"On water" catalytic enantioselective sulfenylation of deconjugated butyrolactams

Article abstract of DOI:10.1039/c7ob01714f

The first catalytic enantioselective α-sulfenylation of deconjugated butyrolactams has been developed using dimeric cinchona alkaloids as catalysts in a water-enriched reaction medium. Highly substituted and densely functionalized γ-lactams, bearing a qua

Full text of DOI:10.1039/c7ob01714f

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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“On water” catalytic enantioselective sulfenylation of
deconjugated butyrolactams
Received 00th January 20xx,
Accepted 00th January 20xx
Soumya Jyoti Singha Roy and Santanu Mukherjee*
The first catalytic enantioselective α-sulfenylation of deconjugated butyrolactams has been developed using dimeric
cinchona alkaloids as the catalyst in water-enriched reaction medium. Highly substituted and densely functionalized
γ-lactams, bearing a quaternary stereogenic center, are produced with up to 99.5:0.5 er. The applicability of the same
catalyst system for enantioselective α-selenylation and formal vinylogous γ hydroxylation of deconjugated butyrolactam
has also been described.
DOI: 10.1039/x0xx00000x
www.rsc.org/
fact, enantioselective reactions involving deconjugated
butyrolactams remain sporadic.8
Introduction
γ
-Lactams and their derivatives are structural motifs often
found in a wide variety of natural products and are of principal
interests in medicinal chemistry.1 Therefore, enantioselective
construction of structurally diverse γꢀlactams remain a subject
of general interest. 2-Silyloxypyrroles2 and N-Boc α,β-
unsaturated
γ
-butyrolactam3 are the two most commonly
employed building blocks for enantioselective synthesis of
γꢀlactams. In spite of their widespread utility, the methods
involving the former suffer from narrow substrate scope while
the applicability of the latter is limited to the synthesis of
γ
γ
-monosubstituted
-monosubstituted
γꢀlactams.
α,β-unsaturated butyrolactams (Scheme
The
corresponding
1A), owing to their considerably lower acidity, have not found
application in catalytic enantioselective synthesis until a very
recent report by Maruoka and co-workers.4 Although the
problem of low acidity of
γ-monosubstituted conjugated
butyrolactams has been addressed by the Maruoka group
using a fairly strong base under phase-transfer catalysis, the
scope of this vinylogous Michael reaction is restricted to
or heteroaryl substituted butyrolactams.4
γ-aryl
Scheme 1 Direct enantioselective functionalization of γ-lactams.
In the domain of butenolides – the oxo-analogs of
γꢀlactams, a similar problem has been tackled through the
introduction of γ-substituted β,γ-unsaturated butyrolactones
(deconjugated butenolides) as a highly reactive class of
In analogy with deconjugated butenolides, we postulated
that enolization of butyrolactam should be possible under the
influence of a Brønsted basic tertiary amine catalyst. The
resulting pyrrolyl dienolate could then react with an
nucleophiles (Scheme 1A).5
A wide range of catalytic
enantioselective reactions involving deconjugated butenolides
have been reported during the past few years.6 In contrast, the
corresponding deconjugated butyrolactams, although reported
in the literature,7 have rarely been used as nucleophile.4 In
electrophile in a regiodivergent manner either through α- or γ-
position (Scheme 1B). This strategy holds the potential of
generating highly substituted and densely functionalized
γ
-lactams containing
Controlling regioselectivity would obviously be the key since a
mixture of - and -addition products would be of little value.
a quaternary stereogenic center.
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012,
India. E-mail: sm@orgchem.iisc.ernet.in; Tel: +91-80-2293-2850; Fax: +91-80-2360-
0529.
α
γ
Prevalence of sulfur-containing compounds in many
synthetic drugs and bioactive natural products9 inspired the
†Electronic Supplementary Information (ESI) available: Experimental details,
characterization and analytical data. CCDC 1549957
1549959 ( ). See DOI: 10.1039/x0xx00000x
(3aa), 1549958 (5) and
development of numerous enantioselective C–S bond forming
6
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reactions.10 However, to the best of our knowledge, was observed with quinidine as the catalyst in CH2Cl2 at 25
°C,
sulfenylation of butyrolactams has never been reported, let 10 mol% of its dimer (QD)2PHAL furnished the product with
alone an enantioselective variant.11 Herein we report the first significantly improved er under the same reaction conditions
example of the use of deconjugated butyrolactams as (entries 2-3). A comprehensive solvent survey revealed tert-
nucleophile in catalytic enantioselective carbon
bond forming transformations (Scheme 1B).
–
heteroatom butylbenzene (t-BuPh) as the optimal in terms of
enantioselectivity, even though the reaction rate suffered
drastically (entry 5). In an attempt to address the sluggish rate
of this sulfenylation process, we speculated the use of water
as a co-solvent which is known to accelerate certain organic
reactions through “hydrophobic hydration”.14 Additionally, the
Results and discussion
At the outset of our investigation, sulfenylation of
tetrasubstituted -unsaturated butyrolactam 1a with
N-(phenylsulfanyl)succinimide (2a) was selected as the model
reaction (Table 1). Our initial focus was on the Br nsted basic
stoichiometric by-product succinimide may be removed from
the organic phase because of its high water solubility.
β,γ
The use of a 1:1 mixture of t-BuPh and water as the
reaction medium was indeed found to enhance the rate of the
reaction considerably without affecting the enantioselectivity,
and led to complete conversion of 1a within 36 h (entry 7).
This effect appeared to be more prominent in water-enriched
media, and a 1:9 ratio of t-BuPh/water turned out to be the
optimal with respect to both rate and enantioselectivity (entry
10). On performing the reaction on 0.1 M initial concentration,
an improved enantioselectivity (98:2 er) was observed within a
reasonable time scale (entry 11). However, further dilution
showed a deleterious influence on both reaction rate and er
(entry 12). It is interesting to note that the sulfenylation
reaction proceeds even more rapidly in aqueous medium15 and
also in brine while maintaining high level of enantioselectivity
(entries 13-14). The rate deceleration in antihydrophobic
aqueous LiClO4 is often used as an indication for hydrophobic
hydration effect.14 The sulfenylation reaction, when conducted
in saturated aqueous LiClO4 solution, required 80 h for
completion as opposed to 6 h in water and 12 h in brine (cf.
entry 15 with entries 13-14). These observations tentatively
point to the possible hydrophobic hydration effect operative
for this sulfenylation reaction in water-enriched media.
ø
catalysts, employed earlier for various enantioselective
reactions of deconjugated butenolides by our group and
others.5,6 Bifunctional tertiary amino(thio)ureas and
squaramides, although displayed decent catalytic activity for
the sulfenylation of 1a, failed to induce appreciable level of
enantioselectivity.12 Nevertheless, we were delighted to
observe that in all these cases,
formed exclusively without
α
-sulfenylation product was
a
trace of γ-addition. This
regioselectivity trend stands in sharp contrast to those
reported earlier for deconjugated butenolides, where
γ
-addition predominates.5,6,13 While modest enantioselectivity
Table 1 Optimization of reaction conditions: effect of water
O
SPh
EtO2C
Me
4-ClC6H4
+
EtO2C
Me
catalyst
(10 mol %)
N
SPh
O
O
N
solvent
(0.13 M)
25 °C
N
Cl
Bn
O
2a
(1.5 equiv)
Bn
3aa
1a
(1.0 equiv)
OMe
N
OH
MeO
R
N
N
N
N
O
O
The scope and limitations of this sulfenylation protocol was
then tested under the catalyst and reaction conditions
optimized for 1a and 2a (Table 1, entry 11). As shown in Table
N
H
N
R
quinidine
OMe
N
(QD)2PHAL: R = CH=CH2
(DHQD)2PHAL: R = Et
2A, an array of butyrolactams having
diverse steric and electronic demands were well
accommodated to deliver the -sulfenylated products
generally with high yields and excellent enantioselectivities.
These substituents include benzylic (3aa ia) as well as linear
3ja and branched 3la aliphatics. However, α-isobutyl
α-substituent with
Entry
Catalyst
Solvent
t/ha
erb
-
α
1
None
CH2Cl2
48c
72
24
96
96
96
36
72
36
12
24
48
6
2
quinidine
CH2Cl2
62.5:37.5
88:12
-
3
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
(DHQD)2PHAL
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
(QD)2PHAL
CH2Cl2
(
)
(
)
4
PhMe
93.5:6.5
96.5:3.5
93.5:6.5
96.5:3.5
97:3
containing butyrolactam (3k) remained unreacted under our
reaction conditions, presumably due to increased steric
crowding near the reaction site. Our protocol was amenable to
different aromatic sulfenyl donors (2b-f), furnishing products
with good to high yields and excellent enantioselectivities
5
t-BuPh
6
t-BuPh
7
t-BuPh/H2O (1:1)
t-BuPh/H2O (3:1)
t-BuPh/H2O (1:3)
t-BuPh/H2O (1:9)
t-BuPh/H2O (1:9)
t-BuPh/H2O (1:9)
H2O
8
9
97:3
10
11d
12e
13
14
15
96.5:3.5
98:2
(Table 2B). The aliphatic sulfenyl donors (2g-h) were found to
be unreactive under our reaction conditions and hence mark a
current limitation of our protocol. Deconjugated
97.5:2.5
96:4
butyrolactams bearing various substituents at the γ-position
1m ), on ester (1q) as well as on nitrogen were also tolerated
Brine
12
80
97:3
(
-p
Sat. aq. LiClO4
86.5:13.5
and afforded the products generally in good yields and with
moderate to high er (Table 2C). In fact, the product derived
from N-unprotected butyrolactam (3ta) was also obtained in
high yield, albeit with moderate er.
a
b
Time required for complete consumption of 1a. Enantiomeric ratio (er) was
determined by HPLC analysis on a chiral stationary phase. c No conversion of 1a. d
Reaction at 0.1 M concentration. e Reaction at 0.05 M concentration.
2 | J. Name., 2012, 00, 1-3
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The single crystal X-ray diffraction analysis of 3aa 3aa, which was initially formed with 96.5:3.5 er, could be
confirmed its absolute configuration (Table 2A).16 The recrystallized to deliver essentially enantiopure product.
configurations of the other products were assigned the same
by analogy.
An important aspect of a catalytic enantioselective
protocol is to provide access to both product enantiomers. To
our delight, the opposite product antipode ent-3aa was
formed in 89% yield and with 97:3 er using the related
pseudoenantiomeric quinine dimer (Q)2PHAL as the catalyst
under otherwise standard reaction conditions (Scheme 2).
Table 2 Substrate scope of catalytic enantioselective sulfenylationa
R3O2C
R2
R4
R3O2C
R2
O
SR
R4
(QD)2PHAL
(10 mol %)
+
N
SR
O
O
t-BuPh/H2O (1:9)
N
N
R1
O
R1
25 °C
1
2
3
(1.0 equiv)
(1.5 equiv)
(A)
EtO2C
R
t [h]
Yield [%] er
4-ClC6H4 (3aa)
Ph (3ba)
24
20
88
87
89
98:2
SPh
>98:2
98:2
R
4-CF3C6H4 (3ca)
4-OMeC6H4 (3da)
3-ClC6H4 (3ea)
2-FC6H4 (3fa)
20
32
24
22
Me
O
N
92
87
90
97.5:2.5
96:4
Bn
98:2
O
(3ga)
24
87
98:2
O
2-Furyl (3ha)
2-Thienyl (3ia)
n-Pent (3ja)
i-Pr (3ka)
i-Bu (3la)
24
20
40
48
48
80
90
82
<5
81
97.5:2.5
97.5:2.5
96.5:3.5
-
Scheme 2 “On water” sulfenylation in mmol scale and access to enantiomeric
product.
3aa
We reasoned that the catalyst and reaction conditions
developed for the enantioselective sulfenylation reaction
(CCDC 1549957)
97.5:2.5
(B)
Ar
R
3
t [h] Yield [%] er
(Table 2) might be suitable for an analogous
deconjugated butyrolactams. However, the instability of the
related reagent N-(phenylseleno)succinimide in water forced
us to use t-BuPh as the reaction medium. Under these
modified conditions, the -selenyl butyrolactam was formed
α-selenylation of
2-FC6H4
Ph
3bb 20
3bc 24
86
84
76
93
99.5:0.5
98.5:1.5
96.5:3.5
97.5:2.5
R
S
3-OMeC6H4
Ph
EtO2C
Me
4
4-ClC6H4
2-Naphthyl 3ad 44
Ar
4-ClC6H4 4-BrC6H4
4-ClC6H4 4-NO2C6H4
3ae 24
3af 20
3ag 48
3ah 48
O
N
85
<5
<5
97.5:2.5
α
5
Bn
4-ClC6H4
4-ClC6H4
Bn
-
-
in 72% yield and with 94.5:5.5 er (Scheme 3). A single
c-Hex
recrystallization from n-hexane/CHCl3 not only led to nearly
(C)
SPh
EtO2C
enantiopure 5, but also allowed us to establish its absolute
R =
(3na)
R = n-Pr (3ma)
36 h, 67% yield
93:7 er
4-ClC6H4
2
configuration through X-ray diffraction analysis.16 The same
sense of stereoinduction observed for sulfenylation and
selenylation points to a mechanistic resemblance of these two
reactions.
R
O
48 h, 82% yield
91:9 er
N
PMB
Ar
O
S
EtO2C
Bn
Ar = Ph (3oa)
60 h, 56% yield
88:12 er
Ar = 3-OMeC6H4 (3oc)
48 h, 61% yield
95.5:4.5 er
4-BrC6H4
N
PMB
SPh
SPh
SPh
EtO2C
Ph
BnO2C
Me
EtO2C
Me
4-ClC6H4
4-ClC6H4
4-ClC6H4
O
O
O
N
N
N
Bn
3pa
PMB
DMB
3ra
3qa
42 h, 70% yield
73.5:26.5 er
24 h, 82% yield
98:2 er
240 h, 70% yield
92:8 er
4-BrC6H4
S
SPh
Scheme 3 Catalytic enantioselective selenylation of deconjugated butyrolactam.
EtO2C
Me
EtO2C
Me
Me
4-ClC6H4
O
O
N
N
Having successfully developed the protocols for catalytic
H
Bn
3se
enantioselective
α
-sulfenylation and
α-selenylation of
3ta
deconjugated butyrolactams, we turned our attention to
demonstrate the synthetic usefulness of the respective
20 h, 80% yield
94:6 er
24 h, 94% yield
80.5:19.5 er
a
products. The
mCPBA in a mixture of CH2Cl2 and water, rapidly rearranged to
γ-hydroxylated -unsaturated butyrolactam with
α-selenyl butyrolactam 5, upon exposure to
Reactions were carried out on a 0.1 mmol scale. Yields correspond to the
isolated yield. Er was determined by HPLC analysis on a chiral stationary phase.
PMB = p-methoxybenzyl. DMB = 2,4-dimethoxybenzyl.
a
α
,
β
6
retention of configuration, as confirmed by X-ray diffraction
analysis (Scheme 4A).16 This enantiospecific heteroatom
transposition reaction most likely proceeds via the oxidation of
the allylic selenide moiety, embedded in the butyrolactam
The scalability of this protocol is demonstrated by
conducting the reaction between 1a and 2a in 1.0 mmol scale
using water as the reaction medium (Scheme 2). The product
framework, to the corresponding selenoxide (7) followed by a
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[2,3]-sigmatropic rearrangement.17 It is possible to perform Prodip Howlader (IPC, IISc, Bangalore) for their help with
this reaction in a one-pot manner starting from 1a, when the the X-ray structure analysis.
product
enantioselectivity as the stepwise route. This process
represents formal enantioselective vinylogous
-hydroxylation of deconjugated butyrolactam – a hitherto
unknown transformation. Notably, -hydroxy -lactams
represent the core structure of numerous natural products.18
Even though sulfides are known to undergo the same type of
rearrangement,19 the similar oxidation conditions, when
(6) was formed in 66% yield with comparable
Notes and references
a
γ
1
J. Caruano, G. G. Muccioli and R. Robiette, Org. Biomol.
Chem., 2016, 14, 10134.
γ
γ
2
(a) X. Li, M. Lu, Y. Dong, W. Wu, Q. Qian and J. Y. D. J. Dixon,
Nat. Commun., 2014, 4479; (b) C. Curti, B. Ranieri, L.
Battistini, G. Rassu, V. Zambrano, G. Pelosi, G. Casiraghi and
F. Zanardi, Adv. Synth. Catal., 2010, 352, 2011; (c) H. Suga, H.
Takemoto and A. Kakehi, Heterocycles, 2007, 71, 361; (d) M.
Frings, I. Atodiresei, Y. Wang, J. Runsink, G. Raabe and C.
Bolm, Chem.–Eur. J., 2010, 16, 4577. For reviews, see: (e) X.
applied to α-sulfenyl butyrolactam 3aa, led to sulfone 8 in high
yield, without any trace of hydroxylation product (Scheme 4B).
Jusseau, L. Chabaud and C. Guillou, Tetrahedron, 2014, 70
,
2595; (f) Z. Miao and F. Chen, Synthesis, 2012, 44, 2506; (g)
G. Casiraghi, F. Zanardi, L. Battistini and G. Rassu, Synlett,
2009, 1525.
3
For selected examples, see: (a) D. Li, Y. Wang, L. Wang, J.
Wang, P. Wang, K. Wang, L. Lin, D. Liu, X. Jiang and D. Yang,
Chem. Commun., 2016, 52, 9640; (b) X. Gu, T. Guo, Y. Dai, A.
Franchino, J. Fei, C. Zou, D. J. Dixon and J. Ye, Angew. Chem.,
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Wang, Org. Lett., 2014, 16, 5358; (d) X. Jiang, L. Liu, P. Zhang,
Y. Zhong and R. Wang, Angew. Chem., Int. Ed., 2013, 52
,
11329; (e) J. Zhang, X. Liu, X. Ma and R. Wang, Chem.
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Wang, Chem. Commun., 2013, 49, 3300; (g) A. Ray
Choudhury and S. Mukherjee, Org. Biomol. Chem., 2012, 10
,
7313; (h) H. Huang, Z. Jin, K. Zhu, X. Liang and J. Ye, Angew.
Chem. Int. Ed., 2011, 50, 3232; (i) X. Feng, H.-L. Cui, S. Xu, L.
Wu and Y.-C. Chen, Chem.–Eur. J., 2010, 16, 10309; (j) N. E.
Shepherd, H. Tanabe, Y. Xu, S. Matsunaga and M. Shibasaki,
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4
5
6
A. Arlt, H. Toyama, K. Takada, T. Hashimoto and K. Maruoka,
Chem. Commun., 2017, 53, 4779.
H.-L. Cui, J.-R. Huang, J. Lei, Z.-F. Wang, S. Chen, L. Wu and
Y.-C. Chen, Org. Lett., 2010, 12, 720.
For selected examples, see: (a) M. S. Manna, R. Sarkar and S.
Scheme 4 Synthetic elaboration of the products.
Mukherjee, Chem.–Eur. J., 2016, 22, 14912; (b) A. K.
Simlandy and S. Mukherjee, Org. Biomol. Chem., 2016, 14
,
Conclusions
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49, 11203; (d) Y. Wu, R. P. Singh and L. Deng, J. Am. Chem.
Soc., 2011, 133, 12458; (e) L. Zhou, L. Lin, J. Ji, M. Xie, X. Liu
and X. Feng, Org. Lett., 2011, 13, 3056; (f) A. Quintard, A.
Lefranc and A. Alexakis, Org. Lett., 2011, 13, 1540. Also see:
ref. 2a and 2e.
(a) Á. Cores, V. Estévez, M. Villacampa and J. C. Menéndez,
RSC Adv., 2016, 6, 39433; (b) D. Kalaitzakis, E. Antonatou and
G. Vassilikogiannakis, Chem. Commun., 2014, 50, 400; (c) D.
Kalaitzakis, T. Montagnon, E. Antonatou, N. Bardají and G.
In conclusion, deconjugated butyrolactams have been applied
for the first time as nucleophile in catalytic enantioselective
carbon
by dimeric cinchona alkaloids in a water-enriched reaction
media, this -sulfenylation protocol delivers highly substituted
and densely functionalized -lactams, bearing a quaternary
–heteroatom bond forming transformations. Catalyzed
α
7
8
γ
stereogenic center, in moderate to high yields and generally
with high enantioselectivities. Substantial rate acceleration
was observed in water-enriched or aqueous media, possibly
due to hydrophobic hydration effect. The suitability of the
Vassilikogiannakis, Chem.–Eur. J., 2013, 19, 10119.
(a) Q. Yuan, D. Liu and W. Zhang, Org. Lett., 2017, 19, 1144;
(b) H. R. Campello, J. Parker, M. Perry, P. Ryberg and T.
Gallagher, Org. Lett., 2016, 18, 4124. For reactions of in situ
generated deconjugated butyrolactams, see: (c) M. E.
Muratore, C. A. Holloway, A. W. Pilling, R. I. Storer, G. Trevitt
and D. J. Dixon, J. Am. Chem. Soc., 2009, 131, 10796.
same catalyst system for enantioselective
α-selenylation and
formal vinylogous γ-hydroxylation of deconjugated
butyrolactam has also been demonstrated.
9
(a) J.-S. Yu, H.-M. Huang, P.-G. Ding, X.-S. Hu, F. Zhou and J.
Zhou, ACS Catal., 2016, 6, 5319; (b) P. Chauhan, S. Mahajan
and D. Enders, Chem. Rev., 2014, 114, 8807.
Acknowledgements
10 For pioneering studies, see: (a) S. Sobhani, D. Fielenbach, M.
Marigo, T. C. Wabnitz and K. A. Jørgensen, Chem.–Eur. J.,
2005, 11, 5689; (b) M. Marigo, T. C. Wabnitz, D. Fielenbach
and K. A. Jørgensen, Angew. Chem., Int. Ed., 2005, 44, 794.
11 For enantioselective sulfenylation of a related class of
nucleophiles - oxindoles, see: (a) C. Wang, X. Yang, C. C. J.
Financial supports from SERB [Grant No. SB/S1/OC-
63/2013] and CSIR [Grant No. 02(0207)/14/EMR-II] are
gratefully acknowledged. S.J.S.R. thanks CSIR for a doctoral
fellowship. We wish to thank Mr. Rupak Saha and Mr.
4 | J. Name., 2012, 00, 1-3
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Organic & Biomolecular Chemistry
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DOI: 10.1039/C7OB01714F
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Loh, G. Raabe and D. Enders, Chem.–Eur. J., 2012, 18, 11531;
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12 For comprehensive optimization studies, see the Supporting
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16 CCDC 1549957-1549959 contain the crystallographic data for
3aa, 5 and 6, respectively. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
17 L. C. Hess, and G. H. Posner, Org. Lett., 2010, 12, 2120.
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