Total synthesis of (-)-uniflorine A

Article abstract of DOI:10.1021/np900435d

Total synthesis of (-)-uniflorine A (3) has been accomplished in nine steps and 11% overall yield from carbohydrate-based nitrone 5. The key steps of the synthetic strategy were a high regio- and complete stereoselective 1,3-dipolar cycloaddition of alken

Full text of DOI:10.1021/np900435d

2058
J. Nat. Prod. 2009, 72, 2058–2060
Total Synthesis of (-)-Uniflorine A⊥
Camilla Parmeggiani, Daniele Martella, Francesca Cardona, and Andrea Goti*
Dipartimento di Chimica Organica “Ugo Schiff”, Laboratorio di Progettazione, Sintesi e Studio di Eterocicli Biologicamente AttiVi
(HeteroBioLab), UniVersita` di Firenze, Via della Lastruccia 13, I-50019 Sesto Fiorentino (FI), Italy
ReceiVed July 17, 2009
Total synthesis of (-)-uniflorine A (3) has been accomplished in nine steps and 11% overall yield from carbohydrate-
based nitrone 5. The key steps of the synthetic strategy were a high regio- and complete stereoselective 1,3-dipolar
cycloaddition of alkene 6 with nitrone 5, a Tamao-Fleming reaction for replacing the silicon substituent with a hydroxy
group with retention of configuration, and a Mitsunobu reaction to establish the correct configuration of the target
molecule at C-6.
The leaves of Eugenia uniflora L. (Myrtaceae), an evergreen tree
widely distributed in Paraguay, Uruguay, Argentina, and Brazil,
furnish a natural Paraguayan medicine named Nangapiry. Infusions
prepared from Nangapiry are used in folk medicines as antidiarrheic,
diuretic, antirheumatic, antifebrile, and antidiabetic preparations.1
Water-soluble extracts obtained from leaves of E. uniflora were
found to inhibit the R-glucosidases maltase and sucrase, thus
accounting for the antidiabetic effect induced by Nangapiry
preparations.1 Two iminosugar alkaloids have been isolated from
the water-soluble material, namely, uniflorines A and B, for which
the structures of pentahydroxyindolizidines 1 and 2, respectively,
were proposed on the basis of NMR analyses. Compounds 1 and
2 were found to inhibit rat intestinal maltase (IC50 values of 12
and 4.0 µM, respectively) and sucrase (IC50 values of 3.1 and 1.8
µM, respectively).1
In 2004, Pyne and co-workers reported the total synthesis of
putative uniflorine A (1) from L-xylose.2 However, the NMR
spectroscopic data for synthetic 1 did not match those reported for
natural uniflorine A, thus proving that the original structural
intermediate dihydroxy pyrrolidine derivative as key steps (Scheme
1).11 The pyrrolizidine obtained ent-3 ([R]22D +6.6 (c 0.35, H2O))
(lit.1 for (-)-uniflorine A, [R]D -4.4 (c 1.2, H2O)) displayed
spectroscopic data in good agreement with those of natural
assignment to uniflorine A was incorrect. Other total syntheses of
1 from the group of Dhavale3 and of diastereoisomers of 14-7
uniflorine A, thus confirming the postulated structural assignment
showed that the NMR data of 1,2,6,7,8-pentahydroxyindolizidines
as 6-epi-casuarine (3). Herein we report the first total synthesis of
(-)-uniflorine A (3) via inversion of configuration of the OH group
differ significantly from those of the natural product named
uniflorine A. Pyne and co-workers also recognized that the NMR
at C-6 achieved through a Mitsunobu reaction with benzoic acid
data reported for uniflorine B and its optical rotation were in good
agreement with those of the known alkaloid casuarine 4,6 a known
on a key intermediate previously synthesized in our recent synthesis
of casuarine (4).9,12
inhibitor of R-glucosidases.8,9 Thus, they suggested that uniflorine
The synthesis began with a highly regio- and completely
stereoselective cycloaddition of the nitrone 513 with the alkene 6
to yield the isoxazolidine 7 (Scheme 2).9 Cleavage of the N-O
bond and protection of the OH group at C-6 as an acetate afforded
the lactam 8, which was used in the Tamao-Fleming reaction, in
order to convert the silicon group into an OH group with complete
retention of configuration, to give the lactam 9, which was
orthogonally protected at C-7 as a benzyloxy derivative and then
deprotected at C-6 to liberate the OH in 10 (Scheme 2).9 The
intermediate 10 bears a free OH group at C-6, which can be further
manipulated independently from the other OH groups. For instance,
it could be suitably employed for the first total synthesis of
casuarine-6-O-R-glucoside.9
In order to access (-)-uniflorine A, a Mitsunobu reaction was
performed on 10 using THF as solvent, benzoic acid (BzOH) as a
nucleophile, triphenylphosphine, and, alternatively, diisopropyla-
zodicarboxylate (DIAD) or diethylazodicarboxylate (DEAD). Reac-
tion of 10 with 1.2 equiv of BzOH, 1.2 equiv of PPh3, and 1.2
equiv of DIAD at room temperature for 2 h allowed no conversion
of starting material. When the mixture was heated at 50 °C for
4 h, a complex mixture was obtained, but isolation of the desired
product was unsuccessful. The use of DEAD rather than DIAD
B was actually the known alkaloid casuarine and that uniflorine A
could possess a similar 1,2,6,7-tetrahydroxy-3-hydroxymethylpyr-
rolizidine structure, most likely being 6-epi-casuarine 3, on the basis
of the greatest difference of the chemical shifts at C-6 in the NMR
spectra of uniflorines A and B.6
As a part of our program concerning the synthesis of polyhy-
droxylated indolizidine and pyrrolizidine alkaloids,10 and in order
to shed light on the uniflorine issue, we decided to embark on the
total synthesis of the compound proposed to be (-)-uniflorine A
(3) by Pyne, namely, 6-epi-casuarine, by exploiting a strategy that
allowed the recent total synthesis of casuarine (4).9 While this work
was in progress, Pyne and Ritthiwigrom reported a total synthesis
of the non-natural enantiomer of 3, (+)-uniflorine A, which
supported their structural hypothesis.11 Their synthesis started from
D-xylose and involved a boronic acid-Mannich reaction (Petasis
reaction) and a ring-closing metathesis (RCM), followed by
osmium-catalyzed syn-dihydroxylation and cyclization of the
⊥ Dedicated to Professor Francesco De Sarlo on the occasion of his 70th
birthday.
* To whom correspondence should be addressed. Tel: +39 0554573505.
Fax: +39 0554573531. E-mail: andrea.goti@unifi.it.
10.1021/np900435d CCC: $40.75  2009 American Chemical Society and American Society of Pharmacognosy
Published on Web 10/16/2009

Notes
Journal of Natural Products, 2009, Vol. 72, No. 11 2059
Scheme 1
Figure 1. Compound 11. Irradiation of H-6 gave a NOE at H-7,
and irradiation of H-7 gave a NOE at H-1.
Scheme 2
Figure 2. Compound 3. Irradiation of H-6 gave NOEs at H-7 and
H-1, and irradiation of H-7 gave a NOE at H-1.
3, as shown in Figure 2. Indeed, irradiation of H-6 gave NOEs at
H-7 and H-1, and irradiation of H-7 gave a NOE at H-1.
The 1H NMR spectra (D2O) of 3 and those of natural uniflorine
A were essentially identical (∆δ ) 0.01-0.13 ppm, see Supporting
Information, Table 1). All of the 13C NMR signals of 3 (in D2O
with MeCN as an internal reference at δ 1.47) were 1.6-3.0 ppm
upfield compared to those reported for the natural product (see
Supporting Information); however, a similar difference had been
previously noted for synthetic (+)-uniflorine by Pyne and co-
workers, who attributed this systematic error to a different reference
(not reported) in the spectrum of the isolated natural product. Indeed,
the 13C NMR chemical shifts obtained for the compound synthesized
by us were in good agreement with those reported for ent-3
synthesized by Pyne (∆δ ) (1 ppm, see Supporting Information,
Table 1).11 The specific optical rotation of synthetic 3 ([R]21D -6.9
(H2O, c 0.42)) was essentially identical to that reported for synthetic
ent-3 ([R]21D + 6.6 (H2O, c 0.35)) and matched in sign that of the
Scheme 3a
natural product ([R]21 -4.4 (H2O, c 1.2)). Further confirmation
D
that natural (-)-uniflorine A is actually 6-epi-casuarine (3) was
furnished by the melting point of synthetic 3 (177-180 °C), which
was a near match to that reported for natural (-)-uniflorine A
(174-178 °C).1
In conclusion, we report the first total synthesis of (-)-uniflorine
A in nine steps and 11% overall yield from the carbohydrate-derived
nitrone 5 using an efficient and straightforward synthetic strategy
that employed a 1,3-dipolar cycloaddition reaction, a Tamao-Fleming
reaction, and a Mitsunobu reaction as key steps. These results
confirm Pyne’s hypothesis on the identity of natural (-)-uniflorine
A with 6-epi-casuarine (3).
a Reagents and conditions: (a) BzOH (1.2 equiv), PPh3 (1.2 equiv), DIAD
(1.2 equiv), THF, rt (75%); (b) LiAlH4 (4 equiv), THF, reflux (45%); (c) H2,
10% Pd/C, MeOH, HCl, rt then Dowex 50WX8, 6% NH4OH (71%).
Experimental Section
General Experimental Procedures. Commercial reagents were used
as received. All reactions were magnetically stirred and monitored by TLC
on 0.25 mm silica gel plates (Merck F254). Column chromatography was
carried out on silica gel 60 (32-63 µm). Yields refer to spectroscopically
and analytically pure compounds unless otherwise stated. NMR spectra
were recorded on Varian Mercury-400, Varian INOVA-400 (1H, 400 MHz;
13C, 100 MHz), or Varian Gemini-200 (13C, 50 MHz) spectrometers.
Infrared spectra were recorded with a Perkin-Elmer Spectrum BX FT-IR
System spectrophotometer. Mass spectra were recorded by direct inlet on
a ThermoScientific LCQ Fleet ion trap spectrometer with a Surveyor Plus
LC System; relative percentages are shown in brackets. ESI full MS were
recorded on a Thermo LTQ instrument by direct inlet; relative percentages
are shown in brackets. Elemental analyses were performed with a Perkin-
Elmer 2400 analyzer. Optical rotation measurements were performed on
a JASCO DIP-370 polarimeter.
and the same reaction conditions for 18 h gave compound 11 in
63% yield. However, the best yield (75% of isolated product after
purification on silica gel) was achieved performing the reaction with
DIAD at room temperature for 18 h (Scheme 3). Inversion of
configuration at C-6 was confirmed by 1D NOESY experiments
performed on compound 11. Indeed, irradiation of H-6 gave NOE
enhancement at H-7, and irradiation of H-7 gave a NOE at H-1
(Figure 1).
Concomitant reduction of the CdO bond and deprotection of
the OH group at C-6 with LiAlH4 in refluxing THF gave compound
12 in 45% yield. Finally, catalytic hydrogenation of 12 using 10%
Pd on activated carbon in methanol in the presence of 0.1 mL of
concentrated HCl (37%), followed by absorption on an acid resin
and elution of the free base with 6% aqueous ammonium hydroxide,
afforded (-)-uniflorine A (3) in 71% yield (Scheme 3). The
observed NOEs were consistent with the configuration assigned to
(1R,2R,3R,6S,7S,7aS)-1,2,7-Tri(benzyloxy)-3-[(benzyloxy)methyl]-
5-oxo-hexahydro-5H-pyrrolizin-6-yl Benzoate (11). Triphenylphos-
phine (68 mg, 0.26 mmol) and benzoic acid (31.7 mg, 0.26 mmol)

2060 Journal of Natural Products, 2009, Vol. 72, No. 11
Notes
were added to a solution of 10 (125 mg, 0.216 mmol) in dry THF (4
mL); then DIAD (51 µL, 0.26 mmol) was added at 0 °C. The mixture
was stirred at rt for 18 h; then the solvent was removed at reduced
pressure and the crude product purified by FCC (eluent petroleum ether/
Acknowledgment. We thank MiUR and Ente Cassa di Risparmio
di Firenze, Italy, for financial support. Ente CRF is also acknowledged
for granting a 400 MHz NMR spectrometer. Brunella Innocenti and
Maurizio Passaponti (Dipartimento di Chimica Organica “U. Schiff”)
are acknowledged for technical assistance. CINMPIS is gratefully
acknowledged for a grant to C.P.
EtOAc, 5:1, then 1:1): 111 mg, 0.162 mmol, 75%; [R]28 -55.6 (c
D
0.25, CH2Cl2); IR (CDCl3) cm-1 1726; 1H NMR (CDCl3) δ 8.00-7.98
(m, 2H), 7.52-7.48 (m, 1H), 7.37-7.33 (m, 2H), 7.28-7.13 (m, 20H,
Ar), 5.63 (d, J ) 5.4 Hz, H-6), 4.60 (d, J ) 11.2 Hz, 1H, Bn),
4.53-4.29 (m, 7H, Bn), 4.23-4.15 (m, 2H, H-2 + H-3), 4.01 (dd, J
) 6.8, 5.4 Hz, 1H, H-7a), 3.90 (dd, J ) 6.8, 5.4 Hz, 1H, H-7), 3.70
Note Added after ASAP Publication: The version of this paper
published on October 16, 2009, had refs 12 and 13 mislabeled. The
corrected version was published on October 21, 2009.
(dd, J ) 5.4, 3.4 Hz, 1H, H-1), 3.63-3.52 (m, 2H, Ha-8 + Hb-8); 13
C
Supporting Information Available: Comparison of 1H and 13C
NMR data for synthetic and isolated (-)-uniflorine A and synthetic
(+)-uniflorine A (Table 1). Synthetic procedures and characterization
for compounds 8, 9, and 10. Copies of 1H, 13C NMR and 1D NOESY
spectra for compounds 11 and 3 and 1H and 13C NMR spectra for
compounds 8, 9, 10, and 12. This material is available free of charge
via the Internet at http://pubs.acs.org.
NMR (CDCl3) δ 168.4 (s), 165.1 (s, C-5), 137.6-136.5 (s, 5C), 133.0
(d, 1C), 139.7-128.9 (d, 2C), 128.1-127.2 (d, 22C), 85.4 (d, 2C, C-1
+ C-2), 77.8 (d, C-7), 72.9-71.5 (t, Bn), 72.3 (d, C-6), 68.4 (d, C-7a),
67.9 (t, C-8), 59.2 (d, C-3); MS (ESI) m/z 706 (100, [M + Na]+);
anal. C, 75.16%; H, 6.37%; N, 1.95%, calcd for C43H41NO7, C, 75.53%;
H, 6.04%; N, 2.05%.
(1R,2R,3R,6S,7S,7aS)-1,2,7-Tri(benzyloxy)-3-[(benzyloxy)meth-
yl]hexahydro-1H-pyrrolizin-6-ol (12). LiAlH4 (25 mg, 0.66 mmol)
was added to a solution of 11 (111 mg, 0.162 mmol) in dry THF (3
mL). The mixture was stirred at reflux under nitrogen atmosphere for
5 h; then 0.30 mL of a saturated solution of Na2SO4 was added dropwise
at room temperature. The resulting mixture was filtered through Celite
and washed with EtOAc, and then the solvent was evaporated.
Purification of the crude product by FCC (eluent petroleum ether/
EtOAc, 1:1) gave pure 12 as a pale yellow oil (42 mg, 0.074 mmol,
References and Notes
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Arai, I.; Amagaya, S.; Komatsu, Y. Pharm. Biol. 2000, 38, 302–307.
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(3) Karanjule, N. S.; Markad, S. D.; Dhavale, D. D. J. Org. Chem. 2006,
71, 6273–6276.
(4) 1-epi-1 and 1,2-di-epi-1: Zhao, Z.; Song, L.; Mariano, P. S. Tetrahe-
dron 2005, 61, 8888–8894.
(5) 1,2-Di-epi-1: Bell, A. W.; Pickering, L.; Watson, A. A.; Nash, R. J.;
Griffiths, R. C.; Jones, M. G.; Fleet, G. W. J. Tetrahedron Lett. 1996,
37, 8561–8564.
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Tetrahedron 2008, 64, 4868–4879.
(7) 8a-epi-1 and 1,2,8a-tri-epi-1: see ref 3.
(8) Asano, N.; Oseki, K.; Kaneko, E.; Matsui, K. Carbohydr. Res. 1994,
258, 255–266.
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45%): [R]22 -6.0 (c 0.25, CH2Cl2); IR (CDCl3) cm-1 3689, 3605,
D
2956, 2926, 2854, 1732, 161, 1457, 1378; 1H NMR (CDCl3) δ
7.35-7.18 (m, 20H, Ar), 4.62-4.44 (m, 8H, Bn), 4.29-4.26 (m, 1H,
H-6), 4.03 (pseudo t, J ) 4.9 Hz, 1H, H-2), 3.89 (t, J ) 4.2 Hz, 1H,
H-1), 3.83 (dd, J ) 6.9, 4.6 Hz, 1H, H-7), 3.62 (m, 1H, H-7a),
3.57-3.49 (m, 2H, H-8), 3.33-3.30 (m, 1H, Ha-5), 3.03-2.99 (m,
2H, H-3 + Hb-5); 13C NMR (CDCl3) δ 137.6-137.1 (s, 4C, Ar),
128.3-127.4 (d, 20C, Ar), 85.4 (d, 2C, C-1 + C-2), 82.5 (d, C-7),
73.0-71.3 (5C, 4Bn + C-8), 70.5 and 70.3 (d, 2C, C-6 + C-7a), 69.8
(d, C-3), 60.2 (t, C-5); MS (ESI) m/z 566 (100, [M + H]+); anal. C,
76.40%; H, 7.37%; N, 2.75%, calcd for C36H39NO5, C, 76.43%; H,
6.95%; N, 2.48%.
(1R,2R,3R,6S,7S,7aS)-3-(Hydroxymethyl)hexahydro-1H-pyrrolizin-
1,2,6,7-tetraol ((-)-uniflorine A, 3). HCl 37% (0.1 mL) and 10% Pd
on C (33 mg) were added to a solution of 12 (35 mg, 0.062 mmol) in
MeOH (4 mL). The suspension was stirred under H2 atmosphere for 4
days, then filtered through Celite and washed with MeOH. Evaporation
of the solvent under reduced pressure afforded crude 3, which was
transferred to a column of Dowex 50WX8 and washed with MeOH
(10 mL) and H2O (10 mL) to remove nonbasic impurities, then with
6% NH4OH (15 mL) to elute (-)-uniflorine A (3) as a yellowish solid
(10) Brandi, A.; Cardona, F.; Cicchi, S.; Cordero, F. M.; Goti, A.
Chem.sEur. J. 2009, 15, 7808–7821.
(11) Ritthiwigrom, T.; Pyne, S. G. Org. Lett. 2008, 10, 2769–2771.
(12) For other syntheses of casuarine, see: (a) Denmark, S. E.; Hurd, A. R.
Org. Lett. 1999, 1, 1311–1314. (b) Denmark, S. E.; Hurd, A. R. J.
Org. Chem. 2000, 65, 2875–2886. (c) Izquierdo, I.; Plaza, M. T.;
Tamayo, J. A. Tetrhedron 2005, 61, 6527–6533. (d) Van Ameijde, J.;
Horne, G.; Wormald, M. R.; Dwek, R. A.; Nash, R. J.; Jones, P. W.;
Evinson, E. L.; Fleet, G. W. J. Tetrahedron: Asymmetry 2006, 17,
2702–2712. (e) Bell, A. A.; Pickering, L.; Watson, A. A.; Robert,
J. N.; Pan, Y. T.; Elbein, A. D.; Fleet, G. W. J. Tetrahedron Lett.
1997, 38, 5869–5872.
(13) Cardona, F.; Faggi, E.; Liguori, F.; Cacciarini, M.; Goti, A. Tetrahe-
dron Lett. 2003, 44, 2315–2318. (b) Carmona, A. T.; Wightman, R. H.;
Robina, I.; Vogel, P. HelV. Chim. Acta 2003, 86, 3066–3073. (c)
Desvergnes, S.; Py, S.; Valle´e, Y. J. Org. Chem. 2005, 70, 1459–
1462. (d) Revuelta, J.; Cicchi, S.; Goti, A.; Brandi, A. Synthesis 2007,
4, 485–504. (e) Cicchi, S.; Marradi, M.; Vogel, P.; Goti, A. J. Org.
Chem. 2006, 71, 1614–1619. (f) Tsou, E.-L.; Yeh, Y.-T.; Liang, P.-
H.; Cheng, W.-C. Tetrahedron 2009, 65, 93–100.
(9 mg, 0.044 mmol, 71%): mp 177-180 °C; [R]21 -6.9 (c 0.415,
D
1
H2O); H NMR (D2O) δ 4.23 (q, J ) 5.2 Hz, 1H, H-6), 4.08 (pseudo
t, J ) 4.7 Hz, 1H, H-7), 3.83 (t, J ) 7.4 Hz, 1H, H-1), 3.71 (dd, J )
8.8, 7.6 Hz, 1H, H-2), 3.65 (dd, J ) 11.9, 3.7 Hz, 1H, Ha-8), 3.50 (dd,
J ) 11.9, 6.4 Hz, 1H, Hb-8), 3.08 (dd, J ) 7.2, 5.0 Hz, 1H, H-7a),
2.97 (dd, J ) 12.1, 5.4 Hz, 1H, Ha-5), 2.89 (dd, J ) 12.1, 5.0 Hz, 1H,
Hb-5), 2.70 (ddd, J ) 9.1, 6.4, 3.7 Hz, 1H, H-3); 13C NMR (D2O) δ
78.7 (d, C-1), 77.5 (d, C-2), 75.7 (d, C-7), 72.0 (d, C-6), 71.8 (d, C-7a),
70.9 (d, C-3), 62.3 (t, C-8), 58.1 (t, C-5); MS (ESI) m/z 206 (100, [M
+ H]+), 158 (10), 132 (13), 118 (22), 95 (30), 79 (73), 55 (17); anal.
C, 46.85%; H, 7.68%; N, 7.11%, calcd for C8H15NO5, C, 46.82%; H,
7.37%; N, 6.83%.
NP900435D