Use of a semihollow-shaped triethynylphosphane ligand for efficient formation of six- and seven-membered ring ethers through gold(I)-catalyzed cyclization of hydroxy-tethered propargylic esters

Article abstract of DOI:10.1002/adsc.201200949

The formation of six- and seven-membered ring ethers from hydroxy-tethered propargylic esters was efficiently catalyzed by a cationic gold(I) complex with a semihollow-shaped triethynylphosphane ligand. This gold catalysis showed a tolerance toward the reactions of primary, secondary, and tertiary alcohol substrates with various substitution patterns. A sterically congested 2,2,6,6-tetraalkyl-substituted tetrahydropyran derivative as well as 6,6- and 7,6-fused bicyclic diethers were obtained in useful yields. In addition, the gold catalysis was applicable to the reaction of a sulfonamide-tethered propargylic ester to give a piperidine derivative. Copyright

Full text of DOI:10.1002/adsc.201200949

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DOI: 10.1002/adsc.201200949
Use of a Semihollow-Shaped Triethynylphosphane Ligand for
Efficient Formation of Six- and Seven-Membered Ring Ethers
through Gold(I)-Catalyzed Cyclization of Hydroxy-Tethered
Propargylic Esters
Hideto Ito,a Ayumi Harada,a Hirohisa Ohmiya,a and Masaya Sawamuraa,*
a
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
Fax : (+81)-11-706-3749; phone: (+81)-11-706-3434; e-mail: sawamura@sci.hokudai.ac.jp
Received: October 29, 2012; Revised: December 17, 2012; Published online: February 22, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200949.
Abstract: The formation of six- and seven-mem-
bered ring ethers from hydroxy-tethered propargyl-
ic esters was efficiently catalyzed by a cationic
gold(I) complex with a semihollow-shaped triethyn-
Previously, we showed that a semihollow-shaped
triethynylphosphane (L1) ligand is useful for gold(I)-
catalyzed cyclization of various alkynes tethered to
different nucleophilic functional groups such as b-
keto esters,[6a,b] alkenes,[6a] silyl enol ethers[6c] and sul-
fonamides,[6d] allowing otherwise difficult ring forma-
tions (Figure 1). The most characteristic feature of the
catalysis with L1 is its effectiveness toward the forma-
tion of six- and seven-membered rings, which are en-
tropically unfavored – unlike the formation of the cor-
responding five-membered rings. We propose that the
gold-bound alkyne substrate adopts a bent conforma-
tion to fit in the cavity of the semihollow ligand, re-
sulting in entropy-based rate enhancement of the re-
action between the nucleophile and the ionized
alkyne moiety due to close proximity between the
two reaction centers.
ACHTUNGTRENNUNGylphosphane ligand. This gold catalysis showed a tol-
erance toward the reactions of primary, secondary,
and tertiary alcohol substrates with various substitu-
tion patterns. A sterically congested 2,2,6,6-tetraalk-
yl-substituted tetrahydropyran derivative as well as
6,6- and 7,6-fused bicyclic diethers were obtained in
useful yields. In addition, the gold catalysis was ap-
plicable to the reaction of a sulfonamide-tethered
propargylic ester to give a piperidine derivative.
Keywords: cyclization; gold; propargylic esters;
seven-membered rings; triethnylphosphane
Here, we report use of the semihollow-shaped tri-
AHCTUNGTREGeNNUN thynylphosphane (L1) for efficient formation of six-
and seven-membered ring ethers through gold(I)-cat-
Gold-catalyzed cyclization has emerged as a powerful
strategy for constructing carbo- and heterocyclic com-
pounds.[1] Recently, Brabander and co-workers report-
ed that [PtCl2ACHTUNGTRENNUNG(C2H4)]2 and AuCl catalysts can pro-
mote cyclization of hydroxy-tethered propargylic
esters to give tetrahydropyran derivatives (THPs) 2, 3
and 4 in good yields.[2] This reaction likely proceeds
via platinum- or gold-catalyzed 6-exo-trig intramolec-
ular hydroalkoxylation of e-hydroxylalkylallenyl car-
boxylate A, which is formed through gold- or plati-
num-catalyzed [3,3]sigmatropic rearrangement[4] of
propargylic ester 1 (Scheme 1).[5] While this novel cy-
ACHTUNGTRENNUNGclization affords facile synthetic routes to cyclic
ethers[2] and amides,[3] the relatively high catalyst
loading and the limitation to the formation of five-
and six-membered rings remain issues to be over-
come.
Scheme 1. Brabanderꢀs platinum- and gold-catalyzed cycliza-
tion of hydroxy-tethered propargylic esters.
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to Brabanderꢀs cyclic ether synthesis was conducted
for the six-membered ring formation with a propargyl-
ic benzoate (1a, 0.2 mmol) having a 4-hydroxybutyl
substituent at the propargylic position. The results are
summarized in Table 1 (see the Supporting Informa-
tion for details). The cationic gold(I) complex – Au-
AHCTUNGRTEG(NNUN NTf2)(L1) – with 0.5 mol% loading in CH2Cl2 (4 mL)
caused a rapid and complete conversion of 1a at
room temperature (<15 min) to afford a mixture of
tetrahydropyran derivatives bearing alkenyl benzoate
(2a, E/Z=80:20), acylmethyl (3a), or alkynyl (4a)
pendant moieties in 35%, 19%, and 46% yields, re-
spectively (entry 1). After alcoholysis of the benzoyl
group of 2a (K2CO3/MeOH, room temperature),
ketone 3a and alkyne 4a were isolated in 55% and
40% yields, respectively. Notably, the reported reac-
Figure 1. Semihollow-shaped triethynylphosphane L1.
tion conditions with AuCl or [PtCl2ACHTUNRGTNEUNG(C2H4)]2 catalysts
alyzed cyclization of hydroxy-tethered propargylic (THF, room tempertaure) produced non-alkyne-type
esters. Mild reaction conditions, a broad substrate compounds (2 or 3) or an alkyne-type compound (4)
scope for the synthesis of various tetrahydropyran in chemoselective manners, respectively.[2]
(THP) derivatives with different substitution patterns,
and applicability toward seven-membered ring forma- SbF6 affected neither the rate of the reaction nor the
Changing the counteranion to OTfÀ or
À
tion are noteworthy aspects of this catalysis.
An initial screening of reaction conditions for the other hand, AuAHCTUNTGRENNUNG
product distribution (Table 1, entries 2 and 3). On the
(BF4)(L1) caused almost no reaction
application of the semihollow phosphane ligand (L1) (entry 4). A marked chemoselectivity change in pref-
Table 1. Screening of reaction conditions for the gold-catalyzed cyclization of 1a.[a]
Entry Ligand
X
Temp. [8C] Time [h] Conv. [%] 1H NMR yield, [%]
Isolated yield after methanolysis, [%]
2a (E/Z)
3a[c] 4a 3a
4a
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L1
L1
PPh3
NTf2 25
OTf 25
SbF6 25
0.25
0.25
0.25
3
1
4
100
100
100
0
100
100
100
75
75
93
100
44
35 (80:20) 19
41 (73:27) 11
34 (86:14) 10
46 55
40
[b]
[b]
[b]
48
56
0
20
12
0
0
0
3
[b]
BF4
NTf2
25
0
0
0
N.A.[c]
N.A.[c]
[b]
[b]
70 (88:12) 10
[b]
[b]
NTf2 À20
NTf2 À40
NTf2 À40
80 (82:18)
97 (87:13)
8
3
4
96
0
[b]
[b]
24
24
24
24
4
58 (49:51) 16
61 (49:51) 14
73 (49:51) 10
[b]
[b]
[b]
[b]
9
P
G
10
11
12
X-Phos
IPr
NTf2 À40
NTf2 À40
NTf2 À40
95 (51:49)
43 (55:45)
5
8
0
0
93
0
[b]
[b]
IPr
[a]
[b]
[c]
Reaction conditions: 1a (0.20 mmol), Au(X)(L1) (0.5 mol%), CH2Cl2 (4.0 mL).
Not isolated.
N.A.: not applicable.
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Use of a Semihollow-Shaped Triethynylphosphane Ligand for Efficient Formation
erence to the non-alkyne-type compounds 2a and 3a 49% due to the formation of a significant amount of
over the alkyne 4a was observed upon reducing the alkynylated THP 4j (entry 9). Interestingly, the sub-
reaction temperature (entries 5–7). The alkyne forma- strate 1k, which contains tertiary alcohol and tertiary
tion was completely inhibited by conducting the reac- propargylic benzoate moieties, furnished a sterically
tion at À408C for 4 h, and the pure ketone 3a was ob- congested 2,2,6,6-tetrasubstituted THP derivative (3k)
tained in 96% isolated yield after alcoholysis in 65% isolated yield along with the alkyne 4k in
(entry 7).[7] Under these reaction conditions, E/Z 14% yield (entry 10).
ratios were in the range of 73:27 to 90:10. Next, annulation of THP-tethered propargylic ben-
Next, various cationic gold complexes [AuHCATUNGTENRN(GNU NTf2) zoate 1l for synthesizing a fused bicyclic diether com-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(OPh)3, 3l) (ꢀ9 h), and 3l was obtained in 91% isolated yield
X-Phos, and bulky NHC ligand IPr (Table 1, en- with excellent diastereoselectivity (>20:1) after alco-
tries 7–12). For example, the reaction with PPh3 or holysis. This reaction could also be conducted on
PACHTUNGTRENNUNG(OPh)3 did not reach full conversion even after 24 h a scale 10 times larger (2 mmol) with a 2.5 mol% cat-
(entries 8 and 9). Since the electron-donating power alyst loading, affording 3l in 80% isolated yield
of the latter is as low as that of L1,[6e] these results (entry 12).
(entries 7 vs. 9) suggest that the rate enhancement
This strategy for heterocycle synthesis could be ex-
with L1 is not due to an electronic effect. The reac- tended to the synthesis of N-heterocyclic compounds.
tion with sterically demanding and strongly electron- The reaction of propargylic ester 1m, bearing a p-tol-
donating ligands such as X-Phos and IPr gave higher uenesulfonamide moiety as a nucleophile, underwent
conversions than those with PPh3 and PACTHNURGTNEUNG(OPh)3 under cyclization in a similar fashion with a 5 mol% catalyst
otherwise identical conditions, but these reactions loading, affording piperidine derivative 3m in 79%
were much slower than that with L1 (entries 10 and yield (Table 2, entry 13).
11). In fact, the reaction with the IPr ligand showed
only 44% conversion of 1a after 4 h (entry 12).
Encouraged by the successful six-membered ring
formation with the hydroxy-tethered propargylic
Next, we investigated the substrate scope of the esters, we next explored the corresponding seven-
gold catalysis with L1 for various substitution patterns membered ring formation with substrates having an
(Table 2). Effects of substituents at the alkyne termini additional carbon in the tether. In general, seven-
are shown in entries 1–5. As the steric demand of the membered ring formation is more difficult than five-
substituents increased, reaction times became longer and six-membered ring formations due to entropic
(entries 1–4). However, the substrates (1b–e) with reasons. Seven-membered ring forming gold catalysis
Me, n-Bu, i-Bu, or Cy groups were quantitatively con- is particularly rare.[8] Results of the gold catalysis with
verted into the corresponding THP-substituted alken- semihollow-shaped triethynylphosphane L1 for seven-
yl benzoates (2b–e) with reasonable reaction times membered ring ether formation are summarized in
with a 0.5 mol% catalyst loading, and the ketones 3b– Table 3. To our delight, a simple substrate (1n) with
e were obtained in high yields after alcoholysis with a primary alcohol moiety underwent seven-membered
K2CO3/MeOH. Although an acetal-containing sub- ring formation in CH2Cl2 at room temperature with
strate 1f showed somewhat lower reactivity, the cycli- 2.5 mol% catalyst loading. The substrate consumption
zation proceeded efficiently with a 2.5 mol% catalyst under the gold catalysis was completed in 24 h, but
loading to give 3f in 89% isolated yield (entry 5). In the isolated yield for seven-membered ring ether 3n
all cases, traces (<3%) of alkyne products 4b–f were was only 49%, probably due to the formation of oli-
detected in the crude product mixtures.
gomeric by-products (entry 1). We found that a clean-
The steric effect around the nucleophilic oxygen er reaction occurred in THF to improve the yield to
atom was also investigated (Table 2, entries 6–8). The 80% (entry 2). High dilution conditions (0.02M, 1n)
reactions of the methyl- or isopropyl-substituted sec- increased the yield of 3n further to 88%, but a pro-
ondary alcohols (1g, 1h) were slower than that of pri- longed reaction time (18 h) was necessary (entry 3).
mary alcohol 1a, but completed in 24 h to give THPs The catalyst loading could be reduced to 1 mol%
3g and 3h in 86% and 67% yields as diastereomeric without a significant decrease in the yield (83%,
mixtures (87:13 for both cases) (entries 6 and 7). Ter- entry 4).
tiary alcohol 1i cyclized more rapidly than the pri-
mary and secondary alcohols (1a, g, h) despite in- proceeded with the IPr ligand [1 mol% Au
creased bulkiness around the nucleophilic center (IPr)] or with the ligand-free AuCl system (2 mol%)
The seven-membered ring formation with 1n also
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(entry 8). The reaction of the primary alcohol-teth- under high dilution conditions, but the conversion ef-
ered tertiary propargylic ester 1j was even more ficiencies and the product yields were not satisfactory,
rapid, but the yield of ketone (3j) was decreased to
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Table 2. Substrate scope of the hydroxy- or sulfonamide-tethered propargylic esters catalyzed by triethynylphosphane (L1)-
gold complex.[a]
[a]
Reaction conditions: 1 (0.20 mmol), Au
coholysis with K2CO3 (5 equiv.) in MeOH (4 mL) at room temperature for 2 h.
Isolated yield.
Diastereomeric ratio (dr) was determined by GC.
(NTf2)(L1) (0.5 mol%), CH2Cl2 (4.0 mL) at À408C. Ketone 3 was isolated after al-
[b]
[c]
[d]
[e]
[f]
Au
Corresponding alkynylated THP derivative was obtained: entry 9, 4j, 7%; entry 10, 4k, 14%.
Au(NTf2)(L1) (4 mol%).
1l (2.0 mmol), Au(NTf2)(L1) (2.5 mol%), CH2Cl2 (10 mL).
AuNTf2(L1) (5 mol%).
ACHTUNGTRENNUNG(NTf2)(L1) (2.5 mol%).
ACHTUNGTRENNUNG
[g]
[h]
AHCTUNGTRENNUNG
being much lower than those with the L1 ligand to afford 6,7-fused bicyclic diether 3o in 70% yield as
(Table 3, entries 5 and 6). a 1:1 diastereomeric mixture (Table 3, entry 7). The
Annulation of THP-tethered substrate 1o was con- relatively low yield and the demand of catalyst load-
ducted with a 5 mol% loading of the Au-L1 catalyst ing as high as 5 mol% may be due to impurities that
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Use of a Semihollow-Shaped Triethynylphosphane Ligand for Efficient Formation
Table 3. Seven-membered ring formation through gold-catalyzed cyclization of hydroxy-tethered propargylic esters.[a]
[a]
Reaction conditions: 1 (0.20 mmol), Au catalyst in the indicated solvent at room temperature for the indicated time.
Ketone 3 was isolated after alcoholysis with K2CO3 (5 equiv.) in MeOH (4 mL) at room temperature for 2 h.
[b]
Isolated yield.
[c]
1
Determined by H NMR.
remained in the starting material 1o (see the Support- markably, a sterically congested 2,2,6,6-tetraalkyl-sub-
ing Information for details). Cyclic tertiary alcohol stituted THP derivative was obtained in useful yield.
substrate 1p underwent efficient cyclization to afford Furthermore, 6,6- and 7,6-fused bicyclic diethers with
a 5,6-spiro compound 3p in good yield (82%, entry 8). a carbonyl-containing pendant were obtained through
The primary alcohol substrate (1q) with an aromatic annulation of THP-tethered propargylic esters. In ad-
ring in the tether showed relatively high reactivity, dition, the gold catalysis was applicable to the reac-
probably due to the conformational constraint in the tion of a sulfonamide-tethered propargylic ester to
tether, to give the benzo-fused seven-membered ring give a piperidine derivative. It is proposed that the
ether (benzoxepin) 3q in useful yield (61%, entry 9). enhanced efficiency of these reactions with the semi-
In summary, we have demonstrated the utility of hollow ligand is due to close proximity between the
a semihollow-shaped triethynylphosphane ligand for nucleophilic center and the ionized alkene moiety of
formation of six- and seven-membered ring ethers the gold-bound substrate that adopts a bent confor-
through gold(I)-catalyzed cyclization of hydroxy-teth- mation to fit in the cavity of the ligand. Further appli-
ered propargylic esters, which was triggered by a prop- cation of this concept is in progress.
argylic 1,3-acyl shift reaction. The gold catalysis with
the semihollow phosphane showed a tolerance toward
the reactions of primary, secondary, and tertiary alco-
hol substrates with various substitution patterns. Re-
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Hideto Ito et al.
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Experimental Section
[2] a) J. K. De Brabander, B. Liu, M. Qian, Org. Lett. 2008,
10, 2533–2536; b) Q. Liang, J. K. De Brabander, Tetrahe-
dron 2011, 67, 5046–5053; c) Y.-M. Wang, C. N. Kuz-
niewski, V. Rauniyar, C. Hoong, F. D. Toste, J. Am.
Chem. Soc. 2011, 133, 12972–12975.
Typical Procedure for the Ether Ring Formation with
Hydroxy-Tethered Propargylic Benzoate Catalyzed
by Gold Complex with Semihollow-Shaped Phos-
phane L1 (Table 1, Entry 7)
[3] J. Huang, X. Huang, B. Liu, Org. Biomol. Chem. 2010, 8,
2697–2699.
[4] For selected recent examples of Au-catalyzed reactions
initiated by 1,3-shift of propargylic esters not listed in
refs.[2a,3], see: a) A. S. Dudnik, T. Schwier, V. Gevorgyan,
Org. Lett. 2008, 10, 1465–1468; b) A. Correa, N. Marion,
L. Fensterbank, M. Maracria, S. P. Nolan, L. Cavallo,
Angew. Chem. 2008, 120, 730–733; Angew. Chem. Int.
Ed. 2008, 47, 718–721; c) N. Marion, G. Lemiꢄre, A.
Correa, C. Costabile, R. S. Ramꢅn, X. Moreau, P. de
Frꢃmont, R. Dahmane, A. Hours, D. Lesage, J.-C. Tabet,
J.-P. Goddard, V. Gandon, L. Cavallo, L. Fensterbank,
M. Malacria, S. P. Nolan, Chem. Eur. J. 2009, 15, 3243–
3260; d) P. Mauleꢅn, J. L. Krinsky, F. D. Toste, J. Am.
Chem. Soc. 2009, 131, 4513–4520; e) Y. Peng, L. Cui, G.
Zhang, L. Zhang, J. Am. Chem. Soc. 2009, 131, 5062–
5063; f) G. Zhang, Y. Peng, L. Cui, L. Zhang, Angew.
Chem. 2009, 121, 3158–3161; Angew. Chem. Int. Ed.
2009, 48, 3112–3115; g) D. Garayalde, E. Gꢅmez-
Bengoa, X. Huang, A. Goeke, C. Nevado, J. Am. Chem.
Soc. 2010, 132, 4720–4730; h) P. Num, S. Gaillard, A.
Poater, L. Cavallo, S. P. Nolan, Org. Biomol. Chem.
2011, 9, 101–104.
[5] The formation of 2a would cleary suggest that the pres-
ent reaction does not proceed through intramlecular 1,4-
addition of O-nucleophile to a,b-unsaturated ketone
which is produced by Au- and Pt-catalyzed Meyer–Shus-
ter rearrangement of propargylic alcohol nor hydration
of allenyl benzoate A. For corresponding works on gold-
catalyzed Meyer–Shuster rearrangement and intramolec-
ular 1,4-addition of O-nucleophile, see: a) H. H. Jung,
P. F. Floreancig, J. Org. Chem. 2007, 72, 7359–7366;
b) H. H. Jung, P. F. Floreancig, Org. Lett. 2006, 8, 1949–
1951; c) Qi. Liang, M. Qian, M. Razzak, J. K. De
Brabander, Chem. Asian J. 2011, 6, 1958–1960.
[6] a) A. Ochida, H. Ito, M. Sawamura, J. Am. Chem. Soc.
2006, 128, 16486–16487; b) H. Ito, Y. Makida, A.
Ochida, H. Ohmiya, M. Sawamura, Org. Lett. 2008, 10,
5051–5054; c) H. Ito, H. Ohmiya, M. Sawamura, Org.
Lett. 2010, 12, 4380–4383; d) H. Ito, T. Harada, H.
Ohmiya, M. Sawamura, Beilstein J. Org. Chem. 2011, 7,
951–959; e) A. Ochida, M. Sawamura, Chem. Asian J.
2007, 2, 609–618.
[7] We also examined whether alkyne product 4a is further
reacted with benzoic acid or H2O under the reaction
condition of entry 7. However, no alkenyl benzoate 2a
or ketone 3a was obtained at all.
[8] To the best of our knowledge, gold-catalyzed hydroal-
koxylation of alkenes, allenes and alkynes in a 7-exo
manner has not been reported. For examples of gold-
catalyzed 7-endo-dig hydroalkoxylation of alkynes, see:
K. Wilckens, M. Uhlemann, C. Czekelius, Chem. Eur. J.
2009, 15, 13323–13326.
To a stirred solution of hydroxy-tethered propargylic ester
1a (66.6 mg, 0.198 mmol) in degassed dry CH2Cl2 (3 mL),
which was cooled to À408C, a solution of Au
(NTf2)(L1)[5a]
ACHTUNGTRENNUNG
(2.6 mg, 1.0 mmol, 0.5 mol%) in CH2Cl2 (0.25 mL) was
added dropwise. Liquids remaining in the micro tube and
the syringe were washed into the reaction mixture with
CH2Cl2 (3ꢂ0.25 mL). After the reaction was complete
(monitored by TLC), the reaction mixture was passed
through a pad of silica gel and concentrated to dryness in an
open vial. The yield of THP-substituted alkenyl benzoate 2a
1
was determined by H NMR integration with Cl2CHCHCl2
as an internal standard. Next, the crude product was treated
with K2CO3 (140 mg, 1.0 mmol) in MeOH (4 mL) under air
at room temperature for 2 h. After consumption of 2a was
confirmed by TLC, the mixture was concentrated into half
its original volume, and ether (5 mL) and H2O (10 mL)
were added. The organic layer was separated, and the aque-
ous layer was extracted with Et2O (3ꢂ5 mL). The organic
layers were combined, dried over MgSO4, filtered, and con-
centrated under reduced pressure. The residue was purified
by flash chromatography on silica gel (hexane/EtOAc 99:1
to 83:17) to give ketone 3a as a colorless oil; yield: 44.0 mg
(96%).
Acknowledgements
This work was supported by a Grant-in-Aid for Challenging
Exploratory Research. H.I. thanks JSPS for a fellowship.
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