Enantioselective concomitant creation of vicinal quaternary stereogenic centers via cyclization of alkynols triggered addition of azlactones

Article abstract of DOI:10.1016/j.tetlet.2011.08.123

An asymmetric cyclization of alkynols triggered addition of azlactones catalyzed by a combined catalyst system consisting of a chiral gold phosphate and a phosphoric acid produces conformationally restricted amino acid precursors bearing vicinal quaternar

Full text of DOI:10.1016/j.tetlet.2011.08.123

Tetrahedron Letters 52 (2011) 5963–5967
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Tetrahedron Letters
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Enantioselective concomitant creation of vicinal quaternary stereogenic centers
via cyclization of alkynols triggered addition of azlactones
⇑
Zhi-Yong Han, Rui Guo, Pu-Sheng Wang, Dian-Feng Chen, Han Xiao, Liu-Zhu Gong
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2011
Revised 19 August 2011
Accepted 23 August 2011
Available online 6 September 2011
An asymmetric cyclization of alkynols triggered addition of azlactones catalyzed by a combined catalyst
system consisting of a chiral gold phosphate and a phosphoric acid produces conformationally restricted
amino acid precursors bearing vicinal quaternary stereogenic centers in high levels of stereoselectivity.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Relay catalysis
Intramolecular hydroalkoxylation
Azlactone
Brønsted acid
Gold catalyst
Previous Strategy for Concomitant Creation of Vicinal Quaternary
Stereogenic Centers
The enantioselective construction of quaternary stereogenic
centers has long been holding great importance, but accepted as
a challenging task in organic synthesis. As a result, great efforts
have been made in this field, leading to elegant advances on the
enantioselective formation of the single quaternary stereogenic
carbon and of adjacent quaternary and tertiary stereogenic cen-
ters.1 Unarguably, the concomitant creation of the vicinal quater-
nary stereogenic centers in highly enantioselective manner is an
even more formidable challenge owing to the huge steric repulsion
between the two tetrasubstituted carbons. So far, very few success-
ful methods have been available to forge the skeletons of this type,
including intramolecular double Heck reaction, aldol reaction,
[3+2] cycloaddition, and others.2 All these reactions involve sub-
strates with pre-quaternary carbon centers where the reactions
take place (Eq. 1). Herein, we propose an alternative strategy,
which consists of a conversion of a linear functionality, carbon–
carbon triple-bond, to an electrophile with a pre-quaternary car-
bon center able to subsequently undergo an addition reaction, to
enantioselectively afford vicinal quaternary stereogenic centers
(Eq. 2). As proof-of-principle, we will present a highly enantiose-
lective intramolecular cyclization of alkynols triggered addition
reaction of azlactones catalyzed by a combined catalyst system
of a gold complex and a chiral Brønsted acid.
X
X
Chiral Cat
+
Y
ð1Þ
ð2Þ
pre-quaternary carbon centers: X = C (Heck and 3+2),
O (aldol, alkylation)
Y = Proton or leaving group
This Strategy
Y
HX
X
Cat-1
Cat-2
X
pre-quaternary carbon
center in situ generated
Linear
Functionality
Alkynols of type 1 under the catalysis of p-Lewis acids undergo
an intramolecular hydroxylation to give enol ethers, which proved
to be reactive toward nucleophiles, providing a robust platform for
the creation of a variety of important transformations,3 whereas no
enantioselective variants have been established, yet. Tepe, Terada,
and co-workers have disclosed a Brønsted acid-catalyzed aldol
reaction of vinyl ethers with azlactones, creating adjacent quater-
nary and tertiary stereogenic centers with excellent stereocontrol.4
Recently, we and others have demonstrated the excellent compat-
ibility of gold complexes and Brønsted acids in catalysis.5 These
achievements prompted us to speculate about the possibility to
⇑
Corresponding author.
E-mail address: gonglz@ustc.edu.cn (L.-Z. Gong).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.123

5964
Z.-Y. Han et al. / Tetrahedron Letters 52 (2011) 5963–5967
OR*
*RO
P
O
OR*
O
O
H
O
B*-H
O
R1
O
R1
P
R1
O
OR*
O
N
O
I
H
Ar
II
R2
O
HO
Ar
2'
III
Au(I)
OH
N
-B*-H
O
R2
O
R2
O
Au(I)/ B*-H
OH
+
R1
O
R1
N
N
R2
Ar
H
O
1
3
Ar
2
-B*-Au
R2
OR*
Au(I)
O
*RO
P
N
O
OR*
P
O
2'
O
Ar
H
O
O
[Au]-
O
B*-Au
R1
R1
O
OR*
O
N
[Au]-
Ar
I
R2
O
R1
II'
III'
Scheme 1. The possibility to initiate the titled reaction.
initiate a cyclization of alkynols triggered addition of azlactones by
using a combined catalyst system consisting of a chiral phosphoric
acid6 and a gold complex for the concomitant creation of vicinal
quaternary stereocenters (Scheme 1). This proposed enantioselec-
tive protocol presumably proceeds initially with an intramolecular
hydroalkoxylation with a suitable gold catalyst to generate an enol
ether I. Theoretically, the enol ether I participates in the addition
reaction of the azlactone through two possible intermediates
including (1) an ion pair of a chiral conjugate base and an oxonium
ion II formed from protonation of the enol ether with Brønsted acid
4 and (2) a chiral oxonium intermediate II0 generated from coordi-
nation of the chiral gold catalyst to the double bond of the enol
Table 1
Identification of optimal gold catalyst and azlactonea
O
O
O
OH
5 mol% Au(I)
O
10 mol% 4
+
Ar
Ph
N
H
Toluene, RT
N
Ar
2a, Ar = Ph,
O
Ph
2b, Ar = 4-(t-Bu)C6H4
1a
3
2c, Ar = 3,5-(MeO)2C6H3
2d, Ar = 3,5-(EtO)2C6H3
2e, Ar =3,4,5-(MeO)3C6H3
Entry
Ar (2)
Au(I)
Yield (%)b
drc
ee (%)d
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
(Ph3P)AuMe
(Ph3P)AuMe
(Ph3P)AuMe
(Ph3P)AuMe
(Ph3P)AuMe
L1AuMe
L2AuMe
L3AuMe
(Ph3P)AuNTf2
(Ph3P)AuOBz
L3AuMe
95
93
60
91
88
91
94
91
95
93
96
99
86
99
67/33
68/32
67/33
72/28
80/20
81/19
80/20
80/20
63/37
78/22
84/16
85/15
84/16
84/16
63 (36)
69 (30)
50 (35)
68 (26)
91 (37)
90 (34)
91(30)
8
ether.3a,b,7 Either the intermediate II4 or II0 is principally able to
participate in the enantioselective addition of the azlactones via
the transition state III or III0, furnishing the products 3, which
can be readily transformed into a new type of amino acids poten-
tially useful in the construction of conformationally constraint
peptides for the medicinal purpose.9
92 (35)
16 (45)
89 (34)
95 (33)e
9
The implementation of our hypothesis began with a reaction of
2,2-dimethyl-4-pentynol (1a) with 2,4-diphenyloxazol-5(4H)-one
(2a) in the presence of 10 mol % of phosphoric acid 4 and 5 mol %
of (Ph3P)AuMe (Fig. 1).5a Encouragingly, the reaction proceeded
smoothly in toluene at room temperature to give the desired prod-
uct 3a in 95% yield, but with moderate diastereo- and enantioselec-
tivities (Table 1, entry 1). The variation of substitution pattern at
C4 of the azlactone found that enlarging the substituent was ben-
eficial to the stereoslectivity while the reactivity was maintained
(entries 2–5). As a consequence, the use of the azlactone 2e pro-
vided the highest stereoselectivity (entry 5, 80/20 dr, 91% ee). With
the optimal azlactone substrate 2e in hand, we then evaluated the
gold complexes. Basically, the variation of the phosphine ligands10
had very little influence on the stereoselectivity (entries 6–8), but
10
11
12
13
14
L3AuMe
L3AuMe
L3AuMe
95 (31)e,f
93 (24)g
94 (29)h
a
The reaction of 1a (0.2 mmol) and 2 (0.1 mmol) was carried out with 3 Å MS
(100 mg) in toluene (1 mL) for 15 h.
b
Isolated yield.
c
Determined by 1H NMR.
d
Determined by HPLC and the ee values in parenthesis are observed for minor
diastereomer.
e
The reaction was conducted at 0 °C for 48 h.
3 Å MS (50 mg) and toluene (2 mL) were used.
5 mol % of 4 is used.
15 mol % of 4 was used.
f
g
h
i-Pr
Ar
t-Bu
P
Cy
O
O
P
i-Pr
i-Pr
t-Bu
Cy
P
OH
P(t-Bu)2
O
Ar
(S)-4, Ar = 2,4,6-(i-Pr)3C6H2
L3
L1
L2
Figure 1. The organocatalyst and ligands used in this study.

Z.-Y. Han et al. / Tetrahedron Letters 52 (2011) 5963–5967
5965
the gold complex with a sterically bulky phosphine L311 offered
the best results in terms of enantioselectivity (entry 8). In contrast,
the counter anion exerted great impact on the reaction (entries 9
and 10). Although a high conversion was observed with the highly
cationic (Ph3P)AuNTf2,12 the stereoselectivity dropped dramati-
cally (entry 9), presumably because (Ph3P)AuNTf2 offered a non-
enantioselective background reaction to compete with phosphoric
acid-catalyzed variant. Interestingly, less cationic (Ph3P)AuOBz
proved to be a good partner of 4, providing a high stereoselectivity
(entry 10), probably because the (Ph3P)AuOBz reacted with 4 to
generate a gold phosphate as (Ph3P)AuMe did.5c The screening of
solvents revealed that non-polar solvents such as toluene and ben-
zene were suitable reaction media (see Table S1, Supplementary
data). Delightfully, the stereoselectivity could be further enhanced
by lowering the temperature and concentration (entries 11 and 12,
95% ee). Tuning the stoichiometry of 4 to the gold complex
revealed that an equimolar amount of phosphoric acid still ren-
dered a smooth reaction albeit in a lower yield and slightly eroded
ee (entry 13 vs 14), indicating that the chiral gold phosphate was
capable of effectively catalyzing the addition reaction.
Having established the optimal conditions, we explored the
generality for the substrates (Table 2). In addition to 1a, 2,2-disub-
stituted 4-pentynols reacted cleanly with the azlactone 2e to gen-
erate aldol-like products 3f–3h with vicinal quaternary stereogenic
centers in high levels of enantioselectivity for the major diastereo-
mers (90–95% ee). 4-Bromophenyl substituted azlactone 2f partic-
ipated in a smooth cyclization triggered addition reaction with 2,2-
disubstituted 4-pentynols to furnish the desired products 3i–3k in
high yields and high enantioselectivities (up to 86% ee). The meta-
methylphenyl substituted azlactone 2g underwent clean reactions
with 2,2-disubstituted 4-pentynol, giving rise to a cyclic product 3l
with a moderate enantioselectivity (78% ee). The highly electroni-
cally rich 3,4-dimethoxyphenyl substituted azlactone 2h could also
be tolerated with a high enantioselectivity. The para-substitution
of the aromatic ring with a highly electron-donating group enabled
a highly enantioselective reaction to give products 3n–3q in exqui-
site yields.
The further exploration of reaction generality revealed that 5-
hexynol was much less reactive toward the azlactone 2e. Thus,
5-hexynol 7 was unable to participate in the desired reaction under
the optimal conditions. However, the reaction proceeded smoothly
at 60 °C in the presence of 10 mol % of L3AuMe and 20 mol % of 4,
and after alcoholysis with sodium methoxide in methanol to fur-
nish 8 in 73% yield (Eq. 4), albeit with unsatisfactory stereoselectiv-
ity (1.5/1dr, 48%, and 41% ee for the diastereomers, respectively).
As the gold phosphate proved to be an efficient catalyst for the
cascade reaction (Table 1, entry 13), we investigated the kinetic
profile of the cascade reaction of 1a with 2e under the promotion
of the combined catalyst with varying ratios of 4 to L3AuMe, to
clarify if it was necessary to use excess amounts of phosphoric
O
OH
(1) 10 mol% L3AuMe
O
CO2Me
4
20 mol% , 3A MS
O
OMe
OMe
Ph
Toluene, 60 oC, 2.5 d
N
Ph NHCOAr
+
ð4Þ
8
, Ar =3,4,5-(MeO)3C6H3
(2) NaOMe/MeOH
RT, 2 h
73%, 1.5/1 dr,
48% ee (major)
41% ee (minor)
OMe
2e
7
acid. As shown in Figure 2, when the second-order kinetic was ap-
plied, the plot of Ln([3e]/[2e] + 2) against time gave a linear rela-
tionship, from which rate coefficient could be determined,
respectively, from the slope of the plot. The analysis of the plot
indicated that the cationic gold phosphate5c generated from
L3AuMe and 4 served as a good catalyst for the cascade reaction.
However, the presence of excess amounts of phosphoric acid 4 sig-
nificantly speeded up the reaction, in addition to offering a slightly
higher ee (entry 12 vs 13, Table 1). The fact that the combination of
5 mol % L3AuMe and 10 mol % phosphoric acid 4 provided a faster
reaction than 10 mol % of gold phosphate by a factor of 1.8 led to
the conclusion that the phosphoric acid afforded a faster addition
reaction of azlactone to enol ether than its gold salt. Moreover,
these results also suggested that both the intermediates II and II0
were involved in the asymmetric nucleophilic addition of azlac-
tone to the enol ether. In addition, a control reaction between enol
ether 5 and the azlactone 2e proceeded smoothly to give 6 in sim-
ilar yields and stereoselectivities in the presence of 5 mol % of
either gold phosphate or 4 (Eq. 3). These results strongly supported
that the addition reaction occurred via intermediates II and II0.
Notably, a scale-up reaction of 2e (1.0 g) with 1a also proceeded
smoothly to give the syn-3e in a high yield with slight erosion of
the enantioselectivity (93% ee). The reduction of syn-3e with LiAlH4
generated amido alcohol 9 in 84% yield. The Parikh–Doering oxida-
tion13 of
9 and followed by a condensation with benze-
nesulfonohydrazide furnished a hydrazone 10 (Scheme 2). The X-
ray crystallography analysis of the optically pure 10 indicates that
the configuration of syn-3e was assigned to be (R,R) (also see Sup-
plementary data).
In summary, we have established an asymmetric cyclization of
alkynols triggered addition of azlactones catalyzed by a combined
catalyst system consisting of a chiral gold phosphate and a phos-
phoric acid, which results in a new strategy to convert the linear
carbon–carbon triple bond functionality to a quaternary stereogen-
ic center. This process provides a unique entry to access conform-
ationally restricted amino acid precursors bearing vicinal
quaternary stereogenic centers in high levels of stereoselectivity.
In addition, this work first demonstrated that the chiral gold
phosphate is able to catalyze highly enantioselective addition of
O
O
O
OMe
O
5 mol% catalyst
Toluene, RT
+
OMe
N
O
Ph
N
O
Ph
OMe
OMe
ð3Þ
5
OMe
2e
6
endo/exo = 91/9
OMe
5 mol% L3AuMe+ 5 mol% 4: 94%, 1.4/1 dr, 73(64%)ee
5 mol% 4: 94%, 1.4/1 dr, 73(66%)ee

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Z.-Y. Han et al. / Tetrahedron Letters 52 (2011) 5963–5967
O
O
O
OH
O
OMe
X mol% L3AuMe
+
OMe
OMe
Y mol% 4
N
O
Ph
Ph
N
H
OMe
OMe
C6D6CD3, RT
1a
2e
3e
OMe
Figure 2. Kinetic studies on the cascade reaction of 1a and 2e: (j) the reaction catalyzed by 5 mol % L3AuMe and 5 mol % 4; (d) the reaction catalyzed by 5 mol % L3AuMe
and 10 mol % 4; (N) the reaction catalyzed by 5 mol % L3AuMe and 15 mol % 4; (.) the reaction catalyzed by 10 mol % L3AuMe and 10 mol % 4. Two equivalents of ynol to
azalactone 2e were used for all these reactions. The reaction was monitored by 1H NMR at 10 °C.
Table 2
Generality for the substratesa,b,c,d
O
OMe
OMe
5 mol% L3AuMe
O
OH
O
O
N
10 mol% 4
+
OMe
OMe
R
R
N
O
Toluene, 0 °C
H
Ar
Ar
OMe
1
2e, Ar = Ph
2f, Ar = 4-BrC6H4
OMe
3
2g, Ar = 3-MeC6H4
2h, Ar = (3,4-MeO)2C6H3
2i, Ar = 4-MeOC6H4
O
O
O
O
O
O
N
N
N
Ar'
Ar'
Ar'
O
O
O
Ph
3f
Ph
3g
Ph
3h
98%, 87/13 dr, 95% (10%) ee
98%, 87/13 dr, 90% (15%) ee
96%, 86/14 dr, 93% (8%) ee
O
O
O
O
O
O
N
N
N
Ar'
Ar'
Ar'
O
O
O
Ar
Ar
Ar
3i, Ar = 4-BrC6H4
75%, 78/22 dr, 80% (39%) ee
3j, Ar = 4-BrC6H4
90%, 86/14 dr, 86% (13%) ee
3k, Ar = 4-BrC6H4
88%, 81/19 dr, 81% (19%) ee
O
O
O
O
O
O
N
N
N
Ar'
Ar'
Ar'
O
O
O
Ar
Ar
Ar
3l, Ar = 3-MeC6H4
3m, Ar = 3,4-(MeO)2C6H3
3n, Ar = 4-MeOC6H4
78%, 81/19 dr, 78% (9%) ee
91%, 66/34 dr, 83% (15%) ee
99%, 80/20 dr, 91% (27%) ee
0 °C, 2.5 d
O
0 °C, 3 d
O
0 °C, 2.5 d
O
O
O
O
N
N
N
Ar'
Ar'
Ar'
O
O
O
Ar
Ar
Ar
3o, Ar = 4-MeOC6H4
95%, 83/17 dr, 90% (14%) ee
0 °C, 3 d
3p, Ar = 4-MeOC6H4
97%, 80/20 dr, 92% (5%) ee
0 °C, 3 d
3q, Ar = 4-MeOC6H4
94%, 82/18 dr, 90% (10%) ee
0 °C, 3 d
a
b
c
The reaction of 1a (0.2 mmol) and 2 (0.1 mmol) was carried out with 3 Å MS (100 mg) in toluene (1 mL) for 15 h.
Isolated yield.
Determined by 1H NMR.
d
Determined by HPLC and the ee values in parenthesis are observed for minor diastereomers.

Z.-Y. Han et al. / Tetrahedron Letters 52 (2011) 5963–5967
5967
5 mol% L3AuMe
O
O
10 mol% 4
+
1a
+
2e
O
O
O
O
toluene, 3 Å MS, 0 °C, 3 d
Ph
1.0 g
N
Ph
N
Ar
anti-3e
Ar
syn-3e
880 mg
93% ee
Ar = 3,4,5-(MeO)3C6H3
LiAlH4, Et2O
84%
.
(1) SO3 Py, Et3N
NNHSO2Ph
O
OH
DMSO, RT, 91%
O
O
(2) PhSO2NHNH2
MgSO4, AcOH
RT, 71%
O
Ph
Ph
HN
HN
Ar
Ar
9
10
Scheme 2. Scale-up of the reaction and identification of the stereochemsitry of syn-3e.
A.-N. R.; Rios, R. Chem. Asian J. 2011, 6, 720; (e) Hewlett, M.; Hupp, C. D.; Tepe, J.
J. Synthesis 2009, 2825.
azlactones to enol ethers by using chiral anion to control the ste-
reochemistry.8,14,15 Further investigations focused on the related
transformations by using the binary catalyst system are underway.
5. (a) Han, Z.-Y.; Xiao, H.; Chen, X.-H.; Gong, L.-Z. J. Am. Chem. Soc. 2009, 131,
9182; (b) Liu, X.-Y.; Che, C.-M. Org. Lett. 2009, 11, 4204; (c) Wang, C.; Han, Z.-Y.;
Luo, H.-W.; Gong, L.-Z. Org. Lett. 2010, 12, 2266; (d) Muratore, M. E.; Holloway,
C. A.; Pilling, A. W.; Storer, R. L.; Trevitt, G.; Dixon, D. J. J. Am. Chem. Soc. 2009,
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Gong, L.-Z. J. Am. Chem. Soc. 2008, 130, 7782; (h) Mukherjee, S.; List, B. J. Am.
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2009, 131, 6962; (j) Terada, M.; Toda, Y. J. Am. Chem. Soc. 2009, 131, 6354; (k)
Zhao, Z.-A.; You, S.-L. Angew. Chem., Int. Ed. 2009, 48, 7428; (l) Cai, Q.; Zhang, C.;
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Acknowledgments
We are grateful for the financial support from NSFC (20732006),
and the Ministry of Health (2009ZX09501-017), MOST (973 Project
2010CB833300), and the Ministry of Education.
Supplementary data
Supplementary data (experimental details and characterization
of new compounds) associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2011.08.123.
6. For reviews, see: (a) Akiyama, T. Chem. Rev. 2007, 107, 5744; (b) Terada, M.
Chem. Commun. 2008, 4097; (c) Terada, M. Synthesis 2010, 1929.
7. For a review on gold-g
2-coordination to unsaturated carbon–carbon bonds,
see: Schmidbaur, H.; Schier, A. Organometallics 2010, 29, 2.
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