Assembling heterocycle-tethered C-glycosyl and α-amino acid residues via 1,3-dipolar cycloaddition reactions

Article abstract of DOI:10.1021/ol048963g

(Equation Presented) The 1,3-dipolar cycloadditions of C-glycosyl nitrile oxides and acetylenes to an alkyne and an azide, respectively, bearing a masked glycinyl moiety furnished disubstituted isoxazoles and triazoles. Unveiling the glycinyl group in the

Full text of DOI:10.1021/ol048963g

ORGANIC
LETTERS
2004
Vol. 6, No. 17
2929-2932
Assembling Heterocycle-Tethered
C-Glycosyl and r-Amino Acid Residues
via 1,3-Dipolar Cycloaddition Reactions
Alessandro Dondoni,* Pier Paolo Giovannini, and Alessandro Massi
Laboratorio di Chimica Organica, Dipartimento di Chimica, UniVersita` di Ferrara,
Via L. Borsari 46, I-44100 Ferrara, Italy
adn@dns.unife.it
Received June 4, 2004
ABSTRACT
The 1,3-dipolar cycloadditions of C-glycosyl nitrile oxides and acetylenes to an alkyne and an azide, respectively, bearing a masked glycinyl
moiety furnished disubstituted isoxazoles and triazoles. Unveiling the glycinyl group in these cycloadducts afforded C-glycosyl r-amino acids
in which the two bioactive entities were tethered through rigid five-membered heterocycles. Optimized entries to the same compounds involved
the use of unmasked but protected alkyne- and azide-containing amino acids as the partners of 1,3-dipolar cycloadditions.
In respect to native O- and N-glycosides, anomeric carbon-
linked glycoconjugates are impervious to chemical and
enzymatic degradation and do not undergo hydrogen bonding
at the former anomeric position. Therefore there has been
over the past 2 decades a great deal of effort by organic
chemists to develop expedient syntheses of various C-gly-
cosides.1 In the realm of this rapidly growing field of
carbohydrate chemistry, C-glycosyl amino acids such as
isosteres of O-glycosyl serines and N-glycosyl asparagines
have been attracting intense interest2 because of their
potential service as probes in studies of glycopeptide bio-
logical activity3 and as leads for the development of new
drugs against carbohydrate-based metabolic disorders.4 A
special family of C-glycosyl R-amino acid are the C-gly-
cosylacetylene-phenylalanine residues reported by Meldal
and Vasella and their co-workers.5 These compounds were
designed as rigidified systems by virtue of their acetylene
bridges, which were expected to confer new chemical,
structural, and biological characteristics on glycopeptides in
which they were eventually incorporated. As a further
contribution to this area, we thought it interesting to prepare
another class of rigidified C-glycosyl R-amino acids in which
the tether holding the sugar and amino acid residue was
constituted of a heteroaromatic ring. Needless to say, such
a structural motif is present in a natural product, i.e., the
C-mannopyranosyltryptophan, the only native C-glycosyl
amino acid so far discovered in which the heterocycle is
represented by the rigid indole ring.6 The method of
executing our plan became readily apparent after considering
the increasing attention that is being paid to the development
of chemoselective ligation methodologies of biologically
active entities via 1,3-dipolar cycloaddition reactions (1,3-
DCRs).7 Therefore we will present below efficient ap-
proaches to C-glycosylated isoxazole and triazole alanines
(1) Postema, M. H. D. C-Glycoside Synthesis; CRC Press: Boca Raton,
1995.
(2) Dondoni, A.; Marra, A. Chem. ReV. 2000, 100, 4395-4421.
(3) (a) Varki, A. Glycobiology 1993, 3, 97-130. (b) Dwek, R. A. Chem.
ReV. 1996, 96, 683-720. (c) Varki A, Cummings, R.; Esko, J.; Freeze, H.;
Hart, G.; Marth, J. Essentials of Glycobiology; Cold Spring Harbor
Laboratory Press: New York, 1999.
(5) Lowary, T.; Meldal, M.; Helmboldt, A.; Vasella, A.; Bock, K. J.
Org. Chem. 1998, 63, 9657-9668.
(6) (a) Beer, T.; Vliegenthart, J. F. G.; Lo¨ffler, A.; Hofsteenge, J.
Biochemistry 1995, 34, 11785-11789. (b) Lo¨ffler, A.; Doucey, M.-A.;
Jansson, A. M.; Mu¨ller, D. R.; Beer, T.; Hess, D.; Meldal, M.; Richter, W.
J.; Vliegenthart, J. F. G.; Hofsteenge, J. Biochemistry 1996, 35, 12005-
12014. (c) Doucey, M.-A.; Hess, D.; Blommers, M. J. J.; Hofsteenge, J.
Glycobiology 1999, 9, 435-441.
(4) Wong, C.-H. Carbohydrate-Based Drug DiscoVery; Wiley-VCH:
Weinheim, 2003.
10.1021/ol048963g CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/30/2004

(see graphical abstract) via the venerable Huisgen 1,3-DCRs
of nitrile oxides and azides to acetylenes.8 This work stems
from our recent studies on C-glycosyl amino acid2 and
heterocycle amino acid9 synthesis as the basic operation
toward the construction of designed bioactive molecules,
peptides in primis, with new biological properties.
Scheme 3. Synthesis of Masked and Protected Azido
Functionalized Glycines 5 and 11
Toward such a target as the sugar and amino acid
functionalized isoxazole 3 and triazole 6 systems via 1,3-
DCRs, a retrosynthetic blueprint (Scheme 1) was carried out
Scheme 1. Synthetic Strategy
Taking advantage of our earlier experience in nitrile oxide
generation from C-glycosyl aldoximes,10b we first treated a
DMF solution of the C-galactosyl oxime 12a and alkyne 1
(10.0 equiv) with N-bromosuccinimide (NBS) and then added
Et3N dropwise (Scheme 4). Chromatography of the reaction
Scheme 4. Initial Approach toward Synthesis of C-Glycosyl
Isoxazole Alanines
by taking into account the ready access to anomeric sugar
nitrile oxides 2 and acetylenes 4 from recent chemistry
developed in our and other laboratories.10,11 Appropriate
partners for the planned cycloadditions were the acetylene
1 and the azide 5, both carrying the N-Boc oxazolidine ring
as a valuable masked glycinyl moiety.9a,12 The preparation
of these new reagents was straightforward starting from the
protected homoserinal9 7 and the phosphonate13 8 for the
synthesis of 1 (Scheme 2), and the protected serinol14 10
Scheme 2. Synthesis of Masked and Protected Ethynyl
Functionalized Glycines 1 and 9
mixture furnished the 3,5-disubstituted isoxazole cycloadduct
13a (4-H δ 6.23 ppm, DMSO-d6, 120 °C) in good yield
(68%) as the sole regioisomer,15 alongside the furoxan side
product 14a.10b Noteworthy is that the unreacted dipolaro-
phile 1 was recovered almost completely in very pure form.
Under the same conditions, the C-glucosyl oxime 12b and
(9) (a) Dondoni, A.; Massi, A.; Minghini, E.; Sabbatini, S.; Bertolasi,
V. J. Org. Chem. 2003, 68, 6172-6183. (b) Dondoni, A.; Massi, A.;
Minghini, E.; Bertolasi, V. Tetrahedron 2004, 60, 2311-2326.
(10) C-Glycosyl nitrile oxides: (a) Baker, K. W. J.; March, A. R.;
Parsons, S.; Paton, M.; Stewart, G. W. Tetrahedron 2002, 58, 8505-8513.
(b) Dondoni, A.; Giovannini, P. P. Synthesis 2002, 1701-1706.
(11) Ethynyl C-glycosides: (a) Nishika, T.; Ishikawa, M.; Isobe, M.
Synlett 1999, 123-125. (b) Dondoni, A.; Mariotti, G.; Marra, A. J. Org.
Chem. 2002, 67, 4475-4486.
and commercially available reagents for the synthesis of 5
(Scheme 3).
(7) (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2001, 40, 2004-2021. (b) Fazio, F.; Bryan, M. C.; Blixt, O.; Paulson,
J. C.; Wong, C.-H. J. Am. Chem. Soc. 2002, 124, 14397-14402. (c) Perez-
Balderas, F.; Ortega-Munoz, M.; Morales-Sanfrutos, J.; Hernandez-Mateo,
F.; Calvo-Flores, F. G.; Calvo-Asin, J. A.; Isac-Garcia, J.; Santoyo-Gonzalez,
F. Org. Lett. 2003, 5, 1951-1954. (d) Lee, L. V.; Mitchell, M. L.; Huang,
S.-J.; Fokin, V. V.; Sharpless, K. B.; Wong, C.-H. J. Am. Chem. Soc. 2003,
125, 9588-9589.
(8) Huisgen, R. Angew. Chem., Int. Ed. Engl. 1963, 2, 565-598; 633-
645. For reviews, see: (a) Padwa, A. 1,3-Dipolar Cycloaddition Chemistry;
Wiley: New York, 1984. (b) Padwa, A.; Pearson, W. H. Synthetic
Applications of 1,3-Dipolar Cycloaddition Chemistry Toward Heterocycles
and Natural Products; Wiley: New York, 2003.
(12) (a) Garner, P.; Yoo, J. U.; Sarubu, R.; Kennedy, V. O.; Youngs,
W. J. Tetrahedron 1998, 54, 9303-9316. (b) Dondoni, A.; Marra, A.; Massi,
A. J. Org. Chem. 1999, 64, 933-944.
(13) Callant, P.; D’Haenens, L.; Vandewall, M. Synth. Commun. 1984,
14, 155-161.
(14) Dondoni, A.; Perrone, D. Org. Synth. 1999, 77, 64-77.
(15) Gru¨nanger, P.; Vita-Finzi, P. The Chemistry of Heterocyclic
Compounds. Isoxazoles; Wiley: New York, 1991; Vol. 49, Part. 1, pp 24-
55 and 183-186.
2930
Org. Lett., Vol. 6, No. 17, 2004

the dipolarophile 1 afforded the isoxazole 13b (4-H δ 6.42
ppm, DMSO-d6, 100 °C) in yield comparable to that for 13a
(75%).
Table 1. Unveiling of Glycinyl Moiety in Cycloadducts 13a,b,
16a,b, and 17a,b
Next, we examined the triazole synthesis by coupling the
azide 5 with the ethynyl C-galactoside 15a11b in 1:1 molar
ratio at 120 °C under solvent-free conditions (Scheme 5).
Scheme 5. Initial Approach toward Synthesis of C-Glycosyl
Triazole Alanines
This fusion process furnished a mixture of 1,5- and 1,4-
disubstituted triazoles 16a and 17a in 5:1 ratio (as determined
by 1H NMR analysis) and 80% overall yield after chromato-
graphic purification. Under the same conditions the ethynyl
C-glucoside 15b11b and the azide 5 afforded the correspond-
ing triazole regioisomers 16b and 17b in 1.5:1 ratio and 80%
yield. The above product ratios were established after a
1
careful characterization of each isomer. H NMR spectra
(DMSO-d6, 100 °C) of 1,4-regioisomers 17a,b exhibited the
triazole proton (δ ∼8.0 ppm) sensibly shifted downfield
compared to the corresponding 1,5-isomers 16a,b (δ ∼7.6
ppm). This finding is in agreement with earlier assignments
of 1,4- and 1,5-disubstitued triazole derivatives based on 1H
NMR spectroscopy.16
The final conversion of isoxazole and triazole cycloadducts
13a,b, 16a,b, and 17a,b into the target R-amino acids was
then achieved by acetonide removal and subsequent oxidation
(see Supporting Information).17 All carboxylic acids thus
prepared were conveniently characterized as their methyl
esters 18a,b, 19a,b, and 20a,b shown in Table 1. It is
important to point out that the yields of isolated 19a,b and
20a,b were substantially altered by the loss of material upon
chromatography on silica gel of the crude reaction mixtures.
It is also worth noting that 18a can also be regarded as a
a Isolated yields.
conformationally constrained analogue of a C-galactosyl
hydroxynorvaline methylene isostere (â-D-Gal-CH2-Hnv).
This amino acid has been recently synthesized18,19 and used
in immunological studies, such as for the induction of
tolerance in autoimmune rheumatoid arthritis.
Of relevance to the elaboration of these amino acid esters
into appropriate building blocks for peptide synthesis, was
the demonstration that their O-benzyl protecting groups could
be reductively removed without affecting the heterocyclic
rings. Addressing this issue was especially important in the
case of the isoxazole derivatives because the isoxazole ring
can be easily cleaved by reducing agents.20 Hence, the
debenzylation of compounds 18a and 19b was carried out
by Pd(OH)2-catalyzed hydrogenation under mild conditions
to afford the corresponding compounds with free hydroxy
groups in very high yields (see Supporting Information).21
Aiming at simplifying access to the heterocycle amino
esters of Table 1, we sought a more direct approach to these
(16) (a) Tornoe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002,
67, 3057-3064. (b) Wang, Z.-X.; Qin, H.-L. Chem. Commun. 2003, 2450-
2451 and references therein. Unfortunately, unambiguous structural assign-
ment of cycloadducts 16a,b and 17a,b or their derivatives by NOE was
unsuccessful. Nevertheless, the structures of all compounds derived from
16a,b and 17a,b were consistent with their respective 1H and also 13C NMR
spectra (4-C, δ ∼132 ppm; 5-C, δ ∼135 ppm in 1,5-regioisomers. 4-C, δ
∼144 ppm; 5-C δ ∼124 ppm in 1,4-regioisomers). For 13C NMR spectral
data of 1,4- and 1,5-disubstituted triazoles, see: Crandall. J. K.; Crawley,
L. C.; Komin, J. B. J. Org. Chem. 1975, 40, 2045-2047. Katritzky, A. R.;
Lagowski, J. M. In ComprehensiVe Heterocyclic Chemistry; Potts, K. T.,
Ed.; Pergamon Press: Oxford, 1984; pp 15-16.
(18) Dondoni, A.; Giovannini, P. P.; Marra, A. J. Chem. Soc., Perkin
Trans. 1 2001, 2380-2388.
(19) Wellner, E.; Gustafsson, T.; Ba¨cklund, J.; Holmdahl, R.; Kihlberg,
J. ChemBioChem 2000, 1, 272-280.
(17) Attempts to carry out the one-pot oxidative cleavage of the
oxazolidine ring with the Jones reagent as described (see ref 12b) turned
out to be fruitless.
(20) Gru¨nanger, P.; Vita-Finzi, P. The Chemistry of Heterocyclic
Compounds. Isoxazoles; Wiley: New York, 1991; Vol. 49, Part 1, p 278.
Org. Lett., Vol. 6, No. 17, 2004
2931

compounds using the readily available ethynyl and azido
functionalized amino esters 9 and 11 (Schemes 2 and 3) as
partners of the 1,3-DCRs. A matter of concern, however,
was the stereochemical integrity of the unmasked glycinate
moiety under the conditions of the 1,3-DCRs. Pressing
forward nonetheless, nitrile oxide generation from the sugar
oximes 12a and 12b in the presence of excess of the alkyne
9 at ambient temperature (Scheme 6) furnished the 3,5-
Table 2. Cycloadditions of Sugar Alkynes 15a and 15b with
the Azide 11 (1.0 equiv)
Scheme 6. Optimized Synthesis of C-Glycosyl Isoxazole
Alanines
alkyne
thermala
catalyzedb
15a
15b
19a (68)c + 20a (13)c
19b (47)c + 20b (31)c
20a (80)c
20b (82)c
a Neat, 120 °C, 2 h. b Toluene, CuI (0.1 equiv), DIPEA (1.0 equiv), rt,
15 h. c Isolated yields.
performing nonconcerted Cu(I)-catalyzed reactions. This
produced only 1,4-disubstituted regioisomers.16a,23 Thus,
treatment of the sugar alkynes 15a and 15b with 1.0 equiv
of the azide 11 and CuI (0.1 equiv) in toluene and addition
of diisopropylethylamine (DIPEA)7b afforded high yields
(Table 2) of the expected 1,4-disubstitutd triazoles 20a and
20b as single products.
In summary, we have developed synthetic routes based
on effective 1,3-DCRs that provide a rapid entry to a new
class of unnatural C-glycosyl amino acids featuring a five-
membered heterocyclic moiety (isoxazole or triazole) be-
tween the sugar and amino acid entities. A primary service
of these amino acids can be foreseen as building blocks for
new glycopeptides.
disubstituted isoxazoles 18a and 18b in comparable overall
yields (∼50% from 1).
Next, we examined the synthesis of the triazoles 19 and
20. These were achieved by heating the sugar acetylenes 15a
and 15b with the azide 11 at 120 °C without solvent for 2
h (Table 2).22 The corresponding 1,5- and 1,4-disubstituted
triazoles 19a/20a and 19b/20b were obtained in almost
identical ratios but in better overall yields (∼60% vs ∼40%
from 5) in respect to the previous route. This improved
procedure reduced, in fact, the loss of material caused by
the chromatographic purification of intermediates.
Looking at the above results from the viewpoint of a
preparative efficiency, a disadvantage lies in the formation
of mixtures of 1,4- and 1,5-disubstituted triazoles.8 A simple
solution to this longstanding problem has relied upon
adopting the latest advance in this chemistry, that is,
Acknowledgment. We gratefully acknowledge MIUR
(COFIN 2002) for financial support.
Supporting Information Available: Experimental pro-
cedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
(21) Crucial for the selective hydrogenolysis of benzyl groups in 18a
was the presence of acetic acid in the reaction medium. For a wider
discussion on the effect of pH in the hydrogenation of isoxazole derivatives,
see: Stork, G.; Danishefsky, S.; Ohashi, M. J. Am. Chem. Soc. 1967, 89,
5459-5460.
(22) A prolonged reaction time caused partial epimerization at the
R-carbon of the target C-glycosyl R-amino acids.
OL048963G
(23) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V. Sharpless, K. B.
Angew. Chem., Int. Ed. 2002, 41, 2596-2599.
2932
Org. Lett., Vol. 6, No. 17, 2004