Total synthesis of ent-(-)-azonazine using a biomimetic direct oxidative cyclization and structural reassignment of natural product

Article abstract of DOI:10.1016/j.tet.2014.03.061

A biomimetic approach has been investigated and developed for the total synthesis of azonazine, an unusual marine natural cyclopeptide containing a rigid transannular 10-membered ring. A hypervalent iodine-mediated direct oxidative cyclization was success

Full text of DOI:10.1016/j.tet.2014.03.061

Tetrahedron 70 (2014) 3197e3210
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Total synthesis of ent-(ꢀ)-azonazine using a biomimetic direct
oxidative cyclization and structural reassignment of natural product
*
Ji-Chen Zhao, Shun-Ming Yu, Hai-Bo Qiu, Zhu-Jun Yao
State Key Laboratory of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A biomimetic approach has been investigated and developed for the total synthesis of azonazine, an
unusual marine natural cyclopeptide containing a rigid transannular 10-membered ring. A hypervalent
iodine-mediated direct oxidative cyclization was successfully developed and applied to construct the
highly strained core, which was the key step in the first total synthesis of ent-(ꢀ)-azonazine. Based on the
physical evidences of synthesized diastereomer and enantiomer of azonazine, both the relative and
absolute configurations of the natural product were revised. Two fluorinated azonazine derivatives were
also synthesized in short convenient steps utilizing the same intermediate in this work. The established
total synthesis opens a potential opportunity to study the structureeactivity relationship of natural
azonazine.
Received 11 February 2014
Accepted 10 March 2014
Available online 22 March 2014
Keywords:
Biomimetic synthesis
Azonazine
Stereochemical assignment
Oxidative cyclization
Hypervalent iodine
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
dipeptide from Aspergillus insulicola, a Hawaiian marine sediment-
derived fungus.2 This compound was found to exhibit anti-
Fungi within the genus Aspergillus provides an abundant source
of bioactive small molecules possessing diverse structures.1 Azo-
nazine (1a, Fig. 1) was recently reported as a novel hexacyclic
inflammatory activity by inhibiting NF-
k
B
luciferase (IC50
8.37 M) and nitrite production (IC50 13.70
m
mM). The originally
proposed structure of azonazine (1a) contains a benzofuranoindo-
line ring system bearing a quaternary center at C10 position, which
presents similarity to the core of diazonamide A3 (2, Fig. 1). How-
ever, a unique transannular 10-membered ring (F) connecting the
tetracyclic moiety and the diketopiperazine unit (E) makes the
skeleton of azonazine much more rigid than diazonamide A. It
could be deduced that such tension would distorted the benzene
ring of dihydrobenzofuran out of a planar structure. The unique
challenging structure and interesting biological activity make
azonazine an attractive target for total synthesis.4e6 Bisai and co-
workers recently developed a straightforward access to the core
of azonazine (1a) by a Lewis acid catalyzed FriedeleCrafts reaction
of electron rich aromatics with 3-alkyl-3-hydroxy-2-oxoindole.5
Vincent and co-workers also reported a close method to con-
struct the benzofuroindolines and chromenoindoline derivatives by
FeCl3-promoted regioselective hydroarylation of N-acetyl indoles
with phenols, followed by an oxidation step.6 Very recently, our
group disclosed the first total synthesis of (ꢀ)-azonazine,7 in which
a hypervalent iodine-mediated biomimetic oxidative cyclization
process was applied to construct the highly strained 10-membered
ring. Both the relative and absolute structure of natural (þ)-azo-
nazine were reassigned by assistance of the synthetic compounds
and their physical characterizations. Herein, we wish to detail our
campaigns toward the original proposed structure (1a) as well as
Fig. 1. Retrosynthetic analysis of the proposed azonazine (1a).
* Corresponding author. Tel.: þ86 21 54925133; fax: þ86 21 64166128; e-mail
addresses: yaoz@sioc.ac.cn, yaoz@nju.edu.cn (Z.-J. Yao).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.03.061

3198
J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
the structure revision, which finally led to first total synthesis of
ent-(ꢀ)-azonazine.
2. Results and discussion
Construction of the benzofuranoindoline ring system (Fig. 1, A/
B/C/D rings of azonazine 1a) bearing a highly strained transannular
10-membered ring (F ring) is the most arduous task in the whole
synthesis. In order to achieve a higher efficiency in our synthesis, an
oxidative cyclization mimicking the biogenesis pathway2,4b was
considered as a preferred method. According to our retrosynthesis
(Fig. 1), the de-N-acetyl precursor indoline 3 could be prepared
from the corresponding diketopiperazine 4. The key C8eC10 bond
were devised to form through a direct biomimetic oxidative cycli-
zation of the indole and phenol functionalities. Diketopiperazine 4
could be conveniently prepared from commercially available
and -Trp derivatives.
-Tyrosine derivative 5 and
applied as the starting materials in our synthesis (Scheme 1).
Double methylation of
-N-Boc-Tyr-OH (5) provided compound 6,8
which was then transformed to -N-Me-Tyr(OMe)-OMe hydro-
chloride salt (7) by treatment with thionyl chloride in methanol.
Coupling of -N-Boc-Trp-OH (8) with 7 was carried out under BEP-
D-Tyr
D
D
D
-tryptophan derivative 8 were
D
D
Scheme 2. Oxidative cyclization of diketopiperazine 4 with PhI(OAc)2. Reagents and
conditions: (a) PhI(OAc)2 (1 equiv), LiOAc, TFE, ꢀ15 ꢁC, 30 min, 11a (<5% yield), 11b
(<5% yield).
D
mediated conditions,9,10 providing the dipeptide 9. Cyclization of 9
to diketopiperazine 10 was achieved by deprotection of N-Boc
functionality with 3 M HCl in EtOAc and subsequent treatment with
aqueous NaHCO3. O-Demethylation of 10 was achieved by treat-
ment with BBr3 in dichloromethane, affording precursor 4 for ex-
amination of the key oxidative cyclization.
Table 1
Optimization of oxidative cyclization of 4a
Conditions
Results or yield (%)
4
11a
11b
1
2
3
4
5
6
7
8
9
PIDA (1 equiv), LiOAc, TFE, ꢀ15 ꢁC, 30 min
PIFA (1 equiv), LiOAc, TFE, ꢀ15 ꢁC, 30 min
HTIB (1 equiv), LiOAc, TFE, ꢀ15 ꢁC, 30 min
PIDA (1 equiv), LiOAc, HFIP, ꢀ15 ꢁC, 30 min
PIFA (1 equiv), LiOAc, HFIP, ꢀ15 ꢁC, 30 min
HTIB (1 equiv), LiOAc, HFIP, ꢀ15 ꢁC, 30 min
Fe(acac)3 (2 equiv), t-BuOK, THF, ꢀ40 ꢁC to rt
PIDA (2 equiv), LiOAc, TFE, ꢀ15 ꢁC, 30 min
PIDA (2 equiv), LiOAc, TFE, ꢀ30 ꢁC, 30 min
>30
<5
<5
Complicated
Complicated
Complicated
Complicated
Complicated
N.R.
0
0
16
15
12
11
a
PIDA: (diacetoxyiodo)benzene; PIFA: [bis(trifluoroacetoxy)iodo]-benzene;
HTIB: [hydroxy(tosyloxy)iodo]benzene; TFE: 2,2,2-trifluoroethanol; HFIP:
hexafluoroisopropanol.
the presence of t-BuOK, and no reaction was detected (entry 7).
Final optimizations were achieved by increasing the amount of
PIDA (up to 2 equiv) in TFE, affording the best results at ꢀ15 ꢁC
(entry 8). Further extending the reaction times or using more PIDA
were not helpful to this reaction, giving more complicated
situations.
Scheme 1. Preparation of diketopiperazine 4. Reagents and conditions: (a) MeI, NaH,
THF, 0 ꢁC to rt; (b) SOCl2, MeOH, 0 ꢁC, then reflux, 98% for two steps; (c) N-Boc-D-Trp-
OH (8), BEP, DIEA, CH2Cl2, 0 ꢁC to rt, 76%; (d) 3 M HCl in EtOAc, rt, then aqueous
NaHCO3; (e) BBr3, CH2Cl2, 0 ꢁC to rt, 90% for two steps. (BEP¼2-bromo-1-ethyl pyr-
idinium tetrafluoroborate.)
Oxidative construction of the benzofuranoindoline ring system
of azonazine was then examined using diketopiperazine 4 as the
substrate (Scheme 2). Among the available mild oxidants, hyper-
valent iodine reagents and their applications have attracted great
attentions in organic synthesis in recent years. Various oxidative
transformations mediated by hypervalent iodines have been suc-
cessfully applied to the synthesis of complex organic molecules.4b,11
Treatment of diketopiperazine 4 with 1 equiv of PhI(OAc)2 in the
presence of LiOAc in a cold trifluoroethanol solution was found to
be workable. Two diastereomeric hexacyclic products 11a and 11b,
whose structures were determined by NMR methods and con-
firmed by the X-ray single crystallographic analyses, were afforded
in very low isolated yields (Scheme 2; and entry 1, Table 1). Several
other hypervalent iodine (III) reagents were also examined in this
study (Table 1). Unfortunately, all the attempts using PIFA or HTIB in
either TFE or HFIP, or PIDA in HFIP gave undesirable results (entries
2e6). An iron(III) oxidant, Fe(acac)3, was also examined in THF in
The products 11a and 11b clearly mentioned that an over-
oxidation happened at the C14 position. Though the detailed
mechanism is unclear yet, intermediates 3 and 3b (Scheme 2,
a diastereomer of 3) are possibly produced at the first stage of the
oxidative cyclization (C, Fig. 2). Unfortunately, these two highly
strained structures (3 and 3b) make a further PhI(OAc)2-oxidation
easily happen on the indoline nitrogen (D, Fig. 2). A subsequent
TFE-addition takes place at the C14 position to release certain
amount of molecular tension of the intermediates. As a net result,
an O,O-di(trifluoroethoxy)-ketal was formed at the C14 position of
11a and 11b after these PhI(OAc)2-mediated oxidations.
To avoid the above over-oxidation, modification of the func-
tional group(s) on the indole moiety of diketopiperazine 4 was
attempted. Four derivatives 12aed were thus designed and syn-
thesized, either to change the electron density of the indole sub-
structure (12aec) or to block the C14 position where the previous
over-oxidation took place (12d). Nind-Ac diketopiperazine 12a was

J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
3199
Scheme 4. Preparation of diketopiperazine 12b. Reagents and conditions: (a) NaOH,
H2O/MeOH; (b) N-Me-D-Tyr(Me)-OMe HCl (7), BEP, DIEA, CH2Cl2, 0 ꢁC to rt, 71% for two
steps; (c) 3 M HCl in EtOAc; (d) NaHCO3 (aqueous); (e) BBr3, DCM, 0 ꢁC to rt, 79% for
three steps.
was transformed to rac-N-Boc-Trp derivative 19, which was reacted
with -Tyr derivative 7 to yield a mixture of two dipeptide di-
D
astereomers 20a and 20b. Both of them were further converted into
two separable diketopiperazine 21 and 22 by a sequence of acidic
deprotection and alkalic cyclization. Diketopiperazine substrate 12c
was finally obtained by O-demethylation of 22 (Scheme 5).
Fig. 2. A proposed mechanism for PIDA-oxidative generation of 11b.
synthesized using Nind-Ac-
D
-Trp derivative 1312 and N-Me-Boc-
D-
Try(tBu)-OH (14) as the starting materials. Compound 13 was
deprotected and coupled with 14 to yield dipeptide 15. Depro-
tection of the N-Boc functionality followed by ring-closing under
microwave-assisted conditions13 afforded 12a in satisfactory yields
(Scheme 3).
Scheme 5. Preparation of diketopiperazine 12c. Reagents and conditions: (a) 6 M HCl,
reflux; (b) Boc2O, NaOH, THF/H2O, 70% for two steps; (c) N-Me-D-Tyr(OMe)-OMe HCl
(7), BEP, HOBt, CH2Cl2, 65%; (d) 3 M HCl in EtOAc, then aqueous NaHCO3, 21 (41%), and
22 (43%); (e) BBr3, CH2Cl2, 0 ꢁC to rt, 85%.
50-Bromo-diketopiperazine 12d was designed to block the C14
position where the over-oxidation happened. Starting with rac-N-
Boc-(50-Br-Trp)-OH (23)16 and 7, a mixture of two dipeptide di-
astereomers 24a and 24b were synthesized. After similar depro-
tection and cyclization, diketopiperazine 25 and 26 were obtained.
O-Demethylation of 26 led to the desired substrate 12d
(Scheme 6).
All four cyclopeptide derivatives 12aed were examined under
oxidative cyclization conditions, but the results turned out to be
disappointing (Table 2). When the indole nitrogen was protected
either by an electron-withdrawing group (Ac, entry 1, 12a) or an
electron-donating group (Bn, entry 2, 12b), the corresponding
substrates displayed no reactivity under the oxidation conditions.
This indicated that the free indole nitrogen (NH) was an essential
group for the oxidative cyclization process initiated by PIDA. When
a bromine atom was introduced into 70 position of the indole
moiety to decrease the electron density (entry 3, 12c), the reaction
turned out to be very complicated. Furthermore, when 50 position
of the indole moiety was substituted by a bromine atom to block
Scheme 3. Synthesis of diketopiperazine 12a. Reagents and conditions: (a) 3 M HCl in
EtOAc; (b) N-Me-Boc-D-Tyr(tBu)-OH (14), EDCI, HOBt, DIEA, CH2Cl2, 0 ꢁC to rt, 76% for
two steps; (c) 3 M HCl in EtOAc; (d) nBuOH/NMM/HOAc, MW, 100 ꢁC, 83% for two steps.
Accordingly, Nind-Bn diketopiperazine 12b was obtained using
D
-Nind-Bn Trp derivative 1614 and
-Tyr derivative 7 as the starting
D
materials. Hydrolysis of 16 followed by coupling with 7 gave di-
peptide 17, which was further transformed to 12b by a sequence of
deprotection, cyclization, and O-demethylation (Scheme 4).
A racemic material of N-Ac-(70-Br-Trp)-OH (18)15 was used as
the starting material to synthesize 70-Br substituted diketopiper-
azine 12c. After amino-protecting group exchange, compound 18

3200
J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
Table 3
Electrochemical oxidative cyclization of diketopiperazine 4
Scheme 6. Preparation of diketopiperazine 12d. Reagents and conditions: (a) N-Me-D-
Tyr(Me)-OMe HCl (7), BEP, HOAt, DIEA, CH2Cl2, 85%; (b) 3 M HCl in EtOAc, then
aqueous NaHCO3, 25 (46%) and 26 (47%); (c) BBr3, CH2Cl2, 0 ꢁC to rt, 90%.
Entry
Voltage
Electrolyte
Base/acid
Solvent
Results
1a
2a
3b
4b
0.8 V
1.0 V
0.8 V
1.0 V
LiClO4
LiClO4
Et4NClO4
Et4NClO4
HOAc
HOAc
NaOH
NaOH
CH3NO2
CH3NO2
CH3CN/H2O
CH3CN/H2O
dimerization
dimerization
N.R.
Table 2
Oxidative cyclization of diketopiperazine derivatives 12aeda
27 (19%)
a
LiClO4 (3 M); HOAc (0.8 M).
Et4NClO4 (0.15 M); NaOH (2e4 equiv); CH3CN/H2O¼10:1.
b
Entry
Substrate
Oxidant
Results
1
2
3
4
5
6
12a (R1¼Ac, R2¼H, R3¼H)
12a (R1¼Ac, R2¼H, R3¼H)
12b (R1¼Bn, R2¼H, R3¼H)
12b (R1¼Bn, R2¼H, R3¼H)
12c (R1¼H, R2¼H, R3¼Br)
12d (R1¼H, R2¼Br, R3¼H)
PIDA
PIFA
PIDA
PIFA
PIDA
PIDA
NR
NR
NR
NR
Complicated
Complicated
a
PIDA: (diacetoxyiodo)benzene; PIFA: [bis(trifluoroacetoxy)iodo]benzene.
Scheme 7. Preparation of oxoindole 29a/b. Reagents and conditions: (a) DMSO, HOAc,
concd HCl, 40% (dr w2:1).
the over-oxidation site (entry 4, 12d), no positive result was ob-
served either.
With 29a/29b in hand, a variety of oxidative coupling conditions
were then screened for the intramolecular cyclization (Table 4).
Several different oxidants were firstly tested, including Fe(III),
Ce(IV), Mn(IV), Mn(III), Cu(I), Ir(IV), I2, and air. Under most
Since all these substrate modifications couldn’t improve the
results, we further decided to attempt other oxidation methods,
such as electrochemical oxidation reaction,17 using the previous
substrate 4. Electrolysis of 4 with a constant voltage was examined
under both acidic and basic conditions (Table 3). Under acidic
conditions, only dimerized products were detected with different
voltages (entries 1 and 2). When the reaction was performed under
basic conditions at 0.8 V, no apparent reaction was found to happen
(entry 3). But when the voltage of the electrolysis reaction was
raised up to 1.0 V, an unexpected cyclization product 27 was ob-
served, structure of which was further confirmed by X-ray dif-
fraction analysis of its O-Ac derivative 28. To our surprise, the
carbocation generated by anode oxidation was intramolecularly
captured by the indole nitrogen atom, instead of its 30-position of
the indole moiety. As the result, a new 12-membered ring was
given via the newly formed CeN bond. More likely, the high strain
within the molecule blocks the formation of the required 10-
membered ring.
Table 4
Oxidative coupling of 29a/b
Oxidants
Solvent
Additive
Results (yield)
1a
2a
3a
4a
5a
6a
7a
8a
9b
K3Fe(CN)6
K3Fe(CN)6
MnO2
Mn(OAc)3,Cu(OAc)2
[Cu(TMEDA)OHCl]2
K2IrCl6
H2O/CHCl3
HOAc
Toluene
HOAc
CF3CH2OH
H2O/CHCl3
H2O
KOH
d
30 (31%)
30 (25%)
30 (ND)
30 (ND)
30 (ND)
30 (ND)
30 (ND)
Complicated
Complicated
KOH
d
In order to further enhance the nucleophilicity of 30-carbon of
the indole moiety of 4, we then oxidized the corresponding indole
moiety to oxindole 29 (Scheme 7). It was expected that an oxidative
coupling reaction would happen to construct the C10 stereocenter
of azonazine (1a). Diketopiperazine 4 was oxidized by DMSO under
acidic condition to yield a mixture of inseparable diastereomer 29a
and 29b (w2:1 dr according to the 1H NMR).
KOH
KOH
KOH
d
Air
(NH4)2Ce(NO3)6
I2
MeCN
THF
LiHMDS
a
Reaction was carried out from 0 ꢁC to rt.
Reaction was carried out from ꢀ40 ꢁC to rt.
b

J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
3201
conditions, substrate 29a/29b was transformed to a ring-opening
product 30, structure of which was confirmed by X-ray diffraction
analysis (Table 4, entries 1e7). When Ce(IV) and I2 (entries 8 and 9)
were used as the oxidants, the reactions turned out to be very
complicated. A probable mechanism of this transformation is
shown in Fig. 3. The C30 position of the oxindole is firstly oxidized to
a 30-hydroxy-20-oxindole, which undergoes an intramolecular
fragmentation leading to 30 after releasing one molecule of carbon
monoxide.
transformed to 1a by N-acetylation. On the other hand, the ether
group of 33a can also be deprotected and triflated to yield 36a,
which can be converted to 1a by hydrogenation and N-acetylation.
In method 3, hydrolysis of 11a would give quinone 34a as a key
intermediate. Azonazine 1a could be provided by a sequence of
reduction, phenol O-triflation, hydrogenation, and N-acetylation
transformation from 34a.
Ketal exchange of 11a was attempted, and the results turned out
to be disappointing. Under the catalysis of a variety of Lewis acids,
including Et2O$BF3, ZnI2, and Sc(OTf)3, the expected reaction didn’t
happen. We then had to move our attention to alternative methods.
A number of different reductive conditions were screened (Table 5),
and 11a was finally reduced with activated zinc powder in the
presence of Et3SiH and HOAc in tetrahydrofuran, giving the desired
product 33a in 87% yield (entry 4).
Table 5
Reduction of benzoquinone functionality of 11a
Entry
Conditions
Temp
Yield (%)
1
2
3
4
Zn (activated), HOAc
SmI2, THF
NaBH4, MeOH
rt
rt
0
52
34
42
87
ꢁC to rt
Zn (activated)/Et3SiH/HOAc, THF
rt
Fig. 3. A proposed mechanism for the oxidative fragmentation.
Considering the existence of a naked nitrogen atom on the
indoline ring of 33a might make it unstable, we decided to convert
it into the N-Ac derivative 37a at first. Unfortunately, further
cleavage of the trifluoroethyl aryl ether of 37a was proven to be
problematic (Table 6). Substrate 37a remained unchanged using
Lewis acid BBr3 or TMSI (entries 1e2) even at high temperatures.
When EtSNa was applied as a nucleophilic reagent (entry 3), the
reaction turned out to be complicated. When 37a was exposed to
NaOH in methanol (entry 4), only hydrolysis of N-acetyl group
happened. Cleavage of the aryl ether bond of 37a by nickel cata-
lyzed CeO activation and hydrogenation18 also gave negative re-
sults (Scheme 8).
Since the previous attempts to avoid over-oxidation were un-
successful, we turned back to the intermediate 11a, which contains
the similar hexacyclic ring system of natural product with the right
stereochemistry at C10 position. Starting from 11a, three possible
sequences could reach the natural product (Fig. 4). Method 1 in-
volves a ketal exchange of 11a with 1,3-propanedithiol, giving
dithioacetal 31a. Further hydrogenation and acetylation of 31a will
afford 1a finally. The imine functionality is reduced at first, by the
second method, to yield C-14 trifluoroethoxy derivative 33a. Re-
ductive cleavage of the ether bond affords 32a, which is further
Table 6
Attempts for conversion of 37a into 36a
Entry
Conditions
Temp (ꢁC)
Results
1
2
3
4
BBr3, CH2Cl2
TMSI, DMF
EtSNa, DMF
NaOH (1 M), MeOH
ꢀ40 to 40
rt to 100
rt to 100
rt
N.R.
N.R.
Complicated
33a
Fig. 4. Three possible methods to convert 11a into azonazine (1a).

3202
J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
Scheme 8. Failed attempts for aryl CeO reductive cleavage with nickel-catalyzed ac-
tivation and hydrogenative reduction. Reagents and conditions: (a) Metal: NiII; ligands:
Ph3P, or Cy3P; solvents: PhMe, or iPr2O, or THF, or (EtO)2CH2; H donors: Et3SiH, or
DIBAl-H, or 9-BBN, or BH3, or Bu3SnH.
Hydrolysis of the O,O-ditrifluoroethyl acetal of 11a was even-
tually explored (Fig. 4, method 3; Table 7). While strong acids were
used, slow decomposition of the substrate was observed during the
reaction process (Table 7, entries 1e5). Careful adjustment of the
combinations of Lewis acid and Brønsted acid found that the hy-
drolysis provided the best results when HOAc was employed as the
solvent (entry 6). If it was carried out under microwave irradiation,
the reaction was greatly accelerated (entries 9 and 10). Further-
more, addition of a small amount of water would facilitate the
hydrolysis, but would lead to several side-products. The ratio of
water and HOAc was therefore carefully optimized (entries 11e13).
When HOAc/H2O (15:1, v/v) was applied as the solvent, the reaction
turned out to be fast and clean under microwave-assisted condi-
tions, giving the desired quinone 34a (59% yield).
Scheme 9. Synthesis of the proposed structure of azonazine. Reagents and conditions:
(a) NaBH4, MeOH, 0 ꢁC, 67%; (b) Tf2O, Et3N, EtOAc, ꢀ15 ꢁC, 53%; (c) H2, cat. Pd(OH)2/C,
Et3N, MeOH/EtOAc, rt, 80%; (d) Ac2O, pyridine, reflux, 25%.
natural product. After analyzing the original report carefully,2 we
found that the assignment of the relative configurations at C10 and
C11 might be ambiguous.19 We found the 1H NMR of the previous
intermediate 11b was much more similar to that of natural product
than its counterpart 11a. Therefore, we decided to synthesize the
other C10/C11 diastereomer 1b from 11b for further analysis.
The diastereomeric target 1b was synthesized with similar steps
as those for 1a (Scheme 10). Hydrolysis of 11b under microwave-
assisted conditions followed by NaBH4 reduction of quinone 34b
provided phenol 35b. Deoxygenation of the C14 phenol group of
35b was accomplished by O-triflation and subsequent hydrogena-
tion, giving 3b in good yield. However, problems were met in the
final N-acetylation step. Previously used basic conditions (Ac2O,
pyridine, reflux) didn’t work well in this substrate, leading to very
low yield and several inseparable side-products. A variety of acet-
ylation reagents, additives, and solvents were then screened
(Table 8). Finally, we found the unusual acidic conditions (Ac2O in
HOAc at rt) provided much higher yield (89%) of the final product
1b, whose stereochemical structure was determined by NMR
Table 7
Hydrolysis of 11a to quinone 34a
Entry
Conditions
Temp (ꢁC)
Time (h)
Results (yield)
1
2
3
4
5
6
1 M HCl, MeOH
PTSA, acetone
PPTS, acetone
HCOOH
TFA
HOAc
rt
rt
2
2
2
0.25
0.16
7
Decomposition
Decomposition
Decomposition
Decomposition
Decomposition
34a (41%)
ꢀ30 to rt
rt
rt
rt
7
8
Zn(OTf)2, acetone
MgI2, acetone
HOAc
rt to reflux
rt to reflux
1
1
Decomposition
Decomposition
34a (38%)
34a (40%)
34a (59%)
9a
10a
11a
12a
13a
70
50
50
50
50
0.033
0.4
0.1
0.1
0.1
HOAc
HOAc/H2O(15:1)
HOAc/H2O(5:1)
HOAc/H2O(2:1)
34a (42%)
34a (31%)
a
Microwave irradiation.
Availability of quinone 34a enabled us to continue our synthesis
of azonazine (1a). As shown in Scheme 9, quinone 34a was im-
mediately reduced with NaBH4 to afford stable phenol 35a, which
was then converted into the corresponding O-triflate 36a under the
optimized conditions (Tf2O/Et3N in EtOAc, ꢀ15 ꢁC). Hydrogenative
removal of the triflate functionality of 36a (in the presence of cat-
alytic amount of Pd(OH)2 on charcoal) followed by N-acetylation
finally afforded the originally proposed structure of (þ)-azonazine
(1a).
Scheme 10. Completion of the total synthesis of ent-(ꢀ)-azonazine (1b). Reagents and
conditions: (a) HOAc, microwave irradiation, dioxane/H2O, 65 ꢁC, 45e55%; (b) NaBH4,
MeOH, 0 ꢁC, 80%; (c) Tf2O, Et3N, EtOAc, ꢀ15 ꢁC, 70%; (d) H2, cat. Pd(OH)2/C, Et3N,
MeOH/EtOAc, rt, 96%; (e) Ac2O, HOAc, rt, 1b (89%); 1a (67%); see Table 8 for more
details of optimization of the final N-acetylation step.
To our surprise, multiple apparent differences were found in the
1H NMR spectrum of synthetic 1a from those reported for the

J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
3203
Table 8
step to construct the highly strained hexacyclic core of azonazine.
An unusual product 28 contains a 12-membered ring via new sp2
CeN bond formation in the electrochemical oxidation further
confirmed that high strain exists within the natural skeleton. By
analysis of spectroscopic evidences and X-ray single crystal studies
of the synthesized enantiopure diastereomers, the absolute ste-
reochemistry of natural (þ)-azonazine was unambiguously revised
as (2S,10R,11S,19S)-1. In addition, two intermediates produced in
this work were utilized to the synthesis of corresponding fluori-
nated azonazine derivatives in short convenient steps. Biological
properties of these unnatural products including the two-fluori-
nated derivatives are currently under investigation in our labora-
tory and will be reported in due course.
Conditions for selective N-acetylation of 3b
Entry
Conditions
Temp (ꢁC)
1b (%)
1
Ac2O, DCM/Py (5:1), 48 h
Ac2O, Py, 4 h
Ac2O, Py, 30 min
Ac2O, TEA, DCM, 11 h
AcCl, TEA, DMF
Ac2O, TEA, DMF, DMAP (0.05 equiv)
HOAc, HATU, DIEA, DMF
HOAc, BEP, DIEA, DMF
1-Acetylimidazole, TEA, DMF
Ac2O, AcOH
70
70
90
30
40
2
3a
4
33
Reflux
rt
db
5
6
7
8
9
10
Complicated
ꢀ30 ꢁC to rt
db
0
0
0
rt
ꢁC to rt
ꢁC to rt
ꢁC to rt
N.R.
N.R.
N.R.
89
a
Microwave irradiation.
No desired product were found.
b
4. Experimental section
4.1. General
methods and further confirmed by an X-ray single crystal diffrac-
tion experiment. The optimized acidic N-acetylation conditions
were also workable to convert 32a into 1a (previously in Scheme 9,
25% yield) in an improved yield (67%, Scheme 10).
All reactions were carried out in flame-dried glassware with
magnetic stirring and monitored by thin-layer chromatography
(TLC). THF was distilled from sodium; dichloromethane was dis-
tilled from CaH2; and ethyl acetate was dried over K2CO3 and dis-
tilled. All melting points were uncorrected. 1H NMR was recorded
on a Varian instrument (300 or 500 MHz) and a Bruker instrument
(400 or 600 MHz); 13C NMR were recorded on a Varian instrument
(75 or 125 MHz) and a Bruker instrument (100 or 150 MHz). IR
spectra were recorded on Nicolet 380 FT-IR instrument. Optical
rotations were measured on a Jasco P-1030 digital polarimeter.
High resolution mass spectra (HRMS) were measured on an Ion-
Spec 4.7 Tesla FTMS spectrometer. Flash column chromatographies
With the above physical characterizations in hand, the absolute
chemistry of 1b was unambiguously assigned as (2R,10S,11R,19R)
(Scheme 10). Except the opposite optical rotation {synthetic: ½a D27
ꢂ
ꢀ299.3 (c 0.11, MeOH); natural: ½a D23
þ295.0 (c 0.1, MeOH)}, all
ꢂ
other physical evidences of 1b were in good agreement with those
reported for the natural product.2 Therefore, the absolute config-
uration of natural (þ)-azonazine should be revised as
(2S,10R,11S,19S)-1.
Advantages of modern natural product synthesis include its
ability in the development of important derivatives, among which
some are difficult to be achieved by traditional functionality
modifications. Selective introduction of fluorine atom(s) to bio-
active natural and unnatural products is of extreme interest in to-
day’s pharmaceutical research.20 Some fluorine-containing
intermediates accidentally generated in this synthesis undoudt-
edly provide us a particular opportunity to explore fluorinated
analogues and derivatives of azonazine. As an extra income of this
study, we easily synthesize the C14-trifluoroethoxy analogues 33a/
33b of ent-azonazine in two steps from the common ketal in-
termediates 11a/11b by reduction with zinc/HOAc under mild
conditions21 and subsequent N-acetylation (Scheme 11).
were performed on silica gel H (10e40 mm). MPLC purification was
performed using a Biotage Isolera system and KP-SIL SNAP flash
cartridge column packed with silica gel (particle size 50e60 mi-
ꢀ
cron, 50e52 A pore size). Microwave reactions were run on a CEM
Discover S Microwave Synthesizer. X-ray intensity data were col-
lected on a Bruker APEX-II CCD area detector employing graphite-
ꢀ
monochromated Cu-Ka radiation (l¼1.54178 A) at a temperature
of 296(2) K.
4.2.
D-N-Me-Tyr(Me)-OMe$HCl (7)
To a solution of
D
-Boc-Tyr-OH (5) (50.0 g, 178 mmol) in 500 ml of
dry THF under N2 atmosphere at 0 ꢁC was added sodium hydride
(60% oil dispersion, 35.6 g, 890 mmol, 5 equiv) in portions. The
resulting reaction mixture was stirred at 0 ꢁC for 15 min, and
methyl iodide (44.3 ml, 712 mmol, 4 equiv) was then added. The
reaction was stirred overnight at rt. The excess sodium hydride was
quenched by the careful addition of water, and the solvents were
removed in vacuo. The residue was diluted with 300 ml of water
and washed with Et2O. The aqueous phase was then acidified with
KHSO4 (to pH 3) and was extracted with EtOAc. The combined
extracts were successively washed with aqueous Na2S2O3 and
brine, dried over Na2SO4, and concentrated in vacuo. The resulting
crude compound 6 was used directly for the next step.
The above crude product was dissolved in methanol (400 ml)
and cooled down to 0 ꢁC under N2 atmosphere. Thionyl chloride
(14.3 ml, 196 mmol, 1.1 equiv) was slowly added to the reaction
system. The resulting mixture was stirred at 0 ꢁC for 0.5 h, and then
refluxed for 2 h. The solvents were removed in vacuo to give 7 as
white solid (45.3 g, 174 mmol, 98% for two steps). Mp: 152e155 ꢁC.
Scheme 11. Convenient synthesis of fluorinated azonazine analogues 33a and 33b.
Reagents and conditions: (a) Et3SiH, Zn, HOAc, THF, rt; (b) Ac2O, AcOH, rt. Compound
33a (70% for two steps); 33b (67% for two steps).
½
a 2D5:2
1745, 1612, 1514, 1249, 1029, 835 cmꢀ1; 1H NMR (500 MHz, DMSO-
d6)
9.88 (br s, 2H), 7.22 and 7.16 (two d, J¼8.5 Hz, 2H), 6.87 (d,
J¼8.5 Hz, 2H), 4.22 and 4.08 (two dd, J¼7.5, 4.8 Hz, 1H), 3.71 (s, 3H),
3.63 (s, 3H), 3.27 (m, 1H), 3.14 (m, 1H), 2.54 and 2.52 (two s, 3H); 13
NMR (125 MHz, DMSO-d6) 169.45 and 168.62 (1C), 158.48 and
ꢂ
ꢀ31.3 (c 1.0, MeOH); IR (KBr): 3417, 2954, 2834, 2691, 2433,
3. Conclusions
d
In summary, we have successfully accomplished the first total
synthesis of ent-(ꢀ)-azonazine. A biomimetic direct oxidative cy-
clization has been successfully developed and applied as the key
C
d

3204
J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
158.44 (1C), 130.61 and 130.47 (2C), 126.55 and 126.18 (1C), 114.00
and 113.91 (2C), 61.39 and 61.20 (1C), 55.07, 52.59, 33.91 and 33.84
(1C), 31.79 and 31.52 (1C); ESI-MS (m/z): 224 (MꢀHClþHþ); HRMS
(ESI) calcd for C12H18NO3 (MꢀHClþHþ) 224.1287, found 224.1281.
31.75; ESI-MS (m/z): 364 (MþHþ); HRMS (ESI) calcd for C21H22N3O2
(MþHþ) 364.1648, found 364.1656.
4.5. Ketals 11a and 11b
To a solution of PhI(OAc)2 (3.86 g, 12.0 mmol, 2 equiv) and LiOAc
(2.38 g, 6 equiv) in CF3CH2OH (550 ml) was added 4 (2.18 g,
6.0 mmol) in 50 ml CF3CH2OH slowly at ꢀ15 ꢁC. The reaction
mixture was stirred for 30 min and quenched with aqueous
Na2S2O3 (10 ml). The solvents were removed, and MeOH/CH2Cl2
(1:20) was added. The sediment was filtered and washed three
times with MeOH/CH2Cl2 (1:20). The filtrates were concentrated in
vacuo. The resulting residue was purified by MPLC on silica gel
(MeOH in CH2Cl2: 0e5%) to give a mixture of 11a/b, which was
further purified by preparative TLC (EtOAc) to give 11a (535 mg,
16%) and 11b (401 mg, 12%) as slightly yellow solids.
4.3. Dipeptide 9
To a solution of 7 (12.7 g, 49.0 mmol, 1.05 equiv), N-Boc-D-Trp-
OH (8) (14.2 g, 46.7 mmol, 1 equiv), and BEP (16.6 g, 60.6 mmol,
1.3 equiv) in dry CH2Cl2 (500 ml) under N2 atmosphere at 0 ꢁC was
added N-ethyldiisopropylamine (24.3 ml, 140 mmol, 3 equiv)
slowly via a syringe pump (flow rate 25 ml/h). The resulting mix-
ture was stirred at 0 ꢁC for 0.5 h and then stirred at rt overnight. The
organic solvent was removed in vacuo, and the resulting mixture
was diluted with EtOAc. The organic layer was washed with H2O,
brine, dried over Na2SO4, and concentrated. The crude residue was
purified by MPLC on silica gel (MeOH in CH2Cl2: 0e5%) to give 9
Compound 11a: Single crystal for X-ray analysis was prepared by
slow evaporation from DMSO. Mp: >270 ꢁC (decomposition); ½a 2D5:0
ꢂ
(18.0 g, 76%) as a white foam. ½a D25:8
þ48.0 (c 1.02, CHCl3). IR (KBr):
ꢂ
ꢀ56.0 (c 1.0, THF). IR (KBr): 3445, 3218, 2945, 1678, 1621, 1580,
3337, 3005, 2976, 2952, 2834, 1738, 1701, 1641, 1513, 1247, 1032,
1483, 1282, 1170, 1107, 992 cmꢀ1; 1H NMR (500 MHz, CD3CN)
d 7.05
747 cmꢀ1
; d 8.34e8.20 (m, 1H),
1H NMR (500 MHz, CDCl3)
(s, 1H), 6.93 (d, J¼8.2 Hz, 1H), 6.89 (s, 1H), 6.85 (d, J¼10.2 Hz, 1H),
6.78 (d, J¼8.1 Hz, 1H), 6.58 (dd, J¼10.2, 2.4 Hz, 1H), 6.11 (s, 1H), 5.47
(s, 1H), 4.19e4.00 (m, 6H), 3.30 (d, J¼14.0 Hz, 1H), 3.06 (s, 3H), 2.89
(dd, J¼14.1, 4.3 Hz, 1H), 2.81 (dd, J¼16.1, 3.2 Hz, 1H), 2.00 (dd,
7.68e7.50 (m, 1H), 7.34e7.27 (m, 1H), 7.16 (t, J¼7.4 Hz, 1H),
7.12e7.08 (m, 1H), 7.03 (d, J¼1.8 Hz, 1H), 6.94e6.76 (m, 2H),
6.72e6.62 (m, 2H), 5.16 (d, J¼9.0 Hz, 1H), 5.02 (dd, J¼9.9, 5.7 Hz,
1H), 4.92e4.78 (m, 1H), 3.73e3.59 (m, 6H), 3.29e3.13 (m, 2H),
3.09e3.03 (m, 1H), 2.87e2.81 (m, 1H), 2.88 and 2.66 (two s, 3H),
J¼16.2, 3.8 Hz, 1H); 13C NMR (125 MHz, CD3CN)
d 168.76, 165.87,
164.86, 158.89, 148.41, 139.26, 133.55, 133.25, 128.13, 127.52, 126.05,
125.33 (q, J¼275 Hz, 1C), 125.28 (q, J¼275 Hz, 1C), 122.28, 116.86,
111.37, 98.84, 66.32, 62.08 (q, J¼34.6 Hz, 2C), 57.42, 56.12, 38.38,
37.60, 33.86; ESI-MS (m/z): 558 (MþHþ); HRMS (ESI) calcd for
1.43e1.33 (d, J¼21.6 Hz, 9H); 13C NMR (125 MHz, CDCl3)
d 173.17
and 172.75 (1C), 171.10 and 170.57 (1C), 158.63 and 158.48 (1C),
155.30 and 155.12 (1C), 136.28, 130.15 and 129.97 (2C), 128.97,
127.93, 123.44 and 122.67 (1C), 122.12 and 122.09 (1C), 119.69 and
119.60 (1C), 119.03 and 118.93 (1C), 114.25 and 114.03 (2C), 111.28
and 111.06 (1C), 110.52 and 110.15 (1C), 79.73, 61.19 and 59.64 (1C),
55.31, 52.56 and 52.32 (1C), 50.90 and 50.47 (1C), 34.61 and 33.92
(1C), 33.39, 29.18 and 29.11 (1C), 28.49 and 28.41 (3C); ESI-MS (m/
z): 532 (MþNaþ); HRMS (ESI) calcd for C28H35N3O6 (MþNaþ)
532.2427, found 532.2418.
C
25H22N3O5F6 (MþHþ) 558.1469, found 558.1458.
Compound 11b: Single crystals for X-ray analysis was prepared
by vapor diffusion (THF/hexanes). Mp: >250 ꢁC (decomposition);
½
a 2D5:1
1484, 1287, 1155, 1112, 988, 752 cmꢀ1
ꢂ
ꢀ20.4 (c 1.02, THF). IR (KBr): 3260, 2933, 1682, 1655, 1579,
;
1H NMR (500 MHz, CD3CN)
d
7.32 (s, 1H), 6.97 (dd, J¼8.1, 1.7 Hz, 1H), 6.94 (s, 1H), 6.87 (d,
J¼10.2 Hz, 1H), 6.66 (d, J¼8.1 Hz, 1H), 6.58 (dd, J¼10.2, 2.4 Hz, 1H),
6.46 (d, J¼2.4 Hz, 1H), 6.25 (s, 1H), 4.17e3.99 (m, 6H), 3.48 (d,
J¼14.0 Hz, 1H), 3.05 (dd, J¼14.1, 6.4 Hz, 1H), 2.71 (dd, J¼16.4, 3.9 Hz,
1H), 2.44 (s, 3H), 2.28 (dd, J¼16.4, 3.6 Hz, 1H); 13C NMR (125 MHz,
4.4. Diketopiperazine 4
To a solution of 3 M HCl in EtOAc (100 ml) was added 9 (17.9 g,
35.1 mmol) at rt. The mixture was stirred at rt for 2 h. The solvent
was removed in vacuo. The resulting residue was diluted with
EtOAc, and the organic layer was successively washed with aqueous
NaHCO3, water and brine, dried over Na2SO4, and concentrated. The
crude product 10 was used directly for the next step without fur-
ther purification.
CD3CN) d 169.87, 165.19, 164.78, 159.56, 149.67, 139.18, 132.92,
132.31, 129.16, 127.40, 126.46, 125.37 (q, J¼275 Hz, 1C), 125.23 (q,
J¼275 Hz, 1C), 121.23, 110.86, 118.18, 98.71, 66.84, 62.25 (q, J¼35 Hz,
1C), 62.10 (q, J¼35 Hz, 1C), 57.17, 55.62, 39.29, 38.66, 32.93; ESI-MS
(m/z): 558 (MþHþ). HRMS (ESI) calcd for C25H22N3O5F6 (MþHþ)
558.1472, found 558.1458.
To a solution of crude 10 (13.0 g, 34.4 mmol, 1 equiv) in dry
CH2Cl2 (400 ml) was added BBr3 (1.9 M in CH2Cl2, 90 ml, 171 mmol,
5 equiv) at 0 ꢁC. The resulting mixture was stirred at rt overnight.
Water was slowly added at 0 ꢁC to quench the reaction. The
resulting solution was filtered, and the solid was washed with
CH2Cl2. The organic phases were concentrated (to remove CH2Cl2),
and then diluted with EtOAc. The organic layer was washed with
aqueous NaHCO3, water, and brine, dried over Na2SO4, and con-
centrated. The residue combined with the previously filtered solid
was purified by MPLC on silica gel (MeOH in CH2Cl2: 0e5%) to give 4
4.6. Dipeptide 15
To a solution of 3 M HCl in dioxane (30 ml) was added com-
pound 13 (1.62 g, 4.5 mmol) at rt. The mixture was stirred at rt for
3 h. The solvent was removed in vacuo. The resulting residue was
diluted with EtOAc, and the organic layer was successively washed
with aqueous NaHCO3, water, and brine, dried over Na2SO4, and
concentrated. The crude product was used directly for the next step
without further purification.
To a solution of the above crude product in dry CH2Cl2 (40 ml)
under N2 atmosphere at 0 ꢁC was added EDCI (0.920 g, 4.8 mmol),
HOBt (0.648 g, 4.8 mmol). The reaction mixture was stirred at 0 ꢁC
for 0.5 h, and then 14 (1.405 g, 4.0 mmol) was added. N-ethyl-
diisopropylamine (1.31 ml, 8.0 mmol)was slowly added. The
resulting mixture was stirred at 0 ꢁC for 0.5 h and then stirred at rt
overnight. The organic solvent was removed in vacuo, and the
resulting mixture was diluted with EtOAc. The organic layer was
washed with H2O, brine, dried over Na2SO4, and concentrated. The
crude residue was purified by flash chromatograph on silica gel
(EtOAc/PE: 0e1/2), to give 15 as a white foam (1.80 g, 76% for two
(11.3 g, 90% for two steps) as a white solid. Mp: 128e132 ꢁC. ½a D25
ꢂ
þ157.1 (c 0.65, CH3CN); IR (KBr): 3291, 2926, 2685, 2600, 2261,
1664, 1632, 1516, 1458, 1233, 1100, 746 cmꢀ1
CD3CN)
;
1H NMR (400 MHz,
9.15 (s, 1H), 7.44 (d, J¼8.1 Hz, 1H), 7.38 (d, J¼8.1 Hz, 1H),
d
7.13 (t, J¼7.2 Hz, 1H), 7.05 (t, J¼7.5 Hz, 1H), 6.96e6.84 (m, 4H), 6.78
(d, J¼8.5 Hz, 2H), 6.19 (s, 1H), 4.02 (t, J¼4.5 Hz, 1H), 3.92 (d,
J¼9.6 Hz, 1H), 2.95e2.81 (m, 5H), 2.73 (dd, J¼14.3, 4.3 Hz, 1H), 1.56
(dd, J¼14.3, 9.7 Hz, 1H); 13C NMR (100 MHz, CD3CN)
d 167.00,
166.96, 157.28, 137.63, 132.25 (2C), 128.14, 128.05, 125.12, 122.51,
119.89, 119.50, 116.41 (2C), 112.35, 110.48, 64.09, 56.45, 36.91, 33.41,

J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
3205
steps). ½a 2D6
ꢂ
þ31.75 (c 1.20, CHCl3); IR (KBr): 3334, 2976, 2932, 1743,
1H NMR
158.45, 155.22, 137.71, 136.56, 129.93 (2C), 128.93, 128.80 (2C),
128.63, 127.63, 127.43, 126.88 (2C), 121.86, 119.42, 119.18, 113.98
(2C), 109.85, 109.74, 79.59, 59.39, 55.25, 52.23, 50.98, 50.07, 33.90,
33.28, 28.95, 28.42 (3C); HRMS (MALDI): calcd for C35H41N3O6Na
(MþNaþ) 622.2903; found 622.2888.
1685, 1507, 1452, 1388, 1366, 1327, 1162, 1015, 750 cmꢀ1
;
(500 MHz, CDCl3)
d
8.41 (d, J¼7.7 Hz, 1H), 7.48 (d, J¼7.7 Hz, 1H), 7.28
(m, 3H), 7.05 (m, 2H), 6.88 (d, J¼8.2 Hz, 2H), 6.70 (m, 1H), 4.83 (m,
2H), 3.66 (m, 3H), 3.23 (m, 3H), 2.88 (m, 1H), 2.64 (s, 3H), 2.56 (s,
3H), 1.28 (m 18H). 13C NMR (125 MHz, CDCl3)
d 171.77, 170.88,
170.41, 168.66, 156.41, 155.14, 154.03, 135.83, 132.22, 129.49, 129.42
(2C), 125.51, 124.30 (2C), 123.79, 118.65, 116.80, 80.52, 78.39, 60.81,
52.60, 52.08, 33.64, 31.77, 28.90 (3C), 28.23 (3C), 27.61, 24.10. HRMS
4.9. Diketopiperazine 12b
To a solution of 3 M HCl in dioxane (3 ml) was added compound
17 (163 mg, 0.27 mmol) at rt. The mixture was stirred at rt for 2 h.
The solvent was removed in vacuo. The resulting residue was di-
luted with EtOAc, and the organic layer was successively washed
with aqueous NaHCO3, water, and brine, dried over Na2SO4, and
concentrated. To a solution of the crude product in CH2Cl2 (3 ml)
under N2 atmosphere at 0 ꢁC was slowly added BBr3 (1.9 M in
CH2Cl2, 0.6 ml, 1.14 mmol). The resulting mixture stirred at rt
overnight. The reaction was quenched by slow addition of water
and extracted by DCM/MeOH (20:1). The organic layer was washed
with H2O and brine, dried over Na2SO4, and concentrated. The
crude residue was purified by flash chromatograph on silica gel
(DCMeDCM/MeOH¼20:1) to give 12b (97 mg, 79% for three steps)
(MALDI): calcd for
616.2993.
C
33H43N3O7Na (MþNaþ) 616.3003; found
4.7. Diketopiperazine 12a
To a solution of 3 M HCl in dioxane (5 ml) was added compound
15 (0.10 g, 0.17 mmol) at rt. The mixture was stirred at rt for 3 h. The
solvent was removed in vacuo. The resulting residue was diluted
with EtOAc, and the organic layer was successively washed with
aqueous NaHCO3, water, and brine, dried over Na2SO4, and con-
centrated. The crude product was used directly for the next step
without further purification.
a 26
ꢁ
To a solution of the above crude product in n-butyl alcohol
(1 ml) was added HOAc (0.15 ml) and N-methylmorpholine (0.1 ml).
The resulting mixture was irradiated by microwave (150 W) at
100 ꢁC for 15 min. The organic solvent was removed in vacuo, the
crude residue was purified by MPLC on silica gel (MeOH in CH2Cl2:
0e6%) to give 12a (57 mg, 83% for two steps) as a white solid. Mp:
as a white solid. Mp: 122e125 C; ½ ꢂD þ118.44 (c 1.60, MeOH); IR
(KBr): 3340, 3245, 3062, 3028, 2926, 1669, 1637, 1515, 1466, 1331,
1234,1026, 742 cmꢀ1, 1H NMR (400 MHz, CD3CN)
d
7.46 (d, J¼7.8 Hz,
1H), 7.28 (tt, J¼6.9, 6.1 Hz, 4H), 7.19 (m, 2H), 7.12 (m, 1H), 7.05 (m,
1H), 6.97 (s, 1H), 6.94 (s, 1H), 6.84 (m, 2H), 6.77 (m, 2H), 6.19 (s, 1H),
5.31 (s, 2H), 4.02 (t, J¼4.5 Hz, 1H), 3.93 (dt, J¼9.5, 3.1 Hz, 1H), 2.89
(m, 4H), 2.77 (ddd, J¼34.0, 14.2, 4.6 Hz, 2H), 1.58 (dd, J¼14.3, 9.6 Hz,
179e185 ꢁC. ½a 2D3
þ153.82 (c 0.50, MeOH); IR (KBr): 3250, 3016,
ꢂ
2938, 2814, 1676, 1637, 1612, 1452, 1329, 1245, 1227, 1017, 748 cmꢀ1
;
1H). 13C NMR (100 MHz, CD3CN)
d 167.08, 167.02, 157.47, 139.25,
1H NMR (400 MHz, CD3CN)
d
8.20 (m, 1H), 7.99 (s, 1H), 7.45 (m, 1H),
137.62, 132.32 (2C), 129.63 (2C), 129.04, 129.01, 128.48, 128.09 (2C),
127.93, 122.62, 120.08, 120.06, 116.54 (2C), 110.99, 110.25, 64.13,
56.51, 50.52, 37.07, 33.52, 31.73. HRMS (MALDI): calcd for
7.26 (m, 2H), 7.00 (m, 2H), 6.88 (dd, J¼6.5, 2.0 Hz, 3H), 6.21 (s, 1H),
4.12 (t, J¼4.0 Hz, 1H), 3.88 (dt, J¼10.9, 2.8 Hz, 1H), 3.06 (s, 5H), 2.73
(ddd, J¼14.1, 3.0, 1.1 Hz, 1H), 2.47 (s, 3H), 0.95 (dd, J¼14.1, 11.0 Hz,
C
28H28N3O3 (MþHþ) 454.2135; found 454.2125.
1H). 13C NMR (100 MHz, CD3CN)
d 169.76, 166.92, 166.52, 157.62,
136.84, 132.62 (2C), 130.69, 127.59, 125.83, 125.79, 124.04, 119.84,
117.25, 117.15, 116.44 (2C), 63.93, 55.28, 36.35, 33.34, 31.50, 24.37.
HRMS (MALDI): calcd for C233H243N3O4 (MþHþ) 406.1770; found
406.1761.
4.10. Compound rac-19
To a solution of rac-18 in dioxane (60 ml) was added HCl (6 M,
60 ml). The reaction mixture was heated to reflux for 6 h before the
solvent was removed in vacuo. The crude residue was dissolved in
NaOH (aqueous, 3 M, 50 ml). Then water (100 ml), THF (150 ml),
Boc2O (9.2 g, 42.2 mmol) was added and the reaction mixture was
stirred at rt overnight. The solvent was removed in vacuo and the
resulting mixture was diluted with EtOAc. The organic layer was
washed with H2O and brine, dried over Na2SO4, and concentrated
to give rac-19 (9.90 g, 70%) was brown solid. The crude residue used
directly for the next step without further purification. Mp:
116e120 ꢁC; IR (KBr): 3418, 3334, 2976, 2926, 1698, 1506, 1435,
4.8. Dipeptide 17
To a solution of 16 (300 mg, 0.62 mmol) in methanol (6 ml) was
added 2 M NaOH solution (1 ml) at 0 ꢁC. The resulting mixture was
slowly warmed to rt and stirred until the consumption of the
starting material (monitored by TLC). The resulting residue was
diluted with EtOAc, and the water layer was acidified by 1 M HCl (to
pH w3). The organic layer was successively washed with water and
brine, dried over Na2SO4, and concentrated. The crude product was
used directly for the next step without further purification.
To a solution of the above crude product, 7 (169 mg, 0.65 mmol)
and BEP (221.7 mg, 0.81 mmol) in dry CH2Cl2 (40 ml) under N2
atmosphere at 0 ꢁC was slowly added N-ethyldiisopropylamine
(0.30 ml, 1.86 mmol), the reaction mixture was stirred at 0 ꢁC for
0.5 h and then stirred at rt overnight. The organic solvent was re-
moved in vacuo, and the resulting mixture was diluted with EtOAc.
The organic layer was washed with H2O, brine, dried over Na2SO4,
and concentrated. The crude residue was purified by flash chro-
matograph on silica gel (EtOAc/PE: 0e1/2), to give 17 as a white
1161, 1073, 1023, 823 cmꢀ1, 1H NMR (400 MHz, DMSO-d6)
d 12.62 (s,
1H),11.08 (s,1H), 7.55 (d, J¼7.8 Hz,1H), 7.29 (d, J¼7.5 Hz,1H), 7.23 (s,
1H), 7.04 (d, J¼7.9 Hz, 1H), 6.95 (t, J¼7.7 Hz, 1H), 4.15 (m, 1H), 3.14
(dd, J¼14.4, 4.2 Hz, 1H), 2.98 (dd, J¼14.3, 9.7 Hz, 1H), 1.26 (d,
J¼51.2 Hz, 8H). 13C NMR (100 MHz, DMSO-d6)
d 173.83, 155.41,
134.33, 129.00, 125.07, 123.48, 119.88, 117.84, 111.78, 104.21, 78.05,
54.43, 28.17, 26.78. HRMS (MALDI): calcd for C16H19N2O4BrNa
(MþNaþ) 405.0431; found 405.0420.
4.11. Diketopiperazines 21 and 22
foam (227 mg, 71%). ½a D26
ꢂ
þ38.37 (c 1.75, CHCl3). IR (KBr): 3430,
To a solution of rac-19 (3.1 g, 8.10 mmol) in dry CH2Cl2 (80 ml)
under N2 atmosphere at 0 ꢁC was slowly added BEP (2.41 g,
8.8 mmol), HOAt (1.43 g, 8.8 mmol). The reaction mixture was
3302, 2999, 2976, 2932, 2831, 1740, 1707, 1647, 1513, 1364, 1247,
1227, 1175, 741 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
7.66 (d, J¼7.7 Hz,
1H), 7.14 (m, 9H), 6.94 (d, J¼8.6 Hz, 2H), 6.70 (d, J¼8.6 Hz, 2H), 5.24
(t, J¼10.4 Hz, 2H), 5.15 (d, J¼9.1 Hz, 1H), 5.05 (dd, J¼9.9, 5.7 Hz, 1H),
4.88 (m, 1H), 3.69 (d, J¼32.1 Hz, 3H), 3.57 (d, J¼14.7 Hz, 3H), 3.21
(m, 2H), 3.05 (dd, J¼14.4, 6.0 Hz, 1H), 2.86 (m, 1H), 2.72 (s, 3H), 1.48
stirred at 0 ꢁC for 0.5 h, then N-Me-
D-Tyr(Me)-OMe HCl (2.18 g,
8.4 mmol) was added followed by slowly addition of N-ethyl-
diisopropylamine (4.42 ml, 26.8 mmol). The reaction mixture was
stirred at rt overnight. The solvent was removed in vacuo. The
resulting residue was diluted with EtOAc, and the organic layer was
(d, J¼77.5 Hz, 9H); 13C NMR (100 MHz, CDCl3)
d 172.66, 171.06,

3206
J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
successively washed with water and brine, dried over Na2SO4, and
concentrated. The crude residue was purified by flash chromato-
graph on silica gel (PEePE/EA¼2:1) to give 20a/20b (3.11 g, 65%) as
a mixture of two diastereomers, which was used directly for the
next step without further purification.
concentrated. The crude residue was purified by flash chromato-
graph on silica gel (PEePE/EA¼2:1) to give 24a/24b (4.05 g, 85%) as
a mixture of two diastereomers, which was used directly for the
next step without further purification.
To a solution of 3 M HCl in EtOAc (30 ml) was added compound
24a/24b (2.3 g, 3.91 mmol) at rt. The mixture was stirred at rt for
2 h. The solvent was removed in vacuo. The above residue was
dissolved in methanol (100 ml) and ammonium hydroxide (6 ml)
was added. The reaction mixture was stirred at rt for 0.5 h before
the solvent was removed in vacuo. The crude residue was purified
by flash chromatograph on silica gel (DCMeMeOH/DCM¼20:1) to
give 25 (0.84 g, 47%) and 26 (0.80 g, 45%) as white foams.
To a solution of 3 M HCl in EtOAc (30 ml) was added compounds
20a/20b (2.3 g, 3.91 mmol) at rt. The mixture was stirred at rt for
2 h. The solvent was removed in vacuo. The above residue was
dissolved in methanol (100 ml) and ammonium hydroxide (6 ml)
was added. The reaction mixture was stirred at rt for 0.5 h before
the solvent was removed in vacuo. The crude residue was purified
by flash chromatograph on silica gel (DCMeMeOH/DCM¼20:1) to
give 21 (0.77 g, 43%) and 22 (0.73 g, 41%) as white foams.
Compound 25: ½a 2D3
ꢀ21.05 (c 0.68, MeOH); IR (KBr): 3266, 2932,
ꢂ
Compound 21: ½a 2D7
ꢂ
ꢀ35.61 (c 0.95, MeOH); IR (KBr): 3256, 2928,
2829, 1674, 1644, 1511, 1455, 1397, 1248, 1180, 1107, 1030, 881,
2830, 1676, 1653, 1511, 1437, 1325, 1248, 1111, 1030, 780 cmꢀ1
,
1H
793 cmꢀ1 1H NMR (400 MHz, CD3CN)
, d 9.39 (s, 1H), 7.62 (d,
NMR (400 MHz, CD3CN)
d
9.33 (s, 1H), 7.36 (d, J¼7.9 Hz, 1H), 7.32 (d,
J¼1.4 Hz, 1H), 7.29 (d, J¼8.6 Hz, 1H), 7.19 (dd, J¼8.6, 1.9 Hz, 1H), 7.01
(d, J¼2.3 Hz, 1H), 6.97 (d, J¼8.6 Hz, 2H), 6.78 (d, J¼8.6 Hz, 2H), 6.40
(s, 1H), 3.87 (t, J¼4.0 Hz, 1H), 3.71 (s, 3H), 3.01 (m, 4H), 2.96 (s, 1H),
J¼7.6 Hz, 1H), 7.07 (d, J¼2.4 Hz, 1H), 6.96 (t, J¼7.5 Hz, 3H), 6.77 (d,
J¼8.0 Hz, 2H), 6.07 (s, 1H), 3.93 (t, J¼4.1 Hz, 1H), 3.74 (s, 3H), 3.13
(dd, J¼11.0, 0.8 Hz, 1H), 3.03 (dd, J¼5.9, 4.2 Hz, 2H), 2.89 (m, 5H). 13
C
2.88 (s, 3H). 13C NMR (100 MHz, CD3CN)
d 168.30, 167.38, 160.12,
NMR (100 MHz, CD3CN)
d
168.65, 167.35, 159.91, 135.70, 131.80 (2C),
135.98, 131.94, 130.39, 128.05, 126.71, 125.05, 122.17, 114.66, 114.10,
112.75, 110.06, 64.18, 55.89, 54.44, 36.40, 32.99, 28.80. HRMS
129.82, 127.75, 126.20, 124.94, 121.12, 119.16, 114.59 (2C), 111.54,
105.21, 64.15, 55.74, 54.11, 36.26, 33.02, 29.05. HRMS (ESI): calcd for
(MALDI): calcd for
456.09173.
C
22H23N3O3Br (MþHþ) 456.0915; found
C
22H23N3O3Br (MþHþ) 456.0928; found 456.0917.
Compound 22: ½a 2D7
ꢂ
þ129.54 (c 1.15, MeOH); IR (KBr): 3250,
Compound 26: ½a 2D6
ꢂ
þ80.20 (c 1.80, MeOH); IR (KBr): 3261, 2934,
1H
9.31 (s, 1H), 7.58 (d, J¼1.8 Hz, 1H), 7.32 (d,
2929, 2834, 1676, 1652, 1512, 1460, 1330, 1249, 1032, 832, 743 cmꢀ1
.
2827, 1673, 1644, 1512, 1456, 1335, 1248, 1030, 882, 798 cmꢀ1
,
1H NMR (400 MHz, CD3CN)
d
9.38 (s, 1H), 7.41 (d, J¼7.9 Hz, 1H), 7.32
NMR (400 MHz, CD3CN) d
(d, J¼7.6 Hz, 1H), 6.99 (dd, J¼5.0, 2.7 Hz, 2H), 6.95 (m, 2H), 6.89 (d,
J¼8.7 Hz, 2H), 6.36 (s, 1H), 4.03 (t, J¼4.5 Hz, 1H), 3.92 (m, 1H), 3.70
(s, 3H), 2.86 (m, 5H), 2.76 (dd, J¼14.2, 4.3 Hz, 1H), 1.54 (dd, J¼14.3,
J¼8.6 Hz,1H), 7.23 (dd, J¼8.6, 1.8 Hz,1H), 6.97 (d, J¼2.4 Hz,1H), 6.92
(q, J¼8.8 Hz, 4H), 6.24 (s, 1H), 4.03 (t, J¼4.6 Hz, 1H), 3.91 (dt, J¼¼9.1,
3.2 Hz, 1H), 3.71 (s, 3H), 2.89 (s, 3H), 2.81 (dt, J¼14.3, 3.5 Hz, 2H),
2.71 (dd, J¼14.2, 4.4 Hz, 1H), 1.59 (dd, J¼14.4, 9.2 Hz, 1H). 13C NMR
9.6 Hz, 1H). 13C NMR (100 MHz, CD3CN)
d 165.53, 165.25, 158.67,
134.70, 130.88 (2C), 128.55, 127.91, 124.61, 123.81, 120.03, 117.85,
113.72 (2C), 111.14, 103.88, 62.76, 55.16, 54.62, 35.71, 32.03, 30.67.
HRMS (ESI): calcd for C22H23N3O3Br (MþHþ) 456.0904; found
456.0917.
(100 MHz, CD3CN) d 166.80, 166.56, 159.93, 136.28, 132.13, 130.20,
129.28, 126.70, 125.14, 122.05, 115.02, 114.22, 112.77, 110.66, 64.08,
56.57, 55.93, 37.12, 33.37, 31.68. HRMS (MALDI): calcd for
C
22H23N3O3Br (MþHþ) 456.0919; found 456.09173.
4.12. Diketopiperazine 12c
4.14. Diketopiperazine 12d
To a solution of 22 in CH2Cl2 (60 ml) under N2 atmosphere at
0 ꢁC was slowly added BBr3 (1.9 M in CH2Cl2, 10 ml). The resulting
mixture stirred at rt for 12 h. The reaction was quenched by slow
addition of water and extracted by DCM. The crude residue was
purified by flash chromatograph on silica gel (DCMeDCM/
MeOH¼20:1) to give 12c (4.20 g, 85%) as a white solid. Mp:
To a solution of 26 (5.84 g, 12.8 mmol) in CH2Cl2 (60 ml) under
N2 atmosphere at 0 ꢁC was slowly added BBr3 (1.9 M in CH2Cl2,
10 ml). The resulting mixture stirred at rt for 12 h. The reaction was
quenched by slow addition of water and extracted by DCM. The
crude residue was purified by flash chromatograph on silica gel
(DCMeDCM/MeOH¼20:1) to give 12d (5.38 g, 95%) as a white solid.
152e155 ꢁC; ½a 2D3
ꢂ
þ136.95 (c 1.25, MeOH); IR (KBr): 3256, 3016,
Mp: 161e165 ꢁC; ½a D27
ꢂ
þ112.40 (c 0.65, MeOH). IR (KBr): 3272, 2935,
1H NMR
(400 MHz, CD3CN) d 9.08 (s, 1H), 7.91 (s, 1H), 7.58 (s, 1H), 7.17 (m,
2938, 2362, 1667, 1643, 1515, 1471, 1334, 1237, 1046, 1026, 835,
2825, 1670, 1639, 1515, 1460, 1341, 1233, 1046, 878 cmꢀ1
;
737 cmꢀ1
;
1H NMR (400 MHz, CD3CN)
d
9.27 (s, 1H), 7.42 (d,
J¼7.9 Hz, 1H), 7.32 (d, J¼7.6 Hz, 1H), 7.06 (s, 1H), 6.97 (dd, J¼10.5,
5.1 Hz, 2H), 6.89 (d, J¼8.6 Hz, 2H), 6.80 (d, J¼8.6 Hz, 2H), 6.21 (s,1H),
4.04 (t, J¼4.3 Hz, 1H), 3.89 (dt, J¼9.8, 3.1 Hz, 1H), 2.92 (s, 3H), 2.83
(m, 3H), 1.47 (dd, J¼14.3, 9.9 Hz, 1H). 13C NMR (100 MHz, CD3CN)
2H), 6.89 (d, J¼8.4 Hz, 2H), 6.83 (d, J¼8.6 Hz, 2H), 6.75 (d, J¼1.8 Hz,
1H), 6.24 (s,1H), 4.04 (t, J¼4.3 Hz,1H), 3.80 (d, J¼9.8 Hz,1H), 2.95 (s,
3H), 2.90 (dd, J¼14.3, 4.7 Hz, 1H), 2.79 (dd, J¼14.3, 3.9 Hz, 1H), 2.71
(dd, J¼14.2, 2.9 Hz, 1H), 1.23 (m, 1H); 13C NMR (100 MHz, CD3CN)
d
166.79, 166.68, 157.26, 135.98, 132.29, 129.74, 128.16, 125.93,
d 167.21, 166.81, 157.44, 136.09, 132.29 (2C), 129.78, 127.79, 126.56,
125.08, 121.29, 119.14, 116.40, 112.33, 105.14, 64.08, 56.43, 36.90,
33.34, 31.89. HRMS (MALDI): calcd for C21H21N3O3Br (MþHþ)
442.0769; found 442.0761.
124.90, 121.97, 116.49 (2C), 113.99, 112.59, 110.10, 64.02, 56.36,
36.73, 33.47, 31.36; HRMS (MALDI): calcd for C21H21N3O3Br (MþHþ)
442.0775; found 442.0761.
4.13. Diketopiperazines 25 and 26
4.15. Cyclopeptide 28
To a solution of rac-23 (3.1 g, 8.10 mmol) in dry CH2Cl2 (40 ml)
under N2 atmosphere at 0 ꢁC was slowly added BEP (2.41 g,
8.8 mmol), HOAt (1.43 g, 8.8 mmol). The reaction mixture was
A solution of 4 (200 mg, 0.55 mmol), NEt4ClO4 (3 g, 13.1 mmol)
in CH3CN (70 ml) and aqueous NaOH (0.6 M, 10 ml) was transferred
into an undivided electrolysis cell equipped with a graphite felt
anode and a platinum cathode. Electrolysis with a constant voltage
of 1.0 V was performed at rt, using a standard calomel electrode as
a reference electrode. After the consumption of the starting mate-
rial, the electrolysis was stopped and the reaction mixture was
neutralized by KHSO4 (aqueous) and the solvent was removed in
vacuo. The resulting residue was diluted with EtOAc, and the
stirred at 0 ꢁC for 0.5 h, then N-Me-
D-Tyr(Me)-OMe HCl (2.18 g,
8.4 mmol) was added followed by slowly addition of N-ethyl-
diisopropylamine (4.42 ml, 26.8 mmol). The reaction mixture was
stirred at rt overnight. The solvent was removed in vacuo. The
resulting residue was diluted with EtOAc, and the organic layer was
successively washed with water and brine, dried over Na2SO4, and

J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
3207
organic layer was successively washed with water and brine, dried
over Na2SO4, and concentrated. The crude residue was purified by
preparative TLC (CH2Cl2/MeOH¼20:1) to give 27 (38 mg, 19%).
To a solution of 27 (19 mg, 0.053 mmol) in CH2Cl2 (2 ml) was
added pyridinium (0.2 ml) and acetic anhydride (20 mg,
0.20 mmol), the reaction mixture was heated to reflux until the
consumption of the starting material. The solvent was removed in
vacuo. The resulting residue was diluted with EtOAc, and the or-
ganic layer was successively washed with 1 M HCl, water, aqueous
NaHCO3 and brine, dried over Na2SO4, and concentrated. The crude
residue was purified by preparative TLC (CH2Cl2/MeOH¼20:1) to
give 28 (17 mg, 80%) as a white solid. Single crystals for X-ray
analysis were prepared by vapor diffusion (THF/hexanes). Mp
Solvents were removed in vacuo, and the residue was diluted with
EtOAc. The organic layer was washed by aqueous NaHCO3, water
and brine, dried over Na2SO4, and concentrated. The crude residue
was purified by MPLC on silica gel (MeOH in CH2Cl2: 0e5%) to give
34a (24 mg, 59%) as a yellow solid. Mp: 135e137 ꢁC; ½a D24:8
ꢀ197.3 (c
ꢂ
0.50, THF); IR (KBr): 3244, 3062, 2922, 2851, 1679, 1638, 1481, 1235,
1030, 733 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d
7.42 (d, J¼10.0 Hz, 1H),
7.32 (s, 1H), 7.03 (d, J¼8.0 Hz,1H), 6.91 (d, J¼8.2 Hz,1H), 6.88 (s, 1H),
6.67 (d, J¼9.9 Hz, 1H), 6.17 (s, 1H), 5.39 (s, 1H), 4.24 (s, 1H), 4.20 (d,
J¼3.7 Hz, 1H), 3.29 (d, J¼13.9 Hz, 1H), 3.13 (s, 3H), 3.10e3.01 (m,
2H), 1.99 (d, J¼15.6 Hz, 1H); 13C NMR (100 MHz, CDCl3)
d 187.94,
166.97, 166.32, 165.74, 157.75, 154.02, 137.33, 133.39, 133.15, 131.76,
125.79, 125.17, 123.85, 115.62, 111.61, 65.35, 57.18, 54.95, 37.69,
36.08, 33.83; ESI-MS (m/z): 376 (MþHþ); HRMS (ESI): calcd for
>270 ꢁC (decomposition). ½a 2D3
ꢀ93.54 (c 0.40, MeOH); IR (KBr):
ꢂ
3451, 3248, 2932, 1761, 1673, 1651, 1504, 1449, 1324, 1198, 1101,
C
21H18N3O4 (MþHþ) 376.1298; found 376.1292.
1016, 910, 755 cmꢀ1, 1H NMR (400 MHz, CD3CN)
d
7.68 (d, J¼7.7 Hz,
1H), 7.40 (d, J¼7.8 Hz, 1H), 7.31 (td, J¼7.5, 1.1 Hz, 1H), 7.25 (m, 1H),
7.05 (dd, J¼8.3, 1.6 Hz, 1H), 7.01 (d, J¼8.3 Hz, 1H), 6.48 (s, 1H), 6.45
(s, 1H), 5.54 (s, 1H), 4.41 (dd, J¼4.5, 1.9 Hz, 1H), 4.17 (dd, J¼5.9,
1.7 Hz, 1H), 3.50 (ddd, J¼17.3, 15.7, 0.9 Hz, 2H), 3.05 (dd, J¼14.8,
4.18. Quinone 34b
Ketal 11b (40 mg, 0.072 mmol), HOAc (1 ml), and dioxane (1 ml),
water (0.1 ml) were introduced into an oven-dried 10 ml CEMÒ vial.
The vial was then sealed with a Teflon-lined septum. The resulting
mixture was irradiated by microwave (50 W) at 65 ꢁC for 1.2 h.
Solvent was removed in vacuo, and the residue was diluted with
EtOAc. The organic layer was washed by aqueous NaHCO3, water
and brine, dried over Na2SO4, and concentrated. The crude residue
was purified by MPLC on silica gel (MeOH in CH2Cl2: 0e5%) to give
4.7 Hz,1H), 2.93 (dd, J¼16.5, 7.1 Hz,1H), 2.41 (s, 3H), 2.34 (s, 3H). 13
C
NMR (100 MHz, CD3CN)
d 170.79, 168.12, 166.47, 153.49, 148.25,
144.33, 138.31, 138.26, 135.57, 130.42, 130.28, 126.01, 124.66, 124.06,
122.64, 120.93, 117.77, 61.26, 58.99, 34.22, 31.55, 26.69, 21.04. HRMS
(MALDI): calcd for C23H22N3O4 (MþHþ) 404.1611; found 404.1605.
4.16. Compound 30
34b (13 mg, 48%) as a yellow solid. Mp: 160e165 ꢁC; ½a D25:6
ꢀ1.72 (c
ꢂ
0.275, CHCl3); IR (KBr): 3473, 3223, 2928, 1672, 1637, 1483, 1274,
To a solution of 4 (100 mg, 0.27 mmol) in HCl (concd, 0.3 ml) and
1027, 753 cmꢀ1 1H NMR (500 MHz, CDCl3)
; d 8.95 (s, 1H), 7.46 (d,
HOAc (2.7 ml) was slowly added DMSO (48.8
m
l), the reaction
J¼9.9 Hz,1H), 7.36 (s, 1H), 7.08 (s, 1H), 6.99 (d, J¼8.1 Hz,1H), 6.83 (d,
J¼8.1 Hz,1H), 6.63 (d, J¼9.9 Hz,1H), 6.40 (s, 1H), 4.28 (s, 1H), 4.16 (d,
J¼6.1 Hz, 1H), 3.90 (d, J¼14.3 Hz, 1H), 3.06 (dd, J¼14.4, 6.3 Hz, 1H),
2.97 (dd, J¼16.4, 3.7 Hz, 1H), 2.56 (s, 3H), 2.48 (dd, J¼16.3, 3.4 Hz,
mixture was stirred at rt until the consumption of the starting
material. The solvent was removed in vacuo. The resulting residue
was diluted with EtOAc, and the organic layer was successively
washed with aqueous NaHCO3, water and brine, dried over Na2SO4,
and concentrated. The crude residue was purified by flash chro-
matograph on silica gel (DCM/MeOH¼100:1eDCM/MeOH¼20:1)
to give a mixture of 29a/29b (40 mg, 39%) as a yellow solid, which
was used directly for the next step without further purification.
A solution of 29a/29b in aqueous KOH (1 M, 0.15 ml) was diluted
with water (6 ml). The above solution was added to aqueous so-
lution of K3Fe(CN)6 (52 mg, 0.158 mmol) at 0 ꢁC, then CHCl3 (10 ml)
was added and the reaction mixture was stirred at rt until the
consumption of the starting material. The reaction mixture was
neutralized by KHSO4 (aqueous) and extracted with CH2Cl2 for two
times. The organic layer was washed with brine and dried over
Na2SO4, and concentrated. The crude residue was purified by pre-
parative TLC (CH2Cl2/MeOH¼20:1) to give 30 (3 mg, 31%) as
a slightly yellow solid. Single crystals for X-ray analysis was pre-
1H); 13C NMR (125 MHz, CDCl3)
d 187.77, 170.58, 165.65, 163.93,
158.51, 155.48, 136.79, 133.65, 131.31, 131.28, 126.34, 125.12, 122.11,
116.32, 110.58, 65.84, 56.86, 54.34, 38.54, 36.19, 32.62; ESI-MS (m/
z): 376 (MþHþ); HRMS (ESI): calcd for C21H18N3O4 (MþHþ)
376.1295; found 376.1292.
4.19. Phenol 35a
To a solution of 34a (86 mg, 0.23 mmol) in MeOH (5 ml) was
added NaBH4 (10 mg, 0.26 mmol, 1.14 equiv) at 0 ꢁC. The reaction
was monitored by TLC until the starting material was consumed.
Silica gel was added and the solvent was removed in vacuo. The
residue was purified by flash column chromatography (CH2Cl2/
MeOH, 100:1e20:1) to give 35a (58 mg, 67%) as a white solid. Mp:
>240 ꢁC (decomposition); ½a 2D9
þ193.7 (c 0.108, MeOH); IR (KBr):
ꢂ
pared by vapor diffusion (THF/hexanes). ½a D27
ꢂ
þ70.37 (c 0.20,
3350, 2970, 2937, 2835, 1661, 1635, 1481, 1236, 1058, 925 cmꢀ1
;
MeOH); IR (KBr): 3444, 3345, 2923, 1675, 1638, 1617, 1590, 1515,
1H NMR (500 MHz, CD3CN)
d
7.52 (d, J¼2.6 Hz, 1H), 6.93 (d,
1450, 1338, 1253, 1026, 980, 832, 754 cmꢀ1
DMSO-d6)
;
1H NMR (400 MHz,
J¼1.6 Hz, 1H), 6.85 (dd, J¼8.0, 1.7 Hz, 1H), 6.71 (d, J¼8.0 Hz, 1H),
6.53 (dd, J¼8.3, 2.5 Hz, 1H), 6.49e6.44 (m, 2H), 6.08 (d, J¼5.3 Hz,
1H), 5.50 (s, 1H), 5.27 (d, J¼5.3 Hz, 1H), 4.10 (d, J¼4.2 Hz, 2H), 3.26
(d, J¼14.0 Hz, 1H), 3.08 (s, 3H), 2.96 (dd, J¼16.1, 3.2 Hz, 1H), 2.86
(dd, J¼14.0, 4.6 Hz, 1H), 1.91 (d, J¼4.3 Hz, 1H); 13C NMR (125 MHz,
d
9.48 (s, 1H), 7.83 (d, J¼2.1 Hz, 1H), 7.24 (t, J¼7.2 Hz, 1H),
7.16 (d, J¼9.1 Hz, 3H), 6.88 (d, J¼8.4 Hz, 2H), 6.73 (t, J¼8.1 Hz, 3H),
6.58 (t, J¼7.5 Hz, 1H), 4.34e4.26 (m, 1H), 4.21 (s, 1H), 3.09e2.99 (m,
2H), 2.98 (s, 3H), 2.54 (d, J¼3.4 Hz, 1H), 1.62 (dd, J¼17.4, 8.2 Hz, 1H).
13C NMR (100 MHz, DMSO-d6)
d
197.35, 165.84, 165.54, 156.89,
CD3CN) d 168.18, 165.84, 158.50, 151.61, 142.54, 133.81, 132.57,
151.10, 134.32, 131.35 (2C), 131.06, 125.36, 116.78, 115.95, 115.26
(2C), 114.65, 62.21, 50.18, 43.95, 34.80, 32.19. HRMS (MALDI): calcd
for C20H22N3O4 (MþHþ) 368.1614; found 368.1605.
132.46, 130.61, 125.53, 115.18, 113.25, 110.87, 110.73, 110.32, 65.97,
59.95, 56.18, 41.15, 38.15, 33.86; ESI-MS (m/z): 378 (MþHþ);
HRMS (ESI): calcd for C21H20N3O4 (MþHþ) 378.1459; found
378.1448.
4.17. Quinone 34a
4.20. Phenol 35b
Ketal 11a (60 mg, 0.108 mmol), HOAc (4.5 ml), and water
(0.03 ml) were introduced into an oven-dried 20 ml CEMÒ vial. The
vial was then sealed with a Teflon-lined septum. The resulting
mixture was irradiated by microwave (50 W) at 50 ꢁC for 15 min.
To a solution of 34b (110 mg, 0.29 mmol) in MeOH (5 ml) was
added NaBH4 (12 mg, 0.32 mmol, 1.1 equiv) at 0 ꢁC. The reaction
was monitored by TLC until consumption of the starting material.

3208
J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
Silica gel was added and the solvent was removed in vacuo. The
residue was purified by flash column chromatography (CH2Cl2/
MeOH, 100:1e20:1) to give 35b (88 mg, 80%) as a white solid. Mp:
4.23. Compound 32a
A mixture of 36a (35 mg, 0.069 mmol), triethylamine (30 ml,
>220 ꢁC (decomposition); ½a 2D4
ꢂ
ꢀ253.6 (c 0.33, MeOH); IR (KBr):
0.22 mmol), and 15 mg Pd(OH)2/C (20% Pd) in MeOH (1.5 ml) and
EtOAc (1.5 ml) was stirred at rt under hydrogen atmosphere (1 atm)
at rt for 2 h. The reaction mixture was filtered through a pad of
Celite. The solid was washed with 5% MeOH/CH2Cl2. The combined
organic phases were concentrated in vacuo. The residue was puri-
fied by flash column chromatography (CH2Cl2/MeOH, 100:1e20:1)
to afford 32a (20 mg, 80%) as a white solid. Mp: >300 ꢁC (de-
3336, 2935, 2852, 1662, 1632, 1482, 1454, 1200, 1019, 819 cmꢀ1; 1H
NMR (500 MHz, CD3CN)
d
7.33 (d, J¼1.5 Hz, 1H), 7.00 (s, 1H), 6.96
(d, J¼2.3 Hz, 1H), 6.89 (dd, J¼8.0, 1.7 Hz, 1H), 6.62e6.48 (m, 4H),
6.17 (d, J¼4.6 Hz, 1H), 5.40 (d, J¼4.1 Hz, 1H), 4.12 (s, 1H), 4.07 (d,
J¼6.5 Hz, 1H), 3.46 (d, J¼14.0 Hz, 1H), 3.02 (dd, J¼14.1, 6.6 Hz, 1H),
2.62 (dd, J¼16.3, 4.5 Hz, 1H), 2.43e2.38 (m, 4H); 13C NMR
(125 MHz, CD3CN)
d
169.94, 165.32, 159.23, 151.88, 142.32, 134.20,
composition); ½a D29
ꢂ
þ154.8 (c 0.14, CHCl3); IR (KBr): 3330, 2925,
132.19, 131.81, 130.82, 126.00, 115.60, 111.53, 111.00, 110.74, 110.12,
66.52, 59.88, 55.18, 42.19, 38.92, 32.76; ESI-MS (m/z): 378 (MþHþ);
HRMS (ESI): calcd for C21H20N3O4 (MþHþ) 378.1455; found
378.1448.
2848, 1670, 1650, 1485, 1316, 1233, 1057, 944, 752 cmꢀ1 1H NMR
;
(500 MHz, CDCl3)
d
8.01 (d, J¼7.5 Hz, 1H), 7.12 (t, J¼7.7 Hz, 1H),
6.97e6.89 (m, 3H), 6.83 (d, J¼8.5 Hz, 1H), 6.67 (d, J¼7.8 Hz, 1H), 6.14
(d, J¼3.6 Hz,1H), 5.44 (s, 1H), 4.91 (d, J¼3.6 Hz,1H), 4.22 (s, 1H), 4.17
(d, J¼4.5 Hz, 1H), 3.29e3.24 (m, 2H), 3.16 (s, 3H), 3.02 (dd, J¼14.0,
4.6 Hz, 1H), 1.89 (dd, J¼16.2, 4.3 Hz, 1H); 13C NMR (125 MHz, CDCl3)
4.21. Triflate 36a
d
166.95,165.57,156.95,147.47,131.93,131.39,130.82,129.55,128.51,
124.82, 124.75, 120.59, 110.94, 109.20, 109.07, 65.35, 58.93, 55.48,
40.58, 37.61, 33.84; ESI-MS (m/z): 362 (MþHþ); HRMS (ESI): calcd
for C21H20N3O3 (MþHþ) 362.1496; found 362.1499.
To a solution of 35a (50 mg, 0.13 mmol) in dry EtOAc (8 ml)
under N2 atmosphere was added triethylamine (0.3 ml). The so-
lution was cooled down to ꢀ15 ꢁC for 10 min, and Tf2O (48
ml,
0.28 mmol, 2.15 equiv) was slowly added. The solution was stirred
for 3 h at ꢀ15 ꢁC until aqueous NH4Cl solution was added. The
resulting mixture was diluted with EtOAc and washed with water
and brine, dried over Na2SO4, and concentrated. The crude residue
was purified by flash column chromatograph (CH2Cl2/MeOH,
100:1e20:1) to give 36a (36 mg, 53%) as a white solid. Mp:
4.24. Compound 3b
A mixture of 36b (62.6 mg, 0.123 mmol), triethylamine (34 ml,
0.246 mmol), and 15 mg Pd(OH)2/C (20% Pd) in MeOH (3 ml) and
EtOAc (1 ml) was stirred at rt under hydrogen atmosphere (1 bar) at
rt for 2 h. The reaction mixture was filtered through a pad of Celite.
The solid was rinsed with 5% methanol/dichloromethane. The
combined filtrates were concentrated in vacuo. The residue was
purified by flash column chromatography (CH2Cl2/MeOH,
100:1e20:1) to afford 3b (43.0 mg, 96%) as a white solid. Mp:
>270 ꢁC (dec); ½a 2D8
ꢂ
þ217.0 (c 0.16, MeOH); IR (KBr): 3285, 3053,
1H NMR
8.03 (d, J¼2.7 Hz, 1H), 7.36 (d, J¼3.7 Hz, 1H),
2928, 2845, 1673, 1654, 1488, 1418, 1213, 1060, 893 cmꢀ1
;
(500 MHz, DMSO-d6)
d
7.26 (d, J¼1.6 Hz, 1H), 7.12 (dd, J¼8.6, 2.7 Hz, 1H), 6.86 (dd, J¼8.0,
1.6 Hz, 1H), 6.75 (dd, J¼6.1, 4.8 Hz, 2H), 6.62 (d, J¼8.6 Hz, 1H), 6.20
(d, J¼3.8 Hz, 1H), 4.18 (d, J¼4.2 Hz, 1H), 4.14 (s, 1H), 3.26 (d,
J¼13.8 Hz, 1H), 3.03 (s, 3H), 2.79 (dd, J¼14.0, 4.6 Hz, 1H), 2.74 (dd,
J¼15.9, 3.6 Hz, 1H), 1.94 (dd, J¼16.1, 3.9 Hz, 1H); 13C NMR (125 MHz,
>300 ꢁC (dec); ½a D28:7
ꢂ
ꢀ416.2 (c 0.125, MeOH); IR (KBr): 3430, 2979,
1H NMR
8.78 (s, 1H), 7.44 (d, J¼7.2 Hz, 1H), 7.22 (d,
2858, 1664, 1634, 1479, 1055, 1012, 810, 744 cmꢀ1
;
(400 MHz, DMSO-d6)
d
DMSO-d6)
d
167.41, 164.24, 156.49, 148.89, 141.32, 132.82, 131.60,
J¼1.4 Hz, 1H), 7.14 (d, J¼3.5 Hz, 1H), 7.03 (td, J¼7.7, 1.2 Hz, 1H), 6.88
(dd, J¼8.0, 1.7 Hz, 1H), 6.73 (td, J¼7.4, 0.8 Hz, 1H), 6.60e6.54 (m,
2H), 6.19 (d, J¼3.6 Hz, 1H), 4.12 (m, 2H), 3.36 (m, 1H), 2.99 (dd,
J¼14.0, 6.4 Hz, 1H), 2.55 (m, 1H), 2.41e2.34 (m, 4H); 13C NMR
131.50, 128.47, 123.91, 121.09, 118.38 (q, J¼319 Hz, 1C), 117.26,
109.93, 108.68, 107.95, 64.43, 58.13, 54.20, 39.09, 36.71, 32.67; ESI-
MS (m/z): 510 (MþHþ); HRMS (ESI): calcd for C22H19N3O6F3S
(MþHþ) 510.0957; found 510.0941.
(100 MHz, DMSO)
d 168.28, 164.18, 157.70, 148.45, 131.67, 131.11,
130.30, 129.66, 127.96, 124.51, 122.92, 118.20, 108.87, 108.58, 108.06,
64.92, 58.02, 53.60, 40.47, 37.49, 31.63; ESI-MS (m/z): 362 (MþHþ);
4.22. Triflate 36b
HRMS (ESI): calcd for
362.1499.
C
21H20N3O3 (MþHþ) 362.1504; found
To a solution of 35b (28 mg, 0.074 mmol) in dry EtOAc (2 ml)
under N2 atmosphere was added triethylamine (0.05 ml). The so-
lution was cooled down to ꢀ15 ꢁC for 10 min, and Tf2O (25
m
l,
4.25. Azonazine diastereomer 1a (the originally proposed
structure)
0.15 mmol, 2 equiv) was slowly added. The solution was stirred for
1 h at ꢀ15 ꢁC until aqueous NH4Cl solution was added. The resulting
mixture was diluted with EtOAc and washed with water and brine,
dried over Na2SO4, and concentrated. The crude residue was puri-
fied by flash column chromatograph (CH2Cl2/MeOH, 100:1e20:1)
to give 36b (26.5 mg, 70%) as a white solid. Mp: >250 ꢁC (de-
To a solution of 32a (14 mg, 0.039 mmol) in HOAc (2 ml) was
added Ac2O (0.01 ml, 0.106 mmol) at rt. The resulting mixture was
stirred at rt for 6 h. The solvent was removed, and the residue was
diluted by EtOAc. The organic layer was washed with aqueous
NaHCO3, water and brine, dried over Na2SO4, and concentrated. The
crude residue was purified by preparative TLC (EtOAc/MeOH, 50:1)
to afford 1a (10.5 mg, 67%) as a white solid. Mp: >300 ꢁC (dec);
composition); ½a D25
ꢂ
ꢀ368.4 (c 0.115, MeOH); IR (KBr): 3444, 2934,
1H NMR
8.83 (s, 1H), 7.58 (d, J¼2.6 Hz, 1H), 7.52 (d,
2848, 1673, 1636, 1490, 1419, 1217, 1137, 884 cmꢀ1
;
(500 MHz, DMSO-d6)
d
J¼3.1 Hz, 1H), 7.22 (d, J¼1.5 Hz, 1H), 7.10 (dd, J¼8.6, 2.6 Hz, 1H), 6.93
(dd, J¼8.0, 1.7 Hz, 1H), 6.63 (d, J¼3.7 Hz, 1H), 6.62 (d, J¼3.1 Hz, 1H),
6.28 (d, J¼3.2 Hz, 1H), 4.12 (d, J¼6.5 Hz, 2H), 3.35 (d, J¼13.7 Hz, 1H),
3.02 (dd, J¼13.9, 6.4 Hz,1H), 2.55 (dd, J¼16.2, 4.5 Hz,1H), 2.39e2.32
½
a 2D8:2
2848, 1677, 1649, 1485, 1391, 1241, 971, 756 cmꢀ1
(500 MHz, CD3CN)
ꢂ
þ342.7 (c 0.175, MeOH); IR (KBr): 3518, 3232, 3012, 2928,
1H NMR
8.10 (s, 1H), 8.06 (d, J¼7.1 Hz, 1H), 7.26 (t,
;
d
J¼7.3 Hz, 1H), 7.19 (t, J¼7.4 Hz, 1H), 7.01 (s, 1H), 6.91 (d, J¼8.1 Hz,
1H), 6.80 (d, J¼8.1 Hz, 1H), 6.46 (s, 1H), 5.61 (s, 1H), 4.18 (s, 1H), 4.14
(d, J¼4.5 Hz, 1H), 3.29 (d, J¼14.1 Hz, 1H), 3.13 (dd, J¼16.3, 2.8 Hz,
1H), 3.09 (s, 3H), 2.89 (dd, J¼14.1, 4.7 Hz, 1H), 2.41 (s, 3H), 2.18 (dd,
(m, 4H); 13C NMR (125 MHz, DMSO-d6)
d 168.19, 164.00, 157.60,
148.64, 141.29, 133.21, 130.74, 130.15, 129.93, 124.52, 120.89, 118.28
(q, J¼319 Hz, 1C), 116.78, 109.16, 108.58, 108.01, 64.83, 58.04, 53.41,
39.59 (DEPT, HMQC), 37.55, 31.62; ESI-MS (m/z): 510 (MþHþ);
HRMS (ESI): calcd for C22H18N3O6F3SNa (MþNaþ) 532.0760; found
532.0761.
J¼16.3, 4.8 Hz, 1H); 13C NMR (125 MHz, CD3CN)
d 171.74, 168.24,
165.77, 157.67, 142.95, 134.78, 133.77, 132.82, 130.35, 129.28, 125.35
(2C), 124.95, 116.52, 111.25, 107.01, 65.97, 58.79, 56.18, 40.98, 38.20,

J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
3209
33.94, 24.37; ESI-MS (m/z): 404 (MþHþ); HRMS (ESI): calcd for
with EtOAc. The organic layers were washed with aqueous NaHCO3,
water, and brine, dried over Na2SO4, and concentrated. The crude
residue was directly used for the next step without further
purification.
C
23H21N3O4Na (MþNaþ) 426.1429; found 426.1424.
4.26. ent-(L)-Azonazine (1b)
The above crude product was treated with HOAc (2.0 ml) and
Ac2O (0.01 ml, 0.106 mmol), and the resulting mixture was stirred
at rt for 10 h. The mixture was concentrated in vacuo, and the
residue was then diluted by EtOAc. The solution was washed with
aqueous NaHCO3, water, and brine, dried over Na2SO4, and con-
centrated. The crude residue was purified by preparative TLC
(EtOAc/MeOH, 50:1) to afford 33b (12.6 mg, 70% for two steps) as
To a solution of 3b (13 mg, 0.036 mmol) in HOAc (1.5 ml) was
added Ac2O (0.01 ml, 0.106 mmol) at rt. The resulting mixture was
stirred at rt for 4 h. The solvent was removed and the residue was
diluted by EtOAc. The organic layer was washed with aqueous
NaHCO3, water and brine, dried over Na2SO4, and concentrated. The
crude residue was purified by preparative TLC (EtOAc/MeOH, 50:1)
to afford 1b (13 mg, 89%). The single crystal for X-ray analysis was
prepared by slow evaporation from CH3CN. Mp: >350 ꢁC (dec);
a white solid. Mp: >270 ꢁC (dec); ½a D26
ꢀ251.4 (c 0.725, THF); IR
ꢂ
(KBr): 3451, 2964, 2917, 2844, 1673, 1656, 1492, 1391, 1259, 1154,
½
a 2D6:5
1661, 1626, 1485, 1394, 1283, 1027, 740 cmꢀ1
CD3CN)
ꢂ
ꢀ299.3 (c 0.11, MeOH); IR (KBr): 3430, 3219, 2926, 2849,
1077, 1018, 797 cmꢀ1; 1H NMR (400 MHz, DMSO-d6)
d 8.82 (s, 1H),
;
1H NMR (600 MHz,
8.00 (d, J¼8.7 Hz,1H), 7.39 (s,1H), 7.35 (d, J¼2.5 Hz,1H), 6.99 (s,1H),
6.97 (d, J¼1.8 Hz, 1H), 6.74e6.68 (d, J¼8.5 Hz, 2H), 4.87e4.71 (m,
2H), 4.25 (s, 1H), 4.16 (d, J¼6.3 Hz, 1H), 3.40 (d, J¼13.9 Hz, 1H), 3.06
(dd, J¼14.0, 6.6 Hz, 1H), 2.80 (dd, J¼16.2, 5.1 Hz, 1H), 2.50e2.43 (m,
1H), 2.38 (s, 3H), 2.35 (s, 3H); 13C NMR (100 MHz, DMSO-d6)
d
8.13 (s, 1H), 7.60 (dd, J¼7.5, 0.8 Hz, 1H), 7.51 (d, J¼1.6 Hz,
1H), 7.29 (td, J¼7.8, 1.3 Hz, 1H), 7.19 (td, J¼7.5, 1.1 Hz, 1H), 6.96 (m,
2H), 6.67 (d, J¼8.1 Hz, 1H), 6.59 (s, 1H), 4.27 (d, J¼5.1 Hz, 1H), 4.09
(d, J¼6.5 Hz, 1H), 3.50 (d, J¼14.2 Hz, 1H), 3.08 (dd, J¼14.2, 6.9 Hz,
1H), 2.84 (dd, J¼16.5, 5.8 Hz, 1H), 2.49 (dd, J¼16.5, 2.2 Hz, 1H), 2.42
d
169.64,168.07,164.17,157.14,153.98,135.95,135.82,131.76,130.44,
(s, 3H), 2.40 (s, 3H); 13C NMR (150 MHz, CD3CN)
d
171.29, 169.43,
129.98, 124.92, 124.02 (q, J¼277 Hz, 1C), 116.20, 113.55, 110.82,
109.49, 105.49, 65.13 (q, J¼34 Hz, 1C), 64.73, 57.37, 53.33, 40.64,
37.73, 31.67, 23.36; ESI-MS (m/z): 502 (MþHþ); HRMS (ESI): calcd
for C25H23N3O5F3 (MþHþ) 502.1576; found 502.1584.
165.49, 159.05, 142.52, 135.37, 133.22, 132.02, 131.51, 129.62, 126.46,
125.71, 124.15, 117.31, 110.84, 106.98, 66.32, 59.08, 54.84, 43.41,
39.24, 32.83, 24.17; ESI-MS (m/z): 404 (MþHþ); HRMS (ESI): calcd
for C23H22N3O4 (MþHþ) 404.1610; found 404.1605.
Acknowledgements
4.27. Azonazine analogue 33a
Financial support from the Ministry of Science and Technology
of the People’s Republic of China (2010CB833200, SS2013AA
090203) and National Natural Science Foundation of China grant
(21032002) is greatly appreciated. The authors would like to thank
Mr. Lan-De Zhang at Chemistry Department of Nanjing University
for his assistance in the 1H NMR analysis, and Dr. Gang Chen and
Jian-Feng Mou at SIOC for their help in performing the electrolysis
chemistry.
To a solution of 11a (26 mg, 0.047 mmol) in THF (3 ml) was
added Zn powder (9.0 mg, 0.14 mmol), Et3SiH (29
ml, 0.19 mmol),
and AcOH (30 l, 0.52 mmol) at rt under N2 atmosphere. The re-
m
action mixture was stirred at rt until consumption of the starting
material (monitored by TLC). The reaction mixture was diluted with
EtOAc and filtered through a pad of Celite. The solid was washed
with EtOAc. The organic layers were washed with aqueous NaHCO3,
water, and brine, dried over Na2SO4, and concentrated. The crude
residue was directly used for the next step without further
purification.
Supplementary data
The above crude product was treated with HOAc (1.5 ml) and
Ac2O (0.01 ml, 0.106 mmol), and the resulting mixture was stirred
at rt for 10 h. The mixture was concentrated in vacuo, and the
residue was then diluted by EtOAc. The solution was washed with
aqueous NaHCO3, water, and brine, dried over Na2SO4, and con-
centrated. The crude residue was purified by preparative TLC
(EtOAc/MeOH, 50:1) to afford 33a (16 mg, 70% for two steps) as
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.03.061.
References and notes
1. Cole, R. J.; Jarvis, B. B.; Schweikert, M. A. Handbook of Secondary Fungal Me-
tabolites; Academic: San Diego, USA, 2003, Vols. IeIII.
2. Wu, Q. X.; Crews, M. S.; Draskovic, M.; Sohn, J.; Johnson, T. A.; Tenney, K.;
Valeriote, F. A.; Yao, X. J.; Bjeldanes, L. F.; Crews, P. Org. Lett. 2010, 12, 4458.
3. Lindquist, N.; Fenical, W.; Van Duyne, G. D.; Clardy, J. J. Am. Chem. Soc. 1991, 113,
2303.
4. For total syntheses of diazonamide A: (a) Nicolaou, K. C.; Bella, M.; Chen, D. Y.
K.; Huang, X.; Ling, T.; Snyder, S. A. Angew. Chem., Int. Ed. 2002, 41, 3495; (b)
Burgett, A. W. G.; Li, Q.; Wei, Q.; Harran, P. G. Angew. Chem., Int. Ed. 2003, 42,
4961; (c) Nicolaou, K. C.; Bheema Rao, P.; Hao, J.; Reddy, M. V.; Rassias, G.;
Huang, X.; Chen, D. Y. K.; Snyder, S. A. Angew. Chem., Int. Ed. 2003, 42, 1753; (d)
Cheung, C.-M.; Goldberg, F. W.; Magnus, P.; Russell, C. J.; Turnbull, R.; Lynch, V. J.
Am. Chem. Soc. 2007, 129, 12320; (e) Mai, C.-K.; Sammons, M. F.; Sammakia, T.
Angew. Chem., Int. Ed. 2010, 49, 2397; (f) Knowles, R. R.; Carpenter, J.; Blakey, S.
B.; Kayano, A.; Mangion, I. K.; Sinz, C. J.; MacMillan, D. W. C. Chem. Sci. 2011, 2,
308.
a white solid. ½a D27
þ282.10 (c 0.20, THF); IR (KBr): 3224, 2924,
ꢂ
2854, 1677, 1651, 1619, 1490, 1390, 1261, 1162, 1066, 971, 818 cmꢀ1
;
1H NMR (400 MHz, CD3CN)
d
8.05 (s, 1H), 7.83 (d, J¼2.7 Hz, 1H), 7.00
(s, 1H), 6.92 (d, J¼8.1 Hz, 1H), 6.84 (m, 2H), 6.45 (s, 1H), 5.63 (s, 1H),
4.58 (q, J¼8.4 Hz, 2H), 4.19 (s, 1H), 4.15 (d, J¼4.3 Hz, 1H), 3.28 (d,
J¼14.0 Hz, 1H), 3.07 (m, 4H), 2.89 (dd, J¼14.1, 4.6 Hz, 1H), 2.39 (s,
3H), 2.12 (s, 1H). 13C NMR (100 MHz, CD3CN)
d 171.37, 168.37,165.76,
157.59, 155.33, 138.00, 133.81, 131.38 (q, J¼329 Hz), 125.37, 123.71,
118.38, 117.26, 114.37, 112.57, 111.29, 106.99, 66.7 (q, J¼35 Hz), 65.97,
65.93, 58.84, 56.10, 40.60, 38.19, 34.95, 33.93, 30.44, 24.12. HRMS
(MALDI): calcd for
524.1404.
C
25H22N3O5Na (MþNaþ) 524.1428; found
5. For a recent study on the synthesis of azonazine, see: Ghosh, S.; Kinthada, L. K.;
Bhunia, S.; Bisai, A. Chem. Commun. 2012, 10132.
6. For a recent synthesis of benzofuroindolines: Beaud, R.; Guillot, R.; Kouklovsky,
C.; Vincent, G. Angew. Chem., Int. Ed. 2012, 51, 12546.
7. Zhao, J.-C.; Yu, S.-M.; Yao, Z.-J. Org. Lett. 2013, 15, 4300.
8. Boger, D. L.; Yohannes, D. J. Org. Chem. 1988, 53, 487.
4.28. Azonazine analogue 33b
9. BEP¼2-bromo-1-ethyl pyridinium tetrafluoroborate.
To a solution of 11b (20 mg, 0.036 mmol) in THF (1 ml) was
10. (a) Li, P.; Xu, J. C. Tetrahedron 2000, 56, 8119; (b) Li, P.; Xu, J. C. Chem. Lett. 2000,
29, 204.
added Zn powder (5.0 mg, 0.077 mmol), Et3SiH (22.9
ml, 0.14 mmol),
11. For recent reviews on hypervalent iodines in organic synthesis, see: (a) Tohma,
H.; Kita, Y. Adv. Synth. Catal. 2004, 346, 111; (b) Zhdankin, V. V.; Stang, P. J. Chem.
Rev. 2008, 108, 5299; (c) Merritt, E. A.; Olofsson, B. Angew. Chem., Int. Ed. 2009,
48, 9052; (d) Dohi, T.; Ito, M.; Yamaoka, N.; Morimoto, K.; Fujioka, H.; Kita, Y.
Tetrahedron 2009, 65, 10797; (e) Zhdankin, V. V. J. Org. Chem. 2011, 76, 1185; (f)
and AcOH (23 l, 0.40 mmol) at rt under N2 atmosphere. The re-
m
action mixture was stirred at rt until consumption of the starting
material (monitored by TLC). The reaction mixture was diluted with
EtOAc and filtered through a pad of Celite. The solid was washed

3210
J.-C. Zhao et al. / Tetrahedron 70 (2014) 3197e3210
Silva, L. F.; Olofsson, B. Nat. Prod. Rep. 2011, 28, 1722; (g) Duschek, A.; Kirsch, S. F.
Angew. Chem., Int. Ed. 2011, 50, 1524.
J.; Mikami, Y.; Chiba, K. Chem.dEur. J. 2012, 18, 6284; (d) Enache, T. A.; Oliveira-
Brett, A. M. Electroanalysis 2011, 23, 1337.
12. Coste, A.; Toumi, M.; Wright, K.; Razafimahaleo, V.; Couty, F.; Marrot, J.; Evano,
18. (a) Dankwardt, J. W. Angew. Chem., Int. Ed. 2004, 43, 2428; (b) Guan, B. T.; Xiang,
S. K.; Wu, T.; Sun, Z. P.; Wang, B. Q.; Zhao, K. Q.; Shi, Z. J. Chem. Commun. 2008,
1437.
G. Org. Lett. 2008, 10, 3841.
13. (a) Suzuki, K.; Sasaki, Y.; Endo, N.; Mihara, Y. Chem. Pharm. Bull. 1981, 29, 233;
(b) Durow, A. C.; Long, G. C.; O’Connel, S. J.; Willis, C. L. Org. Lett. 2006, 8, 5401.
14. Leathen, M. L.; Rosen, B. R.; Wolfe, J. P. J. Org. Chem. 2009, 74, 5107.
15. Konda-Yamada, Y.; Okada, C.; Yoshida, K.; Umeda, Y.; Arima, S.; Sato, N.; Kai, T.;
Takayanagi, H.; Harigaya, Y. Tetrahedron 2002, 58, 7851.
19. The authors (Ref. 2) favored to assign the absolute configuration of azonazine in
parallel to that of diazonamide A (R10/S11), since the chemical shifts of dihy-
drobenzofuran of diazonamide A (Ref. 3) were virtually identical to those of
(þ)-azonazine.
16. Blaser, G.; Sanderson, J. M.; Batsanov, A. S.; Howard, J. A. K. Tetrahedron Lett.
20. A review on fluorine in medicinal chemistry, see: Purser, S.; Moore, P. R.;
Swallow, S.; Gouverneur, V. Chem. Soc. Rev. 2008, 37, 320.
21. Ghaffarzadeh, M.; Joghan, S. S.; Faraji, F. Tetrahedron Lett. 2012, 53, 203.
2008, 49, 2795.
17. (a) Sainsbury, M.; Wyatt, J. J. Chem. Soc., Perkin Trans. 1 1979, 108; (b) Keech, P.
G.; Bunce, N. J. J. Appl. Electrochem. 2003, 33, 79; (c) Kim, S.; Hirose, K.; Uematsu,