Stereoselective synthesis of enynones via base-catalyzed isomerization of 1,5-disubstituted-2,4-pentadiynyl silyl ethers or their alcohol derivatives

Article abstract of DOI:10.1039/c0ob00344a

1,5-Disubstituted-2,4-pentadiynyl silyl ethers undergo smooth desilylative isomerization to afford cis-enynones as major products with moderate stereoselectivities in the presence of a catalytic amount of KO^tBu or DBU. While the isomerization reactions of their alcohol derivatives catalyzed by KOH, KO^tBu or NaH take place efficiently to produce trans-enynones with high stereoselectivities. These reactions provide convenient and practical routes for the synthesis of enynones with a wide range of substitution groups.

Full text of DOI:10.1039/c0ob00344a

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COMMUNICATION
www.rsc.org/obc | Organic & Biomolecular Chemistry
Stereoselective synthesis of enynones via base-catalyzed isomerization of
1,5-disubstituted-2,4-pentadiynyl silyl ethers or their alcohol derivatives†
Jingjin Chen, Guoqin Fan and Yuanhong Liu*
Received 29th June 2010, Accepted 25th August 2010
DOI: 10.1039/c0ob00344a
1,5-Disubstituted-2,4-pentadiynyl silyl ethers undergo
smooth desilylative isomerization to afford cis-enynones
as major products with moderate stereoselectivities in the
presence of a catalytic amount of KOtBu or DBU. While the
isomerization reactions of their alcohol derivatives catalyzed
by KOH, KOtBu or NaH take place efficiently to produce
trans-enynones with high stereoselectivities. These reactions
provide convenient and practical routes for the synthesis of
enynones with a wide range of substitution groups.
clear yet, we wondered whether enynones might be constructed
stereoselectively in a metal-free process by using a base as catalyst.
As already known, isomerization of propargyl alcohols or its silyl
ethers to either trans- or cis- chalcone derivatives in the presence
of various bases have been reported. For example, Yoshizawa10
reported that (Z)-chalcone could be stereoselectively synthesized
from siloxypropynes via siloxyallenes using KOtBu. Zimmerman
isolated the siloxyallenes and found that the kinetic protonation
during the allenolate formation with fluoride salt resulted in
a general preference for the formation of the (Z)-chalcone.11
Mu¨ller12 and others13 reported that the isomerization of propar-
gylic alcohols to (E)-chalcones occurred under triethylamine,12
triton B13a or potassium hydroxide.13b–d These methods for (E)-
chalcones usually required one or more equivalents of base except
in the case of triton B. Transition metals such as Pd,14 Ru,15 Rh16
and Ir17 have also been found to induce the redox isomerization
of propargylic alcohols to enones or enals.18 However, there is no
report for the corresponding reactions of pentadiynoic substrates,
to the best of our knowledge. Herein we’d like to report the
base-catalyzed stereoselective synthesis of both the Z and the E
isomers of enynones via the isomerization of 1,5-disubstituted-
2,4-pentadiynyl silyl ethers or their alcohol derivatives
(Scheme 2).
cis- and trans-Enynones are important building blocks in organic
synthesis, e.g., in conjugated addition with organocuprates,1
phosphine2a,b or acid/base2c mediated furan synthesis, metal-
catalyzed furan formations via (2-furyl)carbene complexes,3 or
as substrate precursors in [3,3]-sigmatropic rearrangement,4 as
an intermediacy in IBX-induced cascade oxidation/cyclization
of enynols to 2-acyl furans,5 in total synthesis of natural
products6 and so on. Therefore, efficient procedures leading to
these substrates, especially with good levels of stereocontrol,
are highly desirable. The synthetic methods for cis-selective
enynones are achieved mainly by Sonogashira coupling reactions
as key steps,5,7 for example, coupling of the terminal alkynes
with (Z)-3-iodo-propenoates followed by reduction/Grignard
addition/oxidation,5 or with 3-chloro-cis-2-propen-1-one with
low overall yields.8 trans-Enynones can be prepared by the reaction
of ynals with Wittig reagents,2a,b however, a large amount of
phosphine oxide is produced as waste which is hard to reduce.
Recently, we have developed a convenient method for the selective
titanation of 1,3-butadiynes using Ti(OiPr)4/n-BuLi reagent.9
During our further investigation of the coordination behavior
of 2,4-pentadiynyl silyl ethers with this low valent titanium
reagent, we found that under the reaction conditions shown in
Scheme 1, (Z)-enynone could be isolated in 59% yield, while
analysis of the crude reaction mixture revealed that a mixture of
(Z)- and (E)-isomers existed in a ratio of 88 : 12 as determined
by 1H NMR. Although the real reaction mechanism was not
Scheme 2
We started our investigation with TBS-substituted pentadiynoic
ether 1a (Table 1), which was conveniently prepared in a one-
pot procedure through iodination/Brook rearrangement of a-
alkynyloxatitanacyclopentenes developed by us.9a A wide range
of reaction conditions were screened and some of the results are
shown in Table 1. We found that addition of 0.3 equiv KOtBu to
a solution of 1a in THF at -78 ◦C resulted in a gradual color
change to dark blue. The resulting solution was stirred at the same
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu,
Shanghai 200032, People’s Republic of China. E-mail: yhliu@mail.sioc.ac.cn
† Electronic supplementary information (ESI) available: Experimen-
tal section and NMR spectra of all new compounds. See DOI:
10.1039/c0ob00344a
Scheme 1
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Table 1 Optimization studies for the isomerization of 2,4-pentadiynyl silyl ethers
Entry
Base
Equiv
Solvent
Temp.
Time
Yield (%) of 2aa
Z/E ratiob
1
2
3
4
5
6
7
8
9
KOtBu
KOtBu
KOtBu
KOtBu
DBU
DBU
NaH
n-BuLi
KOH
NaOH
DMAP
0.3
0.3
0.3
0.1
0.15
0.05
1.2
0.1
0.1
0.1
0.1
THF
toluene
DCM
THF
THF
THF
THF
THF
THF
THF
THF
-78 ◦
rt
C
25 min
12 h
12 h
1 h
4 h
11 h
6 h
7 h
5 h
1.5 h
8 h
70 (17)
78/22
c
—
c
rt
—
-78 ◦
C
54d
66/34
74/26
77/23
0 ◦
0 ◦
C
C
C
70 (19)
55
50 ◦
rt
—
c
c
—
0 ◦
rt
C
messy
messy
NRe
10
11
50 ◦
C
a Isolated yield of the pure cis isomer of 2a, the isolated yields of the trans isomer 3a are shown in parentheses. b Determined by 1H NMR of the crude
mixture. c Most of the 1a remained. d NMR yield. e No reaction.
Table 2 KOtBu or DBU promoted isomerization of 1,5-disubstituted-2,4-pentadiynyl silyl ethers
Entry
R1
R2
Methoda
Yield of 2 (%)b
Yield of 3 (%)b
2/3c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
TBS
TBS
TBS
TBS
TBS
TBS
TBS
TBS
TBS
TBDPS
TMS
TMS
Bu
Ph (1a)
A
B
A
B
A
Bd
A
A
A
A
A
B
70
70
66
70
56
43
63
70
46
61
67
68
63
44
2a
2a
2b
2b
2c
2c
2d
2e
2f
17
19
18
17
18
39e
17
17
23
20
15
31e
27
34
3a
3a
3b
3b
3c
3c
3d
3e
3f
78/22
74/26
76/24
77/23
79/21
51/49
75/25
81/19
66/34
78/22
81/19
76/24
72/28
54/46
Ph (1a)
p-ClC6H4 (1b)
p-ClC6H4 (1b)
p-MeOC6H4 (1c)
p-MeOC6H4 (1c)
2-thienyl (1d)
2-furyl (1e)
trans-C6H5CH CH (1f)
Ph (1g)
2g
2h
2h
2i
3g
3h
3h
3i
Ph (1h)
Ph (1h)
Ph (1i)
A
B
A
Ph
TBS
Ph (1j)
2j
3j
f
n-C3H7 (1k)
—
a Method A: 30 mol% KOtBu, in THF, -78 ◦C, 0.25–4 h. Method B: 15 mol% DBU, in THF, 0 ◦C, 40 min–5 h. b Isolated yield. c Determined by 1H NMR
of the crude mixture. d 2.0 equiv of DBU was used, reaction time was 10 h. e Containing small amount of impurity. f No reaction.
temperature for 25 min, and it was observed that the silyl ether
was consumed completely. After quenching the mixture by 3 N
HCl at -78 ◦C, cis-enynone 2a was isolated in 70% yield, together
with 17% of the trans-isomer of 3a. The cis/trans ratio was 78/22
as indicated by 1H NMR of the crude reaction mixture (Table 1,
entry 1). The results showed that the isomerization of the silyl
ether 1 was cis-selective. It should be noted that quenching the
reaction with H2O or NaHCO3 resulted in a complex mixture.
Reactions in toluene or DCM only led to low conversions (entries
2–3). Decreasing the catalyst loading to 0.1 equiv resulted in lower
yield and stereoselectivity (entry 4). As for base catalyst, DBU
is also effective for the isomerization at 0 ◦C, leading to similar
yields and stereoselectivities as that of KOtBu, although with a
longer reaction time (4–11 h) (entries 5 and 6). Other bases, such
as NaH, BuLi, KOH, NaOH or DMAP did not afford good results
(Table 1, entries 7–11).
We chose the reaction conditions shown in Table 1, entry 1
(method A) or entry 5 (method B) to examine the scope of this
reaction. The method is applicable to a wide range of suitably
substituted 2,4-pentadiynyl silyl ethers, and the corresponding
enynones were formed in good to high overall yields as illustrated
in Table 2. In most cases, (Z)-enynone 2 was obtained as a major
product, which could be separated from trans-isomers by column
chromatography. We first investigated the electronic effects of the
arene substituent of R2 at C-1. It was found that an electron-
withdrawing group (-Cl) or an electron-donating group (-OMe)
did not give a significant differences in the yields or selectivity
under the catalysis of KOtBu, furnishing the corresponding
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(Z)-enynones 2b and 2c in 66% and 56% yields, respectively (entries
3, 5). However, DBU was not as efficient as KOtBu when 1c bearing
an electron-donating group (-OMe) was used (entry 6). In this case,
2.0 equiv of DBU and a longer reaction time (10 h) was needed to
achieve the higher yield. A thienyl or furyl group of R2 was also
compatible with this isomerization to generate the desired (Z)-2d
and (Z)-2e in 63–70% yields (entries 7–8). When R2 is a cinnamyl
group, the corresponding 2f was obtained in 46% yield with lower
stereoselectivity (entry 9). We then investigated the effect of the
R1 group at the alkyne terminus for this isomerization. Butadiyne
1g bearing a sterically hindered TBDPS (tert-butyldiphenylsilyl)
group afforded the desired (Z)-enynone 2g smoothly in 61%
yield, however, the stereoselectivity was not improved (entry 10),
indicating that the TBDPS group is too remote from the reactive
site to influence the stereochemical course. Similar results were
achieved when using 1h with a TMS group (entries 11–12). Alkyl
substituted diyne 1i also provided excellent conversion with a Z/E
ratio of 72 : 28 (entry 13). However, the presence of a phenyl group
on R1 resulted in a 54 : 46 mixture of Z/E isomers (entry 14).
No reaction occurred even at refluxing temperature when R2 is
an alkyl group such as n-propyl group (entry 15), due to the low
acidity of pentadiynoic proton, in which the proton abstraction is
difficult to proceed.
Scheme 3
as a base in a solvent of DCE. As shown in Table 3, entry 1, the
desired trans-enynone 3a was obtained in 87% yield with 96 : 4 ratio
of E/Z isomer using 15 mol% KOH at 30 ◦C (it was noted that if
the temperature was below 20 ◦C, the reaction could not complete).
The reaction proceeded also very well using 10 mol% KOtBu, to
generate 3a in 83% yield with high stereoselectivity (entry 2). NaH
could induce the isomerization, however, with a lower yield of 3a
(entry 3). With the optimal conditions in hand, we investigated the
scope of this reaction. The reaction was found to be quite general
and proceeded cleanly in most cases to give the trans-enynones 3 in
good yields with excellent selectivity (Table 3). Substrates bearing
aryl and heteroaryl groups at the propargylic position were all
compatible under the reaction conditions, furnishing 3a–3e in 57–
87% yields (entries 1–7). However, a vinyl group at C-1 resulted in
a lower stereoselectivity (entry 8). A sterically hindered naphthyl
group at C-1 did not affect the efficiency of the isomerization,
and the corresponding product 3p was obtained in 76% yield with
95/5 selectivity (entry 9). In the case of butyl-substituted 7i, a
satisfactory result was obtained by increasing the amount of KOH
to 0.8 equiv (entry 10). The reaction was also efficient with aryl
(R1) substituted substrates 7j–7o to afford the similar results as
that obtained from TBS-substituted ones (entries 11–16). These
We propose the following mechanism for this reaction10,11
(Scheme 3). Deprotonation of a-H in 1 by base affords anion
4, which can be described as an allenic anion 5. Protonation of
5 either by remaining 1 or t-BuOH generated in the first step
furnishes linear allenic enolate 6. Protonation occurs preferentially
from the less hindered side of 6, that means, trans to the larger
alkynyl group, leading to the thermodynamically less stable (Z)-
isomer 2.
We next proceeded to investigate the trans-selective formation of
enynones from penta-2,4-diyn-1-ol 7. After much effort, we were
delighted to find that the best results were achieved by using KOH
Table 3 Stereoselective synthesis of trans-enynones
Entry
R1
R2
Equiv of KOH
Temp.
Product
Yield of 3 (%)a
3/2b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
TBS
TBS
TBS
TBS
TBS
TBS
TBS
TBS
TBS
Bu
Ph
Ph
Ph
Ph
Ph (7a)
0.15
0.1c
0.1d
0.15
0.3
0.3
0.3
0.3
0.3
0.8
0.1
0.1
0.3
0.1
0.1
0.3
30 ◦
rt
C
3a
3a
3a
3b
3c
3d
3e
3f
3p
3i
3j
3k
3l
3m
3n
3o
87 (4)
83
70
79 (5)
57
84
57
40 (18)
76
63
77
96/4
97/3
99/1
95/5
94/6
97/3
94/6
68/32
95/5
96/4
96/4
95/5
93/7
70/30
92/8
89/11
Ph (7a)
Ph (7a)
0 ◦
C
p-ClC6H4 (7b)
p-MeOC6H4 (7c)
2-thienyl (7d)
2-furyl (7e)
30 ◦
50 ◦
30 ◦
30 ◦
30 ◦
C
C
C
C
C
trans-C6H5CH CH (7f)
1-naphthyl (7g)
Ph (7i)
50 ◦C
50 ◦
rt
C
Ph (7j)
p-ClC6H4 (7k)
p-MeOC6H4 (7l)
trans-C6H5CH CH (7m)
2-thienyl (7n)
2-furyl (7o)
rt
77
66
50 ◦
30 ◦
30 ◦
30 ◦
C
C
C
C
47e(11)
68
Ph
Ph
66
a Isolated yield of the pure isomer 3. The isolated yields of the cis isomer 2 are shown in the◦parentheses. b Determined by 1H NMR of the crude mixture.
c KOtBu was used instead of KOH, rt, 40 min. d NaH was used instead of KOH, in THF, 0 C, 15 min. e Isomeric purity: 97 : 3.
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reactions likely proceed through tautomerization of an allenol
intermediate,12a,13a which leads to a thermodynamically stable trans
product 3.
and the residue was purified by flash chromatography on silica gel
(eluent: petroleum ether/dichloromethane = 3 : 1) to afford trans-
enynones 3a in 87% yield.
In conclusion, we have developed an efficient and metal-free
process for the stereoselective synthesis of cis- or trans-enynones
via base-catalyzed isomerization of 2,4-pentadiynyl silyl ethers or
their alcohol derivatives. These reactions provide convenient and
practical routes for enynones with a wide range of substitution
groups. The resulting enynone derivatives are attractive substrates
for further synthetic manipulations.
(Z)-5-(tert-Butyldimethylsilyl)-1-phenylpent-2-en-4-yn-1-one (2a)
Pale yellow solid. mp 43-44 ◦C. 1H NMR (CDCl3, Me4Si,
400 MHz) d 0.05 (s, 6H), 0.88 (s, 9H), 6.21 (d, J = 11.6 Hz, 1H),
6.93 (d, J = 12.0 Hz, 1H), 7.43–7.48 (m, 2H), 7.53–7.58 (m, 1H),
7.93–7.96 (m, 2H); 13C NMR (CDCl3, Me4Si, 100 MHz) d -5.0,
16.5, 26.0, 102.0, 105.8, 120.2, 128.5, 128.8, 133.0, 134.9, 137.4,
190.6. IR (neat) 3062, 3029, 2954, 2929, 2857, 2147, 1666, 1598,
1581, 1250, 1231, 839, 824, 776, 749, 691 cm-1. HRMS (EI) for
C17H22OSi [M]+: calcd 270.1440, found 270.1436.
Experimental section
Typical procedure for the selective formation of cis-enynones from
1,5-disubstituted-2,4-pentadiynyl silyl ethers catalyzed by KOtBu
(Method A)
(E)-5-(tert-Butyldimethylsilyl)-1-phenylpent-2-en-4-yn-1-one (3a)
1
Pale yellow oil. H NMR (CDCl3, Me4Si, 400 MHz) d 0.18 (s,
To a stirred solution of tert-butyl(5-(tert-butyldimethylsilyl)-1-
phenylpenta-2,4- diynyloxy)-dimethylsilane 1a (0.3 mmol, 115 mg)
in THF (3 mL) was added KOtBu powder (30 mol%, 0.09 mmol,
10 mg) at -78 ◦C under argon. After stirring for 25 min at the same
temperature, the mixture was quenched at -78 ◦C with 3 N HCl (ca.
0.5 mL). At this stage the color of the solution changed from dark
blue to pale yellow, then the suspension was warmed up to room
temperature and stirred for several minutes (during this process,
more 3 N HCl (ca. 5 mL) was added to the solution). The mixture
was extracted with EtOAc, and the combined organic extracts were
washed with brine, dried over MgSO4. The solvent was evaporated
in vacuo and the residue was purified by flash chromatography
on silica gel (eluent: petroleum ether/dichloromethane = 3 : 1) to
afford cis-enynones 2a and trans isomer 3a in 70% and 17% yields,
respectively.
6H), 0.98 (s, 9H), 6.88 (d, J = 15.6 Hz, 1H), 7.36 (d, J = 16.0 Hz,
1H), 7.44–7.48 (m, 2H), 7.54–7.58 (m, 1H), 7.93–7.96 (m, 2H); 13
C
NMR (CDCl3, Me4Si, 100 MHz) d -4.9, 16.6, 26.0, 103.2, 104.5,
124.8, 128.5, 128.7, 133.2, 134.1, 137.0, 188.8. IR (neat) 3063,
2954, 2930, 2857, 2119, 1663, 1599, 1586, 1282, 1209, 1005, 846,
824, 776 cm-1. HRMS (EI) for C17H22OSi [M]+: calcd 270.1440,
found 270.1442.
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant No. 20732008, 20821002), Chinese Academy of Science,
Science and Technology Commission of Shanghai Municipality
(Grant No. 08QH14030) and the Major State Basic Research
Development Program (Grant No. 2006CB806105) for financial
support.
Typical procedure for the selective formation of cis-enynones from
1,5-disubstituted-2,4-pentadiynyl silyl ethers catalyzed by DBU
(Method B)
Notes and references
To a solution of 1a (0.4 mmol, 154 mg) in THF (4 mL) at 0 ◦C was
added DBU (15 mol%, 0.06 mmol, 9.0 mL, in some cases, DBU
was used as a 0.15 M solution in THF). After stirring for 4 h◦at
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◦
0 C, the reaction mixture was quenched with 3 N HCl at 0 C
and extracted with EtOAc. The combined organic extracts were
washed with brine, dried over MgSO4. The solvent was evaporated
in vacuo and the residue was purified by flash chromatography
on silica gel (eluent: petroleum ether/dichloromethane = 3 : 1) to
afford cis-enynones 2a and trans isomer 3a in 70% and 19% yields,
respectively.
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Typical procedure for the selective formation of trans-enynones
from penta-2,4-diyn-1-ol catalyzed by KOH
To a stirred solution of KOH (15 mol%, 0.06 mmol, 3.4 mg) in
DCE (4 mL) at 30 ◦C was added 5-(tert-butyldimethylsilyl)-1-
phenylpenta-2,4-diyn-1-ol 7a (0.4 mmol, 108 mg) under argon
(KOH was weighed in a glove box due to its hygroscopic property).
After stirring for 1 h at the same temperature, the reaction mixture
was quenched with 3 N HCl and extracted with EtOAc (for the
cases of 3j–3o, the reactions were quenched by saturated NH4Cl
solution). The combined organic extracts were washed with brine,
and then dried over MgSO4. The solvent was evaporated in vacuo
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4810 | Org. Biomol. Chem., 2010, 8, 4806–4810
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