Chemical resolution of (±)-calanolide A, (±)-cordatolide A and their 11-demethyl analogues

Article abstract of DOI:10.1016/j.bmcl.2007.12.008

The chemical resolution of (±)-calanolide A and (±)-cordatolide A into their corresponding optically active enantiomers is described. Their inhibitory activities against HIV-1 are tested in vitro.

Full text of DOI:10.1016/j.bmcl.2007.12.008

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1079–1083
Chemical resolution of ( )-calanolide A, ( )-cordatolide A
and their 11-demethyl analogues
Tao Ma,a Qi Gao,a Zhiwei Chen,b Lin Wanga and Gang Liua,*
aDepartment of Synthetic Medicinal Chemistry, Institute of Materia Medica,
Chinese Academy of Medical Sciences & Peking Union Medical College, 2# Nan Wei Road, Beijing 100050, China
bAIDS Institute, Li Ka Shing Faculty of Medicine, The University of Hong Kong, 21 Sassoon Road, Pokfulam, Hong Kong, China
Received 17 July 2007; revised 15 November 2007; accepted 6 December 2007
Available online 21 December 2007
Abstract—The chemical resolution of ( )-calanolide A and ( )-cordatolide A into their corresponding optically active enantiomers
is described. Their inhibitory activities against HIV-1 are tested in vitro.
Ó 2007 Elsevier Ltd. All rights reserved.
Coumarin is one of the main natural product scaffolds
displaying a broad range of biological activities. Its
derivatives have been widely used as therapeutic agents,
active media for tunable dye lasers, optical bleaching
agents, luminescent probes, and triplet sensitizers.1 Since
1992, medicinal scientists have been interested in some
Calophyllum coumarins (1–3, Fig. 1) because of their po-
tent inhibitory activity against HIV-1.2 Especially, (+)-
calanolide A (1) was found to inhibit not only the wild
type of HIV-1, but also clinically isolated resistant
strains such as A17 (Y181C mutant).3 Recently, it was
demonstrated that (+)-calanolide A could also inhibit
Mycobacterium tuberculosis (TB) at MIC 3.13 lg/ml
level.4
The total synthesis of racemic calanolide A has been re-
ported by Chenera,5 Kucherenko,6 and us7,8 with phlor-
oglucinol as the starting material. Its 4-ring system was
consecutively constructed through the three skeletal
rings, coumarin (ring A and B), 2,3-dimethylchromanol
(ring C), and 2,2-dimethyl chromene (ring D). In 1994,
Palmer9 reported general synthetic routes for racemic
calanolide A, inophyllum B, and cordatolide A through
a ten-step approach with lower total yields. Further-
more, optically active (+)-calanolide A has been success-
fully synthesized either by the chiral borane-participated
allylation of 8-formyl coumarin derivative,10 or by the
Pd-catalyzed asymmetric O-allylation of 7-hydroxy-
coumarin,11 or by the application of a (ꢀ)-quinine-cata-
lyzed intramolecular oxo-Michael addition (IMA),12,13
or by optically enzymatic resolution of 8-(3-hydroxy-2-
methyl propionyl)coumarin,14 or by resolution through
chiral preparative HPLC.15,16 However, their chemical
resolution has not been previously reported.
7
O
O
O
A
D
4
8
B
O
O
O
O
O
O
O
O
O
1
C
O
O
O
11
10
12
OH
2 (+)-inophyllumB
OH
3 (+)-cordatolide A
OH
1 (+)-calanolide A
O
O
O
O
O
O
O
O
O
Figure 1. The chemical structures of the biologically active Calophyl-
lum coumarins.
11
OH
OH
OH
5 ( )-11-demethyl
inophyllumB
6 ( )-11-demethyl
cordatolide A
4 ( )-11-demethyl
calanolide A
Keywords: HIV-1; Chemical resolution; Calanolide A; Cordatolide A.
*
Figure 2. The racemic 11-demethyl analogues of Calophyllum
coumarins.
Corresponding author. Tel./fax: +86 10 63167165; e-mail: gangliu27@
yahoo.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.12.008

1080
T. Ma et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1079–1083
Table 1. The chemical shifts (d) of characteristic 1H signals and their
ratios of the two diastereoisomers
O
O
O
O
O
N
O
i
ii
S
N
O
S
COCl
COOH
H
S-(ꢀ)-a-phthalyl-L-
alanine-(+)-11-
demethyl
S-(ꢀ)-a-phthalyl-L-
alanine-(ꢀ)-11-
demethyl
D
8
7
calanolide A
calanolide A
O
O
O
O
8-H
7-H
6.616 (52.5%)
5.536 (62.6%)
6.314 (65.1%)
5.112 (53.5%)
6.532 (37.5%)
5.486 (37.4%)
6.387 (34.9%)
5.022 (46.5%)
0.084
0.050
0.073
0.090
iii
12-H
O
O
O
O
O
O
+
O
N
O
N
CO-CH-N
CH3
O
O
S
S
O
O
10-H
4.474 (59.3%)
4.357 (40.7%)
0.117
9
Scheme 1. Reagents and conditions: (i) L-alanine, 160 °C, 2 h (68%);
(ii) SOCl2, toluene, reflux, 5 h (88%); (iii) racemic 11-demethyl
calanolide A, 4-dimethylaminopyridine (DMAP), dichloromethane
(DCM), room temperature (rt) (41%).
plored to obtain their corresponding optically active
enantiomers.
S-(ꢀ)-a-Phthalyl-L-alanine (7) was first selected as the
chemical resolution reagent, because it could be conve-
niently prepared through condensation of commercially
available phthalic anhydride with L-alanine. With the
racemic 11-demethyl calanolide A as the template com-
pound, S-(ꢀ)-a-phthalyl-L-alanine was converted readily
into its corresponding acyl chloride (8) in the presence of
SOCl2 with toluene as the solvent (Scheme 1). A pair of
diastereoisomers (9) was obtained subsequently. How-
In our laboratory, the concise total synthesis of the race-
mic compounds and their 11-demethyl analogues (1–6,
Figs. 1 and 2) has been developed with fewer reaction
steps and higher total yields.17,18 Moreover, their corre-
sponding demethyl analogues were also found to be
anti-HIV active at a similar level to (+)-calanolide A
in vitro. Therefore, the chemical resolution was then ex-
Figure 3. The magnified 1H NMR spectrum (300 MHz, CDCl3, ppm) of the mixed diastereoisomers (9, only downfield is shown). The two sets of 1H
signals were assigned to the mixed diastereoisomers, respectively. The chemical shifts of 12-H signals in compound 9 range from 6.3 to 6.4 ppm (t,
J = 6.6 Hz), compared to 5.239 ppm (dd, J = 7.35 Hz, 8.7 Hz) in compound ( )-4. That difference apparently indicated that the 12-OH was acylated
by the resolution reagent (8).

T. Ma et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1079–1083
1081
tified by 1H NMR analysis (Fig. 3). Their relative ratios
could be consequently approximately calculated through
their corresponding integration areas (Table 1).
i
ii
OH
OCH2COOH
OCH2COCl
An alternative approach was then developed. (ꢀ)-Men-
thol-acetic acid (10) was finally selected as the resolution
reagent, which had three asymmetric carbon centers in
its scaffold. (ꢀ)-Menthol-acetyl chloride (11) was readily
reacted with racemic 11-demethyl calanolide A to pro-
duce the anticipative pair of diastereoisomers (12, 13),
which were effectively separated and purified using silica
gel H column chromatography (Scheme 2, Fig. 4). The
two separated diastereoisomers (12, 13), which were
11
10
O
O
O
O
iii
O
O
O
O
O
O
+
O
O
O
O
1
characterized by H NMR spectra (Table 2), were dis-
solved in tetrahydrofuran (THF), respectively, and were
treated with 0.1 mol/L NaOH, respectively, to eventu-
ally gain their corresponding optical enantiomers [(+)-
4 and (ꢀ)-4] (Fig. 5).19
12
13
iv'
iv
O
O
O
O
O
O
O
O
Table 2. The chemical shifts (d) of characteristic 1H signals between
the purified diastereoisomers (12 and 13)
OH
OH
(+)-4
(-)-4
H
(ꢀ)-Menthol-acetyl- (ꢀ)-Menthol-acetyl-
D
Scheme 2. Reagents and conditions: (i) ClCH2COOH, NaH, 1,4-
dioxane, reflux, 4 h (41%); (ii) SOCl2, toluene, reflux, 4 h (86%); (iii)
racemic 11-demethyl calanolide A, DMAP, DCM, room temperature
(42% and 37%); (iv and iv0) 0.1 mol/L NaOH, THF, rt (86% and 77%).
(+)-11-demethyl
calanolide A (12)
(ꢀ)-11-demethyl
calanolide A (13)
12-H
10-H
OCH2COO 4.196
6.346
4.356
6.335
4.336
4.179
3.296
0.894
0.888
0.831
0.011
0.020
0.017
0.145
0.024
0.002
0.189
10-H
3.151
0.918
0.886
0.642
ever, they could not be effectively separated from each
other through classical purification methods such as silica
gel column chromatography and preparative thin layer
chromatography. The mixed diastereoisomers were iden-
a-CH3
b-CH3
c-CH3
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OR
OR
OR
OR
12 (-)-Menthol acetyl-
(+)-11-demethyl-
calanolide A
14 (-)-Menthol acetyl-
(+)-calanolide A
16 (-)-Menthol acetyl-
(+)-cordatolide A
18 (-)-Menthol acetyl-
(+)-11-demethyl-
cordatolide A
[α ]20
D = - 13.2 º
[α ]2D0
[α ]2D0 = + 44.32 º
= - 3.05 º
[α ]2D0
= - 26.1 º
(c 14.45, CH2Cl2)
(c 2.0, CHCl3)
(c 0.88, CH2Cl2)
(c 11.2, CH2Cl2 )
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OR
OH
OR
OR
13 (-)-Menthol acetyl-
(-)-11-demethyl-
calanolide A
15 (-)-Menthol acetyl-
(-)-calanolide A
19 (-)-Menthol acetyl-
(-)-11-demethyl-
cordatolide A
17 (-)-Menthol acetyl-
(-)-cordatolide A
20 = - 36.6 º
[α ]2D0 = - 47.74 º
20 = - 48.6 º
[α ]2D0
[α ] D
[α ]D
= - 130.2 º
(c 0.775, CH2Cl2)
(c 4.9, CH2Cl2)
(c 2.15, CH2Cl2)
(c 2.1, CHCl3)
Figure 4. The four pairs of optically active 12-O-(ꢀ)-menthol acetyl derivatives. R represents (ꢀ)-menthol acetyl group.

1082
T. Ma et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1079–1083
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
OH
OH
OH
(+)-4, (+)-11-
demethyl-calanolide A
(+)-6, (+)-11-
demethyl-cordatolide A
(+)-3, (+)-cordatolide A
(+)-1, (+)-calanolide A
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
OH
(-)-1, (-)-calanolide A
OH
(-)-3, (-)-cordatolide A
OH
(-)-4, (-)-11-
(-)-6, (-)-11-
demethyl-cordatolide A
demethyl-calanolide A
Figure 5. The four pairs of optically active enantiomers.
Table 3. The anti-HIV-1 activities and cytotoxicities of the optically active calanolide A, cordatolide A, and their corresponding 11-demethyl
analogues
20
½aꢁD
Entry
Inhibition (%) 10 lM Inhibition (%) 1.0 lM Cytotoxicity (%) (10 lM) Cytotoxicity (%) (1.0 lM)
( )ꢀ1
0
97
81
86
67
44
0
0
6
0
1
0
0
0
0
0
0
0
(+)ꢀ1 +58.1° (c 0.93, CH2Cl2)a 98
(ꢀ)ꢀ1 ꢀ56.6° (c 0.60, CH2Cl2)b 97
0
(+)ꢀ3 +51.8° (c 1.3, CHCl3)
(ꢀ)ꢀ3 ꢀ58.1° (c 1.1, CHCl3)
(+)ꢀ4 +38.4° (c 3.2, CH3OH)
(ꢀ)ꢀ4 ꢀ33.3° (c 0.9, CH3OH)
(+)ꢀ6 +46.2° (c 1.3, CH2Cl2)
(ꢀ)ꢀ6 ꢀ44.1° (c 1.7, CH2Cl2)
25
85
0
0
0
98
0
75
0
54
0
0
0
0
0
0
0
½aꢁD ¼ þ60ꢂ (c 0.5, CHCl3),2 +68.8° (c 0.7, CHCl3),16 +66° (c 0.5, CHCl3).10
a
25
½aꢁD ¼ ꢀ75:6ꢂ (c 0.7, CHCl3),16 ꢀ66° (c 0.5, CHCl3),10 ꢀ68° (c 1.36, CHCl3).22
b
Using the same strategy, the other three racemic
Calophyllum coumarins (1, 3, and 6) were chemically
by (+)-4 or (+)-1 at 10 lM. The findings indicate that
the removal of a methyl group at the 11-position of
calanolide A increases the cytotoxicity of this type of
optical enantiomer.
resolved into their corresponding optical enantiomers
a resolution
with (ꢀ)-menthol-acetyl chloride as
reagent. All the four pairs of optical diastereoisomers
and enantiomers are summarized in Figures 4 and 5,
respectively.
In summary, the concise total synthesis of the racemic
Calophyllum coumarins and their 11-demethyl ana-
logues has been developed in our laboratory with fewer
reaction steps and higher total yields. The chemical res-
olution of ( )-calanolide A and ( )-cordatolide A into
their corresponding optically active enantiomers is de-
scribed in this paper. Their inhibitory activities against
HIV-1 were tested in vitro.
The inhibitory activities against HIV-1 of all the enanti-
omers were tested in vitro using a pseudotyped viral as-
say as previously described.20 The assay results in cell
culture and their cytotoxicity are outlined in Table 3.21
Obviously, (ꢀ)-11-demethyl calanolide A [(ꢀ)-4], (+)
and (ꢀ)-11-demethyl cordatolide A [(+)-6 and (ꢀ)-6],
and (ꢀ)-cordatolide A [(ꢀ)-3] enantiomers lost their
activities against HIV-1. (+)-Cordatolide A [(+)-3] had
lower inhibitory ability compared with (+)-, or (ꢀ)-
calanolide [(+)-1, or [(ꢀ)-1]. Interestingly, (ꢀ)-calano-
lide A [(ꢀ)-1] also showed inhibitory activity against
HIV-1 in the assay that was different from previous
studies.15,16 The racemic calanolide A [( )-1] was also
tested in the assay, and it had very close inhibitory activ-
ity to (+)-1. This difference will be further investigated to
determine the impalpable structure–activity relationship
of the stereo configurations of calanolide A. We also
determined the cytotoxicity to the host cells induced
Acknowledgments
This research was financially supported by the National
Natural Science Foundation of China (No. 3970868).
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2007.12.008.

T. Ma et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1079–1083
1083
J = 6.0 Hz, 10-CH3), 0.989 (t, 3H, J = 7.5 Hz, 4-
CH2CH2CH3), 0.918 (d, 3H, J = 6.5 Hz, a-CH3), 0.886
(d, 3H, J = 6.5 Hz, b-CH3), 0.642 (d, 3H, J = 6.5 Hz, c-
CH3).
References and notes
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Analytical data for 13: colorless oil; H NMR (500 MHz,
CDCl3, ppm): 6.620 (d, 1H, J = 10.0 Hz, 8-H), 6.335 (t,
1H, J = 7.0 Hz, 12-H), 5.940 (s, 1H, 3-H), 5.540 (d, 1H,
J = 10.0 Hz, 7-H), 4.336 (m, 1H, 10-H), 4.179 (q, 2H,
OCH2COO), 3.296 (dt, 1H, J = 4.0 Hz, 10.5 Hz, 10-H),
2.880 (m, 2H, 4-CH2CH2CH3), 2.222 (m, 2H, 11-CH2),
2.014 (m, 2H, CH2), 1.638 (m, 2H, CH2), 1.508 (m, 2H, 4-
CH2CH2CH3), 1.472, 1.464 (2s, 6H, 6-CH3), 1.310 (d, 3H,
J = 6.0 Hz, 10-CH3), 0.979 (t, 3H, J = 7.5 Hz, 4-
CH2CH2CH3), 0.894 (d, 3H, J = 7.0 Hz, a-CH3), 0.888
(d, 3H, J = 7.5 Hz, b-CH3), 0.831 (d, 3H, J = 7.0 Hz, c-
CH3). Analytical data for (+)-4: off-white powder; 1H
NMR (500 MHz, CDCl3, ppm): 6.620 (d, 1H, J = 10.0 Hz,
8-H), 5.949 (s, 1H, 3-H), 5.543 (d, 1H, J = 10.0 Hz, 7-H),
5.247 (t, 1H, J = 8.0 Hz, 12-H), 4.270 (m, 1H, 10-H), 2.890
(m, 2H, 13-CH2), 2.369 (m, 1H, 11-He), 1.934 (m, 1H, 11-
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(500 MHz, CDCl3, ppm): 6.620 (d, 1H, J = 10.5 Hz, 8-H),
5.947 (s, 1H, 3-H), 5.542 (d, 1H, J = 10.5 Hz, 7-H), 5.247
(t, 1H, J = 7.5 Hz, 12-H), 4.270 (m, 1H, 10-H), 2.895 (m,
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19. Analytical data for 12: colorless oil; H NMR (500 MHz,
CDCl3, ppm): 6.620 (d, 1H, J = 10.0 Hz, 8-H), 6.346 (t,
1H, J = 7.0 Hz, 12-H), 5.936 (s, 1H, 3-H), 5.540 (d, 1H,
J = 10.0 Hz, 7-H), 4.356 (m, 1H, 10-H), 4.196 (q, 2H,
OCH2COO), 3.151 (dt, 1H, J = 4.0 Hz, 10.5 Hz, 10-H),
2.864 (m, 2H, 4-CH2CH2CH3), 2.220 (m, 2H, 11-CH2),
2.016 (m, 2H, CH2), 1.637 (m, 2H, CH2), 1.508 (m, 2H, 4-
CH2CH2CH3), 1.471, 1.451 (2s, 6H, 6-CH3), 1.313 (d, 3H,
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