Telomerase Inhibitors from Cyanobacteria
FULL PAPER
droxylation of the allyl group in 6 afforded the desired diol
7 in a yield of 75% as a mixture of two diastereoisomers
(d.r.=1.2:1). As the biological activity of the mixture was
almost identical to that of the single diastereoisomers (see
below), we proceeded with the synthesis. The acylation
of diol 7 with different fatty acids in the presence of 1-ethyl-
organisms, such as cyanobacteria, higher plants, mosses,
ferns, or algae.[21] Recently it was suggested that they inter-
fere with DNA synthesis in the producing cyanobacteri-
um.[35] It is reasonable to assume that the telomerase activity
found in the lipophilic fractions of Synechococus sp. PCC
6911 and in the cyanobacterial crude extracts might general-
ly be caused by SQDGs. However, the presence of other
metabolites with telomerase activity cannot be ruled out at
this point. This study should therefore encourage further
screening of extracts of photosynthetic organisms for more
potent telomerase inhibitors.
3-(3-dimethylaminopropyl)carbodiimide
hydrochloride
(EDCI) and 4-dimethylaminopyridine (DMAP) provided
diesters 8a–i. All of the fatty acids used were commercially
available except for 12a,b and 13a,b, which were prepared
from the corresponding aldehydes 11a and 11b in two steps.
Thus, carboxylic acids 12a and 12b were obtained by a
Knoevenagel condensation reaction between the aldehyde
and malonic acid followed by hydrogenation of the resulting
a,b-unsaturated carboxylic acid with Pd/C in ethanol. Fatty
acids 13a and 13b were synthesized by a Wittig reaction fol-
lowed by catalytic hydrogenation of the resulting alkene in-
termediates. Following the acylation of 7 with the fatty
acids, oxidation of the thioacetate group in 8a–i with Oxone
in acetic acid gave the sodium sulfonate derivatives 9a–i. Fi-
nally, deprotection of the benzyl groups in 9a–i by hydro-
Conclusion
This broad study has encompassed different aspects, such as
screening, fermentation of cyanobacteria, isolation of me-
tabolites, structural characterization, and an SAR study car-
ried out by the synthesis of natural product analogues to
identify inhibitors of telomerase. In particular, we have
demonstrated that the Telospot assay is well suited to the
screening of natural product libraries. Among the 27 differ-
ent strains of cyanobacteria evaluated, we have identified
and characterized sulfoquinovosyl diacylglycerols (SQDG)
obtained from Microcystis aeruguinosa PCC 7806 by 2D
NMR and MS/MS analyses. We also prepared pure SQDG 2
by chemical synthesis, which showed an activity against telo-
merase of 17 mm. In addition, to enable SAR studies on this
class of natural products, we synthesized a series of com-
pounds of which 10i proved to be the most active. Interest-
ingly, the introduction of more bulky and more lipophilic
substituents led to higher activity. Although SQDGs are
known to inhibit various DNA polymerases, such as HIV re-
verse transcriptase, their mode of action is poorly under-
stood. In addition, the presence of SQDGs in Spirulina, a
cyanobacterium used as a food supplement, as well as in
many other plant-based food ingredients, and their role in
telomerase inhibition and the chemoprevention of cancer
should be investigated.
ACHTUNGTRENNUNGgenACHTUNGTRENNUNGation with palladium in ethanol yielded the target deriv-
atives 10a–i.
These analogues were tested in the telomerase assay (see
Figure S2 in the Supporting Information). Compounds 10a
and 10b bearing C6 and C7 acyl chains, respectively, did not
show any inhibition up to a concentration of 200 mm. In fact,
a fatty acid with at least nine carbon atoms is necessary for
the inhibition of telomerase, as displayed by compound 10c
(IC50 =50 mm). Increasing the number of carbon atoms in
the side-chain led to greater inhibition (10d: IC50 =29 mm;
10e: IC50 =36 mm). The influence of a terminal alkylphenyl
group strongly depended on the chain length: A shorter
alkyl chain yielded a less potent inhibitor (10 f: IC50 =
31 mm), whereas a significant increase in potency was ob-
served for 10g (IC50 =20 mm). The absolute configuration at
C-2 of the glycerol unit does not appear to influence the in-
hibition values significantly. Compound (2S)-10 f was ob-
tained by the stereoselective dihydroxylation of olefin 6
with AD-mix-a (d.r.>8:1) and subsequent transformations
as detailed in Scheme 1. Similar inhibition values were ob-
tained for (2S)-10 f as for the mixture. Finally, IC50 values of
14 and 11 mm were measured for 10h and 10i, respectively,
which shows that a two-fold improvement in the inhibition
was obtained by the introduction of a biphenyl motif.
The structure of the putative Tribolium castaneum telo-
merase catalytic subunit TERT was recently solved by X-ray
crystallographic analysis,[32] which showed that this TERT
candidate is similar to HIV reverse transcriptases, viral
RNA polymerases, and b-family DNA polymerases. It is
therefore not surprising that SQDGs were also found to in-
hibit HIV reverse transcriptases.[19a,b,33] On the other hand,
free fatty acids, particularly polyunsaturated ones, have
shown telomerase inhibition.[25,28] SQDGs, first isolated from
Chlorella by Benson et al. in 1959,[34] are associated with the
thylakoid membrane and are found in most photosynthetic
Experimental Section
General procedure for the acylation of glycol 7: EDCI (2.38 mmol,
2.5 equiv), DMAP (1.52 mmol, 1.6 equiv), and the corresponding fatty
acid (2 equiv) were added to a solution of diol 7 (0.95 mmol, 1 equiv) in
dry dichloromethane (40 mL). The reaction mixture was stirred at room
temperature for 20 h and then diluted with water (20 mL). The aqueous
phase was extracted with dichloromethane (3ꢃ20 mL) and the combined
organic layers were washed with brine (1ꢃ20 mL), dried over Na2SO4,
and concentrated under reduced pressure. The crude product was puri-
fied by flash column chromatography (SiO2, pentane/AcOEt) to afford a
colorless oil.
General procedure for oxidation and deprotection: Potassium acetate
(14.9 mmol, 20 equiv) and Oxone (1.86 mmol, 2.5 equiv) were added to a
solution of acylated glycol 8 (0.75 mmol, 1 equiv) in glacial acetic acid
(10 mL). After 16 h at room temperature, the resulting mixture was dilut-
ed with water (40 mL). The aqueous phase was extracted with ethyl ace-
tate (3ꢃ80 mL) and the combined organic layers were washed with satu-
Chem. Eur. J. 2013, 00, 0 – 0
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Brahmi, Malika Makhlouf
