Synthesis, biology, and modeling of a C-4 carbonyl C,D-seco-taxoid

Article abstract of DOI:10.1016/j.tet.2006.06.080

A C,D-seco-paclitaxel derivative 26 was prepared from taxine and tested for biological activity. Chemical reactivity of the seco-compounds proved to be substantially modified, with respects to taxoids. The corresponding C,D-seco-taxoid does not show tubul

Full text of DOI:10.1016/j.tet.2006.06.080

Tetrahedron 62 (2006) 8503–8514
Synthesis, biology, and modeling of a C-4 carbonyl
C,D-seco-taxoid
Zorana Ferjancic,a,b Radomir Matovic,b Zivorad Cekovic,b Yi Jiang,c
James P. Snyder,c Vladimir Trajkovicd and Radomir N. Saicica,b,
*
aFaculty of Chemistry, University of Belgrade, Studentski trg 16, PO Box 158, 11000 Belgrade, Serbia and Montenegro
bICTM, Center for Chemistry, Njegosꢀeva 12, 11000 Belgrade, Serbia and Montenegro
cDepartment of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, GA 30322, USA
dSchool of Medicine, University of Belgrade, Dr Subotica 1, 11000 Belgrade, Serbia and Montenegro
Received 7 February 2006; revised 6 June 2006; accepted 22 June 2006
Available online 20 July 2006
Dedicated with profound respect and affection to the memory of Professor Pierre Potier
Abstract—A C,D-seco-paclitaxel derivative 26 was prepared from taxine and tested for biological activity. Chemical reactivity of the seco-
compounds proved to be substantially modified, with respects to taxoids. The corresponding C,D-seco-taxoid does not show tubulin
stabilizing activity or cytotoxicity. Explanation of these observations based on molecular modeling is provided.
Ó 2006 Elsevier Ltd. All rights reserved.
O
1. Introduction
OR2
9
18
R1
NH O
OH
O
10
Boc
Ph
19 OH
12
O
Taxol 11,2 and Taxotere 22,3 are currently considered as the
most efficient therapeutic agents for the treatment of several
types of cancer. In order to understand better their unique
mechanism of action, which would allow for design of
the analogs of reduced structural complexity, yet with
improved properties (i.e., higher activity, better solubility,
activity against multidrug-resistant (MDR) tumors, etc.), ex-
tensive structure–activity relationship (SAR) studies have
been performed. These studies indicated that the oxetane
ring may be one of the crucial structural units of biologically
active taxoids; its exact role, however, remained a matter of
debate.4 According to one explanation, the four-membered
ring may serve purely as a rigidifying element, that imposes
a proper conformational bias to the taxane core, thus forcing
the functional groups at C-2, C-4, and C-13 to assume the ap-
propriate positions for productive interactions with tubulin
receptor. Alternatively, electronic effects may be important,
with the heteroatom being involved in stabilizing dipolar,
or hydrogen bonding, interactions with tubulin protein.
For both hypotheses—‘conformational’ and ‘electronic’—
experimental support exists, as well as some contradictory
data. Thus, substitution of oxygen in ring D by nitrogen,5
OH
NH
O
17
O 13
7
15
1
Ph
16
8
6
O
OH
1H4 O
4
5
3
2
OH
BzO
O
HO
AcO 20
BzO
AcO
1 Paclitaxel R1=Ph, R2=Ac
2 Docetaxel R1=OtBu, R2=H
3
OH
O
Bz
OAc
Bz
Ph
NH
O
O
OH
NH
O
OH
O
Ph
O
O
OH
OH
HO
HO
BzO
AcO
BzO
AcO
4
5 C,D-seco-Paclitaxel
sulfur,6 or selenium6b results in the loss of activity, although
the geometries of the corresponding azetidine, thietane, and
selenetane derivatives do not differ very much, as compared
to the parent oxetane. These findings infer the prevalence of
the electronic factors. On the other hand, the lack of bio-
activity of most of D-seco-taxoids with various degree of
oxygenation at positions C-4, C-5, and C-20, suggests that
the role of the oxetane ring may be ‘the conformational lock’
of the diterpene scaffold.7,4b In line with this hypothesis,
a taxoid 3, containing cyclopropane ring in place of oxetane,
showed the tubulin polymerizing activity comparable to that
of docetaxel.8 Recently, it was shown that, contrary to long
Keywords: Taxane antitumor agents; Radicals and radical reactions;
Medium sized rings; Natural products; Molecular modeling.
* Corresponding author. Tel.: +381 113282537; fax: +381 11636061;
e-mail: rsaicic@chem.bg.ac.yu
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.06.080

8504
Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
lasting belief, no ring D is required for tubulin polymerizing
activity of taxoids: on the basis of molecular modeling it
was predicted that 4 would have a strong microtubule stabi-
lizing activity, which was subsequently experimentally con-
firmed.9 However, it should be noted that both 3 and 4 are at
least by two orders of magnitude less cytotoxic with respect
to paclitaxel or docetaxel; thus, while not necessary for the
tubulin stabilizing activity, the oxetane ring still might be
essential for cytotoxicity.
with TBTH, rearrangements occur and unidentified products
are formed). This skeletal transformation reversed the order
of reactivity of carbonyl groups, with respect to the parent
taxane: while C-9 carbonyl group in paclitaxel is resistant
even to LiAlH4,16 in 4,5-seco-derivative it underwent rapid,
but stereorandom reduction with NaBH4, as well as C-13
carbonyl. On the contrary, the newly formed ketone at C-4
proved resistant toward NaBH4, NaBH4/CeCl3, or
NaBH3CN, while a large excess of NaBH4 resulted in
decomposition of material.
2. Results and discussion
In order to overcome this difficulty, the target structure was
modified by substituting the C-9 a-acetate for the C-9 car-
bonyl group: a change that should simplify the synthetic pro-
cedure without affecting the biological activity of the final
product (Scheme 2).17 After the acetylation of a mixture of
cinnamoyltaxicines 9 and 10, selective removal of the cinna-
moyl side chain in 10, in the presence of two acetate units
and a cyclic carbonate, was accomplished using our previ-
ously developed procedure:12 dihydroxylation of 11 with
OsO4 resulted in stereoselective introduction of two requi-
site hydroxyl groups into the taxane core, while simulta-
neously activating (by intramolecular hydrogen bonding in
12) the intermediate 2,3-dihydroxy-3-phenylpropanoate
side chain toward alcoholysis in a buffered methanolic solu-
tion. The resulting triol 13 was converted into oxetane 17 by
a slightly modified four-step protocol developed by the
CNRS group11a (the cyclization step had to be performed
with DIPEA in refluxing toluene; the reagents described in
the literature proved unsuccessful). Alternatively, the con-
version of 11 into the advanced cyclization precursor 16
could be effected via a three-step procedure based on hy-
droxylamine promoted selective cinnamate cleavage.6a,18
The latter route is shorter and avoids the use of protective
groups; however, the overall yield of 17 is lower (calculated
yields for the transformation 11/17, via tetraol 12: 38%;
via allylic alcohol 18: 23%).
In order to discern between the conformational and elec-
tronic effects of the oxetane ring, we endeavored to synthe-
size a C,D-seco-taxoid of type 5 and to evaluate its
biological activity. While conformationally more flexible,
the seco-derivative maintains the functionalization pattern
of paclitaxel. The lack of biological activity of the com-
pound would point to a conformational constraint as the
principal (if not the sole) effect of the oxetane ring; on the
contrary, eventual cytotoxicity of the seco-derivative would
indicate the importance of the oxygenation pattern (i.e., the
electronic effect of the oxetane oxygen atom). We decided
to perform our study on 7-deoxy-taxoids: given that a
7-hydroxyl group is not required for cytotoxicity,10 synthetic
transformations could be performed on derivatives that
require less protective group manipulation. In addition,
others11 and we12 have shown that taxine B13—a pseudo-
alkaloid readily isolated from the renewable needles of
the European yew (Taxus baccata)—constitutes a suitable
starting material, amenable to structural patterns relevant
for this study.14
Our initial plan called for the scission of the central C-4–C-5
bond in previously described alcohol 6,12 via a fragmentation
of the corresponding alkoxyl radical (Scheme 1). After some
experimentation, the iodine-transfer fragmentation was
cleanly effected using diacetoxyiodobenzene (DIB)/iodine
reagent under photolytic conditions.15 Subsequent reduction
with tributyltin hydride (TBTH) afforded the desired inter-
mediate 8 in good yield. For the reaction to proceed effi-
ciently, a high concentration and an excess of TBTH are
needed (under the conventional conditions for reductions
With compound 17 in hand, several reagents for the genera-
tion of tertiary alkoxyl radicals were tried for the fragmenta-
tion step. Surprisingly, the major product in the reaction with
DIB/iodine was acetate 19 (29%), accompanied by the de-
sired iodide 20 (17%); this is in sharp contrast to the same
reaction with 6, which proceeded in 73% yield without
R4O
AcO
OR3
AcO
O
O
6 steps
56% overall yield
Ref. 12
O
O
O
O
O
R1O
R2O
Ref. 12
Ph
O
O
O
NMe2
Ph
O
O
O
O
O
HO
6
O
Taxine: R1, R2, R3, R4=H or Ac
i
H-
AcO
AcO
11
O
O
OAc
OH
O
9
7
12
iii
ii
15
O 1
O
8
13
6
O
O
HO
HO
2
5
I
3
4
14
O
O
O
O
O
O
O
O
O
20
BzO
7
8
Scheme 1. Reagents and conditions: (i) PhI(OAc)2 (1.5 equiv), I2 (1.5 equiv), benzene, 15 ꢀC, 250 W Xenophot lamp, 73%; (ii) Bu3SnH (10 equiv, 0.4 M),
benzene, 15 ꢀC, 250 W Xenophot lamp, 80%; (iii) NaBH4, CeCl3, EtOH, rt.

Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
8505
R4O
OR3
O
O
R1O
R2O
Taxine: R1, R2, R3, R4=H or Ac
O
NMe2
Ph
Ref. 12
OR1
AcO
AcO
OAc
OR2
OAc
i
vii
O
O
O
O
O
OH
O
O
OCinn
OCinn
O
O
O
18
O
O
11
9 R1=H; R2=Ac
v, ii
ii
10 R1=Ac; R2=H
AcO
AcO
OAc
AcO
OAc
OAc
iii
vi
O
O
O
O
OR2
OR1
O
O
O
O
OMs
OH
OH
Ph
O
O
O
HO
HO
HO
HO
O
O
O
iv
HO
13 R1=R2=H
14 R1=TBDMS; R2=H
15 R1=TBDMS; R2=Ms
16
12
viii
v
AcO
AcO
AcO
OAc
OAc
O
OAc
ix
x
O
O
O
X
O
O
O
O
O
O
O
O
O
O
O
HO
O
O
22
xi
19 X=OAc
20 X=I
17
21 X=Br
nOe
nOe
OAc
AcO
O
AcO
OAc
OAc
H
H3C
HO
H3C
NH O
OAc
Ph
H
xiii, xiv
xii
O
Ph
O
or
H
xiii, xv
OH
HO
O
HO
O
O
O
O
O
HO
O
O
O
BzO
O
23
Ph
24
Ph
H
26
nOe
xiv, 12 h
OAc
PMP
O
O
OAc
Ph
NH O
BzN
Ph
Ph
O
COOH
OH
27
25
O
HO
O
BzO
Scheme 2. Reagents and conditions: (i) Ac2O, pyridine, CH2Cl2, DMAP, 93%; (ii) OsO4, NMO, THF, H2O, t-BuOH; (iii) KOAc, MeOH, 73% over two steps;
(iv) TBDMSCl, DMAP, Et3N, CH2Cl2; (v) MsCl, pyridine; (vi) TBAF, THF, 58% from 13; (vii) NH2OH, HCl, Et3N, THF, EtOH, H2O, 50%; (viii) DIPEA,
PhMe, D, 24 h, 90%; (ix) AgOAc, Br2, PhH, hv, see text for conditions, 19 (60%) or 21 (61%); (x) Bu3SnH, AIBN, PhH, hv, 61% over two steps; (xi) (a)
PhLi, THF, ꢁ80 ꢀC; (b) Ac2O, DMAP, CH2Cl2, 62%; (xii) NaBH4, MeOH, 72%; (xiii) 25, DCC, DMAP, PhMe, 75 ꢀC; (xiv) 5% p-TsOH, MeOH, 30 min,
70% from 24; (xv) CAN, H2O, CH3CN, 40 ꢀC, 2.5 h, 61% from 24.
complication. Reagents such as mercuric oxide/iodine,19 or
silver oxide/bromine,20 were also unsuccessful, afforded
20, or 21, in very low yield (<10%), along with some un-
identified products. Finally, we found that the fragmentation
step could be accomplished with silver acetate/bromine
reagent,19a,b under carefully controlled conditions. As a
result both bromide 21 and acetate 19 could be obtained
chemoselectively, depending on the stoichiometry of
reactants. When the reaction was performed with 1.4 equiv
of AgOAc and 2.5 equiv of Br2, the required bromide 21
was the predominant product isolated as a 1:1 mixture of
stereoisomers. With 4 equiv of AgOAc and 2 equiv of Br2 the
chemoselectivity of the reaction is reversed and only acetate
19 is obtained. Interestingly, the acetate was formed stereo-
selectively, with strong predominance of the b-isomer
(b:a¼10.5:1), as determined by NOESY experiment.

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Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
The formation of bromide mixture 21 and acetate 19 can be
understood as the consequence of different mechanistic
pathways. In the first case, the AgOAc/Br2 reagent produces
a hypobromite at C-4 (C4OBr), which is cleaved to an
alkoxyl radical 29 under the influence of light (Scheme 3).
As part of the chain reaction process, hypobromite 28 de-
livers bromine to both faces of the ring-opened radical 30
(path a) accountable for the product mixture. An MM3*
Monte Carlo conformational search21,22 of the flexible eight-
membered ring radical corresponding to 30 with the C-5
carbon center modeled as a carbon free radical shows that
both faces of the ring are free to react with 28 to give 21
(Fig. 1).23
moiety introduces an element of rigidity in the seco-C,D
eight-membered ring as suggested by a Monte Carlo confor-
mational search of the analog of 32 modeled as the corre-
sponding alkene. Unlike in the radical, the ring adopts
a low energy conformation that presents only the face corre-
sponding to the b-isomer as available for capturing the ace-
tate nucleophile (32, Scheme 3 and Fig. 2). Bromide does not
reassociate with the cation as it is removed as insoluble
AgBr.
Br2
4
5
hν
AgOAc
Br
O
HO
O
-AgBr
O
O
O
Br
28
17
29
a
path a
OR
5
O
O
Br
O
O
Ag+
28
21
30
Ag0
Figure 2. Low energy conformer of oxonium cation 32 (Scheme 3) modeled
as the corresponding alkene-21. Only the b-face is exposed to nucleophilic
attack. The O]C+ bond modeled by C]C is highlighted in yellow.
path b
H
OAc
5
O
O
O
+
Br AgOAc
H
O
O
O
_OAc
Due to its instability on silica-gel, the crude bromide 21 was
not purified, but directly reduced with TBTH to afford ke-
tone 22 in 60% yield (from 17). The installation of benzoate
ester into C-2 position was effected by treatment with phe-
nyllithium,25 followed by reacetylation and the reduction
of the C-13 carbonyl group in 23 with NaBH4. At this stage
we planned the reduction of the C-4 carbonyl group in 24
followed by acetylation of the so formed secondary alcohol.
To our surprise, 24 proved unreactive toward a variety of re-
ducing agents, such as NaBH4 (20 equiv, rt), NaBH4/CeCl3,
BH3$Me2S (rt), L-Selectride (ꢁ70/0 ꢀC), Red-Al (ꢁ60/
+50 ꢀC), or Noyori transfer-hydrogenation (performed sepa-
rately with both (R,R)- and (S,S)-enantiomers of TsDPEN
ligand).26 Under the conditions of reduction with Et3SiH/
BF3 only the formation of products of the Lewis acid-cata-
lyzed rearrangement were observed. With LiAlH4 a complex
mixture of products was obtained. At low temperatures,
DIBALH in excess induced only the reductive hydrolysis
of the acetate groups, while a very complex mixture of prod-
ucts was formed at rt. The attempted Noyori reduction of 22
was very slow and resulted only in reductive deprotection
of the cyclic carbonate unit. With other reducing reagents
occasional reduction of the C-13 carbonyl group was
observed, but the C-4 carbonyl remained intact. Thus,
similar to compound 8, fragmentation of the central bond
in the condensed C,D-system of 17 brought about a major
conformational change in the taxane core and rendered the
newly formed carbonyl group inaccessible to reducing
agents.
31
19
32
(β-face only)
Scheme 3. Proposed hypobromite reaction of 17 to give 21 and subsequent
formation of oxonium cation 32 in the presence of excess AgOAc. Inter-
mediate 32 is proposed to lead chemoselectively to b-acetate 19.
The stereospecific formation of acetate 19 proceeds with
a two-fold excess of AgOAc over Br2. We speculate that
under these conditions the 50:50 mixture of bromides 21
(Schemes 2 and 3) is likewise formed. However, the Lewis
acid silver ion or silver acetate then coordinates with
bromine to give complex 31 (Scheme 3, path a). Facile
elimination can then provide the stabilized oxonium cation
32. Alternatively, in the presence of excess AgOAc electron-
transfer may predominate over the bromine transfer reaction,
which results in direct oxidation of radical intermediate 30
to carbocation 32 (Scheme 3, path b).24 The C–C]O+–CH
To understand the remarkable lack of reactivity at the C-4
carbonyls of 22 and 24, we examined the conformational
profile of the molecules to learn what structural features
might contribute to sequester the C]O groups at C-4. In a
preliminary communication,14 we performed a 10,000-step
Figure 1. The first (a) and second (b) low energy MM3* conformers of the
radical intermediate 30 (Scheme 3). The C-5 free radical carbon is high-
lighted in yellow.

Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
8507
28
˚
(van der Waals sum¼3.5 A). The steric repulsion might,
nonetheless, be mediated by a LP/p* interaction from O
to C(]O). Similar to the benzoyl ester 24, the solvent acces-
sible surface area for C-4 in 22 is estimated to be a diminutive
2
˚
0.35 A . One implication of the C4]O shielding from C-2 is
that reduction of the C-4 carbonyl in the absence of either the
carbonate or the C-2 phenyl ring, namely the bare C-2 OH, is
likewise predicted to fail.
Thus, the reluctance of 24 toward reduction hampered the
introduction of acetoxy substituent into C-4 position. Al-
though this structural feature is known to be important for
the biological activity of taxoids, we proceeded further
with the appendage of the side chain and the conversion of
24 into a derivative suitable for biological evaluation. Esteri-
fication of the C-13 hydroxyl group was achieved through
the coupling of 24 with the protected side chain 25, using
a previously described procedure.29,12 Acidic hydrolysis of
p-methoxybenzylidene protective group afforded the re-
quired C,D-seco-derivative 26 in good yield (70% over
two steps). However, prolonged reaction times (12 h instead
of 30 min) lead to the rearrangement of the A-ring in 26 and
formation of 11(15-1)-abeotaxane 27. We found that, alter-
natively, the deprotection step could be performed oxida-
tively, with CAN in aqueous acetonitrile, which obviates
the need for close monitoring of the progress of the reaction.
Figure 3. The lowest energy conformations for taxanes 24 and 22 and the
disposition of the C-4 carbonyl. (a) For 24, the C4]O (yellow arrow) is
surrounded by the C-2 phenyl (left), the C-8 methylene (red arrow) and
the C-19 methyl (top, magenta arrow); (b) for 22, the cyclic carbonate
effectively replaces the C-2 phenyl in 24 as a shielding unit for C4]O.
MMFF/GBSA/H2O conformational search for ketone 24
and located a boat-chair (BC) conformation as the global
minimum with a predicted population of 99.5% at 298 K.
This form is identical to the lowest energy torsional isomer
of cyclooctanone.27 The BC conformer, embedded in the
taxane architecture, occludes approach to either face of the
C4]O group (center, yellow arrow) by reducing agents
(Fig. 3a). The plane of the carbonyl is shielded on three sides
by the C-2 phenyl group (left), the C-8 CH2 group (right, red
arrow), and the C-19 Me group (top, magenta arrow) causing
the C-4 carbon to be buried within the folds of the taxane.
This observation is substantiated by calculation of zero
solvent accessible surface for C-4.
For the evaluation of biological activity, compound 26 was
submitted to the tubulin test,30 as well as to cytotoxicity as-
says against rat glioma C6 cell lines,31 and these results were
compared to those obtained with paclitaxel. The result of the
tubulin test showed 26 to be devoid of microtubule stabiliz-
ing activity. At 11.9 mM concentration, 26 effected only 7%
inhibition of the microtubule disassembly process (for com-
parison, paclitaxel showed 50% inhibition at 1 mM concen-
tration). The staining with crystal violet showed that
paclitaxel reduced the number of C6 glioma cells in a dose-
dependent manner, with an LC50 value of approx. 5 nmol
(Fig. 4a). In contrast, compound 26 failed to display signifi-
cant cytotoxicity in the same concentration range (Fig. 4a).
The absence of cytotoxic activity of 26 was also confirmed
in the MTT assay for mitochondrial respiration (Fig. 4b).
Therefore, the observed lack of microtubule stabilizing
In Figure 3a the C-2 phenyl of 24 is shown as a bulky imped-
iment to the approach of a reducing agent to the a-face of the
C-4 carbonyl. By comparison, the cyclic carbonate in 22 is
both smaller and, to some extent, twisted away from the
C-4 position by virtue of attachment at C-1 (Fig. 3b). Yet,
it too prevents reduction. To understand this, we performed
an MMFF conformational search of the C,D-ring of 22
similar to that of 24. The lowest energy conformer likewise
sustains the BC shape. Surprisingly, the cyclic carbonate
oxygen, though much smaller than a phenyl ring, blocks
approach to C-4 equally effectively. The distance between
the C-4 carbon and the C-2 carbonate oxygen atom is
˚
2.7 A, indicating that the two atoms are most likely crowded
Figure 4. Cytotoxicity of paclitaxel and compound 26. (a) C6 cells were incubated alone (control) or with various doses of paclitaxel or 26; (b) C6 cells were
incubated in the absence (control) or presence of 40 nmol of paclitaxel or 26. Cell viability was assessed after 48 h by crystal violet (a) or MTT assay (b). The
results from the representative of three separate experiments are presented as meanꢂSD of triplicate observations.

8508
Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
activity of 26 can be associated with the complete loss of
cytotoxicity at pharmacologically relevant concentrations.
baccatin cavity. The relative docking poses for PTX and
26 (Fig. 5) reflect the inside-out relationship operating
within the C,D region of the molecules. One critical differ-
ence is that the C-4 acetate of PTX enjoying close contact
with the C-30 phenyl ring in the hydrophobic basin of the
tubulin binding site32 is lost in 26. Another stems from the
latter’s a orientation of the bulky seco-C,D moiety. As a
result of steric clashes with binding pocket residues
Leu371, Pro274, and Phe272, the molecule is obligated to
sit much higher in the pocket than PTX as depicted in
Figure 5. Unfortunately, this results in the loss of pivotal
hydrophobic interactions between ligand and protein. For
example, instead of surrounding His229 in a stacked
arrangement, the C-30 benzamido and C-2 benzoyl moieties
of 26 are moved much further up in the pocket and well out-
side of van der Waals contact. The situation is reminiscent of
bridged taxanes proposed to be similarly dislodged from
deep residence in the tubulin binding cleft as a result of
ligand/protein steric encounters. The compounds are 10 to
30-fold less active than PTX.35
These findings confirmed the importance of the precise con-
formation of the C,D-ring system in taxoids for the biologi-
cal activity. However, the fact that compound 26 lacks the
C-4 acetate group makes interpretations of the biological
tests somewhat ambiguous, as it is difficult to estimate,
which of the two structural features in 26 (the conforma-
tional change, or the absence of the C-4 acetate) contributes
more to the loss of biological activity.
Another factor concerns the relocation of the oxetane oxy-
gen and the adjacent carbons. A conformation of compound
26 was constructed to incorporate the global minimum BC
conformation of the seco-C,D unit as depicted in
Figure 3a. In addition, the C-13 side chain was oriented in
the T-Taxol conformation.32,33 The corresponding structure
was rigidly docked with the Glide protocol34,22 into the
taxane pocket of b-tubulin. The pose that resembles most
closely the electron crystallographic structure of paclitaxel
is depicted in Figure 5.
The proposed conformation of 26 (Fig. 5) is corroborated
with the results of NOESYexperiments (Scheme 2). A cross
peak between H-20a and H-30 is in agreement with the mod-
eled conformation with C-20 curled into the concave region
of the baccatin core and with a H-20a to H-30 distance of
A significant difference between the structures of T-Taxol
and 26 is the conformational reorganization of atoms in
the seco-C,D rings. In paclitaxel (PTX), the C-4 acetate is
found on the a-face of the molecule and directed into the
concave region of the baccatin core. Simultaneously, the
oxygen of the oxetane ring is oriented upward toward
the b-face and outward forming weak contact with Thr274
of b-tubulin.4a,32 In striking contrast, compound 26, to a first
approximation, inverts the roles of these two functionalities.
The C]O replacement for the C-4 acetate is b oriented,
while the oxetane oxygen along with its adjacent methylene
groups is folded a and found deep within the concave
˚
2.8 A in the docked conformation. In addition, NOESY
cross peaks between H-7a and H-18 are observed for both
26 and 24, in accord with the concave conformation of the
molecules. Taken together these observations support the
proposal that the molecule adopts the pose pictured in
Figure 5 and consequently experiences the steric clash
suggested.
To summarize, a method is developed for the efficient
preparation of C,D-seco-taxoids. The conformational
change, induced by the scission of the C-4–C-5 bond in
the taxane core, brings about important modifications in
chemical reactivity of the 4,20-seco-derivatives and results
in the loss of biological activity of the corresponding C,D-
seco-taxoid.
3. Experimental
3.1. General
All chromatographic separations were performed on silica,
10–18 mm, 60A, ICN Biomedicals. Standard techniques
were used for the purification of reagents and solvents.
NMR spectra were recorded on a Varian Gemini 200 spec-
1
trometer, H NMR at 200 MHz, 13C NMR at 50 MHz, for
samples in deuterated chloroform. Chemical shifts are
expressed in parts per million using tetramethylsilane as
internal standard, coupling constants (J) are in hertz. The
NOESY spectra were acquired on a Varian Inova Unity
600 MHz spectrometer in deuterated chloroform at 25 ꢀC,
with mixing times of 400 ms and 500 ms for compounds
26 and 24, respectively. IR spectra were recorded on a
Perkin–Elmer 457 grating FT instrument, and are expressed
in cmꢁ1. Mass spectrometric analysis was provided by the
Emory University Mass Spectrometry Center using an
ACQ Advantage mass spectrometer. Microanalyses were
Figure 5. Docking poses of Taxol (blue) and 26 (cyan) within the taxane/
tubulin binding pocket.

Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
8509
performed at the Vario EL III instrument CHNOS Elementar
Analyzer, Elementar Analysensysteme GmbH, Hanau,
Germany. Melting points were determined on a Kofler hot-
stage apparatus and are uncorrected.
(3H, s), 2.10–1.60 (4H, m), 1.70 (3H, s), 1.35 (3H, s), 1.01
(3H, s). 13C NMR (50 MHz, CDCl3) d: 196.38 (C), 169.87
(C), 169.43 (C), 165.76 (C), 152.25 (C), 150.14 (C),
146.00 (CH), 141.96 (C), 139.49 (C), 134.22 (C), 130.42
(CH), 128.85 (CH), 128.31 (CH), 118.60 (CH2), 117.16
(CH), 88.60 (C), 79.06 (CH), 76.93 (CH), 75.29 (CH),
72.74 (CH), 44.46 (C), 42.99 (CH), 40.93 (CH2), 40.86
(C), 32.54 (CH3), 27.26 (CH2), 27.02 (CH2), 20.60 (CH3),
20.41 (CH3), 19.94 (CH3), 17.19 (CH3), 14.24 (CH3). IR
(KBr) nmax: 2998, 2958, 1818, 1750, 1714, 1686, 1638,
1374, 1310, 1270, 1202, 1163, 1032. HRMS (FAB) calcd
for C34H39O10 (MH+) 607.2543, found 607.2579. [a]2D5
+251 (c 1.0, CH2Cl2).
3.1.1. Compound 8. In a Pyrex, external water-cooled reac-
tor, a deaerated suspension of 6 (30 mg, 0.067 mmol), di-
acetoxyiodobenzene (32 mg, 0.1 mmol), and iodine (25.4 mg,
0.1 mmol) in benzene (3 mL) was irradiated for 20 min with
a 250 W Xenophot sun-lamp focalized light, with stirring
under an argon atmosphere. The reaction mixture was
diluted with CH2Cl2, washed successively with 5% aq
Na2S2O3 and water, and dried over anhyd MgSO4. After
the solvent was removed under reduced pressure, the crude
7 (60 mg) was used in the next step without further purifica-
tion.
3.1.3. Compound 13. To a solution of 11 (800 mg,
1.32 mmol) in a mixture of THF (19 mL) and water
(9.5 mL) was added NMO (910 mg, 6.74 mmol), followed
by osmium tetraoxide (1.27 mL, 2.5% solution in
t-BuOH). The reaction mixture was stirred for 6 h at rt,
then Florisil (900 mg), sodium dithionite (2.5 g), and water
(10 mL) were added, and stirring was continued for 30 min.
The reaction mixture was filtered and the filtrate was
extracted with CH2Cl2. The organic extract was dried over
anhyd MgSO4 and concentrated under reduced pressure.
Crude 12 (800 mg, 90%) was used in the next step without
further purification.
In a Pyrex, external water-cooled reactor, a deaerated
solution of crude iodide 7 (60 mg), TBTH (101 mg,
0.348 mmol), AIBN (2 mg), and benzene (0.9 mL) was irra-
diated for 10 min with a 250 W Xenophot sun-lamp focal-
ized light, with stirring under an argon atmosphere. After
removal of the solvent under reduced pressure, the crude
product was purified by dry flash chromatography (eluent:
benzene/ethyl acetate¼8/2) to give compound 8 (17.8 mg,
1
59%) as a colorless film. H NMR (200 MHz, CDCl3) d:
6.72 (1H, s, H-10), 4.42 (2H, br s, H-2 superimposed with
H-3), 3.94 (1H, d, J¼17.2 Hz, H-20), 3.86–3.72 (1H, m,
H-5), 3.64–3.62 (1H, m, H-5), 3.52 (1H, d, J¼17.2 Hz,
H-20), 2.99 (1H, d, J¼19.6 Hz, H-14), 2.76 (1H, d,
J¼19.4 Hz, H-14), 2.29 (3H, s, H-18), 2.25 (3H, s, Ac),
1.97–1.30 (4H, m, H-6 and H-7), 1.35 (3H, s, H-17 or
H-16), 1.28 (3H, s, H-16 or H-17), 1.25 (3H, s, H-19). 13C
NMR (50 MHz, CDCl3) d: 205.24 (C, C-4), 202.15 (C,
C-9), 195.31 (C, C-13), 169.30 (C, Ac), 151.55 (C, CO),
147.02 (C, C-11), 141.99 (C, C-12), 88.41 (C, C-1), 78.77
(CH, C-2), 76.18 (CH2, C-20), 75.99 (CH, C-10), 72.55
(CH2, C-5), 57.28 (C, C-8), 48.49 (CH, C-3), 41.41 (CH2,
C-15), 40.68 (C, C-14), 36.00 (CH2, C-7), 31.26 (CH3,
C-17 or C-16), 21.93 (CH2, C-6), 20.74 (CH3, Ac), 18.34
(CH3, C-16 or C-17), 16.96 (CH3, C-19), 15.28 (CH3,
C-18). IR (film) nmax: 2960, 1820, 1749, 1694, 1270, 1236,
1028. HRMS (MALDI-FTMS) calcd for C23H27O9Na
(M+Na+) 471.1631, found 471.1641.
To a solution of 12 (800 mg) in methanol (104 mL) was
added methanolic KOAc (4.93 mL, 0.1 M solution) and
the reaction mixture was stirred and heated to reflux for
15 min. The solvent was evaporated under reduced pressure
and the residue was diluted with water, extracted with
CH2Cl2, the extract was dried over anhyd MgSO4 and con-
centrated under reduced pressure. Purification by dry flash
chromatography (eluent: benzene/ethyl acetate¼1/1) af-
forded compound 13 (490 mg, 81%) as a colorless film. 1H
NMR (200 MHz, CDCl3) d: 6.04 (1H, d, J¼10.4 Hz), 5.62
(1H, d, J¼10.2 Hz), 4.86 (1H, d, J¼4.6 Hz), 4.05 (1H, br
s), 3.98 (1H, d, J¼19.3 Hz), 3.98 (1H, d, J¼10.6 Hz),
3.80–3.60 (1H, m), 3.65 (1H, br s), 3.54 (1H, d,
J¼10.6 Hz), 3.01 (1H, br s), 2.93 (1H, d, J¼4.6 Hz), 2.81
(1H, d, J¼19.2 Hz), 2.21 (3H, s), 2.10 (3H, s), 2.04 (3H,
s), 1.97–1.48 (4H, m), 1.67 (3H, s), 1.32 (3H, s), 0.89 (3H,
s). 13C NMR (50 MHz, CDCl3) d: 198.07 (C), 170.33 (C),
169.36 (C), 153.38 (C), 148.73 (C), 143.25 (C), 89.09 (C),
80.20 (CH), 75.36 (CH), 74.85 (C), 72.58 (CH), 70.21
(CH), 62.34 (CH2), 43.50 (C), 41.59 (CH), 41.39 (C),
40.93 (CH2), 32.50 (CH3), 24.40 (CH2), 23.58 (CH2),
20.69 (CH3), 20.56 (CH3), 20.03 (CH3), 18.63 (CH3),
13.80 (CH3). IR (film) nmax: 3611, 3477, 2995, 2960,
1808, 1748, 1688, 1474, 1401, 1375, 1272, 1238, 1025.
HRMS (FAB) calcd for C25H35O11 (MH+) 511.2179, found
511.2170. [a]2D5 +178 (c 1.0, CH2Cl2).
3.1.2. Compound 11. A mixture of 9-O-acetyl-5-O-cinna-
moyltaxicine 1,2-carbonate 9 and 10-O-acetyl-5-O-cinna-
moyltaxicine 1,2-carbonate 10 (200 mg, 0.37 mmol) was
dissolved in CH2Cl2 (6 mL) and treated with acetic anhy-
dride (160 mg, 1.57 mmol), pyridine (80 mg, 1.01 mmol),
and DMAP (5 mg, 0.04 mmol). The reaction mixture was
stirred at rt for 2.5 h, diluted with CH2Cl2, washed with
1.5 N HCl and water, dried over anhyd MgSO4, and evapo-
rated under reduced pressure. Purification of the residue by
dry flash chromatography (eluent: petroleum ether/ethyl
acetate¼7/3) afforded 9-O,10-O-diacetyl-5-O-cinnamoyl-
taxicine 1,2-carbonate 11 (210 mg, 93%) as a white amor-
phous solid (mp 233–235 ꢀC). 1H NMR (200 MHz,
CDCl3) d: 7.76–7.72 (2H, m), 7.67 (1H, d, J¼16.0 Hz),
7.46–7.42 (3H, m), 6.31 (1H, d, J¼15.9 Hz), 6.07 (1H, d,
J¼10.2 Hz), 5.77 (1H, d, J¼10.2 Hz), 5.55 (1H, br s), 5.39
(1H, s), 5.37 (1H, s), 5.02 (1H, d, J¼5.8 Hz), 3.40 (1H, d,
J¼5.5 Hz), 2.96 (2H, br s), 2.32 (3H, s), 2.11 (3H, s), 2.07
3.1.4. Compound 14.
A solution of 13 (470 mg,
0.92 mmol), triethylamine (540 mg, 5.35 mmol), DMAP
(15 mg, 0.12 mmol), and TBDMSCl (660 mg, 4.41 mmol)
in CH2Cl2 (13 mL) was stirred for 5 days at rt under an argon
atmosphere. The reaction mixture was diluted with CH2Cl2,
washed with ice-cold satd aq NH4Cl and water, and dried
over anhyd MgSO4. The solvent removal under reduced
pressure followed by purification by dry flash chromato-
graphy (eluent: benzene/ethyl acetate¼8/2) afforded silyl

8510
Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
ether 14 (523 mg, 91%) as a white foam. 1H NMR
(200 MHz, CDCl3) d: 6.05 (1H, d, J¼10.3 Hz), 5.62 (1H,
d, J¼10.3 Hz), 4.81 (1H, d, J¼4.7 Hz), 4.01 (1H, d,
J¼19.4 Hz), 3.98 (1H, d, J¼9.4 Hz), 3.94 (1H, br s), 3.56
(1H, br s), 3.49 (1H, d, J¼9.2 Hz), 2.95 (1H, d, J¼4.5 Hz),
2.79 (1H, d, J¼19.2 Hz), 2.70 (1H, br s), 2.22 (3H, s),
2.09 (3H, s), 2.04 (3H, s), 2.00–1.50 (4H, m), 1.65 (3H, s),
1.31 (3H, s), 0.90 (9H, s), 0.86 (3H, s), 0.14 (3H, s), 0.10
(3H, s). 13C NMR (50 MHz, CDCl3) d: 197.96 (C), 170.24
(C), 169.29 (C), 152.70 (C), 148.57 (C), 143.27 (C), 88.45
(C), 79.80 (CH), 75.38 (CH), 74.76 (C), 72.56 (CH), 70.01
(CH), 63.29 (CH2), 43.48 (C), 41.41 (C), 41.11 (CH),
40.99 (CH2), 32.48 (CH3), 25.69 (CH3), 24.44 (CH2),
23.49 (CH2), 20.69 (CH3), 20.54 (CH3), 20.01 (CH3),
18.59 (CH3), 18.12 (C), 13.82 (CH3), ꢁ5.58 (CH3). IR
(film) nmax: 3479, 2956, 2934, 1813, 1750, 1690, 1470,
1373, 1237, 1091, 1023. HRMS (FAB) calcd for
C31H49O11Si (MH+) 625.3044, found 625.3028.
2.25 (3H, s), 2.11 (3H, s), 2.05 (3H, s), 2.10–1.65 (4H, m),
1.66 (3H, s), 1.33 (3H, s), 0.92 (3H, s). 13C NMR
(50 MHz, CDCl3) d: 197.64 (C), 170.29 (C), 169.31 (C),
153.12 (C), 149.68 (C), 143.33 (C), 88.94 (C), 81.75 (CH),
79.57 (CH), 75.20 (CH), 73.85 (C), 72.63 (CH), 62.16
(CH2), 43.44 (C), 43.01 (CH), 41.44 (C), 40.91 (CH2),
38.60 (CH3), 32.66 (CH3), 24.93 (2ꢃCH2), 20.63 (CH3),
20.54 (CH3), 19.96 (CH3), 18.90 (CH3), 13.75 (CH3). IR
(film) nmax: 3563, 3455, 2967, 2934, 1806, 1750, 1685,
1414, 1375, 1346, 1273, 1238, 1033. HRMS (FAB) calcd
for C26H37O13S (MH+) 589.1955, found 589.1998. [a]2D5
+152 (c 1.0, ethyl acetate).
3.1.7. Compound 17. A solution of 16 (62 mg, 0.1 mmol)
and DIPEA (95 mg, 0.74 mmol) in anhyd toluene
(15.2 mL) was heated to reflux with stirring for 24 h. The
solvent was removed under reduced pressure and the residue
was purified by dry flash chromatography (eluent: benzene/
ethyl acetate¼6/4) to give compound 17 (47 mg, 90%) as
1
3.1.5. Compound 15. To a cold (0 ꢀC) solution of 14 (40 mg,
0.064 mmol) in pyridine (2 mL) was added mesyl chloride
(110 mg, 0.96 mmol). The reaction mixture was stirred at
0 ꢀC for 15 min and then for 24 h at rt. The reaction mixture
was diluted with CH2Cl2, washed successively with ice-cold
2.5% HCl, aq NaHCO3, and water, and dried over anhyd
MgSO4. After removal of the solvent under reduced pres-
sure, the residue was purified by dry flash chromatography
(eluent: benzene/ethyl acetate¼8/2) to give mesylate 15
(34 mg, 77%) as a colorless film. 1H NMR (200 MHz,
CDCl3) d: 6.04 (1H, d, J¼10.4 Hz), 5.64 (1H, d,
J¼10.2 Hz), 4.79 (1H, d, J¼4.6 Hz), 4.75 (1H, br s), 4.01
(1H, d, J¼19.1 Hz), 3.99 (1H, d, J¼9.8 Hz), 3.92 (1H, s),
3.54 (1H, d, J¼9.7 Hz), 2.96 (3H, s), 2.89 (1H, d,
J¼4.9 Hz), 2.83 (1H, d, J¼19.9 Hz), 2.27 (3H, s), 2.11
(3H, s), 2.10–1.50 (4H, m), 2.05 (3H, s), 1.65 (3H, s), 1.33
(3H, s), 0.92 (9H, s), 0.90 (3H, s), 0.16 (3H, s), 0.12 (3H,
s). 13C NMR (50 MHz, CDCl3) d: 197.56 (C), 170.20 (C),
169.22 (C), 152.43 (C), 149.52 (C), 143.44 (C), 88.36 (C),
81.22 (CH), 79.24 (CH), 75.20 (CH), 73.87 (C), 72.63
(CH), 63.18 (CH2), 43.46 (C), 42.61 (CH), 41.48 (C),
41.01 (CH2), 38.66 (CH3), 32.67 (CH3), 25.68 (CH3),
25.13 (CH2), 25.06 (CH2), 20.65 (CH3), 20.54 (CH3),
19.99 (CH3), 18.99 (CH3), 18.06 (C), 13.77 (CH3), ꢁ5.51
(CH3), ꢁ5.57 (CH3). IR (film) nmax: 3481, 2957, 2935,
1815, 1750, 1688, 1472, 1370, 1265, 1235, 1174, 1094,
1025. HRMS (FAB) calcd for C32H51O13SSi (MH+)
703.2820, found 703.2814.
a colorless film. H NMR (200 MHz, CDCl3) d: 5.96 (1H,
d, J¼10.4 Hz), 5.73 (1H, d, J¼10.6 Hz), 4.94 (1H, d,
J¼5.1 Hz), 4.81–4.76 (1H, m), 4.64 (1H, d, J¼9.0 Hz),
4.47 (1H, d, J¼9.2 Hz), 3.73 (1H, d, J¼19.1 Hz), 3.12
(1H, s), 2.80 (1H, d, J¼19.3 Hz), 2.19 (1H, d, J¼5.3 Hz),
2.14 (3H, s), 2.11 (3H, s), 2.40–1.60 (4H, m), 2.06 (3H, s),
1.70 (3H, s), 1.43 (3H, s), 1.32 (3H, s). 13C NMR
(50 MHz, CDCl3) d: 197.47 (C), 170.22 (C), 169.42 (C),
152.92 (C), 149.53 (C), 142.52 (C), 88.42 (C), 87.83 (CH),
80.00 (CH2), 79.42 (CH), 75.25 (CH), 74.52 (C), 72.67
(CH), 46.56 (CH), 42.68 (C), 41.35 (C), 40.71 (CH2),
32.41 (CH3), 27.82 (CH2), 26.40 (CH2), 20.70 (CH3),
20.60 (CH3), 20.05 (CH3), 16.32 (CH3),13.93 (CH3). IR
(film) nmax: 3479, 2993, 2965, 1812, 1750, 1690, 1375,
1271, 1238, 1022. HRMS (FAB) calcd for C25H33O10
(MH+) 493.2074, found 493.2317. [a]2D5 +147 (c 1.0, ethyl
acetate).
3.1.8. Compound 18. To a solution of 11 (300 mg,
0.49 mmol) in a mixture of THF, EtOH, and water (1:1:1,
54 mL) were added hydroxylamine hydrochloride (170 mg,
2.47 mmol) and triethylamine (250 mg, 2.47 mmol). The so-
lution was stirred and heated at 80 ꢀC for 30 h. The reaction
mixture was diluted with CH2Cl2 and water, the organic layer
was separated and dried over anhyd MgSO4. After evapora-
tion of the solvent under reduced pressure, the residue was
purified by dry flash chromatography (eluent: petroleum
ether/ethyl acetate¼6/4) to give compound 18 (120 mg,
1
50%) as a colorless film. H NMR (200 MHz, CDCl3) d:
3.1.6. Compound 16. To a solution of 15 (320 mg,
0.45 mmol) in THF (27 mL) was added a solution of tetra-
butylammonium fluoride trihydrate (156 mg, 0.49 mmol)
in THF (3 mL). The reaction mixture was stirred at rt for
5 min and ethyl acetate was added. The resulting solution
was washed with aq NaHCO3 and water and dried over
anhyd MgSO4. The solvent was removed under reduced
pressure and the residue was purified by dry flash chromato-
graphy (eluent: benzene/ethyl acetate¼6/4) to give 16
6.11 (1H, d, J¼10.2 Hz), 5.71 (1H, d, J¼10.2 Hz), 5.39
(1H, br s), 5.18 (1H, br s), 4.95 (1H, d, J¼5.7 Hz), 4.20
(1H, br s), 3.61 (1H, d, J¼5.5 Hz), 2.90 (2H, br s), 2.25
(3H, s), 2.10 (3H, s), 2.07 (3H, s), 1.85–1.60 (4H, m), 1.68
(3H, s), 1.32 (3H, s), 0.96 (3H, s). 13C NMR (50 MHz,
CDCl3) d: 196.86 (C), 170.09 (C), 169.62 (C), 152.74 (C),
148.74 (C), 144.79 (C), 142.56 (C), 114.87 (CH2), 88.78
(C), 79.71 (CH), 75.52 (CH), 74.49 (CH), 72.63 (CH),
44.72 (2ꢃC), 40.80 (CH and CH2), 32.41 (CH3), 29.42
(CH2), 26.16 (CH2), 20.63 (CH3), 20.45 (CH3), 19.96
(CH3), 17.04 (CH3), 14.28 (CH3). IR (KBr) nmax: 2998,
2958, 1818, 1750, 1714, 1686, 1638, 1374, 1310, 1270,
1202, 1163, 1032. HRMS (FAB) calcd for C25H32O9Na
(MNa+) 499.1944, found 499.1930. [a]2D5 +179 (c 1.1,
CH2Cl2).
1
(216 mg, 83%) as a colorless film. H NMR (200 MHz,
CDCl3) d: 6.03 (1H, d, J¼10.2 Hz), 5.63 (1H, d,
J¼10.4 Hz), 4.87 (1H, d, J¼4.4 Hz), 4.79 (1H, br s), 4.11
(1H, br s), 3.99 (1H, d, J¼19.3 Hz), 3.99 (1H, d,
J¼11.3 Hz), 3.85 (1H, br s), 3.62 (1H, d, J¼10.9 Hz), 3.00
(3H, s), 2.85 (1H, d, J¼4.4 Hz), 2.83 (1H, d, J¼19.3 Hz),

Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
8511
3.1.9. Compound 19. In a Pyrex, external water-cooled
reactor, to deaerated suspension of 17 (96 mg,
(1H, d, J¼19.3 Hz, H-14), 2.73 (1H, d, J¼19.4 Hz, H-14),
2.26 (3H, s, H-18), 2.12 (3H, s, Ac), 2.25–2.10 (1H, m, H-
7), 2.07 (3H, s, Ac), 1.69 (3H, s, H-16 or H-17), 1.72–1.30
(3H, m, H-6 and H-7), 1.33 (3H, s, H-17 or H-16), 0.99
(3H, s, H-19). 13C NMR (50 MHz, CDCl3) d: 207.14 (C,
C-4), 195.49 (C, C-13), 169.80 (C, Ac), 169.38 (C, Ac),
151.81 (C, CO), 148.93 (C, C-11), 141.91 (C, C-12), 88.45
(C, C-1), 77.38 (CH, C-2), 75.91 (CH2, C-20), 74.80 (CH,
C-9), 72.47 (CH2, C-5), 71.97 (CH, C-10), 46.90 (CH, C-
3), 44.97 (C, C-8), 40.82 (CH2, C-14), 40.70 (C, C-15),
32.19 (CH3, C-16 or C-17), 31.63 (CH2, C-7), 21.20 (CH2,
C-6), 20.56 (CH3, Ac), 20.32 (CH3, Ac), 19.90 (CH3, C-17
or C-16), 17.83 (CH3, C-19), 14.66 (CH3, C-18). IR (film)
nmax: 2958, 1818, 1747, 1692, 1373, 1268, 1234, 1026.
HRMS (MALDI-FTMS) calcd for C25H32O10Na (M+Na+)
515.1888, found 515.1885. [a]2D5 +180 (c 1.0, ethyl acetate).
a
0.195 mmol), silver acetate (130 mg, 0.778 mmol), and
benzene (8.8 mL) was added a solution of bromine (62 mg,
0.387 mmol) in benzene (0.7 mL). The reaction mixture
was irradiated for 5 min with a 250 W Xenophot sun-lamp
focalized light, with stirring under an argon atmosphere.
The reaction mixture was filtered and diluted with CH2Cl2,
washed successively with 5% Na2S2O3, aq NaHCO3, and
water, and dried over anhyd MgSO4. The solvent was re-
moved under reduced pressure and the crude product was
purified by dry flash chromatography (eluent: benzene/ethyl
acetate¼7/3) to give acetate 19 (b/a¼10.5/1) (65 mg, 60%)
as a colorless film. Spectroscopic data for major isomer: 1H
NMR (500 MHz, CDCl3) d: 6.08 (1H, d, J¼10.5 Hz, H-10),
5.96–5.94 (1H, m, H-5), 5.75 (1H, d, J¼10.5 Hz, H-9), 4.94
(1H, d, J¼7.1 Hz, H-2), 4.16 (1H, d, J¼17.6 Hz, H-20), 3.95
(1H, d, J¼7.1 Hz, H-3), 3.74 (1H, d, J¼17.6 Hz, H-20), 2.98
(1H, d, J¼19.4 Hz, H-14), 2.73 (1H, d, J¼19.4 Hz, H-14),
2.29 (3H, s, H-18), 2.15–2.05 (1H, m, H-7), 2.13 (3H, s,
Ac), 2.09 (3H, s, Ac), 2.08 (3H, s, Ac), 1.85–1.72 (1H, m,
H-6), 1.70 (3H, s, H-16 or H-17), 1.60–1.50 (1H, m, H-7),
1.50–1.40 (1H, m, H-6), 1.34 (3H, s, H-17 or H-16), 1.02
(3H, s, H-19). 13C NMR (125 MHz, CDCl3) d: 204.94 (C,
C-4), 195.06 (C, C-13), 169.65 (C, Ac), 169.33 (C, Ac),
168.82 (C, Ac), 151.60 (C, CO), 148.75 (C, C-11), 142.09
(C, C-12), 95.71 (CH, C-5), 88.49 (C, C-1), 77.37 (CH, C-
2), 74.54 (CH, C-9), 71.99 (CH, C-10), 69.12 (CH2, C-20),
47.52 (CH, C-3), 45.76 (C, C-8), 40.99 (CH2, C-14), 40.82
(C, C-15), 32.30 (CH3, C-16 or C-17), 28.73 (CH2, C-7),
24.98 (CH2, C-6), 20.91 (CH3, Ac), 20.71 (CH3, Ac),
20.52 (CH3, Ac), 20.09 (CH3, C-17 or C-16), 18.25 (CH3,
C-19), 14.94 (CH3, C-18). IR (film) nmax: 2998, 1821,
1750, 1693, 1375, 1235, 1025. HRMS: C27H34O12Na
573.1948, found 573.1959.
3.1.11. Compound 23. A cold (ꢁ80 ꢀC) solution of 22
(70 mg, 0.142 mmol) in THF (15 mL) was treated with
PhLi (240 mg, 1.5 mL of a 1.9 M solution in toluene) and
stirred at ꢁ80 ꢀC for 6 h under an argon atmosphere. The re-
action mixture was quenched with satd aq NH4Cl (10 mL)
and then allowed to warm to rt. After dilution with
CH2Cl2, the organic layer was separated, dried over anhyd
MgSO4, and concentrated under reduced pressure. The resi-
due was dissolved in CH2Cl2 (6 mL) and acetic anhydride
(147 mg, 1.42 mmol) and DMAP (87 mg, 0.71 mmol) was
added. The mixture was stirred for 30 min at rt and diluted
with CH2Cl2. The resulting solution was washed with satd
aq NaHCO3, 1.5 M HCl and water, and dried over anhyd
MgSO4. After removal of the solvent under reduced pressure
the residue was purified by column chromatography (eluent:
petroleum ether/ethyl acetate¼7/4) to give benzoate 23
(47 mg, 58%, 62% based on recovered carbonate 22) as a
colorless film, followed by the starting compound 22
(4 mg). 1H NMR (200 MHz, CDCl3) d: 7.86–7.82 (2H, m),
7.61–7.53 (1H, m), 7.47–7.39 (2H, m), 6.20 (1H, d, J¼
10.5 Hz), 6.01 (1H, d, J¼10.6 Hz), 5.88 (1H, d, J¼7.7 Hz),
4.32 (1H, d, J¼7.7 Hz), 3.87 (1H, d, J¼16.0 Hz), 3.78–
3.64 (1H, m), 3.53–3.45 (1H, m), 3.37 (1H, d, J¼16.1 Hz),
2.94 (1H, d, J¼19.6 Hz), 2.80 (1H, d, J¼19.4 Hz), 2.26
(3H, s), 2.11 (3H, s), 2.09 (3H, s), 2.05–1.45 (4H, m), 1.76
(3H, s), 1.27 (3H, s), 1.00 (3H, s). 13C NMR (50 MHz,
CDCl3) d: 211.00 (C), 198.55 (C), 169.73 (C), 169.62 (C),
165.14 (C), 151.45 (C), 140.03 (C), 133.28 (CH), 129.53
(C), 129.47 (CH), 128.45 (CH), 77.98 (C), 77.36 (CH2),
74.71 (CH), 72.72 (CH2), 72.21 (CH), 72.17 (CH), 50.84
(CH), 44.17 (CH2 and C), 42.50 (C), 33.83 (CH3), 31.45
(CH2), 21.54 (CH2), 20.78 (CH3), 20.54 (CH3), 20.01
(CH3), 17.88 (CH3), 13.99 (CH3). IR (film) nmax: 3497,
2986, 1746, 1680, 1451, 1374, 1269, 1231, 1096, 1026.
HRMS (TOF MS ES+) calcd for C31H39O10 (MH+)
571.2543, found 571.2548. [a]2D5 +112 (c 1.0, ethyl acetate).
3.1.10. Compound 22. In a Pyrex, external water-cooled
reactor, to a deaerated suspension of 17 (220 mg,
0.447 mmol), silver acetate (112 mg, 0.670 mmol), and
benzene (21 mL) was added a solution of bromine (143 mg,
0.894 mmol) in benzene (1.9 mL). The reaction mixture
was irradiated for 5 min with a 250 W Xenophot sun-lamp
focalized light, with stirring under an argon atmosphere.
The reaction mixture was filtered and diluted with CH2Cl2,
washed successively with 5% Na2S2O3, aq NaHCO3, and
water, and dried over anhyd MgSO4. After concentration
under reduced pressure, the crude 21 (260 mg, equimolar
mixture of diastereoisomers) was used in the next step
without further purification.
In a Pyrex, external water-cooled reactor, a deaerated solu-
tion of bromides 21 (260 mg, 0.447 mmol), TBTH (1.3 g,
4.47 mmol), AIBN, and benzene (8 mL) was irradiated for
15 min with a 250 W Xenophot sun-lamp focalized light,
with stirring under an argon atmosphere. After removal of
the solvent under reduced pressure, the crude product was
purified by dry flash chromatography (eluent: benzene/ethyl
acetate¼8/2) to give compound 22 (135 mg, 61%) as a color-
less film. 1H NMR (200 MHz, CDCl3) d: 6.10 (1H, d,
J¼10.5 Hz, H-10), 5.74 (1H, d, J¼10.5 Hz, H-9), 4.94
(1H, d, J¼7.0 Hz, H-2), 4.06 (1H, d, J¼7.0 Hz, H-3), 3.91
(1H, d, J¼17.2 Hz, H-20), 3.78–3.64 (1H, m, H-5), 3.59–
3.52 (1H, m, H-5), 3.48 (1H, d, J¼17.2 Hz, H-20), 2.98
3.1.12. Compound 24.
A solution of 23 (40 mg,
0.070 mmol) in methanol (5.7 mL) was treated with excess
of NaBH4 (53 mg, 1.40 mmol) for 1 h at rt. The reaction
was quenched with satd aq NH4Cl and the resulting mixture
was stirred for 10 min. After dilution with water, the reaction
mixture was extracted twice with CH2Cl2 and dried over
anhyd MgSO4. Concentration under reduced pressure, fol-
lowed by purification of the residue by column chromato-
graphy (eluent: benzene/ethyl acetate¼6/4) gave alcohol

8512
Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
24 (29 mg, 72%) as a white amorphous solid (mp 139–
Ac), 2.05 (3H, s, Ac), 1.96 (3H, s, H-18), 1.70 (3H, s, H-
17 or H-16), 1.65–1.50 (1H, m, H-7), 1.40–1.20 (2H, m,
H-6), 1.13 (3H, s, H-16 or H-17), 0.99 (3H, s, H-19). 13C
NMR (50 MHz, CDCl3) d: 211.21 (C, C-4), 171.79 (C-10),
169.93 (C, Ac), 169.75 (C, Ac), 166.51 (C, Ar), 165.51 (C,
Ar), 138.70 (C, Ar), 137.99 (C, C-12), 135.39 (C, C-11),
134.12 (C, Ar), 133.10 (CH, Ar), 131.79 (CH, Ar), 129.95
(CH, Ar), 129.67 (C, Ar), 128.69 (CH, Ar), 128.65 (CH,
Ar), 128.40 (CH, Ar), 127.94 (CH, Ar), 127.00 (CH, Ar),
126.92 (CH, Ar), 77.31 (CH2, C-20), 75.47 (CH, C-9),
74.09 (CH, C-20), 73.07 (CH, C-2), 72.87 (CH2, C-5),
72.32 (CH, C-13), 71.68 (CH, C-10), 54.20 (CH, C-30),
51.55 (CH, C-3), 44.15 (C, C-8), 41.53 (C, C-15), 36.72
(CH2, C-14), 31.24 (CH2, C-7), 28.91 (CH3, C-16 or C-
17), 21.61 (CH2, C-6), 20.94 (CH3, Ac), 20.63 (2ꢃCH3,
Ac and C-17 or C-16), 17.94 (CH3, C-19), 16.04 (CH3, C-
18). The signal corresponding to C-1 could not be detected
under the recording conditions. IR (film) nmax: 3443, 2931,
1737, 1662, 1517, 1487, 1452, 1373, 1243, 1096, 1025.
HRMS (ESI-TOF high acc) calcd for C47H54NO13 (MH+)
840.3589, found 840.3575. [a]2D5 +95 (c 1.6, ethyl acetate).
1
141 ꢀC). H NMR (200 MHz, CDCl3) d: 7.87–7.83 (2H,
m), 7.59–7.51 (1H, m), 7.45–7.37 (2H, m), 6.11 (1H, d,
J¼10.4 Hz), 5.89 (1H, d, J¼10.4 Hz), 5.76 (1H, d,
J¼7.1 Hz), 4.73–4.61 (1H, m), 4.40 (1H, d, J¼6.9 Hz),
4.07 (1H, d, J¼16.0 Hz), 3.95–3.77 (1H, m), 3.69–3.55
(1H, m), 3.47 (1H, d, J¼16.2 Hz), 3.14 (1H, d,
J¼10.9 Hz), 2.63 (1H, dd, J1¼15.8 Hz, J2¼10.4 Hz), 2.29
(3H, s), 2.23–2.16 (1H, m), 2.10–1.28 (4H, m), 2.07 (3H,
s), 2.05 (3H, s), 1.66 (3H, s), 1.07 (3H, s), 0.99 (3H, s).
13C NMR (50 MHz, CDCl3) d: 210.69 (C), 169.96 (C),
169.76 (C), 165.19 (C), 142.73 (C), 133.99 (C), 133.17
(CH), 129.85 (C), 129.51 (CH), 128.45 (CH), 77.09 (C),
76.49 (CH2), 75.85 (CH), 73.56 (CH2), 72.88 (CH), 72.41
(CH), 69.35 (CH), 51.11 (CH), 43.95 (C), 41.92 (CH2),
41.53 (C), 31.87 (CH2), 28.98 (CH3), 21.62 (CH2), 20.96
(CH3), 20.61 (CH3), 20.38 (CH3), 17.94 (CH3), 16.74
(CH3). IR (film) nmax: 3474, 2932, 2875, 1737, 1630,
1452, 1374, 1244, 1094, 1024. HRMS (TOF MS ES+) calcd
for C31H40O10Na (M+Na+) 595.2519, found 595.2493. [a]2D5
+41 (c 1.0, ethyl acetate).
1
3.1.13. Compound 26. To a solution of carboxylic acid 25
(73.0 mg, 0.181 mmol), alcohol 24 (26.0 mg, 0.045 mmol)
and DMAP (22.2 mg, 0.181 mmol) in toluene (11.0 mL)
was added DCC (37.4 mg, 0.181 mmol) in toluene
(1.4 mL), at rt, under an argon atmosphere. The reaction
mixture was stirred at 75 ꢀC for 30 min. After filtration
and removal of the solvent under reduced pressure, the prod-
uct was roughly purified by short column chromatography
(eluent: benzene/ethyl acetate¼7/3) to give the protected
ester (41.3 mg, 93%).
3.1.14. Spectroscopic data for 27. H NMR (500 MHz,
CDCl3) d: 8.01 (2H, dd, J1¼7.0 Hz, J2¼1.5 Hz, Ar), 7.78–
7.75 (2H, m, Ar), 7.54–7.50 (3H, m, Ar), 7.46–7.36 (4H,
m, Ar), 7.34–7.29 (2H, m, Ar), 7.20–7.17 (2H, m, Ar),
6.95 (1H, d, J¼9.5 Hz, N–H), 6.41 (1H, d, J¼9.5 Hz,
H-2), 6.19 (1H, d, J¼10.0 Hz, H-10), 5.89–5.87 (1H, m,
H-30), 5.88 (1H, d, J¼10.0 Hz, H-9), 5.79 (1H, br t,
J¼6.5 Hz, H-13), 4.69 (1H, d, J¼2.0 Hz, H-20), 4.38 (1H,
d, J¼17.0 Hz, H-20), 4.34 (1H, d, J¼9.0 Hz, H-3), 3.83–
3.77 (1H, m, H-5), 3.60–3.57 (1H, m, H-5), 3.37 (1H, d,
J¼17.5 Hz, H-20), 2.52 (1H, dd, J1¼15.5 Hz, J2¼6.0 Hz,
H-14), 2.44 (1H, dd, J1¼15.5 Hz, J2¼7.5 Hz, H-14), 2.08–
2.03 (1H, m, H-7), 2.05 (3H, s, Ac), 2.03 (3H, s, Ac), 1.87
(3H, s, H-18), 1.65–1.20 (3H, m, H-7 and H-6), 1.13 (3H,
s, H-17 or H-16), 1.11 (3H, s, H-16 or H-17), 1.05 (3H, s,
H-19). 13C NMR (50 MHz, CDCl3) d: 212.12 (C, C-4),
172.51 (C-10), 169.51 (C, Ac), 168.64 (C, Ac), 166.68 (C,
Ar), 165.09 (C, Ar), 145.82 (C, C-11), 139.28 (C, Ar),
137.51 (C, C-12), 135.00 (C, Ar), 132.63 (CH, Ar), 131.65
(CH, Ar), 130.29 (C, Ar), 129.75 (CH, Ar), 128.78 (CH,
Ar), 128.33 (CH, Ar), 128.00 (CH, Ar), 127.11 (CH, Ar),
127.06 (CH, Ar), 127.04 (CH, Ar), 81.68 (CH, C-13),
76.34 (CH, C-9), 76.17 (CH2, C-20), 74.97 (C, C-15),
73.49 (CH, C-20), 72.58 (CH2, C-5), 69.11 (CH, C-10),
68.85 (C, C-1), 67.95 (CH, C-2), 54.38 (CH, C-30), 50.28
(CH, C-3), 42.28 (C, C-8), 36.46 (CH2, C-14), 32.36 (CH2,
C-7), 27.48 (CH3, C-16 or C-17), 25.25 (CH3, C-16 or C-
17), 21.82 (CH2, C-6), 20.86 (CH3, Ac), 20.68 (CH3, Ac),
Acidic deprotection: the product from the previous step was
treated with 5% methanolic p-TsOH (7.0 mL) at rt for
30 min. The reaction mixture was diluted with ethyl acetate,
washed successively with aq NaHCO3 and brine, dried over
anhyd MgSO4, and concentrated under reduced pressure.
The residue was purified by column chromatography (elu-
ent: hexane/ethyl acetate¼1/1) to give ester 26 (26.9 mg,
75%) as a colorless film.
Deprotection with CAN: to a stirred solution of protected
compound (15 mg, 0.015 mmol) in acetonitrile (0.9 mL)
was added a solution of CAN (25.2 mg, 0.046 mmol) in
water (0.9 mL). The resulting yellow mixture was stirred
at 40 ꢀC for 2.5 h, diluted with CH2Cl2, organic layer was
separated and dried over anhyd MgSO4. The residue was
purified by column chromatography (eluent: hexane/ethyl
acetate¼1/1) to give compound 26 (8.7 mg, 66%) as a color-
1
less film. H NMR (500 MHz, CDCl3) d: 8.03 (2H, dd,
18.14 (CH3, C-19), 12.05 (CH3, C-18). IR (film) nmax:
J1¼8.0 Hz, J2¼1.0 Hz, Ar), 7.81 (2H, dd, J1¼8.5 Hz,
J2¼1.5 Hz, Ar), 7.54–7.48 (4H, m, Ar), 7.43 (2H, t,
J¼7.5 Hz, Ar), 7.40–7.30 (5H, m, Ar), 7.25 (1H, d super-
imposed with CDCl3, J¼9.5 Hz, N–H), 6.07 (1H, d,
J¼10.5 Hz, H-10), 6.06–6.03 (1H, m, H-13), 5.93 (1H, d,
J¼9.5 Hz, H-30), 5.93 (1H, d, J¼10.5 Hz, H-9), 5.79 (1H,
d, J¼7.5 Hz, H-2), 4.73 (1H, br s, H-20), 4.65 (1H, d,
J¼16.0 Hz, H-20), 4.41 (1H, d, J¼7.5 Hz, H-3), 3.88 (1H,
br s, OH), 3.86–3.78 (1H, m, H-5), 3.68–3.65 (1H, m,
H-5), 3.48 (1H, d, J¼16.0 Hz, H-20), 2.46 (1H, dd,
J1¼16.0 Hz, J2¼9.5 Hz, H-14), 2.39 (1H, dd, J1¼16.0 Hz,
J2¼5.0 Hz, H-14), 2.15–2.00 (1H, m, H-7), 2.08 (3H, s,
3450, 2921, 1730, 1663, 1517, 1486, 1454, 1372, 1233,
1100, 1030. HRMS (ESI-TOF high acc) calcd for
C47H53NO13Na (MH+) 862.3409, found 862.3416. [a]2D5
ꢁ1.9 (c 0.53, ethyl acetate).
3.2. Biological assays
Tubulin test was performed according to the described
procedure.30
3.2.1. Cell culture and determination of cell viability. The
cells of the rat glioma cell line C6 (ATCC) were seeded in

Z. Ferjancic et al. / Tetrahedron 62 (2006) 8503–8514
8513
96-well flat-bottom plates (2ꢃ104 cells/well) and incubated
with paclitaxel or compound 26 for 48 h. The cells were cul-
tivated at 37 ꢀC in a humidified atmosphere with 5% CO2, in
a HEPES-buffered RPMI 1640 cell culture medium supple-
mented with 5% fetal calf serum, 2 mmol L-glutamine,
50 mmol 2-mercaptoethanol, 10 mmol sodium pyruvate
and penicillin/streptomycin (all from Sigma, St. Louis,
MO). Cell viability was analyzed by staining the viable cells
with crystal violet, or by measuring the mitochondrial-
dependent reduction of 3-[4,5-dimethyl thiazol-2-yl]-2,5-
diphenyl-tetrazolium bromide (MTT) to formazan. Both
crystal violet and MTT test were performed exactly as
previously described,31 and the results were presented as
% viability relative to untreated control.
2. (a) Kingston, D. G. I.; Jagtap, P. G.; Yuan, H.; Samala, L. Prog.
Chem. Org. Nat. Prod. 2002, 84, 53–225; (b) Gueritte, F. Curr.
Pharm. Des. 2001, 7, 933–953; (c) The Chemistry and
Pharmacology of Taxol and its Derivatives, Pharmaco-
chemistry Library; Farina, V., Ed.; Elsevier: Amsterdam,
1995; Vol. 22.
3. Guenard, D.; Gueritte-Voegelin, F.; Potier, P. Acc. Chem. Res.
1993, 26, 160–167.
4. (a) Wang, M.; Cornett, B.; Nettles, J.; Liotta, D. C.; Snyder, J. P.
J. Org. Chem. 2000, 65, 1059–1068; (b) Boge, T. C.; Hepperle,
M.; Vander Velde, D. G.; Gunn, C. W.; Grunewald, G. L.;
Georg, G. I. Bioorg. Med. Chem. Lett. 1999, 9, 3041–3046.
5. Marder-Karsenti, R.; Dubois, J.; Bricard, L.; Guenard, D.;
Gueritte, F. J. Org. Chem. 1997, 62, 6631–6637.
6. (a) Merckle, L.; Dubois, J.; Place, E.; Thoret, S.; Gueritte, F.;
Guenard, D.; Poupat, C.; Ahond, A.; Potier, P. J. Org. Chem.
2001, 66, 5058–5065; (b) Gunatilaka, A. L.; Ramdayal,
F. D.; Sarragiotto, M. H.; Kingston, D. G. I.; Sackett, D. L.;
Hamel, E. J. Org. Chem. 1999, 64, 2694–2703.
7. (a) Barboni, L.; Data, A.; Gutta, D.; Georg, G. I.; Vander Velde,
D. G.; Himes, R. H.; Wang, M.; Snyder, J. P. J. Org. Chem.
2001, 66, 3321–3329; (b) Deka, V.; Dubois, J.; Thoret, S.;
Gueritte, F.; Guenard, D. Org. Lett. 2003, 5, 5031–5034; (c)
Beusker, P. H.; Veldhuis, H.; Brinkhorst, J.; Hetterscheid,
D. G. H.; Feichter, N.; Bugaut, A.; Scheeren, H. W. Eur. J.
Org. Chem. 2003, 689–705; (d) Samaranayake, G.; Magri,
N. F.; Jitrangsrei, C.; Kingston, D. G. I. J. Org. Chem. 1991,
56, 5114–5119.
3.3. Modeling
All calculations including energy minimization, confor-
mational searches, and Glide Docking were performed in
22
€
Maestro (Schordinger Inc.). With the exception of the
free radical intermediate 30, 10,000-step MMFF/GBSA/
H2O conformational searches were performed for taxane
analogs within 7.0 kcal/mol of the global minimum. For
C-5 of the C–O–Cꢄ–C radical unit of intermediate 30
(Scheme 3 and Fig. 2), a set of provisional force field para-
meters were employed in a 10,000-step gas phase MM3*
conformational search.
8. Dubois, J.; Thoret, S.; Gueritte, F.; Guenard, D. Tetrahedron
Lett. 2000, 41, 3331–3334.
A local modification of the refined electron crystallo-
graphic complex of PTX/b-tubulin32 was made prior to
docking. Thus, Arg284 on the M-loop was relocated from
the electron crystallographic position so as to form a hydro-
9. Barboni, L.; Giarlo, G.; Ricciutelli, M.; Ballini, R.; Georg,
G. I.; Vander Velde, D. G.; Himes, R. H.; Wang, M.;
Lakdawala, A.; Snyder, J. P. Org. Lett. 2004, 4, 461–464.
10. (a) Chaudhary, A. G.; Rimoldi, J. M.; Kingston, D. G. I. J. Org.
Chem. 1993, 58, 3798–3799; (b) Chen, S.-H.; Huang, S.; Kant,
J.; Fairchild, C.; Wei, J.; Farina, V. J. Org. Chem. 1993, 58,
5028–5029.
11. (a) Ettouati, L.; Ahond, A.; Poupat, C.; Potier, P. Tetrahedron
1991, 47, 9823–9838; (b) Wiegerinck, P. H. G.; Fluks, F.;
Hammink, J. B.; Mulders, S. J. E.; de Groot, F. M. H.;
van Rozendaal, H. L. M.; Scheeren, H. W. J. Org. Chem.
1996, 61, 7092–7100; (c) Poujol, H.; Ahond, A.; Al Mourabit,
A.; Chiaroni, A.; Poupat, C.; Riche, C.; Potier, P. Tetrahedron
1997, 53, 5169–5184; (d) Poujol, H.; Al Mourabit, A.;
Ahond, A.; Poupat, C.; Potier, P. Tetrahedron 1997, 53,
12575–12594.
gen bond with PTX’s C-10 acetyl group. Without this
€
34,22
modification, the Glide method (Schrodinger Inc.)
docks the taxane structures in an inverted binding pose
that directs the two C-13 side chain phenyl rings out into
solvent instead of deep within the hydrophobic pocket.
With the M-loop modified protein, however, Glide is able
to accurately predict the binding mode for PTX conforma-
tion as observed in the refined electron crystallographic
complex.32 Accordingly, taxane analogs 22 and 26 were
constructed with the C-13 T-taxane geometry and the
seco-C,D ring conformer illustrated in Figure 3 followed
by rigid Glide Docking into the complex. The best docking
poses were chosen on the basis of the Emodel scoring
function together with visualization to ensure reasonable
binding modes.
12. (a) Saicic, R. N.; Matovic, R. J. Chem. Soc., Perkin Trans. 1
2000, 59–65; (b) Matovic, R.; Saicic, R. N. J. Chem. Soc.,
Chem. Commun. 1998, 1745–1746.
13. (a) Lucas, H. Arch. Pharmacol. 1856, 85, 145–149; (b)
Appendino, G. Alkaloids: Chemical and Biological Perspec-
tives; Pelletier, S. W., Ed.; Pergamon: 1996; Vol. 11, pp 237–
268; (c) Ettouati, L.; Ahond, A.; Poupat, C.; Potier, P. J. Nat.
Prod. 1991, 54, 1455–1458; (d) Jenniskens, L. H. D.;
van Rosendaal, E. L. M.; van Beek, T. A.; Wiegerinck,
P. H. G.; Scheeren, H. W. J. Nat. Prod. 1996, 59, 117–123.
14. For a preliminary report, see: Ferjancic, Z.; Matovic, R.;
Cekovic, Z.; Snyder, J. P.; Saicic, R. N. Tetrahedron Lett.
2005, 46, 5049–5052.
Acknowledgements
This work was financially supported by the Ministry of sci-
ence, technology and development (Projects No. 1719 and
2020) and by the National Institutes of Health (Grant CA-
69571). R.N.S. is grateful to Dr. Sylviane Thoret (Institut
de Chimie des Substances Naturelles, CNRS, France) for
performing the tubulin test on 26.
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