Enantioselective enzymatic desymmetrization of prochiral allenic diols

Article abstract of DOI:10.1002/anie.201103227

Crude lipase from porcine pancreas acts as a highly efficient biocatalyst in the enantioselective transesterification of prochiral allendiols. Following a simple synthetic protocol, highly functionalized axially chiral allenes are obtained in high yields and excellent enantiopurity (see scheme). Copyright

Full text of DOI:10.1002/anie.201103227

DOI: 10.1002/anie.201103227
Axial Chirality
Enantioselective Enzymatic Desymmetrization of Prochiral
Allenic Diols**
Chicco Manzuna Sapu, Jan-E. Bꢀckvall, and Jan Deska*
The employment of allenes as versatile synthetic building
blocks represents an emerging field in modern organic
chemistry.[1] Numerous novel methods, mainly based on
transition metal catalysis, are opening up a toolbox for
synthetic chemists to exploit the unprecedented reactivity of
this class of axially chiral compounds.[2] With the increasing
number of applications dealing with chiral allenes, both as
synthetic intermediates and in the form of allene-based
natural product target structures,[3] stereoselective methods
for their preparation are becoming more and more important.
Despite the fact that optically active allenes are often
synthesized from enantioenriched, mainly propargylic, non-
allene precursors conserving their stereochemical informa-
tion,[4] there is a growing interest in the kinetic resolution of
allenes.[5,6] In this context, we recently identified porcine
pancreatic lipase (PPL) as an outstanding biocatalyst for the
kinetic resolution of axially chiral primary allenic alcohols.[6]
The crude enzyme preparation catalyzed the transesterifica-
tion in organic solvents with good to excellent enantioselec-
tivities (Scheme 1, top). However, starting from a racemic
Scheme 1. Enantioselective biocatalytic transformations involving
mixture, kinetic resolution is always limited to a maximum
yield of 50% of the enantiopure material.[7] By means of
additional racemization catalysts, allowing for a constant
equilibration between both enantiomers of the starting
material, dynamic kinetic resolution is obtained.[8] Using
this approach, with the aid of a palladium complex we were
able to further increase the yield of the allene resolution in a
dynamic fashion while maintaining good optical purity of the
product (Scheme 1, top).[9]
An alternative strategy to overcome yield limitations in
enzyme-catalyzed transformations is to use prochiral sub-
strates. Thus, by selective transformation of one out of two
enantiotopic groups, elements of symmetry are eliminated
and optically active products can be obtained in high yield.[10]
Herein, we report on our investigations regarding a novel
protocol for the lipase-catalyzed desymmetrization of pro-
axially chiral allenes. IPr=1,3-bis-(2,6-diisopropylphenyl)imidazol-2-yli-
dene.
chiral allenic diols through selective transesterification,[11]
which yields highly enantioenriched axially chiral monoesters
(Scheme 1, bottom).
Initially, we screened a series of lipases and esterases to
identify potential catalysts for the title reaction. Since good
results were obtained in the kinetic resolution of allenols
employing porcine pancreatic lipase, we suspected this
biocatalyst to be suitable also for the stereoselective transes-
terification of related diol structures. And indeed, treatment
of prochiral diol 1a with vinyl butyrate in presence of PPL led
to the formation of monobutyrate (R)-2a with excellent
enantioselectivity, and only minor amounts of the corre-
sponding dibutyrate 3a from overacylation were detected
(Table 1, entry 1). 1,4-Dioxane was chosen as solvent since
most of the crystalline diols showed inadequate solubility in
other organic solvents.
Working with lipase from porcine pancreas, inevitably the
question arises whether PPL can be considered the actual
catalyst or if the reaction might be influenced by hydrolase
impurities present in commercial PPL preparations. As shown
by Hermetter, Faber, and co-workers, both crude and pure
PPL from different suppliers contain contaminations of a
series of lipolytically active enzymes, typically cholesterol
esterase (ChE), a-chymotrypsin (a-CT), and carboxypepti-
dase B.[12] Thus, we also conducted the desymmetrization
reaction of 1a in the presence of possible contaminant
[*] Dipl.-Chem. C. Manzuna Sapu, Dr. J. Deska
Department Chemie, Universitꢀt zu Kçln
Greinstrasse 4, 50939 Kçln (Germany)
E-mail: jan.deska@uni-koeln.de
Homepage: http://www.oc.uni-koeln.de/deska
Prof. J.-E. Bꢀckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University, 10691 Stockholm (Sweden)
[**] This work was supported by the Fonds der Chemischen Industrie
(Liebig Fellowship), the German Academic Exchange Service DAAD,
the Swedish Research Council, and the K&W Wallenberg Founda-
tion as well as by Carbolution Chemicals.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103227.
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9731

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Table 1: Lipases and esterases in the desymmetrization of 1a.[a]
Table 2: Desymmetrization of allendiols.[a]
Entry
Diol
R
t [h]
Yield [%]
ee [%]
Entry
Enzyme
Conv. [%]
ee (2a) [%]
2a/3a
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1 f
1g
1h
1i
phenyl
24
96
48
48
30
32
36
32
46
36
96
95 (2a)
69 (2b)
90 (2c)
78 (2d)
93 (2e)
96 (2 f)
92 (2g)
93 (2h)
89 (2i)
98
76
98
84
99
99
99
99
2-methylphenyl
3-methylphenyl
4-methylphenyl
4-chlorophenyl
4-methoxyphenyl
3,4-methylendioxyphenyl
2-naphthyl
2-trimethylsilylethynyl
n-heptyl
cyclohexyl
1
2
3
porcine pancreatic lipase
a-chymotrypsin
cholesterol esterase
C. antarctica lipase B
P. cepacia lipase
P. fluorescens lipase
A. niger lipase
C. rugosa lipase
96
<1
36
94
78
98
1
1
2
4
2
98
n.d.
71
91
63
68
n.d.
39
25
9
98:2
n.d.
92:8
84:16
97:3
71:29
4:96
40:60
64:36
87:13
46:54
4[b]
5
6[c]
7
9
10
11
99
8
9
10
11
1j
1k
77 (2j)
<5 (2k)
92[b]
n.d.
M. javanicus lipase
P. stutzeri lipase
R. oryzae lipase
11
[a] Reaction conditions: allendiol 1 (0.2 mmol), vinyl butyrate (114 mL,
1.0 mmol), and PPL (20 mg) in 1,4-dioxane (1 mL) were incubated at
408C for 24–96 h. ee values were determined by chiral HPLC. n.d.=not
determined. [b] ee value determined by chiral HPLC after derivatization
to the corresponding 3,5-dinitrobenzoate.
[a] Reaction conditions: 1a (8.8 mg, 50 mmol), vinyl butyrate (13 mL,
100 mmol), and enzyme (2.5 mg) in 1,4-dioxane (1 mL) were incubated
for 72 h at 408C. Conversion and ee were determined by chiral HPLC.
n.d.=not determined. [b] 1 h incubation time. [c] 16 h incubation time.
selectivity over reference diol 1a, and monobutyrates 2e and
2 f were obtained in nearly enantiopure form and very high
yields (Table 2, entries 5 and 6). In the same way, allenic diols
bearing bicyclic (1g and 1h) or acetylenic substituents (1i)
were desymmetrized with a high degree of enantioselectivity
(Table 2, entries 7–9). While alkyl-substituted allenols
showed rather low selectivity in the kinetic resolution using
PPL,[6] in the case of allene desymmetrization even n-heptyl
derivative 1j was isolated with an enantiomeric excess of 92%
(Table 2, entry 10). Only cyclohexyl-substituted diol 1k did
not react under these conditions.
As none of the tested proteins exhibited inverse selectivity
compared to PPL (see Table 1), we were also interested in the
lipase-catalyzed hydrolysis of the prochiral dibutyrates 3 as an
enantiocomplementary approach yielding (S)-configured
monobutyrates 2. To our surprise, PPL-catalyzed saponifica-
tion of 3a in aqueous buffer with acetone as cosolvent at 408C
led to fast formation of (S)-2a, albeit at a low level of
enantioselectivity (Scheme 2). This result was in sharp con-
trast to previous studies, where we used similar conditions for
the hydrolysis of racemic allenyl butyrates with excellent
stereocontrol.[9] However, treatment of 3a with lipase from
porcine pancreas under non-aqueous conditions (1-butanol in
heptane) efficiently solved this selectivity issue, and highly
enantioenriched (S)-2a was isolated in good yield. Thus, a
single enzyme can be used to produce either (S)- or (R)-
configured, axially chiral allenic monobutyrates in high
enantiomeric purity by biocatalytic desymmetrization by
simply choosing between synthetic or digestive reaction
conditions.[15]
proteins. While a-CT gave only marginal conversion (Table 1,
entry 2), ChE catalyzed the transesterification with consid-
erable effect, although with inferior selectivity (Table 1,
entry 3). These results indicate that PPL does represent the
catalytically active protein; however, traces of ChE might be
responsible for a certain erosion of enantioselectivity.[13]
Furthermore, we also tested other commercially available
lipases for their ability to catalyze the transesterification of
1a. Among the proteins tested, only lipases from Candida
antarctica (lipase B), Pseudomonas cepacia, and Pseudomo-
nas fluorescens showed synthetically useful conversions
(Table 1, entries 4–6). The outstanding activity of Candida
antarctica lipase B (94% conversion after 60 min), however,
was qualified by a slightly lower enantioselectivity than PPL,
accompanied by a significant degree of overacylation. Other
lipases, namely from Aspergillus niger, Candida rugosa,
Mucor javanicus, Pseudomonas stutzeri, or Rhizopus oryzae,
did not reveal substantial catalytic activity (Table 1, entries 7–
11). In all cases, the (R)-enantiomer was formed preferen-
tially.[14]
To study the scope of the reaction, we conducted
desymmetrizations of a variety of substituted allendiols
through enzymatic transesterification. As the reaction pro-
ceeded relatively slowly under the reaction conditions used in
the enzyme screening (Table 1), substrate concentration,
enzyme loading, and excess of the acylating agent were
further increased. Under these modified conditions, the
desymmetrization of phenyl-substituted diol 1a reached full
conversion after 24 h, and monoester 2a was isolated in 95%
yield and 98% ee (Table 2, entry 1). Tolyl derivatives 1b–1d
showed an interesting phenomenon, as substitution in ortho
or para position of the aryl moiety led to a considerable
decrease in enantioselectivity (Table 2, entries 2 and 4), while
meta derivative 2c was isolated with an excellent enantio-
meric excess of 98% (Table 2, entry 3). In contrast, hetero-
substituents in para position of the arene led to improved
The spectrum of interesting reactions employing axially
chiral allenols is broad; hence, these compounds are increas-
ingly found as intermediates in the synthesis of complex
natural products.[6,16] Exemplarily, the synthetic potential of
the optically active monoesters formed in the allene desym-
metrization is demonstrated in the silver-mediated cyclo-
isomerization of piperonyl derivative 2g, where dihydrofuran
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(w), 2933 (w), 2875 (w), 1952 (w), 1732 (s), 1589 (w), 1496 (w), 1460
(w), 1415 (w), 1381 (w), 1249 (w), 1168 (s), 1024 (m), 746 (m),
692 cmÀ1 (s). Elemental analysis (%) calcd for C15H18O3: C 73.15,
H 7.37; found: C 73.02, H 7.51. HPLC (Chiralpak AD-H, hexane/2-
propanol 95:5, 1.0 mLminÀ1, 250 nm): tR ((R)-2a) = 12.9 min, tR ((S)-
2a) = 14.2 min.
Representative procedure for the solvolytic desymmetrization:
Dibutyrat 3a (63.2 mg, 200 mmol) was dissolved in n-heptane
(3.6 mL) and n-butanol (0.4 mL), porcine pancreatic lipase (100 mg)
was added, and the reaction mixture was incubated for 48 h at 408C.
Monoester (S)-2a (39.0 mg, 158 mmol, 79%, 97% ee) was obtained
after column chromatography (SiO2, cyclohexane/ethyl acetate 8:2–
6:4) as colorless oil. [a]20D: + 34.98 (c = 0.5, CHCl3).
Scheme 2. Enantioselective solvolysis of dibutyrate 3a.
4 is obtained in 90% yield and with complete transfer of
chirality (Scheme 3). Thus, the combination of desymmetri-
zation and 5-endo-trig-cyclization opens up a fast asymmetric
synthetic access towards the core structure of the hyperiones
(5 and 6), two norlignanes which have recently been isolated
from the roots of Hypericum chinense.[17]
Received: May 11, 2011
Revised: June 16, 2011
Keywords: allenes · axial chirality · desymmetrization ·
.
enzyme catalysis · lipases
[1] a) N. Krause, A. S. K. Hashmi, Modern Allene Chemistry, 1st ed.,
Wiley-VCH, Weinheim, 2004; b) A. S. K. Hashmi in Organic
Synthesis Highlights V, 1st ed. (Eds.: H.-G. Schmalz, T. Wirth),
Wiley-VCH, Weinheim, 2003, pp. 56 – 67.
[2] Selected recent examples: Pd catalysis: a) R. Zimmer, C. U.
Dinesh, E. Nandanan, F. A. Khan, Chem. Rev. 2000, 100, 3067;
b) S. Ma, Z. Yu, Angew. Chem. 2002, 114, 1853; Angew. Chem.
Int. Ed. 2002, 41, 1775; c) J. Franzꢀn, J.-E. Bꢁckvall, J. Am.
Chem. Soc. 2003, 125, 6056; d) A. K. ꢂ. Persson, J.-E. Bꢁckvall,
Angew. Chem. 2010, 122, 4728; Angew. Chem. Int. Ed. 2010, 49,
4624; Ru catalysis: e) B. M. Trost, A. B. Pinkerton, J. Am. Chem.
Soc. 1999, 121, 10842; f) B. M. Trost, D. R. Fandrick, D. C. Dinh,
J. Am. Chem. Soc. 2005, 127, 14186; Rh catalysis: g) K. M.
Brummond, H. Chen, B. Mitasev, A. D. Casarez, Org. Lett. 2004,
6, 2161; h) T. Nishimura, S. Hirabayashi, Y. Yasuhara, T.
Hayashi, J. Am. Chem. Soc. 2006, 128, 2556; i) T. Seiser, N.
Cramer, Angew. Chem. 2008, 120, 9435; Angew. Chem. Int. Ed.
2008, 47, 9294; j) A. H. Stoll, S. B. Blakey, J. Am. Chem. Soc.
2010, 132, 2108; Ag catalysis: k) J. A. Marshall, M. A. Wolf,
E. M. Wallace, J. Org. Chem. 1997, 62, 367; l) M. P. VanBrunt,
R. F. Standaert, Org. Lett. 2000, 2, 705; Au catalysis: m) N.
Nishina, Y. Yamamoto, Angew. Chem. 2006, 118, 3392; Angew.
Chem. Int. Ed. 2006, 45, 3314; n) N. Bongers, N. Krause, Angew.
Chem. 2008, 120, 2208; Angew. Chem. Int. Ed. 2008, 47, 2178;
o) C. Winter, N. Krause, Angew. Chem. 2009, 121, 6457; Angew.
Chem. Int. Ed. 2009, 48, 6339.
Scheme 3. Allene cycloisomerization as access towards the core struc-
ture of the hyperiones.
In summary, a powerful chemoenzymatic protocol for the
synthesis of optically active, highly functionalized allenes has
been developed, combining excellent enantiopurity and high
yields. Depending on the reaction conditions and prochiral
allene source chosen, either enantiomer of the axially chiral
allenyl monoesters is easily accessible in highly enantioen-
riched form. Currently, we are investigating the extended
scope of this reaction as well as its implementation in natural
product synthesis.
[3] A. Hoffmann-Rçder, N. Krause, Angew. Chem. 2004, 116, 1216;
Angew. Chem. Int. Ed. 2004, 43, 1196.
[4] Selected examples: a) N. Krause, A. Hoffmann-Rçder, Tetrahe-
dron 2004, 60, 11671; b) B. D. Sherry, F. D. Toste, J. Am. Chem.
Soc. 2004, 126, 15978; c) C. Deutsch, B. H. Lipshutz, N. Krause,
Org. Lett. 2009, 11, 5010.
Experimental Section
Representative procedure for the desymmetrization by esterification:
Allendiol 1a (35.2 mg, 200 mmol) and vinyl butyrate (114 mg,
1.0 mmol) were dissolved in 1,4-dioxane (1.0 mL), porcine pancreatic
lipase (20 mg) was added, and the reaction mixture was incubated for
24 h at 408C. After filtration and concentration of the filtrate in
vacuo, purification by column chromatography (SiO2, cyclohexane/
ethyl acetate 8:2–6:4) delivered monoester (R)-2a (46.8 mg,
190 mmol, 95%, 98% ee) as colorless oil. [a]20D: À35.68 (c = 0.5,
CHCl3). Rf (cyclohexane/ethyl acetate 7:3): 0.24. 1H NMR (300 MHz,
CDCl3): d = 7.22–7.33 (m 5H), 6.40 (m, 1H), 4.81 (d, J = 2.1 Hz, 2H),
4.26 (d, J = 2.2 Hz, 2H), 2.34 (t, J = 7.4 Hz, 2H), 1.66 (tq, J = 7.4 Hz,
2H), 1.28 (br s, 1H), 0.95 ppm (t, J = 7.4 Hz, 3H). 13C NMR
(100 MHz, CDCl3): d = 202.8, 173.8, 133.3, 128.7, 127.5, 127.1, 105.1,
98.0, 62.0, 61.3, 36.1, 18.4, 13.7 ppm. FTIR (ATR): n = 3402 (br), 2964
[5] Review on chemical and enzymatic resolution of allenes: a) M.
Ogasawara, Tetrahedron: Asymmetry 2009, 20, 259; see also:
b) G. Kresze, W. Runge, E. Ruch, Liebigs Ann. Chem. 1972, 756,
112; c) J. Nyhlꢀn, L. Eriksson, J.-E. Bꢁckvall, Chirality 2007, 20,
47.
[6] J. Deska, J.-E. Bꢁckvall, Org. Biomol. Chem. 2009, 7, 3379.
[7] a) U. T. Bornscheuer, R. J. Kazlauskas, Hydrolases in Organic
Synthesis: Regio- and Stereoselective Biotransformations, 2nd
ed., Wiley-VCH, Weinheim, 2005; b) K. Faber, Biotransforma-
tions in Organic Chemistry, 5th ed., Springer, Berlin, 2008; c) V.
Gotor, I. Alfonso, E. Garcꢃa-Urdiales, Asymmetric Organic
Synthesis with Enzymes, 1st ed., Wiley-VCH, Weinheim, 2008.
[8] Reviews: a) A. Stecher, K. Faber, Synthesis 1997, 1; b) O.
Pꢄmies, J.-E. Bꢁckvall, Chem. Rev. 2003, 103, 3247; c) N. J.
Angew. Chem. Int. Ed. 2011, 50, 9731 –9734
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Communications
Turner, Curr. Opin. Chem. Biol. 2004, 8, 114; d) B. Martꢃn-
contain a substantial number of contaminant proteins,[12] signifi-
cantly lower enantioselectivities were obtained.
Matute, J.-E. Bꢁckvall, Curr. Opin. Chem. Biol. 2007, 11, 226;
e) J. H. Lee, K. Han, M.-J. Kim, J. Park, Eur. J. Org. Chem. 2010,
999; f) H. Pellissier, Tetrahedron 2011, 67, 3769.
[14] The absolute configuration of the resulting monoester was
confirmed by degradation to 4-methyl-2-phenyl-2,5-dihydro-
furan and comparison of its optical rotation with a stereodefined
sample (see the Supporting Information). All monoesters were
configurationally stable, and even after storage for three month
at 48C only marginal loss in optical purity was detected.
[15] While diester 3a showed acceptable conversion rates in the
transesterification with butanol, other allenyl dibutyrates suf-
fered from significantly extended reaction times. Owing to the
reversibility of the transesterification, full conversion was not
achieved; further studies, especially with other nucleophiles, will
be necessary. Nevertheless, the excellent enantioselectivity was
obtained in all of the solvolytic desymmetrizations.
[16] Selected recent examples: a) M. K. Gurjar, S. Nayak, C. V.
Ramana, Tetrahedron Lett. 2005, 46, 1881; b) S. Inuki, S. Oishi,
N. Fujii, H. Ohno, Org. Lett. 2008, 10, 5239; c) Y. Sawama, Y.
Sawama, N. Krause, Org. Biomol. Chem. 2008, 6, 3573; d) Z.
Gao, Y. Li, J. P. Cooksey, T. N. Snaddon, S. Schunk, E. M. E.
Viseux, S. M. McAteer, P. J. Kocienski, Angew. Chem. 2009, 121,
5122; Angew. Chem. Int. Ed. 2009, 48, 5022; e) F. Volz, S. H.
Wadman, A. Hoffmann-Rçder, N. Krause, Tetrahedron 2009, 65,
1902.
[9] J. Deska, C. del Pozo Ochoa, J.-E. Bꢁckvall, Chem. Eur. J. 2010,
16, 4447.
[10] Reviews: a) E. Schoffers, A. Golebiowski, C. R. Johnson,
Tetrahedron 1996, 52, 3769; b) E. Garcꢃa-Urdiales, A. Ignacio,
V. Gotor, Chem. Rev. 2005, 105, 313.
[11] Guanti et al. reported preliminary results of an allene desym-
metrization; however, they did not pursue the trail they blazed:
a) G. Guanti, L. Banfi, E. Narisano, J. Org. Chem. 1992, 57, 1540;
other examples of enzymatic desymmetrizations yielding axially
chiral compounds (biaryls und spiro[3.3]heptanes): b) K. Nae-
mura, A. Furutani, J. Chem. Soc. Perkin Trans. 1 1990, 3215; c) T.
Matsumoto, T. Konegawa, T. Nakamura, K. Suzuki, Synlett 2002,
122; d) K. Okuyama, K. Shingubara, S.-I. Tsujiyama, K. Suzuki,
T. Matsumoto, Synlett 2009, 941; e) B. Yuan, A. Page, C. P.
Worrall, F. Escalettes, S. C. Willies, J. J. W. McDouall, N. J.
Turner, J. Clayden, Angew. Chem. 2010, 122, 7164; Angew.
Chem. Int. Ed. 2010, 49, 7010.
[12] R. Birner-Grꢅnberger, H. Scholze, K. Faber, A. Hermetter,
Biotechnol. Bioeng. 2004, 85, 147.
[13] ”Lipase from porcine pancreas” from MP Biomedicals was used
in our studies, and the results gave no indication for this effect.
However, using “PPL type II” from Sigma, which was shown to
[17] W. Wang, Y. H. Zeng, K. Osman, K. Shinde, M. Rahman, S.
Gibbons, Q. Mu, J. Nat. Prod. 2010, 73, 1815.
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