Catalytic regio- And stereoselective intermolecular [5+2] cycloaddition: Via conjugative activation of oxidopyrylium

Article abstract of DOI:10.1039/d0cc04309e

A catalytic stereodivergent intermolecular [5+2] cycloaddition of maltol-type oxidopyrylium through conjugative activation was reported, which featured high stereoselectivity, good compatibility and mild conditions, providing a convenient access route to various seven-membered heterocycles in moderate to excellent yields. In addition, a discrete mechanism was proposed to illustrate the stereoselectivity. This journal is

Full text of DOI:10.1039/d0cc04309e

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Catalytic Regio- and Stereoselective Intermolecular [5+2]
Cycloaddition via Conjugative Activation of Oxidopyrylium
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
Ling Zhang , Qiu Shi , Tongxiang Cao* and Shifa Zhu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
1e, 2
A catalytic stereodivergent intermolecular [5+2] cycloaddition of contribution;
(2) Despite the alkylating activation strategy
maltol-type oxidopyrylium through conjugative activation was could decrease the reaction temperature dramatically, which
reported, which featured high stereoselectivity, good was introduced by Wender in their landmark total syntheses of
8
compatibility and mild conditions, providing a convenient access phorbol, it typically needed super-stoichiometric alkylating
9
10
to various seven-membered heterocycles in moderate to excellent reagents, excess bases and dipolarphiles ; what’s more, a
yields. In addition, a discrete mechanism was proposed to bad selectivity was furnished, especially for the intermolecular
1e, 2
illustrate the stereoselectivity.
version.
Collectively, a new strategy which could lower the
enthalpy barrier to ensure a stereoselective intermolecular
cycloaddition of maltol-type oxidopyrylium was highly
desirable.
Cycloaddition reactions can effectively build different sized1
carbon- or heterocyclic rings with high stereochemical fidelity,
among which the dipolar [5+2] cycloaddition of oxidopyrylium
is a powerful method to construct stereochemically dense
bridged seven-membered bicycles, which are difficult available
otherwise. Due to the structure variation of the precursors,
different activation strategies for the oxidopyrylium species
generation were developed (Scheme 1a). For acetoxypyranone
As our continuous interesting in developing novel cascade
11
reaction based on maltol-type pyrones, we are intend to
exploiting new tactic for oxidopyrylium generation and
activation to meeting aforementioned challenges (Scheme 1a).
Comparing the substrates scope of specie I-a and I-b mediated
2
[
5+2] cycloadditions, it was found that the latter preferred
12,13
(A), the most successful method was thermal-driven ester
electron-rich dipolarphiles,
which might arise from the low-
group elimination, which was extensively exploited in the past
laying LUMO of the extended π-system (Scheme 1b). Inspired
by such observations, we postulated if the oxidopyrylium II-a
was conjugated with an in situ formed arene, such as furan in
oxidopyrylium II-b, the reaction barrier might decrease, and
eventually resulted in better selectivity and compatibility.
2
decades. Recently, an organocatalyst promoted asymmetric
3
variant of this reaction was presented by Jacobsen. In
addition,
a palladium-catalyzed efficient generation of
oxidopyrylium intermediate through a π-allyl palladium species
4
was reported. Moreover, a stereoselective hetero-[5+2]
cycloaddition between oxidopyrylium ylide and imine was also
5
developed. Notably, several highlighting profiles about its
2, 6
utility in total synthesis, such as type-II [5+2] cycloaddition
7
enabled concise synthesis of vinigrol and cyclocitrinol, further
demonstrated the power of this unique reaction. In contrast,
methods based on maltol-type pyranone precursor (B) haven’t
experienced much development presumably due to the
following issues: (1) the group transfer strategy often needed
high temperature or intramolecular precursors to compensate
the high enthalpy barrier by cutting unfavorable entropy
a
Key Lab of Functional Molecular Engineering of Guangdong Province, School of
Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou 510640, P. R. China, E-mail: caotx@scut.edu.cn, zhusf@scut.edu.cn.
‡
Electronic Supplementary Information (ESI) available: [experimental details and
1 13
characterization of all compounds, copies of H and C NMR spectra for selected
compounds, CCDC: 1984385, 1984647, 1997501.] See DOI: 10.1039/x0xx00000x
Scheme 1 Oxidopyrylium-based [5+2] cycloaddition.
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Theoretically, this conjugative activation strategy has the exo configuration of 3aa was unambiguously confirmed by X-
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DOI: 10.1039/D0CC04309E
following benefits: elongating the lifetime of actived ray diffraction analysis.
oxidopyrylium and speeding up the reaction by partially
transfering the loosed resonance energy into a newly formed hand, the substrate scope was then explored. As shown in
arene. Herein we report catalytic stereodivergent Scheme 2, various indoles were tested firstly, which
intermolecular [5+2] cycloaddition reaction via an unprecedent underwent the [5+2] cycloaddition reaction smoothly to give
conjugative activation strategy. 3aa 3aj in moderate to good yield (56-88%) and selectivity
With the optimized reaction conditions (Table 1, entry 10) in
a
-
We started the investigation by using maltol derivative 1a (5.5:1 to 10:1). Besides, the electronic nature of substituents
and N-methyl indole for systematically conditions optimization. didn’t exert a dramatic impact on the reaction outcomes. For
Initially, the privileged catalyst In(OTf) in our previous work example, the 5-chloro (3ad) and 5-methoxyl (3ag) products
3
11a
was chosen, which gave the desired [5+2] product 3aa in were obtained in almost identical yields (74% vs 76%) and
8% yield and 1.4:1 exo/endo selectivity (Table 1, entry 1). In selectivity (6.7:1 vs 6.6:1). Varying the functional groups on the
addition, other Lewis acids, such as Cu(OTf) , ZnI and PtCl , aryl of alkyne had little influence on reaction results as well,
3
2
2
2
didn’t furnish satisfactory results (entries 2-4). Then, the giving the corresponding products 3ca-3ga in 68-75% yields
combinations of Ph PAuCl with different silver salts were and 5:1 to 8.3:1 diastereoselectivities. The naphthyl
3
tested as catalysts (entries 5-7). To our delight, when 10 substituted product 3ha was obtained in 49% yield and a
mmol% Ph PAuCl and AgBF were used, both the yield and dismal diastereomeric ratio. This strategy was also applicable
3
4
stereoselectivity have a great improvement, giving 3aa in 86% to thiophene-containing substrate, and furnished 3ia in 72%
yield and 10:1 exo/endo ratio (entry 5). Significant decrease in yield and 5.6:1 dr. The practicability of alkyl-capped substrates
terms of yield and selectivity was observed when lower was discussed as well, and the desired products 3ja-3ma were
catalyst loading was applied (entries 8-9). Further investigation obtained in moderate yield and selectivity. Interestingly, the
on solvents revealed that acetonitrile was a superior one, in products 3ak and 3dk from N-benzyl indole were obtained in
which 3aa was isolated in 81% yield and 10:1 exo/endo ratio good yields and better diastereoselectivities comparing with
with 5 mmol% catalyst (entry 10). When Ph PAuCl was the N-methyl indole. The improved selectivity indicated that
3
replaced by IMesAuCl, the reaction occurred smoothly in DCM, bulkier benzyl on nitrogen might have beneficial effect in
giving 3aa in moderate yield and selectivity (entry 11); while in stereoselectivity, which supported the proposed sterically
acetonitrile, the reaction was almost retarded and elevated demanding model (see Scheme 4b). The N-allyl indole worked
temperature was necessary to restore the catalytic capacity smoothly to give 3al and 3fl in promising yield and selectivity.
(entry 12). Control experiments showed that the adventitious
water in solvent was critical which might participate in
nucleophilic desilylation and AgBF alone could promote this
4
transformation but gave an inferior result (entries 13-14). The
a
Table 1 Optimization of reaction conditions
Catalyst
[%])
Additive
([%])
Conv. Yield 3aa [%]
1a [%] (exo:endo)
Entry
Solvent
b
(
1
2
3
4
5
6
7
8
In(OTf) (10)
----
----
----
----
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
100
100
100
100
100
60
38 (1.4:1)
32 (1:1)
41 (1:1)
39 (5:1)
86 (10:1)
32 (2.5:1).
28 (2.5:1).
24 (5:1)
54 (5:1)
81 (10:1)
60 (5.6:1)
46 (1.5:1)
n.r.
3
Cu(OTf) (10)
2
ZnI (10)
2
PtCl (10)
2
Ph PAuCl (10) AgBF (10)
3
4
Ph PAuCl (10) AgNTf (10)
3
Ph PAuCl (10) AgOTf (10)
50
42
3
Ph PAuCl (3)
AgBF (3)
3
4
9
Ph PAuCl (5)
AgBF (5)
100
100
100
100
100
100
3
4
c
1
1
1
1
0
1
2
3
4
Ph PAuCl (5)
AgBF (5) CH CN
3
4
3
IMesAuCl (5)
IMesAuCl (5)
AgBF (5)
DCE
4
AgBF (5) CH CN
4
3
d
Ph PAuCl (5)
AgBF (5) CH CN
3
4
3
1
a
----
AgBF (5)
DCE
38 (5:1)
4
Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), catalyst and additive in 2
mL solvent were stirred at 25 C for 8 h. The yield and ratio were based on H-
NMR analysis. Isolated yield. 4Å M.S. (50 mg) was added.
Scheme 2 Scope with respect to the indoles. Reaction conditions:
1 (0.1 mmol), 2
◦
b
1
(0.15 mmol), 5 mmol% Ph PAuCl and AgBF in 2 mL acetonitrile were stirred at
3
4
c
d
◦
25 C for 8 h. Isolated yield.
2
| J. Name., 2012, 00, 1-3
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Notably, the alkene was outcompeted entirely herein, which 88% as well. Interestingly, the vinyl ether, wVhieiwchArticulesOunallinlye
underwent polymerization in the pre
furnished 5ps in 87% yield. Finally, 1, 1-disubstituted alkenes
were also competent for cycloaddition, resulting the sterically
hindered vicinal quaternary center adducts (5at-5aw) in good
yield. It’s noteworthy that a large spectrum of functional
groups, such as nitro, carbonic aicd, ester, thiophene, ether
and alkyl bromide were well-tolerated and the
diastereoselectivity were greater than 10:1 in these cases.
Comparing with the results of TBS-protected substrate 1a
the hydroxyl free substrate 1r gave the product 3aa in an
inferior yield (57% vs 81%) and diastereoselectivity (2:1 vs
D
se
O
n
I:
c
1
e
0.10
o
3
f
9/
L
D
e
0
w
CCis04
a
3
cid,
0
9
E
implied the aggressive property of indole. In addition, the
unprotected indole afforded 3am in 70% yield, but in an
inferior diastereoselectivity. Finally, when other heteroarenes
were used, the catalytic system was out of function (See
Supplementary Information).
After reevaluation of the reaction parameters, it was found
that the ZnI was an optimal catalyst for alkene-participated
2
[
1
5+2] cycloaddition reactions (Scheme 3). The maltol derived
and 1b furnished the adduct 5aa and 5ba in 99% and 86%
,
a
yield, respectively. Interestingly, the stereochemistry was
entirely reversed by favoring the endo configuration (exo for
cycloaddition of indole). The yield and stereoselectivity were
insensitive to the capping groups on the alkyne terminal,
afforded cycloadduts in high yields (72-89%). Besides,
substrate with terminal alkyne was also well tolerated,
producing 5ma in 75% yield. Tuning electronic substitution
1
0:1), which implied that the TBS group should preserve during
the cycloaddition course to ensure good stereoselectivity (See
control experiments in Supplementary Information, page 25).
The O-methylated counterpart 1s couldn’t furnish 3aa, which
indicated that the leaving of protecting group might be
synchronous with the cycloaddition process, or at least not
later than the latter, because once a post-cycloaddition
oxonium species was formed, it would funnel to 3aa inevitably.
It’s not surprising that the Ts-protected indole couldn’t
participate in this reaction due to the negative inductive effect
of Ts, which makes the indole less nucleophilic. The C2 and C3-
substitued indoles only gave trace mount of the desired
products, which indicated that the reaction might not a FMO-
governing concerted process which was usually insensitive to
ordinary steric perturbation. The electron-deficient maleimide,
an competent oxidopyrylium dipolarphile, was unable to react
with the furan-conjugated oxidopyrylium, which was in line
with our initial speculation that HOMO-rised dipolarphiles
were preferred.
pattern on the styrene (5ac-5an) resulted in a large variation
of yields (66-98%). The results clearly show that the electron-
rich styrenes worked better than the electron-deficient ones.
The endo configuration was established by the X-ray structure
analysis of 5ad. Notably, the internal alkenes were found to be
effective dipolarphile, affording the cycloaddition products in
4
6% (5ao, dr = 6.7:1) and 86% (5ap+5ap’) respectively, but bad
in stereoselectivity, especially for 5ap and 5ap’ (dr = 1:1). The
relative configuration of these diastereoisomers was
distinguished by X-ray analysis of 5ap’. To our delight, the
unactivated alkenes provided the products 5aq and 5ar in 56%,
Based on the above investigations, a tentative working
mechanism was proposed (Scheme 4a). Initially, the catalyst
coordinated with 1a to form complex
endo-dig cyclization to give . The oxonium
counterpart was then attacked by dipolarphiles via a
transition state, which eventually collapsed to furnish the [5+2]
cycloaddition product or with the assistant of external
A
, which underwent 5-
or its resonance
-like
B
B
C
D
3
5
nucleophiles (water or anion) and regenerated the catalyst to
close the catalytic cycle.
The observed stereoselectivity could be rationalized from
the putative models (Scheme 4b). Considering the mild and
14
neutral conditions, the protodeauration is not so fast, thus it
is reasonable to postulate that the vinyl-Au-L species is
survived in cycloaddition event. Consequently, an opening
groove is formed wherein two bulky groups (TBS, Au-L)
flanking two sides of the planar oxidopyrylium core, which will
block the attack of indole from the “in-groove” trajectory and
therefore give the exo-adducts dominantly (Scheme 4b). It’s
noteworthy that the reverse selectivity observed by Li in their
oxidopyrydylium ylide mediated [5+2] cycloaddition somehow
15
supports this sterically demanding model. In addition, indoles
are more nucleophilic than alkenes, so they are eager to
nucleophilically attack
B or C at C5 position; meanwhile, the
built-up positive charge could be well-stabilized by the
adjacent nitrogen, which together makes the exo-cycloaddition
kinetically favorable to give exo-adducts via a stepwise-like
Scheme 3 Scope with respect to the alkenes. Reaction conditions:
1 (0.1 mmol),
◦
4
(0.15 mmol) and 20 mmol% ZnI in 2 mL DCE were stirred at 25 C for 12 h.
2
a
Isolated yield. Most of values of dr are larger than 10:1, unless otherwise stated. process. This scenario is reminiscent of the [5+2] cycloaddition
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5
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(
1
Comparatively, the less sterically hindered and less
nucleophilic alkenes prefer a concerted process under the ZnI
7
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16
catalysis, where the phenyl group of styrene posed away
from the methyl group of oxidopyrydylium to obviate the
steric clash (Scheme 4b). What’s more, the pocket-like
microenvironment of the oxidopyrydylium might promote the
endo-selectivity by minimizing activation entropy after alkenes
fitting into the opening groove. This selectivity is congruent
with the precedent observations in the concerted [5+2]
2
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Chem., 1991, 56, 6267.
9
1
A unique base-catalyzed [5+2] cycloaddition See: Y. Toda, M.
17
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2
377.
In summary, we have established a catalytic regio- and
stereoselective intermolecular [5+2] cycloaddition via
0 M. P. Derasmo, C. Meck, C. A. Lewis and R. P. Murelli, J. Org.
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high stereoselectivity, good compatibility and mild conditions,
providing a convenient access to various seven-membered
heterocycles with diverse functional groups in moderate to
excellent yields. In addition, a discrete mechanism was
proposed to illustrate the stereoselectivity. Specifically, under
the gold catalysis, the indoles favored a “out-groove”
stepwise-like mechanism; under zinc catalysis, however, the
alkenes were inclined to an “in-groove” concerted process.
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Conflicts of interest
1
There are no conflicts to declare.
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Chem. Int. Ed., 2014, 53, 11051.
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Acknowledgements
4
We appreciate financial support from Ministry of Science and
Technology of the People’s Republic of China
in Transition-Metal Mediated Heterocyclic Synthesis, eds. D.
Solé and I. B. T.-A. in T.-M. M. H. S. Fernández, Elsevier, 2018,
pp. 285–310.
(
2016YFA0602900), the NSFC (21871096, 21672071),
Guangdong Science and Technology Department
2018B030308007, 2018A030310359, 2017B090903003),
China Postdoctoral Science Foundation (2018M643062,
019T120723).
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A catalytic stereodivergent intermolecular [5+2] cycloaddition of maltol-type oxidopyrylium
through conjugative activation was reported.