Synthesis of an isomer of the renieramycin skeleton from L -tyrosine

Article abstract of DOI:10.1002/jhet.609

A new approach that tried to obviate the use of bromine protection groups was studied to synthesize (-)-renieramycin G from L-tyrosine. It was found that the first intermolecular Pictet-Spengler reaction proceeded successfully to give the correct tetrahyd

Full text of DOI:10.1002/jhet.609

414
Synthesis of an Isomer of the Renieramycin Skeleton
from L-Tyrosine
Vol 48
Wei Liu, Wen Fang Dong, Xiang Wei Liao, Bao He Guan,
and Zhan Zhu Liu*
Key Laboratory of Bioactive Substances and Resources Utilization of Chinese Herbal Medicine
(Ministry of Education), Institute of Materia Medica, Chinese Academy of Medical Sciences and
Peking Union Medical College, Beijing 100050, People’s Republic of China
*E-mail: liuzhanzhu@imm.ac.cn
Received April 6, 2010
DOI 10.1002/jhet.609
Published online 21 January 2011 in Wiley Online Library (wileyonlinelibrary.com).
A new approach that tried to obviate the use of bromine protection groups was studied to synthesize
(ꢀ)-renieramycin G from L-tyrosine. It was found that the first intermolecular Pictet–Spengler reaction
proceeded successfully to give the correct tetrahydroisoquinoline precursor 6. However, the second
intramolecular Pictet–Spengler cyclization step failed to give the desired product, and an isomer of the
skeleton of the renieramycins was obtained via 12 steps starting from L-tyrosine.
J. Heterocyclic Chem., 48, 414 (2011).
INTRODUCTION
rosine as the chiral starting material [25]. Now as a con-
tinuation of this research, we investigated a more effi-
cient synthetic route of (ꢀ)-renieramycin G, which tried
to avoid the use of the bromine protection groups in the
two benzene rings.
Members of the tetrahydroisoquinoline family of alka-
loids including saframycins, ecteinascidins, renieramy-
cins, quinocarcins, and lemonomycin display a wide
range of biological properties such as antitumor and
antimicrobial activities [1]. Renieramycins are marine
natural products that are structurally and biologically
related to the isoquinoline natural products. Renieramy-
cin G (Fig. 1) was isolated in 1992 by Davidson [2]
from the Fijian sponge Xestospongia caycedoi. Several
studies on the total synthesis of the renieramycin natural
products have been reported [3–5].
RESULTS AND DISCUSSIONS
The synthesis of amino acid 4 (Scheme 1) and the
key 1,2,3,4-tetrahydroisoquinoline precursor 6 (Scheme
2) basically followed our published procedures. The dif-
ference was that the use of the bromine protecting
groups on the benzene rings was obviated. The Baeyer–
Villiger oxidation of 1 using m-chloroperoxybenzoic
acid (m-CPBA) in chloroform at room temperature,
followed by hydrolysis of the resulting formate interme-
diate, provided phenol 2. The N-acetyl group of 2 was
removed with SOCl2 in methanol, and the resulting free
amine was reprotected as the corresponding Boc carba-
mate to afford compound 4. Finally, hydrolysis of the
methyl ester with LiOH provided amino acid 4. Amino
ester 2 was reduced to the corresponding alcohol by
LiBH4 in 91% yield. The N-acetyl group was removed
The stereospecific intramolecular Pictet–Spengler cy-
clization is one of the key steps for the construction of
the pentacyclic skeleton of the bistetrahydroisoquinoline
alkaloids. Organic acids such as HCOOH, MeSO3H,
CF3SO3H, and CF3COOH have been used to realize the
cyclization [6–23]. Previous research in our group
mainly focused on the study of new approaches to con-
struct the pentacyclic skeleton and the discovery of sim-
plified derivatives of the bistetrahydroisoquinoline alka-
loids [24]. Recently, our group reported a new approach
for the total synthesis of (ꢀ)-renieramycin G using L-ty-
C
V
2011 HeteroCorporation

March 2011
Synthesis of an Isomer of the Renieramycin Skeleton from L-Tyrosine
415
Thus it was confirmed that the intramolecular Pictet–
Spengler cyclization occurred para instead of ortho to the
hydroxyl group of the right benzene ring. It is supposed
that the relatively strong condition of this reaction
(CF3CO2H/r.t.) other than the mild one of the first inter-
molecular Pictet–Spengler reaction (acetic acid/ꢀ10ꢁC)
failed to give the desired product.
CONCLUSION
Figure 1. Structure of (ꢀ)-renieramycin G.
In conclusion, we studied a new approach for the total
synthesis of (ꢀ)-renieramycin G without the use of the
bromine protection groups. The first intermolecular Pic-
tet–Spengler reaction proceeded smoothly to give the
desired tetrahydroisoquinoline product. However, the sec-
ond intramolecular Pictet–Spengler reaction did not give
the correct cyclization product, and an isomer of the skel-
eton of renieramycin natural products was obtained
through 12 steps for the longest linear route.
with 6 N aqueous HCl in CH3OH to give amino alcohol
5 in 90% yield. The highly diastereoselective Pictet–
Spengler cyclization reaction between amino alcohol 5
and benzyloxyacetaldehyde at ꢀ10ꢁC regioselectively
provided (1R,3S)-1,2,3,4-tetrahydroisoquinoline 6 in 87%
yield. No product from the cylization occurring para to
the hydroxyl group on the benzene ring was found. The
stereochemistry of compound 6 was verified through the
analysis of the stereochemistry of (ꢀ)-MY 336a, which
was obtained from the hydrogenation of compound 6 [26].
Thus it proved that the intermolecular Pictet–Spengler reac-
tion was successful without the bromine blocking group.
Next, 1,2,3,4-tetrahydroisoquinoline 6 was coupled
with 4 through the action of bis(2-oxo-3-oxazolidinyl)
phosphinic chloride (BOPCl) to afford amide 7. Then,
silylation of compound 7 with tert-butyldimethylsilyl chlo-
ride (TBSCl) and subsequent cleavage of TBS group
selectively provided the primary alcohol 8. Oxidation of
compound 8 with Dess–Martin periodinane provided hem-
iaminal as a single diastereomer. Cleavage of the aryl
TBS ether using tetrabutylammonium fluoride (TBAF)
afforded compound 9 (Scheme 3).
With 9 in hand, we then investigated the key intramo-
lecular Pictet–Spengler reaction to construct the penta-
cyclic skeleton. However, the pentacyclic product 10
was not obtained from the treatment of 9 with HCOOH,
CF3SO3H, or MeSO3H. Finally, treatment of compound
9 with trifluoroacetic acid (TFA) at room temperature
provided pentacyclic compound 10 with the Boc group
having been removed simultaneously. Without purifica-
tion, crude compound 10 was N-methylated to give prod-
uct 11 through the reductive methylation. The characteris-
tic nuclear Overhauser effects (NOEs) between 5-H and
6-CH3, and between 15-H and 16-OH in compound 11
confirmed that 15-H was ortho to 16-OH (Scheme 3).
EXPERIMENTAL
1
General. H-NMR spectra were recorded on a Bruker AM
600 or 300 instrument (Bruker BioSpin Corporation, MA) at
24ꢁC in the indicated solvent and are reported in parts per
million relative to tetramethylsilane and referenced internally
to the residually protonated solvent. 13C-NMR spectra were
recorded at 150 or 75 MHz spectrometer at 24ꢁC in the
solvent indicated and are reported in parts per million relative
to tetramethylsilane and referenced internally to the residually
protonated solvent. HRMS were carried out by Agilent LC/MSD
TOF (Agilent Technologies, CA). Optical rotations were meas-
ured on
a Perkin Elmer Polarimeter 341LC (PerkinElmer
Incorporation, MA) using 10-cm cells and the sodium D line
(589 nm) at 20ꢁC and concentration was indicated. All reagents
were obtained from commercial suppliers unless otherwise stated.
Synthesis of compound 4. To a solution of 3 (1.56 g, 4.6
mmol) in CH3OH (10 mL), a solution of lithium hydroxide
(0.44 g, 18.4 mmol, 4 equiv.) in water (5 mL) was added. The
solution was stirred at room temperature for 4 h. The methanol
was removed in vacuo, and the aqueous phase was acidified to pH
1.5. The aqueous phase was extracted with EtOAc (50 mL ꢂ 2),
and the combined organic extracts were washed with brine and
dried over Na2SO4. The organic phase was concentrated, and the
residue was purified by flash column chromatography (EtOAc) to
afford 4 (1.39 g, 93%) as a white solid. m.p.: 77–80ꢁC.
1H-NMR (300 MHz, dimethyl sulfoxide-d6): r 12.53 (s,
1H), 9.04 (s, 1H), 7.03 (d, J ¼ 9.7 Hz, 1H), 6.55 (s, 1H), 6.46
(s, 1H), 4.02 (m, 1H), 3.63 (s, 3H), 2.85 (dd, J ¼ 13.8, 4.2
Hz, 1H), 2.68 (dd, J ¼ 13.2, 9.9 Hz, 1H), 2.13 (s, 3H), 1.34
(s, 9H).
Scheme 1. Reagents and conditions. (a) m-chloroperoxybenzoic acid, CHCl3, rt, 6 h; (b) 12 N aq HCl, CH3OH, 10 h, 91%; (c) SOCl2, CH3OH,
reflux, 24 h, 94%; (d) Boc2O, Et3N, CH2Cl2, 91%; and (e) LiOH, CH3OH–H2O, 93%.
Journal of Heterocyclic Chemistry
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W. Liu, W. F. Dong, X. W. Liao, B. H. Guan, and Z. Z. Liu
Vol 48
˚
Scheme 2. Reagents and conditions. (a) LiBH4, THF, 24 h, 91%; (b) 6 N aq HCl, CH3OH, reflux, 6 h, 87%; (c) BnCH2CHO, HOAc, 4 A molecu-
lar sieves, CH2Cl2–CF3CH2OH, ꢀ10ꢁC, 87%; and (d) H2 (50 psi), Pd(OH)2, CH3OH, 12 h, 86%.
Synthesis of compound 6. To a solution of compound 5
(0.63 g, 3.0 mmol), acetic acid (0.45 g, 0.44 mL, 7.5 mmol,
˚
2.5 equiv.), and the 4 A molecular sieves (0.6 g) in CH2Cl2–
Synthesis of compound 7. To a solution of tetrahydroiso-
quinoline 6 (1.16 g, 3.37 mmol) and triethylamine (1.17 mL,
8.42 mmol, 2.5 equiv.) in CH2Cl2 (70 mL) at 0ꢁC, N-Boc
amino acid 4 (1.20 g, 3.71 mmol, 1.1 equiv.) was added fol-
lowed by BOPCl (0.94 g, 3.71 mmol, 1.1 equiv.) in portions.
The mixture was aged for 6 h at room temperature. Water (40
mL) and 2M HCl were added to pH 1.5, and the organic phase
was separated. The organic phase was washed with saturated
aqueous NaHCO3, brine, and dried over Na2SO4, and concen-
trated by rotary evaporation. The residue was purified by col-
umn chromatography (CHCl3) to provide 7 (1.64 g, 75%) as a
CF3CH2OH (7:1, v/v, 12 mL), a solution of benzyloxyacetal-
dehyde (495 mg, 3.3 mmol, 1.1 equiv.) in dichloromethane
(4 mL) was added slowly via syringe over 60 min at ꢀ10ꢁC.
After being stirred at ꢀ10ꢁC for 8 h, the reaction mixture was
diluted with dichloromethane and filtered. The filtrate was con-
centrated under reduced pressure, and the residue was purified
by column chromatography (CHCl3:CH3OH:NEt3 ¼ 100:1:0.2)
to afford compound 6 (0.89 g, 87%) as a white solid. [a]2D0:
ꢀ115.2 (c 0.5, CH3OH). m.p: 109–111ꢁC. HRMS calcd. for
C20H26NO4 (M þ Hþ) 344.1856 Da, Found 344.1885 Da.
1H-NMR (300 MHz, dimethyl sulfoxide-d6): d 8.65 (s, 1H),
7.32 (m, 5H), 6.37 (s, 1H), 4.70 (t, 1H), 4.49 (dd, J ¼ 17.1,
12.6 Hz, 2H), 4.31 (brd, 1H), 4.13 (dd, J ¼ 8.7, 2.7 Hz, 1H),
3.59 (s, 3H), 3.46 (m, 1H), 3.42 (d, J ¼ 8.4 Hz, 1H), 3.36
(m, 1H), 3.33 (s, 1H), 2.68 (m, 1H), 2.42 (brd, J ¼ 15 Hz,
1H), 2.28 (dd, J ¼ 14.1, 11.1 Hz, 1H), 2.14 (s, 3H). 13C-
NMR (75 MHz, dimethyl sulfoxide-d6): d 146.7, 143.8,
138.7, 132.6, 128.1, 128.0, 127.3, 127.2, 120.9, 73.8, 72.0,
65.2, 59.9, 54.9, 53.9, 53.0, 33.0, 15.3.
white solid. HRMS calcd. for C36H46N2O9 (M
651.3281 Da, Found 651.3282 Da.
þ
Hþ)
1H-NMR (300 MHz, CDCl3): d 7.81 (s, 1H), 7.29 (m, 5H),
6.67–6.26 (m, 3H), 6.18 (m, 1H), 5.93 (d, J ¼ 5.1 Hz, 1H),
5.56 (d, J ¼ 7.2 Hz, 1H), 5.28 (d, J ¼ 7.2 Hz, 1H), 5.93 (dd,
J ¼ 7.5 Hz, 1H), 4.98 (m, 1H), 4.67 (m, 1H), 4.48 (m, 1H),
4.43 (m, 1H), 3.94 (m, 1H), 3.88 (m, 1H), 3.75 (s, 3H), 3.61
(s, 3H), 3.54 (m, 1H), 3.45(m, 1H), 3.10–2.74 (m, 4H), 2.23
(s, 3H), 2.22 (s, 3H), 1.42–1.34(s, 9H).
Synthesis of compound 8. To a solution of compound 7
(0.786 g, 1.21 mmol) in CH2Cl2 (45 mL), TBSCl (1.002 g, 5.4
Scheme 3. Reagents and conditions. (a) BOPCl, Et3N, CH2Cl2, 88%; (b) TBSCl, Et3N, DMAP, CH2Cl2, rt, 88%; (c) HCOOH, THF, H2O, 92%;
(d) Dess–Martin periodinane, CH2Cl2, 94%; (e) TBAF, THF, 2 h, 90%; (f) CF3COOH, 82%; and (g) HCHO, NaBH3CN, HOAc, CH3OH, 83%.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

March 2011
Synthesis of an Isomer of the Renieramycin Skeleton from L-Tyrosine
417
mmol, 4 equiv.), triethylamine (1.9 mL, 7.26 mmol, 6 equiv.),
and 4-dimethylamino pyridine (DMAP) (111 mg, 0.6 mmol,
0.5 equiv.) were added. The solution was stirred for 24 h and
quenched with saturated aqueous NH4Cl (30 mL). The phases
were separated, and the aqueous phase was extracted with
CH2Cl2 (30 mL ꢂ 2). The combined organic phase was washed
with brine, dried over Na2SO4, and concentrated under reduced
pressure. The residue was purified by column chromatography
(5% EtOAc in n-hexane) to provide a yellow oil (694 mg, 88%).
The yellow oil (546 mg, 0.55 mmol) was dissolved in THF–
HCO2H–H2O (6:3:1; 20 mL), the solution was stirred for 2 h
at room temperature. The solution was concentrated by rotary
evaporation, and the residue was dissolved in EtOAc (50 mL).
Then, the organic phase was washed with saturated aqueous
NaHCO3, brine, and dried over Na2SO4. After concentration of
the solution by rotary evaporation, the residue was purified by
column chromatography (10% EtOAc in n-hexanes) to provide
8 (445 mg, 92%) as a white solid.
MeOH (3 mL). To the solution, HCHO (0.22 mL, 37%),
NaBH3CN (25 mg, 0.40 mmol), and CH3COOH (0.4 mL) were
added, and the mixture was stirred at room temperature for 2 h
and concentrated. The residue was dissolved in EtOAc (20 mL),
and saturated aqueous NaHCO3 was added. Then the organic
layer was separated and washed with saturated aqueous NaCl
and dried over anhydrous Na2SO4 for 10 h. The organic layer
was concentrated under reduced pressure, and the resultant resi-
due was purified by flash column chromatography (EtOAc:-
CH3OH:Et3N ¼ 100:2:0.2) to afford compound 11 (8 mg,
47.7%). [a]2D0: ꢀ115.4 (c 4.8, CH3OH). HRMS calcd. for 15 (M
þ Hþ) 545.2646 Da, Found 545.2668 Da.
1H-NMR (600 MHz, dimethyl sulfoxide-d6): d 9.01 (s, 1H,
16-OH), 8.86 (s, 1H, 8-OH), 7.20 (dd, 2H, Bn-H, J ¼ 7.2),
7.16 (t, 1H, Bn-H, J ¼ 7.2), 6.81 (d, 2H, Bn-H, J ¼ 7.2), 6.49
(s, 1H, 5-H), 6.49 (s, 1H, 15-H), 5.55 (dd, 1H, 1-H, J ¼ 3.6),
4.30 (d, 1H, 11-H, J ¼ 3.0), 3.848 (m, 1H, 3-H), 3.82 (brs, 2H,
22-H), 3.625 (d, 1H, 21-H, J ¼ 7.2), 3.59 (s, 3H, 17-H), 3.57
(s, 3H, 7-H), 3.39 (dd, 1H, 21-H, J ¼ 10.2, 3.6), 3.30 (m, 1H,
14-H), 3.03 (dd, 1H, 14-H, J ¼ 16.8, 7.2), 2.79 (m, 1H, 13-H),
2.79 (m, 1H, 4-H), 2.37 (dd, 1H, 4-H, J ¼ 13.8), 2.21 (s, 3H,
N-CH3), 2.15 (s, 3H, 6-CH3), 2.07 (s, 3H, 16-CH3); 13C-NMR
(150 MHz, dimethyl sulfoxide-d6): d 70.61, 146.54, 145.85,
144.25, 144.16, 138.58, 132.89, 130.09, 129.11, 128.80, 127.97,
126.89, 126.68, 123.44, 119.83, 119.28, 114.12, 72.36, 71.08,
61.81, 59.90, 59.04, 53.18, 50.02, 48.84, 34.46, 31.99, 30.67,
15.44, 13.43.
1H-NMR (300 MHz, dimethyl sulfoxide-d6): d 7.23 (m, 5H),
6.63 (s, 1H), 6.55 (s, 1H), 6.47 (s, 1H), 6.07 (m, 1H), 4.87
(brs, 1H), 4.65 (brd, J ¼ 5.7 Hz, 1H), 4.50 (d, J ¼ 12.6 Hz,
1H), 4.46 (d, J ¼ 10.5 Hz, 1H), 4.35 (brs, 1H), 3.88 (brs, 1H),
3.61 (m, 2H), 3.57 (s, 3H), 3.54 (m, 2H), 3.50 (s, 3H), 2.86 (m,
2H), 2.74 (d, J ¼ 4.8 Hz, 2H), 2.17 (s, 3H), 2.15 (s, 3H), 1.32
(s, 9H), 0.96 (s, 9H), 0.94 (s, 9H), 0.12 (s, 6H), 0.11(s, 6H).
Synthesis of compound 9. To a solution of compound 8
(298 mg, 0.34 mmol) in CH2Cl2 (25 mL) at 0ꢁC, Dess–Martin
periodinane (228 mg, 0.51 mmol, 1.5 equiv.) was added, and
the solution was stirred for 2 h at room temperature. The reac-
tion was quenched with two drops of 2-propanol. The solution
was washed with saturated aqueous NaHCO3, brine, and dried
over Na2SO4, and concentrated by rotary evaporation. The res-
idue was purified by column chromatography (5% EtOAc in
n-hexanes) to provide 12 (271 mg, 91%) as a white solid.
The white solid (245 mg, 0.28 mmol) was dissolved in THF
(25 mL). To this solution, tetrabutylammonium fluoride
(TBAF) was added (1.0M in THF, 0.84 mL, 0.84 mmol, 3.0
equiv.) at 0ꢁC, and the solution was stirred for 1 h. The reac-
tion was quenched with saturated aqueous NH4Cl (20 mL) and
extracted with EtOAc (30 mL ꢂ 3). The combined organic
phase was washed with saturated brine, dried over Na2SO4,
and concentrated by rotary evaporation. The residue was puri-
fied by column chromatography (1% CH3OH in CH2Cl2) to
provide 9 (181 mg, 90%) as a white solid. HRMS calcd. For
C36H45N2O9 (M þ Hþ) 649.3125 Da, Found 649.3148 Da.
1H-NMR (300 MHz, dimethyl sulfoxide-d6): d 8.94 (s, 3H),
7.24 (m, 5H), 6.64 (s, 1H), 6.60 (d, J ¼ 4.8 Hz, 1H), 6.56 (s,
1H), 6.52 (s, 1H), 5.77 (m, 1H), 5.66 (m, 1H), 4.49 (d, J ¼
12.6 Hz, 2H), 4.38 (d, J ¼ 12.6 Hz, 1H), 3.75 (dd, J ¼ 9.9,
5.4 Hz, 1H), 3.64 (s, 3H), 3.62 (s, 3H), 3.59 (m, 1H), 3.12 (d,
J ¼ 12.6, 1H), 3.04 (d, J ¼ 9.6 Hz, 2H), 2.62 (d, J ¼ 12.3,
1H), 2.19 (s, 3H), 2.14 (s, 3H), 1.07 (s, 9H).
Acknowledgments. The authors thank the National Natural Sci-
ence Foundation of China (No. 30672518) and Specialized
Research Fund for the Doctoral Program of Higher Education
(No. 20060023025) for financial support.
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