Palladium(II)- and mercury(II)-catalyzed rearrangements of propargyl acetates

Article abstract of DOI:10.1016/j.tet.2007.08.008

The scope and utility of the metal-catalyzed rearrangement of propargyl acetates first reported by Rautenstrauch were expanded. Treatment of a series of appropriate acetate substrates with Pd(II)- and Hg(II)-catalysts afforded synthetically useful fused 5

Full text of DOI:10.1016/j.tet.2007.08.008

Tetrahedron 63 (2007) 10646–10656
Palladium(II)- and mercury(II)-catalyzed rearrangements
of propargyl acetates
*
Patrick A. Caruana and Alison J. Frontier
Department of Chemistry, University of Rochester, 414 Hutchison Hall, Rochester, NY 14627, USA
Received 15 June 2007; revised 31 July 2007; accepted 1 August 2007
Available online 8 August 2007
Abstract—The scope and utility of the metal-catalyzed rearrangement of propargyl acetates first reported by Rautenstrauch were expanded.
Treatment of a series of appropriate acetate substrates with Pd(II)- and Hg(II)-catalysts afforded synthetically useful fused 5,6-bicyclic-1,4-
cyclopentadienyl acetates and 2-cyclopentenones. It was found that the substituents at the terminal alkynyl and alkenyl positions of the acetate
substrate had a significant impact on the outcome of the reaction.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
This rearrangement came to our attention because one of the
potential mechanisms proposed for the reaction was a 4p-
electrocyclization, with generation of the pentadienyl cation
intermediate via an unusual reaction pathway. In contrast
to the usual Lewis or Bronsted acid-promoted Nazarov elec-
trocyclization, the oxypalladation creates a transient vinyl
palladium species at one of the carbon termini involved in
the putative electrocyclization. The ability to place the cata-
lyst so close to the site of bond formation led us to wonder
whether enantioselectivity could be controlled using chiral
palladium(II) complexes.2,3 Several studies that appeared
during the course of our work (vide infra) have prompted
us to report our findings.4
In 1984, Rautenstrauch reported a unique Pd(II)-mediated
rearrangement of 1-ethynyl-2-propenyl acetates to give
1,4-cyclopentadienyl acetates, which were cleaved in situ
to 2-cyclopentenones (Scheme 1).1 It was proposed that
coordination of the Pd(II) species to the alkyne effects 1,2-
acetoxy migration to give a metal carbene species that is
in equilibrium with a pentadienyl cation. Both intermediates
can undergo cyclization and ester cleavage to generate the
cyclopentenone products. The study described the optimiza-
tion of reaction conditions and the rearrangement results for
five different substrates.
In 2002, Fensterbank and co-workers reported the PtCl2-
catalyzed tandem rearrangement/cyclopropanation of dien-
ynes to give fused bicyclic cyclopropanes (see Eq. 1).5 The
reaction mechanism proposed involved a metal carbene in-
termediate generated through 1,2-acetoxy migration, similar
to the reaction pathway proposed by Rautenstrauch in 1984
for palladium(II). Sarpong and co-workers have also re-
ported platinum(II)-catalyzed pentannulation reactions of
propargylic esters involving the putative formation of metal
carbene intermediates by a similar pathway.6
O
O
R1
PdCl
O
2
OAc
R2
2.5-10 mol %
PdCl2(MeCN)2
AcO
R1
R2
R1
0.7M MeCN,
60 - 80 °C
R2
R1 = alkyl or H
O
R
2 = alkyl or H
R1
R2
+
O
5 mol % PtCl2,
PdCl
R
OX
R
toluene, 80 °C
R1
2-9 hrs
For X= COCH3,
p-COC6H4NO2
OX
R2
50 - 89%
isolated yield
62-88% yield
(1)
R
Scheme 1. Rautenstrauch rearrangement of propargyl acetates.
via
OX
PtCl2
* Corresponding author. E-mail: frontier@chem.rochester.edu
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.08.008

P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
10647
O
Ruthenium-7 and gold-catalyzed reactions8 involving 1,2-
Table 1. Optimization of the reaction conditions
acetoxy migration have also been reported. Recently, Toste
and co-workers reported a gold(I)-catalyzed transformation
of propargylic pivaloates to 2-cyclopentenones (Eq. 2).9 Sig-
nificantly, a high degree of chirality transfer was observed
for reactions of chiral substrates (Eq. 3). Computational
studies focused on Toste’s work indicate that reactions that
occur with chirality transfer proceed through helical, penta-
dienyl cationic intermediates.10
OAc
AcO
Ph
Conditions
H
MeOH/
cat. p-TsOH
quench
Ph
Ph
7a
30
8a
Entrya Catalyst
Temp (ꢀC) Time (min) Yieldb (%)
1
2
3
4
5
6
7
8
9
5 mol % Cl2Pd(PhCN)2 70
5 mol % Cl2Pd(MeCN)2 60
10 mol % Cl2Pd(PPh3)2 60
5 mol % PdCl2
15 mol % PdCl2
5 mol % PdCl2
5 mol % PdCl2
5 mol % PdCl2
5 mol % Pd(OAc)2
40
15
960
20
30
15
360
120
15
44c
68
OPiv
d
2 - 5 mol % Ph3PAuOTf,
R3
R4
R3
R4
O
R3
R4
CH3CN, r.t.
60/70
65
17
71
92
(2)
(3)
45-80%
R5
R5
R5
60
rt
67
OPiv
5 mol% Ph3PAuSbF6,
CH3CN, -20 °C
e
60/75
60
O
R3
R4
21
80-88%
a
R5
77-96% ee
H
Reactions were run in MeCN (0.7 M) unless otherwise specified.
Yields indicate isolated product formed upon quenching the reaction with
MeOH, cat. p-TsOH.
b
79-98% ee
c
d
e
AcOH (1 equiv) was added.
Reaction run in DCE; starting material was recovered.
Reaction run in DCE; crude NMR showed 1:1 mixture of starting mate-
rial/enol ether.
2. Results
Initial studies focused on extending the limited scope of
Rautenstrauch’s pioneering work. In one of his five cases,
an unusual cyclopentenone fused to a macrocycle was gen-
erated. This result suggested that it should be possible to
achieve the synthesis of fused bicyclic ring systems with
an improved protocol. Furthermore, the introduction of sub-
stituents at the terminal alkenyl and alkynyl positions should
lead to new arrays of substitution.
thetemperaturefrom60to70 ꢀCinacetonitrileinthepresence
of PdCl2 correlated with a substantial decrease in isolated
yield (entries 4–6). The reaction did proceed in a sluggish
manner at room temperature, albeit in lower yield (entry 7).
Other Pd(II) complexes (entries 1–3 and 9) and solvents (en-
tries 3 and 8) gave inferior results in comparison to PdCl2 in
acetonitrile. The optimal conditions found were treatment of
7a in acetonitrile with 5 mol % PdCl2 for 15 min at 60 ꢀC,
which afforded 8a in 92% isolated yield upon p-TsOH/
MeOH quench and silica gel chromatography (entry 6).12
Thus, substrates 7a–d were synthesized as shown in
Scheme 2. Ketones 1 and 2 were subjected to both conditions
A and B, leading to the formation of substituted propargyl
alcohols 3–6. Reactions of ketones with lithiated ethyl pro-
piolate led to alkynoates, while reactions with lithiated trime-
thylsilylacetylene ultimately led to terminal alkynes. The
resulting tertiary alcohols were acetylated under standard
conditions to give the desired propargyl acetates 7a–d in
moderate to good yields. Purified 7a could only be attained
by recrystallization, as efforts at column chromatography
resulted in quantitative conversion to allylic acetate 9 (entry
2, Table 2) via a known allylic rearrangement process.11
With optimized conditions in hand,13 propargyl acetates
7b–d were subjected to the reaction conditions (Table 2).
The reaction tolerated various substituents at the unsaturated
positions, leading to the formation of fused 5,6-bicyclic
compounds 8b–d in moderate to excellent yields. The effi-
ciency of the reaction was largely dependent on substitution
patterns at the terminal alkenyl and alkynyl moieties. The
alkyl cases were more sluggish in comparison to the phenyl
cases (entries 1 vs 4; 3 vs 5). Reactions with alkynyl esters
led to the isolation of stabilized enol acetates in excellent
yields (entries 3 and 5). Reaction times and yields for alky-
noates were similar to those of their terminal alkyne analogs
(entries 1 vs 3; 4 vs 5).
Optimized conditions for the rearrangement of 7a to 8a were
established after extensive experimentation (Table 1). Tem-
peraturewasfoundtobeakeyvariable inthisstudy. Increasing
R'
R'
A
Li
CO2Et,
THF, -78 °C
or
Ac2O, NEt3,
cat. DMAP,
CH2Cl2
O
HO
AcO
R
R
R
i)
B
Li
TMS
ii) K2CO3, MeOH
1: R = Ph
2: R = iPr
3: R = Ph, R' = H;
B; 71%, 2 steps
7a: R = Ph, R' = H;
49%
4: R = Ph, R' = CO2Et; 7b: R = Ph, R' = CO2Et;
A; 95%
89%
5: R = iPr, R' = H;
B; 70%, 2 steps
7c: R = iPr, R' = H;
61%
6: R = iPr, R' = CO2Et; 7d: R = iPr, R' = CO2Et;
A; 89%
93%
Scheme 2. Synthesis of propargylic acetates 7a–d.

10648
P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
Table 3. Rearrangement of propargyl acetates 14a–f
Table 2. Rearrangement of propargyl acetates 7a–d
Entry
1
Acetate
AcO
Time (min)
15
Producta
Yieldb (%)
Entry
Acetate
AcO
Time (min)
Producta
Yieldb (%)
O
O
92c
56c
Ph
Ph
1
35
Ph
14a
7a
8a
15a
AcO
CO2Et
CO2Et
OAc
AcO
AcO
>99d
Ph
2
35
88c
2
3
1
7a
9
14b
15b
CO2Et
Ph
OAc
AcO
OBn
OBn
n-Bu
AcO
AcO
AcO
AcO
15
97e
CO2Et
3
4
5
5
64d
Ph
8b
14c
7b
15c
O
d,e
57e
—
—
—
4
50
45
14d
7c
8c
OBn
AcO
OAc
CO2Et
d,e
5
5
5
AcO
CO2Et
5
60e
14e
7d
OBn
8d
AcO
3
a
In cases where ketone is the product, yield refers to the product formed
after quenching the reaction with MeOH, cat. p-TsOH.
Higher catalyst loadings (10 mol %) were sometimes required to drive the
reaction to completion.
d,e
6
b
14f
c
d
e
Conditions: 5 mol % PdCl2, MeCN (0.7 M), 60 ꢀC.
Conditions: silica gel, rt.
a
Compound 15a formed from quenching the reaction with MeOH, cat.
p-TsOH.
Higher catalyst loadings (10–20 mol %) were required to drive the reac-
tion to completion in certain cases.
Conditions: 10 mol % PdCl2, MeCN (0.7 M), 60 ꢀC.
Conditions: 20 mol % PdCl2, MeCN (0.7 M), 60 ꢀC.
Reaction yielded predominantly starting material+complex mixture that
appeared to contain target enol acetate.
Conditions: 10 mol % PdCl2, MeCN (0.7 M), 60 ꢀC.
b
c
d
e
In an effort to broaden the scope of the reaction, another class
of substrates expected to deliver a different type of fused
5,6-bicyclic system was examined. As shown in Scheme 3,
propargyl acetates 14a–f were synthesized in good yields
according to the routes used in the synthesis of 7a–d and
then subjected to the rearrangement conditions (Table 3).
Substrates 14a and 14b cyclized smoothly to give products
ketone 15a and enol acetate 15b,14 respectively, in moderate
to excellent yields (entries 1 and 2). A search for other sub-
stituents that would accelerate and/or facilitate the reaction
led us to study the cyclization of 14c–f. Propargyl acetate
14c, when treated with 20 mol % PdCl2 in acetonitrile at
60 ꢀC, underwent complete conversion in 5 min to deliver
enol acetate 15c, which was isolated in 64% yield. In
contrast, reactions of butyl-substituted analog 14d and
benzyloxy derivatives 14e and 14f afforded complex
A
Li
R
R
R
Ac2O, NEt3,
cat. DMAP,
CH2Cl2
O
AcO
HO
THF, -78 °C
or
i)
B
Li
TMS
10: R = H;
14a: R = H;
93%
ii) K2CO3, MeOH
B; 80%, 2 steps
11: R = CO2Et;
14b: R = CO2Et;
86%
A; 52%
12: R = CH2OBn;
A; 91%
14c: R = CH2OBn;
99%
13a: R =butyl;
A; 69%
14d: R = butyl;
99%
13b: R = (CH2)2OBn;
A; 91%
14e: R = (CH2)2OBn;
97%
13c: R = (CH2)4OBn;
A; 76%
14f: R = (CH2)4OBn;
89%
Scheme 3. Synthesis of propargylic acetates 14a–f.

P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
10649
mixtures containing significant amounts of starting mate-
rial.15
A more detailed view of the cyclization step is depicted in
Scheme 5. 4p-Conrotatory electrocyclization of polarized
pentadienyl cation 19 (Nazarov cyclization)17 would give
stabilized allylic cation 23, which upon elimination of the
palladium moiety would afford the desired product 21 and
the regenerated Pd(II) catalyst.18 If palladium carbene 20
is the reactive intermediate, then a few different pathways
to enol acetate 21 exist. Oxidative cyclization of 20 to give
palladium(IV) intermediate 24 followed by reductive elimi-
nation would lead to the desired species, as originally
proposed by Rautenstrauch. Another feasible pathway in-
volves intramolecular cyclopropanation of carbene 20 to
give fused [2.1.0] bicyclic intermediate 25, which upon
ring opening would yield enol acetate 21. Since both the car-
bene-based routes to enol acetate 21 invoke high-energy
intermediates 24 and 25, the 4p-electrocyclic mechanism
is widely accepted.10
Unfortunately, the chiral cationic palladium(II) complexes
we tested were not effective catalysts for the rearrangement,16
so this strategy did not appear to be viable for the develop-
ment of a catalytic, enantioselective variant of the reaction.
3. Discussion
While the mechanism is not fully understood, a scheme de-
picting several possibilities is shown below (Scheme 4). Co-
ordination of the palladium(II) species to the electron-rich
alkyne effects 1,2-acetoxy migration to give cationic inter-
mediate 18, which can also exist as the pentadienyl cation
19 or palladium carbene 20. Both the intermediates 19 and
20 can then undergo cyclization to give 1,4-cyclopenta-
dienyl acetate 21, which upon ester cleavage would give
the 2-cyclopentenone species 22.
Three factors that appear to contribute to reactivity trends
are outlined below.
O
O
+
AcO
R1
R2
O
(1) Electron-withdrawing ability of the alkyne. Rearrange-
ment of alkynoate 14b occurred more efficiently than
the rearrangement of either terminal or alkyl-substituted
substrates 14a and 14d–f. This outcome can be rational-
ized in terms of the electronic and coordinative proper-
ties of the ester moiety. The electron-withdrawing
nature of the ester in conjunction with its ability to direct
coordination of the palladium(II) species to the alkyne
likely promotes the 1,2-acetoxy shift, thereby facilitat-
ing the cyclization (Fig. 1, A).
(2) Coordinative ability of the propargylic substituent. The
rearrangement of substrate 14c, which bears a benzyloxy
group at the propargylic position, was greatly acceler-
ated in comparison to the rearrangement of 14d, 14e,
and 14f, which have only alkyl substitution at the prop-
argylic position (Table 3). The benzyloxy group of 14c
might be able to direct the catalyst to the alkyne as pro-
posed for alkynoates (Fig. 1, B). The unique positioning
of both the ester carbonyl and benzyloxy functionalities
adjacent to the alkyne terminus appears to be vital to the
O
R1
R2
+ Cl-
PdCl
R3 18
Pd
Cl
Cl
R1
R2
R3
16
R3
17
O
O
O
OAc
ester
cleavage
R1
R2
+ Cl-
R1
R1
+
PdCl
R2
R3
R2
R3
21
19
R3
O
22
O
R1
R2
PdCl2
20
R3
Scheme 4. General mechanistic scheme.
OAc
OAc
+
OAc
+
R1
R2
+ Cl-
PdCl
19
R1
R1
PdCl
-Cl
4π
electrocyclization
R2
R2
R3
R3
R3
PdCl2
21
23
OAc
R1
R2
reductive
oxidative
elimination
cyclization
PdCl2
OAc
OAc
24
R1
R2
R3
R1
PdCl2
OAc
R2
R3
20
R3
21
cyclopropanation
H
R1
ring opening
25
R2
R3
Scheme 5. Proposed mechanisms for the formation of enol acetate 21.

10650
P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
A
B
for the exclusive formation of 33 from pure enol acetate
OEt
Ph
Cl
30. Indeed, treatment of 7a with 10 mol % Cl2Pd(MeCN)2
in DCE at 60 ꢀC afforded acetate 30 with only trace amounts
of ketone 8a. Enol acetate 30 could then be converted to con-
jugated enol acetate 33 after aqueous workup in the presence
of 50 mol % NEt3 in CH2Cl2 at room temperature. Enol ace-
tates 33 and 31 are both thought to arise from [1,5] hydride
shifts of enol acetate 30.21
R3
O
R3
O
O
n O
Pd
Cl
O
R1
R2
O
R1
R2
Pd
Cl
Cl
Figure 1. Esters and benzyloxy groups may promote 1,2-acetoxy migration.
efficiency of the 1,2-acetoxy migration. Formation of
a reactive Pd(II)–oxygen–alkyne complex appears opti-
mal for 14c, where the length of the carbon tether n¼1
(entry 3, Table 3). Surprisingly, when n¼2, the reaction
was extremely slow (entry 5). It is possible that the five-
membered Pd(II) chelate derived from 14e is too stable
to allow catalyst turnover. Extension of the tether to n¼4
(14f) would require the formation of an unlikely seven-
membered chelate, and might be expected to behave like
simple alkyl substrate 14d. This prediction appeared to
be consistent with our experimental results (entries 4
and 6).
i) aqueous wkup
ii) 50 mol % NEt3,
OAc
CH2Cl2, r.t.
H
[1,5] H shift
Ph
33
major product
under conditions c;
AcO
OAc
33% isolated yield
Conditions
Ph
H
Ph
7a
30
Conditions:
(3) Impact of substituents at the alkene terminus. Substitu-
tionattheterminalpositionofthealkenealsohadasignif-
icant effect on the outcome of the reaction. Sluggish
reactions and lower yields of rearrangements were ob-
served for substrates 7c and 7d, in comparison to the re-
actions of 7a and 7b (Table 2). This can be ascribed to
either an electronic effect or steric factors in the transition
state of the cyclization step (Scheme 6). It is possible that
an aromatic substituent in the R position would better sta-
bilize the developing positive charge a to the R group rel-
ative to alkyl, facilitating the desired cyclization pathway
and minimizing unwanted side reactions. Alternatively,
steric interactions between the R group and vinyl palla-
dium group in the transition state 28 would be expected
to be greater for R¼isopropyl than for the more planar
R¼phenyl, which could affect the rate of conrotatory
4p-electrocyclization of pentadienyl cation 27. There-
fore, either electronic or steric factors could account for
the observed differences in reaction efficiency evident
in Table 2.
a) 10 mol % Cl2Pd(PPh3)2,
THF, 60 °C
OAc
i) aqueous wkup
b) 5 mol % PdCl2, DME, 60 °C
c) 10 mol % Cl2Pd(MeCN)2,
DCE, 60 °C
[1,5] H shift
H
Ph
31
major product
under conditions
a and b
O
pTsOH
32
Ph
known compound
Scheme 7. [1,5] Hydride shifts of enol acetate 30.
Since the scope of the rearrangement was found to be limited
in experiments with palladium(II) catalysts, we began to
consider alternatives. Mercury(II) species have a long his-
tory of effectiveness in oxymercuration reactions with al-
kenes and alkynes,22 and recently it has been reported that
mercury(II) complexes can catalyze intramolecular cycliza-
tions of carboxylates23 and polyenes.24 The mechanistic
similarity to the Pd(II)-mediated 1,2-acetoxy shifts led us
to wonder whether catalysis of the Rautenstrauch rearrange-
ment could be achieved with mercury(II) salts.
The unique reactivity patterns of enol acetate 30 were uncov-
ered during purification and while screening reaction condi-
tions. During some experiments, enol acetate 30 underwent
unexpected further rearrangements to give alternative cyclo-
pentadienes (Scheme 7). When substrate 7a was treated with
Cl2Pd(PPh3) in THF or PdCl2 in dimethoxyethane at 60 ꢀC,
enol acetate 3119 was the major product. Its structure was
confirmed by treatment with p-TsOH, which gave the known
unsaturated ketone 32.20 Furthermore, during purification of
enol acetate 30 via triethylamine-deactivated silica gel chro-
matography, a third enol acetate 33 was generated. We be-
lieved that triethylamine was mediating the rearrangement
and therefore sought reaction conditions that would allow
Indeed, treatment of propargyl acetates 7a–c with 5 mol %
HgCl2 did effect the rearrangement under mild conditions
(Table 4). To our surprise, treatment of 7a with 5 mol %
HgCl2 in MeCN gave exclusively the dimer 34 as a 1.3:1
mixture of diastereomers (entry 1).25 This product was
apparently the result of a mercuric chloride-promoted SN20
reaction between the desired product enol acetate 30 and
a molecule of the starting material. Fortunately, when
R'
R'
PdCl2
AcO
R'
R'
AcO
AcO
PdCl
R
AcO
PdCl
PdCl
R
R
R
26
Scheme 6. Steric interactions at the alkene terminus.
27
28
29

P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
10651
Table 4. Reactions catalyzed by HgCl2
reaction. Substrates with ester substituents at the alkyne ter-
minus rearranged smoothly to give relatively stable enol ace-
tate products, as compared to those with alkyl substituents,
which exhibited poor reactivity. Aryl substituents on the al-
kene terminus gave better results than alkyl substituents.
Thus, alkene substituents, able to stabilize the pentadienyl
cation intermediate, may improve reactivity. Conversely,
nonbonding interactions between the terminal substituent
and the internally situated vinyl palladium may hinder reac-
tivity. Lastly, electron-withdrawing substituents at the prop-
argyl position (i.e., either carboethoxy (14b) or benzyloxy
(14c)) seem to improve the reactivity. While it is likely
that the 1,2-acetoxy migration is more favorable in a sub-
strate with an electron deficient alkyne, it is also possible
that the oxygen lone pairs at the propargylic position im-
prove the ability of these substrates to complex the Pd(II)
catalyst.
Entry
Acetate
Time (h)
Product
Yield (%)
H
H
AcO
AcO
Ph
O
1
1
88a,b
Ph
7a
Ph
34
O
H
2
3
1.5
>99c
>99b
44c
Ph
Ph
7a
7b
7c
8a
OAc
CO2Et
Ph
AcO
CO2Et
2
Ph
8b
5. Experimental
5.1. General
O
H
AcO
4
1.5
Reactions were done in oven-dried glassware under an argon
atmosphere unless otherwise noted. Acetonitrile, tetrahydro-
furan, dichloromethane, 1,2-dichloroethane, and diethyl
ether were purchased from Fisher or VWR and dispensed us-
ing the Glass Contour solvent purification system. Reactions
were heated in a mineral oil bath using an IKA Werke RCT
basic magnetic stirrer/hot plate, and the temperature was
monitored with an ETS-D4 fuzzy IKA Werke digital ther-
mometer. TLC visualization was done with UV light and/
or potassium permanganate, p-anisaldehyde, or ceric ammo-
nium molybdate solution followed by heating. Column chro-
matography was performed on EM Science silica gel 60
(230–400 mesh). 1H NMR and 13C NMR spectra were col-
lected on a Bruker Amax 400, Avance 400, or Avance
500 MHz spectrometer at ambient temperature. Chemical
shifts d are reported in parts per million (ppm) relative to
tetramethylsilane. Multiplicity is abbreviated as follows: s
(singlet), d (doublet), t (triplet), q (quartet), qn (quintet), m
(multiplet), dd (doublet of doublets), dt (doublet of triplets),
qd (quartet of doublets), qt (quartet of triplets), br (broad).
High resolution mass spectra were done on a ThermoFinni-
gan MAT 95XL at the Chemistry Instrumentation Center of
the University of Buffalo. Infrared spectra were recorded on
an 8400S Shimadzu FTIR spectrometer.
8c
a
Yield reflects mixture of 1.3:1 ratio of diastereomers.
Conditions: 5 mol % HgCl2, MeCN, rt.
Conditions: 5 mol % HgCl2, CH2Cl2, rt, then p-TsOH, MeOH.
b
c
dichloromethane was used as solvent instead of acetonitrile,
the monomer was obtained in quantitative yield (entry 2).
Monomer product 8b was also obtained in quantitative yield
from the alkynyl ester 7b in the presence of 5 mol % HgCl2
in acetonitrile. In this case, the stability of the enol acetate
product 8b probably suppresses the mercuric chloride-medi-
ated dimerization event. The rearrangement of propargyl
acetate 7c to 8c was also carried out in the presence of
5 mol % HgCl2 and CH2Cl2 at room temperature, but the iso-
lated yield (44%) proved to be lower than the palladium(II)-
catalyzed reaction (entry 4).
4. Conclusions
In summary, we have successfully extended the scope and util-
ity of the palladium(II)-catalyzed rearrangement of 1-ethy-
nyl-2-propenyl acetates first reported by Rautenstrauch in
1984. Our methodology resulted in the novel synthesis of
synthetically useful fused 5,6-bicyclic-1,4-cyclopentadienyl
acetates and 2-cyclopentenones via treatment of various
substituted propargyl acetates with Pd(II) and Hg(II) cata-
lysts. Although our results are consistent with a Nazarov-type
mechanism, a mechanism involving a palladium carbene in-
termediate cannot be ruled out. The unexpected reactivity of
some substrates and enol acetate products hindered our ef-
forts at further generalizing the reaction. We were, however,
successful in elucidating the interesting reactivity of enol
acetate 30, which rearranged via [1,5] H shifts to give enol
acetates 31 and 33 under specific reaction conditions.
PdCl2 was purchased from Strem Chemicals and stored on
the benchtop. Cl2Pd(MeCN)2 was prepared according to
a literature procedure and stored in a desiccator. Pd(TFA)2
was purchased from Aldrich and stored in a desiccator.
[MeCN]4Pd(BF4)2 was purchased from Aldrich and stored
in an argon-filled glovebox. NEt3 and diisopropylamine
were distilled from calcium hydride and stored under argon.
n-Butyl lithium in hexanes was purchased from Aldrich and
stored in a refrigerator.
5.2. General procedures for the preparation of
1-ethynyl-2-propenyl alcohols 3–6, 10–13c
The substituents at the terminal positions of both the alkyne
and the alkene had a strong impact on the outcome of the
5.2.1. Terminal alkynes 3, 5, 10. To a stirred solution of
TMS acetylene (2.0 equiv) in THF (1.5 M) at ꢁ60 ꢀC was

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P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
added n-BuLi (1.6 M in hexanes, 2.0 equiv) dropwise. After
stirring for 30 min at this temperature, the solution was
cooled to ꢁ78 ꢀC and ketones 1, 2, or 1-acetyl cyclohexene
(1 equiv) was added dropwise either neat or in solution
(1.3 M). The reaction was allowed to warm slowly to
ꢁ50/ꢁ40 ꢀC, and progress was monitored by TLC.
Upon completion or near-completion, the reaction mixture
was quenched with water and allowed to warm to room tem-
perature. The solution was then diluted with ether, and the
aqueous and organic layers were separated. The aqueous
layer was washed with ether (2ꢂ), and the combined organic
portions were washed with brine (1ꢂ). The combined or-
ganic extracts were then dried with anhydrous magnesium
sulfate and concentrated in vacuo. Silica gel chromato-
graphy (7/1 hexanes/ethyl acetates) afforded pure TMS-
alkyne products as precursors to the terminal alkyne alcohols
3, 5, and 10. The TMS-alkyne precursor to 10 was purified
by simply washing the crude material with 7/1 hexanes/ethyl
acetate. The TMS-alkyne derivatives were then dissolved in
MeOH (0.3 M), treated with anhydrous K2CO3 (2.0 equiv),
and stirred at room temperature until judged complete by
TLC. The reaction mixture was concentrated in vacuo and
redissolved in ether. Water was added to the organic phase,
and the layers were separated. The aqueous layer was
washed with ether (2ꢂ), and the combined organic portions
were washed with water (1ꢂ) and brine (1ꢂ). The combined
organic extracts were then dried with anhydrous magnesium
sulfate and concentrated in vacuo to afford pure propargylic
alcohols 5 and 10. Silica gel chromatography (3/1 hexane/
ethyl acetate) was used to obtain pure 3.
5.3. Representative procedure for the synthesis of
propargyl ester substrates 7a–d, 14a–f from tertiary
alcohols
To a solution of 4 (0.74 g, 2.6 mmol) in CH2Cl2 (5.2 mL)
was added Ac2O (0.37 mL, 3.9 mmol, 1.5 equiv), NEt3
(1.4 mL, 10.4 mmol, 4.0 equiv), and several crystals of
DMAP. The resulting solution was stirred overnight at
room temperature. Upon completion of the reaction as
judged by TLC, the solution was then quenched with satu-
rated ammonium chloride (aq) and the solvent was removed
in vacuo. The resulting oil was diluted with ether and the
aqueous layer was separated. After washing the aqueous
layer with ether (2ꢂ), the organics were combined and
washed with saturated (aq) ammonium chloride (2ꢂ), water
(5–7ꢂ), saturated (aq) NaHCO3 (2ꢂ), and brine (1ꢂ). The
combined organic extracts were then dried with anhydrous
MgSO4 and concentrated in vacuo. Silica gel chromato-
graphy (3/1 hexanes/ethyl acetate) afforded acetate 7b
(0.75 g, 89% yield) as a light yellow oil. Crude acetate 7a
was recrystallized with 10/1 hexanes/MeOH and washed
with ice-cold hexanes (silica gel chromatography was
avoided as it promoted quantitative allylic rearrangement).
Acetates 14a, c–f were used following aqueous workup
without further purification.
5.3.1. Spectroscopic data for propargyl acetates.
5.3.1.1. Propargyl acetate 7a.
AcO
Ph
5.2.2. Ethyl propiolates 4, 6, 11. To a stirred solution of
ethyl propiolate (2.5 equiv) in THF (1.7 M) at ꢁ78 ꢀC was
added n-BuLi (1.6 M in hexanes, 2.5 equiv) dropwise. After
stirring for 10 min at ꢁ78 ꢀC, ketones 1, 2, or 1-acetyl cyclo-
hexene (1 equiv) was added dropwise either neat or in solu-
tion (1.3 M). The reaction mixture was stirred at ꢁ78 ꢀC
until judged complete by TLC. The solution was then
quenched with saturated NH4Cl and diluted with ether.
The organic and aqueous phases were separated, and the
aqueous layer was washed with ether (2ꢂ). The combined
organic extracts were then dried with anhydrous magnesium
sulfate and concentrated in vacuo. Silica gel chromato-
graphy afforded clean ethyl propiolates 4, 6, and 11 in excel-
lent yields.
7a
1H NMR (500 MHz, CDCl3): d 7.35–7.32 (m, 2H), 7.25–
7.21 (m, 3H), 7.11 (s, 1H), 2.82 (s, 1H), 2.71 (dt, J¼13.5,
4.5 Hz, 1H), 2.45 (dt, J¼13.5, 4.5 Hz, 1H), 2.24–2.04 (m,
5H), 1.83–1.67 (m, 3H), 1.50 (m, 1H); 13C NMR
(100 MHz, CDCl3): d 168.7, 138.5, 137.3, 128.9, 128.1,
126.7, 125.8, 82.0, 77.7, 76.8, 40.2, 27.1, 25.2, 21.81,
21.77; IR (neat, cmꢁ1) 3285, 3022, 2935, 2860, 2116,
1747, 1493, 1445, 1366, 1227; HRMS (EI+): calculated for
C17H18O2 (M+): 254.1301, found: 254.1311.
5.3.1.2. Propargyl acetate 7b.
5.2.3. Alkyl-substituted alkynes 12, 13a–c. To a stirred so-
lution of benzyl-protected yn-ol or 1-hexyne (1.5 equiv) in
THF (1.2 M) at ꢁ78 ꢀC was added n-BuLi (1.6 M in hex-
anes, 1.5 equiv) dropwise, and the resulting solution was
warmed to ꢁ60 ꢀC. After stirring for 1 h at ꢁ60 ꢀC, the
solution was cooled back down to ꢁ78 ꢀC and 1-acetyl
cyclohexene (1.0 equiv) was added dropwise. The reaction
mixture was then allowed to warm slowly to ꢁ40 ꢀC.
Upon completion or near-completion of the reaction as
judged by TLC, the solution was quenched with saturated
NH4Cl and diluted with ether. The organic and aqueous
phases were separated, and the aqueous layer was washed
with ether (2ꢂ). The combined organic extracts were then
dried with anhydrous magnesium sulfate and concentrated
in vacuo. Silica gel chromatography (gradient systems) af-
forded clean alkyl-substituted alkynes 12, 13a–c in excellent
yields.
CO2Et
AcO
Ph
7b
1H NMR (500 MHz, CDCl3): d 7.35–7.32 (m, 2H), 7.26–
7.22 (m, 3H), 7.01 (s, 1H), 4.24 (q, J¼7 Hz, 2H), 2.65 (dt,
J¼14, 4.5 Hz, 1H), 2.38 (dt, J¼14, 4.5 Hz, 1H), 2.22–2.10
(m, 5H), 1.84–1.66 (m, 3H), 1.51 (m, 1H), 1.31 (t,
J¼7 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 168.3,
153.2, 137.3, 136.7, 128.7, 128.0, 126.7, 126.3, 84.6, 80.1,
76.8, 62.0, 39.4, 26.8, 25.1, 21.6, 21.3, 13.9; IR (neat,
cmꢁ1) 3080, 3055, 3022, 2980, 2937, 2860, 2239, 1751,

P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
10653
1709, 1599, 1445, 1367, 1254, 1221; HRMS (EI+): calcu-
lated for C20H22O4 (M+): 326.1513, found: 326.1524.
CDCl3): d 168.6, 153.5, 135.9, 124.6, 86.1, 78.3, 76.0,
62.0, 27.0, 25.0, 23.5, 22.6, 21.9, 21.5, 14.0; IR (neat,
cmꢁ1) 2989, 2935, 2860, 2839, 2245, 1749, 1713, 1447,
1367, 1259, 1229; HRMS (EI+): calculated for C15H20O4
(M+): 264.1356, found: 264.1363.
5.3.1.3. Propargyl acetate 7c.
AcO
5.3.1.7. Propargyl acetate 14c.
OBn
AcO
7c
1H NMR (400 MHz, CDCl3): d 5.78 (d, J¼9.2 Hz, 1H), 2.73
(s, 1H), 2.55 (m, 1H), 2.45 (dt, J¼14, 4.8 Hz, 1H), 2.35 (dt,
J¼14, 4.8 Hz, 1H), 2.08–2.03 (m, 4H), 1.88 (m, 1H), 1.68
(m, 3H), 1.39 (m, 1H), 0.98 (m, 6H); 13C NMR (100 MHz,
CDCl3): d 168.4, 133.3, 133.2, 82.0, 77.6, 76.1, 40.0, 27.0,
26.4, 24.7, 23.1, 22.9, 21.7, 21.6; IR (neat, cmꢁ1) 3286,
3267, 2959, 2935, 2866, 1749, 1464, 1445, 1366, 1229;
HRMS (EI+): calculated for C14H19O2 (M+): 219.1380,
found: 219.1377.
14c
1H NMR (400 MHz, CDCl3): d 7.38–7.27 (m, 5H), 6.07 (m,
1H), 4.62 (s, 2H), 4.26 (s, 2H), 2.16–1.95 (m, 7H), 1.70–1.53
(m, 7H); 13C NMR (100 MHz, CDCl3): d 168.8, 137.5,
137.1, 128.4, 128.2, 127.8, 123.6, 86.3, 82.3, 77.1, 71.3,
57.4, 27.8, 25.0, 23.6, 22.7, 22.0, 21.8; IR (neat, cmꢁ1
)
3062, 3030, 2990, 2932, 2856, 1747, 1497, 1454, 1366,
1238; HRMS (EI+): calculated for C20H24O3 (M+):
312.1720, found: 312.1720.
5.3.1.4. Propargyl acetate 7d.
5.3.1.8. Propargyl acetate 14d.
CO2Et
AcO
n-Bu
AcO
7d
14d
1H NMR (400 MHz, CDCl3): d 5.68 (d, J¼9.1 Hz, 1H), 4.22
(q, J¼7.1 Hz, 2H), 2.53 (m, 1H), 2.42 (m, 1H), 2.27 (m, 1H),
2.07–1.97 (m, 5H), 1.71–1.67 (m, 3H), 1.41 (m, 1H), 1.30 (t,
J¼7.1 Hz, 3H), 0.97 (m, 6H); 13C NMR (100 MHz, CDCl3):
d 168.2, 153.3, 1133.8, 132.2, 85.2, 79.7, 61.8, 39.2, 26.8,
1H NMR (500 MHz, CDCl3): d 6.04 (m, 1H), 2.24 (t,
J¼7.5 Hz, 2H), 2.16–1.91 (m, 7H), 1.66–1.39 (m, 11H),
0.90 (t, J¼7.5 Hz, 3H); 13C NMR (100 MHz, CDCl3):
d 168.8, 137.8, 123.0, 86.7, 79.7, 77.6, 30.7, 28.1, 25.0,
23.5, 22.8, 22.0, 21.9, 21.8, 18.5, 13.6; IR (neat, cmꢁ1
)
26.5, 24.7, 23.0, 22.8, 21.7, 21.4, 13.9; IR (neat, cmꢁ1
)
2988, 2954, 2931, 2860, 2839, 2241, 1747, 1680, 1447,
1366, 1236; HRMS (EI+): calculated for C16H24O2 (M+):
248.1771, found: 248.1772.
2959, 2935, 2866, 2239, 1751, 1715, 1464, 1447, 1367,
1259, 1223; HRMS (ES+): calculated for C17H24O4Na
(M+Na)+: 315.1567, found: 315.1577.
5.3.1.9. Propargyl acetate 14e.
OBn
AcO
5.3.1.5. Propargyl acetate 14a.
AcO
14e
14a
1H NMR (400 MHz, CDCl3): d 7.34–7.26 (m, 5H), 6.04 (m,
1H), 4.55 (s, 2H), 3.61 (t, J¼7.2 Hz, 2H), 2.58 (t, J¼7.2 Hz,
2H), 2.10–1.90 (m, 7H), 1.66–1.54 (m, 7H); 13C NMR
(100 MHz, CDCl3): d 168.8, 138.3, 137.6, 128.4, 127.6,
123.2, 83.6, 80.8, 77.5, 72.9, 68.5, 28.0, 25.0, 23.5, 22.8,
22.1, 21.8, 20.3; IR (neat, cmꢁ1) 3062, 3029, 2988, 2931,
2857, 2246, 1746, 1496, 1453, 1437, 1364, 1238; HRMS
(ES+): calculated for C21H26O3Na (M+Na)+: 349.1774,
found: 349.1764.
1H NMR (400 MHz, CDCl3): d 6.08 (m, 1H), 2.63 (s, 1H),
2.13–2.02 (m, 6H), 1.93 (m, 1H), 1.67–1.51 (m, 7H); 13C
NMR (100 MHz, CDCl3): d 168.8, 136.8, 123.7, 83.2,
76.8, 74.4, 27.8, 25.0, 23.4, 22.7, 22.0, 21.7; IR (neat,
cmꢁ1) 3262, 2990, 2931, 2858, 2120, 1748, 1447, 1437,
1366, 1236; HRMS (EI+): calculated for C12H16O2 (M+):
192.1145, found: 192.1148.
5.3.1.6. Propargyl acetate 14b.
5.3.1.10. Propargyl acetate 14f.
CO2Et
AcO
OBn
AcO
3
14b
14f
1H NMR (400 MHz, CDCl3): d 6.05 (m, 1H), 4.22 (q,
J¼8 Hz, 2H), 2.13–2.02 (m, 6H), 1.92 (m, 1H), 1.72–1.56
(m, 7H), 1.30 (t, J¼8 Hz, 3H); 13C NMR (100 MHz,
1H NMR (400 MHz, CDCl3): d 7.36–7.27 (m, 5H), 6.04 (m,
1H), 4.50 (s, 2H), 3.50 (t, J¼6.4 Hz, 2H), 2.28 (t, J¼7.2 Hz,

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P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
2H), 2.17–1.90 (m, 7H), 1.76–1.51 (m, 11H); 13C NMR
(100 MHz, CDCl3): d 168.8, 138.6, 137.8, 128.3, 127.6,
127.5, 123.1, 86.6, 80.0, 77.6, 72.8, 69.8, 28.8, 28.1, 25.4,
25.0, 23.6, 22.8, 22.1, 21.9, 18.6; IR (neat, cmꢁ1) 3087,
3062, 3029, 2932, 2857, 2793, 2242, 1742, 1453, 1365,
1236; HRMS (ES+): calculated for C23H30O3Na (M+Na)+:
377.2087, found: 377.2088.
5.4.2.2. Allyl acetate 9.
OAc
Ph
9
1H NMR (500 MHz, CDCl3): d 7.41–7.26 (m, 5H), 7.14 (s,
1H), 3.27 (s, 1H), 2.32–2.17 (m, 6H), 1.92 (m, 1H), 1.63–
1.49 (m, 4H); 13C NMR (100 MHz, CDCl3): d 169.5,
143.9, 139.2, 128.3, 127.4, 125.4, 117.9, 83.0, 81.4, 76.1,
30.1, 23.3, 22.0, 21.7, 21.0; IR (neat, cmꢁ1) 3283, 3061,
3030, 2934, 2860, 2839, 2091, 1495, 1450, 1435, 1369,
1232; HRMS (EI+): calculated for C17H18O2 (M+):
254.1301, found: 254.1310.
5.4. Representative procedure for Pd(II)-catalyzed
rearrangement of propargyl acetates
PdCl2 (4 mg, 0.02 mmol) was added to a solution of prop-
argyl acetate 7a (0.100 g, 0.39 mmol) in MeCN (0.6 mL),
and the solution was heated to 60 ꢀC (ꢃ62 ꢀC!) until TLC in-
dicated complete consumption of the starting material
(15 min in this case). The reaction was then cooled to
room temperature and quenched with either a catalytic
amount of p-TsOH$H2O and MeOH, or water (reactions
with non-terminal olefin substrates). After stirring the mix-
ture for 5–10 min, ether and water were added and the aque-
ous layer was washed with ether (3ꢂ). The organics were
combined, washed with brine (1ꢂ), dried with anhydrous
MgSO4, and concentrated in vacuo. Silica gel chromato-
graphy (4/1 hexanes/ethyl acetate) afforded ketone 8a
(77 mg, 92%) as a yellow oil. Cat. HgCl2 reactions were
carried out in the same fashion at room temperature.
5.4.2.3. Enol acetate 8b.
OAc
CO2Et
Ph
8b
1H NMR (400 MHz, CDCl3): d 7.26–7.05 (m, 5H), 4.26 (br
s, 1H), 4.011 (m, 2H), 2.33, (s, 3H), 2.18–2.11 (m, 3H), 1.89
(m, 1H), 1.68–1.57 (m, 4H), 1.06 (t, J¼7.1 Hz, 3H); 13C
NMR (100 MHz, CDCl3): d 167.5, 162.1, 160.2, 152.9,
136.9, 134.2, 128.3, 127.7, 126.5, 122.1, 59.3, 56.6, 24.0,
22.2, 21.8, 20.8, 20.5, 13.9; IR (neat, cmꢁ1) 3059, 3026,
2980, 2934, 2858, 2833, 1771, 1697, 1645, 1574, 1493,
1437, 1394, 1369, 1339, 1288; HRMS (EI+): calculated for
C20H22O4 (M+): 326.1513, found: 326.1520.
5.4.1. Synthesis of enol acetate 33 from propargyl acetate
7a. To a stirred solution of 7a (50 mg, 0.2 mmol) in dichlo-
roethane (0.4 mL) was added Cl2Pd(MeCN)2 (5 mg,
0.02 mmol, 0.1 equiv), and the resulting mixture was then
stirred at 60 ꢀC for 12 min. The mixture was then quenched
with water and the resulting aqueous layer was washed with
ether (2ꢂ). The combined organics were washed with brine
(1ꢂ), dried with anhydrous MgSO4, and concentrated in va-
cuo. The crude material was redissolved in CH2Cl2 (1–
2 mL), NEt3 (14 mL, 0.1 mmol, 0.5 equiv) was added, and
the mixture was stirred until TLC indicated total consump-
tion of the enol acetate starting material (5–15 min). The
reaction mixture was then concentrated in vacuo, and
purification by alumina chromatography (hexane with min-
imal methylene chloride for solubility/97.5/2.5 hexanes/
ethyl acetate) afforded enol acetate 33 (17 mg, 34%) as
a light yellow solid.
5.4.2.4. Unsaturated ketone 8c.
O
8c
1H NMR (400 MHz, CDCl3): d 2.74 (br s, 1H), 2.31–2.10
(m, 7H), 1.76–1.58 (m, 4H), 0.98 (d, J¼6.9 Hz, 3H), 0.62
(d, J¼6.9 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 208.3,
175.3, 139.2, 47.3, 35.4, 27.6, 26.5, 22.1, 21.6, 21.2, 19.8,
15.0; IR (neat, cmꢁ1) 2954, 2932, 2870, 1697, 1647, 1391,
1277; HRMS (EI+): calculated for C12H18O (M+):
178.1352, found: 178.1357.
5.4.2. Spectroscopic data for rearranged products.
5.4.2.1. Unsaturated ketone 8a.
O
Ph
5.4.2.5. Enol acetate 8d.
8a
OAc
1H NMR (400 MHz, CDCl3): d 7.32–7.22 (m, 3H), 7.08 (m,
2H), 3.82 (m, 1H), 2.88 (dd, J¼18.9, 6.9 Hz, 1H), 2.37–1.98
(m, 5H), 1.65 (m, 4H); 13C NMR (100 MHz, CDCl3):
d 207.9, 174.9, 141.8, 139.1, 128.8, 127.1, 126.7, 47.9,
44.8, 26.3, 22.0, 21.5, 20.0; IR (neat, cmꢁ1) 3026, 2930,
2858, 1697, 1647, 1601, 1493, 1454, 1389, 1277, 1244;
HRMS (EI+): calculated for C15H16O (M+): 212.1196,
found: 212.1200.
CO2Et
8d
1H NMR (400 MHz, CDCl3): d 4.14 (m, 2H), 3.24 (br s, 1H),
2.60 (m, 1H), 2.33–2.27 (m, 5H), 2.17–2.05 (m, 2H), 1.77
(m, 2H), 1.58 (m, 2H), 1.27 (t, J¼7.1 Hz, 3H), 1.08 (d,

P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
10655
J¼6.9 Hz, 3H), 0.69 (d, J¼6.9 Hz, 3H); 13C NMR
5.4.2.9. Enol acetate 33.
(100 MHz, CDCl3): d 167.5, 162.8, 159.0, 151.3, 135.0,
120.0, 59.4, 56.9, 28.0, 27.1, 22.5, 21.8, 20.8, 20.6, 20.5,
17.0, 14.2; IR (neat, cmꢁ1) 2933, 1772, 1695, 1571, 1458,
1383, 1371, 1321, 1288, 1261, 1232; HRMS (EI+): calcu-
lated for C17H24O3Na (M+Na)+: 315.1567, found: 315.1560.
OAc
H
Ph
33
5.4.2.6. Unsaturated ketone 15a.
1H NMR (400 MHz, C6D6): d 7.38–7.28 (m, 4H), 7.17 (m,
1H), 3.67 (m, 2H), 2.56 (m, 2H), 2.45 (m, 2H), 1.84 (s,
3H), 1.49–1.38 (m, 4H); 13C NMR (100 MHz, CDCl3):
d 168.8, 146.8, 138.0, 137.0, 130.1, 128.3, 127.9, 126.8,
O
125.5, 40.4, 27.0, 23.6, 22.29, 22.26, 20.9; IR (neat, cmꢁ1
)
3053, 3025, 2932, 2856, 1758, 1734, 1710, 1649, 1493,
1445, 1368, 1250, 1236, 1209; HRMS (EI+): calculated for
C17H18O2 (M+): 254.1301, found: 254.1302.
15a
1H NMR (400 MHz, CDCl3): d 2.83 (m, 1H), 2.53 (m, 2H),
2.14–1.83 (m, 5H), 1.68 (s, 3H), 1.50 (qt, J¼13.2, 3.2 Hz,
1H), 1.30 (qt, J¼13.2, 3.6 Hz, 1H), 1.04 (qd, J¼12,
3.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d 209.0, 175.8,
132.5, 41.3, 40.2, 35.0, 28.6, 26.6, 25.5, 7.5; IR (neat,
cmꢁ1) 2930, 2855, 1699, 1652, 1446, 1410, 1382, 1356,
1320, 1298; HRMS (EI+): calculated for C10H14O (M+):
150.1039, found: 150.1038.
5.4.2.10. Dimer product (higher Rf spot) 34.
H
Ph
O
Ph
34
5.4.2.7. Enol acetate 15b.
1H NMR (400 MHz, CDCl3): d 7.23–7.11 (m, 8H), 6.75 (m,
2H), 4.42 (d, J¼12 Hz, 1H), 3.32 (br s, 1H), 3.02 (m, 2H),
2.36–1.40 (m, 16H); 13C NMR (100 MHz, CDCl3):
d 207.7, 172.4, 147.1, 141.8, 141.4, 137.9, 128.5, 128.0,
127.4, 126.7, 126.5, 115.1, 84.9, 79.2, 55.6, 55.3, 54.3,
30.2, 26.4, 24.8, 22.2, 22.1, 22.0, 21.6, 20.0; IR (neat,
cmꢁ1) 3302, 3250, 3084, 3059, 3026, 2932, 2858, 2085,
1699, 1649, 1599, 1492, 1450, 1434, 1391, 1275, 1246;
HRMS (EI+): calculated for C30H30O (M+): 406.2291,
found: 406.2294.
AcO
CO2Et
15b
1H NMR (400 MHz, CDCl3): d 4.17 (m, 2H), 2.96 (ddd,
J¼12, 5.9, 0.9 Hz, 1H), 2.74–2.67 (m, 2H), 2.30 (s, 3H),
2.10 (m, 1H), 2.00–1.96 (m, 1H), 1.81–1.73 (m, 4H), 1.47
(qt, J¼13.2, 3.2 Hz, 1H), 1.27–1.12 (m, 4H), 0.91 (qd,
J¼12.9, 3.4 Hz, 1H); 13C NMR (100 MHz, CDCl3):
d 167.7, 162.6, 160.0, 152.7, 127.1, 120.8, 59.3, 49.3,
5.4.2.11. Dimer product (lower Rf spot) 34.
32.4, 28.2, 26.6, 25.0, 20.6, 14.3, 8.5; IR (neat, cmꢁ1
)
2978, 2933, 2856, 1772, 1695, 1396, 1447, 1396, 1371,
1360, 1345, 1259, 1247, 1226; HRMS (EI+): calculated for
C15H20O4 (M+): 264.1356, found: 264.1368.
H
Ph
O
5.4.2.8. Enol acetate 15c.
Ph
34
AcO
OBn
1H NMR (400 MHz, CDCl3): d 7.34–7.10 (m, 10H), 4.57 (d,
J¼11.6 Hz, 1H), 3.71 (br s, 1H), 3.19 (dd, J¼11.6, 2.4 Hz,
1H), 3.14 (s, 1H), 2.27–1.01 (m, 16H); 13C NMR
(100 MHz, CDCl3): d 207.5, 172.5, 148.3, 141.8, 140.3,
138.0, 128.8, 128.6, 127.8, 127.7, 126.9, 126.3, 115.1,
84.6, 80.4, 54.8, 54.0, 52.3, 30.2, 26.2, 24.7, 22.3, 21.8,
21.6, 21.5, 20.1; IR (neat, cmꢁ1) 3302, 3059, 3026, 2932,
2858, 2087, 1699, 1651, 1601, 1493, 1452, 1389, 1275,
1244; HRMS (EI+): calculated for C30H30O (M+):
406.2291, found: 406.2299.
15c
1H NMR (400 MHz, CDCl3): d 7.39–7.25 (m, 5H), 4.43 (m,
2H), 4.15 (m, 2H), 2.75–2.64 (m, 2H), 2.38 (m, 1H), 2.19 (s,
3H), 2.09–1.78 (m, 3H), 1.71 (s, 3H), 1.43 (dt, J¼13.2,
3.2 Hz, 1H), 1.17 (qt, J¼13.2, 3.2 Hz, 1H), 0.88 (qd,
J¼11.2, 3.2 Hz, 1H); 13C NMR (100 MHz, CDCl3):
d 169.0, 150.1, 144.9, 138.6, 128.3, 127.8, 127.4, 126.6,
125.6, 71.2, 62.0, 48.9, 31.4, 28.3, 26.1, 25.4, 20.5, 9.2; IR
(neat, cmꢁ1) 3086, 3063, 3030, 2928, 2852, 1762, 1663,
1609, 1497, 1445, 1367, 1329, 1294, 1205; HRMS (ES+):
calculated for C20H24O3Na (M+Na)+: 335.1618, found:
335.1621.
Acknowledgements
We thank the University of Rochester for generous financial
support, and we are grateful to Dr. Alice Bergmann (Director

10656
P. A. Caruana, A. J. Frontier / Tetrahedron 63 (2007) 10646–10656
phenomenon in the absence of AcOH and suggested that it
occurred via formal loss of ketene.
of the Chemistry Instrumentation Center at the University of
Buffalo) for performing HRMS analysis.
13. For most other substrates, ꢄ10% PdCl2 was optimal.
14. 1H NMR shows similar peaks to that of a structurally-related 2-
ethoxycyclopentadiene: Hatanaka, M.; Himeda, Y.; Imashiro,
R.; Tanaka, Y.; Ueda, I. J. Org. Chem. 1994, 59, 111.
15. Longer reaction times or higher catalyst loadings led to com-
plex mixtures minus starting material. Crude NMR indicated
that some cyclized product was likely formed, but it could
not be isolated free of other compounds.
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1
25. COSY data in conjunction with H NMR J values for each
diastereomer indicate a trans-relationship between the cyclo-
pentenone stereocenters.