Search tips
Search criteria 


Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
Bioorg Med Chem Lett. Author manuscript; available in PMC 2010 May 1.
Published in final edited form as:
PMCID: PMC2677726

(±)-Nantenine analogs as antagonists at human 5-HT2A receptors: C1 and flexible congeners


C1 and flexible analogs of (±)-nantenine were synthesized and evaluated for antagonist activity at human 5-HT2A receptors in a calcium mobilization assay. This work has resulted in the identification of the most potent 5-HT2A antagonist known based on an aporphine. Our results also suggest that the C1 position may be a key site for increasing 5-HT2A antagonist activity in this compound series. In addition, the structural rigidity of the aporphine core appears to be required for nantenine to function as a 5-HT2A antagonist.

The monoamine neurotransmitter, serotonin (5-hydroxytryptamine, 5-HT, 1) is known to play a significant role in the central nervous system (CNS) modulation of appetite, mood, body temperature and sleep in humans.1 There are fourteen serotonin receptors presently known, of which all except one (5-HT3) are G-protein coupled receptors.2 Ligands for the 5-HT2A receptor are constantly being developed as chemical tools to study the functional role of this receptor in hallucinations, depression, anxiety and psychosis.37 The role of 5-HT2A receptor blockade in the neuropharmacological processes of addiction is also of growing interest.813

Aporphines are a diverse group of tetracyclic alkaloids found in several plant species and have been found to show a range of interesting biological activities such as antiplasmoidal, antihelminthic and antitumor activities.1418 As a result of their biological activities, new and facile synthetic methodologies to prepare these compounds are always being explored.19, 20

Others have reported the 5-HT2A antagonist properties of the aporphine alkaloid nantenine (2). 21, 22 This pharmacodynamic property seems to be relevant to it’s in vivo activity as an antagonist of the designer drug MDMA (methylenedioxymethamphetamine, “Ecstasy”).23 Although aporphines have been evaluated as 5-HT1A, α-adrenergic, and dopaminergic D1 and D2 ligands, very little SAR work has been performed on aporphines as 5-HT2A antagonists.21, 2428 Part of our program is geared towards understanding the importance of selective receptor blockade as well as multi-potent antagonism involving 5-HT2A receptors in the antagonism of MDMA-induced effects. Aporphines may be a valuable structural template for such a study, given the apparent promiscuity of these compounds across various CNS targets including 5-HT subtypes. As such, we have embarked on a study to evaluate the 5-HT2A antagonist activity of aporphines using 2 as a lead molecule. In this communication, we report results on the synthesis and evaluation of C1 and less rigid nantenine analogs.

Preparation of C1 analogs commenced with readily available29 4-benzyloxy-3-methoxy phenethylamine (3) which was condensed with bromoacid (4) under standard peptide coupling conditions (Scheme 1). The amide (5) thus produced was cyclized under Bischler-Napieralski30 conditions to afford imine 6, which was immediately reduced to give 7 without purification (due to apparent instability of the imine).

Scheme 1
Reagents and conditions: (a) GDI, THF, rt, 20 h, 90% (b) PCI5, DCM, 0 °C - rt, 96% (c) NaBH4, MeOH, 0 °C, 4h, 99% (d) ethyl chloroformate, K2CO3, DCM, rt, 12h, 76% (e) Pd(OAc)2, ligand A, K2CO3, (CH3)3CCOOH, DMA, 130 °C,17h, 76% ...

Following protection of amine 7 as the ethyl carbamate, we were now in a position to attempt the direct biaryl cyclization31 of 8 to give the aporphine core - a key step in this synthetic route. Following similar methodology to that previously described31 with some optimization of reaction conditions, we obtained 9 in 76% yield. Hydrogenolysis of the C1 benzyl group gave key intermediate 10 which served as a precursor for the C1 alkyl intermediates 11a11f.

Our plan at this juncture was to reduce the carbamate functionalities of 11a11f with LAH to provide the C1 target analogs 12a12e and nantenine (2).

However, when LAH reduction was attempted, cleavage of the carbamate group occurred, giving the corresponding secondary amines as major products. This result was surprising and is in need of further investigation. Nevertheless, subsequent reductive amination of the derived secondary amines with formaldehyde afforded the target analogs 12a12e and 2. The C1 benzyl derivative 13 was accessed via LAH reduction of 9.

To begin to evaluate the role of molecular rigidity on the 5-HT2A activity of nantenine, we prepared the benzyltetrahydroisoquinoline 16 and tertiary amine 18 (Scheme 2). Thus, following similar procedures as in Scheme 1, the readily available amide 14 was cyclized under Bischler-Napieralski conditions. Subsequently, the imine prepared was reduced and then subjected to reductive amination conditions providing the seco-ring C derivative 16. Borane reduction of amide 14 followed by N-alkylation gave compound 18. All compounds were characterized with routine spectroscopic techniques including 1H NMR, 13C NMR and HRMS.

Scheme 2
Reagents and conditions:(a) PCI5, DCM, 0 °C - rt (b) NaBH4, MeOH, rt, (c) aq. HCHO, NaBH(OAc)3, DCM, rt, 62% over 3 steps (d) BF3 OEt2, BH3THF, THF, 88%

Compounds 2, 12a12e, 13, 16 and 18 were evaluated for functional activity at human 5-HT2A receptors using a calcium mobilization assay.32 Results are presented in Table 1. The less rigid analogs, ie compounds 16 and 18 had significantly reduced antagonist activities as compared to nantenine, suggesting that the structural rigidity of the aporphine nucleus is required for 5-HT2A activity. Increasing the C1 alkyl chain length by one carbon (compound 12a) had little effect on 5-HT2A antagonist activity. However, incremental additions of 2, 3 and 4 carbons gave a progressive increase in 5-HT2A antagonist activity (12b, 12c, 12d). Replacement of the C1 methyl group of nantenine with a methylenecyclopropyl moiety (compound 12e) resulted in a twelve-fold enhancement in activity as compared to nantenine. Interestingly, the C1 benzyl analog was found to be a negative allosteric modulator (IC50 = 4600 nM). In keeping with attributes of successful CNS agents, 33 the ClogP values of the C1 analogs represent reasonable starting points for simultaneous optimization of pharmacodynamic properties and blood-brain barrier penetrability in this compound series.

Table 1
Apparent affinity of nantenine analogs at human 5-HT2A receptors and ClogP values32

In conclusion, our biological results indicate that increasing the length of the C1 alkyl chain beyond two carbon atoms, results in an increase in 5-HT2A antagonist activity in this series of aporphines. The C1 site may thus be a key position for further structural modifications to increase 5-HT2A antagonist activity. Compound 12e 34 was the most active compound identified and showed a 12-fold increase in activity as compared to nantenine. This compound is the most potent 5-HT2A antagonist known with an aporphine skeleton. Our work has also identified a low activity negative allosteric modulator, 13. We are continuing to explore the SAR of other nantenine-derived aporphines at human 5-HT2A receptors and will report our findings in due course.

Figure 1
Structures of serotonin (1) and Nantenine (2)


OL acknowledges the RISE program at Hunter College for financial support. This publication was made possible by Grant Number RR03037 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health.


Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

References and Notes

1. Nichols DE, Nichols CD. Chem Rev. 2008;108:1614. [PubMed]
2. Costall B, Naylor RJ. Curr Drug Targets. 2004;3:27. [PubMed]
3. Schmidt CJ, Sorensen SM, Kehne JH, Carr AA, Palfreyman MG. Life Sci. 1995;56:2209. [PubMed]
4. Olivier B, Mos J, van Oorschot R, Hen R. Pharmacopsychiatry. 1995;282(Suppl):80. [PubMed]
5. Pae CU, Serretti A, Patkar AA, Masand PS. CNS Drugs. 2008;22:367. [PubMed]
6. Moller HJ. Expert Opin Pharmacother. 2005;6:803. [PubMed]
7. Graeff FG, Guimaraes FS, De Andrade TG, Deakin JF. Pharmacol,, Biochem Behav. 1996;54:129. [PubMed]
8. Auclair A, Drouin C, Cotecchia S, Glowinski J, Tassin JP. Eur J Neurosci. 2004;20:3073. [PubMed]
9. Herin DV, Liu S, Ullrich T, Rice KC, Cunningham KA. Psychopharmacology. 2005;178:505. [PubMed]
10. Szucs RP, Frankel PS, McMahon LR, Cunningham KA. Behav Neurosci. 2005;119:1173. [PubMed]
11. Zaniewska M, McCreary AC, Przegalinski E, Filip M. Eur J Pharmacol. 2007;571:156. [PubMed]
12. Lanteri C, Salomon L, Torrens Y, Glowinski J, Tassin JP. Neuropsychopharmacology. 2008;33:1724. [PubMed]
13. Levin ED, Slade S, Johnson M, Petro A, Horton K, Williams P, Rezvani AH, Rose JE. Eur J Pharmacol. 2008;600:93. [PMC free article] [PubMed]
14. Rasoanaivo P, Ratsimamanga-Urverg S, Rafatro H, Ramanitrahasimbola D, Palazzino G, Galeffi C, Nicoletti M. Planta Med. 1998;64:58. [PubMed]
15. Ayers S, Zink DL, Mohn K, Powell JS, Brown CM, Murphy T, Brand R, Pretorius S, Stevenson D, Thompson D, Singh SB. Planta Med. 2007;73:296. [PubMed]
16. Abdalla S, al-Khalil S, Afifi F. Gen Pharmacol. 1991;22:253. [PubMed]
17. Stevigny C, Bailly C, Quetin-Leclercq J. Curr Med Chem Anticancer Agents. 2005;5:173. [PubMed]
18. Huang RL, Chen CC, Huang YL, Ou JC, Hu CP, Chen CF, Chang C. Planta Med. 1998;64:212. [PubMed]
19. Hamamoto H, Shiozaki Y, Nambu H, Hata K, Tohma H, Kita Y. Chem Eur J. 2004;10:4977. [PubMed]
20. Orito K, Uchiito S, Satoh Y, Tatsuzawa T, Harada R, Tokuda M. Org Lett. 2000;2:307. [PubMed]
21. Indra B, Matsunaga K, Hoshino O, Suzuki M, Ogasawara H, Ishiguro M, Ohizumi Y. Can J Physiol Pharmacol. 2002;80:198. [PubMed]
22. Tsuchida H, Ohizumi Y. Eur J Pharmacol. 2003;477:53. [PubMed]
23. Fantegrossi WE, Kiessel CL, Leach PT, Van Martin C, Karabenick RL, Chen X, Ohizumi Y, Ullrich T, Rice KC, Woods JH. Psychopharmacology. 2004;173:270. [PubMed]
24. Zhang A, Zhang Y, Branfman AR, Baldessarini RJ, Neumeyer JL. J Med Chem. 2007;50:171. [PubMed]
25. Si YG, Gardner MP, Tarazi FI, Baldessarini RJ, Neumeyer JL. Bioorg Med Chem Lett. 2007;17:4128. [PubMed]
26. Cannon JG, Flaherty PT, Ozkutlu U, Long JP. J Med Chem. 1995;38:1841. [PubMed]
27. Ivorra MD, Valiente M, Martinez S, Madrero Y, Noguera MA, Cassels BK, Sobarzo EM, D’Ocon P. Planta Med. 2005;71:897. [PubMed]
28. Westkaemper RB, Yousif M, Teitler M, Glennon RA. Med Chem Res. 1992;2:482.
29. Batra S, Sabnis YA, Rosenthal PJ, Avery MA. Bioorg Med Chem. 2003;11:2293. [PubMed]
30. Wang YC, Georghiou PE. Org Lett. 2002;4:2675. [PubMed]
31. Lafrance M, Blaquiere N, Fagnou K. Chem Commun. 2004:2874. [PubMed]
32. Calcium Mobilization Human 5-HT2A Receptor Functional Assays. Stably expressed human 5-HT2A receptor in CHO-K1 cells (ATCC), were used for the calcium mobilization functional assay. The calcium 4 dye assays (Molecular Devices, Sunnyvale, CA) were run according to manufacturer’s specifications. Briefly, wells of black clear-bottom 96-well tissue culture-treated plates were seeded with 20,000 cells the afternoon before assay. On the day of assay, the cells were incubated with the calcium indicator dye for 1 hr @ 37 °C. The test compounds were preincubated with the cells during the last 15 min of the dye incubation. The plate was then placed into a FlexStation pre-warmed to 37 °C. Basal or unstimulated fluorescence intensity was recorded for 13 sec followed by the addition of 5-HT (antagonist and Ke assays). Fluorescence intensity was recorded for an additional 47 s. The effect of test compound was determined by subtracting the minimum from the maximum fluorescence recorded for each well during the 47 s recording period. Thus, each well served as its own control. All samples were run in duplicate.
33. Pajouhesh H, Lenz GR. Neuro Rx. 2005;2:541. [PubMed]
34. Compound 12e. 1H NMR (CDCl3, 500 MHz): δ0.12 (m, 2H), 0.48 (m, 2H), 1.15 (m, 1H), 2.50 –2.54 (obscured, 2H), 2.53 (s, 3H), 2.67 (dd, J = 3.2, 16.3 Hz, 1H), 2.97 (m, 2H), 3.03 (dd, J = 5.7, 11.4 Hz, 1H), 3.13 (m, 1H), 3.40 (dd, J = 7.6, 10.0 Hz, 1H), 3.71 (dd, J = 7.0, 10.0 Hz, 1H), 3.86 (s, 3H), 5.96 (d, J = 1.1 Hz, 1H), 5.98 (d, J = 1.1 Hz, 1H), 6.57 (s, 1H), 6.74 (s, 1H), 8.07 (s, 1H); 13C NMR (CDCl3, 125 MHz): δ 3.1, 3.4, 11.0, 29.1, 35.1, 44.0, 53.2, 55.8, 62.5, 77.8, 100.8, 108.2, 109.5, 110.4, 125.9, 127.1, 127.5, 128.5, 130.6, 143.3, 146.2, 146.3, 152.1; HRESIMS Calcd for C23H25NO4: 379.1784. Found: 379.1783.