Facile Orthoester Formation in a Model Compound of the Taxol Oxetane: Are Biologically Active Epoxy Esters, Orthoesters, and Oxetanyl Esters Latent Electrophiles?

Article abstract of DOI:10.1002/hlca.200390306

A steroidal oxetanyl ester was synthesized in eight steps as a biomimetic model of taxol oxetane. The model compound was surprisingly reactive under acidic conditions, rearranging in the absence of H₂O to a [2.2.1]-bicyclic orthoester. Both the

Full text of DOI:10.1002/hlca.200390306

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Facile Orthoester Formation in a Model Compound of the Taxol Oxetane:
Are Biologically Active Epoxy Esters, Orthoesters, and Oxetanyl Esters
Latent Electrophiles?
by Jose¬-Luis Giner* and Juan A. Faraldos
Department of Chemistry, State University of New York-ESF, 1 Forestry Drive, Syracuse, NY 13210, USA
(phone: ‡13154706895; fax: ‡13154706856; e-mail: jlginer@syr.edu)
Dedicated to Professor Duilio Arigoni on the occasion of his 75th birthday.
A steroidal oxetanyl ester was synthesized in eight steps as a biomimetic model of taxol oxetane. The model
compound was surprisingly reactive under acidic conditions, rearranging in the absence of H2O to a [2.2.1]-
bicyclic orthoester. Both the oxetanyl ester and the orthoester readily hydrolyze to produce the same triol
monoacetate. On the basis of the oxetanyl ester/orthoester rearrangement, a novel biochemical function is
suggested for the epoxy esters and oxetanyl esters found in taxoids whereby dioxonium ions, generated from
these functional groups, react with cellular proteins to form mixed orthoesters or ethers. A similar process could
be involved in the mechanism of action of natural orthoesters such as resiniferatoxin.
Introduction. The oxetane ring and its associated b-AcO group are considered
important molecular features for the anticancer activity of taxol1) (1; for a review, see
[1]), a valuable medicinal natural product found in the yew tree Taxus brevifolia [2].
Co-occurring epoxy alcohols and esters (e.g., 3, Scheme 1) are considered to represent
the biosynthetic precursors of the oxetane system, and a variety of possible mechanisms
have been proposed for this interconversion [3].
Scheme 1. Proposed Biosynthesis of Taxol (1) Oxetane by Epoxy-Ester/Cyclic-Ether Rearrangement
We assume that the biosynthetic reaction proceeds via an acid-catalyzed epoxy-
ester rearrangement of the type we have recently investigated in the formation of
[3.2.1]-bicyclic [4] and [2.2.1]-bicyclic [5] orthoesters. By our proposed mechanism
(Scheme 1), intramolecular nucleophilic displacement with inversion at the spiro-
epoxide center gives rise to a dioxonium-ion intermediate (2). Subsequent intra-
molecular displacement of the acetoxonium ion by the newly generated OH group
1
)
Taxol is a registered trademark of Bristol-Myers Squibb Co., Princeton, NJ.

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gives rise to the oxetane ring, again with inversion of configuration. The second step of
this rearrangement is analogous to the mechanism of cyclic-ether formation from
bicyclic orthoesters, a reaction that typically produces tetrahydrofurans [4c][6], but
which may also extend to oxetanes [7].
As part of a biomimetic study to explore this rearrangement, a steroidal model of
the taxol oxetane was required as a reference compound. This compound proved to be
remarkably reactive, undergoing facile rearrangement in the presence of acid to a
[2.2.1]-bicyclic orthoester.
Results. The model oxetane 12 was obtained from the known steroid 4 [8] in eight
steps (56% overall yield, Scheme 2) by means of a modification of a strategy developed
for the synthesis of taxol [9].
Scheme 2. Synthesis of Model Oxetane 12
a) OsO4, Py, Et2O; 84%. b) Ac2O, Py, DMAP, CH2Cl2; 99%. c) i-Pr2NEt, EtOCH2Cl, reflux; 95%. d) LiAlH4,
Et2O; 98%. e) TsCl, Py, DMAP; 89%. f) NaH, THF, reflux. g) cat. H2SO4, MeOH/H2O 4 :1, reflux; 83%
(2 steps). h) Ac2O, Py, DMAP, CH2Cl2; 98%. Abbrev.: DMAP ˆ 4-(dimethylamino)pyridine, EOM ˆ ethoxy-
methyl, Py ˆ pyridine, Ts ˆ tosyl.
Oxetane 12 partially decomposed during an NMR measurement in CDCl3,
hydrolyzing to the secondary acetate 13 (Scheme 3). Upon standing in 6.6 mm
trifluoroacetic acid (TFA) in CDCl3, the initial hydrolysis product (13) equilibrated
to a 1:4 mixture of acetates 13 and 14, favoring the primary acetate 14. Acetylation of
either 13 or 14 yielded the diacetate 17. The inversion of configuration at C(3) of the
hydrolysis products was established by the conversion of 13 to the known epoxy alcohol
16 [10].
Under anhydrous conditions, acidic treatment (7.9 mm TFA in anh. CDCl3) resulted
in the rearrangement of oxetane 12 to the [2.2.1]-bicyclic orthoester 15 (Scheme 3).
This process was followed by 1H-NMR via the gradual replacement (t1/2 ˆ 70 min, 258)
of the AcO Me signal at 2.05 ppm with that of an orthoacetate Me signal at 1.71 ppm.
Orthoester 15 proved very susceptible to hydrolysis, typically decomposing to mixtures
of 13 and 14. However, only the secondary acetate 13 was observed in a sample of 15

Helvetica Chimica Acta Vol. 86 (2003)
Scheme 3. Reactions of Model Oxetane 12
3615
a) MsCl, Py; 97%. b) K2CO3, MeOH; quant. c) Ac2O, Py, quant. Abbrev.: Ms ˆ mesyl (methanesulfonyl
chloride), Py ˆ pyridine.
that had been frozen in C6D6 for four weeks. Despite the reactivity of the orthoester, it
was possible to isolate pure 15 in 70% yield by means of silica-gel chromatography,
scrupulously avoiding acidic conditions by addition of Et3N to the solvent. The
presence of an orthoester was confirmed by an HMBC-NMR experiment, which
showed correlations of HÀC(3) (3.97 ppm), HÀC(28) (3.70 ppm), and the orthoa-
cetate Me (1.71 ppm) with a 13C-NMR signal (diagnostic for an orthoester) at
‡
118.1 ppm. In addition, HR-EI-MS showed a molecular ion at m/z 458.3752 (M ,
‡
C30H50O3 ; calc. 458.3762).
In contrast to the steroidal model 15, the oxetane ring of taxol (1) proved to be quite
resistant. When treated with 1m TFA in CDCl3, 1 remained largely unchanged after 3h
at 258.
Discussion. The intermediacy of [2.2.1]-bicyclic orthoesters of type 18 has long
been proposed in Lewis acid catalyzed hydrolysis of taxoid oxetanes such as 1 [11].
However, this is unnecessary, since direct hydrolysis of the intermediate dioxonium ion
2 will lead to the observed hydrolysis products 19 and 20 (Scheme 4). To date, no taxoid
[2.2.1]-bicyclic orthoesters 18 have been reported. However, we believe that, by
applying the precautions used to prepare the model orthoester 15 (strictly anhydrous
reaction conditions and purification under basic conditions), it should also be possible
to prepare such compounds.
The isomerization of the steroidal oxetane 12 to the orthoester 15 has implications
for the proposed biosynthesis of taxol (1). We have recently shown that the formation

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Scheme 4. Reactions of Taxol (1) Oxetane
of tetrahydrofurans of type 25 by the rearrangement of the corresponding epoxy esters
21 proceeds via the [3.2.1]-bicyclic orthoesters 23 and the two different dioxonium-ion
intermediates 22 and 24 (Scheme 5) [4c] (see also [6b]). If the taxoid orthoesters 18,
like the model orthoester, prove to be thermodynamically more stable than the
corresponding oxetanes (e.g., 1), then the proposed biosynthetic epoxy ester/cyclic
ether rearrangement of taxoids (Scheme 1) will, in contrast to the formation of
tetrahydrofurans, not involve the intermediacy of orthoesters, but should proceed
directly via the dioxonium ion 2.
Scheme 5. Epoxy Ester/Orthoester/Cyclic-Ether Rearrangement
The disparity in the reactivity of taxol (1) compared to the model oxetane 12 is an
interesting result. Perhaps the enhanced reactivity of 12 relative to 1 is due to increased
ring strain imparted by the more-rigid steroidal system. Alternatively, 1 may be
stabilized relative to 12 by the influence of a nearby functional group in the taxoid
system that somehow stabilizes the oxetanyl ester towards acid-catalyzed rearrange-
ment. However, while the reactivities of 1 and 12 are very different, both yield the same
types of hydrolysis products (13/14 and 19/20, resp.). These products are best
rationalized by the intermediacy of dioxonium ions 2 and 26, which are generated via 5-
exo ring closure with inversion of configuration at the secondary center of the oxetane.
In both cases, despite steric hindrance, oxetane-ring opening takes place at the
secondary (Scheme 6, Path a) instead of the primary center (Path b). No evidence
for the formation of an alternative dioxonium ion 27, with retention of configuration,
was detected. This is a similar pattern to that seen in epoxides, where enhanced
reactivity at the more-substituted site is believed to be due to the ability of electron-
donating alkyl groups to stabilize a partial positive charge in a −borderline SN2
mechanism× [12].

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Dioxonium ion 26 is also the most likely intermediate in the hydrolysis of orthoester
15, based on the initial formation of secondary acetate 13. The two alternative
dioxonium ions (28 and 29) are less likely intermediates, since they would be expected
to hydrolyze to the more stable primary acetate 14 (Scheme 6).
Scheme 6. Alternative Dioxycarbenium Ions in the Reaction Mechanisms of 12 and 15
It is interesting to speculate on the biological implications of the rearrangement of
the model oxetane 12 to the orthoester 15. Although the oxetanyl ester of taxol (1) is
relatively stable, it could be activated in a protein-bound form by conformational
distortion or changes in H-bonding to more closely approximate the high reactivity
seen with the model compound 12. The generated dioxonium ion 2 is a reactive species
that might react with a protein OH group to form a mixed taxoidÀprotein orthoester
(30, Scheme 7). This type of covalent linkage would have limited stability, being
susceptible to hydrolysis, leading to the generation of ring-opened products, either with
retention of the Ac group by the taxoid (e.g., 19) or with transfer to the protein (32).
Alternatively, the protein nucleophile could displace the acetoxonium group leading to
the formation of a more-stable ether linkage (31) (S- and N-nucleophiles are
also possible). The same scenario also applies to taxoids having the epoxy ester
substructure 3.
It is unlikely that the latent electrophilicity of the oxetane ester is of relevance to
the anticancer mechanism of taxol in tubulin binding, which has been shown to be
noncovalent based on the ability of taxol to displace [3H]-labeled taxol from
microtubules [13]. However, the biological action remains unknown for the majority
of taxoids that do not bind to tubulin and are generated by the yew tree as chemical
defenses against insects [14]. Little is also known about the binding mechanism of taxol
(1) itself in its interactions with nontubulin proteins [15]. In these cases, it is possible

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that the oxetanyl esters and epoxy esters of taxoids function as latent electrophiles
capable of reacting with cellular proteins.
Scheme 7. Possible Reactions of Taxoid Oxetanyl Esters (1) and Epoxy Esters (3) with Proteins
The speculation that taxoids containing epoxy esters and oxetanyl esters can act as
latent electrophiles raises the possibility that other natural products with similar
functionalities might act in the same way. The orthoester moieties of both the
GABAergic antagonist petuniasterone D (33) [16] and the potent vanilloid-receptor
ligand resiniferatoxin (34) [17] have been shown to be essential for their biological
activities. These biologically active orthoesters can also be regarded as latent
dioxonium ions, potentially capable of undergoing covalent reactions with proteins.

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We thank Dr. Jeffrey M. Evans (Bristol-Myers Squibb) for providing a sample of taxol.
Experimental Part
General. TLC was performed on aluminum-backed plates coated with a 0.25-mm layer of silica gel 60 F254
.
1H- and 13C-NMR spectra were acquired at 300 and 75 MHz, resp., using CDCl3 as the solvent, unless specified
1
otherwise; chemical shifts d in ppm rel. to Me4Si, coupling constants J in Hz. Only selected H-NMR signals
relevant to the structural alterations of the steroidal skeleton are given.
1. Synthesis of Steroidal Oxetane 12. 3b-Acetoxy-4b-(hydroxymethyl)-5a-cholestan-4a-ol (5). A soln. of 4
[8] (376.6 mg, 0.85 mmol) in Et2O (13ml) and pyridine (0.5 ml) was treated at r.t. with a soln. of OsO 4 (250 mg,
0.98 mmol) in benzene (3ml). After 6 h, the solvents were evaporated under reduced pressure, and the
resulting osmate was immediately hydrolyzed by adding pyridine (20 ml), H2O (15 ml), and Na2SO5 (2.5 g,
13.17 mmol). After 12 h at 208, the soln. was extracted with AcOEt. The org. layer was washed with 10% aq.
HCl and brine, dried (Na2SO4), and evaporated under reduced pressure. Column chromatography (CC) (SiO2;
AcOEt/hexanes 1:4 and 3 :2 afforded 338.7 mg (84%) of 5. Rf 0.43(hexanes/AcOEt 2 :1). 1H-NMR: 4.74
(dd, J ˆ 12.3, 4.2, 1 H); 4.03 (d, J ˆ 11.7, 1 H); 3.47 (d, J ˆ 11.7, 1 H); 2.08 (s, 3H); 0.89 ( d, J ˆ 6.6, 3H); 0.86
(d, J ˆ 6.6, 3H); 0.85 ( d, J ˆ 6.6, 3H); 0.76 ( s, 3H); 0.62 ( s, 3 H). 13C-NMR: 170.5; 83.6; 73.7; 61.8; 56.3; 56.1;
55.8; 54.4; 42.4; 39.8; 39.5; 36.8; 36.5; 36.1; 35.8; 35.2; 32.3; 28.2; 28.0; 25.1; 24.2; 23.8; 22.8; 22.6; 21.4; 21.0; 20.4;
18.6; 14.4; 12.0.
3-b-Acetoxy-4b-(acetoxymethyl)-5a-cholestan-4a-ol (6). A soln. of DMAP (4-(dimethylamino)pyridine;
30 mg, 0.25 mmol) in Ac2O/pyridine 1 :2 (1.5 ml) was added to a stirred soln. of 5 (216.7 mg, 0.46 mmol) in anh.
CH2Cl2 (5 ml) at 208. After 45 min, the solvents were evaporated under reduced pressure, the residue was taken
up in Et2O and filtered through SiO2 to afford, after evaporation, 234.1 mg (99%) of 6. Rf 0.51 (hexanes/AcOEt
2 :1). 1H-NMR: 4.69 (dd, J ˆ 12.1, 4.8, 1 H); 4.52 (d, J ˆ 12, 1 H); 4.10 (d, J ˆ 12, 1 H); 2.08 (s, 3H); 2.06
(s, 3H); 0.89 ( d, J ˆ 6.3, 3 H); 0.86 (d, J ˆ 6.6, 3H); 0.85 ( d, J ˆ 6.6, 3H); 0.84 ( s, 3H); 0.63( s, 3 H). 13C-NMR:
171.4; 170.6; 81.7; 74.0; 63.7; 56.2; 56.1; 55.9; 55.3; 42.4; 39.7; 39.5; 36.7; 36.6; 36.1; 35.7; 35.1; 32.5; 28.2; 28.0;
25.1; 24.2; 23.8; 22.8; 22.5; 21.2; 21.0; 20.9; 20.8; 18.6; 14.2; 12.0. Anal. calc. for C32H54O5: C 74.08, H 10.49;
found: C 73.83, H 10.41.
3b-Acetoxy-4b-(acetoxymethyl)-4a-(ethoxymethoxy)-5a-cholestane (7). To
a soln. of 6 (234.1 mg,
0.45 mmol) in 10 ml of i-Pr2NEt was added, dropwise, EtOCH2Cl (1 ml, 1.02 g, 10.8 mmol) at 208 under N2.
The mixture was stirred for 0.5 h and then refluxed for 1.5 h. It was quenched at 208 with 10% aq. HCl. (100 ml)
and extracted with Et2O. The org. phase was washed with brine and dried (Na2SO4). After evaporation of the
solvents, the residue was purified by flash chromatography (FC; hexanes/AcOEt 9 :1) to afford 240.3mg (95%)
of 7. Rf 0.41 (hexanes/AcOEt 4 :1). 1H-NMR: 4.92 (dd, J ˆ 12.0, 4.8, 1 H); 4.87 (d, J ˆ 7.8, 1 H); 4.72 (d, J ˆ 7.8,
1 H); 4.66 (d, J ˆ 12.9, 1 H); 4.33 (d, J ˆ 12.9, 1 H); 3.63 (m, 1 H); 3.51 (m, 1 H); 2.09 (s, 3H); 2.03( s, 3H); 1.15
(t, J ˆ 6.9, 2 H); 0.90 (s, 3H); 0.89 ( d, J ˆ 6.7, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.85 ( d, J ˆ 6.6, 3H); 0.64 ( s, 3 H).
13C-NMR: 170.8; 170.1; 89.2; 79.1; 76.9; 64.2; 63.8; 56.6; 56.2; 56.1; 53.2; 42.5; 39.8; 39.5; 37.4; 36.7; 36.6; 36.1;
35.7; 35.4; 32.9; 28.2; 28.0; 25.4; 24.2; 23.8; 22.8; 22.5; 22.0; 21.3; 21.2; 21.0; 18.6; 14.9; 14.2; 12.0.
4a-(Ethoxymethoxy)-4b-(hydroxymethyl)-5a-cholestan-3b-ol (8). LiAlH4 (200 mg, 5.3mmol) was added
to a stirred soln. of 7 (230.5 mg, 0.41 mmol) in 10 ml of anh. Et2O at 208. After 15 min, the reaction was
quenched by careful addition of 10% aq. HCl acid (20 ml), and the mixture was extracted with Et2O. The org.
layer was washed with brine, dried (Na2SO4), and evaporated to afford 207.0 mg (98%) of 8. Rf 0.43(hexanes/
1
AcOEt 1:1). H-NMR: 5.03( d, J ˆ 10.8, 1 H); 5.00 (d, J ˆ 10.8, 1 H); 4.35 (d, J ˆ 5.1, OH); 4.22 (dd, J ˆ 12.3,
5.1, 1 H); 3.82 3.64 (m, 4 H); 3.52 (t, J ˆ 6, OH); 1.24 (t, J ˆ 7.2, 2 H); 0.89 (d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6,
3H); 0.85 ( d, J ˆ 6.6, 3H); 0.84 ( s, 3H); 0.62 ( s, 3 H). 13C-NMR: 89.6; 82.0; 76.8; 64.1; 63.6; 56.5; 56.4; 56.1;
52.5; 42.4; 39.7; 39.5; 37.4; 36.9; 36.1; 35.7; 35.1; 32.6; 28.2; 28.2; 28.0; 24.2; 23.8; 22.8; 22.5; 20.9; 20.8; 18.6; 15.2;
14.8; 12.0.
4a-(Ethoxymethoxy)-4b-({[(4-methylphenyl)sulfonyl]oxy}methyl)-5a-cholestan-3b-ol (9). To a stirred
soln. of 8 (200.1 mg, 0.41 mmol) and DMAP (15 mg, 0.12 mmol) in anh. pyridine (15 ml) was added 4-
methylbenzenesulfonyl chloride (TsCl; 1.5 g, 7.87 mmol). After 17 h at 208 under N2, the reaction was quenched
by adding H2O, and the mixture was extracted with hexanes/AcOEt 2 :1. Drying (Na2SO4), removal of solvents
under reduced pressure, and CC (hexanes/AcOEt 19 :1, 9 :1, and 4 :1) afforded 222.7 mg (89%) of 9. Rf 0.58
(hexanes/AcOEt 2 :1). 1H-NMR: 7.80 (d, J ˆ 8.4, 2 H); 7.34 (d, J ˆ 8.1, 2 H); 4.68 (d, J ˆ 10.2, 1 H); 4.66 (d, J ˆ
10.2, 1 H); 4.53( d, J ˆ 12.0, 1 H); 4.29 (s, OH); 4.27 (d, J ˆ 12.0, 1 H); 3.78 3.64 (m, 1 H); 3.57 3.47 (m, 1 H);
1.18 (t, J ˆ 6.9, 3H); 0.88 ( d, J ˆ 6.9, 3H); 0.86 ( d, J ˆ 6.9, 3H); 0.85 ( d, J ˆ 6.9, 3H); 0.76 ( s, 3H); 0.63( s, 3 H).

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13C-NMR: 144.8; 132.6; 129.7 (2 C); 128.1 (2 C); 88.6; 81.3; 75.8; 68.1; 63.9; 56.7; 56.4; 56.2; 53.7; 42.3; 39.8; 39.5;
37.3; 36.9; 36.1; 35.7; 35.2; 33.1; 28.2; 28.0; 27.0; 24.2; 23.8; 22.8; 22.5; 22.0; 21.6; 20.8; 18.6; 15.0; 14.0; 11.9.
3b,4b-(Epoxymethano)-4a-(ethoxymethoxy)-5a-cholestane (ˆ(2aS,4aR,4bR,6aR,9aS,9bS,11aR,11bR)-7-
[(1R)-1,5-dimethylhexyl]-11b-(ethoxymethoxy)octadecahydro-4a,6a-dimethylcyclopenta[7,8]phenanthro[2,1-
b]oxete; 10). A soln. of 9 (222.7 mg, 0.36 mmol) in anh. THF (10 ml) was added under N2 to a suspension of
NaH (96 mg, 4 mmol) in THF (10 ml) at 208. After 30 min at reflux, H2O was added dropwise, and the mixture
was extracted with Et2O. The org. layer was dried (Na2SO4) and evaporated under reduced pressure to afford
1
crude 10. Rf 0.76 (hexanes/AcOEt 2 :1). H-NMR: 4.94 (d, J ˆ 8.1, 1 H); 4.91 (d, J ˆ 7.5, 1 H); 4.78 (d, J ˆ 7.5,
1 H); 4.45 (d, J ˆ 14.1, 1 H); 4.43( d, J ˆ 14.1, 1 H); 3.65 (q, J ˆ 6.9, 2 H); 1.21 (t, J ˆ 6.9, 3H); 1.18 ( s, 3H); 0.90
(d, J ˆ 6.6, 3H); 0.87 ( d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.66 ( s, 3 H). 13C-NMR: 90.2; 83.8; 80.4; 75.4; 63.5;
56.6; 56.2; 55.0; 48.9; 42.4; 39.9; 39.5; 36.1; 36.0; 35.8; 35.4; 33.4; 31.6; 28.2; 28.0; 27.4; 24.2; 23.8; 22.8; 22.6; 20.9;
20.2; 18.6; 15.2; 12.1; 12.0.
3b,4b-(Epoxymethano)-5a-cholestan-4a-ol (ˆ(2aS,4aR,4bR,6aR,9aS,9bS,11aR,11bR)-7-[(1R-1,5-dimeth-
ylhexyl)-11b-hydroxyoctadecahydro-4a,6a-dimethylcyclopenta[7,8]phenanthro[2,1-b]oxete; 11). Removal of the
ethoxymethyl group was accomplished by dissolving 10 in MeOH/H2O 4 :1 (15 ml) and adding a 1% soln. of
H2SO4 in THF (2 ml). The resulting mixture was stirred at reflux for 2.5 h and then quenched with pyridine
(0.5 ml). Partial evaporation under reduced pressure gave an aq. soln., which was extracted with Et2O. The org.
layer was dried (Na2SO4), and the solvent was removed under reduced pressure. FC (SiO2; AcOEt/hexanes 1:9
and 1:4) afforded 123.7 mg (83% from 9) of 11. Rf 0.41 (hexanes/AcOEt 2 :1). 1H-NMR: 4.75 (dd, J ˆ 9.0, 2.7,
1 H); 4.54 (d, J ˆ 7.5, 1 H); 4.30 (d, J ˆ 7.5, 1 H); 1.15 (s, 3H); 0.90 ( d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.86
(d, J ˆ 6.6, 3H); 0.66 ( s, 3 H). 13C-NMR: 87.8; 78.1; 75.4; 56.6; 56.2; 54.8; 52.3; 42.4; 39.9; 39.5; 36.1; 35.9; 35.8;
35.3; 33.3; 31.6; 28.2; 28.0; 27.2; 24.1; 23.8; 22.8; 22.5; 20.8; 20.4; 18.6; 12.0; 11.7. Anal. calc. for C28H48O2: C 80.71,
H 11.61; found: C 80.56, H 11.70.
4a-Acetoy-3b,4b-(epoxymethano)-5a-cholestane (ˆ(2aS,4aR,4bR,6aR,9aS,9bS,11aR,11bR)-11b-acetoxy-7-
(1R-1,5-dimethylhexyl)octadecahydro-4a,6a-dimethylcyclopenta[7,8]phenanthro[2,1-b]oxete; 12). A soln. of 11
(113.1 mg, 0.27 mmol) in CH2Cl2 (7 ml) and pyridine (2 ml) was treated with Ac2O (0.9 ml) and DMAP (15 mg,
0.12 mmol). After 16 h at 208, the mixture was partitioned between Et2O and H2O. The org. layer was filtered
through SiO2 and dried (Na2SO4). Solvents were removed under reduced pressure to afford 121.8 mg (98%) of
12. Rf 0.43(hexanes/AcOEt 9 :1). 1H-NMR (600 MHz): 5.03( d, J ˆ 8.5, 1 H); 4.42 (br. s, 2 H); 2.25 (m, 1 H);
2.05 (s, 3H); 1.98 ( dt, J ˆ 12.6, 3.4, 1 H); 1.74 (dd, J ˆ 13.2, 3.4, 1 H); 1.20 (s, 3H); 0.91 ( d, J ˆ 6.5, 3H); 0.87
(d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.67 ( s, 3 H). 13C-NMR: 169.7; 84.0; 81.8; 73.9; 56.5; 56.2; 54.4; 47.4; 42.4;
39.9; 39.5; 36.3; 36.2; 35.8; 35.3; 32.9; 31.5; 28.2; 28.0; 27.4; 23.8; 22.8; 22.6; 21.8; 20.8; 20.2; 18.6; 12.1; 11.7. Anal.
calc. for C30H50O3: C 78.55, H 10.99; found: C 78.44, H 11.23.
2. Reactions of Model Steroidal Oxetane 12. 2.1. Hydrolysis of (12). 3a-Acetoxy-4b-(hydroxymethyl)-5a-
cholestan-4a-ol (13). According to NMR measurement of 12 (5.6 mg, 0.01 mmol) in alumina-filtered CDCl3
(0.75 ml), 20% of the starting oxetane was converted to a new compound after 20 min (1H-NMR). Evaporation
of the solvent with a stream of N2 followed by prep. TLC (SiO2; hexanes/AcOEt 9 :1) afforded starting material
(4.4 mg, 79%) and 13 (1.1 mg, 20%). 1H-NMR: 5.21 (t, J ˆ 2.4, 1 H); 3.63 (br. s, 2 H); 2.32 (s, OH); 2.14 (s, 3 H);
0.90 (d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.80 ( s, 3H); 0.64 ( s, 3 H). 13C-NMR: 170.9;
74.4; 72.2; 62.8; 56.5; 56.2; 55.9; 50.7; 42.4; 39.8; 36.8; 36.2; 35.8; 35.2; 32.5; 31.8; 28.2; 28.0; 24.2; 23.8; 23.6; 22.8;
22.6; 21.4; 20.9; 20.2; 18.7; 14.1; 12.0.
2.1.1. Stereochemical Assignment of 13. 3a-Acetoxy-4b-{[(methylsulfonyl)oxy]methyl}-5a-cholestan-4a-ol.
A soln. of 13 (1.5 mg, 0.003mmol) in anh. pyridine (0.5 ml) was treated with methanesulfonyl chloride (MsCl;
10 ml, 0.13mmol) at 20 8 under N2. After 0.5 h, the reaction was quenched by adding 10% aq. HCl (1 ml), and the
mixture was extracted with Et2O. The org. layer was filtered through a pad of SiO2, evaporated with a stream of
N2, and purified by prep. TLC (SiO2; hexanes/AcOEt 2 :1) to afford 1.7 mg (97%) of the mesylate (precursor of
1
16). H-NMR: 5.17 (t, J ˆ 2.4, 1 H); 4.43( d, J ˆ 10.8, 1 H); 4.21 (d, J ˆ 10.8, 1 H); 3.10 (s, 3H); 2.34 ( s, OH);
2.14 (s, 3H); 0.90 ( d, J ˆ 6.6, 3H); 0.87 ( d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.84 ( s, 3H); 0.64 ( s, 3 H).
(4R)-Spiro[5a-cholestan-4,2'-oxiran]-3a-ol (16). Treatment of the above mesylate (1.7 mg, 0.003mmol)
with K2CO3 in MeOH at 208 overnight gave, after evaporation of the solvent with a stream of N2, extraction of
the residue with Et2O, and filtration through SiO2, 16 [10] in quant. yield: 1H-NMR: 3.36 (br. s, 1 H); 2.87
(d, J ˆ 4.5, 1 H); 2.61 (d, J ˆ 4.5, 1 H); 0.90 (d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.83
(s, 3H); 0.65 ( s, 3 H).
2.1.2. Isomerization of 13 to 14. 4b-(Acetoxymethyl)-5a-cholestane-3a,4a-diol (14). A soln. of 13 (3.6 mg,
0.008 mmol) in CDCl3 (0.75 ml) was treated with 10 ml of a 0.5m soln. of trifluoroacetic acid (TFA) in CDCl3
(final conc.: 6.6 mm TFA). After 3.75 h at 208, the solvents were evaporated with a stream of N2 to afford a 1 :4

Helvetica Chimica Acta Vol. 86 (2003)
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mixture of 13 and 14 in quant. yield. Prep. TLC (SiO2; hexanes/AcOEt 9 :1) gave pure 14. 1H-NMR: 4.23( d, J ˆ
12, 1 H); 3.98 (d, J ˆ 12, 1 H); 3.69 (t, J ˆ 2.4, 1 H); 2.45 (s, OH); 2.38 (s, OH); 2.01 (s, 3H); 0.79 ( d, J ˆ 6.6,
3H); 0.76 ( d, J ˆ 6.6, 3H); 0.76 ( d, J ˆ 6.6, 3H); 0.74 ( s, 3H); 0.54 ( s, 3 H). 13C-NMR: 171.1; 74.3; 68.4; 65.4;
56.4; 56.1; 55.9; 48.6; 42.4; 39.8; 39.5; 36.7; 36.2; 35.8; 35.1; 32.5; 30.7; 28.2; 28.0; 25.0; 24.2; 23.8; 22.8; 22.6; 20.9;
20.8; 20.4; 18.7; 14.1; 12.0.
3a-Acetoxy-4b-(acetoxymethyl)-5a-cholestan-4a-ol (17). Acetylation of either 13 or 14 with Ac2O/pyridine
gave 17 in quant. yield. 1H-NMR: 5.09 (t, J ˆ 2.8, 1 H); 4.38 (d, J ˆ 11.7, 1 H); 4.01 (d, J ˆ 11.7, 1 H); 2.30
(s, OH); 2.13( s, 3H); 2.13 ( s, 3H); 0.90 ( d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.86 ( d, J ˆ 6.6, 3H); 0.86
(s, 3H); 0.64 ( s, 3 H). 13C-NMR: 171.5; 171.0; 73.5; 71.4; 65.1; 56.4; 56.2; 55.9; 50.1; 42.4; 39.7; 39.5; 36.7; 36.1;
35.8; 35.1; 32.4; 31.6; 28.2; 28.0; 24.1; 23.8; 23.7; 22.8; 22.6; 21.4; 20.9; 20.8; 20.2; 18.7; 14.2; 12.0. Anal. calc. for
C32H54O5: C 74.08, H 10.49; found: C 73.95, H 10.55.
2.2. Rearrangement of 12. Steroidal Orthoester 15. A soln. of 12 (12.1 mg, 0.03mmol) in anh. CDCl 3
(0.75 ml) was treated with 10 ml of a 0.5m soln. of TFA in CDCl3 (final conc.: 6.6 mm TFA) at 258 under N2. After
150 min, the reaction was quenched with 10 ml of Et3N, and the mixture was directly applied to a prep. TLC plate
of SiO2 (SiO2, pretreated with 5% Et3N in Et2O). Development with 2% Et3N in hexanes and elution with 2%
Et3N in Et2O gave a 1:2 mixture of 13 and 14 (1.5 mg, 12%), and orthoester 15 (8.4 mg, 70%). 1H-NMR
(600 MHz, C6D6): 3.89 (dd, J ˆ 6.1, 11.4, 1 H); 3.69 (d, J ˆ 6.2, 1 H); 3.57 (d, J ˆ 6.2, 1 H); 1.86 (s, 3H); 1.01
‡
(d, J ˆ 6.5, 3H); 0.93( d, J ˆ 6.6, 3H); 0.93( d, J ˆ 6.6, 3H); 0.63( s, 3H); 0.48 ( s, 3H). EI-MS: 458 (10, M ),
‡
‡
443 (9), 416 (18), 399 (31), 398 (100), 370 (48), 243 (62), 95 (37). HR-EI-MS: 458.3752 (M , C30H50O3 ; calc.
458.3762).
Hydrolysis of Orthoester 15. A soln. of 15 (3.5 mg, 0.008 mmol) in CDCl3 (0.75 ml) was treated with 10 ml of
a 10% soln. of TFA in H2O. After 10 min at 258, 1H-NMR (600 MHz) showed a ca. 1:1 mixture of 13 and 14.
Spontaneous hydrolysis was observed in a sample of 15 that had been frozen in C6D6 for four weeks. In this case,
only the secondary acetate 13 was observed.
3. Acid Treatment of Taxol (1). Compound 1 (10.2 mg, 0.012 mmol) was treated in 0.8 ml of a 1m soln. of
TFA in CDCl3 at 258 and under N2. After 3h, the reaction was quenched with Et 3N, and the mixture was
evaporated with a stream of N2 to recover mainly starting material.
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Received August 26, 2003