Diversity-oriented syntheses of 7-substituted lentiginosines

Article abstract of DOI:10.1016/j.tet.2011.10.008

Diversity-oriented synthesis of derivatives of the potent glycosidase inhibitor lentiginosine can be achieved in an efficient and versatile way by two modular approaches on key intermediates. After assembling the indolizidine ring system through 1,3-dipol

Full text of DOI:10.1016/j.tet.2011.10.008

Tetrahedron 67 (2011) 9555e9564
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Diversity-oriented syntheses of 7-substituted lentiginosines
,
a
b
b
,
c
Franca M. Cordero a , Paola Bonanno , Matteo Chioccioli , Paola Gratteri , Inmaculada Robina ,
*
*
Antonio J. Moreno Vargas c, Alberto Brandi a
a Department of Chemistry ‘Ugo Schiff’ and HeteroBioLab, University of Firenze, via della Lastruccia 13, I-50019 Sesto Fiorentino (FI), Italy
b Department of Pharmaceutical Sciences, Laboratory of Molecular Modeling Cheminformatics & QSAR and HeteroBioLab, University of Firenze, via U. Schiff 6, I-50019 Sesto
Fiorentino (FI), Italy
c
ꢀ
Department of Organic Chemistry, University of Seville, Prof. García Gonzalez, 1, E-41012-Seville, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 June 2011
Received in revised form 13 September 2011
Accepted 3 October 2011
Available online 14 October 2011
Diversity-oriented synthesis of derivatives of the potent glycosidase inhibitor lentiginosine can be ach-
ieved in an efficient and versatile way by two modular approaches on key intermediates. After assem-
bling the indolizidine ring system through 1,3-dipolar cycloaddition of a dihydroxylated pyrroline N-
oxide with 4-butenol followed by elaboration of the isoxazolidine moiety, the 7-amino and 7-azido
derivatives synthesized can be conjugated with functionalised chains by coupling, respectively, with
an amino acid, or an alkyne in copper-catalyzed Huisgen cycloadditions.
Dedicated to Professor Albert Padwa on the
occasion of his 75th birthday
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
1,3-Dipolar cycloaddition
Iminosugar
Indolizidine
Glycosidase
Triazole
1. Introduction
a small molecule (MW 157) exhibiting very interesting properties.
In particular, (þ)-1, with only two hydroxyl groups, is the least
oxygenated iminosugar that mimics glucosidase natural substrates
(þ)-Lentiginosine
[(1S,2S,8aS)-octahydro-1,2-indolizinediol,
(þ)-1, Fig. 1], an indolizidine alkaloid isolated for the first time by
and indeed it is a selective inhibitor of amyloglucosidases, i.e., en-
zymes that hydrolyze 1,4- and 1,6- -glucosidic linkages. The in-
hibitory power is good (Ki¼2.0 and 3.0 M toward Aspergillus
Elbein et al. from the leaves of Astragalus lentiginosus in 1990,1 is
a
m
niger amyloglucosidase and Rhizopus mold amyloglucosidase,
respectively)2 and the selectivity is remarkably high in compa-
rison with other important glucosidase inhibitors such as
1-deoxynojirimycin and castanospermine.3 The non-natural en-
antiomer, (ꢀ)-lentiginosine [(ꢀ)-1, Fig. 1], is a weaker inhibitor of
amyloglucosidases compared to (þ)-lentiginosine (Ki¼70 and
98 mM toward A. niger amyloglucosidase and Rhizopus mold amy-
loglucosidase, respectively),2 and was recently shown to be an
appealing caspase-dependent apoptosis inducer on tumor cells of
different origin.4 The high interest in (þ)- and (ꢀ)-lentiginosine5 is
also testified by the large number of their syntheses published since
the discovery of 1.6e9
Fig. 1. Natural (þ)-lentiginosine, non-natural (ꢀ)-lentiginosine, and general structure
of their 7-substituted derivatives.
Based on the biological properties of (þ)-1 and (ꢀ)-1, our goal
is to design diversity-oriented syntheses for the preparation of
7-substituted lentiginosine in both enantiomeric forms to get
a deeper insight into ligand receptor interactions regarding their
glycosidase inhibition and pro-apoptotic activity.
* Corresponding authors. Tel.: þ39 055 4574398; fax: þ39 055 4573531 (F.R.M);
tel.: þ39 055 4573702; fax: þ39 055 4573780 (P.G.); e-mail addresses: franca.
cordero@unifi.it (F.M. Cordero), paola.gratteri@unifi.it (P. Gratteri).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.008

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F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
The 1,3-dipolar cycloaddition (1,3-DC) of nitrones with dipo-
chains, natural amino acids, and markers such as biotin. The len-
tiginosine derivatives were also tested as inhibitors toward some
commercially available glycosidases. Finally, docking studies into
the active site of glucoamylase from Aspergillus awamori were
carried out to investigate the interaction mode of the new syn-
thesized lentiginosine derivatives.
larophiles containing the 3-buten-1-ol moiety followed by con-
version of cycloadducts into 4-piperidinols was proved to be
a reliable approach to azaheterocyclic compounds. The method,
first proposed by Tufariello and Tegeler,10 has been applied to the
preparation of functionalized piperidines,11 indolizidines,12 and
quinolizidines13 (Scheme 1).
2. Results and discussion
The indolizidinols 4 (a: R¼Bz; b: R¼t-Bu; c: R¼TBDMS) used in
this study were prepared according to the general synthetic
methodology previously described18,19 (see also Supplementary
data). The introduction of the azido group in position 7 was ach-
ieved either through mesylation followed by nucleophilic sub-
stitution or directly by a Mitsunobu reaction.
The 7-mesyl derivatives 5a and 5b were obtained from the
corresponding alcohols 4a and 4b under standard conditions in
good yields (73e87%) (Scheme 3). The nature of the R protecting
group proved to be crucial during the nucleophilic substitution of
the mesylate group. Treatment of 5a with NaN3 in DMF at 80 ꢁC
afforded the corresponding azide 6a with complete inversion of
configuration at C-7 in 56% yield (Scheme 3). Under the same
conditions, the tert-butyl protected mesylate 5b gave the expected
azide 6b in only 13% yield along with 1-(but-3-enyl)-3-tert-butoxy-
1H-pyrrole18 (21% yield), which is believed to form through
a Grob fragmentation followed by aromatization of the tert-butyl
derivative 5b.
Scheme 1.
Taking advantage of the very well-known, highly reliable, and
selective nitrone 1,3-DC reaction, the planning of the synthesis of
these compounds is quite simple, and a nice control of the relative
and absolute configuration of up to three new stereocenters in the
final products can be achieved employing a suitable nitrone, an (E)-
or (Z)-disubstituted dipolarophile and the inter- or intramolecular
approach between the two reagents.14
The use of pyrroline N-oxides derived from chiral pool mole-
cules such as malic acid and tartaric acid allowed the stereo-
selective synthesis of optically active hydroxyindolizidines14,15
including (þ)-lentiginosine [(þ)-1] (Scheme 2).2b,16
Scheme 2.
Scheme 3.
The glycosidase inhibitor activity of (þ)-1 combined with the
discovery of the pro-apoptotic action of its enantiomer (ꢀ)-1 sug-
gested the undertaking of a program to derivatize the lentiginosine
structure by conjugation with aminoacidic, lipidic, aromatic and
heteroaromatic, and fluorescent structures. In fact, the design,
synthesis, and biological evaluation of analogs of glycosidase in-
hibitors are subjected to extensive studies due to the many po-
tential medical applications of these compounds,17 and general
methods able to introduce groups having different structural and
functional properties are highly desirable. Our intention is to create
a class of possible bioactive compounds based on the lentiginosine
structure that may be useful to shed light on the type of receptors
involved and on the ligandereceptor interactions.
As a starting point of this ongoing programme, we envisioned
that the exploitation of the 7-hydroxy group of intermediate 4 in
the synthesis of lentiginosine (Scheme 2) would provide a rapid
and flexible access to variously decorated final products.
Herein, we report the preparation of 7-azido and 7-amino de-
rivatives of 1 and their use as common intermediates in the di-
versity-oriented syntheses of 7-substituted lentiginosines (Fig. 1).18
Two modular and reliable processes, such as Cu(I)-catalyzed
Huisgen cycloaddition and amino acylation, were used to conjugate
lentiginosine and its enantiomer with polar moieties, lipophilic
The relative configuration of C-7 in azides 6 was confirmed by
the 1H NMR resonance of 7-H (6a: 4.05 ppm; 6b: 3.99 ppm), which
appears as a pseudo quintet with a small coupling constant
(J¼3 Hz) consistent with an equatorial orientation. In compounds 4
and 5, proton 7-H experiences a strong diaxial coupling generating
a triplet of triplet (Jaxeax¼10.9e11.2; Jaxeeq¼4.5e4.8) in the 1H NMR
spectrum. The observed stereochemical result is in accordance with
analogous C-7 nucleophilic substitutions performed by Tyler et al.20
on the indolizidine alkaloid castanospermine, whereas displace-
ments with retention occur at C-8 and C-6 of castanospermine
because of the participation of the ring nitrogen.
The one-step substitution of the 7-OH group under Mitsunobu
conditions was tested on indolizidinols 4a and 4c (Scheme 4).
Treatment of 4a with diphenylphosphoryl azide (DPPA), diisopro-
pylazodicarboxylate (DIAD), and Ph3P in THF afforded 6a with
a better yield compared with the two-step procedure (71% vs 49%
overall yield). On the contrary, purification of the crude azide 6c
obtained under the same conditions proved to be tricky, and the
product was recovered with a very low purity grade after chro-
matography on silica gel (<65% yield). Accordingly, the benzoylated
7-azido lentiginosine was used as substrate in subsequent
reactions.

F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
9557
Analogously, the CuAAc reaction of 6a with alkyne 12,24 pre-
pared by condensation of 2-propynylamine with N-Boc protected
Trp (Scheme 7), followed by hydrolysis of the carbamate and the
benzoyl groups afforded triazole 13 in 53% overall yield (Scheme 8).
Scheme 4.
The best procedure to convert azide 6a into the corresponding
amine 7 proved to be hydrogenation in the presence of Raney-Ni
(Scheme 5). Other catalysts and methods, such as Pd/C, Zn/AcOH,
and PPh3/H2O afforded the desired amine 7 in lower yield and poor
reproducibility. Hydrolysis of the benzoyl groups with the basic
resin Ambersep 900 OH in MeOH gave the 7-amino lentiginosine 8
in 66% yield (Scheme 5).
Scheme 7.
Scheme 8.
To address the pro-apoptotic activity of new derivatives also the
enantiomeric protected indolizidinol ent-4a was prepared and
converted into the 7-substituted
-lentiginosines5 ent-6a and ent-8
D
starting from
D
-tartaric acid (Scheme 9).
Scheme 5. (a): Raney-Ni, H2, MeOH, 71%; or Pd/C, H2, AcOH, MeOH, 26%; or Zn, AcOH,
CH2Cl2, 37%; or Ph3P, H2O, THF, 65e75 ꢁC, 61%; (b) Ambersep 900 OH, MeOH, 85%; (c)
FmocHNeArg(Pbf)eCO2H, DIPEA, PyBrop, 75%; (d) i: TFA, ii: piperidine, iii: Ambersep
900 OH, MeOH, 49%.
To test the ability of the 7-amino group to undergo coupling
with natural amino acids, the synthesis of the 7-arginilated lenti-
ginosine 9 was carried out. The orthogonally protected arginine
FmoceArg(Pbf)eOH smoothly reacted with the benzoylated amine
7 under standard coupling conditions to give the expected amide in
75% yield (Scheme 5). The three protecting groups (Pbf, Fmoc, and
Bz) could be sequentially removed with complete selectivity under
the usual conditions and eventually the rather polar product 9 was
obtained in 49% overall yield.
Scheme 9.
These results indicate the possibility of linking peptides and
simple amino acids with the iminosugar lentiginosine and making
conjugated amino acids suitably protected for peptide syntheses.
Azide 6a was then evaluated as a tool for derivatization of len-
tiginosine, via the synthesis of triazole derivatives. In recent years,
1,4-disubstituted 1,2,3-triazoles have become very popular as sta-
ble linking units to connect molecular fragments through a modu-
lar approach. The reliability of the copper-(I)-catalyzed addition of
azides with terminal alkynes (CuAAc)21,22 and the structural anal-
ogies with the amide bond21c,22a,22f,22m,23 makes the triazole moi-
ety a privileged tether in drug discovery. This approach could be
applied to azide 6a to get different 7-substituted lentiginosines. An
aliphatic chain, a polar chain, and an amino acid such as tryptophan
were chosen as different model fragments to be appended on
lentiginosine. The two commercial alkynes 1-octyne and 3-butyn-
1-ol were reacted with 6a in the presence of in situ generated Cu(I)
to achieve, after hydrolysis of the benzoyl groups, the triazoles 10
and 11 in 80% and 45% overall yield, respectively (Scheme 6).
Moreover, to allow a study of the interaction of D-lentiginosines
with cellular receptors, a biotinylated lentiginosine was synthe-
sized. The amide of biotin and 2-propynylamine 1425 was prepared
in high yield using the coupling reagents HOBt and EDCI in an
acetonitrile/water mixture as a solvent and reacted with azide ent-
5a under the usual conditions. After basic hydrolysis, the bio-
tinylated
D
-lentiginosine 15 was obtained in 52% overall yield
(Scheme 10).
Scheme 6.
Scheme 10.

9558
F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
The 7-substituted
L
-lentiginosines 8, 9, 10, 11, and 13 were
calculations show a slight change of position of the indolizidine
ring in the binding cavity compared to (þ)-1. In all these cases, the
charged NeH/OWat501 hydrogen bond loses the optimal geo-
evaluated as glycosidase inhibitors toward 12 commercially avail-
able enzymes.26 All the inhibition values different from zero at
1 mM concentration are given in Table 1. At 1 mM concentration,
the arginylated lentiginosine 9 did not inhibit any of the examined
enzymes. Moderate inhibition was displayed by indolizidines 8, 10,
and 11 toward amyloglucosidase (16e40%) and
(17e67%) and by 13 toward amyloglucosidase and
(Table 1).
metrical requirements both in term of distance d and angles
q
(donoreH/acceptor) and (H/acceptoreY) (Table 2). In most
4
cases, the C1eOH does not H-bond to the D55 residue and un-
favorable van der Waals contacts are formed between atoms of the
7-substituent and the binding site W120, W178, E179, E180, and
Y311 residues. The increase in the internal torsional energy (Einter-
nal), one of the terms of the docking scoring function, reflects the
difficulty in the adaptation of the indolizidine ring into the binding
cavity (Table 2, Fig. 2bee). In summary, calculation results are
consistent with a lower affinity of indolizidines 8, 10, 11, and 13 for
the enzyme amyloglucosidase and reaffirm the importance of the
formation of a NeH/O hydrogen bond between the iminosugar
and the water 501 molecule for inhibitory activity.
b
b
-glucosidase
-glucosidase
Table 1
Inhibition (in %) toward glycosidases at 1 mM concentration of 7-substituted
-lentiginosines 8, 10, 11, and 13
L
1
8
10
11
13
Amyloglucosidase EC 3.2.1.3
Aspergillus niger
-Glucosidase EC 3.2.1.21
Almonds
-Xylosidase EC 3.2.1.37
Aspergillus niger
100a
40
16
26
40
b
b
0
0
35
0
67
0
17
0
0
15
Table 2
a
Ki¼2.0
m
M2.
Interaction features between amyloglucosidase and (þ)-lentiginosine and its
7-substituted analogs 8, 9, 10, 11, and 13
NeH/OWat501
Having established a rapid, reliable, and general functionaliza-
ꢁ
Compd
d (A)
q
(ꢁ)
4
(ꢁ)
Einternal
Residue in
tion on C-7 of lentiginosine through two modular approaches based
on 7-amino and 7-azido-lentiginosine as building blocks and in
order to interpret the biological data from a structural point of view,
we decided to perform a modeling investigation to analyze the
interactions between these new lentiginosine derivatives and the
amyloglucosidase binding site. Docking calculations carried out on
the N-protonated form of the (þ)-lentiginosine revealed a good fit
of the indolizidine ring into the binding cavity of the enzyme
(Fig. 2a), thus confirming what was already shown with previous
(DeH/Ae)
(H/AeY)
(kJ/mol)
unfavorable van
der Waal contacts
(þ)-1
8
9
10
11
13
2.172
2.261
2.55
2.55
2.6
163.3
156.8
148.9
138.1
135.9
121.4
94.7
85.4
75.1
90.9
88.3
76.7
1.15
1.48
4.6
5.41
5.25
8.5
W178
E180
E179, E180
E180, Y311, W120
3.5
Fig. 2. Docking orientation of (a) (þ)-lentiginosine and (bee) 7-lentiginosine derivatives (8, 10, 11, and 13) into the 1DOG glucoamylase binding site.
molecular dynamics simulations.27 A favorable network of in-
teractions are established involving a three center C1eOeH/(D55,
R54) hydrogen bond and a C2eOH H-bond interaction with the D55
active site residue. Moreover (þ)-1 donates a NeH/O hydrogen
bond to the water 501 molecule, which is known to have a catalytic
role in the hydrolysis mechanism. The resulting charged H-bond is
characterized by good geometrical parameters (Table 2). In the case
of the 7-substituted derivatives 8, 10, 11, and 13, docking
3. Conclusion
A general method for the functionalization of lentiginosine on
the7-position of the indolizidine ring has been reported using the
7-azido and 7-amino derivatives as common intermediates. The
study has set the basis for the construction of a new class of len-
tiginosines conjugated with different groups for the study of their
glycosidase inhibition (1S,2S,7S,8aS series) and pro-apoptotic

F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
9559
activity (1R,2R,7R,8aR series). None of the synthesized derivatives
of (þ)-1 has reached the glycosidase inhibitor activity and selec-
tivity of the parent lentiginosine. A preliminary computational
study of the interaction of the new compounds with the amylo-
glucosidase enzyme cavity gives some hints to justify the reduced
activity and sets the basis for selection of active candidates.
Method B: Diisopropyl azodicarboxylate (DIAD, 0.62 mL,
3.15 mmol) was added dropwise to a solution of triphenylphos-
phine (708 mg, 2.7 mmol) in dry THF (45 mL) under N2 atmo-
sphere. After 5 min under magnetic stirring at rt, the solution was
added via cannula to a solution of 4a (835 mg, 2.19 mmol, see
Supplementary data) in dry THF (45 mL). Diphenyl phosphoryl
azide (DPPA, 0.681 mL, 3.15 mmol) was added dropwise and the
reaction mixture was stirred overnight at rt under N2 atmosphere.
After concentration under reduced pressure, the crude product was
purified by chromatography on silica gel (eluent: first petroleum
ether/EtOAc 10:1, then petroleum ether/EtOAc 4:1) to obtain 6a as
4. Experimental section
4.1. General experimental methods
All the reactions requiring anhydrous conditions were carried
out under N2, and the solvents were appropriately dried before use.
Rf values refer to TLC on 0.25 mm silica gel plates. The NMR data are
a colorless oil (694 mg, 71%).
25
Compound 6a: Rf¼0.34 (eluent: petroleum ether/EtOAc 4:1); [
a]
D
þ109.7 (c 0.500, CHCl3); 1H NMR (CDCl3, 400 MHz):
¼8.10e8.03 (m,
d
reported in
d
(ppm) from TMS at 25 ꢁC, and peak assignments were
4H; HPh), 7.60e7.52 (m, 2H; HPh), 7.48e7.40 (m, 4H; HPh), 5.43e5.36
(m, 2H; 1-Hþ2-H), 4.07e4.02 (m, 1H; 7-H), 3.19 (d, J¼11.2 Hz, 1H;
3-Ha), 2.95e2.86 (m, 2H; 3-Hbþ5-Ha), 2.55 (ddd, J¼11.1, 8.4, 2.4 Hz,
1H; 8a-H), 2.42 (dt, J¼3.0,11.6 Hz,1H; 5-Hb), 2.13 (dm, J¼13.6 Hz,1H;
8-Ha), 1.98e1.77 (m, 3H; 6-Hþ8-Hb) ppm; 13C NMR (CDCl3,
made on the basis of 1He1H COSY, HMQC, HSQC, and HMBC ex-
periments. Mass spectra: MS (ESI) were recorded on a LCQ Fleet Ion
Trap Mass Spectrometer with Surveyor Plus LC System (Thermo
Scientific); MS (EI) were recorded on a QP5050 Shimadzu spec-
trometer with a GC; relative percentages are shown in parentheses.
Accurate mass spectra were recorded on a LTQ-Orbitrap high-res-
olution mass spectrometer (Thermo, San Jose, CA, USA), equipped
with a conventional ESI source. Microwave-assisted reactions were
carried out in a CEM Discover (TM) singlemode microwave reactor
with IR temperature sensor. The two series of enantiomeric com-
pounds were synthesized starting from, respectively, (2R,3R)-
(þ)-tartaric acid (ee ꢂ99.5%) and (2S,3S)-(ꢀ)-tartaric acid (ee 99%).
100 MHz):
d
¼166.4 (s; C]O), 165.9 (s; C]O), 133.3 (d; CHPh), 133.1
(d; CHPh),129.9 (d, 2C; CHPh),129.8 (d, 2C; CHPh),129.7 (s; CPh),129.5
(s; CPh), 128.4 (d, 2C; CHPh), 128.3 (d, 2C; CHPh), 81.9 (d; C-1), 77.4 (d;
C-2), 61.7 (d; C-8a), 59.5 (t; C-3), 55.5 (d; C-7), 47.3 (t; C-5), 33.2 (t; C-
8), 28.6(t;C-6) ppm;IR(CDCl3):
n
¼2930, 2815, 2098,1718,1602,1451,
1276, 1112 cmꢀ1; MS (EI): m/z (%)¼405 (Mþꢀ1, 3), 364 (2), 284 (2),
163 (23), 105 (100), 77 (31); HRMS: calcd for C22H23N4O4 [MþH]þ
407.17138; found 407.17102; C22H22N4O4 (406.4): calcd C, 65.01; H,
5.46; N, 13.78; found C, 64.70; H, 5.08; N, 13.55.
4.2. (1S,2S,7S,8aS)-7-Azido-2-(benzoyloxy)octahydro-1-
indolizinyl benzoate (6a)
4.3. (1R,2R,7R,8aR)-7-Azido-2-(benzoyloxy)octahydro-1-
indolizinyl benzoate (ent-6a)
Method A: (1S,2S,7R,8aS)-2-(Benzoyloxy)-7-[(methylsulfonyl)oxy]
octahydro-1-indolizinyl benzoate (5a): MsCl (0.327 mL, 4.24 mmol)
was added dropwise to a solution of 4a (809 mg, 2.12 mmol, see
Supplementary data) and TEA (1.462 mL, 10.5 mmol) in CH2Cl2
(3 mL) at 0 ꢁC. The mixture was stirred under N2 at rt for 2 h and the
resulting suspension was diluted with CH2Cl2 (9 mL) and H2O
(9 mL). The two phases were separated and the aqueous phase
extracted with CH2Cl2 (2ꢃ9 mL). The collected organic phases were
washed with brine, dried over Na2SO4, and concentrated under re-
duced pressure. The residue was filtered through a short pad of silica
gel (eluent: EtOAc/petroleum ether 1:1) and evaporation of the
solvent afforded 5a (848 mg, 87%) as a white solid. Compound 5a:
Azide ent-6a was prepared applying the same two procedures as
for its enantiomer 6a.
Method A: Starting from ent-4a (272 mg, 0.715 mmol),
(1R,2R,7S,8aR)-2-(benzoyloxy)-7-[(methylsulfonyl)oxy]octahydro-
1-indolizinyl benzoate (ent-5a) was obtained in 93% yield (306 mg).
Mesylate ent-5a (283 mg, 0.6 mmol) was then converted into azide
ent-6a (132 mg, 54% yield).
Method B: Starting from indolizidinol ent-4a (964 mg,
2.53 mmol), azide ent-6a was obtained in 75% yield (764 mg). ent-
6a: [
a
]
23 ꢀ111.4 (c 0.460, CHCl3); C22H22N4O4 (406.4): calcd C 65.01,
D
H 5.46, N 13.78; found C 64.73, H 5.26, N 13.55.
Rf¼0.35; mp 134.4e134.7 ꢁC; [
a
]
D
25 þ97.5 (c 0.325, CHCl3); 1H NMR
(CDCl3, 400 MHz):
d
¼8.11e8.02 (m, 4H; HPh), 7.61e7.54 (m, 2H; HPh),
4.4. (1S,2S,7S,8aS)-7-Azido-1,2-bis{[tert-butyl(dimethyl)silyl]
oxy}octahydroindolizine (6c)
7.48e7.41 (m, 4H; HPh), 5.67e5.40 (m, 2H; 1-H, 2-H), 4.72e4.62 (m,
1H; 7-H), 3.19 (br d, J¼11.3 Hz, 1H; 3-Ha), 3.14e3.08 (m, 1H; 5-Ha),
3.03 (s, 3H; CH3), 2.87 (dd, J¼11.3, 6.5 Hz, 1H; 3-Hb), 2.47 (dm,
J¼11.6 Hz, 1H; 8-Ha), 2.41e2.33 (m, 1H; 8a-H), 2.27e2.12 (m, 2H; 5-
Hbþ6-Ha), 1.95 (dq, J¼4.5, 12.1 Hz, 1H; 6-Hb), 1.88 (q, J¼11.6 Hz, 1H;
Azide 6c was prepared starting from alcohol 4c2b (611 mg,
1.5 mmol) using Method B. Chromatography on silica gel (eluent:
petroleum ether/EtOAc 4:1) of the crude product afforded 6c
8-Hb) ppm; 13C NMR (CDCl3, 100 MHz):
d
¼166.3 (s; C]O), 165.8 (s;
(435 mg, 68%) as a colorless oil. Compound 6c: Rf: 0.60; [
a
]
24 þ23.4 (c
D
C]O), 133.4 (d; CHPh), 133.2 (d; CHPh), 129.9 (d, 2C; CHPh), 129.8 (d,
2C; CHPh), 129.6 (s; CPh), 129.3 (s; CPh), 128.4 (d, 2C; CHPh), 128.3 (d,
2C; CHPh), 81.5 (d; C-1), 78.3 (d; C-7), 78.1 (d; C-2), 65.6 (d; C-8a),
58.6 (t; C-3), 49.3 (t; C-5), 38.9 (q, CH3), 35.5 (t; C-8), 31.5 (t; C-6)
0.780, CHCl3); 1H NMR (CDCl3, 400 MHz):
d¼4.05e3.97 (m, 2H, 2-
Hþ7-H), 3.71 (dd, J¼8.4, 3.8 Hz, 1H, 1-H), 2.84 (dd, J¼10.1, 1.3 Hz,
1H, 3-Ha), 2.75 (dm, J¼11.0 Hz, 1H, 5-Ha), 2.64e2.56 (m, 1H, 3-Hb),
2.30e2.20 (m, 1H, 5-Hb), 2.15 (br t, J¼9.2 Hz, 1H, 8a-H), 2.00 (dq,
J¼13.3, 2.5 Hz,1H, 8-Ha),1.94e1.80 (m,1H, 6-Ha),1.74 (dm, J¼14.0 Hz,
1H, 6-Hb), 1.57e1.47 (m, 1H, 8-Hb), 0.89 [br s, 18H, C(CH3)3], 0.08 (s,
3H, SiCH3), 0.06 (s, 3H, SiCH3), 0.05 (s, 3H, SiCH3), 0.04 (s, 3H, SiCH3)
ppm. IR (CDCl3):
n
¼3050, 2948, 2942, 1720, 1332, 1279, 1178,
1112 cmꢀ1; C23H25NO7S (459.5): calcd C 60.12, H 5.48, N 3.05; found
C 60.33, H 5.49, N 3.03. A mixture of the mesylate 5a (591 mg,
1.29 mmol) and NaN3 (209 mg, 3.2 mmol) in DMF (3.1 mL) was
heated at 80 ꢁC for 22 h. The reaction mixture was diluted with
EtOAc (8 mL) and H2O (9 mL), the two phases were separated, and
the aqueous phase was extracted with EtOAc (9ꢃ8 mL). The col-
lectedorganicphaseswerewashedwith brine, dried overanhydrous
Na2SO4, filtered, and concentrated under reduced pressure. The
crude product was purified by column chromatography on silica gel
(eluent: CH2Cl2) to afford azide 6a (296 mg, 56%) as a colorless oil.
ppm; 13C NMR (CDCl3, 100 MHz):
d
¼85.1 (d; C-1), 78.0 (d; C-2), 62.4
(d; C-8a), 61.8 (t; C-3), 55.5 (d; C-7), 47.7 (t; C-5), 32.5 (t; C-8), 28.7 (t;
C-6), 25.9 [q; 3C, C(CH3)3], 25.8 [q; 3C, C(CH3)3], 17.9 (s; SiCMe3), 17.8
(s; SiCMe3), ꢀ4.1 (q; SiCH3), ꢀ4.28 (q; SiCH3), ꢀ4.31 (q; SiCH3), ꢀ4.7
(q; SiCH3) ppm; IR (CDCl3):
n
¼2955, 2929, 2856, 2813, 2100, 1471,
1360,1258,1150 cmꢀ1; MS (EI): m/z (%)¼426 (Mþ, 6), 411 (3), 398 (3),
384 (11), 369 (11),138 (76), 73 (100); C20H42N4O2Si2 (426.74): calcd C,
56.29; H, 9.92; N, 13.13; found C, 56.10; H, 9.77; N, 13.52.

9560
F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
4.5. (1S,2S,7S,8aS)-7-Amino-2-(benzoyloxy)octahydro-1-
indolizinyl benzoate (7)
4.9. (1S,2S,7S,8aS)-2-(Benzoyloxy)-7-({(2S)-2-{[(9H-fluoren-9-
ylmethoxy)carbonyl]amino}-5-[(imino{[(2,2,4,5,6-
pentamethyl-2,3-dihydro-1-benzofuran-7-yl)sulfonyl]amino}
methyl)amino]pentanoyl}amino)octahydro-1-indolizinyl
benzoate [FmocHNeArg(Pbf)eHN-7]
A water suspension of activated Raney-Ni was added dropwise
to an ice bath cooled solution of 6a (395 mg, 0.97 mmol) in MeOH
(11 mL). The mixture was stirred at 0 ꢁC for 1 h and then at rt for
45 min under H2 atmosphere (1 atm). The reaction mixture was
filtered through a short pad of Celite and MeOH was evaporated
under reduced pressure. The aqueous phase was extracted with
CH2Cl2 (6ꢃ4 mL). The combined organic phases were dried on
Na2SO4, filtered, and concentrated under reduced pressure. The
residue was purified by chromatography on silica gel (eluent:
CH2Cl2/MeOH 7:1) to give 7 in 71% yield (262 mg) as a colorless oil.
Bromotripyrrolidinophosphonium hexafluorophosphate (PyBroP,
289 mg, 0.62 mmol) was added to a mixture of 7 (235.6 mg,
0.62 mmol), FmocHNeArg(Pbf)eCO2H (441.2 mg, 0.68 mmol), and
N,N-diisopropylethylamine (0.106 mL, 0.62 mmol) in CH2Cl2
(0.62 mL, freshly distilled from P2O5) at 0 ꢁC (ice/H2O bath). The re-
action mixture was stirred at rt under N2 atmosphere overnight and
then concentrated under reduced pressure. The residue was dis-
solved in EtOAc (5 mL), filtered, and concentrated under reduced
pressure. Purification bychromatography on silicagel (eluent: EtOAc/
TEA 98:2) afforded the title compound FmocHNeArg(Pbf)eHN-7 in
Compound 7: Rf: 0.24; [
400 MHz):
a
]
24þ77.5 (c 0.390, CHCl3); 1H NMR (CDCl3,
D
d
¼8.11e8.01 (m, 4H, HPh), 7.59e7.51 (m, 2H, HPh),
7.47e7.39 (m, 4H, HPh), 5.44e5.36 (m, 2H, 1-Hþ2-H), 3.47e3.39 (m,
1H, 7-H), 3.19 (d, J¼11.3 Hz,1H, 3-Ha), 2.95 (dd, J¼11.3, 6.6 Hz,1H, 3-
Hb), 2.83 (ddd, J¼11.2, 4.7, 2.2 Hz, 1H, 5-Ha), 2.70 (ddd, J¼11.0, 8.4,
2.7 Hz, 1H, 8a-H), 2.57e2.49 (m, 1H, 5-Hb), 2.07e1.75 (m, 3H, 6-
Haþ8-H), 2.70 (dm, J¼13.9 Hz, 1H, 6-Hb) ppm; 13C NMR (CDCl3,
75% yield (471 mg) as a white solid. FmocHNeArg(Pbf)eHN-7: Rf:
22
0.17; mp 127e130 ꢁC; [
a]
D
þ44.2 (c 0.445, CHCl3); 1H NMR
(400 MHz, CDCl3):
d
¼8.08e8.03 (m, 2H, HAr), 7.97e7.91 (m, 2H, HAr),
7.76e7.67 (m, 2H, HAr), 7.57e7.50 (m, 3H, HAr), 7.45e7.17 (m, 10H,
9HArþNH), 6.51e6.09 (m, 4H, NH), 5.32 (dd, J¼8.1, 3.2 Hz, 1H, 1-H),
5.28 (dd, J¼6.4, 2.9 Hz, 1H, 2-H), 4.29 (d, J¼7.2 Hz, 2H, CH2O),
4.31e418 (m, 2H, 10-Hþ7-H), 4.11 (t, J¼7.2 Hz, 1H, CHCH2O),
3.28e3.16 (m, 2H, 40-H), 3.05 (d, J¼11.2 Hz, 1H, 3-Ha), 2.88 (s, 2H,
CH2CMe2), 2.79e2.63 (m, 2H, 3-Hbþ5-Ha), 2.58 (s, 3H, CH3), 2.50 (s,
3H, CH3), 2.53e2.40 (m, 1H, 8a-H), 2.36e2.24 (m, 1H, 5-Hb),
2.13e1.99 (m, 1H, 8-Ha), 2.05 (s, 3H, CH3),1.91e1.46 (m, 7H, 20-Hþ30-
Hþ6-Hþ8-Hb),1.40 (s, 6H, CH3ꢃ2) ppm; 13C NMR (100 MHz, CDCl3):
100 MHz):
d
¼166.4 (s, C]O), 165.9 (s, C]O), 133.2 (d, CHPh), 133.1
(d, CHPh), 129.9 (d, 2C, CHPh), 129.8 (d, 2C, CHPhþs, CPh), 129.6 (s,
CPh), 128.4 (d, 2C, CHPh), 128.3 (d, 2C, CHPh), 82.2 (d, C-1), 77.6 (d, C-
2), 61.0 (d, C-8a), 59.7 (t, C-3), 46.9 (t, C-5), 43.7 (d, C-7), 36.0 (t, C-
8), 31.8(t, C-6) ppm; IR (CDCl3):
n
¼3393, 3063, 2927, 2853, 1717,
1603, 1451, 1280, 1114 cmꢀ1; MS (EI): m/z (%)¼362 (Mþꢀ18, 1), 258
(6), 137 (75), 120 (77), 105 (100), 77 (60); HRMS: calcd for
C22H25N2O4 [MþH]þ 381.18088, found 381.18190.
d¼171.6 (s; C]O),166.3 (s; C]O),165.9 (s; C]O),158.8 (s; CAr),156.7
(s) and 156.4 (s) (C]N and C]O), 143.6 (s; 2C, CAr), 141.2 (s; 2C, CAr),
138.3 (s; CAr),133.1 (d; 2C, CHAr),132.6 (s; CAr),132.2 (s; CAr),129.8 (d;
2C, CHAr),129.7 (d; 2C, CHAr),129.6 (s; CAr),129.4 (s; CAr),128.3 (d; 4C,
CHAr),127.7 (d; 2C, CHAr),127.0 (d; 2C, CHAr),125.1 (d; 2C, CHAr),124.7
(s; CAr),120.0 (d; 2C, CHAr),117.6 (s; CAr), 86.4 (s; CMe2), 82.2 (d; C-1),
77.4 (d; C-2), 67.2 (t; CH2O), 61.8 (d; C-8a), 59.2 (t; C-3), 54.6 (d; C-10),
47.3 (t; C-5), 47.0 (d; CHCH2O), 43.2(d; C-7), 43.1 (t; CH2CMe2), 40.5(t;
C-40), 33.0 (t; C-8), 29.6 (t; C-20), 28.7 (t; C-6), 28.5 (q; 2C, CH3ꢃ2), 25.6
(t; C-30), 19.4 (q; CH3), 18.0 (q; CH3), 12.4 (q; CH3) ppm; IR (CDCl3):
4.6. (1R,2R,7R,8aR)-7-Amino-2-(benzoyloxy)octahydro-1-
indolizinyl benzoate (ent-7)
Following the same procedure as for amine 7, the enantiomer
ent-7 was obtained in 71% yield (32 mg) starting from ent-6a
23
(48 mg, 0.118 mmol). ent-7: [
a
]
ꢀ79.3 (c 0.265, CHCl3); spectral
D
properties are identical to those of 7.
n
¼3431, 3340, 3067, 2931, 2248, 1718, 1666, 1554, 1451, 1281 cmꢀ1
;
4.7. (1S,2S,7S,8aS)-7-Aminooctahydro-1,2-indolizinediol (7S-
amino-lentiginosine, 8)
MS (ESI): m/z¼1011 [MþH]þ, 1033 [MþNa]þ.
Ambersep 900 OH was added to a solution of 7 (40 mg,
0.10 mmol) in MeOH (2 mL) and the mixture was shaken at rt for
1 h on a flat shaker at 150 rpm. The reaction mixture was filtered
through cotton wool and concentrated under reduced pressure.
4.10. (2S)-N-[(1S,2S,7S,8aS)-1,2-Dihydroxyoctahydro-7-
indolizinyl]-2-amino-5-{[amino(imino)methyl]amino}
pentanamide {N1-[(1S,2S,7S,8aS)-1,2-
dihydroxyoctahydroindolizin-7-yl]-L-argininamide, 9}
Chromatography on silica gel [eluent: MeOH (1% NH4OH)] afforded
24
8 in 85% yield (15 mg). Compound 8 Rf: 0.19; [
a
]
D
ꢀ2.4 (c 0.340,
A solution of FmocHNeArg(Pbf)eHN-7 (187.5 mg, 0.185 mmol)
in a 95:5 mixture of TFA and H2O (3 mL) was stirred at rt for 30 min.
Et2O (8 mL) was added to the reaction mixture, the precipitated
white solid was filtered, and washed with Et2O. MS-ESI analysis of
the residue showed the complete removal of the Pbf protecting
group {FmocHNeArgeHN-7: MS (ESI): m/z¼759 [MþH]þ, 781
[MþNa]þ}. The residue was dissolved in CH2Cl2 (2.2 mL), cooled in
an ice bath, and treated with freshly distilled piperidine (0.43 mL,
0.004 mmol). The reaction mixture was stirred at 0 ꢁC for 1 h and
30 min, concentrated under reduced pressure, and the residue was
sequentially washed with petroleum ether (5 mL), Et2O (5 mL), and
EtOAc (5 mL). MS-ESI analysis of the residue showed the complete
removal of the Fmoc protecting group {ArgeHN-7: MS (ESI): m/
z¼537 [MþH]þ}. The crude intermediate was dissolved in MeOH
(15 mL) and treated with Ambersep 900 OH at rt for 2 h. Then, the
mixture was filtered through a short pad of Celite, concentrated
under reduced pressure, diluted with H2O (0.5 mL), and acidified
with 3 N HCl to pH¼1 under magnetic stirring at 0 ꢁC. The aqueous
phase was sequentially washed with petroleum ether (0.5 mLꢃ3),
EtOAc (0.5 mLꢃ3), and CH2Cl2(0.5 mLꢃ3) and then concentrated
CH3OH); 1H NMR (CD3OD, 400 MHz):
d
¼4.00 (ddd, J¼7.2, 3.5,
1.5 Hz, 1H; 2-H), 3.62 (dd, J¼8.5, 3.5 Hz, 1H; 1-H), 3.33e3.29 (ob-
scured by solvent residual peak, 1H; 7-H), 2.85 (dd, J¼10.8, 1.5 Hz,
1H; 3-Ha), 2.78 (ddd, J¼11.5, 4.7, 2.7 Hz, 1H; 5-Ha), 2.66 (dd, J¼10.8,
7.2 Hz, 1H; 3-Hb), 2.42e2.33 (m, 1H; 5-Hb), 2.21 (ddd, J¼11.5, 8.5,
2.7 Hz, 1H; 8a-H), 1.97e1.82 (m, 2H; 6-Haþ8-Ha), 1.66e1.57 (m, 2H;
6-Hbþ8-Hb) ppm; 13C NMR (CD3OD,100 MHz):
¼85.1 (d; C-1), 77.8
d
(d; C-2), 64.1 (d; C-8a), 62.6 (t; C-3), 48.2 (t; C-5), 45.1 (d; C-7), 35.9
(t; C-8), 32.2 (t; C-6) ppm; MS (EI): m/z (%)¼154 (MþꢀH2O, 27), 137
(49), 120 (33), 95 (33), 56 (100); HRMS: calcd for C8H17N2O2
[MþH]þ 173.1294, found 173.1290.
4.8. (1R,2R,7R,8aR)-7-Aminooctahydro-1,2-indolizinediol
(ent-8)
Following the same procedure as for 8, the enantiomeric com-
pound ent-8 was obtained in 83% yield (19 mg) starting from ent-7
22
(50 mg, 0.13 mmol). ent-8: [
a]
þ3.7 (c 0.500, CH3OH); spectral
D
properties are identical to those of 8.

F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
9561
under reduced pressure. MS-ESI analysis of the residue showed the
complete removal of the Bz protecting groups. The residue was
dissolved in MeOH and the solution was basified with Ambersep
900 OH to pH¼10, filtered through a short pad of Celite, and con-
centrated under reduced pressure. The product 9 was obtained in
49% overall yield over three steps (30 mg) as a white waxy solid and
was characterized without further purification. 1H NMR spectrum
showed the presence of two isomers in ca. 1.7:1 ratio, which were
silica gel (eluent: MeOH) of the residue afforded 10 in 82% yield
24
(31 mg) as a waxy solid. Compound 10: Rf: 0.43; [
a
]
þ14.4 (c
D
0.965, CH3OH); 1H NMR (CD3OD, 400 MHz):
d¼7.82 (s, 1H, Htriazole),
4.83e4.78 (m, 1H, 7-H), 3.94 (ddd, J¼7.0, 3.4, 1.4 Hz, 1H, 2-H), 3.64
(dd, J¼8.2, 3.4 Hz, 1H, 1-H), 2.95e2.88 (m, 1H, 5-Ha), 3.88 (dd,
J¼10.6, 1.4 Hz, 1H, 3-Ha), 2.69 (t, J¼7.7 Hz, 2H, 10-H), 2.70e2.63 (m,
1H, 8-Ha), 2.56 (dd, J¼10.6, 7.0 Hz, 1H, 3-Hb), 2.47e2.37 (m, 2H, 6-
Haþ5-Hb), 2.26e2.16 (m, 1H, 6-Hb), 2.08 (ddd, J¼11.5, 8.2, 2.3 Hz,
1H, 8a-H), 1.95 (ddd, J¼13.9, 11.5, 4.1 Hz, 1H, 8-Hb), 1.67 (pseudo
quintet, J¼7.5 Hz, 2H, 20-H), 1.41e1.27 (m, 6H, 30-Hþ40-Hþ50-H),
0.90 (t, J¼6.9 Hz, 3H, 60-H) ppm; 13C NMR (CD3OD, 50 MHz):
tentatively assigned as the two epimers at Ca of Arg. Compound 9:
24
(isomer mixture) [
a
]
ꢀ16.0 (c 0.760, CH3OH/H2O 1:1); 1H NMR
D
(400 MHz, D2O) major isomer:
d
¼4.04e3.95 (m, 2H, 2-Hþ7-H),
3.57 (dd, J¼8.4, 3.9 Hz, 1H, 1-H), 3.34e3.27 (m, 1H, 10-H), 3.12e3.04
(m, 2H, 40-H), 2.81e2.72 (m, 2H, 3-Haþ5-Ha), 2.56 (dd, J¼11.2,
7.3 Hz, 1H, 3-Hb) 2.24e2.14 (m, 1H, 5-Hb), 2.05e1.91 (m, 2H,
8-Haþ8a-H), 1.80e1.36 (m, 7H, 20-Hþ30-Hþ6-Hþ8-Hb) ppm; 13C
d¼149.0 (s; Ctriazole), 122.5 (d; CHtriazole), 85.1 (d; C-1), 77.7 (d; C-2),
64.8 (d; C-8a), 62.5 (t; C-3), 55.3 (d; C-7), 49.4 (t; C-5), 34.1 (t; C-8),
32.7 (t; C-30 or C-40 or C-50), 30.6 (t; C-20), 30.0 (t; C-30 or C-40 or C-
50), 29.7 (t; C-6), 26.4 (t; C-10), 23.6 (t; C-30 or C-40 or C-50), 14.4 (q;
C-60) ppm; MS (EI): m/z (%)¼290 (MþꢀH2O, 2), 206 (2), 191 (1), 154
(14), 152 (2), 137 (61), 120 (100); MS (ESI): m/z¼309 [MþH]þ, 331
[MþNa]þ, 639 [2MþNa]þ. HRMS-ESI: calcd for C16H29N4O2 [MþH]þ
309.22786; found 309.22850.
NMR (D2O, 100 MHz) major isomer:
d
¼162.6 (s, C]O), 156.7 (s, C]
N), 82.5 (d; C-1), 75.4 (d; C-2), 63.2 (d; C-8a), 60.0 (t; C-3), 53.9 (d,
C-10), 47.4 (t; C-5), 43.1 (d; C-7), 40.7 (t, C-40), 31.4 (t; C-8), 28.9 (t, C-
20), 27.7 (t; C-6), 24.3 (t, C-30) ppm; MS (ESI): m/z¼329 [MþH]þ;
HRMS-ESI: calcd for C14H29N6O3 [MþH]þ 329.22957; found
329.22899.
4.13. (1S,2S,7S,8aS)-2-(Benzoyloxy)-7-[4-(2-hydroxyethyl)-1H-
1,2,3-triazol-1-yl]octahydro-1-indolizinyl benzoate (11-Bz2)
4.11. (1S,2S,7S,8aS)-2-(Benzoyloxy)-7-(4-hexyl-1H-1,2,3-
triazol-1-yl)octahydro-1-indolizinyl benzoate (10-Bz2)
Azide 6a (70 mg, 0.172 mmol), 97% pure 3-butin-1-ol
(0.012 mL, 0.155 mmol), H2O (0.4 mL), t-BuOH (0.4 mL), THF
(0.1 mL), copper powder (4.4 mg, 0.07 mmol), and copper sulfate
(19.5 mg, 0.12 mmol) were mixed in a microwave reaction tube.
The reaction mixture was heated at 80 ꢁC (MW irradiation power
of 100 W) under stirring for 30 min. Then, another portion of
butanol (0.006 mL) was added and the mixture was heated at
80 ꢁC for 15 min. The reaction mixture was diluted with CH2Cl2
(2 mL), dried over Na2SO4, filtered, and concentrated under re-
duced pressure. The crude product was purified by chromatog-
raphy on silica gel (EtOAc/MeOH 14:1) to afford the title
Azide 6a (61 mg, 0.150 mmol), 98% pure 1-octyne (0.020 mL,
0.135 mmol), H2O (0.4 mL), t-BuOH (0.4 mL), THF (0.1 mL), copper
powder (3.8 mg, 0.06 mmol), and copper sulfate (16.8 mg,
0.10 mmol) were mixed in a microwave reaction tube. The re-
action mixture was heated at 80 ꢁC (MW irradiation power of
100 W) under stirring for 30 min. Then, other three portions of the
octyne (0.020 mL) were added and the mixture was heated at
80 ꢁC for 30 min after each addition. The reaction mixture was
filtered and concentrated under reduced pressure. The crude
product was purified by chromatography on silica gel (EtOAc/pe-
troleum ether 1:1) to afford the title compound 10-Bz2 in ana-
compound 11-Bz2 in 85% yield (70 mg) as a colorless oil. Com-
24
pound 11-Bz2: Rf¼0.26; [
a
]
þ119.0 (c 0.49, CHCl3); 1H NMR
D
lytically pure form (76 mg, 98%) as a white solid. Compound 10-
(CDCl3, 400 MHz):
d
¼8.10e8.06 (m, 2H; HPh), 8.04e8.01 (m, 2H;
22
Bz2: Rf: 0.26; mp 119.5e120.7 ꢁC; [
a
]
þ83.1 (c 0.76, CHCl3); 1H
HPh), 7.59e7.54 (m, 2H; HPh), 7.46e7.41 (m, 5H; HPhþHtriazole),
5.46e5.39 (m, 2H; 1-Hþ2-H), 4.80e4.74 (m, 1H; 7-H), 3.94 (t,
J¼5.9 Hz, 2H; CH2OH), 3.22 (d, J¼11.2 Hz, 1H; 3-Ha), 3.01 (ddd,
J¼11.3, 4.7, 2.4 Hz, 1H; 5-Ha), 2.98e2.91 (m, 3H; 3-HbþCH2CH2OH),
2.75e2.67 (m, 2H; 8-Haþ8a-H), 2.63 (dt, J¼2.9, 11.8 Hz, 1H; 5-Hb),
2.43e2.27 (m, 2H; 6-H), 2.22 (ddd, J¼14.5, 11.8, 4.4 Hz, 1H; 8-Hb)
D
NMR (CDCl3, 400 MHz):
d
¼8.10e8.05 (m, 2H, HPh), 8.04e8.00 (m,
2H, HPh), 7.60e7.51 (m, 2H, HPh), 7.47e7.37 (m, 4H, HPh), 7.30 (s,
1H, Htriazole), 5.45 (dd, J¼8.1, 3.1 Hz, 1H, 1-H), 5.41 (ddd, J¼6.7, 3.1,
0.9 Hz, 1H, 2-H), 4.78e4.73 (m, 1H, 7-H), 3.24 (d, J¼11.4 Hz, 1H, 3-
Ha), 3.04 (ddd, J¼11.3, 4.5, 2.5 Hz, 1H, 5-Ha), 2.96 (dd, J¼11.4,
6.7 Hz, 1H, 3-Hb), 2.79e2.61 (m, 5H, 5-Hb, 8a-H, 8-Ha, 10-H),
2.45e2.28 (m, 2H, 6-H), 2.24 (ddd, J¼14.1, 11.3, 4.1 Hz, 1H, 8-Hb),
1.65 (pseudo quintet, J¼7.7 Hz, 2H, 20-H), 1.39e1.22 (m, 6H, 30-
Hþ40-Hþ50-H), 0.87 (t, J¼7.0 Hz, 3H, 60-H) ppm; 13C NMR (CDCl3,
ppm; 13C NMR (CDCl3, 100 MHz):
d
¼166.3 (s; C]O), 166.0 (s; C]
O), 145.4 (s, Ctriazole), 133.3 (d; CHPh), 133.2 (d; CHPh), 129.8 (d, 4C;
CHPh), 129.6 (s; CPh), 129.3 (s; CPh), 128.4 (d, 2C; CHPh), 128.3 (d, 2C;
CHPh), 121.0 (d, CHtriazole), 82.1 (d; C-1), 77.4 (d; C-2), 61.6 (t;
CH2OH), 61.5 (d; C-8a), 59.3 (t; C-3), 53.8 (d; C-7), 47.6 (t; C-5),
33.5 (t; C-8), 29.2 (t; C-6), 28.6 (t, CH2CH2OH) ppm; IR (CDCl3):
50 MHz):
d
¼166.2 (s; C]O), 165.9 (s; C]O), 148.3 (s; Ctriazole),
133.3 (d; CHPh), 133.1 (d; CHPh), 129.8 (d, 4C, CHPh), 129.6 (s; CPh),
129.3 (s; CPh), 128.3 (d; 2C, CHPh), 128.2 (d; 2C, CHPh), 119.9 (d;
CHtriazole), 82.0 (d; C-1), 77.3 (d; C-2), 61.6 (d; C-8a), 59.3 (t; C-3),
53.5 (d; C-7), 47.7 (t; C-5), 33.5 (t; C-8), 31.5 (t; C-30 or C-40 or C-50),
29.3 (t; C-20), 29.1 (t; C-6), 28.9 (t; C-30 or C-40 or C-50), 25.6 (t; C-
10), 22.6 (t; C-30 or C-40 or C-50), 14.1 (q; C-60) ppm; IR (CDCl3):
n
¼3627, 3456 br, 3064, 2957, 2836, 1718, 1603, 1451, 1280,
1113 cmꢀ1; HRMS-ESI: calcd for C26H29N5O4 [MþH]þ 477.21325;
found 477.21286; C26H28N4O5 (476.52): calcd C 65.53, H 5.92, N
11.76; found C 65.17, H 5.95, N 11.40.
n
¼3064, 2957, 2929, 2857, 1716, 1603, 1451, 1316, 1280, 1177, 1114,
4.14. (1S,2S,7S,8aS)-7-[4-(2-Hydroxyethyl)-1H-1,2,3-triazol-1-
yl]octahydro-1,2-indolizinediol (11)
1027 cmꢀ1; MS (EI): m/z 394 (MþꢀBzOH, 2), 273 (5), 241 (2), 120
(100), 105 (38), 77 (15); C30H36N4O4 (516.63): calcd C 69.74, H 7.02,
N 10.84; found C 69.46, H 6.92, N 10.64.
Ambersep 900 OH was added to a solution of the diester 11-Bz2
(70 mg, 0.15 mmol) in MeOH (5 mL) and the mixture was stirred at
rt for 2 h, filtered, and the solution was concentrated under reduced
pressure. Chromatography on silica gel [eluent: MeOH (2% NH4OH)]
of the residue afforded 11 in 52% yield (23.92 mg). Compound 11:
4.12. (1S,2S,7S,8aS)-7-(4-Hexyl-1H-1,2,3-triazol-1-yl)
octahydro-1,2-indolizinediol (10)
Ambersep 900 OH was added to a solution of the diester 10-Bz2
(63 mg, 0.122 mmol) in MeOH (5 mL) and the mixture was stirred
at rt for 4 h. Then, the reaction mixture was filtered and the solution
was concentrated under reduced pressure. Chromatography on
Rf: 0.40; [
a
]
24 þ13.5 (c 0.215, CH3OH); 1H NMR (CD3OD, 400 MHz):
D
d¼7.87 (s, 1H, Htriazole), 4.84e4.80 (m partially overlapped with
water peak, 1H, 7-H), 3.94 (ddd, J¼7.0, 3.4, 1.5 Hz, 1H, 2-H), 3.81 (t,
J¼6.7 Hz, 2H, CH2OH), 3.65 (dd, J¼8.2, 3.4 Hz,1H, 1-H), 2.96e2.87

9562
F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
(m, 2H, 5-Haþ3-Ha), 2.91 (t, J¼6.7 Hz, 2H, CH2CH2OH), 2.65 (dq,
J¼13.9, 2.4 Hz, 1H, 8-Ha), 2.58 (dd, J¼10.6, 7.0 Hz, 1H, 3-Hb),
2.49e2.40 (m, 2H, 6-Haþ5-Hb), 2.22 (ddt, J¼15.0, 12.8, 4.7 Hz, 1H,
6-Hb), 2.12 (ddd, J¼11.5, 8.2, 2.4 Hz, 1H, 8a-H), 1.97 (ddd, J¼13.9,
concentrated under reduced pressure and the product was puri-
fied by chromatography on silica gel [eluent: CH2Cl2/MeOH (1%
NH4OH) 1:1] to give 13 in 73% yield (35.3 mg) as a white waxy
24
solid. Compound 13: Rf: 0.21, [
NMR (400 MHz, CD3OD):
a
]
þ17.0 (c 0.995, CH3OH); 1H
D
11.5, 4.2 Hz,1H, 8-Hb) ppm; 13C NMR (CD3OD, 50 MHz):
d¼145.9 (s;
d
¼7.60 (d, J¼7.9 Hz, 1H, Hindole), 7.35 (d,
Ctriazole), 123.4 (d; CHtriazole), 85.0 (d; C-1), 77.7 (d; C-2), 64.8 (d; C-
8a), 62.5 (t; C-3), 62.1 (t; CH2OH), 55.3 (d; C-7), 49.4 (t; C-5), 34.0 (t;
C-8), 30.0 (t; CH2CH2OH), 29.7 (t; C-6) ppm; MS (EI): m/z (%)¼250
(MþꢀH2O, 8), 233 (2),154 (19),137 (60),120 (100); HRMS-ESI: calcd
for C12H21N4O3 [MþH]þ 269.16082; found 269.16125.
J¼8.2 Hz, 1H, Hindole), 7.33 (s, 1H, Htriazole), 7.13e7.08 (m, 1H, Hin-
dole), 7.06 (s, 1H, Hindole), 7.03e6.98 (m, 1H, Hindole), 4.72e4.67 (m,
1H, 7-H), 4.39 (A part of an AB system, J¼15.3 Hz, 1H, CHHNH),
4.32 (B part of an AB system, J¼15.3 Hz, 1H, CHHNH), 3.93 (ddd,
J¼7.0, 3.3, 1.5 Hz, 1H, 2-H), 3.66e3.61 (m, 2H, 1-HþCHNH2), 3.15 (A
part of an ABX system, J¼14.1, 6.8 Hz, 1H, CHHCHNH2), 3.04 (B part
of an ABX system, J¼14.1, 6.4 Hz, 1H, CHHCHNH2), 2.92e2.84 (m,
2H, 3-Haþ5-Ha), 2.58e2.51 (m, 2H, 3-Hbþ8-Ha), 2.44e2.27 (m,
2H, 5-Hbþ6-Ha), 2.24e2.12 (m, 1H, 6-Hb), 2.06 (ddd, J¼11.5, 8.2,
2.3 Hz, 1H, 8a-H), 1.93 (ddd, J¼13.9, 11.5, 4.2 Hz, 1H, 8-Hb) ppm;
4.15. (1S,2S,7S,8aS)-2-(Benzoyloxy)-7-[4-({[(2S)-2-[(tert-
butoxycarbonyl)amino]-3-(1H-indol-3-yl)propanoyl]amino}
methyl)-1H-1,2,3-triazol-1-yl]octahydro-1-indolizinyl
benzoate (13-NBoc,OBz)
13C NMR (50 MHz, CD3OD):
d
¼177.2 (s; C]O), 145.8 (s; Ctriazole),
Azide 6a (80.5 mg, 0.198 mmol), alkyne 12 (24 mg, 0.07 mmol,
see Supplementary data), H2O (0.5 mL), t-BuOH (0.5 mL), THF
(0.3 mL), copper powder (5.2 mg, 0.08 mmol), and copper sulfate
(22 mg, 0.14 mmol) were mixed in a microwave reaction tube. The
reaction mixture was heated at 80 ꢁC (MW irradiation power of
100 W) under stirring for 30 min. Then, other two portions of 12
(24 mg) were added and the mixture was heated at 80 ꢁC for
30 min after each addition. The reaction mixture was filtered and
concentrated under reduced pressure. The crude product was
purified by chromatography on silica gel (eluent: CH2Cl2/MeOH
10:1) to afford the title compound 13-NBoc,OBz in 61% yield
138.0(s; Cindole), 128.9 (s; Cindole), 124.7 (d; C-2indole), 123.4 (d;
CHtriazole), 122.5 (d; CHindole), 119.8 (d; CHindole), 119.6 (d; CHindole),
112.3 (d; CHindole), 111.2 (s; Cindole), 85.0 (d; C-1), 77.7 (d; C-2), 64.7
(d; C-8a), 62.6 (t; C-3), 57.0 (d; CHNH2), 55.4 (d; C-7), 49.3 (t; C-5),
35.7 (t; CH2NH), 34.2 (t; C-8), 32.2 (t; CH2CHNH2), 29.7 (t; C-6)
ppm; MS (ESI): m/z¼440 [MþH]þ, 462 [MþNa]þ, 901 [2MþNa]þ;
C22H29N7O3 (439.5): calcd C 60.12, H 6.65; N 22.31; found C 60.34,
H 7.04, N 22.51.
4.17. (1R,2R,7R,8aR)-7-{4-[({5-[(3aS,4S,6aR)-2-Oxohexahydro-
1H-thieno[3,4-d]imidazol-4-yl]pentanoyl}amino)methyl]-1H-
1,2,3-triazol-1-yl}-2-(benzoyloxy)octahydro-1-indolizinyl
benzoate (15-Bz2)
(90 mg,) as a white solid. Compound 13-NBoc,OBz: Rf: 0.26; mp
25
223e224 ꢁC; [
a
]
þ74.7 (c 0.745, CHCl3); 1H NMR (400 MHz,
D
CDCl3):
d
¼8.40 (br s, 1H, NHindole), 8.10e8.06 (m, 2H, HPh),
8.03e7.99 (m, 2H, HPh), 7.61 (d, J¼7.9 Hz, 1H, Hindole), 7.58e7.52
(m, 2H, HPh), 7.46e7.39 (m, 4H, HPh), 7.33 (d, J¼8.1 Hz, 1H, Hindole),
7.19e7.14 (m, 1H, Hindole), 7.12e7.06 (m, 2H, HindoleþHtriazole), 6.88
(d, J¼2.3 Hz, 1H, Hindole), 6.38 (br s, 1H, NH), 5.45e5.38 (m, 2H, 1-
Hþ2-H), 5.17 (br s, 1H, NH), 4.62 (br s, 1H, 7-H), 4.42e4.30 (m, 3H,
CH2NHþCH2CHNH), 3.26 (dd, J¼14.5, 4.9 Hz, 1H, CHHCHNH), 3.20
(d, J¼11.4 Hz, 1H, 3-Ha), 3.13 (dd, J¼14.5, 7.4 Hz, 1H, CHHCHNH),
3.00e2.90 (m, 1H, 5-Ha), 2.93 (dd, J¼11.4, 6.5 Hz, 1H, 3-Hb),
2.70e2.55 (m, 3H, 5-Hbþ8-Haþ8a-H), 2.30e2.13 (m, 3H, 6-Hþ8-
Azide ent-6a (141 mg, 0.347 mmol), alkyne 14 (35 mg,
0.123 mmol, see Supplementary data), H2O (0.75 mL), t-BuOH
(0.75 mL), THF (0.1 mL), copper powder (9 mg, 0.141), and copper
sulfate (39 mg, 0.246 mmol) were mixed in a microwave reaction
tube. The reaction mixture was heated at 80 ꢁC (MW irradiation
power of 100 W) under stirring for 30 min. Then, other three
portions of 14 (35 mg) were added and the mixture was heated at
80 ꢁC for 30 min after each addition. The reaction mixture was
filtered and concentrated under reduced pressure. The crude
product was purified by chromatography on silica gel (eluent:
Hb), 1.38 (s, 9H, CH3ꢃ3) ppm; 13C NMR (50 MHz, CDCl3):
¼171.7
d
(s; C]O), 166.3 (s; C]O), 166.2 (s; C]O), 155.3 (s; C]O), 144.2 (s;
Ctriazole), 136.2 (s; Cindole), 133.4 (d; CHPh), 133.2 (d; CHPh), 129.8 (d;
4C, CHPh), 129.7 (s; CPh), 129.3 (s; CPh), 128.5 (d; 2C, CHPh), 128.3 (d;
2C, CHPh), 127.5 (s; Cindole), 123.1 (d; CHindole), 122.2 (d; CHindole),
121.7 (d; CHtriazole), 119.7 (d; CHindole), 118.9 (d; CHindole), 111.2(d;
CHindole), 110.5 (s; Cindole), 82.3 (d; C-1), 80.2 (s; CMe3) 77.4 (d; C-
2), 61.5 (d; C-8a), 59.3 (t; C-3), 55.5 (d; CH2CHNH), 53.8 (d; C-7),
47.5 (t; C-5), 35.0 (t; CH2NH), 33.7 (t; C-8), 29.1 (t; C-6), 28.4 (t;
CH2Cl2/MeOH 10:1) to afford the title compound 15-Bz2 in 53%
25
yield (126.3 mg) as a waxy solid. Compound 15-Bz2: Rf: 0.28; [a]
D
ꢀ44.4 (c 0.565, CHCl3); 1H NMR (400 MHz, CDCl3):
¼8.09e8.04
d
(m, 2H, HPh), 8.04e7.99 (m, 2H, HPh), 7.71 (t, J¼5.8 Hz,1H, NH), 7.65
(s, 1H, Htriazole), 7.58e7.52 (m, 2H, HPh), 7.46e7.39 (m, 4H, HPh),
7.05 (br s, 1H, NH), 6.42 (br s, 1H, NH), 5.44e5.37 (m, 2H, 1-Hþ2-
H), 4.79e4.73 (m, 1H, 7-H), 4.52 (dd, J¼15.0, 6.0 Hz, 1H, CHHNH),
4.48 (dd, J¼7.6, 4.9 Hz, 1H, 6a00-H), 4.37 (dd, J¼15.0, 5.6 Hz, 1H,
CHHNH), 4.26 (dd, J¼7.6, 4.6 Hz, 1H, 3a00-H), 3.20 (d, J¼11.2 Hz, 1H,
3-Ha), 3.05 (dt, J¼4.6, 7.2 Hz, 1H, 400-H), 3.02e2.94 (m, 1H, 5-Ha),
2.93 (dd, J¼11.2, 6.5 Hz, 1H, 3-Hb), 2.82 (dd, J¼12.8, 4.8 Hz, 1H, 600-
Ha), 2.76e2.62 (m, 2H, 8-Haþ8a-H), 2.70 (d, J¼12.8 Hz, 1H, 600-Hb),
2.52 (dt, J¼3.1, 11.4 Hz, 1H, 5-Hb), 2.38e2.12 (m, 5H, 6-H, 8-Hb, 20-
H), 1.77e1.55 (m, 4H, 30-H, 50-H), 1.46e1.32 (m, 2H, 40-H) ppm; 13C
CH2CHNH), 28.2 (q; 3C, CH3ꢃ3) ppm; IR (CDCl3): ¼3464, 3411,
n
3054, 2980, 2933, 1716, 1672, 1491, 1316, 1281, 1114 cmꢀ1; MS (EI):
m/z (%)¼647 (MþꢀC4H8ꢀCO2, 1), 441 (1), 359 (1), 316 (1), 134 (21),
98 (45), 57 (65), 43 (100); HRMS-ESI: calcd for C41H46N7O7
[MþH]þ 748.34532; found 748.34467.
4.16. (2S)-N-({1-[(1S,2S,7S,8aS)-1,2-Dihydroxyoctahydro-7-
indolizinyl]-1H-1,2,3-triazol-4-yl}methyl)-2-amino-3-(1H-
indol-3-yl)propanamide (13)
NMR (50 MHz, CDCl3):
d
¼173.3 (s; C]O), 166.2 (s; C]O), 165.9 (s;
C]O), 164.5 (s; C]O), 144.8 (s; Ctriazole), 133.3 (d; CHPh), 133.1 (d;
CHPh), 129.8 (d; 4C, CHPh), 129.5 (s; CPh), 129.3 (s; CPh), 128.4 (d; 2C,
CHPh), 128.3 (d; 2C, CHPh), 121.9 (d; CHtriazole), 82.0 (d; C-1), 77.3
(d; C-2), 61.6 (d; C-3a00), 61.5 (d; C-8a), 60.3 (d; C-6a00), 59.3 (t; C-3),
55.7 (d; C-400), 53.9 (d; C-7), 47.5 (t; C-5), 40.6 (t; C-600), 35.7 (t; C-
20), 34.3 (t; CH2NH), 33.2 (t; C-8), 29.2 (t; C-6), 28.1 (t; C-40), 27.9 (t;
A 1% solution of thiophenol in TFA (3 mL) was added to the
protected intermediate 13-NBoc,OBz (85 mg, 0.11 mmol) at 0 ꢁC.
The mixture was stirred at rt for 40 min and concentrated under
reduced pressure. MS-ESI analysis of the residue showed the
complete removal of the Boc protecting group {13-OBz: MS-ESI:
m/z¼562 [MþNa]þ}. Ambersep 900 OH was added to the solu-
tion of the residue in MeOH (10 mL) at 0 ꢁC and the mixture was
stirred overnight at rt and then filtered. The solution was
C-50), 25.4 (t; C-30) ppm; IR (CDCl3):
n
¼3257 br, 2932, 1717, 1699,
1659, 1521, 1450, 1280, 1113 cmꢀ1; MS (ESI): m/z¼710 [MþNa]þ;
HRMS-ESI: calcd for C35H42N7O6S [MþH]þ 688.29118; found
688.29065.

F.M. Cordero et al. / Tetrahedron 67 (2011) 9555e9564
9563
ꢀ
4.18. 5-[(3aS,4S,6aR)-2-Oxohexahydro-1H-thieno[3,4-d]
imidazol-4-yl]-N-({1-[(1R,2R,7R,8aR)-1,2-
dihydroxyoctahydro-7-indolizinyl]-1H-1,2,3-triazol-4-yl}
methyl)pentanamide (15)
de Educacion y Ciencia of Spain (CTQ2004-00649/BQU; CTQ2008-
01565/BQU) and the Junta de Andalucía (FQM-345) are also
gratefully acknowledged for financial support.
Supplementary data
Ambersep 900 OH was added to a solution of 15-Bz2 (75.2 mg,
0.109 mmol) in MeOH (10 mL) and the mixture was stirred at rt for
3 h. The reaction mixture was filtered and the solution was con-
centrated under reduced pressure. Purification of the crude product
bychromatography on silica gel (eluent: CH2Cl2/MeOH 5:1) afforded
15 in 98% yield (51.3 mg) as a waxy solid. Compound 15: Rf: 0.29
Preparations of 2a, ent-2a, 4a, ent-4a, 12, and 14; 1H and 13C
NMR spectra. Supplementary data associated with this article can
be found in the online version, at doi:10.1016/j.tet.2011.10.008.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
(MeOH); [
a
]
22 þ29.9 (c 0.750, CH3OH); 1H NMR (CD3OD, 400 MHz):
D
d
¼7.94 (s,1H, Htriazole), 4.86e4.82 (obscured by water peak,1H, 7-H),
4.49 (dd, J¼7.9, 4.9 Hz, 1H, 6a00-H), 4.43 (s, 2H, CH2NH), 4.30 (dd,
J¼7.9, 4.6 Hz, 1H, 3a00-H), 3.94 (ddd, J¼7.1, 3.3, 1.4 Hz, 1H, 2-H), 3.64
(dd, J¼8.2, 3.3 Hz, 1H, 1-H), 3.19 (ddd, J¼8.7, 5.7, 4.6 Hz, 1H, 400-H),
2.95e2.88 (m, 1H, 5-Ha), 2.93 (dd, J¼12.7, 4.9 Hz, 1H, 600-Ha), 2.88
(dd, J¼10.6,1.4 Hz,1H, 3-Ha), 2.70 (d, J¼12.7 Hz,1H, 600-Hb), 2.65 (dq,
J¼13.9, 2.3 Hz, 1H, 8-Ha), 2.56 (dd, J¼10.6, 7.1 Hz, 1H, 3-Hb),
2.46e2.38 (m, 2H, 5-Hbþ6-Ha), 2.27e2.19 (m, 1H, 6-Hb), 2.24 (t,
J¼7.3 Hz, 2H, 20-H), 2.09 (ddd, J¼11.5, 8.2, 2.3 Hz, 1H, 8a-H), 1.96
(ddd, J¼13.9, 11.5, 4.2 Hz, 1H, 8-Hb), 1.78e1.53 (m, 4H, 30-Hþ50-H),
References and notes
1. Pastuszak, I.; Molyneux, R. J.; James, L. F.; Elbein, A. D. Biochemistry 1990, 29,
1886e1891.
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Brandi, A. Glycobiology 2010, 20, 500e506.
5. (þ)-Lentiginosine and (ꢀ)-lentiginosine are also indicated as
L-(þ)-lentigino-
1.48e1.36 (m, 2H, 40-H); 13C NMR (CD3OD, 50 MHz):
d
¼175.9 (s; C]
sine and
D
-(ꢀ)-lentiginosine, respectively, in consideration of the enantiomer of
O), 166.0 (s; C]O), 145.9 (s; Ctriazole), 123.7 (d; CHtriazole), 85.0 (d; C-
1), 77.6 (d; C-2), 64.8 (d; C-8a), 63.3 (d; C-3a00), 62.6 (t; C-3), 61.6 (d;
C-6a00), 57.0 (d; C-400), 55.5 (d; C-7), 49.4 (t; C-5), 41.1 (t; C-600), 36.6 (t;
C-20), 35.6 (t; CH2NH), 34.1 (t; C-8), 29.8 (t), 29.7 (t), and 29.5 (t) (C-6,
C-40, and C-50), 26.8 (t; C-30) ppm; MS (EI): m/z (%)¼325 (1), 267 (2),
239 (2), 226 (1),154 (2),143 (3),134 (6), 98 (13); HRMS-ESI: calcd for
C21H34N7O4S [MþH]þ 480.23875; found 480.23767.
the tartaric acid used as chiral precursor in most of their syntheses (see Refs.
6e8).
6. Cardona, F.; Goti, A.; Brandi, A. Eur. J. Org. Chem. 2007, 1551e1565 and refer-
ences cited therein.
7. For some other recent syntheses of (þ)-lentiginosine, see: (a) Chen, M.-J.; Tsai,
Y.-M. Tetrahedron Lett. 2007, 48, 6271e6274; (b) Alam, M. A.; Vankar, Y. D.
Tetrahedron Lett. 2008, 49, 5534e5536; (c) Lauzon, S.; Tremblay, F.; Gagnon, D.;
ꢂ
Godbout, C.; Chabot, C.; Mercier-Shanks, C.; Perreault, S.; DeSeve, H.; Spino, C. J.
Org. Chem. 2008, 73, 6239e6250; (d) Shaikh, T. M.; Sudalai, A. Tetrahedron:
Asymmetry 2009, 20, 2287e2292; (e) Cui, L.; Zhang, L.-M. Sci. China Chem. 2010,
53, 113e118; (f) Prasad, K. R.; Pawar, A. B. Arkivoc 2010, vi, 39e46.
8. For some other recent syntheses of (ꢀ)-lentiginosine, see: (a) Ayad, T.; Gen-
isson, Y.; Baltas, M. Org. Biomol. Chem. 2005, 3, 2626e2631; (b) Kim, I. S.; Zee,
O. P.; Jung, Y. H. Org. Lett. 2006, 8, 4101e4104; (c) Angle, S. R.; Bensa, D.; Be-
langer, D. S. J. Org. Chem. 2007, 72, 5592e5597; (d) Azzouz, R.; Fruit, C.; Bischoff,
L.; Marsais, F. J. Org. Chem. 2008, 73, 1154e1157; (e) Chandrasekhar, S.; Vijay-
kumar, B. V. D.; Pratap, T. V. Tetrahedron: Asymmetry 2008, 19, 746e750; (f) Liu,
S.-W.; Hsu, H.-C.; Chang, C.-H.; Gavin Tsai, H.-H.; Hou, D.-R. Eur. J. Org. Chem.
2010, 4771e4773.
9. For some lentiginosine analogs, see: (a) Muramatsu, T.; Yamashita, S.; Naka-
mura, Y.; Suzuki, M.; Mase, N.; Yoda, H.; Takabe, K. Tetrahedron Lett. 2007, 48,
8956e8959; (b) Kumar, K. S. A.; Chaudhari, V. D.; Dhavale, D. D. Org. Biomol.
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4.19. Molecular modeling
The ligand structures were built by using Maestro v8.5.28 The
protonation state for all the ligands was evaluated by means of Epik
1.629 (solvent water and pH¼7.0ꢄ0.4) in order to better simulate the
behavior at physiological conditions. All the molecules were sub-
jected to conformational search and clusterization with Macromodel
9.630 in order to sample the most accessible conformations. All the
docking calculationswere performed by usingGlide 5.0.31 The crystal
structure of glucoamylase-471 from A. awamori complexed with
1-deoxynojirimycin, available in the PDB (PDB ID: 1DOG),32 was used
in the docking calculations. This crystal structure corresponds to
a proteolytic fragment of glucoamylase G2 from A. awamori, which
has 95% sequence identity with A. niger amyloglucosidase and makes
it possible the investigation of the interaction between ligands and
the glucoamylase at atomic level. The structure 1DOG was prepared
according to the recommended Protein Preparation module in
Maestro 8.5 by using default input parameters (no scaling factors for
the van der Waals radii of non-polar protein atoms, 0.8 scaling factor
for non-polar ligand atoms). This procedure was used to remove
watermolecules(exceptfor molecule Wat501, which is considered as
part of the target structure), to assign missing hydrogen atoms, to
optimize hydrogen-bonding interactions, and to reduce structural
problems. The grids were prepared with the centerof the site defined
by the center of the complexed ligand. All the relevant conformations
forthe ligands were dockedin thebinding sitebyusingtheSPscoring
functionto score theligandposes. Afterdocking calculations, foreach
ligand, the poses with the lowest value of Emodel (docking scoring
functions) were chosen. Finally, a post-docking minimization was
applied to the selected poses in order to remove ligand strains.
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C-7 of the corresponding indolizidinol 4 (R¼Bz) was incorrectly assigned (see
Supplementary data).
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Acknowledgements
The Ministry of Research and University (MiUR-Rome, Italy) is
acknowledged for financial support (PRIN project). The Ministerio

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a
-
L
-Fucosidase from bovine kidney,
lactosidase from Escherichia coli and from Aspergillus oryzae,
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dase from 15-almonds, -mannosidase from Jack beans, -mannosidase from
snail, -xylosidase from A. niger, -N-acetylglucosaminidase from Jack beans.
a
-galactosidase from coffee beans,
-glucosidase
-glucosi-
b-ga-
a
b
a
b
b
b
The experiments were performed essentially as previously described: (a) Saul,
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