Generalized anomeric effect in gem-diamines: Stereoselective synthesis of α-N-linked disaccharide mimics

Article abstract of DOI:10.1021/ol901125n

The orbital (negative hyperconjugation) contribution to the generalized anomeric effect is highly increased in bicyclic gem-diamines with a pseudoamide-type endocyclic nitrogen atom, which has been exploited for the stereoselective synthesis of configurat

Full text of DOI:10.1021/ol901125n

ORGANIC
LETTERS
2009
Vol. 11, No. 15
3306-3309
Generalized Anomeric Effect in
gem-Diamines: Stereoselective
Synthesis of r-N-Linked Disaccharide
Mimics
Elena M. Sa´nchez-Ferna´ndez,† Roc´ıo R´ısquez-Cuadro,‡ Matilde Aguilar-Moncayo,‡
M. Isabel Garc´ıa-Moreno,‡ Carmen Ortiz Mellet,*,‡ and
Jose´ M. Garc´ıa Ferna´ndez*,†
Instituto de InVestigaciones Qu´ımicas, CSIC and UniVersidad de SeVilla,
Ame´rico Vespucio 49, Isla de la Cartuja, E-41092 SeVilla, Spain, and
Departamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, UniVersidad de SeVilla,
Apartado 1203, E-41071 SeVilla, Spain
jogarcia@iiq.csic.es; mellet@us.es
Received May 21, 2009
ABSTRACT
The orbital (negative hyperconjugation) contribution to the generalized anomeric effect is highly increased in bicyclic gem-diamines with a
pseudoamide-type endocyclic nitrogen atom, which has been exploited for the stereoselective synthesis of configurationally stable r-N-linked
azadisaccharide heteroanalogues of the natural disaccharides maltose and isomaltose as aglycon-sensitive inhibitors of isomaltase.
Since the structure elucidation of nephritogenoside (1), a
nephritogenic glycopeptide isolated from the rat glomerular
basement membrane,1 the construction of the R-N-glycosidic
bond has been a major challenge in carbohydrate chemistry.
Although R-glycosylamines are direct intermediates toward
this goal, anomerization of 1-aminoglycopyranosyl deriva-
tives is so problematic that a number of alternative ap-
proaches have been developed which furnish glycosylamides2
and other N-glycosyl derivatives3 avoiding intermediate
formation of the free amine. The lability of the aminoacetal
functionality makes glycosylamines hydrolytically unstable
and the preparation of R-configured diastereomers particu-
larly problematic, due to the overwhelming thermodynamic
preference for the ꢀ-configuration as determined by the
reverse anomeric effect.4 Replacing the O,N-acetal segment
by an S,N-acetal (5-thiosugar glycosylamines) has been
shown to weaken the reverse anomeric effect contribution
to the anomeric equilibria, resulting in significant proportions
of the axial isomers. Thus, 5-thioglucopyranosylamine (2)
† Instituto de Investigaciones Qu´ımicas.
‡ Departamento de Qu´ımica Orga´nica, Universidad de Sevilla.
(1) (a) Shibata, S.; Miura, K. J. Biochem. 1987, 89, 1737–1749. (b)
Shibata, S.; Takeda, T.; Natori, Y. J. Biol. Chem. 1988, 263, 12483–12485.
(c) Takeda, T.; Sawaki, M.; Ogihara, Y.; Shibata, S. Chem. Pharm. Bull.
1989, 37, 54–56.
(2) (a) Damkaci, F.; DeShong, P. J. Am. Chem. Soc. 2003, 125, 4408–
4409. Ratcliffe, A. J.; Konradsson, P.; Fraser-Reid, B. J. Am. Chem. Soc.
1990, 112, 5665–5667. (b) Nair, L. G.; Fraser-Reid, B.; Szardenings, A. K.
Org. Lett. 2001, 3, 317–319.
(3) Jime´nez Blanco, J. L.; Sylla, B.; Ortiz Mellet, C.; Garc´ıa Ferna´ndez,
J. M. J. Org. Chem. 2007, 72, 4547–4550.
(4) (a) Chandrasekhar, S. ARKIVOC 2005, 37–66. (b) Box, V. G. J.
Mol. Struct. 2000, 522, 145–164.
10.1021/ol901125n CCC: $40.75
Published on Web 07/16/2009
 2009 American Chemical Society

and 5-thiomannopyranosylamine (3) exist as 1:2 and 1:3 R:ꢀ
mixtures in water solution, respectively.5 Particularly inter-
esting is the possibility of accessing R-N-linked disaccharide
mimics,6 a goal largely pursued in the design of inhibitors
of the enzymes that process R-linked oligo or polysaccharides
as drug candidates. Actually, the S,N-acetal analogue of
maltose 4r behaved as a low micromolar inhibitor of
glucoamylase G2.7 However, compound 4r was in equilib-
rium with the corresponding ꢀ-anomer 4ꢀ in aqueous
solution, with a relative proportion 1:2.5 in favor of the latter
(Figure 1).
mimetics9,10 with a hemiaminal portion anchored in the
R-configuration, such as the bicyclic R-D-glucopyranose and
R-D-mannopyranose analogues 5 and 6, were prepared and
shown to behave as anomer-selective glycosidase inhibitors.11
We hypothesized that a similar strategy might be applied to
stabilize the elusive R-diastereomer in sp2-iminosugar gly-
cosylamine analogues by replacing the N,O-acetal group by
a gem-diamine functionality (7, Figure 2). This concept has
Figure 2.
Structures of sp2-iminosugars with hemiaminal (5, 6) and
gem-diamine-type pseudoanomeric centers (7). The orbitals involved
in the negative hyperconjugation contribution to the GAE and the
presumably structure of the azacarbonium ion intermediate are
shown.
now been translated into the synthesis of the first mono- and
disaccharide analogues bearing a configurationally and
conformationally stable R-N-glycosidic linkage.
Figure 1. Partial structure of the R-Ν-glycopeptide nephritogenoside
(1) and structures of 5-thiogluco(manno)pyranosylamines (2 and
3) and of the N-linked pseudodisaccharide 4, with indication of
the anomeric ratio.
The gem-diamine structural motif is found in several
biologically active natural and synthetic 2-acylaminopyrro-
lidine and -piperidine derivatives, including sugar-like com-
pounds.12 However, the incorporation of the 2-acylamino
group in these molecules in a diastereoselective manner
continues to be a major problem. Moreover, the resulting
gem-diamines suffer from instability under physiological
conditions.13 In our prototype 7, we expected that the
preformed endocyclic carbamate functionality would direct
Shifting the anomeric equilibrium in C-X-C-Y seg-
ments, where X represents an element with lone pairs, toward
the gauche (R) orientation can be achieved by enhanc-
ing the generalized anomeric effect (GAE).8 We conceived
that the negative hyperconjugation contribution to the GAE
would be particularly favorable when X is an sp2-hybridized
nitrogen atom, due to a very efficient overlapping of the
π-type orbital hosting the lone pair in this case and the σ*
antibonding orbital of the contiguous C-Y bond. As a proof
of principle, configurationally stable sp2-iminosugar glyco-
(10) For recent publications on the synthesis and biological activity of
sp2-iminosugars, see: (a) Aguilar-Moncayo, M.; Ortiz Mellet, C.; Garc´ıa
Ferna´ndez, J. M.; Garc´ıa-Moreno, M. I. J. Org. Chem. 2009, 74, 3585–
3598. (b) Aguilar-Moncayo, M.; Gloster, T. M.; Turkenburg, J. P.; Garc´ıa-
Moreno, M. I.; Ortiz Mellet, C.; Davies, G. J.; Garc´ıa Ferna´ndez, J. M.
Org. Biomol. Chem. 2009, 7, 2738–2747. (c) Brumshtein, B.; Aguilar-
Moncayo, M.; Garc´ıa-Moreno, M. I.; Ortiz Mellet, C.; Garc´ıa Fernandez,
J. M.; Silman, I.; Shaaltiel, Y.; Aviezer, D.; Sussman, J. L.; Futerman, A. H.
ChemBioChem 2009, 10, 1480–1485. (d) Aguilar, M.; D´ıaz-Pe´rez, P.;
Garc´ıa-Moreno, M. I.; Ortiz Mellet, C.; Garc´ıa Ferna´ndez, J. M. J. Org.
Chem. 2008, 73, 1995–1998. (e) Aguilar, M.; Gloster, T. M.; Garc´ıa-Moreno,
M. I.; Ortiz Mellet, C.; Davies, G. J.; Llebaria, A.; Casas, J.; Egido-Gaba´s,
M.; Garc´ıa Fernandez, J. M. ChemBiochem, 2008, 9, 2612–2618. (f)
Benltifa, M; Garc´ıa-Moreno, M. I.; Ortiz Mellet, C.; Garc´ıa Ferna´ndez, J. M.;
Wadouachi, A. Bioorg. Med. Chem. Lett. 2008, 18, 2805–2808. (g) Garc´ıa-
Moreno, M. I.; Ortiz Mellet, C.; Garc´ıa Ferna´ndez, J. M. Tetrahedron 2007,
63, 7879–7884.
(5) Kavlekar, L. M.; Kuntz, D. A.; Wen, X.; Johnston, B. D.; Svensson,
B.; Rose, D. R.; Pinto, B. M. Tetrahedron: Asymmetry 2005, 16, 1035–
1046.
(6) (a) Shing, T. K. M.; Kwong, C. S. K.; Cheung, A. W. C.; Kok,
S. H.-L.; Yu, J.; Cheng, C. H. K. J. Am. Chem. Soc. 2004, 126, 15990–
15992. (b) Gravier-Pelletier, C.; Maton, W.; Le Merrer, Y. Tetrahedron
Lett. 2002, 43, 8285–8288. (c) Johnston, B. D.; Pinto, B. M. J. Org. Chem.
1998, 63, 5797–5800. (d) Dong, W.; Jespersen, T.; Bols, M.; Skrydstrup,
T.; Sierks, M. R. Biochemistry 1996, 35, 2788–2795. (e) Ble´riot, Y.;
Dintinger, T.; Guilo, N.; Tellier, C. Tetrahedron Lett. 1995, 36, 5175–5178.
(f) Knapp, S.; Purabdare, A.; Rupitz, K.; Withers, S. G. J. Am. Chem. Soc.
1994, 116, 7461–7462. (g) Cottaz, S.; Brimacombe, J. S.; Ferguson, M. A. J.
Carbohydr. Res. 1993, 247, 341–345.
(11) (a) Jime´nez Blanco, J. L.; D´ıaz Pe´rez, V. M.; Ortiz Mellet, C.;
Fuentes, J.; Garc´ıa Ferna´ndez, J. M. Chem. Commun. 1997, 1969–1970.
(b) D´ıaz Pe´rez, V. M.; Garc´ıa Moreno, M. I.; Ortiz Mellet, C.; Fuentes, J.;
D´ıaz Arribas, J. C.; Can˜ada, F. J.; Garc´ıa Ferna´ndez, J. M. J. Org. Chem.
2000, 65, 136–143. (c) D´ıaz-Pe´rez, P.; Garc´ıa-Moreno, M. I.; Ortiz Mellet,
C.; Garc´ıa Ferna´ndez, J. M. Eur. J. Org. Chem. 2005, 2903–2913.
(12) For selected references, see: (a) Morgan, I. R.; Yazici, A.; Pyne,
S. G.; Skelton, B. W. J. Org. Chem. 2008, 73, 2943–2946. (b) Nishimura,
Y. Heterocycles 2006, 67, 461–488. (c) Knapp, S.; Yang, C.; Pabbaraja,
S.; Rempel, B.; Reid, S.; Withers, S. G. J. Org. Chem. 2005, 70, 7715–
7720. (d) Brown, J. R.; Nishimura, Y.; Esko, J. D. Bioorg. Med. Chem.
Lett. 2006, 16, 532–536.
(7) Andrews, J. S.; Weimar, T.; Frandsen, T. P.; Svensson, B.; Pinto,
B. M. J. Am. Chem. Soc. 1995, 117, 10799–10804.
(8) Carballeira, L.; Pe´rez-Juste, I. J. Phys. Chem. A 2000, 104, 9362–
9369.
(9) By analogy with the accepted name “iminosugar”, we are using here
the term “sp2-iminosugar” to refer to glycomimetics where the endocyclic
oxygen atom has been replaced by a nitrogen atom with substantial sp2
character, typically a pseudoamide-type nitrogen. For recent monographs
on iminosugars as glycosidase inhibitors, see: (a) Iminosugars: From
Synthesis to Therapeutic Applications; Compain, P., Martin, O. R., Eds.;
Wiley-VCH; Weinheim, 2007. (b) Iminosugars as Glycosidase Inhibitors;
Stu¨tz, A. E., Ed.; Wiley-VCH: Weinheim, 1999.
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Bioorg. Med. Chem. Lett. 2001, 9, 1091–1095.
Org. Lett., Vol. 11, No. 15, 2009
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the stereochemical outcome of the reaction to the axial
derivative by virtue of the incipient GAE in an azacarbonium
ion intermediate (Figure 2). Because the ring sp2-nitrogen
should activate the pseudoanomeric C-1 carbon toward
glycosylation-type reactions in a manner not unlike that of
a pyranosyl anomeric carbon, formation of the target gem-
diamine functionality by reaction of 5 and 6 with amine
nucleophiles seemed feasible.
Scheme 2. Synthesis of Configurationally Stable
R-D-Mannopyranosylamine Mimics with gem-Diamine Structure
Treatment of crude 5, accessible in only two steps from
5-amino-5-deoxy-1,2-O-isopropylidende-R-D-glucofura-
nose 8,11 with ammonium bicarbonate and ammonium
hydroxide at 42 °C resulted in slow formation of the
corresponding ammonium glycosylcarbamate14 9. The trans-
formation was complete in 15 days (TLC), and the unstable
carbamate salt could be isolated by evaporation of the
Interestingly, pure 10r and 12r remained conformationally
and configurationally stable after 48 h in water solution under
slightly basic, neutral, or acidic conditions (pH 8-4), which
is drastically different from the situation encountered in
glycosylamines or glycosylamine mimics such as 2 and 3.
Reversion to the starting amino acetals 5 and 6 was only
observed after heating a solution of 10r or 12r, respectively,
in water at 60 °C for several hours. The ꢀ-anomers 10ꢀ and
12ꢀ were comparatively less stable under acidic conditions
and underwent hydrolysis in the presence of silica gel during
column chromatography using aqueous eluents (40:10:1
CH2Cl2-MeOH-H2O), which facilitated purification of the
R-anomers. Altogether, these results strongly support the
prevalence of the anomeric versus the reverse anomeric effect
in gem-diamine sp2-iminosugar systems.
1
solvents and fast purification on a silica gel filter. H NMR
(500 MHz) analysis evidenced the presence of a single
anomer having the R-configuration in D2O solution. Attempts
to further purify 9 by SiO2 column chromatography resulted
in elimination of carbon dioxide and ammonia to give the
corresponding R-glucosylamine analogue 10r as the sole
product. In a separate experiment, the reaction mixture
containing the carbamate salt was concentrated, lyophilized
twice from water, and directly chromatographed to afford a
mixture of the gem-diamine diastereomers 10r and 10ꢀ (72%
yield) in 11:1 relative proportion (H-1 integration). Further
SiO2 column chromatography afforded the R-anomer in pure
form (Scheme 1). An analogous transformation of the
Compounds 10r and 12r can, in principle, mimic the
developing gluco- and mannopyranosyl cations in the reac-
tion coordinate of enzymatic hydrolysis of the corresponding
R-glycosides by matching not only the hydroxylation pattern
but also the R-anomeric configuration and the (partial)
positive charge at the anomeric region. Notwithstanding, they
exhibited a rather broad glycosidase inhibition profile and a
poor anomeric selectivity.15 Previous results have shown that
the presence of an anomeric pseudoaglyconic hydroxyl group
anchored in the axial position in glucomimetic structures does
not warrant an increased selectivity toward R-glucosidases
either.11 In the absence of an aglycon portion, the glycomi-
metic can adapt to the active site of different glycosidases.16
The existence of additional nonglyconic interactions can
result even in totally reverse anomeric selectivity. In
Scheme 1. Synthesis of Configurationally Stable
R-D-Glucopyranosylamine Mimics with gem-Diamine Structure
(15) Inhibition constants (Ki) for 10r and 12r against commercial
enzymes: ꢀ-glucosidase from almonds, 26 ( 2 and 49 ( 5 µM; ꢀ-glucosi-
dase/ꢀ-galactosidase from bovine liver, 4 ( 1 and 217 ( 15 µM; naringinase
(ꢀ-glucosidase/ꢀ-rhamnosidase) from Penicillium decumbes, 11 µM and
no inhibition; isomaltase from yeast, 8 and 184 µM; amyloglucosidase from
Aspergillus niger, 70 µM and no inhibition; trehalase from pig kidney, 241
( 15 and 462 ( 20 µM; R-mannosidase from jack bean, 45 ( 3 and 7 (
1 mM; ꢀ-mannosidase from Helix pomatia, 430 ( 20 and 25 ( 3 µM,
respectively. Neither 10r nor 12r inhibited ꢀ-galactosidase (E. coli) and
R-galactosidase (coffee beans) at 1 mM concentration.
bicyclic D-mannopyranose mimic 6, obtained from the Vic-
amino alcohol precursor 11 by carbonylation and further
furanose f piperidine rearrangement,11 afforded a mixture
of the gem-diamines 12r and 12ꢀ in 17:1 relative proportion
(H-6b integration), from which the R-anomer was isolated
in pure form (Scheme 2).
Compounds 10r, 12r and 10ꢀ, 12ꢀ existed in water
(16) In principle, the negative hyperconjugation contribution to the
anomeric effect in nonprotonated 10r and 12r must be weaker as compared
with the oxygen analogues 5 and 6, while it would be expected to be stronger
in the protonated state. The anomerization barriers at pH 7.3, used in the
inhibition experiments toward ꢀ-glucosidases, are therefore expected to be
lower, which might be at the origin of the observed low inhibition selectivity.
Actually, traces of the ꢀ-anomers were observable after dissolving the
R-configured free bases in D2O (pH ∼8.5) by NMR, while this was not the
case for the corresponding hydrochloride salts (pH ∼5).
4
solution in the C1 conformation, with the anomeric amino
group in axial and equatorial dispositions, respectively, as
the anomeric substituents in natural R- and ꢀ-glucosides.
(14) Likhorchestov, L. M.; Novikova, O. S.; Derevistkaja, V. A.;
Kochetkov, N. K. Carbohydr. Res. 1986, 146, C1.
3308
Org. Lett., Vol. 11, No. 15, 2009

principle, effective and selective inhibition of a particular
glycosidase requires designing mimics of both portions,
glyconic and aglyconic, of the natural substrate. To test this
notion, the synthesis of the gem-diamine disaccharide mimics
of isomaltose and maltose 15 and 18, respectively, was next
attempted (Scheme 3).
of the glycosylamine-like derivative 16 to the open-chain
1-amino-1-deoxy-D-fructose derivative 20. Spontaneous oxi-
dation of the ꢀ-aminocarbonyl functionality to the transient
iminoketone 21 seems then to occur. Subsequent double
intramolecular attack of the carbamate nitrogen to the imine
carbon and of the OH-3 hydroxyl at the glucose moiety to
the carbonyl group takes place with total stereoselectivity,
leading to the diastereomer having the heteroatom substit-
uents at the central 1,4-oxazine ring, in a chair conformation,
in axial disposition, fitting the anomeric effect.
Scheme 3. Synthesis of R-N-Linked Disaccharide Mimics
Compounds 15 and 18 represent examples of designed,
linkage-spanning glycosidase inhibitors. Actually, they be-
haved as highly selective, though modest inhibitors of
isomaltase,19 an enzyme involved in the hydrolysis of
R(1f6) linkages in polysaccharides.20 This is attributable
to the interplay of a configurationally anchored “R” gem-
diamine moiety, isosteric of the acetal functionality in the
natural substrate, and the discriminating effect of the aglycon
portion,21 suggesting that enzyme inhibition may be generally
tunable by this kind of structural modification. Exploitation
of the generalized anomeric effect in sp2-iminosugar systems
as a tool to investigate the effect of other modes of linkage
on glycosidase inhibitory action and selectivity is currently
underway in our laboratories.
Acknowledgment. This work was supported by the
Spanish MEC (contract nos. CTQ2006-15515-C02-01/BQU
and CTQ2007-61180/PPQ) and the Junta de Andaluc´ıa (P08-
FQM-03711).
Supporting Information Available: Experimental details
and copies of the NMR spectra for the new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
We found that heating a 1:1.5 molar solution of 5 and
methyl 6-amino-6-deoxy-R-D-glucopyranoside (13)17 in metha-
nol at 60 °C for 4 h afforded the target R-N-linked isomaltose
mimic 15 in 47% yield (68% considering the recovered 5)
as a single diastereomer. A similar reaction using instead
the 4-amino-4-deoxy derivative 1418 as the glycosidating
partner led to a mixture of two compounds that could be
separated, after acetylation, into the R(1f4)-N-linked pseudo-
disaccharide 16 (25%) and the tetracyclic gem-diamine 17
(28%). Further deacetylation provided the fully unprotected
derivatives 18 and 19.
OL901125N
(19) The disaccharide mimics 15 and 18 exhibited total selectivity
towards isomaltase among the enzymes listed in ref 15 with Ki values 53
( 5 and 83 ( 8 mM, respectively.
(20) Nichols, B. L.; Avery, S.; Sen, P.; Swallow, D. M.; Hanh, D.;
Sterchi, E. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 1432–1437.
(21) Rather than deriving increased affinity, the presence of the “right”
aglycon moiety is compromising the binding to the “wrong” enzymes.
Recent crystallographic evidence on structurally related R-glucosyl hydro-
lases suggests that the productive contacts of the aglyconic glucopyranosyl
moiety in the substrate at the +1 site are, actually, very limited. See : (a)
Sim, L.; Quezada-Calvillo, R.; Sterchi, E. E.; Nichols, B. L.; Rose, D. R.
J. Mol. Biol. 2008, 375, 782–792. (b) Ernst, H. A.; Leggio, L. L.; Willemoe¨s,
M.; Leonard, G.; Blum, P.; Larsen, S. J. Mol. Biol. 2006, 358, 1106–1124.
(c) Lovering, A. L.; Lee, S. S.; Kim, Y.-W.; Withers, S. G.; Strynadka,
N. C. J. J. Biol. Chem. 2005, 280, 2105–2115.
Formation of the tetracyclic derivative 19 in the reaction
of 5 and 14 is intriguing. It involves Amadori rearrangement
(17) Garc´ıa Ferna´ndez, J. M.; Ortiz Mellet, C.; Fuentes, J. J. Org. Chem.
1993, 58, 5192–5199.
(18) Reist, E. J.; Spencer, R. R.; Calkins, D. F.; Baker, B. R.; Goodman,
L. J. Org. Chem. 1965, 30, 2312–2317.
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