Diastereoselective Intramolecular Aldol-Type Trapping of Zwitterionic Intermediates by Ketones for the Synthesis of Spiro[chroman-4,3′-oxindole] Derivatives

Article abstract of DOI:10.1002/adsc.201600998

A simple, mild and efficient rhodium-catalyzed aromatic C–H functionalization of α-phenoxy ketones by 3-diazooxindoles for the synthesis of the spiro[chroman-4,3′-oxindole] ring system is described. A series of functionalized spiro[chroman-4,3′-oxindole] derivatives bearing two adjacent quaternary carbon centers have been attained in a highly diastereoselective manner with very good yields. Control experiments suggested the possibility of an intramolecular aldol-type trapping of zwitterionic intermediates after the C–H functionalization. Furthermore, the applicability of this method has been tested by a gram-scale synthesis. A facile treatment with base enabled the access to both syn- and anti-diastereomers via isomerization. (Figure presented.).

Full text of DOI:10.1002/adsc.201600998

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DOI: 10.1002/adsc.201600998
Very Important Publication
Diastereoselective Intramolecular Aldol-Type Trapping of
Zwitterionic Intermediates by Ketones for the Synthesis of
Spiro[chroman-4,3’-oxindole] Derivatives
Shikun Jia,a Yubing Lei,a Longlong Song,a A. Gopi Krishna Reddy,a Dong Xing,a
and Wenhao Hua,*
a
Shanghai Engineering Research Center of Molecular Therapeutics and New Drug Development, School of Chemistry and
Chemical Engineering, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, Peopleꢀs Republic
of China
E-mail: whu@chem.ecnu.edu.cn
Received: September 9, 2016; Revised: October 24, 2016; Published online: && &&, 0000
Abstract: A simple, mild and efficient rhodium-cat-
alyzed aromatic C–H functionalization of a-phen-
oxy ketones by 3-diazooxindoles for the synthesis of
the spiro[chroman-4,3’-oxindole] ring system is de-
scribed. A series of functionalized spiro[chroman-
4,3’-oxindole] derivatives bearing two adjacent qua-
ternary carbon centers have been attained in
a highly diastereoselective manner with very good
yields. Control experiments suggested the possibility
of an intramolecular aldol-type trapping of zwitter-
ionic intermediates after the C–H functionalization.
Furthermore, the applicability of this method has
been tested by a gram-scale synthesis. A facile
treatment with base enabled the access to both syn-
and anti-diastereomers via isomerization.
reactions (MCRs).[6] Nevertheless, a thorough litera-
ture survey indicated that there were limited develop-
ments in order to access the fused chroman-3-ol
moiety at the C-3 position, although chroman-3-ol is
also an important skeleton in natural/unnatural com-
pounds of biological interest (Figure 1b).[7] It is worth
mentioning that the two adjacent quaternary stereo-
centers present in the oxindole fused chroman-3-ol
moiety at the C-3 position make this moiety difficult
to construct. A few methods comprising a tandem
Friedel–Crafts-type reaction of 2-naphthol[8a] or cas-
Keywords: aromatic C–H functionalization; rhodi-
um; spiro[chroman-4,3’-oxindole] derivatives; trap-
ping; zwitterionic intermediates
Heterocyclic spirooxindoles architectures have drawn
the attention of many medicinal chemists in organic
synthesis due to their existence as privileged scaffolds
of important pharmacophores in pharmaceuticals and
also as the core structure of alkaloid natural prod-
ucts.[1] Among them the oxindole structure with a tet-
rahydropyran ring junction at the C-3 position (Fig-
ure 1a) is one of the synthetic targets.[2] Recently,
some efficient strategies have been successfully devel-
oped towards the synthesis of these spirooxindole an-
alogues including [Pd]-catalyzed carbosilylation fol-
lowed by cyclization of N-[2-(1,3-carbamoylbuten-
ACHTUNGTRENNUNG
yl)aryl] chlorides,[3] formal [5+1] annulation of oxin-
doles and ester-linked bisenones,[4] the Prins cascade
reaction of 4-hydroxy-N-methyl-2-methylene-N-phe-
Figure 1. Spiro[tetrahydropyran-4,3’-oxindole] and chroman-
nylbutanamide and aldehydes,[5] and multi-component 3-ol skeletons in natural products and drugs.
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cade Michael-addition of 8-hydroxyquinoline[8b] have well as zwitterionic intermediates[11] (generated from
been targeted to accomplish the key chroman ring in indoles, pyrroles, N-substituded anilines etc.,) could
spiro[chroman-4,3’-oxindole] (Scheme 1a). However, be trapped by electrophiles such as aldehydes, ke-
the development of a straightforward and sensible tones, imines, or a,b-unsaturated carbonyl com-
method to access such skeletons from simple starting pounds. Among them, we have achieved the asym-
materials is highly desirable for drug development.
metric functionalization of aromatic C–H bonds
In this context multi-component reactions have through a three-component reaction of N,N-disubsti-
become powerful tools in recent years to achieve tuted anilines with diazo compounds and imines in
complex molecules from corresponding simple units. the presence of Rh(II)/chiral phosphoric acid cocata-
Our research group has developed some important lysts (Scheme 1b).[12] Recently, Moody[13] and our
multi-component reactions through a metal carbene- group[14] reported an intramolecular aldol-type trap-
induced active intermediate trapping process in which ping of oxonium/ammonium ylides for the synthesis
oxonium ylides[9] (generated from water or alcohols) of tetrahydrofurans/pyrrolidines from diazo com-
or ammonium ylides[10] (generated from amines) as pounds and b-hydroxy ketones/b-amino ketones
(Scheme 1c). Similarly a gold-catalyzed sequential C–
H functionalization and aldol annulation strategy was
developed by Zhang[15] for the synthesis of indanol
derivatives. However, in application to this method,
only one example of a spirooxindole has been ob-
tained, albeit possessing low diastereoselectivity
(Scheme 1d). With our ongoing interest in the con-
struction of polyfunctional heterocycles on the basis
of these trapping tactics and aromatic C–H function-
alizations, herein we have designed an intramolecular
aldol-type trapping (not a sequential) of metal car-
bene-induced zwitterionic intermediates which could
be generated in-situ from 3-diazooxindole and a-
phenoxy ketones, to establish the spirooxindole fused
chroman-3-ol moiety (Scheme 1e). This strategy
shows the dual role of a-phenoxy ketones as both
zwitterionic intermediate precursor and also as the in-
tramolecular trapping reagent. It has been observed
that the results with N,N-dibenzylaniline are better
than those with other N-substituted aniline substrates
in an intermolecular zwitterionic intermediate trap-
ping process.[12] Hence, we decided to introduce di-
benzylamino substitution on the phenyl ring of the a-
phenoxy ketone to enrich the electronic density of the
aromatic C–H bond.
Thus the synthetic strategy was initiated with the
reaction of 2-[3-(dibenzylamino)phenoxy]-1-phenyl-
AHCTUNGTREGeNNUN thanone 1a and 1-benzyl-3-diazoindolin-2-one 2a in
the presence of [Rh2(OAc)4] catalyst and 4ꢂ molecu-
lar sieves at 08C in dichloromethane. To our delight,
the desired product 3a was obtained in 52% yield and
with 90:10 diastereoselectivity (Table 1, entry 1). Both
the yield and diastereoselectivity of 3a were improved
with an increase in temperature, but further increases
in temperature reduced the yield (Table 1, entries 2
and 3). Various solvents were also screened in order
to further improve the yield of 3a. The reactions in
toluene, xylene and CHCl3 showed decrements in the
yield of 3a (Table 1, entries 4, 5 and 7). However, the
desired product 3a was obtained in 90% yield and
95:5 diastereoselectivity when the reaction was per-
formed in 1, 2-dichloroethane (DCE) at 258C in the
presence of 4ꢂ molecular sieves (Table 1, entry 6).
Scheme 1. Strategies for the synthesis of spiro[chroman-4,3’-
oxindole] derivatives.
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Table 1. Optimization of the reaction conditions.[a]
result of partial isomerization of syn-3h under the
standard reaction conditions. Crucially, methyl- and
ethyl-substitution were also compatible with the stan-
dard conditions (Table 2, 3i, 3j). The more bulky
butyl-substitution afforded the corresponding spiro-
AHCTUNGTREG[NNNU chroman-4,3’-oxindole] in good yield with relatively
low diastereoselectivity (Table 2, 3k). In addition to
other spectroscopic studies, the relative configuration
of the desired product was unambiguously determined
by X-ray analysis of 3b.[16]
Furthermore, the scope with respect to the 3-diazo-
oxindoles 2 was further investigated. Substituents on
the C-5 and C-6 positions of 3-diazooxindoles 2 gave
the desired products 3l–q in good yields with high dia-
stereoselectivities (Table 3, entries 1–6). Replacing
the N-benzyl substituent of 3-diazooxindole by N-Cbz
and N-Me slightly reduced the yield and diastereose-
lectivity (3r vs. 3a, 3s vs. 3a, 3t vs. 3l).
To gain some insight into the reaction pathway of
this intramolecular aldol-type trapping process, a con-
trol experiment was conducted to verify whether the
current reaction proceeds via a stepwise or a concert-
ed reaction pathway.[17] Thus the C–H insertion prod-
uct 4a was isolated and then treated under the identi-
cal reaction conditions of the current reaction, but no
3a was observed. In this manner the control experi-
ment excluded the possibility of a stepwise C–H inser-
Entry Solvent Temp. [oC] Yield [%][b] dr (syn:anti)[c]
1
2
3
4
5
6
7
DCM
DCM
DCM
PhCH3
Xylene 25
DCE
0
52
89
81
77
57
90
66
90:10
94:6
94:6
94:6
93:7
95:5
83:17
25
40
25
25
25
CHCl3
[a]
Reactions were conducted by adding 2a (0.11 mmol) in
1 mL of the solvent to the mixture of 1a (0.1 mmol) and
catalyst in 1 mL of the solvent via syringe pump over 1 h.
Isolated yield after column chromatography.
[b]
[c]
1
Determined by H NMR of crude mixture.
With the optimized reaction conditions in hand, the tion/aldol-type addition pathway. To demonstrate the
substrate scope of this transformation for the synthe- synthetic efficiency of this process, a gram-scale reac-
sis of spiro[chroman-4,3’-oxindole] derivatives 3 was tion was carried out by using 2 mmol of 1a and
examined. The current strategy showed very good 2.2 mmol 2a. To our delight the reaction proceeded
substrate scope tolerating different aryl (from elec- smoothly under standard conditions to give the de-
tron-donating to electron-withdrawing and heterocy- sired product 3a in 86% yield (1.09 g) with 95:5 dia-
clic) substituted a-phenoxyarylethanones 1 and result- stereoselectivity (Scheme 2). Interestingly, on treat-
ed in the desired products 3 in very good yields with ment with an equivalent of K2CO3 in toluene and
excellent diastereoselectivities. For example, excellent MeOH at room temperature, syn-3a isomerized to its
yield and diastereoselectivity were obtained in the diastereomer anti-3a totally in 95% yield. The isomer-
case of electron-donating substituents on the aryl ization of syn-3a might be possibly due to an intramo-
rings (Table 2, 3b, 3c, 3d). A heterocyclic substituted lecular retro-aldol reaction via transition state I to
substrate gave the corresponding product 3e in 92% give the thermodynamically stable anti-3a (Scheme 3).
yield along with >95:5 diastereoselectivity. Naphthyl- On the other hand, similar K2CO3 treatment on inser-
substituted substrates also resulted in good yields tion product 4a did not result anti-3a, which revealed
(Scheme 2, 3f). Meanwhile, the 2,4-dichlorophenyl that the anti-3a was not produced from the insertion
substrate was also tolerated well, affording 79% yield
and >95:5 diastereoselectivity (Table 2, 3g). Notably,
4-CF3-phenyl-substitution gave 72% yield and 24:76
diastereoselectivity (Table 2, 3h). For this substrate,
anti-3h was the major product, which perhaps was the
Scheme 2. Gram-scale reaction.
Scheme 3. Diastereomer switching experiment.
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Table 2. Scope of the a-phenoxy ketones.[a],[b],[c]
[a]
Reactions were conducted by adding 2a (0.11 mmol) in 1 mL of the solvent to the mixture of 1a (0.1 mmol) and catalyst
in 1 mL of the solvent via syringe pump over 1 h.
[b]
[c]
Isolated yield after column chromatography.
Determined by 1H NMR of crude mixture.[a] Unless otherwise noted, all reactions were conducted by adding 2a
(0.33 mmol) and 1 (0.3 mmol) under standard conditions via syringe pump over 1 h, and the reaction mixture was stirred
for another 10 min.
[b]
[c]
Isolated yield after column chromatography.
1
The dr was determined by H NMR of the crude mixture.
product 4a.[17] Furthermore, this syn to anti isomeriza-
The observed stereoselective control of the reaction
tion broadens the scope of the current strategy as we could be explained by comparing the transition states
could accomplish both the diastereomers conveniently I and II. The interaction between the rhodium species
in a highly diastereoselctive manner.
and the carbonyl of 1a facilitated the favored TS I
A plausible reaction pathway is proposed in compared to TS II which was disfavored.
Scheme 4 based on the control experiments. Firstly, 3-
In summary, we have developed an efficient
diazooxindole 2a formed metal carbene A in the pres- method for the construction of spiro[chroman-4,3’-ox-
ence of Rh2(OAc)4. Then active metal carbene-in- indole] derivatives in very good yields with excellent
duced zwitterionic intermediate B-1[18] or its enolate diastereoselectivities through aromatic C–H function-
form B-2 was generated by the interaction between alization. Notably, a-phenoxyalkyl ketone substrates
1a and metal carbene A. A traditional 1,2-H shift also underwent the reaction effectively and resulted
gave the C–H insertion product 4a, while a novel in- in corresponding spiro systems in very good yields.
tramolecular electrophilic trapping went ahead to This transformation offers a straightforward way for
afford the intermediate C. Finally, the desired product the construction of both the diastereomers of the spi-
3a was obtained by a delayed H-transfer and aromati- ro[chroman-4,3’-oxindole] skeleton and also enriches
zation of the aromatic ring.
our trapping strategy in the functionalization of aro-
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Scheme 4. Plausible reaction mechanism.
Table 3. Scope of 3-diazooxindoles[a]
matic C–H bonds. The asymmetric version based on
a Brønsted acid catalyst to activate the carbonyl
group is currently investigated in our laboratory.
Experimental Section
Typical Procedure for Intramolecular Aldol-Type
Trapping of Zwitterionic Intermediates
A mixture of Rh2(OAc)4 (0.003 mmol), 1 (0.3 mmol),and
4ꢂ MS (100 mg) in 1 mL of DCE under an argon atmos-
phere was stirred at room temperature. Diazo compound 2
(0.33 mmol) in 1 mL or 2 mL of DCE was then added over
1 h via a syringe pump. After completion of the addition,
the reaction mixture was stirred for another 0.5 h, then fil-
tered and evaporated under vacuum to give the crude prod-
uct. The crude product was purified by flash chromatogra-
phy on silica gel (EtOAc/light petroleum ether=1:10–1:5)
to give the pure product.
Entry
3
R1
R2
Yield [%][b]
dr (syn:anti)[c]
1
2
3
4
5
6
7
8
9
3l
Bn
Bn
Bn
Bn
Bn
Bn
Cbz
Me
Me
5-Me
5-F
87
83
93
86
88
85
76
82
79
95:5
3m
3n
3o
3p
3q
3r
>95:5
>95:5
94:6
>95:5
93:7
93:7
94:6
91:9
5-Cl
5-Br
6-Cl
6-Br
H
3s
3t
H
5-Br
[a]
Reactions were conducted by adding 2 (0.33 mmol) in
1 mL of the solvent to the mixture of 1a (0.3 mmol) and
catalyst in 1 mL of the solvent via syringe pump over 1 h.
Isolated yield after column chromatography.
Acknowledgements
[b]
[c]
We are grateful for financial support from NSFC of China
(21332003 and 21402051).
1
Determined by H NMR of crude mixture.
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Diastereoselective Intramolecular Aldol-Type Trapping of
Zwitterionic Intermediates by Ketones for the Synthesis of
Spiro[chroman-4,3’-oxindole] Derivatives
7
Adv. Synth. Catal. 2016, 358, 1 – 7
Shikun Jia, Yubing Lei, Longlong Song,
A. Gopi Krishna Reddy, Dong Xing, Wenhao Hu*
Adv. Synth. Catal. 0000, 000, 0 – 0
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These are not the final page numbers!