Highly stereoselective chemoenzymatic synthesis of the 3 h -isobenzofuran skeleton. access to enantiopure 3-methylphthalides

Article abstract of DOI:10.1021/ol300191s

A straightforward synthesis of (S)-3-methylphthalides has been developed, with the key asymmetric step being the bioreduction of 2-acetylbenzonitriles. Enzymatic processes have been found to be highly dependent on the pH value, with acidic conditions being required to avoid undesired side reactions. Baker's yeast was found to be the best biocatalyst acting in a highly stereoselective fashion. The simple treatment of the reaction crudes with aqueous HCl has provided access to enantiopure (S)-3-methylphthalides in moderate to excellent yields.

Full text of DOI:10.1021/ol300191s

ORGANIC
LETTERS
2012
Vol. 14, No. 6
1444–1447
Highly Stereoselective Chemoenzymatic
Synthesis of the 3H-Isobenzofuran
Skeleton. Access to Enantiopure
3-Methylphthalides
ꢀ
ꢀ
Juan Mangas-Sanchez, Eduardo Busto, Vicente Gotor-Fernandez,* and
Vicente Gotor*
´
Organic and Inorganic Department, Instituto Universitario de Biotecnologıa de Asturias,
ꢀ
´
Universidad de Oviedo Calle Julian Claverıa s/n, Oviedo 33006, Spain
vicgotfer@uniovi.es; vgs@uniovi.es
Received January 24, 2012
ABSTRACT
A straightforward synthesis of (S)-3-methylphthalides has been developed, with the key asymmetric step being the bioreduction of
2-acetylbenzonitriles. Enzymatic processes have been found to be highly dependent on the pH value, with acidic conditions being required to
avoid undesired side reactions. Baker’s yeast was found to be the best biocatalyst acting in a highly stereoselective fashion. The simple treatment
of the reaction crudes with aqueous HCl has provided access to enantiopure (S)-3-methylphthalides in moderate to excellent yields.
In recent decades, the syntheses of heterocyclic com-
pounds have attracted the attention of a vast audience of
organic chemists,1 with oxygenated and nitrogenated
systems being the most common synthetic targets. In
particular, phthalides, also known as 1-(3H)-isobenzofur-
anones, are five-membered ring lactones frequently found
as scaffolds in basic bioactive molecules that have
traditionally been used as medicines or fragrances.2 Access
to racemic phthalides has been reported with high success
in recent years and applied to the synthesis of more
complex organic structures.3 In contrast, development of
asymmetric routes remains as a highly demanding goal.4
Examples of metal-catalyzed transformations,5 chemi-
cal enantioresolutions,6 or organocatalytic processes have
been extensively reported.7 Alternatively biocatalytic
methods have been scarcely studied for the production of
optically active 3-alkylphthalide derivatives8 through
the selective action of oxidoreductases toward adequate
intermediates such as 20-iodoacetophenone8a or methyl
2-acetylbenzoate,8b while lipases have been efficiently used
in the hydrolysis of esters8a or the acetylation of alcohols.8c
Unfortunately a lack of general methods is currently
(1) Targets in Heterocyclic Systems. Chemistry and Properties,
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Vol. 10; Attanasi, O. A., Spinelli, D., Eds.; Societa Chimica Italiana:
Rome, 2010.
(2) Beck, J. J.; Chou, S.-C. J. Nat. Prod. 2007, 70, 891.
(3) Xiong, M. J.; Li, Z. H. Curr. Org. Chem. 2007, 11, 833.
(4) See for instance: (a) Witulski, B.; Zimmermann, A. Synlett 2002,
1855. (b) Ye, Z.; Lv, G.; Wang, W.; Zhang, M.; Cheng, J. Angew. Chem.,
Int. Ed. 2010, 49, 3671. (c) Singh, M.; Argade, N. P. J. Org. Chem. 2010,
75, 3121. (d) Lv, G.; Huang, G.; Zhang, G.; Pan, C.; Chen, F.; Cheng, J.
Tetrahedron 2011, 67, 4879.
(5) See for instance: (a) Lei, J.-G.; Hong, R.; Yuan, S.-G.; Lin, G.-Q.
Synlett 2002, 927. (b) Tanaka, K.; Nishida, G.; Wada, A.; Noguchi, K.
Angew. Chem., Int. Ed. 2004, 43, 6510. (c) Chang, H.-T.; Jeganmohan,
M.; Cheng, C.-H. Chem.;Eur. J. 2007, 13, 4356. (d) Chen, W.-W.; Xu,
M.-H.; Lin, G.-Q. Tetrahedron Lett. 2007, 48, 7508. (e) Phan, D. H. T.;
Kim., B.; Dong, V. M. J. Am. Chem. Soc. 2009, 131, 15608. (f) Zhang, B.;
Xu, M.-H.; Lin, G.-Q. Org. Lett. 2009, 11, 4712. (g) Willis, M. C. Angew.
Chem., Int. Ed. 2010, 49, 6026. (h) Zhang, H.; Zhang, S.; Liu, L.; Luo, G.;
Duan, W.; Wang, W. J. Org. Chem. 2010, 75, 368. (i) Zhang, Z.-B.; Lu,
Y.-Q.; Duan, X.-F. Synthesis 2011, 3435.
(6) Kosaka, M.; Sekiguchi, S.; Naito, J.; Uemura, M.; Kuwahara, S.;
Watanabe, M.; Harada, N.; Hiroi, K. Chirality 2005, 17, 218.
(7) (a) Xing, C.-H.; Liao, Y.-X.; He, P.; Hu, Q.-S. Chem. Commun.
2010, 46, 3010. (b) Chang, C.-W.; Chein, R.-J. J. Org. Chem. 2011, 76,
4154.
(8) (a) Izumi, T.; Itou, O.; Kodera, K. J. Chem. Technol. Biotechnol.
1996, 67, 89. (b) Kitayama, T. Tetrahedron: Asymmetry 1997, 8, 3765.
(c) Oguro, D.; Watanabe, H. Tetrahedron 2011, 67, 777.
r
10.1021/ol300191s
Published on Web 03/06/2012
2012 American Chemical Society

Scheme 1. Preparation of 3-Methylphthalide Precursorsa
a (a) Synthesis and spontaneous intramolecular cyclization of 2-(1-hydroxyethyl)benzonitrile; (b) Chemical synthesis of 2-acetylbenzonitriles 8aꢀf
from the adequate precursor.
noticed. Herein we wish to report a general and straight-
forward chemoenzymatic asymmetric approach for the
synthesis of 3-methylphthalides in enantiopure form
based on the synthesis and bioreduction of substituted
2-acetylbenzonitriles.
aminoacetophenones 7bꢀf in moderate to good yields,11
being lower when electronegative atoms were bonded to
the C-4 position (OMe, F, and Cl, 43ꢀ53%).
Finally the amino ketones were converted into the
desired 2-acetylbenzonitriles by Sandmeyer’s reaction,12
obtaining 8aꢀf in moderate yields, attaining in 70% yield
the methoxy and the chlorinated derivatives 8d,f while the
others, 8a,b,c,e, were isolated in 45ꢀ50% yields.
On the basis of our previous experience in the chemo-
enzymatic synthesis of optically active 2,3-dihydro-
benzofuranes,9 we decided to react 2-bromobenzonitrile
(1) with acetaldehyde in order to develop a synthetic
pathway for the preparation of 2-(1-hydroxyethyl)-
benzonitrile (2a, Scheme 1a). As previously observed by
other research groups,10 the strongly basic reaction me-
dium promoted the spontaneous intramolecular cycliza-
tion of the alcohol. The corresponding imidate 3a was
isolated as the sole product, which slowly evolved to the
complete formation of the phthalide 4a under atmospheric
conditions impeding the use of enzymatic transformations.
This fact was clearly demonstrated by infrared data of the
With 2-acetylbenzonitrile (8a) selected as a model sub-
strate, the first focus was on the development of bioreduc-
tion experiments catalyzed by a panel of commercially
available alcoholdehydrogenasesinTris-HCl buffer work-
ing at their optimum pH (around 7.5), but in all cases
3-hydroxy-3-methyl-2-benzofuran-1(3H)-imine (9a) was
observed as the unique final product. Product formation
may be explained by the instability of the ketone at pH > 7
as outlined in Scheme 2a. First, the formation of the
hemiacetal is highly favored at high pH’s. Then, the
intramolecular cyclization followed by the protonation
of the imide anion led to the formation of the imidate 3a.
At the same time a sample of the racemic alcohol 2a was
searched for analytical purposes, so the chemical reduction
of ketone 8a was attempted with sodium borohydride
(Scheme 2b). Nevertheless, the imidate 3a was obtained
as a unique product instead of the alcohol 2a due to the
instability of the alcohol at basic pH, which lately evolved
to the racemic phthalide 4a under atmospheric conditions
or more rapidly with acidic catalysis.
nonsubstituted imidate CdNH band (around 1680 cmꢀ1
)
and the phthalide CdO band (around 1760 cmꢀ1). IR
spectra are shown in Figure 1a. As the racemic alcohol was
not accessible, an alternative synthetic approach based on
the chemical preparation of ketonitriles 8aꢀf and latter
stereoselective reduction of the carbonyl group with alco-
hol dehydrogenases was designed (Scheme 1b).
2-Acetylbenzonitriles 8aꢀf were prepared from com-
mercially available 20-amino-acetophenone (7a), amino
acids 6b,dꢀf, or 4-methyl-2-nitro-benzoic acid (5c) respec-
tively. Therefore, 4-methyl-2-nitro-benzoic acid was
hydrogenated in the presence of platinum(IV) oxide ob-
taining the amino acid 6c in quantitative yield. Treat-
ment of 6bꢀf with a methyl lithium solution afforded
At this point we moved forward to the use of other
oxidoreductases capable of reacting at neutral or acidic
pH. Among them Baker’s yeast (Saccharomyces cerevisiae)
is possibly one of the most frequently employed micro-
organisms in CdO or CdC bond reductions due to its easy
ꢀ
ꢀ
(9) Mangas-Sanchez, J.; Busto, E.; Gotor-Fernandez, V.; Gotor, V.
Org. Lett. 2010, 12, 3498.
(12) (a) Kushner, S.; Morton, J., II; Boothe, J. H.; Williams, J. H.
J. Am. Chem. Soc. 1953, 75, 1097. (b) Radziejewski, C.; Ghosh, S.;
Kaiser, E. T. Heterocycles 1987, 26, 1227. (c) Vogl, M.; Kratzer, R.;
Nidetzky, B.; Brecker, L. Org. Biomol. Chem. 2011, 9, 5863.
(13) (a) Servi, S. Synthesis 1990, 1. (b) Csuk, R.; Glaenzer, B. I. Chem.
Rev. 1991, 91, 49. (c) Komentani, T.; Yoshii, H.; Matsuno, R. J. Mol.
Catal. B: Enzym. 1996, 1, 45.
(10) (a) Parham, W. E.; Jones, L. D. J. Org. Chem. 1976, 41, 1187. (b)
Kobayashi, K.; Matsumoto, K.; Konishi, H. Heterocycles 2011, 83, 99.
(11) (a) Lee, J. I.; Youn, J. S. Bull. Korean Chem. Soc. 2008, 29, 1853.
(b) Kern, J. C.; Terefenko, E. A.; Fensome, A.; Unwalla, R.; Wrobel, J.;
Cohen, J.; Zhu, Y.; Berrodin, T. J.; Yudt, M. R.; Winneker, R. C.;
Zhang, Z.; Zhang, P. Bioorg. Med. Chem. Lett. 2008, 18, 5015.
Org. Lett., Vol. 14, No. 6, 2012
1445

Figure 1. (a) Infrared spectra of ketone 8a and alcohol (()-2a, imidate (()-3a, and phthalide (()-4a. (b) Chemoenzymatic route for the
preparation of (S)-4a.
Scheme 2. Approach for the Preparation of 3-Methyl Phthalide (4a)a
a (a) Instability of ketone 8a at pH above 7; (b) synthesis of racemic 4a.
handling and high availability.13 The bioreduction of 8a
was conducted in an aqueous medium, and the complete
disappearance of the starting material was observed after
16 h, obtaining the (S)-alcohol 2a as sole product and in
quantitative yield. Then Baker’s yeast was identified as
a suitable biocatalyst within this context. Notably, when
the reaction was carried out in the absence of glucose, the
alcohol was also obtained although in lower isolated yield
(83%). Undesired products were not detected in any case
as the bioreduction occurs at the pH range 3.9ꢀ4.4 due to
the Baker’s yeast metabolism. The treatment of (S)-2a with
NaBH4 afforded the enantiopure imidate (S)-3a, a very
interesting compound with potential applications in asym-
metric catalysis.14 Subsequently the imidate was trans-
formed into the phthalide in the presence of hydrochloric
acid during 48 h at room temperature. In this manner
the (S)-phthalide 4a was finally isolated in quantitative
yield and in enantiopure form. The complete reaction
pathway is represented in Figure 1b together with the IR
spectrum of each of the species involved in the sequential
strategy.
Once the best experimental conditions were found, we
decided to explore the bioreduction of other 2-acetyl-
benzonitriles 8bꢀf previously synthesized following the
strategy showed in Scheme 1. For this study 5- and
6-substituted derivatives were considered due to the
commercial availability of amino acids 6b,dꢀf and benzoic
acid 5c. Satisfyingly, a similar reactivity compared to that
with 8a (Table 1, entry 1) was observed for the 6-methyl
(14) N-Protected chiral imidates have been recently synthesized from
chiral amines and later applied as ligands in asymmetric aziridination
€
reactions or diethylzinc additions to benzaldehyde: Noel, T.; Vandyck,
K.; Robeyns, K.; Van Meervelt, L.; Van der Eycken, J. Tetrahedron
2009, 65, 8879.
(15) General procedure for the synthesis of phthalides (S)-4aꢀf.
Baker’s yeast (2.2 g) was added to a solution of glucose (282 mg) in
H2O (19 mL) with stirring of the resulting suspension for 15 min at 30 °C
and 250 rpm. After this time the corresponding ketone 8aꢀf (0.29 mmol)
was added (dissolved in 453 μL of IPA for 8d), and the suspension stirred
for the required time (see Table 1). Then the reaction was centrifuged,
and the supernatant was extracted with Et2O (3 ꢁ 20 mL). Organic
phases were combined, dried over Na2SO4, and filtered, and the solvent
was evaporated under reduced pressure affording a reaction crude
containing a mixture of alcohol (S)-2aꢀf and phthalide (S)-4aꢀf. The
reaction crude was dissolved in HCl 1 M (2.0 mL), and the solution was
stirred at room temperature for 48 h. After this time the solution was
extracted with CH2Cl2 (3 ꢁ 5 mL). Organic layers were combined, dried
over Na2SO4, and filtered, and the solvent was evaporated by distillation
under reduced pressure. Finally the reaction crude was purified by flash
chromatography (30% EtOAc/hexane) affording the corresponding
enantiomerically pure phthalides (S)-4aꢀf (42ꢀ98%).
1446
Org. Lett., Vol. 14, No. 6, 2012

tially to the formation of the alcohol (entry 3), while strong
electron-donating groups as the 5-methoxy group (entry 4)
favored the formation of the phthalide although with
moderate yields due to the low solubility of the ketone in
water. On the other hand weak electron-withdrawing
groups such as fluoride or chloride led to the isolation of
mixtures, where the formation of phthalides was slightly
favored (entries 5 and 6). The (S)-stereochemistry for the
phthalide chiral center was assigned in accordance with the
data already reported in the literature.16
Table 1. Enzymatic Reduction of Ketones 8aꢀf Using Baker’s
Yeast in H2O at 30 °C and 250 rpm for the Asymmetric
Synthesis of Enantiopure Phthalides (S)-4aꢀf15
Insummary a straightforwardsynthesis of (S)-3-methyl-
phthalides has been designed from commercially available
nitro acids, amino acids, or amino ketones where the key
step is the bioreduction of 2-acetylbenzonitriles in a highly
stereoselective fashion. Baker’s yeast was found to be
the optimal biocatalyst displaying excellent degrees of
activity and stereoselectivity, fullfilling the prerequisites
to act at acidic pH’s toward these panels of substrates.
Different treatments of the reaction crudes enable the
production of highly valuable compounds in enantiopure
form, such as the alcohol (S)-2a, the imidate (S)-3a, or the
phthalides (S)-4aꢀf, the last ones being isolated in mod-
erate to excellent yields depending on the ring pattern
substitution.
t
c
(S)-2
(S)-4
(S)-4
entry
ketone
8a (H)
(h)
(%)a
(%)b
(%)b
(% ee)c
1
16
72
48
67
54
48
>97
85
>97
85
88
ꢀ
<3 (99)
<3 (61)
12 (81)
50 (42)
50 (92)
71 (72)
>99
99
2
8b (6-Me)
8c (5-Me)
8d (5-OMe)
8e (5-F)
3
>97
50
>99
>99
>99
>99
4d
5
95
45
29
6
8f (5-Cl)
>97
a Conversion value of the reaction related to the dissapearance of
starting material and the formation of alcohol and phthalide (unique
reaction products). b Ratio of alcohol and phthalide calculated by 1H
NMR of the reaction crude. Isolated yields of phthalides in brackets
after acidic treatment. c Enantiomeric excess determined by HPLC. d 2-
Propanol (IPA, 453 μL) used to dissolve the starting ketone (0.29 mmol).
Acknowledgment. Financial support of this work by the
ꢀ
Spanish Ministerio de Ciencia e Innovacion (MICINN)
through CTQ 2007-61126 projects is gratefully acknowl-
edged. V.G.-F. thanks MICINN for a postdoctoral grant
substituted ketone 8b, yielding preferentially the alcohol
although with a lower reaction rate (entry 2).
Different trends were observed for the 5-substituted
derivatives in terms of alcohol/phthalide ratio although
in all cases phthalides (S)-4cꢀf were obtained in enantio-
pure form. Weak electron-donating groups led preferen-
ꢀ
(Ramon y Cajal Program). J.M.-S. thanks MICINN for a
predoctoral fellowship (FPU Program).
Supporting Information Available. General methods,
experimental procedures, characterization data for new
1
compounds, and copies of H, 13C, and DEPT NMR
(16) (a) Optical rotation power obtained for (S)-4a: [R]20D ꢀ39.5 (c 1,
CHCl3) (>99% ee). Described [R]20D ꢀ42.1 (c 0.95, CHCl3) in ref 5f. (b)
experiments. This material is available free of charge via
the Internet at http://pubs.acs.org.
Optical rotation power obtained for (S)-4c: [R]20 ꢀ36.0 (c 1, CHCl3)
D
(>99% ee). Described [R]20 ꢀ35.0 (c 1, CHCl3) in ref 5e. (c) Optical
D
rotation power obtained for (S)-4f: [R]20 ꢀ33.7 (c 1, CHCl3) (>99%
D
ee). Described [R]20D ꢀ36.1 (c 0.3, CHCl3) in ref 5e.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 6, 2012
1447