Lipase-Catalyzed Condensation Reaction of 4-Nitrobenzaldehyde with Acetyl Acetone in Aqueous-Organic Cosolvent Mixtures and in Nearly Anhydrous Media

Article abstract of DOI:10.1080/00397911.2013.834059

The nature of the product(s) in lipase-catalyzed reaction of acetyl acetone with 4-nitrobenzaldehyde was found to depend upon the source of lipase and the reaction medium. Mucor javanicus lipase was found to give 70% aldol with 80% enantiomeric excess in anhydrous t-amyl alcohol. A 2:2 adduct was formed by the dimerization of the aldol along with an unsaturated cyclic ether as the side products in varying proportions depending upon the reaction medium and the lipase used. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]

Full text of DOI:10.1080/00397911.2013.834059

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Lipase-Catalyzed Condensation Reaction
of 4-Nitrobenzaldehyde with Acetyl
Acetone in Aqueous–Organic Cosolvent
Mixtures and in Nearly Anhydrous Media
Abir B. Majumder a & Munishwar N. Gupta b
a Department of Chemistry , Rajiv Gandhi University of Knowledge
Technologies, Basar Campus , Andhra Pradesh , India
b Department of Biochemical Engineering & Biotechnology , Indian
Institute of Technology Delhi , New Delhi , India
Published online: 20 Feb 2014.
To cite this article: Abir B. Majumder & Munishwar N. Gupta (2014) Lipase-Catalyzed Condensation
Reaction of 4-Nitrobenzaldehyde with Acetyl Acetone in Aqueous–Organic Cosolvent Mixtures
and in Nearly Anhydrous Media, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 44:6, 818-826, DOI: 10.1080/00397911.2013.834059
To link to this article: http://dx.doi.org/10.1080/00397911.2013.834059
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Synthetic Communications1, 44: 818–826, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.834059
LIPASE-CATALYZED CONDENSATION REACTION
OF 4-NITROBENZALDEHYDE WITH ACETYL ACETONE
IN AQUEOUS–ORGANIC COSOLVENT MIXTURES
AND IN NEARLY ANHYDROUS MEDIA
Abir B. Majumder1 and Munishwar N. Gupta2
1Department of Chemistry, Rajiv Gandhi University of Knowledge
Technologies, Basar Campus, Andhra Pradesh, India
2Department of Biochemical Engineering & Biotechnology, Indian Institute of
Technology Delhi, New Delhi, India
GRAPHICAL ABSTRACT
Received May 11, 2013.
Address correspondence to Munishwar N. Gupta, Department of Biochemical Engineering &
Biotechnology, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India. E-mail:
munishwar48@yahoo.co.uk
818

LIPASE-CATALYZED CONDENSATION REACTION
819
Abstract The nature of the product(s) in lipase-catalyzed reaction of acetyl acetone with
4-nitrobenzaldehyde was found to depend upon the source of lipase and the reaction
medium. Mucor javanicus lipase was found to give 70% aldol with 80% enantiomeric excess
in anhydrous t-amyl alcohol. A 2:2 adduct was formed by the dimerization of the aldol along
with an unsaturated cyclic ether as the side products in varying proportions depending upon
the reaction medium and the lipase used.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for the following free supplemental resource(s):
Full experimental and spectral details.]
Keywords Aldol condensation; asymmetric synthesis; biocatalytic promiscuity; 1,3-dike-
tones; lipases; regioselectivity
INTRODUCTION
Replacement of chemical catalysts with biocatalysts is an important area in
biotechnology.[1–3] The recent results on catalytic promiscuity of enzymes have con-
siderably enhanced the versatility of biocatalysts.[4–7] So far, such observations in this
area have been mostly limited to showing that some specific reactions, not expected
from the known biological activity of the enzymes in vivo condition, can take place
in in vitro. In the present work, we show that depending upon the source of the enzyme
and the reaction medium, such promiscuous reaction can give a larger variety of pro-
ducts. This opens up the possibility of creating larger chemical libraries using enzymes.
The utility of such approach in the synthesis of novel molecules of pharmaceutical
importance is obvious. Carbon–carbon bond formation reactions are very important
in organic synthesis.[6,7] Of special interest are the C-C bond-formation reactions
involving 1,3-diketones, which have considerable synthetic utility.[8,9] The aldol reac-
tion with 1,3-diketones with biocatalysts has been especially challenging. Both cata-
lytic antibodies 33F12 and 38C2 raised with a 1,3-diketone as haptens (by reactive
immunization) failed to carry out this reaction as e-amino group of the active site
lysine reacted with the diketone.[10,11] More recently, a designer aldolase grafted from
phase–display libraries also failed to catalyze aldol reaction with 1,3-diketones for a
similar reason.[11] Even by chemical methods, it requires a very long time to obtain
the aldol of acetylacetone (the simplest 1,3-diketone) and substituted benzalde-
hydes.[8,9] In this work we show that biocatalytic promiscuity of lipases can be well uti-
lized for the aldol condensation of 4-nitrobenzaldehyde with acetylacetone, which acts
as a donor, and also for the preparation of an acyclic and eight-membered unsaturated
cyclic ether. It can be added that while the condensed products involving more than
one free carbonyl functional groups are important synthons, the synthesis of cyclic
ethers has been a matter of great interest to the organic chemists because the latter
finds importance in the synthesis of allylic and homoallylic alcohols.[12] The present
work shows an enzymatic approach to prepare these three types of compounds.
RESULTS AND DISCUSSION
The aldol condensation reaction of 4-nitrobenzaldehyde (1, Scheme 1,
100 mM) and acetyl acetone (2, Scheme 1, 100 mM) was tried with lipases in phos-
phate buffer (50 mM, pH 7.0) containing 30% v=v water miscible organic cosolvent.

820
A. B. MAJUMDER AND M. N. GUPTA
Scheme 1. Proposed lipase-catalyzed aldol condensation.
Three different organic solvents [viz., N,N0-dimethylfomamide (DMF), dioxan, and
dimethylsulfoxide (DMSO)] were tried and 30% v=v was found to be the minimum
concentration of any of these organic cosolvents that was necessary to dissolve the
substrates at these concentrations. Of these three cosolvents (at this concentration)
DMSO gave the best initial rate as well as conversion (details given as Fig. S1, sup-
plementary information). It was found that different commercially available lipases
(some free and some in immobilized forms), in aqueous–organic cosolvent mixture,
did not give the expected aldol (3, Scheme 1) but yielded two different products
(detected by TLC of RF 0.25 and 0.4 in hexane–ethylacetate ¼ 3:1) in varying pro-
portions (quantified by HPLC, Table 1). The products were purified by column chro-
1
matography and characterized by H NMR analysis, 13C NMR (including DEPT
spectra), and high-resolution mass spectral analysis (given as Figs. S2–S6, sup-
plementary information). The structure of one of the two products was found to
1
be 4 (Scheme 2). In case of the second product (RF 0.4), it was found that the H
NMR spectra exactly matched with the spectra recorded for deuterated 4 [i.e., the
1H NMR spectra of 4 when taken after a deuterium exchange (Fig. S7, supplemen-
tary information)]. This confirmed that the two protons of the two enolic OH groups
of 4 are no longer present in it and the structure of this product was anticipated as 5
(Scheme 2). This was further confirmed by the fact that when purified 4 was taken as
the starting material, by Novozym 435, 5 was formed exclusively. The two enolic
protons of 4 were removed during the ring closure to form 5 (Scheme 3). Compounds
4 and 5 were formed by an abstraction of a water molecule.
Table 1. Products formed in aqueous-organic cosolvent mixture by different lipases: Screening based
upon higher product selectivity
Products
formed
Initial rates for overall
conversion (mM mgꢀ1 hꢀ1 a
Time
(h)
Yield
4(%)b
Yield
5(%)b
Entry
Lipase
B. cepacia
)
1
2
3
4, 5
4, 5
4, 5
280
1500
170
72
24
72
82
70
15
12
25
50
C. antarctica
C. antarctica
(immobilized)
M. javanicus
M. miehei
4
5
6
4
230
280
300
72
72
72
77
80
83
—
5
4, 5
4, 5
M. miehei
(immobilized)
7
a4-Nitrobenzaldehyde (100 mM), acetylacetone (100 mM), 20 mg lipase formulations were taken in 1 mL
of phosphate buffer (50 mM, pH 7.0) containing 30% DMSO and were shaken at 200 rpm at 30 ꢁC. Ali-
quots were taken out at different time intervals and were analyzed by HPLC.
bAfter purification by silica-gel column.

LIPASE-CATALYZED CONDENSATION REACTION
821
Scheme 2. Lipase-catalysed promiscuous reaction: formation of acyclic and cyclic 2:2 adducts.
A lipase from Burkholderia cepacia was found to catalyze the formation of 4 as
a major product, resulting in an 82% yield within 72 h (entry 1, Table 1). Compound
5 was formed as the minor product. On the other hand, the use of immobilized
formulation of Candida antarctica lipase B, Novozym 435, led to the formation of
5 as a predominant product (entry 3, Table 1). The free form of Candida antarctica
lipase B catalyzed the formation of 4 in 70% yield within 24 h (entry 2, Table 1).
Immobilization is known to influence the selectivity of the biocatalyst.[13] However,
no such effect was observed in the case of Mucor miehei lipase: Both the free and the
immobilized forms catalyzed the formation of the product 4 as the major product
(entry 5,6 Table 1). Interestingly Mucor javanicus lipase was found to give only
Scheme 3. Probable mechanism for the formation of cyclized product.

822
A. B. MAJUMDER AND M. N. GUPTA
one product, 4 (entry 4, Table 1). Presumably with Mucor javanicus lipase, the
reaction stops at the dimerization stage and hence only one product was obtained
exclusively. While medium engineering is an established approach in use of enzymes
for organic synthesis in low water media,[14] this aspect has so far not been explored
in the context of promiscuous reactions. It was decided to also explore nearly anhy-
drous conditions as the reaction medium. Enzyme reactions in such media have been
extensively studied. However, promiscous reactions have been more often studied
just in water.[5,6] In an earlier work with a cyclic and more complex diketone we
had used nearly anhydrous media for the lipase-catalyzed promiscuous reactions.[4]
The reaction in different dry organic solvents was investigated in the presence
of M. javanicus lipase (Table 2, detailed conversion vs time plots have been given as
Fig. S8, supplemenatry information). Interestingly product 5 was not formed in these
dry solvents. In addition to product 4, another product was also formed (observed
with an RF of 0.53 on thin-layer chromatography, TLC) in all the organic solvents
that were tried, but in varying proportions (as found by high-performance liquid
chromatography, HPLC). The products were purified by column chromatography
for characterization. The structure of this product was confirmed as 3 (Scheme 1),
1
the expected aldol, by H NMR spectra only as it showed a similar pattern as
described in the literature.[15] When the reaction was carried out in solvent-free
medium (i.e., when the acetylacetone itself was used as the solvent), almost equal
percentages of 3 and 4 were formed (entry 2, Table 2).
When t-alcohols were used as the solvent, the formation of 4 was minimum and
mostly aldol 3 was obtained (entries 6,7, Table 2). The greatest ratio of the aldol 3 to
the 2:2 uncyclized product 4 (12:1, entry 7, Table 2) was obtained when t-amyl alco-
hol was used as the reaction medium. It may be added that t-alcohols are generally
not the substrates of Mucor javanicus lipase.[16]
Table 2 gives the initial rates for the enzyme catalyzed formation of 3 and 4 and
also shows that in all the cases [except cosolvents cyclopentyl methyl ether (CPME)
Table 2. Optimization for the synthesis of 3: Effect of low-water media on initial rates and product nature
as exhibited by Mucor javanicus lipase
Initial rates for 3
(mM mgꢀ1 hꢀ1 a
Initial rates for 4
(mM mgꢀ1 hꢀ1 a
Time Yield Yield ee (þ)
(h) (%) 3 (%) 4 (%), 3
Entry
Solvent
)
)
1
2
3
4
5
6
7
Acetonitrile
Acetylacetone
CPME
10
180
5
72
390
32
72
48
72
72
72
72
72
10b
45
6
48
52
35
0
<10c
24a
0
DCM
DMSO
5
3
0
280
128
140
80
14
15
72
64
70
18
8
45
68
80
t-Butanol
t-Amyl
alcohol
6
aDetermined by analysis of the crude reaction mixture (by analytical HPLC using Zobax Sil C18 col-
umn) of hexane and isopropanol (4:1, v=v) at a flow rate of 1 mL=min and was monitored by UV-diode
array detector at 254 nm.
bAfter column purification.
cDetermined by chiral HPLC using Chiralcel-ODRH column of the column purified aldol product, and
polarimetric studies confirmed a dextrorotation in all the cases.

LIPASE-CATALYZED CONDENSATION REACTION
823
Figure 1. Chiral HPLC analysis of the column purified aldol product (3, Scheme 1): (a) the racemic mix-
ture (obtained by using CPME), (b) at 50% conversion in t-amyl alcohol exhibiting 38% ee of d form (by
polarimetric studies of the injected sample) after 24 h, and (c) at 70% conversion in t-amyl alcohol corre-
sponding to 80% ee of the d form after 72 h.
and dichloromethane (DCM)], enantiopreference of the d form (confirmed by polari-
metric analysis) of the aldol product 3 was observed. The racemic aldol (3) was pro-
duced as a result of enzymatic reaction using CPME (entry 3, Table 2) as a green
solvent.[17] Its resolution on the chiral column is shown in Fig. 1a [efforts to improve
this resolution by changing the flow rate did not succeed]. Figure 1b shows the chro-
matogram after 24 h of the reaction in t-amyl alcohol. This corresponded to 50%
conversion and 38% ee of d-form. Figure 1c gives the chromatogram after 72 h
and corresponds to 70% conversion and 80% ee of the d-form. The best enantiomeric
excess, ee (80%) was obtained in the case of t-amyl alcohol (Fig. 1). The compound 3
has not been reported in the literature so far. Hence, determination of its absolute
configuration by crystallographic analysis or NMR studies may be interesting but
was not pursued in the present work. Products 4 and 5 were found to be optically
inactive. This raised several possibilities. These could be both meso compounds.
Alternatively, these could be racemic compounds. The efforts to resolve either of
these on chiral column did not succeed and only a single peak was obtained in each
case (data not shown). Hence, it looks more likely that these were meso forms
(Scheme 2) but further work would be needed to settle this issue. Also, as these
are new compounds the assignment of R and S configuration around chiral center
cannot be done without detailed structural characterization.
CONCLUSION
To sum up, this first-ever study of lipase-catalyzed condensation of acyclic
1,3-diketone acetylacetone with 4-nitrobenzaldehyde showed that multiple products
are formed, including an unsaturated cyclic ether. However, choice of the lipase and
the solvent conditions dictate the nature of the products. It is interesting to note that
in aqueous medium lipases were found to give the 2:2 adduct 4 as the major product

824
A. B. MAJUMDER AND M. N. GUPTA
by a fast dimerization of 3 and 5 as a minor product. The dimerization of the aldol 3
was restricted in the low-water media. It is also of interest to note that there was a
regioselectivity in this catalytic promiscuity because the reaction occurred only via
C-3 atom of acetylacetone. Similar regioselectivity was observed with a cyclic
1,3-diketone as well.[4]
EXPERIMENTAL
Lipases from Mucor javanicus, Mucor miehei (both free and immobilized on
anion exchange resin), and Candida antarctica (B, both free and immobilized on
acrylic resin) were obtained from Novozymes A=S, Bagsvaerd, Denmark. Burkhol-
deria cepacia lipase (Lipase PS) was a gift from Amano Enzymes Inc., Nagoya,
Japan. 4-Nitrobenzaldehyde (>99%) and acetylacetone (>99.5%, GC) were pur-
chased from Spectrochem, Mumbai, India.
Condensation Reaction
4-Nitrobenzaldehyde (15 mg, 0.1 M), acetylacetone (10 mg, 0.1 M) were taken
in phosphate buffer (50 mM, pH 7.0) containing 30% v=v organic cosolvent (up to
1 mL) with 20 mg lipase formulations and were shaken at 200 rpm at 30 ꢁC. For
the reactions in low-water media, 4-nitrobenzaldehyde (15 mg, 0.1 M), acetylacetone
(10 mg, 0.1 M or in excess in case of solvent free conditions), and pH-tuned (in
50 mM phosphate buffer, pH 7) freeze-dried lipase formulations were taken in
1 mL of various organic solvents (distilled and dried by molecular sieves) and were
shaken at 200 rpm at 30 ꢁC.
HPLC Analysis
The aliquots were taken at different time intervals, diluted with acetonitrile (10
times to precipitate the enzyme, which was required especially in cases of the reac-
tions in aqueous–organic media), and centrifuged, and the supernatant was analyzed
on Zorbax SIL C-18 column (fitted in Agilent 1100 GC system) with an eluent con-
sisting of a mixture of hexane and isopropanol (4:1, v=v) at a flow rate of 1 mL minꢀ1
and was monitored by a UV-diode array detector at 254 nm.
Workup Details and Analysis
The crude product was extracted from the aqueous solutions by DCM and was
purified by column chromatography (column length 20 cm, 2.5 cm diameter) with
hexane and ethyl acetetate (5:1) as eluent, and the isolated products were recrystal-
lized from HPLC-grade hexane and ethyl acetate. The purity was cross-checked by
TLC and confirmed by HPLC.
Chiral HPLC Analysis
The column-purified, NMR-tested product 3 was dissolved in isopropanol and
analyzed on Chiralcel ODRH (fitted in Agilent 1100 GC system) with an eluent

LIPASE-CATALYZED CONDENSATION REACTION
825
composed of hexane and isopropanol (5: 2, v=v ratio) at a flow rate of 1 mL minꢀ1
and was monitored by a UV-diode array detector at 254 nm. The l and d enantiomers
were detected at 5.5 and 5.9 min respectively. Enantiomeric excess were calculated
from the corresponding peak areas.
Polarimetric Analysis
The column-purified characterized (by NMR) product was dissolved in ethanol
(conc. 0.7–1.2%) and its optical rotation was measured by a digital polarimeter,
Autopol V. A dextrorotation was observed for 3, while 4 and 5 were found to be
optically inactive.
Product Characterization
(Z)-4-Hydroxy-3-(hydroxy(4-nitrophenyl)methyl)pent-3-en-2-one
(3). C12H13NO5 yellow, semisolid mass, RF ¼ 0.53 in hexane–ethylacetate (3:1). IR
(KBr), n (cmꢀ1): 735, 1059, 1344, 1537, 1572, 1640, 1690, 3200 (b). 1H NMR (CDCl3,
300 MHz), d (ppm): 1.85 (3H, s), 2.2 (3H, s), 5.12 (1H, bs, exchangeable with
D2O), 5.3 (1H, s), 8.08 (2H, d, J ¼ 8.4 Hz), 8.40 (2H, d, J ¼ 8.4 Hz), 10.16 (1H,
exchangeable with D2O).
(3Z,3’Z)-3,3’-(Oxybis((4-nitrophenyl)methylene))bis(4-hydroxypent-3-en-
2-one) (4). C24H24N2O9, orange yellow solid, mp 72–73.5 ꢁC. RF ¼ 0.25 in hexane–
ethylacetate (3:1). IR (KBr), n (cmꢀ1): 737, 1059, 1210, 1344,1537, 1572, 1640, 3300
1
(b). H NMR (CDCl3, 300 MHz), d (ppm): 2.25 (6H, s), 2.46 (6H, s), 5.3 (2H, s;
exchangeable with D2O), 7.49 (2H, s), 7.55 (4H, d, J ¼ 8.4 Hz), 8.24 (4H, d,
J ¼ 8.4 Hz). 13C NMR: 26.72, 31.78, 124.16, 130.25, 136.5, 139.24, 145.59, 148.48,
196.01, 204. 36. DEPT: 26.7, 31.79, 124.17, 130.26, 136.51; m=z 509.3036 (Mþ
NaC24H22 D2 N2O9 requires 509.2027).
1,1’-((3E,6E)-4,6-Dimethyl-2,8-bis(4-nitrophenyl)-2,8-dihydro-1,5-dioxocine-
3,7diyl)diethanone (5). C24H22N2O9, orange-yellow solid (mp 60–62 ꢁC), RF ¼ 0.4
in hexane–ethylacetate (3:1). IR (KBr), n (cmꢀ1): 735, 1059, 1215, 1270, 1348,
1
1537, 1572, 1690. H NMR (CDCl3, 300 MHz), d (ppm): 2.25 (6H, s), 2.46 (6H,
s), 7.49 (2H, s), 7.55 (4H, d, J ¼ 8.4 Hz), 8.24 (4H, d, J ¼ 8.4 Hz).
SUPPORTING INFORMATION
Screening of organic cosolvents, NMR spectral data of new compounds, and
mass spectra (HRMS) can be found via the Supplementary Content section of this
article’s Web page.
ACKNOWLEDGMENTS
The financial support by the Department of Science and Technology, govern-
ment of India (Grant No. SR=SO=BB-68=2010) is gratefully acknowledged. We
thank P. S. Pandey, Department of Chemistry, IIT Delhi, for his suggestions and
helpful discussions during this work.

826
A. B. MAJUMDER AND M. N. GUPTA
REFERENCES
1. Bommarius, A. R.; Riebel, B. R. (Eds.). Biocatalysis; Wiley-VCH: Weinheim, 2008.
2. (a) Carrea, G.; Riva, S. (Eds.). Organic Synthesis with Enzymes in Non-aqueous Media;
Wiley-VCH Verlag: Weinheim, 2008; (b) Adlercreutz, P. Immobilisation and application
of lipases in organic media. Chem. Soc. Rev. 2013, 42, 6406–6436.
3. Gotor, V.; Alfonso, I.; Uradiales, E. G. Asymmetric Organic Synthesis with Enzymes;
Wiley-VCH Verlag: Weinheim, 2008.
4. Majumder, A. B.; Ramesh, N. G.; Gupta, M. N. A lipase-catalyzed condensation reaction
with a tricyclic diketone: Yet another example of biocatalytic promiscuity. Tetrahedron
Lett. 2009, 50, 5190–5193.
5. Busto, E.; Gotor-Fernandez, V.; Gotor, V. Hydrolases: Catalytically promiscous enzymes
for non-conventional reactions in organic synthesis. Chem. Soc. Rev. 2012, 39, 4504–4523.
6. (a) Khersonsky, O.; Tawfik, D. S. Enzyme promiscuity: A mechanistic and evolutionary
perspective. Ann. Rev. Biochem. 2010, 79, 471–505; (b) Gupta, M. N.; Kapoor, M.;
Majumder, A. B.; Singh, V. Isozymes, moonlighting proteins, and promiscuous enzymes.
Curr. Sci. 2011, 100, 1152–1163.
7. Li, C.; Feng, X.-W.; Wang, N.; Zhou, Y.-J.; Yu, X.-Q. Biocatalytic promiscuity: The first
lipase-catalysed asymmetric aldol reaction. Green Chem. 2008, 10, 616–618.
8. Echavarren, A. M.; Stille, J. K. On the preparation of tris(tribenzylideneacetylacetone)-
tripalladium: A correction. J. Organomet. Chem. 1988, 356, C35–C37.
9. Shao, W.-Y.; Cao, Y.-N.; Yu, Z.-W.; Pan, W.-J.; Qiu, X.; Bu, X.-Z.; An, L.-K.; Huang,
Z.-S.; Gu, L.-Q.; Chan, A. S. C. Facile preparation of new unsymmetrical curcumin deri-
vatives by solid-phase synthetic strategy. Tetrahedron Lett. 2006, 47, 4085–4089.
10. Zhong, G.; Hoffmann, T.; Lerner, R. A.; Danishefsky, S.; Barbas, C. F. Antibody-
catalyzed enantioselective Robinson annulation. J. Am. Chem. Soc. 1997, 119, 8131–8132.
11. Tanaka, F.; Fuller, R.; Barbas III, C. F. Development of small designer aldolase enzymes:
Catalytic activity, folding, and substrate specificity. Biochemistry 2005, 44, 7583–7592.
12. (a) Rathman, T. L.; Greenwood, T. D.; Wolfe, J. F. Reactions of b-diketones with aromatic
aldehydes and ketones in the presence of potassium hydride. J. Org. Chem. 1980, 45, 1086–
1091; (b) Overman, L. E.; Blumenkopf, T. A.; Castaneda, A.; Thompson, A. S. Efficient
preparation of 8- and 9-membered cyclic ethers. J. Am. Chem. Soc. 1986, 108, 3516–3517;
(c) Ortega, N.; Martin, T.; Martin, V. S. Stereoselective synthesis of eight-membered cyclic
ethers by tandem Nicholas reaction = ring-closing metathesis: A short synthesis of (þ)-cis-
Lauthisan. Org. Lett. 2006, 8, 871–873; (d) Mack, D. J.; Guo, B.; Njardarson, J. T. Synthesis
of allylic and homoallylic alcohols from unsaturated cyclic ethers using mild and selective
C-O reduction approach. Chem. Commun. 2012, 48, 7844–7846.
13. Cao, L. Carrier-Bound Immobilized Enzymes; Wiley-VCH: Weinheim, 2005.
14. (a) Laane, C. Medium engineering for bio-organic synthesis. Biocatalysis 1987, 30, 17–22; (b)
Reslow, M.; Adlercreutz, P.; Mattiasson, B. Organic solvents for bioorganic synthesis: Optimi-
zation of parameters for a chymotrypsin–catalyzed process. Appl. Microbiol. Biotechnol. 1988,
26, 1–8; (c) Carrea, G.; Riva, S. Properties and synthetic applications of enzymes in organic sol-
vents. Angew. Chem. Int. Ed. 2000, 39, 2226–2254; (d) Roy, I.; Gupta, M. N. Enzymes in
organic media: Forms, function, and applications. Eur. J. Biochem. 2004, 271, 2575–2583.
15. Noguchi, T.; Fujii, Y.; Nakano, Y. An electrophilic reaction on tris(2,4-pentanedionato)-
cobalt in Lewis acidic conditions. Inorg. Chim. Acta 2006, 359, 2268–2270.
16. Majumder, A. B.; Gupta, M. N. Increasing catalytic efficiency of Candida rugosa lipase for
the synthesis of tert-alkyl butyrates in low-water media. Biocatal Biotransform. 2011, 29,
238–245.
17. Watanabe, K.; Yamagiwa, N.; Torisawa, Y. Cyclopentyl methyl ether as a new and alter-
native process solvent. Org. Process. Res. Dev. 2007, 11, 251–258.