Convergent, stereoselective syntheses of the glycosidase inhibitors broussonetines C, O and P

Article abstract of DOI:10.1016/j.tet.2009.10.066

The first syntheses of the polyhydroxylated alkaloids (iminosugars) broussonetines O and P, glycosidase inhibitors of the pyrrolidine class, have been performed in a convergent, stereocontrolled way from d-serine as the chiral starting material. The synth

Full text of DOI:10.1016/j.tet.2009.10.066

Tetrahedron 65 (2009) 10612–10616
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Convergent, stereoselective syntheses of the glycosidase inhibitors
broussonetines C, O and P
,
a
a
b,
*
Celia Ribes a, Eva Falomir a , Juan Murga , Miguel Carda , J. Alberto Marco
*
a
´
´
´
´
Depart. de Q. Inorganica y Organica, Univ. Jaume I, Castellon, E-12080 Castellon, Spain
Depart. de Q. Organica, Univ. de Valencia, E-46100 Burjassot, Valencia, Spain
b
´
a r t i c l e i n f o
a b s t r a c t
Article history:
The first syntheses of the polyhydroxylated alkaloids (iminosugars) broussonetines O and P, glycosidase
inhibitors of the pyrrolidine class, have been performed in a convergent, stereocontrolled way from
Received 8 September 2009
Received in revised form 15 October 2009
Accepted 20 October 2009
Available online 23 October 2009
D-serine as the chiral starting material. The synthesis of broussonetin C, a further member of this com-
pound family, is also reported. A cross-metathesis step was one key feature of the synthesis. The versatility
of the synthetic concept chosen permits the access to many members of this compound family, both
natural ones and analogues thereof.
Dedicated to Professor P. Camps, from the
University of Barcelona, on the occasion of
his 65th birthday
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
often exhibit inhibitory activity on some glycosidases.5 This prop-
erty, which relies on structural similarities with sugars, is frequently
Pyrrolidine alkaloids have been isolated from species of many
plant families,1 as well as from some animal organisms.2 Members
of this compound class are known to exhibit a broad range of bi-
ological activities.1d,3 Those bearing several hydroxyl functions
display a structural similarity to monosaccharides and are thus in-
cluded into the general group of the iminosugars.4 These com-
pounds display various types of heterocyclic systems (Fig. 1) and
associated to pharmacological utility.6 This therefore has led to
a significant synthetic effort on members of this compound class.7
Within naturally occurring pyrrolidines, the broussonetines
constitute a particular subgroup with about 30 reported examples.
They all have been isolated in recent years from the terrestrial plant
species Broussonetia kazinoki Siebold (Moraceae), a deciduous tree
growing in several Far East countries, mainly China and Japan, where
it is used in folk medicine.8,9 Most broussonetines have been found to
display marked inhibitoryactivities on various glycosidase types (the
IC50 values are situated in the micromolar to nanomolar range).
Furthermore, their selectivity has proven both qualitatively and
quantitatively different from that of other standard iminosugars
such as DNJ (Fig. 1). Broussonetine D, for instance, was found to
display a strong inhibitory activity (IC50, 29 nmol) against bovine
OH
OH
OH
OH
OH
OH
HO
HO
N
H
N
H
1-Deoxynojirimycin (DNJ) 1-Deoxygalactonojirimycin
liver b-galactosidase, an enzyme not inhibited by DNJ. The imino-
(DGJ)
sugar DGJ, which is structurally related to DNJ, does inhibit this en-
zyme but at a concentration 4.5 times higher than broussonetine D.
The majority of the reported broussonetines show the general
structure depicted in Figure 2. A common, polyhydroxylated pyr-
rolidine core is bound to a variable 13-carbon chain that displays
various types and degrees of functionalization. Figure 2 shows six
selected examples.
HO
HO
HO
OH
OH
HO
H
N
H
OH
HO
OH
N
N
H
HO
OH
Swainsonine
Australine
DMDP
Synthetic activity in the field of the broussonetines has been
very scarce until now.10 The first total synthesis of a member of the
broussonetine group in enantiopure form was carried out in 1999
by Yoda and co-workers,10a who prepared broussonetine C (1) from
Figure 1. Structures of some representative iminosugars with inhibitory ability on
glycosidases.
D
-tartaric acid as the chiral starting material. Four years later,
a second total synthesis of 1 was reported by Perlmutter and co-
workers, once again with a member of chiral pool, -arabinose, as
* Corresponding authors. Tel.: þ34 96 3544337; fax: þ34 96 3544328.
E-mail address: alberto.marco@uv.es (J.A. Marco).
D
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.10.066

C. Ribes et al. / Tetrahedron 65 (2009) 10612–10616
10613
HO
BnO
O
OH
OBn
functional groups
HO
OH
1´
HO
N
N
R
R
H
HO
N
H
1, 5, 6
O
13´
Broussonetines C-I, J-J2, K-M, M1, O, P, R-T, W, Y, Z
CRM
R
BnO OBn
(22 compounds)
+
N
O
O
8 or 9
OH
C
7
O
10 ´
13´
1
O
9´
OH
8
9
D
OTPS
OTPS
O
2
O
6´
10´
O
O
G
M
O
13´
(TPS = tert-butyldiphenylsilyl)
OH
3
OH
Scheme 1. Retrosynthetic analysis of 1, 5 and 6.
OH
OH
10´
4
O
Li
MeO
OTPS
O
N
THF, 0 ºC (80%)
10´
Me
3´
3´
10
5
OTPS
9´
OH
P
O
9
O
6
Scheme 2. Synthesis of ketone 9.
Figure 2. General structure of most broussonetines and some specific examples.
the presence of ruthenium catalyst 11.13 This gave the desired CRM
product 12 in 61% overall yield as a 4:1 E/Z mixture. Other products
obtained in the same reaction were the two expected homodimers
13 and 14.11,14 As observed in our previous synthesis, the reaction
could be also conducted under microwave (MW) irradiation with
practically unchanged yields but much shorter reaction times.
For the preparation of 1, separation of the E/Z mixture is not
necessary. Thus, treatment of (EþZ)-12 with ethanolic NaOH caused
sequential cleavage of the silyl and cyclic carbamate groups.15 The
resulting product 15, still as an E/Z mixture, was then subjected to
hydrogenation in an acidic medium16 to yield a product identical in
its physical and spectral properties to broussonetine C (1).
The preparation of broussonetine O (5) proved laborious. Only
after repeated column chromatography on silica gel of (EþZ)-15 we
were able to obtain reasonably pure (E)-15. Cleavage of the two
benzyl groups in the latter compound was achieved through
treatment with BCl3 in CH2Cl2 at low temperature17 and afforded 5
in 70% yield. As expected, hydrogenation of 5 in a neutral medium
yielded 1.
the starting material.10b Almost simultaneously, the third total
synthesis was reported by Trost and co-workers, who synthesized
broussonetine G (3) by means of a palladium-based, asymmetric
catalytic procedure.10c Finally, we have recently published the first
syntheses of broussonetines D (2) and M (4). The commercial
amino acid D
-serine was the chiral starting material.11
In the present article, we describe the first total syntheses of
broussonetines O (5) and P (6). In addition, we also report a further
synthesis of broussonetine C (1). For that purpose, we have fol-
lowed the same retrosynthetic analysis of our previous syntheses of
2 and 4.11 Thus, the olefinic bond of the side chain in 5 may be
hydrogenated to the saturated side chain of 1 and, at the same time,
allows for a cleavage via retro-cross-metathesis (retro-CRM) to the
common building block 711 and the appropriate side chain frag-
ment (Scheme 1). In the present case, the side chain fragment is one
of the two unsaturated ketones 811 (for the synthesis of 6) or 9 (for
the synthesis of 1 and 5).
2. Results and discussion
In our previous paper,11 we reported the cross-metathesis re-
action between oxazolidinone 7 and ketone 8 catalyzed by ruthe-
nium complex 11. After cleavage of the silyl and cyclic carbamate
groups in the resulting compound with ethanolic NaOH, compound
16 (EþZ) was obtained. Hydrogenation of the mixture furnished
broussonetine D (2).11 For the preparation of broussonetine P (6),
the isolation of pure (E)-16 was necessary. As in the previous case,
this required a lengthy chromatographic separation. Finally, treat-
ment of (E)-16 with BCl3 in CH2Cl2 at low temperature17 gave 6 in
80% yield.
The synthesis of ketone 9, depicted in Scheme 2, followed the
same methodology used in our previous preparation of the
structurally close 8.11 Thus, the known Weinreb amide 10, pre-
pared as reported12 from
g-butyrolactone, was treated with 6-
heptenyllithium, obtained by means of halogen–lithium exchange
in the corresponding bromide, to yield 9.
In the same manner as in our previous synthesis of 2 and 4,11
a 1:2 mixture of 7 and 9 was heated at reflux in CH2Cl2 for 24 h in

10614
C. Ribes et al. / Tetrahedron 65 (2009) 10612–10616
In summary, the three bioactive pyrrolidine alkaloids brousso-
netine C (1), broussonetine O (5) and broussonetine P (6) have been
required an inert atmosphere were carried out under N2 with flame-
dried glassware. Et2O and THF were freshly distilled from sodium/
benzophenone ketyl and transferred via syringe. Dichloromethane
was freshly distilled from CaH2. Tertiaryamines were freshly distilled
from KOH. Toluene was freshly distilled from sodium wire. Com-
mercially available reagents were used as received. Unless detailed
otherwise, ‘work-up’ means pouring the reaction mixture into brine,
followed by extraction with the solvent indicated in parenthesis. If
the reaction medium was acidic, an additional washing with 5% aq
NaHCO3 was performed. If the reaction medium was basic, an ad-
ditional washing with aq NH4Cl was performed. New washing with
brine, drying over anhydrous Na2SO4 and elimination of the solvent
underreducedpressure were followed bychromatography on a silica
prepared in enantiopure form from the commercially available
aminoacid
is the first synthesis of both 5 and 6, as well as the third synthesis of 1.
D
-serineastheultimatechiralitysource(Scheme3).11 This
BnO
OBn
10% 11
OTPS
O
7 + 9
N
O
CH2Cl2,
(1:2)
O
MW, 100ºC
12 (60%, E/Z 4:1)
O
+
BnO
OBn
O
gel column (60–200 mm) and elution with the indicated solvent
Mes
N
N Mes
Ph
mixture. Where solutions were filtered through a Celite pad, the pad
was additionally washed with the same solvent used, and the
washings incorporated to the main organic layer.
N
Cl
Cl
N
O
Ru
11
O
PCy3
13
BnO
OBn
3.1.1. 1-(tert-Butyldiphenylsilyloxy)undec-10-en-4-one (9). A 1.7 M
pentane solution of tert-butyllithium (11.8 mL, ca. 20 mmol) was
added under N2 to THF (10 mL) cooled to ꢁ78 ꢀC. Subsequently,
a solution of 7-bromo-1-heptene (1.53 mL, ca. 10 mmol) in dry THF
(6 mL) was added dropwise. The mixture was stirred for 3 h at the
same temperature. Weinreb amide 10 (1.93 g, 5 mmol) was then
dissolved in dry THF (8 mL) and added dropwise during 10 min. The
cooling bath was then removed and the solution was left to reach
room temperature and stirred for further 30 min. Work-up (extrac-
tion with EtOAc, 3ꢂ25 mL) and column chromatography on silica gel
(elution with hexanes/EtOAc, 95:5) gave ketone 9 (1.69 g, 80%): oil;
+
O
OTPS
3
TPSO
3
O
14
BnO
OH
OBn
NaOH, EtOH,
Δ, 8 h (72%)
12
OH
N
H
O
15
1H NMR (CDCl3)
d 7.70–7.65 (4H, m), 7.45–7.35 (6H, m), 5.81 (1H, ddt,
J¼17, 10.3, 6.5 Hz), 5.02 (1H, br dd, J¼17, 1.5 Hz), 4.96 (1H, br d,
J¼10.3 Hz), 3.69 (2H, t, J¼6 Hz), 2.53 (2H, t, J¼7.3 Hz), 2.40 (2H, t,
J¼7.4 Hz), 2.07 (2H, br q, Jw7 Hz), 1.85 (2H, br quint, Jw6.7 Hz), 1.60
(2H, br quint, Jw7.5 Hz), 1.42 (2H, br quint, Jw7.5 Hz), 1.32 (2H, br
1: Isomer separation
H2, 10% Pd/C, MeOH,
6M HCl, RT, 16 h (92%)
2: BCl3, CH2Cl2, -78 ºC,
6 h (70%)
H2, 10% Pd/C, MeOH
RT, 5 h (97%)
quint, Jw7.5 Hz), 1.05 (9H, s); 13C NMR (CDCl3)
d
211.0, 133.9 (ꢂ2),
5
1
19.2 (C), 138.9, 135.6 (ꢂ4), 129.6 (ꢂ2), 127.7 (ꢂ4) (CH), 114.4, 63.1,
42.8, 39.1, 33.6, 28.7 (ꢂ2), 26.7, 23.7 (CH2), 26.9 (ꢂ3) (CH3); IR nmax
t
1715 (C]O) cmꢁ1; HR EIMS m/z (rel int.) 365.1935 (Mþꢁ Bu, 100),
t
199 (64), calcd for C27H38O2Siꢁ Bu 365.1936.
BnO
OBn
3.1.2. (5R,6R,7R,7aR)-6,7-Bis(benzyloxy)-5-(13-tert-butyldiphenylsi-
lyloxy-10-oxotridec-3E,Z-enyl)tetrahydro-pyrrolo[1,2-c]oxazol-
3(1H)-one (12). Procedure A: compounds 7 (197 mg, 0.5 mmol), 9
(422 mg, 1 mmol) and Grubbs catalyst 11 (42 mg, 0.05 mmol) were
dissolved in dry, deoxygenated CH2Cl2 (8 mL). The mixture was
heated at reflux under N2 for 24 h. Solvent removal under reduced
pressure gave a residue, which was dissolved in Et2O (25 mL). The
organic layer was washed three times with water and then dried on
sodium sulfate. After this, charcoal (1 g) was added and the sus-
pension stirred for 24 h at room temperature. Solvent removal
under reduced pressure and column chromatography on silica gel
(elution with hexanes/EtOAc, 8:2) yielded compound 12 (240 mg,
61% as a 4:1 E/Z mixture). In addition, homodimers 1311 (57 mg, 30%
based on 7) and 14 (265 mg, 65% based on 9) were isolated, both as
E/Z mixtures. Procedure B: compounds 7 (197 mg, 0.5 mmol), 9
(422 mg, 1 mmol) and Grubbs catalyst 11 (42 mg, 0.05 mmol) were
dissolved in dry, deoxygenated CH2Cl2 (8 mL). The reaction mixture
was placed in a CEM Discover microwave oven and heated for 1 h at
100 ꢀC (100 W power). Work-up as above gave 12 (236 mg, 60%)
together with 13 and 14 in yields similar to those of procedure A.
Homodimer 14 could be recycled to 12 through reaction with 7
under the same metathesis conditions. It gave compound 12 in 56%
yield, together with excess 14 and homodimer 13.
2 steps
OH
7 + 8
(1:2)
N
H
ref. 11
OH
O
16
1: Isomer separation
2: BCl3, CH2Cl2, -78 ºC,
6 h (80%)
6
Scheme 3. Stereoselective synthesis of 1, 5 and 6.
3. Experimental
3.1. General
1H/13C NMR spectra were recorded at 500/125 MHz in the in-
dicated solvent at 30 ꢀC. Signals of the deuterated solvent were taken
as the reference (for CDCl3,
respectively; for pyridine-d5, the signals at
d
7.25 and 77.0 ppm for 1H and 13C NMR,
d
7.20 and 123.5 ppm for
1H and 13C NMR, respectively). Carbon atom types (C, CH, CH2, CH3)
were determined with the DEPT pulse sequence. Mass spectra were
run by the electron impact mode (EIMS) or by the fast atom bom-
bardment mode (FABMS, m-nitrobenzyl alcohol matrix). IR data are
given only for compounds with significant functions (OH, C]O) and
were recorded as oily films on NaCl plates (oils) or as KBr pellets
(solids). Optical rotations were measured at 25 ꢀC. Reactions, which
3.1.2.1. Compound 12. Oil; 1H NMR (CDCl3) (signals from the
major E isomer)
d 7.65 (4H, br d, Jw7.5 Hz), 7.45–7.25 (16H, br m),

C. Ribes et al. / Tetrahedron 65 (2009) 10612–10616
10615
5.50–5.40 (2H, m), 4.62 (2H, br d, J¼11.6 Hz), 4.50–4.40 (3H, m),
4.10–4.00 (2H, m), 4.00–3.90 (2H, m), 3.85 (1H, m), 3.68 (2H, t,
J¼6 Hz), 2.50 (2H, t, J¼7.3 Hz), 2.39 (2H, t, J¼7.3 Hz), 2.20–2.10 (2H,
m), 2.05–2.00 (2H, m), 1.85 (2H, br quint, Jw7 Hz), 1.70–1.50 (4H,
m), 1.40–1.25 (4H, m), 1.08 (9H, s); 13C NMR (CDCl3) (signals from
1.60–1.50 (2H, m), 1.40–1.20 (4H, br m), five exchangeable protons
(4OH, NH) were not detected under these conditions; 13C NMR
(pyridine-d5)
d
209.3 (C), 131.5, 129.2, 81.5, 77.4, 65.2 (ꢂ2), 62.3
(CH), 60.5, 45.0, 42.6, 39.4, 32.5, 29.8 (ꢂ2), 28.0, 26.9, 23.9 (CH2).
Hydrogenation of 5 under standard conditions (Pd/C, H2) gave 1.
Even though no natural sample of broussonetine O was available
for direct comparison with synthetic compound 5, the hydrogena-
tion of the latter to 1 (broussonetine C), for which an authentic
sample was available, gives support to our structural assignment.
Besides, the reaction sequence, which leads to 5 is almost identical
to that giving 6, the structure of which could be secured (see below).
There are differences in the chemical shifts of some signals
between the synthetic and the natural sample,8d even though their
identity has been secured as described above. Most likely, these
differences are due to the presence of minute amounts of acid and/
or metal impurities, which markedly affect the position of some
signals, as already reported for basic nitrogen-containing natural
products.18
the major E isomer)
d
211.0, 161.2, 137.3, 137.2, 133.9 (ꢂ2), 19.2 (C),
135.5 (ꢂ4), 129.6 (ꢂ2), 128.7, 128.5 (ꢂ2), 128.2, 128.0, 127.9, 127.7
(ꢂ4),127.6 (ꢂ4), 88.8, 87.8, 62.8, 62.3 (CH), 72.7, 71.6, 67.2, 63.1, 42.7,
42.6, 39.0 (ꢂ2), 35.2, 32.3, 32.1, 29.2, 28.8, 23.7, 23.3 (CH2), 26.9
(ꢂ3) (CH3); IR nmax 1761, 1714 (C]O) cmꢁ1; HR FABMS m/z
788.4323 (MþHþ), calcd for C49H62NO6Si 788.4346.
3.1.3. 1,20-Bis(tert-butyldiphenylsilyloxy)eicos-10E,Z-ene-4,17-dione
(14). Obtained as described above as an E/Z mixture: oil; 1H NMR
(CDCl3, signals from the major E isomer)
d 7.65 (8H, br d, Jw7.5 Hz),
7.45–7.30 (12H, br m), 5.40–5.35 (2H, m), 3.69 (4H, t, J¼6.5 Hz), 2.54
(4H, t, J¼7.3 Hz), 2.40 (4H, t, J¼7.3 Hz), 2.00 (4H, m),1.86 (4H, brquint,
Jw7 Hz), 1.60 (4H, br quint, Jw7 Hz), 1.40–1.30 (8H, m), 1.06 (18H, s).
3.1.4. 13-[(2R,3R,4R,5R)-3,4-Bis(benzyloxy)-5-(hydroxy-methyl)-
pyrrolidin-2-yl]-1-hydroxytridec-10E-en-4-one (15). A solution of
compound 12 (315 mg, 0.4 mmol) in 3:1 EtOH/H2O (10 mL) was
treated with NaOH (400 mg, 10 mmol). The reaction mixture was
stirred at reflux for 8 h. After this, all volatiles were removed under
reduced pressure and the residue was taken into brine, followed by
extraction with EtOAc (3ꢂ10 mL). The organic layer was then dried
over anhydrous MgSO4 and evaporated under reduced pressure. The
residue was subjected to column chromatography on silica gel (elu-
tion with CHCl3/MeOH, from 95:5 to 90:10) to furnish compound 15
(150 mg, 72%) as a 4:1 E/Z mixture. For the synthesis of broussonetine
C, the E/Z mixture was used as such in the hydrogenation step (see
below). For the synthesis of broussonetine O, the E/Z mixture was
subjected to repeated column chromatography on silica gel. In this
way, a fraction containing almost pure (E)-15 was obtained.
3.1.6. Broussonetine C (1). Compound 15 (26 mg, 0.05 mmol) as an
E/Z mixture (see above) was dissolved in MeOH (5 mL) and treated
with 6 M HCl (0.5 mL). After addition of 10% Degussa-type Pd/C
catalyst (30 mg), the mixture was placed under a H2 atmosphere
and stirred for 16 h at room temperature. The mixture was then
filtered through Celite (washing with MeOH). Removal of all vola-
tiles under reduced pressure gave a residue, which was dissolved in
MeOH (2 mL) and treated dropwise with 33% aq ammonia until
basic pH. Removal of all volatiles under reduced pressure gave
a residue, which was subjected to column chromatography on silica
gel (elution with CHCl3/MeOH/NH4OH, from 95:4:1 to 70:29:1).
The eluted material was subsequently purified in an ion-exchange
column (Dowex 5Wx4-400 Aldrich, acidified with 0.5 M HCl).
Elution was performed first with distilled water (50 mL) and then
with 1 M aq ammonia (until elution of the product). This gave 1
(15 mg, 90%). The identity of the synthetic sample was confirmed
by direct comparison with an authentic sample by means of co-
3.1.4.1. Compound (E)-15. Oil; [
(CDCl3)
a
]
þ8.2 (c 1.7, CHCl3); 1H NMR
D
d
7.40–7.25 (10H, br m), 5.40–5.30 (2H, m), 4.60–4.50 (4H,
chromatography in an HPLC-MS system: amorphous solid; [a]
D
m), 3.85 (1H, m), 3.80–3.70 (4H, m), 3.65–3.55 (4H, br m), 3.44 (1H,
m), 3.25–3.20 (1H, m), 2.50 (2H, t, J¼7 Hz), 2.40 (2H, t, J¼7 Hz),
2.20–1.95 (4H, br m), 1.80 (2H, br quint, Jw7 Hz), 1.75–1.60 (2H, m),
1.55 (2H, br quint, Jw7 Hz), 1.40–1.20 (4H, br m); 13C NMR (CDCl3)
þ26.3 (c 0.85, MeOH), lit.8c
[
a
]
þ25 (c 0.96, MeOH); 1H NMR
D
(pyridine-d5)
d
4.92 (1H, t, J¼6.3 Hz), 4.66 (1H, br t, J¼6.5 Hz),
4.50–4.40 (2H, m), 4.30 (1H, m), 4.08 (1H, m), 3.88 (2H, t, J¼6.5 Hz),
2.67 (2H, t, J¼7 Hz), 2.38 (2H, t, J¼7 Hz), 2.35–2.25 (2H, m), 2.07
(2H, br quint, Jw7.3 Hz), 1.85–1.60 (4H, br m), 1.60–1.50 (2H, m),
1.40–1.10 (8H, br m), five exchangeable protons (4OH, NH) were not
d
212.2, 137.8, 137.7 (C), 131.1, 129.2, 128.5 (ꢂ2), 128.4 (ꢂ2), 127.8
(ꢂ3),127.7 (ꢂ3), 88.4, 85.3, 61.9, 61.4 (CH), 72.0 (ꢂ2), 63.7, 61.5, 42.7,
39.4, 33.0, 32.2, 29.5, 29.1, 28.5, 26.5, 23.6 (CH2); IR nmax 3380 (br,
OH), 1709 (C]O) cmꢁ1; HR FABMS m/z 524.3379 (MþHþ), calcd for
C32H46NO5 524.3376.
detected under these conditions; 13C NMR (pyridine-d5)
d 210.6 (C),
80.5, 76.5, 65.1, 61.2 (CH), 63.0, 59.5, 42.7, 39.5, 31.7, 29.5 (five
overlapped signals), 27.7, 26.6, 24.0 (CH2); HR FABMS m/z 346.2581
(MþHþ), calcd for C18H36NO5 346.2593. The identity of the natural
and the synthetic sample was secured by comparison with an au-
thentic sample.
As in the previous case, there are differences in the chemical
shifts of some signals between the synthetic and the natural sam-
ple8d and for the same reasons.18
3.1.5. Broussonetine O (5). A solution of (E)-15 (26 mg, 0.05 mmol)
in dry CH2Cl2 (2 mL) was cooled to ꢁ78 ꢀC under N2 and treated
with BCl3 (0.4 mL of a 1 M solution in CH2Cl2, 0.4 mmol). The re-
action mixture was stirred at ꢁ78 ꢀC for 5 h and then at ꢁ30 ꢀC for
1 h. The reaction was quenched through addition of MeOH (1 mL)
followed by stirring at room temp for 1 h. After removal of all
volatiles under reduced pressure, the crude residue was dissolved
in MeOH (1 mL) and evaporated again to dryness. The yellowish
oily residue was then subjected to column chromatography on
silica gel (elution with CHCl3/MeOH/NH4OH, from 95:4:1 to
70:29:1). The eluted material was subsequently purified in an ion-
exchange column (Dowex 5Wx4-400 Aldrich, acidified with 0.5 M
HCl). Elution was performed first with distilled water (50 mL) and
then with 1 M aq ammonia (until elution of the product). This
3.1.7. Broussonetine P (6). Compound 16 (EþZ) was obtained as
reported in our previous paper.11 For the preparation of (E)-16, the
mixture was subjected to a repeated column chromatography on
silica gel using the same elution mixtures as for (E)-15. In this way,
a fraction containing almost pure (E)-16 was obtained.
A solution of (E)-16 (26 mg, 0.05 mmol) in dry CH2Cl2 (2 mL)
was cooled to ꢁ78 ꢀC under N2 and treated with BCl3 (0.4 mL of
a 1 M solution in CH2Cl2, 0.4 mmol). The reaction mixture was
stirred at ꢁ78 ꢀC for 5 h and then at ꢁ30 ꢀC for 1 h. The reaction
was quenched through addition of MeOH (1 mL) and stirring at
room temp for 1 h. After removal of all volatiles under reduced
pressure, the crude residue was dissolved in MeOH (1 mL) and
evaporated again to dryness. The yellowish oily residue was then
provided 5 (14 mg, 80%): amorphous solid; [
a
]
þ24.8 (c 0.35,
D
MeOH), lit.8d
[
a]
þ22.7 (c 0.37, MeOH); 1H NMR (pyridine-d5)
D
d
5.55–5.45 (2H, m), 4.73 (1H, br t, J¼6.3 Hz), 4.46 (1H, m),
4.30–4.20 (2H, m), 3.90–3.85 (3H, m), 3.68 (1H, m), 2.65 (2H, br t,
J¼7.3 Hz), 2.40 (3H, m), 2.20 (1H, m), 2.08 (3H, m), 1.94 (3H, m),

10616
C. Ribes et al. / Tetrahedron 65 (2009) 10612–10616
4. Iminosugars. From Synthesis to Therapeutic Applications; Compain, P., Martin,
O. R., Eds.; John Wiley and Sons: Chichester, UK, 2007.
subjected to column chromatography on silica gel (elution with
CHCl3/MeOH/NH4OH, from 95:4:1 to 70:29:1). The eluted material
was subsequently purified in an ion-exchange column (Dowex
5Wx4-400 Aldrich, acidified with 0.5 M HCl). Elution was per-
formed first with distilled water (50 mL) and then with 1 M aq
ammonia (until elution of the product). This afforded 6 (13 mg,
5. (a) Jacob, G. S. Curr. Opin. Struct. Biol. 1995, 5, 605–611; (b) Ganem, B. Acc. Chem.
Res. 1996, 29, 340–347; (c) Sears, P.; Wong, C.-H. Chem. Commun. 1998,
1161–1170; (d) Compain, P.; Martin, O. R. Bioorg. Med. Chem. 2001, 9,
3077–3092; (e) Asano, N. Glycobiology 2003, 13, 93R–104R; (f) Borges de Melo,
E.; da Silveira Gomes, A.; Carvalho, I. Tetrahedron 2006, 62, 10277–10302.
6. Cox, T. M.; Platt, F. M.; Aerts, J. M. F. G., in Ref. 4, pp 295–326.
75%): amorphous solid; [
a]
D þ29.7 (c 0.3, CHCl3), lit.8d
[
a
]
D þ28.8 (c
7. For various methods of synthesis of pyrrolidines (including other heterocyclic
systems), see: (a) Look, G. C.; Fotsch, C. H.; Wong, C.-H. Acc. Chem. Res. 1993, 26,
182–190; (b) Pichon, M.; Figade`re, B. Tetrahedron: Asymmetry 1996, 7, 927–964;
(c) Enders, D.; Thiebes, C. Pure Appl. Chem. 2001, 73, 573–578; (d) Yoda, H. Curr.
Org. Chem. 2002, 6, 223–243; (e) Cipolla, L.; La Ferla, B.; Nicotra, F. Curr. Top.
Med. Chem. 2003, 3, 485–511; (f) El-Ashry, E. H.; El-Nemr, A. Carbohydr. Res.
2003, 338, 2265–2290; (g) Merino, P. Targets Heterocycl. Chem. 2003, 7,
140–156; (h) Pyne, S. G.; Davis, A. S.; Gates, N. J.; Hartley, J. P.; Lindsay, K. B.;
Machan, T.; Tang, M. Synlett 2004, 2670–2680; (i) Popowycz, F.; Gerber-Lem-
aire, S.; Schu¨tz, C.; Vogel, P. Helv. Chim. Acta 2004, 87, 800–810; (j) Cren, S.;
Gurcha, S. S.; Blake, A. J.; Besra, G. S.; Thomas, N. R. Org. Biomol. Chem. 2004, 2,
2418–2420; (k) Ayad, T.; Genisson, Y.; Baltas, M. Curr. Org. Chem. 2004, 8,
1211–1233; (l) Izquierdo, I.; Plaza, M. T.; Rodrı´guez, M.; Franco, F.; Martos, A.
Tetrahedron 2005, 61, 11697–11704; (m) Brinner, K. M.; Ellman, J. A. Org. Biomol.
Chem. 2005, 3, 2109–2113; (n) Ballini, R.; Bosica, G.; Fiorini, D.; Palmieri, A.;
Petrini, M. Chem. Rev. 2005, 105, 933–971; (o) Huang, P.-Q. Synlett 2006,
1133–1149; (p) Bellina, F.; Rossi, R. Tetrahedron 2006, 62, 7213–7256; (q) Pan-
dey, G.; Banerjee, P.; Gadre, S. R. Chem. Rev. 2006, 106, 4484–4517; (r) Murruzzu,
C.; Riera, A. Tetrahedron: Asymmetry 2007, 18, 149–154; (s) Håkansson, A. E.; van
Ameijde, J.; Guglielmini, L.; Horne, G.; Nash, R. J.; Evinson, E. L.; Kato, A.; Fleet,
G. W. J. Tetrahedron: Asymmetry 2007, 18, 282–289; (t) Martin, O. Ann. Pharm. Fr.
2007, 65, 5–13; (u) Minatti, A.; Mun˜iz, K. Chem. Soc. Rev. 2007, 36, 1142–1152; (v)
Wolfe, J. P. Eur. J. Org. Chem. 2007, 571–582.
8. (a) Shibano, M.; Tsukamoto, D.; Kusano, G. Heterocycles 2002, 57, 1539–1553;
(b) Tsukamoto, D.; Shibano, M.; Kusano, G. Nat. Med. 2003, 57, 68–72; (c) for
broussonetine C, see: Shibano, M.; Kitagawa, S.; Kusano, G. Chem. Pharm. Bull.
1997, 45, 505–508; (d) for broussonetines O and P, see: Shibano, M.; Tsukamoto,
D.; Fujimoto, R.; Masui, Y.; Sugimoto, H.; Kusano, G. Chem. Pharm. Bull. 2000, 48,
1281–1285.
9. For other bioactive compounds from the genus Broussonetia, see: Lee, D.;
Kinghorn, A. D. In Studies in Natural Products Chemistry; Atta-ur-Rahman, Ed.;
Elsevier Science B.V: 2003; Vol. 28; Part I, pp 3–33.
10. (a) Yoda, H.; Shimojo, T.; Takabe, K. Tetrahedron Lett. 1999, 40, 1335–1336; (b)
Perlmutter, P.; Vounatsos, F. J. Carbohydr. Chem. 2003, 22, 719–732; (c) Trost,
B. M.; Horne, D. B.; Woltering, M. J. Chem.dEur. J. 2006, 12, 6607–6620; (d) for
the synthesis of the side chain fragment of broussonetine H, see: Brimble, M.
A.; Parka, J. H.; Taylor, C. M. Tetrahedron 2003, 59, 5861–5868.
0.96, MeOH); 1H NMR (pyridine-d5)
d 5.55–5.45 (2H, m), 4.90 (1H,
br t, Jw6.5 Hz), 4.66 (1H, br t, Jw6.5 Hz), 4.50–4.40 (2H, m), 4.26
(1H, m), 4.10 (1H, m), 3.85 (2H, t, J¼6.5 Hz), 2.60–2.50 (2H, m), 2.45
(2H, t, J¼7.5 Hz), 2.40 (2H, m), 2.33 (2H, t, J¼7.5 Hz), 1.95–1.80 (4H,
br m), 1.75 (2H, m), 1.60–1.50 (2H, m), 1.40–1.20 (2H, br m), five
exchangeable protons (4OH, NH) were not detected under these
conditions; 13C NMR (pyridine-d5)
d 210.5 (C), 131.7, 129.0, 80.6,
76.4, 65.2, 61.7 (CH), 62.2, 59.5, 42.5, 42.4, 33.0, 32.5, 31.9, 29.7, 29.0,
23.5, 21.0 (CH2); HR FABMS m/z 344.2443 (MþHþ), calcd for
C18H34NO5 344.2437. The identity of the natural and the synthetic
sample was secured by comparison with an authentic sample.
Acknowledgements
Financial support has been granted by the Spanish Ministry of
Science and Technology (projects CTQ2005-06688-C02-01 and
CTQ2005-06688-C02-02), by the BANCAJA-UJI foundation (projects
P1-1A-2005-15 and P1-1B-2005-30) and by the Conselleria de
´
Educacion, Ciencia y Empresa de la Generalitat Valenciana (project
ACOMP07/023). C.R. thanks the Spanish Ministry of Education and
Science for a predoctoral fellowship (FPU program). The authors
further thank Professor M. Shibano, from the Osaka University of
Pharmaceutical Sciences, Japan, for the kind sending of authentic
samples of broussonetines and for advice related to the purification
of these compounds.
Supplementary data
11. Ribes, C.; Falomir, E.; Murga, J.; Carda, M.; Marco, J. A. Org. Biomol. Chem. 2009, 7,
1355–1360.
Supplementary data contain the graphical NMR spectra. Sup-
plementary data associated with this article can be found in online
version at doi:10.1016/j.tet.2009.10.066.
12. Fukuda, Y.; Shindo, M.; Shishido, K. Org. Lett. 2003, 5, 749–751.
13. (a) Connon, S. J.; Blechert, S. Angew. Chem., Int. Ed. 2003, 42, 1900–1923; (b)
Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003,
125, 11360–11370; (c) Vernall, A. J.; Abell, A. D. Aldrichimica Acta 2003, 36,
93–105; (d) Schrodi, Y.; Pederson, R. L. Aldrichimica Acta 2007, 40, 45–52.
14. Homodimer 14 could be recycled to 12 by subjecting it to CM with pyrrolidine 7
under the same conditions as above. In contrast, homodimer 13 did not give 12
when allowed to react with 9. In relation to such issues, see Ref. 13b and:
Goldup, S. M.; Pilkington, C. J.; White, A. J. P.; Burton, A.; Barrett, A. G. M. J. Org.
Chem. 2006, 71, 6185–6191.
15. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 3rd ed.; John
Wiley: New York, NY, 1999; p 142.
16. Hydrogenation in a neutral medium caused saturation of the olefinic bond but
no hydrogenolysis of the benzyl groups.
17. Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 3rd ed.; John
Wiley: New York, NY, 1999; p 82.
18. (a) Wormald, M. R.; Nash, R. J.; Hrnciar, P.; White, J. D.; Molyneux, R. J.; Fleet,
G. W. J. Tetrahedron: Asymmetry 1998, 9, 2549–2558; (b) Donohoe, T. J.; Sintim,
H. O.; Hollinshead, J. J. Org. Chem. 2005, 70, 7297–7304; (c) Magolan, J.; Car-
son, C. A.; Kerr, M. A. Org. Lett. 2008, 10, 1437–1440.
References and notes
1. For reviews on various alkaloidal types, including pyrrolidines, see: (a) Plun-
´
kett, A. O. Nat. Prod. Rep. 1994, 11, 581–590; (b) OHagan, D. Nat. Prod. Rep. 1997,
14, 637–651; (c) O’Hagan, D. Nat. Prod. Rep. 2000, 17, 435–446; (d) Watson, A. A.;
Fleet, G. W. J.; Asano, N.; Molyneux, R. J.; Nash, R. J. Phytochemistry 2001, 56,
265–295; (e) Dembitsky, V. M. Lipids 2005, 40, 1081–1105.
2. Daly, J. W.; Spande, T. F.; Garraffo, H. M. J. Nat. Prod. 2005, 68, 1556–1575.
3. (a) Evans, S. V.; Fellows, L. E.; Shing, T. K. M.; Fleet, G. W. J. Phytochemistry 1985,
24, 1953–1955; (b) Molyneux, R. J.; Pan, Y. T.; Tropea, J. E.; Elbein, A. D.; Lawyer,
C. H.; Hughes, D. J.; Fleet, G. W. J. J. Nat. Prod. 1993, 56, 1356–1364; (c) Tsuchiya,
K.; Kobayashi, S.; Kurokawa, T.; Nakagawa, T.; Shimada, N. J. Antibiot. 1995, 48,
630–634; (d) Asano, N. Curr. Top. Med. Chem. 2003, 3, 471–484; (e) Greimel, P.;
Spreitz, J.; Stu¨tz, A. E.; Wrodnigg, T. M. Curr. Top. Med. Chem. 2003, 3, 513–523;
(f) Compain, P.; Martin, O. R. Curr. Top. Med. Chem. 2003, 3, 541–560; (g) Butters,
T. D.; Dwek, R. A.; Platt, F. M. Curr. Top. Med. Chem. 2003, 3, 561–574.