Stereoselective chemoenzymatic synthesis of enantiopure 1-(Heteroaryl)ethanamines by lipase-Catalysed kinetic resolutions

Article abstract of DOI:10.1002/ejoc.200900150

The efficient chemical synthesis and enzymatic kinetic resolution of a family of 1-(heteroaryl)ethanamines have been performed with lipases responsible for the preparation of nitrogenated compounds in high optical purity. Thus, Candida antarctica lipase type B has been identified as an excellent biocatalyst for the stereoselective production of the corresponding enantiomerically enriched (B)-acetamides and (S)-amines. A similar effect of the heteroatom in the cyclic ring has been observed in terms of reactivity and enantio- selectivity, with benzoxazole, benzothiazole and benzimidazole derivatives being obtained with excellent enantiopurities after one day of reaction. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009.

Full text of DOI:10.1002/ejoc.200900150

FULL PAPER
DOI: 10.1002/ejoc.200900150
Stereoselective Chemoenzymatic Synthesis of Enantiopure
1-(Heteroaryl)ethanamines by Lipase-Catalysed Kinetic Resolutions
Sergio Alatorre-Santamaría,[a] Vicente Gotor-Fernández,[a] and Vicente Gotor*[a]
Keywords: Amines / Chirality / Enzyme catalysis / Heterocycles / Kinetic resolution
The efficient chemical synthesis and enzymatic kinetic reso-
lution of a family of 1-(heteroaryl)ethanamines have been
performed with lipases responsible for the preparation of
nitrogenated compounds in high optical purity. Thus, Can-
dida antarctica lipase type B has been identified as an excel-
lent biocatalyst for the stereoselective production of the cor-
responding enantiomerically enriched (R)-acetamides and
(S)-amines. A similar effect of the heteroatom in the cyclic
ring has been observed in terms of reactivity and enantio-
selectivity, with benzoxazole, benzothiazole and benzimid-
azole derivatives being obtained with excellent enantiopuri-
ties after one day of reaction.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
molecules of biological interest, we report herein the chemi-
cal preparation of three racemic heteroarylethanamines and
a comparison of their reactivity towards different enzymatic
preparations such as Candida antarctica lipase (CAL-B)
and Pseudomonas cepacia lipase (PSL-C I) in their enzy-
matic kinetic resolution. In addition, non-activating esters
have traditionally been used as resolving agents in the prep-
aration of optically active amines. In this manner, param-
eters that influence the enzymatic catalysis have been
exhaustively studied, looking for the best conditions to iso-
late the desired chiral targets.
Introduction
Optically active amines are an important set of com-
pounds because of their practical applications as chiral
building blocks in the preparation of more complex struc-
tures. In particular, enantiomerically pure heteroarylamines
are employed in medicinal chemistry,[1] agriculture[2] or
asymmetric organocatalysis, in some cases being a part of
metal complexes commonly used for the catalytic hydrogen-
ation of aromatic ketones.[3]
Although a wide range of synthetic procedures have been
adapted for industry, the convenient synthesis of suitable
enantiomerically pure amines by traditional chemical meth-
ods is sometimes associated with several protection and de-
protection steps, which present serious drawbacks such as
low isolated yields, use of air-sensitive catalysts or extremely
long reaction times. Biocatalysis has presented itself as a
practical alternative for asymmetric synthesis in the last two
decades,[4] attracting the attention of industry as a result of
the mild and environmentally friendly conditions employed
in the production of enantiopure compounds.[5] Lipase-me-
diated kinetic resolution processes are the most common
approach to the synthesis of enantiopure compounds[6] even
though just a few of these studies involve the preparation
of optically active 1-(heteroaryl)ethanamines,[7] in contrast
to the vast number of chemoenzymatic syntheses described
for 1-(heteroaryl)ethanol analogues.[8]
Results and Discussion
In this study, we first explored different chemical routes
to the suitable production of a family of racemic heterocy-
clic compounds based on the presence of a benzene ring
fused to a heterocycle. Hence, we describe the chemoenzymat-
ic synthesis of three compounds with potential biological
interest, as occurs with their alcohol-derived analogues: 1-
(benzo[d]imidazol-2-yl)ethanamine (1a), 1-(benzo[d]thiazol-
2-yl)ethanamine (1b) and 1-(benz[d]oxazol-2-yl)ethanamine
(1c). The chemical synthesis of all of these compounds has
been studied starting from commercially available low-cost
compounds.
Our first target compound was the benzimidazole deriva-
tive 1a because of the importance of the benzimidazole ring
as a pharmacophore in modern drug discovery.[9] Thus, we
prepared 1-(benzo[d]imidazol-2-yl)ethanamine (1a) by
modifying a previously reported process,[10] subjecting 1,2-
phenylendiamine (2a) to a direct acidic condensation with
,-alanine (3) at reflux in an aqueous solution of 6  HCl.
A long reaction time was required due to the low kinetics
of the process and the desired heterocyclic compound was
Therefore, as part of our ongoing project devoted
towards the stereoselective preparation of heterocyclic
[a] Departamento de Química Orgánica e Inorgánica. Instituto
Universitario de Biotecnología de Asturias, Universidad de
Oviedo,
C./ Julián Clavería s/n, 33071 Oviedo, Spain
Fax: +34-98-5103448
E-mail: vgs@fq.uniovi.es
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2009, 2533–2538
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2533

S. Alatorre-Santamaría, V. Gotor-Fernández, V. Gotor
FULL PAPER
finally isolated in 63% yield after 144 h in a straightforward
manner (Scheme 1). The reaction was next carried out with
commercially available -alanine, isolating the (S)-amine 1a
in enantiopure form as previously observed by Zang and
Chow.[11]
Scheme 2. Chemical synthesis of 1-(benzo[d]thiazol-2-yl)ethan-
amine (1b).
Scheme 1. Initial attempts at the chemical synthesis of amines 1a–
c.
Following the same approach, the synthesis of 1-(benz[d]-
oxazol-2-yl)ethanamine (1c) was attempted, but poor reac-
Then, encouraged by the ease of preparation of the benz- tivity in the Mitsunobu reaction of the corresponding
imidazole derivative 1a, we decided to synthesize the race- alcohol, in conjunction with several problems encountered
mic heteroarylamines (Ϯ)-1b,c by the same methodology. in the isolation of both the phthalimide and the deprotec-
Thus, 2-mercaptoaniline (2b) and 2-aminophenol (2c) were tion product, led us to develop an alternative method. 2-
treated with ,-alanine at reflux in an aqueous solution of Aminophenol (2c) was treated with lactic acid, employing
6  HCl. Unfortunately, no reaction was observed with p-toluenesulfonic acid monohydrate as the catalyst in a
either of the reactants, even though the reaction times were Dean–Stark apparatus and toluene as the solvent, to obtain
extended to several days. Therefore we decided to search the racemic alcohol 7 after 8 h in 67% isolated yield
for new synthetic methods starting from other commercial (Scheme 3). Next, Mitsunobu reaction with phthalimide
reagents.
and deprotection with hydrazine monohydrate allowed the
Racemic 1-(benzo[d]thiazol-2-yl)ethanamine (1b) was preparation of racemic 1-(benz[d]oxazol-2-yl)ethanamine
prepared through a three-step route starting from benzo- (1c). However, low yields and several problems in both puri-
thiazole (4), as described in Scheme 2. First, benzothiazole fication steps led us to search for another synthetic route.
was acylated by using nBuLi and acetaldehyde following the
To establish the optimal conditions for the preparation
method previously described by Poppe and co-workers[8c] of (Ϯ)-1c, the racemic alcohol 7 was subjected to an acti-
to give the alcohol 5, which was then subjected to a nucleo- vation step with mesyl chloride in the presence of pyridine
philic substitution reaction under Mitsunobu conditions and DMAP in dry dichloromethane (CH2Cl2) as the sol-
with phthalimide, triphenylphosphane (PPh3) and diethyl vent at room temperature. The mesylated alcohol 8 was
azodicarboxylate (DEAD) in dry THF at room tempera- then converted into the azide 9 by using sodium azide and
ture. After 6 h, racemic compound 6 was isolated in 69% N,N-dimethylformamide (DMF) as solvent at 55 °C, iso-
yield after flash chromatography. Next, deprotection of the lated by flash chromatography in 77% yield. Finally, the
phthalimide using hydrazine monohydrate in a mixture of azide was transformed into the benzoxazolylamine 1c by
THF/EtOH at 70 °C afforded the amine (Ϯ)-1b in 75% hydrogenation using Pd over activated carbon in deoxy-
yield after 2 h.
genated MeOH at room temperature. Thus, after 15 h the
Scheme 3. Chemical synthesis of 1-(benzo[d]oxazol-2-yl)ethanamine (1c).
2534
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 2533–2538

Synthesis of Enantiopure 1-(Heteroaryl)Ethanamines
desired racemic amine was finally recovered in a 74% iso- using low temperatures to avoid a decrease in enantio-
lated yield.
selectivity. However, the latter was observed, maybe because
With the three desired substrates in hand, we undertook of a reduction in enzyme activity at times over 24 h (en-
the enzymatic studies. We decided to use the two lipases tries 7 and 8). Because the solubility of the substrate maybe
CAL-B and PSL-C I due to their well-documented capacity an important factor in the kinetics of the reaction, the con-
to produce chiral nitrogenated compounds such as amines centration of 1b was decreased to 0.1  (entries 9 and 10).
and amides.[12] A non-activated ester such as ethyl acetate Again CAL-B showed the best results and, although the
(EtOAc, 10) was selected as the resolving agent, which ad- enantiomeric excesses of (R)-11b fell to 97%, the enantio-
ditionally offers may behave as an acyl donor and solvent selectivity was still excellent and the total conversion
at the same time. Various reaction parameters that could reached 50%, also yielding the substrate in a nearly
influence the enzymatic catalysis were then studied with 1b enantiopure form (entry 9).
as the model substrate (Scheme 4). The reaction data are
summarized in Table 1.
Once we had established a methodology for the resolu-
tion of benzothiazole 1b, we extended the study to the enzy-
matic resolution of racemic amines 1a and 1c using the
same successful experimental conditions (Scheme 5). The
reaction data are summarized in Table 2. First we tested the
benzimidazole derivative 1a, but it was poorly soluble in
TBME, so we decided to explore the possibilities of using
THF as solvent for the enzymatic kinetic resolution of 1a
(entries 3 and 4). As was observed with 1b, CAL-B gave the
best results in the stereoselective preparation of the acet-
amide (R)-11a and the amine (S)-1a, yielding Ͼ99 and 98%
ee, respectively, after 24 h and 51% conversion (entry 3). On
the other hand, PSL-C I showed a lower reaction rate,
reaching 29% conversion in the same reaction time and al-
lowing isolation of the amide in enantiopure form (entry 4).
Scheme 4. Lipase-mediated resolution of benzothiazolylethan-
amine 1b.
The use of the acyl donor as both reactant and solvent
was first studied at 0.15  substrate concentration, both
biocatalysts displaying high activity towards the formation
of the acetamide (R)-11b in high enantiomeric excess after
24 h (Table 1, entries 1 and 2). Although the lipases showed
excellent stereopreference, our main goal was not ac-
complished because conversion values over 50% were at-
tained. To isolate both the amine and the amide in enantio-
pure form, we next decided to perform the enzymatic ki-
netic resolutions using different solvents such as tetra-
hydrofuran (THF) or tert-butyl methyl ether (TBME) and
only 2 or 3 equiv. of the acyl donor (entries 3–6). Both
CAL-B and PSL-C I showed good enantioselectivity, higher
activity being observed with a combination of CAL-B and
TBME (entry 5). The reactions in THF presented lower re-
action rates, the drop being more noticeable with PSL-C I.
Scheme 5. Enzymatic kinetic resolution of racemic amines 1a and
1c.
The benzoxazole derivative (Ϯ)-1c showed similar behav-
After the solvent had been chosen, we focused our efforts iour to 1b. Both CAL-B and PSL-C I showed an excellent
on improving both the conversion and the reaction rates, enantiopreference for the acetylation of the (R)-amine en-
increasing the reaction times of the enzymatic processes, but antiomer, the kinetics of the CAL-B process being much
Table 1. Lipase-mediated kinetic resolution of 1b using EtOAc as the acyl donor at 250 r.p.m. in the production of acetamide (R)-11b
and amine (S)-1b.
Entry
Enzyme
Solvent
EtOAc [equiv.] T [°C] [Substrate] []
t [h]
eeS [%][a]
eeP [%][a] Conv. [%][b]
E[c]
1
2
3
4
5
6
7
8
9
10
CAL-B
PSL-C I
CAL-B
PSL-C I
CAL-B
PSL-C I
CAL-B
PSL-C I
CAL-B
PSL-C I
EtOAc
EtOAc
THF
69
69
2
2
3
3
3
3
3
30
30
30
30
30
30
20
20
30
30
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.1
24
24
24
24
24
24
48
48
24
24
98
95
64
10
91
78
85
50
97
58
82
90
Ͼ99
Ͼ99
Ͼ99
Ͼ99
96
98
97
Ͼ99
55
51
39
9
48
44
47
34
50
37
45
70
Ͼ200
Ͼ200
Ͼ200
Ͼ200
118
168
Ͼ200
Ͼ200
THF
TBME
TBME
TBME
TBME
TBME
TBME
3
0.1
[a] Measured by HPLC, see details in the Supporting Information. [b] c = eeS/(eeS + eeP). [c] E = ln[(1 – c)ϫ(1 – eeP)]/ln[(1 – c)ϫ(1 +
eeP)].
Eur. J. Org. Chem. 2009, 2533–2538
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2535

S. Alatorre-Santamaría, V. Gotor-Fernández, V. Gotor
FULL PAPER
Table 2. Enzymatic kinetic resolution of racemic amines 1a–c using reactions of these substrates have been exhaustively tested
CAL-B or PSL-C I as the biocatalyst and 3 equiv. of EtOAc as the
using ethyl acetate as the acyl donor and different lipases,
acyl donor over 24 h at 30 °C and 250 r.p.m. The starting amine
obtaining optically enriched (R)-acetamides and (S)-amines
was used at a concentration of 0.1 .
in high-to-excellent yields. CAL-B was found to be the best
[a]
[a]
Entry
X
Enzyme
Solvent % eeS
% eeP
% Conv.[b]
E[c]
enzyme for the preparation of enantiomerically pure com-
pounds with TBME or THF as solvent, depending on the
solubility properties of each starting material.
1
2
3
4
5
6
S (1b) CAL-B
S (1b) PSL-C I
N (1a) CAL-B
N (1a) PSL-C I
O (1c) CAL-B
O (1c) PSL-C I
TBME
TBME
THF
97
58
97
50
37
51
29
49
42
Ͼ200
Ͼ200
Ͼ200
Ͼ200
Ͼ200
Ͼ200
Ͼ99
98
Ͼ99
Ͼ99
Ͼ99
Ͼ99
39
THF
TBME
TBME
97
72
Experimental Section
General: Candida antarctica lipase type B (CAL-B, Novozyme 435,
7300 PLU/g) was a gift from Novo Nordisk Co. Pseudomonas cepa-
cia lipase was acquired from Sigma–Aldrich as Amano Lipase
PSL-C I immobilized on ceramic (1061 U/g). All other reagents
were purchased from different commercial sources (Acros, Fluka
or Sigma–Aldrich) and used without further purification. Solvents
were distilled from a suitable desiccant under nitrogen. Flash
chromatography was performed using silica gel 60 (230–240 mesh).
Melting points were measured on samples in open capillary tubes
and are uncorrected. IR spectra were recorded by using NaCl
plates or KBr pellets with a Perkin–Elmer 1720-X F7 spectrometer.
1H and 13C NMR, DEPT and 1H-13C heteronuclear experiments
were performed with a Bruker AC-300 (1H: 300.13 MHz; 13C:
75.5 MHz), DPX-300 (1H: 300.13 MHz; 13C: 75.5 MHz) or AV-400
(1H: 400.13 MHz; 13C: 100.6 MHz) spectrometer. The chemical
shifts are given in delta (δ) values and the coupling constants (J)
in Hertz (Hz). Mass spectra in APCI+, EI+ and ESI+ modes were
recorded with a HP1100 chromatograph mass detector. High-reso-
lution mass spectra were obtained with a Bruker Microtof-Q spec-
[a] Measured by HPLC, see details in the Supporting Information.
[b] c = eeS/(eeS + eeP). [c] E = ln[(1 – c)ϫ(1 – eeP)]/ln[(1 – c)ϫ(1
+ eeP)].
higher than the PSL-C I process. The amine was completely
soluble in TBME and after 24 h, 49% conversion was at-
tained, affording the (R)-acetamide 11c in enantiopure
form.
To demonstrate the stereopreference shown by lipases,
we designed a simple synthetic methodology to obtain com-
pounds of known stereochemistry and we compared the sig-
nal of their optical rotations with those obtained by the
lipase-mediated kinetic resolution of racemic amines. First,
we performed the CAL-B kinetic resolution of the racemic
alcohol 5, as described elsewhere.[8c] Once this enantiopure
alcohol (S)-5 had been isolated, it was subjected to the Mit-
sunobu inversion reaction with PPh3, phthalimide and
DEAD in THF followed by deprotection with N2H4 in a trometer. Optical rotations were measured with a Perkin–Elmer 241
polarimeter. High-performance liquid chromatography (HPLC)
was performed with a Hewlett–Packard 1100 chromatograph with
a UV detector at 210 nm using a Daicel CHIRALCEL OD, CHI-
RALPAK IC or CHIRALPAK IA column (25 cmϫ4.6 mm i.d.),
varying the conditions depending on the specific substrate. Condi-
tions and retention times are given in the Supporting Information.
mixture of THF and EtOH, which gave the (R)-(+)-amine
1b. This amine showed an optical rotation sign opposite to
that observed for the amine obtained directly from the ki-
netic resolution of the racemic amine catalysed by CAL-B
{[α]2D0 = +10.3 (c = 0.98, MeOH) for 98% ee of (R)-1b and
[α]2D0 = –9.3 (c = 0.88, MeOH) for 95% ee of (S)-1b}. The
opposite sign was the result of the Mitsunobu inversion in
the synthesis of the nitrogenated compound, so the lipases
act with the same stereopreference for these amine and
alcohol derivatives.
Similarly, it has recently been reported that lipases such
as CAL-B or PSL have successfully catalysed the acety-
lation of the (R)-alcohol of the corresponding benzimid-
azole derivative, structurally analogous to the amino com-
pound 1a object of our enzymatic study.[8b]
Thus, CAL-B and PSL-C I have made possible the reso-
lution of a family of 1-(heteroaryl)amines under very mild
reaction conditions, allowing in all cases the recovery of the
(R)-acetamides and the (S)-amines with excellent enantio-
purities. The appropriate choice of biocatalyst and organic
solvent is highly important for the success of the enzymatic
synthetic method.
Procedure for the Synthesis of Racemic 1-(1H-Benzimidazol-2-yl)-
ethanamine (1a): o-Phenylenediamine (2a; 432 mg, 4.0 mmol) and
an aqueous solution of 6  HCl (4 mL) were mixed and stirred in
a round-bottomed flask. After a solution had formed, ,-alanine
(534 mg, 6.0 mmol) was added and the mixture heated at reflux for
144 h, when a considerable proportion of the product was detected
by TLC (70% EtOAc/MeOH). The reaction was quenched by add-
ing a saturated aqueous solution of NH3 until the pH of the mix-
ture turned slightly basic (pH 8–9) and then it was extracted with
EtOAc (6ϫ5 mL). The organic phases were combined and dried
with Na2SO4 and the solvent was evaporated under reduced pres-
sure to give a crude product that was purified by flash chromatog-
raphy (100% CH2Cl2 to 5% MeOH/CH2Cl2). Racemic amine 1a
was finally recovered as a pale-yellow solid (409 mg, 63% isolated
yield). R (5% MeOH/CH Cl ) = 0.21. IR (NaCl): ν = 3367, 3050,
˜
f
2
2
2974, 2890, 2733, 1932, 1889, 1772, 1622, 1590, 1457, 1419, 1309,
1273, 1221, 1061, 1020, 991, 933, 850, 769, 751 cm–1. 1H NMR
(MeOD, 300.13 MHz): δ = 7.60 (m, 2 Harom), 7.20 (m, 2 Harom),
4.31 (q, J = 6.6 Hz, 1 H, CH), 1.59 (d, J = 6.8 Hz, 3 H, CH3) ppm.
13C NMR (MeOD, 75.5 MHz): δ = 23.3 (CH3), 47.4 (CH-NH2),
112.2 (CHarom), 121.0 (CHarom), 126.1 (CHarom), 126.8 (CHarom),
142.3 (Carom), 152.6 (Carom), 160.9 (NCN) ppm. HRMS (ESI+):
calcd. for [M + H]+ 162.1031; found 162.1026.
Conclusions
We have chemically synthesized a family of racemic het-
eroarylamines by using the most suitable approach in each
case for the isolation of the benzimidazole, benzothiazole or
Procedure for the Synthesis of Racemic 1-(1,3-Benzothiazol-2-yl)eth-
anol (5): Ethanol 5 was prepared from benzothiazole (4) following
benzoxazole derivatives. The enzymatic kinetic resolution the same approach described by Poppe and co-workers.[8c]
2536
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 2533–2538

Synthesis of Enantiopure 1-(Heteroaryl)Ethanamines
Procedure for the Synthesis of 2-[1-(1,3-Benzothiazol-2-yl)ethyl]-1H- Procedure for the Synthesis of Racemic 1-(1,3-Benzoxazol-2-yl)ethyl
isoindole-1,3(2H)-dione (6): PPh3 (179 mg, 0.68 mmol) and phthal-
imide (83 mg, 0.57 mmol) were successively added to a solution of
racemic alcohol 5 (100 mg, 0.57 mmol) in dry THF (3 mL) under
nitrogen. The resulting solution was cooled to 0 °C and DEAD
(124 µL, 0.68 mmol) dissolved in dry THF (0.9 mL) was added
dropwise. The mixture was allowed to warm to room temperature
and stirred for 6 h; no starting material was detected after this time
by TLC analysis (90% CH2Cl2/MeOH). The organic solvent was
evaporated under reduced pressure and the crude purified by flash
chromatography (eluent gradient 50% CH2Cl2/hexane to 5%
MeOH/CH2Cl2), isolating 6 as a pale-yellow oil (121 mg, 69% iso-
Methanesulfonate (8): Pyridine (370 µL, 4.6 mmol) and DMAP
(38.0 mg, 0.30 mmol) were added to a solution of racemic alcohol
7 (250 mg, 1.5 mmol) in dry CH2Cl2 (19.1 mL) under nitrogen and
the resulting mixture was cooled to 0 °C. Then mesyl chloride
(665 µL, 9 mmol) was added and the mixture was allowed to warm
to room temperature and stirred for 30 h; no starting material was
detected after this time by TLC analysis (50% EtOAc/hexane). The
organic solvent was evaporated under reduced pressure and the
crude purified by flash chromatography (30–50% EtOAc/hexane),
isolating compound 8 as a pale-yellow oil (301 mg, 83% isolated
yield). R (50% EtOAc/hexane) = 0.37. IR (NaCl): ν = 3001, 2992,
˜
f
lated yield). R (5% MeOH/CH Cl ) = 0.85. IR (NaCl): ν = 3057,
2941, 2358, 2340, 1615, 1575, 1477, 1455, 1415, 1351, 1241, 1175,
1106, 1077, 973, 918, 813, 766, 751 cm–1. 1H NMR (CDCl3,
300.13 MHz): δ = 7.73 (d, J = 8.4 Hz, 1 Harom), 7.55 (d, J = 8.5 Hz,
1 Harom), 7.37 (m, 2 Harom), 5.92 (q, J = 6.8 Hz, 1 H, CH), 3.10 [s,
3 H, (SO2)CH3], 1.90 (d, J = 6.8 Hz, 3 H, CH3) ppm. 13C NMR
˜
f
2
2
2984, 2940, 1995, 1778, 1718, 1612, 1520, 1469, 1437, 1383, 1352,
1335, 1265, 1208, 1127, 1030, 898, 880 cm–1. 1H NMR (CDCl3,
300.13 MHz): δ = 7.98 (d, J = 8.2 Hz, 1 Harom), 7.86 (m, 2 Harom),
7.80 (d, J = 8.5 Hz, 1 Harom), 7.72 (m, 2 Harom), 7.43 (t, J = 7.0 Hz,
1 Harom), 7.33 (t, J = 7.0 Hz, 1 Harom), 5.92 (q, J = 7.2 Hz, 1 H, (CDCl3, 100.6 MHz): δ = 19.3 (CH3), 37.3 (CH3), 71.8 (CH), 110.9
CH), 2.09 (d, J = 7.1 Hz, 3 H, CH3) ppm. 13C NMR (CDCl3,
100.6 MHz): δ = 17.6 (CH3), 48.3 (CH), 121.5 (CHarom), 123.2
(CHarom), 120.6 (CHarom), 124.8 (CHarom), 126.0 (CHarom), 140.3
(Carom), 150.6 (Carom), 161.7 (NCO) ppm. MS (EI+): m/z (%) = 241
(CHarom), 123.4 (2 CHarom), 125.1 (CHarom), 126.0 (CHarom), 131.8 (60) [M]+, 162 (100) [M – CH3SO2]+.
(2 Carom), 134.1 (2 Carom), 152.8 (CHarom), 167.4 (2 CO), 170.0
Procedure for the Preparation of the Azide 9: Sodium azide (175 mg,
(NCS) ppm. MS (ESI+): m/z (%) = 331 (100) [M + Na]+, 309 (10)
2.7 mmol) was added to a solution of 8 (433 mg, 1.8 mmol) in
DMF (7.2 mL) and the mixture was heated at reflux at 55 °C over-
night. After no more starting material was detected by TLC (50%
EtOAc/hexane), water (5 mL) was added to the mixture and then
extracted with Et2O (3ϫ5 mL). The organic layers were combined,
dried with Na2SO4 and evaporated under reduced pressure. The
crude was purified by flash chromatography (10% EtOAc/hexane),
isolating the azide 9 as a colourless oil (262 mg, 77% isolated yield).
[M + H]+.
Procedure for the Synthesis of Racemic 1-(1,3-Benzothiazol-2-yl)-
ethanamine (1b): Hydrazine monohydrate (137 µL, 2.9 mmol) was
added to a solution of 6 (120 mg, 0.39 mmol) in THF (5.6 mL) and
EtOH (0.9 mL) and the mixture was stirred at 70 °C for 2 h. The
white suspension formed after this time was filtered off and washed
with THF. The organic solvents were evaporated under reduced
pressure to give a crude that was purified by flash chromatography
(gradient eluent 100% CH2Cl2 to 5% MeOH/CH2Cl2) to give the
corresponding racemic amine 1b as a yellowish oil (52 mg, 75%
R (50% EtOAc/hexane) = 0.60. IR (NaCl): ν = 3336, 3057, 2991,
˜
f
2939, 2475, 2112, 1617, 1569, 1475, 1455, 1241, 1074, 748 cm–1. 1H
NMR (CDCl3, 300.13 MHz): δ = 7.75 (m, 1 Harom), 7.56 (m, 1
isolated yield). R (5% MeOH/CH Cl ) = 0.34. IR (NaCl): ν =
˜
f
2
2
Harom), 7.37 (m, 2 Harom), 4.79 (q, J = 7.0 Hz, 1 H, CH), 1.77 (d,
J = 7.0 Hz, 3 H, CH3) ppm. 13C NMR (CDCl3, 100.6 MHz): δ
= 17.6 (CH3), 53.9 (CH), 110.7 (CHarom), 120.3 (CHarom), 124.6
(CHarom), 125.5 (CHarom), 140.5 (Carom), 150.7 (Carom), 163.8
(NCO) ppm. MS (EI+): m/z (%) = 188 (50) [M]+, 146 (100) [M –
N3]+.
3366, 3297, 2967, 2915, 2863, 2351, 1650, 1590, 1512, 1433, 1308,
756, 726 cm–1. 1H NMR (CDCl3, 300.13 MHz): δ = 7.95 (d, J =
7.9 Hz, 1 Harom), 7.85 (d, J = 7.9 Hz, 1 Harom), 7.43 (t, J = 7.5 Hz,
1 Harom), 7.33 (t, J = 7.3 Hz, 1 Harom), 4.50 (br. d, 1 H, CH), 2.46
(br. s, NH2), 1.60 (d, J = 6.4 Hz, 3 H, CH3) ppm. 13C NMR
(CDCl3, 100.6 MHz): δ = 24.3 (CH3), 50.2 (CH), 121.6 (CHarom),
122.6 (CHarom), 124.6 (CHarom), 125.8 (CHarom), 134.1 (Carom),
153.3 (Carom), 178.8 (NCS) ppm. MS (EI+): m/z (%) = 178 (90)
[M]+, 136 (100) [M – C2H4N]+.
Procedure for the Preparation of the Racemic 1-(1,3-Benzoxazol-2-
yl)ethanamine (1c): The azide 9 (265 mg, 1.4 mmol) and activated
Pd/C (105 mg, 0.14 mmol) were placed in a round-bottom flask
under vacuum. The mixture was dissolved in deoxygenated MeOH
(5.6 mL) injecting at the same time a H2 balloon. The suspension
formed was stirred vigorously overnight, then filtered through Ce-
lite and concentrated under reduced pressure. The crude was puri-
fied by flash chromatography (5–10% MeOH/EtOAc), isolating the
racemic amine 1c as a yellow oil (169 mg, 74% isolated yield). Rf
Procedure for the Synthesis of Racemic 1-(1,3-Benzoxazol-2-yl)etha-
nol (7): Lactic acid (5 mL, 55 mmol) and p-toluenesulfonic acid
(570 mg, 5.5 mmol) were successively added to a solution of o-ami-
nophenol (2c; 6 g, 55 mmol) in toluene (25 mL) and the suspension
formed was heated at reflux in a Dean–Stark apparatus for 8 h.
After this time, the two phases were separated and the aqueous
layer was extracted with EtOAc (4ϫ10 mL). Subsequently, the or-
ganic layers were combined and dried with Na2SO4 and, after that,
the solvents were evaporated. The crude was purified by flash
chromatography (20–40% EtOAc/hexane) to give the alcohol 7
(6.04 g, 67% isolated yield) as a brownish solid. Rf (50% EtOAc/
(10% MeOH/EtOAc) = 0.32. IR (NaCl): ν = 3336, 3290, 2972,
˜
2929, 1611, 1562, 1452, 1058, 749 cm–1. 1H NMR (CDCl3,
300.13 MHz): δ = 7.68 (m, 1 Harom), 7.59 (m, 1 Harom), 7.36 (m, 2
Harom), 4.31 (q, J = 7.0 Hz, 1 H, CH), 1.60 (d, J = 6.9 Hz, 3 H,
CH3) ppm. 13C NMR (CDCl3, 100.6 MHz): δ = 21.9 (CH3), 47.2
(CH), 112.2 (CHarom), 121.0 (CHarom), 126.1 (CHarom), 126.8
(CHarom), 142.3 (Carom), 152.6 (Carom), 171.8 (NCO) ppm. HRMS
(ESI): calcd. for [M + H]+ 163.0871; found 163.0866.
hexane) = 0.37; m.p. 29–31 °C. IR (KBr): ν = 3421, 3284, 2986,
˜
2931, 2871, 1649, 1593, 1512, 1460, 1431, 1365, 1275, 1240, 1185,
1139, 1099, 1028, 1015, 771, 714 cm–1. 1H NMR (CDCl3,
300.13 MHz): δ = 7.67 (m, 1 Harom), 7.46 (m, 1 Harom), 7.29 (m, 2
Harom), 5.17 (q, J = 6.6 Hz, 1 H, CH), 1.72 (d, J = 6.8 Hz, 3 H,
General Procedure for the Enzymatic Acylation Reactions: The ap-
propriate dry solvent (2 mL; TBME for X = S or O; THF for X =
CH3) ppm. 13C NMR (CDCl3, 100.6 MHz): δ = 21.1 (CH3), 63.8 NH) and EtOAc (59 µL, 0.60 mmol) were added to a suspension
(CH), 110.5 (CHarom), 119.6 (CHarom), 124.3 (CHarom), 125.0
(CHarom), 140.1 (CHarom), 150.5 (CHarom), 168.5 (NCO) ppm. enzyme/substrate by weight) under nitrogen. The reaction mixture
HRMS (ESI+): calcd. for [M + H]+ 164.0712; found 164.0706.
was placed in an orbital shaker (250 rpm) for 24 h at 30 °C and
of racemic amine 1a–c (0.20 mmol) and the enzyme (ratio 2:1 of
Eur. J. Org. Chem. 2009, 2533–2538
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2537

S. Alatorre-Santamaría, V. Gotor-Fernández, V. Gotor
FULL PAPER
250 r.p.m., and after this time the enzyme was filtered off, the sol-
vent evaporated under reduced pressure and the crude purified by
flash chromatography (10% MeOH/EtOAc for 1b,c or 30–50%
MeOH/EtOAc for 1a). The amide formed was then directly injected
Supporting Information (see also the footnote on the first page of
this article): Conditions for chiral high-performance liquid
chromatography (HPLC).
into the chiral HPLC apparatus {optical rotation values: [α]2D0
=
Acknowledgments
+99.5 (c = 0.99, MeOH) for 98% ee of (R)-11a, [α]2D0 = +119.6 (c
= 0.96, MeOH) for 99% ee of (R)-11b and [α]2D0 = +145.5 (c = 0.98,
CH2Cl2) for Ͼ99% ee of (R)-11c} and the enantiopure amines were
subjected to a derivatization procedure to determine their enantio-
meric excesses by HPLC. The optical rotations for the enantiomer-
ically enriched amines are: [α]2D0 = –8.6 (c = 0.98, MeOH) for Ͼ99%
ee of (S)-1a, [α]2D0 = –9.3 (c = 0.88, MeOH) for 95% ee of (S)-1b
and [α]2D0 = –12.2 (c = 0.59, MeOH) for 96% ee of (S)-1c.
Financial support by the Spanish Ministerio de Ciencia e Innova-
ción (MICINN) (project CTQ 2007-61126) is acknowledged. V. G.-
F. (Ramón y Cajal Program), and S. A.-S. thank the Mexican Con-
sejo Nacional de Ciencia y Tecnología (CONACYT) for a predoc-
toral fellowship.
[1] a) H. Y. Lo, J. Bentzien, R. W. Fleck, S. S. Pullen, H. H. Khine,
J. Woska Jr., S. Z. Kugler, M. A. Kashem, H. Takahashi,
Chem. Lett. 2008, 18, 6218–6221; b) H. Y. Lo, J. Bentzien, A.
White, C. C. Man, R. W. Fleck, S. S. Pullen, H. H. Khine, J.
King, J. Woska Jr., J. P. Wolak, M. A. Kashem, G. P. Roth, H.
Takahashi, Tetrahedron Lett. 2008, 49, 7337–7340.
[2] H. López-Sandoval, M. E. Londoño-Lemos, R. Garza-Vela-
sco, I. Poblano-Meléndez, P. Granada-Macías, I. Gracia-Mora,
N. Barba-Behrens, J. Inorg. Chem. 2008, 102, 1267–1276.
[3] B. Machura, R. Kruszynski, J. Kusz, Polyhedron 2008, 27,
1679–1689.
General Procedure for the Chemical Acetylation of Racemic or Op-
tically Active Amines 1a–c: Acetyl chloride (6.7 mmol, 478 µL) was
added to a solution of the corresponding amine (1.7 mmol) in dry
CH2Cl2 (2.5 mL) and pyridine (4.2 mmol, 340 µL) at 0 °C. The re-
sulting mixture was stirred at room temperature until no more
amine was detected by TLC. After that time, the solvent was evapo-
rated under reduced pressure and the residue purified by flash
chromatography using different proportions of MeOH/EtOAc as
eluent to give the corresponding amides 11a–c.
[4] V. Gotor, I. Alfonso, E. García-Urdiales (Ed.), Asymmetric Or-
ganic Synthesis with Enzymes, Wiley-VCH, Weinheim, Ger-
many, 2008.
N-[1-(1H-Benzimidazol-2-yl)ethyl]acetamide (11a): Rf (30% MeOH/
[5] a) J. S. Carey, D. Laffan, C. Thomson, M. T. Williams, Org.
Biomol. Chem. 2006, 4, 2337–2347; b) N. Ran, L. Zhao, Z.
Chen, J. Tao, Green Chem. 2008, 10, 361–372; c) J. M. Woodley,
Trends Biotechnol. 2008, 26, 321–327.
[6] a) U. T. Bornscheuer, R. J. Kazlauskas (Eds.), Hydrolases in
Organic Synthesis: Regio- and Stereoselective Biotransforma-
tions, 2nd ed., Wiley-VCH, Weinheim, Germany, 2005; b) V.
Gotor-Fernández, R. Brieva, V. Gotor, J. Mol. Catal. B: En-
zym. 2006, 40, 111–120.
[7] a) L. E. Iglesias, V. M. Sánchez, F. Rebolledo, V. Gotor, Tetra-
hedron: Asymmetry 1997, 8, 2675–2677; b) K. A. Skupinska,
E. J. McEachern, I. R. Baird, R. T. Skerlj, G. J. Bridger, J. Org.
Chem. 2003, 68, 3546–3551; c) A. E. Sigmund, R. DiCosimo,
Tetrahedron: Asymmetry 2004, 15, 2797–2799; d) O. Torre, E.
Busto, V. Gotor-Fernández, V. Gotor, Adv. Synth. Catal. 2007,
349, 1481–1488; e) J. B. Crawford, R. T. Skerlj, G. J. Bridger, J.
Org. Chem. 2007, 72, 669–671; f) K. Ditrich, Synthesis 2008,
2283–2287.
[8] See for example: a) C. Paizs, M. Tos¸a, V. Bódai, G. Szakács, I.
Kmecz, B. Simándi, C. Madjik, L. Novák, F.-D. Irimie, L.
Poppe, Tetrahedron: Asymmetry 2003, 14, 1943–1949; b) R. K.
Cheedral, R. Sachwani, R. Krishna, Tetrahedron: Asymmetry
2008, 19, 901–905; c) M. Tos¸a, S. Pilbák, M. Moldovan, C.
Paizs, G. Szatzker, G. Szakács, L. Novák, F.-D. Irimie, L.
Poppe, Tetrahedron: Asymmetry 2008, 19, 1844–1852; d) M. I.
Tos¸a, P. V. Podea, C. Paizs, F.-D. Irimie, Tetrahedron: Asym-
metry 2008, 19, 2068–2071.
EtOAc) = 0.65; m.p. 245–247 °C. IR (KBr): ν = 3172, 2296, 2968,
˜
2848, 2303, 2269, 1651, 1568, 1456, 1440, 1375, 1319, 1276, 1147,
1
833, 746, 737 cm–1. H NMR (MeOD, 300.13 MHz): δ = 7.56 (m,
2 Harom), 7.23 (m, 2 Harom), 5.30 (q, J = 7.1 Hz, 1 H, CH), 2.07 [s,
3 H, (CO)CH3], 1.66 (d, J = 7.1 Hz, 3 H, CH3) ppm. 13C NMR
(MeOD, 100.6 MHz): δ = 20.2 (CH3), 23.1 [(CO)CH3], 45.8 (CH),
116.2 (2 CHarom), 124.0 (2 CHarom), 139.8 (Carom), 140.0 (Carom),
157.8 (NCN), 173.6 (CO) ppm. HRMS (ESI+): calcd. for [M +
H]+ 204.1137; found 204.1131.
N-[1-(1,3-Benzothiazol-2-yl)ethyl]acetamide (11b): Rf (30% MeOH/
EtOAc) = 0.63; m.p. 139–141 °C. IR (KBr): ν = 3283, 3109, 3042,
˜
2976, 2939, 2426, 1624, 1569, 1472, 1435, 1378, 1156, 1203, 942,
1
825, 743, 715 cm–1. H NMR (MeOD, 300.13 MHz): δ = 7.98 (d,
J = 8.1 Hz, Harom), 7.85 (d, J = 7.7 Hz, Harom), 7.48 (t, J = 7.4 Hz,
Harom), 7.37 (d, J = 7.4 Hz, Harom), 6.63 (br. s, NH), 5.49 (q, J =
7.1 Hz, 1 H, CH), 2.07 [s, 3 H, (CO)CH3], 1.67 (d, J = 7.0 Hz, 3
H, CH3) ppm. 13C NMR (MeOD, 100.6 MHz): δ = 21.8 (CH3),
23.2 [(CO)CH3], 47.7 (CH), 121.7 (CHarom), 122.7 (CHarom), 125.2
(CHarom), 126.2 (CHarom), 134.1 (Carom), 152.5 (Carom), 169.4
(NCS), 172.9 (CO) ppm. MS (APCI+): m/z (%) = 222 (10) [M +
2H]+, 221 (100) [M + H]+.
N-[1-(1,3-Benzoxazol-2-yl)ethyl]acetamide (11c): Rf (10% MeOH/
EtOAc) = 0.51; m.p. 126–128 °C. IR (KBr): ν = 3278, 3096, 3042, [9] M. J. Tebbe, W. A. Spitzer, F. Victor, S. C. Miller, C. C. Lee,
˜
T. R. Sattelberg, E. McKinney, C. J. Tang, J. Med. Chem. 1997,
2985, 2979, 2932, 2426, 1611, 1572, 1472, 1458, 1378, 1245, 1168,
1
1113, 942, 825, 764, 753 cm–1. H NMR (MeOD, 300.13 MHz): δ
40, 3937–3946.
[10] H. Skolnik, J. G. Miller, A. R. Day, J. Am. Chem. Soc. 1943,
65, 1854–1858.
[11] E. Zang, C. Chow, Chin. Chem. Lett. 1991, 2, 169–170.
[12] a) V. Gotor-Fernández, V. Gotor, Curr. Org. Chem. 2006, 10,
1125–1143; b) V. Gotor-Fernández, E. Busto, V. Gotor, Adv.
Synth. Catal. 2006, 348, 797–812.
= 7.69 (m, Harom), 7.62 (m, 1 Harom), 7.40 (m, 2 Harom), 5.32 (q, J
= 7.1 Hz, 1 H, CH), 2.07 [s, 3 H, (CO)CH3], 1.66 (d, J = 7.2 Hz,
3 H, CH3) ppm. 13C NMR (MeOD, 100.6 MHz): δ = 19.3 (CH3),
22.9 [(CO)CH3], 45.6 (CH), 112.3 (CHarom), 121.0 (CHarom), 126.3
(CHarom), 127.0 (CHarom), 142.3 (Carom), 152.6 (Carom), 169.1
(NCO), 173.4 (CO) ppm. HRMS (ESI+): calcd. for [M + H]+
205.0977; found 205.0972.
Received: February 12, 2009
Published Online: April 8, 2009
2538
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 2533–2538