Ru-MACHO-Catalyzed Highly Chemoselective Hydrogenation of α-Keto Esters to 1,2-Diols or α-Hydroxy Esters

Article abstract of DOI:10.1055/s-0035-1561971

A ruthenium pincer catalyst has been shown to be highly effective for the hydrogenation of a wide range of α-keto esters, affording either diols or hydroxy esters depending on the choice of reaction conditions. Strong base, high temperature, and pressure favor the formation of diols whilst the opposite is true for the hydroxy esters.

Full text of DOI:10.1055/s-0035-1561971

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
S. Gao et al.
Letter
Synlett
Ru-MACHO-Catalyzed Highly Chemoselective Hydrogenation of
α-Keto Esters to 1,2-Diols or α-Hydroxy Esters
Shaochan Gaoa
OH
Weijun Tang*a
Minghui Zhanga
Chao Wanga
R
81–98% yield
89–99% yield
H
H
Ru
Cl
A
OH
PPh2
CO
N
O
1 mol% Ru-MACHO
H2, 10 mol% base
OMe
Jianliang Xiao*a,b
R
OH
P
MeOH
O
Ph2
OMe
B
a Key Laboratory of Applied Surface and Colloid Chemistry of
Ministry of Education, and School of Chemistry & Chemical
Engineering, Shaanxi Normal University, Xi’an, 710119,
P. R. of China
R
19 examples
R = Me, i-Pr, Cy, Ph, Ar, Naph
O
Ru-MACHO catalyst
conditions A: 50 bar H2, 80 °C, t-BuONa
tangwj@snnu.edu.cn
conditions B: 10 bar H2, 25 °C, NaHCO3
j.xiao@liv.ac.uk
b Department of Chemistry, University of Liverpool, Liverpool,
L69 7ZD, UK
Received: 30.01.2016
Accepted after revision: 14.03.2016
Published online: 05.04.2016
Given the importance of 1,2-diols as building blocks in
synthetic applications,11 the pursuit of new methods for
their synthesis remains an interesting topic. In the litera-
ture, 1,2-diols are synthesized generally by the following
methods: osmium-catalyzed dihydroxylation of olefins,12
diboration of olefins,13 hydrogenation of α-hydroxy esters8f
and/or α-hydroxy ketones14 and/or 1,2-diketones,15 ring-
opening reactions of epoxides,16 enzymatic reactions,17 and
the reduction of α-keto esters with NaBH4.18 Among these
methods, hydrogenation with H2 is perhaps the most
straightforward, efficient, and atom-economic method.
Herein, we report our results on the first hydrogenation of
α-keto esters to produce 1,2-diols directly with the Ru-
MACHO catalyst. It is noted that by simply changing the
base, this catalytic system affords α-hydroxy esters without
the formation of diol products.
Initially, we investigated the hydrogenation of α-keto
esters with the Ru-MACHO catalyst under the reaction con-
ditions reported by Kuriyama.8f Unfortunately, an unsatis-
factory chemoselectivity was observed, with the diol 2a
and hydroxy ester 3a formed in a ratio of 20:80 (Table 1, en-
try 1). The ratio of 2a/3a increased slightly in favor of 2a in
a prolonged reaction time (Table 1, entry 2). Considering
the remarkable impact of base on the catalytic activity in
the hydrogenation of ketones and esters,19 a series of inor-
ganic bases was tested (Table 1, entries 3–7).20 To our de-
light, excellent conversion and selectivity for 2a were
achieved with the strong base sodium tert-butoxide (Table
1, entry 3). A lower H2 pressure or temperature did not have
much effect on the conversion, but decreased the ratio of
2a/3a (Table 1, entries 8–12). Further experiments demon-
strated that decreasing the amount of base improves the ra-
tio of 2a/3a and up to 99:1 was observed (Table 1, entries
13 and 14). However, under the reaction conditions em-
DOI: 10.1055/s-0035-1561971; Art ID: st-2016-w0067-l
Abstract A ruthenium pincer catalyst has been shown to be highly ef-
fective for the hydrogenation of a wide range of α-keto esters, affording
either diols or hydroxy esters depending on the choice of reaction con-
ditions. Strong base, high temperature, and pressure favor the forma-
tion of diols whilst the opposite is true for the hydroxy esters.
Key words hydrogenation, α-ketoesters, α-hydroxy esters, diols, ru-
thenium catalyst
The transformation of ketones and esters into the corre-
sponding alcohols is one of the most important chemical
reactions in organic synthesis.1 Traditionally, production of
these chemicals is based on processes using stoichiometric
amounts of reducing reagents, such as LiAlH4, NaBH4, and
BH3, which lead to risk in operation and tedious workup.2
From the viewpoint of both atom economy and practical
applications, the direct catalytic hydrogenation of ketones
and esters with hydrogen gas affords a clean and efficient
route for the preparation of alcohols.3 Indeed, over the past
three decades, the hydrogenation of ketones for the prepa-
ration of alcohols has achieved a great deal of success.4
However, for esters, the hydrogenation is still a challenging
task due to their poor reactivity and selectivity.5 In recent
years, excellent catalytic systems have been developed,
which are mainly based on pincer complexes containing
iron, cobalt, ruthenium, iridium, and osmium, allowing for
the hydrogenation of esters under much milder condi-
tions.6–10 One of the successful examples is Takasago’s Ru-
MACHO catalyst, which has been applied to the industrial
hydrogenation of (R)-lactate in a large scale to produce (R)-
1,2-propanediol.8f
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
S. Gao et al.
Letter
Synlett
ployed but with a lower catalyst loading of 0.2 mol% and 0.1
mol%, the ratio of 2a/3a decreased to 90:10 (Table 1, entry
15) and 62:38 (Table 1, entry 16), respectively.
0.1% catalyst loading and 25 °C (Table 1, entry 20). These re-
sults are consistent with ketone reduction being much easi-
er than that of esters.
Based on the optimal reaction conditions, the scope of
substrates was explored subsequently and the results are
listed in Table 2. In general, most of α-keto esters were fully
hydrogenated with a high chemoselectivity, affording ei-
ther diols under the conditions A or α-hydroxy esters under
the conditions B with good to excellent isolated yields.
Thus, for substrates 1a–k and hydrogenated diols 2a–k un-
der the reaction conditions A, it was found that the reaction
was only slightly sensitive to the substituent position (Table
2, compare entry 3 with entries 4 and 5 and entry 6 with
entry 7). For the para-substituted esters, the reduction
worked equally well for both electron-donating and elec-
tron-withdrawing groups (Table 2, entries 2, 3, 6, 8, 10, 11).
For disubstituted aromatic substrates 1l–o, the hydrogena-
tion reaction proceeded smoothly, affording excellent iso-
lated yields for both the 2,4-disubstituted substrates (Table
2, entries 12 and 13) and 3,5-disubstituted ones (Table 2,
entries 14 and 15). Naphthyl-substituted substrate 1p and
aliphatic substrates 1q–s were both hydrogenated to diol
products with high isolated yields (Table 2, entries 16–19).
Table 1 Optimization of Reaction Conditions for Hydrogenation of α-
Keto Ester 1aa
OH
O
OH
Ru-MACHO
base, H2
OMe
OMe
OH
O
O
1a
2a
3a
Entry
Base
Temp (°C)
Conversion (%)b 2a/3ab
1
2c
MeONa
MeONa
t-BuONa
NaOH
80
80
80
80
80
80
80
80
80
60
40
25
80
80
80
80
25
25
25
25
100
100
100
100
93
20:80
23:77
97:3
3
4
6:94
5
KOH
0:93
6
t-BuOK
K2CO3
100
100
100
100
100
100
100
100
100
100
100
100
91
92:8
7
30:70
96:4
8d
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
NaOH
9e
92:7
10
11
12
13f
14g
15f,h
16f,i
17f,j
18f,j
19f,j
20f,i,j
95:5
Table 2 Hydrogenation of α-Keto Esters to Diols and α-Hydroxy Estersa
89:11
77:23
99:1
O
OH
OH
1 mol% Ru-MACHO
10 mol% base, H2
OMe
OMe
R
or
R
R
O
O
OH
95:5
1a–s
2a–s
3a–s
90:10
62:38
5:95
Entry
Substrate
Product 2 (%)b,d Product 3(%)c,d
O
OMe
KOH
0:91
1
2
3
4
5
91 (99:1)e
90 (99:1)e
96 (100:0)e
81 (86:14)e
89 (90:10)e
98 (0:100)e
95 (0:100)e
99 (0:100)e
98 (2:98)e
91 (3:97)e
O
O
NaHCO3
NaHCO3
100
100
0:100
0:100
1a
a Reaction conditions: 1a (0.5 mmol), Ru-MACHO (5 umol), base (0.1
mmol), toluene (0.5 mL), 50 bar H2, 12 h.
b The conversion and ratio of 2a/3a were determined by 1H NMR spectros-
copy of the crude reaction mixture.
c The reaction time was 24 h.
OMe
O
1b
d The pressure was 30 bar.
e The pressure was 10 bar.
O
f With 0.05 mmol base.
OMe
OMe
g With 0.15 mmol base.
h S/C was 500.
O
O
MeO
MeO
i S/C was 1000.
1c
j The pressure was 10 bar.
O
The chemoselectivity of this catalytic system is strongly
affected by the strength of the base. Thus, using a weak
base benefited the formation of α-hydroxy esters under the
hydrogenation conditions (Table 1, entries 4 and 5).21 Fur-
ther studies demonstrated that a weaker base was better,
affording excellent chemoselectivity under much milder
conditions (Table 1, entries 17–19). In particular, high activ-
ity and chemoselectivity were achieved with NaHCO3 at
1d
OMe
O
OMe
O
1e
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
S. Gao et al.
Letter
Synlett
Table 2 (continued)
Entry
17
Substrate
Product 2 (%)b,d Product 3(%)c,d
Entry
Substrate
Product 2 (%)b,d Product 3(%)c,d
O
O
OMe
OMe
93 (96:4)e
94 (99:1)e
91 (95:5)e
98 (0:100)e
92 (8:92)f
6
94 (99:1)e
98 (0:100)e
89 (8:92)e
98 (0:100)e
90 (5:95)e
99 (0:100)e
95 (0:100)e
97 (0:100)e
99 (0:100)e
O
1q
O
Cl
Cl
1f
O
O
OMe
OMe
18
19
7
8
90 (93:7)e
92 (100:0)e
91 (98:2)e
97 (100:0)e
97 (100:0)e
95 (99:1)e
98 (99:1)e
O
O
1r
1g
O
O
OMe
94 (0:100)e
OMe
O
O
1s
F
1h
a The reaction was carried out with 0.5 mmol substrate (1a–1s), H2, 0.05
mmol base, MeOH (0.5ml), reaction time 12 h.
b Conditions A: 50 bar H2, t-BuONa (0.05 mmol), 80 °C.
c Conditions B: 10 bar H2, NaHCO3 (0.05 mmol), 25 °C.
d Isolated yields.
F
O
OMe
9
O
e The ratio of 2/3was determined by 1H NMR spectroscopy of the crude
reaction mixture.
1i
f The ratio of 1/3was determined by 1H NMR.
O
OMe
10
11
12
13
Under the conditions B where a weaker base, lower H2
pressure, and lower temperature were employed, the sub-
strates 1a–s were all hydrogenated to the α-hydroxy esters
3a–s. Excellent yields were obtained for almost all the sub-
strates examined.
In summary, we have developed an efficient method for
the preparation of diol and α-hydroxy esters via the chemo-
selective hydrogenation of α-keto esters catalyzed by a pin-
cer Ru-MACHO catalyst. It was found that the chemoselec-
tivity can be readily altered by simply varying the reaction
conditions, including particularly the strength of the base.
Exploration of related chiral pincer catalysts for asymmetric
hydrogenation is ongoing in our group.
O
Br
1j
O
OMe
O
F3C
1k
O
OMe
O
1l
Cl
O
OMe
O
Cl
1m
Acknowledgment
O
We are grateful for the financial support of the National Science Foun-
dation of China (21302116) and the Fundamental Research Funds for
the Central Universities (GK201503035).
OMe
14
94 (100:0)e
95 (0:100)e
O
1n
O
Supporting Information
F3C
OMe
95 (100:0)e
96 (100:0)e
98 (0:100)e
99 (0:100)e
Supporting information for this article is available online at
15
16
O
O
http://dx.doi.org/10.1055/s-0035-1561971.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
1o
CF3
O
References and Notes
OMe
(1) Carruthers, W.; Coldham, I. In Modern Methods of Organic Syn-
thesis; Cambridge University Press: Cambridge, 2004, 4th ed.
405.
1p
(2) (a) Greeves, N. In Comprehensive Organic Synthesis: Selectivity,
Strategy, and Efficiency in Modern Organic Chemistry; Trost, B.
M.; Fleming, I., Eds.; Pergamon Press: Oxford, NY, 1991, 1st ed.,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
S. Gao et al.
Letter
Synlett
Vol. 8 1. (b) Seyden-Penne, J. Reductions by the Alumino- and
Borohydrides in Organic Synthesis; Wiley-VCH: New York, 1997,
2nd ed..
E.; Kliman, L. T.; Morken, J. P. J. Am. Chem. Soc. 2009, 131, 9134.
(e) Kliman, L. T.; Mlynarski, S. N.; Ferris, G. E.; Morken, J. P.
Angew. Chem. Int. Ed. 2012, 51, 521. (f) Ferris, G. E.; Hong, K.;
Roundtree, I. A.; Morken, J. P. J. Am. Chem. Soc. 2013, 135, 2501.
(g) Toribatake, K.; Nishiyama, H. Angew. Chem. Int. Ed. 2013, 52,
11011.
(3) Bullock, R. M. In The Handbook of Homogeneous Hydrogenation;
de Vries, J. G.; Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, 2008.
(4) (a) Bullock, R. M. Chem. Eur. J. 2004, 10, 2366. (b) Noyori, R.;
Ohkuma, T. Angew. Chem. Int. Ed. 2001, 40, 40.
(14) For typical examples, see: (a) Hodgkinson, R.; Jurčík, V.;
Zanotti-Gerosa, A.; Nedden, H. G.; Blackaby, A.; Clarkson, G. J.;
Wills, M. Organometallics 2014, 33, 5517. (b) Touge, T.;
Hakamata, T.; Nara, H.; Kobayashi, T.; Sayo, N.; Saito, T.; Kayaki,
Y.; Ikariya, T. J. Am. Chem. Soc. 2011, 133, 14960. (c) Kadyrov, R.;
Koenigs, R. M.; Brinkmann, C.; Voigtlaender, D.; Rueping, M.
Angew. Chem. Int. Ed. 2009, 48, 7556. (d) Ohkuma, T.; Utsumi,
N.; Watanabe, M.; Tsutsumi, K.; Arai, N.; Murata, N. Org. Lett.
2007, 9, 2565. (e) Saito, T.; Yokozawa, T.; Ishizaki, T.; Moroi, T.;
Sayo, N.; Miura, T.; Kumobayashi, H. Adv. Synth. Catal. 2001, 343,
264.
(15) (a) Monnereau, L.; Cartigny, D.; Scalone, M.; Ayad, T.;
Ratovelomanana-Vidal, V. Chem. Eur. J. 2015, 21, 11799.
(b) Zhang, H.; Feng, D. D.; Sheng, H. B.; Ma, X. B.; Wan, J. W.;
Tang, Q. RSC Adv. 2014, 4, 6417. (c) Xu, H.; Meng, Q. H.; Zhang, Z.
G. Chin. J. Chem. 2008, 26, 1656. (d) Clarke, M. L.; France, M. B.;
Knight, F. R.; Frew, J. J. R.; Roff, G. J. Synlett 2007, 1739.
(16) (a) Yudin, K. A. In Aziridines and Epoxides in Organic Synthesis;
Wiley-VCH: Weinheim, 2006. (b) Tokunaga, M.; Larrow, J. F.;
Kakiuchi, F.; Jacobsen, E. N. Science 1997, 277, 936. (c) Schaus, S.
E.; Brandes, B. D.; Larrow, J. F.; Tokunaga, M.; Hansen, K. B.;
Gould, A. E.; Furrow, M. E.; Jacobsen, E. N. J. Am. Chem. Soc. 2002,
124, 1307.
(5) (a) Rieke, R. D.; Thakur, D. S.; Roberts, B. D.; White, G. T. J. Am.
Chem. Soc. 1997, 74, 333. (b) Pouilloux, Y.; Autin, F.; Barrault, J.
Catal. Today 2000, 63, 87. (c) Adkins, H. Org. React. 1954, 8, 1.
(6) For recent examples on Fe-catalytic hydrogenation of esters,
see: (a) Dupau, P.; Do, M. L. T.; Gaillard, S.; Renaud, J. L. Angew.
Chem. Int. Ed. 2014, 53, 13004. (b) Werkmeister, S.; Junge, K.;
Wendt, B.; Alberico, E.; Jiao, H. J.; Baumann, W.; Junge, H.;
Gallou, F.; Beller, M. Angew. Chem. Int. Ed. 2014, 53, 8722.
(c) Zell, T.; Ben-David, Y.; Milstein, D. Angew. Chem. Int. Ed.
2014, 53, 4685. (d) Chakraborty, S.; Dai, H. G.; Bhattacharya, P.;
Fairweather, N. T.; Gibson, M. S.; Krause, J. A.; Guan, H. R. J. Am.
Chem. Soc. 2014, 136, 7869. (e) Lu, L. Q.; Li, Y. H.; Junge, K.;
Beller, M. Angew. Chem. Int. Ed. 2013, 52, 8382. (f) Junge, K.;
Wendt, B.; Zhou, S. L.; Beller, M. Eur. J. Org. Chem. 2013, 2061.
(7) For Co-catalytic hydrogenation of esters, see: Srimani, D.;
Mukherjee, A.; Goldberg, A. F. G.; Leitus, G.; Diskin-Posner, Y.;
Shimon, L. J. W.; Ben David, Y.; Milstein, D. Angew. Chem. Int. Ed.
2015, 54, 12357.
(8) For recent examples on Ru-catalytic hydrogenation of esters,
see: (a) Filonenko, G. A.; Aguila, M. J. B.; Schulpen, E. N.; van
Putten, R.; Wiecko, J.; Muller, C.; Lefort, L.; Hensen, E. J. M.;
Pidko, E. A. J. Am. Chem. Soc. 2015, 137, 7620. (b) Tan, X. F.;
Wang, Y.; Liu, Y. H.; Wang, F. Y.; Shi, L. Y.; Lee, K. H.; Lin, Z. Y.; Lv,
H.; Zhang, X. M. Org. Lett. 2015, 17, 454. (c) Tan, X. F.; Wang, Q.
L.; Liu, Y. H.; Wang, F. Y.; Lv, H.; Zhang, X. M. Chem. Commun.
2015, 51, 12193. (d) Chen, T.; Li, H. F.; Qu, S. L.; Zheng, B.; He, L.
P.; Lai, Z. P.; Wang, Z. X.; Huang, K. W. Organometallics 2014, 33,
4152. (e) Westerhaus, F. A.; Wendt, B.; Dumrath, A.; Wienhofer,
G.; Junge, K.; Beller, M. ChemSusChem 2013, 6, 1001.
(f) Kuriyama, W.; Matsumoto, T.; Ogata, O.; Ino, Y.; Aoki, K.;
Tanaka, S.; Ishida, K.; Kobayashi, T.; Sayo, N.; Saito, T. Org.
Process Res. Dev. 2012, 16, 166. (g) Zhang, J.; Leitus, G.; Ben-
David, Y.; Milstein, D. Angew. Chem. Int. Ed. 2006, 45, 1113.
(h) Saudan, L. A.; Saudan, C. M.; Debieux, C.; Wyss, P. Angew.
Chem. Int. Ed. 2007, 46, 7473.
(17) For a review, see: Bala, N.; Chimni, S. S. Tetrahedron: Asymmetry
2010, 21, 2879.
(18) (a) Shi, L.; Liu, Y. Y.; Liu, Q. F.; Wei, B.; Zhang, G. S. Green Chem.
2012, 14, 1372. (b) Kim, J.; De Castro, K. A.; Lim, M.; Rhee, H.
Tetrahedron 2010, 66, 3995. (c) Wang, G. Y.; Hu, J. B.; Zhao, G.
Tetrahedron: Asymmetry 2004, 15, 807. (d) Collot, V.; Schmitt,
M.; Marwah, P.; Bourguignon, J. Heterocycles 1999, 51, 2823.
(e) Meijer, L. H. P.; Pandit, U. K. Tetrahedron 1985, 41, 467.
(19) (a) Hayes, J. M.; Deydier, E.; Ujaque, G.; Lledos, A.; Malacea-
Kabbara, R.; Manoury, E.; Vincendeau, S.; Poli, R. ACS Catal.
2015, 5, 4368. (b) Sandoval, C. A.; Ohkuma, T.; Muniz, K.;
Noyori, R. J. Am. Chem. Soc. 2003, 125, 13490. (c) Abdur-Rashid,
K.; Clapham, S. E.; Hadzovic, A.; Harvey, J. N.; Lough, A. J.;
Morris, R. H. J. Am. Chem. Soc. 2002, 124, 15104.
(9) For Ir-catalytic hydrogenation of esters, see: Junge, K.; Wendt,
B.; Jiao, H. J.; Beller, M. ChemCatChem 2014, 6, 2810.
(20) Typical Procedures for Hydrogenation of α-Keto Esters with
Ru-MACHO
(10) For Os-catalytic hydrogenation of esters, see: (a) Spasyuk, D.;
Vicent, C.; Gusev, D. G. J. Am. Chem. Soc. 2015, 137, 3743.
(b) Spasyuk, D.; Smith, S.; Gusev, D. G. Angew. Chem. Int. Ed.
2012, 51, 2772. (c) Spasyuk, D.; Gusev, D. G. Organometallics
2012, 31, 5239. (d) Acosta-Ramirez, A.; Bertoli, M.; Gusev, D. G.;
Schlaf, M. Green Chem. 2012, 14, 1178.
(11) Typical examples, see: (a) Smith, A. B. III.; Chen, S. S.-Y.; Nelson,
F. C.; Reichert, J. M.; Salvatore, B. A. J. Am. Chem. Soc. 1995, 117,
12013. (b) Pye, P. J.; Rossen, K.; Weissman, S. A.; Maliakal, A.;
Reamer, R. A.; Ball, R.; Tsou, N. N.; Volante, R. P.; Reider, P. J.
Chem. Eur. J. 2002, 8, 1372. (c) Kang, S. H.; Jeong, J. W.; Hwang,
Y. S.; Lee, S. B. Angew. Chem. Int. Ed. 2002, 41, 1392. (d) Gupta,
P.; Naidu, S. V.; Kumar, P. Tetrahedron Lett. 2004, 45, 849.
(e) Edagwa, B. J.; Taylor, C. M. J. Org. Chem. 2009, 74, 4132.
(12) Schröder, M. Chem. Rev. 1980, 80, 187.
A glass liner containing a stir bar was charged with substrate
(0.5 mmol), base (0.05 mmol), Ru-MACHO (5 μmol) and MeOH
(0.5 mL) in a glove box. The glass liner was then placed into an
autoclave followed by degassing with H2 three times. The
hydrogenation was carried out at 10–50 bar H2 with stirring at
80 °C for 12–24 h. After the reaction finished, the autoclave was
allowed to cool down to r.t. The hydrogen gas was then carefully
released in a fume hood, and the solution transferred to a flask
with H2O (2 mL), extracted with CH2Cl2 (3 × 5 mL), dried with
Na2SO4, and concentrated in vacuo to afford the pure product
2a–s or 3a–s (see Supporting Information).
Compound 2a: 91% yield, white solid; mp 56–58 °C. 1H NMR
(400 MHz, CDCl3): δ = 7.28–7.38 (m, 5 H), 4.83 (dd, J = 3.6, 8.1
Hz, 1 H), 3.76–3.79 (m, 1 H), 3.65–3.70 (m, 1 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 140.6, 128.7, 128.2, 126.2, 74.8, 68.2 ppm.
MS (EI): m/z calcd for C8H10O2 [M + Na]+: 161.0578; found:
161.0561.
(13) For reviews, see: (a) Beletskaya, I.; Moberg, C. Chem. Rev. 2006,
106, 2320. (b) Burks, H. E.; Morken, J. P. Chem. Commun. 2007,
4717. For typical examples, see: (c) Kliman, L. T.; Mlynarski, S.
N.; Morken, J. P. J. Am. Chem. Soc. 2009, 131, 13210. (d) Burks, H.
Compound 3a: 98% yield, colorless liquid. 1H NMR (400 MHz,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

E
S. Gao et al.
Letter
Synlett
CDCl3): δ = 7.33–7.43 (m, 1 H), 5.19 (s, 1 H), 3.75 (s, 3 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 174.3, 138.3, 128.7, 128.7, 126.7,
73.0, 53.2 ppm. ESI-HRMS: m/z calcd for C9H10O3 [M + Na]+:
189.0528; found: 189.0510.
(21) (a) Yang, W.; Pan, X. J.; Yang, D. Q. Chin. J. Org. Chem. 2015, 35,
1216. (b) Steward, K. M.; Gentry, E. C.; Johnson, J. S. J. Am. Chem.
Soc. 2012, 134, 7329. (c) Wang, C. J.; Sun, X. F.; Zhang, X. M.
Synlett 2006, 1169.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E