A palladium-catalyzed approach to allenic aromatic ethers and first total synthesis of terricollene A

Article abstract of DOI:10.1039/d1sc01896e

A palladium-catalyzed C-O bond formation reaction between phenols and allenylic carbonates to give 2,3-allenic aromatic ethers with decent to excellent yields under mild reaction conditions has been described. A variety of synthetically useful functional

Full text of DOI:10.1039/d1sc01896e

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A palladium-catalyzed approach to allenic aromatic
ethers and first total synthesis of terricollene A†
Cite this: Chem. Sci., 2021, 12, 9347
Chaofan Huang,a Fuchun Shi,b Yifan Cui,‡b Can Li,‡b Jie Lin,‡a Qi Liu,‡a Anni Qin,‡a
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Huanan Wang,‡a Guolin Wu,‡a Penglin Wu,‡a Junzhe Xiao, ‡b Haibo Xu,‡b
a
b
Yuan Yuan,‡a Yizhan Zhai,‡b Wei-Feng Zheng,‡a Yangguangyan Zheng, ‡ Biao Yu
*
ab
*
and Shengming Ma
A palladium-catalyzed C–O bond formation reaction between phenols and allenylic carbonates to give 2,3-
allenic aromatic ethers with decent to excellent yields under mild reaction conditions has been described. A
variety of synthetically useful functional groups are tolerated and the synthetic utility of this method has
been demonstrated through a series of transformations of the allene moiety. By applying this reaction as
the key step, the total syntheses of naturally occurring allenic aromatic ethers, eucalyptene and
terricollene A (first synthesis; 4.5 g gram scale), have been accomplished.
Received 7th April 2021
Accepted 4th June 2021
DOI: 10.1039/d1sc01896e
rsc.li/chemical-science
Results and discussion
Introduction
In contrast to the well-established metal-catalyzed coupling
reactions between aryl halides and phenols or aliphatic alcohols
(Scheme 1b),9 metal-catalyzed coupling reactions of aryl halides
Aromatic ethers are prevalent and prominent in a variety of
natural products, pharmaceuticals, and agrochemicals.1 Tradi-
tional methods for the preparation of aromatic ethers include
the Williamson ether synthesis,2 direct nucleophilic substitu-
tion reactions,3 Mitsunobu reaction,4 Ullman-type couplings of
alkoxides with aryl halides,5 etc. On the other hand, due to their
unique chemical properties as well as the substituent-loading
capability derived from the distinctive structure, allenes have
been demonstrated as powerful platform molecules for the
efficient syntheses of other functional organic compounds.6,7
Interestingly, many aromatic 2,3-allenylic ethers are also found
in the nature (Scheme 1a).8 Reports on the syntheses of such
aromatic ethers are very limited. Thus, development of new
methods for the efficient synthesis of aromatic 2,3-allenylic
ethers is of high interest currently. Herein, we disclose the
development of palladium-catalyzed C–O bond formation
between aryl phenols and allenylic carbonates to give allenic
aromatic ethers in 70–99% yields under mild reaction condi-
tions (Scheme 1c).
aResearch Center for Molecular Recognition and Synthesis, Department of Chemistry,
Fudan University, 220 Handan Lu, Shanghai 200433, P. R. China
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, P. R.
China. E-mail: masm@sioc.ac.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1sc01896e
Scheme 1 Naturally occurring aromatic 2,3-butadienyl ethers and the
metal-catalyzed C–O bond formation for the synthesis of aromatic
ethers.
‡ These authors contributed equally to this work and are sorted in alphabetical
order of last name.
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Table 1 Optimization of the reaction conditions for Pd-catalyzed reaction of p-methoxyphenol 1a with 2,3-butadienyl carbonate 2aa
NMR yield of
Recovery of 1a
Entry
[Pd]
Ligand
Solvent
3aa (%)
(%)
1
2
3
4
5
6
7
8
[PdCl(p-allyl)]2
Pd2dba3
Pd(PPh3)4
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
BINAP
BINAP
BINAP
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
MTBE
39
68
43
80
28
10
68
96
48
26
49
10
70
89
13
—
—
28
47
Xantphos
DPEPhos
BIPHEP
SPhos
Xantphos
Xantphos
Xantphos
Xantphos
9
10
11
Et2O
Dioxane
THF
99 (95b)
67
49
a
Yield and recovery were determined by 1H NMR analysis using CH3NO2 as the internal standard. b The reaction was carried out with 1 mmol of 1a
and the isolated yield is shown in parentheses.
and allenols have not been developed (Scheme 1c). In highly reactive C–I bond is generally less compatible in
a preliminary survey, the palladium-10 and copper-catalyzed11 palladium-catalyzed reactions.12 We reasoned that the relatively
coupling reactions between 4-iodoanisole and allenols were strong coordination effect of the allene unit brought the palla-
studied; however, only a trace amount of the desired allenyl aryl dium catalyst to the carbonate, so that the oxidative addition of
ether product was obtained (Scheme 1c, for details, see the
ESI‡). Then 4-methoxyphenol 1a was treated with 1.5 equiv. of
Table 2 The substrate scope: phenols or naphtholsa
2,3-butadienyl carbonate 2a in the presence of 2.5 mol%
[PdCl(p-allyl)]2 and 10 mol% BINAP with toluene as the solvent
at room temperature for 12 h. Interestingly, 39% yield of the
expected allenol ether product 3aa was formed with 48%
recovery of 4-methoxyphenol 1a. Various palladium catalysts
and ligands were then screened (Table 1, entries 1–7). The
combination of Pd2dba3 and Xantphos as the catalyst delivered
the best result, affording 80% yield of 3aa with 10% recovery of
1a (Table 1, entry 4). Subsequent solvent screening (Table 1,
entries 8–11) showed that Et2O or MTBE was the best solvent,
affording the product 3aa in 99% or 96% yield, respectively
(entries 8 and 9).
With the optimized conditions in hand, the scope of phenols
was evaluated (Table 2). To our delight, parent phenol 1b and
electron-rich p-cresol 1c gave high yields of 3ba and 3ca; even
the reaction of highly sterically hindered 2,6-dimethyl or 2,6-
diphenyl substituted phenols proceeded smoothly to afford 3da
or 3ea in 71% or 84% yield, respectively. The ortho-allyl
substituent also survived, allowing further modication based
on the reactivity of the C–C double bond. Bromo-substituents at
the ortho- (3ga), meta- (3ha), and para-positions (3ia) were all
well tolerated without much difference, indicating that the
steric effect is negligible and the relatively reactive C–Br bonds
were tolerated. In addition, other synthetically useful halogen
substituents such as F, Cl, and even I were all compatible,
delivering the corresponding products 3ja, 3ka, and 3la in 94%,
87%, and 88% yields, respectively. It is worth noting that the
a
Reaction conditions: 1 (1.0 mmol), 2a (1.5 mmol), Pd2dba3 (2.5 mol%),
and Xantphos (10 mol%) in Et2O (5 mL) at rt; isolated yields are shown.
b
In MTBE at 35 ꢀC. c With 5 mol% Pd(PPh3)4 in dioxane at 35 ꢀC. d With
5 mol% Pd(PPh3)4 in MTBE at 40 ꢀC. e With 2.5 mmol of 2a. f In MTBE at
60 ꢀC in a sealed tube.
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the palladium catalyst occurred exclusively to the carbonate heteroaryl substituents such as a furyl and a thienyl group in
unit, leaving the C–Br/I bond intact.13 Furthermore, phenols allenylic carbonates 2 were also successfully tolerated affording
with electron-withdrawing and synthetically versatile substitu- corresponding products 3bo and 3pp in excellent yields.
ents such as CO2Me and CN could also deliver the corre-
A gram-scale reaction between 1g and 2a delivered 1.0465 g
sponding products 3ma and 3na in good to excellent yields of 3ga in 93% yield with a reduced loading of both the catalyst
upon changing the catalyst to Pd(PPh3)4. Moreover, 1- or 2- and ligand (Scheme 2a). As a class of synthetically useful
naphthols could also afford the corresponding products 3oa or chemicals, the versatile allenyl unit could be transferred into
3pa in 84% and 95% yield, respectively. With 2.5 equiv. of allene different structure units: selenohydroxylation and oxidation of
2a, the coupling reactions of hydroquinone, resorcinol, cate- 3ga with 1-uoropyridinium/1,2-diphenyldiselane could deliver
chol, and 1,10-binaphthol gave the double-coupling products 2-selenoenal 4 with a Z/E selectivity of 94/6;14 bromohydrox-
3qa–3ta in excellent yields. Interestingly, only the phenol ylation of this allenyl group in aqueous dioxane afforded
hydroxy group was exclusively coupled leaving the benzylic terminal 2-bromoallylic alcohol 5 as the sole product in 72%
hydroxyl group untouched to afford 3ua. Such a coupling reac- yield;15 the allenyl group may also be selectively transformed
tion of the hydroxyl group in estrone was also realized by into the corresponding primary allylic alcohol 6 with an E/Z
changing the solvent to MTBE for a reaction at 60 C to afford selectivity of 98/2 by a gold-catalyzed hydration reaction;16
3va in an excellent yield. iodination of the allenyl group exclusively gave diiodide product
ꢀ
We then evaluated the scope of 2,3-allenylic carbonates (Z)-7 in 69% yield.17 It should be noted that these products are
(Table 3): carbonates with 4-alkyl and isopropyl substituents difficult to synthesize through traditional transition-metal
delivered the coupling products 3bb, 3pc, and 3pd in decent to catalyzed coupling reactions.10,18
excellent yields. Benzyl ether, which is a common and practical
Finally this method has been applied to the efficient
protecting group for the hydroxyl group, was also viable in this syntheses of two naturally occurring allenes: eucalyptene A has
transformation affording 3oe in 96% yield. 4-Phenyl-2,3- been successfully synthesized under the catalysis of 5 mol%
butadienyl carbonate 1f reacted with phenol to afford 3bf in Pd(PPh3)4 in 90% yield from methyl 4-hydroxycinnamate and
an almost quantitative yield. Moreover, electron-donating 2,3-butadienyl methyl carbonate (Scheme 2b); 70 mmol scale
groups such as p-methyl and p-methoxy and electron with-
drawing yet potentially useful p-Cl, p-Br,13 p-I,13 p-CF3, p-NO2,
and p-CN were all accommodated yielding the corresponding
products 3pg–3pn in decent yields. Last but not the least, 4-
Table 3 The substrate scope: 2,3-allenylic carbonatesa
a
Reaction conditions: 1 (1.0 mmol), 2 (1.5 mmol), Pd2dba3 (2.5 mol%),
and Xantphos (10 mol%) in Et2O (5 mL) at rt; isolated yield.
Scheme 2 Synthetic applications.
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Author contributions
C. H. and S. M. conceived the idea. C. H. conducted the most of
experiments. B. Y. supervised the total synthesis of terricollene
A. C. H. and F. S. co-synthesized the terricollene A. Y. C., C. L., J.
L., Q. L., A. Q., H. W., G. W., P. W., J. X., H. X., Y. Y., Y. Z., W. Z.,
and Y. Z. co-synthesized part of substrates and they contribute
equally to this paper. C. H. and S. M. co-wrote the paper. All the
authors discussed the results and commented on the
manuscript.
Scheme 3 Proposed mechanism.
Conflicts of interest
reaction of p-hydroxymethylphenol with 1.14 equiv. of 2,3-
butadienyl methyl carbonate 2a afforded the corresponding p-
hydroxymethylphenyl 2,3-butadienyl ether 10. D-Glucose (4.0 g)
was etheried with chlorotrimethylsilane in pyridine to give
penta-O-trimethylsilyl-D-glucopyranose 8, which was then reac-
There are no conicts to declare.
Acknowledgements
ted with iodotrimethylsilane to generate the glucosyl iodide 9 Financial support from the National Natural Science Founda-
within 30 min at room temperature. The reaction of allene 10 tion of China (21690063 & 21988101) is greatly appreciated.
(7.15 g) with 9 in the presence of 2,6-di-tert-butylpyridine in dry
dichloromethane at room temperature for 4 h followed by
Notes and references
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In conclusion, we have described a general method of
palladium-catalyzed C–O bond formation reaction between
phenols and allenylic carbonates. This rst example of such
a C–O bond formation reaction exhibits a broad scope of both
phenols and carbonates under very mild conditions, providing
an attractive complement to traditional C–O bond formation
reactions. The attractive reactivities of the allene unit in the
products have been demonstrated. Moreover, the present cata-
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of naturally occurring aromatic 2,3-allenylic ethers, eucalyptene
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