A two-step biomimetic synthesis of antimalarial robustadials A and B

Article abstract of DOI:10.1016/j.tetlet.2006.07.113

The antimalarial robustadials A and B have been synthesized in two steps starting from commercially available phloroglucinol comprising a key biomimetic three-component reaction that involves in situ generation of an o-quinone methide via Knoevenagel cond

Full text of DOI:10.1016/j.tetlet.2006.07.113

Tetrahedron Letters 47 (2006) 7021–7024
A two-step biomimetic synthesis of antimalarial
robustadials A and B
Sandip B. Bharate and Inder Pal Singh*
Department of Natural Products, National Institute of Pharmaceutical Education and Research (NIPER), Sector-67,
S.A.S. Nagar, Punjab 160 062, India
Received 27 June 2006; revised 11 July 2006; accepted 21 July 2006
Available online 14 August 2006
Abstract—The antimalarial robustadials A and B have been synthesized in two steps starting from commercially available phloro-
glucinol comprising a key biomimetic three-component reaction that involves in situ generation of an o-quinone methide via Knoe-
venagel condensation and subsequent Diels–Alder cycloaddition with (À)-b-pinene.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Robustadials A 1a and B 1b were isolated as antimalar-
ial compounds from the leaves of Eucalyptus robusta.1
They exhibited strong in vivo activity against Plasmo-
dium berghei.1 Several structurally similar phloroglu-
cinol–terpene adducts (euglobals) occur widely in
various species of Eucalyptus and are reported to show
diverse biological activities such as Epstein–Barr Virus
inhibitory, antileishmanial, antimalarial, etc.2 The cor-
rect structure of robustadials was established by total
synthesis of robustadial dimethyl ethers, 2a and 2b.3,4
and B 1b for evaluation of their antimalarial activity.
There have been no further reports on the antimalarial
activity of these compounds or other related
compounds.
There have been some reports on the total synthesis of
robustadials by different routes involving varying num-
ber of steps. Further, most of the syntheses were for di-
methyl ether derivatives of robustadials and a few were
of key precursors required for robustadial synthesis.
Salomon et al. have synthesized robustadial dimethyl
ethers from 2,4-dimethoxy-6-hydroxyacetophenone via
condensation with (+)-nopinone, followed by cycliza-
tion and introduction of isobutyl and diformyl function-
alities (8 steps, overall yield 2.5%).3 Majewski and
Bantle reported a stereoselective synthesis of one of
the key intermediates involved in the synthesis of
robustadials via Prins reaction.7 Furthermore, Koser
and Hoffman synthesized the dimethyl robustadial
skeleton (which lacks the diformyl functionality) via a
five-step procedure starting from 3,5-dioxo-cyclohexan-
ecarboxylic acid methyl ester involving condensation,
Diels–Alder cycloaddition and aromatization to yield
the chroman skeleton.8 Majewski et al. reported stereo-
selective syntheses of dimethyl robustadials via amine
catalyzed cyclization as a key step for formation of the
chroman skeleton (8 steps, overall yield 0.5%).9
Recently, Aukrust and Skattebol reported a synthesis
of robustadial A 1a starting from (À)-nopol involving
Friedel–Crafts condensation of an a,b-unsaturated acid
derivative with phloroglucinol to yield the chromone
skeleton in the key step (6 steps, overall yield 4%).10 In
CHO
CHO
RO
O
RO
O
OHC
OHC
OR
OR
1a: R = H
2a: R = CH3
1b: R = H
2b: R = CH3
As a part of our continuing program to explore natu-
rally occurring phloroglucinol compounds for their bio-
logical potential,5,6 we have developed a short and
efficient method for the synthesis of robustadials A 1a
Keywords: Robustadials A and B; Antimalarial; Phloroglucinol; Bio-
mimetic synthesis.
NIPER Communication Number: 380.
*
Corresponding author. Tel.: +91 172 2214683; fax: +91 172
2214692; e-mail: ipsingh@niper.ac.in
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.113

7022
S. B. Bharate, I. P. Singh / Tetrahedron Letters 47 (2006) 7021–7024
CHO
this letter, we report a simple and efficient two-step
synthesis of robustadials A and B from commercially
available phloroglucinol 9 with an overall yield of 27%
involving Knoevenagel condensation followed by
Diels–Alder cycloaddition as a key biomimetic step.
This strategy should also give ready access to related
euglobals and s-euglobals in higher yields.6
HO
OH
a
+
a
1a + 1b
OHC
OH
6
5
CHO
CHO
HO
OHC
OH
HO
O
Biogenetically, robustadials A 1a and B 1b have been
proposed to be formed from the Diels–Alder cycloaddi-
tion of o-quinone methide, 3 generated from jensenone 4
with (À)-b-pinene 5.2,11 Based on the proposed biogene-
sis, our retrosynthetic analysis of 1 is depicted in
Scheme 1. The ring skeleton of 1 could be constructed
from o-quinone methide, 3 via a Diels–Alder cycloaddi-
tion reaction with 5. The key intermediate, o-quinone
methide, 3 could be generated by the oxidation of
diformylated synthon 6 or by coupling of phenol 7
and aldehyde 8 via a Knoevenagel condensation.
+
OHC
O
OH
12
11
Scheme 2. Reagents and conditions: (a) DDQ, CH3NO2, 60 ꢁC, 2 h,
20%.
HO
OH
O
HO
OH
a
OH
10
OH
9
Our synthetic strategy involved initial synthesis of
diformylated key precursors, 6 or 7. Based on earlier
reports6,12 on the synthesis of several natural as well as
unnatural euglobals which are structurally similar to
robustadials, we initially prepared 3,5-diformyl-isopen-
tyl phloroglucinol 6 which after treatment with DDQ
in the presence of (À)-b-pinene 5 was expected to yield
the desired cycloadduct. The precursor, 6 was synthe-
sized as described earlier.5,13
d
b
CHO
CHO
HO
OH
HO
OH
O
c
OHC
OHC
OH
7
OH
4
Scheme 3. Reagents and conditions: (a) (CH3)2CHCH2COCl, AlCl3,
50 ꢁC, 40%; (b) (CH2)6N4, TFA, 60 ꢁC, 2 h, 40%; (c) 80% H2SO4,
80 ꢁC, 1 h, 45%; (d) POCl3/DMF (3 equiv each), RT, 2 h, 40%.
Treatment of 6 with (À)-b-pinene 5 in the presence of
DDQ in nitromethane, however, did not result in forma-
tion of the desired pair of diastereoisomers 1a and 1b,
instead benzopyran 11 and benzofuran 12, respectively,
were formed via DDQ-mediated intramolecular cycliza-
tion of the isopentyl functionality (Scheme 2).14 A simi-
lar type of DDQ-mediated intramolecular cyclization
was reported earlier for mallotojaponin.15
approach involved direct introduction of both diformyl
groups onto the phloroglucinol nucleus. After investi-
gating
a
number of reagents and experimental
conditions, 7 was synthesized from 9 using the Vilsme-
ier–Haack reagent (3 equiv each of POCl3 and DMF)
in 40% yield (Scheme 3).16 Diformylation of phloroglu-
cinol compounds has been reported earlier with
Zn(CN)2 in 1.5% yield.17
In order to synthesize 7, we adopted two different ap-
proaches. The first involved synthesis of the naturally
occurring diformylated acyl phloroglucinol compound,
jensenone 4 as reported earlier.5 Jensenone 4 upon deac-
ylation by refluxing in 80% sulfuric acid for 1 h resulted
in the formation of 7 in 45% yield (Scheme 3). A second
Treatment of 2,4-diformyl phloroglucinol 7 with isoval-
eraldehyde 8 and (À)-b-pinene 5 in the presence of
CHO
HO
OH
OHC
CHO
OH
6
CHO
Diels-Alder
Cycloaddition
HO
O
HO
O
+
OHC
OHC
OH
OH
CHO
3
5
1
HO
OH
CHO
+
OHC
OH
7
8
Scheme 1. Retrosynthetic analysis of robustadials A 1a and B 1b.

S. B. Bharate, I. P. Singh / Tetrahedron Letters 47 (2006) 7021–7024
7023
CHO
HO
O
5
OHC
CHO
OH
OH
HO
OH
+
robustadial A 1a
3a
CHO
a
+
OHC
+
CHO
7
8
5
robustadial B 1b
HO
O
5
OHC
OH
3b
Scheme 4. Reagents and conditions: (a) CH3COOH, CH3COONa, MW, 1000 W, 4 min, 68%.
sodium acetate in acetic acid under microwave irradia-
tion (1000 W) for 4 min resulted in formation of the
desired products in a combined yield of 68% (Scheme
4). Similar results were obtained when the reaction
was carried out under conventional heating at 80 ꢁC
for 2 h.
References and notes
1. Xu, R.; Snyder, J. K.; Nakanishi, K. J. Am. Chem. Soc.
1984, 106, 734–736.
2. (a) Singh, I. P.; Bharate, S. B. Nat. Prod. Rep. 2006, 23,
558–591; (b) Singh, I. P.; Etoh, H. Nat. Prod. Sci. 1997,
3, 1–7; (c) Ghisalberti, E. L. Phytochemistry 1996, 41, 7–
22.
The o-quinone methide is generated in situ by addition
of isovaleraldehyde to phloroglucinol followed by dehy-
dration (Knoevenagel-like condensation). (À)-b-Pinene
5 can undergo the cycloaddition reaction with o-quinone
methides, 3a and 3b via two distinct orientations and
each can lead to two pairs of diastereoisomers. How-
ever, only one pair of diastereoisomers, 1a and 1b were
formed which indicated selectivity in the Diels–Alder
cycloaddition. This regioselective preference for forma-
tion of two isomers arises because the oxygen of the o-
quinone methide only attacks the more substituted ter-
minus of the pinene double bond.
3. (a) Salomon, R. G.; Lal, K.; Mazza, S. M.; Zarate, E. A.;
Youngs, W. J. J. Am. Chem. Soc. 1988, 110, 5213–5214;
(b) Salomon, R. G.; Mazza, S. M.; Lal, K. J. Org. Chem.
1989, 54, 1562–1570.
4. Cheng, Q.; Snyder, J. K. J. Org. Chem. 1988, 53, 4562–
4567.
5. Bharate, S. B.; Chauthe, S. K.; Bhutani, K. K.; Singh, I. P.
Aust. J. Chem. 2005, 58, 551–555.
6. Bharate, S. B.; Bhutani, K. K.; Khan, S. I.; Tekwani, B.
L.; Jacob, M. R.; Khan, I. A.; Singh, I. P. Bioorg. Med.
Chem. 2006, 14, 1750–1760.
7. Majewski, M.; Bantle, G. Tetrahedron Lett. 1989, 30,
6653–6656.
8. Koser, S.; Hoffman, H. M. R. J. Org. Chem. 1993, 58,
6163–6165.
9. Majewski, M.; Irvine, N. M.; Bantle, G. W. J. Org. Chem.
1994, 59, 6697–6702.
10. Aukrust, I. R.; Skattebol, L. Acta Chem. Scand. 1996, 50,
132–140.
11. Singh, I. P.; Etoh, H.; Takasaki, M.; Konoshima, T. Res.
Adv. Phytochem. 2000, 1, 51–64.
12. (a) Chiba, K.; Arakawa, T.; Tada, M. Chem. Commun.
1996, 1763–1764; (b) Chiba, K.; Arakawa, T.; Tada, M.
J. Chem. Soc., Perkin Trans. 1. 1998, 2939–2942.
13. Tanaka, T.; Mikamiyama, H.; Maeda, K.; Iwata, C.
J. Org. Chem. 1998, 63, 9782–9793.
Diastereoisomers 1a and 1b were separated by reverse
phase preparative HPLC18 and were characterized from
spectral data, viz. UV, NMR, MS, IR and optical rota-
tion.19 All previously published papers reported data for
the dimethyl ethers of robustadials except for that of
Aukrust and Skattebol who reported spectral data for
robustadial A only.10 Hence, the structures were finally
confirmed by synthesis of robustadial dimethyl ethers,
2a and 2b prepared by treatment of a mixture of 1a
and 1b with methyl iodide in the presence of potassium
carbonate in acetone (70% yield). The diastereoisomers
2a and 2b were separated by preparative-TLC20 and
were characterized by comparison of their UV, NMR,
MS, IR and optical rotations with the literature data.3,4
14. Compounds 11 and 12: A mixture of 3,5-diformyl-1-
isopentylphloroglucinol (6, 150 mg, 0.60 mmol), (À)-b-
pinene (5, 0.186 ml, 1.19 mmol) and DDQ (10 mg) in
nitromethane (10 ml) was heated at 60 ꢁC for 2 h. The
solvent was evaporated and the crude product was purified
by silica gel column chromatography (5% EtOAc in
hexane) to yield two products. 5,7-Dihydroxy-6,8-di-
formyl-2H-chromene (11, 30 mg, 21%): Cream white solid;
mp 106–108 ꢁC; IR (KBr): mmax 3369, 2928, 1642, 1440,
In conclusion, we have reported a short and efficient
two-step synthesis of antimalarial robustadials A 1a
and B 1b starting from commercially available phloro-
glucinol 9 in an overall yield of 27%. This methodology
should also be useful for the synthesis of euglobals in
which an isobutyl group is present on the pyran ring.
1314, 1178, 112 cmÀ1 1H NMR (CDCl3, 300 MHz): d
;
13.39 (s, 1H), 13.16 (s, 1H), 10.16 (s, 1H), 10.06 (s, 1H),
6.57 (d, J = 10.1 Hz, 1H), 5.54 (d, J = 10.1 Hz, 1H), 1.25
(s, 6H); 13C NMR (CDCl3, 75 MHz): d 191.9, 191.8, 168.9,
165.7, 163.0, 125.8, 114.5, 104.0, 103.9, 100.9, 80.5,
28.5; EIMS: m/z 248 [M]+, 233, 205, 103, 77, 65, 53.
4,6-Dihydroxy-5,7-diformyl-2-isopropyl-benzofuran (12,
35 mg, 24%): Yellow oil; IR (Neat): mmax 3412, 2927,
Acknowledgements
I.P.S. is thankful to NIPER for start-up funds. S.B.B. is
thankful to NIPER for a fellowship.

7024
S. B. Bharate, I. P. Singh / Tetrahedron Letters 47 (2006) 7021–7024
1647, 1451, 1301, 1212, 1137, 1064 cmÀ1 1H NMR
;
acetic acid (5 ml) was heated in a domestic microwave
oven (1000 W, 4 min). On cooling, the solvent was
removed under reduced pressure, ethyl acetate was added
and the resultant mixture was washed with water and
finally dried over sodium sulfate. The product was purified
by silica gel column chromatography (5% EtOAc in
hexane) to give robustadials A and B as a yellow oil
(0.29 g, 68%). The mixture of diastereoisomers (70 mg)
was separated by semi-preparative HPLC.18 Robustadial
A (1a, 20 mg): Yellow oil, [a]D +71.19 (c 0.45, CHCl3); UV
(CHCl3) kmax 283 (e 19,400), 300 (14,800), 350 (3200); IR
(Neat): mmax 3373, 2922, 1730, 1635, 1541, 1435, 1306,
1162, 1048 cmÀ1; 1H NMR (CDCl3, 300 MHz): d 13.45 (br
s, 2H), 10.14 (s, 1H), 10.04 (s, 1H), 2.90 (m, 1H), 2.26–1.72
(m, 11H), 1.48–1.43 (m, 1H), 1.30–1.22 (m, 1H), 1.24 (s,
3H), 1.01 (s, 3H), 0.96 (d, J = 6.8 Hz, 3H), 0.92 (d,
J = 6.4 Hz, 3H); 13C NMR (CDCl3, 75 MHz): d 192.24,
191.78, 169.42, 168.10, 164.12, 105.85, 104.62, 104.06,
85.90, 48.16, 42.54, 40.51, 38.97, 38.19, 30.49, 27.52, 26.50,
25.92, 25.44, 24.77, 24.02, 23.24, 20.94; CIMS: m/z 387
[M+1]+, 251 (M–C10H16); Robustadial B (1b, 24 mg):
Yellow oil, [a]D À66.17 (c 0.45, CHCl3); UV (CHCl3) kmax
283 (e 17,030), 300 (13,400), 350 (2800); IR (Neat): mmax
3373, 2925, 1733, 1634, 1544, 1440, 1383, 1302, 1178,
(CDCl3, 300 MHz): d 13.43 (s, 1H), 12.83 (s, 1H), 10.32
(s, 1H), 10.21 (s, 1H), 6.47 (s, 1H), 3.06 (m, 1H), 1.34 (d,
J = 6.8 Hz, 6H); 13C NMR (CDCl3, 75 MHz): d 193.4,
189.6, 166.0, 164.5, 164.4, 160.0, 110.8, 105.6, 101.6, 97.7,
28.0, 20.7; EIMS: m/z 248 [M]+, 233, 220, 205, 107, 91, 77,
65, 41.
15. Arisawa, M.; Fujita, A.; Hayashi, T.; Hayashi, K.; Ochiai,
H.; Morita, N. Chem. Pharm. Bull. 1990, 38, 1624–1626.
16. Synthesis of 7: To a solution of phloroglucinol dihydrate 9
(1.0 g, 6.17 mmol) in ethyl acetate (15 ml), dimethyl
formamide (1.43 ml, 18.5 mmol) and phosphoryl chloride
(1.73 ml, 18.5 mmol) were added and the reaction mixture
was allowed to stir at room temperature for 2 h. Water
(10 mL) was added and the reaction mixture was extracted
with ethyl acetate. The organic layer was washed with
brine and finally dried over sodium sulfate. Silica gel
column chromatography using 20% EtOAc in hexane gave
7 (0.45 g, 40%). 1,3-Diformyl-2,4,6-trihydroxybenzene 7:
Cream white solid; mp 218–220 ꢁC; IR (KBr): mmax 2959,
1
2576, 1645, 1441, 1399, 1259, 1195, 1097 cmÀ1; H NMR
(CDCl3: CD3OD-4: 1, 300 MHz): d 10.08 (s, 2H), 5.84 (s,
1H); 13C NMR (CDCl3: CD3OD-4: 1, 75 MHz): d 192.5,
170.1, 104.5, 94.6; CIMS: m/z 183 [M+1]+.17
17. Gruber, W. Chem. Ber. 1942, 75B, 29–33.
1046 cmÀ1 1H NMR (CDCl3, 300 MHz): d 13.48 (br s,
;
18. Diastereoisomers, 1a and 1b were separated by reverse
phase HPLC using a Shimadzu HPLC (USA manufac-
turing Inc.) system consisting of Princeton SPHER-100,
C18 (100 A, 5 l, 250 · 10.0 mm) column using metha-
nol:water:acetic acid 100:5:3 as the mobile phase with a
flow rate of 3.5 ml/min. Diastereoisomers, 1a and 1b
eluted with retention times of 27.42 and 29.61 min,
respectively.
19. Synthesis of robustadials A 1a and B 1b: A mixture of 1,3-
diformyl-2,4,6-trihydroxybenzene (7, 0.2 g, 1.09 mmol),
isovaleraldehyde (8, 0.24 ml, 2.18 mmol), (À)-b-pinene
(5, 0.51 ml, 3.27 mmol) and sodium acetate (10 mg) in
2H), 10.14 (s, 1H), 10.03 (s, 1H), 2.93 (m, 1H), 2.31–2.18
(m, 3H), 2.10–1.85 (m, 6H), 1.78–1.67 (m, 2H), 1.56 (d,
J = 9.1 Hz, 1H), 1.30 (s, 3H), 1.25–1.20 (m, 1H), 1.03 (s,
3H), 0.98 (d, J = 6.7 Hz, 3H), 0.92 (d, J = 6.7 Hz, 3H);
13C NMR (CDCl3, 75 MHz): d 192.29, 191.77, 169.72,
168.11, 164.0, 105.99, 104.94, 103.94, 85.34, 50.58, 42.65,
40.36, 38.94, 38.18, 28.14, 27.16, 26.88, 25.75, 25.58, 24.74,
23.96, 23.42, 21.07; CIMS: m/z 387 [M+1]+, 251 (M–
C10H16).
20. The diastereoisomers 2a and 2b were separated by
preparative TLC using 5% EtOAc in hexane as the mobile
phase (Rf 0.31 and 0.27, respectively, triple run).