Green chemistry approach to the synthesis of N-substituted piperidones

Article abstract of DOI:10.1021/jo026848i

An efficient green chemistry approach to the synthesis of N-substituted piperidones and piperidines was developed and applied to the synthesis of 1-(2-pyridinylmethyl)-piperidin-4-one, 1, a key starting material for the synthesis of LY317615, an antiangio

Full text of DOI:10.1021/jo026848i

CHART 1
Gr een Ch em istr y Ap p r oa ch to th e
Syn th esis of N-Su bstitu ted P ip er id on es
Margaret M. Faul, Michael E. Kobierski, and
Michael E. Kopach*
Lilly Research Laboratories, A Division of Eli Lilly and
Company, Chemical Process Research and
Development Division, Eli Lilly and Co.,
Indianapolis, Indiana 46285-4813
kopach_michael@lilly.com
Received December 13, 2002
Abstr a ct: An efficient green chemistry approach to the
synthesis of N-substituted piperidones and piperidines was
developed and applied to the synthesis of 1-(2-pyridinyl-
methyl)-piperidin-4-one, 1, a key starting material for the
synthesis of LY317615, an antiangiogenic agent currently
under development at Eli Lilly and Company (Chart 1).1 The
general utility of this methodology, which presents signifi-
cant advantages over the classical Dieckman approach to
this class of compounds, was also demonstrated by the direct
synthesis of a series of substituted piperidones and pip-
eridines, including potential dopamine D4 receptor antago-
nists 2 and 3, that have been evaluated in the clinic as
antipsychotic agents (Chart 2).2
CHART 2
SCHEME 1
To support clinical evaluation of LY317615, multiki-
logram quantities of 1-(2-pyridinyl-methyl)-piperidin-4-
one, 1, were required (Chart 1). A synthesis of 1 in 50%
yield was reported by Hosken via a classical three-step
sequence, involving a bis-Michael addition of 2-(amino-
methyl)-pyridine, 4, with ethyl acrylate, 5, to generate 6
followed by Dieckman cyclization and base-catalyzed
decarboxylation (Scheme 1).3 This method represents the
most general approach to the synthesis of N-substituted
piperidones reported in the literature.4
Our initial scale-up of the Dieckman cyclization se-
quence to produce kilogram quantities of 1 resulted in
significant processing problems that included (1) a need
for a large excess of ethyl acrylate (7 equiv) to ensure
complete formation of the bis-Michael adduct 6; (2) long
reaction times (7-10 days); (3) a need to completely
remove residual ethyl acrylate from 6 prior to the
Dieckman cyclization, otherwise the yield and quality of
1 were significantly reduced; and (4) significant problems
in isolation of 1, following decarboxylation, and partition-
ing of 1 into organic solvents proved to be a significant
challenge due its high aqueous solubility. In fact, five
extractions of the aqueous layer with CH2Cl2 were
required to achieve efficient isolation of 1 in 70% yield.5
Furthermore, a competing side reaction with CH2Cl2
produced a troublesome chloroiminium salt impurity.6
Overall the Dieckman process for preparing 1 was
inefficient and required multiple solvents and cumber-
some aqueous workups. In addition, the dilute reaction
conditions required for scale-up resulted in large genera-
tion of solvent and reagent waste streams that were
environmentally undesirable.
An alternate approach to N-substituted piperidones
developed by Kuehne has some advantages over the
classical Dieckman conditions.7 Mainly, the troublesome
bis-Michael addition is avoided since the desired piperi-
done is prepared by an exchange reaction between 4-oxo-
piperidinium iodide, 7, and a primary amine (Scheme 2).
However, shortcomings of this approach are (1) exchange
reactions frequently do not go to completion; (2) the
approach is not applicable to systems where quaternary
amine salts cannot be easily formed; and (3) it lacks the
* Corresponding author.
(1) (a) Faul, M. M., Gillig, J . R.; J irousek, M. R.; Ballas, L. M.;
Schotten, T.; Kahl, A.; Mohr, M. Bioorg. Med. Chem. Lett. 2003, 13,
1857. (b) Faul, M. M.; Grutsch J . L.; Kobierski, M. E.; Kopach, M. E.;
Krumrich, C. A.; Staszak, M. A.; Sullivan, K. A.; Udodong, U.; Vicenzi,
J . T. Tetrahedron. In press.
(2) (a) Belliotti, T. R.; Blankley, C. J .; Kestemn, S. R.; Wise, L. D.;
Wustrow, D. J . U.S. Patent 5,945,421, August 31, 1999. (b) Maryanoff,
C. A.; Reitz, A. B.; Scott, M. K. U.S. Patent, 8 pp, Continuation-in-
part of U.S. Patent 5,314,885.
(3) Hosken, G. D.; Hancock, R. D. J . Chem. Soc., Chem. Comm. 1994,
1363.
(4) (a) Schaefer, J . P.; Bloomfield J . J . Org. React. 1967, 15, 1. (b)
McElvain, S. M. J . Am. Chem. Soc. 1926, 48, 2179. (c) Leonard; N. J .;
Barthel, E., J r. J . Am. Chem. Soc. 1950, 72, 3632. (d) McElvain, S.
M.; Stork, G. J . Am. Chem. Soc. 1946, 68, 1049. (e) Reed; Cook. J .
Chem. Soc. 1945, 399. (f) Dickerman; Lindwall. J . Org. Chem. 1949,
14, 530. (g) Bolyard, N. W.; McElvain, S. M. J . Am. Chem. Soc. 1929,
51, 922. (h) Elpern, B.; Wetterau, W.; Carabateas, P.; Grumbach, L.
J . Am. Chem. Soc. 1958, 80, 4916.
(5) Partition coefficients: CH2Cl2 k ) 8.6, EtOAc k ) 0.9, MTBE k
) 0.8.
(6) Hansen, S. H.; Nordholm, L. J . Chromatogr. 1981, 204, 97.
(7) (a) Kuehne, M.E.; Muth R. S. J . Org. Chem. 1991, 56, 2701. (b)
Kuehne, M.E.; Matson, P. A.; Bornmann, W. G. J . Org. Chem. 1991,
56, 513. (c) Tschaen, D. M.; Abramson, L.; Cai, D.; Desmond, R.;
Dolling, U.; Frey, L.; Karady, S., Shi, Y.; Verhoeven, T. R. J . Org. Chem.
1995, 60, 4324. (d) Tortolani, D.; Poss, M. Org. Lett 1999, 1, 1261.
10.1021/jo026848i CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/06/2003
J . Org. Chem. 2003, 68, 5739-5741
5739

SCHEME 2
SCHEME 3
ments, deprotection of the intermediate ketal was slug-
gish.10 This led to investigation of a more efficient ap-
proach to 1 using commercially available 4-piperidone
hydrochloride monohydrate, 9. Since in situ liberation
of the free base of 11a was successful vide infra, it was
anticipated that this approach would also be effective
using 9. The transformation proceeded smoothly using
3-4 equiv of powdered Na2CO3 or K2CO3 in acetonitrile
at 70 °C.11 Under these conditions, alkylation was com-
plete in 4-6 h and 1 was isolated in 90% yield after
filtration of the salts and removal of solvent in vacuo.
Powdered Na2CO3 or K2CO3 was comparable and clearly
produced the best results, while reactions using granular
forms of the above reagents were sluggish and produced
1 in lower yield and purity.12 The alkylation was suc-
cessful with a variety of other bases such as Hunig’s base
and NaHCO3 but was unsuccessful with triethylamine,
DBU, pyridine, and morpholine.13 However, use of amine
bases requires aqueous workup, which on the basis of
our experience would be difficult for 1. In addition, the
main waste byproduct via the carbonate base alkylation
strategy was CO2, which was desirable from an environ-
mental perspective relative to amine bases and facilitates
workup. The best solvents for the alkylation were polar
aprotic solvents such as acetonitrile or DMF as per lit-
erature precedent.14 Ultimately, acetonitrile was chosen
as the solvent for scale-up because waste streams were
minimized and an aqueous workup was not required.15
Alternate leaving groups (X ) Br, I, OMs) were
evaluated in the alkylation but afforded 1 in reduced
yield and with increased levels of impurities due mainly
to quaternization. Reduction of the reaction temperature
broad generality of the Dieckman condensation. In ad-
dition, hindered amines are known to produce cross-
coupled side products.8 However, utilizing Kuehne’s
methodology successfully afforded 1 in 70% yield via an
exchange reaction between 7 and 4 (Scheme 2).7a,b Iodide
salt 7 was prepared in 75% yield by treatment of
commercially available 4-methyl-piperidone with CH3I in
diethyl ether. A mixture of 4 and 7 was then stirred in
aqueous ethanol at 90 °C in the presence of excess K2-
CO3 until the exchange reaction was complete. The
disadvantage to Kuehne’s approach was that aqueous
workup was tedious and required multiple extractions
with CH2Cl2 to isolate 1 from the aqueous layers as per
the Dieckman process. The third approach evaluated for
preparation of 1 was reductive amination of 2-pyridine-
carboxaldehyde, 8, with 4-piperidone, 9, or ketal 10,
which produced impure mixtures of 1 due to competitive
reduction of 8 to 2-(hydroxymethyl)-pyridine and self-
condensation of 9.9
Although the above modifications have been beneficial
in the specific examples for which they were developed,
a direct approach to N-substituted piperidones that is
applicable to a wide range of substrates has not been
demonstrated. In this paper, we describe a new general
green chemistry approach to the synthesis of a variety
of N-substituted piperidones and piperidines. An alter-
nate synthesis of N-substituted piperidones, e.g., 1, that
has not been previously reported in the literature,
involves direct alkylation of 2-picolyl chloride with 4-pi-
peridone or the keto-protected derivative thereof. Thus,
it was anticipated that a single-step, one-pot process to
produce 1 could be achieved under mildly basic conditions
(Scheme 3). This new approach to 1 potentially provides
improved atom economy, as well as solvent and waste
stream minimization, and the potential to deliver multi-
kilogram quantities of 1 to support clinical development
of LY317615.
(10) Meth od A. To a solution of 11 (2 mmol) in 2:1 acetic acid/water
(7.5 mL) was added 0.1 mL of concentrated HCl, and the solution was
heated to 60 °C for 18 h. The solution was poured into ethyl acetate
and treated with saturated sodium carbonate solution until the
aqueous layer was basic. The layers were separated, and the aqueous
layer was extracted with additional ethyl acetate. The organic layer
was washed with water followed by saturated NaCl solution and then
concentrated to an oil. NMR showed an 8:1 ratio of product to starting
material. Meth od B. To a solution of 11 (4 mmol) in acetone were
added pyridinium p-toluenesulfonate (1.3 mmol) and a few drops of
water. The resulting solution was heated to reflux for 3 h. NMR of a
reaction aliquot showed complete disappearance of starting material
but also no desired product and a complex mixture of other products.
(11) Na2CO3 (2 equiv) is required to free base 2-picolyl chloride
hydrochloride and 4-piperidone monohydrate hydrochloride in situ.
(12) Typically, 1 is produced in >99% purity by HPLC A.N. analysis
when powdered carbonate base is used. Granular carbonate bases
require longer alkylation reaction times at 70 °C, and up to 10%
quaternization is observed. 2-(hydroxymethyl)pyridine is typically
observed at <0.5% for this reaction.
Initially, construction of 1 was attempted by alkylation
of ketal 10 with 2-picolyl chloride hydrochloride, 11, in
the presence of Na2CO3 in refluxing acetonitrile. While
these conditions quantitatively coupled the above frag-
(8) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C.
A.; Shah, R. D. J . Org. Chem. 1996, 61, 3849.
(9) Aqueous workup was problematic due to the large neutralization
volumes required and the difficulty in separating 1 from residual
2-(hydroxymethyl)-alcohol and related polar impurities. The best
results were achieved with ketal 10, but competitive reduction of 8
(15-30%) was not suppressed. See ref 10 for deprotection conditions.
(13) Main byproducts observed are the amine base/2-picolyl chloride
direct addition product.
(14) Bellouard, F.; Chuburu, F.; Kervarec, N.; Toupet, L.; Triki, S..;
LemMest, Y.; Handel, H. J . Chem. Soc., Perkin Trans. 1 1999, 3499.
5740 J . Org. Chem., Vol. 68, No. 14, 2003

TABLE 1. Exa m p les of Alk yla tion Scop ea
SCHEME 4
yield
(%)
compound
alkylating agent
2-picolyl chloride
2-chloromethyl quinoline 1-phenyl piperazine
2-chloromethylquinoline
2-picolyl chloride
amine
4-piperidone
1
2
3
12
90
80
80
92
4-phenyl-1,2,3,6-THP
4-piperazino aceto-
phenone
13
14
15
16
17
18
19
2-picolyl chloride
2-picolyl chloride
3-picolyl chloride
4-picolyl chloride
2-chloromethyl-quinoline 4-piperidone
2-methoxy-benzyl chloride 4-piperidone
2-chloromethyl-
benzimidazole
2-nitrobenzyl chloride
4-piperidino piperidine 84
methyl)quinone and 2-nitrobenzyl chloride required over-
night reaction at 70 °C for complete reactions to occur.
With these data in hand, we envisioned that this meth-
odology could be extended to the synthesis of potential
dopamine D4 receptor antagonists 2 and 3 (Scheme 4).2
In this manner, commercially available 2-(chloromethyl)-
quinoline and 1-phenyl piperazine were treated with
powdered Na2CO3 in acetonitrile to produce 2 in 80%
yield.17 In addition, 3 was also prepared in 80% yield via
the direct alkylation of 2-(chloromethyl)quinoline with
4-phenyl-1,2,3,6-tetrahydropyridinehydrochloride.18
isonipecotamide
4-piperidone
4-piperidone
35
81
84
86
88
86
4-piperidone
20
4-piperidone
90
a
All amines and alkylating reagents were used as their
hydrochloride salts unless otherwise specified.
to 23 °C produced higher impurity levels than the picolyl
chloride system at 70 °C, which is consistent with a
carbocation mechanism. Thus, in the systems with
stronger activation, the carbocation is generated faster
and has a propensity to undergo side reactions such as
quaternization or reaction with water to produce 2-(hy-
droxymethyl)pyridine. In fact, control experiments of the
title reaction revealed that 2-picolyl chloride free base is
formed at 23 °C with powdered Na2CO3, while the
4-piperidone free base is slowly formed once the reaction
temperature reaches >50 °C. Thus, when picolyl chloride
is used, slow generation of a transient carbocation occurs,
which is an important feature in obtaining a productive
alkylation reaction, but equally important appears to be
rate-limiting formation of the 4-piperidone free base.
Overall, the chemical transformation to produce 1 via
picolyl chloride was excellent (90% yield, single-solvent
system). However, the physical properties of 1 were quite
challenging in that it existed as a low-melting oily solid
in pure form (mp 23-25 °C). In addition, 1 was sensitive
to hydration under ambient conditions and the solid was
converted to an oil as it hydrated. For these reasons, 1
was converted to its CSA salt, which is a stable under
ambient conditions.
Utilizing the standard conditions developed for the
preparation of 1 allowed the preparation of a series of
N-substituted piperidones and piperidines to demon-
strate the general utility of this approach (80-92%, 2,
3, 12-20; Table 1).16 As one might expect, the structural
isomers to 1 (15 and 16) were prepared from the reaction
of 3- and 4-picolyl chloride, respectively, with 4-piperi-
done monohydrate hydrochloride. Predictably, electron-
rich chloromethylarene alkylating reagents such as 2-
methoxybenzyl chloride and 2-(chloromethyl)benzimida-
zole reacted rapidly with 4-piperidone at room temper-
ature to produce piperidines 18 and 19. In contrast, elec-
tron-deficient chloromethyl-arenes such as 2-(chloro-
In summary, we have demonstrated that N-substituted
piperidones can be prepared via a simple one-pot process
using carbonate bases. A straightforward green chemical
process was utilized that appears to have widespread
utility for synthesis of 4-piperidones and piperidines,
including those containing both electron-donating and
-withdrawing groups. For the synthesis of 1, this new
practical approach provides the best yield, atom economy,
and waste stream minimization.
Exp er im en ta l Section
Rep r esen ta tive P r oced u r e for P r ep a r a tion of N-Su b-
stitu ted P ip er id on es (1-3) a n d P ip er id in es (12-20). To a
suspension of 1.0 equiv of alkylating agent and 1.05 equiv of
amine in 10 volumes of acetonitrile (based upon alkylating agent)
was added 3.0 or 4.0 equiv of powdered Na2CO3 (depending upon
the amount of acid equivalents to be neutralized). The mixture
was stirred for 45 min at ambient temperature, 45 min at 40
°C, 45 min at 50 °C, and 45 min at 60 °C and then heated to 70
°C (unless otherwise noted) with vigorous stirring until complete
disappearance of the alkylating agent was noted by TLC or
HPLC. The reaction mixture was allowed to cool to room
temperature and filtered to remove the insoluble solids and then
the filter cake was washed with acetonitrile. The crude products
were purified by flash chromatography and/or crystallization.
Ack n ow led gm en t. The authors would like to thank
Mr. Curtis Miller and Mr. David Anderson for their
contributions to this project.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
and HRMS data for compounds 1-3 and 12-20 are sum-
marized. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O026848I
(15) Throughput for the scale-up process was excellent; 10 volumes
(L/kg of 2-picolyl chloride hydrochloride) were used for the alkylation.
(16) Yield for 14 was 35% due to competitive alkylation of the amide
functionality.
(17) Powdered Na2CO3 (1 equiv) was employed.
(18) Powdered Na2CO3 (2 equiv) was employed.
J . Org. Chem, Vol. 68, No. 14, 2003 5741