Dynamic Reductive Kinetic Resolution of Benzyl Ketones using Alcohol Dehydrogenases and Anion Exchange Resins

Article abstract of DOI:10.1002/adsc.201500801

Dynamic reductive kinetic resolutions of racemic 3-arylalkanones have been performed by the proper combination of an alcohol dehydrogenase and a basic anionic resin. The best results were found for the bioreduction with the alcohol dehydrogenase type A from Rhodococcus ruber DSM 44541 overexpressed in Escherichia coli (E. coli/ADH-A) and the commercially available evo-1.1.200, while the Amberlite IRA-440 C and the DOWEX-MWA-1 resins allowed efficient in situ racemizations. Reaction conditions were optimized in terms of enzyme source and loading, type and amount of resin, pH, temperature and reaction times, obtaining a series of (R,R)-substituted propan-2-ols with good conversions and both diastereoselectivity and stereoselectivity. As a proof of concept, the subsequent intramolecular cyclization of a selected propan-2-ol substrate afforded a valuable isochroman heterocycle without any loss of the optical purity.

Full text of DOI:10.1002/adsc.201500801

FULL PAPERS
DOI: 10.1002/adsc.201500801
Dynamic Reductive Kinetic Resolution of Benzyl Ketones using
Alcohol Dehydrogenases and Anion Exchange Resins
Daniel MØndez-Sµnchez,a Juan Mangas-Sµnchez,a,b Eduardo Busto,a,c
Vicente Gotor,a and Vicente Gotor-Fernµndeza,*
a
Organic and Inorganic Chemistry Department, Biotechnology Institute of Asturias (IUBA), University of Oviedo,
Avenida Juliµn Clavería s/n, 33006 Oviedo, Spain
Fax: (+34)-98-510-3446; phone: (+34)-98-510-3454; e-mail: vicgotfer@uniovi.es
Department of Biotechnology, University of Lund, PO Box 124, SE-221 00 Lund, Sweden
Organic Chemistry Department I, Faculty of Chemistry, Complutense University of Madrid, 28040-Madrid, Spain
b
c
Received: August 27, 2015; Revised: October 18, 2015; Published online: January 5, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500801.
Abstract: Dynamic reductive kinetic resolutions of temperature and reaction times, obtaining a series of
racemic 3-arylalkanones have been performed by the (R,R)-substituted propan-2-ols with good conversions
proper combination of an alcohol dehydrogenase and both diastereoselectivity and stereoselectivity.
and a basic anionic resin. The best results were As a proof of concept, the subsequent intramolecular
found for the bioreduction with the alcohol dehydro- cyclization of a selected propan-2-ol substrate afford-
genase type A from Rhodococcus ruber DSM 44541 ed a valuable isochroman heterocycle without any
overexpressed in Escherichia coli (E. coli/ADH-A) loss of the optical purity.
and the commercially available evo-1.1.200, while
the Amberlite IRA-440 C and the DOWEX-MWA-
1 resins allowed efficient in situ racemizations. Reac- Keywords: alcohol dehydrogenases; dynamic pro-
tion conditions were optimized in terms of enzyme cesses; dynamic reductive kinetic resolutions; ion ex-
source and loading, type and amount of resin, pH, change resins; ketones
Introduction
ed in an adjacent position to the carbonyl group, bear-
ing an acidic proton that facilitates the racemization,
Dynamic kinetic resolutions (DKRs) provide signifi- leading to the development of the so-called dynamic
cant advantages in synthetic chemistry leading to reductive kinetic resolutions (DYRKRs).[5]
enantiopure compounds in the theoretically maximum
In our continuous efforts to develop efficient one-
100% yield.[1] In this context, the use of enzymes has pot transformations for the design of dynamic pro-
been extensively explored for the selective modifica- cesses using redox enzymes, the synthesis of a series
tion of one enantiomer while the unreacted substrate of racemic 3-arylalkan-2-ones was performed, to later
enantiomer is racemized using either biological or explore the combination of ADHs under basic condi-
chemical catalysts, or even through a spontaneous tions for the development of DYRKRs.[6] This chemo-
process. Thus, the DKRs of racemic alcohols, amines enzymatic strategy provides access to enantioenriched
and their derivatives have been achieved with excel- alcohols, immediate precursors of enantio- and diaste-
lent selectivity levels mainly using hydrolases.[2] reomerically pure 3,4-dihydroisocoumarins by intra-
Redox enzymes have also been presented in recent molecular cyclization processes (Scheme 1). Herein,
years as ideal catalysts for the development of dynam- we wish to expand the synthetic possibilities of this
ic processes.[3] In this context, alcohol dehydrogenases methodology using different benzyl ketones as start-
are valuable tools for the selective bioreduction of ke- ing materials, in order to obtain new families of privi-
tones, obtaining valuable alcohols with one or multi- leged heterocyclic structures such as isochromanes,[7]
ple stereocenters.[4] Consequently, the racemization of through a chemoenzymatic and asymmetric strategy.
the untouched stereocenter is possible, enabling the Isochromanes represent an interesting family of com-
generation of multiple stereocenters in a single reac- pounds due to their cytotoxicity properties and high
tion. This epimerizable stereocenter is normally locat- value as building blocks for more complex structures.
122
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Adv. Synth. Catal. 2016, 358, 122 – 131

FULL PAPERS
Dynamic Reductive Kinetic Resolution of Benzyl Ketones
ered for the aromatic ring such as methoxy, hydroxy,
nitro or fluoro rests in ortho and para positions (en-
tries 1–7), but also a broad selection of alkyl iodides
was tested from linear aliphatic reagents (methyl,
ethyl, propyl, allyl and butyl chains, entries 1–7, 8, 9,
10 and 11) to cyclohexyl (entry 12) and benzyl
(entry 13). After 2 h at 408C, the corresponding alky-
lated ketones 2a–m were obtained with moderate to
high yields after an extraction and a column chroma-
tography purification (30–84%). Then, in order to
find suitable conditions for the measurement of con-
version and enantiomeric excess values for the biore-
duction processes, the chemical reduction of the so-
obtained benzyl ketones was performed by reaction
with sodium borohydride in methanol, yielding the al-
cohols 3a–m in very high yields (90–99%). The anti-
Scheme 1. Synthesis of different heterocyclic scaffolds from
benzyl ketones in a sequential dynamic bioreduction and in-
tramolecular cyclization.
Results and Discussion
A series of racemic a-substituted benzyl ketones 2a– configurations were favoured in all cases (up to 13:87
m was prepared by reacting the prochiral ketones 1a– diastereomeric ratio syn:anti).
g with alkyl iodides in the presence of equimolecular
The less substituted substrate from this series, 3-
amounts of tetrabutylammonium bisulfate (Table 1). phenylbutan-2-one (2a) was chosen as model sub-
Based on the commercial availability of benzyl ke- strate for the development of bioreduction processes.
tones, different patterns of substitution were consid- Initially, KR and racemization experiments were inde-
pendently conducted trying to find adequate condi-
tions aiming for the later combination of both pro-
Table 1. Synthesis of ketones 2a–m through alkylation reac-
tions of 1a–g with alkyl iodides, and subsequent chemical re-
duction towards the formation of racemic alcohols 3a–m.
cesses in order to explore and compare the possibili-
ties of DYRKR with previous benzyl ketones,[6] but
now lacking of a cyano group in the C-2’ position
(Table 2). Based on our previous findings, the Rhodo-
coccus ruber DSM 44541 ADH overexpressed in Es-
cherichia coli (E. coli/ADH-A),[8] was used consider-
ing its potential for the bioreduction of those benzyl
ketones.[6] Surprisingly, high conversion values and
poor diasteroselectivities were found after 24 h at
308C using either 5 or 15 mg of enzyme loading (en-
tries 1 and 2). This is in contrast with the moderate
conversion associated to an excelent enantio- and dia-
stereoselectivity found for the C-2 substituted ketone,
Entry
3
R1
H
R2
2a–m [%][a] 3a–m [%][a,b]
namely 2-(3-oxobutan-2-yl)benzonitrile (41% conver-
sion, >99:<1 dr).[6]
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3g
3h
3i
Me
67
60
45
67
55
55
84
56
49
54
93 (20:80)
96 (16:84)
93 (14:86)
96 (23:77)
99 (13:87)
90 (13:87)
97 (16:84)
97 (19:81)
98 (15:85)
98 (21:79)
95 (14:86)
90
A series of experiments was carried out to under-
stand the reaction outcome, considering shorter reac-
tion times for a selectivity improvement. As it can be
seen the use of 15 mg (entries 3–8) or 5 mg of enzyme
(entries 9–12) at 4 (entries 13–20) or 308C (entries 3–
12), led to good selectivity values at short reaction
times (up to 83:17 dr) for conversion values close to
50% (entries 5, 12 and 19). Reducing the bioreduction
kinetics would allow a good DYRKR always in that
much faster racemization kinetics could be achieved
(see enantiomeric excess of ketone 2a in the Support-
ing information for the reaction at 48C). In contrast
with the non-enzymatic reduction performed with
NaBH4, now the formation of the syn-diastereomer
was favoured. In all cases the (S,S)-alcohol was identi-
fied as the major diasteroisomer by HPLC analysis.
Next, conditions for the dynamic process were im-
plemented based on the presence of an acidic a-hy-
4-OMe Me
4-OH
4-NO2
2-OMe Me
2-Me
2-F
H
H
H
Me
Me
Me
Me
Et
n-Pr
allyl
n-Bu 60
Cy
Bn
9
10
11
12
13
3j
3k
3l
H
H
H
30
40
3m
96
[a]
Isolated yields (see further details in the Experimental
Section).
[b]
Diastereomeric ratios syn:anti measured by GC analysis
appear in parentheses (see the Supporting Information
for further details). No data are reported for 3l and 3m
as adequate analytics were not found for the determina-
tion of the dr.
Adv. Synth. Catal. 2016, 358, 122 – 131
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123

FULL PAPERS
Daniel MØndez-Sµnchez et al.
Table 2. Bioreduction of ketone 2a using E. coli/ADH-A in
a Tris-HCl buffer pH 7.5 at 250 rpm.
drogen next to the carbonyl group. On one hand, the
addition of a hydrophobic co-solvent such as hexane
was tested in the presence of triethylamine as basic
catalyst for the racemization. The organic solvent acts
as a reservoir for both product and substrate, reducing
their presence in the aqueous phase where the biore-
duction occurs, thus favouring the solubilization of
the substrate and avoiding the enzyme inhibition at
high substrate concentrations. On the other hand, the
use of an anion exchange resin was considered due to
the good racemization obtained in the development
of the dynamic Baeyer–Villiger oxidation of racemic
benzyl ketones catalyzed by Baeyer–Villiger
monooxygenases.[10] Identical results were achieved
using the DOWEX-MWA-1 and the Amberlite IRA-
Entry
Temp.
[8C]
Time
ADH-A
[mg]
Conv.
[%][a]
dr[b]
1
2
3
4
5
6
7
8
30
30
30
30
30
30
30
30
30
30
30
30
4
4
4
4
4
4
4
4
24 h
24 h
15
5
96
98
19
41
52
70
75
88
12
21
30
49
8
12
16
20
26
41
53
64
53:47
53:47
91:9
87:13
82:18
70:30
5 min
10 min
20 min
40 min
1 h
15
15
15
15
15
15
5
5
5
5
5
5
5
5
5
5
5
5
63:37 440C (data not shown) in comparison with the previ-
54:46
93:7
2 h
ous kinetic resolution experiments (Table 2). Aiming
for a higher racemization rate, the dynamic process
was tested at higher pH values (9–10). Unfortunately,
only a slight racemization was found when using the
IRA-440C resin at pH 9 (data not shown). For that
reason, the reactions at pH 10 were immediately ana-
lyzed in depth (Table 3).
9
5 min
10 min
20 min
40 min
5 min
10 min
20 min
40 min
1 h
10
11
12
13
14
15
16
17
18
19
20
91:9
89:11
82:18
90:10
90:10
90:10
90:10
90:10
Partial racemization was observed with the triethyl-
amine/hexane and the DOWEX systems at 48C (en-
2 h
4 h
6 h
89:11 tries 1–3). Nevertheless, the racemization of the re-
83:17
76:24
maining ketone was more pronounced with the IRA-
440C as it occurred at pH 9, but now in a higher
extent (entry 4). Unfortunately the enzyme was
highly inactivated under these conditions, either at
a higher enzyme loading (entry 5). For that reason,
a higher loading of E. coli/ADH-A was considered
through two different strategies: by using a higher re-
action temperature and adding the enzyme in one
[a]
Conversion values measured by GC analysis (see the
Supporting Information for further details).
Diastereomeric ratios syn:anti measured by GC analysis
(see the Supporting Information for further details).
[b]
Table 3. DYRKR of racemic ketone 2a using E. coli/ADH-A in a 50 mM Tris-HCl buffer pH 10 at 250 rpm, and subsequent
intramolecular cyclization catalyzed by anhydrous ZnCl2.
Entry
ADH-A [mg]
Racemization system
Time [h]
Temp. [8C]
Conv. [%][a]
ee of 2a [%][a]
dr of 3a[b]
1
2
3
4
5
6
7
8
5
5
5
5
15
15
–
24
24
24
24
24
24
72
72
4
4
4
4
4
30
4
63
59
58
11
15
43
27
84
86
60
72
6
10
6
80:20
85:15
83:17
95:5
94:6
88:12
95:5
Et3N/hexane
DOWEX-MWA-1
IRA-440C
IRA-440C
IRA-440C
IRA-440C
IRA-440C
5+5+5[c]
5+5+5[c]
22
23
30
88:12
[a]
Conversion values measured by GC (see the Supporting Information for further details).
Enantiomeric excess values and diastereomeric ratios syn:anti measured by HPLC (see the Supporting Information for
[b]
further details).
5 mg of ADH-A added at 24 and 48 h of reaction.
[c]
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FULL PAPERS
Dynamic Reductive Kinetic Resolution of Benzyl Ketones
Table 4. DYRKR of racemic ketones 2a–m using E. coli/ADH-A in a 50 mM Tris-HCl buffer pH 10 at 250 rpm.
Entry 2a–m R1
R2
ADH-A [mg][a] Resin
Time [h] Conv. [%][b] ee of 3a–m [%][c] dr of 3a–m[c]
1
2
3
4
5
6
7
8
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
2f
2g
2g
2h
2h
2h
2h
2i
H
H
Me
Me
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5
5+5+5+5
10+10+10
5+5+5
IRA-440C
DOWEX-MWA-1 72
72
84
94
34
79
47
38
67
91
4
52
14
36
24
90
67
83
83
87
51
86
74
89
92
25
86
<3
18
>99
>99
>99
>99
>99
>99
10
88:12
70:30
91:9
82:18
69:31
86:14
95:5
4-OMe Me
4-OMe Me
4-OH
4-OH
4-NO2 Me
4-NO2 Me
2-OMe Me
2-OMe Me
2-Me
2-Me
2-F
2-F
H
H
H
H
H
H
H
H
H
IRA-440C
DOWEX-MWA-1 72
IRA-440C 72
DOWEX-MWA-1 72
IRA-440C 72
DOWEX-MWA-1 72
IRA-440C 72
DOWEX-MWA-1 72
IRA-440C 72
DOWEX-MWA-1 72
IRA-440C 72
DOWEX-MWA-1 72
IRA-440C
IRA-440C
IRA-440C
DOWEX-MWA-1 72
IRA-440C 72
DOWEX-MWA-1 72
72
Me
Me
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
n.d.
95:5
9
>99:<1
>99:<1
>99:<1
>99:<1
97:3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Me
Me
Me
Me
Et
Et
Et
Et
96:4
95:5
72
96
72
89:11
79:21
68:32
96:4
65:35
92:8
76:24
72:28
76:24
57:43
n.d.
n-Pr 5+5+5
n-Pr 5+5+5
allyl 5+5+5
allyl 5+5+5+5
allyl 10+10+10
n-Bu 5+5+5
n-Bu 5+5+5
2i
2j
2j
2j
2k
2k
2l
IRA-440C
IRA-440C
IRA-440C
IRA-440C
DOWEX-MWA-1 72
IRA-440C
IRA-440C
72
96
72
72
H
H
H
H
Cy
Bn
5+5+5
5+5+5
72
72
2m
n.d.
n.d.
[a]
[b]
[c]
Five or ten mg of ADH-A were added at the start of the reaction and then further five or ten mg every 24 h.
Conversion values measured by GC (see the Supporting Information for further details).
Enantiomeric excess values and diastereomeric ratios syn:anti measured by HPLC (see the Supporting Information for
further details). n.d.=not determined.
portion (entry 6) or stepwise addition of the ADH-A tern, we only observed some side reactions for the
(entries 7 and 8), affording in the latter case a 27% IRA-440C resin and the 3-(4’-nitrophenyl)butan-2-
conversion at 48C and an 84% conversion at 308C one (2d), affecting the enantiomeric excess of the re-
with a good selectivity after 3 days. It must be men- sulting alcohol (entry 7).
tioned that the addition of an external nicotinamide
cofactor was not required for the correct action of the found for the C-2 substituted 3-arylbutan-2-ones (2e–
E. coli/ADH-A.[8c]
g, entries 9–14). For the IRA-440C resin, the biore-
Excellent enantio- and diastereoselectivities were
At this point, DYRKR experiments were conducted duction of ketones with alkyl chains longer than
over a series of benzyl ketones 2b–m. The bioreduc- butan-2-ones (R2 different from the methyl group) led
tion experiments were performed at 308C in a Tris- to a significant decrease of the reactivity (entries 15,
HCl buffer pH 10 using the IRA-440C or the 19, 21, 24, 26 and 27), requiring the addition of higher
DOWEX-MWA-1 resin, and adding in all cases the enzyme loadings and longer reaction times to reach
ADH-A in portions over 3 or 4 days. Data are depict- high conversions into the corresponding alcohols (en-
ed in Table 4. As occurred with the formation of alco- tries 16, 17, 22 and 23). On the contrary, the use of
hol 3a, the diastereoisomers syn-(S,S)-3b–m were ob- DOWEX-MWA-1 provided better conversion values
tained in all cases as major compounds and with a per- (86–87% for 2h, 2i and 2k, entries 18, 20 and 25) al-
fect enantiomeric excess. In all cases, the resins and though modest diastereomeric ratios were found in
the ADH were compatible with the substitution pat- these cases.
Adv. Synth. Catal. 2016, 358, 122 – 131
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125

FULL PAPERS
Daniel MØndez-Sµnchez et al.
Table 5. DYRKR of racemic ketones 2a–m using anti-Prelog ADHs in a 50 mM Tris-HCl buffer pH 10 at 250 rpm.
Entry
2a–m
R1
R2
ADH[a]
Resin
Conv. [%][b]
ee of 3a–m [%][c]
dr of 3a–m[c]
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
H
H
H
H
H
4-OMe
4-OH
4-NO2
2-OMe
2-Me
2-F
H
H
H
H
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
LBADH
LBADH
LBADH
–
47
2
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
n.d.
95:5
n.d.
97:3
80:20
97:3
80:20
85:15
87:13
>99: <1
>99: <1
>99: <1
>99: <1
96:4
IRA-440C
DOWEX-MWA-1
–
14
59
14
72
85
30
96
22
16
78
71
65
21
82
<3
27
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
evo-1.1.200
IRA-440C
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
DOWEX-MWA-1
9
10
11
12
13
14
15
16
17
18
n-Pr
allyl
n-Bu
Cy
97:3
91:9
98:2
n.d.
2j
2k
2l
2m
H
Bn
n.d.
n.d.
[a]
Three units of enzyme were added at the start of the reaction and every 24 h thereafter (total 9 units of enzyme).
Conversion values measured by GC (see the Supporting Information for further details).
Enantiomeric excess values and diastereomeric ratios syn:anti measured by HPLC (see the Supporting Information for
further details). n.d.=not determined.
[b]
[c]
Encouraged by the excellent selectivities attained mic ketone 2a was scaled-up from an Eppendorf tube
with the ADH-A, other alcohol dehydrogenases such to a 1 mmol scale at 308C, developing later the intra-
as commercially available Lactobacillus brevis molecular cyclization of the resulting optically active
(LBADH) and evo-1.1.200 were tested, enzymes that alcohol (S,S)-3a (Scheme 2). A sequential addition of
usually act with an opposite anti-Prelog selectivity in the enzyme and the presence of external cofactor
comparison with the ADH-A.[9] Thus, their compati- from the beginning was required, observing a slower
bility with the racemization systems (IRA-440C and reaction rate in comparison with the bioprocess in an
DOWEX-MWA-1 anion resins) was investigated for Eppendorf tube. Nevertheless, after 7 days and the
the development of efficient DYRKR experiments addition of 100 mg of enzyme every 24 h, the alcohol
(Table 5). Firstly, control experiments in the absence (S,S)-3a was obtained with 77% conversion (55% iso-
of resin and the use of LBADH and evo-1.1.200 were
studied in the bioreduction of 3-phenylbutan-2-one
(2a) to explore the compatibility of these two ADHs
with the anion exchange resins (entries 1–6), finding
a considerable inhibition when using the Amberlite
IRA-440C, while the combination of DOWEX-
MWA-1 with the evo-1.1.200 provided a 72% conver-
sion into the syn-alcohol (R,R)-3a with excellent
enantioselectivity and good diastereoselectivity
(entry 6). Then, the racemic ketones 2b–n were sub-
jected to the DYRKR using the same catalytic
system, affording the corresponding (R,R)-3b–m with
moderate to good conversions and every high selec-
tivities for most of the cases (entries 7–18).
Finally, this chemoenzymatic strategy was applied
to the synthesis of a representative member of the
Scheme 2. Chemoenzymatic strategy for the synthesis of 3,4-
dimethylisochroman by a dynamic bioreduction followed by
isochroman family. The bioreduction reaction of race- intramolecular cyclization.
126
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Dynamic Reductive Kinetic Resolution of Benzyl Ketones
3-Phenylbutan-2-one (2a): Colourless oil; yield: 67%; Rf
(50% Et2O/hexane): 0.48; IR (NaCl): n=3061, 2954, 1713,
lated yield) and an 88:12 syn:anti diastereomeric
ratio. Subsequent intramolecular cyclization catalyzed
by anhydrous zinc chloride in methoxymethyl chlo-
ride, led to the desired isochroman (S,S)-4a in 42%
isolated yield and without any loss of the optical
purity (see the Supporting Information).
1484, 1242, 967 cmÀ1
;
1H NMR (300.13 MHz, CDCl3): d=
3
1.39 (d, 3JH,H =7.0 Hz, 3H), 2.04 (s, 3H), 3.74 (q, JH,H
=
7.0 Hz, 1H), 6.83–7.75 (m, 5H); 13C NMR (75.5 MHz,
CDCl3): d=17.2 (CH3), 28.3 (CH3), 53.7 (CH), 127.1 (CH),
127.8 (2CH), 128.9 (2CH), 140.6 (C), 208.8 (C); HR-MS
(ESI+): m/z=171.07864, calcd. for (C10H12NaO)+ (M+Na)+:
171.07858.
Conclusions
3-(4’-Methoxyphenyl)butan-2-one (2b): Colourless oil;
yield: 60%; Rf (50% Et2O/hexane): 0.43; IR (NaCl): n=
3045, 2978, 1716, 1367, 1200, 1083 cmÀ1
;
1H NMR
In summary, the chemical synthesis of a series of
benzyl ketones was developed by reaction of 1-aryl-
propan-2-ones with alkyl halides, to later explore
their DYRKR using alcohol dehydrogenases and
anion exchange resins. After optimization of the bio-
reduction and racemization steps, the dynamic pro-
cesses were performed, affording optically active alco-
hols in good conversions, diastereo- and stereoselec-
tivities. Remarkably, the creation of two stereogenic
centers was possible starting from structurally simple
racemic ketones.
3
(300.13 MHz, CDCl3): d=1.34 (d, JH,H =7.0 Hz, 3H), 2.01
3
(s, 3H), 3.67 (q, JH,H =7.0 Hz, 1H), 3.76 (s, 3H), 6.85 (d,
3JH,H =8.6 Hz, 2H), 7.11 (d, 3JH,H =8.7 Hz, 2H); 13C NMR
(75.5 MHz, CDCl3): d=17.0 (CH3), 27.9 (CH3), 52.5 (CH),
54.9 (CH3), 114.1 (2CH), 128.6 (2CH), 132.4 (C), 158.6 (C),
208.8 (C); HR-MS (ESI+): m/z=201.0892, calcd. for
(C11H14NaO2)+ (M+Na)+: 201.08915.
3-(4’-Hydroxyphenyl)butan-2-one (2c): Yellow oil; yield.
45%; Rf (50% Et2O/hexane): 0.59; IR (NaCl): n=3361,
3056, 2944, 1710, 1489, 1075, 796 cmÀ1
;
1H NMR
3
(300.13 MHz, CDCl3): d=1.38 (d, JH,H =7.0 Hz, 3H), 2.07
3
3
The applicability of these valuable compounds was
(s, 3H), 3.70 (q, JH,H =7.0 Hz, 1H), 6.82 (d, JH,H =8.4 Hz,
demonstrated through the development of a chemoen- 2H), 7.10 (d, 3JH,H =8.4 Hz, 2H); 13C NMR (75.5 MHz,
CDCl3): d=17.6 (CH3), 28.6 (CH3), 53.3 (CH), 116.2 (CH),
129.4 (CH), 132.9 (C), 155.3 (C), 210.3 (C); HR-MS (ESI+):
m/z=187.07344, calcd. for (C10H12NaO)+ (M+Na)+:
187.07350.
zymatic strategy based on the stereocontrolled biore-
duction of 3-phenylbutan-2-one under basic condi-
tions, followed by intramolecular cyclization of the re-
sulting optically active alcohol. Thus, (S,S)-3,4-dime-
thylisochroman was obtained as a representative
member of this family of oxygenated heterocycles.
3-(4’-Nitrophenyl)butan-2-one (2d): Yellow solid; yield:
67%; Rf (50% Et2O/hexane): 0.60; IR (NaCl): n=3061,
2954, 1720, 1537, 1359, 800 cmÀ1
;
1H NMR (300.13 MHz,
3
CDCl3): d=1.46 (d, JH,H =4.6 Hz, 3H), 2.12 (s, 3H), 3.92 (q,
3JH,H =7.0 Hz, 1H), 7.42 (d, 3JH,H =8.8 Hz, 2H), 8.22 (d,
3JH,H =8.8 Hz, 2H); 13C NMR (75.5 MHz, CDCl3): d=17.8
(CH3), 29.1 (CH3), 53.8 (CH), 124.5 (2CH), 129.2 (2CH),
148.2 (CH), 162.7 (C), 207.4 (C); HR-MS (ESI+): m/z=
216.06358, calcd. for (C10H11NNaO3)+ (M+Na)+: 216.06366.
3-(2’-Methoxyphenyl)butan-2-one (2e): Yellow oil; yield:
55%; Rf (50% Et2O/hexane): 0.68; IR (NaCl): n=3031,
Experimental Section
General Remarks
ADH from Lactobacillus brevis (LBADH, 300 U/mL), was
obtained from Codexis Inc. ADH evo-1.1.200 (0.42 U/mg)
was purchased from Evocatal GmbH. ADH-A from Rhodo-
coccus ruber was overexpressed in E. coli BL21 cells and
later lyophilized.[8] The Amberlite IRA-440C and the
DOWEX-MWA-1 anion exchange resins were purchased
from Sigma–Aldrich.
1
2924, 1722, 1055, 739 cmÀ1; H NMR (300.13 MHz, CDCl3):
3
d=1.37 (d, JH,H =7.0 Hz, 3H), 2.04 (s, 3H), 3.85 (s, 3H),
4.07 (q, 3JH,H =7.0 Hz, 1H), 6.89–6.98 (m, 2H), 7.14 (dd,
3JH,H =7.5 Hz; 4JH,H =1.8 Hz, 1H), 7.23–7.29 (m, 1H);
13C NMR (75.5 MHz, CDCl3): d=16.1 (CH3), 28.5 (CH3),
47.3 (CH), 55.7 (CH3), 111.7 (CH), 121.4 (CH), 128.6 (CH),
130.0 (C), 157.1 (C), 210.1 (C); HR-MS (ESI+): m/z=
201.08923, calcd. for (C11H14NaO2)+ (M+Na)+: 201.08915.
3-(2’-Methyphenyl)butan-2-one (2f): Yellow oil; yield:
55%; Rf (50% Et2O/hexane): 0.68; IR (NaCl): n=3058,
2951, 1708, 1490, 1222, 742 cmÀ1. 1H NMR (300.13 MHz,
General Procedure for the Synthesis of Racemic
Ketones 2a–m[11]
To a solution of the corresponding ketone 1a–g (1 mmol) in
a biphasic mixture composed of CH2Cl2 (500 mL) and an
aqueous 2M NaOH solution (500 mL), tetrabutylammonium
bisulfate (340 mg, 1 mmol) and the corresponding alkyl
iodide (1.2 mmol) were added. The reaction mixture was
stirred at 408C for 2 h until no starting material was detect-
ed by TLC analysis. The mixture was extracted afterwards
with CH2Cl2 (3”10 mL), combining the organic layers that
were dried over Na2SO4, filtered and the solvent evaporated
under reduced pressure. The resulting reaction crude was
purified by column chromatography on silica gel (20–50%
Et2O/hexane), affording the corresponding alkyl ketones
2a–m; yield: 30–84%.
3
CDCl3): d=1.37 (d, JH,H =6.9 Hz, 3H), 2.02 (s, 3H), 2.40 (s,
3H), 3.95 (q, 3JH,H =6.9 Hz 1H), 7.05–7.09 (m, 1H), 7.16–
7.22 (m, 3H); 13C NMR (75.5 MHz, CDCl3): d=17.0 (CH3),
20.1 (CH3), 28.7 (CH3), 50.2 (CH), 127.1 (CH), 127.3 (CH),
127.4 (CH), 131.2 (CH), 139.5 (C), 209.6 (C); HR-MS
(ESI+): m/z=185.09418, calcd. for (C11H14NaO)+ (M+Na)+:
185.09423.
3-(2’-Fluorophenyl)butan-2-one (2g): Colourless oil; yield:
84%; Rf (50% Et2O/hexane): 0.58; IR (NaCl): n=3029,
2924, 1718, 1484, 1237, 750 cmÀ1
CDCl3): d=1.42 (d, JH,H =7.0 Hz, 3H), 2.11 (s, 3H), 4.08 (q,
;
1H NMR (300.13 MHz,
3
Adv. Synth. Catal. 2016, 358, 122 – 131
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
127

FULL PAPERS
Daniel MØndez-Sµnchez et al.
3JH,H =7.0 Hz, 1H), 7.07–7.29 (m, 4H); 13C NMR (75.5 MHz,
CDCl3): d=16.0 (CH3), 28.4 (CH3), 45.9 (CH) 115.5 (d,
7.09 (m, 2H), 7.17–7.24 (m, 4H), 7.29–7.36 (m, 3H);
13C NMR (75.5 MHz, CDCl3): d=29.9 (CH2), 38.7 (CH2),
61.9 (CH), 126.5 (CH), 127.8 (CH), 128.6 (CH), 128.7 (CH),
129.3 (CH), 129.4 (CH), 138.9 (C), 140.1 (C), 208.8 (C);
HR-MS (ESI+): m/z=247.11001, calcd. for (C16H16NaO)+
(M+Na)+: 247.10988.
4
2JC,F =22.4 Hz, CH,), 124.6 (CH, d, JC,F =3.3 Hz), 128.8 (d,
3JC,F =7.5 Hz, CH), 129.1 (d, 3JC,F =4.9 Hz, CH,), 162.1 (d,
1JC,F =244.6 Hz, C), 207.9 (C); HR-MS (ESI+): m/z=
189.06922, calcd. for (C10H11FNaO)+ (M+Na)+: 189.06916.
3-Phenylpentan-2-one (2h): Yellow oil; yield: 56%; Rf
(50% Et2O/hexane): 0.69; IR (NaCl): n=3027, 2933, 1712,
1
1493, 759, 701 cmÀ1; H NMR (300.13 MHz, CDCl3): d=0.86
General Procedure for the Synthesis of Racemic
(t, 3JH,H =7.4 Hz, 3H), 1.69–1.78 (m, 1H), 2.04–2.14 (m, Alcohols 3a–m
3
3H), 2.07 (s, 3H), 3.54 (t, JH,H =7.4 Hz, 2H), 7.22–7.35 (m,
To a solution of racemic ketones 2a–m (1 mmol) in dry
5H); 13C NMR (75.5 MHz, CDCl3): d=12.0 (CH3), 24.9
(CH2), 29.0 (CH3), 61.5 (CH), 127.2 (CH), 128.3 (CH), 128.9
(CH), 139.0 (C), 208.6 (C); HR-MS (ESI+): m/z=185.09427,
calcd. for (C11H14NaO)+ (M+Na)+: 185.09423.
MeOH (10 mL), sodium borohydride (38 mg, 1 mmol) was
carefully added under a nitrogen atmosphere. The mixture
was stirred for 2 h until no starting material was detected by
TLC analysis (50% Et2O/hexane). The reaction was
quenched afterwards with H2O (10 mL) and extracted with
CH2Cl2 (3”10 mL). The organic layers were combined,
dried over Na2SO4, filtered and the solvent evaporated
under reduced pressure. The resulting reaction crude was
purified by column chromatography on silica gel (50%
Et2O/hexane), affording the corresponding racemic alcohols
3a–m; yield: 90–99%.
3-Phenylhexan-2-one (2i): Colourless oil; yield. 49%; Rf
(50% Et2O/hexane): 0.70; IR (NaCl): n=3060, 2957, 1715,
1491, 1161, 746, 701 cmÀ1
d=0.90 (t, JH,H =7.3 Hz, 3H), 1.15–1.27 (m, 2H) 1.63–1.76
;
1H NMR (300.13 MHz, CDCl3):
3
3
(m, 1H), 1.96–2.08 (m, 1H), 2.07 (s, 3H), 3.63 (t, JH,H
=
7.4 Hz, 1H), 7.21–7.37 (m, 5H); 13C NMR (75.5 MHz,
CDCl3): d=14.4 (CH3), 21.0 (CH2), 29.4 (CH3), 34.3 (CH2),
60.0 (CH), 127.5 (CH), 128.6 (CH), 129.2 (CH), 139.5 (C),
209.0 (C); HR-MS (ESI+): m/z=199.10991, calcd. for
(C12H16NaO)+ (M+Na)+: 199.10988.
3-Phenylbutan-2-ol (3a): Colourless oil; yield: 93%; Rf
(50% Et2O/hexane): 0.32; diastereomeric ratio for (Æ)-3a
(syn:anti)=20:80; IR (NaCl): n=3373, 3065, 2969, 1472,
3-Phenylhex-5-en-2-one (2j): Yellow oil; yield: 54%; Rf
(50% Et2O/hexane): 0.71; IR (NaCl): n=3063, 2977, 1716,
1
1374, 935 cmÀ1; H NMR (300.13 MHz, CDCl3): d=1.10 (d,
3JH,H =6.3 Hz, 3H, syn), 1.24 (d, 3JH,H =6.2 Hz, 3H, anti),
1641, 1161, 749, 700 cmÀ1 1H NMR (300.13 MHz, CDCl3):
;
3
3
1.28 (d, JH,H =7.0 Hz, 3H, anti), 1.34 (d, JH,H =7.1 Hz, 3H,
syn), 1.56 (br s, 1H, syn+anti), 2.44–3.29 (m, 1H, syn+
anti), 3.49–4.30 (m, 1H, syn+anti), 6.86–7.68 (m, 5H, syn+
anti); 13C NMR (75.5 MHz, CDCl3): d=16.0 (CH3, syn), 17.8
(CH3, anti), 20.5 (CH3, anti), 21.0 (CH3, syn), 47.1 (CH, syn),
47.9 (CH, anti), 72.3 (CH, syn+anti), 126.4 (CH, syn), 126.7
(CH, anti), 127.8 (2CH, syn), 128.0 (2CH, anti), 128.3 (2CH,
syn), 128.5 (2CH, anti), 143.5 (C, anti), 144.2 (C, syn); HR-
MS (ESI+): m/z=173.0390, calcd. for (C10H14NaO)+ (M+
Na)+: 173.0397.
d=2.07, (s, 3H), 2.40–2.50 (m, 1H), 2.77–2.86 (m, 1H), 3.71
(t, 3JH,H =7.5 Hz, 1H), 4.94–5.06 (m, 2H), 5.62–5.75 (m,
2H), 7.21–7.38 (m, 5H); 13C NMR (75.5 MHz, CDCl3): d=
29.5 (CH3), 36.5 (CH2), 59.8 (CH), 116.9 (CH2), 127.7 (CH),
128.7 (CH), 129.3 (CH), 136.1 (CH), 138.7 (C), 208.0 (C);
HR-MS (ESI+): m/z=197.09421, calcd. for (C12H14NaO)+
(M+Na)+: 197.09423.
3-Phenylheptan-2-one (2k): Yellow oil; yield: 60%; Rf
(50% Et2O/hexane): 0.70; IR (NaCl): n=3056, 2977, 1710,
1355, 1117, 731, 698 cmÀ1
d=0.86 (t, JH,H =7.1 Hz, 3H), 1.10–1.35 (m, 4H), 1.62–1.77
;
1H NMR (300.13 MHz, CDCl3):
3-(4’-Methoxyphenyl)butan-2-ol (3b): Colourless oil;
yield: 96%; Rf (50% Et2O/hexane): 0.28; diastereomeric
ratio for (Æ)-3b (syn:anti)=16:84; IR (NaCl): n=3390,
3
3
(m, 2H), 2.06 (s, 3H), 3.61 (t, JH,H =7.4 Hz, 3H), 7.21–7.38
(m, 5H); 13C NMR (75.5 MHz, CDCl3): d=14.3 (CH3), 22.7
(CH2), 29.4 (CH3), 30.0 (CH2), 31.9 (CH2), 60.2 (CH), 127.5
(CH), 128.6 (CH), 129.2 (CH), 139.5 (C), 209.0 (C); HR-MS
(ESI+): m/z=213.12558, calcd. for (C13H18NaO)+ (M+Na)+:
213.12553.
1
3045, 2967, 1477, 1267 and 990 cmÀ1; H NMR (300.13 MHz,
3
3
CDCl3): d=1.07 (d, JH,H =6.3 Hz, 3H, syn), 1.21 (d, JH,H
=
3
6.2 Hz, 3H, anti), 1.24 (d, JH,H =7.1 Hz, 3H, anti), 1.29 (d,
3JH,H =7.1 Hz, 3H, syn), 1.50 (br s, 1H, syn+anti), 2.52–2.78
(m, 1H, syn+anti), 3.63–3.95 (m, 4H, syn+anti), 6.77–6.97
(m, 2H, syn+anti), 7.06–7.20 (m, 2H, syn+anti); 13C NMR
(75.5 MHz, CDCl3): d=16.2 (CH3, syn), 18.0 (CH3, anti),
20.6 (CH3, anti), 20.9 (CH, syn), 46.2 (CH, syn), 47.1 (CH,
anti), 55.3 (CH3, syn+anti), 72.4 (CH, syn+anti), 113.8
(2CH, syn), 114.0 (2CH, anti), 128.7 (CH, syn), 129.0 (CH,
anti), 135.3 (C, anti),136.1 (C, syn), 158.0 (C, syn), 158.2 (C,
anti); HR-MS (ESI+): m/z=203.1053, calcd. for
(C11H16NaO2)+ (M+Na)+: 203.1043.
1-Cyclohexyl-1-phenylpropan-2-one (2l): Yellow oil;
yield: 30%; Rf (50% Et2O/hexane): 0.69. IR (NaCl): n=
3061, 2964, 1719, 1355, 732, 696 cmÀ1
;
1H NMR
(300.13 MHz, CDCl3): d=0.68–0.81 (m, 1H), 0.91–1.04 (m,
1H), 1.11–1.21 (m, 2H), 1.25–1.40 (m, 2H), 1.60–1.75 (m,
3H), 1.79–1.89 (m, 1H), 2.10 (s, 3H), 2.05–2.19 (m, 1H),
3.42 (d, 3JH,H =10.4 Hz 1H), 7.22–7.38 (m, 5H); 13C NMR
(75.5 MHz, CDCl3): d=26.1 (CH2), 26.4 (CH2), 30.4 (CH3),
30.5 (CH2), 32.2 (CH2), 39.5 (CH), 66.5 (CH), 127.1 (CH),
128.7 (CH), 128.8 (CH), 137.5 (C), 208.8 (C); HR-MS
(ESI+): m/z=239.14113, calcd. for (C15H20NaO)+ (M+Na)+:
239.14118.
3-(4’-Hydroxybutan-2-yl)phenol (3c): Colourless oil; yield:
93%; Rf (50% Et2O/hexane): 0.39; diastereomeric ratio for
(Æ)-3c (syn:anti)=14:86; IR (NaCl): n=3361, 3056, 2944,
1488, 1076, 797 cmÀ1 1H NMR (300.13 MHz, CDCl3): d=
;
3,4-Diphenylbutan-2-one (2m): Yellow oil; yield: 40%; Rf
(50% Et2O/hexane): 0.63; IR (NaCl): n=3058, 2958, 1735,
1.24–1.32 (m, 6H, anti+syn), 1.50 (br s, 1H, anti+syn),
2.59–2.75 (m, 1H, syn+anti), 3.77–3.89 (m, 1H, syn+anti),
6.78–6.82 (m, 2H, syn+anti), 7.09–7.13 (m, 2H, syn+anti);
13C NMR (75.5 MHz, CDCl3): d=16.4 (CH3, syn), 18.4
(CH3, anti), 20.1 (CH3, syn+anti), 47.5 (CH3, syn+anti), 73.1
1
696 cmÀ1; H NMR (300.13 MHz, CDCl3): d=2.06 (s, 3H),
2
3
3
2.93 (dd, JH,H =13.8 Hz, JH,H =7.4 Hz 1H), 3.46 (dd, JH,H
=
2
3
13.8 Hz, JH,H =7.4 Hz 1H), 3.95 (t, JH,H =7.4 Hz, 1H), 7.05–
128
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FULL PAPERS
Dynamic Reductive Kinetic Resolution of Benzyl Ketones
3
(CH, syn+anti), 115.9 (2CH, syn+anti), 129.5 (2CH, syn+
anti), 135.6 (C, syn+anti), 155.0 (C, syn+anti); HR-MS
(ESI+): m/z=189.08920, calcd. for (C10H14NaO2)+ (M+
Na)+: 189.08915.
syn+anti), 129.1 (d, JC,F =4.9 Hz, CH, syn+anti), 130.3 (C,
1
syn+anti), 161.1 (d, JC,F =244.6 Hz, C, syn+anti); HR-MS
(ESI+): m/z=191.19786, calcd. for (C10H13FNaO)+ (M+
Na)+: 191.19779.
3-(4’-Nitrophenyl)butan-2-ol (3d): Colourless oil; yield:
96%; Rf (50% Et2O/hexane): 0.53; diastereomeric ratio for
(Æ)-3d (syn:anti)=23:77; IR (NaCl): n=3360, 3055, 2969,
3-Phenylpentan-2-ol (3h): Colourless oil; yield: 97%; Rf
(50% Et2O/hexane): 0.57; diastereomeric ratio for (Æ)-3h
(syn:anti)=19:81; IR (NaCl): n=3227, 1493, 1050, 740,
1
1
3
1473, 1374, 1222, 860 cmÀ1; H NMR (300.13 MHz, CDCl3):
703 cmÀ1; H NMR (300.13 MHz, CDCl3): d=0.78 (t, JH,H
=
3
3
d=1.11 (d, JH,H =6.3 Hz, 3H, syn), 1.23 (d, JH,H =6.2 Hz,
7.4 Hz, 3H, syn+anti), 1.23 (d, 3JH,H =6.2 Hz, 3H, syn+
anti), 1.37 (br s, 1H, syn+anti), 1.61–1.71 (m, 1H, syn+
anti), 1.76–1.86 (m, 1H, syn+anti), 2.39–2.47 (m, 1H, syn+
anti), 3.90–3.99 (m, 1H, syn+anti), 7.22–7.29 (m, 3H, syn+
anti), 7.33–7.38 (m, 2H, syn+anti); 13C NMR (75.5 MHz,
CDCl3): d=12.6 (CH3, syn+anti), 21.5 (CH3, syn+anti),
25.2 (CH2, syn+anti), 56.4 (CH, syn+anti), 71.5 (CH, syn+
anti), 127.1 (CH, syn+anti), 128.9 (CH, syn+anti), 129.3
(CH, syn+anti), 141.7 (C, syn+anti); HR-MS (ESI+): m/z=
187.10993, calcd. for (C11H16NaO)+ (M+Na)+: 187.10988.
3-Phenylhexan-2-ol (3i): Yellow oil; yield: 98%; Rf (50%
Et2O/hexane): 0.60; diastereomeric ratio for (Æ)-3i (syn:an-
ti)=15:85; IR (NaCl): n=3350, 2957, 1487, 1051, 748,
3
3
3H, anti), 1.33 (d, JH,H =7.1 Hz, 3H, anti), 1.37 (d, JH,H
=
7.0 Hz, 3H, syn), 1.50 (br s, 1H, syn+anti), 2.80–2.92 (m,
1H, syn+anti), 3.93–3.99 (m, 1H, syn+anti), 7.38–7.45 (m,
2H, syn+anti), 8.16–8.20 (m, 2H, syn+anti); 13C NMR
(75.5 MHz, CDCl3): d=15.6 (CH3, syn), 17.7 (CH3, anti),
21.2 (CH3, anti), 21.3 (CH3, syn), 47.1 (CH, syn), 47.5 (CH,
anti), 71.8 (CH, syn), 71.9 (CH, anti), 123.6 (2CH, syn+
anti), 128.7 (2CH, syn), 129.0 (2CH, anti), 146.7 (C, syn+
anti), 151.8 (C, syn+anti); HR-MS (ESI+) m/z=218.07928,
calcd. for (C10H13NNaO3)+ (M+Na)+: 218.07931.
3-(2’-Methoxyphenyl)butan-2-ol (3e): Yellow oil (99%);
Rf (50% Et2O/hexane): 0.47; diastereomeric ratio for (Æ)-3e
(syn:anti): 13:87; IR (NaCl): n=3390, 3045, 2967, 1477,
1
3
703 cmÀ1; H NMR (300.13 MHz, CDCl3): d=0.85 (t, JH,H
=
1
1267, 742 cmÀ1; H NMR (300 MHz, CDCl3): d=1.12 (d, J=
7.3 Hz, 3H, syn+anti), 1.10–1.20 (m, 2H, syn+anti), 1.23 (d,
3JH,H =6.2 Hz, 3H, syn+anti), 1.37 (br s, 1H, syn+anti),
1.61–1.71 (m, 1H, syn+anti), 1.76–1.86 (m, 1H, syn+anti),
2.50–2.57 (m, 1H, syn+anti), 3.90–3.99 (m, 1H, syn+anti),
7.22–7.29 (m, 3H, syn+anti), 7.33–7.38 (m, 2H, syn+anti);
13C NMR (75.5 MHz, CDCl3): d=14.5 (CH3, syn+anti), 21.1
(CH2, syn+anti), 21.5 (CH3, syn+anti), 33.7 (CH2, syn), 34.5
(CH2, anti), 54.0 (CH, syn), 54.2 (CH, anti), 71.7 (CH, anti),
72.2 (CH, syn), 126.8 (CH, syn), 127.1 (CH, anti), 128.7
(CH, syn), 128.9 (CH, anti), 129.2 (CH, syn+anti), 142.0 (C,
syn+anti); HR-MS (ESI+): m/z=201.12556, calcd. for
(C12H18NaO)+ (M+Na)+: 201.12553.
3-Phenyl-hex-5-en-2-ol (3j): Yellow oil; yield: 98%; Rf
(50% Et2O/hexane): 0.48; IR (NaCl): n=3323, 2977, 1641,
1161, 760, 702 cmÀ1; diastereomeric ratio for (Æ)-3j (syn:an-
ti)=21:79; 1H NMR (300.13 MHz, CDCl3): d=1.07, (d,
3JH,H =6.3 Hz, 3H, syn), 1.21, (d, 3JH,H =6.3 Hz, 3H, anti)
1.53 (brs, 1H), 2.46–2.69 (m, 2H, syn+anti), 3.98–4.02 (m,
1H, syn+anti), 4.92–5.06 (m, 2H, syn+anti), 5.59–5.70 (m,
1H, syn+anti), 7.17–7.38 (m, 5H); 13C NMR (75.5 MHz,
CDCl3): d 21.5 (CH3, syn+anti), 35.9 (CH2, syn), 36.8 (CH2,
anti), 53.8 (CH, anti), 53.9 (CH, syn), 70.4 (CH, anti), 70.5
(CH, syn), 116.3 (CH2,syn), 116.6 (CH2, anti), 127.0 (CH,
syn), 127.2 (CH, anti), 128.8 (CH, syn), 128.9 (CH, anti),
129.0 (CH, syn) 129.4 (CH, anti), 137.0 (CH, anti), 137.4
(CH, syn), 141.1 (C); HR-MS (ESI+): m/z=199.10991,
calcd. for (C12H16NaO)+ (M+Na)+: 199.10988.
6.3 Hz, 3H, syn), 1.22 (d, J=6.2 Hz, 3H, anti), 1.26 (d, J=
7.2 Hz, 3H, anti), 1.30 (d, J=7.2 Hz, 3H, syn), 1.86 (brs, 1H,
syn+anti), 3.21–3.33 (m, 1H, syn+anti), 3.85 (s, 3H, syn+
anti) 3.92–4.01 (m, 1H, syn+anti), 6.89–7.00 (m, 2H, syn+
anti), 7.19–7.26 (m, 2H, syn+anti); 13C NMR (75.5 MHz,
CDCl3): d=15.2 (CH3, syn), 17.1 (CH3, anti), 21.0 (CH3,
syn), 21.1 (CH3, anti), 39.8 (CH, syn), 40.8 (CH, anti), 55.8
(CH, anti+syn), 111.0 (CH, syn), 111.1 (CH, anti), 121.1
(CH, syn), 121.3 (CH, anti), 127.6 (CH, syn), 127.8 (CH,
anti), 128.6 (anti+syn), 132.1 (C, syn), 132.3 (C, anti), 157.4
(C, syn), 157.8 (C, anti); HR-MS (ESI+): m/z=203.10487,
calcd. for (C11H16NaO2)+ (M+Na)+: 203.10480.
3-(2’-Methylphenyl)butan-2-ol (3f): Colourless oil; yield:
90%; Rf (50% Et2O/hexane): 0.47; diastereomeric ratio for
(Æ)-3f (syn:anti)=13:87; IR (NaCl): n=3203, 2951, 1490,
1222, 1035, 742 cmÀ1
;
1H NMR (300.13 MHz, CDCl3): d=
3
3
1.14 (d, JH,H =6.3 Hz, 3H, syn), 1.22 (d, JH,H =7.0 Hz, 3H,
3
anti), 1.30 (d, JH,H =6.1 Hz, 3H, syn+anti), 1.69 (br s, 1H,
syn+anti), 2.36 (s, 3H, syn) 2.39 (s, 3H, anti), 2.91–3.24 (m,
1H, syn+anti), 3.92–3.96 (m, 1H, syn+anti), 7.13–7.30 (m,
4H, syn+anti); 13C NMR (75.5 MHz, CDCl3): d=18.3 (CH3,
syn+anti), 20.3 (CH3, syn+anti), 20.9 (CH3, syn+anti), 40.0
(CH, syn+anti), 72.7 (CH, syn+anti), 126.2 (CH, syn+anti),
126.6 (CH, syn+anti), 126.9 (CH, syn+anti), 137.2 (C, syn+
anti), 142.5 (C, syn+anti); HR-MS (ESI+): m/z=calcd. for
(C11H16NaO)+ (M+Na)+: 187.10988.
3-(2’-Fluorophenyl)butan-2-ol (3g): Colourless oil; yield:
97%; Rf (50% Et2O/hexane): 0.51; diastereomeric ratio for
(Æ)-3g (syn:anti)=16:84; IR (NaCl): n=3350, 2920, 1489,
3-Phenylheptan-2-ol (3k): Colourless oil; yield: 95%; Rf
(50% Et2O/hexane): 0.62; diastereomeric ratio for (Æ)-3k
(syn:anti)=14:86; IR (NaCl): n=3366, 2917, 1399, 1061,
1237, 752 cmÀ1
;
1H NMR (300.13 MHz, CDCl3): d=
743, 700 cmÀ1 1H NMR (300.13 MHz, CDCl3): d=0.85 (t,
;
1H NMR (300 MHz, CDCl3): d=1.14 (d, J=6.3 Hz, 3H,
syn), 1.23 (d, J=6.2 Hz, 3H, anti), 1.31 (d, J=7.2 Hz, 3H,
anti), 1.36 (d, J=7.1 Hz, 3H, syn), 1.55 (brs, 1H, syn+anti),
3.09–3.18 (m, 1H, syn+anti), 3.95–4.03 (m, 1H, syn+anti),
7.03–7.34 (m, 4H, syn+anti); 13C NMR (75.5 MHz, CDCl3):
d=15.1 (CH3, syn), 16.8 (CH3, anti), 20.8 (CH3, anti), 21.1
(CH3, syn) 40.0 (CH, syn), 40.3 (CH, anti), 71.3 (CH, anti),
3JH,H =7.2 Hz, 3H, syn+anti), 1.04–1.18 (m, 2H, syn+anti),
1.23 (d, 3JH,H =6.3 Hz, 3H, syn+anti), 1.26–1.34 (m, 2H,
syn+anti) 1.45 (br s, 1H, syn+anti), 1.64–1.76 (m, 2H,
syn+anti), 2.48–2.55 (m, 1H, syn+anti), 3.89–3.97 (m, 1H,
syn+anti), 7.22–7.29 (m, 3H, syn+anti), 7.33–7.41 (m, 2H,
syn+anti); 13C NMR (75.5 MHz, CDCl3): d=14.3 (CH3,
syn+anti), 21.5 (CH3, syn+anti), 23.1 (CH2, syn+anti), 30.1
(CH2, syn+anti), 32.0 (CH2, syn+anti), 54.5 (CH, syn+
anti), 71.7 (CH, syn+anti), 127.1 (CH, syn+anti), 128.7
2
71.4 (CH, syn) 115.5 (d, JC,F =23.2 Hz, CH, syn+anti), 124.2
4
3
(CH, d, JC,F =3.2 Hz syn+anti), 127.9 (d, JC,F =8.4 Hz, CH,
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Daniel MØndez-Sµnchez et al.
(CH, syn+anti), 129.2 (CH, syn+anti), 142.0 (C, syn+anti); over Na2SO4, filtered and the solvent was removed under re-
HR-MS (ESI+): m/z=215.14109, calcd. for (C13H20NaO)+ duced pressure. The resulting reaction crude was purified by
(M+Na)+: 215.14118.
column chromatography on silica gel (50% Et2O/hexane),
1-Cyclohexyl-1-phenylpropan-2-ol (3l): Yellow oil; yield: affording the corresponding optically active (S,S)-alcohol
90%; Rf (50% Et2O/hexane): 0.55; IR (NaCl): n=3389, 3a; [a]20: +8.7 (c=1, CHCl3), >99% ee (diastereomeric
2964, 1352, 729, 695 cmÀ1. 1H NMR (300.13 MHz, CDCl3): ratio syDn:anti=88:12).
3
d=0.78–2.05 (m, 10H), 1.08 (d, JH,H =6.3 Hz, 3H), 2.27 (dd,
3
3JH,H =8.4 Hz, JH,H =5.1 Hz, 1H), 4.30–4.38 (m, 1H), 7.20–
General Procedure for the Dynamic Reductive
Kinetic Resolution of Racemic Ketones 2a–m using
Evo.1.1.200 ADH
7.35 (m, 5H); 13C NMR (75.5 MHz, CDCl3): d=22.5 (CH3),
26.5 (CH2), 26.6 (CH2), 30.8 (CH2), 31.9 (CH2) 59.3 (CH),
66.6 (CH), 126.8 (CH), 128.5 (CH), 130.2 (CH), 140.5 (C);
HR-MS (ESI+): m/z= 241.15868, calcd. for (C15H22NaO)+
To
a
solution of the corresponding ketone 2a–m
(M+Na)+: 241.15683.
(0.01 mmol) in a Tris-HCl buffer (325 mL) at different pHs
in an Eppendorf tube, 2-propanol (25 mL), IRA-440C resin
(12 mg) or DOWEX-MWA-1 resin (12 mg), a 10 mM MgCl2
solution (50 mL), a 10 mM NADH solution (50 mL) and
a stock evo-1.1.200 solution (3 U every 24 h) were succes-
sively added. The mixture was shaken at 250 rpm and 308C
for 72 h. After this time, the reaction mixture was extracted
with EtOAc (3”0.5 mL). Organic layers were collected,
dried over Na2SO4 and filtered, measuring the conversions
values into the corresponding alcohols (R,R)-3a–m by GC
analysis, and their optical purity by HPLC analysis.
3,4-Diphenylbutan-2-ol (3m): Yellow oil; yield: 96%; Rf
(50% Et2O/Hhexane): 0.50; IR (NaCl): n=3395, 2961, 1352,
729, 699, 695 cmÀ1; H NMR (300.13 MHz, CDCl3): d=1.22
1
(d, 3JH,H =6.3 Hz, 3H), 1.41 (brs, 1H),2.84–3.02 (m, 2H),
3.22 (dd, 2JH,H =13.3 Hz, 3JH,H =6.5 Hz, 1H), 4.02–4.06 (m,
1H), 7.10–7.36 (m, 9H); 13C NMR (75.5 MHz, CDCl3): d=
22.0 (CH3), 39.0 (CH2), 55.7 (CH), 69.9 (CH), 126.3 (CH),
127.2 (CH), 128.6 (CH), 128.8 (CH), 129.5 (CH), 129.6
(CH), 140.8 (C), 141.0 (C); HR-MS (ESI+): m/z=249.12558,
calcd. for (C16H18NaO)+ (M+Na)+: 249.12553.
General Procedure for the Dynamic Reductive
Kinetic Resolution of Racemic Ketones 2a–m using
ADH from Lactobacillus brevis
General Procedure for the Kinetic Resolution of
Racemic Ketones 2a using E. coli/ADH-A
E. coli/ADH-A cells (15 mg) were rehydrated in an Eppen-
dorf tube with a 50 mM Tris-HCl buffer (500 mL) at differ-
ent pHs. The mixture was shaken at 250 rpm for 30 min and
after this time the corresponding ketone 2a (0.01 mmol), 2-
propanol (25 mL), hexane (25 mL) and a 10 mM solution of
NADH in the corresponding Tris-HCl buffer (50 mL) were
successively added. The reaction was shaken at 250 rpm and
308C, measuring the conversion values by GC analysis.
To
a
solution of the corresponding ketone 2a–m
(0.01 mmol) in a 50 mM Tris-HCl buffer (336 mL) at differ-
ent pHs in an Eppendorf tube, 2-propanol (32 mL), IRA-
440C resin (12 mg) or DOWEX-MWA-1 resin (12 mg),
a 10 mM MgCl2 solution (50 mL), a 10 mM NADPH solution
(60 mL) and a LBADH solution (3 U every 24 h) were suc-
cessively added. The mixture was shaken at 250 rpm and
308C for 72 h. After this time, the reaction mixture was ex-
tracted with EtOAc (3”0.5 mL). Organic layers were col-
lected, dried over Na2SO4, filtered, measuring the conver-
sions values into the corresponding alcohols (R,R)-3a–m by
GC analysis, and their optical purity by HPLC analysis.
General Procedure for the Dynamic Reductive
Kinetic Resolution of Racemic Ketones 2a–m using
E. coli/ADH-A
In an Eppendorf tube containing rehydrated E. coli/ADH-A
cells (5 or 10 mg) in a 50 mM Tris-HCl buffer (500 mL) at
different pHs, the corresponding ketone 2a–m (0.01 mmol),
2-propanol (25 mL), IRA-440C resin (12 mg) or DOWEX-
MWA-1 resin (12 mg) were successively added. The reaction
mixture was shaken at 250 rpm and 308C, adding additional
enzyme (5 or 10 mg) every 24 h. The conversion values into
the corresponding alcohols (S,S)-3a–m were measured by
GC analysis, and their optical purity by HPLC analysis.
General Procedure for the Cyclization of Alcohol 3a
To a solution of the alcohol racemic or (S,S)-3a (100 mg,
0.74 mmol) in methoxymethyl chloride (1.64 mL, 19 mmol),
anhydrous ZnCl2 (43 mg, 0.32 mmol) was added at room
temperature under a nitrogen atmosphere. The mixture was
stirred for 15 min until no starting material was detected by
TLC analysis (50% Et2O/hexane). The reaction was
quenched afterwards with H2O (10 mL) at 08C and extract-
ed with Et2O (3”10 mL). Organic layers were collected,
dried over Na2SO4, filtered, and the solvent evaporated
under reduced pressure. The reaction crude was finally puri-
Scaled-Up Dynamic Reductive Kinetic Resolution of
Racemic Ketones 2a using E. coli/ADH-A
To a solution containing rehydrated E. coli/ADH-A cells fied by column chromatography on silica gel (5% Et2O/
(100 mgmmolÀ1, 100 mg) in a 50 mM Tris-HCl buffer pH 10 hexane), affording the 3,4-dimethylisochroman (4a) as a col-
(0.05M, 19 mL), 2-propanol (5% v/v, 1 mL), IRA-440C orless oil; yield: 53 mg (44%); Rf (5% Et2O/hexane): 0.22;
resin (100 mg), NADH (1 mM, 14 mg) and the racemic diastereomeric ratio for (Æ)-4a syn:anti=88:12. IR (NaCl):
ketone 2a (1 mmol) were successively added. The reaction n=3068, 2978, 1616, 1380, 1035, 809 cmÀ1
;
1H NMR
mixture was shaken for 24 h and 250 rpm. Then, additional (300.13 MHz, CDCl3): d=1.24 (d, 3JH,H =7.1 Hz, 3H, syn),
3
3
E. coli/ADH-A cells (100 mg) were added every 24 h during 1.31 (d, JH,H =6.9 Hz, 3H, anti), 1.32 (d, JH,H =6.4 Hz, 3H,
7 days. The process was monitored by GC analysis. After syn), 1.40 (d, 3JH,H =6.2 Hz, 3H, anti), 2.57–2.83 (m, 1H,
seven days, the mixture was centrifuged and extracted with syn+anti), 3.53 (dq, JH,H =8.7, 6.1 Hz, 1H, anti), 3.92 (qd,
3
EtOAc (3”15 mL). Organic layers were combined, dried 3JH,H =6.4, 2.8 Hz, 1H, syn), 4.82 (s, 3H, anti), 4.87 (s, 3H,
130
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FULL PAPERS
Dynamic Reductive Kinetic Resolution of Benzyl Ketones
syn), 6.97–7.04 (m, 1H, syn+anti), 7.13–7.34 (m, 3H, syn+
anti); 13C NMR (75.5 MHz, CDCl3): d=16.3 (CH3, syn), 17.6
(CH3, anti), 18.1 (CH3, syn), 19.7 (CH3, anti), 36.6 (CH, syn),
38.0 (CH, anti), 67.5 (CH2, anti), 68.4 (CH2, syn), 72.8 (CH,
syn), 77.0 (CH, anti), 123.9 (CH, anti), 124.0 (CH, syn), 125.7
(CH, anti), 126.0 (CH, syn), 126.4 (CH, syn), 126.7 (CH,
anti), 127.2 (CH, anti), 128.8 (CH, syn), 134.0 (C, syn), 134.3
(C, anti), 138.1 (C, anti), 140.6 (C, syn); HR-MS (ESI+):
m/z=185.0943, calcd. for (C11H14NaO)+ (M+Na)+:
185.0937; [a]2D0: +85.8 (c=1, CHCl3), >99% ee for the (S,S)-
diasteroisomer (diastereomeric ratio syn:anti=88:12).
c) G. de Gonzalo, M. Mihovilovic, M. W. Fraaije,
ChemBioChem 2010, 11, 2208–2231; d) T. Matsuda, R.
Yamanaka, K. Nakamura, Tetrahedron: Asymmetry
2009, 20, 513–557.
[4] C. M. Nealon, M. M. Musa, J. M. Patel, R. S. Phillips,
ACS Catal. 2015, 5, 2100–2114.
[5] G. A. Applegate, D. B. Berkowitz, Adv. Synth. Catal.
2015, 357, 1619–1632.
[6] J. Mangas-Sµnchez, E. Busto, V. Gotor, V. Gotor-
Fernµndez, Org. Lett. 2013, 15, 3872–3875.
[7] For recent articles regarding the synthesis and biologi-
cal properties of isochromanes, see: a) E. L. Largui,
T. S. Kaufan, Eur. J. Org. Chem. 2011, 5195–5231; b) R.
Bai, X. Yang, Y. Zhu, Z. Zhou, W. Xie, H. Yao, J.
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[8] This (S)-selective enzyme has a strong preference for
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3881.
Acknowledgements
We thank Prof. Wolfgang Kroutil (University of Graz, Aus-
tria) for providing us with ADH-A overexpressed in E. coli
cells. Financial support from the Spanish Ministerio de Cien-
cia e Innovación (CTQ-2013-44153-P) is gratefully acknowl-
edged.
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