Core-structure-oriented asymmetric organocatalytic substitution of 3-hydroxyoxindoles: Application in the enantioselective total synthesis of (+)-folicanthine

Article abstract of DOI:10.1002/anie.201107079

Something constructive: The title reaction involving 3-hydroxyoxindoles gives 3,3′-disubstituted oxindoles with concomitant generation of an all-carbon quaternary stereogenic center in high yield and excellent enantioselectivity. This reaction enabled the

Full text of DOI:10.1002/anie.201107079

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DOI: 10.1002/anie.201107079
Synthetic Methods
Core-Structure-Oriented Asymmetric Organocatalytic Substitution of
3-Hydroxyoxindoles: Application in the Enantioselective Total
Synthesis of (+)-Folicanthine**
Chang Guo, Jin Song, Jian-Zhou Huang, Peng-Hao Chen, Shi-Wei Luo, and Liu-Zhu Gong*
Cyclotryptamine alkaloids constitute a large family of natural
products (Figure 1) which show fascinating biological activ-
ities.[1] For example, (+)-chaetocin not only shows antibacte-
have also been found to exhibit important biological and
pharmaceutical activities.[5]
In addition to their wide range of bioactivities, the
cyclotryptamine alkaloids contain an octahydro-3a,3’a-
bispyrrolo[2,3-b]indole subunit (core structure A, Figure 1),
which is characterized by vicinal all-carbon quaternary
stereogenic centers. These compounds have been a long-
standing challenge in organic synthesis.[6] The unique struc-
tural arrays and interesting biological activities displayed by
these alkaloids have led to a demand for efficient asymmetric
synthetic methods. Much effort has been directed toward the
development of new synthetic methods for the construction of
hexahydropyrroloindole skeletons from research groups
around the world.[7] Overman and co-workers reported the
enantioselective total synthesis of optically pure chimonan-
thines, wherein intramolecular double Heck and dialkylation
reactions were exploited to construct the cyclotryptamine
core, and the stereochemical control came from a tartrate
derivative.[8] Movassaghi and co-workers have established a
reductive homodimerization of 3-bromo-hexahydropyrrolo-
indole, readily derived from l-tryptophan, which provided
facile construction of the vicinal quaternary stereogenic
centers that led to the enantioselective total synthesis of
several optically pure cyclotryptamine alkaloids.[9] Very
recently, Sodeoka and co-workers applied a related strategy
to the total synthesis of (+)-chaetocin.[10] In addition, Over-
man and co-workers have described the catalyst-controlled
enantioselective total syntheses of cyclotryptamine alkaloids
from meso derivates of the core structure A.[7,11] In spite of
these elegant achievements, the development of an enantio-
selective catalytic method to access cyclotryptamine struc-
tures of type A still holds great importance in the total
synthesis of the hexahydropyrroloindole alkaloid family.
Over the past several years, numerous endeavors have
been directed toward the enantioselective synthesis of all-
carbon quaternary 3,3’-disubstituted oxindoles,[12] but these
protocols have not provided a chiral intermediate for the
synthesis of the 3a,3a’-bispyrrolidino[2,3-b]indoline skeleton.
The unmet challenge of this catalytic enantioselective syn-
thesis prompted us to consider a new approach. Our strategy
to synthesize the core structure A, as indicated by the
retrosynthetic analysis in Scheme 1, involves accessing struc-
tures of type A from diamide 1 by the Rodrigo protocol.[13]
The diamide 1 would be prepared from 2 through oxidation/
alkylation reactions. A Beckmann rearrangement reaction
would give 2 from 3, which is considered to be the key
intermediate for the synthesis of the core structure A, and
could be obtained from an enantioselective substitution
Figure 1. Representative cyclotryptamine alkaloids. Bn=benzyl.
rial and cytostatic activity, but is also a potent inhibitor of a
lysine-specific histone methyltransferase.[2] The compound
WIN 64821, first isolated from Aspergillus sp.,[3] is a potent
competitive substance P antagonist with respect to human
neurokinin-1 and the cholecystokinin B receptor.[4] More-
over, other members of the cyclotryptamine alkaloid family
[*] C. Guo, J. Song, J.-Z. Huang, P.-H. Chen, S.-W. Luo, Prof. L.-Z. Gong
Hefei National Laboratory for Physical Sciences at the Microscale
and Department of Chemistry
University of Science and Technology of China
Hefei, 230026 (China)
E-mail: gonglz@ustc.edu.cn
Homepage: http://staff.ustc.edu.cn/~gonglz/links.htm
[**] We are grateful for financial support from the NSFC (20732006),
MOST (973 program 2009CB825300), the Ministry of Education,
and CAS.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107079.
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Table 1: Catalyst screening and optimization of the reaction conditions.[a]
Entry
6
4
5
3
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
6a
6b
6c
6d
6e
6 f
6d
6d
6d
6d
6d
6d
6d
6d
6d
6d
6d
6d
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
4a
4a
4a
4a
4a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
5c
5d
5c
5c
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3a
3a
3a
3a
3a
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
1,2-DCE
toluene
Et2O
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
63
71
57
71
62
60
37
43
51
75
76
70
53
50
73
40
75
92
77
86
65
90
85
85
73
88
85
89
89
90
60
87
93
60
94[d]
94[d,e]
Scheme 1. Retrosynthetic analysis of the core structure A.
reaction of 3-hydroxyoxindoles [Eq. (1)]. Herein, we report
the development of a highly enantioselective organocatalytic
substitution reaction of 3-hydroxyoxindoles [Eq. (1)] and its
application to the first catalytic enantioselective total syn-
thesis of (+)-folicanthine.[14]
9
10
11
12
13
14
15
16
17
18
[a] Reaction conditions: 4 (1.0 equiv), 5 (1.5 equiv), 10 mol% catalyst,
with solvent indicated 0.1m. [b] Yield of the isolated product. [c] The
ee value was determined by HPLC analysis using a chiral stationary
phase. [d] The reaction was performed at 0.05m. [e] In the presence of
Na2SO4 (100 mg). Boc=tert-butoxycarbonyl, 1,2-DCE=1,2-dichloro-
ethane.
bamate 5c gave higher enantioselectivity (Table 1, entries 4
and 14–16). Additional optimization of the reaction condi-
tions revealed that lowering the concentration resulted in a
slightly enhanced enantioselectivity of 94% ee (Table 1,
entry 17). Interestingly, the highest product yield at which
the ee value was maintained was obtained when the reaction
was conducted in the presence of anhydrous sodium sulfate
(Table 1, entry 18). The anhydrous sodium sulfate presumably
serves as a scavenger of water generated from the dehydra-
tion reaction and thereby inhibits the decomposition of the
enamide.
Under the optimized reaction conditions, we investigated
the generalities for both reaction components (Table 2). The
introduction of either electron-withdrawing or electron-
donating substituents on the indole moiety was tolerated
and provided products with 90–95% ee (Table 2, entries 1–6).
Moreover, the substitution pattern on the oxindole subunit
exerted little effect. Thus, excellent ee values were obtained
for the reaction involving substrates 4k and 4l, which possess
a fluoro and a methyl substituent, respectively, at the 5-
position (Table 2, entries 7 and 8). The presence of substitu-
ents on both the indole and oxindole moieties were also
amenable to the reaction, thus affording high levels of
stereochemical control (Table 2, entries 9–11). The generality
Our preliminary studies were focused on evaluating a
substitution reaction[15] of the 3-hydroxy-3,3’-bisindolin-2-one
4a[16] with methyl 1-phenylvinylcarbamate 5a catalyzed by a
chiral phosphoric acid 6a[17] (see Eq. (1) for catalyst structure)
in dichloromethane at room temperature (Table 1, entry 1).
To our delight, the reaction proceeded smoothly to afford the
desired product 3a in 63% yield and 77% enantiomeric
excess (ee). Encouraged by these preliminary results, a variety
of chiral phosphoric acids derived from binol were evaluated
(Table 1, entries 2–6). We determined that the 3,3’-di(b-
naphthyl) phosphoric acid 6d was the optimal catalyst, thus
providing a fairly good yield and excellent enantioselectivity
(Table 1, entry 4). Trials using other solvents led to no
improvement in the reaction performance (Table 1,
entries 7–10). The N substituents in the oxindole moiety
were tolerated (Table 1, entries 11–12), whereas the intro-
duction of an N substituent to the indole subunit led to less
satisfactory results (Table 1, entry 13). The variation of the
N substituent of the enamides exerted considerable influence
on the enantioselectivity (Table 1, entries 14–16). As com-
pared to other analogues of 5, the benzyl 1-phenylvinylcar-
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Table 2: Asymmetric alkylation of 3-hydroxyoxindoles with enecarba-
mates catalyzed by 6d.[a]
phosphoric acid 6d acts as a bifunctional catalyst to simulta-
neously activate the enecarbamate and the vinylogous
iminium compound through hydrogen-bonding interactions,
and thereby facilitates the enantioselective conjugate addi-
tion.[21] TS-1 was predicted to be more stable than TS-2 by
4.52 kcalmolÀ1 because of the steric repulsion between the
phenyl ring of enecarbamate 5a and the b-naphthyl ring of the
catalyst 6d. As such, the R-configured product that is
observed experimentally was favorably formed (see
Scheme 2).
Entry
4 (R1, R2)
5 (R3)
3
Yield [%][b]
ee [%][c]
We demonstrated the importance of this reaction in the
construction of the 3a,3a’-bispyrrolidino[2,3-b]indoline core
structure by applying it to the catalytic enantioselective total
synthesis of (+)-folicanthine (Scheme 2). Under optimized
reaction conditions, the enantioselective substitution reaction
gave 3r in 82% yield and 90% ee. The dimethylation of 3r
furnished 7 in 76% yield. When initial attempts to conduct
the Baeyer–Villiger oxidation of 7 and its derivatives failed
under various reaction conditions, we turned our attention to
the Beckmann rearrangement, a reliable tool to convert
ketones into amides. The treatment of compound 7 with
NH2OH·HCl in EtOH under basic conditions gave rise to the
desired ketoximes 8a and 8b (8a/8b = 1:2) in 93% yield.
Interestingly, 8a could be completely converted into 8b in the
presence of p-toluenesulfonic acid (PTSA). A single recrys-
tallization provided enantiopure 8b in 75% overall yield from
ketone 7, and the absolute configuration of 8b was assigned as
R by X-ray analysis.[22] We screened numerous reaction
conditions and found that mercury(II) chloride promoted
8b to undergo a clean Beckmann rearrangement at 808C to
furnish amide 9 in 90% yield.[23] After introducing a methyl
group to the amide nitrogen atom, the resulting product 10 in
the crude reaction mixture was oxidized by DMSO/HCl, thus
producing bis(oxindole) 11 in high yield. Compound 11 then
underwent an alkylation to afford 12 in 66% yield. The
removal of the p-methoxyphenyl (PMP) group by means of
ceric ammonium nitrate (CAN) furnished the compound 13.
The amidation of 13 with methyl amine afforded the
intermediate 14 in 77% yield. Following the procedure
reported by Rodrigo and co-workers,[13] compound 14 was
transformed into (+)-folicanthine by a three-step sequence in
26% overall yield. All of the spectroscopic and optical
rotation data of the synthetic (+)-folicanthine were in
agreement with those reported previously.[9a,13]
In summary, we have developed a highly enantioselective
nucleophilic substitution reaction of 3-hydroxyoxindoles with
enecarbamates catalyzed by chiral phosphoric acids (up to
96% ee), thus providing a new entry into 3,3’-disubstituted
oxindoles with the creation of a quaternary all-carbon
stereogenic center at C3. Theoretical studies on the transition
states using DFT calculations suggested that the reaction
might proceed through a sequential dehydration/Michael
addition reaction with the phosphoric acid activating the
unsaturated iminium species and the enecarbamate through
hydrogen-bonding interactions. This method holds great
potential in asymmetric total syntheses of hexahydropyrro-
loindole alkaloids. In this context, we used this protocol to
prepare a key chiral building block, and accomplished the first
catalytic enantioselective total synthesis of (+)-folicanthine:
1
2
3
4
5
6
7
8
4e (5’-Br, H)
4 f (5’-MeO, H)
4g (5’-Cl, H)
4h (6’-Me, H)
4i (5’-F, H)
5c (H)
5c (H)
5c (H)
5c (H)
5c (H)
5c (H)
5c (H)
5c (H)
5c (H)
5c (H)
5c (H)
5e (Me)
5 f (Cl)
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
72
76
78
85
78
92
79
84
75
88
70
87
92
90
91
92
95
93
94
92
92
94
94
92
91
96
4j (6’-F, H)
4k (H, 5-F)
4l (H, 5-Me)
4m (6’-Me, 5-Me)
4n (6’-Me, 5-F)
4o (5’-F, 5-F)
4a (H, H)
9
10
11
12
13
4a (H, H)
[a] Reaction conditions: 4 (1.0 equiv), 5 (1.5 equiv), 10 mol% 6d in
CH2Cl2 (2 mL), were performed on a 1 mmol scale with 100 mg Na2SO4.
[b] Yield of the isolated product. [c] The ee value was determined by
HPLC analysis using a chiral stationary phase.
of the protocol for the enecarbamate nucleophiles was also
examined. Variation of the para substituent on the phenyl
group led to smooth reactions with high levels of enantiose-
lectivity. Notably, the benzyl 1-(4-chlorophenyl)vinylcarba-
mate 5 f offered the highest enantioselectivity (Table 2,
entry 13).
To gain insight into the origin of the enantioselectivity
observed in the substitution reaction, we performed theoret-
ical calculations on the transition state of the 6d-catalyzed
substitution of 3-hydroxyoxindole 4a with enecarbamate 5a
by the density-functional theory (DFT) method.[18] The fully
optimized structures of the vinylogous iminium intermediate
generated from the dehydration[15,19] suggested that the
cis isomer was more stable than the trans isomer by approx-
imately 1 kcalmolÀ1.[20] Consequently, enecarbamate 5a and
the cis-vinylogous iminium species were employed in the next
set of calculations. As a result, two transition structures, TS-1
and TS-2, were identified (Figure 2), thus illustrating that the
Figure 2. Located transition-state structures with distance parameters
in angstroms and relative energies in enthalpy and free energy,
respectively in kcalmolÀ1
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Scheme 2. Total synthesis of (+)-folicanthine. Reaction conditions: a) nBu4NHSO4, KOH, MeI, RT
(258C); b) NH2OH·HCl, pyridine, EtOH, RT, 8a/8b=1:2.1. (90% ee, >99% ee after single
recrystallization); c) PTSA, EtOH, RT; d) HgCl2, MeCN, 808C; e) tBuOK, MeI, tetrahydrofuran
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Received: October 6, 2011
Published online: December 7, 2011
Keywords: alkaloids · asymmetric catalysis · Brønsted acids ·
.
organocatalysis · total synthesis
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Angew. Chem. Int. Ed. 2012, 51, 1046 –1050