Catalytic synthesis of peptide-derived thiazolines and oxazolines using bis(quinolinolato)dioxomolybdenum(VI) complexes

Article abstract of DOI:10.1002/adsc.200700068

Bis(2-ethyl-8-quinolinolato)dioxomolybdenum(VI) (9) (1 mol %) shows remarkable catalytic activity for the dehydrative cyclization of cysteine-containing dipeptides 1 to give the corresponding thiazolines 2 with less than 6 % epimerization at the C2-exomet

Full text of DOI:10.1002/adsc.200700068

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DOI: 10.1002/adsc.200700068
Catalytic Synthesis of Peptide-Derived Thiazolines and Oxazo-
lines using Bis(quinolinolato)dioxomolybdenum(VI) Complexes
Akira Sakakura,a Rei Kondo,a Shuhei Umemura,a and Kazuaki Ishiharaa,*
a
Graduate School of Engineering, Nagoya University, Chikusa, Nagoya, 464-8603 Japan
Fax : (+81)-52-789-3222; e-mail: ishihara@cc.nagoya-u.ac.jp
Received: January 26, 2007
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Bis(2-ethyl-8-quinolinolato)dioxomolyb-
denum(VI) (9) (1 mol%) shows remarkable catalyt-
ic activity for the dehydrative cyclization of cys-
teine-containing dipeptides 1 to give the corre-
sponding thiazolines 2 with less than 6% epimeriza-
tion at the C2-exomethine position. For the dehy-
syntheses of thiazolines start from N-(b-hydroxy-
ethyl)thioamides using stoichiometric amounts of de-
hydrating reagents,[2] while a few methods have been
reported for the biomimetic synthesis of thiazolines
from cysteine derivatives.[2a,3] For the chemical synthe-
sis of l-threonine-derived oxazolines using stoichio-
metric amounts of dehydrating reagents, l-allo-threo-
nine, which is much more expensive than l-threonine,
is needed,[4,5] since the reaction proceeds with an in-
version of configuration at the 5-position.[6]
drative
cyclization
of
threonine-containing
dipeptides 4, 1 mol% of bis(2-phenyl-8-quinolinola-
to)dioxomolybdenum(VI) (10) gives the corre-
sponding oxazolines 5 with retention of configura-
tion at the 5-position.
Recently, we reported molybdenum oxides (10
mol%) as effective acid-base monoconjugate cata-
lysts[7] for the biomimetic dehydrative cyclization of
N-acylcysteines, N-acylthreonines and N-acylserines
Keywords: catalysis; dehydrative cyclization; mo-
lybdenum; oxazolines; quinolinolato species; thia-
zolines
(Scheme 1).[8] MoO2
(acac)2 has good catalytic activity
A
for the dehydrative cyclization of cysteine-containing
dipeptide 1a to thiazoline 2a. For the synthesis of oxa-
zoline 5a from dipeptide 4a that includes a threonine
residue, the ammonium salts (NH4)6Mo7O24·4H2O
Thiazolines and oxazolines have been found in many and (NH4)2MoO4 show good catalytic activities. Since
biologically active natural products of peptide origin. the molybdenum oxide-catalyzed dehydrative cycliza-
Their wide range of antitumor, antiviral and antibiotic tion of l-threonine derivatives proceeds with a reten-
activities has fueled numerous synthetic investiga- tion of configuration at the 5-position, the molybde-
tions.[1] Thiazolines and oxazolines are thought to be num oxide-catalyzed method is very useful for the
biosynthesized via the dehydrative cyclization of cys- synthesis of naturally occurring oxazolines derived
teine, threonine and serine residues.[1c] Most chemical from l-threonine. This method is the first successful
Scheme 1. Molybdenum oxide-catalyzed dehydrative cyclization of peptides (our previous work[8]).
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Akira Sakakura et al.
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example of the catalytic dehydrative cyclization of di-
peptides that include cysteine, threonine and serine
residues. Thiazolines and oxazolines such as 2a and 5a
are useful building blocks for the synthesis of various
bioactive natural products.[4–9]
Although the MoO2(acac)2-catalyzed dehydrative
G
cyclization of Cbz-l-Ala-l-Cys-OMe (1a) proceeds
well, epimerization product 3a is also obtained in sig-
nificant yield (2a:3a=82:18) probably because of the
acidity of MoO2(acac)2. Thiazolines are generally
U
more susceptible to epimerization than oxazolines
under both acidic and basic conditions.[2d,10] We con-
sidered that it was important to control the Lewis
acidities and Brønsted basicities of molybdenum cata-
lysts by more suitable ligands for the efficient design
of dehydration catalysts.[7] Furthermore, homogeneous
monomeric molybdenum complexes were expected to
exhibit higher catalytic activities even under lower
catalyst-loading conditions, while MoO2(acac)2,
R
(NH4)6Mo7O24·4H2O and (NH4)2MoO4 are heteroge-
neous oligomeric species and require rather higher
catalyst loading (10 mol%).[8] Through the intensive
examination of molybdenum complexes as dehydra-
tive cyclization catalysts, we found that molybdenum
(VI) complexes with 8-quinolinols showed good cata-
lytic activities.[11,12] We report here bis(quinolinolato)-
Figure 1. X-ray single-crystal structures of 9 (top) and 10
(bottom).
dioxomolybdenum(VI) complexes as efficient cata- are cis to both oxo groups (N-cis); b) each nitrogen
lysts for the dehydrative cyclization of dipeptides in- atom is trans to one oxo group (N-trans) and c) one
cluding a cysteine or threonine residue to thiazolines nitrogen atom is cis to the oxo groups and the other is
and oxazolines.
trans to one oxo group (N-cis,trans).[13] X-ray crystal-
Bis(quinolinolato)dioxomolybdenum(VI)
plexes 7–12 were easily prepared from MoO
com- lographic analyses revealed that both 9 and 10 had N-
A
and known 8-quinolinols (2 equivs.) in EtOH in yields crystal structure of 10 is more distorted than that of 9
À
À
of 69–99% (Scheme 2). The structures of the bis(qui- (O1 Mo O2 bond angle of 9=163.698 and that of
1
nolinolato)dioxomolybdenum(VI) complexes were 10=147.968). H NMR spectra indicated that 9 was a
1
confirmed by H NMR, IR, HR-MS, and X-ray crys- 44:56 isomeric mixture in toluene-d8 at ambient tem-
tallographic analyses. The X-ray single-crystal struc- perature. When the solution was heated at 1008C, the
tures of 9 and 10 are shown in Figure 1. These hexa- ratio changed to 100:0. For 10, the ratio of the iso-
coordinated complexes may have a total of three ar- mers in toluene-d8 was 77:23 at ambient temperature
rangements: a) the two quinolinolato nitrogen atoms and 100:0 at 608C. The major isomers at high temper-
ature are thought to be N-trans, and the minor iso-
mers are N-cis,trans.
With the bis(quinolinolato)dioxomolybdenum(VI)
complexes 7–12 in hand, we examined their catalytic
activities for the dehydrative cyclization of 1a to thia-
zoline 2a (Table 1). The reaction was conducted in the
presence of a bis(quinolinolato)dioxomolybdenum
(VI) complex in toluene under azeotropic reflux con-
ditions with the removal of water. Molybdenum(VI)
complexes 7–12 could be dissolved in toluene and ap-
peared to be stable under these reaction conditions.
After removal of the solvent, the resulting crude
product was analyzed by HPLC. 8-Quinolinolato
complex 7 (10 mol%) showed good catalytic activity
(80% yield), and the generation of epimer 3a was ef-
fectively reduced, as expected (2a:3a=96:4) (entry 1).
We then tried to reduce the catalyst loading, but un-
Scheme 2. Preparation of bis(quinolinolato)dioxomolybde-
num(VI) complexes.
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Catalytic Synthesis of Peptide-Derived Thiazolines and Oxazolines
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Table 1. Catalytic activities of bis(quinolinolato)dioxomolyb-
denum(VI) complexes for the dehydrative cyclization of
1a.[a]
Table 2. Dehydrative cyclization of dipeptides 1 to thiazo-
lines 2 catalyzed by 9.[a]
Entry Mo(VI)=O
Time
[h]
Yield[b]
[%]
dr[c]
(2a:3a)
[mol%]
1
2
3
4
5
6
7
8
7 [10]
7 [1]
8 [1]
5
5
5
2
5
5
2
8
80
40
93
96
80
96
94
85
96:4
89:11
95:5
97:3
85:15
96:4
Entry
Dipeptide 1
Yield[b] [%]
dr[c] (2:3)
9 [1]
10 [1]
11 [1]
12 [1]
PG
R
1
2
3
4
C
C
Boc
bz
bz
Me
1aMe 83
1bBn 85
99:1
98:2
94:6
96:4
86:14
82:18
MoO2(acac)2 [10]
G
1c
1d
82
91
[a]
The reaction of 1a (0.10 mmol) was conducted in the
presence of an Mo(VI)=O catalyst in toluene (10 mL)
under azeotropic reflux conditions.
Fmoc
Me
[a]
The reaction of dipeptides 1 (0.10 mmol) was conducted
in the presence of 9 (1 mol%) in toluene (10 mL) under
azeotropic reflux conditions for 1 h.
Isolated yield.
[b]
[c]
1
Yieds of 2a and 3a were determined by H NMR analy-
sis.
[b]
[c]
Determined by HPLCanalysis.
Determined by HPLCanalysis.
fortunately the use of 1 mol% of 7 decreased the re- of the C2-exomethine position was suppressed to less
activity (entry 2). Interestingly, we found that the in- than 6%.
troduction of an alkyl group to the 2-position of the
We previously reported the dehydrative cyclization
8-quinolinol significantly increased the catalytic activi- of 4a to oxazoline 5a in good yields using
ties of the quinolinolato complexes (entries 3–6). In (NH4)6Mo7O24·4H2O and (NH4)2MoO4 as catalysts.[8]
particular, 2-ethyl-8-quinolinolato complex 9 and 2,4- This reaction was accompanied by epimerization at
dimethyl-8-quinolinol complex 11 exhibited remarka- the C2-exomethine position (95:5 dr) (Table 3,
bly higher catalytic activities than MoO2(acac)2, to entry 8). We then compared the catalytic activities of
A
give 2a in 96% yield despite the lower catalyst load- bis(quinolinolato)dioxomolybdenum complexes for
ing (1 mol%) (entries 4 and 5 versus entry 8). Fur- the dehydrative cyclization of threonine-containing
thermore, the use of complexes 9 and 11 suppressed dipeptides Cbz-l-Ala-l-Thr-OMe (4a), Cbz-l-Phe-l-
epimerization at the C2-exomethine position of the Thr-OMe (4b), Boc-l-Ala-l-Thr-OMe (4c) and Fmoc-
product to less than 4%. Although 2,4-dimethyl-5,7- l-Ala-l-Thr-OMe (4d) to the corresponding oxazo-
dibromo-8-quinolinol complex 12 showed good cata- lines 5a–d (Table 3). Although 7 and 8 showed moder-
lytic activity, epimerization increased to 86:14 dr ate catalytic activities (entries 1 and 2), 2-phenyl-8-
(entry 7). It is conceivable that the stronger acidity of quinolinolato complex 10 gave excellent results (en-
complex 12 due to two electronegative bromine tries 3–6). The catalytic activity of the homogeneous
atoms promoted epimerization of the C2-exomethine complex 10 was higher than those of heterogeneous
position. In contrast, the introduction of an alkyl (NH4)2MoO4 and (NH4)6Mo7O24·4H2O, and the
group to the 2-position increased the basicity of the amount of 10 could be reduced to 1 mol%. Further-
quinolinolato-nitrogen to suppress epimerization.
more, the yield of epimers 6a–d was less than 6%.
We then examined the dehydrative cyclization of The present reactions also showed a complete reten-
other cysteine-containing dipeptides, Cbz-l-Phe-l- tion of configuration at the 5-position.
Cys-OMe (1b), Boc-l-Ala-l-Cys-OMe (1c) and
In conclusion, bis(2-ethyl-8-quinolinolato)dioxomo-
Fmoc-l-Ala-l-Cys-OMe (1d) (Table 2). Dipeptides lybdenum(VI) (9) promoted the catalytic dehydrative
1a–d could be converted to the corresponding thiazo- cyclization of cysteine-containing dipeptides 1 to thia-
lines 2a–d in good isolated yields (82–91%). tert-Bu- zolines 2 in high yield without a significant loss of ste-
toxycarbonyl (Boc) and 9-fluorenylmethoxycarbonyl reochemical integrity at the C2-exomethine positions.
(Fmoc) groups, which are useful protecting groups for For the synthesis of threonine-derived oxazolines 5,
the synthesis of peptides and peptide-containing natu- bis(2-phenyl-8-quinolinolato)dioxomolybdenum(VI)
ral products, were also compatible with the reaction (10) showed excellent catalytic activity, and the reac-
conditions (entries 3 and 4). In all cases, epimerization tion proceeded with a retention at the 5-position. In
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Table 3. Dehydrative cyclization of dipeptides 4 to oxazolines 5 catalyzed by bis(quinolinolato)dioxomolybdenum(VI) com-
plexes.[a]
Entry
Mo(VI)=O [mol%]
Dipeptide 4
Time [h]
Yield[b] [%]
dr[c] (5:6)
PG
R
1
2
3
4
5
6
7
8
7 [10]
8 [10]
10 [1]
10 [1]
10 [1]
10 [1]
12 [10]
Cbz
Cbz
Cbz
Cbz
Boc
Fmoc
Cbz
Cbz
Me
Me
Me
Bn
Me
Me
Me
Me
4a
4a
4a
4b
4c
4d
4a
4a
5
5
1
1
2
1
6
2
70[d]
51[d]
85
92
89
81:19
98:2
94:6
96:4
98:2
95:5
nd
88
42[d]
91[d]
(NH4)6Mo7O24·4H2O [10]
95:5
[a]
The reaction of dipeptides 4 (0.10 mmol) was conducted in the presence of an Mo(VI)=O catalyst in toluene (10 mL)
under azeotropic reflux conditions.
Isolated yield.
[b]
[c]
[d]
Determined by HPLCanalysis.
1
Determined by H NMR analysis.
7.58 (d, J=8.1 Hz, 0.4H), 7.91 (d, J=8.1 Hz, 0.4H), 8.06 (d,
both reactions, catalyst loadings of 1 mol% were suf-
ficient to give thiazolines and oxazolines in good
yields. In addition to the total synthesis of thiazoline-
and/or oxazoline-containing natural products, the
present method should also be applicable to the syn-
thesis of thiazole- and/or oxazole-containing natural
products,[15] since thiazoles and oxazoles could be pre-
pared from thiazolines and oxazolines by oxidation.[16]
J=8.1 Hz, 0.6H), 8.15 (d, J=8.1 Hz, 0.4H), 8.29 (d, J=
8.7 Hz, 0.6H); 13C NMR (125 MHz, CDCl3): d=13.0, 14.0,
14.8, 27.4, 29.1, 29.2, 111.7, 113.4, 114.7, 115.9, 116.0, 118.2,
122.2, 123.0, 123.3, 124.7, 126.4, 127.0, 127.7, 127.8, 130.2,
132.8, 136.8, 138.1, 138.5, 140.1, 144.6, 150.3, 156.5, 159.0,
166.4; 9 was a ca. 3:2 isomeric mixture in CDCl3; HR-MS
(FAB): m/z=475.0563, calcd. for C22H21MoN2O4 [M+H]+:
475.0555.
Preparation of cis-Bis(2-phenyl-8-quinolinolato-N,O)-
dioxomolybdenum(VI) (10)
Experimental Section
To a solution of MoO2(acac)2 (65.2 mg, 0.20 mmol) in EtOH
A
(0.50 mL) was added a solution of 2-phenyl-8-quinolinol
(88.5 mg, 0.40 mmol) in EtOH (1.0 mL). After being stirred
at ambient temperature for 15 min, 10 was obtained by fil-
tration; yield: 91 mg (80%). Single crystals suitable for X-
ray analysis were obtained from EtOH. IR (KBr): n=900
Preparation of cis-Bis(2-ethyl-8-quinolinolato-
N,O)dioxomolybdenum(VI) (9)
To a solution of MoO2(acac)2 (44.2 mg, 0.135 mmol) in
G
EtOH (0.50 mL) was added a solution of 2-ethyl-8-quinoli-
nol (47.0 mg, 0.27 mmol) in EtOH (1.0 mL). After being
stirred at ambient temperature for 12 h, 9 was obtained by
filtration; yield: 58 mg (91%). Single crystals suitable for X-
ray analysis were obtained from CH2Cl2. IR (KBr): n=908
1
cmÀ1 (Mo=O); H NMR (300 MHz, CDCl3): d=5.50 (d, J=
7.5 Hz, 0.2H), 5.75 (d, J=5.1 Hz, 0.1H), 5.76 (d, J=5.1 Hz,
0.1H), 5.96 (dd, J=1.8, 7.2 Hz, 0.9H), 6.24 (d, J=7.8 Hz,
0.2H), 6.48 (s, 0.2H), 6.49 (d, J=2.1 Hz, 0.2H), 6.76 (t, J=
7.8 Hz, 0.2H), 6.97 (d, J=7.8 Hz, 0.2H), 7.05 (t, J=6.6 Hz,
0.6H), 7.13–7.25 (m, 4.2H), 7.35 (d, J=6.9 Hz, 0.4H), 7.42–
7.64 (m, 6.2H), 7.66–7.82 (m, 3.2H), 7.94 (d, J=8.7 Hz,
0.2H), 8.00 (d, J=8.1 Hz, 0.9H), 8.16 (m, 0.9H), 8.23 (m,
0.9H), 8.43 (d, J=8.4 Hz, 0.2H); 13CNMR (125 MHz,
CDCl3): d=115.3, 117.5, 124.8, 127.7, 128.0, 129.5, 130.5,
137.8, 138.0, 138.2, 139.9, 155.0, 158.2, 160.1; 10 was a ca. 9:1
1
cmÀ1 (Mo=O); H NMR (300 MHz, CDCl3): d=1.14 (t, J=
7.5 Hz, 3H), 1.25 (t, J=7.2 Hz, 1.7H), 1.32 (t, J=7.5 Hz,
1.3H), 2.62 (m, 0.8H), 3.14 (m, 1.2H), 3.49 (m, 1.2H), 4.08
(m, 0.8H), 5.18 (d, J=7.5 Hz, 0.4H), 5.74 (d, J=7.5 Hz,
0.4H), 6.21 (dd, J=2.4, 6.3 Hz, 0.4H), 6.46 (m, 0.8H), 6.62
(t, J=7.5 Hz, 0.4H), 6.95–7.05 (m, 2.4H), 7.20–7.30 (m,
1.2H), 7.48 (t, J=8.1 Hz, 1.2H), 7.53 (d, J=8.1 Hz, 0.4H),
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Catalytic Synthesis of Peptide-Derived Thiazolines and Oxazolines
COMMUNICATIONS
isomeric mixture in CDCl3; HRMS (FAB): m/z=571.0542,
calcd. for C30H21MoN2O4 [M+H]+: 571.0555.
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General Procedure for the Dehydrative Cyclization
of Dipeptides 1 and 4
A 20-mL, single-necked, round-bottomed flask equipped
with a Teflon-coated magnetic stirring bar and a 5-mL pres-
sure-equalized addition funnel [containing a cotton plug and
ca. 0.1 g of CaH2] surmounted by a reflux condenser was
charged with a dipeptide 1 or 4 (0.10 mmol) and a molybde-
num(VI) complex (1 mol%) in toluene (10 mL). The mix-
ture was heated for several hours under azeotropic reflux
conditions with the removal of water. The reaction mixture
was cooled to ambient temperature, washed with saturated
aqueous solution of NaHCO3 (10 mL) and brine (10 mL),
and the organic solvent was then removed to give a crude
product. The obtained crude product was purified by
column chromatography on silica gel using toluene-acetone
(for 2) or hexane-EtOAc (for 5), to give a corresponding
thiazoline 2 or oxazoline 5.
[6] Corey reported the dehydrative cyclization of a threo-
nine derivative using TsOH (10 mol%) as a catalyst.
The reaction proceeds with a retention of configuration
at the 5-position: L. R. Reddy, P. Saravanan, E. J.
Corey, J. Am. Chem. Soc. 2004, 126, 6230–6231; in our
experiment, dehydrative cyclization of 4a using TsOH
gave a 1:1 mixture of 5a and 6a due to its strong acidi-
ty.
Acknowledgements
Financial support for this project was provided by JSPS KA-
KENHI (15205021 and 18750082), the 21 st Century COE
Program “Nature-Guided Materials Processing” ofMEXT
and The Japan Securities Scholarship Foundation.
[7] K. Ishihara, A. Sakakura, M. Hatano, Synlett 2007, in
press.
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