An iminophosphorane-based approach for the synthesis of spiropyrrolidine-imidazole derivatives

Article abstract of DOI:10.1016/j.tet.2006.12.021

A method based on the reaction of an E-phosphazide, an intermediate in the Staüdinger reaction between triphenylphosphine and an azide, with heterocumulenes allows the one-pot, two-component synthesis of a number of pyrrole-imidazole derivatives. The proc

Full text of DOI:10.1016/j.tet.2006.12.021

Tetrahedron 63 (2007) 1849–1856
An iminophosphorane-based approach for the synthesis of
spiropyrrolidine–imidazole derivatives
a
a
b
a,
Pilar M. Fresneda,a, Marta Castaneda, Miguel Angel Sanz, Delia Bautista and Pedro Molina
*
*
˜
aDepartamento de Quꢀımica Orgaꢀnica, Facultad de Quꢀımica, Universidad de Murcia, Campus de Espinardo, E-30100 Murcia, Spain
bServicio de Apoyo a las Ciencias Experimentales, Campus de Espinardo, E-30100 Murcia, Spain
Received 2 October 2006; revised 7 December 2006; accepted 11 December 2006
Available online 8 January 2007
Abstract—A method based on the reaction of an E-phosphazide, an intermediate in the Sta€udinger reaction between triphenylphosphine and
an azide, with heterocumulenes allows the one-pot, two-component synthesis of a number of pyrrole–imidazole derivatives. The procedure,
which involves sequential treatment of the appropriate a-azido ester with triphenylphosphine and isocyanate leads to the hydantoin product
after aqueous work-up. The cyclization conditions can also be adapted for the synthesis of thiohydantoins by using isothiocyanates. These
hybrids pyrrole–thiohydantoins undergo a novel oxidative spirocyclization by action of DDQ to give a tricyclic derivative (pyrrole–pyrrol-
idine–imidazole), which displays an interesting cytotoxic activity.
Ó 2006 Elsevier Ltd. All rights reserved.
Br
O
N
1. Introduction
Br
NH2
NH
HN
H
N
N
Br
CH3
H
N
Marine sponges have been recognized as a rich source of a
structurally diverse and pharmacologically interesting class
of C11N5 pyrrole–imidazole alkaloids.1 Common structural
features of this group of secondary metabolites are a bromi-
nated or nonbrominated pyrrole carboxamide unit connected
to a functionalized imidazole ring either through a function-
alized or unfunctionalized three-membered carbon bridge or
spiroimidazoline ring. Among the former are midpacamide
12 and dispacamide 2,3 which have been isolated from ma-
rine sponges of the genus Agelas and display antihistaminic
activity.3 In midpacamide 1 the imidazole ring appears in the
guise of an unusual 3-methylhydantoyl moiety, whereas in
dispacamide 2 it is present as a 2-aminoimidazolinone ring.
In the second group, the spirocyclic guanidine molecular
framework defines many biologically active sponge metabo-
lites as exemplified by dibromophakellin 34 and dibromoiso-
phakellin5 4. It is therefore not surprising that their general
biological activity, antibacterial action against Bacillus sub-
tilis and Escherichia coli,4 combined with their exquisite
structure, has made them a formidable challenge to synthe-
ses6 (Fig. 1).
N
CH3
Br
N
H
O
O
O
O
Midpacamide 1
Dispacamide 2
Br
Br
Br
Br
H
H
A
A
NH
N
N
N
B
B
D
D
H2N
H2N
N
N
N
O
N
O
C
C
Dibromophakellin 3
Dibromoisophakellin 4
Figure 1. Biologically active molecules containing the pyrrole–imidazole
(1 and 2) and spirocyclic guanidine moieties (3 and 4).
2. Results and discussion
The N-acylated amino ester 5 was prepared in 90% yield
from the reaction of 2-pyrrolyltrichloromethyl ketone with
ethyl 5-aminovalerate hydrochloride in acetonitrile in the
presence of triethylamine. The common intermediate a-azido
ester 6 was prepared in 70% yield from 5 by the enolate
azidation procedure9 using LDA/2,4,6-triisopropylbenzene-
In conjunction with our synthetic efforts on the synthesis of
a number of imidazole-containing alkaloids of marine ori-
gin,7 we have devised a reliable approach to the synthesis
of the tricyclic ACD ring system of the dibromophakellin
3 and dibromoisophakellin 4.8
€
sulfonyl azide (trisyl azide)/HMPA. Staudinger reaction be-
tween the a-azido ester 6 and triphenylphosphine was very
slow with no detectable nitrogen evolution. When ethyl iso-
cyanate was added to this mixture and the resulting solution
was stirred at room temperature no triphenylphosphine
oxide was detected. Surprisingly, after aqueous work-up the
* Corresponding authors. Tel.: +34 9 68 36 74 84; fax: +34 9 68 36 41 49;
e-mail: fresneda@um.es
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.12.021

1850
P. M. Fresneda et al. / Tetrahedron 63 (2007) 1849–1856
hydantoin 10a, instead of the expected urea derivative, was
formed cleanly in 75% yield. The assignment of 10a as the
product was determined by spectroscopic methods and mass
spectrometry. Similar results were obtained when propyl-
isopropyl, and benzyl isocyanate were used, and the corre-
sponding hydantoins 10b–d were obtained in yields ranging
from 55 to 65%.
oxide (Scheme 1). The Z-phosphazide required for elimi-
nation of nitrogen and concurrent iminophosphorane for-
mation is essentially never formed, perhaps due to steric
hindrance.
€
Mechanistic studies on the Staudinger reaction, formation of
iminophosphoranes from a tertiary phosphine and an organic
azide, revealed that the nucleophilic attack of the tertiary
phosphine on the azide occurs with the formation of an
E-phosphazide, which displays zwitterionic character. After
E to Z isomerization this decomposes rather easily into the
iminophosphorane and nitrogen.13 In spite of the important
role of iminophosphoranes in organic synthesis, they can
react with carbonyl compounds either via aza-Wittig14 or ab-
normal aza-Wittig15 pathways, little attention has been paid
to their alusive precursors phosphazides, primarily due to
their rapid conversion to iminophosphoranes. It has only
been reported11 that an isolated Z-phosphazide reacts with
isocyanates in a ‘normal’ aza-Wittig type fashion to give
the expected carbodiimide. So, the results given here de-
scribe a case where a phosphazide intermediate may partici-
pate in a new cyclization process, thus bypassing the usual
We were intrigued by our inability to observe the uncyclized
urea intermediates despite the mild and neutral reaction con-
ditions employed. Because in the reaction of a-amino acid
esters with isocyanates to give urea derivatives cyclization
usually involves heating, acid catalysis,10 or prolonged treat-
ment with triethylamine at room temperature.11
Monitoring the reaction sequence by 31P NMR revealed a
number of details: (a) the spectrum recorded on the mixture
of triphenylphosphine and compound 6 showed a signal at
d¼17.70 ppm, which is in good agreement with the previ-
ously reported values for phosphazides,12 (b) the spectrum
recorded after the addition of ethyl isocyanate exhibited
a signal at d¼42.40 ppm characteristic of aminotriphenyl-
phosphonium salts, and (c) the spectrum recorded on the
final solution after aqueous work-up showed a signal at
d¼26.12 ppm due to triphenylphosphine oxide. In addition,
the IR spectrum of the solution after addition of ethyl iso-
cyanate did not show the characteristic carbodiimide absorp-
tion band.
€
Staudinger pathway: nitrogen elimination to give the imino-
phosphorane.
It has been described that oxidation with bromine,7d NBS,7e
and PhI(CN)OTf16 of 2-substituted imidazoles (amino, hy-
droxy, phenylthio) takes place with concomitant cyclization
of a tethered nucleophile giving rise to spirocyclic 4,4-disub-
stituted imidazoles, a molecular framework that defines
many biologically active sponge metabolites.17
Taking into account these observations, we believe that this
process is likely to take place through the initially formed
E-phosphazide 7, which reacts with ethyl isocyanate to give a
betaine 8, which undergoes cyclization across the ester func-
tionality with concomitant nitrogen evolution. Eventually,
hydrolytic cleavage of the resulting cyclized phosphonium
salt 9 provided the final product 10 and triphenylphosphine
In this context, we decided to prepare the tricyclic ACD
system of the dibromophakellin 3 and dibromoisophakel-
lin 4, by oxidative spirocyclization of the readily available
hydantoins 10. However, all attempts to promote the
H
a
b
HCl.H2N
CCl3
N
+
COOEt
COOEt
N
N
H
H
O
O
5
Ph3P+
Ph3P+
N
N
N
N
O
O
N-
N
N- R1
OEt
N3
H
H
N
H
N
c
d
N
OEt
COOEt
N
N
H
N
H
H
O
O
O
O
7
6
8
O
N
O
Ph3P+
N
HN
H
H
e
N
R1
N
N
R
N
H
N
H
O
O
O
O
a R1= Et (75%)
b R1= Pr (65%)
10
9
c R1= iPr (58%)
d R1= Bn (55%)
Scheme 1. (a) CH3CN, Et3N, rt (90%); (b) i: LDA, THF, ꢀ30 ꢁC, ii: trisyl azide, HMPA, ꢀ78 ꢁC, iii: AcOH, rt (70%); (c) Ph3P, Et2O, 0 ꢁC/rt;
(d) R1–N]C]O, Et2O, 0 ꢁC/rt; and (e) THF/H2O (9:1), rt.

P. M. Fresneda et al. / Tetrahedron 63 (2007) 1849–1856
1851
1
R
O
N
N
3
H
N
H
N
N
a
b
N
H
O
1
S
COOEt
N
HN
N
H
N
H
O
H
N
O
O
R
11
12
6
S
a R1= Me (75%)
b R1= Et (65%)
c R1= Pr (65%)
d R1= Bn (60%)
a R1= Me (85%)
1
1
b R = Et (70%)
d R = Bn (60%)
N
N
O
N
H
O
c
N
R1
HN
R2
13
a R1= Me; R2=H (60%)
b R1= Me; R2=Bn (60%)
2
c R1= Bn; R =Bn (68%)
Scheme 2. (a) i: Ph3P, Et2O, 0 ꢁC/rt; ii: R1–N]C]S, Et2O, 0 ꢁC/rt, iii: THF/H2O (9:1), rt; (b) DDQ, THF, rt (60–85%); (c) R2–NH2, TBHP, MeOH,
rt (60–68%).
spirocyclization of 10 using the previously mentioned
oxidizing agents failed and only complex mixtures were
obtained.
by treatment of the resulting sulfinic acid18 with ammonia
or alkylamines.
These compounds have been characterized by means of stan-
dard spectroscopic techniques (1H NMR and 13C NMR),
mass spectrometry, and elemental analyses, all data being
in agreement with the proposed structure. Compound 12
was unambiguously determined through a single-crystal
X-ray diffraction analyses19 (Fig. 2).
Therefore, the thiohydantoins 11 were the starting materials
of choice, which were prepared in 60–75% yields by sequen-
tial treatment of the a-azido ester 2 with triphenylphosphine,
and isothiocyanates under the same conditions used for the
preparation of hydantoins 10. These results clearly show
the wide scope of the iminophosphorane-promoted (thio)-
hydantoin cyclization process. When compounds 11 were
treated with DDQ in THF at room temperature spirocycliza-
tion took place and spirothiohydantoins 12 were isolated in
yields ranging from 60 to 85% (Scheme 2).
The structure of 12a (R0¼Me) consists of three-ring systems,
two of them with a common spiro carbon atom C(6) (Tables
1 and 2). The dihedral angle between the mean plane of the
pyrrolidine and the imidazole rings is 96.1ꢁ. With the excep-
˚
tion of N(2)–C(6) [1.4522(19) A], which is a little bit longer
than the mean value reported for Csp3ꢀN in tetrahydro-
The conversion of 11/12 could be understood by initial de-
hydrogenation of the thiohydantoin ring followed by con-
comitant nucleophilic attack of the NH group on the C]N
bond formed. Finally, replacement of the sulfur atom in 12
by a nitrogen functionality to afford 13 was achieved by
oxidation with tert-butylhydroperoxide (TBHP) followed
pyrrole (1.475 A)20 the bond lengths are normal. The inter-
˚
molecular contact N(1)–H(01)/O(1) (ꢀx, ꢀy+1, ꢀꢁz)
˚
[N(1)/O(1)¼2.8130(18) A, N(1)–H(01)/O(1)¼156(2) ]
may indicate hydrogen bonding, these contacts link the
molecules into dimers (Fig. 2b) (Table 3).
Figure 2. ORTEP view of the molecular structure of 12a showing the atom labeling. Thermal ellipsoids are drawn at 50% probability level.

1852
P. M. Fresneda et al. / Tetrahedron 63 (2007) 1849–1856
Table 1. Crystal data and structure refinement of compound 12a
in 13a explained the difference in the activity. Surprisingly,
compound 13a has the best inhibitory activity against A-549
(lung carcinoma NSCL) cells.
Empirical formula
Formula weight
Temperature
C12H14N4O2S
278.33
173(2) K
0.71073 A
Monoclinic
P2(1)/c
˚
Wavelength
Crystal system
Space group
Unit cell dimensions
3. Conclusion
ꢁ
˚
a¼8.9217(5) A; a¼90
ꢁ
˚
˚
In conclusion a method based on unprecedented reaction of
€
b¼12.5387(7) A; b¼102.282(5)
ꢁ
c¼11.5862(7) A; g¼90
an E-phosphazide, an intermediate in the Staudinger reac-
3
˚
1266.44(13) A
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta¼25.00ꢁ
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2sigma(I)]
R indices (all data)
Largest diff. peak and hole
tion, between a tertiary phosphine and organic azide with
isocyanates or isothiocyanates allows the formation of
hydantoins or thiohydantoins under mild reaction conditions.
The prepared thiohydantoin derivatives undergo a new oxi-
dative spirocyclization reaction promoted by DDQ leading
to a spiropyrrolidine–imidazole hybrids, which display
interesting cytotoxic activity.
4
1.460 mg mꢀ3
0.260 mmꢀ1
584
0.60ꢂ0.42ꢂ0.20 mm3
3.09–25.00ꢁ
ꢀ10ꢃhꢃ10, ꢀ14ꢃkꢃ14, 0ꢃlꢃ13
4570
2233 [R(int)¼0.0177]
99.9%
Full-matrix least-squares on F2
2233/0/181
1.028
4. Experimental
4.1. General
R1¼0.0308, wR2¼0.0752
R1¼0.0377, wR2¼0.0781
ꢀ3
˚
0.246 and ꢀ0.231 eA
All reactionswere carried under N2 and solventswere dried by
standard procedures. Column chromatography purifications
˚
were performed using silica gel (60 AC-C, 70–200 mm, SDS)
ꢁ
˚
Table 2. Hydrogen bonds [A and ]
as the stationary phase. All melting points were determined
on a hot-plate melting point apparatus and are uncorrected.
NMR spectra were recorded at 200, 300 or 400 MHz using
CDCl3 or DMSO-d6. Chemical shifts were reported in parts
per million (d scale) relative to Me4Si as an internal standard,
and all J values were in Hertz, assignments were made by
DEPT or two-dimensional NMR experiments. The FAB pos-
itive mass spectra were recorded using 3-nitrobenzylalcohol
as matrix.
D–H/A
d(D–H) d(H/A) d(D/A)
<(DHA)
N(1)–H(01)/O(1)a 0.83(2)
2.04(2)
2.8130(18) 156(2)
a
Symmetry transformations used to generate equivalent atoms: ꢀx,
ꢀy+1, ꢀz.
Due to the wide array of biological activity associated with
the pyrrolidine moiety as well as the 2-aminoimidazole
ring, the biological activity of this hybrid template has been
studied. A panel of 13 human tumor cell lines was used to
evaluate the cytotoxic potential of compounds 12a and 13a:
prostate carcinoma tumor cells (DU-145 and LN-CaP),
SKOV-3 ovary adenocarcinoma, ovarian cells sensitive
(IGROV) or resistant (IGROV-ET) to ET-743, SK-BR-3
breast adenocarcinoma, MEL-28 malignant melanoma,
H-MEC-1 endothelium cells, A-549 lung carcinoma NSCL,
PANC-1 pancreatic epitheloid carcinoma, HT-29 colon carci-
noma cells, and LoVo lymph node metathesis cells and the
corresponding LoVo-Dox cells resistant to Doxorubicin. A
conventional colorimetric assay was set up to estimate GI50
values, i.e. the drug concentration, which causes 50% cell
growth inhibition after 72 h of continuous exposure to the
test molecule. The results obtained are shown in Table 3.
4.1.1. Ethyl 5-(1H-pyrrole-2-carboxamido)pentanoate
(5). A mixture of ethyl 5-aminovaleriato hydrochloride
(2.68 g, 14.7 mmol), 2-trichloroacetylpyrrole (3.92 g,
18.5 mmol), and triethylamine (1.87 g, 18.5 mmol) in anhy-
drous acetonitrile (80 mL), was stirred at room temperature
for 24 h. The solvent was removed under reduced pressure
and the residue was purified by column chromatography
on silica gel, using CH2Cl2/MeOH (9.5:0.5) as an eluent,
to give compound 5 (3.15 g, 90% yield); mp 74–75 ꢁC
(white prisms from EtOAc/n-hexane). 1H NMR (200 MHz,
CDCl3) d 1.25 (t, J¼7.1 Hz, 3H), 1.50–1.75 (m, 4H), 2.34
(t, J¼6.9 Hz, 2H), 3.41 (dt, 2H, J¼6.3, 6.1 Hz, 2H), 4.12
(q, J¼7.1 Hz, 2H), 6.10–6.25 (m, 1H), 6.43 (t, J¼5.6 Hz,
1H), 6.60–6.65 (m, 1H), 6.85–6.95 (m, 1H), 10.34 (s, 1H).
13C NMR (50 MHz, CDCl3) d 14.1, 22.0, 29.0, 33.7, 38.8,
60.3, 109.0, 109.4, 121.6, 125.9, 161.4, 173.6. EIMS m/z
(%): 238 (M+, 7), 144 (12), 110 (11), 94 (pyrroleCO+,
100). Anal. Calcd for C12H18N2O3: C, 60.49; H, 7.61; N,
11.76. Found: C, 60.65; H, 7.73; N, 11.64.
In general, compounds 12a and 13a have cytotoxic activity
against the 13 human tumor cell lines being 13a more active
than 12a. Perhaps the presence of the 2-aminoimidazole ring
Table 3. Data of in vitro cytotoxicity (GI50 mM) of compounds 12a and 13a
CELL LINE
12a
13a
CELL LINE
12a
13a
CELL LINE
12a
13a
DU-145
LN-CaP
SKOV-3
IGROV
35.9
9.7
28.7
15.1
38.3
6.3
11.4
9.2
IGROV-ET
SK-BR-3
MEL-28
35.9
22.7
35.9
35.9
33.9
7.2
18.4
19.8
A-549
31.0
21.7
7.8
0.1
12.8
1.7
PANC-1
LoVo
LoVo-Dox
H-MEC-1
35.9
25.0

P. M. Fresneda et al. / Tetrahedron 63 (2007) 1849–1856
1853
4.1.2. Ethyl 5-(1H-pyrrole-2-carboxamido)-2-azidopen-
tanoate (6). To a ꢀ78 ꢁC cooled solution of diisopropyl-
amine (4.9 mL, 34.6 mmol) in anhydrous THF (75 mL),
n-butyllithium (20.87 mL, 33.4 mmol, 1.6 M in n-hexane)
was added dropwise under N2. The solution was stirred at
ꢀ78 ꢁC for 45 min, then a solution of amidoester 5 (1 g,
4.2 mmol) in THF (50 mL) was added, and the reaction mix-
ture was allowed to warm to ꢀ30 ꢁC, stirred for 1 h and re-
cooled to –78 ꢁC. Hexamethylphosphoric triamide (HMPA)
(11.44 mL, 64.5 mmol) was added in one portion and then
a precooled (ꢀ78 ꢁC) solution of trisyl azide (5.20 g,
16.8 mmol) in THF (20 mL) was added. The reaction mix-
ture was stirred at ꢀ78 ꢁC for 1 h, and then was quenched
with glacial acetic acid (7.2 mL, 126 mmol), allowed to
warm to room temperature, stirred for 12 h, treated with an
saturated solution of NaHCO3 (300 mL) and extracted
with dichloromethane (3ꢂ100 mL). The combined organic
layers were washed with a solution of NaCl (1ꢂ100 mL)
and H2O (2ꢂ100 mL), dried (Na2SO4) and concentrated to
dryness under reduced pressure. The residue was chromato-
graphed on a silica gel column using EtOAc/n-hexane (3:2)
as an eluent to give 6 (0.82 g, 70% yield). 1H NMR
(200 MHz, CDCl3) d 1.28 (t, J¼7.4 Hz, 3H), 1.60–2.00
(m, 4H), 3.44 (td, J¼6.2, 5.8 Hz, 2H), 3.90 (dd, J¼7.6,
5.6 Hz, 1H), 4.24 (q, J¼7.4 Hz, 2H), 6.21 (m, 1H), 6.32 (t,
J¼5.8 Hz, 1H), 6.61 (m, 1H), 7.08 (m, 1H), 10.12 (s, 1H).
13C NMR (75 MHz, CDCl3) d 14.1, 26.0, 28.6, 38.5, 61.6,
61.9, 109.2, 109.6, 121.8, 125.6, 161.5, 170.3. MS: m/z
(%) (FAB positive) 302 ((M+Na)+, 82), 280 ((M+H)+, 85),
267 (43), 251 ((MꢀN2)+, 31), 235 ((MꢀEtOH+H)+, 63),
94 (pyrroleCO+, 100). Anal. Calcd for C12H17N5O3: C,
51.60; H, 6.14; N, 25.08. Found: C, 51.68; H, 6.20; N, 25.01.
1.44–1.59 (m, 6H), 3.31–3.22 (m, 2H), 3.29 (t, J¼7.2 Hz,
2H), 4.04 (t, J¼4.5 Hz, 1H), 6.04–6.07 (m, 1H), 6.70–6.72
(m, 1H), 6.80–6.81 (m, 1H), 7.87 (t, J¼5.5 Hz, 1H), 8.10
(s, 1H), 11.25 (s, 1H). 13C NMR (75 MHz, DMSO-d6)
d 11.0, 20.9, 24.8, 28.9, 38.3, 39.5, 56.0, 108.4, 109.6,
121.1, 126.3, 156.8, 160.6, 174.3. MS: m/z (%) (FAB posi-
tive) 293 (M+H, 19), 292 (M+, 100). Anal. Calcd for
C14H20N4O3: C, 57.52; H, 6.90; N, 19.17. Found: C,
57.58; H, 6.84; N, 19.23.
4.1.3.3. Compound 10c. Yield 58% (R1¼i-Pr, X¼O),
1
mp: 116–117 ꢁC (from EtOAc/EtOH/n-hexane (4:1:5)). H
NMR (300 MHz, DMSO-d6) d 1.27 (d, J¼6.9 Hz, 3H),
1.28 (d, J¼6.9 Hz, 3H), 1.48–1.73 (m, 4H), 3.18–3.19 (m,
2H), 3.98 (t, J¼6.0 Hz, 1H), 4.10 (septet, J¼6.9 Hz, 1H),
6.04–6.06 (m, 1H), 6.70–6.72 (m, 1H), 6.80–6.81 (m, 1H),
7.99 (t, J¼6 Hz, 1H), 8.17 (s, 1H), 11.38 (s, 1H). 13C
NMR (75 MHz, DMSO-d6) d 19.88, 19.78, 24.8, 29.1,
38.4, 42.6, 55.7, 108.6, 109.8, 121.3, 126.5, 156.8, 160.8,
174.3. MS: m/z (%) (FAB positive) 293 (M+H, 100), 292
(M+, 24). Anal. Calcd for C14H20N4O3: C, 57.52; H, 6.90;
N, 19.17. Found: C, 57.60; H, 6.81; N, 19.25.
4.1.3.4. Compound 10d. Yield 55% (R1¼Bn, X¼O),
1
mp: 157–158 ꢁC (from EtOAc/EtOH/n-hexane (4:1:5)). H
NMR (400 MHz, DMSO-d6) d 1.52–1.74 (m, 4H), 3.20 (t,
J¼5.7 Hz, 2H), 4.15 (t, J¼5.3 Hz, 1H), 4.50 (s, 2H), 6.04–
6.06 (m, 1H), 6.71–6.72 (m, 1H), 6.80–6.82 (m, 1H),
7.20–7.31 (m, 5H), 7.97 (t, J¼5.7 Hz, 1H), 8.35 (s, 1H),
11.36 (s, 1H). 13C NMR (100 MHz, DMSO-d6) d 25.4,
29.4, 38.3, 41.4, 56.6, 108.9, 109.0, 121.5, 126.7, 127.2,
127.7, 128.9, 137.2, 157.0, 161.0, 174.6. MS: m/z (%)
(FAB positive) 341 (M+H, 100), 340 (M+, 27). Anal. Calcd
for C18H20N4O3: C, 63.52; H, 5.92; N, 16.46. Found: C,
63.46; H, 5.96; N, 16.52.
4.1.3. General procedure for the preparation of hydan-
toins (10) and thiohydantoins (11). To a solution of tri-
phenylphosphine (0.47 g, 1.79 mmol) in dry diethyl ether
(15 mL) a solution of the a-azido ester 6 (0.5 g, 1.79 mmol)
in the same solvent (15 mL) at 0 ꢁC was added dropwise
under N2. The resultant solution was allowed to warm to
room temperature and stirred for 12 h. Then, it was cooled
to 0 ꢁC and a solution of the appropriate iso(thio)cyanate
(1.79 mmol) was added. The resultant mixture was stirred
at room temperature for 48 h and then THF/H2O (9:1)
(10 mL) was added and stirring was continued for 1 h. The
solution was concentrated to dryness and the residue was
chromatographed on a silica gel column using EtOAc as
an eluent to give 10 and 11.
4.1.3.5. Compound 11a. Yield 75% (R1¼Me, X¼S), mp:
200–202 ꢁC (from EtOAc). 1H NMR (300 MHz, DMSO-d6)
d 1.50–1.90 (m, 4H), 3.06 (s, 3H), 3.22 (td, J¼6.0, 5.55 Hz,
2H), 4.24–4.33 (m, 1H), 6.05–6.09 (m, 1H), 6.74 (m, 1H),
6.83 (d, J¼0.9 Hz, 1H), 7.98 (t, J¼5.5 Hz, 1H), 10.34 (s,
1H), 11.36 (s, 1H). 13C NMR (75 MHz, DMSO-d6) d 24.8,
26.8, 28.3, 37.9, 58.7, 108.5, 109.7, 121.2, 126.3, 160.7,
174.8, 182.9. EIMS m/z (%): 281 (M+1, 6), 280 (M, 44),
213 (24), 94 (100). Anal. Calcd for C12H16N4O2S: C,
51.41; H, 5.75; N, 19.98. Found: C, 51.35; H, 5.82; N, 19.91.
4.1.3.6. Compound 11b. Yield 65% (R1¼Et, X¼S), mp:
4.1.3.1. Compound 10a. Yield 75% (R1¼Et, X¼O), mp:
138 ꢁC (from EtOAc/EtOH/n-hexane (4:1:5)). 1H NMR
(200 MHz, DMSO-d6) d 1.05 (t, J¼7.2 Hz, 3H), 1.49–1.60
(m, H-9, 4H), 3.10–3.28 (m, 2H), 3.32 (q, J¼7.2 Hz, 2H),
4.04 (t, J¼4.8 Hz, 1H), 6.03–6.07 (m, 1H), 6.70–6.73 (m,
1H), 6.80–6.83 (m, 1H), 8.00 (t, J¼5.5 Hz, 1H), 8.12 (s,
1H), 11.27 (s, 1H). 13C NMR (75 MHz, DMSO-d6) d 13.7,
25.2, 29.3, 32.9, 38.4, 56.4, 108.9, 110.0, 121.5, 126.7,
157.1, 161.0, 174.5. MS: m/z (%) (FAB positive) 279
(M+H, 100), 278 (M+, 24). Anal. Calcd for C13H18N4O3: C,
56.10; H, 6.52; N, 20.13. Found: C, 56.17; H, 6.46; N, 20.06.
177–178 ꢁC (from EtOAc/EtOH/n-hexane (7:0.5:2.5)). H
1
NMR (300 MHz, DMSO-d6) d 1.07 (t, J¼7.1 Hz, 3H),
1.45–1.75 (m, 4H), 3.19 (m, 2H), 3.65 (q, J¼7.1 Hz, 2H),
4.26 (t, J¼5.7 Hz, 1H), 6.04–6.06 (m, 1H), 6.71 (m, 1H),
6.81 (m, 1H), 7.99 (t, J¼5.7 Hz, 1H), 10.34 (s, 1H), 11.38
(s, 1H). 13C NMR (75 MHz, DMSO-d6) d 12.9, 24.6, 28.3,
35.1, 37.9, 58.4, 108.4, 109.7, 121.1, 126.3, 160.6, 174.4,
182.2. MS: m/z (%) (FAB positive) 295 (M+H, 100). Anal.
Calcd for C13H18N2O4S: C, 53.04; H, 6.16; N, 19.03. Found:
C, 53.11; H, 6.10; N, 18.98.
4.1.3.7. Compound 11c. Yield 60% (R1¼Pr, X¼S), mp:
4.1.3.2. Compound 10b. Yield 65% (R1¼Pr, X¼O),
129–130 ꢁC (from EtOAc/EtOH/n-hexane (9:1)). H NMR
1
1
mp: 143–144 ꢁC (from EtOAc/EtOH/n-hexane (5:1:4)). H
(300 MHz, DMSO-d6) d 0.81 (t, J¼7.4 Hz, 3H), 1.45–1.60
NMR (300 MHz, DMSO-d6) d 0.81 (t, J¼7.5 Hz, 3H),
(m, 4H), 3.17 (m, 2H), 3.59 (t, J¼7.4 Hz, 2H), 4.28

1854
P. M. Fresneda et al. / Tetrahedron 63 (2007) 1849–1856
1
(t, J¼5.7 Hz, 1H), 6.03–6.07 (m, 1H), 6.70–6.73 (m, 1H),
6.80–6.83 (m, 1H), 8.0 (t, J¼5.7 Hz, 1H), 10.35 (s, 1H),
11.38 (s, 1H). 13C NMR (75 MHz, DMSO-d6) d 12.9,
20.9, 25.1, 28.7, 38.5, 39.7, 58.8, 108.8, 110.0, 121.5,
126.7, 161.0, 175.2, 183.0. MS: m/z (%) (FAB positive)
309 (M+H, 100), 308 (M+, 26). Anal. Calcd for
C14H20N4O2S: C, 54.52; H, 6.54; N, 18.17. Found: C,
54.57; H, 6.50; N, 18.10.
218–220 ꢁC (from Et2O). H NMR (300 MHz, DMSO-d6)
d 2.11–2.19 (m, 4H), 3.84–3.90 (m, 2H), 4.81 (d,
J¼15.3 Hz, 1H), 5.05 (d, J¼15.3 Hz, 1H), 6.19 (m, 1H),
6.74 (m, 1H), 6.98 (m, 1H), 7.24–7.37 (m, 5H), 10.68 (s,
1H), 11.7 (s, 1H). 13C NMR (75 MHz, DMSO-d6) d 23.6,
38.4, 43.6, 48.6, 78.9, 109.5, 113.5, 123.1, 124.0, 127.1,
127.3, 128.3, 136.5, 159.4, 170.3, 181.8. LC/MS: m/z (%)
355 (M+H, 100). Anal. Calcd for C18H18N4O2S: C, 61.01;
H, 5.12; N, 15.81. Found: C, 59.91; H, 5.00; N, 15.76.
4.1.3.8. Compound 11d. Yield 56% (R1¼Bn, X¼S), mp:
162–163 ꢁC (from EtOAc/EtOH/n-hexane (9:1)).1H NMR
(400 MHz, DMSO-d6) d 1.49–1.78 (m, 4H), 3.18–3.25 (m,
2H, H-8), 4.36 (t, J¼5.7 Hz, 1H), 4.86 (s, 2H), 6.04–6.07
(m, 1H), 6.71–6.72 (m, 1H), 6.81–6.83 (m, 1H), 7.20–7.30
(m, 5H), 7.93 (t, J¼5.7 Hz, 1H), 10.43 (s, 1H), 11.30 (s,
1H). 13C NMR (100 MHz, DMSO-d6) d 24.7, 28.34, 37.8,
43.2, 58.5, 108.3, 109.5, 121.0, 126.3, 127.2, 127.3, 128.2,
136.3, 160.6, 174.6, 182.4. EIMS m/z (%): 358 (M+2, 11),
356 (M+, 91). Anal. Calcd for C18H20N2O4S: C, 60.65; H,
5.66; N, 15.72. Found: C, 60.59; H, 5.62; N, 15.67.
4.1.5. Synthesis of spiro-2-aminohydantoin (13). To a
solution of the corresponding spirothiohydantoin 12
(0.36 mmol) in methanol (14 mL), the appropriate amine
(0.47 mmol) or aqueous ammonia (30%, 4 mL) was added.
After stirring at room temperature for 15 min TBHP (70%,
0.2 mL, 1.44 mmol) was added. The resulting reaction mix-
ture was stirred for 36 h at room temperature, then the same
amount of TBHP was added and stirring was continued for
an additional 36 h. The solvent was removed under reduced
pressure and the residue was purified by column chromato-
graphy (CH2Cl2/EtOH/aqueous NH3 (7:2.5:0.5)) in deacti-
vated silica gel with a EtOH/NH3 mixture (30%) (9:1), to
give the title compound 13.
4.1.4. Preparation of spiro-compounds (12). To a solution
of the appropriate thiohydantoin 11a (0.3 g, 1.07 mmol) in
anhydrous THF (30 mL), DDQ (0.3 g, 1.34 mmol) was
added under N2. After stirring at room temperature for
24 h, the same amount of DDQ was added and the mixture
was stirred for additional 48 h. The solvent was removed un-
der reduced pressure and the residue was treated with EtOAc
(100 mL) and filtered. The organic phase was washed with
an aqueous saturated solution of NaHCO3 (2ꢂ30 mL) and
water (30 mL). The combined organic layers were dried
(MgSO4). The solvent was removed under reduced pressure
and the residue was purified by column chromatography on
silica gel, using EtOAc/n-hexane (3:2) as an eluent, to give
the title compound 12a (Rf ¼0.54, 0.25 g, yield 85%).
4.1.5.1. 2-Amino-3-methyl-6-(1H-pyrrole-2-carbonyl)-
1,3,6-triazaspiro[4.4]non-1-en-4-one (13a). Yield 60%,
mp: 212–214 ꢁC (from Et2O). 1H NMR (300 MHz,
DMSO-d6) d 1.75–2.15 (m, 4H), 2.75 (s, 3H), 3.60–3.95
(m, 2H), 6.10–6.20 (m, 1H), 6.57–6.67 (m, 1H), 6.89–6.99
(m, 1H), 7.15–7.55 (br s, 1H), 7.75–8.20 (br s, 1H), 11.51
(s, 1H). 13C NMR (75 MHz, DMSO-d6) d 23.5, 27.8, 36.9,
48.8, 80.7, 109.1, 112.5, 122.1, 125.4, 158.9, 169.8. MS:
m/z (%) (FAB positive) 523 (2M+H+, 15), 262 (M+H+,
100). Anal. Calcd for C12H15N5O2: C, 55.16; H, 5.79; N,
26.80. Found: C, 55.10; H, 5.73; N, 26.85.
4.1.4.1. 3-Methyl-6-(1H-pyrrole-2-carbonyl)-2-thioxo-
1,3,6-triazaspiro[4.4]nonan-4-one (12a). Mp >264 ꢁC
(dec), yellow prisms (EtOAc/n-hexane). 1H NMR
(300 MHz, DMSO-d6) d 2.01–2.27 (m, 4H), 3.12 (s, 3H),
3.74–3.94 (m, 2H), 6.19 (dt, J¼3.6, 2.4 Hz, 1H), 6.74 (m,
1H), 6.97 (m, 1H), 10.60 (s, 1H), 11.75 (s, 1H). 13C NMR
(75 MHz, DMSO-d6) d 23.6, 27.1, 35.6, 48.5, 78.9, 109.6,
113.5 (C-4), 123.1, 124.0, 159.4, 172.7, 182.3. EIMS m/z
(%): 279 (M++1, 2), 278 (M+, 10), 168 (M+ꢀMeꢀpyrrole–
CO, 9), 94 (pyrroleCO+, 100), 66 (pyrrole+, 26). Anal. Calcd
for C12H14N4O2S: C, 51.78; H, 5.07; N, 20.13. Found: C,
51.66; H, 5.22; N, 20.26.
4.1.5.2. 2-Benzylamino-3-methyl-6-(1H-pyrrole-2-car-
bonyl)-1,3,6-triazaspiro[4.4]non-1-en-4-one (13b). Yield
1
60%, mp: 122–124 ꢁC (from Et2O). H NMR (200 MHz,
CDCl3) d 1.92–2.27 (m, 4H), 3.0 (s, 3H), 3.85–4.01 (m, 2H),
4.29 (d, J¼14.4 Hz, 1H), 4.58 (d, J¼14.4 Hz, 1H), 6.14 (m,
1H), 6.54 (m, 1H), 6.75 (m, 1H), 7.16–7.28 (m, 5H), 9.21 (br
s, 1H). 13C NMR (75 MHz, CDCl3) d 24.3, 25.7, 37.3, 46.2,
49.2, 84.9, 110.0, 113.2, 121.7, 125.1, 127.6, 128.1, 128.6,
138.2, 155.8, 160.0, 178.5. EIMS m/z (%): 352 (M++1, 4),
351 (M+, 13), 241 (55), 150 (51), 94 (pyrroleCO+, 67), 91
(100), 66 (pyrrole+, 19). Anal. Calcd for C19H21N5O2: C,
64.94; H, 6.02; N, 19.93. Found: C, 64.87; H, 5.58; N, 20.01.
4.1.4.2. 3-Ethyl-6-(1H-pyrrole-2-carbonyl)-2-thioxo-
1,3,6-triazaspiro[4.4]nonan-4-one (12b). Yield 70%, mp:
118–120 ꢁC (from Et2O). 1H NMR (400 MHz, CDCl3)
d 1.03 (t, J¼7.1 Hz, 3H), 2.09–2.39 (m, 4H), 3.79–3.72 (m,
2H), 3.86–4.03 (m, 2H), 6.18–6.20 (m, 1H), 6.57–6.59 (m,
1H), 6.84–6.86 (m, 1H), 8.66 (s, 1H), 10.44 (s, 1H). 13C
NMR (75 MHz, CDCl3) d 12.1, 24.2, 36.2, 36.6, 48.9, 78.9,
109.7, 113.3, 122.7, 124.5, 161.0, 172.1, 183.1. EIMS m/z
(%): 293 (M++1, 4), 292 (M+, 22), 94 (pyrroleCO+, 100),
66 (pyrrole+, 28). Anal. Calcd for C13H16N4O2S: C, 53.42;
H, 5.48; N, 19.18. Found: C, 53.36; H, 5.40; N, 20.05.
4.1.5.3. 2-Benzylamino-3-benzyl-6-(1H-pyrrole-2-car-
bonyl)-1,3,6-triazaspiro[4.4]non-1-en-4-one (13c). Yield
1
68%, mp: 208–210 ꢁC (from Et2O). H NMR (400 MHz,
CDCl3) d 2.04–2.05 (m, 1H), 2.17–2.26 (m, 3H), 3.91–
3.95 (m, 1H), 3.99–4.01 (m, 1H), 4.16 (d, J¼14.5 Hz, 1H),
4.56 (d, J¼16.4 Hz, 1H), 4.61–4.66 (m, 1H), 4.97 (d,
J¼16.4 Hz, 1H), 6.12 (m, 1H), 6.54 (m, 1H), 6.60 (m,
1H), 6.90 (m, 2H), 7.09–7.22 (m, 8H), 8.44 (br s, 1H). 13C
NMR (75 MHz, CDCl3) d 24.4, 37.8, 36.9, 42.7, 45.3,
49.4, 85.9, 109.9, 113.1, 121.3, 125.1, 127.0, 127.3, 127.4,
127.7, 128.4, 128.9, 135.8, 138.3, 156.1, 159.9, 179.5.
MS: m/z (%) (FAB positive) 428 (M+H+, 100), 427 (M+,
40), 361, 335. Anal. Calcd for C25H25N5O2: C, 70.24; H,
5.89; N, 16.38. Found: C, 70.19; H, 5.82; N, 16.31.
4.1.4.3. 3-Benzyl-6-(1H-pyrrole-2-carbonyl)-2-thioxo-
1,3,6-triazaspiro[4.4]nonan-4-one (12d). Yield 60%, mp:

P. M. Fresneda et al. / Tetrahedron 63 (2007) 1849–1856
1855
Pietra, F. Helv. Chim. Acta 1966, 79, 727; (c) Williams,
D. H.; Faulkner, D. J. Tetrahedron 1996, 52, 5381; (d)
Tsukamoto, S.; Kato, H.; Hirota, H.; Fusetani, N. J. Nat.
Prod. 1996, 108, 2288; (e) Al Mourabit, A.; Potier, P. Eur. J.
Org. Chem. 2001, 237; (f) Hoffmann, H.; Lindel, T. Synthesis
2003, 1753; (g) Jacquot, D. E. N.; Lindel, T. Curr. Org.
Chem. 2005, 9, 1551; (h) Du, H.; He, Y.; Sivappa, R.; Lovely,
C. J. Synlett 2006, 965.
4.2. Cell growth inhibition assay: screening
A colorimetric assay using sulforhodamine B (SRB) has
been adapted for a quantitative measurement of cell growth
and viability, following a previously described method.21
Cells were seeded in 96 well microtiter plates, at 5ꢂ103 cells
per well in aliquots of 195 mL of RPMI medium, and they are
allowed to attach to the plate surface by growing in drug free
medium for 18 h. Afterward, samples are added in aliquots
of 5 mL (dissolved in DMSO/H2O (3:7)). After 72 h expo-
sure, the antitumor effect is measured by the SRB methodol-
ogy: cells are fixed by adding 50 mL of cold 50% (w/v)
trichloroacetic acid (TCA) and incubating for 60 min at
4 ꢁC. Plates are washed with deionized H2O and dried;
100 mL of SRB solution (0.4% w/v in 1% acetic acid) is
added to each microtiter well and incubated for 10 min at
room temperature. Unbound SRB is removed by washing
with 1% acetic acid. Plates are air-dried and bound stain is
solubilized with Tris buffer. Optical densities are read on
an automated spectrophotometer plate reader at a single
wavelength of 490 nm. Data analyses are generated auto-
matically by LIMS implementation. Using control OD
values (C), test OD values (T), and time zero OD values
(T0), the drug concentration that causes 50% growth in-
hibition (GI50 value) was calculated from the equation:
100[(TꢀT0)/(CꢀT0)]¼50.
2. (a) Chevolot, L.; Padua, S.; Ravi, B. N.; Blyth, P. C.; Scheuer,
P. J. Heterocycles 1977, 7, 891; (b) Fati-Afshar, R.; Allen, T. M.
Can. J. Chem. 1988, 66, 45; (c) Jimenez, C.; Crews, P.
Tetrahedron Lett. 1994, 35, 1375; (d) Lindel, T.; Hoffmann,
€
M.; Hochgurtel, M.; Pawlik, J. R. J. Chem. Ecol. 2000,
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3. (a) Cafieri, F.; Fattorusso, E.; Mangoni, A.; Taglialatela-
Scafati, O. Tetrahedron Lett. 1996, 37, 3587; (b) Cafieri, F.;
Carnuccio, R.; Fattorusso, E.; Taglialatela-Scafati, O.;
Vallefuoco, T. Bioorg. Med. Chem. Lett. 1997, 7, 2283.
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(b) Sharma, G. M.; Burkholder, P. R. J. Chem. Soc., Chem.
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J. Org. Chem. 1977, 32, 4118.
5. Fedoreyer, S. A.; Utkina, N. K.; Ilyin, S. G.; Reshetnyak, M. V.;
Maximov, O. B. Tetrahedron Lett. 1986, 27, 3177.
6. (a) Lindel, T.; Hoffmann, H. Liebigs Ann./Recueil 1997, 1525;
(b) Lindel, T.; Hoffmann, H. Tetrahedron Lett. 1997, 38, 8935;
(c) Olofson, A.; Yakushijin, K.; Horne, D. A. J. Org. Chem.
1998, 63, 1248; (d) Foley, L. H.; B€uchi, G. J. Am. Chem. Soc.
1982, 104, 1776; (e) Wiese, K. J.; Yakushijin, K.; Horne,
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7. (a) Molina, P.; Fresneda, P. M.; Sanz, M. A. Tetrahedron Lett.
1998, 39, 6909; (b) Molina, P.; Fresneda, P. M.; Sanz, M. A.
J. Org. Chem. 1999, 64, 2540; (c) Fresneda, P. M.; Molina,
P.; Sanz, M. A. Synlett 2000, 1190; (d) Fresneda, P. M.;
Molina, P.; Sanz, M. A. Synlett 2001, 218; (e) Fresneda,
P. M.; Molina, P.; Sanz, M. A. Tetrahedron Lett. 2001, 42, 851.
8. Preliminary communication: Fresneda, P. M.; Castan˜eda, M.;
Sanz, M. A.; Molina, P. Tetrahedron Lett. 2004, 45, 1655.
9. Trisyl azide has been shown to be a more effective azido-trans-
fer reagent than the more commonly employed tosyl azide: (a)
Evans, D. A.; Ellman, J. A.; Dorow, R. L. J. Am. Chem. Soc.
1990, 112, 4011; (b) Evans, D. A.; Devries, K. M.
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4.3. X-ray structure determinations
The crystal and molecular structures of compound 12a have
been determined by X-ray diffraction studies (Table 1).
A crystal was mounted on glass fibers and transferred to
the cold gas stream of the diffractometer Siemens P4. Data
˚
were recorded with Mo Ka radiation (l¼0.71073 A) in
w-scan mode. Structure of 12a was solved by the direct
method and refined anisotropically on F2 (program
SHELXL-97).22 The hydrogens at N were located in the
Fourier difference maps and refined freely. Methyl groups
were refined using rigid groups and other hydrogens were
refined using a riding method.
Acknowledgements
ꢀ
Oton, F.; Espinosa, A.; Velasco, M. D.; Molina, P.; Evans,
D. J. Chem. Commun. 2004, 458.
ꢀ
We thank Dr. Carmen Cuevas and Dr. Andres Francesch
from PharmaMar S. A. (Colmenar Viejo, Spain) for the bio-
logical study and Foundation Seneca (CARM) for financial
support (project number 00582/PI/04).
10. (a) Hemichart, J. P. Synthesis 1980, 311; (b) Jacobsen, N.;
Toelberg, J. Synthesis 1986, 559; (c) Stella, V.; Higuchi, T.
J. Org. Chem. 1973, 38, 1527.
11. (a) Sim, M. M.; Ganesan, A. J. Org. Chem. 1997, 62, 3230;
(b) Zhang, W.; Lu, Y. Org. Lett. 2003, 5, 2555.
Supplementary data
12. Molina, P.; Lopez-Leonardo, C.; Llamas-Botia, J.; Foces-
Foces, C.; Fernandez-Castan˜o, C. Tetrahedron 1996, 52,
9629 and references cited therein.
Crystallographic data and structure refinement of compound
12a. Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2006.12.021.
13. (a) Leffler, J. E.; Temple, R. D. J. Am. Chem. Soc. 1967, 89,
€
5235; (b) Wideaur, C.; Grutzmacher, H.; Shevchenko, I.;
Gramlich, V. Eur. J. Inorg. Chem. 1999, 1659.
References and notes
14. For reviews see: (a) Molina, P.; Vilaplana, M. J. Synthesis 1994,
1197; (b) Fresneda, P. M.; Molina, P. Synlett 2004, 1.
15. (a) Itoh, K.; Okamura, M.; Yshii, Y. J. Organomet. Chem. 1974,
1. (a) Gribble, G. W. Fortschr. Chem. Org. Naturst.; Herz, W.,
Kirby, G. W., Moore, R. E., Steglich, W., Taman, C., Eds.;
Springer: Wien, 1966; Vol. 68, p 137; (b) D’Ambrosio, M.;
Guerriero, A.; Ripamonti, M.; Debitus, C.; Waikedre, J.;
ꢀ
65, 327; (b) Bruche, L.; Garanti, L.; Zechi, G. J. Chem.
Soc., Perkin Trans. 1 1986, 2177; (c) Molina, P.; Conesa, C.;
Alias, A.; Arques, A.; Velasco, M. D.; Llamas-Sainz, A. L.;

1856
P. M. Fresneda et al. / Tetrahedron 63 (2007) 1849–1856
Foces-Foces, C. Tetrahedron 1993, 49, 7599; (d) Molina, P.;
19. Crystallographic data for 12a: formula C12H14N4O2S,
monoclinic crystal, space group P2(1)/c, a¼8.9217(5), b¼
Alajarin, M.; Vidal, A. Tetrahedron 1990, 46, 1063; (e)
Alajarin, M.; Molina, P.; Sanchez-Andrada, P.; Foces-Foces,
C. J. Org. Chem. 1999, 64, 1121; (f) Hoesl, C. A.; Nefzi, A.;
Hougten, R. A. Tetrahedron Lett. 2003, 44, 3705.
ꢁ
ꢁ
ꢁ
˚
12.5387(7), c¼11.5862(7) A, a¼90 , b¼102.282(5) , g¼90 ,
3
V¼1266.44(13) A , Dc¼1.460 mg mꢀ3
, 2233 independent
reflections used in refinement, 4570 parameters, R1(I>2(I))¼
˚
16. Feldman, K. S.; Skoumbourdis, A. P. Org. Lett. 2005, 7, 929.
17. (a) Pettit, G. R.; McNulty, J.; Herald, D. L.; Doubek, D. L.;
Chapuis, J.-C.; Schmidt, J. M.; Tackett, L. P.; Boyd, M. R.
J. Nat. Prod. 1997, 60, 180; (b) Fedoreev, S. A.; Utkina,
0.0308, R2 (all data) 0.0377, CCDC 607775.
20. Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J. J. Chem. Soc., Perkin Trans. 2 1987, S1.
21. (a) Skehan, P.; Storeng, R.; Scudiero, D.; Monks, A.;
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