Synthesis of the proposed structure and revision of stereochemistry of kaitocephalin

Article abstract of DOI:10.1016/S0040-4039(01)02254-7

A stereoselective total synthesis of the proposed structure of kaitocephalin (1) was accomplished starting from L-proline and D- and L-serines. However, its ¹H NMR spectral data and retention time on HPLC were not identical with those of authentic natural kaitocephalin. The revised stereochemistry of natural kaitocephalin, (2R)-isomer (16), was inferred from further experiments employing diastereomers and model compounds.

Full text of DOI:10.1016/S0040-4039(01)02254-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 857–860
Synthesis of the proposed structure and revision of
stereochemistry of kaitocephalin
Masayuki Okue,a Hiroyuki Kobayashi,a Kazuo Shin-ya,b Kazuo Furihata,a Yoichi Hayakawa,b
Haruo Seto,b,† Hidenori Watanabea,* and Takeshi Kitaharaa,*
aDepartment of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo,
Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
bInstitute of Molecular and Cellular Biosciences, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
Received 1 November 2001; revised 21 November 2001; accepted 22 November 2001
Abstract—A stereoselective total synthesis of the proposed structure of kaitocephalin (1) was accomplished starting from
L
-proline
1
and D- and L-serines. However, its H NMR spectral data and retention time on HPLC were not identical with those of authentic
natural kaitocephalin. The revised stereochemistry of natural kaitocephalin, (2R)-isomer (16), was inferred from further
experiments employing diastereomers and model compounds. © 2002 Elsevier Science Ltd. All rights reserved.
Kaitocephalin (1) was isolated from Eupenicillium
shearii PF1191 in 1997 as a novel NMDA (N-methyl-
A synthesis of 1 is shown in Scheme 1. Aldol reaction
between Seebach’s lactone 24 and Garner’s aldehyde 35
provided 4a and 4b (58% combined yield, 4a/4b=4:1).
These stereochemistries were determined by NOESY
experiments of 5a and 5b, which were derived from 4a
and 4b, respectively. Major product 4a was converted
into desired 4b by Dess–Martin oxidation followed by
reduction with NaBH4 (43% from 4a). After transac-
etalization of t-butyloxazoline and hydrolysis of
dimethyloxazoline with 10% H2SO4, the liberated car-
boxyl and hydroxyl groups were protected successively
to give 6 in 62% yield over three steps. Hydrolysis of
t-butyloxazoline with 80% AcOH (62%) followed by
protection of the secondary alcohol and oxidation of
D
-
aspartic acid) and AMPA/KA (a-amino-3-hydroxy-5-
methyl-4-isoxazolepropionic acid/kainic acid) receptors
antagonist.1 This compound protected chick primary
telencephalic neurons from kainate toxicity at 500 mM
with EC50 value 0.68 mM,1 which is almost the same
level as that of the well known AMPA/KA antagonist
CNQX (EC50 value 0.53 mM). More recently, the abso-
lute stereochemistry of kaitocephalin (1) was reported
to be 2S,3S,4R,7R,9S.2 We were interested in this
unique structure as well as its potent biological activi-
ties, and have been investigating a stereoselective syn-
thesis of kaitocephalin to confirm the absolute
configuration of 1 (Fig. 1).
6
the secondary amine with MeReO3–urea·H2O2 pro-
vided the desired nitrone 7 (67%, three steps).
Herein, we report a total synthesis of proposed struc-
ture 1 using a novel stereoselective coupling reaction of
a nitrone and a halide as a key step. However, 1H
NMR spectral data of our synthetic 1 and its retention
time on HPLC were not identical with those of authen-
tic natural kaitocephalin.3
When 7, 8, Zn powder and CuI in THF/H2O (3.3:1)
were sonicated at ambient temperature, hydroxylamine
9 was obtained in high yield (85%) as a single isomer.
Although these reaction conditions were originally
Cl
O
H2N
HO
2
COOH
6
5
4
Keywords: kaitocephalin; total synthesis; NMDA antagonist; AMPA/
KA antagonist.
NH
3
7
Cl
* Corresponding authors. Fax: 81-3-5841-8019; e-mail: atkita@
mail.ecc.u-tokyo.ac.jp
9
OH
COOH
HOOC
N
H
8
H
† Present address: Department of Applied Biology and Chemistry,
Faculty of Applied Bio-Science, Tokyo University of Agriculture,
Sakuragaoka, Setagaya-ku, Tokyo 156-8502, Japan.
Figure 1. Proposed structure of kaitocephalin (1).
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02254-7

858
M. Okue et al. / Tetrahedron Letters 43 (2002) 857–860
BzlO
BzlO
O
O
Cbz
N
Cbz
N
O
O
H
Cbz
N
N
Cbz
N
N
H
a
d, e
O
N
N
O
N
H
O
+
then
Cbz
OH
O
OH
O
But
But
H
O
O
O
But
N
H
2
NOE
O
O
O
NOE
5a
But
But
b, c
OHC
5b
4a
4b
3
Cl
Cl
O
Cbz
HN
BzlO
BzlO
NH
O
8
NHCbz
OTBDPS
l
i, j, k
I
OTBDPS
f, g, h
BzlOOC
N
4b
OTMS
N
O
O
COOBzl
Cl
But
6
7
Cl
Cbz
Cbz
O
O
m, n, o
p, q
HN
HN
BzlO
BzlO
NH
NH
OTBDPS
OTMS
COOBzl
OH
7
Cl
Cl
Cl
OH
COOBzl
Cbz
BzlOOC
BzlOOC
N
N
H
9
H
OH
11
Cbz
H
HN
H
H H
O
H
H
H2N
HO
OTBDPS
10
m, r, s
COOH
NH
H
9
7
OH
COOBzl
N
H
Cl
OH
COOH
HOOC
N
H
H
H
N
O
H
solid lines : NOE
1
dashed lines : ROE
Scheme 1. Synthesis of the proposed structure of kaitocephalin (1). (a) LDA, THF, −78°C, 58% (4a/4b=4:1); (b) Dess–Martin
periodinane, CH2Cl2, Py, 0°C, 89%; (c) NaBH4, MeOH/THF (2:1), 0°C, 48%; (d) 10% H2SO4, 1,4-dioxane, rt; (e) BzlOH, Ph3P,
DEAD, THF; (f) 10% H2SO4, 1,4-dioxane, rtꢀ80°C; (g) BzlBr, NaHCO3, THF/DMF (2:1), rt; (h) TBDPSCl, Et3N, DMAP,
CH2Cl2, rt, 62%; (i) 80% AcOH, 60°C, 62%; (j) TMSCl, imidazole, DMF, 0°C, 80%; (k) MeReO3, Urea·H2O2, MeOH, rt, 85%;
(l) Zn (8 equiv.), CuI (3.6 equiv.), THF/H2O (3.3:1), ultrasound, rt, 85%; (m) Zn, sat. NH4Cl, EtOH, 90°C, 71%; (n) CbzCl,
K2CO3, toluene/H2O; (o) TBAF, AcOH, THF, rt, 50%; (p) 4-methoxy–TEMPO, KBr, sat. NaHCO3, NaClO, CH2Cl2, 0°C, 67%;
(q) H2, 20% Pd(OH)2-C, EtOH/CHCl3 (10:1), rt 30%; (r) TMSCl, MeOH; (s) AcOH (cat.), toluene, reflux.
To ascertain the correct stereochemistry of natural
kaitocephalin, we then synthesized 3-epi-1, 9-epi-1 and
simpler analogs (12a–c, Fig. 2).10 Although neither
3-epi-1 nor 9-epi-1 were identical with the natural
compound,11 comparison of 1H NMR data of our
synthetic compounds with natural kaitocephalin sug-
gested a solution of the stereochemistry.
developed by Luche et al. for a coupling reaction of an
alkyl halide and a conjugated enone,7 our result is the
first example of the introduction of an alkyl group to a
nitrone in aqueous media. The stereochemistry at C-7
was determined by assigning NOESY and ROESY
spectra of lactamized compound 10. After hydroxy-
lamine 9 was reduced with zinc and ammonium chlo-
ride (71% yield), protection of the resulting amine
followed by removal of the silyl ethers afforded diol 11
in 50% yield for two steps. Selective oxidation of the
primary alcohol of 11 (4-methoxy–TEMPO, excess
NaClO)8 provided carboxylic acid in 67% yield.
Hydrogenolysis of the five benzylic protective groups
with H2/20% Pd(OH)2-C was successful when CHCl3
was used as a co-solvent to prevent undesired dechlori-
nation, and 1 was obtained in 30% yield after prepara-
tive HPLC purification. However, the 1H NMR
spectral data of synthetic compound 19 and retention
time on HPLC were not identical with those of authen-
tic natural kaitocephalin.
As shown in Fig. 2, stereoisomers of 1 and 12 were
classified into groups A–D according to the stereochem-
istry at C-2, 3 and 4, and chemical shifts of H-2 and 3
and coupling constants between these protons were
compared. Compound 12d has not been synthesized yet
due to unexpected inapplicability of the same approach.
However, we noticed that (1) the chemical shifts and
coupling constants of the compounds in the same group
were quite similar irrespective of the presence or
absence of the left side chain at C-7 (12a versus 1 and
9-epi-1, 12b versus 3-epi-1); (2) the compounds in
groups A and B showed different patterns from that of
the natural compound (as highlighted by underlines);

M. Okue et al. / Tetrahedron Letters 43 (2002) 857–860
859
Cl
A) 2S,3S,4R
H2N
B) 2S,3R,4R
H2N
O
H2N
COOH
COOH
HO
2
COOH
6
5
4
2
2
NH
3
3
3
7
4
4
R
OH
R
OH
Cl
N
H
N
H
9
OH
COOH
HOOC
N
H
COOH
COOH
8
H
1
H-2 : 3.81 (d)
H-3 : 4.30 (d)
----------------
Figure 3. Revised structure of kaitocephalin (16).
(J=4.3 Hz)
------
9-epi-1
H-2 : 3.85 (d)
H-3 : 4.28 (d)
3-epi-1
H-2 : 3.78 (s)
H-3 : 4.30 (s)
(J=0 Hz)
----------------
---------
significant NOE correlation (Scheme 2). We suspected
that an epimerization has taken place during this two-
step conversion, and reproduced the reactions by using
12a and 12c. Surprisingly, both compounds gave the
same product (14) and a deuteration experiment (12c“
15) revealed that the epimerization had taken place at
C-2 in the second methylation step.
(J=5.5 Hz)
------
12a (R=H)
H-2 : 3.75 (d)
H-3 : 4.27 (d)
12b (R=H)
H-2 : 3.75 (s)
---------
H-3 : 4.33 (s)
(J=4.2 Hz)
(J=0 Hz)
------
H-2 : 4.16 (s)
H-3 : 4.41 (s)
(J=0 Hz)
Natural
kaitocephalin
From all of these results, we reached the conclusion
that the stereochemistry of kaitocephalin has been mis-
assigned due to the unexpected and unfortunate
epimerization at C-2 during derivatization and the cor-
rect stereochemistry of kaitocephalin must be
2R,3S,4R,7R,9S (Fig. 3). This revised stereochemistry
was confirmed by a total synthesis, as described in the
following paper.12
C) 2R,3S,4R
D) 2R,3R,4R
H2N
H2N
COOH
OH
COOH
2
2
3
3
4
4
OH
N
H
N
H
COOH
COOH
H-2 : 4.08 (s)
H-3 : 4.45 (s)
(J=0 Hz)
12d
12c
(not synthesized yet)
Figure 2. Comparison of 1H NMR spectral data of the
stereoisomers of kaitocephalin, model compounds 12, and
natural kaitocephalin.
Acknowledgements
This work was supported by a Grant-in-Aid for Scien-
tific Research (Priority Area (A) ‘Exploitation of Multi-
Element Cyclic Molecules’ and General Research (B);
No. 12460050) from the Japanese Ministry of Educa-
tion, Culture, Sports, Science and Technology. We also
thank Sankyo Foundation of Life Science, and Suntory
Institute for Bioorganic Research for financial support.
(3) compound 12c showed quite similar patterns to
those of the natural compound.
Thus, we assumed the correct stereochemistry of kaito-
cephalin to be 2R,3S,4R,7R,9S and verified the previ-
ous assignment. As described in Ref. 2, kaitocephalin
was transformed into protected lactam 13 to observe
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O
NOE
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M. Okue et al. / Tetrahedron Letters 43 (2002) 857–860
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2.20–2.27 (m, 4H), 2.37 (dd, 1H, J=5.5, 11.6 Hz), 3.65
(m, 1H), 3.85 (d, 1H, J=5.5 Hz), 4.28 (d, 1H, J=5.5
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1H), 3.61 (m, 1H), 3.81 (d, 1H, J=4.3 Hz), 4.30 (d,
1H, J=4.3 Hz), 4.33 (dd, 1H, J=5.5, 9.2 Hz), 7.62 (s,
2H). [Lit.1 1H NMR (500 MHz, D2O) l 1.61 (m), 2.01
(m), 2.06 (m), 2.12 (m), 2.28 (ddd, J=2.0, 6.0, 14.0
Hz), 2.41 (ddd, J=6.0, 7.0, 14.5 Hz), 3.70 (m), 4.16
1
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4.30 (brs, 1H), 4.45 (m, 1H), 7.82 (s, 2H).
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