Dehydrative cyclization catalyzed by the combination of molybdenum(VI) oxides and benzoic acids: First synthesis of the antitumour substance BE-70016

Article abstract of DOI:10.1002/adsc.200600550

The dehydrative cyclization of N-(o-hydroxybenzoyl)threonine derivative la is efficiently promoted by the combined use of molybdenum(VI) oxides and benzoic acids bearing electron-with-drawing substituents. In the presence of ammonium molybdate [(NH_4<

Full text of DOI:10.1002/adsc.200600550

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DOI: 10.1002/adsc.200600550
Dehydrative Cyclization Catalyzed by the Combination of
Molybdenum(VI) Oxides and Benzoic Acids: First Synthesis of
the Antitumour Substance BE-70016
Akira Sakakura,a Shuhei Umemura,a Rei Kondo,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: October 24, 2006
Dedicated to Prof. M. Shibasaki on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The dehydrative cyclization of N-(o-hy-
droxybenzoyl)threonine derivative 1a is efficiently
promoted by the combined use of molybdenum(VI)
oxides and benzoic acids bearing electron-with-
drawing substituents. In the presence of ammonium
molybdate [(NH4)2MoO4, 10 mol%] and penta-
fluorobenzoic acid (C6F5CO2H; 10 mol%), dehy-
drative cyclization of 1a was conducted in toluene
under azeotropic reflux conditions to give 2-(o-hy-
droxyphenyl)oxazoline 2a in 76% yield. Further-
more, the first total synthesis of the antitumour sub-
stance BE-70016 was achieved using the catalytic
dehydrative cyclization of 1a as a key reaction.
the control of human and mouse tumours. These 2-(o-
hydroxyphenyl)oxazoline-containing natural products
are generally considered to be siderophores,[3] which
are defined as low molecular weight, Fe(III)-specific
A
transport agents. These compounds are thought to be
derived from N-(o-hydroxybenzoyl)threonine.
Although several stoichiometric reagents are
known to be effective for the chemical dehydrative
cyclization of serine and threonine residues,[4] few suc-
cessful examples of dehydrating catalysts have been
reported.[5] Recently, we reported molybdenum(VI)
oxides as highly effective dehydrative cyclization cata-
lysts for the synthesis of oxazolines and thiazolines
[Eq. (1)].[6]
Keywords: catalysis; dehydrative cyclization; mo-
lybdenum(VI) oxide; oxazolines; pentafluorobenzo-
ic acid
Since the late 1980s, many oxazoline-containing natu-
ral products have been isolated from marine organ-
There are two known methodologies for the chemi-
isms.[1] The biosynthesis of these oxazolines appears cal synthesis of oxazolines: the retentive cyclization of
to involve the dehydrative cyclization of serine and N-acylthreonine derivatives at the b-position (bio-
threonine residues.[1c] Among these oxazoline-contain- mimetic cyclization) [Eq. (2)], and its invertive cycli-
ing natural compounds, 2-(o-hydroxyphenyl)oxazoline
structures are often found. For example, BE-70016 is
an antitumour substance that was isolated from Acti-
noplanes sp.[2] This compound appears to be useful in
zation [Eq. (3)]. As in biosynthesis, the molybdenum
oxide-catalyzed dehydrative cyclization of threonine
derivatives proceeds with retention of configuration
at the b-position, while most reactions that use stoi-
chiometric dehydrating reagents proceed with inver-
sion of configuration at the b-position.[4b–g,i,j] There-
fore, the molybdenum oxide-catalyzed method [Eq.
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Akira Sakakura et al.
7.6 Hz, 1H)].[5g] Amide condensation between 4-oxa-
zolinecarboxylic acid 3 and both enantiomers of orni-
thine methyl esters would give two possible diastereo-
mers of BE-70016. The absolute stereochemistry of
BE-70016 would be determined based on a compari-
(1)] is quite useful for the synthesis of naturally occur- son of the sense of the optical rotation. It was expect-
ring oxazolines derived from an l-threonine residue. ed that compound 3 could be prepared from l-threo-
When we synthesize l-threonine-derived oxazolines nine by molybdenum oxide-catalyzed dehydrative
using stoichiometric dehydrating reagents,[7,8] l-allo- cyclization with a retention of configuration at the b-
threonine, which is much more expensive than l- position.
threonine, is needed.
We initially investigated the dehydrative cyclization
We report here the dehydrative cyclization of N-(o- of 1a to 2a using molybdenum(VI) oxides as catalysts
hydroxybenzoyl)threonine derivative 1a catalyzed by (Table 1). Compound 2a is one of the most important
the combination of molybdenum(VI) oxides and ben-
zoic acids bearing electron-withdrawing substituents.
Furthermore, we have achieved the first total synthe-
sis of BE-70016 using the molybdenum oxide-cata-
lyzed dehydrative cyclization as a key reaction.
Table 1. Dehydrative cyclization of N-(o-hydroxybenzoyl)-
threonine methyl ester (1a).[a]
Scheme 1 shows a retrosynthesis of BE-70016. This
compound is composed of two molecules of salicylic
acid, two molecules of threonine, and one molecule of
ornithine. We planned to synthesize BE-70016 biomi-
Entry Mo(VI)=O Additive
X
Yield[b]
[mol%] [%]
1
2
3
4
5
6
7
8
U
-
-
-
-
17
5
E
10
10
2
10
20
10
19
57[c]
47[d]
76
76
76
(CF3)2C6H3CO2H
(NO2)C6H4CO2H
9
C
10
67
10
11
10
10
76[c]
79
(CF3)2C6H3CO2H
TsOH
C6F5CO2H
3,5-
12
13
14
-
-
-
10
10
10
19
1
0
(CF3)2C6H3CO2H
[a]
The reaction of 1a (1 mmol) was conducted in toluene
(10 mL) under azeotropic reflux conditions.
Evaluated by H NMR analysis.
The reaction was conducted for 10 h.
The reaction was conducted for 11 h.
[b]
[c]
[d]
1
Scheme 1. Retrosynthesis of BE-70016.
metically by the dehydrative cyclization of N-(o-hy- common intermediates for the synthesis of many oxa-
droxybenzoyl)threonine methyl ester[9] followed by zoline-containing bioactive natural products. The de-
condensation with ornithine methyl ester. Although velopment of an efficient and practical method for
the relative and absolute stereochemistries of natural the synthesis of this compound is strongly needed.
BE-70016 are not shown in the original patent,[2] we Unfortunately, however, the catalytic activities of
considered that the relative stereochemistries of the (NH4)2MoO4 and MoO2(acac)2 for the dehydrative
N
two oxazoline rings were trans based on the coupling cyclization of 1a were very low (entries 1 and 2), al-
constants of protons at the 4-positions of the oxazo- though they show excellent catalytic activities for the
line rings [d=4.39 (d, J=7.6 Hz, 1H) and 4.43 (d, J= reaction of Cbz-l-Ala-l-Thr-OCH3 [see Eq. (1)].[6] To
552
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First Synthesis of the Antitumour Substance BE-70016
Table 2. Dehydrative cyclization of m- and p-hydroxy deriv-
atives 1b and 1c.[a]
increase the reactivity, we examined several Brønsted
acids as additives. p-Toluenesulfonic acid (TsOH) did
not promote the reaction of 1a (entry 3). The catalytic
activity of TsOH itself was also very low (entry 12),
although it shows good catalytic activity for the dehy-
drative cyclization of N-(p-methoxybenzoyl)-l-threo-
nine methyl ester.[5g] Very interestingly, some benzoic
acids bearing electron-withdrawing substituents effi-
ciently promoted the molybdenum(VI) oxide-cata-
lyzed dehydrative cyclization of 1a. In particular, pen-
tafluorobenzoic acid (C6F5CO2H), 3,5-bis(trifluorome-
thyl)benzoic acid [3,5-(CF3)2C6H3CO2H] and 4-nitro-
benzoic acid [4-(NO2)C6H4CO2H] gave excellent re-
sults (entries 4, 6 and 8–11). In the presence of
(NH4)2MoO4 (10 mol%) and C6F5CO2H (10 mol%),
a solution of 1a was heated under azeotropic reflux
conditions with the removal of water for 12 h. After
aqueous work-up (washing with a 1M aqueous solu-
tion of citric acid), oxazoline 2a was obtained in 76%
yield. Since these benzoic acids themselves showed
Entry Substrate (NH4)2MoO4 C6F5CO2H Time Yield
A
[mol%]
[mol%]
[h]
[%][b]
1
2
3
4
1c
1c
1b
1a
10
2
2
10
0
0
2
4
1
10
88
87
91
18
10
10
[a]
The reaction of 1b or 1c (1 mmol) was conducted in tolu-
ene-DMF (9:1,v/v, 10 mL) under azeotropic reflux condi-
tions.
[b]
1
Evaluated by H NMR analysis.
very low catalytic activities (entries 13 and 14), they (entries 2 and 3). Only 2 mol% of the catalyst was
primarily promoted the activities of molybdenum(VI) sufficient to obtain the products in good yields. The
oxides. The optimized amount of benzoic acid was 1 coordination of the oxazolyl nitrogen of 2a and 2c to
mol equiv. per molybdenum(VI) oxide (entries 5–7). molybdenum(VI) oxides should be stronger than that
One of the reasons for the low catalytic activities of of 2b due to the resonance effect of the hydroxy
molybdenum(VI) oxides is the tight complexation of group at the o- and p-positions. The higher reactivity
molybdenum(VI) oxide with 2a. Actually, the reaction of 1b compared to those of 1a and 1c can be ex-
of N-(o-methoxybenzoyl)-l-threonine methyl ester plained by the faster release of 2b from the catalyst
(3) proceeded well even in the absence of benzoic compared to 2a and 2c. Since 2a was obtained in 18%
acids to give oxazoline 4 in 75% yield [Eq. (4)]. Ben- yield when the reaction of 1a catalyzed by
(NH4)2MoO4 and C6F5CO2H was conducted in tolu-
ene-DMF (9:1, v/v) (entry 4), DMF did not promote
the reaction. A highly polar solvent such as DMF was
not suitable for 1a which was soluble in toluene.[6]
With the key intermediate (2a) for the synthesis of
BE-70016 in hand, we investigated the synthesis of
BE-70016 (Scheme 2). Hydrolysis of 2a with lithium
hydroxide gave carboxylic acid 5 in quantitative yield.
The condensation of ornithine methyl esters was con-
zoic acids might promote decomposition of the stable ducted with 5 (3.0 mol equivs.) using WSCI·HCl (3.0
and inactivated complexes to regenerate the active mol equivs.) and HOBt (2.0 mol equivs.) in CH2Cl2, to
molybdenum(VI) oxide species. The experimental give (S)-l-ornithine derivative 6 and (R)-d-ornithine
result that the isolated yield of 2a was decreased with- derivative 7 in respective yields of 85 and 88%. As
out an aqueous work-up also supported the formation shown in Table 3, some signals in the 1H NMR spectra
of stable complexes of the molybdenum(VI) oxide of 6 were obviously different from those of natural
1
with 2a.
BE-70016 and 7. Based on a comparison of IR, H
Next, we examined the dehydrative cyclization of and 13CNMR, HRMS and specific rotation ([ a]D), 7
m- and p-hydroxy derivatives 1b and 1c (Table 2). was found to be identical to natural BE-70016. Thus,
Since 1b and 1c did not dissolve in toluene, the reac- we have elucidated the stereochemical structure of
tion was conducted in toluene-DMF (9:1, v/v). When BE-70016 as depicted in formula 7, which was com-
the reaction of 1c was conducted in the presence of posed of salicylic acid, l-threonine and unnatural d-
(NH4)2MoO4 (10 mol%) and C6F5CO2H (10 mol%), ornithine. Furthermore, we have achieved the first
the corresponding oxazoline 2c was obtained in 88% total synthesis of BE-70016 using the retentive cycli-
yield (entry 1). Interestingly, in contrast to the reac- zation of 1a as a key reaction.
tion of 1a, the reactions of 1c and 1b proceeded
In conclusion, we have succeeded in the catalytic
smoothly in the absence of pentafluorobenzoic acid, dehydrative cyclization of N-(o-hydroxybenzoyl)-
to give 2c and 2b in respective yields of 87 and 91% threonine derivatives without protecting the o-hy-
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Scheme 2. Synthesis of BE-70016.
colorless oil; IR (neat): n=1743, 1638, 1614, 1491, 1438,
Table 3. Selected spectral data of natural BE70016, and syn-
thetic products 6 and 7.
1355, 1310, 1259, 1229, 1207, 1157, 1134, 1072, 1038 cmꢀ1
;
1H NMR (300 MHz, CDCl3): d=1.55 (d, J=6.3 Hz, 3H),
3.80 (s, 3H), 4.50 (d, J=6.9 Hz, 1H), 4.98 (qd, J=6.3,
6.9 Hz, 1H), 6.87 (ddd, J=0.9, 7.2, 7.8 Hz, 1H), 7.01 (dd,
J=0.9, 8.4 Hz, 1H), 7.39 (ddd, J=1.5, 7.2, 8.4 Hz, 1H), 7.66
(dd, J=1.5, 7.8 Hz, 1H), 11.8 (s, 1H); 13CNMR (75 MHz,
CDCl3): d=20.9, 52.8, 73.7, 78.5, 110.4, 117.0, 118.8, 128.4,
134.0, 160.1, 167.0, 171.0; HR-MS (FAB): m/z=236.0925,
calcd. for C12H14NO4 [M+H]+: 236.0923.
Natural
6
7
BE-70016
1.76
3.33
1.72 1.76
3.20 3.33
m, 1H (b-position of
ornithine)
m, 1H (d-position of
ornithine)
1H NMR
[ppm]
3.69
3.76 3.69
s, 3H (methyl ester)
[a]D
+10.3
+1.6 +10.6
Preparation of (4S,5R)-2-(o-Hydroxyphenyl)-5-
methyl-4-oxazolinecarboxylic Acid (5)
droxy group. The reaction was efficiently promoted
by the combination of molybdenum(VI) oxides and
benzoic acids bearing electron-withdrawing substitu-
ents, such as C6F5CO2H. Furthermore, we have
achieved the first total synthesis of the antitumor sub-
stance BE-70016 via a biomimetic strategy using mo-
lybdenum(VI) oxide-catalyzed dehydrative cyclization
as a key reaction. The present strategy may be suita-
ble for the efficient and practical synthesis of several
bioactive natural products containing 2-(o-hydroxy-
phenyl)oxazolines.
To a solution of 2a (588 mg, 2.5 mmol) in methanol (10 mL)
was added a 1.0M aqueous solution of LiOH (10 mL,
10 mmol) at ambient temperature, and the mixture was stir-
red for 2.5 h. The reaction mixture was cooled to 08Cand
acidified (pH 2) with concentrated aqueous HCl. After
MeOH was removed under vacuum, the resulting aqueous
layer was extracted with EtOAc (3”15 mL). The combined
extracts were washed with brine, dried over Na2SO4 and
concentrated, to give 5 as a colorless amorphous powder;
yield: 550 mg (99%); IR (KBr): n=1732, 1637, 1491, 1446,
1372, 1308, 1259, 1158, 1133, 1073, 1037 cmꢀ1 1H NMR
;
(300 MHz, CDCl3): d=1.45 (d, J=6.3 Hz, 3H), 4.39 (d, J=
7.5 Hz, 1H), 4.93 (qd, J=6.3, 7.5 Hz, 1H), 6.78 (dd, J=7.8,
8.4 Hz, 1H), 6.93 (d, J=8.4 Hz, 1H), 7.31 (dd, J=8.4,
8.4 Hz, 1H), 7.56 (d, J=7.8 Hz, 1H), 11.8 (s, 1H); 13CNMR
(125 MHz, CDCl3): d=20.7, 73.5, 78.4, 110.3, 116.8, 118.4,
128.2, 133.6, 160.1, 166.6, 171.6; HR-MS (FAB): m/z=
222.0772, calcd. for C11H12NO4 [M+H]+: 222.0766.
Experimental Section
Preparation of Methyl (4S,5R)-2-(o-Hydroxyphenyl)-
5-methyl-4-oxazolinecarboxylate (2a)
Preparation of BE-70016 (7)
A solution of 1a (253 mg, 1 mmol), (NH4)2MoO4 (20 mg,
0.10 mmol) and C6F5CO2H (21 mg, 0.10 mmol) in toluene
(10 mL) was heated at azeotropic reflux with the removal of
water using a Dean–Stark apparatus. After 12 h, the reac-
tion mixture was cooled to ambient temperature, diluted
with EtOAc (10 mL) and washed with 1M citric acid in sa-
turated aqueous NaCl (15 mL), saturated aqueous NaHCO3
and NaCl (15 mL), and brine (15 mL). The organic layer
was dried over Na2SO4 and concentrated to give a crude
product. Yields were determined by HPLCanalysis or
1H NMR analysis. The crude product was purified by
column chromatography on silica gel using a mixture of
hexane-EtOAc (15:1!13:1!10:1) as an eluent to give 2a:
To a solution of 2a (465 mg, 2.1 mmol), (R)-ornithine
methyl ester·2HCl (153 mg, 0.70 mmol), HOBt (189 mg,
1.4 mmol) and Et3N (195 mL, 1.4 mmol) in CH2Cl2 (10 mL)
was added a solution of WSCI·HCl (403 mg, 2.1 mmol) and
Et3N (293 mL, 2.1 mmol) in CH2Cl2 (5 mL) at 08C. After the
mixture had been stirred at 68Cfor 15 h and then at ambi-
ent temperature for 2 h, CH2Cl2 was removed under vacuum
and dissolved in EtOAc (50 mL). The resulting solution was
washed with 1M HCl (40 mL), saturated aqueous NaHCO3
(2”40 mL) and brine (40 mL), dried over Na2SO4 and con-
centrated. The residue was purified by column chromatogra-
phy on silica gel using a mixture of hexane-acetone (3:1!
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First Synthesis of the Antitumour Substance BE-70016
2:1!1:1) as an eluent to give 7 as a colorless amorphous
powder; yield: 340 mg (88%); IR (KBr): n=3352, 1743,
1668, 1637, 1613, 1523, 1490, 1445, 1372, 1350, 1310, 1258,
1992, 33, 907–910; e) N. Galꢂotti, C. Montagne, J.
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1227, 1157, 1134, 1074, 1039 cmꢀ1
;
1H NMR (500 MHz,
CDCl3): d=1.60 (d, J=6.5 Hz, 3H), 1.62 (d, J=6.0 Hz, 3H),
1.6–1.7 (m, 2H), 1.76 (m, 1H), 1.93 (dddd, J=5.5, 8.5, 9.0,
14.0 Hz, 1H), 3.33 (ddd, J=6.5, 7.0, 13.5 Hz, 1H), 3.38 (ddd,
J=6.5, 7.0, 13.5 Hz, 1H), 3.69 (s, 3H), 4.39 (d, J=7.5 Hz,
1H) 4.43 (d, J=8.0 Hz, 1H), 4.58 (dt, J=5.0, 7.5 Hz, 1H),
4.84–4.93 (m, 2H), 6.67 (br s, 1H), 6.90 (ddd, J=1.0, 7.5,
8.0 Hz, 2H), 6.91 (ddd, J=1.0, 7.5, 8.0 Hz, 2H), 7.02 (m,
2H), 7.08 (br d, J=8.0 Hz, 1H), 7.41 (ddd, J=1.0, 7.5,
8.5 Hz, 1H), 7.42 (ddd, J=1.0, 7.5, 8.5 Hz, 1H), 7.69 (dd,
J=1.5, 8.0 Hz, 1H), 7.69 (dd, J=1.5, 8.0 Hz, 1H), 11.5 (br s,
1H), 11.5 (br s, 1H); 13C NMR (125 MHz, CDCl3): d=21.7,
21.8, 25.8, 29.7, 38.7, 51.8, 52.7, 74.3, 74.3, 79.4, 79.7, 110.4,
117.0, 117.1, 119.1, 119.2, 128.7, 128.7, 134.3, 134.3, 159.9,
160.0, 167.2, 167.5 170.7, 170.8, 172.0; HR-MS (FAB): m/z=
553.2294, calcd. for C28H33N4O8 [M+H]+: 553.2298.
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Acknowledgements
We thank Dr. Toshiaki Mase, Banyu Pharmaceutical Co.
Ltd., for his helpful discussions. This project was supported
in part by JSPS.KAKENHI (15205021 and 18750082), the
21 st Century COE Program “Nature-Guided Materials
Processing” of MEXT, the Japan Securities Scholarship
Foundation, and the General Sekiyu Research & Develop-
ment Encouragement & Assistance Foundation.
[8] Wipf reported the conversion of cis-oxazolines, which
were prepared from l-threonine derivatives with Bur-
gess reagent, to trans-oxazolines by sequential treatment
with 0.3M HCl, K2CO3, and Burgess reagent: a) P. Wipf,
C. P. Miller, J. Am. Chem. Soc. 1992, 114, 10975–10977;
b) P. Wipf, P. C. Fritch, J. Am. Chem. Soc. 1996, 118,
12358–12367; c) S. V. Downing, E. Aguilar, A. I.
Meyers, J. Org. Chem. 1999, 64, 826–831.
[9] No examples of the catalytic dehydrative cyclization of
N-(o-hydroxybenzoyl)threonine derivatives have been
reported previously. Some stoichiometric dehydrating
reagents can promote the dehydrative cyclization of N-
(o-hydroxybenzoyl)threonine derivatives. However,
these reactions proceed with an inversion of configura-
tion at the b-position of threonine residues: a) A.
Scheurer, P. Mosset, W. Bauer, R. W. Saalfrank, Eur. J.
Org. Chem. 2001, 3067–3074; b) S. Rajaram, M. S.
Sigman, Org. Lett. 2002, 4, 3399–3401.
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