Straightforward enantioselective access to γ-butyrolactones bearing an all-carbon β-quaternary stereocenter

Article abstract of DOI:10.1021/ol502148a

An enantioselective one-pot aldol/lactonization sequence has been developed to access highly challenging γ-butyrolactones bearing an all-carbon quaternary stereocenter at the β-position by reacting acylated succinic esters with aqueous formaldehyde in the presence of 3 mol % loading of a cinchona alkaloid-derived squaramide providing direct access to paraconic acid derivatives in high yield and fairly good level of enantioselectivity (up to 88% ee).

Full text of DOI:10.1021/ol502148a

Letter
pubs.acs.org/OrgLett
Straightforward Enantioselective Access to γ‑Butyrolactones Bearing
an All-Carbon β‑Quaternary Stereocenter
Sara Meninno, Tiziana Fuoco, Consiglia Tedesco, and Alessandra Lattanzi*
̀
Dipartimento di Chimica e Biologia, Universita di Salerno, Via Giovanni Paolo II, 84084 Fisciano, Italy
S
* Supporting Information
ABSTRACT: An enantioselective one-pot aldol/lactonization
sequence has been developed to access highly challenging γ-
butyrolactones bearing an all-carbon quaternary stereocenter at the
β-position by reacting acylated succinic esters with aqueous
formaldehyde in the presence of 3 mol % loading of a cinchona
alkaloid-derived squaramide providing direct access to paraconic
acid derivatives in high yield and fairly good level of
enantioselectivity (up to 88% ee).
γ-Butyrolactones bearing different stereocenters are extensively
found motives in natural products, displaying a wide range of
biological activities spanning from antifungal and antibiotic to
anticancer.1 Intensive efforts have been devoted to develop
stereoselective approaches to these targets.2 However, among
them, only a few routes to enantioenriched γ-butyrolactones,
bearing a one carbon quaternary stereocenter at the β-position,
were reported, mainly through multistep preparation.3 The
synthesis of these targets is particularly challenging due to (i)
the installation of an all-carbon quaternary stereocenter in an
enantioselective fashion4 and (ii) the remote position of the
stereocenter, far from more easily controllable α- and γ-
positions.
Recently, Cossy and Arseniyadis illustrated a palladium-
catalyzed allylic alkylation of dienol carbonates to enantioen-
riched butenolides, which were then converted, in a two-step
reduction/oxidation sequence, into β,β-disubstituted γ-butyr-
olactones (Figure 1, eq 1).5 At the same time, Sun and Chen
reported a chiral phosphoric acid catalyzed intramolecular
desymmetrization of 1,3-diol tethered to an acetal to give five-
membered acetals as the starting material useful to prepare
enantioenriched β,β-disubstituted γ-butyrolactones (Figure 1,
eq 2).6 In both these elegant approaches, γ-butyrolactones were
obtained from enantioenriched precursors after further
derivatization with fairly good level of enantioselectivity (80−
90% ee). It would be highly desirable to prepare these targets in
a direct manner. In this context, the asymmetric Baeyer−
Villiger reaction of 3-disubstituted cyclobutanones has been
exploited to obtain β,β-disubstituted γ-butyrolactones, although
with modest enantiocontrol (29−61% ee).7
Figure 1. Asymmetric approaches to β,β-disubstituted γ-butyrolac-
tones.
acid catalysis in a variety of mechanistically different trans-
formations.8a,9 We envisaged a simple aldol/lactonization
organocatalytic cascade sequence to access β,β-disubstituted
γ-butyrolactones starting from acylated succinic esters and
formaldehyde (Figure 1, eq 3). A prochiral enolate of acylated
succinic esters would react in a chiral environment with
formaldehyde to give an enantioenriched aldol product
followed by lactonization to the desired γ-butyrolactone.
Herein, we illustrate our success in developing a straightforward
enantioselective route to synthetically useful β,β-disubstituted
We have been interested in the development of asymmetric
methodologies for carbon-heteroatom and carbon−carbon
bond formation, using easily accessible bifunctional organo-
catalysts.8
It is generally accepted that typical promoters, such as chiral
amine−thioureas and amine−squaramides, activate pronucleo-
philes via general base catalysis and electrophiles via general
Received: July 23, 2014
Published: September 9, 2014
© 2014 American Chemical Society
4746
dx.doi.org/10.1021/ol502148a | Org. Lett. 2014, 16, 4746−4749

Organic Letters
Letter
γ-butyrolactones10 starting from easily available reagents
catalyzed by cinchona alkaloid-derived squaramides.
a
Table 1. Screening of Catalysts and Substrates
The asymmetric hydroxymethylation reaction of 2-substi-
tuted 1,3-dicarbonyl compounds has been scarcely investigated,
and limited success has been attained, likely to be due to the
highly reactive nature of formaldehyde as a C1 unit in the aldol
reaction. Surprisingly, only transition-metal-based systems,
based on Pd−BINAP11 and chiral Ni2−Schiff base12 complexes,
have been reported to catalyze this reaction achieving moderate
to high enantioselectivity (60−94% ee).13 We commenced our
study by investigating the reactivity of diethyl 2-benzoylsucci-
nate 1a and paraformaldehyde in toluene at room temperature
with 10 mol % of quinine 3 (Table 1). To our delight, γ-
butyrolactone 2a was formed, although in low yield and as a
racemate (entry 1). The activity markedly improved when
using Takemoto thiourea 4 (entry 2) and epi-quinine derived
thiourea 5 (entry 3), which is in agreement with a better H-
bonding donor ability of this class of organocatalysts. We
reasoned that differentiating the steric features of the two ester
groups in compound 1 would have positively affected the
enantiocontrol of the aldol reaction. Indeed, when reacting
compound 1b, with amine−thioureas 4 and 5 good conversion
to product 2b with significant improvement of the
enantioselectivity were observed (entries 4 and 5). epi-
Quinine-derived squaramide 6a afforded the product with
50% ee (entry 6), showing amine squaramides to be suitable
organocatalysts to check in the process. The reaction of
compounds 1c−f, bearing sterically demanding ester groups,
catalyzed by 6a (entries 7−10) enabled selection of reagent 1f
(entry 10) as the most promising for further catalyst screening.
Pleasingly, squaramide 6b, bearing a phenyl group at 2′-
position of the quinoline residue, afforded product 2f in good
yield and 65% ee (entry 11). A solvent screening using
organocatalyst 6b14 enabled us to improve the enantiomeric
excess of product 2f up to 74% when working in 1,2-
dichloroethane (entry 12). Differently 2′-substituted epi-
quinine- or epi-cinchonidine-derived squaramides catalyzed
the process in a slightly less efficient way (entries 13−16).
Catalyst 7, the pseudoenantiomer of catalyst 6b, gave the
opposite enantiomer of product 2f with comparable perform-
ance (entry 17). Finally, epi-hydroquinine-derived squaramide
6g performed at best as product 2f was isolated in 79% yield
and 78% ee (entry 18).
To improve the reaction outcome, additives, nature of
formaldehyde, and reaction temperature were investigated
(Table 2). Formalin gave a result (entry 1) similar to that
obtained when using paraformaldehyde (entry 18, Table 1). A
slight improvement of the enantioselectivity and activity was
observed when Na2SO4 was added as drying agent (entry 2).
Decreasing the reaction temperature and addition of molecular
sieves were beneficial in terms of conversion and enantiocon-
trol (entries 3−5). However, carrying out the reaction at −30
°C was detrimental in both respects (entry 6). The effect of
acidic additives was also investigated.15
The addition of benzoic acid or less acidic 4-nitrophenol
improved the conversion when working at 0 or −20 °C, and
the enantioselectivity of 2f increased up to 86% ee (entries 7−
9).16
Catalyst loading could be conveniently decreased (entries 10
and 11) to 3 mol %, leading to the product in 95% yield and
87% ee (entry11).
b
c
entry
R1
R3
cat. yield (%) ee (%)
2
1
Et
Et
3
28
80
88
68
64
31
83
74
95
47
74
72
65
75
71
48
64
79
2
−7
2a
2a
2a
2b
2b
2b
2c
2d
2e
2f
2
3
4
5
6
7
8
9
Et
Et
4
Et
Et
5
7
t-Bu
t-Bu
t-Bu
C6H11
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
4
47
5
−35
−50
−42
40
d
d
d
d
,
,
,
,
e
e
e
e
,
,
,
,
f
f
f
f
6a
6a
6a
6a
6a
6b
6b
6c
6d
6e
6f
7
2-adamanthyl
1-carbonaphthyl
cumyl
−36
−59
−65
−74
−69
−69
−68
−70
68
d
d
,
,
e
,
,
f
f
10
11
12
13
14
15
16
17
18
e
cumyl
2f
e
e
e
e
e
e
e
,
g
g
g
g
g
g
g
cumyl
2f
,
,
,
,
,
,
cumyl
2f
cumyl
2f
cumyl
2f
cumyl
2f
cumyl
2f
cumyl
6g
−78
2f
a
Reactions were carried out on a 0.1 mmol scale of 1 (C 0.05 M).
Isolated yield. Determined by chiral HPLC analysis. In CHCl3 as
solvent. 5 mol % of catalyst was used. Reaction carried out at C 0.1
b
c
d
e
f
g
M of 1. In Cl(CH2)2Cl as solvent.
Under optimized reaction conditions, a variety of acylated
succinic esters 1 were screened to study the scope of the one-
pot sequence to prepare γ-butyrolactones 2 (Figure 2).
Electron-donating and electron-withdrawing groups on the
phenyl ring of the aroyl residue or heteroaromatic moieties
were well-tolerated (2f−n), with the exception of the ortho-
substitution (2i), achieving the products with high yield and
fairly good ee values (up to 88%). Interestingly, the sequence
was applicable to compounds 1o−q, bearing either linear or
sterically demanding alkyl groups. The products (2o−q) were
recovered in good yield and only slightly decreased ee values.
Interestingly, 1-cumyl-5-methyl-2-benzoylpentanedioate 1r,
4747
dx.doi.org/10.1021/ol502148a | Org. Lett. 2014, 16, 4746−4749

Organic Letters
Letter
a
Table 2. Optimization of the Aldol/Lactonization Sequence
Scheme 1. Elaboration to β-(Hydroxyalkyl)-γ-butyrolactones
b
c
temp
(°C)
yield
ee
entry
6g (mol %)
additive
Na2SO4
(%)
(%)
1
5
5
5
5
5
5
5
5
5
2
3
rt
rt
77
83
94
70
80
58
52
98
90
81
95
73
75
80
84
83
79
83
84
86
84
87
2
3
4
5
6
7
8
9
d
d
d
d
d
d
d
0
−20
−20
−30
0
,
,
,
,
,
e
e
e
e
e
3 Å MS
3 Å MS
,
,
,
f
f
f
PhCO2H
4-NO2C6H4OH
4-NO2C6H4OH
4-NO2C6H4OH
4-NO2C6H4OH
0
−20
−20
−20
d
d
,
,
e
,
,
f
f
10
11
e
a
Reactions were carried out at 0.1 mmol scale of 1 (C 0.05 M) using
b
c
formalin (2 equiv). Isolated yield. Determined by chiral HPLC
analysis. Reaction carried out at C 0.1 M of 1. 3 Å molecular sieves
were added. 5 mol % of additive was used.
d
e
Derivatives 9 and 11 are a subset of more general class of
hydroxy-γ-butyrolactones, important motives in natural prod-
ucts and synthetically useful building blocks.17 Hydroxy-γ-
butyrolactones are difficult to access, and only a few
stereoselective methodologies, yielding β-(hydroxyalkyl)-γ-
butyrolactones bearing two contiguous tertiary stereocenters,
have been developed.18 Reduction of enantioenriched com-
pounds 2f,j afforded β-(hydroxyalkyl)-γ-butyrolactones 9f,j in
high yield and 90:10 dr. Benzoylation of alcohol 9j gave
product 10j in 75% yield. Single-crystal X-ray analysis on the
major diastereoisomer of compound 10j enabled us to assign
the relative and absolute configuration of the stereocenters as
(R,R).19 By analogy, γ-butyrolactones were assigned as (R)-2.
The diastereoisomeric mixture of enantioenriched compound
9f (90:10 dr) was rearranged to O-protected-β,γ-substituted γ-
butyrolactone 11f using TBDMSOTf and 2,6-lutidine.18b,20 No
erosion of the enantioselectivity was observed as confirmed by
chiral HPLC analysis on major diastereoisomer (R,R)-11f.
In conclusion, a one-pot aldol/lactonization process of
acylated succinic esters with formalin catalyzed by a cinchona
alkaloid-derived squaramide has been developed. Highly
challenging γ-butyrolactones, bearing an all-carbon quaternary
stereocenter at the remote β-position, have been isolated in
high yield and with an enantioselectivity level comparable to
indirect methods. The salient features of this methodology are
readily available reagents, with low catalyst loading and mild
reaction conditions. It has to be noted that this work illustrates
the first example of an enantioselective organocatalyzed
reaction of 2-substituted 1,3-dicarbonyl compounds with a
highly reactive and challenging formaldehyde unit. Finally, the
β,β-disubstituted γ-butyrolactones can be elaborated to stereo-
selectively prepare β-(hydroxyalkyl)-γ-butyrolactones bearing
contiguous tertiary and quaternary stereocenters hitherto not
accessible by alternative methods.
f
Figure 2. Substrate scope of the asymmetric aldol/lactonization
sequence. (a) Reactions were carried out on a 0.1 mmol scale of 1 (C
0.1 M) using formalin (2 equiv), 3 mol % of 6g, 5 mol % of 4-
NO2C6H4OH, and 3 Å MS in Cl(CH2)2Cl at −20 °C (R3 = cumyl).
(b) Isolated yield, in parentheses yield at 1 mmol scale. (c)
Determined by chiral HPLC analysis, in parentheses ee at 1 mmol
scale.
when reacted under usual conditions at 0 °C, afforded the
corresponding δ-valeroactone 8a with an encouraging 70% ee.
γ-Butyrolactones 2 can be stereoselectively transformed into
differently decorated β-(hydroxyalkyl)-γ-butyrolactones 9 and
11 bearing contiguous quaternary and tertiary stereocenters
(Scheme 1).
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details, analytical data, crystal data for 10j (CIF),
1H, 13C NMR spectra, and HPLC traces. This material is
available free of charge via the Internet at http://pubs.acs.org.
4748
dx.doi.org/10.1021/ol502148a | Org. Lett. 2014, 16, 4746−4749

Organic Letters
Letter
List, B., Maruoka, K., Eds.; Thieme: Stuttgart, 2012; Vol. 2, pp 119−
168. (c) Singh, R. P.; Deng, L. In Science of Synthesis, Asymmetric
Organocatalysis; List, B., Maruoka, K., Eds.; Thieme: Stuttgart, 2012;
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: lattanzi@unisa.it.
Notes
Vol. 2, pp 41−117. (d) Palomo, C.; Oiarbide, M.; Lop
́
ez, R. Chem. Soc.
Rev. 2009, 38, 632−653.
(10) For the asymmetric synthesis of paraconic acids bearing
quaternary and tertiary stereocenters, see: (a) Manoni, F.; Cornaggia,
C.; Murray, J.; Tallon, S.; Connon, S. J. Chem. Commun. 2012, 48,
6502−6504.
(11) Fukuchi, I.; Hamashima, Y.; Sodeoka, M. Adv. Synth. Catal.
2007, 349, 509−512.
(12) Mouri, S.; Chen, Z.; Matsunaga, S.; Shibasaki, M. Chem.
Commun. 2009, 5138−5140.
(13) For asymmetric hydroxymethylation of 2-cyanopropionates, see:
(a) Kuwano, R.; Miyazaki, H.; Ito, Y. Chem. Commun. 1998, 71−72.
(b) Shirakawa, S.; Ota, K.; Terao, S. J.; Maruoka, K. Org. Biomol. Chem.
2012, 10, 5753−5755. For an asymmetric aldol reaction of formalin
with aldehydes, see: Yasui, Y.; Benohoud, M.; Sato, I.; Hayashi, Y.
Chem. Lett. 2014, 43, 556−558.
(14) See Tables 1 and 2 in the Supporting Information.
(15) For examples on the use of acidic additives with cinchona-based
organocatalysts and thioureas, see: (a) Liu, L.; Zhang, S.; Xue, F.; Lou,
G.; Zhang, H.; Ma, S.; Duan, W.; Wang, W. Chem.Eur. J. 2011, 17,
7791−7795. (b) Chen, X.; Zhu, W.; Qian, W.; Feng, E.; Zhou, Y.;
Wang, J.; Jiang, H.; Yao, Z.-J.; Liu, H. Adv. Synth. Catal. 2012, 354,
2151−2156. (c) Silvi, M.; Renzi, P.; Rosato, D.; Margarita, C.;
Vecchioni, A.; Bordacchini, I.; Morra, D.; Nicolosi, A.; Cari, R.;
Sciubba, F.; Scarpino Schietroma, D. M.; Bella, M. Chem.Eur. J.
2013, 19, 9973−9978. (d) Zhang, Z.; Lippert, K. M.; Hausmann, H.;
Kotke, M.; Schreiner, P. R. J. Org. Chem. 2011, 76, 9764−9776.
(e) Lee, Y.; Klausen, R. S.; Jacobsen, E. N. Org. Lett. 2011, 13, 5564−
5567.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank MIUR for financial support and Centro di
Tecnologie Integrate per la Salute (Project PONa3_00138),
University of Salerno, for 600 MHz NMR instrumental time.
We thank Dr. P. Iannece (University of Salerno) for assistance
with MS spectra and elemental analyses and Dr. P. Oliva
(University of Salerno) with NMR spectroscopy. A.L. thanks
the European COST Action CM0905-Organocatalysis.
REFERENCES
■
(1) (a) Koch, S. S. C.; Chamberlain, A. R. In Enantiomerically Pure γ-
Butyrolactones in Natural Products Synthesis in Studies in Natural
Products Chemistry; Atta-ur-Rahman, Ed.; Elsevier Science: New York,
1995; Vol. 16, pp 687−725. (b) Seitz, M.; Reiser, O. Curr. Opin. Chem.
Biol. 2005, 9, 285−292. (c) Janecka, A.; Wyrebska, A.; Gach, K.;
Fichna, J.; Janecki, T. Drug Discovery Today 2012, 17, 561−572.
(2) (a) Mulzer, J. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Winterfeldt, E., Eds.; Pergamon: Oxford, 1991; Vol. 6, pp
323−380. For selected examples, see: (b) Fukuzawa, S.-i.; Seki, K.;
Tatsuzawa, M.; Mutoh, K. J. Am. Chem. Soc. 1997, 119, 1482−1483.
(c) Hoppe, D.; Hense, T. Angew. Chem., Int. Ed. 1997, 36, 2282−2316.
(d) Sohn, S. S.; Rosen, E. L.; Bode, J. W. J. Am. Chem. Soc. 2004, 126,
14370−14371. (e) Whitehead, D. C.; Yousefi, R.; Jaganathan, A.;
Borhan, B. J. Am. Chem. Soc. 2010, 132, 3298−3300. (f) Yanagisawa,
A.; Kushihara, N.; Yoshida, K. Org. Lett. 2011, 13, 1576−1578.
(g) Birrell, J. A.; Desrosiers, J.-N.; Jacobsen, E. N. J. Am. Chem. Soc.
2011, 133, 13872−13875. (h) Steward, K. M.; Gentry, E. C.; Johnson,
(16) For investigations on cinchona-based amine−thioureas and
́ ́
amine−squaramides self-aggregation phenomenon, see: (a) Tarkanyi,
G.; Kiral
́
y, P.; Soos
́
, T.; Varga, S. Chem.Eur. J. 2012, 18, 1918−1922.
(b) Jang, H. B.; Rho, H. S.; Oh, J. S.; Nam, E. H.; Park, S. E.; Bae, H.
Y.; Song, C. E. Org. Biomol. Chem. 2010, 8, 3918−3922.
́
J. S. J. Am. Chem. Soc. 2012, 134, 7329−7332. (i) Companyo, X.;
(17) For selected examples, see: (a) Salim, H.; Piva, O. J. Org. Chem.
2009, 74, 2257−2260. (b) Huo, X.; Ren, X.; Xu, Y.; Li, X.; She, X.;
Pan, X. Tetrahedron: Asymmetry 2008, 19, 343−347. (c) Cuzzupe, A.
N.; Di Florio, R.; White, J. M.; Rizzacasa, M. A. Org. Biomol. Chem.
2003, 1, 3572−3577. (d) Pena-Lopez, M.; Martinez, M. M.;
Sarandeses, L. A.; Sestelo, J. P. Chem.Eur. J. 2009, 15, 910−916.
(e) Nicolaou, K. C.; Harrison, S. T. J. Am. Chem. Soc. 2007, 129, 429−
440. (f) Tatsuta, K.; Suzuki, Y.; Furuyama, A.; Ikegami, H. Tetrahedron
Lett. 2006, 47, 3595−3598. (g) Chamberlin, A. R.; Dezube, M.; Reich,
S. H.; Sall, D. J. J. Am. Chem. Soc. 1989, 111, 6247−6256. (h) Xu, X.-
Y.; Tang, Z.; Wang, Y.-Z.; Luo, S.-W.; Cun, L.-F.; Gong, L.-Z. J. Org.
Chem. 2007, 72, 9905−9913.
(18) (a) Hajra, S.; Giri, A. K. J. Org. Chem. 2008, 73, 3935−3937.
(b) Hajra, S.; Giri, A. K.; Hazra, S. J. Org. Chem. 2009, 74, 7978−7981.
(c) Berti, F.; Forzato, C.; Nitti, P.; Pitacco, G.; Valentin, E.
Tetrahedron: Asymmetry 2005, 16, 1091−1102. (d) Yamauchi, S.;
Hayashi, Y.; Nakashima, Y.; Kirikihira, T.; Yamada, K.; Masuda, T. J.
Nat. Prod. 2005, 68, 1459−1470.
(19) Full crystallographic data (CIF) are available as Supporting
Information and have been deposited with the Cambridge Crystallo-
graphic Data Centre (reference code 1000859).
Mazzanti, A.; Moyano, A.; Janecka, A.; Rios, R. Chem. Commun. 2013,
49, 1184−1186. (j) Murahashi, S.; Ono, S.; Imada, Y. Angew. Chem.,
Int. Ed. 2002, 41, 2366−2368. (k) Samarat, A.; Amri, H.; Landais, Y.
Tetrahedron 2004, 60, 8949−8956.
(3) (a) Tomioka, K.; Cho, Y.-S.; Sato, F.; Koga, K. Chem. Lett. 1981,
1621−1624. (b) Tomioka, K.; Cho, Y.-S.; Sato, F.; Koga, K. J. Org.
Chem. 1988, 53, 4094−4098. (c) Canet, J.-L.; Fadel, A.; Salaun, J. J.
̈
Org. Chem. 1992, 57, 3463−3473. (d) Kennedy, J. W. J.; Hall, D. G. J.
Am. Chem. Soc. 2002, 124, 898−899. (e) Wilsily, A.; Fillion, E. Org.
Lett. 2008, 10, 2801−2804.
(4) For selected general reviews, see: (a) Corey, E. J.; Guzman-Perez,
A. Angew. Chem., Int. Ed. 1998, 37, 388−401. (b) Christoffers, J.; Baro,
A. Adv. Synth. Catal. 2005, 347, 1473−1482. (c) Cozzi, P. G.; Hilgraf,
R.; Zimmermann, N. Eur. J. Org. Chem. 2007, 5969−5994. (d) Das, J.
P.; Marek, I. Chem. Commun. 2011, 47, 4593−4623.
(5) Fournier, J.; Lozano, O.; Menozzi, C.; Arseniyadis, S.; Cossy, J.
Angew. Chem., Int. Ed. 2013, 52, 1257−1261.
(6) Chen, Z.; Sun, J. Angew. Chem., Int. Ed. 2013, 52, 13593−13596.
(7) (a) Xu, S.; Wang, Z.; Zhang, X.; Zhang, X.; Ding, K. Angew.
Chem., Int. Ed. 2008, 47, 2840−2843. (b) Petersen, K. S.; Stoltz, B. M.
Tetrahedron 2011, 67, 4352−4357. For alternative approaches, see:
(c) Honda, T.; Kimura, N.; Tsubuki, M. Tetrahedron: Asymmetry 1993,
4, 1475−1478. (d) Bika, K.; Gaertner, P. Eur. J. Org. Chem. 2008,
3453−3456.
(20) Yamauchi, S.; Kinoshita, Y. Biosci. Biotechnol. Biochem. 2001, 65,
1559−1567.
(8) (a) Lattanzi, A. Chem. Commun. 2009, 1452−1463. (b) Russo, A.;
Galdi, G.; Croce, G.; Lattanzi, A. Chem.Eur. J. 2012, 18, 6152−6157.
(c) Meninno, S.; Croce, G.; Lattanzi, A. Org. Lett. 2013, 15, 3436−
3439.
(9) For selected recent reviews, see: (a) Inokuma, T.; Takemoto, Y.
In Science of Synthesis, Asymmetric Organocatalysis; List, B., Maruoka,
K., Eds.; Thieme: Stuttgart, 2012; Vol. 2, pp 437−497. (b) Jang, H. B.;
Oh, J. S.; Song, C. E. In Science of Synthesis, Asymmetric Organocatalysis;
4749
dx.doi.org/10.1021/ol502148a | Org. Lett. 2014, 16, 4746−4749