New concise total synthesis of (+)-lentiginosine and some structural analogues

Article abstract of DOI:10.1021/jo0509408

An efficient and concise total synthesis of (+)-lentiginosine (1) starting from an L-tartaric acid-derived nitrone using organometallic addition, indium-catalyzed reduction, and ring-closing metathesis reaction as the key steps is reported. Structural analogues of (+)-1 have been also synthesized, and their inhibitory activity toward 22 commercially available glycosidases has been evaluated.

Full text of DOI:10.1021/jo0509408

lation performed on the complex of glucoamylase II (471)
from Aspergillus awamori var.X100 with all (S)-lentigi-
nosine showed that this enantiomer perfectly fits within
the enzyme cavity, forming strong H-bonds with the key
residues for bioactivity.5
New Concise Total Synthesis of
(+)-Lentiginosine and Some Structural
Analogues
Francesca Cardona,*,† Guillermo Moreno,†
Francesco Guarna,† Pierre Vogel,‡ Catherine Schuetz,‡
Pedro Merino,§ and Andrea Goti*,†
Dipartimento di Chimica Organica “Ugo Schiff”, Universita`
di Firenze, ICCOM, CNR, via della Lastruccia 13, I-50019
Sesto Fiorentino (FI), Italy, Laboratory of Glycochemistry
and Asymmetric Synthesis, LGSA, Ecole Polytechnique
Fe´de´rale de Lausanne, BCH-LGSA, CH-1015 Lausanne,
Switzerland, and Laboratorio de Sintesis Asimetrica,
Departamento de Quı´mı´ca Organica, Facultad de
Ciencias-ICMA, Universidad de Zaragoza-CSIC,
E-50009 Zaragoza, Aragon, Spain
Apart from a recent controversial article,3 all subse-
quent contributors concurred on the fact that natural
lentiginosine has the (1S,2S,8aS) absolute configuration
and that it is dextrorotatory. The negative rotation
initially reported can be ascribed to impurities present
in the natural product, also evident from the published
1H NMR spectrum.1
Despite the plethora of synthetic methods, the interest
in the synthesis of (+)-lentiginosine and its analogues
remains undiminished. Development of methods having
enough flexibility to allow also the construction of non-
natural analogues continues to be an important topic, in
particular to investigate the structure-activity relation-
ships of this class of inhibitors.
andrea.goti@unifi.it; francesca.cardona@unifi.it
Received May 11, 2005
The majority of the reported enantiospecific syntheses
of lentiginosine relies upon the chiral pool. Among the
chiral starting material used, the low-cost L-tartaric acid
is the most widely employed, since it allows the straight-
forward installation of the (1S,2S) configuration of the
final compound.
As part of our research on organometallic addition to
chiral hydroxylated cyclic nitrones,6,7 we report here a
concise and straightforward synthesis of (+)-lentiginosine
(1) and its structural analogues 2-4 (Figure 1) based on
a highly stereoselective addition of vinylmagnesium
An efficient and concise total synthesis of (+)-lentiginosine
(1) starting from an L-tartaric acid-derived nitrone using
organometallic addition, indium-catalyzed reduction, and
ring-closing metathesis reaction as the key steps is reported.
Structural analogues of (+)-1 have been also synthesized,
and their inhibitory activity toward 22 commercially avail-
able glycosidases has been evaluated.
(2) (a) Yoda, H.; Kitayama, H.; Katagiri, T.; Takabe, K. Tetrahe-
dron: Asymmetry 1993, 4, 1455. (b) Cordero, F. M.; Cicchi, S.; Goti,
A.; Brandi, A. Tetrahedron Lett. 1994, 35, 949. (c) Gurjar, M. K.; Ghosh,
L.; Syamala, M.; Jayasree, V. Tetrahedron Lett. 1994, 35, 8871. (d)
Brandi, A.; Cicchi, S.; Cordero, F. M.; Frignoli, R.; Goti, A.; Picasso,
S.; Vogel, P. J. Org. Chem. 1995, 60, 6806. (e) Giovannini, R.;
Marcantoni, E.; Petrini, M. J. Org. Chem. 1995, 60, 5706. (f) Nukui,
S.; Sodeoka, M.; Sasai, H.; Shibasaki, M. J. Org. Chem. 1995, 60, 398.
(g) Goti, A.; Cardona, F.; Brandi, A. Synlett 1996, 761. (h) Yoda, H.;
Kawauchi, M.; Takabe, K. Synlett 1998, 137. (i) McCaig, A. E.;
Meldrum, K. P.; Wightman, R. H. Tetrahedron 1998, 54, 9429. (j)
Cardona, F.; Goti, A.; Picasso, S.; Vogel, P.; Brandi, A. J. Carbohydr.
Chem. 2000, 19, 585. (k) Ha, D.-C.; Yun, C.-S.; Lee, Y. J. Org. Chem.
2000, 65, 621. (l) Yoda, H.; Katoh, H.; Ujihara, Y.; Takabe, K.
Tetrahedron Lett. 2001, 42, 2509. (m) Klitzke, C. F.; Pilli, R. A.
Tetrahedron Lett. 2001, 41, 5605. (n) Rasmussen, M. O.; Delair, P.;
Greene, A. E. J. Org. Chem. 2001, 66, 5438. (o) El-Nezhawy, A. O. H.;
El-Diwani, H. I.; Schmidt, R. R. Eur. J. Org. Chem. 2002, 4137. (p)
Rabiczko, J.; Urbanczyk-Lipkowska, Z.; Chmielewski, M. Tetrahedron
2002, 58, 1433. (q) Feng, Z.-X.; Zhou, W.-S. Tetrahedron Lett. 2003,
44, 497. (r) Ichikawa, Y.; Ito, T.; Nishiyama, T.; Isobe, M. Synlett 2003,
1034. (s) Raghavan, S.; Sreekanth, T. Tetrahedron: Asymmetry 2004,
15, 565. (t) Ichikawa, Y.; Ito, T.; Isobe, M. Chem.-Eur. J. 2005, 11,
1949.
(+)-Lentiginosine (1), a dihydroxylated indolizidine
alkaloid, was isolated in 1990 from the leaves of Astraga-
lus lentiginosus.1 It was reported to be a potent and
selective inhibitor of amyloglucosidase, an enzyme that
hydrolyzes 1,4- and 1,6-R-glucosidic linkages. Isolated
lentiginosine was found to be weakly levorotatory ([R]D
-3.3 in MeOH). The absolute configuration (1S,2S,8aS)
was tentatively assigned on the basis of biosynthetic
considerations.1 Later on, numerous syntheses of (1S,2S,-
8aS)-lentiginosine have been reported, all leading to
samples with small positive rotations.2-4 We have re-
ported the synthesis of both (+)- and (-)-lentiginosine2b,d,g,j
based on stereoselective 1,3-dipolar cyloaddition reactions
of dihydroxylated pyrroline nitrones to suitable dipolaro-
philes. Comparison of inhibitory activity data of both (+)-
and (-)-lentiginosine allowed us to confirm the assign-
ment of the absolute configuration (1S,2S,8aS) for natu-
ral lentiginosine.2d Moreover, a molecular dynamic simu-
(3) Chandra, K. L.; Chandrasekhar, M.; Singh, V. K. J. Org. Chem.
2002, 67, 4630.
(4) For the synthesis of the enantiomeric (-)-lentiginosine, see: (a)
Ayad, T.; Ge´nisson, Y.; Baltas, M.; Gorrichon, L. Chem. Commun. 2003,
582. (b) Reference 2c. (c) Reference 2d. (d) Reference 2i. (e) Reference
3. For a synthesis of racemic lentiginosine see: (f) Sha, C.-K.; Chan,
C.-M. Tetrahedron Lett. 2003, 44, 499.
(5) Cardona, F.; Goti, A.; Brandi, A.; Scarselli, M.; Niccolai, N.;
Mangani, S. J. Mol. Model. 1997, 3, 249.
* To whom correspondence should be addressed. Phone: +39-
0554573505/04. Fax: +39-0554573531.
† Universita` di Firenze.
(6) (a) Merino, P.; Tejero, T.; Revuelta, J.; Romero, P.; Cicchi, S.;
Mannucci, V.; Brandi, A.; Goti A. Tetrahedron: Asymmetry 2003, 14 ,
367. (b) Goti, A.; Cicchi, S.; Mannucci, V.; Cardona, F.; Guarna, F.;
Merino, P.; Tejero, T. Org. Lett. 2003, 5, 4235. (c) Merino, P.; Revuelta,
J.; Tejero, T.; Cicchi, S.; Goti, A. Eur. J. Org. Chem. 2004, 776. (d)
Marradi, M.; Cicchi, S.; Delso, J. I.; Rosi, L.; Tejero, T.; Merino, P.;
Goti, A. Tetrahedron Lett. 2005, 46, 1287.
‡ Ecole Polytechnique Fe´de´rale de Lausanne.
§ Universidad de Zaragoza.
(1) Pastuszak, I.; Molyneux, R. J.; James, L. F.; Elbein, A. D.
Biochemistry 1990, 29, 1886.
10.1021/jo0509408 CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/14/2005
6552
J. Org. Chem. 2005, 70, 6552-6555

SCHEME 2
FIGURE 1. (+)-Lentiginosine and its structural analogues.
SCHEME 1. Retrosynthetic Analysis for
(+)-Lentiginosine (1)
Product 8 derived from an anti attack of the organome-
tallic reagent with respect to the vicinal alkoxy group,
as a result of combined steric and stereoelectronic ef-
1
fects.11 No other stereoisomers could be detected by H
NMR of the crude reaction mixture. The possible alterna-
tive strategy for the synthesis of the indolizidine skeleton,
namely allylmagnesium bromide addition to nitrone 9
followed by coupling with acrylic acid, was discarded,
since the addition lacked any selectivity (two diasteroi-
somers were formed in ca. 1:1 ratio). This behavior of
allylmagnesium bromide was an exception, since a broad
range of Grignard reagents (either sp3, sp2, or sp type)
invariably gave a high anti diastereoselectivity.6,12 We
have recently recorded a similar inversion of selectivity
in the addition of allylmagnesium bromide to another
class of nitrones.13
Reduction of 8 with powdered Zn (4 equiv) and catalytic
indium (18%), according to a methodology we have
recently introduced for the reduction of hydroxylamines
to amines,14 afforded the pyrrolidine 7 (84% yield). Both
steps did not require purification by chromatography and
could be performed on a multigram scale. Amine 7 was
then reacted with but-3-enoic acid in the presence of
N,N′-dicyclohexylcarbodiimide (DCC) and 1-hydroxyben-
zotriazole (HOBt) as the coupling agents, leading to
amide 6a (71%). This compound consisted of a mixture
of two rotamers, clearly visible in the 1H NMR spectrum
recorded at 25 °C. The RCM reaction was performed on
6a using first-generation Grubbs’ catalyst 10 in refluxing
dichloromethane. Three sequential additions of the cata-
lyst (for a 12% total molar amount) and 50 h of heating
under reflux were necessary to reach complete conversion
of 6a.15 This led to lactam 5a, isolated in 60% yield after
column chromatography. We supposed that the bulky
tert-butoxy groups might be responsible for the low
reactivity of 6a in the RCM reaction. Thus, it was
converted into the acetyl-protected amide 6b by treat-
ment with trifluoroacetic acid, followed by an excess of
bromide on an L-tartaric acid-derived nitrone as the key
step. This allowed the totally diastereoselective instal-
lation of the (8aS) stereogenic center. The cyclization step
has been performed using a ring-closing metathesis
(RCM) reaction that has been diffusely applied in the past
few years for the construction of a variety of nitrogen-
containing ring systems,8,9 including pyrrolizidines, in-
dolizidines, and other compounds related to iminosugars.
Analogues 2-4 of (+)-lentiginosine have been tested
toward 22 commercially available glycosidases. We report
that dehydrolentiginosine 3 is a moderate inhibitor of
amyloglucosidases from Aspergillus niger and from Rhizo-
pus mold, not as potent as (+)-lentiginosine. It is also a
weak inhibitor of â-glucosidases from almonds and from
Saccharomyces cerevisiae.
Our retrosynthetic strategy for the synthesis of (+)-
lentiginosine (1) is outlined in Scheme 1. We envisioned
that indolizidine lactam 5, from which (+)-1 may derive
by simple reduction-deprotection sequences, could be
prepared by RCM reaction of amide 6, in turn accessible
from amine 7. Compound 7, in turn, may be prepared by
indium-catalyzed reduction of hydroxylamine 8, which
may finally derive from a stereoselective addition of
vinylmagnesium bromide to the L-tartaric acid-derived
nitrone 9.
Thus, nitrone 910 reacted with 1.2 equiv of vinylmag-
nesium bromide in diethyl ether at room temperature,
affording hydroxylamine 8 in 96% yield (Scheme 2).
(7) For other examples of additions of organometallic reagents to
similar nitrones, see: (a) Ballini, R.; Marcantoni, E.; Petrini, M. J.
Org. Chem. 1992, 57, 1316. (b) Lombardo, M.; Fabbroni, S.; Trombini,
C. J. Org. Chem. 2001, 66, 1264. (c) Ohtake, H.; Imada, Y.; Murahashi,
S.-I. Bull. Chem. Soc. Jpn. 1999, 72, 2737. (d) Pulz, R.; Cicchi, S.;
Brandi, A.; Reissig, H.-U. Eur. J. Org. Chem. 2003, 1153.
(8) For reviews on RCM reaction, see for example: (a) Pandit, U.
K.; Overkleeft, H. S.; Borer, B. C.; Biera¨ugel, H. Eur. J. Org. Chem.
1999, 959. (b) Phillips, A. J.; Abell, A. D. Aldrichimica Acta 1999, 32,
75. (c) Deiters, A.; Martin, S. F. Chem. Rev. 2004, 104, 2199.
(9) For specific examples applied to the synthesis of lentiginosine
and congeners, see: (a) Paolucci, C.; Musiani, L.; Venturelli, F.; Fava,
A. Synthesis 1997, 1415. (b) Paolucci, M.; Mattioli, L. J. Org. Chem.
2001, 66, 4787. (c) Lim, S. H.; Ma, S.; Beak, P. J. Org. Chem. 2001,
66, 9056. (c) Reference 2m. (d) Reference 2o.
(10) Cicchi, S.; Ho¨ld, I.; Brandi, A. J. Org. Chem. 1993, 58, 5274.
(11) Merino, P.; Revuelta, J.; Tejero, T.; Cicchi, S.; Goti, A. Eur. J.
Org. Chem. 2004, 776.
(12) A full account on nucleophilic additions of organometallic
reagents to 3-alkoxy-substituted pyrroline N-oxides is under prepara-
tion and will be published in due time.
(13) Bonanni, M.; Marradi, M.; Cicchi, S.; Faggi, C.; Goti, A. Org.
Lett. 2005, 7, 319.
(14) Cicchi, S.; Bonanni, M.; Cardona, F.; Revuelta, J.; Goti, A. Org.
Lett. 2003, 5, 1773.
(15) Attempts were made using second-generation Grubb’s catalyst
or even higher temperature (80 °C or refluxing temperature in toluene
as solvent), but the rate of conversion and the recovered yield could
not be improved.
J. Org. Chem, Vol. 70, No. 16, 2005 6553

TABLE 1. Inhibition Rate (in %) toward Glycosidases at
SCHEME 3. Synthesis of (+)-Lentiginosine
SCHEME 4. Synthesis of Analogues 2-4
1 mM Concentration of Inhibitors 2-4a
2
3
4
1
â-galactosidase
EC 3.2.1.23 from bovine liver
0
36%
0
â-glucosidase
56%
0
74%
75%
31%
0
EC 3.2.1.21 from almonds
â-glucosidase
EC 3.2.1.21 from Saccharomyces cerevisiae
amyloglucosidase
0
94%
(63)
28
80%
(41)
25
0
2b
3b
EC 3.2.1.3 from Aspergillus niger
amyloglucosidase
EC 3.2.1.3 from Rhizopus mold
0
0
a IC50 (in Parentheses) and Ki values are given in micromolar
concentrations. b Reference 2d.
3 inhibits amyloglucosidase (1,4-R-D-glucan glucohydro-
lase EC 3.2.1.3), an important industrial enzyme used
to produce glucose from starch.18 The inhibitory activity
of 3 is about 1 order of magnitude lower than that of 1,
thus indicating that the “flattening” of the six-membered
ring of the indolizidine unit due to the unsaturation is
detrimental to the inhibitory activity of 3 toward amy-
loglucosidases.
acetic anhydride in pyridine. To our delight, the RCM
reaction on amide 6b was faster and higher yielding than
that of 6a. A 15% molar amount of the Grubbs’ catalyst
10 was required to react 6b completely after 20 h under
reflux. This provided lactam 5b in 89% yield (Scheme
2).
Nonbasic sugar analogues such as glycals, epoxides,
halides, sulfonium salts, and lactams can be potent
glycosidase inhibitors.19 For instance, D-manno-δ-lactam,
a natural product found in the broth of Streptomyces
lavendulae SF435, inhibits R-mannosidases.20 D-Manno-
δ-lactam is known to be a better glucosidase inhibitor
than D-glucono-δ-lactam.21 The glucono-δ-lactone ana-
logue 6-deoxy-4-O-methyl-D-glucono-δ-lactam is a slightly
better inhibitor than deoxynojirimycin toward R-glucosi-
dases from brewer’s yeast and from sweet almonds.22
Isofagomine lactams have been found to be glycosidase
inhibitors with micro- and nanomolar inhibition con-
stants.23 We were therefore keen to see how our new
lactams 2 and 4 would behave toward the 22 glycosidases
assayed in this work. To our dismay, except for a weak
inhibitory activity of 2 toward â-glucosidase from al-
monds (56% inhibition at 1 mM concentration), these two
bicyclic lactams were devoid of inhibitory activity. The
tautomeric form of the lactams might be involved in the
enzyme recognition by a lactam as shown for a xylobiose-
derived isofagomine lactam.24 It is thus not excluded that
the inability of lactams 2 and 4 to recognize any glycosi-
dases tested here might be due to their inability to
equilibrate with the corresponding tautomers of the
Reduction of 5a with an excess of LiAlH4 (3.2 equiv)
in refluxing THF led to 11 (62%). Catalytic hydrogenation
of the alkene moiety afforded the protected lentiginosine
122g,j (Scheme 3). After treatment with trifluoroacetic acid
at room temperature,2g,l (+)-lentiginosine (1) was ob-
tained in 74% yield. The spectroscopical data of the
product were identical to those of (+)-lentiginosine. Its
25
specific optical rotation ([R]D +2.6, c 0.4, MeOH) was
in excellent agreement to those reported for (+)-1.16 In
conclusion, (+)-lentiginosine was obtained in seven steps
and 14% overall yield starting from nitrone 9.
The analogues 2-4 of (+)-lentiginosine were derived
from lactams 5 in a few steps (Scheme 4). Treatment of
5a with trifluoracetic acid at room temperature or of 5b
with strongly basic Ambersep 900 OH provided 2 in
comparable yields (80 and 78%, respectively). Reduction
of 5b with an excess of LiAlH4 in dry THF afforded 3 in
good yield.17 On its turn, catalytic hydrogenation of 5b
and subsequent treatment with strongly basic Ambersep
900 OH produced dihydroxylactam 4 (86%, overall yield).
At 1 mM concentration, compounds 2-4 did not inhibit
R-fucosidase from bovine epididymis, R-galactosidase
from coffee bean and from Escherichia coli, â-galactosi-
dase from E. coli, A. niger and from Aspergillus orizae,
R-glucosidase from yeast and from rice, R-mannosidase
from jack bean and from almond, â-mannosidase from
Helix pomatia, â-xylosidase from A. niger, R-N-acetylga-
lactosamidase from chicken liver, and â-N-acetylglucosa-
midase from jack bean and from bovine epididymis A and
B. Table 1 reports the inhibitory activities of 7,8-dide-
hydrolentiginosine (3) to five glycosidases together with
those evaluated for lactams 2 and 4 toward the same
enzymes.
(18) (a) Reilly, P. J. Appl. Biochem. Bioeng. 1979, 2, 185. (b) Shaha,
B. C.; Zeikus, J. G. Starch 1989, 41, 57.
(19) Houston, T. A.; Blanchfield, J. T. Mini-Rev. Med. Chem. 2003,
669.
(20) (a) Niwa, T.; Tsuruoka, T.; Goi, H.; Kodama, Y.; Itoh, J.; Inouye,
S.; Yamada, Y.; Niida, T.; Nobe, M.; Ogawa, Y. J. Antibiot. 1984, 37,
1579. (b) For the eight isomeric D-glyconic-δ-lactam, see: Nishimura,
Y.; Adachi, H.; Satoh, T.; Shitara, E.; Nakamura, H.; Kojima, F.;
Takeuchi, T. J. Org. Chem. 2000, 65, 4871.
(21) (a) Fleet, G. W.; Ramsden, N. G.; Witty, D. R. Tetrahedron 1989,
45, 319. (b) Shing, T. K. M. J. Chem. Soc., Chem. Commun. 1988, 121.
(c) Imada, C.; Okami, Y. J. Mar. Biotechnol. 1995, 2, 109.
(22) Pistia-Brueggema, G.; Hollingswoth, R. I. Tetrahedron 2001,
57, 8773.
(23) Lillelund, V. H.; Liu, H.; Liang, X.; Søhoel, H.; Bols, M. Org.
Biomol. Chem. 2003, 1, 282.
(24) (a) Gloster, T.; Williams, S. J.; Tarling, G. A.; Roberts, S.;
Dupont, C.; Jodoin, P.; Shareck, F.; Withers, S. G.; Davies, G. J. J.
Chem. Soc., Chem. Commun. 2003, 944. (b) Williams, S. J.; Notenboom,
V.; Wicki, J.; Rose, D. R.; Withers, S. G. J. Am. Chem. Soc. 2000, 122,
4229.
As for (+)-lentiginosine (1), 3 displays similar specific-
ity as inhibitor toward the 22 glycosidases assayed. Thus,
(16) Literature values: +3.2 (ref 2d), +2.2 (ref 2g), +3.1 (ref 2n),
+2.9 (ref 2s), +1.06 (ref 2t).
(17) For the synthesis of a diastereoisomer of 3 via a RCM-ROM
sequence, see: Ovaa, H.; Stragies, R.; van der Marel, G. A.; van Boom,
J. H.; Blechert, S. Chem. Commun. 2000, 1501.
6554 J. Org. Chem., Vol. 70, No. 16, 2005

(23 mg, 0.09 mmol) in MeOH (1.5 mL), basic Ambersep 900 OH
(25 mg) was added. After 2 h under stirring at 20 °C, the
precipitate was filtered off and washed with MeOH. The solution
was concentrated under vacuum and purified by FC (AcOEt/
MeOH 5:1) to give pure 2 (Rf ) 0.4, 12 mg, 0.07 mmol, 78%) as
a colorless oil.
hydroxyimine type. Alternatively, their lack of inhibitory
activity might be because their conjugate acid is not
stable and cannot be formed on penetration into the
active site of the enzymes.
In conclusion, a practical and enantioselective synthe-
sis of (+)-lentiginosine (1) has been realized. It relies on
a highly stereoselective addition of an organometallic
nucleophile onto the L-tartaric acid-derived nitrone 9, an
indium-catalyzed reduction, and a ring-closing metath-
esis. The method has also afforded novel 7,8-didehydro-
lentiginosine (3) and the analogues of 1, namely lactams
4 and 2. Didehydrolentiginosine 3 is a slightly weaker
inhibitor of amyloglucosidase than (+)-lentiginosine (1).
Both compounds showed, however, the same selectivity
pattern toward 22 glycosidases assayed.
(1S,2S,8aS)-1,2-Dihydroxy-1,2,3,5,6,8a-hexahydroindoliz-
ine (3). To a solution of 5b (116 mg, 0.46 mmol) in dry THF (2
mL) a 1 M solution of LiAlH4 in THF (5.5 mL) was added
dropwise at 0 °C under nitrogen atmosphere. After 2 h under
reflux, the mixture was cooled to 20 °C and an aqueous saturated
solution of Na2SO4 (0.5 mL) was added dropwise. After filtration
through Celite and Na2SO4 (washing with AcOEt), the solvent
was evaporated under reduced pressure. FC (CH2Cl2/MeOH/
NH4OH 41:8:1) gave pure 3 (Rf ) 0.26, 42 mg, 0.27 mmol, 59%)
as a white solid: mp 105-107 °C; [R]25D -39.8 (c 0.99, MeOH);
1H NMR (D2O, 400 MHz) δ 5.81 (ddq, J ) 10.2, 1.0, 2.9 Hz, 1H,
H-7), 5.73 (dq, J ) 10.2, 2.0 Hz, 1H, H-8), 4.06 (dt, J ) 7.2, 4.9
Hz, 1H, H-2), 3.72 (dd, J ) 6.8, 4.5 Hz, 1H, H-1), 3.05 (dquint,
J ) 7.0, 2.5 Hz, 1H, H-8a), 2.98 (dd, J ) 11.1, 7.2 Hz, 1H, Ha-
3), 2.80 (ddd, J ) 12.3, 5.7, 3.9 Hz, 1H, Ha-5), 2.71 (dd, J )
11.1, 4.9 Hz, 1H, Hb-3), 2.61 (ddd, J ) 12.3, 9.0, 5.1 Hz, 1H,
Hb-5), 2.21-2.10 (dm, J ) 18.2 Hz, 1H, Ha-6), 1.98-1.90 (dm,
J ) 18.2 Hz, 1H, Hb-6); 13C NMR (D2O, 50 MHz) δ 129.2 (d,
C-7), 127.7 (d, C-8), 84.6 (d, C-1), 79.4 (d, C-2), 66.6 (d, C-8a),
59.3 (t, C-3), 48.8 (t, C-5), 24.5 (t, C-6); MS (70 eV): m/z (%):
Experimental Section
(1S,2S,8aS)-1,2-Diacetyloxy-2,3,6,8a-tetrahydro-5(1H)-in-
dolizinone (5b). To a stirred solution of 6b (342 mg, 1.22 mmol)
in dry CH2Cl2 (60 mL) at 20 °C, under nitrogen atmosphere, a
solution
of
benzylidene-bis(tricyclohexylphosphine)dich-
lororuthenium (Grubbs’ catalyst, 167 mg, 0.202 mmol) in dry
CH2Cl2 (40 mL) was added. The reaction was heated under
reflux and monitored by TLC. After 22 h, the solvent was
evaporated. FC (AcOEt) afforded pure 5b (Rf ) 0.3, 275 mg, 1.09
155 (M+, 38), 138 (7), 95 (100), 80 (65), 67 (77); IR (KBr) ν (cm-1
)
3369, 3033, 2960, 2938, 2916, 2873, 2835, 1383, 1141. Anal.
Calcd for C8H13NO2: C, 61.91; H, 8.44; N, 9.03. Found: C, 61.48;
H, 8.49; N, 8.80.
mmol, 89%) as a light brown solid. 5b: mp 98-100 °C; [R]26
D
+8.7 (c 0.66, CHCl3); 1H NMR (CDCl3, 400 MHz) δ 5.96 (dq, J )
10.1, 1.8 Hz, 1H, H-7), 5.91-5.86 (m, 1H, H-8), 5.22 (ddd, J )
7.8, 3.9, 3.8 Hz, 1H, H-2), 5.07 (dd, J ) 7.8, 4.3 Hz, 1H, H-1),
4.14-4.12 (m, 1H, H-8a), 4.09 (dd, J ) 13.4, 3.4 Hz, 1H, Ha-3),
3.67 (dd, J ) 13.7, 7.0 Hz, 1H, Hb-3), 2.98-2.96 (m, 2H, H-6),
2.14 (s, 3H), 2.06 (s, 3H); 13C NMR (CDCl3, 50 MHz) δ 170.3 (s),
170.0 (s), 166.1 (s, C-5) 124.1 (d, C-7), 121.9 (d, C-8), 79.1 (d,
C-2), 74.5 (d, C-1), 61.5 (d, C-8a), 48.0 (t, C-3), 32.2 (t, C-6), 20.8
(q), 20.7 (q); MS (70 eV): m/z (%): 253 (M+, 4), 151 (89), 134
(100), 96 (83), 57 (56); IR (KBr) ν (cm-1) 2977, 1755, 1739, 1651,
1636, 1242. Anal. Calcd for C12H15NO5: C, 56.91; H, 5.97; N,
5.53. Found: C, 56.91; H, 5.87; N, 5.50.
(1S,2S,8aS)-1,2-Dihydroxy-hexahydro-5(1H)-indolizino-
ne (4). To a solution of 13 (58 mg, 0.23 mmol) in MeOH (4 mL)
strongly basic Ambersep 900 OH (60 mg) was added, and the
mixture was stirred for 2 h at 20 °C. After filtration and solvent
evaporation under reduced pressure, FC (AcOEt/MeOH 5:1)
afforded pure 4 (Rf ) 0.3, 39 mg, 0.228 mmol, 99%) as a white
solid: mp 109-111 °C; [R]25 -3.7 (c 0.15, MeOH); 1H NMR
D
(D2O, 400 MHz) δ 4.09 (qd, 1H, J ) 7.0, 1.0 Hz, H-2), 3.62 (dd,
J ) 12.6, 8.2, 1H, Ha-3), 3.61 (dd, J ) 8.8, 7.2 Hz, 1H, H-1),
3.29 (td, J ) 10.0, 3.8 Hz, 1H, H-8a), 3.22 (dd, J ) 12.6, 7.0 Hz,
1H, Hb-3), 2.29 (dd, J ) 18.3, 6.5 Hz, 1H, Ha-6), 2.19 (ddd, J )
18.3, 11.3, 7.2 Hz, 1H, Hb-6), 2.10-2.04 (m, 1H, Ha-8), 1.88-
1.81 (m, 1H, Ha-7), 1.63-1.51 (m, 1H, Hb-7), 1.29 (qd, J ) 13.3,
3.3 Hz, 1H, Hb-8); 13C NMR (D2O, 50 MHz) δ 175.1 (s), 82.4 (d),
75.3 (d), 64.2 (d), 51.3 (t), 32.4 (t), 28.1 (t), 21.9 (t); MS (70 eV):
m/z (%): 171 (M+, 21), 128 (72), 111 (71), 83 (60), 56 (63), 55
(100); IR (KBr) ν (cm-1) 3369, 2931, 1694, 1602, 1464, 1105. Anal.
Calcd for C8H13NO3: C, 56.13; H, 7.65; N, 8.18. Found: C, 56.45;
H, 7.35; N, 8.23.
(1S,2S,8aS)-1,2-Dihydroxyoctahydroindolizine ((+)-Len-
tiginosine), (1). Trifluoroacetic acid (3 mL) was added at 0 °C
under stirring to 12 (100 mg, 0.372 mmol). The mixture was
stirred at 20 °C overnight. Then, CF3COOH was evaporated
under reduced pressure, the residue was dissolved in MeOH,
and strongly basic Ambersep 900 OH was added. After filtration,
the solvent was evaporated under reduced pressure. FC (CH2-
Cl2/MeOH/NH4OH 41:8:1) afforded (+)-1 as a white solid (43 mg,
0.274 mmol, 74%): mp 106-107 °C, [R]25 +2.6 (c 0.4, MeOH).
D
Acknowledgment. We thank MIUR, Italy, for fi-
nancial support (PRIN) and Ente Cassa di Risparmio
di Firenze, Italy, for granting a 400 MHz NMR spec-
trometer. Technical assistance from Brunella Innocenti
and Maurizio Passaponti is gratefully acknowledged. We
are grateful also to the State Secretariat for Education
and Research SER, Bern (European TRIoH, FP6), for
financial support. G.M. was the recipient of a fellowship
under the Socrates/Erasmus student exchange program
(Firenze-Zaragoza).
Anal. Calcd for C8H15NO2: C, 61.12; H, 9.62; N, 8.91. Found:
C, 61.32; H, 9.64; N, 8.66.
(1S,2S,8aS)-1,2-Dihydroxy-2,3,6,8a-tetrahydro-5(1H)-in-
dolizinone (2). From 5a: CF3COOH (2 mL) was added at 0 °C
under stirring to 5a (63 mg, 0.22 mmol). The mixture was stirred
at 20 °C overnight. The solvent was evaporated under reduced
pressure. FC (AcOEt/MeOH 5:1) gave pure 2 (Rf ) 0.44, 29.7
mg, 0.176 mmol, 80%) as a white solid: mp 100-104 °C; [R]26
D
-27.6 (c 0.34, MeOH); 1H NMR (D2O, 400 MHz) δ 5.93 (m, 1H,
H-8), 5.82 (dddd, J ) 12.5, 5.0, 2.5, 2.3 Hz, 1H, H-7), 4.18 (dt, J
) 8.0, 6.4 Hz, 1H, H-2), 3.98-3.96 (m, 1H, H-8a), 3.67 (m, 2H,
H-1, Ha-3), 3.39 (dd, J ) 12.5, 6.4 Hz, 1H, Hb-3), 2.96-2.89 (m,
1H, Ha-6), 2.84-2.77 (m, 1H, Hb-6); 13C NMR (D2O, 50 MHz) δ
172.4 (s), 125.6 (d), 124.5 (d), 81.3 (d), 75.6 (d), 64.1 (d), 51.2 (t),
34.1 (t); MS (70 eV): m/z (%): 169 (M+, 61), 151 (3), 150 (6), 109
(84), 96 (92), 80 (100); IR (KBr) ν (cm-1) 3195, 2890, 1635, 1480,
1205. Anal. Calcd for C8H11NO3: C, 56.80; H, 6.55; N, 8.28.
Found: C, 56.92; H, 6.45; N, 8.52. From 5b: to a solution of 5b
Supporting Information Available: Characterization
data and full experimental procedures for compounds 8, 7, 6a,
5a, 6b, 11, 12, and 13. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO0509408
J. Org. Chem, Vol. 70, No. 16, 2005 6555