Synthesis of phidianidines A and B

Article abstract of DOI:10.1021/jo300449n

Reaction of a substituted indole-3-acetyl chloride with N-5-azidopentyl-N′-hydroxyguanidine generated a substituted 3-(5-azidopentylamino)-5-((indol-3-yl)methyl)-1,2,4-oxadiazole. Reduction of the azide with zinc and ammonium formate afforded the amine, which was elaborated to the guanidine, completing short and efficient syntheses of the cytotoxic natural products phidianidines A and B in 19% overall yield by a convergent route that will make analogues readily available for biological evaluation. Initial screening in the NCI 60 cell line at 10^-5 M indicated that the bromine on the indole is necessary for activity and that the amine precursor to phidianidine A is more potent than phidianidine A.

Full text of DOI:10.1021/jo300449n

Note
pubs.acs.org/joc
Synthesis of Phidianidines A and B
Hong-Yu Lin and Barry B. Snider*
Department of Chemistry MS 015, Brandeis University, Waltham, Massachusetts 02454-9110, United States
S
* Supporting Information
ABSTRACT: Reaction of a substituted indole-3-acetyl chloride with N-
5-azidopentyl-N′-hydroxyguanidine generated a substituted 3-(5-azido-
pentylamino)-5-((indol-3-yl)methyl)-1,2,4-oxadiazole. Reduction of the
azide with zinc and ammonium formate afforded the amine, which was
elaborated to the guanidine, completing short and efficient syntheses of
the cytotoxic natural products phidianidines A and B in 19% overall yield
by a convergent route that will make analogues readily available for
biological evaluation. Initial screening in the NCI 60 cell line at 10−5
M
indicated that the bromine on the indole is necessary for activity and that
the amine precursor to phidianidine A is more potent than phidianidine
A.
uo, Gavagnin, and co-workers recently reported the
isolation of the guanidine-containing natural products
phidianidines A (1a) and B (1b) as the trifluoroacetate salts
from the shell-less marine opisthobranch mollusk Phidiana
militaris (see Scheme 1).1 Despite their relatively simple
dodine) has been in widespread use since 1956 as a fungicide
and may act by membrane disruption.3 As part of an ongoing
program in the synthesis of structurally novel guanidine-
containing natural products,4 we planned to prepare
phidianidines A (1a) and B (1b) by coupling of the appropriate
ethyl indole-3-acetate (2a or 2b) with hydroxyguanidine 3
containing a suitably protected nitrogen on the other end of the
five-carbon side chain. There is limited precedent for the
synthesis of 3-amino-1,2,4-oxadiazoles by coupling of esters
with hydroxyguanidines,5 and nature may use a related route for
the biosynthesis of the phidianidines.
G
Scheme 1. Retrosynthesis of Phidianidines A (1a) and B
(1b)
We chose to use hydroxyguanidine 8 with an azidopentyl
side chain because the azide group can be easily elaborated into
the guanidine of the phidianidines and a wide variety of
analogues (see Scheme 2). Furthermore, 5-azido-1-pentan-
amine (6) is readily available in quantity in 85% yield by
selective reduction of diazide 5 by Kim’s procedure with
triphenylphosphine in a two-phase system.6a After mono-
reduction of the diazide in Et2O/EtOAc, the azido amine is
protonated and partitions into the 5% HCl solution, thereby
preventing reduction of the second azide. Reaction of amine 6
with cyanogen bromide7 in CH2Cl2 and aqueous NaHCO3
solution afforded somewhat unstable cyanamide 7, which was
treated with hydroxylamine hydrochloride8 and K2CO3 in
EtOH to give the unstable8d hydroxyguanidine 8, which was
used immediately for the next step.
Our initial attempt at 3-amino-1,2,4-oxadiazole synthesis
using models N-butyl-N′-hydroxyguanidine and ethyl phenyl-
acetate (3 equiv) with NaOEt (2−3 equiv) in EtOH at reflux
for 5 h proceeded in 50−70% yield based on the
hydroxyguanidine. However, the yield dropped to 0−15%
without both excess ester and NaOEt and the reaction
proceeded in only 30% yield with ethyl indole-3-acetate (3
structure, the phidianidines are of interest because they appear
to be the first natural products that contain an 1,2,4-oxadiazole
ring.2 Furthermore, they show significant cytotoxicity at 0.14−
5.42 μM concentrations against the highly proliferating C6 rat
glioma and HeLa epithelial cervical cancer cell lines and the
embryonic 3T3-L1 murine embryonic fibroblast and H9c2 rat
embryonic cardiac myoblast cell lines and are less toxic to the
less rapidly proliferating CaCo-2 epithelial colorectal adeno-
carcinoma cell line (35 to 100 μM).1 The mode of action is not
known, but the closely related dodecylguanidinium acetate (4,
Received: March 1, 2012
© XXXX American Chemical Society
A
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The Journal of Organic Chemistry
Note
ring closure to form the 1,2,4-oxadiazole and the poor solubility
of 8 and 10b in ClCH2CH2Cl.
Scheme 2. Synthesis of Phidianidines A (1a) and B (1b)
Reduction of the azide group of 11b was complicated by the
sensitivity of the 1,2,4-oxadiazole.2 Attempted hydrogenation
over Pd destroyed the 1,2,4-oxadiazole ring. Reduction with
Ph3P was successful, but removal of the phosphine oxide
byproduct was difficult. Eventually we found that reduction of
the azide with activated zinc10 and ammonium formate in
MeOH for 7 h at 25 °C proceeded cleanly to give polar amine
12b that was used without purification.11a Coupling of 12b with
Boc-protected S-methylisothiourea 13 using Et3N and
AgNO34b,12 in DMF for 2 h at 0 °C and 5 h at 25 °C afforded
protected guanidine 14b in 60% yield from 11b. Deprotection
of 14b by stirring in 10:1 CH2Cl2/TFA for 8 h at 25 °C
removed the Boc protecting groups providing phidianidine B
1
(1b) in 92% yield. The H and 13C NMR spectral data of
phidianidine B in both CD3OD and DMSO-d6 are identical to
those reported for the natural product1 confirming the presence
of the 1,2,4-oxadiazole ring.
Phidianidine A (1a) was prepared by an analogous sequence
of steps from 6-bromoindole-3-acetic acid (9a).13 Acid chloride
10b was coupled with hydroxyguanidine 8 to give 11a in 61%
yield from 9a and 39% yield from 6. Reduction of the azide of
11a with zinc and ammonium formate afforded amine 12a
which was coupled with 13 using Et3N and AgNO3 in DMF to
give protected guanidine 14a in 61% yield from 11a.
Deprotection of 14a in 10:1 CH2Cl2/TFA provided
1
phidianidine A (1a) in 93% yield with H and 13C NMR
spectral data in both CD3OD and DMSO-d6 identical to those
reported for the natural product.1
Initial screening in the NCI 60 cell line screen at 10−5
M
showed an average of 33% inhibition of cell growth with
phidianidine A (1a). The amine precursor 12a was more potent
with an average of 81% inhibition of the 60 cell lines.
Phidianidine B (1b) and amine 12b were both much less
effective with an average of 5% inhibition of cell growth. These
results indicate that the bromine substituent on the indole is
important for activity and that the amine of 12a is more
effective than the guanidine of 1a. Full details for these four
compounds with all 60 cell lines are provided in the Supporting
Information.
In conclusion, N-5-azido-1-pentanamine (6) was elaborated
to N-5-azidopentyl-N′-hydroxyguanidine (8) in two steps.
Reaction of 8 with indole-3-acetyl chloride 10a or 10b afforded
3-(5-azidopentylamino)-5-((indol-3-yl)methyl)-1,2,4-oxadia-
zoles 11a and 11b in 61−63% yield. Reduction of the azides
with zinc and ammonium formate afforded amines 12a and
12b, which were elaborated to the guanidine, completing short
and efficient syntheses of the cytotoxic natural products
phidianidines A (1a) and B (1b) in 19% overall yield by a
convergent route that will make analogues readily available for
biological evaluation.
equiv) and NaOEt (3 equiv). These initial results indicated that
an ester is not sufficiently reactive to couple with a
hydroxyguanidine unless it is used in large excess. We therefore
chose use an acid chloride rather than an ester to form the 3-
amino-1,2,4-oxadiazole, a protocol that has been reported in the
recent patent literature.9
Indole-3-acetic acid (9b) was reacted with oxalyl chloride
and catalytic DMF in CH2Cl2 to give acid chloride 10b, which
was treated with 1.5 equiv of freshly prepared 8 and Et3N in
CH2Cl2 for 2 h at 25 °C. The solution was concentrated, and a
solution of the residue containing the initial coupling product
in 1,2-dichloroethane was heated at 80 °C for 2 h to form the 3-
amino-1,2,4-oxadiazole giving 11b in 63% yield from 9b and
40% yield from 6. The change of solvents after the initial
coupling was necessitated by the high temperature needed for
EXPERIMENTAL SECTION
■
General Experimental Methods. Reactions were conducted in
flame- or oven-dried glassware under a nitrogen atmosphere and were
stirred magnetically. The phrase “concentrated” refers to removal of
solvents by means of a rotary-evaporator attached to a diaphragm
pump (15−60 Torr) followed by removal of residual solvents at <1
Torr with an vacuum pump. Flash chromatography was performed on
silica gel 60 (230−400 mesh). Analytical thin-layer chromatography
(TLC) was performed using silica gel 60 F-254 precoated glass plates
(0.25 mm). TLC plates were analyzed by short-wave UV illumination
or by dipping in CAM stain (40 g of ammonium molybdate, 1.6 g of
B
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Note
ceric ammonium molybdate, 80 mL of concentrated sulfuric acid and
720 mL of water) and heating on a hot plate, or by spraying with
permanganate solution (5 g KMnO4 in 495 mL water). THF and ether
were dried and purified by distillation from sodium/benzophenone.
Et3N, pyridine, acetonitrile, and benzene were distilled from CaH2. 1H
and 13C NMR spectra were obtained on a 400 MHz spectrometer in
CDCl3 with tetramethylsilane as internal standard unless specifically
indicated. Chemical shifts are reported in δ (ppm downfield from
tetramethylsilane). Coupling constants are reported in Hz with
multiplicities denoted as s (singlet), d (doublet), t (triplet), q
(quartet), p (pentet), m (multiplet), and br (broad). IR spectra were
acquired on an FT-IR spectrometer and are reported in wave numbers
(cm−1). High-resolution mass spectra were obtained using electrospray
ionization (ESI) analyzed by quadrupole time-of-flight (QTof).
5-Azido-1-pentanamine (5).6 To a solution of 56,14 (1.1 g, 7.1
mmol) in 5 mL of Et2O were added 5 mL of EtOAc and 9 mL of 5%
HCl aqueous solution successively. To the resulting mixture at 0 °C
was added PPh3 (1.9 g, 7.2 mmol, 1.0 equiv) in small portions over 1
h. The mixture was stirred at room temperature for 30 h. HCl solution
(1 M, 10 mL) was added, the layers were separated, and the organic
layer was discarded. The aqueous layer was washed with CH2Cl2 (20
mL × 2), which was discarded, and neutralized with 6 M NaOH until
the pH reached 12. The basic aqueous layer was saturated with NaCl
and extracted with CH2Cl2 (20 mL × 4). The combined organic layers
were dried over Na2SO4 and concentrated to give 780 mg (85% from
EtOAc) gave 410 mg (63% from 9b, 40% from 6) of 11b as a beige
solid: mp 58−59 °C; 1H NMR 8.16 (br, 1, NH) 7.63 (br d, 1, J = 8.0),
7.37 (br d, 1, J = 8.0), 7.22 (br dd, 1, J = 8.0, 8.0), 7.20 (br s, 1), 7.15
(br dd, 1, J = 8.0, 8.0), 4.29 (br, 1, NH), 4.22 (s, 2), 3.25 (t, 2, J = 6.8),
3.23 (dt, 2, J = 6.8, 6.8), 1.68−1.56 (m, 4), 1.49−1.37 (m, 2); 13C
NMR 177.3, 168.6, 136.1, 126.7, 123.0, 122.5, 119.9, 118.7, 111.3,
108.1, 51.2, 43.1, 28.9, 28.5, 23.9, 23.4; IR (neat) 3306 (br), 2092,
1703, 1661, 1594, 1456, 1339, 1247, 740; HRMS (ESI) calcd for
C16H20N7O (MH+) 326.1729, found 326.1727.
N-(5-Azidopentyl)-5-[(6-bromo-1H-indol-3-yl)methyl]-1,2,4-
oxadiazol-3-amine (11a). To a suspension of 9a (254 mg, 1.0
mmol) in 10 mL of CH2Cl2 at 0 °C were added DMF (2 drops) and
oxalyl chloride (0.26 mL, 3.0 mmol, 3.0 equiv) successively. The
mixture was stirred at 0 °C for 1.5 h and concentrated to give 320 mg
of crude 10a, which was used for the synthesis of 11a without further
purification.
To a solution of freshly prepared crude 8 (see preparation of 11b
for procedure) (254 mg, from 200 mg (1.6 mmol) of 6) and NEt3
(0.42 mL, 3.0 mmol) in 5 mL of CH2Cl2 at 0 °C was added a solution
of crude 10a (320 mg, from 254 mg (1.0 mmol) of 9a) in 2 mL of
CH2Cl2. The reaction mixture was stirred at room temperature for 2 h
and concentrated. To the residue was added 8 mL of ClCH2CH2Cl.
The mixture was heated at 80 °C for 2 h and concentrated. To the
residue was added 15 mL of saturated NaHCO3 solution. The mixture
was extracted with CH2Cl2 (15 mL × 4). The combined organic layers
were dried over Na2SO4 and concentrated. Flash chromatography of
the residue on silica gel (6:1 CH2Cl2/EtOAc) gave 246 mg (61% from
9a, 39% from 6) of 11a as a beige solid: mp 86−87 °C; 1H NMR 8.21
(br, 1, NH) 7.51 (br s, 1), 7.47 (br d, 1, J = 8.4), 7.24 (br d, 1, J = 8.4),
7.16 (br s, 1), 4.29 (br, 1, NH), 4.18 (s, 2), 3.26 (t, 2, J = 6.8), 3.23 (dt,
2, J = 6.8, 6.8), 1.68−1.56 (m, 4), 1.49−1.37 (m, 2); 13C NMR 177.0,
168.6, 136.9, 125.6, 123.7, 123.2, 120.0, 116.1, 114.2, 108.3, 51.2, 43.1,
28.9, 28.5, 23.9, 23.3; IR (neat) 3296 (br), 2092, 1594, 1453, 1330,
1236, 893, 803, 735; HRMS (ESI) calcd for C16H19BrN7O (MH+)
404.0834, found 404.0833.
N-[5-[[Bis[[(1,1-dimethylethoxy)carbonyl]amino]-
methylene]amino]pentyl]-5-[(1H-indol-3-yl)methyl]-1,2,4-oxa-
diazol-3-amine (14b). To a solution of 11b (410 mg, 1.26 mmol) in
8 mL of MeOH were added ammonium formate (397 mg, 6.30 mmol,
5.0 equiv) and activated zinc10 (328 mg, 5.04 mmol, 4.0 equiv)
successively. The mixture was stirred at room temperature for 7 h. The
reaction was quenched by addition of 1 M NaOH (15 mL). The
mixture was extracted with CH2Cl2 (15 mL × 4). The combined
organic layers were dried over Na2SO4 and concentrated to give 390
mg of crude 12b, which was used in the next step without purification:
1H NMR 8.14 (br, 1, NH), 7.63 (br d, 1, J = 8.0), 7.38 (br d, 1, J =
1
5) of 6 as a colorless oil: H NMR 3.28 (t, 2, J = 6.8), 2.73 (t, 2, J =
6.8), 2.12 (br, 2, NH2), 1.67−1.58 (m, 2), 1.55−1.45 (m, 2), 1.48−
1.38 (m, 2); 13C NMR 51.3, 41.7, 32.7, 28.6, 24.0; IR (neat) 3318 (br),
2933, 2862, 2089, 1558, 1469, 1390, 1300, 1259. The spectral data are
identical to those previously reported.6
N-(5-Azidopentyl)-5-[(1H-indol-3-yl)methyl]-1,2,4-oxadiazol-
3-amine (11b). To a suspension of 9b (350 mg, 2.0 mmol) in 10 mL
of CH2Cl2 at 0 °C were added DMF (2 drops) and oxalyl chloride
(0.51 mL, 5.9 mmol, 3.0 equiv) successively. The mixture was stirred
at 0 °C for 1.5 h and concentrated to give 450 mg of crude 1H-indole-
3-acetyl chloride (10b), which was used for the synthesis of 11b
without further purification.
To a solution of 6 (400 mg, 3.1 mmol) in 6 mL of CH2Cl2 were
added NaHCO3 (1.6 g, 19 mmol, 6.0 equiv) and 7 mL of H2O
successively. To the resulting mixture was slowly added a solution of
BrCN (394 mg, 3.7 mmol, 1.2 equiv) in 2 mL of CH2Cl2 at 0 °C. The
mixture was stirred at 0 °C for 0.5 h and at room temperature for 1 h.
The reaction was quenched by addition of water (10 mL). The
mixture was extracted with CH2Cl2 (15 mL × 3). The combined
organic layers were dried over Na2SO4 and concentrated to give 460
mg (96% from 6) of >90% pure 7 as a somewhat unstable pale yellow
1
oil that can be stored for a few days at 0 °C: H NMR 3.73 (br, 1,
8.0), 7.22 (br dd, 1, J = 8.0, 8.0), 7.22 (br s, 1), 7.15 (br dd, 1, J = 8.0,
8.0), 4.26 (br, 1, NH), 4.22 (s, 2), 3.23 (dt, 2, J = 6.8, 6.8), 2.68 (t, 2, J
= 6.8), 1.68−1.54 (m, 2), 1.52−1.32 (m, 4).
NH), 3.31 (t, 2, J = 6.8), 3.10 (dt, 2, J = 6.8, 6.8), 1.71−1.59 (m, 4),
1.52−1.42 (m, 2); 13C NMR 116.0, 51.1, 46.0, 29.1, 28.3, 23.4; IR
(neat) 3211 (br), 2937, 2865, 2215, 2090, 1454, 1351, 1246, 1159.
To a solution of crude 7 (460 mg, 3.0 mmol) in 10 mL of dry EtOH
were added NH2OH·HCl (252 mg, 3.6 mmol, 1.2 equiv) and K2CO3
(1.24 g, 9.0 mmol, 3.0 equiv) successively. The reaction mixture was
stirred at room temperature for 5 h, diluted with EtOAc (20 mL), and
filtered through Celite. The filtrate was dried over Na2SO4 and
concentrated to give 502 mg of crude 8 as an unstable pale yellow
sticky oil, which decomposed on storage at 0 °C overnight and should
To a solution of crude 12b (390 mg) in 8 mL of DMF were added
1,3-bis(tert-butoxycarbonyl)-2-methylthiopseudourea (13) (561 mg,
1.89 mmol) and NEt3 (1.40 mL, 10.1 mmol) successively. To the
resulting mixture at 0 °C was added AgNO3 (429 mg, 2.52 mmol) in
small portions. The reaction was stirred at 0 °C for 2 h and at room
temperature for 5 h. The reaction was quenched by addition of EtOAc
(50 mL) and filtered through Celite. The filtrate was washed with
saturated NaHCO3 (50 mL × 4), dried over Na2SO4, and
concentrated. Flash chromatography of the residue on silica gel (2:3
EtOAc/hexanes) gave 417 mg (61% from 11b) of 14b as a pale yellow
sticky oil: 1H NMR 11.49 (br s, 1, NH), 8.30 (br, 2, NH), 7.62 (br d,
1, J = 8.0), 7.37 (br d, 1, J = 8.0), 7.21 (br dd, 1, J = 8.0, 8.0), 7.20 (br
s, 1), 7.14 (br dd, 1, J = 8.0, 8.0), 4.33 (br, 1, NH), 4.22 (s, 2), 3.37
(dt, 2, J = 6.8, 6.8), 3.20 (dt, 2, J = 6.8, 6.8), 1.64−1.54 (m, 4), 1.50 (s,
9), 1.49 (s, 9), 1.42−1.32 (m, 2); 13C NMR 177.2, 168.7, 163.6, 156.1,
153.3, 136.1, 126.7, 123.0, 122.3, 119.8, 118.7, 111.3, 108.0, 83.1, 79.3,
43.1, 40.7, 29.0, 28.6, 28.3 (3 C), 28.0 (3 C), 24.0, 23.4; IR (neat)
3324 (br), 1719, 1597, 1414, 1366, 1325, 1129, 1053, 912, 731;
HRMS (ESI) calcd for C27H40N7O5 (MH+) 542.3091, found
542.3085.
1
be used immediately in the next step: H NMR (DMSO-d6) 9.50 (br,
1, NH or OH), 7.49 (br, 1, NH or OH), 7.32 (br, 2, NH or OH), 3.33
(t, 2, J = 7.2), 3.14−3.06 (m, 2), 1.60−1.42 (m, 4), 1.38−1.26 (m, 2).
To a solution of freshly prepared crude 8 (502 mg, from 400 mg
(3.1 mmol) of 6) and NEt3 (0.83 mL, 6.0 mmol) in 10 mL of CH2Cl2
at 0 °C was added a solution of crude 10b (450 mg, from 350 mg (2.0
mmol) of 9b) in 4 mL of CH2Cl2. The reaction mixture was stirred at
room temperature for 2 h and concentrated. To the residue was added
15 mL of ClCH2CH2Cl. The mixture was heated at 80 °C for 2 h and
concentrated. To the residue was added 15 mL of saturated NaHCO3
solution. The mixture was extracted with CH2Cl2 (15 mL × 4). The
combined organic layers were dried over Na2SO4 and concentrated.
Flash chromatography of the residue on silica gel (6:1 CH2Cl2/
C
dx.doi.org/10.1021/jo300449n | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
N-[5-[[Bis[[(1,1-dimethylethoxy)carbonyl]amino]-
methylene]amino]pentyl]-5-[(6-bromo-1H-indol-3-yl)methyl]-
1,2,4-oxadiazol-3-amine (14a). To a solution of 11a (246 mg, 0.61
mmol) in 5 mL of MeOH were added ammonium formate (192 mg,
3.05 mmol, 5.0 equiv) and activated zinc10 (159 mg, 2.44 mmol, 4.0
equiv) successively. The mixture was stirred at room temperature for 7
h. The reaction was quenched by addition of 1 M NaOH (15 mL).
The mixture was extracted with CH2Cl2 (15 mL × 4). The combined
organic layers were dried over Na2SO4 and concentrated to give 228
mg of crude 12a, which was used in the next step without purification:
1H NMR 8.15 (br, 1, NH) 7.53 (br s, 1), 7.48 (br d, 1, J = 8.4), 7.24
(br d, 1, J = 8.4), 7.19 (br s, 1), 4.29 (br, 1, NH), 4.18 (s, 2), 3.23 (dt,
2, J = 6.8, 6.8), 2.68 (t, 2, J = 6.8), 1.68−1.54 (m, 2), 1.52−1.32 (m, 4).
To a solution of crude 12a (228 mg) in 4 mL of DMF were added
1,3-bis(tert-butoxycarbonyl)-2-methylthiopseudourea (13) (267 mg,
0.92 mmol) and NEt3 (0.70 mL, 5.06 mmol) successively. To the
resulting mixture at 0 °C was added AgNO3 (207 mg, 1.22 mmol) in
small portions. The reaction was stirred at 0 °C for 2 h and at room
temperature for 5 h. The reaction was quenched by addition of EtOAc
(30 mL) and filtered through Celite. The filtrate was washed with
saturated NaHCO3 (30 mL × 4), dried over Na2SO4, and
concentrated. Flash chromatography of the residue on silica gel (2:3
EtOAc/hexanes) gave 226 mg (60% from 11a) of 14a as a pale yellow
sticky oil: 1H NMR 11.50 (br s, 1, NH), 8.70 (br, 1, NH), 8.31 (br, 1,
NH), 7.52 (br s, 1), 7.47 (br d, 1, J = 8.0), 7.23 (br d, 1, J = 8.0), 7.16
(br s, 1), 4.47 (br, 1, NH), 4.18 (s, 2), 3.32 (dt, 2, J = 6.8, 6.8), 3.16
(dt, 2, J = 6.8, 6.8), 1.58−1.46 (m, 4), 1.49 (s, 18), 1.34−1.24 (m, 2);
13C NMR 176.9, 168.7, 163.5, 156.1, 153.3, 136.9, 125.7, 123.7, 123.1,
115.5, 108.8, 43.9, 42.5, 29.9, 29.7, 25.1, 24.1; (DMSO-d6, referenced
to the residual solvent peaks centered at δ 39.51) 176.7, 168.5, 156.7,
137.0, 125.8, 125.3, 121.6, 120.2, 114.2, 114.0, 107.4, 42.3, 40.7, 28.1
(2 C), 23.3, 22.5; IR 3280 (br), 1670, 1601, 1184, 1137; HRMS (ESI)
calcd for C17H23BrN7O (M+) 420.1147, found 420.1135. In both
solvents, the indole carbons near the nitrogen are doubled in various
ratios due to the presence of both NH and ND forms of the indole.15
ASSOCIATED CONTENT
* Supporting Information
■
S
Results from the NCI 60 cell screens of phidianidine A (1a),
phidianidine B (1b), amine 12a, and amine 12b, tables of
1
spectral data, and copies of H and 13C NMR spectral data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
snider@brandeis.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the National Institutes of Health (GM-
50151) for support of this work.
■
120.0, 115.9, 114.3, 108.2, 83.1, 79.4, 43.1, 40.7, 29.0, 28.6, 28.3 (3 C),
28.0 (3 C), 24.0, 23.3; IR (neat) 3322 (br), 1718, 1598, 1413, 1366,
1330, 1131, 1052, 910, 803, 730; HRMS (ESI) calcd for
C27H39BrN7O5 (MH+) 620.2196, found 620.2187.
REFERENCES
■
(1) Carbone, M.; Li, Y.; Irace, C.; Mollo, E.; Castelluccio, F.; Di
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Lett. 2011, 13, 2516−2519.
N-[5-[(Aminoiminomethyl)amino]pentyl]-5-[(1H-indol-3-yl)-
methyl]-1,2,4-oxadiazol-3-amine Trifluoroacetic Acid Salt
(Phidianidine B, 1b). Compound 14b (417 mg, 0.77 mmol) was
taken up in 20 mL of 1:10 TFA/CH2Cl2, and the resulting solution
was stirred at room temperature for 8 h. The mixture was diluted with
25 mL of MeOH and concentrated to give 326 mg (93%) of >95%
pure 1b as a pale yellow oil: 1H NMR (CD3OD) 7.52 (br d, 1, J = 8.0),
7.35 (br d, 1, J = 8.0), 7.21 (br s, 1), 7.11 (br dd, 1, J = 8.0, 8.0), 7.01
(br dd, 1, J = 8.0, 8.0), 4.22 (s, 2), 3.15 (t, 2, J = 6.8), 3.14 (t, 2, J =
6.8), 1.68−1.54 (m, 4), 1.46−1.36 (m, 2); (DMSO-d6) 11.04 (br s,
NH), 7.55 (br t, 1, J = 5.6, NH), 7.51 (br d, 1, J = 8.0), 7.37 (br d, 1, J
= 8.0), 7.31 (br d, 1, J = 2.2), 7.09 (br dd, 1, J = 8.0, 8.0), 6.99 (br dd,
1, J = 8.0, 8.0), 6.72 (br t, 1, J = 5.6, NH), 4.20 (s, 2), 3.06 (dt, 2, J =
5.6, 5.6), 3.01 (dt, 2, J = 5.6, 5.6), 1.56−1.40 (m, 4), 1.35−1.25 (m, 2);
13C NMR (CD3OD, referenced to the residual solvent peaks centered
at δ 49.15) 179.5, 170.2, 158.8, 138.2, 128.3, 124.8, 122.9, 120.2, 119.4,
112.5, 108.3, 43.9, 42.5, 29.9, 29.7, 25.0, 24.2; (DMSO-d6, referenced
to the residual solvent peaks centered at δ 39.51) 176.9, 168.5, 156.7,
136.2, 126.7, 124.2, 121.2, 118.7, 118.4, 111.5, 106.9, 42.3, 40.7, 28.1
(2 C), 23.4, 22.7; IR 3303 (br), 1671, 1599, 1201, 1137; HRMS (ESI)
calcd for C17H24N7O (M+) 342.2042, found 342.2047. In both
solvents, the indole carbons near the nitrogen are doubled in various
ratios due to the presence of both NH and ND forms of the indole.15
N-[5-[(Aminoiminomethyl)amino]pentyl]-5-[(6-bromo-1H-
indol-3-yl)methyl]-1,2,4-oxadiazol-3-amine Trifluoroacetic
Acid Salt (Phidianidine A, 1a). Compound 14a (100 mg, 0.16
mmol) was taken up in 5 mL of 1:10 TFA/CH2Cl2, and the resulting
solution was stirred at room temperature for 8 h. The mixture was
diluted with 5 mL of MeOH and concentrated to give 79 mg (92%) of
>95% pure 1a as a pale yellow oil: 1H NMR (CD3OD) 7.52 (br d, 1, J
= 1.7), 7.45 (br d, 1, J = 8.0), 7.23 (br s, 1), 7.13 (br dd, 1, J = 8.0, 1.7),
4.20 (s, 2), 3.15 (br t, 4, J = 7.2), 1.66−1.54 (m, 6), 1.46−1.36 (m, 2);
(DMSO-d6) 11.18 (br, s, NH), 7.56 (br d, 1, J = 1.7), 7.49 (br s, NH),
7.47 (br d, 1, J = 8.4), 7.35 (br d, 1, J = 2.5), 7.14 (br dd, 1, J = 8.4,
1.7), 6.73 (br t, 1, J = 6.0, NH), 4.20 (s, 2), 3.06 (dt, 2, J = 6.0, 6.0),
3.00 (dt, 2, J = 6.0, 6.0), 1.56−1.42 (m, 4), 1.34−1.22 (m, 2); 13C
NMR (CD3OD, referenced to the residual solvent peaks centered at δ
49.15) 179.1, 170.3, 158.8, 139.1, 127.3, 125.9, 123.4, 121.0, 116.3,
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