Enzymatic kinetic resolution of tert-butyl 2-(1-Hydroxyethyl) phenylcarbamate, a key intermediate to chiral organoselenanes and organotelluranes

Article abstract of DOI:10.3390/molecules16098098

The enzymatic kinetic resolution of tert-butyl 2-(1-hydroxyethyl) phenylcarbamate via lipase-catalyzed transesterification reaction was studied. We investigated several reaction conditions and the carbamate was resolved by Candida antarctica lipase B (CAL

Full text of DOI:10.3390/molecules16098098

Molecules 2011, 16, 8098-8109; doi:10.3390/molecules16098098
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Enzymatic Kinetic Resolution of tert-Butyl
2-(1-Hydroxyethyl)phenylcarbamate, A Key Intermediate to
Chiral Organoselenanes and Organotelluranes
Leandro Piovan, Monica D. Pasquini and Leandro H. Andrade *
Institute of Chemistry, University of São Paulo, Av. Prof. Lineu Prestes 748, SP 05508-900,
São Paulo, Brazil
* Author to whom correspondence should be addressed; E-Mail: leandroh@iq.usp.br;
Tel./Fax: +55-11-3091-2287.
Received: 19 August 2011; in revised form: 14 September 2011 / Accepted: 15 September 2011 /
Published: 20 September 2011
Abstract: The enzymatic kinetic resolution of tert-butyl 2-(1-hydroxyethyl)
phenylcarbamate via lipase-catalyzed transesterification reaction was studied. We
investigated several reaction conditions and the carbamate was resolved by Candida
antarctica lipase B (CAL-B), leading to the optically pure (R)- and (S)-enantiomers. The
enzymatic process showed excellent enantioselectivity (E > 200). (R)- and (S)-tert-butyl
2-(1-hydroxyethyl)phenylcarbamate were easily transformed into the corresponding (R)-
and (S)-1-(2-aminophenyl)ethanols.
Keywords: alcohols; carbamates; lipases; kinetic resolution; enatiopure
1. Introduction
Selenium- and tellurium-containing compounds have drawn the attention of the scientific
community due to their biological properties [1-4]. Notwithstanding the intense activity in the field
of selenium and tellurium chemistry over the last three decades, organometallic reagents are
commonly employed on the preparation of organo-selenium and -tellurium compounds. Moreover,
hypervalent organoselenium(IV) compounds (organoselenanes) and organotellurium(IV) compounds
(organotelluranes) have been investigated as cysteine protease [5-8], protein tyrosine phosphatase [9]
and poliovirus 3C proteinase inhibitors [10]. Considering the biological activities of organoselenanes

Molecules 2011, 16
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and telluranes, we have described chemoenzymatic methodologies to synthesize selenium compounds
without employing organolithium or organomagnesium reagents [11,12]. Herein, we report the
preparation of enantiopure organochalcogenane precursors, (R)- and (S)-tert-butyl 2-(1-hydroxyethyl)
phenylcarbamate, employing enzymatic kinetic resolution (EKR) catalyzed by lipases. The chiral
building blocks [(R)-I and (S)-I] could be applied as advanced synthetic intermediates of
organotelluranes and organoselenanes III, containing an asymmetric center (Scheme 1). It is possible
to transform (R)-I and (S)-I into their respective arene diazonium salts, followed by a reaction with a
nucleophilic selenium/tellurium specie to give selenides/tellurides II, direct precursors of selenanes
and telullaranes [12].
Scheme 1. Synthetic route to bioactive chalcogenanes [5-6,9,12].
2. Results and Discussion
As outlined in Scheme 2, chiral building blocks (R)-3 and (S)-3 could be synthesized from
commercially available 1-(2-aminophenyl)ethanone (1). Initially, the amine protection leads to the
N-Boc-protected arylketone 2, which by reduction of the ketone group affords (R,S)-3. Then, the latter
could be submitted to an enzymatic kinetic resolution (EKR) and, at the end of the process, both
enantiomers could be easily separated.
2.1. Synthesis of the (R,S)-tert-butyl 2-(1-Hydroxyethyl)phenylcarbamate (3)
Several methods were evaluated to synthesize tert-butyl (2-acetylphenyl)carbamate (2) (Table 1).
The protection of amine group was carried out by reacting 1-(2-aminophenyl)ethanone (1) with
tert-butyl dicarbonate [(Boc)2O]. For example, by using dichloromethane (CH2Cl2) as solvent and
DMAP as additive, after 24 h at room temperature, compound 2 was obtained in 60% yield (Entry 1). It is
worth mentioning that the intermediate 1a was observed, then easily transformed to the compound 2 [13].
A second method, employing THF as solvent and under reflux was evaluated. However, a slight yield
improvement (compound 2, 67%) was observed with a shorter reaction time, 12 h (Entry 2). Other

Molecules 2011, 16
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reaction conditions were evaluated; however, the yields were lower than 40% (Entries 3–5). Next, we
decided to apply the second method (Entry 2) to prepare the compound 2 in a preparative scale (5 mmol).
Synthetic route to enantiopure tert-butyl 2-(1-hydroxyethyl)phenylcarbamates.
Scheme 2.
Synthesis of tert-butyl (2-acetylphenyl)carbamate ( ).
Table 1.
2
Entry Additive (amount)
Solvent
CH2Cl2
THF
t (°C)
r.t.
reflux
r.t.
r.t.
0–r.t.
Time (h) Yield 2 (%)
Ref.
[13]
[13,14]
[15]
[16]
[17]
1
2
3
4
5
DMAP (1 equiv.)
DMAP (1 equiv.)
I2 (2 equiv.)
NaHCO3 (2 equiv.) Dioxane
NaOH (2 equiv.) Dioxane
24
12
12
12
12
60
67
--
37 a
traces a
traces a
Reaction conditions: Compound 1 (0.5 mmol), Boc)2O (1 mmol), solvent (5 mL), additive;
a Determined by GC analysis; r.t. = room temperature.
The reduction of tert-butyl (2-acetylphenyl)carbamate (2) with NaBH4 gave (R,S)-tert-butyl 2-(1-
hydroxyethyl)phenylcarbamate (3) in 84% yield (Scheme 3). The acylated derivative (R,S)-4 was
efficiently synthesized from (R,S)-3 and acetic anhydride (92% yield, Scheme 3).
Synthesis of racemic compounds and .
Scheme 3.
3
4

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2.2. Enzymatic Kinetic Resolution of the (R,S)-tert-butyl 2-(1-Hydroxyethyl)phenylcarbamate (3)
2.2.1. Screening of Lipases for Kinetic Resolution of (R,S)-3
A screening set with 12 different lipases was carried out, looking for a enzyme able to mediate the
transesterification of (R,S)-3 with high enantioselectivity and conversion in a short reaction time
(Table 2).
Table 2. Screening of lipases for kinetic resolution of (R,S)-3c.
Time c a
(h) (%)
ee b (%)
Entry
Lipase
E c
(S)-3 (R)-4
1
2
Candida antarctica
(Novozym® 435; immobilized on
acrylic resin)
12
24
48
12
24
48
12
24
48
12
24
48
12
24
48
47
50
51
33
44
49
16
26
36
14
17
21
12
15
18
88
>99
>99
95
>200
>200
>200
>200
>200
>200
>200
>200
>200
10
>99
>99
49
77
95
19
34
56
13
17
21
11
14
17
3
4
>99
>99
>99
>99
>99
>99
81
Pseudomonas cepacia
5
(immobilized on ceramics)
6
7
Pseudomonas cepacia
8
(immobilized on diatomite)
9
10
11
12
13
14
15
Candida rugosa
81
11
80
11
84
12
Candida cylindracea
81
10
80
10
Candida sp.
16
24
<5 d
nd
nd
nd
(Novozymes® CALB L)
Thermomyces lanuginosus
Rhizomucor miehei
17
18
19
20
21
22
23
24
24
24
24
24
24
24
24
24
<5 d
<5 d
<5 d
<5 d
<5 d
<5 d
<5 d
<5 d
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
nd
Porcine pancreas lipase
Aspergillus niger
Pseudomonas fluorescens
Penicillium camemberti
Mucor javanicus
Pseudomonas cepacia
Reaction conditions: Compound (R,S)-3 (0.25 mmol), lipase (20 mg), vinyl acetate (1 mmol), hexane
(1 mL), 35 °C, 160 rpm; a conversion: c = 100 × (ees/ees + eep); b enantiomeric excess: determined by
c
HPLC analysis; Enantiomeric ratio: E = ln{[eeP (1 − eeS)]/(eeP + eeS)}/ln{[eeP (1 + eeS)]/(eeP + eeS)};
d determined by GC analysis; nd: not determined due to low conversion.

Molecules 2011, 16
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Among the different type of lipases that were used as biocatalysts in the transesterification reaction,
CAL-B presented high values of both conversion and enantioselectivity (Entries 1–3). After 12 h CAL-
B-catalyzed reaction showed 47% conversion, high enantiomeric excess (ee) for (S)-3 (88%) and (R)-4
(>99%), and an enantiomeric ratio (E) higher than 200 (Entry 1). After 24 h, the conversion increased
to 50% and both (S)-3 and (R)-4 were obtained with ee > 99% (Entry 2). After 48 h, the conversion
was higher than 50% and consequently the ee of (R)-4 dropped to 95%. Based on these results, 24 h
was selected as the most appropriate time to interrupt the kinetic resolution process.
Pseudomonas cepacia lipases also presented interesting results, but slightly inferior to those of
CAL-B. For P. cepacia immobilized on ceramics, we observed high ee for (R)-4 (>99%) and E > 200
after 12 h (Entry 4). But, even after 48 h the conversion did not reach 50% and consequently the
maximum ee for (S)-3 was 95% (Entry 6). For P. cepacia immobilized on diatomite, the conversion
was lower than 40% and 56% ee for (S)-3 (Entry 9).
Lipases from Candida rugosa, Candida cylindracea, Candida sp., Thermomyces lanuginosus,
Rhizomucor miehei, porcine pancreas, Aspergillus niger, Pseudomonas fluorescens, Penicillium
camemberti, Mucor javanicus, Pseudomonas cepacia showed discouraging results (Entries 10–24),
including cases in which the conversion was lower than 5% (Entries 16–24). Then, the CAL-B was
selected as the appropriate lipase to be applied to the next studies.
2.2.2. Influence of Solvent in the Kinetic Resolution of (R,S)-3
The solvent influence on EKR of (R,S)-3 was also investigated (Table 3). The results demonstrated
that the reaction in hexane presented high conversion (50%) and excellent enantioselectivity. For those
reactions in toluene (Entries 3 and 4) and methyl tert-butyl ether (Entries 5 and 6) slightly inferior
results were observed, in comparison with hexane. On the other hand, THF, CHCl3, i-PrOH and i-BuOH
(Entries 7–10) showed a dramatic influence on the conversion, which resulted in low values, <30%.
2.2.3. Influence of Temperature in the Kinetic Resolution of (R,S)-3.
The temperature influence on EKR of (R,S)-3 (Table 4) was also studied. Different reaction time
was also evaluated (12, 16, 20 and 24 h). In this study, we found that at 25 °C the perfect kinetic
resolution of (R,S)-3 was achieved after 24 h (Entry 4). At 35 °C, the optimal values were achieved
after 16 h (Entry 6). By increasing the temperature to 40 °C, the desired results were observed in 12 h
(Entry 9). The same reaction-time tendency was verified at 50 °C. Therefore, 40 °C was chosen as the
best temperature, since a reasonable reaction time with a relative low temperature can be used to obtain
an excellent KR of (R,S)-3.
Table 3. Influence of solvent in the lipase-catalyzed transesterfication of (R,S)-3.

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Table 3. Cont.
ee b (%)
Entry
Solvent
Hexane
Time (h) c a (%)
E c
(S)-3 (R)-4
1
2
12
24
12
24
12
24
24
24
24
24
47
50
88
>99
>99
>99
>99
>99
>99
nd
>200
>200
>200
>200
>200
>200
nd
99
61
87
73
94
nd
nd
nd
nd
3
38
Toluene
4
47
5
42
Methyl tert-butyl ether (MTE)
6
49
7
Tetrahydrofuran (THF)
Cloroform (CHCl3)
<30 d
<30 d
<30 d
<30 d
8
nd
nd
9
Isobutylic alcohol (i-BuOH)
Diethylic ether (Et2O)
nd
nd
10
nd
nd
Reaction conditions: Compound (R,S)-3 (0.25 mmol), CAL-B (20 mg), vinyl acetate (1 mmol), solvent
(1 mL), 35 °C, 160 rpm; a conversion: c = 100 × (ees/ees + eep); b enantiomeric excess: determined
by HPLC analysis; c Enantiomeric ratio: E = ln{[eeP (1 − eeS)]/(eeP + eeS)}/ln{[eeP (1 + eeS)]/(eeP + eeS)};
d determined by GC analysis; nd: not determined due to low conversion.
Table 4. Influence of temperature in the lipase-catalyzed transesterfication of (R,S)-3.
ee b (%)
(S)-3 (R)-4
80
Entry
Temperature (°C) Tempo (h)
c a (%)
E c
1
2
25
25
25
25
35
35
35
35
40
40
40
40
50
50
50
50
12
16
20
24
12
16
20
24
12
16
20
24
12
16
20
24
45
46
49
50
48
50
50
50
50
50
51
52
50
50
52
53
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
98
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
86
3
95
4
>99
89
5
6
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
7
8
9
10
11
12
13
14
15
16
97
98
98
97
94
Reaction conditions: Compound (R,S)-3 (0,25 mmol), CAL-B (20 mg), vinyl acetate (1 mmol),
hexane (1 mL), 160 rpm; a conversion: c = 100 × (ees/ees + eep); b enantiomeric excess: determined by
HPLC analysis; c Enantiomeric ratio: E = ln{[eeP (1 − eeS)]/(eeP + eeS)}/ln{[eeP (1 + eeS)]/(eeP + eeS)}.

Molecules 2011, 16
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2.2.4. Study of the Ratio of Enzyme to Substrate for Kinetic Resolution of (R,S)-3
The ratio enzyme/substrate was also investigated for EKR of (R,S)-3 at 40 °C and 12 h (Table 5).
It was observed that 10 mg of CAL-B was not enough to reach 50% of conversion (Entry 3). By using
20 and 40 mg the desired result was achieved just at the end of the experiments (Entries 6 and 9).
However, the reaction with 80 mg of CAL-B showed 50% of conversion after 8 h (Entry 11) and for
the reaction with 100 mg only 6 h were needed to obtain the desired results (Entry 13), but we
considered the ratio of 100 mg CAL-B to 0.25 mmol substrate impracticable, so 20 mg was chosen as
the optimal amount to achieve excellent values of conversion and enantiomeric excess.
Table 5. Study of ratio CAL-B/substrate for kinetic resolution of (R,S)-3.
Massa
(mg)
10
Tempo
c
(%) a
30
39
45
40
45
50
43
48
50
48
50
51
50
50
52
ee (%) b
Entrada
E c
(h)
6
3 4
1
2
63
75
>99
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
>200
10
8
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
97
3
10
12
6
90
4
20
84
5
20
8
93
6
20
12
6
>99
86
7
40
8
40
8
97
9
40
12
6
>99
97
10
11
12
13
14
15
80
80
8
>99
>99
>99
>99
>99
80
12
6
100
100
100
>99
>99
95
8
12
Reaction conditions: Compound (R,S)-3 (0.25 mmol), CAL-B, vinyl acetate (1 mmol), hexane (1 mL),
a
b
40 °C, 160 rpm; conversion: c = 100 × (ees/ees + eep); enantiomeric excess: determined by
HPLC analysis; c Enantiomeric ratio: E = ln{[eeP (1 − eeS)]/(eeP + eeS)}/ln{[eeP (1 + eeS)]/(eeP + eeS)}.
In order to obtain the compounds (S)-3 and (R)-3 and to assign the absolute configuration, a
reaction on a preparative scale (5 mmol) was carried out. After quenching the reaction, the compounds
(S)-3 and (R)-4 were separated by flash gel column chromatography. Then, the ester (R)-4 was
submitted to a hydrolysis reaction to give the alcohol (R)-3 (Scheme 4). In this way, both enantiomers
of 3 were obtained in high enantiomeric purity (ee > 99%) and yields (>45%).

Molecules 2011, 16
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Synthesis of (R)- and (S)- .
Scheme 4.
3
The absolute configuration of the compound 3 was indirectly attributed after deprotection of the
amino group of (−)-(S)-3 [18]. Then, the optical rotation of the resulting amino-alcohol 5 was measured,
and by comparison with literature data [18] its absolute configuration was attributed to (S)-5 (Table 6).
Consequently, the configuration of the NH-Boc protected precursor was also attributed to (S)-3.
Assignment of the absolute configuration of tert-butyl 2-(1-
Table 6.
hydroxyethyl)phenylcarbamate ( ).
3
#
ee (%)
93
[α]D
Literature [18]
This work
+52, 5 (c = 1,0; CHCl3)
+63,1 (c = 1,1; CHCl3)
>99
3. Experimental Section
Commercially available materials were used without further purification. Lipase from Candida
antarctica (fraction B, CAL-B) immobilized, and commercially available as Novozym® 435 was
kindly donated by Novozymes Latin America Ltda. All solvents were HPLC or ACS grade. Solvents
used for moisture sensitive operations were distilled from drying reagents under a nitrogen
atmosphere: THF was distilled from Na/benzophenone.
Analytical thin-layer chromatography (TLC) was performed using aluminum-backed silica plates
coated with a 0.25 mm thickness of silica gel 60 F254 (Merck), visualized with an ultraviolet light
(λ = 254 nm), followed by exposure to p-anisaldehyde solution or vanillin solution and heating.
Standard chromatographic purification methods were followed using 35–70 mm (240–400 mesh) silica
gel purchased from Acros Organics®.

Molecules 2011, 16
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Nuclear magnetic resonance (NMR) spectra were recorded on a Bruker AC 200 spectrometer at
1
operating frequencies of 200 (1H-NMR) and 50 MHz (13C-NMR). The H-NMR chemical shifts are
reported in ppm relative to TMS peak. The data are reported as follows: chemical shift (δ), multiplicity
(s = singlet, d = doublet, t = triplet, qd = quadruplet, dd = double dublet, td = triple dublet, m = multiplet),
and coupling constant (J) in Hertz and integrated intensity. The 13C-NMR chemical shifts are reported
in ppm relative to CDCl3 signal.
Reaction products were analyzed by a Shimadzu model GC-17A (FID) gas chromatograph
equipped with a J&W Scientific HP5 column (30 m × 0.25 mm I.D.; 0.25 µm). The chromatographic
conditions were as follows: Oven temperature initiated at 50 °C and increased at 10 °C/min; run time
20 min; injector temperature 230 °C; detector temperature 250 °C; injector split ratio 1:20; hydrogen
carrier gas at a pressure of 100 kPa. The enantiomeric excesses of the products were determined
by HPLC analyses performed in a Shimadzu model SPD-10Av instrument with UV-Vis detector
(deuterium lamp 190–600 nm) and equipped with a Chiralcel® OD-H column (25 cm × 0.46 cm I.D.;
Daicel Chemical Ind.) eluted with n-hexane (60%) and 2-propanol (99:1).
High-resolution mass spectra (HRMS) were acquired using a Bruker Daltonics MicroTOF
instrument, operating in the electrospray ionization (ESI) mode.
Infrared spectra were recorded from KBr discs or from a thin film between NaCl plates on FTIR
spectrometer (Bomem Michelson model 101). Absorption maxima (νmax) are reported in wavenumbers
(cm−1).
Optical rotations were measured on a Perkin Elmer-343 digital polarimeter in a 1 mL cuvette with a
1 dm pathlength. All values are reported in the following format: [α]D(temperature of measurement) =
specific rotation (concentration of the solution reported in units of 10 mg sample per 1 mL solvent used).
3.1. Synthesis of tert-butyl (2-Acetylphenyl)carbamate (2) (Adapted from References [13,14])
To a solution of the 1-(2-aminophenyl)ethanone (1, 1.35 g, 10 mmol) in anhydrous THF (100 mL)
Boc)2O (6.48 g, 30 mmol) was added, followed by DMAP (122 mg, 1 mmol). The solution was stirred
under reflux for 12 h then concentrated to dryness and partitioned between 0.5 mol L−1 HCl (100 mL)
and EtOAc (100 mL). The aqueous layer was extracted with EtOAc (2 × 100 mL) and the combined
organic phases were washed with brine (50 mL), dried over MgSO4, filtered and concentrated to
afford the crude tert-butyl (2-acetylphenyl)carbamate (2) and the di-Boc derivative products. These
compounds were separated by flash silica gel column chromatography eluted with hexane/EtOAc 9:1.
(2-Acetylphenyl)carbamate (2) and the di-Boc derivative were isolated in 45% and 31% yields,
respectively. The di-Boc compound (1.00 g) was dissolved in CH2Cl2 (100 mL) and Amberlyst 15
resin (1.00 g) was added. The mixture was stirred for 24 h in an orbital shaker. Then, the solvent was
removed and the residue filtered through a silica gel column with hexane/EtOAc 9:1. The compound 2
was obtained in 67% yield. 1H-NMR (200 MHz, CDCl3); δ (ppm): 10.95 (s, 1H); 8.46 (d, 1H, J = 8.3 Hz);
7.84 (dd, 1H, JA = 7.9 Hz; JB = 1.32 Hz); 7.50 (td, 1H, JA = 8.3 Hz; JB = 1.3 Hz); 7.01 (t, 1H, J = 7.9 Hz);
13
2.64 (s, 3H); 1.53 (s, 9H). C-NMR (50 MHz, CDCl3); δ (ppm): 202.5; 153.4; 142.0; 135.2; 131.9;
121.6; 121.2; 119.4; 80.7; 28.8. IV (KBr), cm−1: 3432; 2947; 1713; 1656; 1562; 1239; 1108; 739.
HRMS (ESI), [M+Na]+: Calculated for C13H17NO3Na: 258.1106. Found: 258, 1104.

Molecules 2011, 16
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3.2. Synthesis of Racemic tert-butyl (2-(1-Hydroxyethyl)phenyl)carbamate [(R,S)-3]
To a solution of tert-butyl (2-acetylphenyl)carbamate (2, 1.175 g, 5 mmol) in methanol (50 mL)
NaBH4 (0.21 g, 5.5 mmol) at 0 °C was added. After adding NaBH4, the ice bath was removed and the
solution was stirred at room temperature for 2 h then concentrated to dryness. To residue water
(30 mL) was added and the pH adjusted to 6.0. In the sequence the mixture was extracted with
CH2Cl2 (3 × 15 mL), dried over MgSO4, filtered and concentrated to afford the crude tert-butyl (2-(1-
hydroxyethyl)phenyl)carbamate (3). This was purified by flash silica gel column chromatography
eluted with hexane/EtOAc 9:1 to afford 3 in 84% yield. 1H-NMR (200 MHz, CDCl3);δ (ppm): 8.01 (s,
1H); 7.90 (d, 1H, J = 8.3 Hz); 7.26 (t, 1H, J = 7.5 Hz); 7.14 (d, 1H, J = 7.5 Hz); 7,00 (t, 1H, J = 7.5 Hz);
13
4,95 (qd, 1H, J = 6.6 Hz); 1.54 (m, 12H). C-NMR (50 MHz, CDCl3); δ (ppm): 153.5; 136.7; 132.7;
127.9; 126.4; 122.9; 121.4; 80.0; 69.5; 28.2; 22.1. IR (film), cm−1: 3457; 3343; 2979; 1761; 1727;
1524; 1449; 1254. HRMS (ESI), [M+Na]+: Calculated for C13H19NO3Na: 260.1263. Found: 260.1262.
3.3. Synthesis of Racemic 1-(2-((tert-Butoxycarbonyl)amino)phenyl)ethyl acetate [(R,S)-4]
To a solution of the (2-(1-hydroxyethyl)phenyl)carbamate (3, 237 g, 1 mmol) in pyridine (2 mL)
was added Ac2O (0.10 g, 1 mmol). The solution was stirred at room temperature overnight then diluted
in EtOAc (20 mL) and washed with CuSO4 (5 mL portions) to the complete removal of the pyridine.
The organic phase was dried over MgSO4, filtered and concentrated to afford the crude 1-(2-((tert-
butoxycarbonyl)amino)phenyl)ethyl acetate (4). The crude material was purified by flash silica gel
column chromatography eluted with hexane/EtOAc 9:1 to afford 4 in 92% yield. 1H-NMR (200 MHz,
CDCl3); δ (ppm): 7.78 (d, 2H, J = 8.1 Hz); 7.66 (s, 1H); 7.32 (m, 2H); 7.12 (td, 1H, JA = 7.5 Hz;
13
JB = 0.8 Hz); 5.98 (qd, 1H, J = 6.4 Hz); 2.0 (s, 3H); 1.61 (d, 3H, J = 6.4 Hz); 1.5 (s, 9H). C-NMR
(50 MHz, CDCl3); δ (ppm): 170.2; 152.8; 135.1; 130.7; 128.1; 126.3; 123.6; 122.9; 79.3; 68.1; 27.6;
20.3; 20.0. IR (film), cm−1: 3432; 3338; 2980; 1731; 1591; 1519; 1453; 1241; 1160. HRMS (ESI),
[M+Na]+: Calculated for C15H21NO4Na: 302.1368. Found 302.1364.
3.4. Enzymatic Kinetic Resolution of the (R,S)-tert-butyl 2-(1-Hydroxyethyl)phenylcarbamate [(R,S)-3]
To solution of racemic tert-butyl (2-(1-hydroxyethyl)phenyl)carbamate (3, 1.185 g; 5 mmol) in
hexane (20 mL), CAL-B (Novozym® 435; 400 mg) and vinyl acetate (1.72 g; 20 mmol) were added.
The mixture was stirred in an orbital shaker at 40 °C for 12 h (160 rpm). Following that, the enzyme
was filtered off and washed with dichloromethane (3 × 20 mL). The solvent was removed under
reduced pressure and the residue was purified by flash silica gel column chromatography eluted with
hexane/EtOAc 9:1 to afford (S)-3 (ee > 99%) in 47% yield and (R)-4 (ee > 99%) in 45% yield.
3.5. HPLC Analysis of (S)- and (R)-tert-butyl (2-(1-Hydroxyethyl)phenyl)carbamate (3)
HPLC conditions: Chiralcel® OD-H column, n-hexane/i-PrOH (99:1), 1.0 mL min−1, 254 nm UV
detector. (S)-3: isolated yield = 45%; retention time: 23.7 min; ee > 99%; [α]D22 = −7.7 (c = 1.63; CHCl3).
(R)-3: Isolated yield = 45%; retention time: 29.2 min; ee > 99%; [α]D22 = 7.0 (c = 1.22; CHCl3).

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3.6. General Procedure to Remove Boc-Protecting Group (Adapted from Reference [19])
To a mixture of AcOEt:HCl 3 mol L−1 1:1 (5 mL), N-Boc protected compound (1 mmol) was
added. The mixture was stirred at room temperature for 1 h. After that, the solvent was removed under
vacuum. The residue was dissolved in CH2Cl2 (10 mL) and washed with saturated NaHCO3 solution
(3 × 3 mL). Then, the organic layer was dried over MgSO4, filtered and concentrated to dryness under
vacuum. The product was obtained in quantitative yield without further purification.
4. Conclusions
In summary, we have described an efficient methodology to obtain (R)- and (S)-tert-butyl 2-(1-
hydroxyethyl)phenylcarbamates in enantiopure form (ee > 99%), using a kinetic resolution process
mediated by lipase as a biocatalyst. Both enantiomers can be employed in the preparation of
organochalcogenanes for further application in biological studies.
Acknowledgments
The authors thank CNPq, CAPES and FAPESP for their financial support.
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