Synthesis of Benzannulated [6,6]-Spiroketals by a One-Pot Carbonylative Sonogashira Coupling/Double Annulation Reaction

Article abstract of DOI:10.1021/acs.orglett.8b03586

A one-pot Pd-catalyzed carbonylative Sonogashira coupling in tandem with double annulation reaction to synthesize benzannulated [6,6]-spiroketals from o-iodophenols and terminal alkynols or alkynyl phenols was achieved. The protocol provides straightforwa

Full text of DOI:10.1021/acs.orglett.8b03586

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Benzannulated [6,6]-Spiroketals by a One-Pot
Carbonylative Sonogashira Coupling/Double Annulation Reaction
Kuirong Xiang, Pei Tong, Baorun Yan, Lingling Long, Chunbo Zhao, Yuan Zhang,* and Ying Li*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, P. R. China
S
* Supporting Information
ABSTRACT: A one-pot Pd-catalyzed carbonylative Sonogashira coupling in tandem with double annulation reaction to
synthesize benzannulated [6,6]-spiroketals from o-iodophenols and terminal alkynols or alkynyl phenols was achieved. The
protocol provides straightforward and facile access to benzannulated [6,6]-spiroketals in moderate to good yields and excellent
diastereoselectivities under balloon pressure of CO at room temperature.
piroketals are prevalent motifs in a diverse range of
biologically active natural products and pharmaceuticals.1
Among the family of spiroketals, the benzannulated [6,6]-
spiroketal is a key skeleton that exists in many bioactive natural
products and chiral ligands (Figure 1).2 Key examples include
efficient methods for the syntheses of benzannulated [5,6]-
spiroketals.13 In accordance with our interest in developing
synthetic tools for the synthesis of natural products containing
benzannulated [6,6]-spiroketal motifs, we wish to develop a
new and efficient method for the stereocontrolled synthesis of
this skeleton.
S
Employing CO as a cheap and abundant C1 source, the
carbonylation reaction represents an atom-economical and
environmentally benign strategy for the preparation of various
carbonyl-containing compounds and their derivatives.14 Since
first reported by Tanaka in 1981,15a palladium-catalyzed
carbonylative Sonogashira reaction of aryl halides has become
an interesting alternative strategy for the synthesis of
alkynones, and notable improvements have been achieved by
Xia15d Yang,15h Beller,15j etc. (Scheme 1A). Specifically, several
groups, including Ortar,16d Yang,16f and Capretta16e have
developed novel approaches to chromones via the Pd-catalyzed
carbonylation/cyclization reaction between o-iodophenols and
terminal alkynes (Scheme 1B).16 Inspired by these elegant
works, we envisaged that a one-pot Pd-catalyzed carbonylative
Sonogashira coupling/double annulation16i,j process would
result in the straightforward formation of monobenzannulated
[6,6]-spiroketal 5 or bisbenzannulated [6,6]-spiroketal 6 via
the chromone intermediate 4 (Scheme 1C). Herein, we report
a highly efficient route for the rapid synthesis of benzannulated
[6,6]-spiroketals starting from o-iodophenols, CO, and
alkynols or alkynyl phenols under mild conditions.
Figure 1. Representative bioactive natural products and ligand
containing benzannulated [6, 6]-spiroketal cores.
the chaetoquadrin3 and virgatolide families,4 which have
demonstrated compelling bioactivities; (S,S,S)-SKP is a type
of privileged chiral ligands with excellent performance in
catalytic enantioselective transformations.5 In view of the
critical roles of benzannulated [6,6]-spiroketal moiety, a range
of novel spirocyclization methodologies have been developed
in recent years. The preparation involves acid-catalyzed
cyclization of a dihydroxyketone precursor (or equivalent),6
intramolecular hetero-Michael addition,7 [4+2] cycloaddition,8
chromone epoxide spirocyclization,9 allylic rearrangement,10
oxidative radical cyclization,11 and iridium-catalyzed spiroke-
talization of allylic carbonates.12
Despite these advances, the development of more atom- and
step-economic methods starting from easily available substrates
under mild conditions is still highly desirable for the synthesis
of benzannulated [6,6]-spiroketals. Our group has a long-
standing interest in this field and has developed a series of
Received: November 9, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03586
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the drastic reaction conditions (for details, see Supporting
Information). These results indicated that different bases
should be added in two steps. Thanks to this awareness,
chromone 4a could be fully transformed into 5a when 10 mol
% of NaH was added to the reaction system after complete
consumption of alkynol 2a, with an 81% isolated yield (88%
NMR yield) (entry 10). The reaction was also carried out in
the absence of CuI (entry 11). The longer reaction time and
extremely low yield confirmed that CuI was essential for this
reaction as a cocatalyst. Moreover, 5a could also be generated
in 55% and 10% yields, respectively, when Mo(CO)6 and
phenyl formate were applied as the CO sources. To further
evaluate the catalyst effect on reaction outcome, several other
palladium catalysts were screened, and PdCl2(PPh3)2 proved
to be the ideal catalyst (entries 12 and 13).
Scheme 1. Palladium-Catalyzed Carbonylative Sonogashira
Coupling Reaction of Aryl halides
With the optimal conditions in hand (Table 1, entry 10), we
next examined the reaction with a broad range of substrates to
determine the scope of substrates, and the results are
summarized in Scheme 2. First, various o-iodophenols were
reacted with alkynol 2a. From the results, it is evident that all
of the reactions tolerated a variety of functional groups and
provided the corresponding spiroketals in good to excellent
yields. As discussed in Tanak’s15a and Mori’s15c research,
electron-withdrawing substituents on the o-iodophenols
resulted in the slightly lower yields of the products (5f, 5g).
We initiated our studies by using 2-iodophenol 1a and
alkynol 2a as the model substrates to explore the reaction
conditions (Table 1). Initially, a series of bases were screened
a
Table 1. Optimization of Reaction Conditions
b
entry
solvent
DMF
DMF
base
time (h)
yield (%) 5a: 4a: 7a
Scheme 2. Substrate Scope for the Syntheses of
1
2
3
4
5
6
7
8
9
10
11
12
13
DBU
Et3N
Cs2CO3
DBU
DBU
DBU
Et3N
DBU
DBU
DBU
DBU
DBU
DBU
20
30
24
10
24
24
40
48
40
40
60
40
40
0:92:0
0:79:0
0:73:0
0:0:83
0:16:63
0:10:61
25:48:13
15:44:25
30:58:5
81:7:4
4:38:0
a,b
Monobenzannulated Spiroketals
DMF
CH2Cl2
CH3CN
1,4-dioxane
Et3N
Toluene
THF
THF
THF
THF
THF
c
d
c e
,
55:6:17
35:6:8
c f
,
a
All reactions were carried out by using 1a (1.1 mmol), 2a (1.0
mmol), PdCl2(PPh3)2 (0.02 mmol), CuI (0.02 mmol), and solvent (5
b
mL) under CO (1 atm) at 25 °C, except as noted. Isolated yield.
c
NaH (10 mol %) was added after complete consumption of alkynol
d
e
2a. The reaction was performed without CuI. PdCl2(dppf) (0.02
mmol) was used instead of PdCl2(PPh3)2. PdCl2 (0.02 mmol) and
f
BINAP (0.02 mmol) were used instead of PdCl2(PPh3)2.
in the presense of 2 mol % of PdCl2(PPh3)2 and 2 mol % of
CuI in DMF with a CO balloon at 25 °C (entries 1−3).
Although no spiroketal 5a was observed, chromone 4a could
be obtained in 92% yield (entry 1), revealing that DBU is the
ideal base for the carbonylation reaction. We next screened
several common solvents. To our delight, the desired spiroketal
5a could be generated in 15−30% yields jointly with plenty of
chromone 4a remaining when toluene, Et3N (also as the base),
or THF was used as the solvent (entries 7−9). Notably,
though benzofuran 7a was obtained in some cases, no ester
byproduct was detected. We speculated that the insufficient
strength of the base might result in a large amount of the
chromone remaining. However, subsequent experimental
results showed that stronger bases such as LiOH, NaOH, or
MeONa could damage the activity of the palladium catalyst
and the palladium precipitate was formed very rapidly under
a
Reaction conditions: 1 (1.1 mmol), 2 (1.0 mmol), PdCl2(PPh3)2
(0.02 mmol), and CuI (0.02 mmol) in THF (5 mL) under CO (1
atm) at 25 °C for 40 h and then NaH (0.1 mmol) at 0 °C for 1 h,
b
c
d
except as noted. Isolated yield, except as noted. NMR yield. The
reaction was performed on a 10 mmol scale. The reaction was
performed without NaH.
e
B
DOI: 10.1021/acs.orglett.8b03586
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Additionally, secondary alcohol 2b was also used as a substrate
for this reaction, affording the corresponding spiroketals (5h−
5n) in higher yield and excellent diastereoselectivities (dr
>20:1). The relative stereochemistry of 5k was characterized
via X-ray crystallography analysis.17 Surprisingly, when chiral
diol 2c was utilized as substrate, the reactions proceeded
smoothly in the absence of NaH, giving the corresponding
spirokrtals 5o−5t in good yields and excellent diastereose-
lectivities (dr >20:1). Because 5o−5t were obtained as oils, the
steric configurations were assigned on the basis of the analysis
of the CD spectrum of 5s. The electronic circular dichroisms
(ECDs) of two possible candidate isomers (S-5s, R-5s) were
calculated for the purpose of comparison (for details, see the
Supporting Information). From the comparison of CEs at
212.5, 250, and 316 nm (Figure 2), the experimental spectrum
Scheme 3. Substrate Scope for the Syntheses of
Bisbenzannulated Spiroketals
a,b
a
All reactions were carried out by using 1 (1.1 mmol), 3 (1.0 mmol),
PdCl2(PPh3)2 (0.02 mmol), CuI (0.02 mmol), and Et3N (5 mL)
under CO (1 atm) at 25 °C for 24 h, then NaH (0.1 mmol) and
b
CH2Cl2 (5 mL) at 0 C for 30 min. Isolated yield, except as noted.
c
d
NMR yield. The stick model is the most stable conformer on the
basis of the computational works.
Figure 2. Comparison of calculated CD spectra of S-5s and R-5s and
experimental CD spectrum of 5s (EXP-1 and EXP-2).
generated in good yields and excellent diastereoselectivities.
The relative configuration of 6j was assigned on the basis of
the analysis of the NOE spectrum, which could match well
with the computational results of the most stable conformer of
6j (see Supporting Information for full details).
To further explore the possible reasons for the excellent
stereoselectivities in our reactions, the relative Gibbs free
energies of possible stereomers and conformers of 5h and 6j
were computed using the Gaussian 09 suite of program, in view
of the conformation of dihydropyran ring, anomeric effect, and
direction of methyl group (see Supporting Information for full
details). The computational results indicate that the config-
urations obtained from our experiments are significantly more
thermally stable, and the equilibriums tend to them according
to Boltzmann distribution. These results could well explain the
excellent stereoselectivities.
In summary, a novel Pd-catalyzed carbonylative Sonogashira
coupling in tandem with double annulation reaction to
synthesize benzannulated [6,6]-spiroketals was achieved.
Notably, this process was conducted under very mild
conditions (under balloon pressure of CO at room temper-
ature). This approach enables access to a wide range of
benzannulated [6,6]-spiroketals in moderate to good yields,
together with good functional group tolerance and excellent
stereoselectivities. With the advantages of environmental
benignity and atom and step economy, this protocol would
serve as an efficient and convergent way to synthesize
benzannulated [6,6]-spiroketals.
could match well with the calculated ECD curve of S-5s.
Therefore, the spiroketal moiety of 5s prepared by our method
herein is more likely to have the S-configuration.
To demonstrate the utility of this methodology, the model
reaction was conducted on a gram-scale (10 mmol). To our
delight, the desired product 5a was obtained in a slightly higher
yield of 85% (Scheme 2).
Unfortunately, the one-pot reaction of alkynyl phenol 3a
with 1a failed to give bisbenzannulated [6,6]-spiroketal 6a
under the optimal conditions. After a series of careful
conditional optimizations (see Supporting Information for
full details), Et3N (also as the base) and CH2Cl2 were
respectively selected as the optimal solvents for this one-pot
reaction. Then various o-iodophenols were reacted with a
diverse array of alkynyl phenols, and the results are
summarized in Scheme 3. Overall, most of substitution pattern
on the aromatic rings of o-iodophenols and alkynyl phenols
proceeded well in the reaction. In comparison with the results
of the monobenzannulated spiroketals, the corresponding
bisbenzannulated spiroketals were produced in relatively
lower yields. It should be pointed out that the carbonylative
Sonogashira coupling and intramolecular hetero-Michael
addition were performed efficiently in each case, as monitored
by thin-layer chromatography (TLC) and NMR (e.g., the
NMR yield of 6d is 90%, see Figure S7). The relatively low
isolated yields of products could be attributed to their low
stabilities on silica gel. In general, electron-donating groups
contributed to higher yields. In order to examine the
diastereoselectivity of the reaction, the alkynyl phenol with a
methyl group at the homopropargyl position was adopted.
Fortunately, the corresponding spiroketals 6j and 6k were
C
DOI: 10.1021/acs.orglett.8b03586
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03586.
■
S
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Detailed experimental procedures, analytical data, X-ray
crystal structure, DFT calculations, computational de-
tails, stereomer and conformer structures, Gibbs free
energies and structural data, spectral data for all
products, and optimization of reaction for synthesis of
5a and 6a (PDF)
Accession Codes
CCDC 1876072 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
(8) (a) Liu, S.; Chen, K.; Lan, X. C.; Hao, W. J.; Li, G.; Tu, S. J.;
Jiang, B. Chem. Commun. 2017, 53, 10692. (b) Zhou, G.; Zheng, D.;
Da, S.; Xie, Z.; Li, Y. Tetrahedron Lett. 2006, 47, 3349. (c) Liu, S.;
Lan, X. C.; Chen, K.; Hao, W. J.; Li, G.; Tu, S. J.; Jiang, B. Org. Lett.
2017, 19, 3831. (d) Spence, J. T.; George, J. H. Org. Lett. 2015, 17,
5970.
(9) (a) Cremins, P. J.; Wallace, T. W. J. Chem. Soc., Chem. Commun.
1986, 1602. (b) Cremins, P. J.; Hayes, R.; Wallace, T. W. Tetrahedron
1993, 49, 3211.
*E-mail: zhangyuan@lzu.edu.cn.
*E-mail: liying@lzu.edu.cn.
ORCID
Yuan Zhang: 0000-0002-4403-0995
Notes
The authors declare no competing financial interest.
(10) (a) van Hooft, P. A. V.; van Swieten, P. F.; van der Marel, G.
A.; van Boeckel, C. A. A.; van Boom, J. H. Synlett 2001, 0269. (b) Lin,
H.-C.; Chen, Y.-B.; Lin, Z.-P.; Wong, F. F.; Lin, C.-H.; Lin, S.-K.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Natural Science
Foundation of China (No. 21472078, 21772079 and
21572092) for financial support. We thank professor Zhixiang
Xie (Lanzhou University) for helpful discussion.
(12) Hamilton, J. Y.; Rossler, S. L.; Carreira, E. M. J. Am. Chem. Soc.
2017, 139, 8082.
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(17) CCDC 1876072 (5k) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
E
DOI: 10.1021/acs.orglett.8b03586
Org. Lett. XXXX, XXX, XXX−XXX