The synthesis of vinylogous amidine heterocycles

Article abstract of DOI:10.1021/jo401927n

We report herein a convenient synthetic methodology for the conversion of meta-dinitro heterocyclic rings to iminoquinones with vinylogous amidine functionality. These structures are found in nature, particularly in marine organisms, and may be important

Full text of DOI:10.1021/jo401927n

Article
pubs.acs.org/joc
The Synthesis of Vinylogous Amidine Heterocycles
Daniel V. LaBarbera*,† and Edward B. Skibo*,§
†The Skaggs School of Pharmacy and Pharmaceutical Sciences, Department of Pharmaceutical Sciences, University of Colorado
Anschutz Medical Campus, Aurora, Colorado 80045, United States
§Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287, United States
S
* Supporting Information
ABSTRACT: We report herein a convenient synthetic
methodology for the conversion of meta-dinitro heterocyclic
rings to iminoquinones with vinylogous amidine functionality.
These structures are found in nature, particularly in marine
organisms, and may be important for the pigments and
biological activity observed with such marine secondary
metabolites. Using benzimidazole and indole ring systems we show the versatility of these vinylogous amidines for organic
synthesis, including the following: transamination substitution reactions with virtually any primary amine, regional control of the
substitution with substituents between the vinylogous amidine, and hydrolytic properties that can be controlled or optimized
based on the properties of the chosen ring system. Taken together, this versatile chemistry and functionalization of organic
molecules may be useful in the preparation of a variety of chemical products such as drug pharmacophores or assembling
macromolecular structures.
biological evaluation of benzimidazole analogues of the
pyrroloiminoquinone alkaloids, Chart 1.
INTRODUCTION
■
The vinylogous amidine functionality, also referred to as an
extended amidine, is present in alkaloid secondary metabolites
from marine sponges such as the Latrunculia and Zyzzya
species.1−3 Examples of these include makaluvamine A and
Discorhabdin A shown in Chart 1. Initially, the discorhabdins
We reasoned that these analogues could exhibit cytotoxicity
due to the presence of both the vinylogous amidine
functionality and the purine-like benzimidazole ring. Thus far,
we have not identified the exact molecular target(s) for the
imidazoquinoxalinones; however, National Cancer Institute
(NCI) 60-cell line screening in combination with COMPARE15
analysis suggest that some of the benzimidazole-based
analogues may be promoting cancer cell apoptosis by inhibiting
the phosphorylation of BAD (BCL-2 associated death
promoter).13,16 In contrast, the pyrroloiminoquinone natural
products have been reported to induce DNA damage via
topoisomerase IIα inhibition, though the exact mode by which
they inhibit topoisomerase IIα is still unknown.1,3,17
Chart 1. Indole and Benzimidazole Iminoquinones with
Extended Amidine Functionality
As a result of these studies we discovered that the vinylogous
amidine functionality could be substituted with virtually any
primary alkyl amine via a transamination reaction. Intrigued by
our findings, we designed a streamlined synthesis to study the
utility of the extended amidine chemistry sans the ethylene
tether (Chart 1, blue bonds) illustrated in Scheme 1.18−23 In
this article, we describe the synthetic methodology for the
efficient conversion of meta-dinitro benzimidazoles and indoles
to vinylogous amidines, their substitution chemistry, and
hydrolytic stability.
Synthesis starts with readily available meta-dinitro derivatives
of benzimidazoles and indoles, which were prepared and then
reduced to the corresponding diamino derivatives. Without
purification, diamines can be converted to iminoquinone based
extended amidines in good yield using the method of Fremy’s
were isolated and found to possess cytotoxic antitumor
properties,4,5 and a number of naturally occurring analogues
were later identified that display similar antitumor activity.6−12
The vinylogous amidine functionality is responsible for the red
to purple pigments of the pyrroloiminoquinone natural
products and may contribute to their cytotoxic properties. In
two previous publications,13,14 we reported the synthesis and
Received: August 30, 2013
© XXXX American Chemical Society
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Scheme 1. General Synthesis of Iminoquinone Extended
Amidines
RESULTS AND DISCUSSION
■
Synthesis of Series 1 Extended Amidines. First, we
studied simple analogues of series 1 (Scheme 2) to explore the
chemistry of the iminoquinone based extended amidines and
the versatility of the transamination reactions with various
primary amines. Dinitration of 1,2,5-trimethyl benzimidazole 4,
with 90% nitric acid and concentrated sulfuric acid, afforded 5
in 83% yield as pale yellow crystals from ethanol. The dinitro
intermediate 5 was catalytically reduced without isolation of the
amine intermediate, followed by Fremy’s oxidation to give 6 as
a purple crystalline phosphate salt in 70% yield upon reversed
phase purification. Key intermediate 6 was substituted using 5
equiv of various amines stirred in methanol at 30 °C, to give
extended amidines 1a−g in ∼50% yields. The substitution
occurs at the 4-position based on 2D-NMR studies, which are
later discussed in detail.
Once we were successful in elaborating simple analogues of
1, we then introduced more complicated and potentially labile
R′ substituents (Scheme 3). The extended amidines 1h and 1i
were synthesized in five steps from 7 and 8. Nitration of 7 and
8 with 90% nitric acid and concentrated sulfuric acid provided
trinitro derivatives (∼70% yields), including nitration at the 4
and 6 position of the benzimidazole ring and formation of a
nitrate ester of the side chain alcohol. To our benefit,
compound 8 was O-demethylated under these nitration
conditions and the resulting alcohol was converted to the
nitrate ester in situ. The trinitro intermediates were catalytically
reduced to give the diamino intermediates that were isolated as
hydrochloride salts, an ∼90% yield as tan crystals. These salts
were then oxidized with Fremy’s salt to afford 9 and 10 as
purple crystalline phosphate salts in ∼98% yield. Subsequent
sulfonation with methane sulfonyl chloride in pyridine followed
by treatment with 5 equiv of 40% aqueous methylamine gave
1h and 1i in 50% yield as blue crystalline TFA salts.
salt oxidation.13,14,24,25 The extended amidines could then be
derivatized using primary amines via a transamination reaction.
The three broad classes of extended amidines addressed in this
report are shown in Chart 2.
Chart 2. Extended Amidines Series 1−3
Synthesis of Series 2 Extended Amidines. The synthesis
of series 2 was achieved in four steps from picryl chloride
(Scheme 4). Picramides 11a and 11b were catalytically reduced
in the presence of acetic anhydride to give peracetylated tetra-
amino intermediates. Subsequently, refluxing in 48% HBr
resulted in both deacetylation and formation of 4,6-
diaminobenzimidazoles, and then Fremy’s oxidation produced
Finally, hydrolytic stability studies were conducted and a pH-
rate profile revealed that the imonoquinone functionality is
stable to hydrolysis at neutrality but hydrolyzed slowly to form
aminoquinones at pH values >9. Our hydrolysis studies
concluded that resonance stabilization of the protonated
extended amidine is responsible for the hydrolytic stability at
neutrality.
Scheme 2. Synthesis of Series 1 Extended Amidines 1a−1g
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Scheme 3. Synthesis of Series 1 Extended Amidines 1h and 1i
Scheme 4. Synthesis of Series 2 Extended Amidines 2a, b, and c
Scheme 5. Synthesis of Series 3 Extended Amidines 3a and b
2a and 2b. Finally, transamination of 2a with tryptamine
afforded disubstituted 2c.
sodium azide in dimethylformamide. Thermolysis of the azide
derivative in nitrobenzene gave a nitrene intermediate that
provided 4,6-dinitro-2-phenylindole (12) by an insertion
reaction (Scheme 5). Like the benzimidazoles, indole based
extended amidines of series 3 were obtained by catalytic
reduction of 12 with subsequent Fremy’s oxidation giving 3a,
and reaction of 3a with phenethylamine gave only the
disubstituted extended amidine 3b.
Transamination Regioselectivity, and Tautomeric
Equilibria of Extended Amidines. The synthetic studies
cited above indicate that series 1 extended amidines exhibit
regioselective transamination at the 4-position. In contrast,
series 2 and 3 undergo transamination at both the 4- and 6-
positions. In this section, we present HMBC spectroscopic data
that confirm transamination at the 4-position in series 1. We
also provide calculations that explain the substitution
regioselectivity in series 1, and the lack thereof in series 2
and 3. We utilized Heteronuclear Multiple Bond Coherence
(HMBC) to obtain proton−carbon long distance couplings
over 2 to 4 bonds allowing regioselectivity for series 1 to be
established. HMBC spectroscopy of 1a where the delay in pulse
Synthesis of Series 3 Extended Amidines. There are
numerous examples of different heterocyclic marine natural
products containing the extended amidine moiety.26 Since
various benzimidazole extended amidines could be easily
substituted via the transamination reaction we hypothesized
that we could apply this versatile reaction to other heterocyclic
rings. We chose to explore extended amidine chemistry with
the indole ring system because of its prevalence in nature and in
synthetic drug pharmacophores.27 From a synthetic standpoint,
starting with a dinitroindole ring was ideal and it was also
necessary that the nitro substituents were positioned meta to
each other. The nitration of indoles has been well studied,
primarily by Noland, Smith, and Rush.28−31 From these studies
it was clear that the desired 4,6-dinitroindole is not possible via
direct nitration of indole.
However, this indole could be achieved in three steps by the
condensation reaction of trinitrotoluene and benzaldehyde,
using a catalytic amount of piperidine.32,33 The resulting
stilbene ortho nitro group was then selectively substituted with
C
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sequence was optimized for 8 Hz shows both two- and three-
bond coupling cross peaks, which could not distinguish
between transamination at the 4 and 6 positions (Supporting
Information). However, when the pulse sequence delay was
optimized at 4 Hz we could easily discern the 2-, 3-, and 4-bond
coupling of the methyl amino protons (H13) and the 2-methyl
protons (H11) to the C9 carbon, shown as red colored peaks in
Figure 1. These cross peaks clearly indicate that transamination
Scheme 6. Calculated ΔE Values for o-Iminoquinone ⇄ p-
Iminoquinone Tautomeric Equilibria
by −7.21 kcal/mol). We conclude that the 5-methyl group of
series 1 causes steric hindrance and 4-amino tautomer
predominance, enforcing 4-regiospecificity for transamination.
Hydrolysis Studies of Benzimidazole and Indole
Based Extended Amidines. We studied the reaction of
extended amidines in aqueous buffers (μ = 1.0 KCl) over a pH
range of 0 to 11 and thermostatted to 30 °C. These reactions
were accompanied by a substantial color change (purple-
colored extended amidine to red-colored aminoquinone) that
was monitored at 500 nm with a UV−visible spectropho-
tometer. These studies provided mechanisms of hydrolysis for
both the benzimidazole and indole based extended amidines
along with associated rate constants and pKa values. The
hydrolysis of 6 provides a mixture of aminoquinone products
13 and 14 (Scheme 7), which were identified by 1H NMR and
mass spectroscopy.
The pH-rate profile shown in Figure 2 indicates the presence
of two mechanisms for hydrolysis: the addition of hydroxide to
6H+ (kHO) and the addition of hydroxide to neutral 6 (k′OH),
Scheme 7. We interpreted the plateau at pH > pKa as the
addition of hydroxide to 6H+ (kOH) rather than the kinetically
equivalent addition of water to neutral 6 based on the stability
of 6H+ toward hydrolysis in moderate to strong acidic aqueous
media. Surely, the addition of water to 6H+ would occur at a
larger rate constant than the addition of water to neutral 6. In
fact, we purified 6H+ and the series 1 and 2 compounds by
reversed phase chromatography using 1 M hydrochloric acid as
the eluent.
Figure 1. HMBC connectivity spectrum of 1a at 4 Hz (key correlation
peaks are colored red).
occurred at the 4-position rather than at the 6-position. In
contrast, a methyl amino group at the 6-position would exhibit
4-bond coupling to the C-7 carbonyl and not C-8. With the
transamination product unambiguously assigned to the 4-
position, we used three-bond HMBC couplings to assign the
13C chemical shifts of C-4 (157 ppm) and C-6 (149 ppm). All
series 1 products display 3-bond coupling to C-4 confirming
substitution at the 4-position.
The origins of transamination regioselectivity in series 1 are
the apparent steric differences between the 4-position and the
6-position. The 5-methyl and the 7-carbonyl groups prevent
substitution by flanking the 6-position. Consistent with this
interpretation, transamination regioselectivity is no longer
observed in series 2 and 3, with neither possessing the 5-
methyl group. The steric interpretation of regioselectivity was
validated with Hartree−Fock calculations carried out using the
3-21G basis set (SpartanModel 2006, Wavefunction, Inc.).34
These calculations revealed that the 6-methylamino derivative is
1.9 kcal/mol higher in energy compared to that observed with
the 4-aminomethyl derivative. The steric differences between
the 4-position and the 6-position also influence the tautomeric
equilibrium of the extended amidine functional group (Scheme
6). The 4-aminomethyl regioisomer exists predominately as the
6-imino tautomer (o-iminoquinone) because steric congestion
does not favor the 6-amino tautomer (p-iminoquinone). If the
6-aminomethyl regioisomer is considered, the increased steric
congestion due to the methyl group favors the 6-imino
tautomer to an even greater degree.
The pH-rate profile shown in Figure 2 was computer-fit to eq
1 (R2 = 0.98), where aH is the proton activity determined with a
pH meter, KW is the autoprotolysis constant of water (10−13.86
at 30 °C), kH2O = 1.17 × 10−3 M−1 s−1, Ka = 2.047 × 10−8 (pKa
= 7.69), kOH = 7346 M−1 s−1, and k′OH = 5.21 × 10−13 M−1 s−1.
The activity of water is considered to be 1 for second-order rate
In contrast, 2b, which does not have the 5-methyl group,
exists predominately as the p-iminoquinone tautomer (favored
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Scheme 7. Hydrolysis Mechanism of 6 in Aqueous Buffer
Scheme 8. Hydrolysis Mechanism of 15 in Aqueous Buffer
Figure 3. pH-Rate profile for the hydrolysis of 15.
calculations. The variable aH and these constants were used to
generate the solid line in Figure 3.
aHk2
aH + Ka2
aHk1
aH + Ka1
kobsd
=
+
(2)
From the results cited above, the hydrolysis of 15 involves
water addition to either the protonated and diprotonated
2+
species. The formation of the dication 15H2 is attributed to
Figure 2. pH-Rate profile for the hydrolysis of compound 6.
carbonyl O-protonation, which has been documented in our
hydrolytic studies of the A-ring of CC-106536 and of aziridinyl
quinones.37 However, the electron-rich indole ring of 15
precludes hydroxide addition to the neutral species as observed
for more electron-deficient benzimidazole 6. Our hydrolytic
studies of 6 indicate that the benzimidazole-based extended
amidines (series 1 and 2) will have half-lives (∼10 min). In
contrast, the unsubstituted indole based extended amidine 15 is
rapidly hydrolyzed (∼ 3 min half-life) at neutrality.
constant calculations. The variable aH and these constants were
used to generate the solid line in Figure 2.
′
KWkOH
aH + Ka
KWkOH
aH
kobsd
=
+
(1)
The indole-based extended amidine compounds of series 3
were too insoluble in aqueous media for kinetic studies of
hydrolysis, so we studied the hydrolysis of the unsubstituted
analogue 15 (Scheme 8). Compound 15 was prepared from
4,6-dinitroindole35 employing the synthetic methodology
shown in Scheme 5. The pH-rate profile shown in Figure 3
indicates the presence of two mechanisms for hydrolysis: the
addition of water to protonated 15, 15H+ (k1), and the addition
CONCLUSION
■
Versatile late stage functionalization of organic molecules is a
sought after attribute in the preparation of useful chemical
products, whether it is the derivatization of a drug
pharmacophore or assembling macromolecular structures
such as nanoparticles. In this report we have demonstrated
such versatility using heterocyclic rings commonly found in
nature and clinically used drug pharmacophores. Specifically,
benzimidazole and indole based extended amidines can be
readily prepared in high yield from meta-dinitro derivatives by
catalytic reduction and subsequent Fremy’s oxidation. These
2+
of water to diprotonated 15, 15H2 (k2). The pH-rate profile
data shown in Figure 3 were computer-fit to eq 2 (R2 = 0.99),
where aH is the proton activity determined with a pH meter, k1
= 3.06 × 10−3 M−1 s−1, Ka1 = 1.0 × 10−9 (pKa1 = 9.00), k2 =
0.034 M−1 s−1, and Ka2 = 0.24 (pKa2 = 0.615). The activity of
water is considered to be 1 for second-order rate constant
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HMBC and HMQC); HRMS (MALDI-TOF) m/z: [M]+ Calcd for
C10H13N4O 205.108, Found 205.109. Anal. C, H, N, Calcd for
C10H12N4O·TFA·0.5 H2O: C, 44.04; H, 4.31; N, 17.12. Found: C,
44.10; H, 4.20; N, 16.90.
highly functionalized compounds can be further derivatized via
transamination reactions with virtually any primary amine.
Furthermore, substitution can be regioselectively controlled
depending on the functional group directly between the
extended amidine nitrogen atoms. For example, the series 1
benzimidazole scaffold can be substituted at the 4-position
when a 5-methyl substituent is present, due to steric hindrance
at the 6-position.
Hydrolysis studies of unsubstituted series 1 and 3 scaffolds
indicate that the hydrolytic stability of the extended amidine is
dependent on the properties of the ring. For example, the
electron deficient benzimidazole series 1 compounds are stable
in dilute solutions of strong acids. The base-catalyzed hydrolysis
to aminoquinones was only observed in solutions at pH > 8.
Such properties are suitable for use in complex biological
systems. In contrast, the electron rich indole based extended
amidine rapidly hydrolyzed over the pH range of 0 to 9.5 by
water addition to both the mono- and diprotonated species.
Therefore, depending on the desired application of the
extended amidine compound, hydrolytic stability may be
controlled or optimized based on the properties of the chosen
ring system. Finally, we have demonstrated the utility and
versatility of the transamination reaction with three different
heterocyclic extended amidine scaffolds, indicating that this
chemistry is not limited and could be conceivably applied to
any ring system containing the extended amidine moiety.
General Transamination Procedure. To a solution of 0.24
mmol of iminoquinone (e.g., 6*phosphate, 72.5 mg) dissolved in 10
mL of methanol were added 5 equiv of a primary alkyl amine. For
example, 40% aqueous methylamine was used to prepare 1a. The
resulting red solution was heated between 30 and 40 °C and stirred
until complete by TLC (1:1 ethyl acetate/acetone). The solvent was
removed in vacuo, giving a red residue that was taken up in ethyl
acetate and chromatographed on silica gel using an eluent of (1:1)
acetone/ethyl acetate. The fractions containing product were acidified
with a few drops of TFA, concentrated in vacuo, and crystallized from
ethyl acetate and hexanes to afford the 4-alkylimino derivatives as
brownish-purple-to-purple solids 1a−g in 40−60% yield.
6-Amino-4-methylimino-1,2,5-trimethyl-1H-benzimidazole-
7-one (1a). 48 mg (60%); mp 196−197 °C; TLC (5:3:2 butanol/
water/acetic acid) Rf = 0.43; FTIR (KBr pellet) 1134, 1194, 1373,
1460, 1525, 1593, 1672, 2285, 3103, 3412, cm−1; 1H NMR (CD3OD)
δ 3.91 (3H, s), 3.84 (3H, s), 2.50 (3H, s), 1.99 (3H, s); 13C NMR
(D2O) δ 170.6, 157.2, 154.4, 146.9, 139.4, 128.3, 114.8, 97.7, 35.2,
32.1, 11.9, 8.9; (see Supporting Information for 2D NMR: HMBC);
HRMS (MALDI-TOF) m/z: [M+H]+ Calcd for C11H15N4O 219.124,
Found 219.119. Anal. C, H, N, Calcd for C11H14N4O·TFA·0.6
CH2Cl2: C, 42.62; H, 4.26; N, 14.62. Found: C, 42.25; H, 3.94; N,
14.89.
6-Amino-4-(3-methoxypropylimino)-1,2,5-trimethyl-1H-
benzimidazole-7-one (1b). 56 mg (60%); mp, 168−169 °C; TLC
(5:3:2 butanol/water/acetic acid) Rf = 0.39; FTIR (KBr pellet) 1132,
1172, 1381, 1527, 1593, 1637, 1697, 2980, 3090, 3412 cm−1; 1H NMR
(CD3OD): δ 4.56 (2H, t, J = 6.6 Hz) 3.90 (3H, s) 3.60 (2H, t, J = 6.6
Hz) 3.35 (3H, s) 2.49 (3H, s) 2.07 (2H, q, J = 5.7 Hz), 1.90 (3H, s);
13C NMR (D2O) δ 170.6, 156.8, 154.3, 147.3, 139.5, 128.3, 97.6, 70.2,
69.7, 57.9, 57.8, 45.6, 32.1, 28.7, 12.0, 8.9; (see Supporting Information
for 2D NMR: HMBC, HMQC, and TOCSY); (HRMS) MALDI-TOF
m/z: [M+H]+ Calcd for C14H21N4O2 277.166, Found 277.165. Anal.
C, H, N, Calcd for C14H20N4O2·TFA: C, 49.23; H, 5.42; N, 14.35.
Found: C, 48.89; H, 5.28; N, 14.16.
6-Amino-4-cyclopropylimino-1,2,5-trimethyl-1H-benzimida-
zole-7-one (1c). 52 mg (60%); mp dec >260 °C; TLC (5:3:2
butanol/water/acetic acid) Rf = 0.30; FTIR (KBr pellet) 800, 1072,
1384, 1525, 1577, 1639, 3121, 3414 cm−1; 1H NMR (CD3OD): δ 4.79
(1H, m), 3.91(3H, s), 2.49 (3H, s), 2.01 (3H, s), 1.15 (4H, m,
cyclopropyl methylenes); 13C NMR (DMSO-d6) δ 170.4, 157.7, 154.3,
151.9, 141.7, 140.1, 126.3, 95.5, 32.7, 13.2, 13.1, 11.5; (see Supporting
Information for 2D NMR: HMBC); HRMS (MALDI-TOF) m/z: [M
+H]+ Calcd for C13H17N4O 245.140, Found 245.142.
6-Amino-4-cyclohexylimino-1,2,5-trimethyl-1H-benzimida-
zole-7-one (1d). 48 mg (50%); mp 249−250 °C; TLC (5:3:2
butanol/water/acetic acid), Rf = 0.44; FTIR (KBr pellet) 1296, 1359,
1527, 1583, 1624, 1680, 2930, 3068, 3364 cm−1; 1H NMR (CD3OD):
δ 5.72 (1H, m), 3.90 (3H, s), 2.49 (3H, s), 1.97 (3H, s), 2.05−1.21
(10H, m, cyclohexyl methylenes); 13C NMR (DMSO-d6) δ 171.1,
155.5, 153.4, 148.8, 140.0, 128.7, 97.4, 57.0, 32.7, 32.0, 25.1; HRMS
(MALDI-TOF) m/z: [M+H]+ Calcd for C16H23N4O 287.187, Found
287.190.
6-Amino-4-phenethylimino-1,2,5-trimethyl-1H-benzimida-
zole-7-one (1e). 41 mg (40%); mp 179−180 °C; TLC (5:3:2
butanol/water/acetic acid) Rf = 0.50; FTIR (KBr pellet) 1134, 1201,
1379, 1527, 1593, 1689, 3088, 3410 cm−1; 1H NMR (CD3OD): δ 7.31
(5H, m), 4.65 (2H, t, J = 7.2 Hz), 3.91 (3H, s), 3.10 (2H, t, J = 7.8
Hz), 2.54 (3H, s), 1.97 (3H, s) (see Supporting Information for 2D
NMR: HMBC); 13C NMR (DMSO-d6) δ 170.4, 157.0, 153.4, 148.0,
139.8, 137.8, 128.7, 128.3, 128.2, 126.4, 97.0, 48.6, 36.0, 31.4, 11.5, 8.5;
HRMS (MALDI-TOF) m/z: [M+H]+ Calcd for C18H21N4O 309.171,
Found 309.168. Anal. C, H, N, Calcd for C18H20N4O·TFA: C, 56.87;
H, 5.01; N, 13.26. Found: C, 56.75; H, 4.99; N, 13.17.
EXPERIMENTAL SECTION
■
4,6-Dinitro-1,2,5-trimethyl-1H-benzimidazole (5). To a sol-
ution consisting of 25 mL of 90% nitric acid and 10 mL of
concentrated sulfuric acid was added 1 g (6.25 mmol) of 1,2,5-
trimethylbenzimidazole38 4 in small portions at room temperature.
The solution was warmed to 90 °C and maintained at that temperature
with stirring for 24 h. The reaction solution was cooled to room
temperature and poured over cracked ice followed by neutralization
with 30% aqueous sodium hydroxide solution. The resulting
precipitate was filtered, washed with water, dried in the air, and
recrystallized with ethanol to give 1.3 g of pale yellow needles, 83%
yield: mp, 161−163 °C; TLC (ethyl acetate), Rf = 0.26; FTIR (KBr
pellet), 725, 879, 1155, 1251, 1330, 1446, 1531, 2951, 3094 cm−1; H
1
NMR (CDCl3) δ 8.09 (1H, s), 3.83 (3H, s), 2.67 (3H, s), 2.54 (3H,
s); 13C NMR (CDCl3) δ 159.3, 144.2, 141.2, 137.8, 134.6, 119.8,
109.0, 30.8, 14.7, 14.2. Anal. C, H, N, Calcd for C10H10N4O4·0.09
H2O: C, 47.69; H, 4.07; N, 22.25. Found: C, 47.29; H, 3.95; N, 22.23.
6-Amino-4-imino-1,2,5-trimethyl-1H-benzimidazole-7-one
(6). A mixture consisting of 200 mg (0.8 mmol) of 5, 100 mg of 5% Pd
on carbon in 20 mL of methanol was hydrogenated under 50 psi of H2
for 3h. The catalyst was filtered through a bed of Celite and washed
with methanol, and the filtrate was concentrated in vacuo to give a
white residue that was immediately taken up into 15 mL of monobasic
phosphate buffer (pH = 2.1). To this solution were added, in one
portion, 429 mg (1.6 mmol) of Fremy’s salt, with subsequent stirring
at room temperature for 10 min. The resulting dark purple solution
was immediately loaded on a nitrogen push reversed phase column
(Phenyl BakerBond) set at 20 psi and eluted with water. The water
elution removes any buffer or salts remaining from the reaction. After
the initial water elution, the eluent is changed to methanol and the
dark purple band is collected, further acidified with a few drops of
phosphoric acid, and concentrated in vacuo to give 6 as a purple
phosphate salt that was recrystallized with methanol and ethyl acetate,
yielding 134 mg, 56%. Note: by eluting the band with TFA/methanol
or HCl/methanol, the TFA and HCl salts are obtained in 55% and
70% respectively: mp 190−191 °C; TLC (5:3:2 butanol: water: acetic
acid) Rf = 0.37; FTIR (KBr pellet): 796, 906, 1041, 1377, 1506, 1626,
1
1780, 3491 cm−1; H NMR (CD3OD) δ 3.79 (3H, s), 2.43 (3H, s),
1.89 (3H, s); 13C NMR (D2O) δ 168.3, 156.5, 155.6, 151.1, 140.9,
6-Amino-4-[2-(4-hydroxyphenyl)-ethylimino]-1,2,5-trimeth-
yl-1H-benzimidazole-7-one (1f). 42 mg (40%); mp; dec >170 °C;
125.3, 95.6, 32.2, 12.0, 8.2; (see Supporting Information for 2D-NMR:
F
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1
TLC (5:3:2 butanol/water/acetic acid) Rf = 0.47; FTIR (KBr pellet)
methylsulfonyl derivative as a blue solid that was pure by NMR: H
1
1383, 1516, 1593, 2184, 3418, cm−1; H NMR (CDCl3): δ 7.19 (2H,
NMR (CD3OD) δ 4.62 (4H, m), 3.05 (3H, s), 2.55 (3H, s), 1.99 (3H,
s).
d, J = 8 Hz), 6.74 (2H, d, J = 8 Hz), 4.68 (2H, t, J = 7.2 Hz), 3.91 (3H,
s) 3.06 (2H, t, J = 7.8 Hz), 2.45 (3H, s), 2.08 (3H, s); 13C NMR
(DMSO-d6) δ 171.0, 157.0, 156.3, 154.0, 148.8, 139.9, 130.1, 128.6,
128.3, 115.7, 97.1, 49.1, 35.5, 32.7, 13.3, 10.5 (see Supporting
Information for 2D NMR: HMBC and HMQC); HRMS (FAB+) m/
z: [M+H]+ Calcd for C18H21N4O2 325.1664; Found 325.1658.
6-Amino-4-[2-(1H-indol-3-yl)-ethylimino]-1,2,5-trimethyl-
1H-benzimidazole-7-one (1g). 44 mg (40%); mp 191−192 °C;
TLC (5:3:2 butanol/water/acetic acid) Rf = 0.37; FTIR (KBr pellet)
To 19 mg of the methylsulfonyl derivative dissolved into 1.5 mL of
methanol were added 24 μL of 40% aqueous methylamine. The
solution, which turns red upon addition of the amine, was stirred for
24 h in a stoppered round-bottom flask at rt. The reaction was
concentrated in vacuo giving a red colored residue, which was purified
in the same manner as 6. The methanolic eluent was first acidified with
a few drops of TFA to aid in the elution. The resulting blue solid was
recrystallized with methanol and ethyl acetate affording 1h as a blue
TFA crystalline salt: mp 158−160 °C; TLC (5:3:2 butanol/water/
acetic acid) Rf = 0.35; FTIR (KBr pellet): 1180, 1354, 1533, 1602,
1676, 3200, 3369, cm−1; 1H NMR (CD3OD) δ 4.65 (2H, t, J = 5 Hz),
4.59 (2H, t, J = 5 Hz), 3.86 (3H, s), 3.04 (3H, s), 2.56 (3H, s), 2.00
(3H, s); 13C NMR (DMSO-d6) δ 171.0, 158.9, 158.6. 156.8, 153.6,
148.4, 148.3, 140.8, 128.3, 97.3, 68.5, 45.1, 37.2, 36.0, 13.5, 10.5; (see
Supporting Information for 2D NMR: HMBC and HMQC); HRMS
(FAB+) m/z: [M+H]+ Calcd for C13H19N4O4S 327.1127, Found
327.1119. Anal. C, H, N, Calcd for C13H18N4O4S·TFA·0.7 H2O: C,
39.77; H, 4.54; N, 12.37. Found: C, 39.59; H, 4.30; N, 12.16.
2,5-Dimethyl-1-(3-methoxypropyl)-1H-benzimidazole (8). A
solution consisting of 5 mL (6.5 g, 38 mmol) of 3-nitro-4-
chlorotoluene and 10 mL of methoxypropyl amine was refluxed
under nitrogen until the reaction was complete by TLC (DCM). The
reaction solution was concentrated in vacuo, and the resulting residue
was taken up in water and extracted with DCM 3× (50 mL). The
organic extracts were washed with aqueous saturated sodium chloride
2× (50 mL), dried with sodium sulfate, and concentrated to give an
orange oil that was purified by silica gel column chromatography
(DCM).
The oil was dissolved in 50 mL of methanol and hydrogenated with
1 g of 5% Pd on carbon under 50 psi of H2 for 3 h. The catalyst was
filtered through a pad of Celite and washed with methanol, and the
filtrate was concentrated in vacuo to give the diamine, 4.75 g, 65%
yield. The diamine was dissolved in a solution consisting of 15 mL of
acetic acid and 15 mL of acetic anhydride and refluxed for 1 h. The
reaction was concentrated in vacuo to give diacetamide in quantitative
yield. The diacetamide (1 g) was refluxed in 25 mL of 4 N HCl for 2 h,
cooled to rt, and neutralized with concentrated ammonium hydroxide
(pH = 8), extracted with DCM 3× (50 mL), dried with sodium sulfate,
and concentrated to give 8 as an amber colored oil that was pure by
NMR: 1H NMR (CDCl3): δ 7.46 (1H, s), 7.17, (1H, d, J = 8 Hz), 7.05
(1H, d, J = 8 Hz), 4.21 (2H, t, J = 6 Hz), 3.32 (3H, s), 3.28 (2H, t, J =
6 Hz), 2.56 (3H, s), 2.46 (3H, s), 2.02 (2H, q, J = 6 Hz); 13C NMR
(CDCl3) δ 151.5, 142.6, 133.0, 131.4, 123.3, 118.7, 68.4, 58.5, 40.3,
28.5, 21.4, 13.5; HRMS (FAB+) m/z: [M+H]+ calcd for C13H19N2O+
219.149, Found 219.150.
1
746, 800, 1128, 1199, 1371, 1525, 1599, 1689, 3254 cm−1; H NMR
(CD3OD) δ 7.65 (1H, d, J = 8.1 Hz), 7.29 (1H, d, J = 8.1 Hz), 7.09−
6.97 (3H, m), 4.78 (2H, t, J = 7.2 Hz), 3.79 (3H, s), 3.23 (2H, t, J =
7.2 Hz), 2.41 (3H, s), 1.94 (3H, s); 13C NMR (DMSO-d6) δ 171.0,
158.4, 158.1, 156.4, 153.5, 148.6, 140.0, 136.7, 128.4, 127.6, 123.9,
121.5, 119.0, 111.9, 110.8, 97.2, 48.4, 32.6, 26.6, 13.1, 10.6; (see
Supporting Information for 2D NMR: HMBC); HRMS (FAB+) m/z:
[M+H]+ Calcd for C20H22N5O 348.1824, Found 348.1819. Anal. C, H,
N, Calcd for C20H21N5O·1.5 H2O·1.7TFA: C, 46.82; H, 4.32; N,
11.67. Found: C, 46.98; H, 3.96; N, 12.08.
Nitration of 7 To Afford 4,6-Dinitro-1-(2-nitrooxyethyl)-2,5-
dimethyl-1H-benzimidazole. To a solution consisting of 10 mL of
90% nitric acid and 5 mL of concentrated sulfuric acid were added 200
mg (1.1 mmol) of 7 (prepared as previously described14) in small
portions over 5 min at rt. After the addition, the reaction was heated to
90 °C and maintained at that temperature for 24 h. The reaction was
cooled to rt and neutralized with 30% aqueous sodium hydroxide with
the addition of ice to keep the solution cool. The resulting pale yellow
precipitate was filtered, washed with water, air-dried, and recystallized
from ethanol to give 202 mg of pale yellow needles, 69% yield: mp
165−166 °C; TLC (ethyl acetate) Rf = 0.43; FTIR (KBr pellet) 754,
1
852, 885, 1039, 1278, 1332, 1527, 1641, 3059, 3443 cm−1; H NMR
(CDCl3) δ 8.15 (1H, s), 4.82 (2H, t, J = 5 Hz), 4.58 (2H, t, J = 5 Hz),
2.74 (3H, s), 2.62 (3H, s). Anal. C, H, N, Calcd for C11H11N5O7·0.5
H2O: C, 39.53, H, 3.62; N, 20.95. Found: C, 39.59; H, 3.33; N, 20.74.
6-Amino-1-(2-hydroxyethyl)-4-imino-2,5-dimethyl-1H-ben-
zimidazole-7-one (9). A mixture consisting of 500 mg (1.5 mmol) of
the nitration product of 7, 500 mg of 5% Pd on carbon, and 30 mL of
methanol was hydrogenated under 50 psi of H2 for 3 h at rt. The
catalyst was filtered off through a pad of Celite, and the filtrate was
acidified with 5 drops of conc HCl. Concentration of the solvent in
vacuo gave the HCl salt of 4,6-diamino-1-(2-hydroxyethyl)-2,5-dimethyl-
1H-benzimidazole from ethyl acetate and methanol as tan crystals, 366
mg, 81% yield.
This HCl salt, 250 mg, was then added to a solution consisting of
688 mg (3 equiv) of Fremy’s salt in 25 mL of monobasic phosphate
buffer (pH = 2.1) and stirred for 30 min. The resulting dark purple
solution was immediately loaded on a nitrogen push reversed phase
column (phenyl BakerBond) set at 20 psi and eluted with water. The
water elution removes any buffer or salts remaining from the reaction.
After the initial water elution the eluent is changed to 100% methanol,
and the dark purple band is collected and further acidified with a few
drops of phosphoric acid. The purple methanolic solution was
concentrated in vacuo resulting in a purple solid that was recrystallized
from methanol and ethyl acetate with cooling at 4 °C for 12 h to afford
9 as a dark purple crystalline phosphate salt, 350 mg, 95% yield: mp
dec >145 °C; TLC (5:3:2 butanol/water/acetic acid), Rf = 0.38; FTIR
Nitration of 8 To Afford 2,5-Dimethyl-4,6-dinitro-1-(3-
nitrooxypropyl)-1H-benzimidazole. Compound 8 was dissolved
in 10 mL of concentrated sulfuric acid and cooled on ice. To the
cooled sulfuric acid solution 10 mL of 90% nitric acid were added
dropwise. The reaction was stirred at rt for 12 h. TLC analysis (ethyl
acetate) showed two nitration products. The reaction was then heated
to 50 °C until TLC showed one product. The reaction was poured
over ice and neutralized with sodium bicarbonate. The neutral solution
was extracted 3× (50 mL) with DCM, dried with sodium sulfate, and
concentrated giving a yellow residue. The residue was recrystallized
with hot ethyl acetate and hexane to yield 500 mg of yellow needles,
41% for two steps: mp; 140−142 °C; TLC (ethyl acetate) Rf = 0.43;
FTIR (KBr pellet): 657, 723, 856, 1020, 1168, 1327, 1440, 1529, 1639,
1
(KBr pellet) 617, 742, 1107, 1400, 1508, 1624, 3095, 3373 cm−1; H
NMR (CD3OD) δ 4.38 (2H, t, J = 6 Hz), 3.86 (2H, t, J = 6 Hz), 3.54
(3H, s) 1.99 (3H, s); 13C NMR (DMSO-d6) δ 168.5, 156.7, 155.8,
151.2, 141.4, 125.1, 95.5, 59.8, 47.9, 12.6, 8.3; HRMS (MALDI-TOF)
m/z: [M+H]+ Calcd for C11H15N4O2 235.119, Found 235.119.
6-Amino-4-methylimino-2,5-dimethyl-1-[2-(methylsulfonyl-
oxy)ethyl]-1H-benzimidazole-7-one (1h). To a solution consisting
of 205 mg (0.62 mmol) of 9 in 10 mL of dry pyridine were added 100
μL of methane sulfonyl chloride (2.1 equiv) under nitrogen at rt, and
the solution was stirred for 12 h. Pyridine was removed in vacuo, and
the resulting blue residue was purified by reversed phase
chromatography in the same manner as described for 6, giving the
3101 cm−1; H NMR (CDCl3) δ: 8.13 (1H, s), 4.49 (2H, t, J = 5.4
Hz), 4.37 (2H, t, J = 5.4 Hz), 2.70 (3H, s), 2.59 (3H, s), 2.31(2H, q, J
= 6 Hz). Nitrated 8 was used to synthesize 10 without any further
purification and characterization.
6-Amino-1-(3-hydroxypropyl)-4-imino-2,5-dimethyl-1H-
benzimidazole-7-one (10). A solution of 0.5 g (1.47 mmol) of the
nitration product of 8 in 100 mL of methanol was reduced under 50
psi H2 in the presence of 150 mg of 5% Pd on carbon for 12 h. The
catalyst was filtered off through a pad of Celite, and the filtrate was
1
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Article
acidified with 5 drops of conc HCl. Concentration of the solvent in
vacuo and crystallization from ethyl acetate and methanol gave the HCl
salt 4,6-diamino-1-(3-hydroxypropyl)-2,5-dimethyl-1H-benzimidazole as
tan crystals, 395 mg, 87% yield.
of DCM and concentrated in vacuo to give 2a and 2b as purple HCl
salts (60% yield).
5-Amino-7-imino-2-methyl-1-phenyl-1H-benzimidazole-4-
one (2a). Mp dec >205 °C; TLC (5:3:2 butanol/water/acetic acid) Rf
= 0.42; FTIR (KBr pellet): 617, 862, 1035, 1172, 1421, 1496, 1637,
3161, 3408 cm−1; 1H NMR (CD3OD): δ 7.77 (3H, m), 7.65 (2H, m),
5.85 (1H, s), 2.26 (3H, s); 13C NMR (DMSO-d6) δ 174.9, 155.6,
153.6, 153.5, 137,9, 133.6, 131.8, 131.5, 131.1, 129.8, 129.0 127.5, 89.5,
13.7; HRMS (FAB+) m/z: calcd for C14H13N4O+, 253.1089; Found
253.1098.
5-Amino-7-imino-2-methyl-1-napthyl-1H-benzimidazole-4-
one (2b). Mp dec >200 °C; TLC (5:3:2 butanol/water/acetic acid) Rf
= 0.21; FTIR (KBr pellet): 584, 829, 1199, 1462, 1612, 3076, cm−1;
1H NMR (CD3OD): δ 8.32 (1H, d, J = 8 Hz), 8.18 (1H, d, J = 8 Hz),
The HCl salt, 223 mg (0.823 mmol), was then added to a solution
consisting of 663 mg (3 equiv) of Fremy’s salt in 25 mL of monobasic
phosphate buffer (pH = 2.1) and stirred for 30 min. The resulting dark
purple solution was immediately loaded onto a nitrogen push reversed
phase column (phenyl BakerBond) set at 20 psi and eluted with water.
The water elution removes any buffer or salts remaining from the
reaction. After the initial water elution, the eluent was changed to 10%
methanol acidified with a few drops of phosphoric acid and the dark
purple band was collected. The purple fraction was concentrated in
vacuo resulting in a purple solid that was recrystallized from methanol
and ethyl acetate with cooling at 4 °C for 12 h to afford 10 as a dark
purple crystalline phosphate salt, 246 mg, 86% yield: mp dec >200 °C;
TLC (5:3:2 butanol/water/acetic acid), Rf = 0.5; 1H NMR (CD3OD)
δ 4.23 (2H, t, J = 6 Hz), 3.48 (2H, t, J = 6 Hz), 2.44 (3H, s), 1.8 (2H,
m), 1.87 (3H, s); 13C NMR (D2O) δ 171.7, 159.8, 157.4, 154.0, 144.2,
127.9, 98.2, 60.9, 45.7, 33.8, 14.7, 10.8; HRMS (FAB+) m/z: [M+H]+
7.85−7.65 (4H, m), 7.38 (1H, d, J = 8 Hz), 5.78 (1H, s), 2.13 (3H, s);
13C NMR (D2O) δ 175.1, 155,3, 153.9, 153.7, 138.5, 134.9, 132.5,
131.7, 129.4, 129.3, 129.2, 128.9, 128,0, 127.3, 126.9, 121.4, 89.5, 13.3;
HRMS (FAB+) m/z: calcd for C18H15N4O+ 303.1246; Found
303.1254.
5, 7-Di-[2-(1H-indol-3-yl)-ethylimino]-2-methyl-1-phenyl-
1H-benzimidazole-4-one (2c). To a solution consisting of 50 mg
of 2a dissolved in methanol were added 5 equiv of tryptamine, and the
reaction was stirred at rt for 12 h. The reaction was concentrated in
vacuo and chromatographed on silica gel (1:1 ethyl acetate/acetone)
to give 39.8 mg (42% yield) of 2c: 1H NMR (CD3OD): δ 7.65 (1H, d,
J = 8 Hz), 7.42−6.98 (14H, m), 6.67 (1H, s), 3.76−3.73 (3H, m),
3.18−3.15 (2H, t), 3.02−2.90 (2H, t), 2.65−2.18 (2H, t), 2.07 (3H,
s); 13C NMR (Methanol-d4) δ 173.9, 155.1, 153.7, 153.3, 138.4, 138.3,
137.5, 133.8, 133.0, 132.9, 132.1, 131.9, 128.7, 128.5, 127.8, 125.0,
123.8, 123.0, 122.8, 120.3, 120.1, 119.2, 119.2, 112.7, 112.3, 110.3,
84.8, 46.1, 45.1 26.3, 24.5, 13.1 (see Supporting Information for
HMBC 2D NMR); HRMS (FAB+) m/z: calcd for C34H31N6O+
539.2559; Found 539.2552.
+
calcd for C12H17N4O2 , 249.1351; Found 249.135.
6-Amino-4-methylimino-2,5-dimethyl-1-[2-(methylsulfonyl-
oxy)propyl]-1H-benzimidazole-7-one (1i). To a solution consist-
ing of 100 mg (0.288 mmol) of 10 and 5 mL of dry pyridine were
added 100 μL of methanesulfonyl chloride (4.5 equiv) under nitrogen
at rt, and the solution was stirred for 12 h. Pyridine was removed in
vacuo, and the resulting blue residue was purified by reversed phase
chromatography in the same manner as described for 10 to give the
methanesulfonoxypropyl derivative as a blue solid, 57 mg (46% yield)
that was pure by TLC (5:3:2 butanol/water/acetic acid), Rf = 0.42,
and NMR.
A solution of the methanesulfonylpropyl derivative (57 mg) in 5 mL
of methanol was combined with 75 μL of 40% aqueous methylamine
in a stoppered flask and stirred for 18 h at rt. The reaction was
concentrated in vacuo to give a red colored residue, which was purified
in the same manner as 10 except 100% methanol acidified with a few
drops of TFA was used to elute the product. The resulting blue solid
was recrystallized with methanol and ethyl acetate affording 1i as a
blue TFA crystalline salt: mp dec >200 °C; TLC (5:3:2 butanol/
6-Amino-4-imino-2-phenyl-1H-indole-7-one (3a). A mixture
consisting of 150 mg (0.53 mmol) of 12 (4,6-dinitro-2-phenyl-1H-
indole32,33) and 75 mg of 5% Pd on carbon, in 15 mL of methanol, was
hydrogenated for 3 h under 50 psi of H2. The catalyst was filtered
through a pad of Celite and washed with methanol, and the filtrate was
acidified with a few drops of conc HCl and dried in vacuo to yield the
HCl salt. This salt was immediately dissolved in 10 mL of pH 7
phosphate buffer containing 3 equiv of Fremy’s salt (426 mg, 1.6
mmol). The solution was stirred for 10 min, and the resulting
amorphous brown solid was filtered off, washed with water, and dried
to give 113 mg (90%) of 3a: mp dec 260 °C; TLC (5:3:2 butanol/
water/acetic acid) Rf = 0.63; FTIR (KBr pellet): 584, 829, 1199, 1462,
1612, 3076, cm−1; 1H NMR (DMSO-d6): δ 13.50 (1H, br s), 10.1−9.9
(1H, br s), 9.06 (1H, br s), 8.43 (1H, br s), 7.84−7.35 (6H, m), 5.71
(1H, s); 13C NMR (DMSO-d6) δ 169.3, 161.4, 154.6, 142.0, 130.1,
129.5, 129.4, 128.3, 126.2, 125.3, 107.7, 89.6, 39.0, 38.0; HRMS
(MALDI-TOF) m/z: calcd for C14H12N3O+ 238.097; Found 238.095.
4,6-Di(phenethylamino)-2-phenyl-1H-indole-7-one (3b). The
general procedure for transamination was employed: dec 260 °C; TLC
1
water/acetic acid) Rf = 0.3; H NMR (CD3OD) δ 4.36 (2H, t, J = 6
Hz), 3.74 (3H, s), 3.54 (2H, t, J = 6 Hz), 2.87 (3H, s), 2.46 (3H, s),
2.10 (3H, s); 13C NMR (D2O) δ 170.9, 156.9, 153.0, 148.5, 140.8,
128.3, 97.2, 68.0, 42.6, 37.1, 36.0, 29.0, 13.2, 10.4; HRMS (FAB+) m/
z: [M+H]+ calcd for C14H21N4O4S+, 341.1283; Found 341.1276
Synthesis of N-(2,4,6-Trinitrophenyl)aniline (11a) and N-
(2,4,6-Trinitrophenyl)-1-naphthylamine (11b). To a solution
consisting of 1 g of picryl chloride dissolved in methanol were
added 2 mol equiv of arylamine, and the mixture was stirred at rt until
complete. The product crystallized from the reaction in high purity
and in quantitative yield when water was added to the reaction mixture
followed by chilling on an ice bath. The melting points of 11a39 and
11b40 matched those reported in the literature.
1
(5:3:2 butanol/water/acetic acid) Rf = 0.70; H NMR (d6-DMSO): δ
Synthesis of 5-Amino-1-aryl-2-methyl-7-imino-1H-benzimi-
dazole-4-one (2a and 2b). To a solution consisting of 100 mL of
acetic acid, 10 mL of acetic anhydride, and 1.8 mmol of 11a or 11b
were added 300 mg of 5% Pd on carbon. This mixture was shaken
under 50 psi of H2 for 12 h, and the catalyst was removed by filtration
through Celite. Evaporation of the acetic acid and acetic anhydride
afforded the peracetylated tetra-amine intermediate as a light yellow
oil. This oil was dissolved in 50 mL of 48% HBr and refluxed for 1 h to
facilitate both deacetylation and benzimidazole formation. The 48%
HBr was removed in vacuo, and the product crystallized from methanol
ethyl acetate: 5,7-diamino-1-1-aryl-2-methyl-1H-benzimidazole·2HBr
was obtained as a light tan solid in 79% yield. This product was
dissolved in 50 mL of phosphate buffer (pH = 7) and combined with 3
equiv of Fremy’s salt. After the reaction was stirred for 10 min, the
neutral product was extracted with DCM 3× (50 mL). The organic
extracts were washed 3× (50 mL) with 0.5 N HCl to extract 2 as the
HCl salt. The acidic aqueous fractions were then washed with 50 mL
7.92 (2H, d, J = 7.5 Hz), 7.43 (3H, t, J = 7.5 Hz), 7.32−7.20 (10H, m),
7.03 (1H, s), 5.51(1H, s), 3.88 (2H, t, 7.5), 3.40 (2H, t, J = 7 Hz), 3.02
(2H, t, J = 7.5 Hz), 2.92 (2H, t, J = 7 Hz); 13C NMR (DMSO-d6) δ
169.0, 142.2, 138.9, 138.5, 137.8, 129.9, 129.6, 129.4, 129.3, 129.2,
129.1, 129.04, 129.0, 128.9, 128.9, 127.9, 127.7, 127.3, 127.1, 127.1,
127.0, 126.9, 126.2, 126.1, 126.0, 86.1, 46.0, 44.8, 33.4, 26.3; (see
Supporting Information for 2D NMR: COSY and HMBC); HRMS
(FAB+) m/z: calcd for C30H28N3O+ 446.2232; Found 446.2227.
ASSOCIATED CONTENT
■
S
* Supporting Information
General methods and materials as well as supporting NMR and
MS spectra for series 1−3 compounds are provided, including
1H, 13C, COSEY, TOCSY, HMQC, and HMBC NMR; high
resolution electron impact bombardment MS, MALDI-TOF
H
dx.doi.org/10.1021/jo401927n | J. Org. Chem. XXXX, XXX, XXX−XXX

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Article
(27) Aniszewski, T. Alkaloids-Secrets of Life: Alkaloid chemistry,
biological significance, applications and ecological role; Elsevier: The
Netherlands, 2007.
MS, and FAB MS. This material is available free of charge via
the Internet at http://pubs.acs.org.
(28) Noland, W. E.; Rush, K. R. J. Org. Chem. 1966, 31, 70−77.
(29) Noland, W. E.; Rush, K. R.; Smith, L. R. J. Org. Chem. 1966, 31,
65−69.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: daniel.labarbera@ucdenver.edu.
*E-mail: eskibo@asu.edu.
■
(30) Noland, W. E.; Smith, L. R.; Johnson, D. C. J. Org. Chem. 1963,
28, 2262−2266.
(31) Noland, W. E.; Smith, L. R.; Rush, K. R. J. Org. Chem. 1965, 30,
3457−3469.
Notes
The authors declare no competing financial interest.
(32) Rozhkov, V. V.; Kuvshinov, A. M.; Gulevskaya, V. I.; Chervin, I.
I.; Shevelev, S. A. Synthesis 1999, 2065−2070.
(33) Rozhkov, V. V.; Kuvshinov, A. M.; Shevelev, S. A. Heterocycl.
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