N-methylation of the C3′ amide of taxanes: Synthesis of N-methyltaxol C and N-methylpaclitaxel

Article abstract of DOI:10.1021/jo800173h

(Chemical Equation Presented) A method has been developed for the methylation of the C3′ amide of taxol C and paclitaxel. Taxol C and paclitaxel were sequentially silylated at the 2′, 7, and 1-hydroxyl groups with tert-butyldimethylsilyl chloride, triethy

Full text of DOI:10.1021/jo800173h

Taxane anticancer agents that are isolated from natural sources
and from cell culture production have to be separated from
structurally closely related taxanes that represent impurities.
Such impurities, if and when present, need to be characterized
in New Drug Applications (NDA) to the FDA. Depending on
the source of the natural taxane, one such impurity is N-
methyltaxol C (Scheme 1). N-methyltaxol C was first reported
by Ma et al. as a constituent of a cell culture of T. baccata in
0.00022% of the biomass and also from the roots of Taxus x
media Rehd. cv Hicksii.7,8 Later, 7-(ꢀ-xylosyl)-N-methyltaxol
C was isolated from T. yunnanensis.9 N-Methyltaxol C was
found to possess similar activity in a tubulin assembly assay
compared to paclitaxel.7
N-Methylation of the C3′ Amide of Taxanes:
Synthesis of N-Methyltaxol C and
N-Methylpaclitaxel
Hari K. R. Santhapuram, Apurba Datta, Oliver E. Hutt, and
Gunda I. Georg*
Department of Medicinal Chemistry, UniVersity of Kansas,
1251 Wescoe Hall DriVe, Lawrence, Kansas 66045-7582
georg239@umn.edu
ReceiVed January 22, 2008
The presence of N-methyltaxol C as a potential impurity in
clinically used taxanes, produced from natural sources and cell
culture, and its low availability from natural sources, prompted
us to investigate the synthesis of this compound via N-alkylation
of the more readily available natural product taxol C. Although
the C13 phenyl isoserinate side chain of paclitaxel has been
the focus of intense structure-activity relationship investiga-
tions,5 to the best of our knowledge, there are no reports
involving N-alkylation studies at the C-3′ amide of the taxane
C13 side chain.
A method has been developed for the methylation of the
C3′ amide of taxol C and paclitaxel. Taxol C and paclitaxel
were sequentially silylated at the 2′, 7, and 1-hydroxyl groups
with tert-butyldimethylsilyl chloride, triethylsilyl chloride,
and dimethylsilyl chloride, respectively. Subsequent reaction
with potassium tert-butoxide and methyl iodide provided the
corresponding N-methylated taxane derivatives. Removal of
the silyl protecting groups furnished N-methyltaxol C and
N-methylpaclitaxel.
In order to avoid undesirable reactions at the C2′ and C7
hydroxy groups of taxol C during the proposed N-alkylation
studies, the prior protection of the above hydroxyl groups was
necessary. Following a known protocol,10 taxol C was converted
to the corresponding 7-TES-2′-TBS derivative 1a in high overall
yield. Subsequent attempts to methylate the 3′-amide of 7-TES-
2′-TBS-taxol C under a variety of conditions did not result in
the formation of any N-alkylated product. Alkylation attempts
were made using different bases such as NaH, LDA, NaHMDS,
LiHMDS, and silver oxide (Ag2O) and methylating agents MeI
or dimethyl sulfate (Me2SO4). The two different solvents used
in these attempts were THF and DMF. The above reactions were
carried out in temperature ranges from -78 °C to room
temperature.
In order to eliminate the possible interference of the (typically
unreactive) free tertiary hydroxyl group at C1 during the
attempted N-deprotonation/alkylation sequence, protection of the
C1 hydroxyl group followed by N-alkylation studies of the
resulting triprotected taxol C was next investigated. Therefore,
the tertiary hydroxyl group at the C1-position of 7-TES-2′-TBS-
taxol C was protected as its dimethylsilyl ether derivative 3
(Scheme 1).11 Subsequent potassium tert-butoxide-assisted
deprotonation at the amide functionality of 3, followed by
quenching of the resulting anion with excess methyl iodide led
to the formation of the corresponding N-methyl derivative 5 in
Tubulin-binding taxanes such as paclitaxel and docetaxel are
important cancer chemotherapeutic agents. However, these drugs
suffer from limitations such as poor aqueous solubility and oral
bioavailability,1,2 emerging drug resistance,3 and the lack of
blood-brain barrier permeability.4 For these reasons, there is a
continuous need for the development of novel taxanes that can
overcome the limitations of the first-generation taxanes.5
Methods for producing taxane anticancer agents include
isolation from the bark of Taxus breVifolia and other taxus
species, plant tissue culture, and the semisynthesis of the taxanes
using naturally occurring baccatin III and related analogues as
building blocks for the diterpene part of the taxanes.6
DeriVatiVes; Farina, V., Ed; Elsevier: Amsterdam, 1995; pp 103-130..and its
(1) Vyas, D. M. In The Chemistry and Pharmacology of Taxol
(2) Sparreboom, A.; Asperen, J. v.; Mayer, U; Schinkel, A. H; Smit, J. W;
Meijer, D. K. F; Borst, P; Nooijen, W. J; Beijnen, J. H; Tellingen, O. v. Proc.
Natl. Acad. Sci. U.S.A. 1997, 94, 1031–1035.
(7) Ma, W.; Park, G. L.; Gomez, G. A.; Nieder, M. H.; Adams, T. L.;
Aynsley, J. S.; Sahai, O. P.; Smith, R. J.; Stahlhut, R. W.; et al. J. Nat. Prod.
1994, 57, 116–122.
(3) Ojima, I.; Slater, J. C.; Michaud, E.; Kuduk, S. D.; Bounaud, P.-Y.;
Vrignaud, P.; Bissery, M. C.; Veith, J. M.; Pera, P.; Bernacki, R. J. J. Med.
Chem. 1996, 39, 3889–3896.
(8) Barboni, L.; Gariboldi, P.; Torregiani, E.; Appendino, G.; Gabetta, B.;
Bombardelli, E. Phytochemistry 1994, 36, 987–990.
(4) Rice, A.; Liu, Y.; Michaelis, M. L.; Himes, R. H.; Georg, G. I.; Audus,
K. J. Med. Chem. 2005, 48, 832–838.
(9) Li, S.-H.; Zhang, H.-J.; Niu, X.-M.; Yao, P.; Sun, H.-D.; Fong, H. H. S.
Tetrahedron 2003, 59, 37–45.
(5) Ganesh, T. Bioorg. Med. Chem. 2007, 15, 3597–3623.
(6) Kingston, D. G. I. In Anticancer Agents from Natural Products; Cragg,
G. M., Kingston, D. G. I., Newman, D. J., Eds; Taylor and Francis: Boca Raton,
2005; pp 89-122..
(10) Georg, G. I.; Ali, S. M.; Boge, T. C.; Datta, A.; Falborg, L.; Himes,
R. H. Tetrahedron Lett. 1994, 35, 8931–8934.
(11) Chen, S.-H.; Farina, V.; Vyas, D. M.; Doyle, T. W.; Long, B. H.;
Fairchild, C. J. Org. Chem. 1996, 61, 2065–2070.
10.1021/jo800173h CCC: $40.75
Published on Web 05/23/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 4705–4708 4705

SCHEME 1. Synthesis of N-Methyltaxol C (7) and N-Methylpaclitaxel (8)
60%. Standard aqueous acidic workup of the above reaction
mixture resulted in clean deprotection of the labile dimethylsilyl
protecting group at C1, directly providing N-Me-7-TES-2′-TBS-
taxol C (5). Simultaneous removal of the TBS and TES
protecting groups with HF in pyridine completed the synthesis
of N-methyltaxol C (7). Multiple gram quantities of N-
methyltaxol C were easily synthesized using the above method.
Experimental Section
2′-tert-Butyldimethylsilyl-7-triethylsilyltaxol C (1a). A solution
of taxol C (3.39 g, 4 mmol), TBSCl (3.02 g, 20 mmol), imidazole
(1.9 g, 28 mmol), and DMAP (25 mg, cat.) in anhydrous CH2Cl2
(125 mL) was stirred at room temperature for 18 h. The mixture
was diluted via addition of anhydrous CH2Cl2 (125 mL) followed
by addition of pyridine (2.4 mL, 30 mmol), DMAP (25 mg, cat.)
and TESCl (3.4 mL, 20 mmol). The resulting mixture was stirred
at room temperature for another 3 h and then quenched by the
addition of water (50 mL). The organic layer was separated, and
the aqueous layer was extracted with chloroform (2 × 50 mL).
The combined organic extract was washed with brine, dried over
anhydrous Na2SO4, and concentrated. The residual oily liquid on
trituration with hexane afforded the pure product 1a as a colorless
solid (4 g, 93%): mp 130-134 °C; 1H NMR (400 MHz, CDCl3) δ
-0.28 and -0.09 (2s, 6H), 0.58-0.62 (m, 6H), 0.78 (s, 9H), 0.83
(t, J ) 4.5 Hz, 3H), 0.94 (t, J ) 7.8 Hz, 9H), 1.22-1.27 (m, 12H),
1.54-1.58 (m, 1H), 1.71 (s, 1H), 1.85 (s, 1H), 1.88-1.92 (m, 1H),
2.04 (s, 3H), 2.12-2.17 (m, 1H), 2.19 (s, 3H), 2.21-2.25 (m, 2H),
2.32-2.41 (m, 1H), 2.47-2.53 (m, 1H), 2.56 (s, 3H), 3.84 (d, J )
7.0 Hz, 1H), 4.20, 4.31 (ABq, J ) 8.4 Hz, 2H), 4.48 (dd, J )
10.5, 6.6 Hz, 1H), 4.57 (d, J ) 2.2 Hz, 1H), 4.95 (br d, J ) 8.3
Hz, 1H), 5.56 (d, J ) 9.5 Hz, 1H), 5.71 (d, J ) 7.1 Hz, 1H), 6.27
(t, J ) 8.9 Hz, 1H), 6.34 (d, J ) 9.1 Hz, 1H), 6.47 (s, 1H),
7.25-7.61 (series of m, 8H), 8.12 (d, J ) 8.1 Hz, 2H); 13C NMR
(100 MHz, CDCl3) δ 201.8, 172.7, 171.4, 170.2, 169.3, 167.1,
140.3, 138.5, 133.6, 130.3, 129.2, 128.8, 128.6, 127.8, 126.5, 84.2,
81.2, 78.9, 75.2, 74.97, 72.2, 58.4, 55.0, 46.7, 43.4, 36.6, 31.3, 26.6,
25.5, 25.4, 23.1, 22.3, 20.9, 18.2, 14.3, 13.9, 10.2, 6.8, 5.3, -5.4,
-5.8. HRMS (FAB-POS) m/e calcd for C58H86NO14Si2 [M+ + 1]:
1076.5587, found 1076.5607.
1-Dimethylsilyl-2′-tert-butyldimethylsilyl-7-triethylsilyltaxol C
(3). To an ice-cold solution of 2′-tert-butyldimethylsilyl-7-trieth-
ylsilyltaxol C (1a) (7.76 g, 7.22 mmol) and imidazole (3.44 g, 50.54
mmol) in DMF (70 mL) was added very slowly dimethylchlorsilane
(4.01 mL, 36.09 mmol). The reaction mixture was allowed to warm
to 20 °C and was stirred overnight. The reaction mixture was
quenched with crushed ice, and extracted with ethyl acetate (3 ×
250 mL). The combined organic layers were washed with brine,
dried over MgSO4, concentrated in vacuo and the residue was
charged onto a silica gel column. Elution with ethyl acetate/hexane
(1:4) furnished 1-dimethylsilyl-2′-tert-butyldimethylsilyl-7-trieth-
ylsilyltaxol-C (6.54 g, 80%) as a colorless solid: mp 116-120 °C;
1H NMR (400 MHz, CDCl3) δ -0.28 (s, 3H), -0.24 (d, J ) 2.8
The optical rotation of our product was higher ([R]D -61
25
(CHCl3, c 0.27) than what has been reported for the natural
product ([R]D25 -53 (CHCl3, c 1.0)8 and [R]D25 -16.5 (MeOH,
c 0.103).9 In addition, we found that the melting point of our
product (146-148 °C, amorphous solid) was different from the
reported melting points of 225-228 °C (dec from MeOAc/
hexane)8 and 134 °C (dec from MeOH). We therefore recrystal-
lized a sample of N-methyltaxol C (7) from MeOAc/hexane
and found the same melting point of 146-148 °C that we had
observed for the amorphous material.
Having developed an efficient route to a 3′-N-methyl taxane,
it was considered of interest to also extend the above method
toward synthesizing the hitherto unreported N-methylated pa-
clitaxel. Following the same procedure as described above, initial
conversion of paclitaxel (2) to the corresponding trisilyl
protected derivative 4, followed by potassium tert-butoxide
mediated deprotonation and subsequent N-methylation resulted
in the formation of the desired 7-TES-2′-TBS-N-methylpacli-
taxel derivative 6 (Scheme 1) in 40% yield (determined by 1H
NMR analysis) accompanied by 20% of 2′-tert-butyldimethyl-
silyl-7-triethylsilylpaclitaxel (2a), obtained from deprotection
of the labile dimethylsilyl protecting group at C1 of unreacted
intermediate 4. We found that it was difficult to separate
N-methylated 6 from 2a by silica gel chromatography. Removal
of the silyl protecting groups of a purified sample of 6 completed
the synthesis of N-methylpaclitaxel (8).
In conclusion, a reaction protocol has been developed for the
syntheses of 3′-N-methylated taxanes. During these studies, prior
protection of the C1 tertiary hydroxyl group at the taxane core
was found to be a critical requirement to achieve the desired
alkylation. The new method is providing access to a new class
of N-alkyltaxanes.
4706 J. Org. Chem. Vol. 73, No. 12, 2008

Hz, 3H), -0.11 (s, 3H), 0.09 (d, J ) 2.7 Hz, 3H), 0.64-0.50 (m,
6H), 0.77 (s, 9H), 0.88-0.79 (m, 2H), 0.93 (t, J ) 7.9 Hz, 9H),
1.18 (s, 3H), 1.24 (s, 3H), 1.21-1.28 (m, 4H), 1.52-1.60 (m, 2H),
1.70 (s, 3H), 1.77 (s, 1H), 1.84-1.98 (m, 1H), 2.01 (s, 3H), 2.18
(s, 3H), 2.22 (t, J ) 7.4 Hz, 2H), 2.35-2.55 (m, 3H), 2.57 (s, 3H),
3.84 (d, J ) 7.0 Hz, 1H), 4.27 (s, 2H), 4.45 (dd, J ) 10.6, 6.6 Hz,
1H), 4.52-4.62 (m, 2H), 4.94 (d with st, J ) 7.9 Hz, 1H), 5.66 (d,
J ) 9.3 Hz, 1H), 5.77 (d, J ) 7.0 Hz, 1H), 6.26 (t, J ) 9.4 Hz,
1H), 6.31 (d, J ) 9.3 Hz, 1H), 6.45 (s, 1H), 7.22-7.42 (series of
m, 5H), 7.44-7.62 (m, 3H), 8.14 (d, J ) 7.1 Hz, 2H); 13C NMR
(100 MHz, CDCl3) δ 201.8, 172.58, 171.5, 170.3, 169.4, 165.6,
139.9, 138.5, 133.6, 133.4, 130.4, 130.1, 128.7, 127.9, 126.5, 84.4,
81.6, 81.4, 75.5, 75.4, 75.0, 72.3, 71.4, 58.4, 54. 9, 46.5, 44.4, 37.3,
36.5, 34.1, 31.4, 27.5, 25.6, 25.3, 23.2, 22.4, 22.2, 21.0, 18.3, 14.3,
13.9, 10.1, 6.8, 5.4, 0.5, -0.04, -5.4, -5.9; IR (KBr) 2956, 2133,
1728, 1684, 1493, 1370, 1245, 1114, 1067, 1025, 989, 904, 838,
809, 782, 745, 710, 618 cm-1; HRMS (FAB-POS) m/e calcd for
C60H92NO14Si3 [M+ + 1]: 1134.5826, found 1134.5861.
N-Methyl-2′-tert-butyldimethylsilyl-7-triethylsilyltaxol C (5).
In a flame-dried RB flask, 1-dimethylsilyl-2′-tert-butyldimethylsilyl-
7-triethylsilytaxol C (2) (5.3 g, 4.68 mmol) was placed under an
argon blanket. THF (50 mL, dried over benzophenone ketyl) was
added and cooled to -78 °C. Potassium tert-butoxide (7.02 mL,
7.02 mmol, 1.0 M solution in THF) was added very slowly and
stirred for 30 min at -78 °C. Excess MeI (7.28 mL, 117 mmol)
was added and stirred for an additional 2 h at -78 °C. The reaction
mixture was warmed slowly to 0 °C over 1 h, and quenched with
saturated NH4Cl solution. The mixture was stirred at 20 °C for 30
min and extracted with ethyl acetate (2 × 150 mL). The combined
organic layers were washed with 10% HCl and brine, dried over
MgSO4, and concentrated under reduced pressure. The residue was
charged onto a silica gel column, and gradient elution with ethyl
acetate/hexane (1:9 to 1:5) furnished N-methyl-2′-tert-butyldim-
ethylsilyl-7-triethylsilyltaxol C (3.06 g, 60%): mp 138-140 °C;
1H NMR (400 MHz, CDCl3) δ 0.14 (s, 3H), 0.15 (s, 3H), 0.52-0.62
(m, 6H), 0.84 (t, J ) 6.9 Hz, 3H), 0.89 (s, 9H), 0.94 (t, J ) 7.9
Hz, 9H), 1.19 (s, 3H), 1.22 (s, 3H), 1.20-1.32 (m, 4H), 1.52-1.62
(m, 2H), 1.70 (s, 3H), 1.64-1.76 (m, 3H), 1.82-2.00 (m, 3H),
2.17 (s, 3H), 2.32 (t, J ) 7.6 Hz, 2H), 2.30-2.40 (m, 1H), 2.46 (s,
3H), 2.45-2.60 (m, 1H), 2.94 (s, 3H), 3.82 (d, J ) 7.0 Hz, 1H),
4.18, 4.28(ABq, J ) 8.3 Hz, 2H), 4.45 (dd, J ) 10.4, 6.7 Hz, 1H),
4.95 (d, J ) 8.3 Hz, 1H), 5.07 (br d, J ) 5.1 Hz, 1H), 5.68 (d, J
) 7.0 Hz, 1H), 6.15 (t, J ) 8.8 Hz, 1H), 6.32 (br s, 1H), 6.41 (s,
1H), 7.25-7.39 (series of m, 5H), 7.48 (m, 2H), 7.60 (m, 1H),
8.12 (d, J ) 7.3 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 202.1,
174.2, 171.8, 170.5, 169.3, 167.0, 140.7, 136.5, 133.6, 133.2, 130.3,
129.4, 128.7, 128.6, 128.1, 128.0, 84.4, 80.8, 78.8, 76.7, 75.2, 75.0,
73.5, 72.3, 71.1, 58.5, 46.7, 43.3, 37.3, 35.4, 33.8, 31.7, 26.5, 25.7,
24.8, 22.8, 22.5, 21.3, 20.9, 18.1, 14.2, 14.0, 10.1, 6.8, 5.4, -4.4,
-5.2; IR (KBr) 3452, 2956, 1727, 1651, 1452, 1392, 1370, 1238,
1110, 1018, 987, 884, 837, 822, 780, 730, 709, 619 cm-1; HRMS
(FAB-POS) m/e calcd for C59H88NO14Si2 [M+ + 1]: 1090.5743,
found 1090.5726.
1H), 4.16, 4.28 (ABq, J ) 8.4 Hz, 2H), 4.35 (d, J ) 8.0 Hz, 1H),
4.42 (m, 1H), 4.89 (dd, J ) 8.0, 3.9 Hz, 1H), 4.93 (d, J ) 9.5 Hz,
1H), 5.68 (d, J ) 7.0 Hz, 1H), 5.76 (d, J ) 3.5 Hz, 1H), 6.16 (t,
J ) 8.7 Hz, 1H), 6.30 (s, 1H), 7.30-7.45 (series of m, 5H), 7.48
(dd, J ) 7.7, 7.7 Hz, 1H), 7.61 (dd, J ) 7.3, 7.3 Hz, 2H), 8.10 (d,
J ) 7.5 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 203.9, 175.1,
173.5, 171.2, 170.2, 166.8, 142.6, 136.6, 133.6, 132.8, 130.2, 129.3,
128.9, 128.6, 128.1, 84.4, 80.8, 78.9, 76.4, 75.7, 75.0, 72.7, 72.1,
72.0, 61.3, 58.5, 45.6, 43.2, 35.7, 35.6, 35.2, 34.0, 31.5, 26.7, 24.7,
22.4, 22.2, 21.9, 20.9, 14.8, 13.9, 9.6; IR (KBr) 3434, 2935, 1726,
1636, 1451, 1371, 1244, 1115, 1025, 982, 619 cm-1; HRMS (FAB-
POS) m/e calcd for C47H60NO14 [M+ + 1]: 862.4014, found
862.4027. [R]D -61 (CHCl3, c 0.27).
25
1-Dimethylsilyl-2′-tert-butyldimethylsilyl-7-triethylsilylpacli-
taxel (4). To an ice-cold solution of 2′-tert-butyldimethylsilyl-7-
triethylsilylpaclitaxel10 (10.34 g, 9.57 mmol) and imidazole (4.55
g, 66.96 mmol) in DMF (90 mL) was added very slowly
dimethylchlorosilane (5.31 mL, 47.83 mmol). The reaction mixture
was allowed to warm to 20 °C and was stirred overnight, quenched
with crushed ice, and extracted with ethyl acetate (3 × 300 mL).
The combined organic layers were washed with brine, dried over
MgSO4, and concentrated in vacuo, and the residue was charged
onto a silica gel column. Elution with ethyl acetate/hexane (1:4)
furnished 1-dimethylsilyl-2′-tert-butyldimethylsilyl-7-triethylsilylpa-
1
clitaxel (10.35 g, 95%) as a colorless solid: mp 208-209 °C; H
NMR (400 MHz, CDCl3) δ -0.44 (d, J ) 2.8 Hz, 3H), -0.29 (s,
3H), -0.14 (d, J ) 2.7 Hz, 3H), -0.03 (s, 3H), 0.50-0.60 (m,
6H), 0.77 (s, 9H), 0.93 (t, J ) 7.9 Hz, 9H), 1.12 (s, 3H), 1.20 (s,
3H), 1.56-1.62 (m, 1H), 1.69 (s, 3H), 1.86-1.96 (m, 1H), 2.03
(s, 3H), 2.17 (s, 3H), 2.26-2.60 (series of m, 2H), 2.63 (s, 3H),
3.83 (d, J ) 7.1 Hz, 1H), 4.26 (s, 2H), 4.33 (t, J ) 2.8 Hz, 1H),
4.45 (dd, J ) 10.6, 6.6 Hz, 1H), 4.70 (d, J ) 2.3 Hz, 1H), 4.94 (d,
J ) 7.7 Hz, 1H), 5.73 (d, J ) 7.1 Hz, 1H), 5.86 (dd, J ) 9.3, 2.0
Hz, 1H), 6.30 (t, J ) 8.7 Hz, 1H), 6.43 (s, 1H), 7.02 (d, J ) 9.3
Hz, 1H), 7.27-7.64 (series of m, 11H), 7.74 (d, J ) 7.1 Hz, 2H),
8.15 (d, J ) 7.1 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 201.6,
171.4, 170.4, 169.4, 166.8, 165.6, 139.8, 138.1, 134.0, 133.6, 133.3,
131.7, 130.4, 130.1, 128.7, 128.6, 127.9, 127.1, 126.5, 84.4, 81.6,
81.3, 76.7, 75.5, 75.3, 74.9, 72.3, 71.1, 58.7, 55.4, 46.5, 44.4, 37.3,
34.2, 27.5, 25.5, 23.2, 22.3, 20.9, 18.2, 14.3, 10.1, 6.8, 5.3, 0.3,
-0.3, -5.2, -5.9; IR (KBr) 3330, 2951, 2877, 2125, 1757, 1724,
1665, 1515, 1483, 1454, 1368, 1262, 1178, 1112, 1067, 986, 911,
899, 837, 797, 780, 747, 714, 698, 619 cm-1; HRMS (FAB-POS)
m/e calcd for C61H86NO14Si3 [M+ + H]: 1140.5356, found
1140.5403.
N-Methyl-2′-tert-butyldimethylsilyl-7-triethylsilylpaclitaxel (6).
To a cooled (-78 °C) solution of 1-dimethylsilyl-2′-tert-butyldim-
ethylsilyl-7-triethylsilylpaclitaxel (4) (9.0 g, 7.9 mmol) in anhydrous
THF (100 mL, dried over benzophenone ketyl) was slowly added
potassium tert-butoxide (11.85 mL, 11.85 mmol, 1.0 M solution
in THF) and allowed to stir for 30 min at the same temperature.
Excess of MeI (12.3 mL, 197.5 mmol) was added, and the reaction
mixture was stirred for an additional 2 h at -78 °C. The reaction
mixture was slowly warmed to 0 °C over 1 h and quenched with
a saturated aq NH4Cl solution (100 mL). The mixture was stirred
at 20 °C for 30 min and extracted with ethyl acetate (3 × 250
mL). The combined organic layers were washed with 10% HCl
and brine, dried over MgSO4, and concentrated under reduced
pressure. The residue was charged onto a silica gel column. Elution
with ethyl acetate/hexane (1:9) furnished a 5.20 g mixture (60%
N-Methyltaxol C (7). N-Methyl-2′-tert-butyldimethylsilyl-7-
triethylsilyltaxol C (4) (5.50 g, 5.05 mmol) was dissolved in pyridine
(35 mL) and cooled to 0 °C. Then, HF ·pyridine (6 mL) was added
very slowly, and the resulting mixture was stirred for 18 h at 20
°C. The reaction mixture was poured into a mixture of ethyl acetate
(500 mL) and a saturated NaHCO3 solution (100 mL). The layers
were separated, and the organic layer was washed with saturated
NaHCO3 solution, water, and brine and dried over MgSO4. The
solvent was removed under reduced pressure, and the residue was
charged onto a silica gel column. Elution with ethyl acetate/hexane
(1:1) furnished N-methyltaxol C (4.20 g, 97%) as a colorless
amorphous solid: mp 146-148 °C; 1H NMR (400 MHz, CDCl3) δ
0.80-0.92 (m, 3H), 1.15 (s, 3H), 1.27 (s, 3H), 1.22-1.28 (m, 4H),
1.53-1.63 (m, 2H), 1.67 (s, 3H), 1.80-1.90 (m, 2H), 1.88 (s, 3H),
2.22 (s, 3H), 2.24 (s, 3H), 2.25-2.42 (m, 4H), 2.48 (d, J ) 4.0
Hz, 1H), 2.48-2.60 (m, 1H), 2.89 (s, 3H), 3.79 (d, J ) 7.0 Hz,
1
total yield of a 2:1 mixture as determined by H NMR) of the
desired N-methyl-2′-tert-butyldimethylsilyl-7-triethylsilyltaxol (6,
1
40% by H NMR analysis) along with 2′-tert-butyldimethylsilyl-
7-triethylsilylpaclitaxel (2a, 20% by 1H NMR analysis). A sample
of pure N-methyl derivative 6 was obtained by additional flash silica
gel chromatographic purification [long column, eluted with ethyl
acetate/hexanes (1:10)] and used for analytical purposes and in the
1
next step: H NMR (400 MHz, CDCl3) δ 0.20 (s, 3H), 0.22 (s,
3H), 0.50-0.68 (m, 6H), 0.94 (s, 9H), 0.95 (t, J ) 7.9 Hz, 9H),
J. Org. Chem. Vol. 73, No. 12, 2008 4707

1.17 (s, 3H), 1.21 (s, 3H), 1.70 (s, 3H), 1.72 (s, 3H), 1.80 (s, 1H),
1.84-1.98 (m, 2H), 2.17 (s, 3H), 2.31-2.41 (m, 1H), 2.50 (s, 3H),
2.48-2.60 (m, 1H), 2.87 (s, 3H), 3.82 (d, J ) 7.0 Hz, 1H), 4.19,
4.29 (ABq, J ) 8.3 Hz, 2H), 4.46 (dd, J ) 10.4, 6.7 Hz, 1H), 4.95
(d, J ) 8.7 Hz, 1H), 5.24 (d, J ) 5.1 Hz, 1H), 5.67 (d, J ) 7.0 Hz,
1H), 6.21 (t, J ) 8.9 Hz, 1H), 6.38 (br s, 1H), 6.41 (s, 1H),
7.26-7.54 (series of m, 12H), 7.59 (dd, J ) 7.4, 7.4 Hz, 1H), 8.09
(d, J ) 7.3 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ 202.0, 172.6,
171.9, 170.5, 169.3, 166.9, 140.6, 136.6, 135.7, 133.6, 133.3, 130.2,
129.6, 129.4, 128.8, 128.6, 128.5, 128.3, 126.7, 84.4, 80.9, 78.7,
76.7, 75.2, 74.9, 73.4, 72.3, 71.1, 58.5, 46.7, 43.2, 37.3, 35.5, 26.5,
25.7, 22.8, 21.2, 20.9, 18.2, 14.2, 10.1, 6.8, 5.4, -4.4, -5.0; HRMS
(FAB-POS) m/e calcd for C60H82NO14Si2 [M+ + H]: 1096.5274,
found 1096.5255.
(m, 2H), 1.89 (s, 3H), 2.25 (s, 3H), 2.27 (s, 3H), 2.34-2.42 (m,
2H), 2.49-2.62 (m, 2H), 2.84 (s, 3H), 3.82 (d, J ) 6.8 Hz, 1H),
4.18, 4.3 (ABq, J ) 8.4 Hz, 2H), 4.40-4.46 (m, 2H), 4.95 (d, J )
9.4 Hz, 1H), 5.02 (dd, J ) 6.8, 3.8 Hz, 1H), 5.69 (d, J ) 6.9 Hz,
1H), 5.90 (s, 1H), 6.25 (t, J ) 8.4 Hz, 1H), 6.30 (s, 1H), 7.37-7.57
(series of m, 12H), 7.62 (dd, J ) 7.2, 7.0 Hz, 1H), 8.10 (d, J ) 7.9
Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 203.9, 173.6, 173.4, 171.4,
170.3, 166.9, 142.7, 136.2, 136.0, 133.7, 132.9, 130.3, 130.1, 129.3,
129.1, 128.7, 128.7, 128.5, 126.9, 84.1, 81.0, 79.1, 76.5, 75.4, 75.1,
72.9, 72.2, 61.5, 58.6, 45.7, 43.3, 37.6, 35.9, 35.6, 26.9, 22.4, 22.0,
21.0, 15.0, 9.6; IR (KBr) 3467, 2940, 1731, 1617, 1451, 1371, 1239,
1114, 1070, 1025, 708, 613 cm-1; HRMS (FAB-POS) m/e calcd
for C48H54NO14 [M+ + H]: 868.3544, found 868.3566. [R]D20 -42
(CHCl3, c 0.12).
N-Methylpaclitaxel (8). N-Methyl-2′-tert-butyldimethylsilyl-7-
triethylsilylpaclitaxel (6) (30 mg, 0.03 mmol) was dissolved in
pyridine (1.0 mL) and cooled to 0 °C. To this solution, HF ·pyridine
(6 drops) was added slowly, and the resulting mixture was stirred
for 24 h at 20 °C. The reaction mixture was poured into an ethyl
acetate (10 mL) and saturated NaHCO3 solution (2 mL) mixture.
The layers were separated, and the organic layer was washed with
a saturated NaHCO3 solution, water, and brine and dried over
MgSO4. The solvent was removed under reduced pressure, and the
residue was charged onto a silica gel column. Elution with ethyl
acetate/hexane (1:1) furnished N-methylpaclitaxel (20 mg, 85%)
Acknowledgment. We gratefully acknowledge the material
(paclitaxel and taxol C) and financial support from Tapestry
Pharmaceuticals, Boulder, Colorado, for this work.
1
Supporting Information Available: Copies of H and 13C
NMR spectra for all new compounds, methods for UPLC
analysis and HPLC purification, and UPLC traces for com-
pounds 7 and 8. This material is available free of charge via
the Internet at http://pubs.acs.org.
1
as a colorless amorphous solid: mp 186-188 °C; H NMR (400
MHz, CDCl3) δ 1.16 (s, 3H), 1.27 (s, 3H), 1.68 (s, 3H), 1.82-1.92
JO800173H
4708 J. Org. Chem. Vol. 73, No. 12, 2008