A facile and general entry to C-glycosyl (R)- and (S)-β-amino acid pairs from glycosyl cyanides through enamino ester intermediates

Article abstract of DOI:10.1055/s-2006-933103

Four O-perbenzylated glycosyl cyanides (α- and β-D- mannopyranosyl, α-D-galactopyranosyl, and α-D-arabinofuranosyl) were converted by treatment with BrCH₂CO₂Et/Zn in THF at reflux (Blaise-Kishi reaction) into the corresponding C-glyc

Full text of DOI:10.1055/s-2006-933103

LETTER
539
A Facile and General Entry to C-Glycosyl (R)- and (S)-b-Amino Acid Pairs
from Glycosyl Cyanides through Enamino Ester Intermediates
C-Glycosyl
b
-Ami
l
no
A
ci
e
Dipartimento di Chimica, Laboratorio di Chimica Organica, Università di Ferrara, Via L. Borsari 46, 44100 Ferrara, Italy
Fax +39(0532)291167; E-mail: adn@dns.unife.it
Received 10 November 2005
diastereoisomer in very good yield from each reaction.
However, the execution of these reactions with such a de-
gree of efficiency appeared to be conditioned by the avail-
ability and stability of the starting sugar aldehydes. Hence
Abstract: Four O-perbenzylated glycosyl cyanides (a- and b-D-
mannopyranosyl, a-D-galactopyranosyl, and a-D-arabinofuranosyl)
were converted by treatment with BrCH2CO2Et/Zn in THF at reflux
(Blaise–Kishi reaction) into the corresponding C-glycosyl b-en-
amino esters which in turn were reduced by NaBH(OAc)3 to give the intrinsic configurational instability of a-linked formyl
C-glycosides5 precluded the synthesis of the correspond-
four pairs of C-glycosyl (R)- and (S)-b-amino esters.
ing a-linked C-glycosyl b-amino acids. To overcome this
limitation we have developed a new reaction sequence
that avoids the use of sugar aldehydes and exploits instead
glycosyl cyanides as starting material. These C-glyco-
sides are readily accessible as both a- or b-anomers via
Lewis acid catalyzed coupling of activated glycosyl do-
nors with trimethylsilyl or tetrabutylammonium cyanide6
and are configurationally stable. Hence, following in part
a route developed in our laboratory and leading to C(2)-
glycosylated Hantzsch dihydropyridines,7 we have
transformed four known glycosyl cyanides 1a–d (a, tetra-
O-benzyl-a-D-mannopyranosyl;6a b, tetra-O-benzyl-b-D-
mannopyranosyl;6a c, tetra-O-benzyl-a-D-galactopyrano-
syl;6a d, tri-O-benzyl-a-D-arabinofuranosyl8) into the
corresponding glycosylated b-amino acrylates 2a–d
(enamino esters) and these were submitted to hydride re-
duction to give pairs of (R)- and (S)-b-amino esters 3a–d
(Scheme 1). The results of this study are presented below.
Key words: amino acids, glycopeptides, glycosides, reductions,
nitriles
It is abundantly demonstrated that oligosaccharide
fragments attached through O- and N-glycosidic linkages
to the polyamide backbone derived from a-amino acid
residues, i.e. in native glycoproteins, exert crucial roles on
the various properties (structure, protease resistance, sta-
bility, and solubility) and biological activities of these
biomolecules.1 Protein glycosylation has been implicated
in a variety of processes such as the immune response,
intracellular targeting, intercellular recognition, infection
by viruses and bacteria adhesion. The introduction of one
or more carbon-linked glycosyl b-amino acid residues in
a natural glycopeptide induces two elements of diversity
at the same time, namely the nature of the linkage holding
the sugar moieties and the primary structure of the peptide
backbone. Both changes may have dramatic effects on the
structure and function of the original a-peptide and there-
fore these artificial products can be used as valuable
probes in mechanistic studies of biological processes at
the molecular level and may serve to modulate one or
more of those processes. In line with these concepts we
began to tackle a project to synthesize C-glycosyl b-ami-
no acids2 which, in contrast with C-glycosyl a-amino
acids,3 are difficult to prepare. Accordingly, we reported
very recently on the three-component Mannich- and
Reformatsky-type syntheses of C-glycosyl b-amino acids
and in this way paved the route for the preparation of
libraries of these compounds.4 Both methods involved, as
the initial step, the coupling of a C-glycosyl formaldehyde
with p-methoxybenzylamine to furnish an intermediate
imine which in turn was captured by a ketene silyl acetal
in the Mannich route and a bromozinc enolate in the Re-
formatsky route. Both processes offered a high degree of
chemical efficiency and an astonishing stereoselectivity
as demonstrated by the formation of a single b-amino acid
NH2
BrCH2CO2Et, Zn
CO2Et
sugar
1a–d
sugar
CO2Et
BnO
CN
THF, 80 °C, 50 min
2a–d
NH2
NaBH(OAc)3
sugar
*
AcOH, r.t., 1 h
3a–d
H
H
O
O
BnO
BnO
OBn
BnO
OBn
OBn
OBn
OBn
a
sugar
b
=
H
H
O
O
BnO
BnO
BnO
BnO
OBn
OBn
c
d
SYNLETT 2006, No. 4, pp 0539–0542
Advanced online publication: 20.02.2006
0
2.
0
3.
2
0
0
6
Scheme 1
DOI: 10.1055/s-2006-933103; Art ID: D33805ST
© Georg Thieme Verlag Stuttgart · New York

540
A. Dondoni et al.
LETTER
Treatment of the glycosyl cyanides 1a–d with a four mo- esters 2a–d to give the corresponding b-amino esters 3a–
lar excess of ethyl bromoacetate in the presence of zinc d, in each case as a mixture of diastereoisomers, in good
dust in THF at reflux according to the Kishi improved isolated yields (Table 1).12 The chiral sugar fragment ad-
conditions of the Blaise reaction9 gave the corresponding jacent to the enamino group exerted a very light asymmet-
C-glycosyl enamino esters 2a–d in yields ranging from ric induction on the formation of the new stereocenter as
86% up to 98% (Table 1), each product being present as a shown by the small to modest diastereomeric excess (de)
single (unidentified) E/Z stereoisomer.10 On the other values of the major products reported in Table 1. Attempts
hand, the retention of the original anomeric configuration to improve the diastereoselectivity of this reaction by the
of starting glycosyl cyanides was confirmed for enamino use of other hydride donor reagents [NaBH4, ZnI2–
esters 2a–d by estimating the J4,5 values in the corre- NaBH4, NaBH3CN, and (i-Bu)2AlH] have been unsuc-
sponding 1H NMR spectra or by the aid of NOE measure- cessful so far. Hence we were forcedly led to consider the
ments, as appropriate. In fact, the a-galactopyranosyl lack of stereoselectivity in a positive sense as a way to ob-
derivative 2c showed a J4,5 value around 6.0 Hz. The a- tain pairs of b-amino esters (R)- 3 and (S)-3 in almost
and b-mannopyranosyl derivatives 2a,b instead displayed equal amounts by the reduction of each individual en-
probing NOE values between H-4 and H-7 or H-4 and H- amino ester 2. This approach may allow the construction
8, respectively, while the a-arabinofuranosyl 2d showed of a collection of these artificial sugar amino acids having
NOE interactions between H-4 and H-6. Next we consid- the configuration of the b-amino acid moiety as one of
ered the key transformation of the enamino esters 2 into the elements of diversity.
the target b-amino esters 3. This kind of process has been
In order to transform the b-amino esters 3 to N-protected
earlier reported by Palmieri and co-workers11 to occur
derivatives suitable for peptide synthesis, the inseparable
readily and chemoselectively in achiral systems by the use
of sodium triacetoxyborohydride [NaBH(OAc)3] in acetic
acid at room temperature. Hence we were delighted to
observe that this hydride releasing agent under the same
pair of a-D-mannosyl (S)-3a and (R)-3a and each individ-
ual compound b-D-mannosyl (S)-3b and (R)-3b, a-D-ga-
lactosyl (S)-3c and (R)-3c, and a-D-arabinosyl (S)-3d and
(R)-3d were transformed by a standard method into the
conditions worked efficiently on our C-glycosyl enamino
corresponding N-Boc derivatives 4a–d in very high yields
Table 1 C-Glycosyl Enamino Esters 2 and C-Glycosyl b-Amino Esters 3 Prepared
Sugar nitrile
Enamino ester
Yield (%)a
87
b-Amino esterb
Yield,c ded (%)
70, 15
1a
NH2
OBn
NH2
NH2
H
H
O
CO2Et
CO2Et
CO2Et
CO2Et
O
CO2Et
BnO
BnO
3
BnO
1
8
4
2
BnO
OBn
OBn
OBn
(S)-3a
2a
1b
1c
97
86
98
70, 25
NH2
H
H
O
O
CO2Et
BnO
BnO
BnO
OBn
NH2
BnO
OBn
NH2
OBn
OBn
2b
(S)-3b
65, 5
H
H
O
O
CO2Et
BnO
BnO
BnO
BnO
OBn
NH2
OBn
NH2
OBn
OBn
2c
(S)-3c
1d
75, 35
H
O
4
H
O
CO2Et
1
3
2
7
BnO
BnO
BnO
BnO
OBn
OBn
2d
(R)-3d
a Isolated yield.
b Only the major diastereoisomer is shown.
c Overall yield of the mixture of diastereoisomers.
d Determined by 1H NMR analysis of the crude reaction mixture.
Synlett 2006, No. 4, 539–542 © Thieme Stuttgart · New York

LETTER
C-Glycosyl b-Amino Acids
541
OMe
(Table 2). Fortunately, the a-D-mannosyl (S)-4a and (R)-
4a were separable by column chromatography and there-
fore the eight compounds reported in Table 2 constitute a
collection of pure C-glycosyl N-Boc b-amino esters.13
Ph
F3C
O
2'R
NH
H
1. (R)-MTPA
2. Pd/C H2
O
2
CO2Et
3
HO
HO
1
The anomeric configuration of the sugar fragment of b-
amino esters 3 and 4 was taken as that established in the
corresponding b-enamino esters 2 (see above), while the
absolute configuration of the carbon atom bearing the
amino group in the side chain of 3 was assigned by the
same NMR-based procedure illustrated in our recent re-
port.4 This procedure follows the protocol developed by
Riguera and co-workers14 for the assignment of the abso-
lute configuration of chiral primary amines. Succinctly,
each pure b-amino ester 3 was transformed into the corre-
sponding hydroxy-free Mosher’s amides (2¢R)-5 and
(2¢S)-5 by treatment with (R)- and (S)-methoxytrifluoro-
methylphenylacetic acid (MTPA) in the presence of di-
cyclohexylcarbodiimide (DCC) followed by benzyl group
8
4
OH
OH
(3R,2'R)-5c
CF3
(R)-3c
Ph
MeO
O
2'S
NH
H
1. (S)-MTPA
2. Pd/C H2
O
2
CO2Et
3
HO
HO
1
8
4
OH
OH
(3R,2'S)-5c
H
∆δRS< 0
1
hydrogenolysis. Next, the H NMR spectra were com-
alkyl
pared to give chemical shift difference values DdRS of pro-
ton signals belonging to the two groups linked to the
asymmetric carbon, i.e. the sugar moiety and the alkyl
chain.15 The positive or negative DdRS values of each set
of signals allowed the spatial disposition of the relevant
H2N
chain
3R-configured 3c
sugar
moiety
∆δRS> 0
Scheme 2
Table 2 Transformation of C-Glycosyl b-Amino Esters 3 into their
Corresponding N-Boc Derivatives 4
groups attached to the stereogenic carbon to be estab-
lished and this allowed the assignment of the absolute
configuration. The example reported in Scheme 2 illus-
trates the various steps of this protocol leading to the
assignment of the 3R configuration of the b-amino ester
(R)-3c.
NHBoc
NH2
Boc2O, NaHCO3
CO2Et
CO2Et
sugar
sugar
*
*
dioxane, r.t., on
90–95%
4a–d
3a–d
In conclusion simple methodology has been delineated
that allows the chain extension of glycosyl cyanides into
b-amino esters with both R and S configuration. This
method owns a special importance for the synthesis of a-
anomers, which are difficult to access via the Mannich-
and Reformatsky-type routes starting from glycosyl
aldehydes. Studies on the stereoselective reduction of
enamino ester 2 using chiral hydride donor reagents are
currently underway in our laboratory.
(S)-Epimer
(R)-Epimer
NHBoc
NHBoc
H
H
O
CO2Et
O
CO2Et
BnO
BnO
BnO
BnO
OBn
OBn
OBn
OBn
(R)-4a
(S)-4a
NHBoc
NHBoc
H
H
O
CO2Et
O
CO2Et
BnO
BnO
BnO
BnO
Acknowledgment
OBn
OBn
We gratefully acknowledge the University of Ferrara and MIUR
(COFIN 2004) for financial support.
OBn
OBn
(S)-4b
(R)-4b
NHBoc
NHBoc
H
H
O
References and Notes
O
CO2Et
CO2Et
BnO
BnO
BnO
BnO
(1) (a) Rocchi, R.; Giordani, V. In Chemistry and Biochemistry
of Amino Acids, Peptides, and Proteins, Weinstein B., Vol.7;
Marcel Dekker, Inc.: New York, 1983. (b) Varki, A.
Glycobiology 1993, 3, 97. (c) Dwek, R. A. Chem. Rev.
1996, 96, 683. (d) Imperiali, B. Acc. Chem. Res. 1997, 30,
452. (e) Varki, A.; Cummings, R.; Esko, J.; Freeze, H.; Hart,
G.; Marth, J. Essentials of Glycobiology; Cold Spring
Harbor Laboratory Press: New York, 1999. (f) Saxon, E.;
Bertozzi, C. R. Annu. Rev. Cell. Dev. Biol. 2001, 17, 1.
(g) Hughes, R. C. Glycoconjugate J. 2001, 18, 567.
OBn
OBn
OBn
OBn
(S)-4c
(R)-4c
NHBoc
NHBoc
H
O
H
O
CO2Et
CO2Et
BnO
BnO
BnO
BnO
OBn
OBn
(R)-4d
(S)-4d
(h) Bertozzi, C. R.; Kiessling, L. L. Science 2001, 291, 2357.
Synlett 2006, No. 4, 539–542 © Thieme Stuttgart · New York

542
A. Dondoni et al.
LETTER
washed with sat. aq NaHCO3 solution (2 × 10 mL). The
organic phase was dried (Na2SO4), concentrated, and
purified by column chromatography on silica gel using a
suitable elution system to give the corresponding (R)- and
(S)-b-amino esters 3 in a variable diastereomeric ratio (see
Table 1).
(2) Palomo, C.; Oiarbide, M.; Landa, A.; González-Rego, M. C.;
García, J. M.; González, A.; Odriozola, J. M.; Martín-Pastor,
M.; Linden, A. J. Am. Chem. Soc. 2002, 124, 8637.
(3) (a) Dondoni, A.; Marra, A. Chem. Rev. 2000, 100, 4395.
(b) For a non-exhaustive list of papers reported after this
review, see ref. 4.
(4) Dondoni, A.; Massi, A.; Sabbatini, S. Chem. Eur. J. 2005,
11, 7110.
(5) (a) Dondoni, A.; Scherrmann, M.-C. J. Org. Chem. 1994, 59,
6404. (b) Dondoni, A.; Zuurmond, H. M.; Boscarato, A. J.
Org. Chem. 1997, 62, 8114.
(6) (a) Igarashi, Y.; Shiozawa, T.; Ichikawa, Y. Bioorg. Med.
Chem. Lett. 1997, 7, 613. (b) Bhat, A. S.; Gervay-Hague, J.
Org. Lett. 2001, 3, 2081.
Analytical data for compound (S)-3b: 1H NMR (CDCl3): d =
7.50–7.10 (m, 20 H, Ph), 5.09 and 4.78 (2 d, 2 H, J = 11.8
Hz, PhCH2), 4.92 and 4.60 (2 d, 2 H, J = 10.8 Hz, PhCH2),
4.86 and 4.77 (2 d, 2 H, J = 11.5 Hz, PhCH2), 4.64 and 4.54
(2 d, 2 H, J = 12.0 Hz, PhCH2), 4.21 (dd, 1 H, J4,5 = ca. 0.5
Hz, J5,6 = 2.5 Hz, H-5), 4.20–4.08 (m, 2 H, OCH2CH3), 3.96
(dd, 1 H, J6,7 = 9.0 Hz, J7,8 = 9.2 Hz, H-7), 3.80–3.70 (m, 2
H, 2 H-9), 3.67 (dd, 1 H, H-6), 3.44 (ddd, 1 H, J2a,3 = 3.2 Hz,
(7) Dondoni, A.; Massi, A.; Minghini, E. Helv. Chim. Acta
2002, 85, 3331.
(8) Acton, E. M.; Fujiwara, A. N.; Goodman, L.; Henry, D. W.
Carbohydr. Res. 1974, 33, 136.
J2b,3 = 8.8 Hz, J3,4 = 8.5 Hz, H-3), 3.42 (m, 1 H, H-8), 3.12
(dd, 1 H, H-4), 2.88 (dd, 1 H, J2a,2b = 16.2 Hz, H-2a), 2.37
(dd, 1 H, H-2b), 2.00 (bs, 2 H, NH2), 1.26 (t, 3 H, J = 7.0 Hz,
OCH2CH3). Anal. Calcd for C39H45NO7: C, 73.22; H, 7.09;
N, 2.19. Found: C, 73.20; H, 7.05; N, 2.15. MALDI-TOF
MS: 640.8 [M+ + H], 662.8 [M+ + Na].
(9) Hannick, S. M.; Kishi, Y. J. Org. Chem. 1983, 48, 3833.
(10) Typical Procedure for Entry to b-Enamino Esters (2).
A suspension of zinc dust (588 mg, 9.00 mmol) in anhyd
THF (8 mL) was heated under reflux then a few drops of
ethyl bromoacetate were added. After a green color had
appeared (ca. 15 min), a solution of glycosyl cyanide 1 (1.50
mmol) in anhyd THF (2 mL) was added in one portion. The
remaining bromoacetate was added dropwise over 50 min
(total amount of bromoacetate: 0.66 mL, 6.0 mmol). The
reaction mixture was cooled to r.t., treated with sat. aq
NaHCO3 solution (10 mL) and filtered through a pad of
Celite®. The filtrate was extracted with Et2O (3 × 75 mL)
and the combined organic layers were dried (Na2SO4) and
concentrated in vacuo. The residue was eluted from a
column of silica gel using a suitable elution system to afford
the corresponding b-enamino ester 2 as a single
stereoisomer.
(13) Typical Procedure for Entry to N-Boc Derivatives (4).
To a stirred mixture of b-amino ester 3 (0.50 mmol), dioxane
(8 mL), and Boc2O (546 mg, 2.50 mmol) a few drops of sat.
aq NaHCO3 solution (until basic pH) were added. The
solution was stirred at r.t. for an additional 12 h then diluted
with Et2O (100 mL) and washed with a 10% aq solution of
citric acid (2 × 10 mL). The organic phase was separated,
washed with brine (2 × 10 mL), dried (Na2SO4) and
concentrated in vacuo. The residue was then purified by
column chromatography on silica gel with the suitable
elution system to give the corresponding N-Boc derivative 4
in almost quantitative yield.
Analytical data for compounds 4: (S)-4a: [a]D 1.7 (c 1.3,
CHCl3). (R)-4a: [a]D –5.8 (c 1.0, CHCl3). (S)-4b: [a]D –12.0
(c 1.5, CHCl3). (R)-4b: [a]D –11.5 (c 1.2, CHCl3). (S)-4c:
[a]D 22.1 (c 0.9, CHCl3). (R)-4c: [a]D 37.0 (c 0.8, CHCl3).
(S)-4d: slightly contaminated by uncharacterized by-
products. (R)-4d: [a]D 8.2 (c 0.7, CHCl3). 1H NMR (CDCl3)
for (S)-4b: d = 7.50–7.10 (m, 20 H, Ph), 5.17 (br d, 1 H,
J3,NH = 8.5 Hz, NH), 5.02 and 4.70 (2 d, 2 H, J = 10.5 Hz,
PhCH2), 4.90 and 4.75 (2 d, 2 H, J = 11.0 Hz, PhCH2), 4.82
and 4.60 (2 d, 2 H, J = 11.5 Hz, PhCH2), 4.62 and 4.51 (2 d,
2 H, J = 12.0 Hz, PhCH2), 4.30–4.20 (m, 1 H, H-3), 4.11 (q,
2 H, J = 7.0 Hz, OCH2CH3), 4.02 (dd, 1 H, J4,5 = ca. 0.5 Hz,
Analytical data for compound 2b: [a]D 12.7 (c 1.4, CHCl3).
1H NMR (CDCl3 + D2O): d = 7.45–7.10 (m, 20 H, Ph), 4.92
and 4.68 (2 d, 2 H, J = 11.2 Hz, PhCH2), 4.84 and 4.59 (2 d,
2 H, J = 11.5 Hz, PhCH2), 4.73 and 4.66 (2 d, 2 H, J = 11.0
Hz, PhCH2), 4.63 and 4.56 (2 d, 2 H, J = 12.0 Hz, PhCH2),
4.47 (s, 1 H, H-2), 4.15 (q, 2 H, J = 7.0 Hz, OCH2CH3), 4.00
(dd, 1 H, J4,5 = ca. 0.5 Hz, J5,6 = 3.0 Hz, H-5), 3.93 (dd, 1 H,
J
6,7 = 9.1 Hz, J7,8 = 9.0 Hz, H-7), 3.91 (d, 1 H, H-4), 3.77 (dd,
1 H, J8,9a = 2.5 Hz, J9a,9b = 10.8 Hz, H-9a), 3.73 (dd, 1 H,
8,9b = 4.8 Hz, H-9b), 3.66 (dd, 1 H, H-6), 3.51 (dd, 1 H, H-
J
J5,6 = 2.5 Hz, H-5), 3.88 (dd, 1 H, J6,7 = 9.2 Hz, J7,8 = 9.5 Hz,
8), 1.30 (t, 3 H, OCH2CH3). Anal. Calcd for C39H43NO7: C,
73.45; H, 6.80; N, 2.20. Found: C, 73.48; H, 6.81; N, 2.24.
MALDI-TOF MS: 638.5 [M+ + H], 660.9 [M+ + Na].
(11) Bartoli, G.; Cimarelli, C.; Marcantoni, E.; Palmieri, G.;
Petrini, M. J. Org. Chem. 1994, 59, 5328.
H-7), 3.76 (dd, 1 H, J8,9a = 3.5 Hz, J9a,9b = 12.0 Hz, H-9a),
3.70 (dd, 1 H, J8,9b = 3.8 Hz, H-9b), 3.65 (dd, 1 H, H-6), 3.53
(dd, 1 H, J3,4 = 8.5 Hz, H-4), 3.42 (ddd, 1 H, H-8), 2.83 (dd,
1 H, J2a,2b = 16.5 Hz, J2a,3 = 6.0 Hz, H-2a), 2.66 (dd, 1 H,
J2b,3 = 4.0 Hz, H-2b), 1.42 (s, 9 H, t-Bu), 1.22 (t, 3 H,
(12) Typical Procedure for Entry to b-Amino Esters (3).
A solution of NaBH(OAc)3 was prepared by adding NaBH4
(57 mg, 1.50 mmol) to glacial AcOH (1.5 mL) while the
temperature was kept at 10 °C. After the H2 evolution ceased
(30 min), a solution of b-enamino ester 2 (0.50 mmol) in
glacial AcOH (0.5 mL) was added slowly. The solution was
stirred at r.t. for an additional 1 h, and then concentrated in
vacuo. The residue was suspended in EtOAc (80 mL) and
OCH2CH3). Anal. Calcd for C44H43NO9: C, 71.43; H, 7.22;
N, 1.89. Found: C, 71.40; H, 7.20; N, 1.80. MALDI-TOF
MS: 762.5 [M+ + Na], 778.5 [M+ + K].
(14) Seco, J. M.; Quinoa, E.; Riguera, R. Tetrahedron:
Asymmetry 2001, 12, 2915.
(15) The removal of benzyl groups was essential to guarantee
DdRS values with a regular sign distribution for all protons of
Mosher’s amides 5.
Synlett 2006, No. 4, 539–542 © Thieme Stuttgart · New York