Stereocontrol in the synthesis of cyclic amino acids: a new ligand for directed hydrogenation through hydrogen bonding

Article abstract of DOI:10.1039/C9OB00003H

A system for the directed hydrogenation of nitrogen heterocycles is described in which hydrogen is delivered cis to a hydroxymethyl group by a rhodium catalyst with a simple phosphine ligand. The chemistry is applied to the synthesis of the hygric acid moiety of lincomycin and the pipecolic acid moiety of Argatroban. A series of control experiments indicate that the stereoselectivity is a result of a combination of both coordination and hydrogen bonding.

Full text of DOI:10.1039/C9OB00003H

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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Organic and Biomolecular Chemistry
ARTICLE
Stereocontrol in the synthesis of cyclic amino acids: a new ligand
for directed hydrogenation through hydrogen bonding
Vasudeva Rao Gandi,a Bao Nguyen Do Doan,a,b Sivarajan Kasinathan,a Roderick W. Batesa*
Received 00th January 20xx,
Accepted 00th January 20xx
A system for the directed hydrogenation of nitrogen heterocycles in described in which hydrogen is delivered cis to a
hydroxymethyl group by a rhodium catalyst with a simple phosphine ligand. The chemistry is applied to synthesis of the
hygric acid moiety of lincomycin and the pipecolic acid moiety of Argatroban. A series of control experiments indicate that
the stereoselectivity is a result of a combination of both coordination and hydrogen bonding.
DOI: 10.1039/x0xx00000x
www.rsc.org/
In both cases, the substituent is trans to the carboxylic acid group. A
possible method for achieving the stereocontrolled synthesis is,
therefore, by directed hydrogenation of an unsaturated heterocycle
Introduction
The stereocontrolled synthesis of substituted cyclic amino acids is an
important topic, as these moieties occur in a variety of natural and
unnatural molecules of significance (Figure 1). For instance, an n-
propyl substituted hygric acid 1 is present in the antibiotic lincomycin
2.1 Methyl substituted hygric acid moieties have also been found in
other natural products, such as the cavinafungins, the griselimycins
and the malacidins.2 An N-methyl substituted pipecolic acid 3 is
present in the anti-thrombotic drug, argatroban 4.3,4 The
corresponding N-methylated amino acid is present in the cytostatic
agents, the tubulysins.5
(Figure 2).
Figure 2. Retrosynthesis through directed hydrogenation.
Directed hydrogenation, the stereochemical direction of catalytic
hydrogenation by interaction between a polar atom and the catalyst,
has been shown to be a useful method for stereocontrol in organic
synthesis.6 Following an early observation concerning palladium on
carbon,7 the method was refined and made general by Evans8 and by
Crabtree.9 Their methods have remained “state of the art” in the
meantime. Our approach to cyclic amino acids outlined above would
complement those of Goodman10 (exo-cyclic alkenes) and Hulme11
(4,5-unsaturation in five membered rings) and should be applicable
to the synthesis of further novel cyclic amino acids and related
compounds.
The Crabtree and Evans methods rely upon coordination of a polar
group, such as an alcohol, to a cationic rhodium or iridium atom,
further stabilised by other ligands. We were interested in how
substituents on the ligand might interact with the substrate to
Figure 1. Lincomycin and Argatroban.
achieve the same directing effect. Thus,
a ligand-substrate
a. Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,
Singapore 637371.
interaction might be able to enhance the substrate-metal interaction
of the Evans and Crabtree systems and provide an alternative
catalytic system to the heterocycles of interest.
b. Singapore University of Technology and Design, 8 Somapah Road, Singapore
487372.
† Electronic Supplementary Information (ESI) available: Experimental procedures
and 1H and 13C{1H} NMR spectra for compounds 6, 9a, 9b, 9c, 10a, 10b, 10c, 10e,
10f, 10g, 11a, 12, 13, 14, 15, 17, 18, 19, 1 and 3. X-ray structures and other data of
the p-nitrobenzoate esters of compounds 11a and 19. Notes for the general
hydrogenation procedure. DFT calculation details. See DOI: 10.1039/x0xx00000x
Results and Discussion
Development of the method
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We chose a simple ligand to investigate this idea: ligand 6 which is potassium carbonate in this reaction. SubsequenVtierwinAgrticcleloOsnilninge
DOI: 10.1039/C9OB00003H
triphenylphosphine modified by the presence of an ortho- metathesis gave the dihydropyrroles 10abc and 12.
methoxymethyl group on one of the rings. Ligand 6 was prepared by Importantly, hydrogenation of dihydropyrrole 10a under
a modification of the reported method (Scheme 1).12 In particular, “traditional” conditions, using Pd/C as the catalyst, gave
we found it to be advantageous to employ palladium catalysed pyrrolidine 11a as a mixture of diastereoisomers (table 1, entry
coupling of aryl iodide 5 with diphenylphosphine,13 rather than to 1), underlining the need for directing methodology. After
employ an organolithium intermediate or a Grignard reagent.
extensive experimentation, we were pleased to find that a
system consisting of bis(norbornadiene)rhodium(I)
tetrafluoroborate and ligand 6, was effective giving pyrrolidine
10a in high yield and high diastereoselectivity (entry 2).14 An
efficient reaction was only achieved when dichloromethane
was employed as the solvent. In THF, toluene and dioxane, high
diastereoselectivity was observed, but only partial conversion
(26-39%). In methanol, complete conversion was achieved, but
with low diastereoselectivity (2:1). Pyrrolidine 10a was shown
to be the anticipated trans isomer by X-Ray crystallographic
analysis of the corresponding p-nitrobenzoate.15 Importantly,
the reaction could be run on a gram scale using a slightly
increased concentration (0.2 – 0.3 M) with no loss of
diastereoselectivity.
Scheme 1. Ligand synthesis.
A series of dihydropyrrole substrates were prepared as test
substrates for the directed hydrogenation reaction (Scheme 2).
For the dihydropyrrole substrates, the p-toluene sulfonamide 7
of 2-aminobut-3-en-1-ol was N-alkylated with 2-substituted
allyl halides 8 in the presence of cesium carbonate to give
dienes 9abc. Cesium carbonate proved to be superior to
Scheme 2. Substrate synthesis and hydrogenation.
Reagents and conditions: (a) R = Me, X = Cl, Cs2CO3 (2 eq.), DMF, rt, 96%; (b) R = CO2Et, X = Br, Cs2CO3 (1 eq.), DMF, rt, 46%; (c) R = Me, Grubbs’ I, toluene, 70°C, 90%;
(d) R = CO2Et, Grubbs’ II, toluene, 70°C, 93%; (e) see table 1; (f) MeI (3 eq.), NaOH (powdered, 2 eq.), n-Bu4NI, toluene, 40° C, 89%; (g) Ac2O (2 eq.), Et3N, CH2Cl2, 95%;
(h) MsCl, Et3N; (i) NaN3, DMSO, 60°C, 58% (2 steps); (j) PhCO2H, PPh3, toluene, reflux, 95%; (k) PPh3, THF/ H2O (10:1 v/v) then MsCl, Et3N, CH2Cl2, 63% (two steps) (l)
Cs2CO3 (2.5 eq.), DMF, 60°C, 91%; (m) Grubbs’ I, toluene, 70°C, 89%.
when hydrogenation of these substrates was carried out, poor
Interestingly, use of chloro(1,5-cyclooctadiene)rhodium(I) conversion and selectivity was again observed (entries 6 and 7).
dimer resulted in no reaction (entry 3). We believe that the It, therefore, seemed likely that the high diastereoselectivity
presence of chloride inhibits the reaction by coordinating to the was due to hydrogen bonding18 between the free alcohol of
rhodium.16 Given the success of the reaction, the question arose substrate 10a and the methoxy group of ligand 6, as the
as to whether the methoxymethyl group had any effect.17 When presence of both of these functional groups is required. We
ligand 2 was replaced by either triphenylphosphine or tri(o- reasoned that an amide would also act as a strong hydrogen
tolyl)phosphine, poor conversion and selectivity was observed bond donor. However, amide substrate 10f, prepared by
(entries 4 and 5), indicating that the methoxy group is, indeed, Staudinger reaction19 of azide 10e, gave disappointing results
essential. Additionally, we prepared the methyl ether 10b and (entry 8). As this result might also be attributed to the amide
the acetate 10c to test whether the selectivity arises from just moiety acting as a 3-ligand, we also prepared sulfonamide 10g,
substrate to metal coordination. Both of these functional which would also be expected to be an effective hydrogen bond
groups would be expected to have the ability to coordinate, yet, donor. Subjecting this compound to the hydrogenation
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conditions resulted in just 37% conversion with poor that hydrogen bonding not only enhances the stereoselectivity
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DOI: 10.1039/C9OB00003H
diastereoselectivity (entry 9). We therefore concluded that by strengthening the directing effect, but also enhances the
hydrogen bonding is necessary but not sufficient in this system. catalytic efficiency by stabilising the intermediates.20
We propose that the powerful and efficient directing effect
observed is due to a cooperative combination of hydrogen
bonding and coordination (Figure 3). Of the substituents tested
in this position, only the alcohol group is able to provide both
effects. It may be noted that high diastereoselectivity may be
obtained in the absence of hydrogen bonding (entries 4 and 7).
This is likely to be due to simple coordination of the polar group
X to the rhodium centre, as in the system of Evans et al.8 In these
examples the yield is low. This is presumably due to catalyst
Figure 3. Proposed catalyst substrate interactions.
decomposition in these examples. Thus, it may be proposed
Table 1. Hydrogenation reactions
entry
1
2
3
4
5
6
7
8
substr.
10a
10a
10a
10a
10a
10b
10c
10f
10g
12
14
18d
X
OH
OH
OH
OH
R
Me
Me
Me
Me
Me
Me
Me
Me
Me
CO2Et
n-Pr
Me
catalyst(s)a
Pd/Cb
ligand
-
6
6
PPh3
P(o-tol)3
yield
99%
99%
0
50%c
75%c
53%c
48%c
0
37% c
89%
87%
88%
trans:cis
1:1
>99:1
-
10:1
2.5:1
1:1
10:1
-
(nbd)2RhBF4
[Rh(COD)Cl]2
(nbd)2RhBF4
(nbd)2RhBF4
(nbd)2RhBF4
(nbd)2RhBF4
(nbd)2RhBF4
(nbd)2RhBF4
(nbd)2RhBF4
(nbd)2RhBF4
(nbd)2RhBF4
OH
OMe
OAc
NHCOPh
NHMs
OH
6
6
6
6
6
6
6
9
2:1
10
11
12
>99:1
95:5
98:2
OH
OH
a) Rhodium catalysed reactions were carried out in CH2Cl2 (0.05 M) at 100 psi H2 pressure, at room temperature, overnight, using 10 mol% of rhodium complex and 12
mol% of ligand 6. b) The reaction was carried out with a balloon of H2 at room temperature. c) Conversion estimated by 1H NMR. The balance of the material is starting
material. d) Six-membered ring substrate: see Scheme 4.
The substrate 12 with an ester substituent was also reduced in hydrogenation of the corresponding exo-cyclic alkene,1b the
high yield and with high diastereoselectivity to give pyrrolidine only reported method for the synthesis of this amino acid, to
13 (entry 10). Similarly, substrate 14 with a larger alkyl the best of our knowledge and to our surprise, is by degradation
substituent was also reduced efficiently (entry 11; see scheme of lincomycin itself!21
3). In addition, tetrahydropyridine 18, prepared analogously To obtain the absolute stereochemistry, sulfinamide 20 was
from sulfonamide 3, behaved similarly upon reduction to give prepared according to the reported method (Scheme 3).22 We
piperidine 19 in 88% yield with a d.r. of 98:2 (entry 12). Again, initially attempted to retain the t-butyl based protecting
trans stereochemistry was demonstrated by X-ray regime, but found a number of problems, beginning with the
crystallographic analysis of the corresponding p- difficulty of achieving N-allylation.23 The t-butylsulfinyl group
nitrobenzoate.15
was, therefore, removed using HCl in dioxane and the nitrogen
was reprotected as a toluenesulfonamide, yielding (S)-7,
allowing straightforward completion of the synthesis. Allylic
bromide 8c24 was prepared from ethyl butyrate using the
Application of the Method to Synthesis
The method was then applied to the two amino acid synthetic Kulinkovich reaction25 and then used to alkylate the nitrogen of
targets. The antibiotic lincomycin contains trans-4-n- sulfonamide (S)-7 under the conditions that we had already
2
propylhygric acid 1 ((2S, 4R)-N-methyl-4-propylproline). With developed in the racemic series. Ring closing metathesis gave
the exception of a single report of a stereo-unselective the dihydropyrrole (S)-14 which was subjected to our
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hydrogenation conditions (entry 12). In this way, the pyrrolidine excellent diastereoselectivity to (R)-19 (Scheme 4). This
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DOI: 10.1039/C9OB00003H
(S,R)-15 was obtained in high yield and with high material was then converted into amino acid 3, isolated as its
diastereoselectivity. The synthesis was continued by oxidation hydrochloride salt, by oxidation to carboxylic acid 21 with
26
of the alcohol to the carboxylic acid 19 using in situ generated RuO4 and deprotection with sodium naphthalenide.27 After
ruthenium tetroxide under Sharpless conditions.26 N- purification by ion exchange chromatography, the desired
Detosylation was carried out using sodium naphthalenide27 as compound 3 was obtained in 68% yield over two steps. Again,
use of neither magnesium nor calcium in methanol was the observed data for the synthetic material was found to be in
effective. The synthesis was completed by reductive N- good agreement with that reported.5,29
methylation of amine 20,1b,28 to give hygric acid 1, which was
purified by ion exchange chromatography. The observed data
for the synthetic material was found to be in good agreement
with that reported.1b, 21
Conclusions
Syntheses of the hygric acid moiety of lincomycin and the
pipecolic acid moiety of Argatroban have been completed,
employing a novel catalyst system for control of the relative
stereochemistry of the substituents. The method promises to
be applicable to further targets. Hydrogen bonding is proposed
as a key factor in the catalytic directed hydrogenation. Famously
invoked as the cause of a directing effect by Henbest,16
hydrogen bonding much more often considered in
organocatalysis31 than transition metal catalysis. There is only a
modest number of examples in which hydrogen bonding is
proposed as the cause of regiochemical or stereochemical
direction.32 It is, therefore, an example of a non-covalent
interaction33 that has under-exploited potential in catalyst
design.
Experimental
Scheme 3. Lincomycin amino acid synthesis.
General procedure for the directed hydrogenation.
Reagents and conditions: (a) MeOH, AcCl, dioxane; (b) TsCl, Et3N, CH2Cl2, 68% (2 steps);
A solution of bis(norbornadiene)rhodium(I) tetrafluoroborate
(10 mol%) in CH2Cl2 was added to a solution of ligand 6 (12
mol%) in CH2Cl2 in a Fischer – Porter tube. The solution was
stirred for 30 min at room temperature under nitrogen. The
hydrogenation substrate (1.0 eq) in CH2Cl2 was then added
dropwise to the solution. The tube was charged with H2 to 100
psi and stirred for 16-60 h at room temperature. The final
concentration of the substrate is ca 0.05 – 0.2 M A higher
concentration may be used for larger scale reactions. The
mixture was filtered through a short pad of Celite and
concentrated. The residue was purified by flash
chromatography, eluting with 20-30% EtOAc/Hexane to give
the hydrogenated product.
(c) 8c, Cs2CO3, DMF, 95%; (d) Grubbs’ II, toluene, 70°C, 90%; (e) see table 1, entry 12; (f)
RuCl3 (10 mol%), NaIO4 (6 eq.), CHCl3/CH3CN/H2O (2:2:3); (g) Na•C10H8, THF, -40°C, then
HCl; (h) aq. HCHO, H2 (75 psi), Pd/C, 52% (3 steps).
Amino acid 3, a moiety present in Argatroban 4, was originally
prepared by a process involving the separation of all four
stereoisomers.3 A diastereoselective synthesis of racemic 3 has
been reported.29 Some asymmetric syntheses involving alkene
hydrogenations with moderate to good stereoselectivity have
also been reported.30
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank the Agency for Science Technology and Research (A-
Star) (PSF grant number 1321202095) and Nanyang
Technological University for financial support of this work. We
thank Dr Richmond Lee (Singapore University of Technology &
Design) for helpful discussions & resources for DFT calculations.
Scheme 4. Synthesis of trans-4-methylpipecolic acid.
Reagents and conditions: (a) see scheme 2 (b) see table 1, entry 12; (c) RuCl3 (10
mol%), NaIO4 (6 eq.), CHCl3/CH3CN/H2O (2:2:3); (d) Na•C10H8, THF, -40°C, then HCl,
68% (two steps).
Thus, alcohol (R)-7 was converted to alkene (R)-18 by the
method described above (see Scheme 2) and reduced with
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20 Preliminary DFT calculations indicate that the hydrogen bond
Notes and references
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provides stabilisation of about 1.3 kcDaOlmI:o10l-.11.0S3e9/eC9thOeB0S0.I0.0f3oHr
further details.
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(f) in organometallic photocatalysis, see K. L., Skubi, J. B. Kidd,
H. Jung, I. A. Guzei, M.-H. Baik, T. P. Yoon, J. Am. Chem. Soc.
2017, 139, 17186-17192.
14 Our original concept was to employ a La3+ ion as a hard Lewis
acid to bridge the two oxygen functional groups, but the
presence of La3+ proved superfluous.
15 Details of all X-ray structure determinations have been
deposited with the Cambridge Crystallographic Data Centre
and may be obtained at http://www.ccdc.cam.ac.uk; CCDC
deposition numbers: p-nitrobenzoate of 11a: 1563706; of 19:
1563707.
16 Confirmation of this idea was obtained by carrying out the
hydrogenation reaction using a catalyst derived from a
combination of [(COD)RhCl]2 and AgBF4. This resulted in 44%
conversion and 8:1 diastereoselectivity.
33 H. J. Davis, R. J. Phipps, Chem. Sci. 2017, 8, 864-877.
17 The reaction was also tested using a ligand with an additional
oxygen atom i.e. methoxyethoxy group in place of the
methoxy group, but this showed no advantages.
18 The use of the acetate to probe for hydrogen bonding was
inspired by H. B. Henbest, R. A. L. Wilson, J. Chem. Soc. 1957,
1958-1965.
19 J. Garcia, F. Urpí, J. Vilarrasa, Tetrahedron Lett. 1984, 25,
4841-4844.
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