Silver(I) and gold(I)-promoted synthesis of alkylidene lactones and 2H-chromenes from salicylic and anthranilic acid derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.06.060

Ag(I) and Au(I) efficiently catalyze the cycloisomerization of terminal alkynoic acids into methylene seven-membered ring lactones. Depending on the metal, divergent reaction pathways were found for non terminal alkynoic acids. While Ag(I) led to lactones, Au(I) led to 2H-chromenes coming from the hydroarylation of the alkyne.

Full text of DOI:10.1016/j.tetlet.2014.06.060

Tetrahedron Letters 55 (2014) 4484–4488
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Silver(I) and gold(I)-promoted synthesis of alkylidene lactones and
2H-chromenes from salicylic and anthranilic acid derivatives
⇑
Roberto Nolla-Saltiel, Elvis Robles-Marín, Susana Porcel
Instituto de Química, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, 04510 México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Ag(I) and Au(I) efficiently catalyze the cycloisomerization of terminal alkynoic acids into methylene
seven-membered ring lactones. Depending on the metal, divergent reaction pathways were found for
non terminal alkynoic acids. While Ag(I) led to lactones, Au(I) led to 2H-chromenes coming from the
hydroarylation of the alkyne.
Received 19 May 2014
Revised 10 June 2014
Accepted 13 June 2014
Available online 20 June 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Alkylidene lactones
2H-Chromenes
Silver(I)
Gold(I)
Catalysis
Introduction
The cycloisomerization of alkynoic acids enables direct access
to alkylidene lactones, which have attracted the interest of
chemists due to their synthetic potential and presence in natural
products with biological activity.1 Several transition metals (Rh,2
Ru,3 Pd,4 Cu,5 Ag,6 and Au7) promote this transformation providing
five- and six-membered ring alkylidene lactones, while examples
of seven-membered ring are scarce.7a,c,e Although much less fre-
quent in nature, some seven-membered ring lactones display
interesting biological properties.8 For this reason we decided to
investigate, the feasibility of synthesizing alkylidene seven-mem-
bered ring lactones by cycloisomerization of alkynoic acids. Previ-
ous work from the group of Kundu showed that in the presence of a
catalytic amount of CuI (5 mol %) and Et3N (2 equiv) arylidene lac-
tones are obtained in moderate to good yields.9 We became inter-
ested in studying the activity of Ag(I) and Au(I) in those reactions
due to their well known carbophilicity and their high performance
in the synthesis of five-membered alkylidene lactones.10
Scheme 1. Synthesis of the alkynoic acid precursors.
of well known organic reactions: esterification, propargylation
and saponification. Non-terminal alkynoic acids were prepared
by propargylation with the corresponding substituted propargyl
bromides or by Sonogashira coupling of a terminal alkynoic acid
with a halo-arene. In the saponification step, certain substrates
when treated with a saturated solution of KOH in MeOH, showed
the tendency to give the allene acid isomer or to lose the propargyl
moiety. For those cases the saponification was performed by
controlling the amount of base with NaH/H2O or by using the
system LiOH/H2O2/Na2SO3.11
In order to establish the optimal reaction conditions, we first
examined the cycloisomerization reaction of 2-(2-propynyl-
oxy)benzoic acid (1a) with Ag(I) and Au(I). Standard and commer-
cially available silver salts like Ag2CO3, AgOAc, or AgNO3 failed to
perform the cycloisomerization reaction in a catalytic amount.
Among these salts better results were obtained using AgNO3/
PPh3 stoichiometrically (Table 1, entry 3). Under these conditions
seven-membered ring methylene lactone 1b was formed
Results and discussion
Salicylic and anthranilic acid derivatives were used as starting
materials to build different alkynoic acid precursors (Scheme 1).
The synthetic pathway of terminal alkynoic acids involved a set
⇑
Corresponding author. Tel.: +52 5556224447; fax: +52 5556162217.
E-mail address: sporcel@unam.mx (S. Porcel).
http://dx.doi.org/10.1016/j.tetlet.2014.06.060
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

R. Nolla-Saltiel et al. / Tetrahedron Letters 55 (2014) 4484–4488
4485
orientation).12,13 Employing AgNO3/PPh3 in (5/5 mol %) together
with 2 equiv of Et3N, the product 1c obtained from a SN2 reaction
with the solvent, was formed in 30% (entry 4). We next turned our
attention to other silver salts with non-coordinating anions. To our
delight, the combination AgSbF6/PPh3 (5/5 mol %) gave 1b in 70%
after heating at 50 °C for 48 h (entry 6). Increasing the temperature
to 70 °C, the same catalytic system furnished 1b in 96% after 16 h.
The presence of PPh3 was essential for the reaction to proceed,
since AgSbF6 alone did not catalyze the formation of 1b (entry
5).14 The addition of Et3N (2 equiv) to the media did not accelerate
the reaction and together with 1b, product 1c was obtained in 27%
yield (entry 8). Other silver salts with non-coordinating anions
such as AgOTf or AgBF4 failed to give the reaction or gave it in
lower yields (entries 12 and 13). Varying the solvent from DCE to
toluene, acetonitrile or THF did not increase the yields (entries
9–11). From all the conditions screened for the cycloisomerization
of 1a with Ag(I), those of entry 7 were the best. For the cycloiso-
merization of 1a with Au(I) we chose the complex AuCl(PPh3) as
the catalyst. Heating 1a at 50 °C for 24 h in the presence of
AuCl(PPh3) (5 mol%) did not afford the desired lactone (entry 16).
Since in many of its application this complex is activated in situ
by chloride abstraction using a silver salt,12f,15 we added AgSbF6
(5 mol %). This adjunction increased the activity of the catalyst
and 1b was obtained in 93% after 15 min (entry 15). The remark-
able difference in reactivity between AgSbF6/PPh3 and AuCl(PPh3)/
AgSbF6 (entry 6 vs 15) points out that the activity of the system
AuCl(PPh3)/AgSbF6 must rely on a Au(I) specie and not on Ag(I).16
Using the system AuCl(PPh3)/AgSbF6 the reaction could also be
performed at rt but the yield was slightly lower and the reaction
time was longer (entry 14).
Table 1
Optimization of the cycloisomerization of alkynoic acid 1a with Ag(I) and Au(I)
catalysts
[M(I)]a
Ad.
Solvent
T (°C)
Time (h)
Yield (%)
1b
2b
3b
4b
Ag2CO3
AgOAc
AgNO3
AgNO3
Ph3P
Ph3P
Ph3P
Ph3P
Et3N
—
Ph3P
Ph3P
Ph3P
Et3N
Ph3P
Ph3P
Ph3P
Ph3P
Ph3P
—
DCE
DCE
DCE
DCE
70
70
70
70
16
16
16
21
—
42
67
59
30d
0
70
96
43
27d
50
<1
0
68
<1
91
93
0
5
6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
DCE
DCE
DCE
DCE
50
50
70
70
18
48
16
16
7
8c
9
AgSbF6
AgSbF6
AgSbF6
AgOTf
Toluene
MeCN
THF
DCE
DCE
DCE
DCE
DCE
70
70
65
70
70
25
50
50
16
16
16
16
16
3
10
11
12
13
14
15
16
AgBF4
AuCl(PPh3)/AgSbF6
AuCl(PPh3)/AgSbF6
AuCl(PPh3)
—
—
0.25
24
a
b
c
AgSbF6: 0.05 equiv, Ph3P: 0.05 equiv, AuCl(PPh3): 0.05 equiv.
[Ag(I)]: 1 equiv, Ph3P: 1 equiv.
AgSbF6: 0.05 equiv, Ph3P: 0.05 equiv, Et3N: 2 equiv.
d
With the optimized reaction conditions for both the cycloiso-
merization with Ag(I) and Au(I) we next examined the reaction
over a variety of alkynoic acids. Results of the cycloisomerization
reaction with Ag(I) are shown in Table 2.
Alkynoic acid derivatives 2a, (R1 = 3-Me, X = O), 3a (R1 = 5-Cl,
X = O), and 6a (R1 = H, X = NTs) behave as 1a and led to the
corresponding lactones in moderate to excellent yields. The
regioselectively in 67% yield. The preference for the 7-exo-dig
cycloisomerization over 8-endo-dig is due to an unsymmetrical
coordination of Ag(I) to the alkyne leading to an enhanced electro-
philicity at the more substituted carbon atom (Markovnikov
Table 2
Ag(I) cycloisomerization of alkynoic acids
Substrate
Conditionsa
Product
1
2a, 3a, 6a: AgSbF6/Ph3P (5/5 mol %) 4a, 5a: AgSbF6/Ph3P (5/5 mol %) Et3N (2 equiv)
2
3
AgSbF6/Ph3P (5/5 mol %) Et3N (2 equiv)
AgSbF6/Ph3P (5/5 mol %) Et3N (2 equiv)
4
AgNO3/Ph3P (1/1 equiv)
(continued on next page)

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R. Nolla-Saltiel et al. / Tetrahedron Letters 55 (2014) 4484–4488
Table 2 (continued)
Substrate
Conditionsa
Product
5
6
AgNO3/Ph3P (1/1 equiv)
AgSbF6/Ph3P (5/5 mol %) Et3N (2 equiv) or AgNO3/Ph3P (1/1 equiv)
—
a
General conditions: [substrate] = 0.1 mmol/ml, DCE, 70 °C, 16 h.
Table 3
Au(I) cycloisomerization of alkynoic acids
Substrate
Conditionsa
Product
1
AuCl(PPh3)/AgSbF6 (5/5 mol %) 50 °C, 15 min (5a, 2 h)
2
3
AuCl(PPh3)/AgSbF6 (5/5 mol %) 50 °C, 15 min
AuCl(PPh3)/AgSbF6 (5/5 mol %) 50 °C, 15 min
4
5
AuCl(PPh3)/AgSbF6 (5/5 mol %) 50 °C, 1 h
25 °C, 1 h
25 °C, 1 h 30 min
6
7
25 °C, 1 h
50 °C, 1 h
8
a
General conditions: [substrate] = 0.1 mmol/ml, DCE.

R. Nolla-Saltiel et al. / Tetrahedron Letters 55 (2014) 4484–4488
4487
presence of
p
donor substituents in the para position with respect
structure of 10c was confirmed by the X-ray diffraction analysis).11
The introduction of electron-withdrawing groups at the terminal
aryl ring of the alkyne favored again the nucleophilic addition over
the alkyne. Thus, in the presence of a CF3 group (12a) lactone 12b
was formed in 35% yield while 2H-chromene 12c was formed in
to the carboxylic group (R1 = 4-OMe, 4-Cl) decreased dramatically
the reactivity of 4a–5a, and the desired lactones were only formed
after the addition of 2 equiv of Et3N to the reaction media. In the
presence of Et3N lactones 4b and 6b were obtained in 99% and
81% yields, respectively (entry 1). Substrate 7a with a naphthalene
core also needed the addition of 2 equiv of Et3N to promote the
reaction (entry 2). Non-terminal alkynoic acids in general were
reluctant to react except for the case of 8a (entries 3–6). The addi-
tion of Et3N in the cycloisomerization of 9a–13a led to an insepa-
rable mixture of the desired lactones, and the derivatives obtained
from the substitution reaction with the solvent. With the aim of
achieving full conversion to the lactones, in these cases the reac-
tion was promoted stoichiometrically with the system AgNO3/
PPh3 (1/1 equiv) (entries 4 and 5). Substrate 14a with a chlorine
substituent in the meta position with respect to the carboxylic
group and a methyl at the terminal position of the alkyne, failed
to undergo the cycloisomerization under all the conditions exam-
ined and only decomposition products were observed. Remarkably,
the nature of the substituents at the end of the alkyne modified the
regioselectivity of the reaction. When R2 was a methyl group, a
mixture of seven- and eight-membered ring lactones was formed,
the seven-membered ring lactone being the major isomer (entries
3 and 4). On the other hand, when R2 was an aryl group only the
seven-membered ring lactones were formed (entry 5). This result
can be explained by a more symmetrical coordination of Ag(I) to
the substituted alkyne which reduces the difference in electrophi-
licity of the alkyne carbons.12 Under such coordination mode,
thermodynamic factors govern the orientation of the nucleophilic
addition. Additionally, the double bond of the so-formed seven-
membered ring lactones was always of Z geometry as confirmed
by the NOESY experiment.11 This is in accord with an anti attack
50%.
A more electron-withdrawing group (NO2, 13a) led
exclusively to lactone 13b in 84% yield.
Conclusion
In summary, after studying the cycloisomerizations of alkynoic
acids with Ag(I) and Au(I), it was shown that Au(I) was a superior
catalyst than Ag(I). Regarding the substrates, whereas terminal
alkynoic acids were regioselectively transformed into methylene
seven-membered ring lactones by both metals, the cycloisomeriza-
tion of non-terminal alkynoic was not always regioselective. In
addition, certain non-terminal alkynoic acids gave a divergent
reaction pathway under Au(I) catalysis that led to 2H-chromenes
obtained from the hydroarylation of the alkyne. The rate of
hydroarylation decreased with the introduction of electron-with-
drawing groups. This difference in reactivity must be attributed
to the special coordination mode of Au(I) to alkynes.12c,d,e,f
Acknowledgments
This work was supported by CONACyT (project No. 153523),
DGAPA (project No.IB202212), and Instituto de Química (UNAM).
The authors would like to thank Ma Angeles Peña-González,
Elisabeth
Huerta-Salazar,
Beatriz
Quiroz-García,
Isabel
Chávez-Uribe, Simón Hernández-Ortega, Ma del Rocío Patiño-
Maya, Luis Velasco-Ibarra, and Francisco Javier Pérez-Flores for
technical support (NMR, X-ray diffraction analysis, IR analysis,
and mass spectra).
of the nucleophile to the
g2-alkyne Ag(I) complex.
Concerning cycloisomerizations catalyzed by Au(I), similarly to
what happened with alkynoic acid 1a, the system AuCl(PPh3)/
AgSbF6 was more active than AgSbF6/PPh3 for the cycloisomeriza-
tion of terminal alkynoic acids (Table 3). Thus substrates 2a–7a
were converted into the corresponding seven-membered ring
methylene lactones (2b–7b) with very good yields (85–99%) in
15 min except for the case of alkynoic acid 6a (with a NTs group),
Supplementary data
Supplementary data (experimental details, characterization
data, crystallographic information, and copies of 1H and 13C NMR
spectra for all new compounds) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.06.060.
which took 2 h to complete the reaction. The presence of p-donor
substituents at the para position of the carboxylic group (4a and
5a), did not significantly affect neither the yields nor the reaction
times (entry 1). With respect to non-terminal alkynoic acids, cer-
tain substrates instead of reacting by attack of the carboxylic group
to the activated alkyne, reacted by a hydroarylation reaction path-
way. Alkynoic acid 9a bearing a NTs and a methyl group at the end
of the alkyne behave as expected and analogously to the catalysis
with Ag(I) gave a mixture of seven and eight-membered ring lac-
tones in 90%. Nonetheless changing the NTs group by an oxygen
atom resulted in the exclusive formation of 2H-chromene 8d in
82% after only 15 min of reaction. The structure of 8d was con-
firmed by the X-ray diffraction analysis.11 Au(I) catalyzed hydro-
arylation of alkynes to give 2H-chromenes has been described
even in the presence of deactivating groups,17 but to the best of
our knowledge the competition between nucleophilic addition
and hydroarylation has not been studied before. Substitution of
the alkyne with an aryl group or the introduction of a chlorine
atom into the salicylic phenyl ring (entries 5–8) decreased the
reactivity of alkynoic acids, not being possible to achieve full reac-
tion conversion. For the cycloisomerization of these substrates,
complex AuCl(PPh3) was changed by a more electrophilic one with
a phosphite ligand.15a Using this complex, cycloisomerizations
were completed in 1–1.5 h, although in some cases the yields were
low.18 Alkynoic acids 10a, 11a, and 14a gave the corresponding
2H-chromenes in 80%, 55%, and 50% yields, respectively (the
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