Base catalyzed racemization of amino acid derivatives

Article abstract of DOI:10.1016/j.tetasy.2011.05.005

Amino acid thioesters used as substrates for a chemo-enzymatic dynamic kinetic resolution (DKR) must be designed with a high enough acidity to be rapidly racemized in the presence of a suitable base. Kinetic data obtained from experimental proton exchange rates are correlated with thermodynamic data for the proton abstraction based on the density functional theory (DFT) calculations. The good correlation obtained allows to evaluate the contribution of different functional groups to the carbon acidity and to define bases able to perform the required racemization.

Full text of DOI:10.1016/j.tetasy.2011.05.005

Tetrahedron: Asymmetry 22 (2011) 851–856
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Base catalyzed racemization of amino acid derivatives
Paola D’Arrigo a,b, Dario Arosio a, Lorenzo Cerioli a, Davide Moscatelli a, Stefano Servi a,b, , Fiorenza Viani c,
⇑
Davide Tessaro a,b
a Department of Chemistry, Materials, and Chemical Engineering ‘Giulio Natta’, Politecnico di Milano, P. zza L. da Vinci 32, 20133 Milano, Italy
b Centro Interuniversitario di Ricerca in Biotecnologie Proteiche ‘The Protein Factory’, Politecnico di Milano and Università degli Studi dell’Insubria,
Via Mancinelli 7, 20131 Milano, Italy
c ICRM, CNR Via Mancinelli 7, 20131 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Amino acid thioesters used as substrates for a chemo-enzymatic dynamic kinetic resolution (DKR) must
be designed with a high enough acidity to be rapidly racemized in the presence of a suitable base. Kinetic
data obtained from experimental proton exchange rates are correlated with thermodynamic data for the
proton abstraction based on the density functional theory (DFT) calculations. The good correlation
obtained allows to evaluate the contribution of different functional groups to the carbon acidity and to
define bases able to perform the required racemization.
Received 24 March 2011
Accepted 12 May 2011
Available online 21 June 2011
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
toward racemization. Numerous methods to achieve this goal have
been described in the past. The DKR of hydroxy compounds such as
Racemization is usually considered as an undesired event in the
production of chiral molecules as single enantiomers. When the
unintentional epimerization of a stereogenic center occurs during
the manipulation of enantiomerically pure chiral molecules, the
mechanism of the racemization is often investigated.1–4
The data obtained can be used to avoid the event. Conversely,
when the production of one single enantiomer from a racemic mix-
ture occurs through a kinetic resolution, then the necessity arises
to racemize the unwanted enantiomer and obtain the starting
racemic mixture.5 Thus the conditions in which racemization oc-
curs are the focus of processes aimed at the production of chiral
compounds as single enantiomers.
Kinetic resolutions of racemates catalyzed by hydrolytic en-
zymes are among the most important routes to enantiomerically
pure compounds and are thus widely employed both in academic
laboratories and for the production of such molecules on an indus-
trial scale. The most effective methods for converting racemates
into single enantiomers (and thereby avoiding isolation, racemiza-
tion or inversion) are processes defined as deracemization.6–10
When the enantioselective resolution of a racemate is combined
with the concomitant racemization of the non, or only partially,
transformed enantiomer, the results are ideally a 100% chemical
yield and enantiomeric excess, namely a dynamic kinetic resolu-
tion (DKR).11–15 For an effective DKR, several requirements have
to be met. The resolution must be highly selective (E P 20–100),
the process irreversible and the final product should be stable
secondary alcohols employs a combination of transition metal cat-
alyzed racemization–lipase catalyzed acetylation, numerous car-
boxylic acids, and their structural analogues can be easily
racemized using simple base-catalysis. Thus, carboxylic acid deriv-
atives, such as hydantoins, azlactones, and aminoacid thioesters
(compare Fig. 1) can be converted into single enantiomers by
DKR employing the aforementioned principles.16,17 The production
of enantiomerically pure D-amino acids from the corresponding
hydantoins is a well established industrial process.
For an efficient DKR of these molecules, the rate of racemization
should ideally be greater than the rate of enzyme-catalyzed trans-
formation. Clearly, the efficiency of the complete deracemization
process will depend on the acidity of the C–H-bond(s) adjacent
to the carbonyl group. This C–H-acidity, in turn is a consequence
of the molecular structure, which can be modified by incorporating
suitable substituents.
H
H
H
O
R
H
O
R
O
R
SR'
N
O
N
N
N
O
H
H
R''
O
O
Ph
hydantoines
oxazolones
amino acid thioesters
c
a
b
⇑
Corresponding author.
E-mail address: stefano.servi@polimi.it (S. Servi).
Figure 1. Amino acid precursors susceptible to base catalyzed racemization in
water.
0957-4166/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2011.05.005

852
P. D’Arrigo et al. / Tetrahedron: Asymmetry 22 (2011) 851–856
The substrate structures in Figure 1 were used in order to obtain
1. The presence of a thioester group.26–28
compounds with an acidity suitable for base catalyzed racemiza-
tion. Thus, the knowledge of the pKa values of the proton abstrac-
tion is central in building a profile for molecule racemization,
either for projecting compounds resistant to racemizing conditions
or conversely, finding conditions for their racemization. However,
even in the most intensively studied case (i.e., proton abstraction
in amino acid derivatives) there are no reliable estimates of the
2. The aromatic group on the
3. The N-protecting groups.
a-carbon.
Further conditions required for successful deracemization in-
clude the compatibility between the protease used and the base
(trioctylamine) and the particular solvent system (water/MTBE).
Extending the DKR method to a wider range of
as the structures depicted in Figure 3 (group B), required us to get
a-amino acids, such
pKa values for the a
-protons.18
The same applies for compounds of other classes. In order to ob-
tain at least estimated values in cases when pKa values cannot be
obtained with an experimental procedure, indirect methods based
on the correlation of kinetic and thermodynamic data have proven
useful.19–23 It is possible for instance to compare enolization reac-
tion rates determined for a given compound with the data for a
similar compound whose pKa value is known.
rid of the a-aromatic ring, whose contribution to the carbon acidity
is important.29
We decided therefore to study the racemization conditions (sol-
vent, form of the hydrolytic enzyme) of a group of N-protected
amino acid thioesters (group B in Figure 3), using a base of high en-
ough strength still compatible with the enzymatic catalyst. The
assumption was that the acids formed in the hydrolytic step would
not be racemized under the experimental conditions.
The thermodynamic parameters of the racemization step were
calculated using computational methods based on density func-
tional theory (DFT).30–32 The rate of racemization was determined
experimentally, both via D-exchange experiments employing 1H
NMR or following the time dependence of the loss of the specific
rotations in enantiomerically enriched molecules.
Keeffe et al.24 measured the enolization rates of the compounds
in Figure 2 (a kinetic parameter) and with reference to the pKa va-
lue of acetone in water25 were able to indirectly assign the relative
pKa values (a thermodynamic parameter). Several examples of
such a procedure can be found in the literature:24–26 this method
allows us to build families of pKa values for classes of compounds
for which experimental data of dissociation in water cannot be col-
lected (substances not significantly soluble in water or with a pKa
near to the one of water).
Alternatively, the effects of such structural modifications on the
C–H-acidity can be correlated with: (a) the results of calculations
using a variety of different approaches and suitable programs
and/or (b) the values directly measured using various techniques
such as deuterium exchange rates (1H NMR) and chiroptical
methods.
2.1. Calculation of the kinetic parameters from thermodynamic
values
Tables of the pKa values, based on
r-correlation, structural
homology, or ab initio calculation, are shown but are limited to
compounds belonging to groups with a strong structural identity.
Extending these calculations to molecules of different complexity
was not possible.33–35 Therefore, the determination of a series of
pKa values requires a structurally-based approach.
Herein we correlate the calculated proton abstraction thermo-
dynamic values with racemization kinetics for a defined set of N-
Boc-amino acid thioesters in order to evaluate the contribution
of different functional groups to the carbon acidity, thus helping
to identify basic compounds able to perform the required
racemization.16
The Brønsted catalysis equation establishes a correlation be-
tween the acidic strength and catalytic activity in general acid
catalysis: log k =
a log(Ka) + b, where k is the kinetic constant, Ka
the dissociation constant and b the Brønsted coefficient relative
to the deprotonation (0 6 b 6 +1). This relationship implies that
the Gibbs free energy for the proton abstraction is proportional
to the activation energy for the catalytic step. If this is the case,
then computational calculations predicting the thermodynamic
parameters should give information about the kinetic constants.
Since in racemization reactions reagents and products have the
2. Results and discussion
In a previous article, we have demonstrated the effectiveness of
the DKR of rac-thioesters derived from amino acids (Fig. 1 com-
pounds of type c). Our success was due to the combination of the
enantioselective activity of an industrial protease and the racemiz-
ing action of the base.16 The application was limited to compounds
with such carbon acidity that the base catalyzed racemization in
the presence of water was possible. Their structures are reported
in Figure 3 (group A). These compounds 1–6 carry aromatic sub-
stituents directly attached to the stereogenic center and owe their
acidity to a combination of at least three structural features:
same energy,
DH values useful for a correlation with kinetic
parameters refer to the reaction in Scheme 1, that is, the reversible
formation of the enolate. Thus by energy minimization techniques
the enthalpy of proton abstraction for the two series of compounds
can be calculated.
The density functional theory (DFT) data, which we used in the
determination of the thermodynamic parameters, were in good
agreement with the limited computational data.36,37 In particular,
the Becke 3 parameters and Lee Yang Parr functional (B3LYP) were
adopted in all the DFT calculations to evaluate the exchange and
correlation energies.38,39 The triple f all electron 6-311 basis set
with added polarization and diffuse functions (6-311 + g(d,p))
was used as the basis set.42 This computational method gave good
results in a study of similar acid–base reactions.40–42 In general, all
geometries were fully optimized with the Berny algorithm and
were followed by frequency calculations. The geometry of each
molecular structure was considered stable only after calculating
vibrational frequencies and force constants and in cases where
no imaginary vibrational frequencies were found.43 Enthalpy
changes were determined as:
O
O
H+
+
H
H
O
H
O
O
H
16
19
12
pKa
X
X
Figure 2. The comparison of the experimental and estimated pKa values allows us
to evaluate the contribution to the carbon-acidity of aromatic ring in acetone
analogues.
D
H ¼
ðEE þ ZPE þ TCÞj ꢀ
ðEE þ ZPE þ TCÞi
j¼products
i¼reactants

P. D’Arrigo et al. / Tetrahedron: Asymmetry 22 (2011) 851–856
853
NHBoc
COR
NHBoc
COSEt
NHCbz
COR
Ph
R
Ph
1a R = SEt; 1b R = SCH2Ph;
1d R= SPh
1c R = SEt
2a R = 4-Cl-Ph; 3a R = 4-F-Ph;
4a R = 2-Cl-Ph; 5a R = 2-F-Ph;
6a R = 2-thienyl
Group A
NHBoc
COR
NHBoc
COR
NHBoc
Ph
Ph
COR
9a R = SEt;
8a R = SEt;
7a R = SEt;
9b R = SCH2Ph
8b R = SCH2Ph
7b R = SCH2Ph
Group B
Figure 3. Amino acid thioester substrates for base catalyzed racemizations.
corresponding values for compounds of group B, where the aro-
matic moiety was remote from the -carbon or not present at all
DDH 10–20 kcal/mol). Thus, the contribution of this (and other)
structural components could be evaluated from these DDH values.
The significance of this becomes evident when the
H1 value for
compound 1a is compared with the
H2 value for compound 7a:
a
(
D
D
that is, they are almost identical, meaning that while base 1
(TOA) is not able to abstract the proton in compound 7a (experi-
mental fact), the same reaction should be possible if base 2
(DBU) is employed. Similar conclusions can be drawn for the other
compounds in Table 1.
Scheme 1. High energy intermediates in the racemization of amino acid
derivatives.
2.2. Measurements of racemization kinetics
Validating the calculated values in Table 1 was carried out by
submitting them to racemizing conditions using base 1 or 2 and
by measuring the relative rates of racemization with two different
methods. Substrates 1–6 of group A were treated with TOA in
DMSO-d6 in the presence of CD3OD and the exchange rate of
Table 1
Calculated proton abstraction enthalpies
Substrate
R1
R2
R3
D
H1
D
H2
1a
1b
7a
7b
8a
8b
9a
9b
Boc
Boc
Boc
Boc
Boc
Boc
Boc
Boc
Ph
Ph
Et
Bn
Et
Bn
Et
Bn
Et
100.52
97.95
88.09
85.52
100.28
96.05
99.26
96.11
103.09
99.01
the
a-proton of the thioester with deuterium was measured by
1H NMR. In a typical experiment, 0.5 equiv of trioctylamine
Ph-CH2
Ph-CH2
Ph-CH2–CH2
Ph-CH2
Bu
112.71
108.48
111.69
108.54
115.52
111.44
(TOA) were added at 31 °C to 20 mg of substrate dissolved in
500 lL of DMSO-d6 and 100 lL of CD3OD. Exchange rates of 50%
varied from a few minutes for compound 6a to several hours
for the 2-substituted phenyl-glycines 4a and 5a. In all cases, no
exchange was observed in the absence of TOA. Control experi-
ments with the corresponding oxoesters and acids showed no
measurable proton exchange under similar conditions. This indi-
cated that the thioester, but not the oxoester, was suitable for
the in situ racemization method and that the product acid was
configurationally stable under thioester-racemization conditions.
Further experiments showed that thioesters were not prone to
spontaneous hydrolysis. The deuterium exchange rate was high-
est for 6a (t1/2 of 10 min) and lowest for the 2-substituted arylgly-
cines 4a–5a (t1/2 of several h).
A comparison of the racemization rates of the group of ring
substituted phenyl-glycines allowed us to quantify the effects of
the electron withdrawing groups present in the molecules. The ra-
tional of the data reported in Figure 4 can be explained as being
caused by electronic effects.
Bu
Bn
D
D
H1 in the presence of TOA (kcal/mol).
H2 in the presence of DBU (kcal/mol).
where EE represents the electron energy, ZPE is the zero point en-
ergy and TC the thermal energy correction. All quantum chemistry
calculations were performed with the GAUSSIAN 03 suite of programs
and all structures were drawn with PYMOL 1.3.44,45
Enthalpy changes for the proton abstraction reaction were cal-
culated in the presence of two organic bases which could be used
in the presence (trioctylamine TOA) or absence (1,8-diazabicy-
clo[5.4.0]undec-7-ene DBU) of water. The values obtained with
the two bases are reported in Table 1. The difference between
the two reported values was constant and equal to about 12 kcal/
mol, which represents the enthalpy change of the reaction:
Substrates 7–9 of group B were expected to have a decreased
TOA þ DBUHþ¡TOAHþ þ DBU
acidity of the hydrogen on the
a-carbon due to the distance or
The proton abstraction enthalpy values highlight the fact that
the absence of the aryl group. In Table 2, the kinetic constants
for the racemization of these compounds are compared with the
values calculated for phenyl glycines 1a and 1b.
substrates of group A bearing an aromatic ring at the
a-position,
and thus stabilizing the carbanion, were markedly lower than the

854
P. D’Arrigo et al. / Tetrahedron: Asymmetry 22 (2011) 851–856
100
90
80
70
60
50
40
30
20
10
0
Ph 1a
4-Cl-Ph 2a
2-Cl-Ph 4a
2-F-Ph 5a
4-F-Ph 3a
Th 6a
Ph-OEt
0
20
40
60
80
100
120
140
time (min)
Figure 4. Racemization kinetic via deuterium exchange for compounds 1a–6a compared with the oxo-ester N-Boc phenylglycine ethylester (lower line).16
Table 2
Table 3
Experimental values of the kinetic constants from
chiroptical measurements
Ratios of the racemization kinetic constants between reference compounds (row/
column)
Substrate
k (minꢀ1
)
1b
7a
7b
8a
8b
9a
9b
1a
1b
7a
7b
8a
8b
9a
9b
26.8
63.9
1a 4.2 ꢂ 10ꢀ1 58.3 25
244 89
581 213
400 134
954 319
1b
7a
7b
8a
8b
9a
139
60
4.63 ꢂ 10ꢀ1
1.07
4.3 ꢂ 10ꢀ1 4.2
1.5
3.5
6.8
16
2.3
8.5
9.7
1.15 ꢂ 10ꢀ1
3.01 ꢂ 10ꢀ1
6.7 ꢂ 10ꢀ2
2.03 ꢂ 10ꢀ1
3.7 ꢂ 10ꢀ1 1.6
5.5 ꢂ 10ꢀ1
1.5
4.5
3.3 ꢂ 10ꢀ1
the amino acid lacking an aromatic ring (norleucines) 9 (1a/9a).
It is interesting to note that aliphatic amino acid thioesters
under these conditions can be racemized at a sensible rate, thus
allowing in principle DKR. Under the same conditions, all of the
corresponding oxo-esters and acids could not be racemized. This
Compounds 7–9 of type a (thioethyl-esters) and of type b (thi-
obenzyl-esters) were prepared in the -form and submitted to race-
mization. Unlike compounds of group A, compounds of group B
were not prone to racemization with TOA, but required the pres-
ence of DBU in order to be racemized.
L
is also
deracemization.
a
condition for
a
successful chemo-enzymatic
Under these conditions, racemization was followed by collect-
ing the rotation values within a polarimeter. In a typical experi-
ment, 20 mg of substrate was dissolved in 2 mL of 2-propanol
and 0.5 equiv of DBU were added at 40 °C. The specific rotation
of the solution was monitored over a 30-min period. No change
in the specific rotation value was observed in the absence of a base.
This method was applied to compounds 1 and 7–9 and the calcu-
lated constants are reported in Table 2.
2.3. Correlation between enthalpy values and kinetic constants
Figure 5 shows the correlation between measured kinetic con-
stants and calculated
D
H values for proton abstraction for com-
shows linear
pounds 1–9. The plot of ꢀln k versus
D
H
a
The ratios obtained between the kinetic constants of com-
pounds 1 and 7–9 allowed us to draw significant considerations
concerning the contribution of a specific substituent and its posi-
correlation, thus making it possible to predict racemization kinet-
ics from calculated proton abstraction enthalpies for a homoge-
neous set of compounds.
tion in the molecule to the acidity of the proton on the
(see Table 3).
a-carbon
4.0
ꢁ Ratios between ka and kb consistently give values around 0.3–
0.5, allowing for the fact that the presence of the benzyl group
in the ester moiety increases the acidity of the proton on the
8a
9a
2.0
8b
9b
7a
0.0
a-carbon by a factor of 2 compared with the ethyl group. This
7b
observation is in agreement with considerations based on elec-
tronic effects or stabilization energy.
-2.0
1a
1c
ꢁ The ratio of the k values for a series of compounds that are
homogeneous for the ester parts a or b but with a different posi-
tion of the aromatic ring in the structure also gives important
data in designing substrates suitable for in situ racemization.
Thus as expected, phenyl glycines 1 are racemized about 60
times faster than phenylalanines 7 (1a/7a), 250 times faster than
homophenyl alanines 8 (1a/8a) and almost 400 times faster than
1b
-4.0
1d
-6.0
82
87
92
97
102
107
delta H (kcal/mol)
Figure 5. Graph of ꢀln k of racemization measured at
calculated H.
a polarimeter versus
D

P. D’Arrigo et al. / Tetrahedron: Asymmetry 22 (2011) 851–856
855
4.2.2. Compound 1b
3. Conclusion
1H NMR d = 1.41 (s, 9H), 4.07–4.16 (m s, 2H), 5.44 (br s, 2H),
7.24 (m, 5H), 7.33 (m, 5H). 13C NMR d = 198.1, 154.5, 136.6,
136.3, 128.8, 128.6, 128.4, 128.3, 127.4, 80.1, 64.4.33.2, 28.1.
The in situ racemization techniques are the basis for a DKR pro-
cess. The pKa values of the proton to be abstracted are generally not
available but racemization rates can be obtained from experimen-
tal values. We have shown that for a number of amino acid esters,
kinetic constants for the proton abstraction reactions can be mea-
sured based on the relative rotation values or 1H NMR proton ex-
change rates. The possibility of obtaining reliable data from these
calculations has obvious advantages over the experimental deter-
mination of kinetic parameters. We have determined thermody-
namic parameters using the density functional theory (DFT)
showing a good correlation with the experimental data, thus
allowing the prediction of useful structural modifications. When
applied to our set of compounds, we concluded that a useful race-
mization rate depends strongly on the nature of the group residue
½
a 2D5
ꢃ
¼ þ73:6 (c 1.1, 2-propanol).
4.2.3. Compound 6a
1H NMR d = 1.16 (t, J = 7.5, 3H), 1.46 (s, 9H), 2.825 (q, J = 7.5 Hz,
2H), 5.49 (br s, 1H), 7.0 (dd, J = 3.4, 4.8 Hz, 1H), 7.15 (m, 1H), 7.490
(m, 1H), 8.13 (m, 1H). 13C NMR, d = 197.8, 154.5, 138.75, 126.95,
126.5, 126.0, 80.5, 59.8, 28.2, 23.6, 14.2. ½a D25
¼ þ28:6 (c 1.0, 2-
ꢃ
propanol).
4.2.4. Compound 7a
1H NMR d = 1.24 (t, J = 7.5 Hz, 3H), 1.4 (s, 9H), 2.87 (q,
J = 7.383 Hz, 2H), 3.04 (br s, 1H), 3.06–3.18 (m, 1H), 4.62 (br s,
1H), 4.87 (br s, 1H), 7.135–7.19 (m, 2H), 7.20–7.31 (m, 3H). 13C
NMR d = 200.8, 154.85, 135.8, 129.2, 128.3, 126.9, 80.0, 60.9,
on the aminoacid a-carbon and on the nature of the thioester moi-
ety. The calculations of the proton abstraction in putative bases
when applying the same method allowed us to define bases which
in principle have sufficient basic behavior to promote the racemi-
zation of the target compounds. When applied to a chemo-enzy-
matic deracemization method, considerations about the
conservation of the catalytic activity in the presence of the selected
base required an experimental validation.
38.35, 28.2, 23.15, 14.3. ½a D25
¼ þ24:4 (c 1.0, 2-propanol).
ꢃ
4.2.5. Compound 7b
1H NMR d = 1.39 (s, 9H), 3.1 (br s, 2H), 4.05–4.16 (m s, 2H), 4.65
(br s, 1H), 4.84 (br s, 1H), 7.02–7.13 (m, 2H), 7.17–7.33 (m, 8H). 13
NMR d = 200.2, 154.8, 137.1, 135.6, 129.2, 128.8, 128.4, 127.1,
C
126.85, 80.1, 60.8, 38.2, 33.1, 28.1.
propanol).
½
a 2D0
ꢃ
¼ þ22:0 (c 1.0, 2-
4. Experimental
4.1. General
4.2.6. Compound 8a
1H NMR d = 1.25 (t, J = 7.415 Hz, 3H), 1.46 (s, 9H), 1.8–2.0 (m,
1H), 2.1–2.26 (m, 1H), 2.6–2.8 (m, 2H), 2.88 (q, J = 7.415 Hz, 2H),
4.38 (br s, 1H), 4.96 (br s, 1H), 7.18 (m, 2,5H), 7.28 (m, 2,5H). 13C
NMR d = 201.1, 155.0, 140.6, 128.438, 128.25, 126.0, 79.9, 60.2,
All chemicals were purchased from Sigma–Aldrich. All solvents
were of analytical grade. Silica Gel 60 F254 plates (Merck) were used
for analytical TLC. Detection was achieved with UV light followed by
I2 staining or ninhydrin or potassium permanganate. HPLC analysis
were performed on a Merck apparatus equipped with a UV detector
34.4, 31.6, 28.2, 23.0, 14.4. ½a D20
¼ þ16:55 (c 1.0, 2-propanol).
ꢃ
and fitted with a Chiracel OD 5 lm column, length/internal
diameter = 250/4.6, eluent hexane/2-propanol. 1H NMR spectra
were recorded on a Bruker ARX 400 instrument operating at the
1H resonance frequency of 400 MHz. Chemical shifts (d, ppm) are re-
ported relative to tetramethylsilane (TMS) as the internal standard.
All spectra were recorded in DMSO-d6 at 304 K. Optical rotations
were determined with a Propol Digital Polarimeter by Dr. Kenchen,
4.2.7. Compound 8b
1H NMR d = 1.46 (s, 9H), 1.92 (br s, 1H), 2.2 (br s, 1H), 2.67 (m,
2H), 4.12 (s, 2H), 4.42 (br s, 1H), 4.95 (br, 1H), 7.12–7.33 (m, 10H).
13C NMR d = 200.5, 155.0, 140.4, 137.1, 128.7, 128.4, 128.4, 128.3,
127.13, 126.1, 80.1, 60.2, 34.3, 33.1, 31.5, 28.2. ½a D25
¼ þ13:4 (c
ꢃ
1.0, 2-propanol).
and [a]
D values are given in units of deg cm2 gꢀ1 at 25 °C.
4.2.8. Compound 9a
1H NMR d = 0.90 (t, J = 7.3 Hz, 3H), 1.25 (t, J = 7.31 Hz, 3H), 1.29–
1.41 (br s, 2H), 1.46 (s, 9H), 1.60 (br s, 1.5H), 2.2 (br s, 0.5H), 2.88 (q,
J = 7.63 Hz, 2H), 4.32 (br s, 1H), 4.90 (br s, 1H). 13C NMR d = 201.35,
4.2. General procedure for
thiobenzyl ester synthesis
L-N-Boc-aminoacid thioethyl and
155.0, 79.7, 60.2, 34.65, 28.1, 22.85, 18.47, 14.3, 13.4. ½a D25
¼ þ20:3
ꢃ
To a stirred solution of L-N-Boc-aminoacid (12 mmol) in dichlo-
(c 1.0, 2-propanol).
romethane (90 mL) immersed in an ice bath, DMAP (0.1 equiv) and
ethanthiol (or benzylthiol; 1.2 equiv) were added. Then a solution
of DCC (1.1 equiv) in dichloromethane (10 mL) was added
dropwise at 0-5 °C. After a short incubation period, a crystalline
white precipitate began to form. After the addition, the ice bath
was removed and the reaction was complete after a further
overnight stirring period at r.t. The mixture was filtered in order
to eliminate the white solid (N,N-dicyclohexylurea) and the filtrate
was evaporated under reduced pressure. The residue was then
purified by column chromatography on silica gel using hexane/
ethyl acetate (95:5) as eluent to give the desired aminoacid thioes-
ter in high yields (from 80 to 90%).
4.2.9. Compound 9b
1H NMR d = 0.863 (t, J = 7.28 Hz, 3H), 1.3 (br s, 4H), 1.42 (s, 9H),
1.56 (br s, 1.5H), 1.825 (br s, 0.5H), 4.09 (s, 2H), 4.33 (br s, 1H),
4.984 (br s, 1H), 7.16–7.29 (m, 5H). 13C NMR d = 200.8, 155.1,
137.2, 128.7, 128.4, 128.1, 79.95, 60.4, 33.0, 32.3, 28.2, 27.3, 22.1,
13.6.
[a]D = +21.58 (c 1.0, 2-propanol, 20 °C).
4.3. General procedure for the determination of the
racemization kinetics of -N-Boc-aminoacid thioesters
L
Approximately 20 mg of substrate were dissolved in 2 mL of 2-
propanol in a polarimetric tube and the rotation value was re-
corded. Next, 0.5 equiv of diaza-bicyclo-undecene (DBU) was
added and the decrease in rotation value were recorded over
30 min at 40 °C.
4.2.1. Compound 1a
1H NMR d = 1.13 (t, 3H), 1.41 (s, 9H), 2.80 (q, 2H), 5.27 (br s, 1H),
7.35 (m, 3H), 7.42 (m, 2H), 7.99 (br s, 1H). 13C NMR (CDCl3)
d = 198.65, 154.65, 136.8, 128.9, 128.5, 127.4, 80.2, 64.5, 28.2,
23.5, 14.25. ½a 2D5
¼ þ151:15 (c 1.1, CHCl3).
ꢃ

856
P. D’Arrigo et al. / Tetrahedron: Asymmetry 22 (2011) 851–856
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