Multicomponent synthesis of peptide-sugar conjugates incorporating hexafluorovaline

Article abstract of DOI:10.1002/adsc.201000489

The development of new methods for linking sugars to peptides or proteins is an active area of research because natural glycopeptides or neoglycoconjugates play important roles in biology and medicine and are indispensable tools for probing several biological processes. Herein we report a novel one-pot, three-component process for the synthesis of peptide-urea conjugates incorporating a hexafluorovaline residue under very mild conditions and high yields using commercially available starting materials such as carbodiimides, α-amino acid derivatives and 4,4,4-trifluoro-3- trifluoromethylcrotonic acid. The reaction has been exploited for the synthesis of a library of structurally diverse peptide-sugar conjugates incorporating hexafluorovaline through a four-component, one-pot sequential process by generating the carbodiimides in situ from easily accessible sugar containing azides and commercial available isocyanates through the Staudinger (aza-Wittig) reaction.

Full text of DOI:10.1002/adsc.201000489

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DOI: 10.1002/adsc.201000489
Multicomponent Synthesis of Peptide-Sugar Conjugates
Incorporating Hexafluorovaline
Maria Cristina Belluccia and Alessandro Volonteriob,*
a
Dipartimento di Scienze Molecolari Agroalimentari, Universitꢀ degli Studi di Milano, via Celoria 2, 20133 Milano, Italy
Dipartimento di Chimica, Materiali, e Ingegneria Chimica “Giulio Natta”, via Mancinelli 7, 20131 Milano, Italy
b
Fax : (+39)-02-2399-3080; phone: (+39)-02-2399-3139; e-mail: alessandro.volonterio@polimi.it
Received: June 23, 2010; Revised: August 7, 2010; Published online: September 30, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000489.
Abstract: The development of new methods for
linking sugars to peptides or proteins is an active
area of research because natural glycopeptides or
neoglycoconjugates play important roles in biology
and medicine and are indispensable tools for prob-
ing several biological processes. Herein we report a
novel one-pot, three-component process for the
synthesis of peptide-urea conjugates incorporating a
hexafluorovaline residue under very mild conditions
and high yields using commercially available start-
ing materials such as carbodiimides, a-amino acid
derivatives and 4,4,4-trifluoro-3-trifluoromethylcro-
tonic acid. The reaction has been exploited for the
synthesis of a library of structurally diverse peptide-
sugar conjugates incorporating hexafluorovaline
through a four-component, one-pot sequential pro-
cess by generating the carbodiimides in situ from
easily accessible sugar containing azides and com-
mercial available isocyanates through the Stauding-
er (aza-Wittig) reaction.
chemistry and diversity-oriented synthesis (DOS) and
thus playing a central role in the development of
modern synthetic methodology for pharmaceutical
and drug discovery research.[2] In this context, the de-
velopment of new methods for linking sugars to pep-
tides or proteins is an active area of research because
natural glycopeptides or neoglycoconjugates play im-
portant roles in biology and medicine and are indis-
pensable tools for probing several biological process-
es.[3] A large number of elegant methods have been
developed for the synthesis and assembly of native N-
and O-linked as well as not-native C- and S-linked
glycopeptides,[4] but these methods may be complicat-
ed by low glycosylation efficiencies and extensive pro-
tection regimes to ensure regioselectivity. Otherwise,
conjugation at primary positions of 6-aminohexoses
and 5-aminopentoses could be an easier way to ac-
complish such an achievement by providing peptide-
sugar conjugates with enzymatically stable artificial
ꢀ
linkages, also considering the fact that the CH2NH2
moiety present in these sugars might mimic some ele-
ments of the glycine structure.[5] For instance, the 6-
position of l-galactose has been conjugated to differ-
ent amino acids in the synthesis of sialyl Lewis X
mimetics[6] as well as the primary 6’-[7] and 5’’-posi-
tions[8] of neomycine in the modification of aminogly-
coside antibiotics targeting RNA. However, glycocon-
Keywords: fluorine; glycoconjugates; multicompo-
nent reactions; peptides
Multicomponent reactions (MCRs)[1] are convergent jugate synthesis involving sugars and amino acids or
transformations, in which three or more starting mate- peptides remains a formidable task since the devel-
rials react to form a product, where basically all or oped synthetic protocols are quite demanding and in-
most of the atoms contribute to the newly formed volve multiple reaction steps. The problem may be in
compound. Applications of MCRs in all areas of ap- part or completely obviated through the use of
plied chemistry are very popular because they offer a MCRs.
wealth of products, while requiring only a minimum
effort combining many elements of an ideal synthesis, when treated with suitable carboxylic acids such as
such as operational simplicity, atom economy, bond- 4,4,4-trifluoro-3-trifluoromethyl(Tfm)-crotonic acid in
Recently, we demonstrated that carbodiimides,
AHCTUNGTRENNUNG
forming efficiency, the access to molecular complexity the absence of a nucleophile, are useful reagents for
from simple starting materials. As such, MCRs have the synthesis of diversely substituted hydantoins
become the cornerstones of both combinatorial through a regiospecific domino condensation/aza-Mi-
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Scheme 1. Three-component synthesis of urea-hfVal amides.
chael/N!O acyl migration process.[9] Herein we wish lower probably because of the steric hindrance of the
to report that the same reaction conducted in the nucleophiles.[16] The process proved to be highly ver-
presence of nucleophiles such as amines or a-amino satile since also a-amino acid amides H-Gly-NH2 5f
acid derivatives give rise to a totally new one-pot, and H-Phe-NHCH3 5g reacted thoroughly producing
three-component process for the synthesis of peptide- urea-peptide conjugates 6f and 6g, respectively, in
urea conjugates incorporating a hexafluorovaline high yields (entries 6 and 7, Table 1). Finally we tried
(hfVal) residue under very mild conditions and high the reaction with dipeptide 5h in order to prove that
yields. The reaction has been exploited for the synthe- such a process could be used for the synthesis of
sis of a library of structurally diverse peptide-sugar longer urea-peptide conjugates. Also in this case, the
conjugates incorporating hfVal through a four-compo- reaction worked efficiently giving rise to the forma-
nent, one-pot sequential process by generating the tion of the expected urea-tripeptide conjugate 6h in
carbodiimides in situ from sugar-containing azides excellent yields.
and isocyanates through the Staudinger (aza-Wittig)
All the experimental data shown before suggest
reaction.[10] It is noteworthy that highly fluorinated that the reaction arises through a domino process in
amino acids such as hfVal and hexafluoroleucine which the acid 1 reacts with carbodiimide 2 forming
(hfLeu) have been used to stabilize proteins[11] for po- an O-acylisourea intermediate 7 which readily cyclizes
tential application in various protein-based biotech- to intermediate 8 through an intramolecular aza-Mi-
nologies, such as protein therapeutics, industrial-scale chael reaction. The cyclic O-acylisourea 8 results to
biotrasformations and biosensors. Moreover, the in- be more reactive toward nucleophiles than 7 and in
corporation of fluorinated amino acids into peptides the presence of amines or a-amino acid derivatives
or proteins can significantly increase diffusion across undergoes ring opening by nucleophilic attack to the
membranes such as the blood-brain barrier,[12] im- more reactive carbonyl moiety leading to the desired
prove the pharmacokinetical properties of the pep- urea-peptide conjugate 6 (Scheme 2).[17]
tide-based drug[13] and allows monitoring of the chem-
ical environment of the fluorine-containing residues the usefulness of this method for the synthesis of
by non-invasive 19F NMR.[14]
more complicated scaffolds by getting regioselectivity
To support the proposed mechanism and to amplify
4,4,4-Trifluoro-3-Tfm-crotonic acid 1 was treated with asymmetric carbodiimides, we thought to per-
with commercially available DIC 2 in the presence of form the reaction with a carbodiimide bearing a pri-
primary and secondary amines, such as benzylamine mary alkyl substituent and a tertiary alkyl substituent
and morpholine, in different conditions of solvents at the two nitrogen atoms. Thus, N-allyl-N’-tert-butyl-
and temperature (data not shown). The best condi- carbodiimide 2 was reacted with 4,4,4-trifluoro-3-
tions found were those outlined in Scheme 1, namely Tfm-crotonic acid 1 in the presence of H-Ala-OBn 5a
the presence of 1 equivalent of a base (TMP) and ace- giving rise to the formation, as expected, of only one
tonitrile as solvent at 08C. In both cases, we were out of the two possible regioisomers 9 which arose
able to recover the target urea-hfVal amides 3 and 4 from the nucleophilic attack of the less congested ni-
in excellent yields.
Thus, in order to demonstrate the feasibility of this tramolecular aza-Michael step (Scheme 3).[18]
methodology for the synthesis of urea-peptide conju- Finally, we investigated the possibility to use this
trogen bearing the primary alkyl substituent in the in-
gates, we explored the reaction with less nuclophilic process for the MC synthesis of sugar-urea-peptide
a-amino acid esters and derivatives (Table 1). Under conjugates by incorporating the glycosyl moiety in the
the same conditions as outlined before, the reaction carbodiimide framework. We considered that by using
worked well with a-amino acid esters 5a–c giving rise the Staudinger reaction starting from easily accessible
to the formation of an equimolar mixture of two, primary glycosyl azide derivatives and commercially
easily separable, urea-hflVal-peptide conjugates 6a–c, available tert-butyl isocyanate we would be able to
respectively, in very good yields (entries 1–3, generate in situ the desired carbodiimides. These, in
Table 1).[15] Also N-alkylated a-amino acid esters 5d, turn, would react with 4,4,4-trifluoro-3-Tfm-crotonic
e reacted smoothly leading to N-alkylpeptides 6d, e acid in the presence of a-amino acid derivatives
respectively. In these cases the yields turned out to be through a regioselective four-component one-pot se-
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Multicomponent Synthesis of Peptide-Sugar Conjugates Incorporating Hexafluorovaline
Table 1. Three-component synthesis of urea-peptide conjugates.
Entry R1
R2
X
Nucleophile
Product[a]
Yield [%][b]
80
1
2
H
H
Me
OCH2Ph
i-Pr
OC
(CH3)3
85
81
3
-(CH2)4-
OCH2Ph
4
5
CH2Ph
allyl
H
OCH2CH3
OCH2Ph
NH2
60
Me
H
53[c]
6
7
H
H
67
84
CH2Ph NHCH3
8
H
CH2Ph
75
[a]
[b]
[c]
Equimolecular ratio of two diastereoisomers.
Isolated yields.
Unseparable diastereoisomers.
Scheme 2. The proposed mechanism.
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Scheme 3. Reaction with asymmetric carbodiimide.
quential process leading to the formation of the target and paromomycin, and that their 5’’ position (5 posi-
conjugates (Table 2). Indeed, when azidogalactose de- tion of ribofuranose) has been often functionalizated
rivative 10a was reacted with tert-butyl isocyanate 11a in order to obtain more potent/selective antibiotics.[21]
in CH3CN in the presence of triphenylphosphine, car- The functionalization of such a position by this pro-
bodiimide 12a and triphenylphosphine oxide were cess is currently being examined in our laboratories.
cleanly formed. By adding to the resulting solution
In conclusion, we have developed a novel and effi-
TMP followed by the hydrochloride salts of H-Ala- cient process for the synthesis of libraries of urea-pep-
OBn 5a or H-Leu-OBn 5i and the acid 1, we were tide and glycosyl-peptide conjugates containing hfVal
able to obtain the desired conjugates 13a and 13b, re- amino acid through a multi-component reaction in-
spectively, in high yields and complete regiocontrol, volving simple and readily accessible starting materi-
in a 3.5 to 1 diastereisomeric ratio (entries 1 and 2, als. The operational simplicity and the good chemical
Table 2).[19] Again, in order to prove the versatility of yields, combined with favourable atom-economy as-
the process, we performed the reaction with a-amino pects and a small number of synthetic steps, render
amide 5j and dipeptide 5k recovering the correspond- this new synthetic strategy attractive and promising
ing hfVal-glyco-dipeptide 13c and -tripeptide 13d in for the preparation of novel classes of glycosyl-pep-
very high yields (entries 3 and 4, Table 2).
tide conjugates and particularly suitable for solid-
It is worth noting that since the diasteroselectivity phase/combinatorial chemistry. The latter issues as
of the process depends on the nucleophilic attack of well as the modification of aminoglycoside antibiotics
the chiral primary glycosyl-amino moiety, we obtained are currently in progress.
the same diasteroisomeric ratio when using the same
carbodiimide, that is, 3.5 to 1 with carbodiimide 12a.
Importantly, the process also worked efficiently with Experimental Section
the symmetrical diglycosylcarbodiimide generated in
situ from azide 10a and thioisocyanate 11b leading to General Methods
the formation of of an equimolar mixture of two dia-
stereoisomers 13e. This is very intriguing because the
Commercially available reagent grade solvents were em-
ployed without purification. TLC analyses were run on silica
product could be seen as the precursor for a new class
of glycomimetics in which two saccharides are teth-
ered through a urea-hfVal-peptide moiety (entry 5,
Table 2). Finally, we tried the reaction starting with
another glycosyl azide, namely methyl 5-azido-5-
gel 60 F254 Merck. Flash chromatographies (FC) were per-
formed with silica gel 60 (60–200 mm, Merck). 1H NMR
spectra were run on spectrometers at 250, 400 or 500 MHz.
Chemical shifts are expressed in ppm (d), using tetramethyl-
1
silane (TMS) as internal standard for H and 13C nuclei (dH
and dC =0.00), while C6F6 was used as external standard
(dF =162.90) for 19F. Glycosyl azides 10a, b and glycosyl iso-
thiocyanate 11b were prepared following reported proce-
dures.[22]
deoxy-2,3-O-isopropylidene-b-ribofuranoside
10b.
Here again, we were able to obtain an almost equi-
molar ratio (1.5:1 dr) of the conjugates 13f, g, respec-
tively, in high yields and with total regiocontrol, after
generating in situ the corresponding carbodiimide
with isocyanate 11a and performing the reaction with
the hydrochloric salts of H-Ala-OBn 5a and H-Val-O-
t-Bu 5b (entries 6 and 7, Table 2). More sterically con-
gested N-alkyl-a-amino acid esters 5d, e reacted with
the same carbodiimide 12c leading to the formation
of the corresponding products 13h, i, respectively, in
lower, although acceptable, yields (entries 8 and 9,
Table 2).[20] As expected, in this case we also obtained
very good results on performing the reaction with di-
peptide 5h affording the hfVal-tetrapeptide conjugate
13j in very good yields (entry 10, Table 2). It is note-
worthy that b-ribofuranose is the central sugar of
General Procedure for the Three-Component
Synthesis of Urea-Peptide Conjugates
To a stirred solution of carbodiimide (1 equiv.) in CH3CN
(0.1M) amino acid derivative (1 equiv.) followed by TMP
(1 equiv. or 2 equiv. when the hydrochloride salts of the
amino acid derivatives were used) and a solution of 4,4,4-tri-
fluoro-3-Tfm-crotonic acid (1 equiv.) in a minimun amount
of CH3CN were added at 08C. The resulting solution was
stirred until the reaction was complete (TLC monitoring) al-
lowing the temperature to slowly reach room temperature.
A 1N HCl aqueous solution was added and the mixture ex-
tracted three times with AcOEt. The combined organic
many aminoglycoside antibiotics, such as neomycin layers were dried over anhydrous Na2SO4, filtered, concen-
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Multicomponent Synthesis of Peptide-Sugar Conjugates Incorporating Hexafluorovaline
Adv. Synth. Catal. 2010, 352, 2791 – 2798
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Multicomponent Synthesis of Peptide-Sugar Conjugates Incorporating Hexafluorovaline
trated under vacuum and the crude material purified by
flash chromatography.
3: Rf =0.34 (hexane:AcOEt, 80:20); H NMR (500 MHz,
CDCl3): d=7.96 (br s, 1H), 7.35 (m, 5H), 5.87 (br s, 1H),
5.49 (d, J=4.7 Hz, 1H), 5.19 (d, J=12.7 Hz, 1H), 5.12 (d,
J=12.7 Hz, 1H), 4.55 (br d, J=6.6 Hz, 1H), 4.44 (septet,
J=7.0 Hz, 1H), 4.27 (dd, J=4.7 and 2.3 Hz, 1H), 4.21 (dd,
J=8.0 and 2.3 Hz, 1H), 4.12 (q, J=7.0 Hz, 1H), 3.97–3.73
(br m, 2H), 3.15 (br m, 1H), 1.45 (s, 6H), 1.33–1.30 (m,
15H), 1.25 (s, 3H); 19F NMR (235.4 MHz, CDCl3): d=ꢀ65.1
(m, 3F), ꢀ64.2 (m, 3F); 13C NMR (125.7 MHz, CDCl3); d=
171.3, 171.1, 167.9, 135.5, 128.6, 128.3, 128.2, 123.7 (q, J=
281.6 Hz), 123.0 (q, J=282.5 Hz), 109.4, 96.2, 70.6, 70.5,
66.9, 60.3, 51.2, 51.1, 49.0, 46.4 (septet, J=29.1 Hz), 28.7,
25.9, 25.8, 25.0, 24.2, 20.9, 14.2; ESI-MS: m/z=750.1 [M+ +
Na, (100)].
(S)-13a (minor diasteroisomer): Rf =0.15 (hexane:AcOEt,
80:20); [a]2D0: ꢀ25.28 (c 0.50, CHCl3); 1H NMR (500 MHz,
CDCl3): d=8.10 (br s, 1H), 7.33 (m, 5H), 6.05 (br s, 1H),
5.51 (d, J=4.5 Hz, 1H), 5.16 (d, J=12.5 Hz, 1H), 5.13 (d,
J=12.5 Hz, 1H), 4.79 (br s, 2H), 4.64 (dd, J=7.0 and
2.0 Hz, 1H), 4.57 (septet, J=7.0 Hz, 1H), 4.32 (dd, J=5.0
and 2.0 Hz, 1H), 4.18 (dd, J=7.5 and 2.0 Hz, 1H), 3.96 (m,
1H), 3.47 (br m, 1H), 3.33 (dd, J=16.5 and 6.0 Hz, 1H),
1.47 (s, 6H), 1.38–1.31 (m, 9H); 19F NMR (235.4 MHz,
CDCl3): d=ꢀ64.9 (m, 3F), ꢀ64.3 (m, 3F); 13C NMR
(62.9 MHz, CDCl3); d=172.1, 167.3, 158.7, 135.6, 128.5,
128.3, 128.2, 109.6, 109.2, 96.4, 70.6, 70.2, 66.8, 51.2, 48.6,
28.9, 26.0, 25.9, 25.6, 24.8, 24.4, 17.5. The CF3 and C-CF3 sig-
nals were obscured due to their low intensity; ESI-MS:
m/z=750.1 [M+ +Na, (100)].
1
CDCl3): d=8.65 (br s, 1H), 7.25 (m, 5H), 4.71 (br s, 1H),
4.58 (br d, J=7.0 Hz, 1H), 4.49 (dd, J=15.0 and 6.0 Hz,
1H), 4.40 (br s, 1H), 4.28 (dd, J=15.0 and 6.0 Hz, 1H), 3.91
(octet, J=6.5 Hz, 1H), 3.77 (septet, J=7.0 Hz, 1H), 1.26 (d,
J=6.5 Hz, 3H), 1.18 (d, J=6.5 Hz, 3H), 1.15 (d, J=6.5 Hz,
3H), 1.13 (d, J=6.5 Hz, 3H); 19F NMR (235.4 MHz,
CDCl3): d=ꢀ64.6 (br s, 3F), ꢀ64.1 (br s, 3F); 13C NMR
(125.7 MHz, CDCl3); d=169.1, 158.4, 137.9, 128.5, 127.6,
127.3, 123.0 (q, J=281.8 Hz), 122.8 (q, J=282.1 Hz), 56.4,
50.4, 47.0 (septet, J=26.8 Hz), 43.6, 43.0, 23.0, 21.1, 20.5;
ESI-MS: m/z=480.0 [M+ +K, (4)], 464.1 [M+ +Na, (100)].
(S)-6a: Rf =0.41 (hexane:AcOEt ,70:30); [a]2D0: +80.58 (c
0.48, CHCl3); 1H NMR (250 MHz, CDCl3): d=9.05 (br d,
J=6.4 Hz, 1H), 7.33 (m, 5H), 5.17 (d, J=12.0 Hz, 1H), 5.11
(d, J=12.0 Hz, 1H), 4.80 (m, 1H), 4.54 (quintet, J=7.2 Hz,
1H), 4.40 (d, J=7.2 Hz, 1H), 4.17 (d, J=11.2 Hz, 1H), 3.94
(octet, J=6.4 Hz, 1H), 3.76 (septet, J=7.2 Hz, 1H), 1.40 (d,
J=7.2 Hz, 3H), 1.28–1.16 (m, 12H); 19F NMR (235.6 MHz,
CDCl3): d=ꢀ64.9 (quintet, J=7.2 Hz, 3F), ꢀ63.9 (quintet,
J=7.2 Hz, 3F); 13C NMR (62.9 MHz, CDCl3); d=172.3,
169.0, 158.4, 135.5, 128.5, 128.3, 128.0, 123.0 (q, J=
281.2 Hz), 122.3 (q, J=282.0 Hz), 66.8, 50.9, 48.2, 46.7
(septet, J=25.9 Hz), 43.0, 23.0, 21.3, 20.0, 17.5; ESI-MS:
m/z=514 [M+, (5)], 222 (89), 91 (100).
(R)-6a: Rf =0.35 (hexane:AcOEt, 70:30); [a]2D0: ꢀ33.18 (c
0.64, CHCl3); 1H NMR (250 MHz, CDCl3): d=8.83 (br s,
1H), 7.35 (m, 5H), 5.20 (d, J=12.0 Hz, 1H), 5.14 (d, J=
12.0 Hz, 1H), 4.71 (m, 1H), 4.54 (m, 2H), 4.34 (br d, J=
9.6 Hz, 1H), 3.94 (octet, J=6.4 Hz, 1H), 3.78 (septet, J=
7.2 Hz, 1H), 1.39 (d, J=7.2 Hz, 3H), 1.28–1.16 (m, 12H);
19F NMR (235.6 MHz, CDCl3): d=ꢀ64.6 (m, 3F), ꢀ64.1 (m,
3F); 13C NMR (62.9 MHz, CDCl3); d=171.9, 168.7, 158.3,
135.4, 128.5, 128.3, 67.0, 56.0, 50.3, 48.4, 46.8 (septet, J=
27.7 Hz), 42.9, 23.0, 21.2, 20.5, 18.0. The CF3 signal was ob-
scured due to its low intensity; ESI-MS: m/z=514 [M+, (9)],
222 (93), 91 (100).
Acknowledgements
Politecnico di Milano is gratefully acknowledged for econom-
ic support. Prof Yitzhak Tor is acknowledged for helpful dis-
cussions and CNR-ISTM for the X-ray experiments.
References
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Sequential Synthesis of Peptide-Sugar Conjugates
To a stirred solution of glycosyl azide (1 equiv.) in CH3CN
(0.1M) neat isoACHTUNGTRENNUNG(thio)cyanate (1 equiv.) followed by a solu-
tion of Ph3P (1 equiv.) in a minimun amount of CH3CN
were added at room temperature. After the formation of
the corresponding carbodiimide was complete (TLC moni-
toring, ca. 3 h.), the solution was cooled to 08C and amino
acid derivative (1 equiv.) followed by TMP (1 equiv. or
2 equiv. when the hydrochloride salts of the amino acid de-
rivatives were used) and a solution of 4,4,4-trifluoro-3-Tfm-
crotonic acid (1 equiv.) in a minimun amount of CH3CN
were added. The resulting solution was stirred until the reac-
tion was complete (TLC monitoring) allowing the tempera-
ture to slowly reach room temperature. A 1N HCl aqueous
solution was added and the mixture extracted three times
with AcOEt. The combined organic layers were dried over
anhydrous Na2SO4, filtered, concentrated under vacuum and
the crude material purified by flash chromatography.
[4] Two reviews: a) A. Dondoni, M. Marra, Chem. Rev.
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(R)-13a (major diasteroisomer): Rf =0.22 (hexane:AcOEt,
80:20); [a]2D0: ꢀ8.38 (c 0.9, CHCl3); 1H NMR (500 MHz,
Adv. Synth. Catal. 2010, 352, 2791 – 2798
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Maria Cristina Bellucci and Alessandro Volonterio
COMMUNICATIONS
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[16] A ca. 20% yield of the corresponding hydantoin deriva-
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[17] The same mechanism has been proposed for a very re-
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[18] The regiochemistry of compounds 9 has been deter-
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action conducted in absence of the a-amino ester nu-
cleophile produced hydantoin arising from the nucleo-
philic attack of the less congested N-allyl moiety (see
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[19] The stereochemistry of the major diastereoisomer was
assessed by X-ray diffraction of sugar-peptide conju-
gate (R)-13f (see Supporting Information) and on the
basis of their spectroscopic and analytical features. Full
details will be given in a full paper.
[20] Also in this case, the diasteroisomeric glycosyl-hydan-
toin conjugates were recovered as by-products. A de-
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published in a forthcoming paper.
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[15] With an achiral carbodiimide the reaction is not stereo-
selective because the stereogenic a-amino acid ester
steps in the process once the new stereogenic center is
already formed (see the proposed mechanism). How-
ACHTUNGTRENNUNGever, from the perspective of combinatorial application,
the formation of both stereoisomers is not necessarily a
drawback. The stereochemistry of the two diastereoiso-
2798
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Adv. Synth. Catal. 2010, 352, 2791 – 2798