Site-Selective and Enantioselective α,β,γ-Functionalization of 5-Alkylidenefuran-2(5 H)-ones: A Route to Polycyclic γ-Lactones

Article abstract of DOI:10.1021/acs.orglett.8b03796

A new strategy for a direct α,β,γ-functionalization of the γ-lactone framework in the corresponding 5-alkylidenefuran-2(5H)-ones is reported. The developed approach is based on a stereocontrolled cascade reaction with 2-mercaptocarbonyl compounds proceedi

Full text of DOI:10.1021/acs.orglett.8b03796

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Site-Selective and Enantioselective α,β,γ-Functionalization of
5‑Alkylidenefuran-2(5H)‑ones: A Route to Polycyclic γ‑Lactones
†
†
́
Anna Skrzynska, Sebastian Frankowski, Marek Moczulski, Piotr Drelich, and Łukasz Albrecht*
̇
́
́
Institute of Organic Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Łodz, Poland
S
* Supporting Information
ABSTRACT: A new strategy for a direct α,β,γ-functionalization of the γ-
lactone framework in the corresponding 5-alkylidenefuran-2(5H)-ones is
reported. The developed approach is based on a stereocontrolled cascade
reaction with 2-mercaptocarbonyl compounds proceeding in a sequence of
thia-Michael/aldol/oxa-Michael reactions. Such a synthetic strategy allows
for a construction of a unique polycyclic architecture containing γ-lactone,
tetrahydrofuran, and tetrahydrothiophene ring systems. Excellent enantioselectivities and diastereoselectivities have been
obtained in the presence of bifunctional catalyst derived from cinchona alkaloids.
he design of new functionalization strategies constitutes
one of the most important goals in the modern organic
Scheme 1. Strategies for the Site-Selective Functionalization
of γ-Lactone Framework
T
synthesis, thereby providing access to novel reaction profiles
and new types of products. The realization of this task has
recently been dominated by the asymmetric catalysis.1 In such
a setup, the chiral catalyst employed serves a dual purpose.
First, it activates the substrate, making the devised strategy
feasible. Second, it provides a well-defined chiral environment,
making it possible for the transformation to proceed in an
enantioselective fashion. Among various asymmetric catalytic
strategies for the stereoselective functionalization of organic
molecules, in the past 20 years, organocatalysis has emerged as
a method of choice.2,3 Notably, organocatalytic reactions can
be realized according to various activation strategies, with
bifunctional catalysis being a highly relevant approach.3
γ-Lactones and related γ-butenolides constitute privileged
structural motifs widely distributed in nature and exhibiting a
wide range of biological activities (Scheme 1, top).4 They are
also employed as key synthetic intermediates in natural
product synthesis. As a consequence, much effort has been
devoted toward the development of methods for their
preparation, in particular in a stereoselective fashion (Scheme
1).5−8 One strategy involves the use of furan-2(5H)-ones as a
template for further structural modifications. These highly
useful building blocks undergo Michael-type addition with
selected nucleophiles, thus introducing new substituents in the
β-position.6 In contrast, site-selective α- or γ-functionalization
of the γ-butenolide architecture is possible via the correspond-
ing dienolate formation or related systems.7 Recently, we have
developed an alternative approach for the synthesis of γ,γ-
disubstituted-γ-butenolides.8 It involves the usage of 5-
alkylidenefuran-2(5H)-ones as a new type of building block.
We have demonstrated that their site-selective α′,γ-function-
alization can be realized under trienamine activation
conditions, providing an excellent route to spirocyclic
derivatives. Interestingly, a literature survey revealed that a
strategy allowing for the direct introduction of substituents at
the α-, β-, and γ-positions of the γ-lactone moiety is missing.
Therefore, we became interested in the development of
organocatalytic strategy for α,β,γ-functionalization of 5-
alkylidenefuran-2(5H)-ones (Scheme 1, bottom).
Organocatalytic cascade reactions where more than one
chemical bond is formed in a single manual operation
Received: November 27, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03796
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
constitute a powerful strategy in modern asymmetric syn-
thesis.9 Enantioselective synthesis of tetrahydrothiophenes is
an interesting example of such an approach to organic
synthesis (Scheme 2, top).10 In such a cascade, 2-
Table 1. Organocatalytic, Stereocontrolled α,β,γ-
Functionalization of 5-Alkylidenefuran-2(5H)-ones 1:
a
Optimization Studies
Scheme 2. Cascade Synthesis of Tetrahydrothiopenes
enantiomeric ratio,
b
c
mercaptocarbonyl compounds are reacted with electron-
deficient double bonds. The reaction is initiated by the thia-
Michael addition, followed by the intramolecular aldol
reaction. Protonation of the originally formed alkoxide results
in the formation of tetrahydrothiophene bearing a hydroxyl
group in the 3-position.
Taking advantage of the formation of the nucleophilic
alkoxide in such a cascade, we decided to use it as a template
for the α,β,γ-functionalization of 5-alkylidenefuran-2(5H)-ones
(Scheme 2, bottom). It was anticipated that the initial thia-
Michael addition should occur at the least sterically demanding
endocyclic double bond in 1. Subsequent intramolecular aldol
reaction introduces oxygen-centered nucleophile into the
molecule capable of undergoing intramolecular oxa-Michael
reaction, thus accomplishing the devised α,β,γ-functionaliza-
tion of 5-alkylidenefuran-2(5H)-ones. Herein, we report our
studies on the development of organocatalytic synthesis of
polycyclic γ-lactones bearing a tetrahydrothiophene and
tetrahydrofuran moieties via stereocontrolled thia-Michael/
aldol/oxa-Michael reaction cascade.
Optimization studies were performed using (E)-ethyl 2-(5-
oxofuran-2(5H)-ylidene)acetate 1a and 2-mercaptoacetophe-
none 2a as model reactants (Table 1). Initially catalyst
screening was performed (Table 1, entries 1−4). To our
delight, it was found that the devised cascade consisting of thia-
Michael/aldol/oxa-Michael reactions proved possible to realize
employing bifunctional organocatalysts 5a−5d derived from
cinchona alkaloids. Notably, the diastereoselectivity of the
transformation was excellent, because γ-lactone 4a was
obtained as a single diastereoisomer in all of the cases, despite
the presence of four stereogenic centers. Interestingly, already
simple quinine 5a promoted the reaction, affording 4a in
unsatisfactory yield and enantioselectivity (Table 1, entry 1).
The use of β-isocupreidine 5b did not enhance the result
(Table 1, entry 2). However, the introduction of a stronger
hydrogen-bonding unit into the structure of the catalyst (Table
1, entries 3 and 4) allowed for significant improvement of the
results with catalyst 5d bearing a squaramide moiety being the
best (Table 1, entry 4). Subsequently, the solvent screening
was initiated (Table 1, entries 4−9) leading to the
identification of chloroform (Table 1, entry 4) as the best-
suited solvent, in terms of enantioselectivity for the developed
α,β,γ-functionalization of furan-2(5H)-one 1a. Further opti-
mization studies were focused on the evaluation of
solvent
cat. (mol %) NMR yield [%]
er
1
2
3
4
5
6
7
8
CHCl3
CHCl3
CHCl3
CHCl3
CH2Cl2
CCl4
toluene
CH3CN
THF
CHCl3
CHCl3
CHCl3
CHCl3
5a (20)
5b (20)
5c (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (20)
5d (5)
31
36
63
56:44
54:46
90:10
97.5:2.5
95:5
96:4
95:5
93:7
94:6
97:3
99:1
99:1
>99:1
75 (74)
83 (75)
61
63
47
54
9
d
e
f
10
11
12
13
73 (69)
73 (71)
48
83 (78)
a
All reactions were performed in a 0.1 mmol scale using 1a (1.0
equiv) and 2a (1.3 equiv) in 0.4 mL of the corresponding solvent (for
b
details, see the Supporting Information). As determined by 1H NMR
of a crude reaction mixture, with respect to the signal of 1,3,5-
tris(trifluoromethyl)benzene as internal standard. Isolated yield is
c
given in parentheses. As determined by the chiral stationary phase
d
e
UPC2. Reaction performed in 0.2 mL of the solvent. Reaction
f
performed in 0.8 mL of the solvent. Reaction performed using 1a
(1.3 equiv) and 2a (1.0 equiv).
concentration effect (Table 1, entries 10 and 11) and relative
ratio of reactants (Table 1, entry 12). Neither of these changes
led to improvement of the results. Finally, the attempt to
reduce the catalyst loading to 5 mol % was made (Table 1,
entry 13). Delightfully, the transformation proceeded with high
chemical and stereochemical efficiency, thus identifying the
final reaction parameters.
With the optimization studies accomplished, studies on the
scope of the methodology were initiated (Table 2). Initially,
various 2-mercaptocarbonyl compounds 2 were employed in
the reaction (Table 2, entries 1−11). To our delight, the
developed, organocatalytic α,β,γ-functionalization of 5-alkyli-
denefuran-2(5H)-ones 1 proved unbiased toward the
electronic character of substituents on the aromatic ring in 2
(compare entries 2, 3, and 4−7 in Table 2). Furthermore, the
position of the substituents on the aromatic ring in 2 had no
significant influence on either the yield of the cascade or its
diastereoselectivity (compare entries 2, 3, and 5−7 in Table 2)
with very good results obtained for ortho-substituted
derivatives (Table 2, entries 3 and 7) being particularly
B
DOI: 10.1021/acs.orglett.8b03796
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
temperature. It was terminated within 3 h providing 6a in
60% yield. To our delight, the transformation proceeded with
preservation of stereochemical information introduced at the
organocatalytic cascade as 6a was obtained as a single
stereoisomer.
Table 2. Organocatalytic, Stereocontrolled α,β,γ-
Functionalization of 5-Alkylidenefuran-2(5H)-ones 1:
Scope Studies
a
In order to assign the absolute configuration of the
polycyclic products obtained, the single-crystal X-ray analysis
was performed. To our delight, 4a provided crystals suitable
for such an analysis (Scheme 4, top). Given the result
Scheme 4. Organocatalytic, Stereocontrolled α,β,γ-
Functionalization of 5-Alkylidenefuran-2(5H)-ones 1:
Mechanistic Considerations
b
electron-withdrawing
group, EWG
yield
[%]
enantiomeric
c
entry
R
ratio, er
1
2
3
4
5
6
7
8
9
Ph
4-FC6H4
2-FC6H4
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Me
CN
4a: 78
4b: 69
4c: 70
4d: 70
4e: 76
4f: 76
4g: 73
4h: 45
4i: 68
4j: 75
4k: 75
4l: 70
4m: 63
4a: 70
>99:1
98:2
98:2
98.5:1.5
99:1
99:1
98.5:1.5
99:1
98:2
99:1
97.5:2.5
98:2
4-CH3C6H4
4-CH3OC6H4
3-CH3OC6H4
2-CH3OC6H4
benzofuran-2-yl
thiophen-3-yl
2-naphthyl
3,4-Cl2C6H3
Ph
10
11
12
13
Ph
Ph
89:11
>99:1
d
14
CO2Et
a
All reactions were performed in a 0.1 mmol scale using 1a (1.0
equiv) and 2a (1.3 equiv) in 0.4 mL of the corresponding solvent (for
b
details, see the Supporting Information). Isolated yields are given.
c
d
As determined by the chiral stationary phase UPC2. Reaction
performed in a gram scale.
noticeable. Interestingly, heteroaromatic substituents were also
well-tolerated with products 4h and 4i efficiently synthesized.
Moreover, disubstituted aromatic rings could also be present in
the structure of 2, as demonstrated in the synthesis of 4j and
4k (Table 2, entries 10 and 11). In the course of further scope
studies, the possibility to modify the structure of olefinic
counterpart was attempted. Delightfully, selected electron-
withdrawing groups proved possible to be present at the
exocyclic double bond in 1 with good results being observed
(Table 2, entries 12 and 13). Finally, a 1-g scale experiment
was performed, leading to the formation of 4a with comparable
results (Table 2, entry 14).
obtained, it was established as 3S,3aR,6S,6aR with the absolute
configuration of remaining products being assigned by analogy.
Based on the stereochemical reaction outcome, a plausible
mechanism of a cascade was proposed (Scheme 4, bottom).
The developed transformation is initiated by the sulfa-Michael
addition of 2-mercaptocarbonyl compound 2 to the endocyclic
double bond of the corresponding 5-alkylidenefuran-2(5H)-
one 1. The reaction is promoted by the bifunctional catalyst 5d
that is responsible for the activation of electrophile 1 through
the hydrogen-bonding interactions. At the same time, 2 is
activated by deprotonation. Such a recognition profile
determines both site selectivity and enantioselectivity of the
cascade. The fact that the Michael reaction occurs at the less-
hindered electrophilic position of the molecule is also of
importance with the observed site selectivity being in contrast
to our previous results concerning the reactivity of 1, where the
nucleophilic attack that occurs at the exocyclic double bond
has been preferred.8 Intramolecular aldol reaction takes place
in the second stage of a cascade, leading to the introduction of
the oxygen-centered nucleophile into the molecule. Notably,
when it is positioned in the pseudoaxial position, it is in the
spatial proximity of the electron-deficient double bond present
in the molecule and the oxa-Michael addition occurs, thus
The cascade product 1a has been subjected to oxidation of
the sulfide moiety into the sulfone 6a (Scheme 3). The
reaction has been efficiently realized using m-chloroperbenzoic
acid (mCPBA) as the oxidant in chloroform at room
Scheme 3. Oxidation of Sulfide Moiety in 4a to the
Corresponding Sulfone 6a
C
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Organic Letters
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α,β,γ-functionalization of the γ-lactone framework. It was
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03796.
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Experimental procedures, characterization of the prod-
ucts, NMR data, and UPC2 traces (PDF)
Accession Codes
CCDC 1879289 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lukasz.albrecht@p.lodz.pl.
ORCID
́
Org. Synth. 2013, 10, 764. (d) Frías, M.; Cieslik, W.; Fraile, A.;
Łukasz Albrecht: 0000-0002-4669-7670
Author Contributions
́
́
Rosado-Abon, A.; Garrido-Castro, A. F.; Yuste, F.; Aleman, J. Chem. -
Eur. J. 2018, 24, 10906. For selected examples, see: (e) Brown, S. P.;
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Org. Lett. 2012, 14, 1086. (l) Chen, J.; Ni, S.; Ma, S. Adv. Synth. Catal.
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All authors have given approval to the final version of the
manuscript.
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project was realized within the Sonata Bis programme
(Grant No. 2015/18/E/ST5/00309) from the National
Science Centre, Poland. Thanks are expressed to Dr. Lesław
́
Sieron (Faculty of Chemistry, Lodz University of Technology)
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E
DOI: 10.1021/acs.orglett.8b03796
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