Total synthesis of diazaquinomycins H and J using double Knorr cyclization in the presence of triisopropylsilane

Article abstract of DOI:10.1039/C8RA09792E

The first total synthesis of diazaquinomycins H (1) and J (2), which are promising anti-tuberculosis natural product leads, has been achieved via selective amidation of diamine 6 with Meldrum's acid derivatives, subsequent EDC coupling with 3-oxobutanoic

Full text of DOI:10.1039/C8RA09792E

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Total synthesis of diazaquinomycins H and J using
double Knorr cyclization in the presence of
triisopropylsilane†
Cite this: RSC Adv., 2019, 9, 1759
*
Allan M. Prior and Dianqing Sun
The first total synthesis of diazaquinomycins H (1) and J (2), which are promising anti-tuberculosis natural
product leads, has been achieved via selective amidation of diamine 6 with Meldrum's acid derivatives,
subsequent EDC coupling with 3-oxobutanoic acid, followed by double Knorr cyclization in the
presence of triisopropylsilane (TIPS). We found that the addition of TIPS was crucial to obtain pure
diazaquinomycins H and J, while preventing isomerization of the terminal iso-branched tail in sulfuric
acid. Our developed synthesis provided diazaquinomycins H (1) and J (2) in 8 steps from commercially
available starting materials in 25% and 21% overall yields, respectively. The spectroscopic data of
synthetic diazaquinomycins H (1) and J (2) agreed very favorably with those of reported natural products.
Received 28th November 2018
Accepted 4th January 2019
DOI: 10.1039/c8ra09792e
rsc.li/rsc-advances
diazaquinomycin A showed excellent in vitro antibacterial
activity against Mycobacterium tuberculosis (M. tuberculosis) with
Introduction
minimum inhibitory concentration (MIC) values of 0.04, 0.07,
and 0.10 mg mLꢀ1, respectively.1 In addition, these analogues
did not display mammalian cytotoxicity against Vero cells when
tested at 28 mM, however, the limited quantity (0.3 mg) of iso-
lated diazaquinomycins H and J prevented further antimicro-
bial assessment.1 More extensive evaluation and proling using
diazaquinomycin A revealed that it also retained potent anti-
tuberculosis activity (MIC ¼ 0.06–0.27 mg mLꢀ1) against drug
resistant M. tuberculosis strains with known resistance to
rifampicin, isoniazid, streptomycin, kanamycin, or cycloserine.1
The specic mechanism of action of diazaquinomycin A against
M. tuberculosis remains unclear, though it was previously re-
ported to interfere with the folate pathway,6 acting as
a competitive inhibitor of thymidylate synthase.7,8 However, M.
tuberculosis does not encode human thymidylate synthase and
M. tuberculosis thymidylate synthase enzymes (ThyA and ThyX)
were not targeted by diazaquinomycin A.1
Diazaquinomycins A and B were rst synthesized in 1988 by
Kelly et al. via a double Knorr cyclization of a symmetrical di-b-
ketoanilide hydroquinone precursor.9 In 1990, Lee et al. re-
ported the synthesis of a diazaanthraquinone scaffold using
hetero Diels–Alder methodology by coupling a dienophile with
a 1-azadiene, followed by dibromohydrin formation and
oxidation.10 In 1998 and 2000, two alternative syntheses of
diazaquinomycin A were reported by Perez et al. and similarly
made use of hetero Diels–Alder chemistry to construct the
tricyclic core.11,12 Subsequent aromatization, N-oxide formation,
and tosyl chloride mediated N-oxide rearrangement afforded
diazaquinomycin A.11,12 The Rinehart group reported the
synthesis of deoxynyboquinone (DNQ) possessing a 1,8-dia-
zaanthraquinone scaffold via the oxidation of natural product
Diazaquinomycins (DAQs) are secondary metabolites produced
by terrestrial and aquatic Actinobacteria, predominantly from
the genus Streptomyces sp.1–5 Spanning the last three and a half
decades, nine DAQs have been reported, however, dia-
zaquinomycins D, F, and G were not obtained in pure form
(Fig. 1).1–5 Specically, diazaquinomycin D was a trace compo-
nent in a sample of diazaquinomycin C that was produced by
Streptomyces sp. GW48/1497 when cultivated in soybean our
and mannitol.3 Its structure was assigned using (ꢀ)-ESI mass
spectrometry.3 The isomeric diazaquinomycins F and G were
isolated as an inseparable mixture;2 their structures were
established using a combination of NMR and UV spectroscopy,
HRMS, and X-ray crystallography.2
Diazaquinomycins H (1) and J (2), bearing a long lipophilic
side chain with a unique terminal isopropyl functionality, are
the latest additions to this family and were isolated by Mul-
lowney et al. from fresh water-derived Actinobacteria in Lake
Michigan.1 Notably, diazaquinomycins H and J along with
Department of Pharmaceutical Sciences, The Daniel K. Inouye College of Pharmacy,
University of Hawaii at Hilo, 34 Rainbow Drive, Hilo, Hawaii, 96720, USA. E-mail:
dianqing@hawaii.edu; Fax: +1 808 933 2974; Tel: +1 808 933 2960
† Electronic supplementary information (ESI) available: 1H and 13C NMR, DEPTQ,
HBMC, HSQC, COSY, HRMS, and HPLC of diazaquinomycins H (1) and J (2); 1H
NMR, DEPTQ spectra and HPLC chromatogram of isomeric mixture of 1 as well
as 2; 1H, 13C NMR spectra and HPLC chromatogram of 3a–c, 4a–b, 5a–b, and
7a–b; 1H, 13C NMR spectra, HRMS, and HPLC chromatogram of 13; synthesis of
14 with a shorter branched side chain and its 1H, 13C NMR spectra; 1H, 13C
NMR spectra comparison of 9-methyldecanoic acid and 4-methylpentanoic acid
before and aer being treated with conc. H2SO4; comparison of NMR spectra
for pure diazaquinomycin H (1) with isomeric mixture of 1; and zoomed 1H
NMR regions of isomeric mixture of 2 with varying concentrations of CF3CO2D
in CDCl3. See DOI: 10.1039/c8ra09792e
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Fig. 1 Chemical structures of naturally occurring diazaquinomycins A–H and J.
deoxynybomycin in hot nitric acid.13 Hergenrother and
Results and discussion
coworkers reported the synthesis of DNQ analogues starting
from a monocyclic dipinacolborane.14,15 Their synthetic route
used Suzuki–Miyaura cross coupling, followed by Buchwald–
Hartwig amidation as key ring forming methodology. Final
oxidation of the penultimate tricyclic methoxy intermediate to
quinone was done using nitric acid15 or a sequence of hydro-
bromic acid, followed by salcomine in the presence of oxygen.14
We became interested in diazaquinomycins due to their
unique 1,8-diazaanthraquinone scaffold and promising antitu-
bercular and antibacterial activities.1,4 Recently, we expanded
the scope of an underexplored double Knorr cyclization and
demonstrated that this methodology can serve as a general and
expedient route to symmetrical or unsymmetrical 1,8-diazaan-
thraquinone scaffolds (Scheme 1).16 We established that the 5-
hydroxy substituent on the di-b-ketoanilide hydroquinone
substrate is essential for successful double Knorr cyclization.
On the other hand, sterically hindered di-b-ketoanilides with
benzyl or isopropyl substitution on the side chains adjacent to
b-carbonyl were poor substrates in this reaction.16 In this work,
we extend this methodology to further develop an efficient and
rst chemical synthesis of diazaquinomycins H (1) and J (2) with
a long iso-branched aliphatic side chain.
Retrosynthetic analysis
The retrosynthetic approach to diazaquinomycins H (1) and J (2)
is outlined in Scheme 2. A key synthetic step would involve the
double Knorr cyclization of di-b-ketoanilide hydroquinones 3a
and 3b having their long iso-branched side chain preinstalled.
Hydroquinones 3a and 3b will be obtained from 4a and 4b aer
removal of MOM protecting groups. The di-b-ketoanilides 4a
and 4b will be obtained in two sequential steps, rstly amida-
tion of diamine 6 with Meldrum's acid derivatives (7a or 7b),
then 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC)
coupling with 3-oxobutanoic acid. The Meldrum's acid deriva-
tives can be obtained through coupling commercially available
iso-branched carboxylic acids 8a and 8b with Meldrum's acid
(9). We envisioned that challenges may reside in the double
Knorr cyclization of di-b-ketoanilide precursor 3a or 3b having
a long, iso-branched aliphatic side chain, as well as the mono-
amidation of diamine intermediate 6.16
Synthesis of key di-b-ketoanilide intermediates 3a–b via
selective amidation of 6 with Meldrum's acid derivatives
The synthesis of di-b-ketoanilide hydroquinone substrates 3a
and 3b started from commercially available 1,4-diacetox-
ybenzene (10) and iso-branched 8-methylnonanoic acid (8a) or
9-methyldecanoic acid (8b) (Scheme 3). Diamine 6 was obtained
in 4 synthetic steps from 10 following our reported synthesis.16
Meldrum's acid derivatives 7a and 7b were synthesized by
coupling 8a and 8b with Meldrum's acid (9) using N,N0-dicy-
clohexylcarbodiimide (DCC) and DMAP in DCM in 78% and
84% yields, respectively, following
protocol.17
a modied literature
Scheme
diazaanthraquinones.
1 Double Knorr cyclization of di-b-ketoanilides to 1,8-
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Scheme 2 Retrosynthetic analysis to diazaquinomycins H and J.
With diamine 6 and Meldrum's acid derivatives 7a and 7b in Unreacted diamine 6 could also be recovered from column
hand, we evaluated the synthesis towards mono-b-ketoanilides purication. To the best of our knowledge, this represents the
5a and 5b. Our previous synthesis of a mono-b-ketoanilide rst example reporting the synthesis of mono-b-ketoanilides
analogue via EDC coupling of 6 with a b-ketoacid was success- directly from a diamine precursor using Meldrum's acid deriv-
ful, but resulted in poor isolated yield (25%).16 Therefore, an atives. In the literature, it was reported that b-ketoanilides or b-
improved synthesis was developed by coupling diamine 6 with ketoamides can form when 5-substituted 2,2-dimethyl-1,3-
Meldrum's acid derivative 7a or 7b to form mono-b-ketoanilide dioxane-4,6-dione (acyl Meldrum's acid) derivatives were
5a or 5b, respectively. Initially, 6 was treated ꢁwith 0.7 molar treated with anilines18,19 or amines,20,21 in solvent such as
equivalents of Meldrum's acid derivative at 45 C in dry aceto- benzene18,19 or acetonitrile.21,22
nitrile. Aer
2
days the Meldrum's acid derivative was
Once 5a–b were synthesized, the second b-ketoanilide side
consumed and HPLC analysis at 254 nm showed a mono- to di- chain was installed via EDC coupling of 5a or 5b with 3-oxo-
b-ketoanilide ratio of 5 : 1, along with unreacted 6. When 0.6 butanoic acid in dichloromethane at 0 ꢁC. We found that a cold
equivalents of Meldrum's acid derivative was used relative to 6, reaction temperature was necessary to slow the decomposition
the mono- to di-b-ketoanilide ratio increased to 9 : 1. This of 3-oxobutanoic acid (via decarboxylation), allowing for reac-
improvement greatly facilitated purication of mono-b-ketoa- tion completion. This afforded the unsymmetrical di-b-ketoa-
nilides using ash column chromatography, affording 5a and nilides 4a and 4b in 83% and 73% yields, respectively, aer
5b in 92% and 83% isolated yields, respectively. It is noteworthy ash column chromatography. Alternatively, crude product 5a
to mention that the yields of 5a and 5b were calculated based on or 5b containing unreacted 6 could be used directly in EDC
Meldrum's acid derivative 7a or 7b, which was used as the coupling with excess 3-oxobutanoic acid since the resulting di-
limiting reagent in order to favor the formation of desired b-ketoanilide reaction mixture can be easily separated by ash
mono-b-ketoanilide, as well as due to the valuable nature of column chromatography. Next, the MOM protecting groups
carboxylic acid precursors 8a and 8b (approx. $900 per gram). were removed by treating 4a or 4b with 1 M HCl : THF : acetone
Scheme 3 Synthesis of unsymmetrical di-b-ketoanilide hydroquinones 3a–b.
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ꢁ
(1 : 1 : 1) at 65 C for 2 h to afford 3a or 3b in 86% and 96% with a linear side chain (n ¼ 5) for comparison and further
yields, respectively, aer trituration with diethyl ether. The optimization studies (Table 1, entries 3–10). Interestingly, when
trituration step was introduced because in our hands the puri- the double Knorr cyclization was performed using 3c in triic
cation of 3a or 3b by ash column chromatography on silica acid, the reaction yielded a mixture of tricyclic hydroquinone
gel led to poor isolated yields. This observation was in contrast (HQ) and 13 in 33% and 33% HPLC yields, respectively, along
to linear chain analogues, which could be isolated in good with unidentied decomposition byproducts (Table 1, entry 3).
yields using ash column chromatography, suggesting the iso- Nevertheless, the fact that linear chain analogue 3c could be
branched functionality in 3a or 3b may play a major role in cyclized using triic acid demonstrated that decomposition of
decomposition on silica gel.
3a and 3b in triic acid is probably accelerated by having an iso-
branched side chain, as this is the only structural difference
between these substrates. Furthermore, the observation of HQ
intermediates in this study was striking because they were not
previously detected in the double Knorr cyclization of shorter
chain analogues.16 The slower rate of oxidation from HQ inter-
mediates to desired quinone products was likely due to steric
effect of the adjacent longer aliphatic chain at the 5 position.
Considering that sulfuric acid is much milder acid than
triic acid, we next investigated if sulfuric acid can perform
better in the double Knorr cyclization of long-chain (e.g., n ¼ 5
or 6) di-b-ketoanilide substrates, while potentially minimizing
the byproduct formation. In contrast to triic acid, cyclization
of 3c using sulfuric acid at 50 ꢁC for 30 min gave a clean reaction
prole and afforded its corresponding HQ intermediate and 13
in 60% and 40% HPLC yields, respectively (Table 1, entry 4).
Double Knorr cyclization and optimization studies
Using our previously optimized conditions,16 3a or 3b was
subjected to double Knorr cyclization in an effort to synthesize
diazaquinomycins H (1) and J (2). Very surprisingly, double
ꢁ
Knorr cyclization of 3a or 3b in triic acid at 50 C resulted in
a complex reaction mixture, and diazaquinomycins H (1) or J (2)
could not be detected and isolated (Table 1, entries 1–2). This
revealed that the longer (n ¼ 5, 6) and/or iso-branched side
chain signicantly impacted this reaction outcome. Presum-
ably, when cyclization rate is lowered, decomposition could
occur through anilide hydrolysis,9,16 4-pyrone formation or
heterolytic cleavage of anilide.23 Additionally, hydrocarbons
have been shown to undergo isomerization24–26 under certain
acidic conditions. Moreover, the length of aliphatic side chain
can have an effect in these processes, with longer chains
showing faster conversion rates.27,28 To probe the effect of
branched functionality on reaction outcome as well as given the
precious nature of 3a–b, we next synthesized model substrate 3c
ꢁ
Continued stirring at 50 C for 3 h (Table 1, entry 5) and 18 h
(Table 1, entry 6) could not complete the oxidation of HQ into
ꢁ
13. At 110 C in sulfuric acid, the oxidation of HQ into 13 was
greatly accelerated, affording 13 in 91% isolated yield aer 1 h
Table 1 Optimization of double Knorr cyclization of di-b-ketoanilides 3a–c
HPLC yield (%)
Entry
Substrate
Reagent/solvent
Temp (ꢁC)
Time (min)
HQ intermediatea
Productb
1
2
3
4
5
6
7
8
3a
3b
3c
3c
3c
3c
3c
3c
3c
3c
3a
3b
TfOH
TfOH
TfOH
50
50
50
50
50
50
30
30
30
30
3 h
18 h
30
60
15
0
0
0
0
33
40
50
67
80
90 (91)
11
33
60
50
33
10
1
89
0
0
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
H2SO4
110
110
25
25
25
25
9
10c
11c
12c
30
30
30
100 (99)
100 (99)d
100 (99)d
0
a
b
c
Hydroquinone (HQ) intermediates were observed using HPLC and MS and not isolated. Isolated yield in parentheses. The reaction was
performed in sulfuric acid for 30 min, then diluted in methanol, and continued to stir for 2–3 h. LC-MS and 13C NMR revealed that 1 or 2
d
contained its corresponding branched side chain isomer(s).
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(Table 1, entries 7–8). The rate limiting step for this trans-
formation was the oxidation of HQ intermediate to 13 since
starting material 3c was already consumed aer 2 min based on
HPLC monitoring. When the double Knorr cyclization of 3c was
performed at room temperature in sulfuric acid, an 8 : 1
mixture of HQ and 13 was formed aer 15 min (Table 1, entry 9)
and additional stirring at room temperature could not complete
the oxidation into 13. In the literature, structurally similar
tricyclic hydroquinone scaffolds have been shown to sponta-
neously be oxidized to anthraquinone in the presence of
oxygen.29–33 We re-analyzed some previous HPLC samples and
found that the HQ intermediate can be oxidized into 13 within
2 h when diluted in methanol. Based on our nding, a milder
and optimized protocol was developed, which involved stirring
3c in sulfuric acid at room temperature for 30 min, followed by
dilution in methanol and continued stirring open to air until
the oxidation of HQ into 13 was complete (Table 1, entry 10).
This provided 13 in 99% isolated yield as a red solid aer
removing methanol in vacuo, precipitation with water, and
collecting solids by centrifugation.
Double Knorr cyclization of 3a or 3b in sulfuric acid leading to
C-5 tail isomerization
Having established a milder double Knorr cyclization protocol,
we proceeded to synthesize diazaquinomycins H (1) and J (2)
from 3a or 3b, respectively. Di-b-ketoanilide 3a or 3b was treated
in sulfuric acid at room temperature for 30 min, then diluted
with methanol and stirred at room temperature to complete the
oxidation of HQ intermediates to diazaquinomycins H (1) and J
(2) (Table 1, entries 11 and 12). In both cases, reactions went to
completion and red solids were obtained in corresponding to
99% isolated yields. Based on their analytical HPLC proles and
1H NMR spectra, the purity of 1 and 2 appeared to be excellent.
However, additional carbon signals were observed in their 13C
NMR spectra, especially in the C-5 aliphatic side chain region.
Further LC-MS analysis revealed that both synthesized dia-
zaquinomycins H (1) and J (2) samples also contained corre-
sponding C-5 aliphatic side chain isomers. The representative
HPLC chromatogram of the isomeric mixture of 1 showed
a single peak (Fig. 2a). However, using a longer HPLC column
under optimized mobile phase conditions, we were able to
detect and partially separate 1 and its side chain isomers, which
were conrmed by (+)-ESI mass spectrometry (Fig. 2b). To
further investigate the isomerization rate during the double
Knorr cyclization from 3a to 1, we closely monitored the reac-
tion using LC-MS and the results are shown in Fig. 2c. For clarity
and improving the peak resolution, the mass range on the MS
detector was set to exclusively monitor m/z ¼ 383 ꢂ 2. Our LC-
MS reaction monitoring data showed the presence of the C-5
chain isomer even aer 2 min, which continued to increase
over the course of 60 min.
Fig. 2 (a) HPLC chromatogram for the isomeric mixture of 1. (b) LC-
MS chromatogram of the isomeric mixture of 1. (c) LC-MS reaction
monitoring of double Knorr cyclization of 3a to 1, which was per-
formed in sulfuric acid only at room temperature. (d) LC-MS reaction
monitoring of double Knorr cyclization of 3a to 1 in sulfuric acid at
room temperature with TIPS added. For clarity, LC-MS analyses shown
in Fig. (c) and (d) were recorded with MS detector set to monitor m/z ¼
383 ꢂ 2.
It was concluded that the C-5 distal iso-branched side chain
of diazaquinomycins H (1) or J (2) must play a critical role in
isomerization since our linear chain model compound 13 was
obtained in pure form. In addition, we found that the length of
the iso-branched alkyl chain also played an important role in
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Scheme 4 Proposed isomerization mechanism for diazaquinomycin J (2) in sulfuric acid.
this isomerization process, with a longer iso-branched alkyl reaction may eliminate isomerization, since the isomerization
chain being more prone to isomerization. For example, for process in acid presumably took place via carbenium ion
comparison to 2, we also synthesized an iso-branched four- formation.25 This hypothesis led to an extremely successful
carbon shorter alkyl chain analogue 14 from 3d (Scheme S1, outcome and adding TIPS in the reaction completely sup-
ESI†), which did not undergo isomerization during double pressed the isomerization of 1 or 2 in sulfuric acid, and no
Knorr cyclization in H2SO4. Moreover, in a side by side isomerization products were detected up to 60 min (Fig. 2d and
comparison, 9-methyldecanoic acid (8b) showed notable isom- 3c). In short, 3a or 3b was treated with H2SO4 and TIPS (5 : 1) at
erization aer being treated in conc. H2SO4 for 30 min at room room temperature for 15 min, followed by stirring in methanol
temperature (Fig. S1, ESI†), whereas 4-methylpentanoic acid for 2 h to complete the oxidation to diazaquinomycin scaffolds
with a ve-carbon shorter chain was stable under the same (Scheme 5). This provided pure samples of diazaquinomycins H
conditions even aer 18 h (Fig. S2, ESI†).
(1) and J (2) in 89% and 84% isolated yields, respectively.
The isomerization of iso-branched analogues may result
A direct comparison of 1H and 13C NMR spectra for isomeric
from carbenium ion formation near the terminal end of the C-5 mixture and pure diazaquinomycin J (2) was made and is shown
aliphatic chain in sulfuric acid, followed by hydride or methide in Fig. 3d–g. When only sulfuric acid was used in double Knorr
migration.24,25 A proposed isomerization mechanism for repre- cyclization, 2 and its isomers were produced. The doublet for
sentative diazaquinomycin J (2) is shown in Scheme 4. Unlike the C-5 iso-branch (H-20/H-21) at 0.87 ppm was seen over-
expected, the isomerization pathway probably proceeds via the lapping with a multiplet (Fig. 3d). In contrast, when TIPS was
initial formation of a secondary carbocation center, in which included in the double Knorr cyclization of 3b in sulfuric acid,
subsequent rearrangement results in anteiso and propyl-methyl pure diazaquinomycin J (2) was obtained, and the multiplet was
isomers. In the case of initial formation of a tertiary carbocation absent from the 1H NMR spectrum with a clean doublet at
center at the iso-branched tail, rearrangement may not take 0.87 ppm observed (Fig. 3e). The 13C NMR spectrum for the
place due to its potential to generate a less favorable primary isomeric mixture of 2 showed multiple sets of carbon signals,
carbocation intermediate. Therefore, the secondary carbenium one of which corresponded to those of reported natural dia-
ion formation in the C-5 side chain is induced instead, upon zaquinomycin J (2)1 (Fig. 3f). When the double Knorr cyclization
exposure of 2 to concentrated sulfuric acid. Skeletal rearrange- was carried out in the presence of TIPS, the extra carbon peaks
ment of the carbenium ion then takes place via a non-classical in the 13C NMR spectrum of isolated pure diazaquinomycin J (2)
protonated cyclopropane intermediate25,27,34,35 to form a rear- were absent (Fig. 3g). It should be noted, the additional carbon
ranged carbenium ion. The newly formed carbenium ion signals in the isomeric mixture (Fig. 3f) could not be completely
further undergoes intermolecular hydride transfer with a reac- assigned; however, characteristic chemical shi patterns in the
tant alkane to form an isomerized alkane (anteiso-isomer), up eld region (<21 ppm) matched extremely well with those of
while generating another carbenium ion and making the anteiso and propyl-methyl isomers (Fig. 3a). The presence of
process catalytic. In a similar manner, the formed anteiso- carbon signals at 11 and 19 ppm strongly suggested the pres-
isomer can also be rearranged to form additional isomers ence of an anteiso-type36–39 substitution pattern and the pres-
(e.g., propyl-methyl isomer).
ence of 14, 19 and 20 ppm signals strongly suggested the
presence of a propyl-methyl isomer form.36,39 In each case, the
terminal methyl group migrated to the C-18 or C-17 position of
the alkyl chain, respectively. The direct comparison of NMR
spectra for the isomeric mixture of 1 and pure synthetic 1 is
provided in ESI (Fig. S3†).
Adding TIPS effectively preventing isomerization in double
Knorr cyclization of 3a or 3b in sulfuric acid
As shown in Fig. 2b, c and 3b, the double Knorr cyclization of 3a
and 3b in sulfuric acid resulted in signicant isomerization of
the terminal C-5 iso-branched aliphatic chain. Attempts to
prevent isomerization by lowering the reaction temperature or
shortening the exposure time to sulfuric acid were unsuccess-
Comparison of 1H and 13C NMR data of natural and synthetic
diazaquinomycins H (1) and J (2)
ful. To overcome this problem, we envisioned that adding Finally, the 1H and 13C NMR spectroscopic data of synthesized
a carbocation scavenger such as triisopropylsilane (TIPS) in the diazaquinomycins H (1) and J (2) compared very favorably with
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Fig. 3 (a) Structures of diazaquinomycin J (2) and proposed anteiso and propyl-methyl isomers. (b) LC-MS chromatogram of isomeric mixture of
2; (c) LC-MS chromatogram of pure synthetic 2; (d) zoomed 1H NMR spectrum of isomeric mixture of 2; (e) zoomed 1H NMR spectrum of pure
synthetic 2; (f) zoomed 13C NMR spectrum of isomeric mixture of 2; (g) zoomed 13C NMR spectrum of pure synthetic 2. For clarity, LC-MS
analyses shown in (b) and (c) were recorded with MS detector set to monitor m/z ¼ 397 ꢂ 2.
reported data for the isolated natural products1 (Tables 2 and 3). slight up eld shi is likely due to a difference in percentage of
It should also be noted, NH resonance was not observed in our CF3CO2D in CDCl3. In our hands, we observed notable varia-
1H NMR spectra, presumably due to overlapping with CF3CO2D tions to the chemical shis of protons that reside close to the
co-solvent or hydrogen/deuterium exchange (Table 2). In addi- 1,8-anthraquinone core (H-3, H-6, H-11, and H-12) when the
tion, for synthetic diazaquinomycin H (1), protons H-3 and H-6 percentage of CF3CO2D varied from 0.4 to 2% (ESI, Fig. S4†).
resonate at 7.00 ppm, agreeing with reported value for 11.
As shown in Table 3, the 13C NMR data for synthetic dia-
However, for the natural sample of diazaquinomycin J (2), zaquinomycin H and J samples also compared favorably with
protons H-3 and H-6 were reported at 6.93 ppm.1 Given the reported data for the isolated natural products.1 However, an
volatile nature of added CF3CO2D co-solvent, we believe, this inconsistency was noticed for the reported chemical shi of
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Scheme 5 Optimized synthesis of diazaquinomycins H (1) and J (2).
amide carbons (C-2/C-7) for 2 that was up eld shied (D z 2.5
and 2.6 ppm), when compared to both synthetic samples of 1
and 2 as well as natural sample of 1. It is worthwhile
mentioning, these reported chemical shi values overlap with
the CF3CO2D co-solvent peaks, which could make their precise
assignments challenging. In our hands, the signals for amide
carbons (C-2/C-7) in 2 clearly resonate at 163.6 or 163.4 ppm and
are consistent with those of other diazaquinomycin samples.16
It appears that the signal intensity for amide carbons (C-2/C-7)
in diazaquinomycin J (2) is signicantly lower in our DEPTQ
spectrum relative to 13C NMR spectrum (ESI†). Notably, these
signals were almost completely absent in our DEPTQ spectrum
for diazaquinomycin J (2). In this light, it is possible that the
amide carbons (C-2/C-7) were not observed in the reported
DEPTQ spectrum of 2 and therefore were assumed to overlap
with the CF3CO2D co-solvent. More extensive two dimensional
HMBC, HSQC and COSY NMR studies (ESI†) of synthetic dia-
zaquinomycins H (1) and J (2) further conrmed their structures
and also matched well with reported spectra of isolated natural
products.1
Experimental section
General methods
Solvents and reagents were purchased from Sigma-Aldrich,
Acros, or Fisher Scientic, and used without further purica-
tion. 8-Methylnonanoic acid (8a) and 9-methyldecanoic acid
(8b) were purchased from BOC Sciences and Synnovator Inc,
respectively. Reactions were monitored either by thin-layer
chromatography (TLC) or by analytical high performance
liquid chromatography (HPLC) employing a Shimadzu LC-20A
series HPLC system. 1H and 13C NMR spectra were recorded
on a Bruker Avance III HD-400 (ultrashield) spectrometer (400
MHz and 100 MHz, respectively). All chemical shis are given as
d value (ppm) and coupling constants, J, are reported in hertz
(Hz). NMR solvent peaks were referenced as follows: (1H NMR)
CDCl3: 7.27 ppm, DMSO-d6: 2.50 ppm. (13C NMR) CDCl3:
77.23 ppm, DMSO-d6: 39.51 ppm. s ¼ singlet; t ¼ triplet; q ¼
quartet; p ¼ pentet; m ¼ multiplet. High-resolution mass
spectroscopy (HRMS) data was obtained using an Agilent 6530
Q-TOF LC/MS. Melting points were determined in open
Table 2 Comparison of 1H NMR data of natural and synthetic diazaquinomycins H (1) and J (2)
Position
Naturala 11 d (ppm)
Syntheticb 1 d (ppm)
Dd (ppm)
Naturala 21 d (ppm)
Syntheticb 2 d (ppm)
Dd (ppm)
1, 8
3, 6
12
11
13
18
14
15
16
17
19
20
21
8.05 (s)
7.00 (s)
3.13 (t)
2.77 (s)
1.60 (p)
1.53 (m)
1.47 (p)
1.35 (m)
1.31 (m)
1.19 (q)
0.87 (d)
0.87 (d)
H/D exc.c
7.00 (s)
3.14 (t)
2.77 (s)
1.66–1.57 (m)
1.57–1.50 (m)
1.50–1.43 (m)
1.40–1.27 (m)
1.40–1.27 (m)
1.22–1.15 (m)
0.88 (d)
—
8.03 (s)
6.93 (s)
3.10 (t)
H/D exc.c
7.00 (s)
3.13 (t)
2.77 (s)
1.66–1.57 (m)
1.21–1.13 (m)
1.50–1.42 (m)
1.40–1.34 (m)
1.33–1.27 (m)
1.33–1.27 (m)
1.57–1.50 (m)
0.87 (d)
—
0.00
0.01
0.00
0.07
0.03
0.03
0.02
0.00
0.01
0.02
2.74 (s)
1.59 (m)
1.17 (m)
1.45 (m)
1.35 (m)
1.31 (m)
1.28 (m)
1.52 (m)
0.86 (d)
0.86 (d)
ꢀ0.01
0.00
0.00
ꢀ0.01
0.03
ꢀ0.01
0.00
0.01
0.01
0.02
0.01
0.01
0.01
0.88 (d)
0.87 (d)
a
b
900 MHz (CDCl3/1% CF3CO2D). 400 MHz (CDCl3/1% CF3CO2D). c Hydrogen/deuterium exchange.
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Table 3 Comparison of 13C NMR data of natural and synthetic diazaquinomycins H (1) and J (2)
Position
Naturala 11 d (ppm)
Syntheticb 1 d (ppm)
Dd (ppm)
Naturala 21 d (ppm)
Syntheticb 2 d (ppm)
Dd (ppm)
10
9
2
7
5
180.1
172.9
163.2
163.0
160.1c
155.6
136.8
136.3
128.6
127.6
118.0
117.5
39.1
35.0
29.7
29.7
29.7
28.1
27.4
23.1
22.8
180.1
172.9
163.7
163.5
160.8c
156.6
137.0
136.5
128.3
127.3
118.6
118.1
39.2
35.1
29.9
29.8
29.8
28.2
27.4
23.1
22.8
0.0
0.0
0.5
0.5
0.7
1.0
0.2
0.2
ꢀ0.3
ꢀ0.3
0.6
0.6
0.1
0.1
0.2
0.1
0.1
0.1
0.0
0.0
0.0
0.0
180.0
173.0
161.0c
160.9c
160.2c
154.6
136.8
136.2
128.9
128.0
118.3
117.8
27.5
35.0
29.9
29.8
29.7
39.1
29.6
23.0
28.1
180.0
172.8
163.6
163.4
161.2c
156.6
136.9
136.4
128.3
127.3
118.6
118.1
27.6
35.1
29.9
29.9
29.7
39.2
29.5
23.2
28.2
0.0
ꢀ0.2
2.6
2.5
1.0
2.0
0.1
0.2
ꢀ0.6
ꢀ0.7
0.3
0.3
0.1
0.1
0.0
0.1
0.0
4
9a
8a
3
6
4a
10a
17
12
13
14
15
18
16
11
19
20
21
0.1
ꢀ0.1
0.2
0.1
0.0
0.0
22.8
22.8
22.8
22.8
22.8
22.8
a
b
c
226.2 MHz (CDCl3/1% CF3CO2D). 100 MHz (CDCl3/2% CF3CO2D). Overlapping with TFA co-solvent peak.
capillary tube using a Buchi B-540 melting point apparatus. mL) was added to a stirring suspension of 3a (12.0 mg, 0.0286
Compounds were puried by ash column chromatography on mmol) and triisopropylsilane (TIPS) (0.12 mL) at room
silica gel using a Biotage Isolera One system. The purity of temperature (rapid evolution of gas was observed). The dark
compounds was determined by analytical HPLC (Shimadzu LC- brown solution was stirred at room temperature for 15 min,
˚
20A series) using a Gemini, 3 mm, C18, 110 A column (50 mm ꢃ then diluted with water (5 mL) to precipitate a sticky beige solid
4.6 mm, Phenomenex) and ow rate of 1 mL minꢀ1. Gradient (became hot). The hot suspension was stirred for 10 min, cooled
conditions: solvent A (0.1% triuoroacetic acid in water) and to room temperature, and the water layer was removed via
solvent B (acetonitrile): 0–2.0 min 100% A, 2.0–7.0 min 0–100% pipette. The beige solid was dissolved in MeOH (150 mL) to give
B (linear gradient), 7.0–8.0 min 100% B, UV detection at 254 nm an orange solution that was stirred at room temperature for 2 h
and 220 nm. LC-MS data was obtained using an Agilent 1260 open to air. Water (10 mL) was added and the solution was
Innity HPLC system coupled with an Agilent G6120A quadru- concentrated in vacuo using a rotary evaporator to precipitate
pole mass spectrometer. HPLC separation was performed using a solid. The mixture was washed with diethyl ether (25 mL ꢃ 3),
˚
a SunFire C18 column (Waters), 100 A, 3.5 mm, 2.1 mm ꢃ 150 and then centrifuged to collect the solid. The resulting solid was
mm, and ow rate of 0.2 mL minꢀ1. Gradient conditions: dried under vacuum to afford 1 (9.70 mg, 0.0254 mmol, 89%) as
solvent A (0.1% formic acid in water) and solvent B (0.1% formic a red solid aer trituration with EtOAc. Rf ¼ 0.41 (DCM/MeOH,
acid in acetonitrile): 0–5.0 min 40% B, 5.0–15.0 min 40–100% B 9 : 1); mp > 200 ꢁC (dec); 1H NMR (400 MHz, CDCl3/1%
(linear gradient), 15.0–18.0 min 100% B. The mass spectrometer CF3CO2D): d ¼ 7.00 (s, 2H), 3.14 (t, J ¼ 7.3 Hz, 2H), 2.77 (s, 3H),
was run in (+)-ESI mode with a scan range (m/z) of 100–1700.
1.66–1.57 (m, 2H), 1.57–1.50 (m, 1H), 1.50–1.43 (m, 2H), 1.40–
1.27 (m, 4H), 1.22–1.15 (m, 2H), 0.88 (d, J ¼ 6.7 Hz, 6H); 1H NMR
(400 MHz, CDCl3/2% CF3CO2D): d ¼ 7.04 (s, 2H), 3.16 (t, J ¼
7.3 Hz, 2H), 2.79 (s, 3H), 1.66–1.57 (m, 2H), 1.57–1.50 (m, 1H),
1.50–1.44 (m, 2H), 1.40–1.27 (m, 4H), 1.22–1.15 (m, 2H), 0.88 (d,
J ¼ 6.7 Hz, 6H); 13C NMR (100 MHz, CDCl3/2% CF3CO2D): d ¼
180.1, 172.9, 163.7, 163.5, 160.8, 156.6, 137.0, 136.5, 128.3,
Double Knorr cyclization using H2SO4/TIPS; syntheses of
diazaquinomycins H (1) and J (2)
4-Methyl-5-(70-methyloctyl)-1H,8H-1,8-diazaanthracene-
2,7,9,10-tetraone (diazaquinomycin H, 1). Sulfuric acid (0.60
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127.3, 118.6, 118.1, 39.2, 35.1, 29.9, 29.8, 29.8, 28.2, 27.4, 23.1, 457.2309, found: 457.2320; HPLC purity: 100% (254 nm), 100%
22.8; HRMS (ESI): m/z [M + H]+ calcd for C22H27N2O4+: 383.1965, (220 nm), tR: 7.19 min.
found: 383.1969; HPLC purity: 100% (254 nm), 100% (220 nm),
tR: 7.18 min.
N-(2,5-Dihydroxy-3-(3-oxobutanamido)phenyl)-3-
oxoundecanamide (3c). N-(2,5-Bis(methoxymethoxy)-3-(3-
oxobutanamido)phenyl)-3-oxoundecanamide (90.0 mg, 0.182
4-Methyl-5-(80-methylnonyl)-1H,8H-1,8-diazaanthracene-
2,7,9,10-tetraone (diazaquinomycin J, 2). Following the opti- mmol) afforded 3c (30.0 mg, 0.0739 mmol, 41%) as an off white
mized double Knorr cyclization procedure using H2SO4/TIPS; solid aer ash column chromatography (silica gel, hexanes/
compound 3b (20.0 mg, 0.0461 mmol) afforded 2 (15.4 mg, EtOAc 9 : 1 to 1 : 2, v/v). Rf ¼ 0.11 (EtOAc/hexanes, 1 : 1); mp
0.0389 mmol, 84%) as an orange-red solid. Rf ¼ 0.45 (DCM/ 159–160 ꢁC; 1H NMR (400 MHz, DMSO-d6): d ¼ 9.77 (s, 2H), 9.01
1
ꢁ
MeOH, 9 : 1); mp > 200 C (dec); H NMR (400 MHz, CDCl3/1% (s, 1H), 8.74 (s, 1H), 6.99 (d, J ¼ 2.9 Hz, 1H), 6.97 (d, J ¼ 2.9 Hz,
CF3CO2D): d ¼ 7.00 (s, 2H), 3.13 (t, J ¼ 7.3 Hz, 2H), 2.77 (s, 3H), 1H), 3.67 (s, 2H), 3.65 (s, 2H), 2.53 (t, J ¼ 7.2 Hz, 2H), 2.19 (s,
1.66–1.57 (m, 2H), 1.57–1.50 (m, 1H), 1.50–1.42 (m, 2H), 1.40– 3H), 1.52–1.43 (m, 2H), 1.29–1.19 (m, 10H), 0.85 (t, J ¼ 7.0 Hz,
1.34 (m, 2H), 1.33–1.27 (m, 4H), 1.21–1.13 (m, 2H), 0.87 (d, J ¼ 3H); 13C NMR (100 MHz, DMSO-d6): d ¼ 205.1, 203.1, 166.0,
6.7 Hz, 6H); 1H NMR (400 MHz, CDCl3/2% CF3CO2D): d ¼ 7.02 (s, 165.9, 149.9, 131.1, 127.87, 127.85, 104.77, 104.74, 51.5, 50.8,
2H), 3.15 (t, J ¼ 7.3 Hz, 2H), 2.79 (s, 3H), 1.66–1.57 (m, 2H), 1.57– 42.2, 31.2, 30.2, 28.8, 28.6, 28.5, 22.9, 22.1, 13.9; HRMS (ESI): m/z
+
1.50 (m, 1H), 1.50–1.43 (m, 2H), 1.41–1.35 (m, 2H), 1.33–1.25 (m, [M + H]+ calcd for C21H31N2O6 : 407.2177, found: 407.2179; [M +
+
4H), 1.21–1.13 (m, 2H), 0.87 (d, J ¼ 6.7 Hz, 6H); 13C NMR (100 Na]+ calcd for C21H30N2NaO6 : 429.1996, found: 429.1998; HPLC
MHz, CDCl3/2% CF3CO2D): d ¼ 180.0, 172.8, 163.6, 163.4, 161.2, purity: 98.3% (254 nm), 98.5% (220 nm), tR: 6.83 min.
156.7, 136.9, 136.4, 128.3, 127.3, 118.6, 118.1, 39.2, 35.1, 30.0,
29.9, 29.7, 29.5, 28.2, 27.6, 23.2, 22.8; HRMS (ESI): m/z [M + H]+
calcd for C23H29N2O4 : 397.2122, found: 397.2126; HPLC purity:
+
Typical EDC coupling procedure for the synthesis of di-b-
ketoanilides
100% (254 nm), 100% (220 nm), tR: 7.40 min.
N-(2,5-Bis(methoxymethoxy)-3-(3-oxobutanamido)phenyl)-
10-methyl-3-oxoundecanamide (4a). EDC$HCl (273 mg, 1.42
mmol) was added to a stirring solution of 5a (200 mg, 0.472
Typical procedure for removal of MOM protecting groups
N-(2,5-Dihydroxy-3-(3-oxobutanamido)phenyl)-10-methyl-3-
mmol), 3-oxobutanoic acid (144 mg, 1.42 mmol) and DMAP
ꢁ
oxoundecanamide (3a). Compound 4a (226 mg, 0.445 mmol) (11.0 mg, 0.0902 mmol) in DCM (10 mL) at 0 C. The solution
was dissolved in 1 M HCl/THF/acetone (1 : 1 : 1, 70 mL) and was stirred at 0–5 ꢁC for 30 min. The reaction was diluted with
stirred at 65 ꢁC for 2 h under nitrogen protection. The reaction DCM (100 mL) and washed with water (50 mL ꢃ 2). The organic
was cooled, diluted with EtOAc (200 mL) and washed with water layer was dried (anhyd. Na2SO4), ltered and concentrated to
(100 mL ꢃ 3). The organic layer was dried (anhydrous Na2SO4), yield 4a (200 mg, 0.394 mmol, 83%) as a sticky pale yellow oil
ltered, and concentrated to give 3a (160 mg, 0.381 mmol, 86%) aer purication by ash column chromatography (silica gel,
as an off white solid aer trituration with diethyl ether (5 mL). Rf hexanes/EtOAc 9 : 1 to 3 : 2, v/v). Rf ¼ 0.23 (EtOAc/hexanes,
1
¼ 0.11 (EtOAc/hexanes, 1 : 1); mp > 150 C (dec); H NMR (400 1 : 1); 1H NMR (400 MHz, CDCl3): d ¼ 9.42 (s, 1H), 9.37 (s,
ꢁ
MHz, DMSO-d6): d ¼ 9.78 (s, 2H), 9.02 (s, 1H), 8.74 (s, 1H), 6.99 1H), 7.79 (s, 2H), 5.15 (s, 2H), 5.07 (s, 2H), 3.65 (s, 3H), 3.58 (s,
(d, J ¼ 2.9 Hz, 1H), 6.97 (d, J ¼ 2.9 Hz, 1H), 3.67 (s, 2H), 3.65 (s, 2H), 3.55 (s, 2H), 3.47 (s, 3H), 2.58 (t, J ¼ 7.5 Hz, 2H), 2.33 (s,
2H), 2.53 (t, J ¼ 7.2 Hz, 2H), 2.19 (s, 3H), 1.53–1.43 (m, 3H), 1.26– 3H), 1.66–1.56 (m, 2H), 1.55–1.45 (m, 1H), 1.32–1.23 (m, 6H),
1.20 (m, 6H), 1.17–1.09 (m, 2H), 0.84 (d, J ¼ 6.6 Hz, 6H); 13C 1.18–1.10 (m, 2H), 0.85 (d, J ¼ 6.5 Hz, 6H); 13C NMR (100 MHz,
NMR (100 MHz, DMSO-d6): d ¼ 205.1, 203.1, 166.0, 165.9, 149.9, CDCl3): d ¼ 206.8, 204.1, 163.8, 163.7, 154.2, 132.4, 131.83,
131.1, 127.9, 104.8, 104.7, 51.5, 50.8, 42.2, 38.4, 30.2, 29.1, 28.5, 131.78, 105.0, 104.9, 101.3, 94.9, 58.4, 56.4, 51.3, 50.4, 44.2, 39.1,
27.4, 26.6, 22.9, 22.5; HRMS (ESI): m/z [M + H]+ calcd for 31.2, 29.8, 29.2, 28.1, 27.4, 23.6, 22.8; HRMS (ESI): m/z [M + H]+
+
C
C
22H33N2O6+: 421.2333, found: 421.2330; [M + Na]+ calcd for calcd for C26H41N2O8 : 509.2857, found: 509.2846; [M + Na]+
22H32N2NaO6+: 443.2153, found: 443.2151; HPLC purity: 100% calcd for C26H40N2NaO8 : 531.2677, found: 531.2673; HPLC
+
(254 nm), 100% (220 nm), tR: 7.00 min.
purity: 99.6% (254 nm), 99.8% (220 nm), tR: 7.40 min.
N-(2,5-Bis(methoxymethoxy)-3-(3-oxobutanamido)phenyl)-
N-(2,5-Dihydroxy-3-(3-oxobutanamido)phenyl)-11-methyl-3-
oxododecanamide (3b). Compound 4b (88 mg, 0.168 mmol) 11-methyl-3-oxododecanamide(4b). Compound 5b (145 mg,
provided 3b (70.0 mg, 0.161 mmol, 96%) as an off white solid 0.331 mmol) afforded 4b (125 mg, 0.239 mmol, 73%) as a sticky
aer trituration with diethyl ether (2 mL). Rf ¼ 0.12 (EtOAc/ pale yellow oil aer ash column chromatography (silica gel,
1
ꢁ
hexanes, 1 : 1); mp > 150 C (dec); H NMR (400 MHz, DMSO- hexanes/EtOAc 9 : 1 to 3 : 2, v/v). Rf ¼ 0.26 (EtOAc/hexanes,
d6): d ¼ 9.77 (s, 2H), 9.00 (s, 1H), 8.75 (s, 1H), 6.99 (d, J ¼ 2.8 Hz, 1 : 1); 1H NMR (400 MHz, CDCl3): d ¼ 9.44 (s, 1H), 9.39 (s,
1H), 6.98 (d, J ¼ 2.8 Hz, 1H), 3.67 (s, 2H), 3.65 (s, 2H), 2.53 (t, J ¼ 1H), 7.80 (s, 2H), 5.15 (s, 2H), 5.07 (s, 2H), 3.66 (s, 3H), 3.59 (s,
7.2 Hz, 2H), 2.20 (s, 3H), 1.53–1.43 (m, 3H), 1.27–1.20 (m, 8H), 2H), 3.56 (s, 2H), 3.47 (s, 3H), 2.59 (t, J ¼ 7.5 Hz, 2H), 2.34 (s,
1.16–1.09 (m, 2H), 0.84 (d, J ¼ 6.6 Hz, 6H); 13C NMR (100 MHz, 3H), 1.66–1.57 (m, 2H), 1.56–1.45 (m, 1H), 1.33–1.22 (m, 8H),
DMSO-d6): d ¼ 205.1, 203.1, 166.00, 165.95, 149.9, 131.1, 127.9, 1.18–1.10 (m, 2H), 0.86 (d, J ¼ 6.5 Hz, 6H); 13C NMR (100 MHz,
104.8, 104.7, 51.6, 50.8, 42.2, 38.5, 30.2, 29.2, 28.9, 28.5, 27.4, CDCl3): d ¼ 206.8, 204.2, 163.8, 163.7, 154.1, 132.4, 131.81,
26.8, 22.9, 22.5; HRMS (ESI): m/z [M + H]+ calcd for C23H35N2O6 : 131.77, 105.0, 104.9, 101.3, 94.9, 58.4, 56.4, 51.3, 50.4, 44.2, 39.2,
+
435.2490, found: 435.2495; [M + Na]+ calcd for C23H34N2NaO6 : 31.2, 29.9, 29.6, 29.2, 28.1, 27.5, 23.6, 22.8; HRMS (ESI): m/z [M +
+
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H]+ calcd for C27H43N2O8+: 523.3014, found: 523.3012; [M + Na]+ 6.6 Hz, 6H); 13C NMR (100 MHz, CDCl3): d ¼ 198.5, 170.8, 160.4,
+
calcd for C27H42N2NaO8 : 545.2833, found: 545.2844; HPLC 104.9, 91.4, 39.1, 35.9, 29.7, 29.6, 28.1, 27.3, 27.0, 26.4, 22.8;
purity: 100% (254 nm), 100% (220 nm), tR: 7.59 min.
HRMS (ESI): m/z [M ꢀ H]ꢀ calcd for C16H25O5ꢀ: 297.1707,
found: 297.1709; HPLC purity: 100% (254 nm), 100% (220 nm),
tR: 7.99 min.
5-(1-Hydroxy-9-methyldecylidene)-2,2-dimethyl-1,3-dioxane-
4,6-dione (7b). The following procedure was modied from
Typical procedure for the synthesis of mono-b-ketoanilides
N-(3-Amino-2,5-bis(methoxymethoxy)phenyl)-10-methyl-3-
oxoundecanamide (5a). To a solution of 6 (200 mg, 0.877 mmol) a literature protocol.17 To a solution of 9-methyldecanoic acid
in dry CH3CN (15 mL) was added 7a (157 mg, 0.527 mmol) (300 mg, 1.61 mmol), Meldrum's acid (278 mg, 1.93 mmol) and
followed by stirring at 45 ꢁC for 48 h under nitrogen protection. DMAP (235 mg, 1.93 mmol) in DCM (8 mL) at 5 C was added
ꢁ
The solvent was removed in vacuo and the remaining residue DCC (398 mg, 1.93 mmol). The solution was allowed to warm to
was puried by ash column chromatography (silica gel, room temperature and stirred for 3 h under nitrogen atmo-
hexanes/EtOAc 9 : 1 to 4 : 1, v/v), affording 5a (205 mg, sphere. The mixture was diluted with DCM (20 mL) and ltered.
0.483 mmol, 92%) as a clear oil. Rf ¼ 0.45 (EtOAc/Hexanes, The ltrate was washed with 1 M HCl (20 mL ꢃ 2), water (20
1
1 : 1); H NMR (400 MHz, CDCl3): d ¼ 9.34 (s, 1H), 7.43 (d, J ¼ mL) and brine (20 mL). The organic layer was dried (Na2SO4),
2.9 Hz, 1H), 6.23 (d, J ¼ 2.9 Hz, 1H), 5.09 (s, 2H), 5.00 (s, 2H), ltered and concentrated to afford 7b (420 mg, 1.35 mmol, 84%)
3.95 (br s, 2H), 3.62 (s, 3H), 3.53 (s, 2H), 3.45 (s, 3H), 2.57 (t, J ¼ as a clear oil aer ash column chromatography (silica gel,
7.5 Hz, 2H), 1.65–1.55 (m, 2H), 1.54–1.44 (m, 1H), 1.32–1.23 (m, hexanes/EtOAc 9 : 1 to 6 : 1, v/v). Rf ¼ 0.28 (EtOAc/hexanes,
1
6H), 1.17–1.10 (m, 2H), 0.85 (d, J ¼ 6.7 Hz, 6H); 13C NMR (100 1 : 4). H NMR (400 MHz, CDCl3): d ¼ 15.29 (s, 1H), 3.09–3.03
MHz, CDCl3): d ¼ 206.8, 163.6, 154.6, 140.4, 132.3, 130.2, 100.3, (m, 2H), 1.73 (s, 6H), 1.72–1.64 (m, 2H), 1.55–1.45 (m, 1H), 1.44–
99.9, 99.2, 94.8, 58.0, 56.2, 50.5, 44.2, 39.1, 29.7, 29.2, 28.1, 27.3, 1.35 (m, 2H), 1.35–1.29 (m, 2H), 1.29–1.23 (m, 4H), 1.18–1.10
+
23.6, 22.8; HRMS (ESI): m/z [M + H]+ calcd for C22H37N2O6 : (m, 2H), 0.86 (d, J ¼ 6.6 Hz, 6H); 13C NMR (100 MHz, CDCl3): d ¼
+
425.2646, found: 425.2645; [M + Na]+ calcd for C22H36N2NaO6 : 198.5, 170.8, 160.4, 104.9, 91.4, 39.2, 35.9, 29.9, 29.6, 29.5, 28.1,
447.2466, found: 447.2460; HPLC purity: 100% (254 nm), 100% 27.5, 27.0, 26.4, 22.8; HRMS (ESI): m/z [M ꢀ H]ꢀ calcd for
(220 nm), tR: 7.30 min.
C
17H27O5ꢀ: 311.1864, found: 311.1860; HPLC purity: 100% (254
N-(3-Amino-2,5-bis(methoxymethoxy)phenyl)-11-methyl-3-
oxododecanamide (5b). Used typical procedure; compound 6
nm), 100% (220 nm), tR: 8.16 min.
4-Methyl-5-octyl-1H,8H-1,8-diazaanthracene-2,7,9,10-tetraone
(365 mg, 1.60 mmol) was coupled with 7b (300 mg, 0.962 mmol) (13). A solution of 3c (21.0 mg, 0.0517 mmol) in sulfuric acid (5
to provide 5b (350 mg, 0.799 mmol, 83%) as a clear oil that mL) was stirred at room temperature for 30 min. The reaction
solidied into an off white solid on standing. Rf ¼ 0.46 (EtOAc/ was chilled using an ice-water bath followed by careful addition
1
ꢁ
hexanes, 1 : 1); mp 64–65 C; H NMR (400 MHz, CDCl3): d ¼ of MeOH (20 mL) over a period of 10 min to keep internal
9.35 (s, 1H), 7.44 (d, J ¼ 2.9 Hz, 1H), 6.24 (d, J ¼ 2.9 Hz, 1H), 5.10 temperature < 30 ꢁC. The resulting solution was allowed to
(s, 2H), 5.01 (s, 2H), 3.98 (br s, 2H), 3.63 (s, 3H), 3.54 (s, 2H), 3.46 warm to room temperature and stirred for 3 h. Water (15 mL)
(s, 3H), 2.58 (t, J ¼ 7.5 Hz, 2H), 1.65–1.56 (m, 2H), 1.56–1.45 (m, was added and MeOH was removed in vacuo using a rotary
1H), 1.33–1.21 (m, 8H), 1.18–1.10 (m, 2H), 0.86 (d, J ¼ 6.7 Hz, evaporator (water bath temperature < 35 ꢁC) to precipitate
6H); 13C NMR (100 MHz, CDCl3): d ¼ 206.9, 163.6, 154.6, 140.4, a solid. The solid was collected by centrifugation and washed
132.4, 130.3, 100.4, 100.0, 99.3, 94.9, 58.0, 56.2, 50.5, 44.2, 39.2, with cold water (2 ꢃ 5 mL) using the centrifuge, carefully
29.9, 29.6, 29.2, 28.1, 27.5, 23.6, 22.8; HRMS (ESI): m/z [M + H]+ removing the supernatant layer between each washing step. The
+
calcd for C23H39N2O6 : 439.2803, found: 439.2782; [M + Na]+ solid was triturated with EtOAc (5 mL) and dried under vacuum
+
calcd for C23H38N2NaO6 : 461.2622, found: 461.2599; HPLC to afford 13 (18.8 mg, 0.0510 mmol, 99%) as a red solid. Rf ¼
purity: 100% (254 nm), 100% (220 nm), tR: 7.52 min.
0.41 (DCM/MeOH, 9 : 1); mp > 200 ꢁC (dec); 1H NMR (400 MHz,
5-(1-Hydroxy-8-methylnonylidene)-2,2-dimethyl-1,3-dioxane- CDCl3/1% CF3CO2D): d ¼ 7.01 (s, 2H), 3.14 (t, J ¼ 7.3 Hz, 2H),
4,6-dione (7a). The following procedure was modied from 2.78 (s, 3H), 1.65–1.57 (m, 2H), 1.51–1.43 (m, 2H), 1.41–1.28 (m,
a literature protocol.17 To a solution of 8-methylnonanoic acid 8H), 0.90 (t, J ¼ 7.0 Hz, 3H); 1H NMR (400 MHz, CDCl3/2%
(500 mg, 2.91 mmol), Meldrum's acid (502 mg, 3.49 mmol) and CF3CO2D): d ¼ 7.09 (s, 2H), 3.15 (t, J ¼ 7.3 Hz, 2H), 2.79 (s, 3H),
DMAP (426 mg, 3.49 mmol) in DCM (10 mL) at 5 ꢁC was added 1.66–1.57 (m, 2H), 1.52–1.43 (m, 2H), 1.41–1.28 (m, 8H), 0.90 (t, J
DCC (719 mg, 3.49 mmol). The solution was allowed to warm to ¼ 7.0 Hz, 3H); 13C NMR (100 MHz, CDCl3/2% CF3CO2D): d ¼
room temperature and stirred for 3 h under nitrogen atmo- 180.0, 172.8, 164.0, 163.9, 162.1, 157.5, 137.0, 136.5, 128.2,
sphere. The mixture was diluted with DCM (20 mL) and ltered. 127.2, 119.0, 118.5, 35.3, 32.1, 30.0, 29.8, 29.5, 29.4, 23.1, 22.9,
The ltrate was washed with 1 M HCl (20 mL ꢃ 2), water (20 14.1; HRMS (ESI): m/z [M + H]+ calcd for C21H25N2O4+: 369.1809,
mL) and brine (20 mL). The organic layer was dried (Na2SO4), found: 369.1816; HPLC purity: 99.2% (254 nm), 99.2% (220 nm),
ltered and concentrated to afford 7a (675 mg, 2.27 mmol, 78%) tR: 7.03 min.
as a clear oil aer ash column chromatography (silica gel,
hexanes/EtOAc 9 : 1 to 6 : 1, v/v). Rf ¼ 0.27 (EtOAc/hexanes,
Conclusions
1
1 : 4). H NMR (400 MHz, CDCl3): d ¼ 15.29 (s, 1H), 3.09–3.03
(m, 2H), 1.73 (s, 6H), 1.72–1.65 (m, 2H), 1.57–1.46 (m, 1H), 1.45– In summary, the rst total synthesis of anti-tuberculosis dia-
1.36 (m, 2H), 1.33–1.24 (m, 4H), 1.18–1.11 (m, 2H), 0.86 (d, J ¼ zaquinomycins H (1) and J (2) has been accomplished via
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¯
selective amidation of diamine 6 with Meldrum's acid deriva-
tives, subsequent EDC coupling with 3-oxobutanoic acid, and
double Knorr cyclization in the presence of TIPS. Adding TIPS
during the double Knorr cyclization was crucial in preventing
isomerization of the C-5 iso-branched tail in sulfuric acid.
Diazaquinomycins H (1) and J (2) were obtained in 8 steps from
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´
´
˜
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´
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´
´
to a terminal isopropyl functionality, we found that the length 12 J. M. Perez, P. Lopez-Alvarado, E. Pascual-Alfonso,
˜
´
of the iso-branched alkyl chain also played an important role in
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ported data for the natural products, with minor discrepancies
noted.
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Conflicts of interest
There are no conicts to declare.
20 H. Kikuchi, K. Sasaki, J. Sekiya, Y. Maeda, A. Amagai,
Y. Kubohara and Y. Oshima, Bioorg. Med. Chem., 2004, 12,
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Acknowledgements
This work was supported by Hawaii Community Foundation 21 M. Syrpas, E. Ruysbergh, L. Blommaert, B. Vanelslander,
LE'AHI FUND for Pulmonary Research (16ADVC-78728) and
UHH DKICP RTRF fund. We thank Justin Reinicke for his expert
K. Sabbe, W. Vyverman, N. De Kimpe and S. Mangelinckx,
Mar. Drugs, 2014, 12, 352–367.
support for developing LC-MS method and acquiring low reso- 22 C. Ma, X. Li, X. liang, K. Jin, J. Cao and W. Xu, Bioorg. Med.
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assistance with the synthesis of intermediate compound 6.
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