Application of Dehydroabietic Acid in Palladium-Catalysed Enyne Cycloisomerisation

Article abstract of DOI:10.1002/adsc.201700061

Dehydroabietic acid (DAA) promotes palladium(0)-catalysed cyclisations of arene-tethered 1,7-enynols and 1,m-enynoates (m=6,7) to give fused carbocyclic dienes. 6,6,6,5-Tetracyclic lactones are accessible by one-pot cycloisomerisation/Diels–Alder reaction

Full text of DOI:10.1002/adsc.201700061

Title: Application of dehydroabietic acid in palladium-catalysed enyne
cycloisomerisation
Authors: Na Wu, Ruikun Li, and Feihu Cui
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10.1002/adsc.201700061
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Application of dehydroabietic acid in palladium-catalysed
enyne cycloisomerisation
Na Wua,b,*, Ruikun Li,a Feihu Cuia and Yingming Pana
a
School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin, China, 541004,
wuna07@gxnu.edu.cn
b
Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University Shenzhen
Graduate School, Shenzhen, China, 518055.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201.
Abstract.
Dehydroabietic
acid
(DAA)
promotes
palladium(0)-catalysed cyclisations of arene-tethered 1,7-
enynols and 1,m-enynoates (m = 6,7) to give fused
carbocyclic dienes. 6,6,6,5-tetracyclic lactones are accessible
by one-pot cycloisomerisation / Diels-Alder reaction /
lactonisation from 1,7-enynols. Furthermore, asymmetric
counteranion-directed catalysis was developed, which
afforded an indene derivative with an all-carbon quaternary
stereogenic center.
Keywords: enynol; dehydroabietic acid; Diels-Alder
reaction-lactonisation; asymmetric counteranion directed
catalysis; cycloisomerisation, palladium
Figure 1. Representative examples of fused polycyclic
lactones.
Introduction
Fused carbocyclic lactones have attracted much
attention from synthetic chemists owing to their
structural diversity, and the existence of these motifs
in a variety of natural products and pharmaceuticals
(Figure 1).1 In this regard, the cascade Diels-Alder
reaction / lactonisation is among the most powerful
routes to access such products.2
Herein, we report that dehydroabietic acid (DAA)
promotes the palladium-catalysed cyclisation of
arene-tethered 1,7-enynols and 1,m-enynoates (m = 6,
7), and avoids these side-reactions to deliver
polycyclic diene products suitable for further
chemistry, including Diels-Alder cycloaddition /
lactonisation.
A cycloisomerisation / Diels-Alder reaction /
lactonisation cascade of enynes equipped with allylic
alcohols3a would offer an attractive, atom-economical
means to convert simple building blocks into this type
of complex polycycle.3b Although palladium-
catalysed aliphatic enyne cycloisomerisation has
undergone extensive studies since the first Alder-ene
reaction,3 there are few reports on the cyclisation of
arene-tethered 1,7-enynols,4 presumably because the
propargylic alcohol can undergo acid-catalysed [1,3]-
shift (the Meyer-Schuster/Rupe rearrangement),5a,5b or
can eliminate under palladium catalysis.5c
Results and Discussion
Initial investigations into palladium-catalysed
cyclisation of 1a (Table 1) revealed that no
cycloisomerisation occurred in absence of an
electron-rich phosphine ligand (Pd2dba3 / AcOH,
entry 1); however, productive cyclisation to 2a was
observed using Pd(PPh3)4 as pre-catalyst in
combination with HOAc, albeit in poor yield (entry 2).
Replacing HOAc by a stronger acid (trifluoroacetic
acid) led to no reaction (entry 3), whilst 2a was
delivered in a moderate yield using formic acid, with
1a consumed completely (entry 4). In contrast, the
cycloisomerisation did not take place in the absence
of acid, nor with catalytic amounts of acid (entries 5
and 6), thus disfavoring a possible pathway through a
palladacyclopentene intermediate, and indicating that
a stoichiometric amount of acid is crucial to initiate
1
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10.1002/adsc.201700061
Advanced Synthesis & Catalysis
the hydropalladation of alkynyols in the The scope of the cyclisation was then investigated
cycloisomerisation. We hypothesised that sterically under the optimised conditions (Table 2). For
unhindered Brønsted acids could react with the substrates 1 bearing secondary (1a-1d) and tertiary
hydroxyl group prior to coordination of Pd to the propargylic alcohols (1e-1g), the corresponding
enyne, thus promoting Meyer-Schuster rearrangement, benzocyclic dienols 2 were obtained in good yields,
and potentially facilitating acid-catalysed elimination regardless of additional steric hindrance at the
of water from dienol 2a.6 By considering the pKa of a propargylic carbon (1f, 1g) or at the homopropargylic
selection of alternative acid additives, including position (1b-1d); however, it was notable that
primary, secondary and tertiary acids (entries 8 to 14), substrates with β-tertiary or quaternary substituents
we were pleased to observe that 1a was converted (1h, 1i) did not afford product. The terminal alkyne 1j,
into 2a in excellent yield in toluene at 80 °C (93%, lacking a propargylic alcohol, also underwent
entry 14), using a bulky acid with a relatively high cycloisomerisation, however the corresponding diene
pKa: dehydroabietic acid (pKa = 7.9).7
2j was not detected: instead, the product of Diels-
Alder dimerization (2j’) was observed.8
Table 1. Optimisation of enynol cyclisation conditions.a
The configuration of the newly-formed alkene was
confirmed by X-ray crystal structure of 2d, which
revealed that the alkene had formed as the Z
stereoisomer (Figure 2).9
Table 2. Scope of the reaction with respect to 1,7-enynols.a
entry
1c
2
acid
HOAc
solvent
toluene
THF
T (℃)
80
yieldb
N.R.d
23
HOAc
50
3
CF3COOH
HCOOH
none
THF
50
N.R.d
60
4
THF
50
5
THF
50
N.R.d
trace
59 (95)f
64
6
DAAe
THF
50
2b, R1 =
Bn 80%
2c, R1 =
CH2-i-Pr
81%
2d, R1 =
CH2-t-Bu
86%
2e, R1 = R2
= Me 95%
7
DAA
THF
50
8
HOAc
toluene
toluene
toluene
toluene
toluene
80
9
HCOOH
PivOH
PhCOOH
80
77
10
11
12
80
88
80
83
2f, R1 = Ph, 2g, R1 = R2 1h, R1 = i-
1i, R1 = t-
Bu, N.R.
80
87
R2 = Me,
85%
= Ph 76%
Pr, N.R.
13
toluene
80
87
14
15
DAA
DAA
DAA
DAA
toluene
toluene
toluene
toluene
80
100
120
80
93
46
66
81
2j’,b 94%,c 84%,d 33%e
16
a)
1 (1 equiv.), Pd(PPh3)4 (5 mol%), DAA (1 equiv.),
17g
toluene (0.04 M), N2, 80 ℃, 4 h; Yields are for the isolated
product; b) diene was not isolated, instead cyclodimerization
a) Reaction conditions, unless otherwise specified: 1a (0.17
mmol), Pd(PPh3)4 (9.8 mg, 5 mol %), acid (0.17 mmol),
solvent (4 mL), N2, specified heating temperature, 4 h;
c)
of diene was observed; Pd(PPh3)4 (2.5 mol%), DAA (1
d)
e)
equiv.);
Pd(PPh3)4-DAA (1:1, 2.5 mol%); Pd(PPh3)4
b)
(2.5 mol %).
c)
d)
isolated yield;
Pd2(dba)3 (2.5 mol %);
N.R. = no
reaction; e) 5 mol %; f) based on recovered starting material;
g) Pd(PPh3)4 (4.9 mg, 2.5 mol %).
2
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10.1002/adsc.201700061
Advanced Synthesis & Catalysis
heating afforded 5n, featuring a quaternary carbon
center (entry 14), in respectable yield (51%).11
Table 3. Scope of the reaction with 1,7-enynoates.a
Figure 2. Crystal structure of 2d
The benzocyclic dienols 2 were prone to decompose
slowly10 upon standing overnight in CDCl3 or d6-
acetone; partial decomposition was also observed
upon silica gel chromatography. To avoid this, as well
as to demonstrate the utility of the products 2, we
applied a one-pot stepwise cycloisomerisation / Diels-
Alder reaction / lactonisation protocol (Scheme 1),
which afforded satisfying overall yields of 3 for the
secondary and tertiary propargylic alcohols 1a and 1e
respectively. The sense of diastereoinduction in the
Diels-Alder reaction was determined by X-ray
crystallographic analysis of analogue 3f (Figure 3).9
entry R3,
product
yield entry R3, product yield
1
Me, 5a
65%b
80%
8
t-Bu, 5h
67%
83%c
81%d
76%
2
3
4
Et, 5b
9
Ph, 5i
78%
52%
To further explore the utility of DAA for
cycloisomerisations of benzene-tethered enynes, we
next attempted DAA-promoted cyclisation of 1,7-
enynoates. We were pleased to observe a high
yielding cyclisation at an optimised loading of 2.5
mol% of Pd(0)/DAA (80%, Table 3, Entry 1). A
range of substrates proved effective, including
n-Bu, 5c 59% 10
Mes, 5j
i-Pentyl, 56% 11
anisole, 5k 86%
5d
5
6
Cy, 5e
77% 12
81% 13
o-OMe-
C6H4, 5l
73%
65%
Cp, 5f
5m
7
i-Pr, 5g
66% 14
51%e
Scheme 1. One-pot cycloisomerised Diels-Alder reaction-
lactonisation of 1.
5n
a)
Unless specified, all entries were conducted with
Pd(PPh3)4-DAA (1:1, 2.5 mol%); b) Pd(PPh3)4-DAA (1:1, 1
c)
d)
mol%);
Pd(PPh3)4-DAA (1:1, 5 mol%);
Pd(PPh3)4-
DAA (1:1, 10 mol%); e) 100 ℃, 36 h.
Asymmetric
counteranion-directed
catalysis
(ACDC)12 has recently emerged as an exciting new
concept for asymmetric induction in reactions that are
sensitive to both the steric and electronic nature of the
reactants. High levels of stereocontrol are often
thought to be achieved through weak binding of the
chiral anion to the 'naked' catalyst through non-
covalent electrostatic interactions.
Figure 3. Crystal structure of 3f
primary, secondary, and tertiary aliphatic alkynoates
(entries 1-8). In addition, electron-rich or sterically
demanding groups at the ortho-, meta-, or para-
position of aryl esters were also tolerated in their
transformations into benzocyclic dienoates 5 (entries
9-13). Cyclisation of 4n, bearing a trisubstituted
alkene,3d,4a was also performed with catalytic DAA /
Pd(PPh3)4, which although requiring additional
In the field of cycloisomerization, the only
examples of the asymmetric construction of
quaternary carbon centers in palladium-catalysed
enyne cycloisomerisation reported to date are the
pioneering study by Mikami,3d,4a which specifically
focused on the use of chiral bidentate phosphanes; in
3
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10.1002/adsc.201700061
Advanced Synthesis & Catalysis
general, asymmetric enyne cycloisomerization has of the moderate instability of these dienols, one-pot
proven difficult to realise.13 Mikami found that to cycloisomerisation / Diels-Alder cycloaddition /
achieve enantioselective cyclisations furnishing lactonisation offers a straightforward route into
quaternary stereocenters via intramolecular Heck tetracyclic lactones with high efficiency. We also
coupling in congested environments, higher achieved the first example of ACDC in palladium-
temperatures and polar solvents were required. catalayded cycloiomerisation, where moderate
However, such conditions pose a challenge to the as enantioselectivities
were
observed.
Further
yet unrealised ACDC approach, as strong ion-pairing investigations to explore and optimise this reactivity
interactions are likely to be favored only in non-polar are ongoing.
solvents.12d Accordingly, cyclisations carried out in
polar solvents or under heating seem destined to
suffer in terms of stereoselectivity due to ion
Experimental Section
dissociation.
General procedure of stoichiometric dehydroabietic
acid (DAA)-mediated 1,7-enynol cycloisomerisation
We probed the applicability of ACDC strategy to
Pd(0)-catalysed 1,6-enyne cycloisomerisation via in
situ generation of ion-paired [H–Pd(II)]+·phosphate
from (S)-TRIP and [H-Pd(II)]+·DAA–.14 We were
delighted to find that the sterically demanding 7 was
furnished in good yields and moderate ee upon
heating to 60 °C (Table 4, entries 1 and 2). The yield
was improved by increasing the reaction temperature,
but with a consequent loss of enantioselectivity
(entries 4 and 5). Generation of ion paired [H–
Pd(II)]+·carboxylate from natural (+)-DAA led only
to low ees (entries 3 and 6).
Toluene (3 mL) was injected under nitrogen into a
Schlenk tube containing Pd(PPh3)4 (10 mg, 0.0085 mmol, 5
mol %) and dehydroabietic acid (51 mg, 0.17 mmol, 1
equiv.), and this suspension was stirred at room
temperature for 20 min. The 1,7-enynol substrate (0.17
mmol, 1 equiv.) in toluene (1 mL) solution was added, and
the tube was sealed with a screw cap. The mixture was
stirred at 80 ℃ for 4-8 hours (monitored by TLC). The
reaction mixture was washed with brine, and extracted with
ethyl acetate. The organic phases were concentrated, and
the residue was purified by column chromatography to
afford 2.
General procedure of one-pot cycloisomerisation /
Diels-Alder reaction / lactonisation of 1
Table 4. ACDC to construct quaternary stereogenic
carbon.a
Toluene (3 mL) was injected under nitrogen into a
Schlenk tube containing Pd(PPh3)4 (9 mg, 0.008 mmol, 5
mol %) and dehydroabietic acid (48 mg, 0.16 mmol, 1
equiv.), and this suspension was stirred at room
temperature for 20 min. The 1,7-enynol substrate (0.16
mmol, 1 equiv.) in toluene (1 mL) solution was added, and
the tube was sealed with a screw cap. The mixture was
stirred at 80 ℃ for 4-8 hours until full consumption of the
starting material 1, monitored by TLC. Then N-
phenylmaleimide (0.16 mmol, 1 equiv.) was added in one-
portion under nitrogen purge and the reaction was stirred at
room temperature for 6 hours, until full consumption of the
starting material by TLC. The reaction mixture was washed
with brine and extracted with ethyl acetate three times. The
combined organic layer was dried by MgSO4, filtered, and
was concentrated in vacuo, and the residue was purified by
column chromatography to afford 3.
entry
1
R3
yield (%)b
ee (%)c
66
Me, 7a
Cp, 7c
Cp, 7c
Cp, 7c
Cy, 7d
Cy, 7d
83
56
93
92
87
80
2
66
3d
4e
-7
General procedure of dehydroabietic acid (DAA)-
catalysed enynoate cycloisomerisation
8
5e
10
Toluene (3 mL) was injected under nitrogen into a
Schlenk tube containing Pd(PPh3)4 (5 mg, 0.004 mmol, 2.5
mol %) and dehydroabietic acid (1.3 mg, 0.004 mmol, 2.5
mol %), and this suspension was stirred at room
temperature for 20 min. The enynoate 4 (0.17 mmol, 1
equiv.) in toluene (1 mL) solution was added, and the tube
was sealed with a screw cap. The mixture was stirred at
80 ℃ for 8 hours as monitored by TLC. The reaction
mixture was washed with brine and extracted with ethyl
acetate. The combined organic layers were evaporated in
vacuo and the residue was purified by column
chromatography to afford corresponding cyclisation
product.
6d
-15
a)
6 (1 equiv.), Pd(PPh3)4 (2.5 mol %), (S)-TRIP (2.5
b)
mol %), toluene (0.04 M), N2, 60 ℃, 5 days; Yields are
for the isolated product; c) Determined by chiral HPLC; d)
6
(1 equiv.), Pd(PPh3)4 (2.5 mol %), DAA (2.5 mol %),
toluene (0.04 M), N2, 80 ℃, 24 h; e) 100 ℃, 24 h.
Conclusion
Supporting Information
In summary, we have found that palladium-
catalysed cycloisomerisations, initiated or promoted
by dehydroabietic acid, convert a wide range of 1,7-
enynols and enynoates to benzocyclic dienes. In spite
General experimental procedures, spectral data, NMR
spectra for all compounds, and the X-ray crystal structures
of 2d and 3f are provided in the Supporting Information.
4
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10.1002/adsc.201700061
Advanced Synthesis & Catalysis
Acknowledgements
[6] In the employed conditions, we did not isolate the
Meyer-Schuster rearranged (MSR) byproduct, or
dehydrated trienes. For fine-tuning pKa of a protic
additive to inhibit MSR and elimination of water from a
β-hydroxy ketone, see M. N. Pennell, M. P. Kyle, S. M.
Gibson, L. Male, P. G. Turner, R. S. Grainger, T. D.
Sheppard, Adv. Synth. Catal. 2016, 358, 1519-1525.
We thank NSFC (21462004), State Key Laboratory for the
Chemistry and Molecular Engineering of Medicinal Resources
(CMEMR2014-A04), 2015 GXNSFBA (139032) and GXNU. We
also thank Prof. Edward Anderson gratefully for fruitful
discussion.
[7] Cyclohexanecarboxylic acid pKa is 4.9, see a)
V. S. Pilyugin, A. N. Mikhailyuk, V. M. Kosareva,
G. E. Chikisheva, E. V. Klimakova, T. P. Vorob'eva,
Russ. J. Gen. Chem. 2003, 9, 1457-1462; 1-
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J. F. J. Dippy, S. R. C. Hughes, J. W. Laxton, J. Chem.
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5
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10.1002/adsc.201700061
Advanced Synthesis & Catalysis
nordehydroabietyl amide derived chiral diene, see b) R.
Li, Z. Wen, N. Wu, Org. Biomol. Chem. 2016, 14,
11080-11084.
6
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10.1002/adsc.201700061
Advanced Synthesis & Catalysis
FULL PAPER
Application of dehydroabietic acid in palladium-
catalysed enyne cycloisomerisation
Adv. Synth. Catal. Year, Volume, Page – Page
Na Wu*, Ruikun Li, Feihu Cui and Yingming Pan
7
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