Neoglycorandomization has led to increases in anticancer
efficacy of the cardenolide digitoxin,9a,c mechanistic alter-
ation and improvements in the synergistic effects of the
nonglycosylated alkaloid colchicine,2 and enhancements in
the potency of the glycopeptide vancomycin against antibiotic-
resistant organisms.9b
Pichette and coworkers revealed that the attachment of
saccharides at C3 moderately improved the antiproliferative
activity (up to 4-fold) and selectivity of BA in a sugar-
dependent manner.15 However, only D-Ara, D-Gal, D-Glc,
D-Man, L-Rha, and D-Xyl were employed in this initial study,
and antiviral activity was not assessed. To more systemati-
cally assess the impact of BA glycosylation upon both
anticancer activity/selectivity and antiviral activity in parallel,
herein we report the synthesis and anticancer/antiviral
activities of a 37-member library of BA C3-neoglycosides.
Our findings indicate that groups of BA derivatives with
improved antitumor or antiviral properties are divergent and
sugar-dependent.
The initial strategy for methoxyamine handle installation
at C3 involved reduction of imine 4 (created from 2)14a using
BH3·t-BuNH2 to give a 3:1 ratio of desired (5) to undesired
diastereomers (see Scheme 1). However, neoglycosylation
Figure 1. Structures of betulinic acid (1), betulin (2), and
3-aminobetulinic acid (3).
Scheme 1. Attempted Direct Neoglycosylation of Betulinic Acid
Many natural products are known to exhibit multiple, diverse
biological activities.10 To assess the impact of differential
glycosylation upon a natural product with known multiple
activities, we selected the lupane-type triterpernoid betulinic acid
(BA, 1) as a model. BA and its reduced form (betulin, 2) exhibit
a wide variety of biological functions, the most prevalent of
which are anticancer and anti-HIV activities.11 In cancer cells,
BA induces apoptosis through multiple mechanisms, including
disruption of the mitochondrial membrane potential and sup-
pression of vascular endothelial growth factor and survivin
proteins.12 Although the exact mechanism of BA anti-HIV
activity has yet to be elucidated, many BA analogs disrupt viral
fusion to host cells through interference with the gp41 viral
glycoprotein or function as inhibitors of the late stage of capsid
protein maturation.13
Although BA derivatization (primarily at C3 and/or C28)
has yielded anti-HIV or antitumor enhancements,11,14 few
glycosylated BAs have been pursued or studied. Studies by
of 5 failed possibly because of the hindered adjacent C4
dimethyl substitution. Consistent with this, aglycon 5 was
also resistant to acetylation conditions (i.e., Ac2O, DMAP,
refluxing pyridine).
(9) (a) Langenhan, J. M.; Peters, N. R.; Guzei, I. A.; Hoffman, M. A.;
Thorson, J. S. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 12305–12310. (b)
Griffith, B. R.; Krepel, C.; Fu, X.; Blanchard, S.; Ahmed, A.; Edmiston,
C. E.; Thorson, J. S. J. Am. Chem. Soc. 2007, 129, 8150–8155. (c)
Langenhan, J. M.; Engle, J. M.; Slevin, L. K.; Fay, L. R.; Lucker, R. W.;
Smith, K. R.; Endo, M. M. Bioorg. Med. Chem. Lett. 2008, 18, 670–673.
(10) (a) Deorukhkar, A.; Krishnan, S.; Sethi, G.; Aggarwal, B. B. Exp.
Opin. InVest. Drugs 2007, 16, 1753–1773. (b) Butler, M. S. Nat. Prod.
Rep. 2008, 25, 475–516. (c) Udenigwe, C. C.; Ramprasath, V. R.; Aluko,
R. E.; Jones, P. J. H. Nutr. ReV. 2008, 8, 445–454. (d) Saladino, R.;
Gualandi, G.; Farina, A.; Crestini, C.; Nencioni, L.; Palamara, A. T. Curr.
Med. Chem. 2008, 15, 1500–1519.
Previously, colchicine neoglycosylation was enabled by
replacing the natural colchicine N-acetyl group with N-(N′-
methoxyglycine).2 While not a direct neoglycosylation of
BA, we postulated that a similar methoxyglycine handle
would distance the hindered BA C4 quaternary center from
the requisite neoglycosylation alkoxyamine. Toward this
goal, 1 (prepared in three steps from 2)16 was esterified at
the C3 hydroxyl group using chloroacetyl chloride in the
presence of DMAP. Under Finklestein conditions, the
chloride (6) was exchanged with iodide to facilitate the SN2
(11) For reviews, see: (a) Cichewicz, R. H.; Kouzi, S. A. Med. Res.
ReV. 2004, 24, 90–114. (b) Yogeeswari, P.; Sriram, D. Curr. Med. Chem.
2005, 12, 657–666. (c) Sami, A.; Taru, M.; Salme, K.; Jari, Y.-K. Eur.
J. Pharm. Sci. 2006, 29, 1–13. (d) Fulda, S. Int. J. Mol. Sci. 2008, 9, 1096–
1107.
(12) (a) Schmidt, M. L.; Kuzmanoff, K. L.; Ling-Indeck, L.; Pezzuto,
J. M. Eur. J. Cancer 1997, 33, 2007–2010. (b) Fulda, S.; Jeremias, I.;
Steiner, H. H.; Pietsch, T.; Debatin, K.-M. Int. J. Cancer 1999, 82, 435–
441. (c) Chintharlapalli, S.; Papineni, S.; Ramaiah, S. K.; Safe, S. Cancer
Res. 2007, 67, 2816–2823.
(14) (a) Kim, D. S. H. L.; Pezzuto, J. M.; Pisha, E. Bioorg. Med. Chem.
Lett. 1998, 8, 1707–1712. (b) Kashiwada, Y.; Chiyo, J.; Ikeshiro, Y.; Nagao,
T.; Okabe, H.; Cosentino, L. M.; Fowke, K.; Lee, K. H. Bioorg. Med. Chem.
Lett. 2001, 11, 183–185.
(15) (a) Gauthier, C.; Legault, J.; Lebrun, M.; Dufour, P.; Pichette, A.
Bioorg. Med. Chem. 2006, 14, 6713–6725. (b) Thibeault, D.; Gauthier, C.;
Legault, J.; Bouchard, J.; Dufour, P.; Pichette, A. Bioorg. Med. Chem. 2007,
15, 6144–6157.
(13) (a) Li, F.; Goila-Gaur, R.; Salzwedel, K.; Kilgore, N. R.; Reddick,
M.; Matallana, C.; Castillo, A.; Zoumplis, D.; Martin, D. E.; Orenstein,
J. M.; Allaway, G. P.; Freed, E. O.; Wild, C. T. Proc. Natl. Acad. Sci.
U.S.A. 2003, 100, 13555–13560. (b) Aiken, C.; Chen, C. H. Trends Mol.
Med. 2005, 11, 31–36.
(16) Kim, D. S. H. L.; Chen, Z.; Nguyen, V. T.; Pezzuto, J. M.; Qiu,
S.; Lu, Z.-Z. Synth. Commun. 1997, 27, 1607–1612.
462
Org. Lett., Vol. 11, No. 2, 2009