Access to 3-Prenylated Oxindoles by α-Regioselective Prenylation: Application to the Synthesis of (±)-Debromoflustramine e

Article abstract of DOI:10.1021/acs.orglett.8b00045

The development of a rapid, highly efficient, and one-pot synthesis of C3-α-prenylated oxindoles with simple reagents is described. The process is based on zinc-mediated α-regioselective prenylation of 3-acylidene-oxindole with commercially available pren

Full text of DOI:10.1021/acs.orglett.8b00045

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Access to 3‑Prenylated Oxindoles by α‑Regioselective Prenylation:
Application to the Synthesis of ( )-Debromoflustramine E
De-Feng Li, Kun Liu, Yi-Xuan Jiang, Yan Gu, Jing-Ru Zhang, and Li-Ming Zhao*
School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou 221116, Jiangsu, China
S
* Supporting Information
ABSTRACT: The development of a rapid, highly efficient, and one-pot synthesis of C3-α-prenylated oxindoles with simple
reagents is described. The process is based on zinc-mediated α-regioselective prenylation of 3-acylidene-oxindole with
commercially available prenyl bromide using inexpensive CeCl3 as the catalyst. The new transformation tolerates a wide range of
3-acylidene-oxindoles, providing easy access to a variety of functionalized 3-prenylated oxindoles. The synthetic utility of the
approach is verified by formal synthesis of the flustramine family alkaloid ( )-debromoflustramine E.
Scheme 1. Methods for the Preparation of 3-Prenylated
Oxindoles
renylated indole alkaloids are present in a large number of
natural products and bioactive molecules (Figure 1).1 They
P
Figure 1. Biologically active 3-prenylated indole alkaloids.
tion−alkylationofprenyl-β-ketoester, whichwaspresentedbythe
same group in 2007 (Scheme 1b).5b Despite great innovation, the
formation of regioisomeric mixtures in these reactions remains
the inherent limitation. Meanwhile, both a presynthesis of the
Pd2dba3·CHCl3 complex and an additional step to access the
prenyl source are required in those transformations, which is not
ideal from a practical perspective. To achieve “ideal synthesis”,6
the use of simple reagents and an easily available prenyl source for
the preparation of 3-prenylated oxindoles with high regioselec-
tivity is highly desirable.
display a variety of interesting biological and pharmacological
properties. 3-Prenylated oxindoles are key intermediates for the
synthesisofthoseindolealkaloidssuchastheflustraminefamilyof
naturalproducts.2 Inprinciple, thedirectincorporationofaprenyl
group into the 3-position of the oxindoles represents the most
reliable method to prepare 3-prenylated oxindoles. However, a
major problem in direct prenylation can be the formation of two
regioisomeric products, i.e., α-product and γ-product.3 In
particular, the selective formation of a α-product has proven to
be challenging.4
In 2011, the Trost group investigated an elegant approach to 3-
prenylated oxindoles via palladium-catalyzed asymmetric pre-
nylation of oxindoles (Scheme 1a).5a Another approach to access
3-prenylated oxindoles was palladium-catalyzed decarboxyla-
On the basis of our continuing interest in the regioselective
prenylation and its application,7 we envisioned that by using 3-
Received: January 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00045
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
acylidene-oxindoles as a synthetic precursor, we might realize the
introduction of a prenyl fragment into the 3-position of oxindoles
with high regioselectivity using prenyl bromide as a prenyl source.
It is expected that an initial addition of the CO bond with
prenylzinc would afford γ-intermediate, which might undergo
subsequent [3,3] sigmatropic rearrangement to afford the target
α-product(Scheme 1c). Herein, wereportour effortsinachieving
such a zinc-mediated α-regioselective prenylation using cheap
and simple reagents without the need for prederivatization to
prenyl sources, allowing for the expedient construction of a wide
range of 3-prenylated oxindoles.
and SnCl4 were less effective (entries 5−7). Other Lewis acids
such as AlCl3, InCl3, and CeCl3 showed higher activity to catalyze
this reaction, among which CeCl3 exhibited the highest catalytic
potency (entries 8−10). Fe (III) catalysts such as FeCl3, FeBr3,
and Fe(caca)3 were also equally effective for the reaction (entries
11−13), but the corresponding workup and purification turned
out to be inconvenient. Therefore, CeCl3 was chosen as the
optimal Lewis acid for further studies. Decreasing the catalyst
loading to 15 mol % led to no loss in yield (entry 14). However,
reduction in the yield was noticed when CeCl3 was decreased to
10 mol % (entry 15). Conducting the reaction in other solvents
such as dioxane (diox) led to no desired α-product formation and
the recovery of the starting material 1a (entry 16). Unlike
previous allylations which usually require the removal of the
excess active zinc powder in advance,10 there is no additional
separate step of prenylzinc reagent needed in all cases presented
here and the excess zinc powder in the reaction system did not
affect the reaction results. This one-pot procedure represents a
practical advantage of the present methodology, which would
greatly facilitate the operation. To demonstrate the synthetic
utility of the one-pot method, the model procedure was
successfully scaled up in comparable yield. The product 2a
(1.21 g, 76%) was prepared when the reaction was run on the 4
mmol scale.
To assess the feasibility for the reaction outlined in Scheme 1c,
compound 1a8 was initially selected as the model substrate to
screen the reaction conditions (Table 1). Treatment of 3-
a
Table 1. Optimization of Reaction Conditions
Under the optimized conditions (Table 1, entry 14), the
generality of this novel α-prenylation was investigated, as shown
in Table 2. Initial investigation of the scope of the reaction was
focused on varying the substituents (R1) on the aromatic ring of
oxindoles. Theresultsshowthatthereactiontookplace efficiently
using substrates that contain a variety of arene ring substituents
(R1). For instance, the substrates possessing −H (1b) as well as
electron-donating groups such as −Me (1c) and −OMe (1d) are
well compatible with this process, delivering the C3-α-prenylated
products 2b−d in good to excellent yields (entries 1−3).
Furthermore, halogen functional groups such as −F (1e), −Cl
(1f), and −Br (1g) are unaltered by and do not affect the process
(entries 4−6). Altering the position of substituents on the phenyl
ring of oxindoles did not significantly affect the reaction behaviors
(entry 6, 1g vs entries 7−9, 1h, 1i, and 1j). Next, we investigated
reactions of 3-acylidene-oxindoles that contain various R3 groups
attached to carbonyl carbon. 3-Acylidene-oxindoles bearing both
electron-donating and -withdrawing functionalities (R3) were
found to be excellent substrates and gave the desired products in
moderate to good yields, such as 4-MeC6H4 (1k), 4-MeOC6H4
(1l), 4-FC6H4 (1m), and 4-ClC6H4 (1n) groups (entries 10−13).
Besides the para-substituted phenyl groups of R3, the substrate
bearing a meta substituent also exhibited good reactivity and
resulted in the formation of 2o in 72% yield (entry 14). However,
no desired C3-α-product 2p or 1,2-γ-adduct was observed when
using the substrate possessing an ortho substituent at the phenyl
ring of R3 (entry 15), probably attributed to the ortho-position
effect of the substituent. To our delight, the trifluoromethyl group
(−CF3), as a useful structural motif in many biologically active
molecules, attached on the phenyl ring of R3 gave the
corresponding product 2q in a reasonable yield (entry 16). 3-
Acylidene-oxindole 1r has been identified as a selective
plasmodial cyclin dependent protein kinase (CDK) inhibitor.11
Webelieve thatthe preparation of itsanalogue wouldbe usefulfor
structure−activity relationship studies. Hence, 1r was subjected
tothesamereactionconditions andthecorresponding product2r
was obtained in 82% yield (entry 17). In addition to substrates
with monosubstituted aryl groups, those bearing disubstituted
aryl groups such as 1s and 1t were also reactive, producing the
Lewis acid
(mol %)
temp
time
(h)
2a
3a
b
b
entry
solvent
(°C)
(%)
(%)
1
none
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
diox
25
2
ND
26
41
37
31
26
20
69
68
77
70
72
75
79
66
ND
>98
49
2
none
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
12
12
12
12
16
16
12
12
8
3
ZnBr2 (25)
BF3·Et2O (25)
CoCl2 (25)
TiCl4 (25)
SnCl4 (25)
AlCl3 (25)
InCl3 (25)
CeCl3 (25)
FeCl3 (25)
FeBr3 (25)
Fe(caca)3 (25)
CeCl3 (15)
CeCl3 (10)
CeCl3 (10)
18
4
27
5
56
6
40
7
44
8
trace
ND
ND
ND
ND
trace
ND
9
9
10
11
12
13
14
15
16
12
12
10
8
12
16
ND
a
Reactions were performed with 1a (0.5 mmol), prenyl bromide (0.75
mmol), and zinc (1.0 mmol) in a one-pot manner. Isolated yield.
b
acylidene-oxindole 1a and prenyl bromide in the presence of zinc
powder in THF at room temperature did not produce the desired
product2aratherthan1,2-γ-adduct3ainalmostquantitativeyield
(entry 1). Pleasingly, when the same reaction was carried out at
reflux, the expected C3-α-prenylated product 2a was observed,
albeit with a low 26% yield (entry 2), indicating a pathway of [3,3]
sigmatropic rearrangement. Encouraged by this result and taking
into consideration the importance of a Lewis acid in [3,3]
rearrangement,9 we envisaged that a Lewis acid could coordinate
to both the oxygen of oxindole and the side chain of 3a, and this
double activation would accelerate the reaction. Hence, various
Lewis acids were screened under similar conditions. The results
indicated that ZnBr2 and BF3·Et2O exhibited moderate catalytic
activity for the reaction (entries 3 and 4), whereas CoCl2, TiCl4,
B
DOI: 10.1021/acs.orglett.8b00045
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
scale (entries 8, 10, 23, and 25), showing the robustness of this
protocol.
Table 2. Substrate Scope
To demonstrate the strength of our method and how this
approach can be very useful as a general method for preparing 3-
prenylated indole alkaloids, we chose debromoflustramine E (a
natural product of flustramine family alkaloids13) to show its
formal synthesis (Scheme 2). To date, several syntheses of the
b
entry
1
R1, R2, R3
2
yield (%)
Scheme 2. Formal Synthesis of ( )-Debromoflustramine E
1
1b
1c
1d
1e
1f
H, H, Ph
2b
2c
2d
2e
2f
89
81
83
83
88
90
80
2
5-Me, H, Ph
3
5-MeO, H, Ph
5-F, H, Ph
4
5
5-Cl, H, Ph
6
1g
1h
1i
5-Br, H, Ph
2g
2h
2i
7
4-Br, H, Ph
c
8
6-Br, H, Ph
83 (82)
9
1j
7-Br, H, Ph
2j
86
d
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1k
1l
H, H, 4-MeC6H4
H, H, 4-MeOC6H4
H, H, 4-FC6H4
H, H, 4-ClC6H4
H, H, 3-BrC6H4
H, H, 2-BrC6H4
H, H, 4-CF3C6H4
5-Br, H, 4-FC6H4
H, H, 3,4-(OCH2O)C6H3
H, H, 3,4-Cl2C6H3
H, H, 2-naphthyl
H, H, 2-thienyl
H, H, Me
2k
2l
85 (83)
78
1m
1n
1o
1p
1q
1r
2m
2n
2o
2p
2q
2r
76
80
72
0
racemic and optically active forms have been reported.3b,14
However, the reported routes to debromoflustramine E mainly
rely on a multistep sequence to construct the prenyl moiety.14a−d
Joseph-Nathan et al. developed a one-step Grignard alkylation to
construct the prenyl moiety and with it as the key step
accomplished the total synthesis of ( )-debromoflustramine
E.3b However, a mixture of regioisomeric products was formed in
the process, which is difficult to separate by column
chromatography. Our formal synthesis of debromoflustramine
E began with the gram-scale prenylation of 3-acylidene-oxindole
1z with prenyl bromide in the presence of zinc powder and CeCl3
to give C3-α-prenylated product 2z in 82% yield (entry 25, Table
2). The treatment of 2z with hydroxylamine hydrochloride in
methanol in the presence of AcONa led to the formation of the
desired ketoxime 4 in 88% yield. We screened numerous reaction
conditions and found that 2,4,6-trichlorobenzenesulfonyl
chloride promoted 4 to undergo a clean Beckmann rearrange-
ment at 25 °C to produce amide 5 in 85% yield. After introducing
a BOC group to the amide nitrogen atom, the resulting N-Boc
derivative was treated with sodium methoxide in methanol to
provide the ester 6, which was then amidated using methylamine
to the methyl amide 7. Following the procedure reported by
Kobayashi et al.,14a compound 7 could be transformed in a few
steps, into indole alkaloid ( )-debromoflustramine E. While we
didnotattempttoassess other flustramine familyalkaloids, weare
confident that the method described here would be equally
effective in the synthesis of debromoflustramine B15 from
compound 2l and flustramine B15 from compound 2i,
respectively.
50
82
1s
1t
2s
2t
77
72
1u
1v
1w
1x
1y
1z
2u
2v
2w
2x
2y
2z
80
42
0
e
H, Bn, Ph
85 (82)
84
H, Bn, 4-BrC6H4
H, Bn, 4-MeOC6H4
f
86 (82)
a
Reactions were performed with 1 (0.5 mmol), prenyl bromide (0.75
mmol), 15 mol % CeCl3, and zinc (1.0 mmol) in THF at reflux under
b
c
N2 for 8 h. Isolated yield. Gram-scale reaction, 1i (1.15 g) was used.
d
e
Gram-scale reaction, 1k (1.05 g) was used. Gram-scale reaction, 1x
f
(1.02 g) was used. Gram-scale reaction, 1z (1.10 g) was used.
corresponding C3-α-prenylated products 2s and 2t in 77% and
72% yields (entries 18 and 19), respectively. More bulky 2-
naphthyl also survived well to afford the desired product 2u in
80% yield (entry 20). The reaction also worked with the
heteroaromatic substrates such as 1v albeit with low yields (entry
21). The low yield of product 2v might be due primarily to the
detrimental interaction between the sulfur atom of the thiophene
ring and the Lewis acid. Unfortunately, the reaction failed to
afford the desired product 2w and only a 1,2-γ-adduct
intermediate was observed when R3 was substituted with an
aliphatic methyl group (entry 22). It is noteworthy that all the
cases mentioned above use N-unprotected 3-acylidene-oxindoles
as the substrate. This is particularly advantageous because the
introduction and removal of protecting groups usually results in a
reduced overall yield.12 Finally, to explore the substrate scope
further, we tested N-protected substrates (R2). Just as expected,
those N-protected substrates such as N-Bn protected ones 1x, 1y,
and 1z were well tolerated in this process, and the corresponding
products 2x, 2y, and 2z were obtained in 85%, 84%, and 86%
yields,respectively(entries23−25). Thesynthesesofproducts2i,
2k, 2x, and 2z were accomplished on a gram scale with yields
almost identical to those obtained in experiments on a smaller
On the basis of the experimental results shown in Table 1,
together with the observation of an ortho-position effect in entry
15 of Table 2, a plausible reaction mechanism for the
transformation is proposed in Scheme 3. Initially, a nucleophilic
attack of the γ-carbon of the prenylzinc bromide on the carbonyl
group attached to R3 produces the γ-prenylated zinc alcoholate.
This can be explained by the formation of the currently accepted
six-membered cyclic transition state between the carbonyl
compound andtheprenylzinc moiety.16 Then, CeCl3 coordinates
to both the oxygen of the zinc alcoholate and the oxindole as a
C
DOI: 10.1021/acs.orglett.8b00045
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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bidentateLewisacidpromotertoformintermediateA.17 Similarly
to the copper(II)- and titanium(IV)-catalyzed Claisen rearrange-
ment,18 this two-point coordination would promote the
subsequent [3,3] thermal rearrangement of A, which resulted in
the formation of intermediate B. Final workup and enolization
delivers the α-adduct 2.
In conclusion, we have developed a conceptually new route for
the introduction of a prenyl fragment at the C-3 position of
oxindoles by the prenylation reaction of 3-acylidene-oxindole
with prenyl bromide catalyzed by CeCl3 in the presence of zinc
powder, which provides easy access to a variety of functionalized
3-prenylated oxindoles. The protocol is direct and operationally
simple, has high step economy and a broad substrate scope, uses
simple materials, and is amenable to gram-scale synthesis of 3-
prenylated oxindoles in high yields. Furthermore, this method
holds great potential in the synthesis of prenylated indole alkaloid
natural products. In this context, we employed this protocol to
finish the formal synthesis of ( )-debromoflustramine E.
Applications of this method toward other prenylated indole
alkaloids are in progress.
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ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.8b00045.
(11) Woodard, C. L.; Li, Z.; Kathcart, A. K.; Terrell, J.; Gerena, L.;
Lopez-Sanchez, M.;Kyle, D. E.; Bhattacharjee, A. K.;Nichols, D. A.;Ellis,
W.; Prigge, S. T.; Geyer, J. A.; Waters, N. C. J. Med. Chem. 2003, 46, 3877.
(12) (a) Hoffmann, R. W. Synthesis 2006, 2006, 3531. (b) Young, I. S.;
Baran, P. S. Nat. Chem. 2009, 1, 193.
Experimental procedures, characterization data, and copies
of 1H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
■
(13) Smith, B. P.; Tyler, M. J.; Kaneko, T.; Garraffo, H. M.; Spande, T.
F.; Daly, J. W. J. Nat. Prod. 2002, 65, 439.
Corresponding Author
*E-mail: lmzhao@jsnu.edu.cn.
ORCID
(14) (a) Miyamoto, H.; Okawa, Y.; Nakazaki, A.; Kobayashi, S.
Tetrahedron Lett. 2007, 48, 1805. (b) Aburano, D.; Yoshida, T.;
Miyakoshi, N.; Mukai, C. J. Org. Chem. 2007, 72, 6878. (c) Ozawa, T.;
Kanematsu, M.; Yokoe, H.; Yoshida, M.; Shishido, K. J. Org. Chem. 2012,
77,9240. (d)Liu,R.;Zhang, J.Chem. -Eur. J. 2013, 19,7319. (e)Zhou, Y.;
Xi, Y.; Zhao, J.; Sheng, X.; Zhang, S.; Zhang, H. Org. Lett. 2012, 14, 3116.
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Nat. Prod. 1994, 57, 997. (b) Zhang, X.; Yang, Z.-P.; Liu, C.; You, S.-L.
Chem. Sci. 2013, 4, 3239. (c) Zhang, X.; Han, L.; You, S.-L. Chem. Sci.
2014, 5, 1059.
Li-Ming Zhao: 0000-0002-7917-8754
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Thiswork was supported bythe Science and TechnologySupport
Program of Jiangsu Province (BE2017643).
■
(16) (a) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207. (b) Fu, J.;
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DOI: 10.1021/acs.orglett.8b00045
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