Enzymatic transesterification of pharmacologically interesting β-aminocycloalkanol precursors

Article abstract of DOI:10.1016/j.tetasy.2013.08.007

Chemoenzymatic syntheses of both enantiomers of cis- and trans-2-aminocyclopentanol as well as cis- and trans-2-aminocyclohexanol, which are valuable building blocks for a plethora of ligands and pharmaceuticals have been efficiently carried out. The strategy involves the stereospecific syntheses of racemic aminocycloalkanol precursors via tagging of a phthalimide as a masking group and subsequent lipase-catalyzed kinetic resolution. Most of the lipases exhibited excellent enantioselectivity (E a‰ 200) in the transesterification reactions of trans-derivatives, with both N-protected (R,R)-amino acetates and (S,S)-amino alcohols being isolated in enantiopure form. With regard to cis-derivatives, lipases were also very selective, even though the biotransformations were significantly slower.

Full text of DOI:10.1016/j.tetasy.2013.08.007

Tetrahedron: Asymmetry xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enzymatic transesterification of pharmacologically interesting
b-aminocycloalkanol precursors
Javier González-Sabín a, Nicolás Ríos-Lombardía a, Vicente Gotor b, Francisco Morís a,
⇑
a EntreChem, SL, Edificio Científico Tecnológico, Campus El Cristo, 33006 Oviedo, Spain
b Departamento de Química Orgánica e Inorgánica, Universidad de Oviedo, 33071 Oviedo, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Chemoenzymatic syntheses of both enantiomers of cis- and trans-2-aminocyclopentanol as well as cis-
and trans-2-aminocyclohexanol, which are valuable building blocks for a plethora of ligands and pharma-
ceuticals have been efficiently carried out. The strategy involves the stereospecific syntheses of racemic
aminocycloalkanol precursors via tagging of a phthalimide as a masking group and subsequent lipase-
catalyzed kinetic resolution. Most of the lipases exhibited excellent enantioselectivity (E ꢀ 200) in the
transesterification reactions of trans-derivatives, with both N-protected (R,R)-amino acetates and (S,S)-
amino alcohols being isolated in enantiopure form. With regard to cis-derivatives, lipases were also very
selective, even though the biotransformations were significantly slower.
Received 2 August 2013
Accepted 13 August 2013
Available online xxxx
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
N
NH2
N
OH
The b-amino alcohol moiety is ubiquitous in nature. The subfam-
ily of b-aminocycloalkanols can be found in many natural products
and synthetic bioactive molecules such as the antiarrhythmic ver-
nakalant or the inhibitors of HIV (indinavir) and spleen tyrosine ki-
nase (Syk) depicted in Figure 1.1 Furthermore, they have also
found great application in asymmetric catalysis as auxiliaries or
ligands.2 Due to their inherent interest, a vast number of methods
have been developed over the last two decades in order to obtain
these compounds in enantiomerically pure form, including the
derivatization of naturally occurring amino acids, asymmetric ami-
no hydroxylation of olefins or resolution of racemic mixtures
employing biocatalysts or via crystallization of diastereomeric salts
prepared from optically active carboxylic and phosphoric acids.
Regarding enzymatic methods, the lipase-catalyzed acylation or
deacylation has proved to be one of the most efficient alternatives
to reach these compounds. Many years ago, the enzymatic acylation
of unprotected b-aminocycloalkanols turned out to be challenging,
undergoing undesired migration processes and leading to poor
enantioselectivies.3 However, these problems could be circum-
vented by employing as substrates different precursors such as
b-azido- or b-nitrocycloalkanols,4,5 or N-protected derivatives (car-
bamates or tertiary amines).3,6 Very recently, Taneja et al.7 reported
O
N
HN
N
N
H
N
NH2
O
O
O
O
O
Spleen tyrosine kinase inhibitor
Vernakalant (antiarrhythmic)
N
OH
OH
H
N
N
N
O
O
NH
Indinavir (inhibitor of HIV protease)
Figure 1. Examples of bioactive molecules.
phthalimide moiety as a masking group for the free amino group
was very advantageous since it allowed easy attachment and
cleavage, and also provided derivatives which were recognized by
the enzymes with excellent selectivity. Herein we report a comple-
mentary approach based on the direct lipase-catalyzed acylation of
trans-2-phthalimidocycloalkanols as well as the corresponding
cis-isomers.
a
efficient enzymatic method to prepare enantiopure trans-
1,2-aminocycloalkanols and trans-1,2-aminoindanols via hydrolysis
of the corresponding N-phthalimido acetates. Thus, the use of the
⇑
Corresponding author. Tel.: +34 985259021.
E-mail address: fmv@entrechem.com (F. Morís).
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.08.007
Please cite this article in press as: González-Sabín, J.; et al. Tetrahedron: Asymmetry (2013), http://dx.doi.org/10.1016/j.tetasy.2013.08.007

2
J. González-Sabín et al. / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
Table 1
2. Results and discussion
Enzymatic transesterification of ( )-trans-3a and ( )-trans-3ba
OH
OAc
OH
N
2.1. Synthesis of racemic compounds ( )-trans-3a,b and
( )-cis-3a,b
AcOCH=CH2
lipase, TBME
O
O
O
+
n
n
n
N
N
The synthesis of racemic trans-2-phthalimidocycloalkanols ( )-
trans-3a and ( )-trans-3b was easily accomplished in two steps
following the procedure previously described (Scheme 1).7 Thus,
the corresponding epoxide 1a,b was opened with ammonia and
the resulting trans-2-aminocycloalkanol ( )-2a,b condensed with
phthalic anhydride. After work-up, ( )-trans-3a and ( )-trans-3b
were isolated in a state of purity (yield >80%) and could be em-
ployed in the enzymatic transesterification without further purifi-
cation. Regarding the cis isomers ( )-cis-3a and ( )-cis-3b, these
were obtained respectively from ( )-trans-3a and ( )-trans-3b via
Mitsunobu inversion8 of the secondary alcohol. Once purified by
flash chromatography, racemic cis-2-phthalimidocycloalkanols
were isolated in moderate yields (40–60%).
O
O
O
3a
3b
n = 1, ( )-trans-
n = 1, (R,R)-trans-4a
n = 2, (R,R)-trans-4b
n = 1, (S,S)-trans-3a
n = 2, (S,S)-trans-3b
n = 2, ( )-trans-
c
c
Entry
Compd
Lipase
t (h)
cb (%)
ees (%)
eep (%)
Ed
1
2
3
4
5
6
7
8
9
10
trans-3a
trans-3a
trans-3a
trans-3a
trans-3a
trans-3b
trans-3b
trans-3b
trans-3b
trans-3b
CAL-B
CAL-A
AK
RM IM
PS IM
CAL-B
CAL-A
AK
24
24
24
24
24
24
24
24
24
24
46
4
85
1
>99
>99
>99
19
2
55
>99
>99
>99
21
97
>200
1
51
51
50
16
2
35
50
50
>200
>200
>200
>200
>200
>200
>200
>200
98
>99
>99
>99
>99
>99
>99
RM IM
PS IM
OH
i
a
O
Reactions carried out at 30 °C and 250 rpm, with a 1:3 molar ratio substrate/
acyl donor.
n
n
NH2
b
c ¼ ees=ðees þ eepÞ.
c
n = 1, 1a
n = 2, 1b
n = 1, ( )-2a
Enantiomeric excess determined by chiral HPLC.
2b
d
n = 2, ( )-
E ¼ ln½ð1 ꢁ cÞ ꢂ ð1 ꢁ eesÞꢃ= ln½ð1 ꢁ cÞ ꢂ ð1 þ eepÞꢃ.
ii
AK led to slightly lower conversion compared to the reactions per-
formed with ( )-trans-3a, and CAL-A showed high enantioselectivity
despite very low conversion (entry 7). The assignment of the (1S,2S)-
configuration for the remaining N-protected aminocycloalkanols
trans-3a,b and (1R,2R) for the acetates formed trans-4a,b was estab-
lished by comparison of the sign of its specific rotation with re-
ported values.7 This means that tested lipases preferentially
catalyzed the acetylation of the (1R)-alcohol, following Kazlauskas’
rule.11 It is worth noting not only the high enantioselectivity but also
the yields obtained after chromatographic separation of the reaction
mixture. Thereby, under the optimal conditions (entries 5, 9, and
10), both (1R,2R)-alcohols and (1R,2R)-acetates were isolated in
enantiopure form (ee >99%) from the same biotransformation with
yields higher than 45% (50% is the limiting yield for a kinetic
resolution).
The PS IM-catalyzed transesterification of ( )-trans-3a (entry 5)
was selected to perform a recycling study of the biocatalyst. Thus,
after five cycles no loss of activity or enantioselectivity was ob-
served, maintaining c = 50% and enantiomeric excesses >99% for
both substrate and product after 24 h of reaction. Likewise, this
process was successfully scaled-up (1 g scale at 0.2 M) and despite
a significant reduction of the lipase amount (1:10 wt/wt ratio to
substrate), the optimal conversion of 50% was reached after 48 h.
OH
OH
N
iii, iv
O
O
n
N
n
O
O
n = 1, ( )-cis-3a
3b
n = 1, ( )-trans-3a
n = 2, ( )-trans-3b
n = 2, ( )-cis-
Scheme 1. Reagents and conditions: (i) concd aq NH3 in MeOH, rt; (ii) phthalic
anhydride, Et3N, toluene, reflux; (iii) DEAD, PPh3, p-nitrobenzoic acid, THF; (iv) 1 N
NaOH in MeOH, rt.
2.2. Enzymatic resolution of racemic trans-2-phthalimidocyclo-
alkanols ( )-trans-3a and ( )-trans-3b
First, we studied the kinetic resolution of trans-2-phthalimido
cycloalkanols ( )-trans-3a and ( )-trans-3b by means of a lipase-cat-
alyzed transesterification reaction. Previously, this methodology has
been successfully applied to other trans-b-substituted cycloalkanols
bearing functional groups such as azides,4 halides,9 carbamates3 or
tertiary amines.5,10 Based on this background, we performed the bio-
transformations using a threefold excess of vinyl acetate and tert-
butyl methyl ether (TBME) as the acyl donor and solvent, respec-
tively, and testing a set of five biocatalysts: lipase B from Candida
antarctica (CAL-B), lipase A from Candida antarctica (CAL-A), lipase
from Rhizomucor miehei (RM IM), lipase from Pseudomonas fluores-
cens (AK), and lipase from Pseudomonas cepacia (PS IM). As can be
deduced from Table 1, in the case of the cyclopentylic derivative
( )-trans-3a all of the lipases were highly enantioselective with
the exception of CAL-A (entries 1–5). Thus, after one day of reaction
the other four biocatalysts led to conversions very close to 50% and
very high enantioselectivities (E >200). For example, PS IM lipase led
to an optimal process in which both the substrate and product were
isolated in enantiopure form (ee >99%) to a conversion of 50% (entry
5). On the contrary, CAL-A showed very low activity and no selectiv-
ity (entry 2). Similarly, all of the selected lipases displayed excellent
enantioselectivities (E >200) toward the homolog trans-2-phthalim-
idocyclohexanol ( )-trans-3b (entries 6–10). Particularly, CAL-B and
2.3. Enzymatic resolution of racemic cis-2-phthalimidocyclo-
alkanols ( )-cis-3a and ( )-cis-3b
After successful resolution of the trans-2-phthalimidocycloalka-
nols, we turned our attention to the cis diastereomers ( )-cis-3a
and ( )-cis-3b. We therefore performed the transesterification pro-
cesses under identical reaction conditions to those described
above. In the case of the cyclopentyl derivative cis-3a, the results
obtained were excellent with all of the lipases (Table 2, entries
1–5). Thereby, with the exception of CAL-B which exhibited lower
activity (entry 1), the other lipases allowed us to reach a conver-
sion value of 50% after 24 h, with both the substrate and product
being isolated with ee >99%. Regarding the cyclohexyl cis-deriva-
tive 3b, the lipases showed a notable decrease in their activity
(c <30%) although they maintained excellent enantioselection
toward the formation of the acetate (R,S)-4b (entries 7–10). In
Please cite this article in press as: González-Sabín, J.; et al. Tetrahedron: Asymmetry (2013), http://dx.doi.org/10.1016/j.tetasy.2013.08.007

J. González-Sabín et al. / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
3
Table 2
purchased from c-LEcta. Lipase PS IM was purchased from Amano
Pharmaceutical Co. Lipases from Pseudomonas fluorescens (AK) and
Rhizomucor miehei (Lipozyme RM IM) were purchased from Sigma–
Aldrich. For the enzymatic reactions, tert-butyl methyl ether of
spectrophotometric grade (stored with 4 Å molecular sieves) and
commercial vinyl acetate were used. Thin-layer chromatography
was performed on precoated TLC plates of Merck silica gel
60F254, using potassium permanganate as a developing reagent.
For column chromatography, Merck silica gel 60 (particle size,
Enzymatic transesterification of ( )-cis-3a and ( )-cis-3ba
OH
OAc
OH
N
AcOCH=CH2
lipase, TBME
O
O
O
+
n
n
n
N
N
O
O
O
3a
n = 1, ( )-cis-
4a
4b
3a
3b
n = 1, (R,S)-cis-
n = 1, (S,R)-cis-
40–63 lm) was used. Optical rotations were measured using a Per-
n = 2, ( )-cis-3b
n = 2, (R,S)-cis-
n = 2, (S,R)-cis-
kin–Elmer 343 polarimeter. ESI+ experiments were carried out to
record mass spectra on a Hewlett-Packard 1100 HPLC/MS (electro-
spray) instrument. High-resolution mass spectra (HRMS) were re-
corded in ESI+ mode. 1H NMR and proton-decoupled 13C NMR
spectra (CDCl3 solutions) were obtained using AV-300 or DPX-
300 (1H, 300.13 MHz and 13C, 75.5 MHz) spectrometers using the
d scale (ppm) for chemical shifts; calibration was made on the
c
c
Entry
Compd
Lipase
t (h)
cb (%)
ees (%)
eep (%)
Ed
1
2
3
4
5
6
7
8
cis-3a
cis-3a
cis-3a
cis-3a
cis-3a
cis-3b
cis-3b
cis-3b
cis-3b
cis-3b
cis-3b
cis-3b
CAL-B
CAL-A
AK
RM IM
PS IM
CAL-B
CAL-A
AK
RM IM
PS IM
PS IM
PS IM
24
24
24
24
24
24
24
24
24
24
72
72
28
50
50
50
50
<1
3
38
>99
>99
>99
>99
—
3
61
17
35
>99
>99
>99
>99
>99
—
>99
>99
>99
>99
>99
>99
>200
>200
>200
>200
>200
—
>200
>200
>200
>200
>200
>200
CDCl3 (
13C; 76.95 ppm) or the residual CHCl3 (1H; 7.26 ppm) sig-
6
nals. Chiral HPLC analyses were performed using Chiralcel OJ-H,
Chiralcel OD and Chiralpak AS (Chiral Technologies) columns.
9
14
26
41
50
10
11
12e
69
>99
4.2. General procedure for the synthesis of ( )-trans-2-phtha-
limidocycloalkanols ( )-trans-3a and ( )-trans-3b
a
Reactions carried out at 30 °C and 250 rpm, with a 1:3 molar ratio substrate/
acyl donor.
b
To a mixture of the corresponding cycloalkene oxide 1a,b
(0.1 mol) and 25% NH3 aq (1 mol), methanol was added until com-
plete dissolution. The reaction mixture was then stirred overnight
for cyclohexene oxide 1b or 5 days for cyclopentene oxide 1a. Next,
the solvents were evaporated under reduced pressure and the
resulting trans-2-aminocycloalkanol 2a,b was submitted to the fol-
lowing step without further purification. Thus, phthalic anhydride
(0.1 mol), toluene (100 mL) and a catalytic amount of triethyl-
amine were added to the previous crude mixture and the solution
was refluxed overnight. After cooling to room temperature, the
reaction mixture was extracted twice with water and brine. The or-
ganic layer was concentrated under reduced pressure to yield
crude trans-2-phthalimidocycloalkanol. Further purification by
flash chromatography (hexane:AcOEt mixtures) provided pure
( )-trans-2-phthalimidocyclopentanol 3a (yield >85%) or ( )-
trans-2-phthalimidocyclohexanol 3b (yield >80%).
c ¼ ees=ðees þ eepÞ.
c
d
e
Enantiomeric excess determined by chiral HPLC.
E ¼ ln½ð1 ꢁ cÞ ꢂ ð1 ꢁ eesÞꢃ= ln½ð1 ꢁ cÞ ꢂ ð1 þ eepÞꢃ.
Reaction temperature: 45 °C.
the particular case of CAL-B, this lipase did not show any activity
(entry 6). In order to reach a higher conversion, the biotransforma-
tion catalyzed by the most active lipase, that is, PS IM, was allowed
to react during 72 h (entry 11) to provide c = 41%. Finally, the opti-
mal conversion of 50% was achieved for 72 h with a higher temper-
ature (45 °C), with both (R,S)-4b and (S,R)-3b being isolated in
enantiomerically pure form. The assignment of the (1R,2S)-config-
uration for the resulting acetates cis-4a,b was established by com-
parison of the sign of its specific rotation with the reported values
for (1R,2S)-2-aminocyclopentanol hydrochloride and (1R,2S)-2-
aminocyclohexanol hydrochloride,12 with previous cleavage of
both acetate and phthalimido groups followed by derivatization
as a hydrochloride salt. Again, the lipases exhibited a preference
for the (1R)-alcohol, according to the Kazlauskas’ rule.11
4.3. General procedure for the synthesis of ( )-cis-2-phthalimi-
docycloalkanols ( )-cis-3a and ( )-cis-3b
The lower reactivity of the lipases toward the cis isomers
compared to the trans-counterpart could be expected according
to previous reports with other b-substituted cycloalkanols, and
could arise from the presence of a bulkier substituent (NH-Phth)
on the same face as the reacting hydroxyl group.10,13
To a solution of the corresponding ( )-trans-2-phthalimidocy-
cloal kanol (10.0 mmol) in dry THF (100 mL) at 0 °C under a nitrogen
atmosphere, the following solutions were successively added:
triphenylphosphine (20.0 mmol), p-nitrobenzoic acid (20.0 mmol)
and diethyl azodicarboxylate (20.0 mmol). The resulting yellow
solution was stirred at room temperature overnight. Next, the or-
ganic solvent was evaporated under reduced pressure and the crude
obtained purified by flash chromatography (hexane:AcOEt mix-
tures), affording the corresponding p-nitrobenzoate ester. This inter-
mediate was subsequently hydrolyzed by treatment with 1 M NaOH
in methanol (30 mL) until the disappearance of the starting material
(TLC control, hexane:AcOEt 2:1). Next, methanol was removed and
the residue extracted with H2O (30 mL) and dichloromethane
(3 ꢂ 20 mL). The organic phases were combined, dried over Na2SO4,
and the solvent evaporated under reduced pressure. Further purifi-
cation by flash chromatography (hexane:AcOEt mixtures) provided
pure ( )-cis-2-phthalimidocyclopentanol 3a (60%) or ( )-cis-2-
phthalimidocyclohexanol 3b (40%).
3. Conclusion
The cis- and trans-diastereomers of 2-phthalimidocyclopenta-
nol and 2-phthalimidocyclohexanol were resolved by lipase-cata-
lyzed transesterification reactions. The processes were highly
effective in terms of both chemical yields and enantiomeric excess
of the final products. The PS IM lipase turned out to be the best
biocatalyst, showing complete selectivity toward the four sub-
strates. Likewise, this lipase was employed satisfactorily for recy-
cling and scaling-up studies with trans-3a as a model substrate.
4. Experimental
4.1. General
4.3.1. ( )-cis-2-Phthalimidocyclopentanol ( )-cis-3a
Yield: 60%; mp 71–73 °C; 1H NMR (300 MHz): d 1.50–1.70 (m,
1H), 1.80–2.10 (m, 4H), 2.40–2.60 (m, 1H), br s (1H, OH), 4.33 (q,
Lipase B from Candida antarctica (CAL-B) was a kind gift from
Novo Nordisk. Lipase A from Candida antarctica (CAL-A) was
Please cite this article in press as: González-Sabín, J.; et al. Tetrahedron: Asymmetry (2013), http://dx.doi.org/10.1016/j.tetasy.2013.08.007

4
J. González-Sabín et al. / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
1H, J = 7.5 Hz), 4.53 (m, 1H), 7.71–7.75 (m, 2H), 7.82–7.90 (m, 2H);
13C NMR (75.5 MHz) d 20.7 (CH2), 25.9 (CH2), 34.3 (CH2), 55.0 (CH),
73.7 (CH), 123.3 (2 ꢂ CH), 131.9 (2 ꢂ C), 134.1 (2 ꢂ CH), 169.8
(2 ꢂ C@O); MS (ESI+) m/z (rel intensity): 232.1 (M+H)+, 100; HRMS
(ESI+) calcd for C13H14NO3 [M+H]+ 232.0968; found 232.0959.
4.4.8. (1R,2S)-cis-2-Phthalimido-1-acetoxycyclohexane (1R,2S)-
cis-4b
Mp 130–132 °C; ½a D20
ꢃ
¼ ꢁ12:0 (c 0.50, CHCl3) >99% ee; 1H NMR
(300 MHz): d 1.35–1.80 (m, 5H), 1.90–2.01 (m, 2H), 2.03 (s, 3H),
3.04 (qd, 1H, J = 3.7, 13.3 Hz), 4.28 (ddd, 1H, J = 2.5, 3.8, 13.3 Hz),
5.27 (br s, 1H), 7.73–7.78 (m, 2H), 7.82–7.90 (m, 2H); 13C NMR
(75.5 MHz) d 19.7 (CH2), 21.2 (CH3), 24.0 (CH2), 25.8 (CH2), 30.1
(CH2), 53.8 (CH), 70.8 (CH), 123.1 (2 ꢂ CH), 131.8 (2 ꢂ C), 133.9
(2 ꢂ CH), 168.6 (C@O), 171.1 (2 ꢂ C@O); MS (ESI+) m/z (rel inten-
sity): 288.2 (M+H)+, 50, 310.1 (M+Na)+, 40; HRMS (ESI+) calcd for
4.3.2. ( )-cis-2-Phthalimidocyclohexanol ( )-cis-3b
Yield: 40%; mp 149–151 °C; 1H NMR (300 MHz): d 1.40–1.65
(m, 4H), 1.75–2.05 (m, 3H), 2.52 (qd, 1H, J = 3.9, 12.3 Hz), 4.19
(br s, 1H), 4.31 (ddd, 1H, J = 1.9, 3.6, 13.1 Hz), 4.40 (br s, 1H, OH),
7.73–7.78 (m, 2H), 7.82–7.90 (m, 2H); 13C NMR (75.5 MHz) d
18.6 (CH2), 24.3 (CH2), 26.0 (CH2), 32.8 (CH2), 56.1 (CH), 69.1
(CH), 123.5 (2 ꢂ CH), 131.7 (2 ꢂ C), 134.3 (2 ꢂ CH), 169.7
(2 ꢂ C@O); MS (ESI+) m/z (rel intensity): 246.1 (M+H)+, 100,
268.2 (M+Na)+, 30; HRMS (ESI+) calcd for C14H16NO3 [M+H]+
246.1125; found 246.1136.
C
16H18NO4 [M+H]+ 288.1230; found 288.1224.
4.5. HPLC analyses
Chiralcel OJ-H, Chiralcel OD and Chiralpak AS columns were
used. UV detection, k = 210 nm.
4.4. Enzymatic transesterification of racemic cis- and trans-2-
phthalimidocycloalkanols. General procedure
4.5.1. ( )-trans-2-Phthalimidocyclopentanol ( )-trans-3a
Chiralcel OJ-H, hexane:2-propanol 95:5; 0.8 mL/min, 30 °C.
tR = 34.2 (R,R) and 39.0 (S,S) min RS = 1.5.
At first, tert-butyl methyl ether (1.0 mL) and vinyl acetate
(3 equiv, 0.6 mmol, 37 lL) were added to a mixture of the racemic
4.5.2. ( )-trans-2-Phthalimido-1-acetoxycyclopentane ( )-
trans-4a
Chiralcel OJ-H, hexane:2-propanol 95:5; 0.8 mL/min, 30 °C.
tR = 15.7 (R,R) and 25.1 (S,S) min RS = 9.3.
N-protected amino alcohol (0.20 mmol), lipase (1:1 wt/wt ratio to
substrate) and 4 Å molecular sieves (5 mg). The suspension was
shaken at 30 °C and 250 rpm. The enzyme was then filtered,
washed with tert-butyl methyl ether and the solvents were evapo-
rated. Both the remaining N-protected amino alcohol and the N-
protected amino acetate present in the residue were separated
by flash chromatography (hexane:AcOEt mixtures) and isolated
in yields higher than 90% (referred to the corresponding conversion
value).
4.5.3. ( )-trans-2-Phthalimidocyclohexanol ( )-trans-3b
Chiralcel OJ-H, hexane:2-propanol 80:20; 0.6 mL/min, 20 °C.
tR = 10.7 (R,R) and 11.5 (S,S) min RS = 1.8.
4.5.4. ( )-trans-2-Phthalimido-1-acetoxycyclohexane ( )-trans-
4b
Chiralcel OJ-H, hexane:2-propanol 80:20; 0.6 mL/min, 20 °C.
tR = 12.7 (S,S) and 14.4 (R,R) min RS = 3.4.
4.4.1. (1S,2S)-trans-2-Phthalimidocyclopentanol (1S,2S)-trans-
3a
½
a 2D0
ꢃ
¼ þ42:1 (c 1.0, CHCl3) >99% ee.
4.5.5. ( )-cis-2-Phthalimidocyclopentanol ( )-cis-3a
Chiralpak AS, hexane:2-propanol 90:10; 0.8 mL/min, 40 °C.
tR = 14.2 (S,R) and 17.6 (R,S) min RS = 8.3.
4.4.2. (1R,2R)-trans-2-Phthalimido-1-acetoxycyclopentane
(1R,2R)-trans-4a
½
a 2D0
ꢃ
¼ ꢁ51:0 (c 1.0, CHCl3) >99% ee.
4.5.6. ( )-cis-2-Phthalimido-1-acetoxycyclopentane ( )-cis-4a
Chiralpak AS, hexane:2-propanol 90:10; 0.8 mL/min, 40 °C.
tR = 10.4 (R,S) and 20.1 (S,R) min RS = 15.0.
4.4.3. (1S,2S)-trans-2-Phthalimidocyclohexanol (1S,2S)-trans-3b
½
a 2D0
ꢃ
¼ þ32:3 (c 1.0, CHCl3) >99% ee.
4.5.7. ( )-cis-2-Phthalimidocyclohexanol ( )-cis-3b
Chiralcel OD, hexane:2-propanol 90:10; 0.8 mL/min, 40 °C.
tR = 9.3 (S,R) and 10.1 (R,S) min RS = 2.0.
4.4.4. (1R,2R)-trans-2-Phthalimido-1-acetoxycyclohexane
(1R,2R)-trans-4b
½
a 2D0
ꢃ
¼ ꢁ10:9 (c 1.0, CHCl3) >99% ee.
4.4.5. (1S,2R)-cis-2-Phthalimidocyclopentanol (1S,2R)-cis-3a
4.5.8. ( )-cis-2-Phthalimido-1-acetoxycyclohexane ( )-cis-4b
Chiralcel OD, hexane:2-propanol 90:10; 0.8 mL/min, 40 °C.
tR = 6.3 (S,R) and 7.0 (R,S) min RS = 1.7.
½
a 2D0
ꢃ
¼ ꢁ3:6 (c 0.50, CHCl3) >99% ee.
4.4.6. (1R,2S)-cis-2-Phthalimido-1-acetoxycyclopentane (1R,2S)-
cis-4a
Acknowledgment
Mp 118–120 °C; ½a D20
ꢃ
¼ þ8:8 (c 0.50, CHCl3) >99% ee; 1H NMR
(300 MHz): d 1.55–1.70 (m, 1H), 1.86 (s, 3H), 1.90–2.20 (m, 4H),
2.82 (m, 1H), 4.65 (m, 1H), 5.27 (q, 1H, J = 6.0 Hz), 7.70–7.76 (m,
2H), 7.82–7.88 (m, 2H); 13C NMR (75.5 MHz) d 20.9 (CH3), 21.2
(CH2), 25.3 (CH2), 30.8 (CH2), 53.0 (CH), 74.6 (CH), 123.1
(2 ꢂ CH), 131.8 (2 ꢂ C), 134.0 (2 ꢂ CH), 168.6 (C@O), 170.5
(2 ꢂ C@O); MS (ESI+) m/z (rel intensity): 274.1 (M+H)+, 80, 296.2
(M+Na)+, 20; HRMS (ESI+) calcd for C15H16NO4 [M+H]+ 274.1074;
found 274.1081.
We thank Novo Nordisk Co. for the generous gift of the CAL-B.
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