Stereodivergent preparation of valuable γ- Or δ-hydroxy esters and lactones through one-pot cascade or tandem chemoenzymatic protocols

Article abstract of DOI:10.1021/cs4010024

A series of enantiopure hydroxy esters and lactones has been synthesized in a chemodivergent manner via alcohol dehydrogenase (ADH) reduction of the corresponding keto esters by means of cascade or tandem protocols. Thus, ADH from Rhodococcus ruber (ADH-A) or Lactobacillus brevis (LBADH) afforded both antipodes in a very selective way when dealing with small derivatives. With bulkier substrates, ADH from Ralstonia sp. (RasADH) was successfully employed to achieve the synthesis of enantioenriched γ- or δ-hydroxy esters. To isolate the corresponding lactones, two different approaches were followed: a cascade reaction by spontaneous cyclization of the hydroxy ester intermediate, or a one-pot two-step tandem protocol. Moreover, a chemoenzymatic route was designed to obtain a chiral brominated lactone, which enabled further modifications in a sequential fashion by Pd-catalyzed reactions, affording relevant functionalized lactones.

Full text of DOI:10.1021/cs4010024

Research Article
pubs.acs.org/acscatalysis
Stereodivergent Preparation of Valuable γ- or δ‑Hydroxy Esters and
Lactones through One-Pot Cascade or Tandem Chemoenzymatic
Protocols
Alba Díaz-Rodríguez,‡ Wioleta Borzęcka,‡ Ivan Lavandera,* and Vicente Gotor*
́
Departamento de Química Organ
́ ́
ica e Inorganica, Universidad de Oviedo, Instituto Universitario de Biotecnología de Asturias,
C/Julian Clavería 8, 33006 Oviedo, Spain
́
S
* Supporting Information
ABSTRACT: A series of enantiopure hydroxy esters and lactones has been
synthesized in a chemodivergent manner via alcohol dehydrogenase (ADH)
reduction of the corresponding keto esters by means of cascade or tandem
protocols. Thus, ADH from Rhodococcus ruber (ADH-A) or Lactobacillus brevis
(LBADH) afforded both antipodes in a very selective way when dealing with
small derivatives. With bulkier substrates, ADH from Ralstonia sp. (RasADH)
was successfully employed to achieve the synthesis of enantioenriched γ- or δ-
hydroxy esters. To isolate the corresponding lactones, two different approaches
were followed: a cascade reaction by spontaneous cyclization of the hydroxy
ester intermediate, or a one-pot two-step tandem protocol. Moreover, a
chemoenzymatic route was designed to obtain a chiral brominated lactone,
which enabled further modifications in a sequential fashion by Pd-catalyzed
reactions, affording relevant functionalized lactones.
KEYWORDS: alcohol dehydrogenases, lactones, cascade reaction, tandem reaction, palladium-catalyzed cross-coupling
traditionally done employing whole-cell systems,11 but
INTRODUCTION
■
generally the selectivities or substrate concentrations were not
satisfactory. Furthermore, in most cases mixtures of the lactone
and the corresponding hydroxy ester were detected. Although
some of these drawbacks were overcome using isolated
enzymes, the hitherto reported methodologies only led to the
(S)-antipodes in an efficient manner.12 More recently,
Pietruszka and co-workers described the enzymatic synthesis
of several γ- or δ-hydroxy esters, and then different lactones
were obtained in a multistep route.13
As a part of our ongoing interest in the field of one-pot
cascade or sequential transformations,14 and because of the rise
of methodologies that have elegantly combined the use of
alcohol dehydrogenases with organo- or metal catalyst(s),15 we
have considered it worthwhile to explore the bioreduction of
several γ- and δ-keto esters to: (i) find a chemoselective and
efficient approach to get access to the lactones or the hydroxy
esters; (ii) search for recombinant ADHs able to afford these
products in a stereodivergent fashion allowing the synthesis of
both antipodes; and (iii) combine a bioreductive cascade with
several Pd-based transformations to expand the scope and
potential of these protocols applied to the formation of highly
valuable lactones.
The preparation of γ- and δ-lactones has attracted increasing
attention, because of their structural implications as basic
chemicals but also as valuable building blocks for polymers and
natural product synthesis. These compounds display a broad
biological profile and are also important flavor and aroma
constituents.1 In particular, γ-valerolactone (GVL) has been
identified as a potential intermediate for the production of
fuels,2 while δ-caprolactone is used as a monomer to synthesize
biodegradable polymers.3 On the other hand, enantioenriched
γ- or δ-hydroxy esters are present in many bioactive molecules,
such as polyketides, prostaglandins, pheromones, and other
important compounds.4
Because of their unsurpassed selectivity, biocatalytic
approaches (Scheme 1) have rapidly gained ground in the
synthesis of these enantioenriched derivatives. Among them,
the Baeyer−Villiger monooxygenase (BVMO) oxidation of
racemic or prochiral cyclic ketones,5 the hydrolase-catalyzed
kinetic resolutions (KRs) starting from the corresponding
racemic lactones6 or the hydroxy ester precursors,7 and the
enantioselective alcohol dehydrogenase (ADH)-catalyzed
oxidation of racemic diols can be mentioned.8,9 Alternatively,
dynamic systems (DKRs) using lipases with a metal-based
racemization have also been studied to quantitatively yield the
enantiopure products.10
Another straightforward route to obtain such enantioen-
riched derivatives is the direct ADH-catalyzed bioreduction of
γ- or δ-keto esters. In this sense, the first protocols were
Received: October 31, 2013
Revised: December 5, 2013
Published: December 18, 2013
© 2013 American Chemical Society
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Scheme 1. General Biocatalytic Approaches To Achieve γ- or δ-Lactones
Scheme 2. Biosynthesis of Enantioenriched γ- and δ-Hydroxy Esters and Lactones Using an ADH-Based One-Pot Cascade or
Tandem Strategy
Figure 1. Effect of the pH in the ADH-catalyzed bioreduction of 1a (50 mM, t = 24 h). Results obtained for ADH-A are shown in blue, and results
obtained for LBADH are shown in red. In all cases, ee values of the final products were >99%.
activity toward the bioreduction of methyl 5-oxohexanoate (1a,
50 mM) at pH 9.0 and 30 °C. For some of them, 2-propanol
(2-PrOH) was used as hydrogen donor to recycle the
nicotinamide cofactor, while for others, glucose and glucose
dehydrogenase (GDH) were employed.
RESULTS AND DISCUSSION
■
Thus, we first focused on the bioreduction of several prochiral
γ- and δ-keto esters 1a−1g (Scheme 2) to study different
biocatalysts and reaction conditions such as pH. In an initial set
of experiments, several recombinant ADHs were screened for
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Figure 2. Effect of the alkyl chain length and the ester moiety in the ADH-catalyzed bioreduction of 1a−d (50 mM) at 30 °C and pH 9.0 for (a)
ADH-A and (b) LBADH. In all cases, the ee values of the final products were >99%.
Although good activities were detected with several alcohol
dehydrogenases (see the Supporting Information), only ADH
from Rhodococcus ruber (ADH-A)16 and ADH from Lactoba-
cillus brevis (LBADH)17 showed excellent stereoselectivities
(enantiomeric excess (ee) > 99%). Furthermore, because of the
fact that these enzymes display opposite selectivities, the
formation of both enantiomers of methyl 5-hydroxyhexanoate
(2a) and δ-caprolactone (3a) was feasible. For these reasons,
ADH-A and LBADH were selected for further investigations. In
a subsequent study, the effect of the pH was examined in order
to check whether this parameter could shift the equilibrium
toward the formation of either of the two possible products.
Thus, these bioreductions were performed at pH 5.0, 7.5, and
9.0 (Figure 1). Gratifyingly, neutral pH was found as the best
one to form preferentially hydroxy ester 2a over lactone 3a,
while a higher pH led to noticeable amounts of 3a with both
ADHs (c > 18%), corresponding to the intramolecular
cyclization of the hydroxy ester intermediate in a cascade
fashion. A lower pH inhibited the formation of lactone 3a also
at the expense of the enzymatic activity.
appeared: (a) the intramolecular cyclization to afford the
desired lactones was highly favored for the 5-membered ring
(compare formation of 3a vs 3c in Figure 2); and (b) the
cyclization rate was enhanced for methyl ester derivatives (thus
producing MeOH), compared to the ethyl esters (where EtOH
was formed), since the transformation into lactone 3a starting
from 1a was faster than from 1b. The same tendency was
observed for γ-valerolactone (3c) starting from 1c or 1d with
both ADHs. In addition, the ee value was >99% in all cases,
getting access to both antipodes of the corresponding hydroxy
esters 2a and 2b or lactones 3a and 3c.
Because of the drastic influence of the pH in the cyclization
process, we designed a one-pot, two-step tandem protocol to
achieve the synthesis of δ-caprolactone 3a. It was envisaged that
acidic treatment of the reaction mixture of 2a and 3a after the
ADH-catalyzed bioreduction of 1a could led to 3a without
losing the excellent stereopreference. Thus, after enzymatic
reduction, a solution of 1 M HCl was added to the reaction
medium, exclusively detecting lactone 3a after 24 h (ee > 99%).
In the case of γ-keto esters 1c and 1d, GVL 3c was obtained as
the major product recycling the cofactor by simple addition of
2-PrOH and without the need of adding HCl. It is important to
remark that these findings could be reproduced on a 300 mg
scale, achieving enantioenriched (R)-GVL employing LBADH
as biocatalyst in 93% conversion and 97% ee (see the
Next, the effect of the chain length (γ- vs δ-keto ester) and
the ester moiety (Me vs Et) were studied with both
biocatalysts. Thus, bioreductions were performed over ethyl
5-oxohexanoate (1b), methyl levulinate (1c), and ethyl
levulinate (1d). From these experiments, two clear trends
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Scheme 3. Chemodivergent and Stereodivergent Synthesis of γ- and δ-Hydroxy Esters and Lactones through ADH-Catalyzed
Reactions
Supporting Information for more details). Using this method-
ology, both enantiomers of 3c could be easily obtained in a
one-pot cascade fashion performing the bioreduction at 30 °C
and pH 9.0. To summarize this part, it is worth mentioning that
depending on the reaction conditions we could control the
synthesis of the hydroxy esters 2a or 2b or the lactone 3a in a
chemo- and stereodivergent manner. In the case of γ-keto
esters, the formation of the lactone was highly favored, thus
synthesizing both enantiomers of GVL 3c with high efficiency
at slightly basic pH. Furthermore, traces coming from the
hydrolysis of the staring material could be just detected.18
At this point, we decided to test a bulkier substrate such as
methyl 5-oxo-5-phenylpentanoate (1e, Scheme 3). For this
purpose, we used ADHs from Sphingobium yanoikuyae
(SyADH)19 and Ralstonia sp. ADH (RasADH)20 overexpressed
in E. coli, since the substrate spectra for these enzymes is known
to cover similar “bulky−bulky” ketones. 2-PrOH was employed
as cosubstrate for E. coli/SyADH, whereas the glucose/GDH
system was chosen in the case of E. coli/RasADH. Based on
previous results, we performed the reduction of 1e at pH 7.5
with both enzymes to obtain hydroxy ester 2e preferentially.
Thus, while E. coli/RasADH led to the enantiomer (R)-2e21
with an excellent conversion (>97%) and ee values (>99%), E.
coli/SyADH led also to (R)-2e with a moderate conversion of
52% with a high enantiomeric excess (95%). In this case, the
cyclization process using HCl 1 M did not work properly,
attaining less than 25% conversion of lactone 3e.
When the aromatic γ-keto ester 1f was used as a substrate, in
contrast to compounds 1c and 1d, only hydroxy ester 2f was
detected. Taken together, these results emphasize the relevance
of the keto ester structure in order to favor (or not favor) the
lactonization process. In this case, E. coli/RasADH was the
most effective in terms of conversion and enantioselectivity
toward (R)-2f (c = 97%, ee = 99%), since SyADH afforded very
low conversions (<20%) at pH 7.5. It is noteworthy that 2f is a
valuable precursor for the preparation of γ-peptides22 and is
also present in the structure of cryptophycin,23 which is a
potent cytotoxic drug. In order to shift the equilibrium toward
the formation of lactone 3f, an acidic treatment with 1 M HCl
was required. When these bioreductions were tried at pH 9.0 to
favor the cyclization, slightly lower ee values (∼90%) were
attained with both biocatalysts toward hydroxy ester 2f, because
of a racemization issue (vide infra), although complete
conversion could be achieved with E. coli/SyADH in this
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case. As a matter of fact, depending on the pH or the substrate
structure, hydroxy esters 2 or lactones 3 can be preferentially
formed in a chemodivergent manner through one-pot cascade
or tandem protocols (see Scheme 3).
to construct more complex and potentially bioactive lactones
(Scheme 4).24,26 We first attempted the Suzuki coupling
starting from racemic 3g with phenylboronic acid using
Pd(PPh3)2Cl2, since this catalyst showed good activities in
aqueous media.15b We were pleased to find that such a reaction
was compatible with the stability of the lactone, achieving
compound 4g with an excellent conversion (>98%). However,
when both steps were combined in one-pot synthesis, only
traces of the coupled product were detected. This result
probably arose from inhibition issues. The reaction was
therefore performed in a sequential manner. After bioreduction,
the crude was simply extracted and then treated under Suzuki
conditions obtaining (R)-4g in 75% conversion. In a similar
manner, Sonogashira and Heck couplings were attempted. In
the first case, Pd(PPh3)2Cl2 and CuI were used to couple
lactone (R)-3g with phenylacetylene at 100 °C affording
derivative (R)-5g in a 60% isolated yield, while for the Heck
transformation, Pd(OAc)2 and styrene were employed at 120
°C to achieve lactone (R)-6g with 69% conversion.
Remarkably, all three Pd-catalyzed reactions proceeded
smoothly getting access to compounds (R)-4g−(R)-6g without
losing enantioselectivity.
Once the appropriate conditions to get access to
enantioenriched hydroxy esters or lactones were found, we
decided to apply this biocatalytic method to the synthesis of
several bioactive derivatives. Therefore, the bioreduction of
brominated compound 1g was studied, since lactone 3g has
been used as an intermediate for the synthesis of γ-
hydroxybutyric acid (GHB) analogues for high-affinity GHB
sites and γ-aminobutyric acid (GABA) receptors.24 Based on
our previous results, E. coli/RasADH was selected as the
biocatalyst to achieve this transformation with glucose and
GDH to recycle the cofactor. In order to obtain the
corresponding lactone, the reduction was first tried under
basic conditions (pH 9.0). Unfortunately, we observed that the
corresponding hydroxy ester 2g and the lactone 3g were
accessed with very low ee values. After confirming that other
ADHs from the E. coli host were not involved in the
nonstereoselective bioreduction of this substrate, we decided
to study this process in more detail following the reduction
within the time. As depicted in Figure 3, ee values from both 2g
CONCLUSIONS
■
In summary, we have demonstrated that ADHs can be used for
the chemodivergent and stereodivergent synthesis of relevant
hydroxy esters and lactones in ee > 97% through direct
bioreduction, or by means of cascade and tandem protocols by
simply tuning the reaction conditions. Also, the substrate
structure was a key factor in the spontaneous cyclization
process. ADH-A or LBADH were used to reduce selectively
methyl ketone substrates, whereas levulinate compounds
afforded the formation of (S)- or (R)-GVL, 5-oxohexanoate
surrogates preferentially gave access to the corresponding (S)-
or (R)-δ-hydroxy esters. In these latter cases, an acid-catalyzed
cyclization step was necessary in order to isolate the six-
membered ring lactones. For bulkier substrates, RasADH or
SyADH were employed, and after bioreduction, the corre-
sponding (R)-γ-hydroxy esters were efficiently obtained. After
acidic treatment, the five-membered ring lactone could be
achieved.
Figure 3. Time study of the E. coli/RasADH-catalyzed bioreduction of
1g at 30 °C and pH 9.0.
These findings have been applied to the preparation of
relevant compounds simplifying the previous established
methods. Furthermore, the viability of this synthetic route
prompted us the one-pot, two-step chemoenzymatic prepara-
tion of an enantioenriched brominated lactone that allowed the
divergent synthesis of several functionalized chiral lactones with
potential biological properties by sequential Pd-catalyzed
transformations. Our approach is efficient, simple, and scalable,
having great potential as a “greener” protocol.
and 3g decreased along the reaction. Control experiments
indicated that racemization of hydroxy ester 2g occurred in
basic medium, while lactone 3g remained unaltered.
With the goal of avoiding this undesired process, the
bioreduction was performed at pH 7.5. Under these conditions,
RasADH showed high activity (c > 99%) and (R)-2g could be
obtained with an excellent selectivity (ee > 97%) after 4 h.
Again, acidic treatment with HCl 1 M led to the corresponding
lactone with identical ee values. Because of the easy handling of
E. coli lyophilized cells, a preparative preparation of (R)-3g
could be readily achieved in a one-pot, two-step tandem
protocol on a 200 mg scale (50 mM), thus attaining the desired
compound with excellent isolated yield (95%) and ee (97%).
Notably, this methodology improves the previously described
protocols for the preparation of brominated lactone (R)-3g.7a,25
Interestingly, SyADH gave access to the opposite enantiomer,
(S)-3g, with complete conversion, although with a lower
stereoselectivity (ee = 60%).
EXPERIMENTAL SECTION
■
General Considerations. Glucose dehydrogenase (GDH 106, 54
U mg−1), ADH-A from Rhodococcus ruber (20 U mg−1), and LBADH
from Lactobacillus brevis (80 U mg−1) were obtained from Codexis.
ADHs from Rhodococcus ruber (ADH-A), Thermoanaerobacter
ethanolicus (TeSADH), Sphingobium yanoikuyae (SyADH) and
Ralstonia sp. (RasADH) have been obtained from Prof. Wolfgang
Kroutil at the University of Graz and have been overexpressed
following the methodology previously described.19,20,27
After these promising results with RasADH, we focused on
the subsequent functionalization of the brominated lactone in
order to expand the applicability of this methodology. Based on
the nature of the substrate, we employed Pd-coupling reactions
NMR spectra were recorded on a 300, 400 or 600 MHz
spectrometers. All chemical shifts (δ) are given in parts per million
(ppm) and are referenced to the residual solvent signal as internal
standard. The following abbreviation is employed: m = multiplet. Gas
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Scheme 4. Chemodiverse Preparation of (R)-4g, (R)-5g, and (R)-6g Combining a One-Pot Enzymatic Protocol with Pd-Based
Catalysis
chromatography (GC) analyses were performed on a standard GC
chromatograph equipped with a flame ionization detection (FID)
device. High-performance liquid chromatography (HPLC) analyses
were carried out in a standard chromatograph coupled to a UV
detector at 210 nm. High-resolution mass spectra (HRMS) were
obtained using a spectrometer by positive electrospray ionization
(ESI+). Infrared (IR) spectra were recorded as thin films on NaCl
plates on a standard Fourier transform infrared (FT-IR) spectroscopy
system and are reported in terms of frequency of absorption (cm−1).
Thin-layer chromatography (TLC) was conducted with silica gel
precoated plates and visualized using a UV lamp and/or potassium
permanganate stain. Column chromatography was performed using
silica gel (230−400 mesh). Microwave reactions were carried out with
a standard microwave apparatus with high stirring. Optical rotations
were measured in a polarimeter and values are reported in units of
10−1 deg cm2 g−1 (concentration, solvent).
Protocol Using ADH from Lactobacillus brevis (LBADH). In a
10-mL vial, LBADH (7.5 U) was dissolved in Tris-HCl buffer (50 mM,
pH 5.0−9.0, 2.28 mL, 1 mM NADPH, 1 mM MgCl2), 2-PrOH (120
μL, 5% v v−1), and the corresponding keto ester (50 mM). The
reaction was shaken at 30 °C and 250 rpm for 24 h. The reaction
mixture was extracted with Et2O (3 × 2 mL). The organic layers were
combined and dried over Na2SO4. The solvent was carefully
evaporated and the conversion of the corresponding hydroxy ester
or lactone was determined by NMR and the ee by GC or HPLC.
Protocol Using ADH from Rhodococcus ruber (ADH-A). In a
10-mL vial, ADH-A (3 U) was dissolved in Tris-HCl buffer (50 mM,
pH 5.0−9.0, 2.28 mL, 1 mM NADH), 2-PrOH (120 μL, 5% v v−1) and
the corresponding keto ester (50 mM). The reaction was shaken at 30
°C and 250 rpm for 24 h. The reaction mixture was extracted with
Et2O (3 × 2 mL). The organic layers were combined and dried over
Na2SO4. The solvent was carefully evaporated and the conversion of
the corresponding hydroxy ester or lactone was determined by NMR
and the ee by GC or HPLC.
mM), glucose dehydrogenase (GDH, 5 U), and the corresponding
keto ester (50 mM). The reaction was shaken at 30 °C and 250 rpm
for 24 h and stopped by extraction with Et2O (3 × 2 mL). The organic
layers were combined and dried over Na2SO4. The solvent was
carefully evaporated and the conversion of the corresponding hydroxy
ester or lactone was determined by NMR and the ee by GC or HPLC.
Chemoenzymatic One-Pot, Two-Step Protocol To Obtain
Enantiopure Lactone 3a. In a 10-mL vial, LBADH (7.5 U) was
dissolved in Tris-HCl buffer (50 mM, pH 9.0, 2.28 mL, 1 mM
NADPH, 1 mM MgCl2), 2-PrOH (120 μL, 5% v v−1), and the
corresponding keto ester 1a−b (50 mM). The reaction was shaken at
30 °C and 250 rpm for 24 h. Then, a solution of 1 M HCl (2 mL) was
added into the reaction mixture and was shaken at 30 °C and 250 rpm
for additional 24 h. The reaction mixture was extracted with Et2O (3 ×
2 mL). The organic layers were combined and dried over Na2SO4. The
solvent was carefully evaporated and the conversion of the reaction
was determined by NMR (>97%) and the ee by GC or HPLC (>97%).
Scaleup of the Preparation of (R)-GVL Using LBADH. In a 50-
mL Erlenmeyer flask, LBADH (140 U) was dissolved in a Tris-HCl
buffer solution (50 mM, pH 9, 17 mL, 1 mM NADPH, 1 mM MgCl2)
containing 2-PrOH (2.4 mL, 5% v v−1) and methyl levulinate 1c (295
μL, 100 mM). The reaction was shaken at 30 °C and 250 rpm for 24 h.
The reaction mixture was then extracted with Et2O (3 × 20 mL). The
organic layers were combined and dried over Na2SO4, and the solvent
was carefully evaporated. (R)-3c was obtained in quantitative yield
with a purity of 93% (ee = 97%).
Scaleup of the Enzymatic Reaction To Obtain (R)-3g with E.
coli/RasADH. In a 50-mL Erlenmeyer flask, E. coli/RasADH cells
(250 mg) were resuspended in a Tris-HCl buffer solution (50 mM, pH
7.5, 11.4 mL, 1 mM NADPH) containing glucose (100 mM), GDH
(200 U), 2-PrOH (600 μL, 5% v v−1), and 1g (200 mg, 50 mM). The
reaction was shaken at 30 °C and 250 rpm for 24 h. After that time, 10
mL of HCl 1 M were added and the reaction mixture was shaken at 30
°C and 250 rpm for additional 24 h. The reaction mixture was then
extracted with Et2O (3 × 10 mL). The organic layers were combined
and dried over Na2SO4. The crude residue was purified by
chromatography column (20% EtOAc/hexane) isolating the corre-
Protocol Using ADH from Ralstonia sp. (E. coli/RasADH). In a
10-mL vial, E. coli/RasADH (50 mg) was resuspended in 50 mM Tris-
HCl buffer pH 7.5−9.0 (2.28 mL, 1 mM NADPH), glucose (100
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sponding lactone (R)-3g in 95% yield (ee = 97%), [α]2D0 = −24.7 (c =
1.23, CHCl3). Spectral data for 3g were consistent with those reported
in the literature.24
Notes
The authors declare no competing financial interest.
Sequential Reaction Using the Suzuki Coupling for the
Synthesis of (R)-4g. In a 50-mL Erlenmeyer flask, E. coli/RasADH
cells (125 mg) were resuspended in Tris-HCl buffer (50 mM, pH 7.5,
4.5 mL, 1 mM NADPH), containing glucose (400 mM), GDH (20
U), 2-PrOH (300 μL, 5% v v−1), and 1g (81.3 mg, 0.3 mmol). The
reaction was shaken at 250 rpm for 24 h at 30 °C. The resulting
mixture was treated with 5 mL of HCl (1 M) and stirred for additional
24 h. After that time, the reaction was centrifuged (5 min, 4000 rpm)
and the cell pellet was washed with EtOAc (3 × 20 mL). The organic
layers were combined and dried over Na2SO4 and the solvent was
evaporated. The resulting crude mixture was dissolved in Tris-HCl
buffer (16 mL, 50 mM, pH 7.5) and 2-PrOH (1.6 mL, 10% v v−1) and
treated with phenylboronic acid (63 mg, 0.50 mmol) and K2CO3 (92
mg, 0.66 mmol). Then, Pd(PPh3)2Cl2 (11.4 mg, 0.016 mmol) was
added and the resulting mixture was stirred for 24 h at 45 °C. The
reaction mixture was extracted with EtOAc (3 × 15 mL) and the
combined organic phases dried over Na2SO4 and concentrated in
vacuo. The crude was analyzed by NMR observing the formation of
lactone (R)-4g in a 75% conversion (ee = 97%). Spectral data for 4g
were consistent with those reported in the literature.28
ACKNOWLEDGMENTS
■
A.D.-R. thanks the European Union for personal funding inside
the 7th Framework Programme (No. FP7 2007-2013, Grant
Agreement No. 266025). W.B. thanks the Ministerio de
Educacion
(FPU Program). I.L. thanks the Spanish Ministerio de Ciencia
e Innovacion (MICINN) for personal funding (Ramon y Cajal
́
, Cultura y Deporte for her predoctoral fellowship
́
́
Program). Financial support of this work by the Spanish
MICINN (Project No. MICINN-12-CTQ2011-24237) and the
Principado de Asturias (No. SV-PA-13-ECOEMP-43) is
gratefully acknowledged.
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1
isolating lactone 5g in 58−64% isolated yield. H NMR (400 MHz,
CDCl3): δ 2.21 (m, 1H), 2.68 (m, 3H), 5.54 (m, 1H), 7.36 (m, 6H,
Ar), 7.55 (m, 4H, Ar); 13C NMR (106 MHz, CDCl3): δ 176.6, 139.4,
132.0 (2C), 131.6 (2C), 128.4 (2C), 126.9, 125.2 (2C), 123.5, 123.0,
90.0, 88.7, 80.8, 30.9, 28.9; IR (neat): 3026, 2920, 2845, 1653, 1600,
1494, 1449, 1260, 1068, 1022 cm−1. MS (APCI+, m/z) 263.0 [(M
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Sequential Reaction Using the Heck Coupling for the
Synthesis of (R)-6g. The corresponding brominated lactone rac- or
(R)-3g (40 mg, 0.166 mmol), Pd(OAc)2 (1.9 mg, 0.008 mmol) and
H3PO4 (2.4 mmol) were added to a sealed tube under a stream of
nitrogen and dissolved in THF (1 mL). Styrene (28 μL, 0.249 mmol)
was added to the stirred solution. The reaction mixture was heated at
120 °C for 16 h. The reaction mixture was then extracted with EtOAc
(3 × 10 mL). The organic layers were combined and dried over
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formation of lactone (R)-6g in 69% conversion (ee = 97%). Spectral
data for 6g were consistent with those reported in the literature.24
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́ ́
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ASSOCIATED CONTENT
■
S
* Supporting Information
(9) Recently, a non-stereoselective version for this oxidative reaction
has been shown in our group using as oxidant a laccase/TEMPO
system. See: Díaz-Rodríguez, A.; Lavandera, I.; Kanbak-Aksu, S.;
Experimental procedures, enzymatic protocols at higher
substrate concentrations, analytical data, and copies of 1H
NMR and 13C NMR for 5g are described. This material is
available free of charge via the Internet at http://pubs.acs.org.
Sheldon, R. A.; Gotor, V.; Gotor-Fernan
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, K.; Backvall, J.-E. Chem.Eur. J. 2013, 19,
13859−13864.
́
dez, V. Adv. Synth. Catal.
̀
̈
́
̀
, L.; Pamies, O.; Backvall, J.-E. J.
̈
AUTHOR INFORMATION
■
Corresponding Authors
́
̈
*E-mail: lavanderaivan@uniovi.es (I.L.).
*E-mail: vgs@uniovi.es (V.G.).
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Author Contributions
‡The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. These authors contributed equally.
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