Gram-scale, chemoselective synthesis of N-[2-(5-hydroxy-1H-indol-3-yl)ethyl]-2-oxopiperidine-3-carboxamide (HIOC)

Article abstract of DOI:10.1016/j.tetlet.2015.01.167

N-[2-(5-Hydroxy-1H-indol-3-yl)ethyl]-2-oxopiperidine-3-carboxamide (HIOC) is a potent activator of the TrkB receptor in mammalian neurons and of interest because of its potential therapeutic uses. In the absence of a commercial supply of HIOC, we sought to produce several grams of material. However, a synthesis of HIOC has never been published. Herein we report the preparation of HIOC by the chemoselective N-acylation of serotonin, without using blocking groups in the key acylation step.

Full text of DOI:10.1016/j.tetlet.2015.01.167

Accepted Manuscript
Gram-scale, chemoselective synthesis of N-[2-(5-hydroxy-1H-indol-3-yl)eth-
yl]-2-oxopiperidine-3-carboxamide (HIOC)
Noah A. Setterholm, Frank E. McDonald, Jeffrey H. Boatright, P. Michael
Iuvone
PII:
S0040-4039(15)00217-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.01.167
TETL 45837
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 December 2014
22 January 2015
26 January 2015
Please cite this article as: Setterholm, N.A., McDonald, F.E., Boatright, J.H., Michael Iuvone, P., Gram-scale,
chemoselective synthesis of N-[2-(5-hydroxy-1H-indol-3-yl)ethyl]-2-oxopiperidine-3-carboxamide (HIOC),
Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.01.167
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Gram-scale, chemoselective synthesis of
N-[2-(5-hydroxy-1H-indol-3-yl)ethyl]-2-
oxopiperidine-3-carboxamide (HIOC)
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Noah A. Setterholm, Frank E. McDonald*, Jeffrey H. Boatright, and P. Michael Iuvone

1
Tetrahedron Letters
journal homepage: www.els evier.com
Gram-scale, chemoselective synthesis of N-[2-(5-hydroxy-1H-indol-3-yl)ethyl]-2-
oxopiperidine-3-carboxamide (HIOC)
Noah A. Setterholma, Frank E. McDonalda,* Jeffrey H. Boatrightb,c, and P. Michael Iuvoneb,d
aDepartment of Chemistry, Emory University, 1515 Dickey Drive NE, Atlanta, GA 30322 USA
bDepartment of Ophthalmology, Emory University School of Medicine, 1365B Clifton Road NE, Atlanta, GA 30322 USA
cRehabilitation R&D Center, Atlanta VA Medical Center, Decatur, GA USA 30033 USA
dDepartment of Pharmacology, Emory University School of Medicine, 1510 Clifton Road NE, Atlanta, GA 30322 USA
ARTICLE INFO
ABSTRACT
Article history:
Received
N-[2-(5-hydroxy-1H-indol-3-yl)ethyl]-2-oxopiperidine-3-carboxamide (HIOC) is a potent
activator of the TrkB receptor in mammalian neurons and of interest because of its potential
therapeutic uses. In the absence of a commercial supply of HIOC, we sought to produce several
grams of material. However, a synthesis of HIOC has never been published. Herein we report
the preparation of HIOC by the chemoselective N-acylation of serotonin, without using blocking
groups in the key acylation step.
Received in revised form
Accepted
Available online
Keywords:
2014 Elsevier Ltd. All rights reserved.
amide synthesis
chemoselectivity
decarboxylation
kinase activation
serotonin
In fond memory of Prof. Harry Wasserman, recognizing his humanity, scholarship, and service.
1. Introduction
Tropomyosin related kinase
B (TrkB) is a neuronal
transmembrane receptor protein in humans and other mammals.
Small proteins such as brain-derived neurotropic factor (BDNF)
bind to the extracellular portion of TrkB, triggering
autophosphorylation of tyrosine residues in its intracellular
domain. This phosphorylation then initiates cascade-signaling
pathways known to promote neuronal differentiation and
survival. Several small molecules have been identified as
agonists of Trk receptors,2 including N-acetylserotonin (NAS, 2,
Figure 1) as a TrkB activator.3 In the course of investigating the
TrkB activity of other serotonin derivatives, N-[2-(5-hydroxy-
1H-indol-3-yl)ethyl]-2-oxopiperidine-3-carboxamide (HIOC, 3)
has displayed greater activation of TrkB, and exhibits a longer in
vivo half-life than NAS.4 HIOC has also demonstrated protective
activity in an animal model for light-induced retinal
degeneration, and can pass the blood-brain and blood-retinal
barriers.4 Thus, HIOC is a compound with high therapeutic
potential, provided that this compound can be reliably prepared
on a scale suitable for animal studies.
Figure 1. Structures of serotonin and derivatives, including HIOC (3)
Figure 2. Retrosynthesis of HIOC
At first glance, HIOC would appear to be trivially prepared
from the N-acylation of serotonin (Figure 2). In practice, there
are a number of plausible challenges:
Although HIOC (3) is described in patents,5 a method for its
preparation has not been disclosed. Moreover, this compound is
not consistently commercially available. Herein we describe a
method for the gram-scale synthesis of HIOC.
1) serotonin can undergo acylation at three sites;
2) the carboxylic acid synthon 4 may potentially undergo
decarboxylation upon standing; and
_________
3) serotonin-HCl 1 and the carboxylic acid 4 both exhibit poor
solubility in most organic solvents.
* Corresponding author.
Email address: frank.mcdonald@emory.edu (F. E. McDonald).

2
Tetrahedron Letters
2. Results and Discussion
Carboxylic acid synthon 4 was prepared by basic hydrolysis of
the commercially available ethyl ester 5.6,7 However, the
literature did not provide a method for isolating this water-
soluble compound. After some optimization we found that
treatment of ester 5 with KOH in water, followed by addition of
concentrated HCl and saturation of the aqueous layer with solid
NaCl allowed for extraction of the desired acid 4 with a solvent
mixture of chloroform and isopropanol (Scheme 1).8
Scheme 1. Preparation and isolation of 2-oxo-3-piperidinecarboxylic acid (4)
We initially attempted to couple the carboxylic acid 4 with
serotonin-HCl (1) by activating the acid using carbodiimide
reagents9 as well as through mixed anhydride formation.10 In the
case of the carbodiimide reagents, a small amount of HIOC was
detected via LC-MS, but could not be isolated. Attempts to
activate 4 with methyl chloroformate furnished the methyl ester
of 4 as the only identifiable product. Efforts using pivaloyl
chloride were also unsuccessful, resulting only in pivaloylation
of the primary amine of serotonin. After screening additional
methods we found that the carboxylic acid could be activated
with carbonyl diimidazole (CDI).11 Addition of CDI to an
opaque suspension of carboxylic acid 4 in dichloromethane
provided complete conversion to the soluble N-acyl imidazole
derivative 6 (Scheme 2). Addition of serotonin-HCl (1) to
intermediate 6 gave some conversion to the desired amide 3.
This acylation procedure was plagued by the poor solubility of
serotonin-HCl in CH2Cl2, and N,O-bisacylated product 7 was
often observed from these heterogeneous reaction conditions,
along with some recovered unacylated serotonin. However,
dimethylformamide (DMF) solubilized all reaction components,
giving 66% conversion to HIOC (3) after 3 hours, along with
only 6% of the bisacylated product 7 (Table 1, entry 1). The
product 3 was very difficult to extract from an aqueous acidic
workup of the reaction mixture, so we sought an alternative to
DMF as the solvent.
Scheme 2. Acylation of serotonin with carboxylic acid 4
Table 1. Optimization of Reaction Conditions
entry
solvents
conversion by 1H NMR
serotonin-HCl
(1)
HIOC
(3)
7
1
2
3
DMFa
25%
68%
33%
66%
29%
66%
6%
3%
<1%
1 : 1 CH2Cl2 : Et3Nb
1 : 1 CH2Cl2 : pyridineb
a CDI was added to a 0.3 M solution of 4 in DMF. After 30 min, another
volume of DMF was added followed by 1 equiv of 1.
b CDI was added to a 0.3 M suspension of 4 in CH2Cl2. After aging 30
min, an equal volume of base (triethylamine or pyridine) was added,
followed by 1 equiv of 1.
Adding an equal volume of triethylamine to the reaction
mixture in CH2Cl2 prior to the addition of serotonin-HCl (1) gave
some conversion to product, however the serotonin-HCl was not
completely solubilized, leading to a small amount of bisacylated
product 7 (entry 2). On the other hand, addition of an equal
volume of pyridine (~40 equivalents prior to addition of
serotonin-HCl 1) resulted in a completely homogeneous reaction
mixture (entry 3). After 3 hours, the reaction had proceeded to
66% conversion and high chemoselectivity favoring HIOC (3),
with only a trace amount of N,O-bisacylated product detected by
NMR.12 The addition of 2 equivalents of triethylamine helped to
push the reaction to completion over the course of 3 hours,
bringing the conversion up to ~85% (Scheme 3). Isolation of the
product from the reaction mixture was non-trivial. Concentration
of the reaction mixture followed by dry loading of the unwieldy
crude onto a silica gel column allowed for chromatographic
separation with ethyl acetate / methanol (9 : 1) as eluent. The
resulting solid was washed with hot diethyl ether to remove
remaining imidazole impurities to furnish HIOC (3) in 71% yield
(Scheme 3).
Scheme 3. Optimized acylation for the synthesis of HIOC (3)
In summary, we have developed a short, scalable, and highly
reproducible synthesis of HIOC without the need for protective
groups. The key discovery was the utility of the additional
volume of pyridine to the acyl-imidazole species, which helped
to solubilize the serotonin hydrochloride and also may have
behaved as a nucleophilic catalyst.

3
Acknowledgments
This work was financially supported by DoD W81XWH-12-1-
0436, NIH R01EY004864, R01EY014026, P30EY006360, the
Abraham J. and Phyllis Katz Foundation, and Research to
Prevent Blindness (RPB). PMI is a recipient of the RPB Senior
Scientific Investigator Award. We also thank Dr. Brooke
Katzman (Emory University) for assistance with LC/MS.
DoD Non Endorsement Disclaimer
The views, opinions and/or findings contained in this research
are those of the authors and do not necessarily reflect the views
of the Department of Defense and should not be construed as an
official DoD/Army position, policy or decision unless so
designated by other documentation. No official endorsement
should be made.
References and notes
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7. Compound 4 (2-oxo-3-piperidinecarboxylic acid) is listed in the
Sigma-Aldrich catalog, CBR01770. We determined that a
commercial sample (lot# B00037799) was merely valerolactam,
corresponding to decarboxylation of compound 4.
8. Continuous extraction of the salted aqueous layer with
dichloromethane was also investigated, but tended to give reduced
yields.
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Bioorg. Med. Chem. 2002, 10, 4143-4154.
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12. The diminished yield of bisacylated product 7 relative to HIOC (3)
may arise from pyridine-promoted transacylation of serotonin-
derived esters, although this has not been proven.
Supplementary Material
Experimental procedures, characterization data, 1H and 13C
NMR spectra for 2-oxopiperidine-3-carboxylic acid (4) and
HIOC (3).