catalyst, internal acetylenes did not give 2-pyranones (Table 2,
entries 7–11).
R. C. Larock, M. J. Doty and X. Han, J. Org. Chem., 1999, 64,
8770.
4 This reaction proceeded using several transition metal complexes.
However, the yields of d-keto esters were low: Re2(CO)10
(1.5 mol%; 3a: 5%, 4a: 15%, 5a: o1%), ReBr(CO)5 (3.0 mol%;
3a: 15%, 4a: 72%, 5a: 3%), Mn2(CO)10 (1.5 mol%; 3a: 1%, 4a:
14%, 5a: o1%), MnBr(CO)5 (3.0 mol%; 3a: o1%, 4a: 36%, 5a:
0%) and Ru3(CO)12 (1.0 mol%; 3a: o1%, 4a: 5%, 5a: o1%).
ð4Þ
5 This reaction did not proceed using RhCl(PPh3)3, Ir4(CO)12
,
Pd(OAc)2/PPh3 (1 : 2), Ni(cod)2/PPh3 (1 : 2), AuCl3 or PtCl2.
6 3a–5a could not be separated by column chromatography on silica
gel.
7 The yields of d-keto esters 3–5 decreased without MS. It is still
unclear why MS accelerate the reaction.
By the treatment of a b-keto ester with an acetylene moiety,
11, with the rhenium catalyst [ReBr(CO)3(thf)]2 and MS,
bicyclic 2-pyranone 12 was produced in 95% yield (eqn (5)).
8 3,3-Dimethyl-1-butyne, ethynyltrimethylsilane and ethyl propio-
late did not give d-keto esters under the same reaction conditions.
9 In this reaction, methylenecyclohexane
9
was formed:
Y. Kuninobu, A. Kawata and K. Takai, Org. Lett., 2005, 7,
4823. See also: M. Nakamura, K. Endo and E. Nakamura,
J. Am. Chem. Soc., 2003, 125, 13002; J. J. Kennedy-Smith,
S. T. Staben and F. D. Toste, J. Am. Chem. Soc., 2004, 126,
4526; S. T. Staben, J. J. Kennedy-Smith and F. D. Toste, Angew.
Chem., Int. Ed., 2004, 43, 5350; Q. Gao, B.-F. Zheng, J.-H. Li and
D. Yang, Org. Lett., 2005, 7, 2185.
ð5Þ
2-Pyranone 6 could be formed by the cyclization of d-keto
ester 3 via the elimination of ethanol (Scheme 2). Another
possible mechanism is an equilibrium between d-keto esters 3,
4 and 5, and intramolecular cyclization from 4.
10
In summary, we have succeeded in the rhenium-catalyzed
regioselective insertion of terminal and internal acetylenes into
a carbon–carbon single bond of b-keto esters. As a result,
d-keto esters were obtained. At higher temperatures, the d-keto
esters were cyclized to 2-pyranone derivatives via the elimina-
tion of ethanol. In addition, by using a manganese catalyst,
2-pyranone derivatives could be obtained under milder condi-
tions. In both the rhenium- and manganese-catalyzed transfor-
mations, the addition of a catalytic amount of TBAF was
effective at promoting the formation of a 2-pyranone frame-
work. We hope that these highly atom-economical trans-
formations will become a powerful tool in synthetic organic
chemistry.
The substrates/catalyst combination in this reaction is almost the
same as that for the insertion of terminal acetylenes into a C–H
bond of the active methylene moieties of b-keto esters, leading to
10 (see ref. 9). The selectivities of 3a–5a over 10 increased when the
reactions were conducted in toluene, at low concentrations, at
higher temperature and by the addition of THF or isocyanide.
11 Only a cyclopentane derivative was formed in 93% yield using
methyl 2-acetylhept-6-ynoate.
12 M. Suginome and Y. Ito, Top. Organomet. Chem., 1999, 3, 131.
13 Y. Kuninobu, A. Kawata and K. Takai, J. Am. Chem. Soc., 2006,
128, 11368.
14 As the reaction time increased, the yield of d-keto ester 3 decreased
and the yield of d-keto ester 4 increased. This result shows that
isomerization of 3 proceeded and that 4 was formed under the
reaction conditions.
15 When the reaction was conducted at 180 1C for 24 h, the
polymerization of 2a took place.
16 The structure of 6f was determined by comparison with reported
1H and 13C NMR data. See: L. S. Liebeskind and J. Wang,
Tetrahedron, 1993, 49, 5461.
17 The addition of MS is important to promote the reaction effi-
ciently. In the absence of MS, 6a was obtained in 78% yield.
18 The reaction required the addition of the rhenium or manganese
catalyst. When a mixture of 1a, 2a, and a catalytic amount of the
rhenium complex and TBAF in toluene, was heated at 80 1C for
8 h, dimerization of phenylacetylene, leading to (E)-1,4-diphenyl-
but-1-en-3-yne, proceeded in 92% yield. On the other hand, the
manganese complex MnBr(CO)5 did not promote any reactions,
and 1a and 2a were recovered.
Scheme 2 Proposed mechanism for the formation of 2-pyranones.
Notes and references
1 A. Ichihara, K. Murakami and S. Sakamura, Tetrahedron, 1987,
43, 5245; X. Shi, W. S. Leal, Z. Liu, E. Schrader and J. Meinwald,
Tetrahedron Lett., 1995, 36, 71; Y. Kamano, T. Nogawa,
A. Yamashita, M. Hayashi, M. Inoue, P. Drasar and
G. R. Pettite, J. Nat. Prod., 2002, 65, 1001.
19 The formation of 2-pyranones
6 were also promoted by
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Therefore, we think
that the role of TBAF is asa base. Other fluoride sources, such
as KF, CsF and AgF, were not as effective as TBAF.
2 N. T. Tam and C.-G. Cho, Org. Lett., 2008, 10, 601.
3 J. Fried and R. C. Elderfield, J. Org. Chem., 1941, 6, 566;
J. D. Bu’Lock and H. G. Smith, J. Chem. Soc., 1960, 502;
ꢀc
This journal is The Royal Society of Chemistry 2008
6362 | Chem. Commun., 2008, 6360–6362