K. J. Am. Chem. Soc. 1952, 74, 2828−2832. (12) Toll, L.; Berzetei-Gurske, I. P.; Polgar, W. E.; Brandt, S. R.; Adapa, I. D.; Rodriguez, L.; Schwartz, R. W.; Haggart, D.; O’Brien, A.; White, A.; Kennedy, J. M.; Craymer, K.; Farrington, L.; Auh, J. S. NIDA Res. Monogr. 1998, 178, 440−466.
well as the yield of the desired product. In vitro data has shown that UMB426 has a profile of μ agonism/δ antagonism, albeit of low potency. We are continuing to investigate the pharmacological effects of UMB426 derivatives, especially the removal of the potentially reactive 7-bromine.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b01025. Full experimental procedures; spectroscopic data (1 H and 13C NMR spectra) of all new compounds; X-ray crystal data of UMB426 (PDF) Accession Codes
CCDC 1814751 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Andrew Coop: 0000-0002-4913-9447 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by the National Institutes of Health (R01 DA013583 and supplement for J.R.H.). The X-ray crystallographic work was supported by NIDA through an Interagency Agreement (IAA) ADA17001 with the Naval Research Laboratory (NRL). The authors also thank the University of Maryland School of Pharmacy Mass Spectrometry Centre for their help with HPLC−MS.
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REFERENCES
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DOI: 10.1021/acs.orglett.8b01025 Org. Lett. XXXX, XXX, XXX−XXX
