Synthesis of enantiopure 2-amino-1-phenyl and 2-amino-2-phenyl ethanols using enantioselective enzymatic epoxidation and regio- and diastereoselective chemical aminolysis

Article abstract of DOI:10.1016/j.tetasy.2006.01.009

Several enantiopure 1,2-amino alcohols have been prepared by combining a stereoselective enzymatic epoxidation of styrenes with regio- and stereoselective chemical reactions. An interesting reactivity has been noted concerning the reaction of epoxides and NH₃ under microwave activation.

Full text of DOI:10.1016/j.tetasy.2006.01.009

Tetrahedron: Asymmetry 17 (2006) 372–376
Synthesis of enantiopure 2-amino-1-phenyl and 2-amino-2-phenyl
ethanols using enantioselective enzymatic epoxidation and
regio- and diastereoselective chemical aminolysis
Guido Sello,a,* Fulvia Orsini,a Silvana Bernasconia and Patrizia Di Gennarob
a
`
Dipartimento di Chimica Organica e Industriale, Universita degli Studi di Milano, via Venezian 21, 20133 Milano, Italy
`
Dipartimento di Scienze dell’Ambiente e del Territorio, Universita di Milano-Bicocca, p.za della Scienza 1, 20126 Milano, Italy
b
Received 30 November 2005; accepted 3 January 2006
Available online 20 February 2006
Abstract—Several enantiopure 1,2-amino alcohols have been prepared by combining a stereoselective enzymatic epoxidation of styr-
enes with regio- and stereoselective chemical reactions. An interesting reactivity has been noted concerning the reaction of epoxides
and NH3 under microwave activation.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
The 1,2-amino alcohol motif is present in a number of
bioactive natural products1 (such as alkaloids,2a–c amino
sugars,2d enzyme inhibitors,2e,3a–c and antibiotics3d,e). In
addition, they are frequently employed in asymmetric
synthesis as chiral auxiliaries or chiral catalysts.4 As a
consequence, synthetic chemists continue to develop
new methods for their efficient asymmetric synthesis.
We have previously reported a very effective biocatalyst
that can transform styrene derivatives into the corre-
sponding epoxides.9 This biotransformation produces
highly enantioenriched compounds, that possess the cor-
rect reactivity for the next synthetic step, that is the reac-
tion of the electrophilic epoxy group with a nucleophilic
partner (Fig. 1). In this case, the reactivity at the benz-
ylic position greatly favors nucleophilic addition, thus
making the preparation of the 2-phenyl, 2-amino
ethanols relatively straightforward. In contrast, the
approach of the nucleophile at the less reactive site
can be problematic.
Enantiopure 1,2-amino alcohols are readily accessible if
they can be derived from proteinogenic amino acids. In
contrast, access to other amino alcohols requires effi-
cient enantioselective routes.5 Among the numerous
synthetic methods developed, there are only a few meth-
ods typically employed to prepare chiral amino alco-
hols.6,7 In particular, we are interested in the
preparation of 2-amino, 2-phenyl ethanols, and their
2-amino-1-phenyl regioisomers.
OH
R
NH2
Recently, some new methods for the preparation of 1,2-
amino alcohols have been reported.8 Their synthetic effi-
ciency is very high considering both regio- and stereo-
selectivity. Nevertheless, the possibility of exploiting
the enantioselectivity that Nature can provide suggests
a different approach to their synthesis.
O
R
R
NH2
R
OH
*
Corresponding author. Tel.: +39 02 50314107; fax: +39 02 50314106;
e-mail: guido.sello@unimi.it
Figure 1. Regioisomeric addition of nitrogen to epoxides.
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.01.009

G. Sello et al. / Tetrahedron: Asymmetry 17 (2006) 372–376
373
Table 1. Substrates, products, and relative activity, of the bioconver-
sion of styrene derivatives
In the literature there are some methods, which have
been used to regioselectively prepare 2-amino-1-phenyl
ethanols from epoxides. However, they show either
low productivity or utilize complex reaction procedures.
The only papers reporting an efficient method of prepar-
ing these compounds employ microwave activation
under different conditions.10 In addition, no preparation
of 2-amino-1-phenyl propanol from the corresponding
epoxide has been reported. Stimulated by these findings
and interested in the enantioselective production of 1,2-
amino alcohols, we investigated the possibility of their
preparation.
Substrate
Product
Relative
activity
(%)a
eeb
H
O
100
166
310
237
38
>95
1a
1d
H
O
>95
The first step in our synthetic plan is the enzymatic oxi-
dation of styrene derivatives. This was accomplished
using a whole cell biocatalyst: a recombinant E. coli con-
taining the monooxygenase activity from P. fluorescens
ST [E. coli JM109(pTAB19)]. Bioconversions were per-
formed in a water buffer using an organic phase to sep-
arate both the starting hydrocarbon and the epoxide
produced. Conversion rates depend on the substrate
and are reported in comparison with styrene reactivity
in Table 1. All the epoxides obtained are enantiopure,
except the epoxide derived from 1-phenyl, 2-methyl pro-
pene that shows a small amount of the second enantio-
mer (ꢀ7%). Compound characteristics are in full
agreement with literature data.
H
1b
1e
H
O
87
1c
2a
2b
2c
3a
1f
H
O
F
F
>95
>95
>95
>95
F
2d
The second step in the synthetic plan involves the reac-
tion of a nitrogen nucleophile with the epoxide. The
reaction of styrene oxides with a nitrogen nucleophile
at the benzylic position is a well-known and efficient
process;11 it is commonly performed in two steps. First
an azide derivative opens the three-membered ring to
give the 1,2-azido alcohol, then the azido group is cata-
lytically hydrogenated to the amino alcohol. Yields are
good, and regio- and stereospecificity is nearly complete
(Fig. 2).
H
O
H
F
2e
H
O
F
17
H
F
In contrast, the preparation of the other regioisomer is
less straightforward. Notwithstanding the reported
methods, the reaction has proven troublesome in our
hands. The direct use of ammonia as a nucleophile gives
quite a low yield of monoderivative (ꢀ30%), always
accompanied by the secondary amine. In addition, the
use of the Gabriel synthesis was not successful. Looking
in the recent literature we found two reports where the
use of microwaves was cited as an important factor to
improve the yield. We did not try to replicate the result
by Favretto et al.,10a because it requires special equip-
ment. Moreover, our attempts to replicate the result
by Sabitha et al.10b (epoxide in the presence of
CH3COONH4, microwave activation, and no solvent)
were unsuccessful. Consequently, we developed a modi-
fied procedure. By reacting a water ammonia solution
and the epoxide in a closed container under microwave
activation, using a commercial home microwave oven, it
was possible to synthesize, in good to optimal yield, the
desired compound (Fig. 3).
2f
H
O
Cl
145
Cl
3d
H
O
H
Cl
78
16
>95
>95
Cl
3b
3e
H
O
Cl
H
Cl
3c
3f
a Relative activity = [Specific activity of substrate (i)]/[Specific activity
of styrene] · 100; Specific activity of (i) = mmol of products formed
in 1 h by 1 g of cells (dry cell weight) in 1 L of broth, as calculated
from the correlation line of the first conversion hours.
This reaction was applied to some 4-substituted styrene
epoxides, obtaining the desired regioisomer as the only
isolated product together with some unreacted starting
b Enantiomeric excess was measured by chiral GLC.

374
G. Sello et al. / Tetrahedron: Asymmetry 17 (2006) 372–376
Mg(ClO4)2
N3
Pd/C, H2
AcOEt
OH
H2N OH
O
NaN3,
H
H
H
4
CH2Cl2
94%
quant.
Figure 2. Amino alcohol preparation via azide addition and reduction.
the 1-amino, 1-phenyl, 2-methyl, 2-propanols (50%),
together with the diol (15%), and the unreacted epoxide
(35%).
X
X
NH4OH,
NH2
OH
O
75-100%
Microwave 100W
X = H, Cl, F
H
H
Concerning the diastereoselectivity, the enantiomeric
excess present in the reactants is clearly unchanged by
the reaction of aminolysis at the homobenzylic position,
because the only stereocenter present is not affected by
the reaction. We can thus prepare enantiopure 2-amino,
1-phenyl ethanols in two highly selective synthetic steps.
5a-c
Figure 3. Reaction of monosubstituted epoxides with NH3 under
microwave activation.
compounds. The reaction is very easy because product
isolation and purification is straightforward, requiring
simple solvent (AcOEt) extraction and solvent evapora-
tion. Interestingly, we did not find the presence of the
1,2-diol that could have also been produced.
In the case of the epoxide reaction at the benzylic posi-
tion, the opening reaction could be partly selective,
with a retention and inversion of configuration, giving
rise to partial racemization. As a matter of fact, we
never noticed this evidence. All the reactions proceeded
smoothly with total inversion of configuration. This
was expected in the azidolysis reaction but, it was also
evident that the aminolysis was completely stereoselec-
tive. Thus, we can also prepare enantiopure 2-amino, 2-
phenyl ethanols.
Successively, we applied the reaction to 1,2-disubstituted
epoxides. In particular, we used the epoxide derived
from trans-b-methyl styrene. Unexpectedly, the recov-
ered product was the 3-amino, 3-phenyl 2-propanol;
the yield obtained was very good (ꢀ75%).12
This reaction has also been applied to compounds 1f, 2e,
2f, 3e, and 3f. The results obtained have proven very
interesting: from trans-epoxides 2f and 3f we obtained
the expected 3-amino, 3-phenyl, 2-propanols;13 in con-
trast, from cis-epoxides 2e and 3e, the products were
the regioisomeric 2-amino, 1-phenyl, 1-propanols
(Fig. 4). The complementary regioisomers were not
3. Conclusion
In conclusion, we were able to prepare 2-amino, 1- and
2-phenyl ethanols in good yields and very high enantio-
purities coupling the enzymatic preparation of enantio-
pure phenyl epoxides with selective chemical ring
opening by nitrogen nucleophiles under convenient con-
ditions. This approach can be extended to the synthesis
of many interesting compounds.
1
visible by using H NMR. Finally, from 1f we obtained
H
H
O
OH
NH4OH
4. Experimental
NH2
H
H
X
X
Microwave 200W
4.1. Analytical methods
6
80-90%
X = H, F, Cl
Bioconversion progress was followed by gas chromato-
graphy (DANI 86.10 gas chromatograph with FID) on
a Chrompack Cp-Sil 8CB column (T = 100 150 ꢁC at
15 ꢁC min, splitless injection) with the appropriate inter-
nal standard (dodecane or hexadecane). At intervals,
2 mL samples were taken from the reaction emulsion.
The two phases were separated by filtration. The water
phase was followed by HPLC on a Merck/Hitachi (L-
6200) system connected to a UV-detector set at 230 nm
on a (C18 Hibar Lichrosorb 50334, 5 lm, 25 cm) column
with 50:50 CH3CN/water. The absolute (S)-configura-
tion of biocatalytically prepared (S)-styrene oxide was
proven via comparison with commercially available,
enantiopure (S)-styrene oxide (Aldrich). Based on the
analogous chromatographic behavior of racemic mix-
tures, we assumed an (S)-configuration for all epoxides.
H
H
O
O
H2N
H
NH4OH
H
OH
Microwave 200W
X
X
7
80-90%
X = F, Cl
H
H
OH
NH4OH
NH2
50%
Microwave 200W
8
Figure 4. Reaction of a,b-disubstituted and a,a,b-trisubstituted epox-
ides with NH3 under microwave activation.

G. Sello et al. / Tetrahedron: Asymmetry 17 (2006) 372–376
375
The enantiomeric excesses of epoxides and amino
Acknowledgments
alcohols were measured using a Chrompack Chiral-
Dex-CB column by comparison to synthetic racemic
mixture. Optical rotations were measured with a Perkin
Elmer 341 polarimeter. NMR spectra were recorded on
a Bruker AC 200 (1H NMR at 200 MHz). All signals
are expressed as parts per million down field from tetra-
methylsilane. All the compounds show spectra in agree-
ment with the literature data.14
Partial financial support by grants from MIUR-PRIN
2004 ‘Use of biocatalytic oxidative process for the
production of intermediates in the chemoenzymatic
synthesis of chiral and/or bioactive compounds’ is
acknowledged.
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