An Alternative to Precious Metals: Hg(ClO4)2·3H2O as a Cheap and Water-Tolerant Catalyst for the Cycloisomerization of Allenols

Article abstract of DOI:10.1021/acs.joc.5b00887

Hg(ClO₄)₂·3H₂O, a cheap, water-tolerant, and stable salt, catalyzes the cycloisomerization reaction or α-allenols to 2,5-dihydrofurans in an efficient and selective manner. The reaction is general and can be applied to dif

Full text of DOI:10.1021/acs.joc.5b00887

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Article
An Alternative to Precious Metals: Hg(ClO4)2.3H2O as a Cheap
and Water-Tolerant Catalyst for the Cycloisomerization of Allenols
Benito Alcaide, Pedro Almendros, Amparo Luna, and Elena Soriano
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00887 • Publication Date (Web): 16 Jun 2015
Downloaded from http://pubs.acs.org on June 22, 2015
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An Alternative to Precious Metals:
Hg(ClO4)2 3H2O as a Cheap and Water-
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Tolerant Catalyst for the Cycloisomerization
of Allenols
Benito Alcaide,* *
,† Pedro Almendros, ,‡ Amparo Luna,† and Elena Soriano‡
†Grupo de Lactamas y Heterociclos Bioactivos, Departamento de Química Orgánica I, Unidad
Asociada al CSIC, Facultad de Química, Universidad Complutense de Madrid, 28040-Madrid,
Spain
‡Instituto de Química Orgánica General, CSIC, Juan de la Cierva 3, 28006-Madrid, Spain
Eꢀmail: alcaideb@quim.ucm.es; Palmendros@iqog.csic.es
Abstract: Hg(ClO4)2·3H2O,
a
cheap, waterꢀtolerant, and stable salt, catalyzes the
cycloisomerization reaction or αꢀallenols to 2,5ꢀdihydrofurans in an efficient and selective manner.
The reaction is general and can be applied to differently functionalized substrates, including alkylꢀ
substituted, arylꢀsubstituted, enantiopure, and tertiary allenols. Besides, density functional theory
(DFT) calculations were performed to obtain an insight into various aspects of the controlled
reactivity of αꢀallenols under mercury catalysis. They suggest a dual activation of the allenol by the
Hgꢀcomplex that drives the reaction to the chemoselective formation of 2,5ꢀdihydrofurans.
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INTRODUCTION
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The last decade has seen an increasing use of allene derivatives as versatile building blocks in
synthetic chemistry, in large part due to their interesting reactivity patterns which allows the
preparation of relevant products in a controlled manner.1 On the other hand, the development of
synthetic methods for the preparation of oxacycles is important because they are present in a wide
range of natural products and biologically active molecules.2 Among the possibilities, transition
metalꢀcatalyzed intramolecular nucleophilic addition of the hydroxyl group across the allene
moiety in allenols is intriguing from the point of view of regioselectivity as well as it being one of
the most rapid and convenient methods for the preparation of oxacycles owing to its atomꢀeconomy
and efficiency.1 Despite that this field was initiated by mercury salts serving as promoters in
stoichiometric or subꢀstoichiometric amounts,3 the discovery of platinumꢀ and especially goldꢀ
based precatalysts4,5 has displaced mercury,6 probably invoking toxicity issues.7 However, from a
scientific point of view, prejudices coming from popularly accepted suppositions must be taken
with care,8 because a particular element can be toxic or not depending on the compound
considered.9,10 From an economic point of view, the common practice of using 5% loading in gold
catalysis makes its use often impractical in larger scale synthesis in fields such as medicinal
chemistry and material science. Compared with nobleꢀmetal catalysts, extremely expensive and
with diminishing reserves, mercuryꢀbased methods have obvious economic attractiveness. In light
of these facts, we decide to investigate the catalytic profile of Hg(ClO4)2·3H2O,11 a cheap, waterꢀ
tolerant, and stable salt,12,13 for the cycloisomerization of allenols.
RESULTS AND DISCUSSION
Starting allenols 1a–l were prepared from the appropriate carbonyl derivative via
regiocontrolled indiumꢀmediated Barbierꢀtype carbonyl–allenylation reaction in aqueous media
adopting our previously reported methodology.14 Allene 1a was chosen as a model substrate for
Hg(II)ꢀcatalyzed cycloetherification reactions. To screen the reactivity of the αꢀallenol moiety,
several conditions were screened. While Hg(ClO4)2·3H2O could catalyze the heterocyclization
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reaction of 1a to 2,5ꢀdihydrofuran 2a in several solvents such as 1,2ꢀdichloroethane, acetonitrile,
and THF, the mercuryꢀbased catalytic system gave the desired product 2a in low yield when DMF
or THF–H2O (1:1) were used (Table S1 in the Supporting Information). Treatment of αꢀallenol 1a
with Hg(ClO4)2·3H2O (10 mol%) in THF at 70oC on a sealed tube afforded the best yield of the
corresponding adduct 2a, from a 5ꢀendo oxycyclization, as a single isomer (Scheme 1).
Interestingly, under gold catalysis αꢀallenol 1a delivered fused bicycle 3a, carbocyclization adduct,
in addition to 2,5ꢀdihydrofuran 2a (Scheme 1). A similar reaction course was encountered through
the use of αꢀallenol 1b. The AuCl3ꢀcatalyzed reaction afforded a separable mixture of 2,5ꢀ
dihydrofuran 2b and naphthalene 3b, while the Hg(ClO4)2·3H2Oꢀcatalyzed reaction was totally
selective towards the oxycyclization adduct 2b (Scheme 1). Thus, it has been shown that
Hg(ClO4)2·3H2O is a more selective cycloetherification catalyst than AuCl3. Different Hg(II) salts
such as HgCl2, Hg(AcO)2, and Hg(TfO)2 were also tested. The corresponding dihydrofuran was
the major reaction product under catalytic Hg(TfO)2 conditions. However, the use of either 10
mol% HgCl2 or 10 mol% Hg(AcO)2 did not get the reaction to completion. Dihydrofuran adducts 2
were afforded as minor products along with degradation products. Comparatively, the use of
mercurium salts different to Hg(ClO4)2.3H2O led to limited reactivity.
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Scheme 1. Gold- versus mercury-catalyzed cyclization reaction of
α-allenols 1a and 1b.
O
OH
MeO
MeO
MeO
5 mol% AuCl3
CH2Cl2, rt, 1 h
10 mol% Hg(ClO4)2—3H2O
THF, 70 oC, 1h
2a
(16%)
•
+
OMe
OMe
OMe
2a (57%)
3a (42%)
1a
OH
O
5 mol% AuCl3
CH2Cl2, rt, 4 h
10 mol% Hg(ClO4)2—3H2O
THF, 70 oC, 2h
2b
•
(35%)
+
OPh
OPh
OPh
1b
3b
(25%)
2b (66%)
Under the optimized reaction conditions, we investigated the generality of the mercuryꢀ
catalyzed transformation of differently substituted αꢀallenols 1c–m. As shown in Scheme 2 and
Scheme 3, the above process serves as a general approach to 2,5ꢀdihydrofurans 2c–m. Nicely,
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bis(αꢀallenol) 1e undergoes the double transformation to give bi(2,5ꢀdihydrofuran) 2e. To assess
scope, the even more challenging enantiopure allenylꢀtethered 2ꢀazetidinones 1f and 1g were tested
as cyclization precursors, giving the desired spirocyclic dihydrofuranꢀβꢀlactams 2f and 2g. Notably,
despite that Lewis acids are well known for their ability to promote acetonide cleavage, no traces of
diols were detected from tertiary αꢀallenols 1f and 1g (Scheme 2).15
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Scheme 2. Controlled mercury-catalyzed oxycyclization of
α-allenols 1c–g.
O
OH
10 mol% Hg(ClO4)2—3H2O
•
Cl
Cl
THF, 70 oC, 8h
2c (70%)
1c
1d
OH
O
Ph
Ph
10 mol% Hg(ClO4)2—3H2O
THF, 70 oC, 30 min
•
2d
(45%)
•
•
10 mol% Hg(ClO4)2—3H2O
O
O
HO
OH
THF, 70 oC, 2 h
Cl
Cl
Cl
Cl
(±)-2e
(±)-1e
(58%)
R2
O
O
O
H
OHH
N
O
O
10 mol% Hg(ClO4)2—3H2O
THF, 70 oC, 2 h
O
•
R2
N
R1
R1
O
(+)-1f R1 = 4MeOC6H4, R2 = Me
(+)-2f
(45%, 30 min)
1
2
(–)-2g (54%, 20 min)
(+)-1g
R = Bn, R = Ph
Encouraged by the above results, 3ꢀallenyl 3ꢀhydroxyoxindoles 1h–m were studied to
determine the applicability of the Hg(II)ꢀcatalyzed method (Table S2 in the Supporting
Information) for the preparation of the spiroindolinone framework,16 which is an important
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structural motif in biologically relevant compounds.17 Nicely, 2ꢀindolinoneꢀtethered allenic
alcohols 1h–m responded well to the oxycyclization reaction, affording reasonable yields of
spiroindolinones 2h–m (Scheme 3).
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Scheme 3. Controlled mercury-catalyzed oxycyclization of 2-indolinone-tethered
α
-allenols 1h–m.
R2
HO
•
O
R3
R3
R2
O
10 mol% Hg(ClO4)2—3H2O
THF, 70 oC
O
N
R1
N
R1
R4
R4
1h R1 = H, R2 = Me, R3 = R4 = H
2h (80%; 1.5 h)
2i (56%; 1.5 h)
1i R1 = H, R2 = 4ꢀMeOC6H4, R3 = R4 = H
1
2
3
4
1j
2j
(75%; 3 h)
R = Me, R = Me, R = R = H
1k R1 = Me, R2 = Ph, R3 = R4 = H
1l R1 = Me, R2 = Me, R3 = Cl, R4 = H
2k (67%; 3 h)
2l (73%; 2 h)
1
2
3
4
1m
2m
(87%; 1 h)
R = Me, R = Me, R = H, R = Cl
Since the above examples bear either electron donating or weakly electron withdrawing
groups, we prepared cyanoꢀ and carboxyethylꢀallenol derivatives 1n and 1o. Pleasingly, exposure
of each of these electronꢀpoor substrates to the mercury salt gave the corresponding 2,5ꢀ
dihydrofurans 2n and 2o in good yields (Scheme 4). By contrast, if an ester group is placed at the
terminal carbon of the allene, such as in allenol 1p, the cycloetherification reaction did not proceed
(Scheme 4). Likewise, exposure of allenol 1q with a monoꢀsubstituted allene moiety under the
optimized conditions did not give rise to the expected heterocycle (Scheme 4). It should be noted
that we observed a similar unselective process for the AuCl3ꢀcatalyzed reaction of unsubstituted
allenol 1q.
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Scheme 4. Mercury-catalyzed reaction of
α
-allenols 1n
–
q.
O
OH
R2
8
10 mol% Hg(ClO4)2—3H2O
THF, 70 oC
9
R2
•
R1
R1
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1n R1 = CN, R2 = Ph
2n (63%, 4 h)
1
2
1o
2o (72%, 1.5 h)
R = COOMe, R = Me
O
OH
10 mol% Hg(ClO4)2—3H2O
THF, 70 oC
R
•
Cl
Cl
R
(degradation products, 8h)
(degradation products, 4h)
1p R = COOEt
1q
R = H
A possible mechanism for the catalytic achievement of dihydrofurans 2 involving a
mercuryꢀbased carbophilic πꢀacid may proceed through initial ηꢀcoordination of the metal to the
distal double bond of allenols 1 leading to species 1-Hg. Next, 5ꢀendoꢀtrig alkoxymercuration
forms zwitterionic dihydrofurans 4. Loss of HClO4 in intermediates 4 generates neutral vinylmetal
species 5. Protonolysis of the carbon–mercury bond of 5 liberates dihydrofurans 2 with
concomitant regeneration of the Hg(II) catalytic species (Scheme 5).
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Scheme 5. Initially proposed mechanistic explanation for the mercury-catalyzed cycloetherification
of allenols 1.
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O
OH
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R1
R2
R2
R1
2
Hg(ClO4)2
•
1
protonolysis
HClO4
OH
Hg(ClO4)
R2
O
R1
R1
R2
•
Hg(ClO4)2
5
1-Hg
H
–HClO4
Hg(ClO4)2
R2
5-endo-trig
O
alkoxymercuration
R1
4
Density functional theory (DFT) calculations have been carried out to rationalize the
divergent chemoselectivity observed for the Hgꢀ and Auꢀcatalyzed reactions of αꢀallenols 1a and
1b.
These metals are known as excellent πꢀactivators and, more generally, soft Lewis acids.
These properties make them superior as catalysts to other transition and nonꢀtransition metals for
the addition of carbon and heteroatom nucleophiles to unsaturated C–C bonds.18 Au(III) complexes
form 16e– squareꢀplanar structures, which is typical of a metal complex with a d8 electron count,
with a vacant coordination site that easily admit extraꢀcoordination to a new molecule (to give an
18e– species). In contrast, the Hg(II) species is a d10 that prefers to form 14e– linear complexes. The
existence of these linear complexes is explained by the high stabilization of the 6s orbital compared
to the 6p: since the LUMO is exclusively composed by the 6s and the 6p orbitals, it has more sꢀ
character. Thus, spꢀhybridization occurs, giving the linear structure for the metal center. In this
case, the access of the reactants must be preceded by the release of the ligands since: a) no
oxidative addition is possible, and b) associative addition hardly occurs since πꢀback donation from
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the Hg(II) cation is disfavored.10 These general trends make important differences in the catalytic
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behavior of both metalꢀcomplexes in the cycloisomerization of αꢀallenols 1.
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Thus, the activation of the allene moiety of 1a by Hg(ClO4)2 proceeds with the release of
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–
ClO4 species from a metal coordination site which is occupied by η2ꢀcoordination of the distal
double bond of the allene to give a linear complex, 1aꢀHg (Figure 1). Remarkably, the inspection of
the transition structure for the 5ꢀendoꢀtrig alkoxymercuration (TS1) reveals that the released
–
counteranion ClO4 forms an hydrogenꢀbond with the nucleophile. It suggests a dual activation by
the catalyst, as both the electrophile and nucleophile are activated for the intramolecular
heterocyclization. This effect is supported by the low activation barrier computed (ꢁG# = 2.6
kcal/mol). Moreover, it should be noted that TS1 leads to 5, not to 4 as proposed in Scheme 4,
since the hydroxylic proton is captured by the counteranion, to form HClO4, simultaneously to the
formation of the C–O bond. Thus, the vinylmetal 5 is Hꢀbonded to HClO4 through the ether
oxygen. The formation of this intermediate structure is highly exothermic (ꢁG = –23.2 kcal/mol),
so this step is probably irreversible. The subsequent protonolysis step of the carbon–mercury bond
of 5 liberates dihydrofuran 2a with regeneration of the linear Hg(ClO4)2 catalytic species. This step
takes place through TS2, –12.9 kcal/mol below 1aꢀHg, and it is strongly exothermic (ꢁG = –31.1
kcal/mol). Therefore, this cycloetherification proceeds through a twoꢀstep mechanism involving
dual activation of the precursor by the catalyst.
In order to verify the ability of the Hgꢀcounteranion to Hꢀbond and its role onto selectivity,
we have performed further calculations with Hg(OTf)2 and Hg(BF4)2. In the former case, slightly
higher barriers than for Hg(ClO4)2 have been computed for the two possible cyclizations (3.3 and
5.9 kcal/mol, for TS1 and TS1’, respectively), which suggest a lower reactivity and the same
ꢀ
selectivity. For the catalyst with the less basic anion BF4 , however, both barriers are similarly
higher (5.9 and 6.5 kcal/mol, for TS1 and TS1’, respectively). Moreover, the evolution of TS1
leads to an intermediate where the H remains partially attached to the oxygen (1.167 Ǻ vs 1.568 for
Hg(OTf)2 and 1.722 Ǻ for Hg(ClO4)2).
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Figure 1. Computed reaction profile (PCM(THF)ꢀM06/6ꢀ31+G(2d,p)/SDD(Hg)//M06/6ꢀ31+G(2d,p)/SDD(Hg) level)
for the cycloisomerization of allenol 1a catalyzed by Hg(ClO4)2. Gasꢀphase results are shown in parenthesis. Relative
free energies are given in kcal/mol and bond lengths in the transition states in angstroms.
The plausible hydroarylation to form naphthalene should take place probably via initial 6ꢀ
endoꢀtrig cyclization of 1aꢀHg following a Friedel–Craftsꢀtype mechanism.19 This carbocyclization
step affords the Whelandꢀtype intermediate I1’ through the transition structure TS1’ (Figure 1), 2.5
kcal/mol higher in energy than TS1. In addition, the intermediate I1’ is 2.1 kcal/mol less stable
than 5. Hence, the 5ꢀendoꢀtrig oxycyclization is the preferred cyclization mode from kinetic and
thermodynamic viewpoints, and these energy differences clearly support the chemoselective
formation of the 2,5ꢀdihydrofuran skeleton under Hg(ClO4)2 catalysis.
On the other hand, the AuCl3 complex presents a vacant coordination site that can be
occupied by the allene moiety to form a squareꢀplanar complex, 1aꢀAu. Thus, the 5ꢀendoꢀtrig
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oxycyclization step takes places with an activation barrier of 4.9 kcal/mol and drives to a
zwitterionic intermediate, I1Au, 6.2 kcal/mol more stable than the reactant complex 1aꢀAu (Figure
2). Amazingly, the calculations show that the nucleophillic addition of the arene to the gold(III)ꢀ
activated allene is more favourable, requiring an activation free energy of only 1.5 kcal/mol via
transition state TS1’Au. Moreover, the formation of the Whelandꢀtype intermediate I1’Au is strongly
exothermic (by 27.7 kcal/mol), and hence likely irreversible. This step leads to a C–C bond
formation between the allene and the aromatic core. The formation of the naphthalene framework
3a then takes place via stepwise rearomatization. Thus, calculations suggest that I1’Au drives to
I2’Au (ꢁG = –27.0 kcal/mol), through a deprotonation process assisted by a chloride ligand via
TS2’Au (ꢁG# = –5.8 kcal/mol). Then, the released HCl promotes the protonation of the hydroxylic
substituent and formation of a water molecule. This step proceeds through TS3’Au (ꢁG# = –10.8
kcal/mol), which shows the advanced formation of the O–H bond (1.033 Å) and breaking of the O–
C bond (2.031 Å). The subsequent step is the aromatization through a deprotonation process
assisted by the chloride anion. The transition state TS4’Au for this step needs a low activation
energy (ꢁG# = –13.0 kcal/mol), and drives to the formation of the aromatic intermediate I4’Au and
HCl in a strongly exothermic step (ꢁG = –54.1 kcal/mol) due to the high stability of the aromatic
bicycle.20 Finally, this intermediate undergoes demetalation by protonolysis to yield the
hydroarylation product and regenerates the gold(III) catalyst. This transformation occurs via
TS5’Au (ꢁG# = –34.2 kcal/mol), and leads to the expected formation of naphthalene 3aꢀAu (ꢁG = –
63.7 kcal/mol). Therefore, the favoured reaction of αꢀallenols 1 under Au(III)ꢀcatalysis proceeds
through initial πꢀcomplexation of the allene moiety, which triggers the nucleophilic attack of the
arene via an endoꢀtrig carbocyclization pathway in a Friedel–Craftsꢀtype mechanism.
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Figure 2. Computed reaction profile (PCM(CH2Cl2)ꢀM06/6ꢀ31+G(2d,p)/SDD(Au)//M06/6ꢀ31+G(2d,p)/SDD(Au)
level) for the cycloisomerization of allenol 1a catalyzed by AuCl3. Gasꢀphase results are shown in parenthesis. Relative
free energies are given in kcal/mol and bond lengths in the transition states in angstroms.
Conclusions
In conclusion, Hg(ClO4)2·3H2O, a cheap, waterꢀtolerant, and stable salt,21 catalyzes the
heterocyclization reaction or αꢀallenols.22 Thus, differently functionalized 2,5ꢀdihydrofurans can be
obtained in an efficient and selective manner. The possibility of using optically active substrates as
well as substrates of increased steric demand, such as tertiary αꢀallenols, expands the attractiveness
of the method. Besides, density functional theory (DFT) calculations were performed to obtain an
insight into various aspects of the controlled reactivity of αꢀallenols under mercury catalysis. They
suggest a dual activation of the allenol by the Hgꢀcomplex that drives the reaction to the
chemoselective formation of 2,5ꢀdihydrofurans.
Experimental Section
o
General methods: NMR spectra were recorded at 25 C on a 300 or a 700 MHz
instruments. Chemical shifts are given in ppm relative to TMS (1H, 0.0 ppm), or CDCl3 (13C, 76.9
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ppm). Low and high resolution mass spectra were taken on a QTOF LC/MS spectrometer using the
electronic impact (EI) or electrospray modes (ES). Specific rotation [α]D is given in 10–1 deg cm2 g–
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at 20 °C, and the concentration (c) is expressed in g per 100 mL. All commercially available
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compounds were used without further purification.
Previously unreported α-allenic alcohols 1b and (±)-1e were prepared adopting our
general procedure.14 2-Methyl-1-(2-phenoxyphenyl)buta-2,3-dien-1-ol 1b. From 235 mg (2.19
mmol) of 2ꢀphenoxybenzaldehyde, and after chromatography of the residue using hexanes/ethyl
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acetate (8:1) as eluent gave compound 1b (337 mg, 61%) as a colorless oil; HꢀNMR (300 MHz,
CDCl3, 25 oC) δ: 7.49 (dd, J = 7.5, 1.8 Hz, 1H), 7.33 (t, J = 7.6 Hz, 2H), 7.25 (td, J = 8.0, 1.8 Hz,
1H), 7.16 (td, J = 7.4, 1.2 Hz, 1H), 7.10 (t, J = 7.4 Hz, 1H), 6.98 (d, J = 7.6 Hz, 2H), 6.88 (dd, J =
8.0, 1.2 Hz, 1H), 5.42 (m, 1H), 4.74 (qu, J = 3.1 Hz, 2H), 2.56 (d, J = 6.0 Hz, 1H), 1.65 (t, J = 3.1
o
Hz, 3H); 13CꢀNMR (75 MHz, CDCl3, 25 C) δ: 205.0, 157.4, 154.3, 133.0, 129.7 (2C), 128.8,
128.3, 123.7, 123.0, 119.1, 118.3 (2C), 102.1, 77.7, 70.0, 15.0; IR (CHCl3, cm–1): ν 3419, 3065,
1483, 1230, 751, 691; HRMS (ES): calcd for C17H16O2 [M]+: 252.1150; found: 252.1147.
(SR)-1-(4-chlorophenyl)-2-(4-((RS)-1-(4-chlorophenyl)-1-hydroxybuta-2,3-dien-2-
yl)phenyl)buta-2,3-dien-1-ol (±)-1e. From 169 mg (1.2 mmol) of 4ꢀchlorobenzaldehyde, and after
chromatography of the residue using hexanes/ethyl acetate (2:1) as eluent, gave compound (±)ꢀ1e
(120 mg, 45%) as a yellow oil; 1HꢀNMR (300 MHz, acetoneꢀδ6, 25 oC) δ: 7.48 (d, J = 8.5 Hz, 4H),
7.31 (d, J = 8.5 Hz, 4H), 5.77 (d, J = 5.0 Hz, 2H), 5.11 (m, 4H), 4.96 (d, J = 5.0 Hz, 2H); 13CꢀNMR
(75 MHz, acetoneꢀδ6, 25 oC) δ: 210.0 (2C), 143.4 (2C), 133.9 (2C), 133.0 (2C), 129.3 (4C), 128.8
(4C), 128.0 (4C), 110.2 (2C), 80.0, 79.9, 72.8 (2C); IR (CHCl3, cm–1): ν 3397, 1937, 1701, 1089,
1014; HRMS (ES): calcd for C26H20Cl2O2 [M]+: 434.0840; found: 434.0829.
4-(1-Hydroxy-2-phenylbuta-2,3-dienyl)benzonitrile 1n. From 200 mg (1.5 mmol) of 4ꢀ
formylbenzonitrile, and after chromatography of the residue using hexanes/ethyl acetate (4:1) as
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eluent, gave compound 1n (157 mg, 44%) as a colorless oil; 1HꢀNMR (300 MHz, CDCl3, 25 oC) δ:
7.62 (d, J = 8.4 Hz, 2H), 7.55 (d, J = 8.4 Hz, 2H), 7.30 (m, 5H), 5.80 (s, 1H), 5.22 (m, 2H), 2.44 (br
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s, 1H); 13CꢀNMR (75 MHz, CDCl3, 25 C) δ: 207.9, 147.2, 133.2, 132.0, 128.6, 127.5, 127.3,
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126.8, 118.7, 111.4, 109.3, 81.3, 72.1; IR (CHCl3, cm–1): ν 3455, 2230, 1711, 853; HRMS (ES):
calcd for C17H13NO [M]+: 247.0997; found: 247.0992.
Methyl 4-(1-hydroxy-2-methylbuta-2,3-dienyl)benzoate 1o. From 330 mg (2.0 mmol) of
methyl 4ꢀformylbenzoate, and after chromatography of the residue using hexanes/ethyl acetate
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(5:1) as eluent, gave compound 1o (284 mg, 65%) as a colorless oil; HꢀNMR (300 MHz, CDCl3,
o
25 C) δ: 7.98 (d, J = 8.2 Hz, 2H), 7.42 (d, J = 8.2 Hz, 2H), 5.18 (s, 1H), 4.84 (m, 2H), 3.88 (s,
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3H), 2.77 (br s, 1H) 1.54 (t, J = 3.1 Hz, 3H); CꢀNMR (75 MHz, CDCl3, 25 C) δ: 205.0, 166.9,
147.0, 129.5, 129.2, 126.3, 101.9, 77.4, 74.3, 51.9, 13.9; IR (CHCl3, cm–1): ν 3471, 1720, 1279,
1112; HRMS (ES): calcd for C13H14O3 [M]+: 218.0943; found: 218.0939.
General procedure for the Hg(ClO4)2⋅3H2O-catalyzed reaction of allenols 1. Preparation
of dihydrofurans 2. Hg(ClO4)2⋅3H2O (0.014 mmol) was added to a solution of the appropriate
allenol 1 (0.14 mmol) in THF (2.5 mL). The reaction mixture was stirred at 70 ºC on a sealed tube
until the starting material disappeared as indicated by TLC. The mixture was concentrated under
vacuum, and purified by flash column chromatography eluting with ethyl acetate/hexanes mixtures.
Spectroscopic and analytical data for pure forms of compounds 2 follow.
2-(3,5-Dimethoxyphenyl)-3-methyl-2,5-dihydrofuran 2a. From 34 mg (0.15 mmol) of
allenol 1a, and after chromatography of the residue using hexanes/ethyl acetate (6:1) as eluent,
gave compound 2a (19 mg, 57%) as a colorless oil; 1HꢀNMR (300 MHz, CDCl3, 25 ºC) δ: 6.44 (d,
J = 2.2 Hz, 2H), 6.40 (m, 1H), 5.63 (m, 1H), 5.41 (m, 1H), 4.84 (m, 1H), 4.70 (m, 1H), 3.79 (s,
6H), 1.58 (m, 3H); 13CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 160.9(2C), 143.9, 138.4, 120.8, 104.7
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(2C), 99.8, 90.5, 75.4, 55.3 (2C), 12.5; IR (CHCl3, cm–1): ν 1596, 1153; HRMS (ES): calcd for
C13H17O3 [M + H]+: 221.1178; found: 221.1175.
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3-Methyl-2-(2-phenoxyphenyl)-2,5-dihydrofuran 2b. From 123 mg (0.49 mmol) of allenol
1b, and after chromatography of the residue using hexanes/ethyl acetate (7:1) as eluent, gave
compound 2b (82 mg, 66%) as a colorless oil; 1HꢀNMR (300 MHz, CDCl3, 25 ºC) δ: 7.30 (dd, J =
7.5, 1.7 Hz, 1H), 7.23 (t, J = 8.0 Hz, 2H), 7.15 (t, J = 7.7 Hz, 1H), 7.06 (t, J = 7.4 Hz, 1H), 6.98
(d, J = 6.9 Hz, 1H), 6.89 (d, J = 7.7 Hz, 2H), 6.80 (d, J = 8.0 Hz, 1H), 5.90 (br s, 1H), 5.51 (m,
1H), 4.74 (m, 1H), 4.60 (m, 1H), 1.55 (s, 3H); 13CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 157.7, 154.1,
138.5, 133.0, 129.6 (2C), 128.9, 128.5, 124.1, 122.8, 120.7, 119.2, 118.0 (2C), 84.1, 75.4, 12.6; IR
(CHCl3, cm–1): ν 3067, 1757, 1483, 1231, 753, 693; HRMS (ES): calcd for C17H16O2 [M]+:
252.1150; found: 252.1147.
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2-(4-Chlorophenyl)-3-methyl-2,5-dihydrofuran 2c. From 35 mg (0.18 mmol) of allenol 1c,
and after chromatography of the residue using hexanes/ethyl acetate (5:1) as eluent, gave
1
compound 2c (25 mg, 70%) as a colorless oil; HꢀNMR (300 MHz, CDCl3, 25 ºC) δ: 7.32 (d, J =
8.5 Hz, 2H), 7.21 (d, J = 8.5 Hz, 2H), 5.65 (m, 1H), 5.46 (m, 1H), 4.84 (m, 1H), 4.71 (m, 1H), 1.55
(m, 3H); 13CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 140.0, 138.1, 133.5, 128.5 (2C), 128.2 (2C), 121.0,
89.8, 75.5, 12.4; IR (CHCl3, cm–1): ν 1730, 1684, 1091; HRMS (ES): calcd for C11H12ClO [M +
H]+: 195.0577; found: 195.0579.
2-Benzyl-3-methyl-2,5-dihydrofuran 2d. From 43 mg (0.24 mmol) of allenol 1d, and after
chromatography of the residue using hexanes/ethyl acetate (9:1) as eluent, gave compound 2d (19
1
mg, 45%) as a colorless oil; HꢀNMR (300 MHz, CDCl3, 25 ºC) δ: 7.27 (m, 5H), 5.44 (q, J = 1.6
Hz, 1H), 4.88 (m, 1H), 4.46 (m, 2H), 3.01 (dd, J = 14.2, 3.9 Hz, 1H), 2.72 (dd, J = 14.2, 6.8 Hz,
1H), 1.74 (q, J = 1.6 Hz, 3H); 13CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 138.2, 137.5, 129.4 (2C),
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128.0 (2C), 126.0, 121.3, 88.1, 74.4, 40.4, 12.7; IR (CHCl3, cm–1): ν 1759, 1081, 1027; HRMS
(ES): calcd for C12H14O [M]+: 174.1045; found: 174.1041.
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1-((RS)-2-(4-Chlorophenyl)-2,5-dihydrofuran-3-yl)-4-((SR)-2-(4-chlorophenyl)-2,5-
dihydrofuran-3-yl)benzene (±)-2e. From 41 mg (0.09 mmol) of allenol (±)ꢀ1e, and after
chromatography of the residue using hexanes/ethyl acetate (4:1) as eluent, gave compound (±)ꢀ2e
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(23 mg, 58%) as a colorless solid; mp 78–80 ºC; HꢀNMR (700 MHz, CDCl3, 25 ºC) δ: 7.27 (m,
8H), 7.09 (s, 4H), 6.45 (d, J = 1.7 Hz, 1H), 6.42 (d, J = 1.7 Hz, 1H), 6.00 (m, 2H), 4.92 (m, 2H),
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4.84 (m, 2H); CꢀNMR (175 MHz, CDCl3, 25 ºC) δ: 139.9 (2C), 139.2 (2C), 134.0 (2C), 131.8,
131.6, 129.0 (2C), 128.9(2C), 128.8 (4C), 126.5 (2C), 126.4 (2C), 123.7, 123.6, 87.3, 87.2, 75.3
(2C); IR (CHCl3, cm–1): ν 1720, 1091, 1014; HRMS (ES): calcd for C26H20Cl2O2 [M]+: 434.0840;
found: 434.0842.
(3S,4R)-3-((S)-2,2-Dimethyl-1,3-dioxolan-4-yl)-2-(4-methoxyphenyl)-8-methyl-5-oxa-2-
azaspiro[3.4]oct-7-en-1-one (+)-2f. From 30 mg (0.087 mmol) of allenol (+)ꢀ1f, and after
chromatography of the residue using hexanes/ethyl acetate (1:1) as eluent, gave compound (+)ꢀ2f
(15 mg, 45%) as a yellow solid; mp 95–97 ºC; [α]D: +6.0 (c 0.53 in CHCl2); 1HꢀNMR (300 MHz,
CDCl3, 25 ºC) δ: 7.72 (d, J = 9.1 Hz, 2H), 6.88 (d, J = 9.1 Hz, 2H), 5.80 (q, J = 1.5 Hz, 1H), 4.82
(dt, J = 13.0, 1.9 Hz, 1H), 4.61 (dt, J = 13.0, 1.9 Hz, 1H), 4.46 (m, 1H), 4.26 (dd, J = 8.6, 7.1 Hz,
1H), 4.08 (d, J = 8.6 Hz, 1H), 3.80 (s, 3H), 3.55 (dd, J = 8.6, 6.3 Hz, 1H), 1.76 (q, J = 1.9 Hz, 3H),
1.54 (s, 3H), 1.34 (s, 3H); 13CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 165.9, 156.5, 132.7, 130.9, 124.8,
119.7 (2C), 114.0 (2C), 109.8, 99.6, 77.2, 76.3, 66.5, 66.3, 55.4, 26.6, 24.6, 10.9; IR (CHCl3, cm–
1): ν 1752; HRMS (ES): calcd for C19H24NO5 [M + H]+: 346.1654; found: 346.1665.
(3SR,4RS)-2-Benzyl-3-((S)-2,2-dimethyl-1,3-dioxolan-4-yl)-8-phenyl-5-oxa-2-
azaspiro[3.4]oct-7-en-1-one (−)-2g. From 29 mg (0.074 mmol) of allenol (+)ꢀ1g, and after
chromatography of the residue using hexanes/ethyl acetate (2:1) as eluent, gave compound (−)ꢀ2g
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(16 mg, 54%) as a colorless oil; [α]D: −22.2 (c 0.5 in CHCl2); 1HꢀNMR (300 MHz, CDCl3, 25 ºC)
δ: 7.20 (m, 10 H), 6.23 (t, J = 1.8 Hz, 1H), 4.97 (dd, J = 14.1, 1.8 Hz, 1H), 4.83 (d, J = 14.4 Hz,
1H), 4.69 (dd, J = 14.1, 1.8 Hz, 1H), 4.46 (m, 1H), 4.25 (d, J = 14.4 Hz, 1H), 4.13 (dd, J = 8.5, 7.0
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Hz, 1H), 3.47 (d, J = 7.9 Hz, 1H), 3.43 (dd, J = 8.5, 6.0 Hz, 1H), 1.33 (s, 3H), 1.31 (s, 3H); Cꢀ
NMR (75 MHz, CDCl3, 25 ºC) δ: 167.9, 158.2, 134.8, 131.2, 129.1, 128.6, 128.4, 128.3, 127.5,
126.9, 126.5, 109.7, 99.8, 77.2, 76.0, 66.3, 64.4, 45.3, 26.5, 24.8; IR (CHCl3, cm–1): ν 1753;
HRMS (ES): calcd for C24H26NO4 [M + H]+: 392.1862; found: 392.1875.
3-Methyl-5H-spiro[furan-2,3'-indolin]-2'-one 2h. From 30 mg (0.15 mmol) of allenol 1h,
and after chromatography of the residue using hexanes/ethyl acetate (2:1) as eluent, gave
compound 2h (24 mg, 80%) as a yellow oil; HꢀNMR (300 MHz, CDCl3, 25 oC) δ: 8.76 (br, 1H),
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7.25 (td, J = 7.7, 1.3 Hz, 1H), 7.19 (m, 1H), 7.05 (td, J = 7.5, 1.0 Hz, 1H), 6.89 (d, J = 7.8 Hz, 1H),
5.98 (m, 1H), 5.03 (dquin, J = 12.4, 2.0 Hz, 1H), 4.94 (dquin, J = 12.4, 2.0 Hz, 1H), 1.50 (q, J =
2.0 Hz, 3H); 13CꢀNMR (75 MHz, CDCl3, 25 oC) δ: 178.3, 141.0, 135.8, 130.0, 128.7, 124.8, 124.7,
123.2, 110.4, 93.0, 76.5, 11.1; IR (CHCl3, cm–1): ν 3256, 1726, 1619; HRMS (ES): calcd for
C12H11NO2 [M]+: 201.0790; found: 201.0793.
3-(4-Methoxyphenyl)-5H-spiro[furan-2,3'-indolin]-2'-one 2i. From 30 mg (0.10 mmol) of
allenol 1i, and after chromatography of the residue using hexanes/ethyl acetate (2:1) as eluent, gave
compound 2i (16.5 mg, 56%) as a yellow oil; 1HꢀNMR (300 MHz, CDCl3, 25 oC) δ: 8.28 (br, 1H),
7.25 (m, 2H), 7.02 (m, 2H), 6.89 (d, J = 7.7 Hz, 1H), 6.68 (d, J = 8.8 Hz, 1H), 6.54 (t, J = 1.8 Hz,
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1H), 5.15 (dd, J = 13.5, 1.8 Hz, 1H), 5.06 (dd, J = 13.5, 1.8 Hz, 1H), 3.70 (s, 3H); CꢀNMR (75
o
MHz, CDCl3, 25 C) δ: 177.5, 159.4, 140.9, 138.4, 130.3, 129.2, 127.4 (2C), 125.6, 125.3, 124.1,
123.4, 113.9 (2C), 110.5, 91.0, 76.0, 55.1; IR (CHCl3, cm–1): ν 1725; HRMS (ES): calcd for
C18H15NO3 [M]+: 293.1052; found: 293.1061.
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1',3-Dimethyl-5H-spiro[furan-2,3'-indolin]-2'-one 2j. From 30 mg (0.14 mmol) of allenol
1j, and after chromatography of the residue using hexanes/ethyl acetate (4:1) as eluent, gave
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compound 2j (22 mg, 75%) as a colorless oil; HꢀNMR (300 MHz, CDCl3, 25 ºC) δ: 7.32 (td, J =
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7.7, 1.2 Hz, 1H), 7.20 (d, J = 6.3 Hz, 1H), 7.07 (t, J = 7.5 Hz, 1H), 6.82 (d, J = 7.7 Hz, 1H), 5.96
(q, J = 1.5 Hz, 1H), 5.01 (dt, J = 12.4, 1.9 Hz, 1H), 4.90 (dt, J = 12.4, 1.9 Hz, 1H), 3.20 (s, 3H),
7.07 (m, 2H), 13CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 175.6, 144.0, 135.8, 130.0, 128.3, 124.9,
124.4, 123.1, 108.2, 92.4,76.3, 26.3, 11.1; IR (CHCl3, cm–1): ν 1715; HRMS (ES): calcd for
C13H14NO2 [M + H]+: 216.1025; found: 216.1031.
1'-Methyl-3-phenyl-5H-spiro[furan-2,3'-indolin]-2'-one 2k. From 25 mg (0.09 mmol) of
allenol 1k, and after chromatography of the residue using hexanes/ethyl acetate (2:1) as eluent,
gave compound 2k (17 mg, 67%) as a yellow solid; mp 144–146 ºC;1HꢀNMR (300 MHz, CDCl3,
25 oC) δ: 7.26 (td, J = 7.7, 1.1 Hz, 1H), 7.19 (m, 2H), 7.07 (m, 2H), 6.97 (td, J = 7.5, 0.8 Hz, 1H),
6.91 (m, 2H), 6.78 (d, J = 7.9 Hz, 1H), 6.57 (q, J = 1.8 Hz, 1H), 5.09 (dd, J = 13.6, 1.8 Hz, 1H),
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4.99 (dd, J = 13.6, 1.8 Hz, 1H), 3.15 (s, 3H); CꢀNMR (75 MHz, CDCl3, 25 C) δ: 175.1, 144.0,
139.0, 131.7, 130.4, 128.7, 128.4 (2C), 128.0, 127.5, 126.1 (2C), 124.9, 123.4, 108.5, 90.7, 75.8,
26.4; IR (CHCl3, cm–1): ν 1725, 1612; HRMS (ES): calcd for C18H16NO2 [M + H]+: 278.1181;
found: 278.1180.
5'-Chloro-1',3-dimethyl-5H-spiro[furan-2,3'-indolin]-2'-one 2l. From 30 mg (0.12 mmol)
of allenol 1l, and after chromatography of the residue using hexanes/ethyl acetate (2:1) as eluent,
gave compound 2l (22 mg, 73%) as a yellow oil; 1HꢀNMR (300 MHz, CDCl3, 25 oC) δ: 7.29 (dd, J
= 8.3, 2.2 Hz, 1H), 7.18 (d, J = 2.2 Hz, 1H), 6.75 (d, J = 8.3 Hz, 1H), 5.97 (q, J = 1.5 Hz, 1H), 5.00
(dt, J = 12.4, 2.0 Hz, 1H), 4.90 (dt, J = 12.4, 2.0 Hz, 1H), 3.18 (s, 3H), 1.44 (q, J = 2.0 Hz, 1H);
13CꢀNMR (75 MHz, CDCl3, 25 oC) δ: 175.2, 142.4, 135.2, 130.0, 129.9, 128.6, 125.3, 124.9, 109.2,
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92.3, 76.5, 26.4, 11.1; IR (CHCl3, cm–1): ν 1729, 1487; HRMS (ES): calcd for C13H13ClNO2 [M +
6
7
H]+: 250.0635; found: 250.0612.
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7'-Chloro-1',3-dimethyl-5H-spiro[furan-2,3'-indolin]-2'-one 2m. From 31 mg (0.12
mmol) of allenol 1m, and after chromatography of the residue using hexanes/ethyl acetate (4:1) as
eluent, gave compound 2m (26 mg, 87%) as a yellow solid; mp 81–83 ºC;1HꢀNMR (300 MHz,
CDCl3, 25 oC) δ: 7.25 (m, 1H), 7.09 (dd, J = 7.2, 1.1 Hz, 1H), 6.99 (t, J = 7.7 Hz, 1H), 5.97 (q, J =
1.5 Hz, 1H), 5.00 (dt, J = 12.4, 1.9 Hz, 1H), 4.90 (dt, J = 12.4, 1.9 Hz, 1H), 3.57 (s, 3H), 1.44 (q, J
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o
= 1.9 Hz, 1H); 13CꢀNMR (75 MHz, CDCl3, 25 C) δ: 175.9, 139.7, 135.5, 132.2, 131.2, 125.1,
123.9, 123.0, 115.7, 91.8, 76.5, 29.7, 11.1; IR (CHCl3, cm–1): ν 1734, 1461; HRMS (ES): calcd for
C13H12ClNO2 [M]+: 249.0557; found: 249.0551.
4-(3-Phenyl-2,5-dihydrofuran-2-yl)benzonitrile 2n. From 40 mg (0.16 mmol) of allenol
1n, and after chromatography of the residue using hexanes/ethyl acetate (7:1) as eluent, gave
compound 2n (24 mg, 63%) as a colorless oil; 1HꢀNMR (300 MHz, CDCl3, 25 ºC) δ: 7.35 (s, 4H),
7.14 (m, 3H), 7.09 (d, J = 3.0 Hz, 2H), 6.05 (td, J = 4.6, 1.8 Hz, 1H), 5.98 (q, J = 1.9 Hz, 1H), 4.76
(d, J = 1.9 Hz, 1H), 4.74 (m, 1H); 13CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 146.7, 141.3, 133.0, 132.7,
129.0, 128.6, 128.5, 127.2, 124.2, 119.0, 112.8, 87.9, 76.1; IR (CHCl3, cm–1): ν 2230, 1758, 843;
HRMS (ES): calcd for C17H13NO [M]+: 247.0997; found: 247.1005.
Methyl 4-(3-methyl-2,5-dihydrofuran-2-yl)benzoate 2o. From 40 mg (0.18 mmol) of
allenol 1o, and after chromatography of the residue using hexanes/ethyl acetate (6:1) as eluent,
gave compound 2o (28 mg, 72%) as a colorless oil; 1HꢀNMR (300 MHz, CDCl3, 25 ºC) δ: 8.16 (d,
J = 8.4 Hz, 2H), 7.17 (m, 2H), 5.38 (m, 1H), 5.13 (q, J = 1.6 Hz, 1H), 4.61 (m, 2H) 3.50 (s, 3H),
13
1.23 (t, J = 1.3 Hz, 3H); CꢀNMR (75 MHz, CDCl3, 25 ºC) δ: 137.0, 147.9, 138.8, 130.7, 130.5,
127.3, 121.7, 90.5, 76.1, 51.9, 12.6; IR (CHCl3, cm–1): ν 1723, 1281, 1112; HRMS (ES): calcd for
C13H14O3 [M]+: 218.0943; found: 218.0938.
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AuCl3-catalyzed reaction of allenol 1a. AuCl3 (2 mg, 0.0065 mmol) was added to a solution
of allenol 1a (30 mg, 0.13 mmol) in dichloromethane (3 mL). The reaction mixture was stirred at
RT until the starting material disappeared as indicated by TLC. Saturated aqueous sodium
bicarbonate (1 mL) was added, before being partitioned between dichloromethane and water. The
organic extract was washed with brine, dried (MgSO4), and concentrated under vacuum.
Chromatography of the residue using hexanes/ethyl acetate (6:1) as eluent gave 12 mg (42%) of the
less polar compound 3a and 5 mg (16%) of the more polar compound 2a.
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1,3-Dimethoxy-6-methylnaphthalene 3a. Colorless oil; 1HꢀNMR (300 MHz, CDCl3, 25 ºC)
δ: 8.02 (d, J = 8.5 Hz, 1H), 7.46 (s, 1H), 7.14 (dd, J = 8.5, 1.2 Hz, 1H), 6.66 (d, J = 1.9 Hz, 1H),
13
6.44 (d, J = 1.9 Hz, 1H), 3.96 (s, 3H), 3.90 (s, 3H), 2.48 (s, 3H); CꢀNMR (75 MHz, CDCl3, 25
ºC) δ: 158.2, 156.5, 136.7, 135.2, 125.6, 125.0, 121.7, 119.7, 97.4. 96.7, 55.4, 55.2, 21.6; IR
(CHCl3, cm–1): ν 2934, 1633, 1154; HRMS (ES): calcd for C13H14O2 [M]+: 202.0994; found:
202.0998.
AuCl3-catalyzed reaction of allenol 1b. AuCl3 (4 mg, 0.013 mmol) was added to a solution
of allenol 1b (62 mg, 0.24 mmol) in dichloromethane (6 mL). The reaction mixture was stirred at
RT until the starting material disappeared as indicated by TLC. Saturated aqueous sodium
bicarbonate (2 mL) was added, before being partitioned between dichloromethane and water. The
organic extract was washed with brine, dried (MgSO4), and concentrated under vacuum.
Chromatography of the residue using hexanes/ethyl acetate (10:1) as eluent gave 22 mg (35%) of
the less polar compound 2b and 14 mg (25%) of the more polar compound 3b.
7-Methyl-1-phenoxynaphthalene 3b. Colorless oil; 1HꢀNMR (300 MHz, CDCl3, 25 ºC) δ:
7.66 (s, 1H), 7.45 (d, J = 7.6 Hz, 1H), 7.32 (m, 3H), 7.16 (m, 3H), 6.97 (m, 3H), 2.36 (s, 3H); 13Cꢀ
NMR (75 MHz, CDCl3, 25 ºC) δ: 157.2, 155.0, 138.6, 134.9, 130.6, 130.0 (2C), 129.8 (2C), 127.7,
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123.3 (2C), 119.3, 119.1, 118.3 (2C), 25.7; IR (CHCl3, cm–1): ν 3065, 1483, 1235, 752, 693;
HRMS (ES): calcd for C17H14O [M]+: 234.1045; found: 234.1041.
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Acknowledgment. Support for this work by MINECO and FEDER (Projects CTQ2012ꢀ33664ꢀ
C02ꢀ01 and CTQ2012ꢀ33664ꢀC02ꢀ02) and UCMꢀBANCO SANTANDER (Project GR3/14) is
gratefully acknowledged. E.S. thanks CESGA for the generous allocation of computational
resources.
Supporting Information Available: Copies of NMR spectra of new compounds and
computational details. This material is available free of charge via the Internet at
http://pubs.acs.org.
References and Notes
(1) For recent reviews on allene chemistry, see: (a) Yang, W.; Hashmi, A. S. K. Chem. Soc. Rev.
2014, 43, 2941. (b) Fensterbank, L.; Malacria, M. Acc. Chem. Res. 2014, 47, 953. (c) Le Bras,
J.; Muzart, J. Chem. Soc. Rev. 2014, 43, 3003. (d) Lechel, T.; Pfrengle, F.; Reissig, H.ꢀU.;
Zimmer, R. ChemCatChem 2013, 5, 2100. (e) Yu, S.; Ma, S. Angew. Chem. Int. Ed. 2012, 51,
3074. (f) Krause, N.; Winter, C. Chem. Rev. 2011, 111, 1994. (g) Alcaide, B.; Almendros, P.
Adv. Synth. Catal. 2011, 353, 2561.
(2) For selected reviews, see: (a) Larrosa, I.; Romea, P.; Urpí, F. Tetrahedron 2008, 64, 2683. (b)
Bremner, J. B.; Samosorn, S. In Progress in Heterocyclic Chemistry; Gribble, G. W., Joule, J.
A., Eds.; Elsevier: Oxford, 2007; Vol. 18, pp 402–429. (c) Newkome, G. R. In Progress in
Heterocyclic Chemistry; Gribble, G. W., Joule, J. A., Eds.; Elsevier: Oxford, 2007; Vol. 18,
pp 430–448. (d) Wolfe, J. P.; Hay, M. B. Tetrahedron 2007, 63, 261. (e) Blunt, J. W.; Copp,
B. R.; Hu, W.ꢀP.; Munro, M. H. G.; Northcote, P. T.; Prinsep, M. R. Nat. Prod. Rep. 2007,
24, 31. (f) Snyder, N. L.; Haines, H. M.; Peczuh, M. W. Tetrahedron 2006, 62, 9301. (g)
Clarke, P. A.; Santos, S. Eur. J. Org. Chem. 2006, 2045. (h) saleem, M.; Kim, H. J.; Ali, M.
S.; Lee, Y. S. Nat. Prod. Rep. 2005, 22, 696. (i) Kilroy, T. G.; O’Sullivan, T.P.; Guiry, P. J.
Eur. J. Org. Chem. 2005, 4929. (j) Faul, M. M.; Huff, B. E. Chem. Rev. 2000, 100, 2407. (k)
Faulkner, D. J. Nat. Prod. Rep. 1996, 13, 75.
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(3) (a) Gelin, R.; Gelin, S.; Albrand, M. Bull. Soc. Chim. Fr. 1972, 1946. (b) Audin, P.;
Doutheau, A.; Gore, J. Bull. Soc. Chim. Fr. II 1984, 297. (c) Bridges, A. J.; Thomas, R. D. J.
Chem. Soc., Chem. Commun. 1984, 694.
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(4) For the firstꢀgold catalyzed reactions of allenes, see: (a) A. S. K. Hashmi, L. Schwarz, J.ꢀH.
Choi, T. M. Frost, Angew. Chem. Int. Ed. 2000, 39, 2285. For the pioneered Auꢀcatalyzed
cycloisomerization of αꢀallenols, see: (b) HoffmannꢀRöder, A.; Krause, N. Org. Lett. 2001, 3,
2537. For selected references, see: (c) Hashmi, A. S. K.; Blanco, M. C.; Fischer, D.; Bats, J.
W. Eur. J. Org. Chem. 2006, 1387. (d) Deutsch, C.; Lipshutz, B. H.; Krause, N. Angew.
Chem. Int. Ed. 2007, 46, 1677. (e) Muzart, J. Tetrahedron 2008, 64, 5815. (f) Asikainen, M.;
Krause, N. Adv. Synth. Catal. 2009, 351, 2305. (g) Alcaide, B.; Almendros, P.; Martínez del
Campo, T.; Soriano, E.; MarcoꢀContelles, J. L. Chem. Eur. J. 2009, 15, 1901. (h) Eom, D.;
Kang, D.; Lee, P. H. J. Org. Chem. 2010, 75, 7447. (i) Brasholz, M.; Dugovič, B.; Reissig,
H.ꢀU. Synthesis 2010, 3855. (j) Alcaide, B.; Almendros, P.; Martínez del Campo, T.;
Redondo, M. C.; Fernández, I. Chem. Eur. J. 2011, 17, 15005. (k) Alcaide, B.; Almendros,
P.; Martínez del Campo, T.; Fernández, I. Chem. Commun. 2011, 47, 9054. (l) Kim, S.; Lee,
P. H. J. Org. Chem. 2012, 77, 215. For goldꢀcatalyzed cyclizations of αꢀ and βꢀallenols, see:
(m) Zriba, R.; Gandon, V.; Aubert, C.; Fensterbank, L.; Malacria, M. Chem. Eur. J. 2008, 14,
1482. (n) Aks n, Ö.; Krause, N. Adv. Synth. Catal. 2008, 350, 1106. For goldꢀcatalyzed
cyclizations of γꢀ and δꢀallenols, see: (o) Zhang, Z.; Widenhoefer, R. A. Angew. Chem. Int.
Ed. 2007, 46, 283.
(5) For the hydroalkoxylation of allenols catalyzed by platinum salts, see: (a) Zhang, Z.; Liu, C.;
Kinder, R. E.; Han, X.; Qian, H.; Widenhoefer, R. A. J. Am. Chem. Soc. 2006, 128, 9066. (b)
Alcaide, B.; Almendros, P.; Martínez del Campo, T.; Soriano, E.; MarcoꢀContelles, J. L.
Chem. Eur. J. 2009, 15, 9127. (c) Alcaide, B.; Almendros, P.; Carrascosa, R.; Martínez del
Campo, T. Chem. Eur. J. 2010, 16, 13243. For the silver(I)ꢀcatalyzed cyclization of allenic
alcohols, see : (d) Olsson, L.ꢀI.; Claesson, A. Synthesis 1979, 743. For the palladiumꢀ
catalyzed cyclization of allenols, see: (e) Xu, D.; Li, Z.; Ma, S. Chem. Eur. J. 2002, 8, 5012.
(f) Ma, S.; Gu, Z.; Yu, Z. J. Org. Chem. 2005, 70, 6291. (g) Deng, Y.; Fu, C.; Ma, S. Chem.
Eur. J. 2011, 17, 4976.
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(6) For the mercury(II)ꢀmediated hydroalkoxylation of δꢀhydroxy allenes installed on a
tetrahydropyran template, see: Suzuki, Y.; Kuwabara, A.; Koizumi, Y.; Mori, Y. Tetrahedron
2013, 69, 9086.
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(7) (a) Hintermann, H. Organomercurials. Their Formation and Pathways in the Environment In
Organometallics in Environment and Toxicology; Sigel, A., Sigel, H., Sigel, R. K. O., Eds.;
Cambridge: RSC publishing, 2010; Vol. 7, pp 365–401. (b) Aschner, M.; Onishchenko, N.;
Ceccatelli, S. Toxicology of Alkylmercury Compounds In Organometallics in Environment
and Toxicology; Sigel, A., Sigel, H., Sigel, R. K. O., Eds.; Cambridge: RSC publishing, 2010;
Vol. 7, pp 403–434. (c) Doja, A.; Roberts, W. Can. J. Neurol. Sci. 2006, 33, 341. (d)
Clarkson, T. W.; Magos, L. Crit. Rev. Toxicol. 2006, 36, 609.
(8) Thiomersal or thimerosal (sodium ethylmercuric thiosalicylate) is used as a bacteriostatic
agent in vaccines, which was supposed to increase autism cases. However, no convincing
proof for autism appearance and the use of mercuryꢀcontaining vaccines has been discovered.
(9) Nowadays the use of mercuryꢀbased compounds is widespread (barometers, batteries,
electrical switches, electrodes, extraction of gold, gauges, highꢀintensity discharge lamps, and
tooth fillings). Blum, J. D. Nat. Chem. 2013, 5, 1066.
(10) For an excellent review on similarities and differences between goldꢀ, platinumꢀ, and
mercuryꢀbased catalysts, see: LeyvaꢀPérez, A.; Corma, A. Angew. Chem. Int. Ed. 2012, 51,
614.
(11) For previous catalytic uses of mercury(II) perchlorate in organic synthesis, see: (a) Hashmi,
A. S. K.; Schwarz, L.; Bolte, M. Tetrahedron Lett. 1998, 39, 8969. (b) Hashmi, A. S. K.;
Schwarz, L.; Bats, J. W. J. Prakt. Chem. 2000, 342, 40. (c) Hashmi, A. S. K.; Schwarz, L.;
Rubenbauer, P.; Blanco, M. C. Adv. Synth. Catal. 2006, 348, 705. (d) Cao, Z.ꢀ Y.; Zhang, Y.;
Ji, C.ꢀB.; Zhou, J. Org. Lett. 2011, 13, 6398. (e) Zhou, F.; Cao, Z.ꢀ Y.; Zhang, J.; Yang, H.ꢀ
B.; Zhou, J. Chem. Asian J. 2012, 7, 233.
(12) Although mercury(II) salts are generally considered to be very toxic by ingestion, the nonꢀ
volatile, crystalline nature of mercury(II) perchlorate, together with its insolubility in lipids,
assures safe manipulations with the reagent when handled in a fumeꢀhood using standard
safety precautions.
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(13) With regards to price, goldꢀ and platinumꢀbased compounds are three orders of magnitude
more expensive that mercury derivatives. Regarding stability, Hg2+ is a stable cation and
contrary to Au+ or Pt2+ does not need stabilizing ligands.
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(14) (a) Alcaide, B.; Almendros, P.; Aragoncillo, C. Org. Lett. 2000, 2, 1411. (b) Alcaide, B.;
Almendros, P.; Aragoncillo, C; Redondo, M. C. Chem. Commun. 2002, 1472. (c) Alcaide, B.;
Almendros, P.; RodríguezꢀAcebes, R. J. Org. Chem. 2005, 70, 3198. Allenols 2p and 2q were
prepared by known literature procedures: (d) Sabbasani, V. R.; Mamidipalli, P.; Lu, H.; Xia,
Y.; Lee, D. Org. Lett. 2013, 15, 1552. (e) Fu, F.; Hoang, K. L. M.; Loh, T.ꢀP. Org.
Lett. 2008, 10, 3437.
(15) Under otherwise similar conditions, the AuCl3ꢀcatalyzed reaction of allenols 1f,g afforded
similar yields of 2,5ꢀdihydrofurans 2f,g in comparison with the Hg(ClO4)2·3H2Oꢀcatalyzed
process, but the cleavage of the acetal moiety was observed in some extension (around 10%).
No acetal cleavage was observed in any case for the Hg(II)ꢀcatalyzed reaction.
(16) A potentially promising subset of the spirooxindole alkaloids are the oxaꢀspirooxindoles,
joining two privileged motifs, the oxacycle and oxindole substructures through a unique
spiroꢀquaternary carbon. Unfortunately, approaches for this interesting hybrid scaffold remain
limited. For selected examples, see: (a) Liu, R.; Yu, C.; Xiao, Z.; Li, T.; Wang, X.; Xie, Y.;
Yao, C. Org. Biomol. Chem. 2014, 12, 1885. (b) Xie, X.; Peng, C.; He, G.; Leng, H.ꢀJ.;
Wang, B.; Huang, W.; Han, B. Chem. Commun. 2012, 48, 10487. (c) Wang, X.ꢀN.; Zhang,
Y.ꢀY.; Ye, S. Adv. Synth. Catal. 2010, 352, 1892. (d) Alcaide, B.; Almendros, P.; Martínez
del Campo, T.; Quirós, M. T. Chem. Eur. J. 2009, 15, 3344.
(17) For reviews, see: (a) Santos, M. M. M. Tetrahedron 2014, 70, 9735. (b) Badillo, J. J.; Hanhan,
N. V.; Franz, A. K. Curr. Opin. Drug. Disc. 2010, 13, 758. (c) Trost, B. M.; Brennan, M. K.
Synthesis 2009, 3003. (d) Marti, C.; Carreira, E. M. Eur. J. Org. Chem. 2003, 2209. (e) For
natural oxacyclic spiroindolones, see: Ma, S.ꢀS.; Mei, W.ꢀL.; Guo, Z.ꢀK.; Liu, S.ꢀB.; Zhao,
Y.ꢀX.; Yang, D.ꢀL.; Zeng, Y.ꢀB.; Jiang, B.; Dai, H.ꢀF. Org. Lett. 2013, 15, 1492.
(18) (a) M. Lein, M. Rudolph, A. S. K. Hashmi, P. Schwerdtfeger, Organometallics 2010, 29,
2206. (b) M. Pernpointner, A. S. K. Hashmi, J. Chem. Theory Comput. 2009, 5, 2717. (c) D.
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J. Gorin, F. D. Toste, Nature 2007, 446, 395. (d) P. Pyykkç, J.ꢀP. Desclaux, Acc. Chem. Res.
1979, 12, 276.
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(19) Soriano, E.; MarcoꢀContelles, J. Organometallics, 2006, 25, 4542.
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(20) Alternatively, some theoretical studies have suggested that residual water molecules may act
as a proton shuttle in proton transfer steps, leading to lower reaction barriers and more
efficient routes, in gold(I)ꢀcatalyzed reactions: (a) Shi, F.ꢀQ.; Li, X.; Xia, Y.; Zhang, L.; Yu,
Z.ꢀX. J. Am. Chem. Soc. 2007, 129, 15503. (b) Krauter, C. M.; Hashmi, A. S. K.;
Pernpointner, M. ChemCatChem 2010, 2, 1226; (c) Mazzone, G.; Russo, N.; Sicilia, E.
Organometallics 2012, 31, 3074, and in gold(III)ꢀcatalyzed reactions: (d) Cordón, J.;
JiménezꢀOsés, G.; LópezꢀdeꢀLuzuriaga, J. M.; Monge, M.; Olmos, M. E.; Pascual, D.
Organometallics 2014, 33, 3823. Accordingly, we have performed calculations on the waterꢀ
assisted formation of the naphthalene framework I4’Au from I2’Au. The results show a higher
barrier for this concerted step (ꢁG# = 16.0 kcal/mol, see S.I. for details) and a less favorable
route than the stepwise mechanism depicted in Figure 2, so it can be ruled out as competitive
path.
(21) CAUTION! Perchlorates are potentially explosive compounds and should be handled very
carefully. We used Hg(ClO4)2·3H2O for our experiments and we encountered no problem
during handling. Hg(ClO4)2·3H2O is a stable salt because: a) 0.006 M solutions (around 22
mg) in different organic solvents (THF, DCE, acetonitrile, and DMF) can be safely heated at
70oC for several hours; b) 10 mg of Hg(ClO4)2·3H2O (neat product) can be heated at 200oC
with decomposition but not explosion; and c) it did not decompose during storage at room
temperature for several weeks. We did not suffer any explosion, but this salt should be
handled with care.
(22) The unpurified reaction mixture of allenol 1j under catalytic HClO4 conditions contained the
corresponding rearranged α,βꢀunsaturated ketone as the major reaction product, along with
unidentified impurities. This result should rule out the possibility of Brønsted acid catalysis
through adventitious perchloric acid with the use of Hg(ClO4)2·3H2O.
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The Journal of Organic Chemistry
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