Catalytic Enantioselective Decarboxylative Allylations of a Mixture of Allyl Carbonates and Allyl Esters: Total Synthesis of (-)- and (+)-Folicanthine

Article abstract of DOI:10.1002/chem.201502878

A highly enantioselective decarboxylative allylation of a mixture of enol carbonates and allyl esters has been achieved. The strategic viability of this methodology has been demonstrated through the total synthesis of cyclotryptamine alkaloids (-)- and (+)-folicanthine (1 a) and the formal total synthesis of (-)-chimonanthine (1 b), (+)-calycanthine (1 c), and (-)-ditryptophenaline (1 d).

Full text of DOI:10.1002/chem.201502878

DOI: 10.1002/chem.201502878
Full Paper
&
Quaternary Stereocenters
Catalytic Enantioselective Decarboxylative Allylations of a Mixture
of Allyl Carbonates and Allyl Esters: Total Synthesis of (À)- and
(+)-Folicanthine
Santanu Ghosh, Saikat Chaudhuri, and Alakesh Bisai*[a]
This work is dedicated to Professor Richmond Sarpong
Abstract: A highly enantioselective decarboxylative allylation
of a mixture of enol carbonates and allyl esters has been
achieved. The strategic viability of this methodology has
been demonstrated through the total synthesis of cyclo-
tryptamine alkaloids (À)- and (+)-folicanthine (1a) and the
formal total synthesis of (À)-chimonanthine (1b), (+)-caly-
canthine (1c), and (À)-ditryptophenaline (1d).
Introduction
Towards this, elegant approaches include: Overman’s first
asymmetric synthesis of alkaloids 1b and 1c (Figure 1) by
either a highly diastereoselective dialkylation or double Heck
cyclizations to establish the quaternary stereocentres,[6] Movas-
saghi’s biosynthetic approach by CoI-promoted reductive ho-
modimerization[7,8] and heterodimerization[9] reactions to con-
struct various hexahydropyrroloindole alkaloids, a similar re-
ductive homodimerization, which is catalyzed by NiII com-
plexes,[10] and coupling reactions via 3a-phenylselenylpyrroloin-
doline.[11]
Cyclotryptamine alkaloids (Figure 1)[1]that possess a bis-pyrroli-
dino-[2,3b]indoline moiety with a labile 3aÀ3a’ s-bond consti-
tute a large family of architecturally interesting alkaloids with
a wide range of biological activities,[2] which makes their effi-
cient total synthesis of notable importance. Recently, consider-
Prominent catalytic, enantioselective approaches include:
the phosphoric-acid-catalyzed reactions of 3-hydroxy-2-oxin-
dole with an enecarbamate by Gong,12a the decarboxylative al-
kylations by Ma,[12b] the Michael addition of indole onto isatyli-
dene-3-acetaldehyde developed by Liu and Zhang,[13] a Michael
reaction of N-Boc-protected bisoxindole with nitroethylene by
Kanai and Matsunaga,[14] and the sequential Pd-catalyzed two-
fold decarboxylative allylations by Trost[15] and from our
group[16] to afford C2-symmetric bis-2-oxindole of type 3
(Scheme 1).
Figure 1. Selected bispyrrolidino-[2,3b]indoline alkaloids.
able effort has been devoted towards the development of oxi-
dative dimerization methods for the construction of the core
structure of these alkaloids.[3] As part of an alternative ap-
proach, the challenge of installing sterically congested vicinal
all-carbon quaternary stereocenters[4] is of great synthetic inter-
est among chemists; to achieve this in an asymmetric fashion
would serve as a novel strategy in the synthesis of these alka-
loids.[5]
Scheme 1. Our retrosynthetic strategy.
[a] Dr. S. Ghosh,+ S. Chaudhuri,+ Prof. Dr. A. Bisai
Department of Chemistry
Indian Institute of Science Education and Research Bhopal
Bhopal Bypass Road, Bhauri
Bhopal - 462066, Madhya Pradesh (India)
E-mail: alakesh@iiserb.ac.in
Results and Discussion
We envisioned that a variety of cyclotryptamine alkaloids could
easily be accessed by synthetic manipulation of a common
enantioenriched intermediate 4. In turn, this can be obtained
through a Pd-catalyzed dynamic kinetic asymmetric transfor-
[+] Both authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502878.
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mation (DYKAT)[17,18] of rac-allyl ester (Æ)-6[19] and allyl carbon-
ate 5, either separately or as a mixture in any ratio. Herein, we
report the use of the aforementioned strategy in the total syn-
thesis of both enantiomers of folicanthine (1a).
Table 1. Optimization of the DYKAT of a 1:1 mixture of allyl carbonate 5
and allyl ester 6.
However, as compounds (Æ)-6 and 5 have different reaction
rates for their Pd-catalyzed deallylation to form a common in-
termediate 7 (Scheme 2), the DYKAT of such a complicated
Scheme 2. DYKAT through rapid interconversion of intermediates 7a and b.
Entry[a] [Pd2(dba)3] Ligand
Solvent Temp. Time Yield ee
[mol%]
([mol%])
[8C]
[%][b] [%][c]
mixture would be challenging and worth testing. The rationale
for the sense of asymmetric induction is shown in Scheme 2.
In our strategy, the DYKAT of compound (Æ)-6 can be accom-
plished through a rapid interconversion of the two diastereo-
meric PdII intermediates 7a–b.[18] Considering that the enantio-
selectivity arises from the kinetic reaction of one intermediate,
this DYKAT requires a Curtin–Hammett condition to be estab-
lished, in which the enantioselectivity is not dependent, in
principle, upon the thermodynamic ratio of the intermediates.
In fact, the asymmetric induction arises from a selective re-
action with one intermediate (7a in the case of [(S,S)-l-Pd0])
through a selective differentiation of the enantiotopic faces of
the nucleophile. In the reaction mixture, the PdII intermediate
7 would exist as an equilibrium of the two diastereomeric PdII
intermediates 7a and b, which have different energies of acti-
vation (Scheme 2). In the presence of an enatioenriched cata-
lyst [(S,S)-l-Pd0], one would expect the allylation of PdII inter-
mediate 7a (when k1 @k2) to afford a product with (R) stereo-
chemistry ((R)-4), whereas, in case of the catalyst [(R,R)-l-Pd0],
the allylation of the PdII intermediate 7b (if k2 @k1) should
afford the enantioenriched product with (S)-stereochemistry
((S)-4).[18]
We chose the well-proved 2-phosphino-oxazoline (PHOX) li-
gands (S)-L1–L4[20] and 2-phosphino-carboxamide ligands L5–
L7[20] alongside the allyl ester (Æ)-6a[21] and allyl carbonate 5a
substrates, which both contain orthogonal protecting groups
(N-Me protection on the 2-oxindole moiety and Boc protection
on the indole nucleus) to test in our Pd-catalyzed enantioselec-
tive DYKAT procedure[22] (Table 1). We carried out optimization
of this reaction through the DYKAT of a mixture of allyl ester
(Æ)-6a and allyl carbonate 5a (1:1) in the presence of
[Pd2(dba)3] (5 mol% ) in combination with ligands L1–L7
(15 mol%) in diethyl ether at room temperature (entries 1–7).
Interestingly, in almost all the cases, DYKAT afforded the ex-
pected product 4a in excellent yield. Among various ligands
that were screened, PHOX ligands (S)-L1–L4 were found to be
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
5
5
5
5
5
5
5
5
5
5
5
5
5
5
2.5
2.5
5
2.5
2.5
1.25
1.25
5
L1 (15)
L2 (15)
L3 (15)
L4 (15)
L5 (15)
L6 (15)
L7 (15)
L7 (15)
L7 (15)
L7 (15)
L7 (15)
L7 (15)
L7 (15)
L7 (15)
L7 (7.5)
L7 (7.5)
L7 (15)
L7 (7.5)
L7 (7.5)
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
DCE
CHCl3
DME
xylene
PhMe
THF
CH2Cl2
Et2O
PhMe
Et2O
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
À25
À25
À25
À25
À25
À40
À40
À60
À60
12 h 90
5
10
11
10 h 89
16 h 91
11 h 87
10 h 89
9 h 92
8 h 93
18 h 52
36 h 30
24 h 10
11 h 86
11 h 87
14 h 78
12 h 88
9 h 94
12 h 88
9 h 92
18 h 86
9 h 93
24 h 74
24 h 87
48 h 86
54 h 65
5 d 75
5 d 46
14
40
31
84
58
50
nd[d]
76
79
61
53
82
74
91
78
90
76
85
90
90
72
81
PhMe
Et2O
L7 (3.75) PhMe
L7 (3.75) Et2O
L7 (15)
L7 (7.5)
L7 (15)
L7 (7.5)
Et2O
Et2O
Et2O
Et2O
2.5
5
2.5
[a] Reactions were carried out on 0.04 mmol of substrates in 3 mL of sol-
vent under an argon atmosphere. [b] Yield of isolated product after purifi-
cation by column chromatography. [c] ee’s were determined by chiral
HPLC analysis. [d] Not determined.
unsuitable because they afforded products in no more that
14% ee, whereas 2-phosphino-carboxamide ligands L5–L6
yielded the product 4a in up to 40% ee (entries 5 and 6). By
using the sterically hindered anthracenyl ligand L7, which is
rigidly held in a specific conformation, we were pleased to find
that product (S)-4a was obtained in 84% ee (entry 7). Solvent
screening showed that diethyl ether was the best choice over
other organic solvents (entries 7–14). Interestingly, reduction of
the catalyst loading to 2.5 mol% led to formation of the prod-
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uct 4a in 82% ee (entry 15). By decreasing the reaction tem-
perature to À258C, we found that catalyst loadings of 5 mol%
and 2.5 mol% provided the product 4a in 91 and 90% ee, re-
spectively (entries 17 and 19). In contrast, when reactions with
a catalyst loading of 2.5 mol% at room temperature and
À258C were carried out in toluene, we observed a reduced
enatioselectivity to afford the product 4a in 74 and 78% ee, re-
spectively (entries 16 and 18). Further decreasing the catalyst
loading of the reaction in diethyl ether to 1.25 mol% led to
a loss in enantioselectivity to give product 4a in 85% ee
(entry 21). We also found that a further decrease in the reac-
tion temperature to À408C and À608C led to slower reaction
rates (entries 22–25). Thus, based on our optimization studies,
we decided to proceed with an investigation into the substrate
scope of this reaction by using 2.5 mol% of [Pd2(dba)3] in com-
bination with 7.5 mol% of the anthracenyl ligand L7 in diethyl
ether at À258C.
Firstly, we checked the effect of differently protected bis-
indole scaffolds on the enantioselectivity (Scheme 3). When we
Scheme 4. Substrate scope of the DYKAT of a mixture of (Æ)-6 and 5 (1:1).
The reactions were carried out on 0.04 mmol of substrates in 3 mL of sol-
vent under an argon atmosphere. The yield is that of the isolated product
after purification by column chromatography. ee values were determined by
chiral HPLC analysis. [a] (S,S)-L7 was used as ligand.
to our delight, products 4l and m were afforded in 93 and
94% ee, respectively, and high yields (Scheme 4).
Next, as per our hypothesis (Scheme 2), the enantioselective
Pd-catalyzed decarboxylative allylation by DYKAT was also car-
ried out with allyl esters (Æ)-6 and carbonates 5, independent-
ly. We observed that in both cases the enantioselectivities of
the reactions were good to excellent (Scheme 5 and 6).
In accordance with our proposal (Scheme 2), we then inves-
tigated the effect that different ratios of the substrates (Æ)-6a
and 5a had on the enantioselectivity of the reaction (Table 2).
Interestingly, all of the possible mixtures that we tried were
equivalently good with respect to the yield and enantioselec-
tivity (entries 1–5). Gratifyingly, a gram-scale reaction of a mix-
ture of substrates (Æ)-6a and 5a also afforded (+)-4a in 89%
yield with 90% ee (entry 6).
Scheme 3. Effect of different protecting groups on the enantioselectivity.
The reactions were carried out on 0.04 mmol of substrates in 3 mL of sol-
vent under an argon atmosphere. The yield is that of the isolated product
after purification by column chromatography. ee values were determined by
chiral HPLC analysis.
used allyl ester (Æ)-6b and allyl carbonate 5b, which have
both N atoms protected with the bulky and electron-deficient
Boc group, unfortunately, we obtained product 4b in poor
enantioselectivity (47% ee). However, when the N-protecting
groups were changed to the less bulky and electron-rich
methyl or benzyl groups, such as allyl esters (Æ)-6c and d and
allyl carbonates 5c and d, the enantioselectivity was enhanced
to give the products 4c and d in 82 and 71% ee, respectively.
Thus, we proceeded to investigate the substrate scope of the
allyl esters by using an electron-donating protecting group at
the 2-oxindole or on the side of enol carbonate, such as sub-
strate (Æ)-6a, and allyl carbonates that have an electron-with-
drawing, bulky Boc group to protect the indole moiety, such
as substrate 5a (see Table 1).
Table 2. Effect of different ratios of 5a and (Æ)-6a on the enantioselec-
tivity of the DYKAT.
A variety of substrates were tested under the optimized con-
ditions to effect the Pd-catalyzed DYKAT of a mixture of sub-
strates (Æ)-6e–k and 5e–k (1:1) in the presence of [Pd2(dba)3]
(2.5 mol%) in combination with ligand L7 (7.5 mol%) in diethyl
ether at À258C to afford the products 4e–k in good to excel-
lent levels of enantioselectivity (up to 91% ee) (Scheme 4). Fur-
thermore, substrates that had different functionality at the
indole moiety, such as allyl esters (Æ)-6l and m and allyl carbo-
nates 5l and m, were tested under the optimized conditions;
Entry
5a/6a
Scale
Time
[h]
Yield
[%]
ee
[%]
1
2
3
4
5
6
50:50
100:0
0:100
25:75
75:25
50:50
100 mg
100 mg
100 mg
100 mg
100 mg
1 g
10
8
10
9
8
11
90
95
94
92
93
89
91
90
92
91
90
90
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Scheme 5. Substrate scope of DYKAT only using allyl carbonate 5. The reac-
tions were carried out on 0.04 mmol of substrates in 3 mL of solvent under
an argon atmosphere. The yield is that of the isolated product after purifica-
tion by column chromatography. ee values were determined by chiral HPLC
analysis. [a] (S,S)-L7 was used as ligand.
Scheme 6. Substrate scope of DYKAT only using (Æ)-allyl ester 6. The reac-
tions were carried out on 0.04 mmol of substrates in 3 mL of solvent under
an argon atmosphere. The yield is that of the isolated product after purifica-
tion by column chromatography. ee values were determined by chiral HPLC
analysis. [a] (S,S)-L7 was used as ligand.
We hypothesized that these reactions might proceed via
either a tight-ion-pair-type PdII-p-allyl complex that is linked
with the nucleophile, such as 7, or a separate PdII-p-allyl com-
plex and an enolate species. To test this hypothesis, we per-
formed a Pd0-catalyzed DYKAT in the presence of two different
enol carbonates 5h and o (1:1) in the presence of [Pd2(dba)3]
(2.5 mol%) in combination with (S,S)-L7 (7.5 mol%), and we
found that no scrambling occurred during the course of the
DYKAT. From this reaction, we isolated only the enantioen-
riched allylated product (+)-4h and methallylated product
(+)-4o in 93% (85% ee) and 92% (98% ee) yield, respectively
without any other side products (Scheme 7). This observation
clearly supports the probable involvement of a tight ion pair in
the DYKAT process and the intramolecular nature of the reac-
tion (Scheme 2).
Scheme 7. Decarboxylative allylation of a mixture of 5h and o (1:1).
To show the synthetic viability of our methodology, we un-
dertook the total synthesis of (À)-folicanthine (1a). Towards
this, we carried out the DYKAT of a mixture of compounds (Æ)-
6a and 5a (1:1) in the presence of Pd0-(S,S)-L7 to afford (R)-4a
in 93% yield with 91% ee (Scheme 8). In a similar manner, we
also synthesized (R)-4e in 90% yield with 86% ee. Starting
from (R)-4a and e, we synthesized the bis-2-oxindoles 8a and
b (up to ꢀ1.8:1 d.r.), respectively, in three steps: removal of
the Boc-group, N-alkylation, and reaction with DMSO in the
presence of HCl. At this stage, we anticipated that the diaste-
reoselective allylation with allyl bromide could afford the C2-
symmetric product (S,S)-9a and b in high d.r. by manipulation
of the reaction temperature (Table in Scheme 8, entries 1–5).
However, we only achieved the product (S,S)-9a in a maximum
5.7:1 d.r. by carrying out the allylation at À508C (entry 3).
Therefore, we adopted an alternative pathway for the syn-
thesis of (S,S)-9a and b, which employs a Pd0-catalyzed diaste-
reoselective decarboxylative allylation of (S)-10a and
b (Scheme 9). These enol carbonates (S)-10a and b were syn-
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>20:1 d.r. and with 99% (79% yield) and 97% ee (72% yield),
respectively (entries 4 and 5).
Following the oxidative degradation of bis-allyl groups, we
synthesized the C2-symmetric bis-aldehydes (S,S)-11 a and b in
95 and 97% yield, respectively (Scheme 10). Then, bis-aldehyde
Scheme 8. Synthesis of C2-symmetric bis-allyl compounds. [a] Reactions were
carried out on a 0.10 mmol scale of 8a and b in 1 mL of DMF. [b] Yield of
isolated product after purification. [c] ee’s were determined by chiral HPLC
1
analysis. [d] d.r.’s were determined by H NMR spectroscopy of unpurified
product. [e] Yields after recrystalization. [f] ee’s after recrystalization. [g] d.r.’s
after recrystalization.
Scheme 10. Total synthesis of (À)-folicanthine 1a.
(S,S)-11 a was reduced with NaBH4 to afford the diol intermedi-
ate (S,S)-12a, which in turn underwent a Mitsunobu reaction in
the presence of diphenylphosphoryl azide to afford the bis-
azide (S,S)-13a in 94% yield over the 2 steps. The latter was
then reduced by using triphenylphosphine in water, and sub-
sequent protection with chloromethylformate provided the
bis-Moc-protected compound (S,S)-14a in 85% yield over two
steps. Finally, compound (S,S)-14a was reduced in the pres-
ence of Red-Al in toluene at 1108C to complete the total syn-
thesis of (À)-folicanthine (1a) in 26.9% overall yield from com-
pound 4a. In fact, the synthesis of bis-aldehyde (S,S)-11 b rep-
resents the formal total synthesis of (À)-chimonanthine (1b),[6b]
(+)-calycanthine (1c),[6b] and (À)-ditryptophenaline (1d)[23]
(Figure 1).
Scheme 9. Highly diastereoselective synthesis of (S,S)-9a and b. [a] Reactions
were carried out on a 0.10 mmol scale of 10a and b in 2 mL of Et2O.
[b] Yield of isolated product after purification. [c] ee’s were determined by
For the total synthesis of (+)-folicanthine (ent-1a), we car-
ried out the Pd-catalyzed DYKAT of (Æ)-6a in the presence of
Pd0-(R,R,)-L7, which afforded (S)-4a in 94% yield with 91% ee.
1
chiral HPLC analysis. [d] d.r.’s were determined by H NMR spectroscopy of
By following
a similar sequence of reactions shown in
unpurified product. [e] Yields after single recrystalization. [f] ee’s after single
recrystalization. [g] d.r.’s were determined in these cases by chiral HPLC anal-
ysis as well. [h] d.r. after single recrystalization.
Scheme 8–10, we also completed the total synthesis of (+)-foli-
canthine (ent-1a) (see the Supporting Information).
Conclusion
thesized from bis-2-oxindoles 8a and b by the reaction of allyl
chloroformate in the presence of KHMDS at À788C. To our de-
light, a highly diastereoselective synthesis of the C2-symmetric
products can be realized in the presence of (S,S)-L7 at À258C
to afford (S,S)-9a and b in high yields with >20:1 and 11:1
d.r., respectively (entries 4 and 5). Furthermore, a single recrys-
talization of these compounds afforded (S,S)-9a and b both in
We have reported an enantioselective, Pd-catalyzed decarboxy-
lative allylation protocol, which proceeds through a DYKAT of
a mixture of allyl ester (Æ)-6 and allyl carbonate 5 (1:1) to
afford the enantioenriched, advanced intermediate (R)-4 in up
to 98% ee with excellent yields. We also achieved intermediate
(S)-4 in similar efficiencies when (R,R)-L7 was used. We applied
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c) L. E. Overman, J. F. Larrow, B. A. Stearns, J. M. Vance, Angew. Chem. Int.
Ed. 2000, 39, 213; Angew. Chem. 2000, 112, 219.
our asymmetric protocol to the enantioselective total synthesis
of (À)- and (+)-folicanthine (1a). Further application of this
strategy for the total synthesis of higher analogues of cyclo-
tryptamine alkaloids is under active investigation.
[7] a) M. Movassaghi, M. A. Schmidt, Angew. Chem. Int. Ed. 2007, 46, 3725;
Angew. Chem. 2007, 119, 3799; b) M. Movassaghi, M. A. Schmidt, J. A.
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238; d) J. Kim, M. Movassaghi, J. Am. Chem. Soc. 2010, 132, 14376.
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[11] K. M. Depew, S. P. Marsden, D. Zatorska, A. Zatorski, W. G. Bornmann,
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Acknowledgements
Financial supports from the Department of Science and Tech-
nology (SR/S1/OC-54/2011) and the Council of Scientific and In-
dustrial Research [02(0013)/11/EMR-II], Govt. of India are grate-
fully acknowledged. S.G. and S.C. thank the CSIR (SRF) and
UGC (JRF), respectively, for research fellowships.
Keywords: alkaloids
·
allylation
·
enantioselectivity
·
palladium · quaternary stereocentres
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Tang, S. Li, R. Guo, J. Nie, J.-A. Ma, Org. Lett. 2015, 17, 1389.
[13] R. Liu, J. Zhang, Org. Lett. 2013, 15, 2266.
[14] H. Mitsunuma, M. Shibasaki, M. Kanai, S. Matsunaga, Angew. Chem. Int.
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Received: July 22, 2015
Published online on October 14, 2015
Chem. Eur. J. 2015, 21, 17479 – 17484
www.chemeurj.org
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