Synthesis and antitumor activity of bisindolylmaleimide and amino acid ester conjugates

Article abstract of DOI:10.1080/10286020903413385

A series of novel bisindolylmaleimide and natural amino acid ester conjugates were synthesized and evaluated for their inhibitory activity against six tumor cell lines. Some compounds displayed interesting cytotoxic profiles. The most active compound 8e s

Full text of DOI:10.1080/10286020903413385

Journal of Asian Natural Products Research
Vol. 12, No. 1, January 2010, 36–42
ORIGINAL ARTICLE
Synthesis and antitumor activity of bisindolylmaleimide and
amino acid ester conjugates
Ke Wang, Xiang-Yan Li, Xiao-Guang Chen and Zhan-Zhu Liu*
Key Laboratory of Bioactive Substances and Resources Utilization of Chinese Herbal Medicine
(Ministry of Education), Institute of Materia Medica, Chinese Academy of Medical Sciences and
Peking Union Medical College, Beijing 100050, China
(Received 9 September 2009; final version received 13 October 2009)
A series of novel bisindolylmaleimide and natural amino acid ester conjugates
were synthesized and evaluated for their inhibitory activity against six tumor cell lines.
Some compounds displayed interesting cytotoxic profiles. The most active compound
8e showed inhibitory activity against several human cancer cell lines.
Keywords: bisindolylmaleimide; amino acid ester; conjugate; antitumor activity
1. Introduction
[5]. Some of the bisindolylmaleimide
derivatives are currently in clinical trial [6].
It is well known that amino acids are
the fundamental building blocks of bio-
logical systems and the biosynthetic
precursors of many natural products [7].
With the aim of increasing the water
solubility of indolocarbazoles and improv-
ing the binding properties to their possible
targets, amino acids have already been
applied in the modification of indolocar-
bazoles [8] and bisindolylmaleimide
derivatives [9]. In this paper, a series of
novel bisindolylmaleimide and natural
amino acid ester conjugate derivatives
are synthesized, and their antiproliferative
activities against six human tumor cell
lines are evaluated.
Staurosporine (Figure 1), a well-known
member of the indolocarbazole alkaloid,
was first isolated from Streptomyces
staurosporeus by Omura et al. [1]. It is an
appealing molecule with interesting bio-
logical properties, including antimicrobial
[1], hypotensive [2], cell cytotoxic [2], and
inhibition of protein kinase C [3]. However,
the lack of specificity limits its utilization
as a tool for studying protein kinase.
In addition, extremely poor water solubility
has plagued the development of stauros-
porine as atherapeutic agent. Staurosporine
has become a lead structure for novel
antitumor agents. For example, bisindolyl-
maleimides [4], in which the planarity of
the indolocarbazole ring system has been
disrupted, have valuable pharmacological
properties. Many derivatives of bisindolyl-
maleimides have been synthesized, and
their biological activities were evaluated
Synthesis of the target compounds
basically followed the Suzuki reaction-
based synthetic route established pre-
viously by our group [10]. Firstly,
*Corresponding author. Email: liuzhanzhu@imm.ac.cn
ISSN 1028-6020 print/ISSN 1477-2213 online
q 2010 Taylor & Francis
DOI: 10.1080/10286020903413385
http://www.informaworld.com

Journal of Asian Natural Products Research
37
HCT-8, and HT-29 cell lines, with the
inhibitory concentration (IC50) values of
14.7, 26.9, and 24.7 mM, respectively.
Compound 8c, which was derived from
L-tyrosine, showed cytotoxicity against
HCT-116, with the IC50 value of 30.1mM.
However, compounds 8b and 8d, which
were the derivativesofL-tryptophane andL-
DOPA, respectively, exhibited no cytotox-
icity against all the tumor cell lines tested.
As for the seven derivatives of the
aliphatic natural amino acids 8e–8k,
compound 8e, which was derived from L-
alanine, exhibited broad-spectrum inhibi-
tory activities against HepG2, HCT-116,
HCT-8, and k562 cell lines. However,
compounds 8f and 8h, which were derived
from L-methionine and L-serine, respect-
ively, were inactive to all tumor cell lines
screened at a concentration of 100 mM.
Compounds 8j and 8k were two achiral
derivatives which were synthesized from
glycine and b-aminopropionic acid,
respectively. Compound 8j exhibited cyto-
toxicity against Bxpc-3, with an IC50 value
of 25 mM, while compound 8k exhibited
inhibitory activities against HCT-116 and
HT-29 cell lines, with IC50 values of 27.7
and 25.4 mM, respectively. Bearing one
more carbonyl group on the side chain,
compound 8g, a derivative of L-glutaminic
acid, exhibited inhibitory activities against
HCT-8 and k562 cell lines, with IC50 values
of 24.8 and 24.9 mM, respectively, while
compound 8i, a derivative of L-asparagic
acid, exhibited inhibitory activities against
HCT-116, Bxpc-3, and k562 cell lines, with
IC50 values of 20.2, 26, and 8.7 mM,
respectively.
Figure 1. Structure of staurosporine.
preparation of the indole boronic acid (4)
from indole (1) was accomplished through
the following steps: protection of the
indole NH with TsCl and then treatment
with Hg(OAc)2 to yield compound 3;
reaction of 3 with BH3·THF followed by
hydrolytic workup gave 4 in a total yield of
82% [11]. Then, bisindolylmaleimide (5)
was synthesized by the Suzuki cross-
coupling of indole boronic acid (4) and N-
methyldibromomaleimide. With com-
pound 5 in hand, cleavage of both indole
nitrogen protective groups was readily
accomplished [12], and basic treatment of
imide 6 led to anhydride 7 [13]. Finally,
treatment of 7 with amino acid esters in
toluene under refluxing gave the desired
product 8 in a reasonable yield (Scheme 1).
2. Results and discussion
By the MTT assay, cytotoxicity of these
derivatives was evaluated against six
human cancer cell lines: human hepato-
cellular liver carcinoma (HepG2), human
colon carcinoma (HCT-116), human ileo-
cecal adenocarcinoma (HCT-8), human
colon carcinoma (HT-29), human pancrea-
tic adenocarcinoma (Bxpc-3), human
erythroleukemia (k562). The results of
the cytotoxicity studies are summarized in
Table 1.
In summary, we have incorporated a
number of natural amino acids into the
bisindolylmaleimide skeleton, and the
derivatives exhibited antiproliferative
activity and selectivity against six human
cancer cell lines. Further studies on the
mechanism of action and the structure–
activity relationship of this type of
compounds are in progress and will be
reported in due course.
Among the four derivatives of aromatic
amino acids 8a–8d, compound 8a, which
was derived from L-phenylalanine, exhib-
ited inhibitory activity against HepG2,

38
K. Wang et al.
Scheme 1. Reagents and conditions: (a) TsCl, KOH, H2O, Bu4NHSO4, 96%; (b) Hg(OAc)2,
AcOH, H2O, HClO4(cat), 99%; (c) 1. BH3·THF; 2. H2O, 85%; (d) N-methyldibromomaleimide,
Pd(PPh3)4, MeOH, PhH, 2M Na2CO3, 10 h, 46%; (e) K2CO3, CH3OH, H2O, 84%; (f) KOH, EtOH,
82%; (g) amino acid ester, toluene, reflux.
3. Experimental
10 cm cells and the sodium D line
(589 nm) at 208C and concentration
indicated. NMR spectra were recorded on
a Varian Oxford 300 (1H: 300 MHz, 13C:
75 MHz) or Varian Oxford 400 (1H:
400 MHz, 13C: 100 MHz), chemical shifts
(d) are expressed in ppm, and the
3.1 General experimental procedures
Melting points were recorded on a Yanaco
melting point apparatus and are uncor-
rected. Optical rotations were measured on
a Perkin-Elmer Polarimeter 341LC using

Journal of Asian Natural Products Research
39
Table 1. In vitro cytotoxicity of compounds 8a–8k (IC50, mM).
Compound
HepG2
HCT-116
HCT-8
HT-29
Bxpc-3
k562
8a
8c
8e
8g
8i
14.7
–
26.9
–
22.8
24.8
–
24.7
–
–
–
–
–
–
–
–
–
6.1
24.9
8.7
–
–
14.0
–
–
–
30.1
31.1
–
20.2
–
–
26.0
25.0
–
8j
8k
–
–
–
25.4
–
27.7
–
Note: The IC50 values represent the compound concentration (mM) required to inhibit tumor cell proliferation
by 50%.
following abbreviations are used: singlet
(s), doublet (d), triplet (t), doubled doublet
(dd), multiplet (m). HRMS were carried
out by Agilent LC/MSD TOF.
3.2.2 Compound 8b
Using 7 and (L)-trytophan methyl ester as
starting materials, the title compound 8b
was obtained as a red solid; mp .2508C;
1
20
½aꢀD : 2123.0 (c ¼ 1.0, CH3OH); H NMR
(300 MHz, CD3OD): d (ppm) 3.57 (d,
J ¼ 3.6 Hz), 3.72 (s, 3H), 5.08 (dd, 1H,
J ¼ 6.3, 9.0 Hz), 6.51(m, 4H), 6.86(m, 5H),
3.2 General procedure for compounds
8a–k
7.21 (m, 3H), 7.42 (s, 1H), 7.44 (s, 2H); 13
C
To a solution of anhydride 7 (30 mmol) in
toluene (15 ml), amino acid ester
(36 mmol) was added. The mixture was
refluxed for 12 h. After cooling, the solvent
was removed under reduced pressure, and
the residue was purified by flash chroma-
tography to give the desired compound.
NMR (75 MHz, DMSO-d6): d (ppm) 25.9,
53.2, 54.5, 107.4, 111.3,112.3, 119.2, 119.9,
120.6, 122.3, 122.6, 123.0, 124.5, 126.9,
128.7, 129.9, 137.7, 172.0 (CvO), 173.1
(CvO); HR-ESI-MS: m/z 528.1779
[MþH]þ (calcd for C32H24N4O4,
528.1792).
3.2.1 Compound 8a
3.2.3 Compound 8c
Using 7 and (L)-phenylalanine methyl ester
as starting materials, the title compound 8a
was obtained as a red solid; mp .2508C;
Using 7 and (L)-tyrosine methyl ester as
startingmaterials, thetitlecompound8cwas
obtained as a red solid; mp 217.1–221.38C;
1
1
20
20
½aꢀD : 293.3 (c ¼ 1.0, CH3OH); H NMR
(300MHz, DMSO-d6): d (ppm) 3.45 (m,
2H), 3.72 (s, 3H), 5.18 (dd, 1H, J ¼ 11.4,
4.8 Hz), 6.67(m, 4H), 6.97(m, 2H), 7.15(m,
1H), 7.22 (m, 4H), 7.34 (d, 2H, J ¼ 8.4 Hz),
7.67 (d, 2H, J ¼ 3.0 Hz), 11.69 (d, 2H,
J ¼ 2.7 Hz); 13C NMR (100MHz, DMSO-
d6): d (ppm) 34.1, 52.6, 59.7, 105.2, 111.8,
119.5, 120.8, 121.7, 125.1, 126.7, 127.2,
128.3, 128.9, 129.4, 136.0, 137.1, 169.6
(CvO), 170.7 (CvO); HR-ESI-MS: m/z
490.1751 [MþH]þ (calcd for C30H24N3O4,
490.1761).
½aꢀD : 2171.0 (c ¼ 1.0, CH3OH); H NMR
(300MHz, CDCl3): d (ppm) 3.49 (m, 2H),
3.81(s, 3H), 5.07(dd, 1H, J ¼ 10.2, 6.0 Hz),
6.67 (d, 2H, J ¼ 8.7 Hz), 6.76 (m, 2H), 6.90
(d, 2H, J ¼ 5.2 Hz), 7.08 (m, 4H), 7.31 (d,
2H, J ¼ 8.4 Hz), 7.65 (d, 2H, J ¼ 2.7 Hz),
8.48 (s, 2H); 13C NMR (75 MHz, CD3OD):
d (ppm) 35.0, 53.2, 54.9, 107.4, 112.4,
116.3, 120.7, 122.5, 123.0, 126.9, 128.6,
129.2, 130.0, 131.3, 137.7, 157.2, 171.7
(CvO), 173.1 (CvO); HR-ESI-MS: m/z
506.1708 [MþH]þ (calcd for C30H24N3O5,
506.1710).

40
K. Wang et al.
13C NMR (100 MHz, CD3OD): d (ppm)
3.2.4 Compound 8d
20.9, 24.8, 29.6, 53.1, 53.2, 107.5, 107.6,
112.3, 112.4, 120.6, 120.7, 122.6, 122.7,
122.9, 123.0, 127.0, 127.0, 128.5, 128.6,
130.0, 130.3, 137.7, 137.7, 172.1 (CvO),
173.3 (CvO), 174.1 (CvO); HR-ESI-MS:
m/z 474.1577 [MþH]þ (calcd for
C26H24N3O4S, 474.1488).
Using 7 and (L)-doba methyl ester as
starting materials, the title compound 8d
was obtained as a red solid; mp .2508C;
1
20
½aꢀD : 2162.4 (c ¼ 1.0, CH3OH); H NMR
(300 MHz, CDCl3): d (ppm) 3.44 (m, 2H),
3.81 (s, 3H), 5.07 (dd, 1H, J ¼ 10.2,
5.7 Hz), 5.18 (s, 1H), 5.31 (s, 1H), 6.72
(m, 5H), 6.91 (d, 2H, J ¼ 8.1 Hz), 7.07 (t,
2H, J ¼ 8.4 Hz), 7.28 (d, 2H, J ¼ 8.1 Hz),
3.2.7 Compound 8g
7.59 (d, 2H, J ¼ 1.5 Hz), 8.49 (s, 2H); 13
C
Using 7 and (L)-glutamic dimethyl ester as
starting materials, the title compound 8g
was obtained as a red solid; mp .2508C;
NMR (75 MHz, CD3OD): d (ppm) 35.2,
53.2, 54.9, 107.4, 112.4, 116.4, 117.3,
120.7, 121.6, 122.6, 123.0, 126.9, 128.6,
130.0, 137.7, 145.1, 146.2, 171.8 (CvO),
173.1 (CvO); HR-ESI-MS: m/z 522.1668
[MþH]þ (calcd for C30H24N3O6,
522.1660).
1
20
½aꢀD : þ 15.0 (c ¼ 1.0, CH3OH); H NMR
(300 MHz, CD3OD): d (ppm) 2.40 (m, 4H),
3.48 (s, 3 H), 3.66 (s, 3H), 4.85 (dd, 1H,
J ¼ 9.0, 5.4 Hz), 6.53 (m, 2H), 6.78 (m,
2H), 6.90 (m, 2H), 7.26 (m, 2H), 7.67 (s,
2H); 13C NMR (75 MHz, CD3OD): d (ppm)
25.5, 31.6, 52.2, 52.5, 53.2, 107.5, 112.4,
120.7, 122.6, 123.1, 127.0, 128.6, 130.2,
137.7, 171.6 (CvO), 173.1 (CvO), 174.8
(CvO); HR-ESI-MS: m/z 486.1750
[MþH]þ (calcd for C27H24N3O6,
486.1660).
3.2.5 Compound 8e
Using 7 and (L)-alanine methyl ester as
starting materials, the title compound 8e
was obtained as a red solid; mp 211.5–
20
214.78C; ½aꢀD : þ 60.0 (c ¼ 1.0, CH3OH);
1H NMR (300 MHz, DMSO-d6): d (ppm)
1.56 (d, 3H, J ¼ 6.9 Hz), 3.67 (s, 3H), 4.96
(q, 1H, J ¼ 7.2 Hz), 6.60 (m, 2H), 6.77 (d,
2H, J ¼ 8.1 Hz), 6.97 (m, 2H), 7.36 (d, 2H,
J ¼ 8.7 Hz), 7.78 (d, 2H, J ¼ 3.0 Hz),
11.72 (d, 2H, J ¼ 2.4 Hz); 13C NMR
(100 MHz, CD3OD): d (ppm) 15.7, 24.2,
53.2, 107.6, 112.4, 120.7, 122.6, 123.0,
127.1, 128.7, 130.1, 137.7, 172.6 (CvO),
173.0 (CvO); HR-ESI-MS: m/z 414.1457
[MþH]þ (calcd for C24H20N3O4,
414.1448).
3.2.8 Compound 8h
Using 7 and (L)-serine methyl ester as
starting materials, the title compound 8h
was obtained as
a red solid; mp
20
202.4–207.58C; ½aꢀD : þ 42.0 (c ¼ 1.0,
CH3OH); 1H NMR (300 MHz, CD3OD): d
(ppm) 3.71 (s, 3H), 4.17 (m, 2H), 4.96 (dd,
1H, J ¼ 8.1, 6.6 Hz), 6.54 (t, 2H,
J ¼ 8.1 Hz), 6.77 (d, 2H, J ¼ 8.1 Hz),
6.92 (t, 2H, J ¼ 8.1 Hz), 7.26 (d, 2H,
J ¼ 8.1 Hz), 7.71 (s, 2H); 13C NMR
(100 MHz, CD3OD): d (ppm) 53.1, 55.5,
60.4, 107.6, 112.4, 120.7, 122.6, 123.0,
127.1, 128.8, 130.1, 137.7, 170.3 (CvO),
173.4 (CvO); HR-ESI-MS: m/z 430.1394
[MþH]þ (calcd for C24H20N3O5,
430.1403).
3.2.6 Compound 8f
Using 7 and (L)-methionine methyl ester as
starting materials, the title compound 8f
was obtained as a red solid; mp 209.0–
20
212.08C; ½aꢀD : þ 6.0 (c ¼ 1.0, CH3OH);
1H NMR (300 MHz, DMSO-d6): d (ppm)
2.04 (s, 3H), 2.38 (m, 2H), 2.56 (m, 2H),
3.68 (s, 3H), 5.02 (dd, 1H, J ¼ 8.7, 6.3 Hz),
6.63 (m, 2H), 6.78 (d, 2H, J ¼ 7.8 Hz), 6.98
(m, 2H), 7.36 (d, 2H, J ¼ 8.1 Hz), 7.79 (d,
2H, J ¼ 2.7 Hz), 11.73 (d, 2H, J ¼ 2.4 Hz);
3.2.9 Compound 8i
Using 7 and (L)-aspartic acid dimethyl
ester as starting materials, the title
compound 8i was obtained as a red solid;

Journal of Asian Natural Products Research
41
20
mp .2508C; ½aꢀD : þ 12.0 (c ¼ 1.0,
3.3 Cell cultures and proliferation
assays
1
CH3OH); H NMR (300 MHz, CDCl3): d
(ppm) 3.41 (q, 2H, J ¼ 6.3 Hz), 3.72 (s,
3H), 3.78 (s, 3H), 5.44 (dd, 1H, J ¼ 6.3,
8.4 Hz), 6.74 (m, 2H), 6.94 (d, 2H,
J ¼ 8.1 Hz), 7.06 (m, 2H), 7.31 (d, 2H,
J ¼ 8.1 Hz), 7.75 (d, 2H, J ¼ 3.0 Hz), 8.62
(s, 2H); 13C NMR (100 MHz, DMSO-d6): d
(ppm) 33.7, 47.7, 51.8, 52.9, 105.4, 111.9,
119.5, 121.0, 121.8, 125.2, 126.8, 129.6,
136.0, 169.1 (CvO), 170.4 (CvO), 170.5
(CvO); HR-ESI-MS: m/z 472.1501
[MþH]þ (calcd for C26H22N3O6,
472.1503).
The tumor cell lines panel consisted of
HepG2, HCT-8, HCT-116, HT-29, Bxpc-
3, and k562. Human cancer cells were
cultured in RPMI-1640 or DMEM/F12
supplemented with 10% fetal bovine
serum, containing penicillin and strepto-
mycin at 378C and humidified at 5% CO2.
Briefly, cells were plated in the appropriate
media on 96-well plates in a total volume
of 100 ml at a density of 1–2.5 £ 104
cells/ml and were allowed to adhere for
24 h before treatment with tested drugs in
DMSO solution (1025, 1026, 1027 mol/l
final concentration). Triplicate wells were
treated with media and agents. Cell
viability was assayed after 96 h continuous
drug exposure with a tetrazolium com-
pound [3-(4,5-dimethylthiazol-2-yl)-5-(3-
carboxymethoxyphenyl)-2-(4-sulfophe-
nyl)-2H-tetrazolium salt; MTT (0.5 mg/
ml, 100 ml); Ameresco Corp., Anchorage,
AK, USA] in a fresh medium. After the
medium was removed, 150 ml of DMSO
was added to each well. The plates were
gently agitated until the color reaction was
uniform and an OD570 was determined
using a microplate reader (Wellscan MK3,
Labsystems Dragon, Helsinki, Finland).
Microsoft_Excel 2003 was used for data
analysis. Media-only treated cells served
as the indicator of 100% cell viability.
The 50% IC50 was defined as the
concentration that reduced the absorbance
of the untreated wells by 50% of the
vehicle in the MTT assay. Assays were
performed in triplicate on three indepen-
dent experiments.
3.2.10 Compound 8j
Using 7 and glycine ethyl ester as starting
materials, the title compound 8j was
obtained as a red solid; mp 241.3–
246.08C; 1H NMR (300 MHz, DMSO-
d6): d (ppm) 1.22 (t, 3H, J ¼ 7.2 Hz), 4.16
(q, 2H, J ¼ 7.2 Hz), 4.39 (s, 2H), 6.63 (m,
2H), 6.77 (d, 2H, J ¼ 8.1 Hz), 6.98 (m,
2H,), 7.36 (d, 2H, J ¼ 8.1 Hz), 7.78 (d, 2H,
J ¼ 2.7 Hz), 11.72 (d, 2H, J ¼ 2.4 Hz);
13C NMR (100 MHz, DMSO-d6): d (ppm)
14.0, 30.6, 61.3, 105.4, 111.8, 119.5,
120.9, 121.7, 125.2, 127.0, 129.4, 136.0,
168.0 (CvO), 170.9 (CvO); HR-ESI-
MS: m/z 414.1457 [MþH]þ (calcd for
C24H20N3O4, 414.1448).
3.2.11 Compound 8k
Using 7 and (b)-alanine methyl ester as
starting materials, the title compound 8k
was obtained as a red solid; mp 214.3–
216.78C; 1H NMR (300 MHz, CD3
COCD3): d (ppm) 2.77 (t, 2H, J ¼
7.5 Hz), 3.64 (s, 3H), 3.93 (t, 2H, J ¼
7.5 Hz), 6.60 (m, 2H), 6.94 (m, 4H), 7.39
(m, 2H), 7.85 (d, 2H, J ¼ 2.1 Hz), 10.83 (s,
2H); 13C NMR (100 MHz, CD3COCD3): d
(ppm) 33.6, 34.7, 51.9, 107.5, 112.4, 120.4,
122.3, 122.7, 126.8, 128.3, 129.9, 127.3,
171.9 (CvO), 172.4 (CvO); HR-ESI-MS:
m/z 414.1451 [MþH]þ (calcd for
C24H20N3O4, 414.1448).
References
[1] S. Omura, Y. Iwai, A. Hirano,
A. Nakagawa, J. Awaya, H. Tsuchiya,
Y. Takahashi, and R. Masumam,
J. Antibiot. 30, 275 (1977).
[2] (a) S. Cesar, M. Carmen, and A.S. Jose,
Nat. Prod. Rep. 23, 1007 (2006).
(b) S. Omura, Y. Iwai, and A. Hirano,
Japan Kokai 78 73,501, Chem. Abstr. 89,

42
K. Wang et al.
178086b (1978). (c) S. Moura and A. Iwai,
Ger Offen. 2,745,326, Chem. Abstr. 89,
(b) M. Sodeoka, M. Katoh, and M. Fujita,
PCT Int. Appl. WO 2005068455 (2005).
(c) H. Osswald and R. Reck, Eur. Patent
Appl. EP 397060 (1990). (d) U.H.
Dhingra, D.M. Huryn, and D.D. Keith,
PCT Int. Appl. WO 9804551 (1998).
[7] M. Zhao, L. Bi, W. Wang, C. Wang, J. Ju,
and S. Peng, Bioorg. Med. Chem. Lett. 14,
6998 (2006).
[8] (a) P. Moreau, M. Sancelme, C. Bailly,
S. Leonce, A. Pierre, J. Hickman,
B. Pfeiffer, and M. Prudhomme, Eur.
J. Med. Chem. 36, 887 (2001). (b) Z. Li,
F. Zhai, L. Zhao, Q. Guo, and Q. You,
Bioorg. Med. Chem. Lett. 19, 406 (2009).
[9] G. Xie, J. Zimmermann, T. Meyer, and
J.W. Lown, Bioorg. Med. Chem. Lett. 5,
497 (1995).
58348 (1978).
[3] (a) H. Kase, K. Iwahashi, and Y. Matsuda,
J. Antibiot. 39, 1059 (1986).
(b) T. Tamaoki, H. Nomoto, I. Takahishi,
Y. Kato, M. Morimoto, and F. Tomita,
Biochem. Biophys. Res. Commun. 135,
397 (1986).
[4] (a) S. Roy, S. Roy, and G.W. Gribble,
Org. Lett. 8, 4975 (2006). (b) S. Bartlett
and A. Nelson, Chem. Commun. 9, 1112
(2004).
[5] (a) M.F. Brana, L. Anorbe, G. Tarrason,
F. Mitjans, and J. Piulats, Bioorg. Med.
Chem. Lett. 11, 2701 (2001).
(b) D. Toullec, P. Pianetti, H. Coste,
P. Bellevergue, P.T. Grand, M. Ajakane,
V. Baudet, P. Boissin, E. Boursier,
F. Loriolle, L. Duhamel, D. Charon, and
J. Kirilovsky, J. Biol. Chem. 266, 15771
(1991). (c) R.A. Bit, P.D. Davis,
L.H. Elliott, W. Harris, C.H. Hill,
E. Keech, H. Kumar, G. Lawton,
A. Maw, J.S. Nixon, J.D.R. Vesey,
J. Wadsworth, and S.E. Wilkinson,
J. Med. Chem. 36, 21 (1993).
[10] K. Wang and Z.Z. Liu, Synth. Commun.
(2009) in press.
[11] N.K. Garg, R. Sarpong, and B.M. Stoltz,
J. Am. Chem. Soc. 124, 13179 (2002).
[12] M.M. Faul, K.A. Sullivan, and
L.L. Winneroski, Synthesis 12, 1511
(1995).
[13] M. Brenner, H. Rexhausen, B. Steffan,
and W. Steglich, Tetrahedron 44, 2887
(1988).
[6] (a) P.D. Davis, C.H. Hill, and G. Lawton,
Eur. Patent Appl. EP 328026 (1989).