Enantioselective alcoholysis of meso-glutaric anhydrides catalyzed by Cinchona-based sulfonamide catalysts

Article abstract of DOI:10.1002/adsc.201000289

The bifunctional Cinchona-based sulfonamide catalysts showed the highest levels of enantioselectivity reported to date in the alcoholytic desymmetrization of meioglutaric anhydrides. Density functional theory (DFT) computational studies provide detailed insight into the observed sense of enantioselectivity. Moreover, detailed experimental studies and single crystal X-ray analysis confirmed that these bifunctional organocatalysts 3 do not form Hbonded self-aggregates in both solution and solid state. The synthetic utility of this methodology was also demonstrated in the synthesis of pharmaceutically important γ-amino acids, such as (S)-pregabalin. Of the many asymmetric syntheses of enantiomerically pure (S)-pregabalin reported to date, our synthesis requires the least number of and the simplest steps.

Full text of DOI:10.1002/adsc.201000289

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DOI: 10.1002/adsc.201000289
Enantioselective Alcoholysis of meso-Glutaric Anhydrides
Catalyzed by Cinchona-Based Sulfonamide Catalysts
Sang Eun Park,a Eun Hye Nam,a Hyeong Bin Jang,a Joong Suk Oh,a Surajit Some,a
Yong Seop Lee,a and Choong Eui Songa,b,*
a
Department of Chemistry, Sungkyunkwan University, 300 Cheoncheon, Jangan, Suwon, Gyeonggi, 440-746, Korea
Fax : (+82)-31-290-7075; e-mail: s1673@skku.edu
Department of Energy Science, Sungkyunkwan University, 300 Cheoncheon, Jangan, Suwon, Gyeonggi, 440-746, Korea
b
Received: April 14, 2010; Revised: June 30, 2010; Published online: August 20, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000289.
Abstract: The bifunctional Cinchona-based sulfon- also demonstrated in the synthesis of pharmaceuti-
ACHTUNGTRENNUNGamide catalysts showed the highest levels of enantio- cally important g-amino acids, such as (S)-pregabalin.
selectivity reported to date in the alcoholytic desym- Of the many asymmetric syntheses of enantiomeri-
metrization of meso-glutaric anhydrides. Density cally pure (S)-pregabalin reported to date, our syn-
functional theory (DFT) computational studies pro- thesis requires the least number of and the simplest
vide detailed insight into the observed sense of enan- steps.
tioselectivity. Moreover, detailed experimental stud-
ies and single crystal X-ray analysis confirmed that
these bifunctional organocatalysts 3 do not form H- Keywords: asymmetric organocatalysis; bifunctional
bonded self-aggregates in both solution and solid catalysis; catalyst self-association; Cinchona-based
state. The synthetic utility of this methodology was sulfonamide catalysts; desymmetrization; pregabalin
Introduction
tagonists[4] while silyl-protected 3-hydroxyglutaric
hemiesters 2 can also be used as chiral synthons[5] in
There has been considerable interest in the stereose- the preparation of HMG-CoA reductase inhibitors
lective ring opening of meso-cyclic anhydrides, since called “statins” which have inhibitory activity that
the resulting hemiesters are used as versatile inter- allows them to suppress the biosynthesis of cholester-
mediates in the construction of many bioactive com- ol (Figure 1). Much effort has, therefore, been made
pounds.[1] Especially, the chiral 3-substituted glutaric to develop efficient enzymatic and non-enzymatic cat-
acid monoesters 2 produced by the alcoholysis of alytic systems for the alcoholysis of the meso-glutaric
meso-glutaric anhydrides 1 (Scheme 1) are used as anhydrides 1.[1] However, the results obtained with
key intermediates for the synthesis of a variety of in- meso-glutaric anhydrides as substrates are still unsat-
dustrially interesting pharmaceutical compounds, e.g., isfactory for practical use, whereas alcoholysis with
the 3-alkyl/arylglutaric acid monoesters 2 are used as other types of meso-anhydrides, such as bi-, tricyclic
key intermediates for the synthesis of g-aminobutyric and succinic anhydrides, usually gives excellent re-
acid (GABA) analogues[2] (e.g., baclofen·HCl[2d,e] and sults. All of the approaches involving the use of meso-
pregabalin[2f]), selective serotonin receptor antagonists glutaric anhydrides as substrates suffer from either a
(e.g., paroxetin·HCl),[3] and potent P2X7 receptor an- narrow substrate scope and/or very long reaction time
and unsatisfactory enantioselectivity.
Cooperative catalysis,[6] the simultaneous binding
and activation of reacting partners resulting in both
pre-organization of the substrates and stabilization of
the transition state structures, is a fundamental princi-
ple in enzymatic catalysis. Therefore, recently, much
effort has focused on the development of efficient bi-
Scheme 1.
functional organocatalysts that effectively orient sub-
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Figure 1. Examples of some pharmaceuticals which can be prepared from chiral 3-substituted glutaric acid monoesters.
strates in an enzyme-like manner.[7] Recently, we[8] selectivity reported to date in the alcoholytic desym-
and another research group[9] employed bifunctional metrization of meso-glutaric anhydrides. DFT compu-
Cinchona-based thioureas as organic chiral catalysts tational studies provide detailed insight into the ob-
for the alcoholytic desymmetrization of meso-glutaric served sense of enantioselectivity. Moreover, detailed
anhydrides. However, their enantioselectivity was still experimental studies and single crystal X-ray analysis
unsatisfactory for the synthetic use. Moreover, it is confirm that these bifunctional organocatalysts 3 do
known that the bifunctional organocatalysts such as not form H-bonded self-aggregates even in solid state.
thiourea-based catalysts can form H-bonded self-ag-
gregates in both solid[10] and solution states,[11] due to
their bifunctional H-donating and accepting nature, Results and Discussion
resulting in the strong dependency of the reactivity
and enantioselectivity on the concentration and tem- To optimize the catalyst structure, we synthesized a
perature. Due to the self-association phenomena of variety of sulfonamide catalysts 3a–k (Figure 2)
this type of catalysts, the enantioselectivity generally simply by the reaction of the 9-amino-(9-deoxy)-epi-
decreases with increasing concentration or decreasing Cinchona alkaloid[14] with an arenesulfonyl chloride
temperature,[8] which can hamper their practical use. (Ar) in the presence of triethylamine,[12] and exam-
We[12] have recently reported that Cinchona-based sul- ined their catalytic efficiency for the asymmetric
fonamide 3a is highly enantioselective for the metha- methanolysis of 3-OTBDPS glutaric anhydride (1a) as
nolytic desymmetrization of meso-bicyclic and tricy-
a model substrate in presence of the catalyst
clic anhydrides[13] and, moreover, the self-association (10 mol%) and MeOH (10 equiv.) in MTBE[15] at
phenomenon is not as significant in the methanolysis room temperature. The results are depicted in
of meso-anhydrides with the bifunctional sulfonamide Table 1, with the data obtained using the thiourea and
catalyst 3a as it is in the process catalyzed by 4 or 5. squaramide catalyst, 4 and 5, respectively. As shown
Thus, we decided to optimize the catalyst structure in Table 1, regardless of the substituent at the C-3 and
and reaction conditions for alcoholysis of meso-gluta- C-6’ positions of alkaloids 3a–d, the methanolysis of
ric anhydrides using the sulfonamide-type catalysts 3. 1a proceeded fast, with the reaction being completed
We have now completed an extensive screening of within 3–4 h affording the corresponding hemiester 2a
a variety of chiral sulfonamide catalysts and report in nearly quantitative yields and excellent ee values
here that bifunctional Cinchona-based sulfonamide (92–93% ee) (Table 1, entries 1–4). The aryl substitu-
organocatalysts 3 show the highest levels of enantio- ents of the sulfonamide moiety of 3a and 3e–3k were
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Enantioselective Alcoholysis of meso-Glutaric Anhydrides
Table 1. Catalytic enantioselective ring opening of 1a with
MeOH in the presence of different catalysts.[a]
Entry
Catalyst
Time
[h]
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
QN-SA (3a)
HQN-SA (3b)
CD-SA (3c)
HCD-SA (3d)
QN-Ph-SA (3e)
QN-1-Np-SA (3f)
QN-2-Np-SA (3g)
QN-o-Tolyl-SA (3h)
QN-m-Tolyl-SA (3i)
QN-p-Tolyl-SA (3j)
QN-m-Xylyl-SA (3k)
QN-TU (4)
4
4
4
3
8
12
6
8
8
8
7
6
5
97
96
94
94
96
92
91
96
99
89
87
95
96
92
92
93
92
91
90
91
90
91
91
91
89
85
9
10
11
12
13
QN-SQA (5)
[a]
Reactions were carried out with 1a (0.5 mmol), 10 equiv.
of MeOH and catalysts (10 mol%), in MTBE (10 mL) at
room temperature.
Isolated yields after chromatographic purification.
Determined by chiral HPLC (see Supporting Informa-
tion).
[b]
[c]
Figure 2. Structures of different Cinchona alkaloid organo-
catalysts examined in this study (QN=quinine; HQN=hy-
droquinine; CD=cinchonidine; HCD=hydrocinchonidine).
also relatively insensitive to the enantioselectivity
(Table 1, entries 1 and 5–11). Even simple phenyl-sub-
stituted sulfonamide catalyst 3e also exhibited compa-
rable reactivity and enantioselectivity with 3a
(entry 5). Only ortho-substituted aryl substituents
gave slightly lower enantioselectivity (Table 1, en-
tries 6 and 8). Moreover, the enantioselectivities of
sulfonamide catalysts 3 are higher compared with
those of other type of catalysts 4 and 5 (Table 1, en-
tries 12 and 13). Much more interestingly, however,
the enantioselectivity of sulfonamide catalyst 3a in-
creases slightly on decreasing the reaction tempera-
ture from 208C to À208C (92% ee to 96% ee)
(Figure 3). In contrast to these results, using other
type of catalysts such as QN-TU (4)[14,16] and QN-
SQA (5),[17] the enantioselectivity decreases signifi-
cantly on decreasing the reaction temperature (e.g.,
80% ee at À208C using 5) (Figure 3 and Supporting
Information, Table S1). On the basis of these experi-
Figure 3. Effect of reaction temperature on the enantioselec-
tivity in the methanolytic desymmetrization of 1a.
mental results, it is clear that the self-association phe- cannot form H-bonded self-aggregates even in the
nomenon is not as significant in the methanolysis of solid state.[18] As shown in Figure 4, there is no inter-
glutaric anhydrides with the bifunctional sulfonamide molecular H-bond interaction between the sulfon-
catalyst 3a as it is in the process catalyzed by 4 or 5.
ACHTUNGTERNUNaNG mide NH group and the sulfone group (SO2). In con-
Furthermore, the single crystal X-ray structure of trast to these results, X-ray crystal structures of bi-
QN-SA (3a) revealed that this type of catalysts functional thiourea derivatives[10] and cinchonine-
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Figure 4. a) ORTEP diagram of QN-SA (3a). b) Schematic drawing of the crystal packing in 3a.
based squaramide[19] show that they form aggregates ent alcohols (Table 3). Almost independently of the
through intermolecular bifurcate H-bonding of NH steric demand of the alcohol, high enantioselectivity
groups of the thiourea or squaramide moiety to the was also obtained in all cases (entries 2–5). The ex-
thiourea sulfur or the squaramide oxygen atoms, re- ceptionally high ee value (98% ee) obtained with
spectively.
benzyl alcohol is particularly noteworthy.[20] To the
Having established that the sulfonamides 3 act as best of our knowledge, this level of reactivity and
highly enantioselective and self-association-free cata- enantioselectivity is unprecedented in the alcoholytic
lysts, we undertook to explore the scope of the sub- desymmetrization of the meso-glutaric anhydrides 1a.
strate under the optimized reaction conditions. As Even enzymatic processes gave much lower enantio-
shown in Table 2, a variety of meso-glutaric anhy- selectivities (e.g., ~14% ee[21d] in the case of 2f).[21]
drides 1a–h, regardless of their substitution patterns The observed sense of stereoselectivity is in agree-
at the 3-position of 1 (3-alkyl, 3-aryl and 3-silyl-pro- ment with our computation-based transition state
tected hydroxy groups), were smoothly converted into models for the desymmetrization of the meso-anhy-
the corresponding hemiester products 2a–h with ex- drides.[8,12a] According to the computational results
cellent enantioselectivity (90–96% ee). The strength (B3LYP at 6-31G* level), the transition structure
and scope of our methodology is further demonstrat- (Figure 5 b and Figure 5 c) leading to the major enan-
ed in the alcoholysis of the anhydride 1a with differ- tiomer 2c is favored relative to that leading to the
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Enantioselective Alcoholysis of meso-Glutaric Anhydrides
Table 2. Enantioselective methanolysis of various glutaric
anhydrides 1a–h.[a]
methyl-5-methylhexanoic acid (pregabalin), an anti-
convulsant drug used for neuropathic pain treatment,
was developed starting from the hemiester (R)-7j via
the Curtius rearrangement (Scheme 2). The sequen-
tial addition of diphenylphosphoryl azide (DPPA)
and NEt3 to the hemiester (R)-7j (90% ee), followed
by benzyl alcohol furnished the benzyl carbamate 8j
in 88% yield. The benzyl carbamate 8j was then de-
protected by hydrogenation with Pd/C to provide (S)-
pregabalin in 100% yield with 90% ee. One recrystal-
lization from aqueous i-PrOH afforded enantiomeri-
cally pure (S)-pregabalin. In fact, of the many asym-
metric syntheses[2f,24] of enantiomerically pure (S)-pre-
gabalin reported to date, our synthesis requires the
least number of steps as well as the simplest steps.
Entry
Substrate
(X=)
T
Time
[h]
Yield
[%][b]
ee
[oC]
[%][c]
1
2
3
4
5
6
7
8
1a (OTBDPS)
1a (OTBDPS)
1b (OTBDMS)
1b (OTBDMS)
1c (Me)
0
11
28
11
28
12
6
14
6
17
1
96
87
97
98
95
95
96
88
91
92
81
92
87
91
95
96
92
93
90
91
92
92
95
91
93
92
92
92
À20
0
À20
À20
0
1d (i-Bu)[23]
1d (i-Bu)[23]
1e (4-Ph)
1e (4-Ph)
1f (4-F-C6H4)
1f (4-F-C6H4)
1g (4-Cl-C6H4)
1g (4-Cl-C6H4)
1h (3,4-Cl-C6H3)
À20
0
Conclusions
9
À20
20
10
11
12
13
14
À20
19
1
18
19
In summary, we describe the alcoholytic desymmetri-
zation of a variety of meso-glutaric anhydrides with
unprecedentedly high enantioselectivity using self-as-
20
À20
À20
sociation-free Cinchona-based sulfonACHTNUTRGNEUNGamide catalysts.
[a]
Alcohols other than methanol were effective nucleo-
philes in the desymmetrization reaction and also pro-
vided excellent enantiomeric excesses. In addition,
the synthetic utility of this methodology was demon-
strated in the synthesis of pharmaceutically important
g-amino acids such as (S)-pregabalin. Of the many
asymmetric syntheses of enantiomerically pure (S)-
pregabalin reported to date, our synthesis requires the
least number of steps as well as the simplest steps.
Reactions were carried out with 1a–h (0.5 mmol),
10 equiv. of MeOH and catalysts 3a (10 mol%) in MTBE
(10 mL) at 208C, 08C or À208C.
[b]
[c]
Isolated yields after chromatographic purification.
Determined by chiral HPLC (see Supporting Informa-
tion).
Table 3. Asymmetric alcoholysis of cyclic anhydride 1a with
different alcohols.[a]
Experimental Section
General Procedure for the Preparation of the
Sulfonamide Catalysts 3a–k
(9-deoxy)-epi-Cinchona alkaloid[14]
and arenesulfonyl chloride
Entry ROH
Main
T
Time Yield ee
[%][b] [%][c]
To a solution of 9-amino
(3.54 g, 10.94 mmol)
isomer [oC] [h]
(10.94 mmol) in CH2Cl2 (100 mL) was added triethylamine
(1.52 mL, 10.94 mmol) at room temperature. The mixture
was stirred at room temperature under an argon atmosphere
overnight. After the addition of 50 mL of water, the reaction
mixture was extracted with CH2Cl2. The organic extract was
dried over MgSO4, filtrated, and then the filtrate was evapo-
rated under vacuum. The residue was purified by column
chromatography on a silica gel column with EtOAc-MeOH
(20:1) to afford the sulfonamide catalyst 3 (usually above
than 85% yield) as a white amorphous solid. Characteriza-
tion data for 3a–k, see Supporting Information.
1
2
3
4
5
MeOH
Allyl-OH 2i
BnOH
BnOH
BnOH
2a
20
20
20
0
4
20
8
96
92
90
98
93
92
91
96
97
98
2j
2j
2j
15
À20 4.5 d
[a]
Reactions were carried out with 1a (0.5 mmol), 10 equiv.
of ROH and catalysts 3a (10 mol%) in MTBE (10 mL)
at 208C, 08C or À208C.
[b]
[c]
Isolated yield.
Determined by chiral HPLC (see Supporting Informa-
tion).
General Procedure for Alcoholytic Desymmetrization
of meso-Glutaric Anhydrides
minor enantiomer ent-2c by 4.3 kcal/mol (see Sup-
porting Information, Table S3 for further details).[22]
To illustrate the synthetic utility of our methodolo-
Alcohol (5 mmol) was added to a stirred solution of the an-
hydrides (0.5 mmol) and catalyst (10 mol%) in MTBE
gy, a short and practical synthesis of (S)-3-amino- (10 mL, 0.05M) at the temperatures indicated in Table 1,
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Figure 5. a) Transition-state analogues 6c and ent-6c for the non-catalyzed methanolysis reaction. b) Schematic representa-
tion of the catalyst-transition-state-analogue complex 3a···6c, which gives the major product. c) Computed structure of
3a···6c
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Windows from Wavefunction Inc. The preliminary
global minimum energy structures were obtained by
molecular mechanics computation and further refined
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from cyanoacetamide and isovaleraldehyde according
[2f].
ˇ
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using the catalyst 3a, but the enantioselectivity was
found to be slightly inferior in comparison to MTBE:
Et2O (90% ee), THF (91% ee), dioxane (91% ee), tolu-
ene (87% ee) and CH2Cl2 (82% ee). For more experi-
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[18] CCDC 771828 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
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Adv. Synth. Catal. 2010, 352, 2211 – 2217
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asc.wiley-vch.de
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