Synthesis and use of new C-glycosyl bicyclic lactones

Article abstract of DOI:10.1016/j.tetlet.2015.07.017

Abstract New C-glycosyl bicyclic lactones have been synthesized in three steps from glycosyl bromides and used as synthons towards C-glycosyl derivatives which can readily be elaborated to 1,2-bisfunctionalized carbohydrate systems. The method was illustrated with several examples of pseudo conjugates prepared from the new C-glycosyl 1,2-fused pyrano-δ-lactonic building blocks, either with groups such as allyl or propargyl with wide subsequent reactivity, or with more elaborated moieties leading to model glycoaminoacids or pseudo nucleotide sugars.

Full text of DOI:10.1016/j.tetlet.2015.07.017

Tetrahedron Letters 56 (2015) 5051–5053
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and use of new C-glycosyl bicyclic lactones
à
⇑
Arkadiusz Listkowski , Otman Otman , Stéphane Chambert, Yves Queneau
Université de Lyon, Institut de Chimie et Biochimie Moléculaires et Supramoléculaires, ICBMS, UMR5246, CNRS, Université Lyon 1, INSA-Lyon, CPE Lyon, INSA Lyon, Bât. Jules Verne,
20 Avenue A. Einstein, Villeurbanne F-69621, France
a r t i c l e i n f o
a b s t r a c t
Article history:
New C-glycosyl bicyclic lactones have been synthesized in three steps from glycosyl bromides and used
as synthons towards C-glycosyl derivatives which can readily be elaborated to 1,2-bisfunctionalized car-
bohydrate systems. The method was illustrated with several examples of pseudo conjugates prepared
from the new C-glycosyl 1,2-fused pyrano-d-lactonic building blocks, either with groups such as allyl
or propargyl with wide subsequent reactivity, or with more elaborated moieties leading to model gly-
coaminoacids or pseudo nucleotide sugars.
Received 25 June 2015
Revised 4 July 2015
Accepted 7 July 2015
Available online 10 July 2015
Keywords:
Carbohydrate
Lactones
Ó 2015 Elsevier Ltd. All rights reserved.
Synthons
C-Glycosides
Introduction
reported,7 though not with the idea to serve as scaffolds. Some of
them have been used as intermediates towards complex structures
1,2-Annulated carbohydrate substructures are found in many
natural products or serve as synthons towards very diverse com-
plex carbohydrate-based systems.1 Those which include a lactone
function can, like carbohydrate-based lactones in general,2 serve
as building blocks towards a wide range of conjugates with appli-
cations in biological or material sciences. Several families of bicyc-
lic systems have been reported, arising from diverse carboxylic
acid containing sugars and involving or not the anomeric position
(Fig. 1).3 One advantage of the bicyclic systems is to keep the sugar
backbone in its closed form while the lactone is used for connec-
tion with the other moiety of the final target by reaction with a
nucleophilic species. The extension of this strategy to form C-gly-
cosyl systems is however much less documented, though C-glyco-
syl variants of glycoconjugates are very interesting compounds,
being insensitive to acid or enzymatic glycosidic bond cleavage
and therefore attractive targets for biological or medicinal pur-
poses.4 C-Glycosyl derivatives can also show interesting properties
in the field of surfactants.5 Apart from aromatic fused systems aris-
ing from C-glycosyl ortho-carboxy arenes, such as the naturally
occurring bergenin and its analogues6 only rare examples of simple
lactones with a C-propanoyl appendage have been previously
by carbon–carbon bond formation on the carbonyl group of the lac-
tone, either by reaction of an intermediate vinyl triflate with a
O
OH
OH
O
O
HO
HO
OH
O
O
OMe
O
OH
HO
O
OH
OH
O
O
galacturonic acid
3,6-lactone
(+)-altholactone
bergenin
HO
O
OBn
O
O
BnO
BnO
HO
O
O
O
Ph
OR
O
O
BnO
O
OMe
Lecourt et al.3e
O
O
Linker et al.3b,c
Rauter et al.3d
OAc
O
OH
O
AcO
AcO
HO
HO
Ph
CO2Me
O
O
O
-CMGL
O
N
H
O
O
Queneau et al.8-12
Dubreuil et al.3h
OBn
H
H
H
H
O
BnO
MeO
O
O
O
O
OTBS
OTBS
O
O
O
H
O
⇑
Corresponding author. Tel.: +33 47243 6169; fax: +33 47243 8896.
E-mail address: yves.queneau@insa-lyon.fr (Y. Queneau).
OBn
Nicolaou et al.7b
Trost et al.7d
Kozikowski et al.7e
Present address: Faculty of Mathematics and Natural Sciences, The Cardinal
Stefan Wyszynski University, Dewajtis 5, 01-815 Warsaw, Poland.
Present address: Department of Chemistry, Faculty of Science, Sebha University,
PO Box: 18758, 71-Sebha, Libya.
Figure 1. Examples of naturally occurring or synthetic carbohydrate-based bicyclic
lactones.
à
http://dx.doi.org/10.1016/j.tetlet.2015.07.017
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

5052
A. Listkowski et al. / Tetrahedron Letters 56 (2015) 5051–5053
borane7b (the IJ fragment on the way to the GHIJKLMNO ring
system of maitotoxin) or by reaction with a Grignard reagent.7e
We have reported earlier that carboxymethyl glycoside
lactones (O-CMGLs,8 Fig. 1) are a family of bicyclolactonic synthons
offering access to various types of pseudo conjugates such as
glycoaminoacids hybrids,9 pseudo-disaccharides,10 pseudo-glycol-
ipids,11 several biologically relevant compounds12 including non-
ionic membrane imaging probes13 and functional glycopolymers.14
The OH-2 group of the sugar moiety, involved in the lactone func-
tion, is released upon lactone opening, and can subsequently be
functionalized leading in a short sequence to 1,2-bisfunctionalized
carbohydrate derivatives.8d,11b,c,12c,14b
Considering the readiness of the method and the importance of
C-glycosyl derivatives in general, we have explored its extension to
C-glycosyl targets, and we report hereafter our preliminary results
on the synthesis and the reactivity of new C-glycosyl bicyclic
lactones.
R2
R2
OAc
O
OAc
O
R1
AcO
R1
BnNH2
O
AcO
O
HO
X
X
NHBn
13: X = O, R1 = OAc, R2 = H
lactone opening
O
relative rate
1
11: X = O, R1 = OAc, R2 = H
3:
5:
X = CH2, R1 = OAc, R2 = H
X = CH2, R1 = OAc, R2 = H
0.12
1
0.2
12: X = O, R1
=
H, R2 = OAc
14: X = O, R1 = H, R2 = OAc
6: X = CH2, R1 = H, R2 = OAc
4: X = CH2, R1 = H, R2 = OAc
Figure 3. Compared rates for the opening of C-glycosyl versus O-glycosyl lactones
with benzylamine in the gluco and galacto series.
given by 1H chemical shifts of H-2 in the range of 4 ppm or lower,
whereas acetylated ones are over 4.2 ppm.
The case of benzylamine was used for comparing the reactivity
of the C- and the O-glycosyl lactones. The rate of the lactone open-
ing reaction was measured by 1H NMR spectroscopy, following the
disappearance of the H-3 signal of the starting lactone 3 and the
appearance of H-3 in amide 5. By comparing with the same reac-
tion using the O-glycosyl lactone 11, it was found that the opening
of the C-glycosyl lactones was ca. 8-fold slower (full data shown in
Supplementary material). The same measurements performed in
the galacto series (rate of formation of benzylamides 6 and 14 from
lactones 4 and 12) showed consistent results, though with a nar-
rower difference (5-fold) in the rates (Fig. 3). These rate differences
are difficult to fully interpret being possibly related to several
parameters (inductive effect of the oxygen atom versus its involve-
ment in the anomeric linkage, chair or partially twisted conforma-
tion of the lactone cycle), however these results suggest that, even
though less reactive than their O-glycosyl counterparts, the C-gly-
cosyl lactones exhibited sufficient reactivity to serve as promising
scaffolds.
The interest of the method for accessing more elaborated tar-
gets is illustrated in Figure 4, first with the preparation of conju-
gates by reaction of the new lactones with aspartic acid and
aminodeoxyuridine, leading to C-glycosyl glycoaminoacid hybrids
15 and 16 in 84 and 90% yields respectively, and C-glycosyl nucleo-
tide-sugar analogue 17. Aminodeoxyuridine, having two unpro-
tected OH groups, could also be used, leading to amide 18 in 63%
yield. In these cases, although no side product was clearly identi-
fied, we can hypothesise that the lower yields are due to compet-
itive transamidification of the 6-OAc group of the starting lactone,
as observed in the reaction of the O-glycosyl lactone with an
aminosugar.10 Finally, the ability for the C-glycosyl adducts to be
further functionalized at OH-2 was illustrated by the transforma-
tion of the galacto allylamide 10 and the gluco allylamide 9 to
the corresponding 2-carbamates, using octyl or dodecyl isocyanate,
Results and discussion
The C-glycosyl lactones were prepared in three steps from
peracetylated glucosyl or galactosyl bromides. Intermediate
C-glycosyl propanoates 1 and 2 [methyl 3-(3,4,6-tri-O-acetyl-a-D-
gluco- and galactopyranosyl)-propanoate]15 were obtained in 88%
and 74% yields, respectively, by anomeric radical addition to
methyl acrylate using Giese–Praly conditions.16 Applying a saponi-
fication-acetylation sequence led to the desired new C-CMGLs 3
(gluco) and
4 (galacto) in 70% and 68% yields, respectively
(Scheme 1). The carboxylate group released in the intermediate
C-glycosyl is first to undergo acylation forming a mixed anhydride
(not isolated) which is readily transformed into a lactone ring by
intramolecular reaction with OH-2, while OH-3, 4 and 6 are also
acetylated. The new lactones 3 and 4 were fully characterized,17,18
and exhibited a very typical 18 Hz geminal coupling constant for
the two hydrogen atoms of the CH2 next to the lactone function
indicating a rather rigid bicyclic lactone ring conformation, as pre-
viously found in the O-glycosyl systems.8
A short preliminary investigation of the ability of the new C-gly-
cosyl lactones to serve as carbohydrate ligation synthons was then
performed by studying their reaction with three simple amines,
benzylamine, allylamine, propargylamine, these two latter offering
wide potential subsequent uses.19 The expected amides 5–10, hav-
ing OH-2 free and all three other positions acetylated were
obtained in 88–94% yields (Fig. 2). The resulting amides exhibited
NMR shifts and coupling constants which suggest a close confor-
mational similarity with their O-glycoside counterparts previously
reported.8,9 Notably, clear evidence for the unprotected OH-2 is
R2
OAc
O
R2
OAc
O
O
1. NaOH
OAc
O
R2
R1
AcO
R1
MeOH-H2O
OAc
O
R1
NH
AcO
AcO
AcO
O
AcO
O
O
2. Ac2O, Py
O
AcO
N
O
HO
OMe
HO
O
N
H
H
NH
O
3:
1:
R1 = OAc, R2 = H
4: R1 = H, R2 = OAc
R
1 = OAc, R2 = H (gluco)
2: R1 = H, R2 = OAc (galacto)
CO2Me
MeO2C
RO
OR
15:
17:
R = OTBDMS
18: R = H
R1 = OAc, R2 = H
Scheme 1. Synthesis of the new C-glycosyl lactones.
16: R1 = H, R2 = OAc
OAc
O
AcO
AcO
OAc
O
R2
R1
AcO
19
OAc
O
R2
R2 OAc
O
O
OAc
O
AcO
AcO
O
R1
AcO
O
R1
AcO
O
O
NH
O
O
O
HO
NH
HO
HO
O
NH
NH
20
HN
NHBn
9: R1 = OAc, R2 = H
10: R1 = H, R2 = OAc
HN
7: R1 = OAc, R2 = H
8: R1 = H, R2 = OAc
5:
6:
R
R
1 = OAc, R2 = H
1 = H, R2 = OAc
Figure 4. Examples of C-glycosyl pseudoglycoconjugates and disubstituted systems
from C-glycosyl lactones.
Figure 2. C-Glycosyl amides from lactones benzyl, propargyl and allyl-amines.

A. Listkowski et al. / Tetrahedron Letters 56 (2015) 5051–5053
5053
Deshpande, P. P.; Cutler, A. B.; Kane, S. A.; Rao, S. P. Carbohydr. Res. 1990, 196,
41–58.
respectively. Thus, the 1,2-disubstituted derivatives 19 and 20,
bearing unsaturated and hydrophobic groups as functional appen-
dages, were obtained in 58% and 60% yields, respectively.
In conclusion, we have described the synthesis and the use of
new C-glycosyl 1,2 annulated lactones which can serve as carbohy-
drate donating systems for the preparation, under mild conditions
and in fair to good yields, of mono or disubstituted C-glycosyl
pseudo-conjugates. The method offers straightforward access to
derivatives with relevance to biological or materials sciences.
7. (a) Yoda, H.; Nakaseko, Y.; Takabe, K. Tetrahedron Lett. 2004, 45, 4217–4220; (b)
Nicolaou, K. C.; Frederick, M. O.; Burtoloso, A. C. B.; Denton, R. M.; Rivas, F.;
Cole, K. P.; Aversa, R. J.; Gibe, R.; Umezawa, T.; Suzuki, T. J. Am. Chem. Soc. 2008,
130, 7466–7476; (c) Nicolaou, K. C.; Cole, K. P.; Frederick, M. O.; Aversa, R. J.;
Denton, R. M. Angew. Chem., Int. Ed. 2007, 46, 8875–8879; (d) Trost, B. M.; Rhee,
Y. H. J. Am. Chem. Soc. 2002, 124, 2528–2533; (e) Kozikowski, A. P.; Ghosh, A. K.
J. Org. Chem. 1985, 50, 3019–3022.
8. (a) Trombotto, S.; Bouchu, A.; Descotes, G.; Queneau, Y. Tetrahedron Lett. 2000,
41, 8273–8277; (b) Pierre, R.; Chambert, S.; Alirachedi, F.; Danel, M.;
Trombotto, S.; Doutheau, A.; Queneau, Y. C. R. Chimie 2008, 11, 61–66; (c)
Listkowski, A.; Ing, P.; Cheaib, R.; Chambert, S.; Doutheau, A.; Queneau, Y.
Tetrahedron: Asymmetry 2007, 18, 2201–2210; (d) Cheaib, R.; Listkowski, A.;
Chambert, S.; Doutheau, A.; Queneau, Y. Tetrahedron: Asymmetry 2008, 19,
1919–1933.
Acknowledgments
Financial support from CNRS and Ministère de l’Education
Nationale, de l’Enseignement Superieur et de la Recherche is grate-
fully acknowledged. We also thank CNRS for a fellowship to AL. AL
and OO contributed equally to the work.
9. Trombotto, S.; Danel, M.; Fitremann, J.; Bouchu, A.; Queneau, Y. J. Org. Chem.
2003, 68, 6672–6678.
10. Le Chevalier, A.; Pierre, R.; Kanso, R.; Chambert, S.; Doutheau, A.; Queneau, Y.
Tetrahedron Lett. 2006, 47, 2431–2434.
11. (a) Chambert, S.; Doutheau, A.; Queneau, Y.; Cowling, S. J.; Goodby, J. W.;
Mackenzie, G. J. Carbohydr. Chem. 2007, 26, 27–39; (b) Alirachedi, F.; Chambert,
S.; Ferkous, F.; Queneau, Y.; Cowling, S. J.; Goodby, J. W. Chem. Commun. 2009,
6355–6357; (c) Xu, R.; Alirachedi, F.; Chambert, S.; Ferkous, F.; Queneau, Y.;
Cowling, S. J.; Davis, E. J.; Goodby, J. W. Org. Biomol. Chem. 2015, 13, 783–792.
12. (a) Sol, V.; Charmot, A.; Krausz, P.; Trombotto, S.; Queneau, Y. J. Carbohydr.
Chem. 2006, 25, 345–360; (b) Ménard, F.; Sol, V.; Ringot, C.; Granet, R.; Alves, S.;
Morvan, C. L.; Queneau, Y.; Ono, N.; Krausz, P. Bioorg. Med. Chem. 2009, 17,
7647–7657; (c) Xavier, N. M.; Goulart, M.; Neves, A.; Justino, J.; Chambert, S.;
Rauter, A. P.; Queneau, Y. Bioorg. Med. Chem. 2011, 19, 926–938.
13. Barsu, C.; Cheaib, R.; Chambert, S.; Queneau, Y.; Maury, O.; Cottet, D.; Wege, H.;
Douady, J.; Bretonniere, Y.; Andraud, C. Org. Biomol. Chem. 2010, 8, 142–150.
14. (a) Chen, J.; Miao, Y.; Chambert, S.; Bernard, J.; Fleury, E.; Queneau, Y. Sci. China
Chem. 2010, 53, 1880–1887; (b) Abdelkader, O.; Moebs-Sanchez, S.; Queneau,
Y.; Bernard, J.; Fleury, E. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1309–1318.
15. (a) Andrews, R. S.; Becker, J. J.; Gagné, M. R. Angew. Chem., Int. Ed. 2010, 49,
7274–7276; (b) Juhasz, Z.; Micskei, K.; Gal, E.; Somsak, L. Tetrahedron Lett.
2007, 48, 7351–7353.
Supplementary data
Supplementary data (experimental details, 1H and 13C NMR
data and spectra of all compounds, lactone opening reaction rates
measurements) associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.tetlet.2015.07.017.
References and notes
1. Awan, S. I.; Werz, D. B. Bioorg. Med. Chem. 2012, 20, 1846–1856.
2. (a) Xavier, N. M.; Rauter, A. P.; Queneau, Y. Top. Curr. Chem. 2010, 295, 19–62;
(b) De Lederkremer, R. M.; Varela, O. Adv. Carbohydr. Chem. Biochem. 1994, 50,
125–209; (c) Lundt, I. Top. Curr. Chem. 1997, 187, 117–156. ibid, 2001, 215, 177–
191; (d) Lundt, I. In Carbohydrate Synthons in Natural Products Chemistry:
Synthesis, Functionalization and Applications; Witzak, Z. J., Tatsuta, K., Eds.; ACS
Symposium Series, American Chemical Society, Washington, DC, 2003; Vol.
841, pp 117–140; (e) Fleet, G. W. J. In Antibiotics and Antiviral Compounds:
Chemical Synthesis and Modifications; Krohn, K., Kirst, H. A., Maas, H., Eds.; VCH:
Weinheim, 1993; pp 333–342.
3. (a) Queneau, Y.; Chambert, S.; Moebs, S.; Listkowski, A.; Cheaib, R. In:
Carbohydrate Chemistry—Chemical and Biological Approaches; Rauter, A. P.,
Lindhorst, T. K., Eds.; SPR, RSC, Cambridge, 2009; Vol. 39, 35, pp 99–126.; (b)
Yin, J.; Linker, T. Chem. Eur. J. 2009, 15, 49–52; (c) Xavier, N. M.; Queneau, Y.;
Rauter, A. P. Eur. J. Org. Chem. 2011, 713–720; (d) Yin, J.; Linker, T. Tetrahedron
2011, 67, 2447–2461; (e) Boultadakis-Arapinis, M.; Lemoine, P.; Turcaud, S.;
Micouin, L.; Lecourt, T. J. Am. Chem. Soc. 2010, 132, 15477–15479; (f) Parkan, K.;
Pohl, R.; Kotora, M. Chem. Eur. J. 2014, 20, 4414–4419; (g) El-Zayat, A. A. E.;
Ferrigni, N. R.; McCloud, T. G.; McKenzie, A. T.; Byrn, S. R.; Cassady, J. M.; Chang,
C. J.; McLaughlin, J. L. Tetrahedron Lett. 1985, 26, 955–956; (h) Naud, S.; Pipelier,
M.; Viault, G.; Adjou, A.; Huet, F.; Legoupy, S.; Aubertin, A. M.; Evain, M.;
Dubreuil, D. Eur. J. Org. Chem. 2007, 3296–3310.
16. (a) Praly, J. P.; Ardakani, A. S.; Bruyère, I.; Marie-Luce, C.; Qin, B. B. Carbohydr.
Res. 2002, 337, 1623–1632; (b) Chen, G. R.; Praly, J. P. C. R. Chimie 2008, 11, 19–
28; (c) Praly, J. P. Adv. Carbohydr. Chem. Biochem. 2001, 56, 65–151; (d) Ling, R.;
Yoshida, M.; Mariano, P. S. J. Org. Chem. 1996, 61, 4439–4449.
17. Lactone 3 (gluco); [a]D +45 (c = 1.05, CHCl3); HRMS (CI) m/z calcd for C15H21O9
(M+H)+ 345.1186, found 345.1183; 1H NMR (CDCl3, 300 MHz) d 5.22 (t, 1H,
J = 4.61 Hz, H-3), 4.85 (t, 1H, J = 4.52 Hz, H-4), 4.62 (dd, 1H, J = 6.96 Hz,
J = 11.2 Hz H-6a), 4.29–4.38 (m, 2H, H-1 and H-2), 4.01–4.15 (m, 2H, H-5 and
H-6b), 2.81 (ddd, 1H, J = 6.8 Hz, J = 9.8 Hz, J = 18.1 Hz, H-8a), 2.53 (ddd, 1H,
J = 5.1 Hz, J = 6.78 Hz, H-8b), 1.90–2.25 (m, 11 H, H-7a, H-7b and 3s  CH3); 13
C
NMR (CDCl3, 75 MHz) d 170.7, 169.9, 168.9, 168.7 (4 Â C@O), 75.2 (C-2), 72.9
(C-5), 68.3 (C-3), 66.1 (C-4), 62.1 (C-1), 60.7 (C-6), 25.9 (C-8), 23.6 (C-7), 20.8,
20.78, 20.75 (3 Â CH3).
18. Lactone 4 (galacto): [a] +76 (c = 1.29, CHCl3); MS (ESI) m/z = 344.9 [C15H21O9
D
(M+H)+]; 1H NMR (CDCl3, 300 MHz) d 5.44 (dd, 1H, J = 3.3 Hz, J = 5.7 Hz, H-4),
5.27 (dd, 1H, J = 5.2 Hz, J = 3.3 Hz, H-3), 4.68 (dd, 1H, J = 9.1 Hz, J = 12.3 Hz, H-
6a), 4.44 (dd, 1H, J = 2.9 Hz, H-2), 4.38–4.25 (m, 2H, H-1, H-5), 4.02 (dd, 1H,
J = 3.9 Hz, J = 12.3 Hz, H-6b), 2.73 (ddd, 1H, J = 7.2 Hz, J = 9.4 Hz, J = 17.7 Hz, H-
8A), 2.48 (ddd, broad, 1H, J = 5.8 Hz, J = 6.0 Hz, H-8b), 2.00–2.15 (m, 11 H, H-7a,
H-7b and 3s  CH3); 13C NMR (CDCl3, 75 MHz) d 171.0 (COO), 169.5, 169.4,
169.1 (3 Â C@O), 77.0 (C-2), 71.8 (C-5), 69.0 (C-3), 65.8 (C-4), 61.9 (C-1), 59.7
(C-6), 26.0 (C-8), 23.7 (C-7), 20.97, 20.96, 20.76 (3 Â CH3). Anal. Calcd. for
4. (a) Compain, P.; Martin, O. R. Bioorg. Med. Chem. 2001, 9, 3077–3092; (b)
Somsak, L. Chem. Rev. 2001, 101, 81–136; (c) Nicotra, F. Top. Curr. Chem. 1997,
187, 55–83; (d) Beau, J. M.; Gallagher, T. Top. Curr. Chem. 1997, 187, 1–54; (e)
Du, Y.; Linhardt, R. J. Tetrahedron 1998, 54, 9913–9959; (f) Miller, G. M.;
Gardiner, J. M. Org. Lett. 2010, 12, 5262–5265.
C
15H20O9: C, 52.32; H, 5.85. Found: C, 52.21; H, 5.98.
19. Typical procedure for lactone opening: mixture of lactone
0.29 mmol) and allylamine (22 L, 1.0 equiv) in CH2Cl2 (1.5 mL) was stirred for
a
3 (100 mg,
5. Foley, P. M.; Phimphachanh, A.; Beach, E. S.; Zimmermann, J. B.; Anastas, P. T.
Green. Chem. 2011, 13, 321–325.
l
72 h at room temperature. After evaporation of the solvent under reduced
pressure, the residue was purified over silica gel (EtOAc–pentane 4/1) to give
amide 7 in 94% yield. Other examples: see the Supplementary material.
6. (a) Sakamaki, S.; Kawanishi, E.; Nomura, S.; Ishikawa, T. Tetrahedron 2012, 68,
5744–5753; (b) Liu, N. N.; Bao, F. K.; Chen, J. B.; Zeng, X. H.; Chi, S. J.; Liu, J. P.
Med. Chem. Res. 2014, 23, 4083–4813; (c) Piacente, S.; Pizza, C.; Tommasi, N. D.;
Mahmood, N. J. Nat. Prod. 1996, 59, 565–569; (d) Martin, O. R.; Hendricks, C. A.;