Synthesis and glycosidase inhibitory activities of 2-(aminoalkyl) pyrrolidine-3,4-diol derivatives

Article abstract of DOI:10.1016/j.bmc.2003.09.020

Several 2-(aminomethyl)-and 2-(2-aminoethyl)-pyrrolidine-3,4-diol derivatives have been assayed for their inhibitory activities towards glycosidases. Good inhibitors of α-mannosidases must have the (2R,3R,4S) configuration and possess 2-(benzylamino)methy

Full text of DOI:10.1016/j.bmc.2003.09.020

Bioorganic & Medicinal Chemistry 11 (2003) 4897–4911
Synthesis and Glycosidase Inhibitory Activities of
2-(aminoalkyl)pyrrolidine-3,4-diol Derivatives
Ana T. Carmona,a Florence Popowycz,b Sandrine Gerber-Lemaire,b
Eliazar Rodrıguez-Garcıa,b Catherine Schutz,b Pierre Vogelb,* and Inmaculada Robinaa,*
a
´
Departamento de Quımica Organica, Facultad de Quımica, Universidad de Sevilla, E-41071 Seville, Spain
´
´
Institut de chimie moleculaire et biologique, Ecole Polytechnique Federale de Lausanne, BCH, 1015 Lausanne, Switzerland
b
´
´ ´
Received 7 July 2003; accepted 12 September 2003
Abstract—Several 2-(aminomethyl)-and 2-(2-aminoethyl)-pyrrolidine-3,4-diol derivatives have been assayed for their inhibitory
activities towards glycosidases. Good inhibitors of a-mannosidases must have the (2R,3R,4S) configuration and possess 2-(benzyl-
amino)methyl substituents. Stereomers with the (2S,3R,4S) configuration are also competitive inhibitors of a-mannosidases, but
less potent as they share the configuration of C(1), C(2), C(3) of b-d-mannosides rather than that of a-d-mannosides. Interestingly,
(2S,3R,4S)-2-{2-[(4-phenyl)phenylamino]ethyl}pyrrolidine-3,4-diol (12g) inhibits several enzymes, for instance a-l-fucosidase from
bovine epididymis (Ki=6.5 mM, competitive), a-galactosidase from bovine liver (Ki=5 mM, mixed) and a-mannosidase from jack
bean (Ki=102 mM, mixed). Diamines such as (2R,3S,4R)-2-[2-(phenylamino) or 2-(benzylamino)ethyl]pyrrolidine-3,4-diol (ent-12a,
ent-12b) inhibit b-glucosidase from almonds (Ki=13–40 mM, competitive).
# 2003 Elsevier Ltd. All rights reserved.
Introduction
could lead to new more potent and more selective
enzyme inhibitors.3 It is also important4 to point out
that the stability of a given compound towards acid
hydrolysis and its permeability through membranes are
important requirements for a compound to become a
useful drug. Hence, hydrolytically stable C–C links and
lipophilic moieties that common sugars lack are impor-
tant structural features for potentially efficient and
selective glycosidase inhibitors. Approaches have been
made to fulfill those requirements such as imino-C-dis-
accharides5 and imino-C-glycosides of heterocycles.6
Additionally, the introduction in the aglycon moiety of a
second amino group able to increase the number of elec-
trostatic interactions with the carboxylic groups of the
active site of the glycosidase, together with the presence
of substituents capable of establishing stabilizing
hydrophobic interactions with other sites of the enzyme,
is an approach that might be useful. Meso-2,3-dihy-
droxypyrrolidine 1 is a weak and non-selective enzyme
inhibitor presenting a wide range of enzymatic inhibi-
tion.7 The introduction of a methyl group in C-2 as in
compound 2 does not improve the enzymatic proper-
ties.8 We have already proposed7 (Fig. 1) a dicationic
mimetic of the transition state (or the intermediate)
corresponding to the hydrolytical process, to account
for an increase of the electrostatic interactions with the
Monosaccharide mimetics such as 1,4-dideoxy-1,4-
iminoalditols (hydroxylated pyrrolidines) are well-
known as glycosidase inhibitors.1,2 Their enzymatic
inhibition arises from their ability, after protonation in
physiological media, to mimick the structure of the
oxycarbeniun ion liberated during the enzyme catalyzed
hydrolytical process. It is observed however, that in
many instances hydroxylated pyrrolidines are not selec-
tive, presenting a wide range of enzymatic inhibition.
This lack of selectivity is probably due to their higher
conformational flexibility compared to their piperidine
analogues.
The selectivity in enzyme inhibition can be improved by
providing the iminosugar with some information of the
aglycon that is liberated in the enzymatic hydrolysis in
addition to the information about the structure of the
glycosyl moiety that is cleaved away and that mimicks
the oxycarbenium ion intermediate. Therefore, the
introduction of additional groups in the iminosugar
*Corresponding authors. Tel.: +34-95-4557150; fax. +34-95-4624960;
e-mail: robina@us.es (I. Robina), Tel.: +41-216939371; fax. +41-
216939375; e-mail: pierre.vogel@epfl.ch (P. Vogel).
0968-0896/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2003.09.020

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A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
carboxylate groups of the enzyme in a-mannosidase
inhibitors. We have demonstrated that the introduction
of an aryl(alkyl)aminomethyl side chain at C-2 of dihy-
droxypyrrolidine 1 causes a remarkable enhancement
on activity and selectivity. Thus, diamines of type 3 can
be highly selective and competitive inhibitors of a-man-
nosidases and we have reported9 a quick combinatorial
approach for their preparation. The best results in this
series towards a-mannosidases are obtained with deriv-
atives containing 4-phenylbenzylamino (Ki=2.5 mM)
and (R)-indenylamino (Ki=2.3 mM) moieties (competi-
tive inhibitors).7b
1,2,5-trideoxy-2,5-imino-d-mannitol. Alkylamino deri-
vatives12 of type 7 (R=6-hydroxyhexyl, 3-aminopropyl)
are good inhibitors of a-glucosidases (Ki=10–25 mM),
while amides of type 813 (R=undecyl, phenylethyl,
naphthyl, coumarinyl) present inhibitory activities
towards b-glucosidases in the nanomolar range (Ki=1.2–
550nM). On their side, Wong and coworkers14 have
reported the synthesis of 8 (R=Me) which is a potent
inhibitor of b-N-acetylglucosaminidases (Ki=0.2 mM).
Additionally, the same authors15 reported the synthesis
and enzymatic activity of derivatives of 1-alkyl(aryl)
amino-1,2,5-trideoxy-2,5-imino-d-galactitol 9 obtained by
reductive amination and Strecker condensation.
Several polyhydroxylated amino and aminoalkyl pyrro-
lidines have also been described. The synthesis of 5-
amino-1,4-dideoxy-1,4-imino-d-mannitol hydrochloride
4, has been reported by Jager and co-workers;10 in con-
trast to its analogue 1,4-dideoxy-1,4-imino-d-mannitol,
which is a potent inhibitor of a-mannosidases, 4 does
not inhibit this enzyme. Reynolds and co-workers11
have prepared 1- and 6-aminoalkyl derivatives of 2,5-
dideoxy-2,5-imino-d-glucitol (5 and 6) that present
antibacterial activity potentially useful in the treatment
of tuberculosis (Fig. 2).
Thus, compound 9a (R=C10H21, R0=H) and 9b
(R=C2H4Ph, R0=H) showed good inhibition towards
a-mannosidases
(Ki=10 mM)
and
a-N-acetyl-
galactosaminidases (Ki=29.4 nM), respectively, while
the same derivatives with R0=CH2NH2 or CONH2
were found to be less active towards the same enzymes.
Recently it has been claimed that the presence of a
hydroxy group at C-2 of the pyrrolidine ring enhances
strongly its enzymatic activity and selectivity. Thus,
hemiaminals 108 are potent inhibitors in the sub-mM
range of a-mannosidases (jack bean), b-glucosidases
(almonds) and b-galactosidases (coffee beans and E.
Coli) while their corresponding enantiomers ent-108,16
are inhibitors of a-mannosidases and a-fucosidases with
Ki between 3 and 20 mM. Other hemiaminals 11 and ent-
1117 have been reported, both enantiomers present
similar inhibitory activities. They are moderate inhibi-
tors of b-glucosidases (almonds) and good competitive
inhibitors of a-mannosidases (jack bean and almonds)
with Ki=0.94–2.5 mM (Fig. 3). Unfortunately, because
of their hemiaminal structure, 10 and 11 present the
inconvenience of their poor stability.
Stutz and co-workers have described the synthesis and
enzymatic inhibitory activity of derivatives of 1-amino-
Looking for new, easily accessible, chemically stable
and selective enzymatic inhibitors, we report here a
Figure 1.
Figure 2.

A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
4899
study that evaluates the importance for the inhibitory
activity towards glycosidases of the configuration of
centers C(2), C(3) and C(4) on the pyrrolidine moiety of
these compounds. We examine also the influence of the
aryl(alkyl) substituent, and of the nature of the spacer
between the nitrogen center of the pyrrolidine ring and
that of the aminoalkyl side chain. With this goal in
mind, we have prepared derivatives of (2S,3R,4S),
(2R,3R,4S), (2R,3S,4R) and (2S,3S,4R)-2-alkyl(aryl)
aminoethyl-3,4-dihydroxypyrrolidines (12, ent-12, 13
and ent-13) and of (2S,3R,4S)-2-(N-alkyl(aryl) amino-
methyl-3,4-dihydroxypyrrolidines (14) and have assayed
these compounds towards 25 commercially available
glycosidases.
proved their efficiency in a variety of antitumor and
antiviral screenings (Fig. 5).20
On their side, a-glucosidase inhibitors are of key
importance in the replication of the AIDS virus (HIV),
particularly in the formation of its envelope glycopro-
tein-mediated fusion at the binding step. N-Butyldeoxy-
nojirimycin (17), one of the most potent a-glucosidase
inhibitors, has been considered as a potential anti-HIV
therapeutic agent.21 Other strong a-glucosidase inhibi-
tors such as castanospermine (18), 1-deoxynojirimycin
(19),22 and bromoconduritol (20)23 reduce HIV-1
infectivity in vitro.24
The best results are observed with systems having aro-
matic substituents. Enzyme specificity depends on the
absolute configuration of the iminosugars. For instance,
while compounds 12, 13 and 14 are good inhibitors of a-
mannosidases, ent-12 and ent-13 are good inhibitors of
b-glucosidases (Fig. 4). In both cases they are moderate
inhibitors of b-galactosidases.
Results and Discussion
The syntheses of 2-alkyl(aryl)-3,4-dihydroxypyrrolidines
12–14, have been carried out by reductive amination of
the corresponding pyrrolidine-carbaldehydes followed
by deprotection. In the case of diamine 12j, reaction
took place between diamine 3625 and biphenyl-4-car-
baldehyde. Alternatively, alkylaminopyrrolidines (12k
and ent-12g) were obtained from the corresponding
pyrrolidine-carboxylic acids after amidation reaction
and subsequent reduction and deprotection. (Scheme 1).
a-Mannosidases inhibitors are of importance in anti-
cancer therapies because they inhibit the biosynthesis of
N-linked glycoproteins associated with metastasis and
cancer progression. Natural compounds such as swain-
sonine (15)18 and mannostatin A (16)19 are among the
most potent inhibitors of a-mannosidases and have
Pyrrolidine-carbaldehydes 25 and 26 were prepared
according to Buchanan’s methodology.26 Starting from
Figure 3.
Figure 4.
Figure 5.

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A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
by in situreduction with NaBH(OAc) and subsequent
3
deprotection with aqueous CF3COOH gave the corres-
ponding diamines 12 in good yields (Scheme 3). In the
case of minor aldehyde 26 (Scheme 4) reductive amina-
tion with aniline gave 28a (45% from 24) which was
subsequently deprotected to give diamine 13a.
The same procedure as described above was applied to
the preparation of diamines ent-12 and ent-13a from the
enantiomerically pure pyrrolidine-carbaldehydes ent-25
and ent-26 (Schemes 5 and 6). For compound ent-25,
reaction with aniline, benzylamine, 1-aminobutanol,
and morpholine gave protected diamines ent-27 in
moderate-to-good yields that, after acidic cleavage,
furnished diamines ent-12 in 66–100% yield. Aldehyde
ent-26 was obtained from minor epimer ent-24, and
made immediately react with aniline in the presence of
NaBH(OAc)3 furnishing protected diamine ent-28a in
55% overall yield from ent-24. Acidic hydrolysis gave
ent-13a in 84% yield.
Scheme 1.
2,3-isopropylidene-l-erythrose,26b a mixture of pyrroli-
dines 23 and 24 in a ratio 9:1 was obtained after Wittig
olefination, addition of ammonia and intramolecular
displacement of the mesylate.
Boc protection and reduction with DIBALH gave the
corresponding aldehydes 255m,5n and 26. In a similar
way enantiomerically pure aldehydes ent-2525 and ent-
26 were obtained from d-arabinose (Scheme 2).
The preparation of compounds 14 was carried out by
reductive amination of aldehyde 30 with benzylamine
and (R)-1-aminoindane and subsequent treatment with
aqueous CF3COOH. Compound 30 was obtained7b by
Swern oxidation of the intermediate primary alcohol 29,
that was obtained from d-ribose following the method
described by Kim and co-workers.27
The minor aldehydes 26 and ent-26 were not isolated
and immediately reacted in the following step. Reaction
of aldehyde 25 with a small library of amines followed
Scheme 2. Reaction conditions: (a) acetone, H2SO4; (b) NalO4, H2O; (c) (i) NaOH 1 M, (ii) HCl 1 M; (d) Ph3P=CHCOOEt, CH2Cl2 refl.; (e) MsCl,
Py; (f) NH3, EtOH, (g) (Boc)2O, Py; (h) DIBALH, CH2Cl2, À78 ꢀC.

A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
4901
Scheme 3.
Scheme 4.
Scheme 5.

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A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
Scheme 6.
Scheme 7.
Diamine 3225 was made react with biphenyl-4-carbalde-
hyde under the same conditions furnishing diamine 33
that, after deprotection with 4 M HCl gave 12j in
quantitative yield (Scheme 7).
compounds 12b (R=Bn) and 12g (R=biphenyl) are
more potent inhibitors suggesting the intervention of
stabilizing hydrophobic interactions between the aro-
matic ring of the inhibitor and the enzyme.
Finally, amidation reaction between pyrrolidine car-
boxylic acids 34 and ent-34 and a-naphthylamine and
biphenyl-4-amine, respectively, afforded compounds 35k
and ent-35g that, after reduction with BH3.SMe2 and
acidic deprotection, gave diamines 12k and ent-12g
(Scheme 8).
In the case of compounds 14 with (aryl)aminomethyl
side chains, the activity and selectivity towards a-man-
nosidases is notably increased. This is an additional
evidence, as it has been reported,7a of the importance of
the size of the spacer between the nitrogen center of the
pyrrolidine moiety and that of the side chain. The opti-
mal results are observed for 1,2-diamines. It is worth
noting that compound 12g with a biphenyl moiety is a
relatively potent inhibitor of a-l-fucosidases (bovine
epididymis) (Ki=6.5 mM). The efficiency in the inhibi-
tory activity of biphenyl derivatives was precedent,7b
having been found that molecular motifs containing a
biphenyl moiety are high affinity-ligands for proteins.28
Compound 12a (R=Ph) presents also moderate inhibi-
tion 40% towards this enzyme. Compounds of this series
were also moderate-to-good inhibitors of b-galacto-
sidases like 12a (R=Ph, 64%), 12c (R=Pri, 33%), 12e
(R=C2H5CH(OH)CH2, 95%), 12f (R=morpholine,
29%), 12g (R=biphenyl, 90%, Ki=5 mM), 12j
(R=biphenylmethyl, 85%), 12k (R=naphthyl, 95%),
14b (R=Bn, 37%) and 14i (R=indenyl, 33%) at 1 mM
concentration (see Table 1).
Enzymatic inhibitory studies
Diamines 12–14, have been tested for their inhibitory
activity towards 25 commercially available glycosidases.
The data are summarised in Tables 1 and 2.
As expected, the absolute configuration of the stereo-
genic centers in the pyrrolidine ring have noticeable
influence on the inhibitory selectivity. Thus, (2S,3R,4S)-
2-alkyl(aryl)aminoethyl and aminomethyl pyrrolidines
12 and 14 have shown to be moderate-to-good inhibi-
tors of a-mannosidases, while derivatives ent-12 of
(2R,3S,4R) configuration were inactive towards these
enzymes, but presented good inhibitory activities
towards b-galactosidases and b-glucosidases.
Diamine 3625 (Fig. 6) is a moderate inhibitor of a-man-
nosidase from jack bean (52% of inhibition at 1 mM)
and is ignored by a-mannosidase from almonds.
Derivatives 12c (R=Pri), 12d (R=2-hydroxypropyl),
12e (R=2-hydroxybutyl), and 12f (R=-morpholine),
showed similar inhibition activities as 36. In contrast,
The inhibition towards b-galactosidases and a-manno-
sidases can be explained taking into account the differ-
ent conformations of the flexible pyrrolidine moiety that
can mimic C-2 and C-3 of d-mannose or C-3 and C-4 of
d-galactose (Fig. 7). The competitive inhibition towards
a-fucosidases can be explained by the resemblance of C-

Table 1. Inhibitory activities of compounds 12a–12g, 12j, 12k, 13a, 14b, 14i and 36. Percentage of inhibition at 1 mM concentration, IC50 and Ki values in mM, when measured at optimal pH, 25 ꢀC. All
inhibitors are competitive (Lineweaver–Burk plots) except when indicated (M=mixed type of inhibition). NI=no inhibition at 1 mM concentration
Enzymes:
12a
12b
12c
12d
12e
12f
12g
12j
12k
13a
14b
14i
36
ꢀ-L-fucosidase
Bovine epididymis
40%
NI
NI
NI
NI
NI
78% (C)
(IC50=63 mM)
Ki=6.5 ꢁM
33%
NI
NI
NI
NI
NI
NI
Human placenta
ꢂ-Galactosidase
Bovine liver
28%
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
64%
(IC50=630 mM)
33%
95%
29%
90% (M)
(IC50=45 mM)
Ki=5 ꢁM
NI
85% 95%
37%
33%
Jack beans
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
30%
60%
47%
66%
NI
ꢀ-Glucosidase
(isomaltase)
Baker yeasts
NI
78%
(IC50=470 mM)
22%
Amyloglucosidase
Aspergillus niger
Rhizopus mold
ꢂ-glucosidase
Almonds
NI
NI
27%
23%
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
24%
NI
NI
NI
NI
NI
NI
NI
NI
22%
NI
NI
NI
48%
NI
NI
NI
NI
NI
NI
NI
47%
NI
31%
25%
NI
NI
caldocellum
saccharolyticum
ꢀ-mannosidase
Jack beans
31%
25%
57% (M)
54%
55%
70% (M)
72% (C)
73% (C)
52%
(IC50=620 mM) 25% (IC50=1 mM) (IC50=830 mM) 41% (IC50=360 mM) 59% 53% 33% (IC50=361 mM) (IC50=397 mM) (IC50=1 mM)
Ki=360 ꢁM
Ki=102 ꢁM
Ki=8.7 ꢁM
Ki=12.1 ꢁM
Almonds
NI
21%
22%
20%
24%
39%
32%
NI
NI
NI
44%
38%
NI
NI towards the following enzymes: ꢀ-galactosidases from coffee beans, Aspergillus niger and Escherchia coli; ꢂ-galactosidase from-Escherichia coli, Aspergillus niger and Aspergillus orizae; ꢀ-glucosidases
(maltase) from yeast and rice; ꢂ-mannosidase from Helix pomatia; ꢂ-xylosidase from Aspergillus niger; ꢀ-N-Acetylgalactosaminidase from chicken liver; ꢂ-N-Acetylglucosaminidases from jack bean, bovine
epididymis A and bovine epididymis B.

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A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
Table 2. Inhibitory activities of compounds ent12a, ent-12b, ent-12f, ent-12g, ent-12h, ent-13a and 37. Percentage of inhibition at 1 mM concen-
tration, IC50 and Ki values in mM, when measured at optimal pH, 25 ꢀC. All inhibitors are competitive (Lineweaver–Burk plots) except when indi-
cated (M=mixed type of inhibition). NI=no inhibition at 1 mM concentration
Enzymes:
ent-12a
ent-12b
ent-12f
ent-12g
ent-12 h
ent-13a
37
ꢀ-Galactosidase
Escherchia coli
Coffee beans
NI
NI
NI
NI
NI
NI
NI
73%
NI
NI
47%
NI
NI
ꢂ-Galactosidase
Escherichia coli
42%
NI
NI
93% (M)
(IC50=73 mM)
Ki=1.5 ꢁM
NI
NI
NI
Bovine liver
86% (M)
(IC50=180 mM)
Ki=20 ꢁM
59%
46%
32%
28%
NI
97%
NI
45%
44%
NI
NI
NI
NI
NI
NI
NI
NI
NI
NI
Aspergillus niger
69%
(IC50=310 mM)
27%
(IC50=640 mM)
NI
Aspergillus orizae
Amyloglucosidase
Rhizopus mold
ꢂ-glucosidase
Almonds
NI
NI
24%
NI
NI
NI
94% (C)
(IC50=180 mM)
Ki=13 ꢁM
36%
87% (C)
(IC50=110 mM)
Ki=40 ꢁM
NI
40%
93% (C)
(IC50=49 mM)
Ki=20 ꢁM
NI
35%
95% (C)
(IC50=30 mM)
Ki=14 ꢁM
NI
Caldocellum
NI
NI
NI
saccharolyticum
ꢀ-mannosidase
Jack beans
NI
NI
NI
NI
23%
NI
NI
NI
NI
NI
58%
(IC50=613 mM)
49%
NI
NI
17-Almonds
(IC50=1 mM
ꢀ-N-Acetylgalacto saminidase
Chicken liver
68%
(IC50=360 mM)
NI
NI
NI
NI
NI
NI
ꢂ-N-Acetylglucosa minidase
Jack bean
NI
51%
NI
NI
NI
NI
NI
NI towards the following enzymes: a-l-fucosidases from bovine epididymis and human placenta; ꢀ-galactosidases from coffee beans and Aspergillus
niger; ꢀ-galactosidase from coffee beans and Aspergillus niger;ꢂ-galactosidase from jack bean; ꢀ-glucosidases (maltase) from yeast and rice; ꢀ-glu-
cosidases (isomaltase) from Baker yeast; amyloglucosidase from Aspergillus niger; ꢂ-mannosidase from Helix pomatia; ꢂ-xylosidase from Aspergillus
niger; ꢂ-N-Acetylglucosaminidases from jack bean, bovine epididymis A and bovine epididymis B
Figure 6.
2,3,4 after rotation of the molecule, with C-3,4,5 of the
fucopyranosyl moiety. Compound 13a, C-2 epimer of
Figure 7.
12a, is totally inactive except for a weak inhibition (33%
at 1 mM concentration) towards a-mannosidases.
(R=biphenyl, 93%, Ki=20 mM) and ent-13a (R=Ph,
Diamine 37 was inactive towards the 25 enzymes ana-
lyzed, however derivatives ent-12a, ent-12b, ent-12f, ent-
12h and ent-12g presented activity towards b-galactosi-
dases from bovine liver, Aspergillus niger and Escher-
iccia coli. Compounds ent-12a and ent-12g presented the
best inhibitory activities values [Ki=20 mM (bovine
liver) and Ki=1.5 mM (E. coli)], respectively. These
compounds are also good inhibitors of b-glucosidases
from almonds, especially derivatives with aromatic sub-
stituents, confirming the importance of the aromatic
ring in the lateral chain: ent-12a (R=Ph, 94%,
Ki=13 mM), ent-12b (R=Bn, 87%, Ki=40 mM), ent-12g
95%, Ki=14 mM), (see Table 2). However, these com-
pounds were inactive towards a-mannosidases.
The inhibition towards b-galactosidases and b-glucosi-
dases can be explained by the resemblance of C-2,3,4
after rotation of the molecule, with C-3,4,5 of the
galactopyranosyl moiety (Fig. 8) The inhibition towards
b-glucosidases can be explained considering that the
pyrrolidine moiety can adopt an envelope conformation
with C-4 in the apical position, then the two hydroxy
groups of the pyrrolidine moiety resemble HO-2 and
HO-3 of the glucopyranosyl cation.

A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
4905
Scheme 8.
all other derivatives of these type of diamine are ignored
by this enzyme. The biphenylamino compound 12g also
inhibits a-l-fucosidase from bovine epididymis
(Ki=5 mM, mixed). The enantiomer of 12g, ent-12g
inhibits b-galactosidase from bovine liver (Ki=1.5 mM,
mixed) and b-glucosidase from almonds (Ki=20 mM,
competitive). The latter enzyme is also inhibited by
analogues (2R,3S,4R)-2-[2-(phenylamino)-ethyl]pyrroli-
dine-3,4-diol (ent-12a),-2-[2-benzylamino)-ethyl]pyrroli-
dine - 3,4 - diol (ent - 12b) and (2S,3S,4R) - 2 - [2 -
(phenylamino)ethyl]pyrrolidine-3,4-diol (ent-13a) with
inhibition constants Ki=13, 40 and 14 mM (all competi-
tive). It should be noted that b-glucosidase from Caldo-
cellum saccharolyticum ignores these diamines.
Figure 8.
Experimental
Compound ent-12a and its C-2 epimer, compound ent-
13a, present similar inhibitory values towards b-gluco-
sidases from almonds. Interestingly, compound ent-12a
is a good inhibitor of b-galactosidases from bovine liver
(Ki=20 mM), while compound ent-13a ignores com-
pletely this enzyme. This fact can be explained by the
configuration of C-2 that does not resemble that of C-5
of d-galactose.
General procedures
Optical rotations were measured in a 1.0 cm tube on
Perkin–Elmer 241 MC and with a JASCO DIP-370
digital polarimeter. 1H NMR and 13C NMR spectra
were obtained for solutions in CDCl3, DMSO-d6,
CD3OD and D2O, J values are given in Hz and d in
ppm. All the assignments were confirmed by two-
dimensional NMR experiments. The IR spectra were
recorded on a Perkin Elmer Paragon 1000 FT-IR spec-
trometer. The FAB mass spectra were obtained with
glycerol or 3-nitrobenzyl alcohol as matrix. TLC was
performed on silica gel 60F254 (Merck), with detection
by UV light and charring with Pancaldi reagent
[(NH4)6MoO4, Ce(SO4)2, H2SO4, H2O]. Silica gel 60
(Merck, 230 mesh) was used for preparative chromato-
graphy. Solvents were dried by standard methods and
were freshly distilled under N2 prior to use.
Conclusion
New 2-(aminomethyl)- and 2-(2-aminoethyl)pyrroli-
dine-3,4-diol derivatives have been prepared and
assayed for their inhibitory activities towards glycosi-
dases.
Although
(2S,3R,4S)-2-[(benzylamino)-
methyl]pyrrolidine-3,4-diol (14b) and (2S,3R,4S)-2-
(inden-1-ylaminomethyl)-pyrrolidine-3,4-diol (14i) share
the configuration of centers C(1), C(2) and C(3) of b-d-
mannosides and not that of a-d-mannosides, they are
moderate competitive inhibitors of a-d-mannosidase
from jack bean (Ki=8.7 mM, 12 mM respectively). As
expected, they are not as good inhibitors as their
(2R,3R,4S)-stereomers (Ki=2–7 mM).7b Apart from
(2S,3R,4S)-2-{2-[(4-phenyl)phenylamino]ethyl}pyrroli-
dine-3,4-diol (12g) which also inhibits a-mannosidase
from jack bean (Ki=102 mM, mixed type of inhibition),
Glycosidase inhibition assays
Well established protocol was applied.29 We verified
that the delay of inhibitor/enzyme incubation did not
affect the inhibition measurements. Under standard
conditions, optimal inhibitory activities were measured
after 5 min of incubation.

4906
A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
The inhibition constants (Ki) and the type of inhibition
(competitive, non-competitive, mixed) were determined
from Lineweaver–Burk plots.30 For each plot, a blank
and two concentrations of inhibitor were used
corresponding to IC50 and IC50/2.
129.4 and 128.7 (Ph), 73.9, 73.3 (C-4, C-3), 61.4 (C-2),
54.3 (CH2-Ph), 51.4 (C-5), 47.3 (C-20), 29.9 (C-10).
CIMS m/z 237 [50%, (M+H)+.]. CIMSHR: m/z
237.1602 (calcd for C13H20N2O2+H: 237.1603).
(2S,3R,4S)-2-[2-(Isopropylamino)ethyl]pyrrolidine-3,4-
diol (12c). Reductive amination of 25 with iso-
propylamine gave, after column chromatography on
silica gel (CH2Cl2–MeOH 30:1!5:1), protected deriv-
Reductive amination, general procedure. To a solution of
N-tert-butoxycarbonyl-3,4-di-O-isopropylidene-3,4-diol-
2-formylmethylpyrrolidine
255m,5n
or
ent-257b
(0.10 mmol) in 1,2-dichloroethane (1 mL), the corre-
sponding amine (0.11 mmol) and NaBH(OAc)3
(0.14 mmol) were added. The reaction mixture was stir-
red at 20 ꢀC under N2 for 3 h. Then, aqueous saturated
solution of NaHCO3, was added and extracted with
AcOEt, dried (Na2SO4) and evaporated in vacuo. Pur-
ification of the residue gave the corresponding N- and
O- protected amino pyrrolidines, all as oils.
ative 27c (40% yield). Conventional deprotection
25
gave 12c (83% yield). ½ꢀŠ +10 (c 0.47, MeOH); IR
589
nmax 3280, 1400, 1095 cmÀ1
;
1H NMR (400 MHz,
MeOD, d ppm, J Hz): d 4.25 (m, 1H, H-4), 3.97 (t, 1H,
J3,2=J3,4=4.3, H-3), 3.06–3.02 (m, 2H, H-2, H-5a),
2.94 (hept, 1H, JH,H=6.4, H-100), 2.87 (dd, 1H,
J5b,4=6.2, J5b,5a=11.4, H-5b), 2.78 (m, 2H, H-20a, H-
2
20b), 1.91 (m, 1H, H-10a), 1.78 (m, 1H, H-10b), 1.16 (d,
6H, 2 CH3); 13C NMR (100.5 MHz, MeOD, d ppm): d
73.9, 73.4 (C-4, C-3), 61.2 (C-2), 51.6 (C-5), 49.6 (C-100),
45.3 (C-20), 29.9 (C-10), 21.9 (2 CH3). CIMS m/z 189
[100%, (M+H)+.]. CIMSHR: m/z 189.1600 (calcd for
C9H20N2O2+H: 189.1603).
Hydrolysis Boc and isopropylidene groups, general pro-
cedure. A solution of N- and O- protected amino pyr-
rolidines (0.05 mmol) in 80% aqueous CF3COOH
(1.5 mL) was left at 20 ꢀC for 2 h. The mixture was pas-
sed through a Dowex 50WX8 column (100–200 mesh)
and eluted, successively with MeOH (30 mL), H2O
(30 mL) and NH4OH 10% (50 mL) and then was con-
centrated by solvent evaporation in vacuo.
(2S,3R,4S)-2-2-[((R and S)-2-Hydroxypropyl)amino]
ethylpyrrolidine-3,4-diol (12d). Reductive amination of
25 with rac-2-hydroxypropylamine gave, after column
chromatography on silica gel (CH2Cl2–MeOH
35:1!10:1), protected derivative 27d (40% yield). Con-
The following compounds were obtained in this
manner.
1
ventional deprotection gave 12d (98% yield). H NMR
(400 MHz, MeOD, d ppm, J Hz): d 4.31 (m, 1H, H-4),
4.10 (t, 1H, J3,2=J3,4=4.3, H-3), 3.96 (m, 1H, H-200),
3.43 (m, 1H, H-2), 3.25 (dd, 1H, J5a,4=7.0,
2J5a,5b=11.6, H-5a), 3.04 (dd, 1H, J5b,4=5.9, H-5b),
3.01–2.94 (m, 2H, H-20a, H-20b), 2.86 (m, 1H, H-100a),
2.72 (m, 1H, H-100b), 2.08–1.97 (m, 2H, H-10a, H-10b),
1.23 (d, 3H, JH,H=6.3, H-300).
(2S,3R,4S)-2-[2-(Phenylamino)ethyl]pyrrolidine-3,4-diol
(12a). Reductive amination of 255m,5n with aniline
gave, after column chromatography on silica gel (ether–
petroleum ether 1:5), the protected derivative 27a (82%
yield). Conventional deprotection gave 12a (83% yield).
25
589
½ꢀŠ +20 (c 1.1, MeOH) ; IR (film) nmax 3335, 1680,
1
1600, 1505, 1095, 755, 695 cmÀ1; H NMR (400 MHz,
MeOD, d ppm, J Hz): d 7.11–7.06 (m, 2H, Ph), 6.66–
6.57 (m, 3H, Ph), 4.33 (m, 1H, H-4), 4.04 (t, 1H,
J3,2=J3,4=3.7, H-3), 3.30 (m, 1H, H-2), 3.24 (t, 2H,
(2S,3R,4S) - 2 - 2 - [(4 - Hydroxybutyl)amino]ethylpyrroli-
dine-3,4-diol (12e). Reductive amination of 25 with rac-
2-hydroxybutylamine gave, after column chromato-
graphy on silica gel (CH2Cl2–MeOH 40:1!5:1), pro-
J2 a,2 b=7.1, H-20a, H-20b), 3.20 (dd, 1H, J5a,4=7.6,
2J5a,5b=11.7, H-5a), 2.96 (dd, 1H, J5b,4=7.0, H-5b),
2.08 (m, 1H, H-10a), 1.91 (m, 1H, H-10b); 13C NMR
(100.5 MHz, MeOD, d ppm): d 150.1 (C-1 of Ph), 130.0,
118.1 and 114.1 (Ph), 73.4, 72.8 (C-4, C-3), 61.2 (C-2),
50.4 (C-5), 42.2 (C-20), 29.1 (C-10). CIMS m/z 223
[100%, (M+H)+.]. CIMSHR: m/z 223.1441 (calcd for
C12H18N2O2+H: 223.1446).
0
0
tected derivative 27e (40% yield). Conventional
25
deprotection gave 12e (83% yield). ½ꢀŠ +18 (c 0.45,
589
MeOH); IR nmax 3300, 1405, 1095 cmÀ1 1H NMR
;
(400 MHz, MeOD, d ppm, J Hz): d 4.26 (m, 1H, H-4),
3.97 (t, 1H, J3,2=J3,4=4.2, H-3), 3.87 (m, 1H, H-200),
3.05 (m, 1H, H-2), 3.05 (dd, 1H, J5a,4=7.2,
2J5a,5b=11.5, H-5a), 2.88 (dd, 1H, J5b,4=6.1, H-5b),
2.78 (m, 2H, H-20a, H-20b), 2.72 (m, 1H, H-100a), 2.58
(m, 1H, H-100b), 1.92 (m, 1H, H-10a), 1.79 (m, 1H,
H-10b), 1.55–1.41 (m, 2H, H-300), 1.00 (t, 3H,
JH,H=7.4, H-400); 13C NMR (100.5 MHz, MeOD, d
ppm): d 73.9, 73.4 (C-4, C-3), 72.2 (C-200), 61.2 (C-2),
55.9 (C-100), 51.5 (C-5), 47.9 (C-20), 29.9 (C-10), 29.3
(C-300), 10.3 (C-400). CIMS m/z 219 [100%,
(M+H)+.]. CIMSHR: m/z 219.1704 (calcd for
C10H22N2O3+H: 219.1709).
(2S,3R,4S)-2-[2-(Benzylamino)ethyl]pyrrolidine-3,4-diol
(12b). Reductive amination of 25 with benzylamine
gave, after column chromatography on silica gel
(CH2Cl2–MeOH 60:1!5:1), protected derivative 27b
(46% yield). Conventional deprotection gave 12b (60%
25
yield). ½ꢀŠ +22 (c 0.8, MeOH); IR: nmax 3330, 1645,
589
1510, 1095, 800, 705 cmÀ1; 1H NMR (400 MHz, MeOD,
d ppm, J Hz): d 7.42–7.36 (m, 5H, Ph), 4.29 (td, 1H,
J4,3=4.6, J4,5a=J4,5b=6.9, H-4), 4.00 (dd, 1H,
J3,2=4.2, H-3), 3.85 (s, 2H, CH2Ph), 3.21 (td, 1H,
(2S,3R,4S)-2-[(Morpholin-4-yl)-ethyl]pyrrolidine-3,4-diol
(12f). Reductive amination of 25 with morpholine gave,
after column chromatography on silica gel (CH2Cl2–
MeOH 50:1!30:1), protected derivative 27f (70%
yield). Conventional deprotection gave 12f (100%
2
0
0
J2,1 a=J2,1 b=6.9, H-2), 3.15 (dd, 1H, J5a,5b=11.5,
H-5a), 2.93 (dd, 1H, H-5b), 2.81 (t, 2H, H-20a, H-20b),
1.99 (m, 1H, H-10a), 1.86 (m, 1H, H-10b); 13C NMR
(75.4 MHz, MeOD, d ppm): d 140.1 (C-1 of Ph), 129.5,

A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
4907
25
yield). ½ꢀŠ +13.6 (c 0.67, MeOH); IR nmax 3360, 1690,
(2R,3R,4S)-2-[2-(Phenylamino)ethyl]pyrrolidine-3,4-diol
(13a). Conventional acidic deprotection of 28a (38 mg,
0.105 mmol), afforded 13a (20.4 mg, 88%) as an oil.
589
1400, 1115 cmÀ1; 1H NMR (400 MHz, MeOD, d ppm, J
Hz): d 4.31 (m, 1H, H-4), 4.02 (t, 1H, J3,2=J3,4=4.0, H-
25
3), 3.74 (t, 4H, JH,H=4.6, H-300), 3.22 (m, 1H, H-2),
½ꢀŠ
+37 (c 1.2, MeOH); IR nmax 3335, 1605, 1505,
3
589
2
3.18 (dd, 1H, J5a,4=7.4, J5a,5b=11.5, H-5a), 2.96 (dd,
1095, 750, 695 cmÀ1 1H NMR (300 MHz, MeOD, d
;
1H, J5b,4=6.6, H-5b), 2.53 (m, 4H, H-200), 2.51 (t, 2H,
H-20a, H-20b), 1.99 (m, 1H, H-10a), 1.84 (m, 1H, H-10b);
13C NMR (100.5 MHz, MeOD, d ppm): d 73.3, 72.8 (C-
4, C-3), 67.7 (C-300), 61.8 (C-2), 56.9 (C-20), 54.7 (C-200),
50.6 (C-5), 26.2 (C-10). CIMS m/z 217 [100%,
(M+H)+.]. CIMSHR: m/z 217.1550 (calcd for
C10H20N2O3+H: 217.1552).
ppm, J Hz): d 7.12–7.06 (m, 2H, Ph), 6.66–6.58 (m, 3H,
Ph), 4.05 (m, 1H, H-4), 3.60 (dd, 1H, J3,2=5.3,
J3,4=7.8, H-3), 3.26–3.18 (m, 3H, H-5a, H-20a, H-20b),
0
0
3.01 (td, 1H, J2,1 a=J2,1 b=7.9, H-2), 2.80 (dd, 1H,
J5b,4=3.4, J5a,5b=12.2, H-5b), 1.96 (m, 1H, H-10a),
2
1.69 (m, 1H, H-10b); 13C NMR (75.4 MHz, MeOD, d
ppm): d 150.1 (C-1 of Ph), 130.0, 118.2 and 114.3 (Ph),
78.3 (C-3), 72.2 (C-4), 61.3 (C-2), 52.4 (C-5), 42.7 (C-20),
33.8 (C-10). CIMS: m/z 223 [100%, (M+H)+.].
CIMSHR: m/z 223.1445 (calcd for C12H18N2O2+H:
223.1446).
(2S,3R,4S) - 2 - [2 - (Biphenylamino)ethyl]pyrrolidine-3,4-
diol (12g). Reductive amination of 25 with 4-bipheny-
lamine gave, after column chromatography on silica
gel (ether–petroleum ether 1:4), protected derivative
27g (92% yield). Conventional deprotection gave 12g
(2R,3S,4R)-2-[2-(Phenylamino)ethyl]pyrrolidine-3,4-diol
(ent-12a). Reductive amination of ent-2525 with aniline
gave, after column chromatography on silica gel (ether–
petroleum ether 1:4), protected derivative ent-27a (91%
25
589
(52% yield). ½ꢀŠ +14 (c 0.54, MeOH); IR nmax 3300,
1
1680, 1610, 1525, 1090, 870, 760, 700 cmÀ1; H NMR
(400 MHz, MeOD, d ppm, J Hz): d 7.50 (dd, 2H,
3
3Jc,d=8.5, H-arom.c ), 7.40 (d, 2H, Jb,a=8.6, H-
yield). Conventional deprotection gave ent-12a (100%
25
arom.b), 7.33 (t, 2H, H-arom.d), 7.19 (tt, 1H, H-arom.
e), 6.72 (d, 2H, H-arom.a), 4.29 (ddd, 1H, J4,3=4.5,
J4,5a=7.5, J4,5b=7.0, H-4), 4.00 (dd, 1H, J3,2=3.8, H-
3), 3.24 (m, 1H, H-2), 3.24 (t, 2H, H-20a, H-20b), 3.13
yield). ½ꢀŠ À24 (c 0.8, MeOH); CIMS: m/z 223 [100%,
589
(M+H)+.]. CIMSHR: m/z 223.1442 (calcd for
C12H18N2O2+H: 223.1446). This product showed
NMR and IR spectra identical to those of its enantio-
mer 12a.
2
(dd, 1H, J5a,5b=11.5, H-5a), 2.90 (dd, 1H, H-5b), 2.07
(m, 1H, H-10a), 1.92 (m, 1H, H-10b); 13C NMR
(100.5 MHz, MeOD, d ppm): d 150.1, 143.2, 131.5,
130.2, 129.1, 127.4 and 114.8 (2Ph), 74.0 (C-3), 73.5 (C-
4), 61.7 (C-2), 51.3 (C-5), 42.7 (C-20), 29.8 (C-10). CIMS:
m/z 299 [70%, (M+H)+.]. CIMSHR m/z 299.1752
(calcd for C18H22N2O2+H: 299.1760).
(2R,3S,4R)-2-[2-(Benzylamino)ethyl]pyrrolidine-3,4-diol
(ent-12b). Reductive amination of ent-25 with benzyla-
mine gave, after column chromatography on silica gel
(CH2Cl2–MeOH 50:1!10:1), protected derivative ent-
27b (54% yield). Conventional deprotection gave ent-
25
12b (66% yield). ½ꢀŠ À26 (c 0.73, MeOH). CIMS: m/z
589
N-(tert-Butoxycarbonyl)-(2R,3R,4S)-2-[2-(Phenylamino)-
ethyl]-3,4-O-isopropylidene-pyrrolidine-3,4-diol (28a). To
a solution of 2426 (80.4 mg, 0.351 mmol) in dry pyridine,
(Boc)2O (84.3 mg, 0.386 mmol) was added. The mixture
was left at 20 ꢀC for 2 h. After evaporation of the sol-
vent, the residue was dissolved in AcOEt and washed
twice with brine. The dried organic phase was evapo-
rated to afford crude protected compound which was
dissolved in dry CH2Cl2 (1.75 mL) and cooled to
À78 ꢀC. 1M DIBALH in CH2Cl2 (475 mL, 0.475 mmol)
was added dropwise under an argon atmosphere. After
stirring for 3 h, MeOH (0.4 mL) was added and the
mixture slowly warmed up to 20 ꢀC. Then, aqueous 1 M
HCl (6 mL) was added in an ice-cold bath and the aqu-
eous phase was extracted with CH2Cl2. The combined
organic phases were washed with saturated aqueous
solution of NaHCO3 and dried (Na2SO4). Evaporation
of the solvent afforded crude protected aldehyde 26
which was dissolved in dry 1,2-dichloroethane (1.5 mL)
and treated with aniline (40 mL, 0.441 mmol) and NaB-
H(OAc)3 (86 mg, 0.385 mmol). The reaction mixture
was left at 20 ꢀC under Ar atmosphere for 2 h. Then, it
was quenched with saturated aqueous solution of
NaHCO3. The aqueous phase was extracted with
AcOEt and the combined organic phases were dried
(MgSO4) and the solvent evaporated in vacuo. The
residue was purified by chromatography on silica gel
(ether–petroleum ether 1:3!1:2) to afford 28a (56 mg,
45%).
237 [100%, (M+H)+.]. CIMSHR: m/z 237.1601 (calcd
for C13H20N2O2+H: 237.1603). This product showed
NMR and IR spectra identical to those of its enantio-
mer 12b.
(2R,3S,4R)-2-[(Morpholin-4-yl)-ethyl]pyrrolidine-3,4-diol
(ent-12f). Reductive amination of ent-25 with morpho-
line gave, after column chromatography on silica gel
(CH2Cl2–MeOH 50:1!20:1), protected derivative ent-
27f (91% yield). Conventional deprotection gave ent-12f
25
(70% yield). ½ꢀŠ À10 (c 0.49, MeOH). CIMS: m/z 217
589
[100%, (M+H)+.]. CIMSHR: m/z 217.1550 (calcd for
C10H20N2O3+H: 217.1552). This product showed NMR
and IR spectra identical to those of its enantiomer 12f.
(2R,3S,4R)-2-[(4-Hydroxybutylamino)ethyl]pyrrolidine-
3,4-diol (ent-12h). Reductive amination of ent-25 with
4-hydroxybutylamine gave, after column chromato-
graphy on silica gel (CH2Cl2–MeOH 30:1), protected
derivative ent-27h (20% yield). Conventional deprotec-
À10 (c 0.48,
25
589
tion gave ent-12h (92% yield). ½ꢀŠ
MeOH); IR nmax 3285, 1400, 1095 cmÀ1 1H NMR
;
(400 MHz, MeOD, d ppm, J Hz): d 4.24 (m, 1H, H-4),
3.97 (t, 1H, J3,2=J3,4=4.3, H-3), 3.61 (t, 2H, JH,H=6.0,
H-400), 3.05 (m, 1H, H-2), 3.05 (dd, 1H, J5a,4=7.2,
2J5a,5b=11.5, H-5a), 2.87 (dd, 1H, J5b,4=6.1, H-5b),
2.80 (t, 2H, H-20a, H-20b), 2.72 (t, 2H, JH,H=6.8, H-
3
100), 1.93 (m, 1H, H-10a), 1.79 (m, 1H, H-10b), 1.70–1.58
(m, 4H, H-200, H-300); 13C NMR (100.5 MHz, MeOD, d

4908
A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
ppm): d 73.8, 73.3 (C-4, C-3), 62.6 (C-400), 61.0 (C-2), 51.5
(C-5), 50.1 (C-100), 47.8 (C-20), 29.5 (C-10), 31.4, 26.7 (C-
200, C-300). CIMS: m/z 219 [45%, (M+H)+.]. CIMSHR:
m/z 219.1706 (calcd for C10H22N2O3+H: 219.1709).
dure starting from 31b (21 mg, 0.06 mmol) in 2 mL of
CF3COOH–H2O (4/1). The crude product was purified
by flash chromatography (MeCN–ammonium hydrox-
25
25
ide 4:1) affording 14b (13 mg, 100%). ½ꢀŠ
+6, ½ꢀŠ
577
+9 (c=0.47, MeOH). UV
546
25
435
25
405
+6, ½ꢀŠ
+7, ½ꢀŠ
N-(tert-Butoxycarbonyl)-(2S,3S,4R)-2-[2-(Phenylamino)-
(MeCN): lmax 312 (e=2506), 260 (e=2268), 212
(e=6919). IR (film) nmax: 3060, 1670, 1435, 1200, 1130,
840, 800, 725 cmÀ1. 1H NMR (400 MHz, CDCl3, d ppm,
J Hz): d 7.57 (m, 5H arom), 4.55 (m, 1H, H- 4), 4.49 (m,
1H, H-3), 4.32 (s, 2H, CH2-Ph), 3.96 (m, 1H, H-2), 3.55
ethyl]-3,4-O-isopropylidene-pyrrolidine-3,4-diol
(ent-
28a). To a solution of ent-2426 (70 mg, 0.306 mmol) in
dry pyridine (1.5 mL), (Boc)2O (73.5 mg, 0.337 mmol)
was added. The mixture was left at 20 ꢀC for 2 h. After
evaporation of the solvent, the residue was dissolved in
AcOEt and washed twice with brine. The dried organic
phase (MgSO4) was evaporated to afford crude pro-
tected compound which was dissolved in dry CH2Cl2
(1.5 mL) and cooled to À78 ꢀC. 1 M DIBALH in
CH2Cl2 (405 mL, 0.405 mmol) was added dropwise
under an Ar atmosphere. After stirring for 3 h, MeOH
(0.35 mL) was added and the mixture slowly warmed up
to 20 ꢀC. Then, aqueous 1 M HCl (3 mL) was added in
an ice-cold bath and the aqueous phase was extracted
with CH2Cl2. The combined organic extracts were
washed with saturated aqueous solution of NaHCO3
and dried (Na2SO4). Evaporation of the solvent affor-
ded crude protected aldehyde ent-26. Reductive amin-
ation with aniline gave, after column chromatography
on silica gel (ether–petroleum ether 1:3!1:2), ent-28a
(61.2 mg, 55%).
(m, 2H, H-10a, H-5a), 3.42 (dd, 1H, J1 b,2=6.3,
3
0
J1 a,1 b=13.8, H-10b), 3.27 (dd, 1H, J5b,4=6.3,
2
3
0
0
2J5b,5a=11.9, H-5b).13C NMR (100.5 MHz, CDCl3, d
ppm): d 129.6 (C arom), 129.4 (C arom), 129.3 (C
arom), 70.6 (C-3), 70.3 (C-4), 57.1 (C-2), 52.0 (CH2-Ph),
48.1 (C-5), 44.7 (C-10). CI-MS (NH3): 223 (M+H+,
100), 222 (M+), 187 (1), 150 (5), 136 (15), 121 (49), 106
(28), 91 (77).
N-(tert-Butoxycarbonyl)-(2S,3R,4S)-2-(inden-1-ylamino)-
methyl-3,4-O-isopropylidene-pyrrolidine-3,4-diol
(31i).
Reductive amination of aldehyde 30 (94 mg,
0.35 mmol) with (R)-1-aminoindane gave, after flash
chromatography (AcOEt), protected derivative 31i
25
589
25
577
25
546
(86 mg, 64% yield). ½ꢀŠ +29, ½ꢀŠ +34, ½ꢀŠ +40,
25
435
25
405
½ꢀŠ +64, ½ꢀŠ +77 (c=0.40, CH2Cl2).UV (MeCN):
lmax 272 (e=3155), 266 (e=3220), 206 (e=19627). IR
(film) nmax: 3340, 2980, 2935, 1695, 1475, 1460, 1395,
1250, 1210, 1165, 1120, 1085, 1000, 860, 750. H NMR
1
(2S,3S,4R)-2-[2-(Phenylamino)ethyl]pyrrolidine-3,4-diol
(ent-13a). Conventional acidic deprotection of ent-28a
(51.5 mg, 0.142 mmol), afforded ent-13a (26.6 mg,
(400 MHz, MeOD, d ppm, J Hz): d 7.34 (m, 1H arom),
7.27–7.16 (m, 3H arom), 4.80 (dd, 1H, J3,4=6.7,
3
25
84%) as an oil. ½ꢀŠ À43 (c 0.9, MeOH). CIMS: m/z
3J3,2=6.7, H-3), 4.72 (m, 1H, H-4), 4.26 (dd, 1H,
589
223 [100%, (M+H)+.]. CIMSHR: m/z 223.1442
(calcd for C12H18N2O2+H: 223.1446). This product
showed NMR and IR spectra identical to those of its
enantiomer 13a.
J1 a,2 a=6.5, J1 a,1 b=6.5, H- 100a), 4.06 (m, 1H, H-2),
3
3
00
00
00
00
3
3.79 (m, 1H, H-5a), 3.36 (dd, 1H, J5b,4=3.6,
2J5b,5a=12.5, H-5b), 2.99 (m, 3H, H-300a, H-10a, H-10b),
2.82 (m, 1H, H-300b), 2.34 (m, 1H, H-200a), 1.89 (m, 1H,
H-200b), 1.45 (s, 3H, CH3), 1.44 (s, 9H, (CH3)3C), 1.33
(s, 3H, CH3). 13C NMR (101 MHz, MeOD, d ppm): d
154.4 (C¼O of Boc), 145.0 (C arom), 143.5 (C arom),
127.3 (C arom), 126.2 (C arom), 124.7 (C arom), 124.1
(C arom), 112.7 (C(CH3)2), 80.3 (C-3), 80.0 (C(CH3)3),
77.9 (C-4), 63.4 (C-100), 59.9 (C-4), 50.7 (C-6), 47.1 (C-
10), 33.2 (C-200), 30.4 (C-300), 28.4 ((CH3)3C), 26.2
((CH3)2C), 25.0 ((CH3)2C). CI-MS (NH3): 389
(M+H+, 100), 388 (M+, 81), 333 (9), 215 (2), 132
(10), 117 (18), 84 (2). Anal. calcd for C22H32N2O4
(388.53): C 68.01, H 8.30, N 7.21 ; found C 68.14, H
8.23.
N-(tert-Butoxycarbonyl)-(2S,3R,4S)-2-(Benzylamino)me-
thyl-3,4-O-isopropylidene-pyrrolidine-3,4-diol
(31b).
Reductive amination of aldehyde 307b (111 mg,
0.41 mmol) with benzylamine gave, after flash chroma-
tography (AcOEt), protected derivative 31b (114 mg,
25
589
77% yield). ½ꢀŠ
24 (c 0.47, CH2Cl2). UV (MeCN):
lmax 217 (e=5918). IR (film) nmax 2980, 2935, 1695,
1495, 1455, 1395, 1245, 1210, 1165, 1115, 1085, 860,
1
735, 700 cmÀ1. H NMR (400 MHz, CDCl3, d ppm, J
3
Hz): d 7.43–7.21 (m, 5H arom), 4.80 (t, 1H, J3,4=6.7,
3J3,2=6.7, H-3), 4.71 (m, 1H, H-4), 4.06 (m, 1H, H-2),
3.83 (m, 2H, CH2-Ph), 3.80 (m, 1H, H-5a), 3.33 (dd, 1H,
3J5a,4=3.7, J5a,5b=12.4, H-5a), 2.88 (m, 2H, H-10a, H-
(2S,3R,4S)-2-[(inden-1-ylamino)methyl]pyrrolidine-3,4-
diol (14b). Following the general deprotection proce-
dure starting from 31i (22 mg, 0.06 mmol) in 2.5 mL
of CF3COOH–H2O (4/1). The crude product was
2
10b), 1.47 (s, 3H, C(CH3)2), 1.42 (s, 9H, (CH3)3C), 1.34
(s, 3H, C(CH3)2); 13C NMR (100.5 MHz, CDCl3, d
ppm): d 154.4 (C¼O of Boc), 140.4, 128.3, 128.1, 126.8
(C arom), 112.7 (C(CH3)2), 80.2 (C-3), 80.0 (C(CH3)3),
77.9 (C-4), 59.3 (C-2), 54.0 (CH2-Ph), 50.6 (C-5), 48.9
(C-10), 28.4 ((CH3)3C), 26.3 (CH3)2C) 25.0 ((CH3)2C)).
CI-MS (NH3): 363 (M+H+, 7), 307 (13), 263 (6), 215
(3), 187 (14), 142 (11), 120 (51), 91 (100). Anal. calcd for
C20H30N2O4 (362.49): C 66.27, H 8.34, N 7.73 ; found C
66.50, H 8.28, N 7.63.
purified by flash chromatography (MeCN–ammonium
25
589
hydroxide 4:1) affording 14i (14 mg, 100%). ½ꢀŠ
25
577
+0.6, ½ꢀŠ
+0.9 (c=0.47, MeOH). UV (MeCN):
lmax 331 (e=389), 200 (e=4355). IR (KBr) nmax
3105, 1670, 1475, 1435, 1400, 1205, 1130, 840, 800,
:
755, 725 cmÀ1. H NMR (400 MHz, MeOD, d ppm, J
1
Hz): d 7.60 (d, 1H, 3J=7.5, H arom), 7.48–7.29 (m,
3
3
00
00
00
00
3H arom), 4.75 (dd, 1H, J1 a,2 a=4.3, J1 a,2 b=7.1,
H-100a), 4.38 (m, 2H, H-3, H-4), 3.94 (ddd, 1H,
0
J2,1 a=5.3, J2,3=5.4, J2,1 =6.3, H-2), 3.59 (dd, 1H,
(2S, 3R, 4S)-2-[(Benzylamino)methyl] pyrrolidine-3,4-
diol (14b). Following the general deprotection proce-
3
3
3
0

A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
4909
J1 a,2=5.3, J1 a,1 b=13.3, H-10a), 3.45 (dd, 1H,
(2S,3R,4S)-2-[2-(Naphtylamino)ethyl]pyrrolidine-3,4-diol
dihydrochloride (12k). To a 0 ꢀC solution of amide 35k
(50 mg, 0.117 mmol) in dry THF (3 mL), BH3.SMe2
(1 M in THF) (0.585 mL) was added dropwise under Ar
atmosphere and the reaction mixture was heated under
reflux for 2 h. After cooling, the excess of reducing agent
was quenched by slow addition of MeOH (3 mL). After
evaporation of the solvent, the residue was purified by
column cromatography (ether–petroleum ether 1:5) to
give the corresponding protected diamine (35 mg, 73%).
Acidic deprotection of the substrate (15 mg,
0.036 mmol) with 1 M HCl in 1:1 H2O/THF (1 mL) gave
12k (12 mg, 100%). 1H NMR (300 MHz, MeOD, d
ppm, J Hz): d 8.13 (dd, 1H, J=0.9, J=7.8, H-arom.),
8.04 (dd, 2H, J=0.6, J=8.1, H-arom.), 7.98 (bd, 1H, H-
arom.), 7.76–7.56 (m, 4H, H-arom.), 4.39 (td, 1H,
J4,3=3.8, J4,5a=J4,5b=7.5, H-4), 4.16 (t, 1H, J3,2=3.8,
H-3), 3.73–3.68 (m, 3H, H-2, H-20a, H-20b), 3.43 (dd,
3
3
2
0
0
0
J1 b,2=6.3, H-10b), 3.42 (m, 1H, H-5a), 3.26–3.18 (m,
0
2H, H-5b, H-300a), 3.00 (ddd, 1H, J3 b,2 a=4.7,
3
00
00
J3 b,2 b=8.7, 2Jgem=13.5, H-300b), 2.58 (m, 1H, H-200a),
2.30 (m, 1H, H-200b).13C NMR (101 MHz, MeOD, d
ppm): d 147.0 (C arom), 139.9 (C arom), 131.7 (C
arom), 128.9 (C arom), 127.3 (C arom), 73.1 (C-3), 72.6
(C-4), 65.7 (C-100), 59.7 (C-2), 50-49 (C-5), 45.1 (C-10),
32.0 (C-300), 31.2 (C-200). CI-MS (NH3): 249 (M+H+,
100), 211 (2), 162 (3), 146 (23), 132 (23), 117 (78), 91
(12).
3
00
00
(2S,3R,4S)-2-[2-(Biphenylmethylamino)ethyl]pyrrolidine-
3,4-diol dihydrochloride (12j). Reductive amination of
diamine 3225 (88 mg, 0.308 mmol) and 4-biphe-
nylcarboxaldehyde (51 mg, 0.28 mmol) gave, after col-
umn chromatography on silica gel (CH2Cl2–MeOH
40:1!30:1), protected derivative 33 (85 mg, 61%).
Acidic deprotection with aqueous 4 M HCl gave 12j
(34 mg, 100% yield). 13C NMR (75.4 MHz, DMSO-d6,
d ppm): d 140.6, 139.4, 131.0, 130.8, 129.0, 127.8,
126.8, 126.7 (2Ph), 70.3, 69.7 (C-3, C-4), 58.0 (CH2-
Ph), 49.6 (C-2), 46.2 (C-5), 43.4 (C-20), 23.2 (C-10).
CIMSHR: m/z 313.1907 (calcd for C19H24N2O2+H:
313.1916).
2
1H, J5a,5b=11.7, H-5a), 3.10 (dd, 1H, H-5b), 2.50 (m,
1H, H-10a), 2.32 (m, 1H, H-10b); 13C NMR (75.4 MHz,
MeOD, d ppm): d 136.1, 133.8, 130.3, 128.9, 128.4,
126.7, 126.4 and 121.4 (Ar), 71.8, 71.4 (C-3, C-4), 60.5
(C-2), 49.1 (C-5), 48.0 (C-20), 25.4 (C-10). FABMS: m/z
273 [100%, (M+H)+], 295 [40%, (M+H)+].
N-(10-Biphenyl)-N-(tert-butoxycarbonyl)-2,3,6-trideoxy-
3,6-imine-4,5-O-isopropylidene-D-arabino-hexanamide
(ent-35g). A solution of ent-23 (565 mg, 1.717 mmol) in
1 M NaOH in 2:1 EtOH–H2O (45 mL) was heated at
40 ꢀC for 5 h. Then, the mixture was neutralized with
IRA-120 (H+) resin, filtered and the filtrate con-
centrated. To a solution of the crude acid ent-34
(200 mg) in DMF (6.5 mL) were added PyBOP (463 mg,
0.863 mmol), (iPr)2NEt (150 mL, 0.863 mmol) and
biphenylamine (149 mg, 0.863 mmol). After stirring at
20 ꢀC for 24 h, the mixture was concentrated and the
residue diluted with CH2Cl2 and sequentially washed
with aqueous 1 M HCl, aqueous saturated solution of
NaHCO3 and brine. The dried organic phase (MgSO4)
was purified by chromatography on silica gel (ether–
N-(ꢀ-Naphtyl)-N-(tert-butoxycarbonyl)-2,3,6-trideoxy-3,6-
imine-4,5-O-isopropylidene-L-arabino-hexanamide (35k). A
solution of 2326b (370 mg, 1.125 mmol) in ethanolic 1 M
NaOH (30 mL) was heated at 40 ꢀC for 5 h. Then, the
mixture was neutralized with IRA-120 (H+) resin, fil-
tered and the filtrate concentrated. Purification of the
residue by chromatography on silica gel (ether–petro-
leum ether 2:1) gave 34 (250 mg, 74%) as an oil. To a
solution of 34 (128 mg, 0.425 mmol) in DMF (3.5 mL)
were added PyBOP (298 mg, 0.552 mmol), (iPr)2NEt
(96 mL, 0.552 mmol) and a-naphthylamine (92 mg,
0.552 mmol). After stirring at 20 ꢀC for 24 h, the mixture
was concentrated and the residue diluted with CH2Cl2
and then washed with aqueous 1 M HCl, then with a
saturated aqueous solution of NaHCO3 and finally with
brine. The dried organic phase (MgSO4) was purified by
petroleum ether 1:1) to give ent-35g (234 mg, 78%) as a
25
589
solid. ½ꢀŠ À52.8 (c 0.7, CH2Cl2); IR nmax 3320, 1695,
1
chromatography on silica gel (ether–petroleum ether
1670, 1530, 1400, 1165, 1120, 990, 860, 765; H NMR
25
589
1:1) to give 35k (116 mg, 64%) as a solid. ½ꢀŠ +59 (c
(300 MHz, DMSO-d6 90 ꢀC, d ppm, J Hz): d 9.62 (bs,
1H, NH), 7.67–7.54 (m, 5H, H-arom.), 7.45–7.40 (m,
3H, H-arom.), 7.30 (m, 1H, H-arom.), 4.80 (t, 1H,
J3,4=J3,2=6.3, H-3), 4.74 (td, 1H, J4,5a=6.6, J4,5b=3.0,
1, CH2Cl2); IR nmax 3290, 1695, 1670, 1535, 1400, 1165,
1
1120, 990, 865, 775 cmÀ1; H NMR (300 MHz, DMSO-
d6 90 ꢀC, d ppm, J Hz): d 9.50 (bs, 1H, NH), 8.13 (m,
1H, H-arom.), 7.90 (m, 1H, H-arom.), 7.72 (d, 1H,
J=8.1, H-arom.), 7.68 (d, 1H, J=8.1, H-arom.), 7.53–
7.43 (m, 3H, H-arom.), 4.81 (t, 1H, J3,4=J3,2=6.1, H-
3), 4.75 (td, 1H, J4,5a=6.3, J4,5b=3.0, H-4), 4.27 (ddd,
0
H-4), 4.25 (ddd, 1H, J2,1 a=4.6, J2,1 b=8.8, H-2), 3.64
0
2
(dd, 1H, J5a,5b=12.6, H-5a), 3.30 (dd, 1H, H-5b), 2.97
(dd, 1H, J1 a,1 b=15.6, H-10a), 2.70 (dd, 1H, H-10b),
1.46 and 1.28 (2s, C(CH3)2), 1.39 (s, C(CH3)3). 13C
NMR (75.4 MHz, DMSO-d6 90ꢀC, d ppm): d 168.7
(NHCO), 153.4 (C¼O of Boc), 139.5 (Cq of Ph), 138.4 (Cq
of Ph), 134.3 (Cq of Ph), 128.2, 126.3, 126.2, 125.7, 119.2
(Ph), 111.1 (C(CH3)2), 79.3 (C-3), 78.6 (C(CH3)3), 76.8 (C-
4), 56.2 (C-2), 50.3 (C-5), 35.9 (C-10), 27.6 (C(CH3)3), 25.8
and 24.5 (C(CH3)2). FABMS: m/z 452 [55%, (M+Na)+].
2
0
0
0
0
1H, J2,1 a=4.8, J2,1 b=9.3, H-2), 3.65 (dd, 1H,
2J5a,5b=12.3, H-5a), 3.31 (dd, 1H, H-5b), 3.13 (dd, 1H,
J1 a,1 b=15.3, H-10a), 2.84 (dd, 1H, H-10b), 1.51 and
1.31 (2s, C(CH3)2), 1.42 (s, C(CH3)3). 13C NMR
(75.4 MHz, DMSO-d6 90 ꢀC, d ppm): d 169.4 (NHCO),
153.5 (C¼O of Boc), 133.6 (Cq of Ph), 133.4 (Cq of Ph),
127.8 (Cq of Ph), 127.4, 125.3, 124.9, 124.5, 122.5, 121.1
(Ph), 111.0 (C(CH3)2), 79.4 (C-3), 78.6 (C(CH3)3), 76.7
(C-4), 56.4 (C-2), 50.5 (C-5), 35.5 (C-10), 27.7 (C(CH3)3),
25.9 and 24.6 (C(CH3)2). FABMS: m/z 426 [100%,
(M+Na)+]. CIMSHR m/z 426.2138 (calcd for
C24H30N2O5: 426.2155).
2
0
0
(2R,3S,4R) - 2 - [2 - (Biphenylamino)ethyl]pyrrolidine - 3,4-
diol dihydrochloride (ent-12g). To a solution of amide
ent-35g (50 mg, 0.111 mmol) in dry THF (3 mL) stirred
at 0 ꢀC, BH3.SMe2 (1 M in THF) (0.553 mL) was added
dropwise under Ar atmosphere and the reaction mixture

4910
A. T. Carmona et al. / Bioorg. Med. Chem. 11 (2003) 4897–4911
was heated under reflux for 1 h. After cooling, the excess
of reducing agent was quenched by slow addition of
MeOH (3 mL). After evaporation of the solvent, the
residue was purified by column cromatography (ether–
petroleum ether 1:4) to afford the corresponding pro-
tected diamine (41.8 mg, 86%). Acidic deprotection of
the substrate (16 mg, 0.036 mmol) with 1 M HCl in 1:1
H2O / THF (1 mL) gave ent-12g (12.8 mg, 94%). 1H
NMR (500 MHz, MeOD, d ppm, J Hz): d 7.82 (bd, 2H,
J=8.5, H-arom.), 7.62 (m, 4H, H-arom.), 7.47 (bt, 2H,
J=8.5, H-arom.), 7.39 (tt, 1H, J=1.5, J=8.5, H-
arom.), 4.42 (td, 1H, J4,3=3.8, J4,5a=J4,5b=7.4, H-4),
4.19 (t, 1H, J3,2=3.8, H-3), 3.72 (m, 1H, H-2), 3.61–3.55
1441. (l) Zhu, Y.-H.; Vogel, P. J. Chem. Soc. Chem. Commun.
1999, 1873. (m) Cardona, F.; Robina, I.; Vogel, P. J. Carbo-
hydr. Chem. 2000, 19, 555. (n) Marquis, C.; Cardona, F.;
Robina, I.; Wurth, G.; Vogel, P. Heterocycles 2002, 56, 181.
(o) Cardona, F.; Valenza, S.; Picasso, S.; Goti, A.; Brandi, A.
J. Org. Chem. 1998, 63, 7311. (p) Navarro, I.; Vogel, P. Helv.
Chem. Acta 2002, 85, 152.
˜
6. (a) Robina, I.; Moreno-Vargas, A. J.; Fernandez-Bolanos,
J. G.; Fuentes, J.; Demange, R.; Vogel, P. Bioorg. Med. Chem.
Lett. 2001, 11, 2555. (b) Moreno-Vargas, A. J.; Demange, R.;
Fuentes, J.; Robina, I.; Vogel, P. Bioorg. Med. Chem. Lett.
2002, 12, 2335. (c) Moreno-Vargas, A. J.; Robina, I.;
Demange, R. ; Vogel, P. Helv. Chim. Acta, in press.
7. (a) Popowycz, F.; Gerber-Lemaire, S.; Demange, R.;
Rodrıguez-Garcıa, E.; Carmona Asenjo, A. T.; Robina, I.;
Vogel, P. Bioorg. Med. Chem. Lett. 2001, 11, 2489. (b) Popo-
wycz, F.; Gerber-Lemaire, S.; Rodrıguez-Garcıa, E.; Schutz,
C.; Vogel, P. Helv. Chim Acta. in press.
8. Behr, J.-B.; Chevrier, C.; Defoin, A.; Tarnus, C.; Streith, J.
Tetrahedron 2003, 59, 543.
9. Gerber-Lemaire, S.; Popowycz, F.; Rodrıguez-Garcıa, E.;
Carmona Asenjo, A. T.; Robina, I.; Vogel, P. ChemBioChem.
2002, 3, 466.
(m, 2H, H-20a, H-20b), 3.45 (dd, 1H, J5a,5b=11.7, H-
2
5a), 3.12 (dd, 1H, H-5b), 2.42 (m, 1H, H-10a), 2.24 (m,
1H, H-10b); 13C NMR (125.5 MHz, MeOD, d ppm): d
143.8, 140.6, 136.1, 130.2, 130.0, 129.3, 128.1 and 123.9
(2Ph), 71.9 (C-4), 71.3 (C-3), 60.4 (C-2), 49.1 (C-5, C-20),
25.0 (C-10). CIMS: m/z 298 [30%, (M+H)+.].
CIMSHR: m/z 298.1670 (calcd for C18H22N2O2:
298.1681).
10. Kieb, F.-M.; Poggendorf, O.; Picasso, S.; Jager, V. J.
Chem. Soc. Chem. Commun. 1998, 119.
11. Reynolds, R. C.; Bansal, N.; Rose, J.; Friedrich, J.; Sul-
ing, W. J.; Maddry, J. A. Carbohydr. Res. 1999, 317, 164.
12. Wrodnigg, T. M.; Gaderbauer, W.; Greimel, P.; Hausler,
H.; Sprenger, F. K.; Stutz, A. E.; Virgona, C.; Withers, S. G.
J. Carbohydr. Chem. 2000, 19, 975.
13. Wrodnigg, T. M.; Withers, S. G.; Stutz, A. E. Biorg. Med.
Chem. Lett. 2001, 11, 1063.
14. Takebayashi, M.; Hiranuma, S.; Kanie, Y.; Kajimoto, T.;
Kanie, O.; Wong, C.-H. J. Org. Chem. 1999, 64, 5280.
15. Saotome, C.; Wong, C.-H.; Kanie, O. Chemistry&Biology
2001, 8, 1061.
Acknowledgements
We thank the Direccion General de Investigacion Cien-
tıfica y Tecnica of Spain for financial support (Grant
No BQU 2001-3740 and BQU 2001-3779) and for the
award of a grant to A. T. C., the Junta de Andalucıa,
the Swiss National Science Foundation and the ‘Office
Federal de l’Education et de la Science’ (Bern). This
work is part of the European COST Program, working
groups COST-D13-0001/98 and D25-0001/02.
16. Joubert, M.; Defoin, A.; Tarnus, C.; Streith, J. Synlett
2000, 1366.
17. Moreno-Vargas, A. J.; Vogel, P. Tetrahedron Lett. 2003,
44, 5069.
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