The first total synthesis of lamellarin α 20-sulfate, a selective inhibitor of HIV-1 integrase

Article abstract of DOI:10.1016/j.tetlet.2006.03.121

The first total synthesis of lamellarin α 20-sulfate (1), a selective inhibitor of HIV-1 integrase, has been completed. The lamellarin α core in which 13-OH and 20-OH were differentially protected by isopropyl and benzyl groups, respectively, was construc

Full text of DOI:10.1016/j.tetlet.2006.03.121

Tetrahedron Letters 47 (2006) 3755–3757
The first total synthesis of lamellarin a 20-sulfate, a selective
inhibitor of HIV-1 integrase
Tomohiro Yamaguchi,a Tsutomu Fukuda,a Fumito Ishibashib and Masatomo Iwaoc,*
aGraduate School of Science and Technology, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan
bDivision of Marine Life Science and Biochemistry, Faculty of Fisheries, Nagasaki University, 1-14 Bunkyo-machi,
Nagasaki 852-8521, Japan
cDepartment of Applied Chemistry, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi,
Nagasaki 852-8521, Japan
Received 8 February 2006; revised 13 March 2006; accepted 17 March 2006
Available online 12 April 2006
Abstract—The first total synthesis of lamellarin a 20-sulfate (1), a selective inhibitor of HIV-1 integrase, has been completed. The
lamellarin a core in which 13-OH and 20-OH were differentially protected by isopropyl and benzyl groups, respectively, was con-
structed by using Hinsberg-type pyrrole synthesis and Suzuki–Miyaura coupling as the key reactions. The 20-sulfate was prepared
by a sequence including debenzylation of 20-OBn, 2,2,2-trichloroethylsulfation of the resulting 20-OH, deprotection of 13-Oi-Pr,
and final reductive cleavage of the 2,2,2-trichloroethyl ester.
Ó 2006 Elsevier Ltd. All rights reserved.
Human immunodeficiency virus (HIV) encodes three
enzymes, namely, reverse transcriptase, protease, and
integrase. Anti-HIV drugs targeting the first two
enzymes have been successfully employed for the treat-
ment of acquired immune deficiency syndrome (AIDS).
Integrase is another attractive and safe target against
HIV because it is essential for HIV replication and,
unlike reverse transcriptase and protease, there is no
similar enzyme in the host cell.1 Unfortunately, how-
ever, no clinically useful integrase inhibitors have been
developed so far.
index. Sulfate 1 inhibited the integrase terminal cleavage
activity with an IC50 of 16 lM, the strand transfer activ-
ity with an IC50 of 22 lM, and growth of the HIV-1
virus in cell culture with an IC50 of 8 lM. The MTT
assay of 1 toward Hela cells displayed the least toxicity
with an LD50 of 274 lM. Protection of the phenolic
hydroxyl group as the sulfate could reduce the cytoto-
xicity of the parental lamellarins in general.
A synthetic approach to lamellarin a 20-sulfate (1) was
reported by Faulkner and co-workers in 2002.5 They
prepared lamellarin a (2) using an intramolecular 1,3-
dipolar cycloaddition strategy developed by Banwell.6
An attempt to synthesize 1 by titration of 2 with a
Lamellarins are polycyclic marine alkaloids having a
unique 14-phenyl-6H-[1]benzopyrano[40,30:4,5]pyrano-
[2,1-a]isoquinolin-6-one ring-system.2 So far, over 30
lamellarins have been isolated from mollusks, tunicates,
and sponges. These alkaloids have received considerable
attention as new leads for anticancer agents.3 In 1999,
Faulkner and co-workers discovered that a series of
lamellarin alkaloids exhibit selective inhibition of HIV-
1 integrase.4 Within the alkaloids tested, lamellarin a
20-sulfate (1) displayed the most favorable therapeutic
MeO
R2O
MeO
OR1
20
13
O
MeO
MeO
N
O
lamellarin α 20-sulfate (1) (R1=SO3Na, R2=H)
lamellarin α (2) (R1=R2=H)
Keywords: HIV-1 integrase inhibitor; Lamellarin; Sulfate; Hinsberg
reaction; Suzuki–Miyaura coupling.
lamellarin α 13,20-disulfate (3) (R1=R2=SO3Na)
*
Corresponding author. Tel./fax: +81 95 819 2681; e-mail: iwao@
net.nagasaki-u.ac.jp
lamellarin α core (4) (R1=Bn, R2=
i-Pr)
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.03.121

3756
T. Yamaguchi et al. / Tetrahedron Letters 47 (2006) 3755–3757
conventional DMF-SO3 complex failed and afforded
only lamellarin a 13,20-disulfate (3) in low yield. Re-
cently, Taylor developed a reliable method to produce
aryl sulfates via mixed aryl 2,2,2-trichloroethyl sulfate
intermediates.7 In this letter, we report the first total syn-
thesis of lamellarin a 20-sulfate (1) using Taylor’s proto-
col for the final steps of sulfate formation.8
lamellarin a 20-sulfate (1) based upon this strategy is
shown in Scheme 1.
Alkylation of the commercially available 2-(3,4-dimeth-
oxyphenyl)ethylamine (5) with 2.2 equiv of methyl bro-
moacetate gave the iminodiacetate 6 in 91% yield.
Hinsberg reaction of 6 with dimethyl oxalate under the
conventional NaOMe/MeOH conditions9,11 provided
3,4-dihydroxypyrrole 7 in only 49% yield. However, the
yield was greatly improved to 85% by carrying out the
reaction in dry THF using sodium hydride as a base.10
Reaction of 7 with 2.2 equiv of trifluoromethanesulfonic
anhydride in pyridine gave the corresponding bistriflate
derivative 8 in good yield. Bistriflate 8 was coupled with
1.0 equiv of boronic acid 910 in the presence of 2 mol %
The pivotal lamellarin a core 4 in which 13-OH and
20-OH are differentially protected for the selective intro-
duction of a sulfate group was constructed by a strategy
developed in our laboratories.9,10 This includes Hins-
berg-type pyrrole synthesis11 and palladium-catalyzed
Suzuki–Miyaura coupling12 of the 3,4-dihydroxypyrrole
bistriflate as the key reactions. The total synthesis of
Scheme 1. Total synthesis of lamellarin a 20-sulfate (1). Reagents and conditions: (a) BrCH2CO2Me (2.2 equiv), NaHCO3, CH3CN, reflux, 2.5 h
(91%); (b) (CO2Me)2 (2.0 equiv), NaH (4.0 equiv), THF, reflux, 4.5 h (85%); (c) (CF3SO2)2O (2.2 equiv), pyridine, 0 °C, 2 h (92%); (d) 9 (1.0 equiv),
Pd(PPh3)4 (2 mol %), aq Na2CO3, THF, reflux, 5 h (80%); (e) 11 (2.0 equiv), Pd(PPh3)4 (8 mol %), aq Na2CO3, THF, reflux, 20 h (90%); (f) concd
HCl, MeOH, reflux, 2 h (93%); (g) (1) 40% aq KOH–EtOH (1:1), reflux, 3 h, (2) cat. p-TsOH, CH2Cl2, reflux, 1 h (77%); (h) Cu2O, quinoline, 220 °C,
10 min (96%); (i) PhI(OCOCF3)2 (1.2 equiv), BF3ÆOEt2 (2.4 equiv), CH2Cl2, ꢀ40 °C, 1.5 h (95%); (j) DDQ (1.0 equiv), CH2Cl2, reflux, 30 h (99%); (k)
H2, 10% Pd–C (20 wt %), AcOEt, rt, 2 h (99%); (l) ClSO3CH2CCl3 (2.0 equiv), DMAP (1.0 equiv), Et3N (2.0 equiv), THF, rt, 4 h (96%); (m) BCl3 (3.0
equiv), CH2Cl2, ꢀ78 °C, 0.5 h, then 0 °C, 4 h (96%); (n) (1) Zn powder (2 equiv), HCO2NH4 (6 equiv), THF–MeOH (1:1), 4 h, (2) Amberlite IRC-50
(Na+ form), MeOH, (3) Sephadex LH-20, MeOH–CH2Cl2 (1:1) (80%).

T. Yamaguchi et al. / Tetrahedron Letters 47 (2006) 3755–3757
3757
´
Res. 2003, 63, 7392–7399; (d) Tardy, C.; Facompre, M.;
of Pd(PPh3)4 and aqueous Na2CO3 in refluxing THF to
give mono-arylated pyrrole 10 in 80% yield. The second
cross-coupling of this product with 2.0 equiv of 1113
using 8 mol % of Pd(PPh3)4 produced 3,4-disubstituted
pyrrole 12 in 90% yield. Deprotection of the MOM
group of 12 with HCl in methanol caused concomitant
lactonization to give 13 in 93% yield. Alkaline hydrolysis
of 13 followed by treatment with p-TsOH in refluxing
dichloromethane gave acid 14 in 77% yield. Decarboxyl-
ation of this compound in hot quinoline in the presence
of Cu2O provided 15 in 96% yield.14 Intramolecular oxi-
dative biaryl coupling of 15 under Kita’s conditions15
[phenyliodine bis(trifluoroacetate) (PIFA)/BF3ÆEt2O]
proceeded cleanly to produce cyclized compound 16 in
95% yield. Dehydrogenation of this compound with
2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) gave
20-benzyl-13-isopropyllamellarin a (4) in 99% yield.
Deprotection of the benzyl group by hydrogenolysis over
palladium on charcoal afforded 17, which was reacted
with trichloroethyl chlorosulfate in pyridine to give
mixed sulfate 18 in 96% yield.6 Selective removal of the
isopropyl protecting group of 18 with boron trichloride16
proceeded cleanly without affecting the trichloroethylsul-
fate moiety to give 19 in 96% yield. Final reductive
deprotection of the trichloroethyl ester with Zn/
HCO2NH4 followed by ion exchange over Amberlite
IRC-50 (Na+ form) and Sephadex purification produced
lamellarin a 20-sulfate (1) in 80% yield.
´
Laine, W.; Baldeyrou, B.; Garcıa-Gravalos, D.; Fran-
cesch, A.; Mateo, C.; Pastor, A.; Jimenez, J. A.; Man-
zanares, I.; Cuevas, C.; Bailly, C. Bioorg. Med. Chem.
2004, 12, 1697–1712; (e) Marco, E.; Laine, W.; Tardy, C.;
Lansiaux, A.; Iwao, M.; Ishibashi, F.; Bailly, C.; Gago, F.
J. Med. Chem. 2005, 48, 3796–3807.
´
4. Reddy, M. V. R.; Rao, M. R.; Rhodes, D.; Hansen, M. S.
T.; Rubins, K.; Bushman, F. D.; Venkateswarlu, Y.;
Faulkner, D. J. J. Med. Chem. 1999, 42, 1901–1907.
5. Ridley, C. P.; Reddy, M. V. R.; Rocha, G.; Bushman, F.
D.; Faulkner, D. J. Bioorg. Med. Chem. 2002, 10, 3285–
3290.
6. Banwell, M.; Flynn, B.; Hockless, D. Chem. Commun.
1997, 2259–2260.
7. Liu, Y.; Lien, I. F.; Ruttgaizer, S.; Dove, P.; Taylor, S. D.
Org. Lett. 2004, 6, 209–212.
8. During the course of our synthesis of lamellarin a 20-
sulfate (1), Furstner completed the total synthesis of the
marine alkaloid dictyodendrin B, in which Taylor’s
protocol was successfully used for the final sulfate
¨
formation, see: Furstner, A.; Domostoj, M. M.; Scheiper,
¨
B. J. Am. Chem. Soc. 2005, 127, 11620–11621.
9. Iwao, M.; Takeuchi, T.; Fujikawa, N.; Fukuda, T.;
Ishibashi, F. Tetrahedron Lett. 2003, 44, 4443–4446.
10. Fujikawa, N.; Ohta, T.; Yamaguchi, T.; Fukuda, T.;
Ishibashi, F.; Iwao, M. Tetrahedron 2006, 62, 594–604.
11. Merz, A.; Schropp, R.; Do¨tterl, E. Synthesis 1995, 795–
800.
12. Oh-e, T.; Miyaura, N.; Suzuki, A. J. Org. Chem. 1993, 58,
2201–2208.
13. The boronic acid 11 was prepared from O-benzylisovanil-
lin in a similar manner as described for the synthesis of 4-
isopropoxy-5-methoxy-2-methoxymethoxyphenylboronic
acid. See Ref. 10.
The spectroscopic data of synthetic 117 were shown to be
identical with those reported for the natural product.4 It
1
is noteworthy that the H NMR absorptions of aro-
14. Boger, D. L.; Soenen, D. R.; Boyce, C. W.; Hedrick, M.
P.; Jin, Q. J. Org. Chem. 2000, 65, 2479–2483.
15. (a) Takada, T.; Arisawa, M.; Gyoten, M.; Hamada, R.;
Tohma, H.; Kita, Y. J. Org. Chem. 1998, 63, 7698–7706;
(b) Tohma, H.; Morioka, H.; Takizawa, S.; Arisawa, M.;
Kita, Y. Tetrahedron 2001, 57, 345–352; (c) Hamamoto,
H.; Anilkumar, G.; Tohma, H.; Kita, Y. Chem. Eur. J.
2002, 8, 5377–5383.
matic (H-5, 6, 7, 15, 16) and hydroxylic protons of 1
shift considerably depending on the concentration of
1
the samples.17 The H NMR data of synthetic 1 ob-
tained at the low concentration (1.0 mg of 1 in 0.7 mL
of DMSO-d6) were found to be identical with those
reported for the natural product.
In conclusion, we have achieved the first total synthesis
of lamellarin a 20-sulfate (1) in 14 steps from the com-
mercially available 2-(3,4-dimethoxyphenyl)ethylamine
(4) in excellent overall yield (24%). This synthesis opens
the way to produce diverse sulfated lamellarins, which
enable us to undertake the structure–activity relationship
studies on integrase-inhibiting and anti-HIV activities.
Studies along this line are in progress in our laboratories.
16. (a) Sala, T.; Sargent, M. V. J. Chem. Soc., Perkin Trans. 1
´
´
1979, 2593–2598; (b) Solladie, G.; Pasturel-Jacope, Y.;
Maignan, J. Tetrahedron 2003, 59, 3315–3321.
17. Lamellarin a 20-sulfate (1). Mp 263–269 °C (dec) (sealed
capillary) (lit.4 mp 145–148 °C); IR (KBr): 3448, 1695,
1
1487, 1419, 1272, 1223, 1167, 1048, 839 cmꢀ1; H NMR
(400 MHz, 17 mg of 1 in 0.7 mL of DMSO-d6): d 3.37 (s,
3H), 3.37 (s, 3H), 3.86 (s, 3H), 3.87 (s, 3H), 6.80 (s, 1H),
6.94 (dd, J = 2.0 and 8.0 Hz, 1H), 7.05 (d, J = 2.0 Hz,
1H), 7.18 (s, 1H), 7.19 (d, J = 8.0 Hz, 1H), 7.29 (d,
J = 7.4 Hz, 1H), 7.38 (s, 1H), 7.57 (s, 1H), 8.48 (br s, 1H),
9.03 (d, J = 7.4 Hz, 1H); 1H NMR (400 MHz, 1.0 mg of 1
in 0.7 mL of DMSO-d6): d 3.38 (s, 3H), 3.38 (s, 3H), 3.87
(s, 3H), 3.89 (s, 3H), 6.81 (s, 1H), 7.03 (d, J = 2.0 Hz, 1H),
7.04 (dd, J = 2.0 and 8.0 Hz, 1H), 7.21 (s, 1H), 7.26 (d,
J = 8.0 Hz, 1H), 7.35 (d, J = 7.4 Hz, 1H), 7.44 (s, 1H),
7.56 (s, 1H), 9.09 (d, J = 7.4 Hz, 1H), 9.45 (br s, 1H); 13C
NMR (100 MHz, 17 mg of 1 in 0.7 mL of DMSO-d6): d
54.36, 54.96, 55.48, 55.97, 104.63, 105.64, 106.80, 108.00,
108.66, 111.13, 111.40, 112.70, 113.42, 118.00, 118.05,
121.36, 121.88, 124.09, 126.83, 127.80, 133.26, 143.00,
144.93, 146.48, 147.87, 147.99, 148.71, 149.71, 153.96.
HRFABMS (positive ion mode) m/z. Calcd for
References and notes
1. For a review of HIV-1 integrase inhibitors, see: Maurin,
C.; Bailly, F.; Cotelle, P. Curr. Med. Chem. 2003, 10,
1795–1810.
2. For reviews of lamellarin alkaloids, see: (a) Cironi, P.;
´
Albericio, F.; Alvarez, M. Prog. Heterocycl. Chem. 2004,
16, 1–26; (b) Bailly, C. Curr. Med. Chem. Anti-Cancer
Agents 2004, 4, 363–378; (c) Handy, S. T.; Zhang, Y. Org.
Prep. Proced. Int. 2005, 37, 411–445.
3. (a) Quesada, A. R.; Gravalos, M. D. G.; Puentes, J. L. F.
Br. J. Cancer 1996, 74, 677–682; (b) Ishibashi, F.; Tanabe,
S.; Oda, T.; Iwao, M. J. Nat. Prod. 2002, 65, 500–504; (c)
C29H22NNa2O11S
[(M+Na)+]:
638.0709.
Found:
´
Facompre, M.; Tardy, C.; Bal-Mahieu, C.; Colson, P.;
638.0710. HRFABMS (negative ion mode) m/z. Calcd
Perez, C.; Manzanares, I.; Cuevas, C.; Bailly, C. Cancer
for C29H22NO11S [(M–Na)ꢀ]: 592.0914. Found: 592.0913.