Synthesis of gossypol atropisomers and derivatives and evaluation of their anti-proliferative and anti-oxidant activity

Article abstract of DOI:10.1016/j.bmc.2005.04.026

Gossypol 1, gossypolone 2, and a series of bis 3 and half Schiff's bases 4 of gossypol were synthesised and tested for anti-proliferative and anti-oxidant activity. (-)-Gossypol (-)-1 was the most potent inhibitor of the proliferation of the HPV-16 kerati

Full text of DOI:10.1016/j.bmc.2005.04.026

Bioorganic & Medicinal Chemistry 13 (2005) 4228–4237
Synthesis of gossypol atropisomers and derivatives and evaluation
of their anti-proliferative and anti-oxidant activity
Kalliopi Dodou,a Rosaleen J. Anderson,a W. John Lough,a David A. P. Small,b
Michael D. Shelleyc and Paul W. Groundwatera,*
aSunderland Pharmacy School, University of Sunderland, Wharncliffe Street, Sunderland SR1 3SD, UK
bStiefel International R&D, Whitebrook Park, 68 Lower Cookham Road, Maidenhead, Berkshire SL6 8XY, UK
cVelindre NHS Trust, Whitchurch, Cardiff CF14 2TL, UK
Received 3 December 2004; revised 7 April 2005; accepted 12 April 2005
Available online 4 May 2005
Abstract—Gossypol 1, gossypolone 2, and a series of bis 3 and half Schiffꢀs bases 4 of gossypol were synthesised and tested for anti-
proliferative and anti-oxidant activity. (ꢀ)-Gossypol (ꢀ)-1 was the most potent inhibitor of the proliferation of the HPV-16 kerat-
inocyte cell line (using an MTT viability assay) with a GI50 of 4.8 lM. The bis Schiffꢀs base of (ꢀ)-gossypol with L-tyrosine ethyl
ester (ꢀ)-3b was the most potent inhibitor of iron/ascorbate dependent lipid peroxidation (using the thiobarbituric acid test), with
an IC50 of 11.7 lM, with (ꢀ)-gossypol being the next most potent of the series, with an IC50 of 13.1 lM. The results from these
initial assays suggest that gossypol, as either a racemic mixture rac-1, or the individual atropisomers (ꢀ)-1 or (+)-1, has potential
for the treatment of psoriasis.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
to exhibit in vitro anti-viral and anti-parasitic activities
at micromolar levels,8,9 and anti-tumour activity.10–12
Gossypol exhibits atropisomerism, as a result of res-
tricted rotation about the C2–C02 binaphthyl bond,
resulting in two optically active forms, l- or (ꢀ), (ꢀ)-1,
and d- or (+), (+)-1. The (ꢀ)-enantiomer is usually more
potent in all these biological systems, when compared to
the (+)-enantiomer and the racemic mixture rac-1. It has
been suggested that the (ꢀ)-enantiomer in low concen-
trations affects cells in a stereospecific manner, whereas
non-stereospecific interactions are found for (+)-gossy-
pol and higher concentrations of (ꢀ)-gossypol.13
Psoriasis is a genetically heterogeneous skin condition,
which is characterised by benign keratinocyte hyperpro-
liferation, skin inflammation, altered dermal vasculature
and defective keratinisation.1,2 The cell-mediated
immune mechanisms, which lead to the increased proli-
feration have been the subject of much attention, but the
antigen responsible remains elusive.3 Therapeutic inter-
ventions for psoriasis are usually based upon anti-proli-
ferative, anti-inflammatory and differentiation-modifying
activity or a combination of these effects,4 but there is
no cure for psoriasis. A number of selective immune
response modifiers are in development.3
Gossypol 1 has been found to be reasonably potent
against melanoma cell lines, with the (ꢀ)-isomer being
more cytotoxic than cisplatin, melphalan and dacarba-
zine.14 In addition, the anti-melanotic activity of gossy-
pol 1, its oxidation product gossypolone 2 (Scheme 2),
the bis 3a and half Schiffꢀs base derivatives 4a (Scheme
1) of gossypol with L-phenylalanine was studied in pig-
mented and non-pigmented melanoma cell lines. These
studies showed that blocking both aldehyde groups [as
bis Schiffꢀs bases 3a, R = Ph, X = (S)-CHCO2CH3] abol-
ished activity against both cell lines, while retaining one
aldehyde group (half Schiffꢀs base with L-phenylalanine
methyl ester 4a) increased the activity against both the
pigmented and non-pigmented cell lines, with the half
Gossypol 1, a yellow anti-inflammatory compound that
is present in members of the Malvaceae family, such as
the cotton plant (Gossypium species) and the tropical
tree Thespesia populnea, has been studied extensively
since the discovery of its in vivo male anti-fertility
activity in the late 1960ꢀs,5–7 and has also been shown
Keywords: Gossypol; Atropisomers; Psoriasis; Anti-oxidant; Anti-
proliferative.
*
Corresponding author. Tel.: +44 (0)191 515 2600; fax: +44 (0)191 515
3405; e-mail: paul.groundwater@sunderland.ac.uk
0968-0896/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2005.04.026

K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
4229
Scheme 1. Reagents and conditions: (i) RCH2XNH2, DCM, IPA; (ii) AcOH, Et2O, H2SO4, 0 ꢁC, 16 h; (iii) AcOH, Et2O, H2SO4, 0 ꢁC, 4 h.
Table 1. % Yield and structure of the individual gossypol bis Schiffꢀs
bases 3
Schiffꢀs base of (ꢀ)-gossypol, (ꢀ)-4a, being the most
potent compound.15
Bis Schiffꢀs base
R
X
% Yield
Having considered the previously reported anti-inflam-
matory activity of gossypol,16,17 we have investigated
the activity of gossypol and its derivatives as novel
agents for the treatment of psoriasis. We have tested
these compounds as inhibitors of keratinocyte hyperpro-
liferation (one of the hallmarks of psoriasis) and their
antioxidant activity (based upon the ꢁoxidative stressꢀ
theory for the pathophysiology of psoriasis18). We have
also investigated the importance of the aldehyde groups
in gossypol 1, in order to observe any structure–activity
relationship correlations between the anti-melanotic and
anti-psoriatic activities of gossypol and its derivatives.
(ꢀ)-3a
Ph
Ph
(S)-CHCO2CH3
(S)-CHCO2CH3
39
31
(+)-3a
(ꢀ)-3b
C6H4OH-4 (S)-CHCO2CH2CH3 16
C6H4OH-4 (S)-CHCO2CH2CH3 21
(+)-3b
(ꢀ)-3c(L)
(+)-3c(L)
(ꢀ)-3c(D)
(+)-3c(D)
3-Indolyl
3-Indolyl
3-Indolyl
3-Indolyl
(S)-CHCO2CH3
(S)-CHCO2CH3
(R)-CHCO2CH3
(R)-CHCO2CH3
25
25
24
24
existing as the imine tautomer (with a singlet for the
imine CH@N) in CDCl3. Interestingly, the Schiffꢀs base
with L-tyrosine ethyl ester exists as the enamine tauto-
mer 3 in DMSO-d6. For the first time, the H and 13C
1
2. Results and discussion
2.1. Chemistry
NMR spectra of the individual diastereoisomers of the
bis 3 and half 4 Schiffꢀs bases were fully assigned, using
a variety of experiments, which included HMQC and
HMBC spectra.
The bis Schiffꢀs base diastereoisomeric mixtures of gos-
sypol with amino acid esters 3 (Scheme 1), including
two novel diastereoisomeric pairs with L-tyrosine ethyl
ester and D-tryptophan methyl ester, were prepared by
a reported method19 and separated by chromatography
on silica gel (Scheme 1, Table 1), using HPLC for reac-
tion monitoring and the determination of diastereoiso-
meric purity. In solution, the bis Schiffꢀs bases can
exist as either the enamine 3 or imine tautomers 30
(Fig. 1),20 depending upon the amino acid ester used
and the solvent, with the L-phenylalanine methyl ester
3a and D- and L-tryptophan methyl ester 3c derivatives
existing as the enamines (with a doublet for the
NHCH@), and the L-tyrosine ethyl ester derivative 3b0
The half Schiffꢀs bases 4 were prepared by the partial
hydrolysis of their respective bis Schiffꢀs bases 3 and
the subsequent separation of the reaction mixtures
(Scheme 1, Table 2). The method for this hydrolysis
was a modification of previously reported methods.15,21
(ꢀ)-Gossypol (ꢀ)-1 was prepared in 95% yield by the
complete hydrolysis of (ꢀ)-gossypol bis(L-tyrosine ethyl
ester) Schiffꢀs base (ꢀ)-3b and (+)-gossypol (+)-1 was
prepared in 91.5% yield by the complete hydrolysis of
(+)-gossypol bis(L-phenylalanine methyl ester) Schiffꢀs
base (+)-3a (Scheme 1). The oxidation of gossypol rac-
1 (Scheme 2) employed ferric chloride hexahydrate22

4230
K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
Figure 1.
the treatment of psoriasis. The GI50 values for metho-
trexate 5 (148.6, 204.9 and 219 lM) and dithranol 6
(0.58 lM), Figure 2, show that methotrexateꢀs inhibitory
efficacy is similar to that of the bis Schiffꢀs bases 3, while
dithranol 6 is an order of magnitude more active than
(ꢀ)-gossypol (ꢀ)-1. Methotrexate 5 is an anti-prolifera-
tive, which is also an immunosuppressive,3 while dithra-
Table 2. % Yield and structure of the gossypol half Schiffꢀs bases 4
Half Schiffꢀs base
R
X
% Yield
(ꢀ)-4a
(+)-4a
Ph
Ph
(S)-CHCO2CH3
(S)-CHCO2CH3
51
13
(ꢀ)-4b
(ꢀ)-4c(L)
C6H4OH-4 (S)-CHCO2CH2CH3 28
3-Indolyl (S)-CHCO2CH3 29
nol
6 is an anti-proliferative, which upregulates
interleukin-10 receptor expression on keratinocytes.4,25
The compounds, which showed increased inhibition
against keratinocyte proliferation were subsequently
tested for their anti-oxidant effect against iron/ascorbate
dependent lipid peroxidation, using the thiobarbituric
acid (TBA) test.26 There is growing evidence supporting
the role of reactive oxygen species in psoriasis and a fail-
ure of the anti-oxidant defence system of the skin has
been suggested.18 In fact, the free fatty acid content is
decreased by 46% in psoriatic scales compared to nor-
mal stratum corneum27 and elevated malondialdehyde
(MDA) levels, a lipid peroxidation product, have been
detected in psoriatic lesions.28 The TBA test utilises
MDA as a marker for assessing the extent of lipid per-
oxidation and is based upon the spectrophotometric
quantitation of the pink complex formed after reaction
of MDA with two molecules of TBA. The absorbance
of each reaction mixture was compared against a control
mixture, consisting of the liposomes and the oxidising
system in the absence of the test compound—the full
Scheme 2. Reagents and conditions: (i) FeCl3, acetone, AcOH, 100 ꢁC,
2 h, then ether, 20% aq H2SO4.
and successfully produced pure gossypolone rac-2 in
59% yield. This has been the most widely used method
for the preparation of gossypolone23,24 and has previ-
ously been shown to give racemic gossypolone rac-2
even when starting with only one of the gossypol enan-
tiomers—this epimerisation at the binaphthyl bond pre-
sumably arising as a result of the intermediacy of a
radical species. The initial product from the gossypol
oxidation is an iron(II) complex with the aldehyde and
the 1-hydroxyl groups of gossypolone, which is decom-
posed after treatment with aqueous H2SO4.22
2.2. Biological evaluation
The in vitro anti-psoriatic activity of gossypol and its
derivatives was evaluated using an anti-proliferative as-
say and an anti-oxidant assay. In the anti-proliferative
study, the sensitivity of an HPV-16 keratinocyte cell line
to each compound was determined using an MTT via-
bility assay. HPV-16 is a rapidly dividing immortalised
human keratinocyte cell line, which mimics the hyper-
proliferation of the epidermis, one of the pathological
features of psoriasis. The assay relies upon the spectro-
photometric measurement of solubilised purple forma-
zan, produced by the mitochondrial reduction of MTT
in viable cells. A comparison was made with the data
from three MTT assays on HPV-16 keratinocyte cell
lines using methotrexate 5, which is used clinically for
Figure 2.

K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
4231
Table 4. Inhibition of iron/ascorbate dependent bovine brain lipid
peroxidation
reaction mixture (FRM). The assay was validated by a
positive anti-oxidant control reaction, which involved
the effect of the established anti-oxidant propyl gallate
7 on the full reaction mixture.
Compound
IC50 (lM)a
(ꢀ)-3b
(ꢀ)-1
rac-1
(+)-1
rac-2
7
11.7
13.1
Racemic gossypol rac-1 and (ꢀ)-gossypol (ꢀ)-1 showed
the highest anti-proliferative activity against HPV-16
keratinocytes, followed by the half Schiffꢀs bases 4,
(+)-gossypol (+)-1 and racemic gossypolone rac-2, and
finally the bis Schiffꢀs bases 3 (Table 3). The bis Schiffꢀs
bases 3 were the least active as inhibitors of keratinocyte
proliferation. (ꢀ)-Gossypol (ꢀ)-1 was tested twice in
order to observe its effect within different concentration
ranges; there is no significant difference between the
two GI50 values when compared by Studentꢀs t-test for
unpaired values (p = 0.1156). The fact that the GI50 val-
ues for the two gossypol enantiomers are different imp-
lies an enantioselective interaction with the site of
action, in common with the other biological activities
exhibited by gossypol, where the (ꢀ)-isomer (ꢀ)-1 is
usually more active than the (+)-isomer (+)-1. There is
a noticeable increase in the GI50 values between the half
4 and the bis Schiffꢀs bases 3. This may indicate either
that the aldehyde groups, which are derivatised upon
Schiffꢀs base formation, are essential for the anti-prolif-
erative effect, or that the bis Schiffꢀs bases have difficulty
in entering the cell due to their size. Both these reasons
could explain why the highest potency was observed for
gossypol 1, which has both aldehyde groups intact and
the lowest molecular weight of all the compounds tested.
Gossypol 1 is believed to exert its anti-proliferative
action in different ways, depending upon the cellular
environment, for example, the enzymes present, the lipi-
dic composition of the cell membrane and the metabolic
(mitochondrial) activity of the cell.9,29–31 In pigmented
(SK-mel-19) and non-pigmented (SK-mel-28) melanoma
cell lines, the activity decreases on going from the half
Schiffꢀs bases to gossypol, with the bis Schiffꢀs bases
again being the least active.15
17.3
17.3
28.0
IC70 = 100
a
n-Butanol was used as an internal standard, and the absorbance of
the MDA–TBA complex was determined at 532 nm. The IC50 values
were obtained from LOWESS analysis of % inhibition of peroxida-
tion versus logarithmic concentration (molar) of each inhibitor. Four
replicates were carried out for each reaction mixture.
The anti-oxidant activity of all compounds tested was
similar and they were all more active than the standard,
propyl gallate 7 (Table 4). Gossypol 1, in common with
other aromatic phenols, is known for its anti-oxidant
activity and its protective effect against lipid peroxida-
tion has been attributed to its partition into the mem-
brane and interruption of the chain reaction
process.32–34 This suggested mechanism is consistent
with the lipophilic nature of the other compounds
tested, and may explain the similarity of their IC50 val-
ues. Gossypol 1 is unable to scavenge superoxide radi-
cals at the low concentrations at which it prevents
peroxidation, and its iron chelating ability has not been
shown to contribute to its anti-peroxidant effect.33
3. Conclusions
Gossypol 1 was shown to be the most potent inhibitor of
keratinocyte proliferation in the anti-proliferative MTT
assay, and a potent anti-oxidant. Considering the inhib-
itory activity of gossypol on cytoplasmic phospholipase-
2,16 its activity against endothelial cell growth35 and its
apoptotic effect on human lymphocytes,36 an overall
anti-psoriatic model of activity can be deduced, which
encompasses both anti-proliferative and anti-inflamma-
tory16,17 features. This in vitro anti-psoriatic activity in
conjunction with the low toxicity in humans,37 lack of
mutagenicity38 and favourable comparison to metho-
trexate and dithranol, makes gossypol a good candidate
for the topical treatment of psoriasis.
Table 3. Inhibition of proliferation of HPV-16 cell line by gossypol and
its derivatives in order of decreasing potency
Compound
GI50 (lM)a
SD
(ꢀ)-1
4.8, 8.6
5.4
16.4
0.48, 0.04
0.33
2.54
rac-1
(+)-1
(ꢀ)-4b
(ꢀ)-4c(L)
rac-2
21.4
37.1
11.15
14.58
9.16
4. Experimental
4.1. Chemistry
47.35
49.4
(ꢀ)-4a
(ꢀ)-3b
(ꢀ)-3a
(+)-3c(D)
(ꢀ)-3c(L)
(ꢀ)-3c(D)
(+)-3a
0.324
28.67
7.67
107.35
155.85
166.5
171.0
188.2
213.25
>219.0
Racemic gossypol acetic acid was a gift from Velindre
NHS Trust, Cardiff, and was converted to the free race-
mic gossypol rac-1 by dissolution in diethyl ether, dou-
ble washing with equal amounts of distilled water to
remove the acetic acid, drying of the ether layer over
MgSO4 and removal of the solvent under reduced pres-
sure. The amino acid ester free bases were obtained by
treatment of their corresponding hydrochloride salts
with saturated NaHCO3 solution and extraction into
dichloromethane followed by removal of the solvent
58.38
23.08
20.57
12.65
—
(+)-3c(L)
a MTT cell viability assay, with absorbance readings at 570 nm. The
GI50 values were obtained from LOWESS analysis of % growth
versus logarithmic concentration (molar) of the compound. Six in-
plate replicates of the eight concentrations were performed.

4232
K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
under reduced pressure. Ferric chloride hexahydrate was
obtained from Aldrich. CDCl3 and DMSO-d6 for NMR
spectroscopy were obtained from Apollo Scientific Ltd.
IR Spectra were recorded on an ATI Mattison Genesis
Series FTIR and samples were examined as potassium
CH3), 3.17 (2H, dd, J = 13.85, 8.6 Hz, CH2-a), 3.34 (2H,
dd, J = 13.85, 4.6 Hz, CH2-b), 3.69 (2H, septet,
J = 7.3 Hz, CH(CH3)2), 3.77 (6H, s, COOCH3), 4.30
(2H, m, a-CH), 5.35 (2H, s, 1,10-OH), 7.11–7.26 (10H,
m, Ar–H), 7.54 (2H, s, 4,40-H), 7.94 (2H, s, 6,60-OH),
9.37 (2H, d, J = 11.9 Hz, HN–CH@), 13.59 (2H, broad
s, HN–CH@); 13C NMR (75 MHz, CDCl3) d 20.4
(4 · CH3), 20.7 (2 · Ar–CH3), 27.8 (2 · CH), 40.4
(2 · CH2), 53.3 (2 · OCH3), 64.9 (2 · a-CH), 104.0
(2 · quat., C-8), 114.8 (2 · quat., C-8a), 116.2 (2 · quat.,
C-2), 118.6 (2 · CH, C-4), 127.7 (2 · CH, C-400), 128.2
(2 · quat., C-4a), 129.3 (4 · CH, 2 · 200,600-CH), 129.6
(2 · quat., C-5), 129.8 (4 · CH, 2 · 300,500-CH), 132.4
(2 · quat., C-3), 135.5 (2 · quat., C-100), 147.4 (2 · quat.,
C-1), 149.3 (2 · quat., C-6), 161.9 (2 · CH–NH), 170.5
(2 · quat., ester C@O), 174.0 (2 · quat., C@O, C-7).
Anal. Calcd for C50H52O10N2: C, 71.4; H, 6.2; N, 3.3.
Found: C, 71.1; H, 6.3; N, 3.3.
1
bromide (KBr) discs. H NMR Spectra were acquired
on a JEOL GSX270, Bruker AVANCE 300 or Bruker
AVANCE 500 at 270, 300 and 500 MHz, respectively.
13C NMR Spectra were acquired on a Bruker AVANCE
300 or Bruker AVANCE 500 at 75 or 125 MHz, respec-
tively. Coupling constants (J) are given in Hertz and all
chemical shifts are relative to the chemical shift of the
residual non-deuterated solvent. Thin-layer chromato-
graphy (TLC) was performed on silica gel-60 coated
plastic plates obtained from Merck. Melting points were
determined on a Reichert hot stage microscope and are
uncorrected. Elemental analysis was performed by
Medac Ltd., Brunel University, Uxbridge, United
Kingdom.
The (+)-gossypol bis(L-phenylalanine methyl ester)
Schiffꢀs base (+)-3a was isolated as a yellow-orange pow-
der (0.57 g, 31%); Rf = 0.40 in 1:3 hexane/ether; Rt(min)
8.96; [a]25 +514.9 (c 0.09, DCM); mp 118–122 ꢁC; mmax
(KBr)/cmꢀ1 3480 (OH, NH), 1745 (C@O), 1612
Analytical HPLC work was performed with a Spectra–
Physics SP8800 ternary HPLC Pump (Spectra–Physics,
San Jose, California, USA) operating at a flow rate of
1.0 mL/min, at room temperature, connected to a Spec-
tra–Physics SP8490 detector, operating at 254 nm for
UV detection. Injections were performed using a Rheo-
dyne 7125 manual injector equipped with a 20 lL loop
(Anachem, Luton, Bedfordshire, UK). Data collection
was performed on a Dionex advanced computer inter-
face (Dionex, Camberley, Surrey, UK) and data pro-
cessing on a computer equipped with Dionex A1-450
software. A C18 column was used, ODS-H15-19724
(15 cm · 3.2 mm i.d.), purchased from CAPITAL
HPLC Ltd. The pH of the mobile phase was determined
with a JENWAY 3030 pH meter by S.H. Scientific.
1
(C@C); H NMR (270 MHz, CDCl3) d 1.52 (12H, d,
J = 7.3 Hz, CH(CH3)2), 2.05 (6H, s, Ar–CH3), 3.12
(2H, dd, J = 13.85, 8.6 Hz, CH2-a), 3.32 (2H, dd,
J = 13.85, 4.6 Hz, CH2-b), 3.68 (2H, septet, J = 7.3 Hz,
CH(CH3)2), 3.76 (6H, s, COOCH3), 4.26 (2H, m, a-
CH), 5.37 (2H, s, 1,10-OH), 7.13–7.26 (10H, m, Ar–H),
7.54 (2H, s, 4,40-H), 7.94 (2H, s, 6,60-OH), 9.24 (2H,
d, J = 11.9 Hz, HN–CH@), 13.60 (2H, broad s, HN–
CH@); 13C NMR (75 MHz, CDCl3) d 20.4 (4 · CH3),
20.7 (2 · Ar–CH3), 27.8 (2 · CH), 40.4 (2 · CH2), 53.3
(2 · OCH3), 64.9 (2 · a-CH), 104.1 (2 · quat., C-8),
114.8 (2 · quat., C-8a), 116.2 (2 · quat., C-2), 118.6
(2 · CH, C-4), 127.7 (2 · CH, C-400), 128.2 (2 · quat.,
C-4a), 129.3 (4 · CH, 2 · 200,600-CH), 129.6 (2 · quat.,
C-5), 129.8 (4 · CH, 2 · 300,500-CH), 132.4 (2 · quat., C-
3), 135.5 (2 · quat., C-100), 147.4 (2 · quat., C-1), 149.3
(2 · quat., C-6), 161.9 (2 · CH–NH), 170.5 (2 · quat.,
ester C@O), 174.0 (2 · quat., C@O, C-7). Anal. Calcd
for C50H52O10N2Æ1/2Et2O: C, 71.1; H, 6.5; N, 3.2.
Found: C, 70.8; H, 6.2; N, 3.3.
4.1.1. General procedure for the synthesis of diastereo-
isomeric gossypol bis Schiffꢀs bases 3a–c. Racemic gossy-
pol rac-1 (1 equiv) was dissolved in DCM (50 mL/1 g of
gossypol) and added to the chiral amino acid ester free
base (6 equiv) and isopropanol (1 mL/1 g of gossypol).
The reaction mixture was left to stir at room tempera-
ture, in the dark. Diastereoisomer formation was moni-
tored by reverse phase HPLC, using a mobile phase of
acetonitrile/aq 0.02 M HCOONH4 (80:20, pH = 6.3) at
k = 254 nm and a flow rate of 1.0 mL/min. The diaste-
reoisomeric reaction mixture was then evaporated to
dryness, adsorbed onto silica and separated by column
chromatography on silica, eluting with hexane/ether
(100:0–33:66) for 3a, and hexane/ethyl acetate (100:0–
50:50) for 3b, 3c(L) and 3c(D).
4.1.1.2. Gossypol bis(L-tyrosine ethyl ester) Schiffꢀs
bases 3b. From the reaction of rac-1 (0.93 g, 1.79 mmol)
with L-Tyr ethyl ester (3.2 g, 15.32 mmol) and separa-
tion of the diastereoisomeric mixture by column chro-
matography, the (ꢀ)-gossypol bis(L-tyrosine ethyl
ester) Schiffꢀs base (ꢀ)-3b was isolated and recrystallised
from CHCl3 as bright yellow crystals (0.27 g, 16%); Rt
(min) 4.11; [a]22 ꢀ824.4 (c 0.09, CH3OH); mp 129–
134 ꢁC; mmax (KBr)/cmꢀ1 3478, 3436 (OH, NH), 1739
4.1.1.1. Gossypol bis(L-phenylalanine methyl ester)
Schiffꢀs bases 3a. From the reaction of rac-1 (1.18 g,
2.21 mmol) with L-Phe methyl ester (2.21 g, 11.83 mmol)
and separation of the diastereoisomeric mixture by col-
umn chromatography, the (ꢀ)-gossypol bis(L-phenylala-
nine methyl ester) Schiffꢀs base (ꢀ)-3a was isolated as a
yellow powder in 39% yield; Rf = 0.53 in 1:3 hexane/
ether; Rt (min) 6.08; [a]25 ꢀ838.9 (c 0.12, DCM); mp
112–116 ꢁC; mmax (KBr)/cmꢀ1 3478, 3438 (OH, NH),
1747 (C@O), 1612 (C@C); 1H NMR (270 MHz, CDCl3)
d 1.51 (12H, d, J = 7.3 Hz, CH(CH3)2), 2.06 (6H, s, Ar–
1
(C@O), 1612 (C@C); H NMR (270 MHz, CDCl3) d
1.26 (6H, t, J = 7.3 Hz, COOCH2CH3), 1.51 (12H, d,
J = 7.3 Hz, (CH3)2CH), 2.04 (6H, s, Ar–CH3), 3.00
(2H, dd, J = 13.85, 9.2 Hz, CH2-a), 3.24 (2H, dd,
J = 13.85, 4.0 Hz, CH2-b), 3.68 (2H, septet, J = 7.3 Hz,
CH(CH3)2), 4.12 (4H, q, J = 7.3 Hz, COOCH2CH3),
4.20 (2H, t, J = 7.3 Hz, a-CH), 5.41 (2H, s, 1,10-OH),
6.10 (2H, s, 2 · 400-OH), 6.63 (4H, d, J = 7.9 Hz,
2 · 300,500-H), 6.96 (4H, d, J = 7.9 Hz, 2 · 200,600-H), 7.54

K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
4233
(2H, s, 4,40-H), 7.88 (2H, broad s, 6,60-OH), 9.31 (2H, s,
CH@N), 13.56 (2H, broad s, 7,70-OH); 1H NMR
(500 MHz, DMSO-d6) d 1.18 (6H, t, J = 7.1 Hz,
COOCH2CH3), 1.44 (12H, 2 · d, J = 7.0 Hz, (CH3)2CH),
1.94 (6H, s, Ar–CH3), 3.11 (4H, m, CH2), 3.69 (2H,
septet, J = 7.0 Hz, CH(CH3)2), 4.15 (4H, q, J = 7.1 Hz,
COOCH2CH3), 4.68 (2H, q, J = 8.0 Hz, a-CH), 6.64
(4H, d, J = 8.5 Hz, 2 · 300,500-H), 7.00 (4H, d, J =
8.5 Hz, 2 · 200,600-H), 7.45 (2H, s, 4,40-H), 7.85 (2H, s,
2 · 400-OH), 8.43 (2H, s, 6,60-OH), 9.25 (2H, s, 1,10-
OH), 9.75 (2H, d, J = 12.5 Hz, @CH–NH), 13.43 (2H,
dd, J = 12.5, 8.0 Hz, @CH–NH); 13C NMR (75 MHz,
CDCl3) d 14.5 (2 · COOCH2CH3) 20.4 (4 · CH3), 20.7
(2 · CH3), 27.8 (2 · CH), 39.5 (2 · CH2), 62.6
(2 · OCH2), 64.8 (2 · a-CH), 104.1 (2 · quat., C-8),
114.8 (2 · quat., C-8a), 116.1 (4 · CH, 2 · 300,500-CH),
116.2 (2 · quat., C-2), 118.2 (2 · CH, C-4), 127.3
(2 · quat., C-400), 128.3 (2 · quat., C-4a), 129.1
(2 · quat., C-5), 131.1 (4 · CH, 2 · 200,600-CH), 132.5
(2 · quat., C-3), 147.4 (2 · quat., C-1), 149.3 (2 · quat.,
C-6), 155.4 (2 · quat., C-100), 156.0 (2 · quat., C-7),
162.0 (2 · CH@N), 170.2 (2 · quat., ester C@O). Anal.
Calcd for C52H56O12N2Æ1/2EtOAc: C, 68.6; H, 6.35; N,
3.0. Found: C, 68.5; H, 6.2; N, 3.05.
cmꢀ1 3475, 3388 (OH, NH), 1751 (ester C@O), 1733
1
(C@O), 1606 (C@C); H NMR (270 MHz, CDCl3) d
1.53 (12H, d, J = 7.3 Hz, (CH3)2CH), 1.99 (6H, s, Ar–
CH3), 3.20 (2H, dd, J = 14.5, 9.9 Hz, CH2-a), 3.60
(2H, dd, J = 14.5, 3.3 Hz, CH2-b), 3.70 (2H, septet,
J = 7.3 Hz, CH(CH3)2), 3.84 (6H, s, COOCH3), 4.46
(2H, m, a-CH–CH2), 4.66 (2H, s, 1,10-OH), 6.73–7.03
(8H, m, Ar–H), 7.50 (2H, s, 4,40-H), 7.57 (2H, d,
J = 7.9 Hz, indole 2-H), 7.82 (2H, s, 6,60-OH), 7.95
(2H, broad s, indole NH), 9.07 (2H, d, J = 9.9 Hz,
@CH–NH), 13.48 (2H, broad, CH–NH); 1H NMR
(300 MHz, DMSO)
d 1.45 (12H, d, J = 6.9 Hz,
(CH3)2CH), 1.95 (6H, s, Ar–CH3), 3.4 (4H, broad m,
CH2), 3.67 (6H, s, COOCH3), 3.73 (2H, m, (CH3)2CH),
4.82 (2H, m, a-CH), 6.95 (2H, t, J = 7.9 Hz, 2 · 500-H),
7.04 (2H, t, J = 8.0 Hz, 2 · 600-H), 7.16 (2H, d,
J = 2.3 Hz, 2 · 200-H), 7.33 (2H, d, J = 8.0 Hz, 2 · 700-
H), 7.46 (2H, s, 4,40-H), 7.49 (2H, d, J = 7.9 Hz,
2 · 400-H), 7.83 (2H, s, 1,10-OH), 8.46 (2H, s, 6,60-OH),
9.79 (2H, d, J = 12.5 Hz, CH–NH), 10.95 (2H, d,
J = 2.1 Hz, indole NH), 13.51 (2H, dd, J = 12.5,
8.0 Hz, CH–NH); 13C NMR (75 MHz, DMSO-d6) d
21.0 (2 · CH3), 21.1 (4 · CH3), 27.4 (2 · CH), 29.8
(2 · CH2), 53.4 (2 · OCH3), 62.9 (2 · a-CH), 104.7
(2 · quat., C-8), 108.6 (2 · quat., C-300), 112.3 (2 · CH,
C-500), 116.5 (2 · quat., C-8a), 117.5 (2 · CH, C-400),
118.9 (2 · CH, C-4), 119.4 (2 · CH, C-600), 121.1
(2 · quat., C-2), 121.9 (2 · CH, C-700), 125.1 (2 · CH,
C-200), 127.8 (2 · quat., C-3a’’), 128.00 (2 · quat., C-5),
128.03 (2 · quat., C-4a), 132.4 (2 · quat., C-3), 136.9
(2 · quat., C-7a00), 147.1 (2 · quat., C-1), 150.5
(2 · quat., C-6), 162.1 (2 · CH–NH), 171.7 (2 · quat.,
ester C@O), 173.5 (2 · quat., C@O, C-7). Anal. Calcd
for C54H54O10N4Æ1/2EtOAc: C, 69.85; H, 6.0; N, 5.8.
Found: C, 69.6; H, 5.9; N, 5.9.
The (+)-gossypol bis(L-tyrosine ethyl ester) Schiffꢀs base
(+)-3b was isolated an orange-brown powder (0.35 g,
21%); Rt (min) 2.56; [a]22 +393.6 (c 0.05, CH3OH); mp
122–127 ꢁC; mmax (KBr)/cmꢀ1 3478 (OH), 1739 (ester
C@O), 1612 (C@C); 1H NMR (270 MHz, CDCl3) d
1.26 (6H, t, J = 7.3 Hz, COOCH2CH3), 1.50 (12H,
2 · d, J = 6.6 Hz, (CH3)2CH), 2.06 (6H, s, Ar–CH3),
3.10 (2H, dd, J = 13.85, 7.9 Hz, CH2-a), 3.20 (2H, dd,
J = 13.85, 5.3 Hz, CH2-b), 3.70 (2H, septet, J = 6.6 Hz,
CH(CH3)2), 4.12 (4H, q, J = 7.3 Hz, COOCH2CH3),
4.20 (2H, dd, J = 7.9, 5.3 Hz, a-CH–CH2), 5.49 (2H, s,
1,10-OH), 6.10 (2H, s, 2 · 400-OH), 6.67 (4H, d,
J = 8.6 Hz, 2 · 300,500-H), 6.97 (4H, d, J = 8.6 Hz,
2 · 200,600-H), 7.55 (2H, s, Ar–H), 7.88 (2H, broad s,
6,60-OH), 9.35 (2H, s, CH@N), 13.51 (2H, broad s,
The (+)-gossypol bis(L-tryptophan methyl ester) Schiffꢀs
base (+)-3c(L) eluted second and recrystallised from
CHCl3 as yellow crystals (0.4 g, 25%); Rt (min) 3.89;
[a]18.5 +250 (c 0.02, CH3OH); mp 148–153 ꢁC; mmax
(KBr)/cmꢀ1 3471, 3434 (OH, NH), 1743 (ester C@O),
7,70-OH); 13C NMR (75 MHz, CDCl3)
d
14.5
1
(2 · COOCH2CH3), 20.4 (4 · CH3), 20.7 (2 · CH3),
27.8 (2 · CH), 39.5 (2 · CH2), 62.6 (2 · O–CH2), 64.8
(2 · a-CH), 104.1 (2 · quat., C-8), 114.8 (2 · quat., C-
8a), 116.1 (4 · CH, 2 · 300,500-CH), 116.2 (2 · quat., C-
2), 118.2 (2 · CH, C-4), 127.3 (2 · quat., C-400), 128.3
(2 · quat., C-4a), 129.5 (2 · quat., C-5), 131.0 (4 · CH,
2 · 200,600-CH), 132.5 (2 · quat., C-3), 147.4 (2 · quat.,
C-1), 149.4 (2 · quat., C-6), 155.5 (2 · quat., C-100),
156.0 (2 · quat., C-7), 162.0 (2 · CH@N), 170.2
(2 · quat., ester C@O). Anal. Calcd for C52H56O12N2Æ
2EtOAc: C, 66.9; H, 6.7; N, 2.6. Found: C, 66.7; H,
6.1; N, 3.0.
1612 (C@O); H NMR (300 MHz, DMSO) d 1.43 (6H,
d, J = 6.65 Hz, (CH3)2CH), 1.45 (6H, d, J = 6.0 Hz,
(CH3)2CH), 1.93 (6H, s, Ar–CH3), 3.38 (4H, broad m,
CH2), 3.67 (6H, s, COOCH3), 3.73 (2H, m, (CH3)2CH),
4.77 (2H, m, a-CH), 6.94 (2H, t, J = 7.9 Hz, 2 · 500-H),
7.00 (2H, t, J = 7.8 Hz, 2 · 600-H), 7.15 (2H, d,
J = 2.3 Hz, 2 · 200-H), 7.30 (2H, d, J = 7.9 Hz, 2 · 700-
H), 7.45 (2H, s, 4,40-H), 7.48 (2H, d, J = 7.8 Hz,
2 · 400-H), 7.55 (2H, s, 1,10-OH), 8.41 (2H, s, 6,60-OH),
9.65 (2H, d, J = 12.5 Hz, CH–NH), 10.90 (2H, d,
J = 2.1 Hz, indole NH), 13.47 (2H, dd, J = 12.5,
8.0 Hz, CH–NH); 13C NMR (75 MHz, DMSO-d6) d
20.7 (2 · CH3), 20.8 (4 · CH3), 27.1 (2 · CH), 29.6
(2 · CH2), 53.1 (2 · OCH3), 62.6 (2 · a-CH), 104.3
(2 · quat., C-8), 108.3 (2 · quat., C-300), 112.0 (2 · CH,
C-500), 116.1 (2 · quat., C-8a), 117.2 (2 · CH, C-400),
118.6 (2 · CH, C-4), 119.1 (2 · CH, C-600), 120.6
(2 · quat., C-2), 121.6 (2 · CH, C-700), 124.9 (2 · CH,
C-200), 127.5 (2 · quat., C-3a00), 127.6 (2 · quat., C-5),
127.7 (2 · quat., C-4a), 132.1 (2 · quat., C-3), 136.7
(2 · quat., C-7a00), 146.8 (2 · quat., C-1), 150.2
(2 · quat., C-6), 161.9 (2 · CH–NH), 171.4 (2 · quat.,
4.1.1.3. Gossypol bis(L-tryptophan methyl ester)
Schiffꢀs bases 3c(L). From the reaction of rac-1 (0.91 g,
1.76 mmol) with L-Trp methyl ester (2.00 g, 9.16 mmol)
and separation of the diastereoisomeric mixture by col-
umn chromatography, the (ꢀ)-gossypol bis(L-trypto-
phan methyl ester) Schiffꢀs base (ꢀ)-3c(L) eluted first
and recrystallised from hexane/ethyl acetate (70:30) as
yellow crystals (0.4 g, 25%); Rt (min) 5.55; [a]21
ꢀ1175.2 (c 0.05, DCM); mp 230–235 ꢁC; mmax (KBr)/

4234
K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
ester C@O), 173.2 (2 · quat., C@O, C-7). Anal. Calcd
for C54H54O10N4Æ1/2EtOAc: C, 69.85; H, 6.0; N, 5.8.
Found: C, 69.8; H, 5.9; N, 5.95.
s, 6,60-OH), 9.65 (2H, d, J = 12.6 Hz, CH–NH), 10.90
(2H, d, J = 2.1 Hz, indole NH), 13.47 (2H, dd,
J = 12.5, 7.9 Hz, CH–NH); 13C NMR (75 MHz,
DMSO-d6) d 21.1 (6 · CH3), 27.4 (2 · CH), 29.8
(2 · CH2), 53.4 (2 · OCH3), 62.9 (2 · a-CH), 104.6
(2 · quat., C-8), 108.5 (2 · quat., C-300), 112.3 (2 · CH,
C-500), 116.4 (2 · quat., C-8a), 117.5 (2 · CH, C-400),
118.9 (2 · CH, C-4), 119.4 (2 · CH, C-600), 121.1
(2 · quat., C-2), 121.9 (2 · CH, C-700), 125.2 (2 · CH,
C-200), 127.8 (2 · quat., C-3a00), 127.9 (2 · quat., C-5),
128.06 (2 · quat., C-4a), 132.4 (2 · quat., C-3), 137.0
(2 · quat., C-7a00), 147.1 (2 · quat., C-1), 156.0
(2 · quat., C-6), 162.2 (2 · CH–NH), 171.7 (2 · quat.,
ester C@O), 173.2 (2 · quat., C@O, C-7). Anal. Calcd
for C54H54O10N4Æ1/2EtOAc: C, 69.85; H, 6.0; N, 5.8.
Found: C, 69.8; H, 5.9; N, 5.9.
4.1.1.4. Gossypol bis(D-tryptophan methyl ester)
Schiffꢀs bases 3c(D). From the reaction of rac-1 (0.98 g,
1.89 mmol) with D-Trp methyl ester (2.37 g,
10.86 mmol) and separation of the diastereoisomeric
mixture by column chromatography, the (+)-gossypol
bis(D-tryptophan methyl ester) Schiffꢀs base (+)-3c(D)
eluted first and recrystallised from hexane/ethyl acetate
(70:30) as yellow crystals (0.4 g, 24%); Rt (min) 5.33;
[a]16.5 +1180 (c 0.05, DCM); mp 230–235 ꢁC; mmax
(KBr)/cmꢀ1 3471, 3421 (OH, NH), 1793 (ester C@O),
1733 (C@O), 1608 (C@C); 1H NMR (270 MHz, CDCl3)
d 1.53 (12H, 2 · d, J = 7.3 Hz, (CH3)2CH), 1.99 (6H, s,
Ar–CH3), 3.24 (2H, dd, J = 14.5, 9.9 Hz, CH2-a), 3.60
(2H, dd, J = 14.5, 4.0 Hz, CH2-b), 3.70 (2H, septet,
J = 7.3 Hz, CH(CH3)2), 3.83 (6H, s, COOCH3), 4.47
(2H, m, a-CH–CH2), 4.67 (2H, s, 1,10-OH), 6.73–7.02
(8H, m, Ar–H), 7.50 (2H, s, 4,40-H), 7.55 (2H, d,
J = 7.9 Hz, indole 2-H), 7.81 (2H, s, 6,60-OH), 7.95
(2H, broad s, indole NH), 9.07 (2H, d, J = 11.2 Hz,
4.1.2. General procedure for the synthesis of gossypol half
Schiffꢀs bases 4a–c by partial hydrolysis and gossypol
atropisomers 1 by complete hydrolysis. The correspond-
ing gossypol bis Schiffꢀs base 3 was dissolved in the min-
imum amount of diethyl ether and glacial acetic acid
and stirred at 0 ꢁC in the dark. To the cooled solution,
concentrated H2SO4 and distilled H2O were added and
the reaction mixture was left to stir at 0 ꢁC in the dark.
The progress of the hydrolysis was monitored by re-
verse-phase HPLC, using a mobile phase of acetoni-
trile/aq HCOONH4 (80:20, pH = 6.3) at k = 254 nm
and a flow rate of 1.0 mL/min. The reaction was stopped
by the addition of aqueous saturated NaHCO3 solution,
in order to neutralise the acid. The ether layer was sep-
arated, washed with distilled H2O, dried over MgSO4,
filtered and evaporated to dryness to give either the par-
tial hydrolysis mixture or the gossypol atropisomer. The
half Schiffꢀs base was separated from the partial hydro-
lysis product mixture by column chromatography on sil-
ica, eluting with hexane/ether (100:0–25:75) for (ꢀ)-4a,
and hexane/ethyl acetate (100:0–0:100) for (+)-4a, (ꢀ)-
4b and (ꢀ)-4c(L).
1
@CH–NH), 13.49 (2H, broad s, CH–NH); H NMR
(300 MHz, DMSO-d6) d 1.44 (12H, d, J = 6.8 Hz,
(CH3)2CH), 1.94 (6H, s, Ar–CH3), 3.37 (4H, broad m,
CH2), 3.67 (6H, s, COOCH3), 3.72 (2H, m, (CH3)2CH),
4.80 (2H, m, a-CH), 6.95 (2H, t, J = 7.8 Hz, 2 · 500-H),
7.04 (2H, t, J = 8.0 Hz, 2 · 600-H), 7.16 (2H, d,
J = 2.2 Hz, 2 · 200-H), 7.32 (2H, d, J = 8.0 Hz, 2 · 700-
H), 7.45 (2H, s, 4,40-H), 7.48 (2H, d, J = 7.8 Hz,
2 · 400-H), 7.82 (2H, s, 1,10-OH), 8.45 (2H, s, 6,60-OH),
9.78 (2H, d, J = 12.6 Hz, CH–NH), 10.94 (2H, d,
J = 2.1 Hz, indole NH), 13.50 (2H, dd, J = 12.5,
7.9 Hz, CH–NH); 13C NMR (75 MHz, DMSO-d6) d
21.1 (6 · CH3), 27.4 (2 · CH), 29.8 (2 · CH2), 53.4
(2 · OCH3), 62.9 (2 · a-CH), 104.7 (2 · quat., C-8),
108.7 (2 · quat., C-300), 112.3 (2 · CH, C-500), 116.5
(2 · quat., C-8a), 117.6 (2 · CH, C-400), 118.9 (2 · CH,
C-4), 119.4 (2 · CH, C-600), 121.1 (2 · quat., C-2),
121.9 (2 · CH, C-700), 125.1 (2 · CH, C-200), 127.8
(2 · quat., C-3a00), 128.01 (2 · quat., C-5), 128.06
(2 · quat., C-4a), 132.4 (2 · quat., C-3), 136.9 (2 · quat.,
C-7a00), 147.1 (2 · quat., C-1), 150.5 (2 · quat., C-6),
162.1 (2 · CH–NH), 171.7 (2 · quat., ester C@O),
173.5 (2 · quat., C@O, C-7). Anal. Calcd for
C54H54O10N4Æ1/2EtOAc: C, 69.85; H, 6.0; N, 5.8.
Found: C, 69.95; H, 5.9; N, 5.9.
4.1.2.1. (ꢀ)-Gossypol half(L-phenylalanine methyl
ester) Schiffꢀs base (ꢀ)-4a. This was obtained from the
reaction of (ꢀ)-3a (0.17 g, 0.20 mmol) in ether
(10 mL), glacial acetic acid (2 mL), concentrated
H2SO4 (0.34 mL) and distilled H2O (0.68 mL), for 7 h.
Yellow solid (0.07 g, 51%); Rt (min) 0.90 (94% purity);
[a]24 ꢀ383.3 (c 0.06, DCM); mp 100–105 ꢁC; mmax
(KBr)/cmꢀ1 3480 and 3436 (OH, NH), 1745 (ester
C@O), 1612 (C@O); 1H NMR (270 MHz, CDCl3) d
ꢀ4.72 (1H, s, 70-OH), 1.54 (6H, d, J = 7.25 Hz,
(CH3)2CH), 1.57 (6H, d, J = 6.6 Hz, (CH3)2CH), 2.08
(3H, s, Ar–CH3), 2.13 (3H, s, Ar–CH3), 3.15 (1H, dd,
J = 13.85, 8.6 Hz, CH2-a), 3.31 (1H, dd, J = 13.85,
8.6 Hz, CH2-b), 3.71 (1H, m, (CH3)2CH), 3.77 (3H, s,
COOCH3), 3.90 (1H, m, (CH3)2CH), 4.30 (1H, m, a-
CH), 5.28 (1H, s, 1-OH), 5.72 (1H, s, 10-OH), 6.45
(1H, s, 60-OH), 7.11–7.27 (5H, m, Ar–H), 7.58 (1H, s,
4-H), 7.75 (1H, s, 40-H), 7.94 (1H, s, 6-OH), 9.31 (1H,
d, J = 11.9 Hz, HN–CH@), 11.18 (1H, s, CHO), 13.61
(1H, broad s, HN–CH@); 13C NMR (75 MHz, CDCl3)
d 20.4 (4 · CH3), 20.7 (Ar–CH3), 20.8 (Ar–CH3), 28.0
(2 · CH), 40.5 (CH2), 53.4 (OCH3), 64.9 (a-CH), 104.1
(quat., C-8), 112.2 (quat., C-80), 114.9 (quat., C-8a),
The (ꢀ)-gossypol bis(D-tryptophan methyl ester) Schiffꢀs
base (ꢀ)-3c(D) eluted second and recrystallised from
CHCl3 as yellow crystals (0.4 g, 24%); Rt (min) 3.68;
[a]18.5 ꢀ250 (c 0.04, CH3OH); mp 147–152 ꢁC; mmax
(KBr)/cmꢀ1 3471, 3442 (OH, NH), 1741 (ester C@O),
1
1612 (C@O); H NMR (300 MHz, DMSO-d6) d 1.43
(6H, d, J = 6.0 Hz, (CH3)2CH), 1.45 (6H, d,
J = 5.9 Hz, (CH3)2CH), 1.93 (6H, s, Ar–CH3), 3.38
(4H, broad m, CH2), 3.68 (6H, s, COOCH3), 3.73 (2H,
m, (CH3)2CH), 4.77 (2H, m, a-CH), 6.95 (2H, t,
J = 7.7 Hz, 2 · 500-H), 7.00 (2H, t, J = 7.8 Hz, 2 · 600-
H), 7.15 (2H, d, J = 2.4 Hz, 2 · 200-H), 7.31 (2H, d,
J = 7.9 Hz, 2 · 700-H), 7.45 (2H, s, 4,40-H), 7.49 (2H, d,
J = 7.8 Hz, 2 · 400-H), 7.55 (2H, s, 1,10-OH), 8.41 (2H,

K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
4235
115.7 (quat., C-80a), 116.5 (2 · quat., 2,20-C), 118.0
(2 · CH, 4,40-CH), 127.7 (CH, C-400), 128.2 (2 · quat.,
C-4a), 129.3 (2 · CH, 200,600-CH), 129.8 (2 · CH, 300,500-
CH), 129.9 (2 · quat., 5,50-C), 132.2 (2 · quat., 3,30-C),
135.5 (quat., C-100), 147.5 (quat., C-1), 149.2 (quat., C-
6), 150.8 (quat., C-10), 150.9 (quat., C-60), 156.5 (quat.,
C-70), 162.0 (CHNH), 170.5 (quat., ester C@O), 173.9
(quat., C@O, C-7), 199.9 (CHO). HRMS: calcd for
C40H42O9N: 680.2854; found 680.2849.
6.97 (2H, d, J = 7.9 Hz, 200,600-H), 7.57 (1H, s, 4-H),
7.75 (1H, s, 40-H), 7.93 (1H, s, 6-OH), 9.30 (1H, d,
J = 9.2 Hz, @CH–NH), 11.15 (1H, s, CHO), 13.55
(1H, broad s, @CH–NH); 13C NMR (75 MHz, CDCl3)
14.5 (COOCH2CH3), 20.4 (4 · CH3), 20.7 (2 · CH3),
28.0 (2 · CH), 42.0 (CH2), 62.6 (OCH2), 67.0 (a-CH),
104.1 (quat., C-8), 112.2 (quat., C-80), 114.8 (quat., C-
8a), 115.8 (quat., C-8a0), 116.1 (2 · CH, 300,500-CH),
116.6 (2 · quat., 2,20-C), 118.0 (2 · CH, 4,40-C), 127.0
(quat., C-400), 128.0 (2 · quat., 4a,4a0-C), 129.1
(2 · quat., 5,50-C), 131.1 (2 · CH, 200,600-CH), 132.0
(2 · quat., 3,30-C), 147.5 (quat., C-1), 149.4 (quat., C-
6), 150.8 (quat., C-10), 150.9 (quat., C-60), 155.5 (quat.,
C-100), 156.0 (quat., C-7), 156.5 (quat., C-70), 162.0
(CH@N), 170.0 (quat., ester C@O), 199.2 (CHO).
HRMS: calcd for C41H44O10N: 710.2959. Found:
710.2957.
4.1.2.2. (+)-Gossypol half(L-phenylalanine methyl
ester) Schiffꢀs base (+)-4a. This was obtained from the
reaction of (+)-3a (0.37 g, 0.44 mmol) in ether (15 mL),
glacial acetic acid (4.5 mL), concentrated H2SO4
(1.5 mL) and distilled H2O (1.5 mL), for 7 h. Dark yel-
low solid (0.04 g, 13%); Rt (min) 1.05 (96% purity);
[a]20 +266.7 (c 0.03, DCM); mp 113–117 ꢁC; mmax
(KBr)/cmꢀ1 3484 and 3434 (OH, NH), 1747 (ester
C@O), 1614 (C@O); 1H NMR (270 MHz, CDCl3) d
ꢀ4.84 (1H, s, 70-OH), 1.53 (12H, 2 · d, J = 7.25 Hz,
(CH3)2CH), 2.07 (3H, s, Ar–CH3), 2.13 (3H, s, Ar–
CH3), 3.11 (1H, dd, J = 13.85, 8.6 Hz, CH2-a), 3.31
(1H, dd, J = 13.85, 4.6 Hz, CH2-b), 3.67 (1H, septet,
J = 7.25 Hz, (CH3)2CH), 3.74 (3H, s, COOCH3), 3.90
(1H, septet, J = 7.25 Hz, (CH3)2CH), 4.28 (1H, m, a-
CH), 5.41 (1H, s, 1-OH), 5.92 (1H, s, 10-OH), 6.45
(1H, s, 60-OH), 7.12–7.27 (5H, m, Ar–H), 7.56 (1H, s,
4-H), 7.78 (1H, s, 40-H), 7.89 (1H, s, 6-OH), 9.26 (1H,
d, J = 7.9 Hz, HN–CH@), 11.11 (1H, s, CHO), 13.55
(1H, broad s, HN–CH@); 13C NMR (75 MHz, CDCl3)
d 20.4 (4 · CH3), 20.7 (Ar–CH3), 20.8 (Ar–CH3), 25.7
(2 · CH), 40.5 (CH2), 53.4 (OCH3), 64.9 (a-CH), 104.1
(quat., C-8), 112.2 (quat., C-80), 114.9 (quat., C-8a),
115.7 (quat., C-80a), 116.5 (2 · quat., 2,20-C), 118.0
(2 · CH, 4,40-CH), 127.7 (CH, C-400), 128.2 (2 · quat.,
C-4a), 129.3 (2 · CH, 200,600-CH), 129.8 (2 · CH, 300,500-
CH), 129.9 (2 · quat., 5,50-C), 132.2 (2 · quat., 3,30-C),
135.5 (quat., C-100), 147.5 (quat., C-1), 149.2 (quat., C-
6), 150.8 (quat., C-10), 150.9 (quat., C-60), 156.5 (quat.,
C-70), 162.0 (CH–NH), 170.5 (quat., ester C@O), 173.9
(quat., C@O, C-7), 199.9 (CHO). Anal. Calcd for
C40H41O9N.3EtOAc: C, 66.2; H, 6.9; N, 1.5. Found:
C, 66.6; H, 6.4; N, 1.65; HRMS: calcd for C40H42O9N:
680.2854. Found: 680.2852.
4.1.2.4. (ꢀ)-Gossypol half(L-tryptophan methyl ester)
Schiffꢀs base (ꢀ)-4c(L). This was obtained from the reac-
tion of (ꢀ)-3c(L) (0.36 g, 0.39 mmol) in ether (7 mL),
glacial acetic acid (4.3 mL), concentrated H2SO4
(1 mL) and distilled H2O (1.8 mL) at ꢀ20 ꢁC for 23 h.
Red-orange powder (0.08 g, 29%); Rt (min) 0.88 (93%
purity); [a]20 ꢀ321 (c 0.02, DCM); mp 115–120 ꢁC; mmax
(KBr)/cmꢀ1 3458 (OH, NH), 1749 (ester C@O), 1631
(aldehyde C@O), 1610 (C@C); 1H NMR (270 MHz,
CDCl3)
d
ꢀ4.76 (1H, s, 70-OH), 1.50 (6H, d,
J = 6.6 Hz, (CH3)2CH), 1.58 (6H, d, J = 7.3 Hz,
(CH3)2CH), 2.04 (6H, s, Ar–CH3), 3.20 (1H, dd,
J = 14.5, 9.2 Hz, CH2-a), 3.50 (1H, dd, J = 14.5,
4.0 Hz, CH2-b), 3.70 (1H, m, (CH3)2CH), 3.80 (3H, s,
OCH3), 3.90 (1H, m, (CH3)2CH), 4.43 (1H, broad, a-
CH), 4.70 (1H, s, 1-OH), 5.69 (1H, s, 10-OH), 6.45
(1H, s, 60-OH), 6.76–7.02 (4H, m, Ar–H), 7.52 (3H, m,
4,40-H and 200-CH), 7.75 (1H, s, 6-OH), 8.0 (1H, broad
s, indole NH), 8.96 (1H, s, CH@N), 11.22 (1H, s,
CHO), 13.43 (1H, s, 7-OH); 13C NMR (75 MHz,
CDCl3) d 20.4 (4 · CH3), 20.7 (Ar–CH3), 20.8 (Ar–
CH3), 28.0 (2 · CH), 30.1 (CH2), 56.0 (OCH3), 65.0
(a-CH), 104.1 (quat., C-8), 112.2 (quat., C-80), 113.0
(quat., C-300), 114.9 (quat., C-8a), 115.8 (quat., C-8a0),
116.1 (CH, C-500), 116.2 (quat., C-2), 116.7 (quat.,
C-20), 118.6 (2 · CH, 4,40-C), 122.5 (CH, C-400), 123.5
(CH, C-600), 127.4 (quat., C-3a00), 128.3 (2 · quat.,
4a,4a0-C), 129.7 (quat., C-5), 129.9 (quat., C-50), 130.1
(CH, C-700), 131.1 (CH, C-200), 132.2 (2 · quat., 3,30-C),
134.5 (quat., C-7a00), 147.5 (quat., C-1), 149.3 (quat.,
C-6), 150.8 (quat., C-10), 150.9 (quat., C-60), 155.4
(quat., C-7), 156.5 (quat., C-70), 162.0 (CH@N), 170.1
(quat., ester C@O), 199.8 (CHO). HRMS: calcd for
C42H43O9N2: 719.2963. Found: 719.2965.
4.1.2.3. (ꢀ)-Gossypol half(L-tyrosine ethyl ester)
Schiffꢀs base (ꢀ)-4b. This was obtained from the reaction
of (ꢀ)-3b (0.63 g, 0.70 mmol) in ether (110 mL), glacial
acetic acid (7.5 mL), concentrated H2SO4 (5.1 mL) and
distilled H2O (7.0 mL), for 7 h. Dark orange solid
(0.14 g, 28%); Rt (min) 0.80 (92% purity); [a]20 ꢀ514 (c
0.14, DCM); mp 90–95 ꢁC; mmax (KBr)/cmꢀ1 3438
(OH, NH), 1739 (ester C@O), 1614 (aldehyde C@O);
1H NMR (270 MHz, CDCl3) d ꢀ4.84 (1H, s, 70-OH),
1.26 (3H, t, J = 7.3 Hz, COOCH2CH3), 1.53 (12H,
2 · d, J = 6.6 Hz, (CH3)2CH), 2.07 (3H, s, Ar–CH3),
2.13 (3H, s, Ar–CH3), 3.00 (1H, dd, J = 13.85, 8.6 Hz,
CH-a), 3.20 (1H, dd, J = 13.85, 4.6 Hz, CH2-b), 3.69
(1H, septet, J = 6.6 Hz, (CH3)2CH), 3.90 (1H, septet,
J = 6.6 Hz, (CH3)2CH), 4.12 (2H, q, J = 7.3 Hz,
COOCH2CH3), 4.22 (1H, m, a-CH), 5.43 (1H, s, 1-
OH), 5.75 (1H, broad, 400-OH), 5.91 (1H, s, 10-OH),
6.46 (1H, s, 60-OH), 6.65 (2H, d, J = 7.9 Hz, 300,500-H),
4.1.2.5. (ꢀ)-Gossypol (ꢀ)-1. This was obtained from
the complete hydrolysis of (ꢀ)-3b (0.35 g, 0.39 mmol)
in ether (80 mL), glacial acetic acid (4.2 mL), concen-
trated H2SO4 (5 mL) and distilled H2O (6 mL), for
21 h. Yellow-orange crystalline solid (0.19 g, 95%);
[a]18.5 ꢀ358.5 (c 0.05, CH3OH) (lit. [a]29 ꢀ363.6; c
0.20, CH3OH39); mp 135–140 ꢁC (lit. mp 166–
167 ꢁC40); (Found: C, 69.3; H, 6.0. C30H30O8 requires
C, 69.5; H, 5.8); mmax (KBr)/cmꢀ1 3498 and 3434
1
(OH), 1617 (C@O), 1614 (C@C); H NMR (270 MHz,

4236
K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
CDCl3) d ꢀ4.86 (2H, s, 7,70-OH), 1.55 (12H, d,
J = 7.25 Hz, CH(CH3)2), 2.15 (6H, s, Ar–CH3), 3.89
(2H, septet, J = 7.25 Hz, CH(CH3)2), 5.86 (2H, s, 1,10-
OH), 6.42 (2H, s, 6,60-OH), 7.78 (2H, s, 4,40-H), 11.13
(2H, s, CHO). Enantiomeric identification was based
upon the sign of the optical rotation, and reverse-phase
HPLC using a packed analytical chiral column (250
mm · 4.6 mm i.d.) with a mobile phase of acetonitrile/
aq 0.01 M KH2PO4 (70:30) at k = 254 nm and a flow
rate of 1.0 mL/min (Rt = 7.09 min). Racemic gossypol
in this HPLC method gave two peaks at Rt = 7.09 min
and Rt = 9.01 min.
of bovine pituitary extract, epidermal growth factor,
hydrocortisone, insulin and transferrin. Cells were al-
lowed to grow to 60–80% confluence under conditions
of 5% CO2 at 37 ꢁC.
A stock solution of each compound was prepared in
1 mL of DMSO followed by dilution with fresh medium.
Two 96 well plates were set up for each compound with
six in-plate replicates of the eight concentrations on the
plate. Cell solution (200 lL, 5 · 104 cells/mL,
1 · 104 cells/well) was plated (rows 2–10, B–G) and ini-
tially incubated for 24 h, after which the media were re-
moved and 100 lL of media containing 1% DMSO was
added. Stock solution (200 lL) was then added to row 2
and serially half diluted across to row 9. Plates were
incubated at 5% CO2 at 37 ꢁC for 3 days, after which
the cells were washed with 200 lL PBS, 200 lL of
0.5 mg/mL MTT solution was added to each well, and
the cells were incubated for 2 h at 37 ꢁC at 5% CO2.
The MTT solution was then removed, the wells were
washed again with PBS, 200 lL of 90% IPA/10%
DMSO solution was added and the plates incubated in
the dark for 10 min. Absorbance readings were taken
at 570 nm using an Anthos III microplate spectropho-
tometer. The data were processed with the aid of Micro-
soft Excel 2000 and the % growth at each compound
concentration was calculated as follows:
4.1.2.6. (+)-Gossypol (+)-1. This is obtained from the
complete hydrolysis of (+)-3a (0.16 g, 0.19 mmol) in
concentrated H2SO4/acetic acid/ether (2.5:1:1) (10 mL)
for 20 h. Dark green solid (0.09 g, 91.5%);
[a]19 +348.25 (c 0.02, CH3OH) (lit. [a]22 +359; c 0.05,
CHCl340); mp. 135–140 ꢁC (lit. mp 166–167 ꢁC40);
(Found: C, 69.2; H, 6.0. C30H30O8 requires C, 69.5; H,
5.8); mma1x (KBr)/cmꢀ1 3496 and 3436 (OH), 1617
(C@O); H NMR (270 MHz, CDCl3) d ꢀ4.84 (2H, s,
7,70-OH), 1.55 (12H, d, J = 7.3 Hz, (CH3)2CH), 2.16
(6H, s, Ar–CH3), 3.90 (2H, septet, J = 7.3 Hz,
CH(CH3)2), 5.84 (2H, s, 1,10-OH), 6.43 (2H, s, 6,60-
OH), 7.79 (2H, s, 4,40-H), 11.14 (2H, s, CHO). Re-
verse-phase HPLC with the chiral column and method
used previously gave an Rt (min.) of 9.01.
ꢀ
ꢁ
ðCompound ꢀ BlankÞ
ðControl ꢀ BlankÞ
%Growth ¼ 100
4.1.3. Preparation of gossypolone rac-2. A solution of
racemic gossypol acetic acid 1 (0.5 g, 0.865 mmol) in
acetone (25 mL) and acetic acid (50 mL) was heated
on a steam bath at 100 ꢁC during the careful addition
of a 10% aq solution of FeCl3Æ6H2O (30 mL, 15 mmol).
The mixture was left to stir at 100 ꢁC for 2 h, then al-
lowed to cool. Distilled water (63 mL) was added to pre-
cipitate a dark iron-containing compound, which was
removed by filtration under gravity, and treated with a
mixture of ether (200 mL) and 20% aq H2SO4
(200 mL). The liberated phenol partitioned into the
ether layer, which was separated, dried over MgSO4
and evaporated to dryness (0.43 g, 91%). This product
was recrystallised from aq acetic acid and the resulting
orange crystals of gossypolone rac-2 were collected by
filtration under vacuum (0.28 g, 59%); mp 256–260 ꢁC
(lit.22 mp 264 ꢁC); (Found: C, 65.5; H, 5.2. C30H26O10
requires C, 65.9; H, 4.8%); mmax (KBr)/cmꢀ1 3492
(OH), 1635 (C@O); 1H NMR (270 MHz, CDCl3) d
1.45 (12H, 2 · d, J = 6.6 Hz, CH(CH3)2), 2.15 (6H, s,
Ar–CH3), 4.11 (2H, septet, J = 6.6 Hz, CH(CH3)2),
6.67 (2H, s, 6,60-OH), 10.57 (2H, s, CHO), 13.00 (2H,
s, 7,70-OH).
Compound is the mean absorbance of the inhibitor and
the cell culture, blank is the absorbance of the media
alone and control is the absorbance of the media with
cells. GI50 was defined as the compound concentration,
which caused 50% inhibition of the growth of the treated
cells compared to the control cells. The GI50 values were
obtained from LOWESS analysis of % growth versus
logarithmic concentration (molar) of the compound,
using PRISM GraphPad.
4.2.2. Anti-oxidant assay. Chemicals were purchased
from BDH and Sigma–Aldrich. The liposomes were pre-
pared as a 5 mg/mL suspension of bovine brain extract
(Sigma) in PBS and were stored at ꢀ80 ꢁC. Ascorbic
acid and ferric chloride (1 mM aqueous solutions) were
freshly prepared before each test. A stock solution of
each compound was prepared in 10 mL ethanol and
six serial 1 in 10 dilutions were made. Four replicates
were carried out for each reaction mixture, resulting in
68 tubes (Sterilin) per assay (blank, full reaction mixture
(FRM), positive control and seven compound concen-
trations with and without the FRM).
4.2. Biological testing
Stock solution (0.1 mL) and each of the six dilutions
were added into eight consecutive tubes each, arranged
into two rows of four replicates. Liposomes (0.2 mL),
0.5 mL PBS, 0.1 mL FeCl3 solution and 0.1 mL of
ascorbic acid solution were added in the first row of each
concentration (test compound). Distilled water (0.4 mL)
and 0.5 mL of PBS were added in the second row of
each concentration (compound alone). The positive con-
trol tubes contained 0.1 mL propyl gallate, 0.2 mL of
liposomes, 0.5 mL of PBS, 0.1 mL FeCl3 solution and
4.2.1. Anti-proliferative MTT assay. Chemicals were
purchased from Sigma–Aldrich and PBS from Gibco.
Manipulation of the cells was conducted in a Class II
Safety Cabinet under aseptic conditions, at Stiefel Lab-
oratories, Maidenhead, UK. The HPV-16 cells (Ameri-
can Tissue Culture Collection) were cultured in a
mixture of Epilife liquid (Cascade Ltd., Mansfield,
UK) with keratinocyte growth supplement, consisting

K. Dodou et al. / Bioorg. Med. Chem. 13 (2005) 4228–4237
4237
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tained 0.1 mL of ethanol, 0.2 mL of liposomes, 0.5 mL
of PBS, 0.1 mL FeCl3 solution and 0.1 mL of ascorbic
acid solution. The blank reaction mixture tubes con-
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%Inhibition
ꢀ
ꢁ
ððFRM ꢀ BlankÞ ꢀ ðTest ꢀ Compound ꢀ BlankÞÞ
ðFRM ꢀ BlankÞ
¼ 100
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