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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.3–7 The role of 5-HT2A receptor blockade in the neuropharmacological processes of addiction is also of growing interest.8–13
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.14–18 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, 24–28 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).
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 11a – 11f.
Our plan at this juncture was to reduce the carbamate functionalities of 11a – 11f with LAH to provide the C1 target analogs 12a – 12e 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 12a – 12e 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.
Compounds 2, 12a–12e, 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.
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.
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.
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