Synthesis of dimeric and tetrameric macrolactams with cytotoxic activity

Article abstract of DOI:10.1139/v00-080

Cyclization of amino acid 14, in turn prepared from 4,4-dimethylcyclohex-2-en-1-one by standard chemistry, yielded the dimeric and tetrameric macrolactams 15 and 16, respectively, representing the first examples of 'taxoid-like' compounds with heterocycli

Full text of DOI:10.1139/v00-080

925
Synthesis of dimeric and tetrameric macrolactams
with cytotoxic activity1
Gabriella Gentile, Daniela Fattori, Maurizio Botta, Federico Corelli,
Domenico Fusar-Bassini, and Doriano Lamba
Abstract: Cyclization of amino acid 14, in turn prepared from 4,4-dimethylcyclohex-2-en-1-one by standard chemistry,
yielded the dimeric and tetrameric macrolactams 15 and 16, respectively, representing the first examples of “taxoid-
like” compounds with heterocyclic B rings. Compounds 15 and 16 exhibited moderate antiproliferative activity in an in
vitro cytotoxicity assay. Consequently, an orthogonally diprotected, dimeric macrolactam 19 was synthesized and, after
selective deprotection of the hydroxy group, was coupled with the docetaxel side chain to give the final compound 2,
showing three-dimensional shape similarity to paclitaxel. Contrary to our expectations, 2 proved to be as active as the
parent compound 15. The structure of 15 has been confirmed by X-ray crystallographic analysis and is also reported.
Key words: macrolactams, macrocyclic compounds, taxane analogs, cytotoxic activity.
Résumé: La cyclisation de l’acide aminé 14, préparé à partir de la 4,4-diméthylcyclohex-2-én-1-one donne des
macrolactames dimère 15 et tétramère 16 qui sont les premiers exemples de composés de type taxoïde avec un noyau
B hétérocyclique. Les composés 15 et 16 exhibent une activité antiprolifère modérée lors d’un essai de cytotoxicité in
vitro. En conséquence, on a synthétisé un macrolactame dimère 19, ayant orthogonalement deux groupes protecteurs, et
après une déprotection sélective du groupe hydroxyle, on l’a couplé avec la chaîne latérale du docétaxel pour obtenir le
produit final 2, dont la forme tridimensionnelle est semblable à celle du paclitaxel. Contrairement à nos attentes, le
composé 2 s’est révélé aussi actif que le composé parent 15. On rapporte la structure du composé 15 que l’on a
confirmée par cristallographie de rayons X.
Mots clés: macrolactames, composés macrocycliques, analogues du taxane, activité cytotoxique.
[Traduit par la Rédaction] Gentile et al. 934
Introduction
has been recently reported in the literature that several
macrocyclic compounds, such as taxuspines, sarcodictyins
(and related eleutherobin and eleuthosides), epothilones,
discodermolide, and rhazinilam (and related nine-membered
macrolactams), though belonging to different chemical fami-
lies, exert antitumor activity through a paclitaxel-like mech-
anism (for taxuspines see ref. (8a); sarcodictyins,
eleutherobin, eleuthosides see ref. (8b); epothilones see
ref. (8c); discodermolide see ref. (8d); rhazinilam and
analogs thereof see ref. (8e)). In the course of our investiga-
tions on taxane systems (9) and on new analogues of
paclitaxel and docetaxel (10), we conceived the possibility
of preparing heterocyclic analogs which are expected to be
more water-soluble (11).
In the last years paclitaxel (1) (Fig. 1) has proven to be a
major achievement in the field of cancer chemotherapy, par-
ticularly for the treatment of advanced ovarian and breast
cancer (for several reviews on paclitaxel see ref. (1)). Due to
the novelty of its mode of action (2) as well as to its unique
structural complexity (3), it has been the objective of total
synthetic (4) and structure–activity relationship studies (for
selected recent contributions see ref. (5)). Intense efforts
have been directed toward structural modifications of
paclitaxel aimed at defining its pharmacophore (6) and de-
veloping analogues with better therapeutic profile and im-
proved water-solubility (7a) (for a review see ref. (7b)). It
Received June 9, 1999. Published on the NRC Research Press website on June 15, 2000.
Dedicated to Professor Stephen Hanessian in recognition of his significant contribution to the art of organic synthesis
G. Gentile,2 D. Fattori,3 M. Botta,4 and F. Corelli.4 Dipartimento Farmaco Chimico Tecnologico, Università degli Studi di Siena,
Banchi di Sotto 55, 53100 Siena, Italy.
D. Fusar-Bassini. Chemistry Department, Pharmacia & Upjohn, Via Giovanni XXIII, 20014 Nerviano, Milano, Italy.
D. Lamba. International Centre for Genetic Engineering and Biotechnology, Area Science Park, Padriciano 99, I-34012 Trieste,
Italy and Istituto di Strutturistica Chimica “G. Giacomello” C.N.R, Sezione di Trieste, Area Science Park, SS 14, Km 163.5,
Basovizza, I-34012 Trieste, Italy.
1Taken in part from: Gentile, G. Ph.D. Thesis, University of Siena, February 1998.
2Present address: GlaxoWellcome, Via Fleming 4, 37135 Verona, Italy.
3Present address: IRBM, Via Pontina km 30,600, 00040 Pomezia, Roma, Italy.
4Authors to whom correspondence may be addressed. Telephone: +39 0577 45393. Fax: +39 0577 232183. e-mail: botta@unisi.it;
corelli@unisi.it
Can. J. Chem. 78: 925–934 (2000)
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Can. J. Chem. Vol. 78, 2000
Fig. 1. Top: Structure of paclitaxel (1) and macrolactam 2. Bottom: stereoview of the superimposition between lowest-energy confor-
mations of 2 (thick) and paclitaxel (thin).
sertion of a side chain at C-5 was realized through TiCl4-cat-
alyzed reaction of allyltrimethylsilane with the enone system
of 6 to give the allyl derivative 7 as the only isomer. The
trans relative stereochemistry was assigned according to the
usual stereochemical outcome of the Sakurai reaction (15a,
b) (for a discussion of the stereochemistry of conjugate addi-
tion reactions to 5-substituted cyclohex-2-ene-1-one deriva-
tives, see ref. (15c). Oxidative cleavage of the double bond
afforded the aldehyde 8, which was in turn subjected to
Wadsworth–Emmons olefination with methyl (triphenyl-
phosphoranylidene)acetate to give in 72% yield the unsatu-
rated esters 9 as a 9:1 E:Z mixture.6 Subsequent ketalization
of 9 under standard conditions afforded compound 10. This
key intermediate (Scheme 2) was hydrogenated to give the
saturated amine 11 and hydrolyzed to the free acid 12 in
92% and 93% yield, respectively. These two synthons were
coupled to afford the amide 13, which was in turn advanced
to the amino acid 14.
In particular, the superimposition between the lowest-energy
conformation of the dimeric macrolactam 2 and the lowest-
energy conformation of 1 (10) highlighted good shape simi-
larity between the partners (Fig. 1) and led us to identify 2
as an interesting heterocyclic mimic of paclitaxel. We wish
to describe herein our synthetic efforts in this direction.
Results and discussion
Enone 3 (12) (Scheme 1) was subjected to hydrocyanation
(13) followed by ketalization and partial hydrolysis of the
nitrile to give the primary amide 4 in 60% overall yield.
Hofmann rearrangement of this amide was best accom-
plished by treatment with bromine and sodium hydroxide.
The resulting amine was protected as the corresponding Cbz
derivative and treated with HCl in THF to give 5. The trans-
formation of 5 into 6 could be efficiently5 performed follow-
ing the Saegusa procedure (14). Reaction of 5 with TMSCl
in the presence of triethylamine afforded a silyl enol ether
mixture, which was treated with palladium(II) acetate in
refluxing acetonitrile in the presence of CaCO3. Chromato-
graphic purification on silica gel led to the enone 6 as the
sole reaction product in an overall yield of 70% from 5. In-
Having secured an access to 14, several procedures based
on the condensing reagents for peptide synthesis were inves-
tigated to accomplish its cyclization to the 16-membered
lactam 15 (Scheme 3). DPPA-mediated cyclization of 14
provided the macrolactam 15 as a colorless solid in a most
5 Attempts to convert 5 to 6 by regioselective bromination-dehydrobromination sequence afforded unsatisfactory results as a complex mixture
of compounds was obtained. On the other hand, direct reaction of 5 with PhSeCl showed little regioselectivity providing, after selenoxide
elimination, a mixture of the enone 6 and its regioisomer.
6 For simplicity, only the predominant (E)-isomer of compounds 9, 10, 12, 13, 17, and 18 are shown in the Schemes.
© 2000 NRC Canada

Gentile et al.
927
Scheme 1. Key: (a) KCN, EtOH, H2O, AcOH, 40°C;
(b) HOCH2CH2OH, CSA, toluene, reflux; (c) NaOH, 30% H2O2,
EtOH, 50°C; (d) Br2, NaOH, 0–80°C; (e) CbzCl, NaHCO3,
EtOH, H2O; (f) Hcl, THF; (g) (i) LiHMDS, TMSCl, Et3N, THF,
–78°C; (ii) Pd(OAc)2, CaCo3, CH3CN, reflux; (h)
allyltrimethylsilane, TiCl4, CH2Cl2, –78 to –30°C; (i) OsO4,
NaIO4, Et2O, H2O, rt; (j) methyl (triphenylphosphoranylidene)ac-
etate, toluene, reflux.
nearly perfect plane, with C-11, in the flap position, dis-
placed out of the plane by 0.337 Å. The cyclohexane moiety
exhibits a chair conformation, with C-3 and C-6 atoms dis-
placed out of the plane defined by C-1, C-2, C-4, and C-5 by
0.647 Å and 0.643 Å, respectively, at opposite sites, the
Cremer and Pople (17) puckering parameters being Q =
0.547 Å, θ = 3.4°, and φ = 42.4°.
Compounds 15 and 16 were evaluated in an in vitro
cytotoxicity assay on a B16-F10 murine melanoma cell line
and in a tubulin polymerization assay (18) performed spec-
trophotometrically. While no activity against tubulin assem-
bly was observed, 15 and 16 displayed an IC50 value of
45 µM and 37 µM, respectively, in the cytotoxicity test. Al-
though no attempt has been made to establish the mecha-
nism of the antiproliferative activity of the tested compounds,
the preliminary biological results indicated that 158 might
have been of interest as a lead compound in the search of
new antitumor agents in spite of its modest cytotoxic activity
(to the best of our knowledge, the tested compounds consti-
tute the first example of “taxoid-like” compounds with
heterocyclic B rings. For an example of analogs oxygenated
in the C ring, see ref. (19)). In fact, it should be kept in mind
that baccatin (i.e., paclitaxel without a C-13 side chain) itself
shows a comparable cytotoxicity, which increases by four
orders of magnitude after esterification at C-13 with the ap-
propriate N-acylphenylisoserine side chain (20).
It now remained to elaborate the dimeric macrolactam 15
by introducing an N-acylphenylisoserine chain to enhance its
cytotoxic activity. Since an attempt to selectively cleave only
one ketal group in 15 proved to be unpractical because of
the formation of mixtures, we turned our attention to ketone
9. Reduction with NaBH4 (Scheme 4) provided two isomers
(in the ratio 7:1)9 of the expected alcohol, whose structures
were assigned on the basis of their 1H NMR spectrum,
which revealed an intramolecular hydrogen bonding between
the OH and NH groups (which must be therefore cis) in the
minor, less polar, isomer (δNH 6.23 vs. 4.88 ppm). Silylation
of the hydroxy group of the major isomer followed by hy-
drolysis of methyl ester gave the acid 17 in 78% overall
yield from 9. Acylation of amine 11 with 17 under mixed
anhydride conditions led to the amide 18, which was ad-
vanced to the orthogonally protected macrolactam 19.
TBAF-mediated desilylation of 19 provided the hydroxy
derivative 20, which by reaction with (4S,5R)-N-(tert-but-
oxycarbonyl)-2-(2,4-dimethoxyphenyl)-4-phenyloxazolidine-5-
carboxylic acid (21) as one (unspecified) stereoisomer (21)
in the presence of N-cyclohexyl-N ′-(2-morpholinoethyl)car-
bodiimide methyl p-toluenesulfonate (CMC) and 4-dimethyl-
aminopyridine (DMAP), followed by selective cleavage of
the aminoacetal group, was converted to the intended target
2 (an inseparable mixture of stereoisomers) in 32% overall
yield from 19.
satisfactory yield of 28%, along with the tetramer 16 (10%).
500 MHz H NMR spectroscopy revealed that compound 15
1
was indeed a mixture of two isomers in the ratio 3:1, as evi-
denced by the presence of two doublets at 7.16 and
7.02 ppm relative to the NH groups and integrating for 1.5
and 0.5 protons, respectively.7 Although attempts to separate
these isomers by HPLC were unsuccessful, the major isomer
was obtained in pure form by two successive recrystalli-
zations from methanol–water.
X-ray crystallographic analysis of this sample (Fig. 2)
showed that the structure is centrosymmetric, with an R-rela-
tive configuration of both the chiral centers at C-2 and C-6
and a trans conformation of the N-1—C-9 amide bond. In
terms of the conformational descriptors (16) (P = 56.5° and
τ = 24.0°), an envelope conformation can be assigned to the
dioxolane ring. The C-4, O-2, O-3, and C-10 atoms define a
In the cytotoxicity and tubulin polymerization assays 2
showed approximately the same biological profile as 15.
This result, though discouraging, is not completely surpris-
ing, since it is well known that neither an intact taxane
7 No coalescence of these signals was observed by raising temperature until 80°C.
8 Although 16 has proven to be the most active compound, it was not further considered as a possible lead owing to its molecular weight
> 1000.
9 Attempts to improve the stereoselectivity of the reduction by using bulkier reagents (L-selectride) or lower reaction temperature
(–30 to –60°C) were unsuccessful.
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Can. J. Chem. Vol. 78, 2000
Scheme 2. Key: (a) H2, 60 psi, 10% Pd/C, EtOH; (b) LiOH, MeOH–H2O, 45°C; (c) i-BuOCOCl, N-methylmorpholine, CH2Cl2, 0°C;
(d) DPPA, NaHCO3, DMF.
Fig. 2. Perspective view and arbitrary atom labelling of com-
pound 15. Thermal ellipsoids are drawn at the 50% probability
level; symmetry transformation used to generate equivalent atoms
(′) is (–x + 1, –y + 1, –z + 1).
Scheme 3. Key: (a) DPPA, NaHCO3, DMF.
paclitaxel side chain may be preorganized through hydrogen
bonding and may fit into a hydrophobic cleft on the taxane
binding site, which stabilizes the drug–tubulin interaction
(5b). Therefore, the moderate cytotoxicity of compounds 2
and 15 is likely to be due to mechanism(s) not involving
microtubule stabilization.
Experimental
Materials
skeleton nor a phenylisoserine side chain are structural ele-
ments per se sufficient for cytotoxic and in vitro microtubule
assembly activity. Thus, even if the inactive baccatin III be-
comes the fully active paclitaxel by esterification at C-13
with the appropriate chain, esterification of simpler cyclic
alcohols with N-acylphenylisoserine does not lead to com-
pounds with paclitaxel-like activities. Although the role of
the paclitaxel side chain in affecting biological activity has
not yet been fully determined, it is thought that the principal
recognition partner is the taxane skeleton itself. The
Reagents were from commercial suppliers and used with-
out further purification. All solvents were dried over stan-
dard drying agents (22) and freshly distilled prior to use.
Instrumentation and techniques
Merck silica gel 60 was used for chromatography (70–230
mesh) and flash chromatography (230–400 mesh) columns.
Analytical and preparative TLC were performed with Merck
silica gel 60 F254 (0.2 mm and 2 mm thickness, respectively)
precoated aluminum sheets with visualization by UV light,
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Gentile et al.
929
Scheme 4. Key: (a) (i) NaBH4, MeOH; (ii) TBMDSCl,
imidazole, DMF, 70°C; (iii) LiOH, MeOH, H2O, 40°C; (b) 11, I-
BuOCOCl, N-methylmorpholine, CH2Cl2, 0°C; (c) see (b), (a) in
Scheme 2 and (a) in Scheme 3; (d) TBAF, THF; (e) CMC,
DMAP, toluene; (f) 0.1 N HCl, MeOH.
After stirring at 40°C for 4 h, the mixture was cooled to rt
and filtered through a short plug of silica gel. The solution
was evaporated and the residue was chromatographed on sil-
ica gel (toluene) to give 6.50 g (71%) of an oil, which crys-
tallized on standing; mp 78–79°C. This compound was
dissolved in a mixture of toluene (200 mL) and ethylene gly-
col (20 mL) containing camphorsulfonic acid (CSA,
400 mg). After stirring for 8 h under reflux with azeotropical
removal of water, the mixture was cooled to rt and washed
with 5% aqueous NaHCO3 and brine, then evaporated under
reduced pressure. The oily residue was dissolved in 95%
EtOH (15 mL) and treated with 6 N NaOH (15 mL) and
30% H2O2 (15 mL). The mixture was heated at 50°C for 6 h,
cooled to rt, and diluted with CH2Cl2 and 5% aqueous
Na2S2O3. The organic layer was extracted twice with
CH2Cl2 and the combined organic solution was washed with
brine, dried, and evaporated to afford 4 (6.42 g, 60% overall
yield from 3) as a white solid, which was purified by
trituration with hexanes–Et2O, mp 178–182°C. IR (KCl):
3628, 3395, 1662, 1546 cm–1. 1H NMR δ: 5.65 (d, 2H), 3.91
(s, 4H), 2.31 (dd, J = 3.5, 13.0 Hz, 1H), 2.00 (dd, J = 3.5,
13.0 Hz, 1H), 1.85–1.25 (m, 5H), 1.17 (s, 3H), 1.02 (s, 3H).
13C NMR δ: 175.6, 108.7, 64.3, 64.1, 51.2, 38.8, 34.0, 32.1,
31.0, 30.3, 19.9. EIMS (m/z): 213 (M+, 7), 169 (25), 99
(100), 86 (55). Anal. calcd. for C11H19NO3: C 61.94, H 8.98,
N 6.57; found: C 62.19, H 9.09, N 6.40.
3-[(Benzyloxycarbonyl)amino]-4,4-dimethylcyclohexanone (5)
Bromine (0.8 mL) was added to 2.5 N NaOH (30 mL)
cooled in an ice bath, followed after 15 min by amide 4
(2.0 g, 9.75 mmol) in small portions. The mixture was
stirred at 0–5°C for 45 min, then at rt for a further 30 min,
and finally heated at 80°C for 1 h. After cooling, the reac-
tion mixture was extracted with Et2O (6 × 20 mL) and the
combined organic solution was washed with brine and evap-
orated. The oily residue was dissolved in 95% EtOH
(15 mL) and 5% aqueous Na2CO3 (10 mL) and benzyl
chloroformate (1.08 mL, 7.55 mmol) was added dropwise.
After stirring at rt for 3 h, the reaction mixture was diluted
with water and extracted with Et2O (4 × 50 mL). The or-
ganic layer was washed with brine, dried and evaporated to
give an oil, which was purified by flash chromatography on
silica gel (hexanes–EtOAc, 4:1). The yellow oil (1.58 g,
4.96 mmol) was dissolved in 30 mL of 1 N HCl–THF (1:1)
and the solution was stirred at rt for two days. After removal
of THF under reduced pressure, the aqueous phase was neu-
tralized with solid NaHCO3 and extracted with Et2O (4 ×
50 mL). The organic layer was washed with brine, dried, and
evaporated. Flash chromatography of the residue on silica
gel (hexanes–EtOAc, 4:1) gave 5 (1.26 g, 45% overall yield
from 4) as a white solid, which was recrystallized from hex-
anes–EtOAc (4:1), mp 106–107°C. IR (KCl): 3450, 3060,
charring with H2SO4 (10% in water), or charring with
anisaldehyde in ethanolic sulfuric acid (23). Melting points
1
2980, 1730, 1520 cm–1. H NMR δ: 7.35 (m, 5H), 5.11 (s,
1
are uncorrected. H and 13C NMR spectra were measured in
2H), 4.88 (d, J = 10.0 Hz, 1H), 3.83 (ddd, J = 5.0, 10.0,
10.0 Hz, 1H), 2.61 (dd, J = 5.0, 15.0 Hz, 1H), 2.33 (m, 2H),
2.27 (dd, J = 10.0, 15.0 Hz, 1H), 1.67 (m, 2H), 1.09 (s, 3H),
1.00 (s, 3H). 13C NMR δ: 208.59, 155.74, 136.27, 128.54,
127.55, 126.93, 66.95, 56.76, 44.64, 37.61, 36.17, 34.14,
27.25, 19.90. EIMS (m/z): 275 (M+, 10), 219 (22), 184 (20),
91 (100). Anal. calcd. for C16H21NO3: C 69.79, H 7.69, N
5.09; found: C 69.58, H 7.83, N 5.25.
CDCl3 at 200 and 50 MHz, respectively, unless otherwise
stated. Mass spectra were obtained at low and high resolu-
tion. Elemental analyses (C, H, N) were performed in house.
2,2-Dimethyl-5-ethylenedioxycyclohexane-1-carboxamide (4)
A solution of KCN (7.88 g, 121 mmol) in water (20 mL)
was added slowly to a solution of 3 (7.51 g, 60 mmol) and
AcOH (3.45 mL) in 95% EtOH (150 mL) warmed to 40°C.
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Can. J. Chem. Vol. 78, 2000
5-[(Benzyloxycarbony)lamino]-4,4-dimethylcyclohex-2-en-1-
one (6)
tracted with Et2O (4 × 100 mL). The combined organic solu-
tion was washed with brine, dried, and concentrated. Flash
chromatography of the resulting oil on silica gel (hexanes–
EtOAc, 7:3) gave the expected aldehyde 8 (0.89 g, 80%) as
a colorless oil. IR (CHCl3): 3064, 1711, 1585, 1468 cm–1.
1H NMR δ: 9.73 (s, 1H), 7.33 (m, 5H), 5.08 (s, 2H), 4.85 (br
d, J = 9.0 Hz, 1H), 3.97 (m, 1H), 2.77–2.12 (m, 7H), 1.09 (s,
3H), 1.07 (s, 3H). 13C NMR δ: 207.86, 200.16, 155.74,
136.05, 128.05, 66.83, 57.18, 44.48, 43.15, 36.21, 34.35,
36.21, 25.41, 23.35, 21.73. EIMS (m/z): 317 (M+, 2), 182
(15), 108 (23), 91 (100). HRMS calcd. for C18H23NO4:
317.16271; found 317.16180. Anal. calcd. for C18H23NO4: C
68.12, H 7.30, N 4.41; found: C 68.36, H 7.26, N 4.29.
To a solution of LHMDS in THF (10.89 mL, 1 M,
10.89 mmol) was added a solution of 5 (1.0 g, 3.36 mmol)
in THF (10 mL) dropwise at –78°C. The solution was stirred
at this temperature for 1.5 h, and then freshly distilled Et3N
(2.50 mL, 18.15 mmol) and TMSCl (1.38 mL, 10.89 mmol)
were added and the reaction was allowed to warm to rt. Af-
ter filtration and evaporation of volatiles, the residual oil was
dissolved in acetonitrile (30 mL) and Pd(OAc)2 (896.4 mg,
3.99 mmol) and CaCO3 (363.3 mg, 3.63 mmol, previously
dried at 120°C overnight) were added. The mixture was
refluxed for 2 h, then evaporated under reduced pressure.
Flash chromatography of the crude residue on silica gel
(hexanes–EtOAc, 3:1) provided 6 (694.9 mg, 70%) as a
white solid, which was recrystallized from hexanes–EtOAc
(4:1), mp 120°C. IR (KCl): 3437, 2969, 1721, 1679, 1512,
4-[3β-[(Benzyloxycarbonyl)amino]-2,2-dimethyl-5-oxocyclo-
hex-1α-yl]-2-butenoic acid methyl ester (9)
To a solution of aldehyde 8 (102 mg, 0.32 mmol) in dry
toluene (10 mL) at rt was added methyl (triphenylphospho-
ranylidene)acetate (131 mg, 0.32 mmol). The reaction mix-
ture was stirred for 8 h at rt. Evaporation of the solvent
followed by flash chromatography on silica gel (hexanes–
EtOAc, 4:1) gave 9 (86 mg, 72%) as a 9:1 mixture of (E–Z)-
isomers. A sample of the pure (E)-isomer was obtained by
1
1466 cm–1. H NMR δ: 7.33 (m, 5H), 6.65 (d, J = 10.0 Hz,
1H), 5.89 (d, J = 10.0 Hz, 1H), 5.09 (s, 2H), 5.01 (br d J =
9 Hz, 1H), 4.11 (m, 1H), 2.65 (dd, J = 4.5, 16.5 Hz, 1H),
2.42 (dd, J = 10.0, 16.5 Hz, 1H), 1.20 (s, 3H), 1.08 (s, 3H).
13C NMR δ: 208.43, 155.57, 136.09, 128.07, 66.08, 44.49,
37.47, 36.02, 33.99, 27.10, 19.73. EIMS (m/z): 273 (M+, 5),
167 (19), 139 (10), 96 (100). Anal. calcd. for C16H19NO3: C
70.31, H 7.01, N 5.13; found: C 70.51, H 6.96, N 5.03.
1
TLC, mp 104–105°C. IR (CHCl3): 1658 cm–1. H NMR δ:
7.32 (m, 5H), 6.75 (ddd, J = 16.0, 9.0, 7.0 Hz, 1H), 5.80 (d,
J = 16.0 Hz, 1H), 5.06 (s, 2H), 4.84 (br d, J = 9.0 Hz, 1H),
4.01 (m, 1H), 3.62 (s, 3H), 2.80–1.80 (m, 7H), 1.10 (s, 3H),
1.08 (s, 3H). EIMS (m/z): 355 (M+–H2O, 70), 281 (2), 223
(12), 95 (100), 81 (42), 67 (95). HRMS calcd. for
C21H27NO5: 373.18892; found: 373.18800. Anal. calcd. for
C21H27NO5: C 67.54, H 7.29, N 3.75; found: C 67.70, H
7.36, N 3.57.
3β-[(Benzyloxycarbonyl)amino]-4,4-dimethyl-5α-(2-prop-
enyl)cyclohexanone (7)
Titanium tetrachloride (1.2 mL, 10.97 mmol) was added
dropwise to a solution of 6 (1.50 g, 5.49 mmol) in dry
CH2Cl2 (112 mL) at –78°C. The solution was stirred at this
temperature for 5 min, then a solution of freshly distilled
allyltrimethylsilane (3.49 mL, 21.96 mmol) in dry CH2Cl2
(27 mL) was added slowly. The dark red mixture was kept at
–78°C for 5 h, then warmed to –30°C. After 2 h the reaction
was quenched with water (50 mL) and allowed to warm to
rt. The organic layer was separated and the aqueous phase
was extracted with Et2O (4 × 100 mL). The combined or-
ganic solution was washed with brine, dried, and concen-
trated to a yellowish oil, which was purified by flash
chromatography on silica gel (hexanes–EtOAc, 4:1) to give
7 (1.25 g, 72%) as white crystals, mp 79–80°C. IR (KCl):
4-[3β-[(Benzyloxycarbonyl)amino]-5-ethylenedioxy-2,2-di-
methylcyclohex-1α-yl]-2-butenoic acid methyl ester (10)
A solution of 9 (267 mg, 0.72 mmol), camphorsulphonic
acid (catalytic amount), and ethylene glycol (0.4 mL,
7.2 mmol) in toluene (30 mL) was stirred at 130°C for 4 h
with azeotropical removal of water. The cooled solution was
washed with 5% aqueous NaHCO3 and brine, then evapo-
rated under reduced pressure. The oily residue was flash
chromatographed on silica gel (hexanes–EtOAc, 7:3) to give
1
10 (289 mg, 98%) as a colorless oil. H NMR δ: 7.34 (m,
1
3065, 1708, 1640, 1586, 1370 cm–1. H NMR δ: 7.33 (m,
5H), 6.87 (ddd, J = 15.0, 8.0, 5.8 Hz, 1H), 6.05 (br d, J =
9.8 Hz, 1H), 5.70 (d, J = 15.0 Hz, 1H), 5.07 (s, 2H), 3.90
(m, 4H), 3.77 (m, 1H), 3.70 (s, 3H), 2.37–1.20 (m, 7H), 0.99
(s, 3H), 0.91 (s, 3H). HRMS calcd. for C23H31NO6:
417.21514; found: 417.21610. Anal. calcd. for C23H31NO6:
C 66.16, H 7.49, N 3.36; found: C 65.80, H 7.59, N 3.11.
5H), 5.61 (m, 1H), 5.07 (s, 2H), 5.00 (m, 2H), 4.70 (br d, J
= 9.0 Hz, 1H), 4.01 (m, 1H), 2.73 (dd, J = 4.5, 16.5 Hz, 1H),
2.47–2.08 (m, 4H), 1.71 (m, 2H), 1.12 (s, 3H), 1.08 (s, 3H).
13C NMR δ: 208.43, 155.62, 136.09, 128.05, 117.09, 66.83,
57.81, 44.46, 42.46, 42.09, 36.78, 36.35, 34.35, 23.69,
21.49. EIMS (m/z): 315 (M+, 5), 272 (12), 224 (22), 180
(15), 114 (33), 91 (100). Anal. calcd. for C19H25NO3: C
72.35, H 7.99, N 4.44; found: C 72.46, H 7.88, N 4.56.
4-(3β-Amino-5-ethylenedioxy-2,2-dimethylcyclohex-1α-yl)-
butanoic acid methyl ester (11)
A mixture of 10 (200 mg, 0.20 mmol) and Pd/C (20 mg,
10%) in EtOH (10 mL) was shaken in a Parr apparatus under
4 atm (1 atm = 101.325 kPa) of pressure of hydrogen for
8 h. Filtration of the mixture through Celite followed by
solvent removal under reduced pressure gave 10 (137 mg,
92%) as a TLC-homogeneous colorless oil. IR (CHCl3):
3β-[(Benzyloxycarbonyl)amino]-4,4-dimethyl-5α-(2-oxo-
ethyl)cyclohexanone (8)
OsO4 (catalytic amount) was added as a single portion to
a stirring solution of 7 (1.10 g, 3.50 mmol) in 70 mL of
Et2O–H2O (1:1). After stirring for 5 min, the solution turned
black and NaIO4 (1.49 g, 7.0 mmol) was added in small por-
tions. Stirring was continued for 8 h until TLC analysis indi-
cated the absence of the starting material. The pale yellow
organic layer was separated and the aqueous phase was ex-
1
3155, 1731, 1580 cm–1. H NMR δ: 3.79 (m, 4H), 3.35 (m,
5H), 2.70 (br s, 2H), 2.16–1.02 (m, 10H), 0.80 (s, 3H), 0.70
(s, 3H). 13C NMR δ: 173.78, 155.67, 108.53, 64.29, 64.18,
63.79, 63.72, 58.15, 56.00, 51.30, 37.05, 34.85, 34.02,
© 2000 NRC Canada

Gentile et al.
931
29.48, 28.78, 28.27, 25.28, 23.11, 19.82. FABMS (m/z): 286
(M+ + H) (100), 269 (449), 215 (22). HRMS calcd. for
C15H27NO4: 285.19401; found: 285.19500. Anal. calcd. for
C15H27NO4: C 63.13, H 9.54, N 4.91; found: C 63.32, H
9.60, N 4.83.
107.96, 64.29, 63.77, 54.23, 53.22, 37.39, 35.82, 29.15,
28.56, 27.80, 25.09, 24.89, 23.56, 20.49, 19.71, 19.47.
FABMS (m/z): 525 (M+
+ H). HRMS calcd. for
C28H48N2O7: 524.34615; found: 524.34790. Anal. calcd. for
C28H48N2O7: C 64.09, H 9.22, N 5.34; found: C 63.88, H
9.30, N 5.50.
4-[3β-[(Benzyloxycarbonyl)amino]-5-ethylenedioxy-2,2-
dimethylcyclohex-1α-yl]-2-butenoic acid (12)
Preparation of macrolactams 15 and 16
To a 5 × 10–4 M solution of 14 (58 mg, 0.11 mmol) in dry
DMF (220 mL) was added NaHCO3 (46 mg, 0.55 mmol)
followed by DPPA (0.24 mL, 1.10 mmol). The reaction mix-
ture was stirred at rt for five days, then the solvent was re-
moved under reduced pressure and the residue was taken up
in EtOAc (50 mL). The solution was washed with water and
brine, dried, and evaporated. Flash chromatography of the
residue on silica gel (CHCl3–MeOH, 19:1) gave 15
(15.7 mg, 28%), followed by 16 (10.8 mg, 10%), as color-
less foams.
To a stirring solution of 10 (58.7 mg, 0.14 mmol) in
MeOH–H2O (3:2, 0.8 mL) was added LiOH (7.0 mg,
0.17 mmol) in one portion at rt. Stirring was continued for
1.5 h at 60°C, then the mixture was cooled to 0–5°C, acidi-
fied to pH - 4 by adding 6 N HCl, and repeatedly extracted
with EtOAc. The combined organic phase was dried, and the
solvent was removed under reduced pressure. The residue
was purified by flash chromatography on silica gel (EtOAc)
to afford 12 (52.8 mg, 93%) as a white foam. A sample of
the pure (E)-isomer was obtained by TLC. IR (CHCl3):
1
3500, 2946, 1698, 1287 cm–1. H NMR δ: 9.11 (br s, 1H),
15: IR (CHCl3): 2421, 1652, 1593, 840 cm–1. 1H NMR
(500 MHz) δ: 7.16 (d, 1.5H), 7.02 (d, 0.5H), 4.02 (m, 10H),
2.46 (m, 4H), 2.36 (m, 4H), 2.20 (m, 8H), 1.56 (m, 12H),
1.03 (m, 6H). 13C NMR (75 MHz) δ: 172.66, 171.16,
109.99, 108.99, 64.42, 64.36, 64.20, 63.92, 55.06, 54.81,
37.83, 37.72, 37.23, 36.96, 34.94, 34.14, 33.63, 33.49,
30.50, 29.69, 25.69, 25.60, 22.98, 22.35, 20.48, 19.64,
19.56. HRMS calcd. for C28H46N2O6: 506.33559; found:
506.33390. Anal. calcd. for C28H46N2O6: C 66.37, H 9.15, N
5.53; found: C 66.52, H 9.06, N 5.38.
7.34 (m, 5H), 6.97 (ddd, J = 15.0, 8.0, 5.8 Hz, 1H), 6.05 (br
d, J = 9.8 Hz, 1H), 5.70 (d, J = 15.0 Hz, 1H), 5.07 (s, 2H),
3.90 (m, 4H), 3.77 (m, 1H), 3.70 (s, 3H), 2.47–1.20 (m, 5H),
1.00 (s, 3H), 0.91 (s, 3H). 13C NMR δ: 156.21, 136.64,
128.31, 108.47, 66.38, 64.30, 63.90, 56.74, 38.07, 37.12,
36.05, 32.72, 24.61, 19.84. FABMS (m/z): 402 (M+ – H),
294 (95), 255 (100), 149, (80). HRMS calcd. for
C22H29NO6: 403.19949; found: 403.20185. Anal. calcd. for
C22H29NO6: C 65.49, H 7.24, N 3.47; found: C 65.29, H
7.29, N 3.38.
16: FD MS (m/z): 1014 (M+ + H). Anal. calcd. for
C56H92N4O12: C 66.37, H 9.15, N 5.53; found: C 66.57, H
9.21, N 5.33.
Synthesis of compound 13
To a solution of 12 (282.1 mg, 0.70 mmol) in dry CH2Cl2
(14 mL) were added (at –15°C) N-methylmorpholine (85 µL,
0.78 mmol) and i-butyl chloroformate (90 µL, 0.70 mmol).
After stirring at –15°C for 1 h, a solution of 11 (201 mg,
0.70 mmol) in dry CH2Cl2 (14 mL) was added slowly. The
reaction was allowed to warm to rt, then was quenched with
water and extracted with CH2Cl2 (4 × 15 mL). The com-
bined organic phase was washed with brine, dried, and evap-
orated to give an oily residue, which after flash
chromatography (SiO2; hexanes–EtOAc, 4:1) afforded 13
(347.1 mg, 74%) as a colorless oil. A sample of the pure
4-[3β-[(Benzyloxycarbonyl)amino]-5β-[(tert-
butyldimethylsilyl)oxy]-2,2-dimethylcyclohex-1α-yl]-2-
butenoic acid (17)
To a solution of 9 (100 mg, 0.27 mmol) in MeOH
(10 mL) was added NaBH4 (41 mg, 1.08 mmol) in one por-
tion. After stirring at rt for 1 h, the solvent was removed un-
der reduced pressure and the residue was taken up in EtOAc.
Usual workup of the organic solution gave an oily mixture,
which was separated by flash chromatography (SiO2; hex-
anes–EtOAc, 3:2) to provide two isomers of the expected al-
cohol as colorless oils. Minor isomer (11 mg, 11%). Rf =
0.4. HRMS calcd. for C21H29NO5: 375.20457; found:
375.20299. Major isomer (81 mg, 81%). Rf = 0.2. HRMS
calcd. for C21H29NO5: 375.20457; found: 375.20310.
TBDMSCl (165 mg, 1.10 mmol) was added to a solution
of the above product (major isomer, 75 mg, 0.20 mmol) and
imidazole (163 mg, 2.40 mmol) in dry DMF (1.5 mL). The
reaction mixture was stirred at 70°C for 1 h, then cooled to
rt, diluted with Et2O (100 mL) and washed with water (5 ×
2 mL). Solvent removal followed by flash chromatography
(SiO2; hexanes–EtOAc, 9:1) gave in quantitative yield the
silylated compound, which was then hydrolyzed with LiOH
using the same procedure described for the preparation of
12. Flash chromatography of the crude product (SiO2,
EtOAc) afforded the expected acid 17 in 98% yield (94 mg,
1
(E)-isomer was obtained by TLC. H NMR δ: 7.33 (m, 5H),
6.78 (d, J = 9.0 Hz, 1H), 6.74 (m, J = 10.0, 7.0, 3.0 Hz, 1H),
6.05 (d, J = 8.5 Hz, 1H), 5.70 (d, J = 10.0 Hz, 1H), 5.08 (q,
2H), 4.01 (m, 1H), 3.93 (m, 10H), 3.87 (m, 1H), 3.63 (s,
3H), 2.35–1.19 (m, 16H), 0.98 (s, 3H), 0.9 (s, 9H). 13C
NMR δ: 186.22, 173.78, 164.66, 156.16, 136.73, 128.28,
127.81, 124.09, 108.56, 63.81, 56.92, 54.33, 37.19, 36.96,
35.82, 34.68, 32.38, 28.72, 24.37, 19.85, 14.02. FABMS
(m/z): 671 (M+ + H) (15), 269 (100), 225 (20), 165 (12), 147
(10). HRMS calcd. for C37H54N2O9: 670.38293; found:
670.38460. Anal. calcd. for C37H54N2O9: C 66.24, H 8.11, N
4.18; found: C 66.50, H 8.00, N 3.93.
Synthesis of amino acid 14
Prepared in 96% overall yield from 13 by hydrolysis fol-
lowed by hydrogenation according to the procedures de-
scribed for the synthesis of 12 and 11, respectively.
1
1
Colorless foam. H NMR δ: 7.24 (d, J = 9.0 Hz, 1H), 5.05
78% overall yield from 9) as a white foam. H NMR δ: 7.35
(br s, 1H), 3.88 (m, 8H), 3.20 (s, 2H), 2.35–1.19 (m, 24H),
(m, 5H), 6.93 (m, 1H), 5.85 (d, J = 10 Hz, 1H), 5.09
(overlapping m, 3H), 3.70 (m, 2H), 2.62–0.91 (m, 13H),
0.85 (s, 3H), 0.83 (s, 9H). 13C NMR δ: 171.72, 109.08,
© 2000 NRC Canada

932
Can. J. Chem. Vol. 78, 2000
0.85 (s, 9H), 0.01 (s, 6H). 13C NMR δ: 171.72, 109.08,
107.96, 64.29, 63.77, 54.23, 53.22, 37.39, 35.82, 29.15, 28.56,
27.80, 25.09, 24.89, 23.56, 20.49, 19.71, 19.47. FABMS
(m/z): 498 (M+ + Na). HRMS calcd. for C26H41NO5SiNa:
498.26517; found: 498.26655. Anal. calcd. for C26H41NO5Si:
C 65.64, H 8.69, N 2.95; found: C 65.90, H 8.77, N 3.14.
DMAP (3.2 mg, 0.026 mmol) in dry toluene (2.0 mL) was
added alcohol 20 (11.1 mg, 0.024 mmol) and the mixture
was stirred at rt for 20 h. The filtered solution was concen-
trated to yield the crude product, which was redissolved in
MeOH (3.0 mL) and 0.1 N HCl (1.5 mL). After stirring at rt
for 2 h, the reaction was neutralized by adding 5% aqueous
NaHCO3 and concentrated to a small volume. Extraction
with EtOAc followed by the usual workup of the organic so-
lution afforded a residue, which was purified by column
chromatography (SiO2; CHCl3–MeOH, 19:1) to give the fi-
nal compound 2 (7.0 mg, 40% overall yield) as a colorless
Synthesis of amide 18
Starting from 17 and following the same procedure de-
scribed for the preparation of 13, the title compound was
prepared in 69% yield after chromatographic purification
(SiO2; hexanes–EtOAc, 7:3). Separation by TLC gave a
sample of the pure geometric isomers.
1
foam. H NMR δ: 7.48 (m, 5H), 6.78 (br d, 2H), 5.59 (de-
formed d, J = 9.5 Hz, 1H), 5.40 (deformed d, J = 9.0 Hz,
1H), 5.03 (m, 1H), 3.98 (superimposed signals, 5H), 3.61
(m, 2H), 2.20–1.24 (m, 22H), 1.12 (s, 9H), 0.90 (s, 6H),
0.84 (s, 6H). HRMS calcd. for C40H61N3O9: 727.44078;
found: 727.43811. Anal. calcd. for C40H61N3O9: C 66.00, H
8.45, N 5.77; found: C 59.73, H 8.58, N 5.95.
1
(E)-18: Rf = 0.3. H NMR δ: 7.32 (m, 5H), 6.75 (superim-
posed signals, 2H), 5.75 (d, J = 15 Hz, 1H), 5.20 (m, 2H),
4.83 (br d, 1H), 4.09 (m, 1H), 3.92 (m, 4H), 3.63 (overlap-
ping signals, 5H), 2.32 (m, 2H), 2.04–1.05 (m, 16H), 0.94
(s, 6H), 0.90 (s, 6H), 0.88 (s, 9H), 0.01 (s, 6H). ES MS
(m/z): 743 (M+ + H). HRMS calcd. for C41H66N2O8Si:
742.45885; found: 742.46555. Anal. calcd. for C41H66N2O8Si:
C 66.36, H 8.95, N 3.77; found: C 66.70, H 8.81, N 3.47.
Single-crystal X-ray diffraction study of 15
Colourless prismatic crystals of 12 were grown from
H2O–MeOH at 277 K. Crystal data: C28H46N2O6; Mr = 506.67;
space group, triclinic P1; a = 9.176(2) Å, b = 9.692(2) Å, γ
= 8.530(2) Å, α = 107.47(2)°, β = 109.82(2)°, γ = 89.38(2)°;
V = 677.3(3) Å3; Z = 1; F(000) = 276; Dx = 1.242 g cm–3;
Cu Kα = 1.5418 Å; µ = 0.697 mm–1. X-ray data were col-
lected on a Rigaku AFC5R automated diffractometer (graph-
ite-monochromated Cu Kα radiation). Intensity data were
1
(Z)-18: Rf = 0.2. H NMR δ: 7.32 (m, 5H), 6.75 (superim-
posed signals, 2H), 5.75 (d, J = 9.0 Hz, 1H), 5.26 (m, 2H),
4.83 (br d, 1H), 4.06 (m, 1H), 3.91 (m, 4H), 3.63 (overlap-
ping signals, 5H), 2.28 (m, 2H), 2.03–1.05 (m, 16H), 0.93
(s, 6H), 0.90 (s, 6H), 0.88 (s, 9H), 0.01 (s, 6H). ES MS
(m/z): 743 (M+ + H). HRMS calcd. for C41H66N2O8Si:
742.45885; found: 742.45690. Anal. calcd. for C41H66N2O8Si:
C 66.36, H 8.95, N 3.77; found: C 66.01, H 8.77, N 3.52.
collected, the experimental conditions being (sin θ/λ)max
=
0.57 Å–1; 2θ – ω scan mode; scan range 1.0°, scan rate 1.0°–
16.0° min–1 (depending on reflection intensity), background
count time a quarter of the scan time. There was no signifi-
cant intensity variation for 3 standard measured every 200
(1.3%). Lorentz, polarization, absorption (min and max
transmission factors 0.91 and 1.00) but no extinction correc-
tions were applied. Of the 2342 reflections measured, 2144
were unique (Rint = 0.067 based on F2) and 1626 with I >
2.0σ(I) were considered as observed. The structure was solved
by direct methods and refined based on F2, using the
SHELXL-97 package (24). Difference Fourier syntheses, us-
ing only data with (sin θ/λ) = 0.50 Å–1, computed at the end
of the anisotropic least-squares refinement, showed all H-
atoms in configurationally feasible positions. The final re-
finements were carried out by full-matrix with the H-atoms
allowed to ride on the corresponding C and N atoms (165
parameters). The final R values are 0.061 for 1626 I > 2σ(I)
and of 0.087 for all 2144 data; the wR are 0.156 and 0.173
respectively. The goodness of fit S is 1.072 for all 2144 data,
Synthesis of macrolactam 19
Prepared in 29% overall yield (after chromatography on
SiO2; CHCl3–MeOH, 19:1) from 18 through a sequence of
hydrolysis-, hydrogenation-, DPPA-mediated macrolactam-
1
ization as described for the conversion of 13 into 15. H
NMR δ: 6.98 (br d, 2H), 3.98 (superimposed signals, 5H),
3.71 (m, 2H), 2.17–1.22 (m, 22H), 0.83 (s, 21H), 0.01 (s,
6H). HRMS calcd. for C32H58N2O5Si: 578.41150; found:
578.41312. Anal. calcd. for C32H58N2O5Si: C 66.39, H
10.10, N 4.84; found: C 66.58, H 10.22, N 4.56.
Preparation of 2
To a solution of 19 (17.3 mg, 0.03 mmol) in THF
(1.0 mL) at 0°C was added a solution of n-Bu4NF (0.12 mL,
1.0 M, 0.12 mmol) in THF, and the mixture was stirred for
3 h at rt, then quenched with saturated aqueous NH4Cl, and
diluted with Et2O and the organic layer was washed with
water and brine and dried. Filtration and evaporation of the
solvent followed by column chromatography (SiO2; CHCl3–
MeOH, 19:1) gave the expected alcohol 20 (12.2 mg, 88%)
as a colorless oil. HRMS calcd. for C26H44N2O5: 464.32502;
found: 464.32622.
based on F2. Heights in final difference Fourier map ρmax
=
0.32 e Å–3 and ρmin = –0.40 e Å–3. A perspective view of the
molecular structure of 1510 was prepared using ORTEP-3 (25).
Superimposition of macrolactam 2 with paclitaxel
Calculations and graphic manipulations were performed on
a SiliconGraphics workstation Indigo R4000. The PCMODEL
software package (26) equipped with the MMX force field
To a solution of (4S,5R)-N-(tert-butoxycarbonyl)-2(2,4-
dimethoxyphenyl)-4-phenyloxazolidine-5-carboxylic acid (21)
(12.0 mg, 0.028 mmol), CMC (20.3 mg, 0.026 mmol) and
10Supplementary material has been deposited and may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI,
National Rsearch Council of Canada, Ottawa, Ontario, Canada, K1A 0S2. Tables of bond lengths and angles and hydrogen coordinates have
also been deposited with the Cambridge Crystallographic Data Centre and can be obtained on request from: The Director, Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, U.K.
© 2000 NRC Canada

Gentile et al.
933
Gränicher, J.B. Houze, J. Jänichen, D. Lee, D.G., Marquess,
P.L. McGrane, W. Meng, T.P. Mucciaro, M. Mühlebach, M.G.
Natchus, H. Paulsen, D.B. Rawlins, J. Satkofsky, A.J. Shuker,
J.C. Sutton, R.E. Taylor, and K. Tomooka. J. Am. Chem. Soc.
119, 2755 (1997); (k) P.A. Wender, N.F. Badham, S.P.
Conway, P.E. Floreancig, T.E. Glass, J.B. Houze, N.E. Krauss,
D. Lee, D.G., Marquess, P.L. McGrane, W. Meng, M.G.
Natchus, A.J. Shuker, J.C. Sutton, and R.E. Taylor. J. Am.
Chem. Soc. 119, 2757 (1997).
was used in this study to perform all the calculations. Solvent
effects were not taken into account. All the graphic manipula-
tions were accomplished using the program InsightII.11 The
lowest-energy conformation of paclitaxel, already deter-
mined in a previous study by some of us (10), was used here
as the reference geometry. The lowest-energy conformation
of 2 was secured by a conformational search performed fol-
lowing the protocol already applied to paclitaxel (10) to per-
form a reliable structural comparison.
5. (a) D. Guénard, F. Guéritte-Voegelein, and P. Potier. Acc.
Chem. Res. 26, 160 (1993); (b) K.C. Nicolaou, W.-M. Dai,
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Horwitz, S.D. Kuduk, and S.J. Danishefsky. Proc. Natl. Acad.
Sci. U.S.A. 96, 4256 (1999).
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Xia, and A. Martinez. J. Med. Chem. 39, 424 (1996), and
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Acknowledgments
Thanks are due to Dr. N. Mongelli, Pharmacia & Upjohn,
Milano (Italy) for helpful discussions. Financial support
from Pharmacia & Upjohn and the program P.O.P. Regione
Calabria 1994–1998 — Misura 4.4 — “Ricerca Scientifica e
Tecnologica” is gratefully acknowledged. M.B. wishes to
thank the NATO International Scientific Exchange
Programme “Bioactive Conformation and Design Mimics
for the Anticancer Agent Taxol,” 1996, for a collaborative
research grant as well as the Merck Research Laboratories
for the 1998 Academic Development Program (ADP) Chem-
istry Award.
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