(R)-(+)-N-Methylbenzoguanidine ((R)-NMBG) catalyzed acylative kinetic resolution of racemic 3-hydroxy-3-aryl-propanoates

Article abstract of DOI:10.1016/j.tetlet.2016.09.014

(R)-(+)-N-methylbenzoguanidine ((R)-NMBG) functioned as an efficient acyl transfer catalyst for the acylative kinetic resolution of racemic β-hydroxy esters using cyclohexane carboxylic anhydride under mild reaction conditions. A tert-butyl ester moiety is necessary to achieve a high selectivity. The effects of the substituents on the aromatic rings of the substrates were systematically investigated, and diverse substrates participated in the reaction with good s-values (>20) irrespective of the type of substituents and their patterns, except for o-methoxy group.

Full text of DOI:10.1016/j.tetlet.2016.09.014

Tetrahedron Letters 57 (2016) 4697–4701
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
(R)-(+)-N-Methylbenzoguanidine ((R)-NMBG) catalyzed acylative
kinetic resolution of racemic 3-hydroxy-3-aryl-propanoates
⇑
Akira Yamada, Kenya Nakata
Interdisciplinary Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue, Shimane 690-8504, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2016
Revised 31 August 2016
Accepted 6 September 2016
Available online 7 September 2016
(R)-(+)-N-methylbenzoguanidine ((R)-NMBG) functioned as an efficient acyl transfer catalyst for the
acylative kinetic resolution of racemic b-hydroxy esters using cyclohexane carboxylic anhydride under
mild reaction conditions. A tert-butyl ester moiety is necessary to achieve a high selectivity. The effects
of the substituents on the aromatic rings of the substrates were systematically investigated, and diverse
substrates participated in the reaction with good s-values (>20) irrespective of the type of substituents
and their patterns, except for o-methoxy group.
Keywords:
Ó 2016 Elsevier Ltd. All rights reserved.
Kinetic resolution
b-Hydroxy esters
Organocatalysis
N-Methylbenzoguanidine
Acylation
The kinetic resolution (KR) of racemic secondary alcohols by
asymmetric acylation is one of the more reliable and facile meth-
ods to obtain optically pure compounds. Therefore, much effort
has been devoted to the development of efficient methods for
KR. Although several enzymatic methods1 have been developed
to progress from laboratory scale to industrial scale, nonenzymatic
chemical methods had not been reported till the middle of 1990s.
The first nonenzymatic acylative KR of racemic alcohols was
reported by Evans2 and Vedejs3 using a stoichiometric amount of
chiral acylating agents. Shortly thereafter, pioneering studies on
the catalytic acylative KR of alcohols were reported by the research
groups of Vedejs,4 Oriyama,5 Fuji and Kawabata,6 Fu,7 and Miller.8
The investigated substrates were mainly limited to racemic sec-
ondary benzylic alcohols, cycloalkanols, propargylic alcohols, and
allylic alcohols till recently, even though several useful protocols
for this purpose have been developed.9 During the last decade,
the KR of other more synthetically valuable substrates such as
Morita–Baylis–Hillman adducts,10 2,2-difluoro-3-hydroxy-3-aryl-
developed for preparing these compounds, such as the asymmetric
Mukaiyama aldol reaction using organometallic complexes19 and
organocatalysts,20 the Reformatsky reaction,21 and the asymmetric
reduction of the corresponding b-keto esters.22 The enzymatic KR
of racemic aromatic b-hydroxy esters has been already devel-
oped.18b,23 The first nonenzymatic acylative KR of racemic
aromatic b-hydroxy esters was recently achieved by Dinér24 using
Fu’s chiral planar ferrocenyl DMAP catalyst.7 Previously, we
achieved the KR of racemic benzylic alcohols ( )-1 with free car-
boxylic acids by asymmetric esterification using pivalic anhydride
as the coupling reagent catalyzed by (R)-N-methylbenzoguanidine
((R)-NMBG; 3)) (Scheme 1, (1)).25 Moreover, we achieved an effi-
cient enantiodivergent synthesis of both the enantiomers of cen-
trolobine using this protocol as the key step.26 Thus, it was
hypothesized that this protocol can also be applied to aromatic
b-hydroxy esters ( )-4 in the same manner (Scheme 1, (2)). Herein,
we report the efficient acylative KR of various racemic 3-hydroxy-
3-aryl-propanoates catalyzed by (R)-NMBG (3) as the chiral acyl
transfer catalyst.
To explore a suitable acyl donor, the KR of racemic b-hydroxy
ester ( )-6 was selected as the model reaction and investigated
using several types of carboxylic anhydrides catalyzed by
(R)-NMBG (3) in Et2O at room temperature for 24 h, the previously
established conditions (Table 1).25 The KR of ( )-6 was carried out
with diphenylacetic anhydride (DPHAA)12b (8a), and cyclohexane
carboxylic anhydride (8b), both of which were efficient acyl
components in a previous study;25 the reactions smoothly pro-
ceeded with good s-values27 (entries 1 and 2). Compared to
propanoates,11 2-hydroxyalkanoates,12 2-hydroxy-
c-butyrolac-
tones,13 2-hydroxyphosphonates,14 1,2-azidoalcohols,15 and
2-hydroxy-2-aryl-ethylphosphonates,16 have been reported. On
one hand, optically active b-hydroxy esters are frequently utilized
as chiral building blocks for the synthesis of biologically active
compounds17 and pharmacologically active compounds.18 There-
fore, several asymmetric syntheses have been intensively
⇑
Corresponding author. Tel.: +81 852 32 6424.
E-mail address: nakata@riko.shimane-u.ac.jp (K. Nakata).
http://dx.doi.org/10.1016/j.tetlet.2016.09.014
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

4698
A. Yamada, K. Nakata / Tetrahedron Letters 57 (2016) 4697–4701
catalyst loading from 5 to 2 mol % (entry 18). In an attempt to
increase the reaction efficiency, the reaction time was prolonged
to 48 h; however, the conversion was not significantly improved
(entry 19).
With the optimized reaction conditions, three other types of
isothiourea catalysts, (R)-BTM (12)28 and (R)-HBTM (13),28b,29
reported by Birman et al., and HBTM-2.1 (14),30 reported by Smith
et al., that served as efficient acyl transfer catalysts, were applied to
the KR of ( )-9f (Table 3). The reaction of ( )-9f was carried out
with 0.75 equiv of anhydride 8b and a catalytic amount of (R)-
BTM (12) to obtain a moderate s-value (entry 2). When the same
reactions were conducted with (R)-HBTM (13) and HBTM-2.1
(14), both the reactions proceeded far beyond 50% conversion,
affording the corresponding chiral alcohols 9f in >99% ee (entries
3 and 5). Because the exact ee of 9f could not be determined by
HPLC analyses, the s-values were not evaluated under the condi-
tions. Thus, the reactions were reinvestigated by reducing the
amount of anhydride 8b to 0.5 equiv. A moderate s-value was
obtained using (R)-HBTM (13) (entry 4), and almost the same
s-value was obtained in the case of (R)-NMBG (3) using
HBTM-2.1 (14) (entry 6 vs entry 1).
To assess the generality of this novel method and systematically
explore the electronic and steric effects on the aromatic rings of the
substrates, a series of racemic b-hydroxy esters ( )-15a–k with
carboxylic anhydride 8b was investigated under the optimized
reaction conditions (Table 4). The KR of 15a–c with methyl sub-
stituents on the aromatic rings of the substrates smoothly pro-
ceeded in almost 50% conversion with good s-values in all the
cases, irrespective of their positions (entries 1–3). Similarly, the
KR of methoxy-substituted substrates 15d–f was investigated;
the reactions were influenced by the substitution patterns. The
reaction of 15d with an ortho-methoxy group resulted in poor reac-
tivity and a low s-value (entry 4). On the other hand, the reactions
of 15e and 15f with meta- and para-methoxy groups smoothly pro-
ceeded with good s-values, respectively (entries 5 and 6). The same
tendency was observed in the case of chloro-substituted substrates
15g–i, and better results were obtained compared to the corre-
sponding cases of methoxy-substituted substrates (entries 7–11).
Scheme 1. Previous study (1) and this study (2).
propionic anhydride (8c) and isobutyric anhydride (8d), the latter
branched substituent showed better results than the former nor-
mal substituent (entries 3 and 4). In contrast, aromatic carboxylic
anhydride 8e with a very low s-value was not effective for the reac-
tion (entry 5).
Next, to evaluate the effect of the structure of ester moiety on
the substrates, the KR of diverse racemic b-hydroxy esters ( )-
9a–h was investigated using both DPHAA (8a), and cyclohexane
carboxylic anhydride (8b) under the abovementioned conditions
(Table 2). First, the reaction was carried out using DPHAA (8a)
(entries 1–8). The KR of 9a and 9b bearing normal substituents
(R2 = Et and Bn) on the ester moiety was carried out to afford good
s-values (entries 1 and 2). On the other hand, the reaction of 9c–e
bearing disubstituted substituents (R2 = iPr, CHPh2, and cC6H11
)
showed better results (entries 3–5). The reaction of 9f with a
trisubstituted substituent (R2 = tBu) smoothly proceeded with a
high s-value regardless of the steric effect (entry 6). When the
methyl group(s) was replaced with a phenyl group(s) at the R2 sub-
stituent of 9f, the reactivity was enhanced; however, the selectivity
decreased (entries 7 and 8). When anhydride 8b was used, the
same tendency was observed, and the selectivity slightly improved
in the corresponding case of 8a (entries 9–17). Among all the
entries, the highest s-value was obtained in the reaction of 9f with
8b (entry 15). Under the conditions in entry 15, it was found that
both the reactivity and selectivity decreased upon decreasing the
The reactions of
a- and b-naphthyl-substituted substrates 15j
and 15k were also investigated; they afforded a high selectivity
(entries 12–15).
In summary, we achieved the nonenzymatic acylative KR of
b-hydroxy esters with cyclohexane carboxylic anhydride in the
Table 1
(R)-NMBG catalyzed KR of ( )-6 using various carboxylic anhydrides 8a–e
Entry
R1
Yield (7; 6)a (%)
ee (7; 6) (%)
s
1
2
3
4
5
Ph2CH (a)
cC6H11 (b)
Et (c)
43; 46
41; 50
32; 57
44; 33
19; 69
83; 66
83; 80
74; 44
72; 79
14; 2
22
26
10
14
1
iPr (d)
Ph (e)
a
Isolated yield.

A. Yamada, K. Nakata / Tetrahedron Letters 57 (2016) 4697–4701
4699
Table 2
Examination of the effect of the ester moiety
Entry
R1, R2
Yield (10/11; 9)a (%)
ee (10/11; 9) (%)
s
1b
2
3
4
5
6
7
8
CHPh2, Et (a)
CHPh2, Bn (b)
CHPh2, Pr (c)
CHPh2, CHPh2 (d)
CHPh2, cC6H11 (e)
CHPh2, tBu (f)
43; 46
43; 52
36; 47
41; 43
41; 50
43; 40
57; 38
55; 43
41; 50
51; 48
50; 35
55; 39
39; 51
50; 38
48; 43
53; 39
57; 30
25; 70
31; 69
84; 66
84; 68
87; 68
84; 91
87; 70
89; 76
68; 95
60; 96
83; 80
82; 93
85; 94
69; >99
82; 76
85; 95
89; 91
76; 94
70; 97
91; 31
87; 38
23
24
29
37
29
39
19
15
26
35
43
NDd
23
44
54
24
22
28
22
i
CHPh2, C(CH3)2Ph (g)
CHPh2, C(CH3)Ph2 (h)
cC6H11, Et (a)
9c
10
11
12
13e
14
15
16
17
18f
19g
cC6H11, Bn (b)
i
cC6H11, Pr (c)
cC6H11, CHPh2 (d)
cC6H11, CHPh2 (d)
cC6H11
cC6H11
,
,
cC6H11 (e)
tBu (f)
cC6H11, C(CH3)2Ph (g)
cC6H11, C(CH3)Ph2 (h)
cC6H11
cC6H11
,
,
tBu (f)
tBu (f)
a
b
c
d
e
f
Isolated yield.
Same as in Table 1, Entry 1.
Same as in Table 1, Entry 2.
Not determined.
The reaction was carried out using 0.55 equiv of 8b.
The reaction was carried out using 2 mol % of 3 for 24 h.
The reaction was carried out using 2 mol % of 3 for 48 h.
g
Table 3
Examination of the catalysts
Entry
Catalyst
Yielda (11f; 9f) (%)
ee (11f; 9f) (%)
s
1b,c
2c
3c
4f
(R)-NMBG (3)
(R)-BTM (12)
(R)-HBTM (13)
(R)-HBTM (13)
HBTM-2.1 (14)
HBTM-2.1 (14)
48; 43
52; 48
70; 29
44; 50
65; 24
38; 50
89; 91
75; 82
54
18
À40; >À99d
À72; À67d
37; >99
92; 71
NDe
12
5c
6f
NDe
51
a
Isolated yield.
Same as in Table 1, Entry 1.
The reaction was carried out using 0.75 equiv of 8b.
Enantiomers.
Not determined.
b
c
d
e
f
The reaction was carried out using 0.50 equiv of 8b.

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A. Yamada, K. Nakata / Tetrahedron Letters 57 (2016) 4697–4701
Table 4
(R)-NMBG catalyzed KR of a variety of racemic aromatic b-hydroxy esters with anhydride 8b
Entry
Ar
Yield (16; 15)a (%)
ee (16; 15) (%)
s
1
2
o-MeC6H4 (a)
m-MeC6H4 (b)
p-MeC6H4 (c)
o-MeOC6H4 (d)
m-MeOC6H4 (e)
p-MeOC6H4 (f)
o-ClC6H4 (g)
m-ClC6H4 (h)
m-ClC6H4 (h)
p-ClC6H4 (i)
47; 48
44; 47
49; 51
16; 81
47; 47
48; 52
44; 51
62; 38
49; 46
59; 35
50; 48
48; 44
48; 44
54; 46
50; 50
91; 84
89; 77
91; 88
77; 13
87; 89
87; 83
83; 75
60; >99
88; 91
67; >99
88; 92
81; >99
90; 99
82; >99
89; 93
55
41
59
9
44
39
3
4b
5
6
7
8
24
NDc
48
9d
10
11d
12
13d
14
15d
NDc
53
p-ClC6H4 (i)
a
a
-Np (j)
-Np (j)
NDc
93
b-Np (k)
b-Np (k)
NDc
60
a
b
c
Isolated yield.
Average of three runs.
Not determined.
d
The reaction was carried out using 0.55 equiv of 8b.
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presence of (R)-NMBG as an efficient chiral acyl transfer catalyst.
Diverse optically active b-hydroxy esters were obtained from the
reaction. Further studies on this method are underway in our lab-
oratory to expand the substrate scope and apply this method to the
synthesis of biologically active chiral compounds.
Acknowledgments
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This study was supported by a Research Grant from The Taka-
hashi Industrial and Economic Research Foundation, and The
Futaba Electronics Memorial Foundation, both Japan.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.09.
014.
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