Mercuric triflate-(TMU)2 catalyzed cyclization of a propargylic ketone into a monosubstituted furan

Article abstract of DOI:10.1055/s-2005-922779

Formation of a furan derivative was observed in the course of a synthetic approach towards a new carotenoid metabolite, starting from a propargylic ketone, in the presence of mercuric triflate-tetramethylurea complex. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-922779

LETTER
57
Mercuric Triflate·(TMU)2 Catalyzed Cyclization of a Propargylic Ketone
into a Monosubstituted Furan
Mercuric
T
riflate·
(
e
T
MU
)
–
C
2 latalyze
p
d Cyclizatio
h
n of
a
P
ropar
i
gylic
K
n
etone e Ménard,a Anne Vidal,a Chantal Barthomeuf,b Jacques Lebreton,c Pascal Gosselin*a
a
Laboratoire de Synthèse Organique – UMR-CNRS 6011, Université du Maine,
Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
Fax +33(2)43833902; E-mail: pascal.gosselin@univ-lemans.fr
b
c
Laboratoire de Pharmacognosie et Biotechnologies, UMR-INSERM U-484, Faculté de Pharmacie, Université d’Auvergne,
pl. H. Dunant, 63001 Clermont-Fd Cedex, France
Laboratoire de Synthèse Organique – UMR-CNRS 6513, Département Chimie, Université de Nantes,
2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France
Received 5 October 2005
In addition, according to the same aldehyde-based strate-
gy (Scheme 1), another route was explored that involved
the hydration of the terminal alkyne of a homopropargylic
alcohol followed by oxidation. The results obtained by
following this route are reported herein.
Abstract: Formation of a furan derivative was observed in the
course of a synthetic approach towards a new carotenoid metabo-
lite, starting from a propargylic ketone, in the presence of mercuric
triflate–tetramethylurea complex.
Key words: alkynes, ketones, furans, catalysis, addition reactions
The homopropargylic alcohol 3 was obtained in 96%
crude yield, as a mixture of four diastereoisomers, by the
addition of aldehyde 2 (ca. 3:2 mixture of cis:trans
isomers) to an ethereal solution of propargylmagnesium
bromide (Scheme 2).9 Hydration of the terminal alkyne in
3 was first attempted following the conventional standard
procedure, which consists in heating an aqueous solution
of alkyne with mercuric oxide and sulfuric acid.10 How-
ever, under these drastic conditions, only degradation of
the starting material was observed. Therefore, milder con-
ditions were tried, using the mercuric triflate-N,N,N¢,N¢-
tetramethylurea (hereafter TMU) complex recently intro-
duced by Nishizawa et al., who extended the scope of this
oxymercuration agent to the hydration of terminal alkynes
to give methyl ketones.11 The reaction is carried out in
acetonitrile, with water, and at room temperature to afford
the corresponding methyl ketone with excellent yields.
A new carotenoid metabolite 1 was isolated in 1999 from
the cultured marine microalgae Skeletonema costatum by
IFREMER.1 This compound exhibits an interesting cyto-
toxicity against various human carcinoma cell lines.2 As
part of our interest in the total synthesis of biologically ac-
tive molecules, we undertook the synthesis of metabolite
1. The retrosynthetic plan is depicted in the following
scheme (Scheme 1), introducing the 1,3-diketone unit in
the final stages on aldehyde 2.3
O
O
O
2
1
AcO
TBSO
OH
TBS = t-BuMe2Si
The hydration reaction was performed as described by
Nishizawa et al.: in situ preparation of mercuric triflate12
was followed by addition of TMU to produce the reacting
Hg(OTf)2·TMU2 complex (0.1 equiv). Then, the ho-
mopropargylic alcohol 3 was added, together with three
equivalents of water. After one day at room temperature,
no methyl ketone was detected. Instead, the aldehydes 4
and 5 were isolated (ca. 1:1 4:5 ratio; Scheme 2).13
Scheme 1 Retrosynthetic analysis of marine natural carotenoid
metabolite 1.
1,3-Diketones are useful building blocks, as illustrated by
the formation of heterocycles4 or their use as chelating
ligands for metals.5 Numerous methods have therefore
been developed for their introduction. One of the most
popular approaches to introduce the 1,3-diketone motif
from the ketone precursor is based on the C-acylation of
the corresponding enolates (or silyl enol ethers) with acyl-
ating agents.6 However, to our knowledge, less attention
has been devoted to aldehydes.7
The following mechanism (Scheme 3) may explain the
formation of these compounds: the initially formed
mercurinium ion 6 may be intramolecularly trapped by the
hydroxyl group according to a 5-endo-dig cyclization.
Protodemercuration14 of the vinyl mercury intermediate 7
would then lead to the dihydrofuran derivative 8. Due to
the presence of in situ generated trifluoroacetic acid, un-
stable protonated 8 may open with concomitant proton
loss, which can occur following two possibilities, ulti-
mately leading to aldehydes 4 and 5 via their enols. The
formation of deprotected aldehydes 4 and 5 can readily be
accounted for by the presence of liberated trifluoroacetic
acid in the reaction medium.15 The formation of 4, with an
endocyclic double bond, may be explained on the same
Within this context, we recently reported a simple meth-
odology to introduce a 1,3-diketone motif from various
aldehyde precursors, using 1-(phenylsulfonyl)propan-2-
one as a masked equivalent of acetone.8
SYNLETT 2006, No. 1, pp 0057–0060
0
5.
0
1.
2
0
0
6
Advanced online publication: 16.12.2005
DOI: 10.1055/s-2005-922779; Art ID: G32105ST
© Georg Thieme Verlag Stuttgart · New York

58
D. Ménard et al.
LETTER
O
OH
rable 3:2 mixture of cis:trans diastereoisomers after flash
chromatography (51% overall yield from 3).18,19 No
methyl ketone signal could be detected in the H NMR
a
1
3
TBSO
TBSO
spectrum of the crude product. Here again, the reaction
was much faster than anticipated from Nishizawa’s re-
sults: furan 11 was already formed after 15 minutes at
0 °C even when no water was added. In all cases, com-
plete deprotection of the silyl ether occurred (Scheme 4).
2
b
O
O
4
5
HO
HO
O
OH
O
+
a
b
9
3
10
TBSO
TBSO
Scheme 2 Reagents and conditions: a) propargyl bromide, Mg,
HgCl2 (cat.), Et2O, –20 °C, 96% (crude yield); b) HgO (red), trifluo-
romethanesulfonic anhydride, tetramethylurea, MeCN, H2O, r.t., 43%
global yield.
O
c
or
9
10
11
HO
Scheme 4 Reagents and conditions: a) Jones’ reagent (80% crude
yield) or DMP (98% crude yield); b) silica gel chromatography 50%
yield from 3; c) HgO (red), Tf2O, TMU, MeCN, H2O, r.t., 51% yield
after chromatography.
grounds. A similar mechanism was recently proposed in
order to explain the electrophilic cyclization of 1,4-diaryl-
but-3-yne-1-ones with N-bromosuccinimide.16 It is note-
worthy that the same products were obtained when the re-
action was effected without addition of water, which
confirms the intramolecular nature of the reaction. More-
over, compared to Nishizawa’s conditions,11 this intra-
molecular reaction was very fast: the same results were
obtained after 30 minutes at 0 °C.
To explain the formation of furan 11, a mechanism similar
to that previously proposed may be envisaged, involving
the enolic form of ketone 9 as the intramolecular nucleo-
phile. Even when a great excess of water (ca. 400 equiv)
was added before the reactant in order to circumvent this
intramolecular attack, no methyl ketone was detected in
the 1H NMR spectrum of the crude product and furan 11
was again formed.
To circumvent this intramolecular attack, oxidation of
alcohol 3 was performed using either Jones’ reagent or
Dess–Martin periodinane (DMP). In both cases, the prop-
argylic ketone 9 was obtained, with excellent crude yield
after one night, as a 3:2 mixture of cis:trans diastereoiso-
mers (ratio identical to that of the starting aldehyde 2).
Only traces of the corresponding allene were detected on
It should be noted that while numerous examples are re-
ported where a-, b- or g-alkynyl ketones and allenyl ke-
tones were cyclized into furans under Pd-,20 Ag-,21 Au-,22
Cu-,23 halonium,16,24 acid-catalyzed25 or base-catalyzed26
conditions, the Hg(II) counterpart was unknown until very
recently: the Hg(OTf)2·TMU2 complex proved to be a
moderately efficient catalyst in the synthesis of 2-methyl-
furans from terminal homopropargylic ketones.27 Further-
more, there are examples in the literature where Pd-, Au-
and Hg catalysts and allenyl ketones give entirely differ-
ent products.28
1
the H NMR spectrum of the crude product. However,
after purification by flash chromatography, the propargyl-
ic ketone 9 was totally transformed into allene 10, as al-
ready described for other terminal propargylic ketones
(Scheme 4).17
When submitted to the same hydration conditions,11–13
crude propargylic ketone 9 yielded furan 11 as an insepa-
OTf
H
H
OTf
Hg(OTf)
–Hg(OTf)2
O
TfOH
O
O
H
Hg(OTf)
H
Cy
Cy
Cy
6
7
8
O
O
– H
H
OH
HO
O
HO
HO
5
4
H
H
– H
H
HO
OH
HO
Scheme 3 Proposed mechanism for the formation of the aldehydes 4 and 5.
Synlett 2006, No. 1, 57–60 © Thieme Stuttgart · New York

LETTER
Mercuric Triflate·(TMU)2 Catalyzed Cyclization of a Propargylic Ketone
59
0.092 mmol, 0.22 equiv) were added in succession at r.t. to
a stirred suspension of mercuric oxide (red, 10 mg, 0.046
mmol, 0.11 equiv) in dry MeCN (1 mL). After 10 min, a
solution of homopropargylic alcohol 3 (132 mg, 0.41 mmol,
1 equiv) in dry CH2Cl2 (0.5 mL) was added, immediately
followed by H2O (22 mL, 1.22 mmol, 3 equiv) and the
mixture was stirred 24 h at r.t. After addition of a 1:1 mixture
of sat. aq NaHCO3 and brine (2 mL), the mixture was
extracted with EtOAc (4 × 10 mL). The combined organic
phases were washed with a 10% HCl solution (10 mL), dried
over anhyd Na2SO4 and the solvent was evaporated.
Purification by flash chromatography (elution with
cyclohexane–EtOAc, 9:1 to 6:4) afforded OTBS-protected
aldehydes (13 mg, 10%, partly separated) and unprotected
aldehydes 4 and 5 (28 mg, 33%, partly separated), as
colorless oils.
In summary, the Hg(OTf)2·TMU2 complex, which was
developed by Nishizawa’s group as a mild and efficient
catalyst for the hydration of terminal alkynes,11 exhibits a
peculiar reactivity with the terminal homopropargylic al-
cohol 3 and propargylic ketone 9 we examined. Therefore,
efforts are currently underway to ascertain whether these
reactions may have any general applicability, especially in
the vast field of furan synthesis.
Acknowledgment
D.M. thanks the Ministère de l’Education Nationale, de la Re-
cherche et de la Technologie and A.V. thanks the Région des Pays
de la Loire for financial support.
Analytical data for aldehyde 4 (ca. 1:2 Z:E mixture):
Rf = 0.40 (eluent: cyclohexane–EtOAc, 6:4). IR (film):
3407, 2953, 2922, 2858, 1667,1458, 1364, 1190, 1117,
1048, 909, 877, 834 cm–1. 1H NMR (400 MHz, CDCl3):
d = 1.00 and 1.03 [2 s, 6 H, (Me)2-C, 1:3], 1.02 and 1.05 [2
s, 6 H, (Me)2-C, 2:3], 1.26 (br s, 1 H, OH), 1.35 (m, 1 H),
1.78 (m, 2 H), 1.91 (br s, 3 H, Me-C=, 1:3), 2.11 (br s, 3 H,
Me-C=, 2:3), 2.19 (m, 1 H), 2.82 (br s, 2 H, CH2CH=, 2:3),
3.21 (br s, 2 H, CH2CH=, 1:3), 3.98 (m, 1 H, CHOH), 5.26
(br s, 1 H, CH2CH=), 5.88 (d, J = 8.1 Hz, 1 H, CH-CHO,
2:3), 5.99 (d, J = 8.1 Hz, 1 H, CHCHO, 1:3), 9.95 (d, J = 8.1
References and Notes
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Tomasoni, C.; Durand, P.; Roussakis, C. Anticancer Res.
1997, 17, 2115.
(3) The preparation of aldehyde 2 from cyclohexane-1,4-diol
will be reported in a forthcoming full paper.
Hz, 1 H, CHO, 1:3), 9.99 (d, J = 8.1 Hz, 1 H, CHO, 2:3). 13
NMR (100 MHz, CDCl3): d = 17.2 (q), 24.8 (q), 29.5 (q),
C
(4) (a) Nagpal, A.; Unny, R.; Joshi, Y. C. Heterocycl. Commun.
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29.6 (q), 29.8 (q), 31.27 (q), 31.31 (q), 34.4 (s), 34.5 (s), 37.7
(t), 37.9 (t), 40.2 (t), 46.0 (t), 48.6 (t), 65.99 (d), 66.03 (d),
128.2 (s), 128.6 (d), 128.7 (s), 129.8 (d), 135.2 (d), 136.1 (d),
161.5, 161.8 (s), 190.9 (d), 191.4 (d). GCMS (EI, 70 eV,
minor diastereomer): m/z (%) = 208 (5), 190 (38), 175 (79),
157 (64), 137 (84), 119 (56), 105 (80), 91 (85), 77 (55), 43
(50), 41 (90), 39 (100). GCMS (EI, 70 eV, major
diastereomer): m/z (%) = 208 (2), 175 (36), 157 (37), 147
(69), 119 (45), 107(100), 105(68), 91 (46), 79 (33), 55 (49),
41 (64), 39 (68). GCMS (CI+, i-C4H10, both diastereomers):
m/z = 209 [MH+], 191, 173, 163, 147, 109, 95, 69. HRMS
(EI): m/z calcd for C13H20O2: 208.1463; found: 208.1454.
Analytical data for OTBS-protected aldehyde 5: Rf = 0.52
(eluent: PE–Et2O, 9:1). IR (film): 2955, 2928, 2857, 1727,
1643, 1471, 1381, 1360, 1256, 1080, 836, 775, 666 cm–1. 1H
NMR (400 MHz, CDCl3): d = 0.06 [s, 6 H (Me)2-Si], 0.88 (s,
9 H, t-Bu-Si), 0.97 and 0.99 [2 s, 2 × 3H, (Me)2-C], 1.34 and
1.62 [2 m, 2 H, CH2-C(Me)2], 1.79 and 2.04 (2 m, 2 H,
CH2C=CH2), 2.68 (br s, 2 H, CH2CH=C), 3.01 (br s, 2 H,
CH2CH=O), 3.90 (dddd, J = 11.3, 9.1, 5.5, 3.6 Hz, 1 H,
CHOSi), 4.95 (br s, 1 H, CH2=C), 5.06 (br s, 1 H, =CHCH2),
5.15 (br s, 1 H, CH2=C), 9.59 (t, J = 2.5 Hz, 1 H, CHO). 13
NMR (100 MHz, CDCl3): d = –4.6 (q), 18.2 (s), 26.0 (q),
C
29.5 (q), 31.2 (q), 34.2 (s), 38.0 (t), 45.5 (t), 46.4 (t), 49.9 (t),
66.8 (d), 116.6 (t), 129.4 (s), 134.9 (d), 138.7 (s), 199.9 (d).
HRMS (ESI): m/z calcd for C19H34O2NaSi: 345.2226.
Found: 345.2221.
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1961, 84, 270.
(10) Stacy, G. W.; Mikulec, R. A. Org. Synth., Coll. Vol. IV 1963,
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(12) The red form of mercuric oxide was used although
Nishizawa et al. (see ref. 11) described the reaction with the
yellow form.
(14) Larock, R. C.; Harrison, L. W. J. Am. Chem. Soc. 1984, 106,
4218.
(15) Depending on reaction time, small amounts of TBS-
protected alcohols 4 and 5 were also isolated.
(16) Sniady, A.; Wheeler, K. A.; Dembinski, R. Org. Lett. 2005,
7, 1769.
(17) Hashmi, A. S. K.; Bats, J. W.; Choi, J.-H.; Schwarz, L.
Tetrahedron Lett. 1998, 39, 7491.
(18) The result is the same when the crude propargylic ketone 9
or the purified allene 10 is used for the hydration reaction.
(13) Typical Experimental Procedure for the Synthesis of
Aldehydes 4 and 5 and Furan 11.
Tf2O (8 mL, 0.046 mmol, 0.11 equiv) and TMU (11 mL,
Synlett 2006, No. 1, 57–60 © Thieme Stuttgart · New York

60
D. Ménard et al.
LETTER
(19) Analytical data for 11 (ca. 3:2 mixture of cis:trans
diastereoisomers): Rf = 0.34 (eluent: PE–Et2O, 6:4). IR
(film): 3352, 2954, 2926, 2866, 1715, 1647, 1588, 1501,
1461, 1364, 1258, 1149, 1044, 1015, 899, 799, 730, 597
cm–1. 1H NMR (400 MHz, CDCl3): d = 0.80, 0.86, 0.98, 0.99
(4 × s, 3 H, Me), 1.42 [br t, J = 12.5 Hz, 1 H, CH2C(Me)2,
2:5], 1.61 [ddt, J = 12.7, 4.7, 1.3 Hz, 1 H, CH2C(Me)2, 3:5],
1.75 (br s, 1 H, OH), 1.80 [br t, J = 12.7 Hz, 1 H,
GCMS (EI, 70 eV, cis-diastereomer, major): m/z (%) = 206
(18), 188 (66), 173 (36), 145 (26), 122 (35), 121 (100), 107
(22), 93 (34), 91 (34), 77 (28), 41 (36), 39 (39). GCMS (EI,
70 eV, trans-diastereomer, minor): m/z (%) = 206 (57), 188
(38), 173 (67), 145 (33), 122 (33), 121 (100), 107 (23), 93
(37), 91 (39), 77 (33), 41 (42), 39 (47). GCMS (CI+, MeCN):
m/z = 207 [MH+], 189, 161, 139, 121, 95, 65. HRMS (EI):
m/z calcd for C13H18O2: 206.1307. Found: 206.1313.
(20) Zeni, G.; Larock, R. C. Chem. Rev. 2004, 104, 2285; and
references therein.
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Angew. Chem. Int. Ed. 2000, 39, 2285. (b) Yao, T.; Zhang,
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Tetrahedron Lett. 1995, 36, 4405.
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Bolte, M. Tetrahedron Lett. 1998, 39, 8969.
CH2C(Me)2, 3:5], 1.87 [ddd, J = 12.5, 4.4, 2.1 Hz, 1 H,
CH2C(Me)2, 2:5], 2.07 (br t, J = 12.0 Hz, 1 H, CH2C=CH2,
2:5), 2.46 (br t, J = 12.7 Hz, 1 H, CH2C=CH2, 3:5), 2.57 (dd,
J = 12.7, 4.5 Hz, 1 H, CH2C=CH2, 3:5), 2.75 (ddd, J = 12.0,
4.9, 2.2 Hz, 1 H, CH2C=CH2, 2:5), 3.14 [s, 1 H, CHC(Me2),
3:5], 3.23 [s, 1 H, CHC(Me2), 2:5], 3.91 (m, 1 H, CHOH),
4.58 and 4.90 (2 × br s, 2 H, CH2=C, 2:5), 4.83 and 4.86
(2 × br s, 2 H, CH2=C, 3:5), 6.06 (d, J = 3.1 Hz, 1 H,
CH=CO, 3:5), 6.08 (d, J = 3.1 Hz, 1 H, CH=CO, 2:5), 6.26
(dd, J = 3.1, 1.8 Hz, 1 H, CH=CHO, 3:5), 6.32 (dd, J = 3.1,
1.8 Hz, 1 H, CH=CHO, 2:5), 7.29 (d, J = 1.8 Hz, 1 H, =CHO,
3:5), 7.34 (d, J = 1.8 Hz, 1 H, =CHO, 2:5). 13C NMR (100
MHz, CDCl3): d = 22.5, 28.1, 28.7, 30.3 (4 q, Me), 35.5 [s,
C(Me)2, major], 36.1 [s, C(Me)2, minor], 41.8 (t, CH2C=,
major), 44.4 [t, CH2C(Me)2, major], 45.5 (t, CH2C=, minor),
50.7 [t, CH2C(Me)2, minor], 53.6 [2 d, CHC(Me)2], 67.6 (d,
CHO, minor), 67.9 (d, CHO, major), 106.4 (d, CH=CO,
major), 108.2 (d, CH=CO, minor), 109.9 (2 d, CH=CHO),
111.9 (t, CH2=C, minor), 113.0 (t, CH2=C, major), 140.7 (d,
=CHO, major), 140.9 (d, =CHO, minor), 144.4 and 145.0 (2
s, C=CH2), 154.0 (s, =CO, minor), 156.3 (s, =CO, major).
Synlett 2006, No. 1, 57–60 © Thieme Stuttgart · New York