From diols to lactones under aerobic conditions using a laccase/TEMPO catalytic system in aqueous medium

Article abstract of DOI:10.1002/adsc.201200892

An efficient catalytic system to oxidize quantitatively aliphatic diols using Trametes versicolor laccase and TEMPO has been developed in aqueous medium. Oxidations have occurred in a non-stereoselective fashion but with complete regio- and/or monoselectivity, obtaining lactones with excellent purity after simple extraction. This catalytic system has been demonstrated to be scalable, compatible with the presence of a variety of functionalities, and also allowed the successful enzyme recycling using a laccase-cross-linked enzyme aggregates (CLEA) preparation.

Full text of DOI:10.1002/adsc.201200892

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DOI: 10.1002/adsc.201200892
From Diols to Lactones under Aerobic Conditions using a
Laccase/TEMPO Catalytic System in Aqueous Medium
Alba Dꢀaz-Rodrꢀguez,a Ivꢁn Lavandera,a Seda Kanbak-Aksu,b Roger A. Sheldon,b
Vicente Gotor,a and Vicente Gotor-Fernꢁndeza,*
a
Organic and Inorganic Chemistry Department, Universidad de Oviedo, Avenida Juliꢁn Claverꢀa s/n., 33006 Oviedo, Spain
Fax : (+34)-985-10-3448; phone: (+34)-985-10-3454; e-mail: vicgotfer@uniovi.es
CLEA Technologies, Delftechpark 34, 2628 XH Delft, The Netherlands
b
Received: August 9, 2012; Revised: October 3, 2012; Published online: December 4, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200892.
Abstract: An efficient catalytic system to oxidize
quantitatively aliphatic diols using Trametes versi-
color laccase and TEMPO has been developed in
aqueous medium. Oxidations have occurred in
a non-stereoselective fashion but with complete
regio- and/or monoselectivity, obtaining lactones
with excellent purity after simple extraction. This
catalytic system has been demonstrated to be scala-
ble, compatible with the presence of a variety of
functionalities, and also allowed the successful
solvent.[4] While a variety of catalytic methods has
been reported for the oxidation of alcohols in organic
solvents, the development of more selective and envi-
ronmentally friendly oxidation procedures remains
nowadays as one of the major challenges for the pro-
duction of fine chemicals.[5] Hence, there is a need for
more efficient, greener and scalable catalytic oxida-
tions in aqueous medium, biocatalytic processes being
considered adequate tools for these types of transfor-
mations.[6]
Copper is a fundamental trace element involved in
many redox reactions occurring in living systems and
can be found in a number of proteins. Laccases are
multicopper enzymes, which are present in many
fungi, plants and bacteria.[7] Functionally, these oxi-
dases reduce O2 into H2O at the expense of the corre-
sponding substrate. Over the last decade, considerable
improvements in protein isolation, expression and pu-
rification methods have provided access to these en-
enzyme recycling using
a
laccase-cross-linked
enzyme aggregates (CLEA) preparation.
Keywords: aerobic oxidation; laccase; lactones; re-
gioselectivity; TEMPO
The regioselective oxidation of alcohols to obtain al- zymes[8] making laccases ideal candidates for biooxi-
dehydes, ketones or carboxylic compounds is one of dation processes.[9] Although phenolic derivatives are
the most relevant and challenging transformations in their natural substrates and have already found in
organic chemistry, due to the utility of these deriva- some cases industrial applications,[10] laccases are not
tives as synthetic precursors for many drugs, fragran- effective towards other non-phenolic substrates, so
ces and natural products. A large variety of stoichio- electron transfer mediators are then required. Recent-
metric reagents have been used for these oxidation ly, the laccase/TEMPO system has proven to be a suit-
reactions such as peroxides, hypervalent organo- able set-up for the oxidation of benzylic alcohols.[11] It
ACHTUNGTRENNUNG
iodane, chromium oxides or sulfur-based reagents.[1] is also noteworthy that laccases are accessible and in-
Particularly interesting are those catalytic oxidations expensive in comparison with other reported copper
using transition metal agents (Pd, Ru, Au, Rh) under complexes and work in water efficiently.
aerobic conditions because oxygen generates water as
As part of our interest in biocatalyzed processes,
the sole by-product.[2] To date, among all organome- we have explored the potential application of a com-
tallic complexes employed to perform aerobic oxida- mercially available laccase from Trametes versicolor
tions, probably copper-containing catalysts have to the selective oxidation reactions of aliphatic diols
emerged as the most efficient ones,[3] as recently re- in aqueous medium.
ported by Hoover and Stahl in the chemoselective ox-
As a first attempt, the laccase/TEMPO system was
idation of a range of primary alcohols in the presence used in a competition experiment for the selective ox-
of other functional groups using (bpy)Cu(I) com- idation of a 1-octanol and 2-octanol mixture. Reac-
plexes, N-methylimidazole as base and acetonitrile as tions were conducted at room temperature, in NaOAc
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nediol as substrate (4, Scheme 1, b) led also to inter-
esting results. When containing an aliphatic primary
and a benzylic alcohol (known substrates for laccas-
es),[11] the primary position was selectively oxidized
providing lactone 7 in 89% conversion after the
chemical cyclization process, while 10% of ketoalde-
hyde 9 was also formed. Only traces of the hemiketal
8 were detected in the crude material coming from
the benzylic oxidation.[14]
Figure 1. Chemical structure of selected diols used with the
laccase/TEMPO system.
Based on the laccase/TEMPO-catalyzed oxidation
mechanism,[15] where the oxoammonium ion acts as
buffer (pH 4.8), open to ambient air, in the absence the actual oxidant, lactone 7 was obtained as a racemic
of exogenous bases, and with the commercial laccase mixture. However, when enantioenriched diol (S)-4
from Trametes versicolor and TEMPO as catalytic was used (>95% ee), lactone (S)-7 was obtained with
system.[12] To our delight, GC analyses revealed the same enantiomeric excess as oxidation takes place
a clear preference for the oxidation of the primary al- with high selectivity for the primary alcohol (see the
cohol. This result led us to pursue the regioselective Supporting Information).
oxidation of a library of diols bearing both primary
Application of this methodology to 3-substituted
and secondary alcohols (Figure 1). Satisfyingly, the 1,5-pentanediols (11–16, Table 1)[16,17] enabled the
same tendency was observed for the reaction of 1,2- facile and efficient preparation of functionalized tet-
octanediol (1) using the laccase/TEMPO system, rahydro-2H-pyran-2-ones in water with potential in-
while the formation of the corresponding hydroxy al- terest for industry.[13] Gratifyingly, monoselective oxi-
dehyde was observed at short reaction times, pro- dation of diols 10–16 was achieved in all cases, and
longed periods resulted in the oxidation of both pri- subsequent in situ cyclization led to lactones 17–23 in
mary and secondary alcohols. In order to improve the very high isolated yields (91–97%, Table 1). Bubbling
selectivity of our system and shift the equilibrium to- O2 through the solution speeded up the reactions
wards the formation of thermodynamically stable achieving complete conversion in 2.5–8 h,[18] isolating
compounds, we decided to employ diols 2–4, which the lactones with excellent purity after a simple ex-
can cyclize in situ leading to the corresponding lac- traction, since this oxidative method produces water
tones.
Remarkably when the reaction was carried out with
as the only by-product.
To further explore the scope of this system, aromat-
1,4-pentanediol (2), oxidation of the primary alcohol ic 1,4-diol 24 was employed as a substrate (Scheme 2).
took place selectively generating a hydroxy aldehyde Interestingly, treatment of benzenemethanediol with
intermediate, which immediately cyclized affording the laccase/TEMPO catalytic system afforded isoben-
a hemiacetal. The subsequent oxidation of the latter zofuranone 25 in excellent yields (93%) and without
gave access to the stable g-valerolactone in quantita- the need of further purification, demonstrating the
tive yield (Scheme 1, a). Excellent regioselectivity was potential of this catalytic system for the production of
also observed when 1,5-hexanediol (3) was treated five-membered ring lactones, interesting building
with the laccase/TEMPO system delivering d-capro- blocks for the preparation of bioactive molecules.[19]
lactone (6) in 92% isolated yield. It is noteworthy to
Some of the reactions were effectively scaled up,
highlight the relevance of these monomers for indus- for instance, 600 mg of 11 and 200 mg of 16 were
try,[13] and that herein excellent yields can be achieved treated with the laccase/TEMPO catalytic system, ob-
in aqueous medium after a simple extraction purifica- taining the corresponding 3-methyl-1,5-lactone (18)
tion step. Additionally, the use of 1-phenyl-1,5-penta- and 3-(2-methoxyphenyl)-1,5-lactone (23) in 90% and
Scheme 1. Catalytic oxidation using the laccase/TEMPO system. Isolated yields appear in brackets: a) 1,4-pentanediol; b) 1-
phenyl-1,5-pentanediol.
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From Diols to Lactones under Aerobic Conditions using a Laccase/TEMPO Catalytic System
Table 1. Oxidation of 1,5-diols by the Trametes versicolor
92% yield, respectively. The scalability of this process
is critical for the development of an economic process
for industry. In this sense, immobilization of enzymes
makes recycling possible while improving stability.[20]
To this end, we used a laccase immobilized as cross-
linked enzyme aggregates (CLEAs)[21] in our system
with diols 11 and 16. To our delight, the laccase-
CLEA resulted in similar product yields (89–92%).
Additionally, this reaction occurred without signifi-
cant loss of enzyme activity after three reuse cycles,
isolating the final products in similar yields compared
to the free-laccase experiments.
laccase/TEMPO catalytic system in aqueous medium.[a]
Entry
Product
Time
[h]
Conversion[b]
(Yield [%])[c]
AHCTUNGTRENNUNG
In conclusion, we have described the use of the Tra-
metes versicolor laccase/TEMPO catalytic system to
efficiently oxidize interesting aliphatic diols in
a regio- and/or monoselective manner, obtaining
highly valuable lactones with excellent purity after
a simple extraction. Due to the oxidation mechanism
of this system, lactones were obtained as a racemic
mixture, but starting from an enantioenriched diol, no
racemization was observed. This is the first report of
an aerobic oxidation of 1,4- and 1,5-diols in aqueous
medium using laccases and in the absence of a base.
This catalytic system is compatible with the presence
of unprotected secondary alcohols, activated benzylic
hydroxy groups and other functionalities such as
methoxy, fluorine or bromine substituents. Further-
more, scalability of the process has been demonstrat-
ed using an inexpensive commercially available lac-
case, which makes this methodology a direct competi-
tor of Cu-organometallic complexes under aerobic ox-
idative conditions and other TEMPO systems. Finally,
the possibility of using immobilized laccases has also
been demonstrated, rendering this chemoenzymatic
methodology as a very attractive tool for industrial
purposes.
1
2
2.5
2.5
>97% (92)
>97% (92)
3
4
8
>97% (91)
>97% (97)
5.5
5
6
7
>97% (92)
7
3
>97% (92)
>97% (93)
7
Experimental Section
General Procedure for the Preparation of 1,5-
Lactones using Laccase from Trametes versicolor
[a]
Small scale reactions were performed in NaOAc buffer
50 mM, pH 4.8 at room temperature, total volume: 5 mL,
[substrate]: 25–30 mM, [TEMPO]: 4–6 mM, 10 U/mL of
laccase and bubbling O2.
Followed by GC.
Isolated yields.
A solution of the corresponding diol 2–4, 10–16 or 24 (25–
30 mM) in an NaOAc buffer pH 4.8 (5 mL), was treated
with TEMPO (4–6 mM) and this mixture was stirred until
complete dissolution. Then, laccase was added (10 U/mL)
and the solution stirred vigorously in an open-to-air tube at
room temperature overnight (oxygen can be bubbled
through the solution to minimize reaction times). The reac-
tion time courses were followed by GC analysis until no
starting material remained. Dichloromethane was added
(2ꢃ5 mL), the layers were separated, the organic phase
washed with brine (10 mL), dried over Na2SO4 and evapo-
rated under reduced pressure to give the corresponding lac-
tone without further purification. Conversion values were
determined by GC analysis, and enantiomeric excess by
HPLC (see Supporting Information).
[b]
[c]
Scheme 2. Synthesis of isobenzofuranone 25 using the lac-
case/TEMPO catalytic system.
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Acknowledgements
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This research is part of BIONEXGEN project (grant agree-
ment 266025) sponsored by the European Union inside the
7th Framework Programme; (FP7 2007-2013). I.L. thanks the
Spanish MICINN for personal funding inside the Ramꢀn y
Cajal Program.
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