Serine and threonine Schiff base esters react with β-anomeric peracetates in the presence of BF3·Et2O to produce β-glycosides

Article abstract of DOI:10.1080/07328303.2010.508295

Improved procedures are reported for the glycosylation of L-serine and L-threonine utilizing activated Schiff base glycosyl acceptors, which are less expensive and more efficient alternatives to published methods. L-serine or L-threonine benzyl ester hydrochloride salts were reacted with the diarylketimine bis-(4-methoxyphenyl)-methanimine in CH₃CN at rt to form the more nucleophilic Schiff bases 3a and 3b in excellent yield. These Schiff bases exhibited ring-chain tautomerism in CDCl₃ as shown by ¹H NMR. Schiff bases 3a and 3b, acting as glycosyl acceptors, reacted at rt with simple sugar peracetate donors with BF₃·OEt ₂ promotion to provide the corresponding L-serine and L-threonine O-linked glycosides in excellent yields and purities. The dipeptide ester Schiff base Ar₂C = N-Ser-Val-OCH₃ 3e also reacted to provide β-glycosides in excellent yields, and without epimerization. With microwave irradiation the reactions were complete in 2 to 5 min. To investigate this reaction further, classical AgOTf-promoted Koenigs-Knorr reaction of D-glucopyranosyl, lactosyl, and maltosyl bromides were examined, providing the β-glycosides with yields ranging from 35% to 68%. The difference in reactivity between α- and β-carbohydrate peracetate donors was remarkable. The less configurationally stable D-xylopyranosyl tetra-acetate (a pentose) showed no selectivity (αvsβ-configuration) toward the Schiff bases. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/07328303.2010.508295

This article was downloaded by: [University of Waterloo]
On: 17 October 2014, At: 09:04
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Carbohydrate Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/lcar20
Serine and Threonine Schiff Base Esters
React with β-Anomeric Peracetates in
the Presence of BF3·Et2O to Produce β-
Glycosides
Charles M. Keyari a & Robin Polt a
a Carl S. Marvel Laboratories, Department of Chemistry &
Biochemistry, BIO5 , The University of Arizona , Tucson, AZ, 85721,
USA
Published online: 26 Aug 2010.
To cite this article: Charles M. Keyari & Robin Polt (2010) Serine and Threonine Schiff Base Esters
React with β-Anomeric Peracetates in the Presence of BF3·Et2O to Produce β-Glycosides, Journal of
Carbohydrate Chemistry, 29:4, 181-206, DOI: 10.1080/07328303.2010.508295
To link to this article: http://dx.doi.org/10.1080/07328303.2010.508295
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Carbohydrate Chemistry, 29:181–206, 2010
ꢀ
C
Copyright Taylor & Francis Group, LLC
ISSN: 0732-8303 print / 1532-2327 online
DOI: 10.1080/07328303.2010.508295
Serine and Threonine Schiff
Base Esters React with
β-Anomeric Peracetates in the
Presence of BF3·Et2O to
Produce β-Glycosides
Charles M. Keyari and Robin Polt
Carl S. Marvel Laboratories, Department of Chemistry & Biochemistry, BIO5,
The University of Arizona, Tucson, AZ 85721, USA
Improved procedures are reported for the glycosylation of L-serine and L-threonine uti-
lizing activated Schiff base glycosyl acceptors, which are less expensive and more effi-
cient alternatives to published methods. L-serine or L-threonine benzyl ester hydrochlo-
ride salts were reacted with the diarylketimine bis-(4-methoxyphenyl)-methanimine in
CH3CN at rt to form the more nucleophilic Schiff bases 3a and 3b in excellent yield.
1
These Schiff bases exhibited ring-chain tautomerism in CDCl3 as shown by H NMR.
Schiff bases 3a and 3b, acting as glycosyl acceptors, reacted at rt with simple sugar
peracetate donors with BF3·OEt2 promotion to provide the corresponding L-serine and
L-threonine O-linked glycosides in excellent yields and purities. The dipeptide ester
Schiff base Ar2C = N-Ser-Val-OCH3 3e also reacted to provide β-glycosides in excel-
lent yields, and without epimerization. With microwave irradiation the reactions were
complete in 2 to 5 min. To investigate this reaction further, classical AgOTf-promoted
Koenigs-Knorr reaction of D-glucopyranosyl, lactosyl, and maltosyl bromides were ex-
amined, providing the β-glycosides with yields ranging from 35% to 68%. The differ-
ence in reactivity between α- and β-carbohydrate peracetate donors was remarkable.
The less configurationally stable D-xylopyranosyl tetra-acetate (a pentose) showed no
selectivity (αvsβ-configuration) toward the Schiff bases.
Keywords Glycosylation; Glycoside; Schiff base; Serine; Glycopeptide; Lewis acid
Received March 19, 2010; accepted July 10, 2010.
Address correspondence to Robin Polt, Carl S. Marvel Laboratories, Department of
Chemistry & Biochemistry, BIO5, The University of Arizona, Tucson, AZ 85721, USA.
E-mail: polt@u.arizona.edu
181

C.M. Keyari & R. Polt
182
Introduction
Many well-documented glycosylation procedures, both old and new, exist for
the synthesis of complex glycosides.[1] For example, in the Koenigs-Knorr[2]
and Helferich[3] reactions, reactive glycosyl bromides and chlorides are re-
acted with alcohols or phenols in the presence of transition metal salts such
as Ag(I) or Hg(II) in a suitable solvent. The AgOTf-catalyzed Koenigs-Knorr
reaction (Hanessian modification) of glycosyl bromides used for preparation of
β-O-glycosylated amino acids requires low temperatures accompanied by slow
introduction of the promoter.[4] The α-product and ortho-ester are often formed
in addition to the β-glycoside. The glycosyl bromides are easily hydrolyzed and
many have short half-lives unless stored in very dry conditions in a freezer.
Sugar fluorides have been exploited by Micheel,[5] Mukaiyama,[6] Nicolaou,[7]
and others.[8] The glycosyl fluoride is a more stable glycosyl donor compared
to the corresponding chlorides or bromides and can be activated in the pres-
ence of different Lewis acids. The more reactive iodides have also been used
recently as glycosyl donors by Gervay.[9] The trichloroacetimidates, introduced
by Schmidt[10] as glycosyl donors in 1980, are generally activated by catalytic
amounts of a Lewis acid, with Me3SiOSO2CF3 (TMSOTf) and BF3·Et2O being
the most commonly used reagents. Numerous methodologies based on alkyl-
and aryl-thioglycosides have also become useful building blocks in oligosac-
charide synthesis.[11] Fugedi et al.[12] developed thiophilic activators such as
dimethyl(methylthio)sulfonium triflate (DMTST) as mildly oxidative promot-
ers. Kahne et al.[13] developed a two-step procedure based on sulfoxide interme-
diates as glycosyl donors activated with triflic anhydride. Fraser-Reid et al.[14]
developed the 4-pentenyl glycosides that can be activated with Br2 or simi-
lar oxidative reagents to produce intermediate bromosugar donors in situ that
undergo further activation with Ag+ or Hg++ salts.
As useful and diverse as the aforementioned methods may be, all of them
require the synthesis of relatively complex donors, which is a disadvantage
when preparing amino acid glycosides on large scale. Often instability (shelf
life) of the donor is an issue. The use of easily prepared and stable sugar ac-
etates as donors avoids these problems. Kilberg’s coupling of sugar acetates
with FMOC-protected amino acids (unprotected carboxyl groups) provided
a direct pathway to glycosyl amino acids, albeit in modest yield.[15] In this
method, BF3·OEt2 or SnCl4 activation of glycosyl acetates in CH2Cl2/CH3CN
at rt was demonstrated to be feasible. The major limitation with this method is
that laborious purification procedures are required, which become increasingly
tedious as the scale of the reaction is increased, and the yields are often low
as a result. Very recently microwave-assisted glycosylation of FMOC-Ser-OBn
˚
using FeCl3 in the presence of 4 A molecular sieves in PhCH3 or CH3CN was
reported to provide β-glycosides in short reaction times and moderate yields.[16]
It was suggested that the α-forms of the sugar acetates investigated were less
reactive due to the anomeric effect.

Serine and threonine Schiff base esters
183
CH3O
COOBn
O
COOBn
N
O
N
H
O
H
O
H
H3CO
E
E
O
COOBn
O
CH3O
O
N
H
H
E
COOBn
N
H
O
E
H3CO
Figure 1: Protection of the amine (Schiff base vs. FMOC) can alter the H-bonding patterns
and influence glycosyl acceptor properties of the serine derivatives. The Schiff base lone
pair (:N ) can act as an internal base to enhance the basicity of the serine OH, whereas
the Fmoc proton (H N) can act as a proton donor to reduce the basicity of the serine OH.
The use of Schiff bases in glycosylation was reported by Szabo´ et al. in 1991
employing glycosyl bromides as donors in the presence of Ag(I) salts.[17] This
chemistry was further extended to amino acid glycosides[18] for use in the syn-
thesis of O-linked glycopeptides that penetrate the blood-brain barrier.[19] The
x-ray crystal structure of the benzophenone Schiff base of methyl L-threonate
determined by Wijayaratne et al.[20] confirmed the existence of the oxazolidine
tautomer, at least in the solid state. The hypothesis is that the Schiff bases
are more effective nucleophiles (Fig. 1) for glycosylation than FMOC-protected
serine or threonine derivatives in the preparation of amino acid glycosides. In
this study, we show that the tautomer population of the Schiff base depends on
whether it is serine or threonine derived and also on the aromatic substituents.
RESULTS
Preparation of Serine and Threonine Schiff Bases
The ketimine, bis-(4-methoxyphenyl)methanimine (1), and serine (2a) or
threonine (2b) benzyl ester hydrochloride salts (Sch. 1) were prepared using
published methods (vide infra). The crystalline diarylketimine (1) was formed

C.M. Keyari & R. Polt
184
CH3O
CH3O
COOBn
NH2• HCl
a
R
COOBn
R
OH
NH
N
OH
H3CO
H3CO
2a R = -H
2b R = -CH3
3a R = -H
88%
1
95%
3b R = -CH3
Scheme 1: Preparation of Schiff bases 3a and 3b. Reagents and conditions: (a) CH3CN, rt,
overnight.
by reacting 1-bromo-4-methoxybenzene and Mg◦ shavings in THF under re-
flux to provide the aryl Grignard reagent, which was subsequently reacted
with p-CH3O-C6H4CN and quenching the reaction with anhydrous CH3OH.
This provided the product 1 in good yield, which crystallized directly from
the reaction mixture.[21] The benzyl ester hydrochloride salts 2a and 2b were
prepared from L-serine or L-threonine and PhCH2OH in the presence of
p-toluenesulfonic acid. The L-serine/L-threonine benzyl ester p-toluenesulfonic
salts formed from the reactions were not isolated, but were evaporated to an
oil in vacuo and dissolved in CH2Cl2. Gaseous HCl was bubbled through this
solution for at least 30 min at 0◦C with stirring followed by stirring for 1 h to
precipitate the product 2a[22,23] as a white crystalline material. The Schiff base
3a was obtained in 88% yield by the reaction of the ketimine with L-serine
benzyl ester hydrochloride salt in CH3CN at rt according to the procedure of
O’Donnell.[24] The threonine Schiff base 3b was prepared using the same pro-
cedure, except the crude benzyl ester formed from L-threonine and benzyl alco-
hol in the presence of p-TsOH was neutralized with NaHCO3 solution. Workup
and crystallization afforded the L-threonine benzyl ester according to a mod-
ification of the procedure reported by Pe´tursson and Baldwin.[25,26] All other
Schiff bases (Fig. 2) were prepared similarly according to the aforementioned
procedures.
Preparation of Dipeptide Schiff Bases
The FMOC-protected dipeptide ester 2c was prepared by coupling FMOC-
L-Ser-OH and L-valine methyl ester HCl salt with HBTU/HOBt and iPrNEt2
in CH2Cl2. The FMOC group was then removed by treatment with 20%
piperidine in CH2Cl2, and the resulting dipeptide ester was acidified with 1N
HCl. The crude HCl salt was reacted with 4,4ꢁ-dimethoxydiphenylketimine in
CH3CN to yield the Schiff base 3e (Sch. 2).

Serine and threonine Schiff base esters
185
Figure 2: Schiff base acceptors.
Preparation of α- and β-acetates of Sugars[27]
Reaction of α-lactose monohydrate with Ac2O/pyridine (0◦ → rt) gave pre-
dominately the more stable (less reactive) α-anomer peracetate, whereas re-
action with Ac2O/NaOAc or Ac2O/KOAc (>100◦C) resulted in the formation of
HO
Fmoc
2c
HO
Fmoc
H
N
a
COOCH3
N
N
COOH
HCl•H2N
COOCH3
H
H
O
HO
HO
3e
CH3O
H
H
b, c
d
N
COOCH3
N
COOCH3
HCl•H2N
N
O
O
CH3O
Scheme 2: Solution phase synthesis of dipeptide Schiff base 3e. Reagents and conditions:
(a) Coupling with HBTU/HOBt, iPrNEt2, CH2Cl2, rt, overnight, 82%; (b) Fmoc deprotection with
C5H10NH/CH2Cl2, rt, 1 h; (c) CH3OH/HCl; (d) Ar2C = NH, CH2Cl2, rt, 16 h, 21% over 3 steps
b–d.

C.M. Keyari & R. Polt
186
Table 1: Acetylation of sugars
Temperature
(◦C)
Reaction
time (h)
Yield
(%)
Sugar
Reagents
β/αa
α-D-Glucose
D-Xylose (α + β)
D-Xylose (α + β)
D-Mannose (α + β)
α-Lactose
Ac2O/pyridine
Ac2O/pyridine
Ac2O/KOAc
Ac2O/pyridine
Ac2O/pyridine
Ac2O/ZnCl2
Ac2O/NaOAc
Ac2O/KOAc
Ac2O/NaOAc
0→rt
0→rt
140
24
24
93
83
50
98
87
97
94
86
92
0.07
0.07
33
0.1
24
0→rt
0→rt
rt
0
24
24
0.25
1.0
0.25
0.10
0.32
4.0
5.9
5.0
α-Lactose
α-Lactose
105
α-Lactose
140
Maltose (α + β)
105
aDetermined by 1 H NMR.
the more reactive β-anomer as the major product. Katsuraya et al. had already
shown that acetylation of oligosaccharides with KOAc as a catalyst gave the
highest β/α ratio.[28] The results are summarized in Table 1.
Schiff base glycosylation
The Schiff base 3a, acting as an acceptor, was reacted with carbohy-
drate peracetate donors in the presence of BF3·OEt2 at rt according to
Scheme 3. These results can be compared with those using the previously
published Koenigs-Knorr method (Sch. 3)[4] with 2,3,4,6-tetra-O-acetyl-α-D-
glucopyranosyl bromide. The glycoside products and yields are given in
Tables 2 and 3. In all cases, the major anomer was isolated, and the tabu-
lated yields are for the anomerically pure glycosides. In Table 3, the difference
in reactivity between α- and β-peracetates is illustrated using both the serine-
or threonine-derived Schiff bases. This reaction was further extended to dis-
accharides (Table 4). The glycoside Schiff bases were purified by flash SiO2
1
column chromatography and analyzed by H NMR, 13C NMR, COSY, HSQC,
and HMBC spectroscopic techniques.
Microwave-assisted glycosylation of Schiff bases
Most of the glycosylation reactions took at least 40 h to react to comple-
tion, and appreciable racemization of the amino acid moiety was also detected
(Tables 5 and 6). The reactions were then subjected to microwave conditions
and products examined by HPLC after very short reaction times (<3 min).
A commercial 1000W kitchen microwave was used to heat the samples. Tem-
perature control was provided by immersing the sealed reaction vessels in ice
water. We estimated the reaction temperatures to be 40◦ to 60◦C by measuring
the temperature of several reactions upon completion of the glycosylation re-
actions. The results are provided in Tables 5 and 6. Glycosides (17–21, 26–27,

Serine and threonine Schiff base esters
187
Scheme 3: Direct glycosylation of acetobromoglucose versus Koenigs-Knorr with the Schiff
base acceptor 3a. (a) BF3·OEt2, CH2Cl2, rt, 24 h, 84%; (b) HBr, HOAc/Ac2O, rt, 1 h; (c) AgOTf,
CH2Cl2, rt, 24 h, 35%.
Fig. 3) from D-serine were included to compare the retention times with those
from L-serine on a chiral column.[29] The products were purified by flash col-
1
umn chromatography using silica gel and analyzed by H NMR, 13C NMR,
COSY, HSQC, and HMBC spectroscopy.
The reaction of β-D-glucose peracetate with the Schiff base 3a with
BF3·OEt2 promotion in a microwave was further investigated to determine the
effect of the solvent on racemization. The results are summarized in Table 7.
OCH3
OCH3
OAc
AcO
BnOOC
O
BnOOC
O
BnOOC
O
AcO
AcO
AcO
AcO
AcO
AcO
N
N
N
O
O
O
AcO
OCH3
OCH3
17
18
19
OAc
OAc
OAc
OCH3
OCH3
AcO
AcO
BnOOC
O
O
BnOOC
O
AcO
AcO
N
N
AcO
AcO
O
OCH3
20
21
OAc
OCH3
OCH3
OAc
OAc
AcO
AcO
BnOOC
O
BnOOC
O
O
AcO
O
AcO
O
AcO
AcO
N
N
O
O
AcO
AcO
OAc
OAc
O
27
OCH3
26
OAc
OAc
Figure 3: D-Serine Schiff base glycosides.

C.M. Keyari & R. Polt
188
Table 2: Glycosylation yields using carbohydrate peracetates
Donor
Reaction time
24 h
Product
Isolated yield
84%
30 h
40 h
82%
64%
48 h
55%
83%
40 h
Imine Hydrolysis with Citric Acid and N-protection
with 9-fluorenylmethylchloroformate (FMOC) Group
Selective removal of the imine was smoothly effected by citric acid and the
NH2 group was then reprotected with FMOC (Sch. 4). Finally, deprotection by

Serine and threonine Schiff base esters
189
Scheme 4: (a) Citric acid (10 equiv), H2O, THF, 2 h; (b) FMOC-Cl (1.2 equiv.) in CH2Cl2 added
dropwise over 20 min, DIEA (1.2 equiv.) 0◦C; (c) 1 Atm. H2\10% Pd-C EtOAc:CH3OH (3:1) 2 h
at rt.
hydrogenolysis cleaves the benzyl ester to provide the required glycosyl amino
acids for FMOC peptide synthesis in excellent yield.
Glycosylation of the Dipeptide Schiff Base
The dipeptide Schiff base 3e was then reacted with β-D-glucose penta-
acetate and β-lactose under microwave conditions in the presence of BF3·OEt2
to provide glycosides 28 and 29 in high yield as shown in Scheme 5. The prod-
1
ucts were analyzed by H NMR, 13C NMR, DQF-COSY, HSQC, and HMBC
spectroscopic techniques after purification by flash column chromatography
using silica gel.
DISCUSSION
Ring-chain Tautomerism in Benzophenone Schiff Bases
The reversible addition of a hydroxyl group to an imine double bond, also
known as ring-chain tautomerism, is a well-known process of the Schiff bases
of amino alcohols and has been studied extensively.[30–32] This is an endo-trig
cyclization, which is disfavored according to the rules proposed by Baldwin.[33]
The ring-chain tautomerism of benzophenone Schiff bases derived from serine/
threonine has not been studied in detail. Immediately after formation, the
Schiff base 3 undergoes tautomerization to give the oxazolidine 4 (Sch. 6). The
NMR spectral data show that the open form (3) and the oxazolidine are ap-
proximately in equal proportions at equilibrium for the serine-derived Schiff

C.M. Keyari & R. Polt
190
OCH3
H3CO
OAc
OAc
OH
O
O
RO
AcO
RO
AcO
O
OAc
O
N
OAc
OAc
N
O
OCH3
HN
H3CO
BF3•OEt2/Toluene:
CH2Cl2, µW
NH
O
O
3e
O
O
R = Ac, 28, 86%
OAc
AcO
R =
O
, 29, 90%
AcO
AcO
Scheme 5: Dipeptide glycosylation.
base (Fig. 4a). The tautomeric ratio determination was based on the integrals
of well-separated protons; the ring protons of the p-methoxy compounds are
1
well-separated doublets (ca 6.75–7.65 ppm) in the 500-MHz H NMR spectra.
In addition, for the threonine Schiff bases the methyl protons on the Cβ posi-
tion are also well-separated doublets at ca 1.13 ppm for the chain and at ca.
1.30 ppm for the ring. Using a combination of 2D COSY.
HSQC, and HMBC, the oxazolidine quaternary carbon (ca 100.61 ppm) is
coupled to the aromatic protons in 1–3 bond correlations (ca 7.53, 7.38 ppm) as
MeO
MeO
R
MeO
H
R1
CO2Bn
O
O
OBn
O
O
CDCl3
N
R1
N
CO2Bn
H
NH
MeO
MeO
4 a,b
3 a,b
MeO
MeO
MeO
R
O
O
OBn
R
OBn
O
O
N
H
NH
MeO
R = H, CH3
MeO
Scheme 6: Ring-chain tautomerism of the Schiff bases.

Serine and threonine Schiff base esters
191
Table 3: Glycosylation with α- versus β-carbohydrate peracetates
Reaction
time
Isolated
yield
Donor
α:β
Product
R
14:1
40 h
H (10)
76%
14:1
1:>33∗
14:1
48 h
37 h
48 h
CH3 (11) 29%
CH3 (11) 66%
H (5)
trace
1:>33∗
24 h
H (5)
84%
14:1
48 h
48 h
CH3 (12) trace
CH3 (12) 34%
1:>33∗
>33:1∗
48 h
CH3 (13) 50%
∗33:1 is the limit of detection using NMR.
shown in Figure 5 for compound 4a. Likewise the open form of 3a exhibits the
quaternary carbon (ca 171.98 ppm) coupled to the aromatic protons (ca 7.59,
7.015 ppm) in 1–3 bond correlations.
In the case of L-threonine, the oxazolidine predominates (64.4%), suggest-
ing that the presence of the methyl group at the Cβ position has a strong effect
on the tautomer population. This is also supported by the Schiff base without
substituents on the aromatic rings in which the proportion of the oxazolidine is
91.4% for threonine compared to 76.8% of the Schiff base derived from serine.
This is closely related to the gem-dialkyl (Thorpe-Ingold) effect,[34–36] which is
defined by an increase in both rate and equilibrium constants of cyclization
reactions resulting from the introduction of substituents in the linking chain.
The effect is attributed to increased strain and a reduction in entropy of rota-
tion in the open-chain compound.

C.M. Keyari & R. Polt
192
Table 4: Glycosylation with disaccharide donors
Acceptor
Purified yields of
β glycosides
Disaccharide donor
α:β
1:4
69% (14)
61% (16)
66% (16)
65%∗ (16)
1:5.9
—
73% (14)
68% (14)
1:5
—
66% (15)
51% (15)
—
40%∗ (15)
∗Mitchell et al.[4]
An interesting observation is made with the Schiff base with p-methoxy
groups in which the proportion of the open form is significantly increased
(Fig. 4a) for both serine and threonine. These results further establish clearly
the positive inductive effect of the methoxy groups on the aromatic rings of the
Schiff base. A similar observation was made with the Schiff bases of amino al-
cohols in which increasing the electron-withdrawing effect of substituents on
the aryl group at the para- or meta-position shifts the equilibrium toward the
cyclic form.[37]
Another observation worth noting in the 2D COSY spectrum in Figure 5
is the four clearly discernable spin systems in the aromatic region belonging

Serine and threonine Schiff base esters
193
Table 5: RT and microwave-assisted glycosylations of monosaccharides
Reaction
conditions
HPLC
%L
Donor
Acceptor
Time
Yield
β-D-Glc(OAc)5
β-D-Glc(OAc)5
β-D-Glc(OAc)5
β-D-Glc(OAc)5
β-D-Glc(OAc)5
β-D-Glc(OAc)5
β-D-Glc(OAc)5
β-D-Glc(OAc)5
β-D-Xyl(OAc)4
β-D-Xyl(OAc)4
α-D-Xyl(OAc)4
α-D-Man(OAc)5
α-D-Man(OAc)5
3a
3a
3a
3f
rt, CH2Cl2
24 h
84%
63%
63%
84%
65%
82%
41%
56%
69%
62%
54%
38%
56%
84%
88%
95%
81%
97%
100%
100%
100%
90%
95%
92%
89%
92%
−78◦C-rt, CH2Cl2
µW, CH3CN
rt, CH2Cl2
51 h
1.75 min
40 h
2.25 min
30 h
2.0 min
2.0 min
2.25 min
2.25 min
2.25 min
2.25 min
2.25 min
3f
µW, CH3CN
rt, CH2Cl2
3c
3c
3g
3a
3f
3a
3a
3f
µW, CH3CN
µW, CH3CN
µW, CH3CN
µW, CH3CN
µW, CH3CN
µW, CH3CN
µW, CH3CN
Table 6: RT and microwave-assisted glycosylation with hepta-O-acetyl-β−lactose
Reaction
conditions
HPLC
%L
Donor
Acceptor
Time
Yield
β-Lac(OAc)8
β-Lac(OAc)8
β-Lac(OAc)8
β-Lac(OAc)8
β-Lac(OAc)8
β-Lac(OAc)8
β-Lac(OAc)8
3a
3a
3f
rt, CH2Cl2
48 h
2.25 min
48 h
82%
68%
82%
52%
70%
67%
62%
91%
96%
µW, CH3CN
rt, CH2Cl2
91%
3f
µW, CH3CN
µW, CH3CN
rt, CH2Cl2
2.25 min
2.25 min
48 h
98%
3c
3c
3g
100%
100%
100%
µW, CH3CN
2.25 min
Table 7: Solvent effects on glycosylation
Reaction
conditions
HPLC
(L-isomer)
Entry
Solvent
1
2
3
4
5
6
7
8
9
CH2Cl2/toluene (1:3)
CH2Cl2/toluene (1:3)
CH2Cl2/toluene (1:3)
CH2Cl2/toluene (1:3)
CH2Cl2/toluene (1:3)
Toluene
0.5 h, 60◦C
1 h, 60◦C
91%
91%
92%
87%
95%
91%
94%
95%
96%
3 h, 60◦C
2.25 min, µW
2.25 min, µW
2.25 min, µW
1.75 min, µW
2.25 min, µW
2.25 min, µW
CH3CN
CH3CN
CH3CN
µW = Microwave oven.

C.M. Keyari & R. Polt
194
Figure 4a: Ring-chain tautomerism and tautomer populations of Schiff bases.
Figure 4b: Ring-chain tautomerism and tautomer populations of Schiff bases derived from
D-serine.
Figure 5: HMBC and COSY spectra of 3a and 4a in the aromatic region, and a and a’
protons on the aromatic rings of each tautomer in a 1–3 bond correlation to the quaternary
carbon.

Serine and threonine Schiff base esters
195
OCH3
OH
H3CO
O
O
H
N
O
O
N
N
CDCl3
H
HN
O
H3CO
O
O
4e (16%)
H3CO
3e (84%)
Figure 6: Elongation of the chain shifts the equilibrium to the left, favoring structure 3e.
to the two tautomers. These data indicate that the two aromatic groups are in
different chemical environments, corroborating the results of Wijayaratne and
coworkers in which the N H bond was shown to be directed toward the center
of a phenyl ring on an adjacent molecule in the x-ray crystal structure of the
benzophenone Schiff base of methyl L-threoninate.
On the other hand, chain elongation shifts the equilibrium to the left,
favoring the open form as shown in Figure 6. On crystallization from ethyl
acetate/hexanes at 0◦C, only compound 3e was isolated and its x-ray crystal is
given in Figure 7. Further, the two aromatic groups are orthogonal to each
other in agreement with the NMR results. The hydrogen bonding network
Figure 7: The molecular structure of 3e with displacement ellipsoids at the 50% probability
level. Hydrogen atoms have been omitted.

C.M. Keyari & R. Polt
196
shows that the amide H (N1) is bonded to the lone pair of the imine nitro-
gen (N2) and the hydroxyl H (O1) is hydrogen bonded to the amide carbonyl
(C5 O3) of a neighboring molecule of the compound 3e. A possible explana-
tion for the equilibrium favoring structure 3e is that whereas carbon C10 is
electron rich due to the positive inductive effect of the para-methoxy groups,
the hydrogen bonding pattern from the imine nitrogen (N2) favors the amide
H (N1) rather than the hydroxyl H (O1). This has the overall effect of decreas-
ing the distance between O1 and C10 for cyclization to occur. The compound
adopts an extended conformation and the molecular geometry is largely un-
exceptional. This conformation is given added stability by an intramolecular
. . .
. . .
N H N C hydrogen bond; O H O hydrogen bonding parallel with the a
. . .
axis, as shown in Figure 7; and C H π interactions found within the crys-
tal packing, although there is no evidence of face-to-face aromatic stacking.
˚
. . .
The N(1) H(1) N(2) hydrogen bonding distance is 2.18(7) A, whereas the
˚
. . .
O(1) H(1) O(2A) hydrogen bonding distance is only 1.83(8) A and the dis-
tance of N(1) N(2) is 2.630(8) (Table 8). This suggests that the interactions
are quite strong when compared to a normal bond length of 1.451(8) A for
˚
N(2) C(2).
Schiff Base Glycosylation—Neighboring Group Participation
Versus the Anomeric Effect
The reaction of the carbohydrate peracetate donors with the serine-derived
Schiff base provided the glycosides in high yields. For the threonine-derived
Schiff base moderate to low yields were obtained because it is a 2◦ alcohol
in which sterics seem to dictate its lower reactivity. The anomeric configu-
rations[38] of the acetates were confirmed by the one-bond coupling constants
1JC1-H1 ≈ 160–170 Hz for the α-glycosides from the 2D HMBC spectra. An in-
teresting reactivity trend emerged between the α- and β-acetates. One might
reasonably assume that since either anomeric acetate should provide the same
Table 8: Hydrogen bonds for compound 3e [Å and ◦]
. . .
. . .
d(H A)
D
H
A
d(D H)
d(D A)
<(DHA)
. . .
O(1) H(1O) O(2A)
0.87(8)
0.90(7)
0.98
1.83(8)
2.18(7)
2.55
2.702(8)
2.630(8)
3.354(10)
3.400(10)
3.220(9)
178(7)
111(5)
139
. . .
N(1) H(1N) N(2)
. . .
C(6) H(6B) O(1B)
. . .
C(17) H(17C) O(3C)
0.98
2.51
151
. . .
C(20) H(20) O(3A)
0.95
2.46
137
Symmetry operations for equivalent atoms: A: x−1, y, z; B: x+1, y−1, z; C: x, y+1, z−1.

Serine and threonine Schiff base esters
197
oxonium ion under the influence of a Lewis acid, the anomeric ratio would have
no bearing on the course of the reaction. This assumption seems to be invalid.
From Table 2, β-D-glucose penta-acetate reacted with the L-serine Schiff
base 3a in CH2Cl2 at rt to form glycoside 5 in 84% yield, whereas the α-anomer
was completely unaffected under the same reaction conditions (Table 3). Reac-
tion of the L-threonine Schiff base (3b) produced glycoside 12 in 34% yield.
Acetylation of D-mannose with acetic anhydride/pyridine at 0◦C rt (Table 1)
gave the α-peracetate exclusively as the major product, which upon reaction
with 3a resulted in the formation of the α-glycoside 9 in 83%. The pentose-
derived D-xylose tetraacetate (α:β = 14:1, Table 3) reacted with 3a to form
76% of the glycoside 10. However, upon reaction of the same peracetate with
3b, only 27% of 11 was obtained; the unreacted starting materials were re-
covered, but not used in calculating the yield. Reaction of the tetra-acetate
(α:β = 1:33) with 3b resulted in more than double the yield (66%). This clearly
demonstrates that the β-anomeric acetate is more reactive than α-anomer in
the presence of the Lewis acids examined, most likely aided by neighboring
group participation of the acetyl in the 2-position. Further proof is illustrated
by glycosylation of peracetylated lactose and maltose (Table 4) in which higher
yields were obtained when the proportion of the β-acetate was increased.
It is believed that the enhanced reactivity of the β-glycosides is mostly
due to participation of the neighboring trans-acetate, but there may also be
a significant β-effect.[39] Qualitatively, the β-effect was explained in terms of
the interaction of the lone pairs resulting in increased nucleophilicity due to
the generation of a “new” highest occupied molecular orbital (HOMO). Strictly
speaking, the β-glycoside experiences no anomeric effect and one γ-repulsion
O-2-O-1 (Fig. 8), while the α-glycoside does experience the anomeric effect, and
still experiences a γ-repulsion. This is also true for the α-mannoside (Tables
2 and 3), which experiences the anomeric effect and one γ-repulsion O-2-O-1,
hence showing good reactivity toward the Schiff base. Hydrogen bonding can
occur in protic solvents, resulting in small lone pair interactions. Hence, the
anomeric effects are predicted to be more pronounced in nonpolar, aprotic sol-
vents. Hudson and Dale[40] also demonstrated that β-glucose acetylates more
readily than the α-anomer in 1915.
Figure 8: Consequence of the “β-effect.”

C.M. Keyari & R. Polt
198
Our results show that the α-xylose tetraacetate has good reactivity to-
ward the serine-derived Schiff base, but lower reactivity with the threo-
nine Schiff base as indicated by the yields. The difference can clearly be
seen on reaction with the β-anomer, where the yield was more than doubled
with the threonine Schiff base. Similar observations have been made with
the penta-O-acetyl derivatives of β-D-glucopyranose and β-D-galactopyranose,
which reacted rapidly at 0◦C in ethyl mercaptan using ZnCl2 as a catalyst to
yield ethyl tetra-O-acetyl-1-deoxy-1-thio-β-D-glycopyranosides; the α-forms of
these sugar acetates reacted only very slowly.[41] Further studies by Lemieux
et al.[42] demonstrated that penta-O-acetyl-β-D-glucopyranose reacted very
rapidly with TiCl4 to yield the unstable tetra-O-acetyl-β-D-glucopyranosyl
chloride, whereas the α-penta-acetate was quite stable under the same reac-
tion conditions. Mechanistically, the reaction proceeds as shown in Scheme 7.
The last three steps in the syntheses included hydrolysis of the imine by citric
acid (mild acid), protection of the free amine by the FMOC group, and hy-
drogenolysis on Pd-C, which proceeded in excellent yields.
Both rt and microwave-assisted glycosylation reactions with the Schiff
base 3a resulted in racemized products (Tables 5 and 6). However, with the
Schiff base 3c, no racemization was detected. And finally, changing the sol-
vent from the less polar toluene/CH2Cl2 to the more polar CH3CN resulted
in a small decrease in racemization (Table 7). Acetonitrile is known to form a
complex with the oxocarbenium ion intermediate, hence affecting the outcome
OAc
O
OAc
O
BF3•OEt2
CH2Cl2
AcO
AcO
OAc
AcO
AcO
O
CH3
OAc
OAc
O
BF3
δ−
δ+
OAc
O
O
AcO
AcO
Ar
O
H
O
Ph
O
O
δ−
N
Ar
δ+
Ar
OBn
OAc
O
N
Ar
AcO
AcO
O
OAc
O
R
O
R'
AcO
O
O
Scheme 7: Mechanism of glycosylation with Schiff base.

Serine and threonine Schiff base esters
199
of glycosylation reactions.[43] Presumably, the more polar CH3CN can also pro-
mote enhanced enolization of the Schiff base esters, leading to epimerization
of the amino acid. It is noteworthy that the dipeptide Schiff base 3e does not
show any epimerization.
CONCLUSIONS
In retrospect, this study clearly shows that Schiff bases are excellent glycosyl
acceptors. This direct glycosylation procedure is ideal for the stereoselective
synthesis of amino acid glycosides for peptide synthesis. The study has also
demonstrated the difference in reactivity between α- and β-peracetylated car-
bohydrate donors in glycosylation reactions. Consequently, the β-peracetylated
carbohydrate donors may be ideal candidates for glycoside syntheses at larger
scales. These results further establish the positive inductive effect of the
methoxy groups on the aromatic rings of the Schiff bases, and that the tau-
tomer population of the Schiff bases depends on the substituents on the aro-
matic rings. The hydrogen bonding pattern from the imine nitrogen (N2) fa-
vors the amide H (N1) rather than the hydroxyl H (O1) in the dipeptide Schiff
base 3e according to the x-ray crystal structure; hence, cyclization is not favor-
able. Glycosylation with the dipeptide Schiff base shows the usefulness of this
method in the preparation of peptide building blocks. Finally, the last three
steps of the synthesis—selective removal of the imine by citric acid, reprotec-
tion of the NH2 group with the FMOC group, and deprotection of the benzyl
ester by hydrogenolysis—provided required glycosyl amino acids for FMOC
peptide synthesis in good yields.
EXPERIMENTAL SECTION
General Methods
All solvents for air- and moisture-sensitive reactions were freshly distilled
before use. Methylene chloride, CH3OH, and CH3CN were distilled over CaH2
under an atmosphere of argon. THF was dried and deoxygenated over Ph2C =
O/Na◦-K◦. The p-methoxybenzonitrile and 4-bromoanisole were distilled in
vacuo prior to use. All other reagents and solvents, which were purchased from
1
commercial sources, were used directly without further purification. The H
and 13C NMR spectra were recorded on Varian 300, Bruker 500, and Bruker
600-MHz spectrometers in CDCl3 or d6DMSO. The 2D experiments, COSY,
HSQC, and HMBC, were obtained at 500 or 600 MHz. Chemical shifts are re-
ported relative to Me4Si (δH 0.0), CHCl3 (δC 77.0, central triplet), or d6DMSO
[δH 2.50 (central pentet), δC 39.5 (central heptet)]. Optical rotations were deter-
mined with a Jasco Model P-1010 polarimeter. Reaction products of glycosides

C.M. Keyari & R. Polt
200
were determined by HPLC on a CHIRALCEL OD [250 × 4.6 mm (LxI.D.)]
column using hexane/2-propanol (70/30, v/v) as the mobile phase, flow rate
0.75 mL/min, detection: UV (254 nm), temperature: rt.
Bis-(4-methoxyphenyl)-methanimine (1)[24,44]
Mg (0.76 g, 31.6 mmol) was suspended in 30 mL dry THF in a flame-dried
three-neck 250 mL round-bottomed flask equipped with a stir bar under argon.
Some drops of 4-bromoanisole were added, and to initiate the reaction, a crys-
tal of I2 was added and the solution heated to a gentle reflux (∼60◦C). Once
the color of the I2 had disappeared, the remaining 1-bromo-4-methoxybenzene
(3.9 mL, 31.1 mmol) in 8 mL THF was added via a syringe to maintain a
gentle reflux. The mixture was refluxed for 45 min. After cooling to rt, 3.75 g
(28.2 mmol) of p-methoxybenzonitrile in 8 mL of dry THF was added over 20
min and reacted for 6 h under reflux. The reaction mixture was cooled and
4.5 mL of MeOH added to quench the reaction. After stirring for 20 min, it was
concentrated to remove THF, redissolved in CH2Cl2, and quickly washed with
H2O in a separatory funnel to remove Mg salts. The CH2Cl2 layer was dried
over MgSO4, concentrated, and redissolved in a minimal amount of CH2Cl2,
and Et2O added to precipitate white shiny crystals of 1 (4.01 g, 16.62 mmol,
59% yield); mp 128–129◦C, lit.[45] 129.5–130◦C; Rf 0.32 (60% EtOAc:hexanes);
1H NMR (CDCl3, 500 MHz) δ 3.84 (6H, s), 6.90 (4H, d, J = 9.0 Hz), 7.52 (4H, d,
J = 8.5 Hz), 8.95 (NH, broad); 13C NMR (CDCl3, 125 MHz) δ 55.3 (2CH3O),
113.4 (4CH, ar), 130.0 (4CH, ar), 132.1(2C, q), 161.1 (2C-O, q), and 176.9
(C N).
L-Serine Benzyl Ester·HCl (2a)
L-Serine (5.25 g, 50 mmol) and p-toluenesulfonic acid (9.46 g, 55 mmol)
were suspended in PhCH2OH (25 mL) in a 250-mL round-bottomed flask
equipped with a Dean Stark apparatus. Benzene (25 mL) was added and the
reaction was brought to reflux for 12 h; the reaction was cooled to rt and con-
centrated. The sample was then redissolved in 5% NaHCO3 solution and ex-
tracted with i-PrOH/CHCl3 (1:4, 3 × 50 mL), dried over MgSO4, filtered, and
evaporated to a residue. The residue was dissolved in 125 mL of CH2Cl2 and
HCl gas bubbled into the solution at 0◦C for 30 min, followed by stirring for 1 h.
The precipitated white solid was washed with diethyl ether and recrystallized
from i-PrOH/CH3OH (13:1) according to the procedure of Holden et al.[13] Yield
6.06 g (26.2 mmol, 52.3%, over two steps); mp 170–171◦C, [α]23.3 = −4.35◦
D
1
(c 1.1, MeOH); H NMR (d6DMSO, 500 MHz) δ 3.87 (d, Jα,β = 3 Hz, 2H), 4.14
(t, Jβ,α = 3 Hz, 1H), 5.21 (s, 2H), 5.70 (1H, brs), 7.36 (m, 5H, ar); 13C NMR
(d6DMSO, 500 MHz) δ 54.6 (β-CH2), 59.5 (α-CH), 66.9 (CH2Ph), 127.9 (CH, ar),
128.2 (2CH, ar), 128.4 (2CH, ar), 135.3 (C, q), and 168.0 ( COO ).

Serine and threonine Schiff base esters
201
L-Threonine Benzyl Ester·HCl Salt (2b)
L-Threonine (4.0 g, 34 mmol) and pTsOH acid monohydrate (7.10 g,
38 mmol) were reacted with benzyl alcohol (40 mL) in 50 mL dry benzene us-
ing a Dean-Stark apparatus according to the procedure of Pe´tursson and Bald-
win.[16] Yield 5.004 g (23.9 mmol, 71.3%); mp 52–54◦C; [α]25.5 = −19.3◦ (c 2.1,
D
CH3OH); 1H NMR (CDCl3, 500 MHz) δ 1.20 (d, Jβ,γ = 6.5 Hz, 3H), 2.30 (broad,
3H), 3.29 (d, Jβ,α = 5.0 Hz, α-H), 3.91 (m, β-H), 5.17 (s, 2H), 7.33–7.36 m, 5H);
13C NMR (CDCl3, 125 MHz) δ 19.7 (γ- CH3), 59.9 (β-CH2), 66.9 (CH2Ph), 68.2
(α-CH), 128.2 (2CH, ar), 128.4 (CH, ar), 128.6 (2CH, ar), 135.3 (C, q), and 173.9
( COO ).
L-Threonine benzyl ester (1.35 g, 6.45 mmol) was dissolved in
CH2Cl2:CH3OH (3:1) and HCl gas bubbled through the solution with stirring
at 0◦C for 1 h. The solvent was removed by rotary evaporation; then on ad-
dition of CH2Cl2 and diethyl ether, white crystals of the salt were precipi-
tated and filtered to provide 1.31 g (5.33 mmol, 83%) of 2b; mp 125–128◦C,
lit.[46] 127–128◦C; [α]27.7 = −27.38◦ (c 1.0, CH3OH),lit.34 [α]22 = –11.1◦ (c 10,
D
D
EtOH);1H NMR (d6DMSO, 500 MHz) δ 1.21 (d, Jβ,γ = 6.5 Hz, 3H), 3.50 (d,
JαH-NH = 12.0, 2H, NH2), 3.96 (d, Jβ,α = 3.5 Hz, 1H), 4.16 (m, H), 5.21 (s, 2H),
5.75 (OH, brs), 7.32–7.43 (m, 5H, ar); 13C NMR (d6DMSO, 125 MHz) δ 20.0
(γ-CH3), 58.1 (β-CH2), 65.3 (α-CH), 67.0 (CH2Ph), 128.1 (2CH, ar), 128.2 (CH,
ar), 128.4 (2CH, ar), 135.2 (C, q), and 168.0 ( COO ).
D-Serine Benzyl Ester·HCl (2c)
This was synthesized according to the procedure reported by Maclaren.[47]
Starting with 2.65 g (25 mmol, 1.0 equiv.), D-serine provided 4.4 g (76%) of com-
pound 2c as white crystals: mp 179–181◦C; [α]25.1 = +11.93◦ (c 1.5, MeOH);
D
1H NMR (d6DMSO, 500 MHz) δ 3.47 (brs, NH2), 3.88 (d, Jα,β = 3 Hz, 2H), 4.15
(t, Jβ,α = 3.5 Hz, 1H), 5.21 (s, CH2Ph), 5.71 (brs, 1OH), 7.31–7.34 (m, 5H, ar);
13C NMR (d6DMSO, 125 MHz) δ 54.5 (β-CH2), 59.5 (α-CH), 66.9 (CH2Ph), 127.9
(2CH, ar), 128.2 (CH, ar), 128.4 (2CH, ar), 135.3 (C, q), and 167.9 ( COO ).
Methyl-N-(9-fluorenylmethoxycarbonyl)-L-seryl-L-valinate (2d)
L-Valine methyl ester HCl salt (1.02 g, 6.11 mmol, 2.0 equiv.), FMOC-
Ser-OH (1.0 g, 3.06 mmol, 1.0 equiv.), HOBt (0.94 g, 6.11 mmol, 2.0 equiv.),
and HBTU (2.32 g, 6.11 mmol, 2.0 equiv.) were weighed into a 250-mL
round-bottomed flask and 5.5 mL DIEA (30.6 mmol, 10.0 equiv.) added.
Dichloromethane (15 mL) was then added and stirred at rt overnight. The re-
action mixture was then washed two times with conc. NaHCO3, one time with
brine, and one time with H2O; dried over MgSO4; and concentrated on a rotary
evaporator. Flash chromatography with 60% EtOAc: hexanes (Rf = 0.3) crys-
tallization from EtOAc: hexanes provided 1.10 g (82%) of 2d as white crystals.

C.M. Keyari & R. Polt
202
1
[α]25.2 = −35.12◦ (c 1.0, CHCl3); H NMR (CHCl3, 500 MHz) δ 0.88 (d, J =
D
7.0 Hz, 3H), 0.92 (d, J = 7.0 Hz, 3H), 2.18 (m, 1H), 3.50 (dd, J1 = 9.3 Hz, J2 =
4.48 Hz, OH), 3.69 (m), 3.72 (s, OCH3), 4.04 (m, 1H), 4.20 (t, J = 7.0 Hz, 1H),
4.34 (m, 1H), 4.39 (d, J = 7.0, 2H), 4.51 (dd, J1 = 9.0 Hz, J2 = 5.0 Hz, 1H), 5.98
(d, J = 7.5 Hz, NH, FMOC), 7.11 (d, J = 8.0 Hz, NH), 7.29 (t, J = 7.5 Hz, 2H,
ar), 7.38 (t, J = 7.5 Hz, 2H, ar), 7.57 (d, J = 7.5 Hz, 2H, ar), 7.74 (d, J = 7.5
Hz, 2H, ar); 13C NMR (CHCl3, 125 MHz) δ 17.6 (CH3), 19.0 (CH3), 30.7 (CH),
47.0 (CH, FMOC), 52.3 (OCH3), 55.2 (CH), 57.5 (CH), 62.9 (CH2), 67.4 (CH2),
119.9 (2CH, ar), 125.0 (2CH, ar), 127.0 (2CH, ar), 127.7 (2CH, ar), 141.2 (C,
q), 143.6 (C, q), 143.6 (C, q), 156.5 ( CONH, FMOC), 171.1 ( CONH ), and
172.3 ( COO ). FABMS: C24H28N2O6, m/z [M + H]+ 441.2.
Benzyl N-bis-(4-methoxyphenyl)methylene-L-serinate (3a)
The procedure adopted was that of O’Donnell.[24] L-Serine benzyl ester·HCl
(2a) (2.535 g, 10.5 mmol, 1.1 equiv.) and the bis (p-methoxy)-diarylketimine
(1) (2.4 g, 10 mmol, 1.0 equiv.) were dried over P2O5 overnight in vacuo and
transferred to a flame-dried 250-mL round-bottomed flask with a stir bar. The
L-serine benzyl ester·HCl was ground into a fine powder before being trans-
ferred to the round-bottomed flask. Dry CH3CN (60 mL) was added and stir-
ring started at rt and reacted for at least 16 h. TLC showed full conversion.
The reaction mixture was filtered, washed quickly with saturated NaHCO3 so-
lution, and then dried over MgSO4. Finally, it was filtered, concentrated, and
flash chromatographed with 55% EtOAc: hexanes: 0.1% NEt3 to afford 3.67 g
(8.75 mmol, 88%) of the Schiff base as a colorless oil; [α]28.1 = −75.16◦ (c =
D
1
3.0, CHCl3); Rf 0.64 (55% EtOAc: hexanes); (3a) H NMR (CDCl3, 500 MHz) δ
2.73 (brs, OH), 3.78 (s, 3H, CH3O), 3.80 (s, 3H, CH3O), 3.97 (dd, Jα,β = 1.5 Hz,
J
α,β‘ = 4.5 Hz, 2H), 4.29 (t, Jβ,α = 5.0 Hz, 1H), 5.15 (d, J = 9.0 Hz, 2 H, CH2Ph),
6.82 (d, J = 8.5 Hz, 2H, ar H’s), 6.85 (d, J = 9.0 Hz, 2H, ar H’s), 7.01 (d, J = 8.5
Hz, 2H, ar H’s), 7.31 (m, 5H, ar), 7.59 (d, J = 9.0 Hz, 2H, ar); 13C NMR (CDCl3,
125 MHz) δ 55.2 (CH3O), 55.1 (CH3O), 64.2 (β-CH2), 66.4 (α-CH), 66.4 (CH2Ph),
113.2 (CH, ar), 113.7 (CH, ar), 127.9 (2CH, ar), 127.9 (CH, ar), 128.4 (CH, ar),
128.5 (CH, ar), 130.6 (CH, ar), 132.1 (C, q), 132.2 (C, q), 135.6 (C, q), 159.6 (C,
q), 161.6 (C, q), 170.8 ( COO ) and 172.0 (C N). ESIMS: C25H25NO5, m/z
[M + H]+ 420.4.
(S)-Benzyl-2,2-bis(4-methoxyphenol)oxazolidine-4-carboxylate
(4a)
1H NMR (CDCl3, 500 MHz) δ 3.06 (brs, NH), 3.73 (s, 3H, CH3O), 3.75 (s,
3H, CH3O), 3.91 (dd, Jα,β = 7.5 Hz, Jβ,β = 12.5 Hz, 2H), 5.16 (d, J = 12.0 Hz,
2H, CH2Ph), 6.78 (d, J = 8.5 Hz, 2H, ar), 6.82 (d, J = 8.5 Hz, 2H, ar), 7.31 (m,
5H, ar), 7.38 (d, J = 8.5 Hz, 2H, ar), 7.52 (d, J = 9.0 Hz, 2H, ar); 13C NMR

Serine and threonine Schiff base esters
203
(CDCl3, 125 MHz) δ 55.1 (2CH3O), 59.8 (β-CH2), 66.4 (α-CH), 67.2 (CH2Ph),
100.6 (C, q), 113.4 (2CH, ar), 127.1 (CH, ar), 128.2 (CH, ar), 134.7 (C, q), 135.1
(C, q), 135.3 (C, q), 159.0 (C, q), 159.0 (C, q), and 172.3 ( COO ).
Benzyl N-bis (4-methoxyphenyl)methylene-L-threoninate (3b)
Reaction of 1 (0.625 g, 2.6 mmol, 1.0 equiv.) and 2b (0.70 g, 2.85 mmol,
1.1 equiv.) in 40 mL CH3CN as described in the preparation of 3a gave 3b as
a colorless oil(1.12 g, 2.58 mmol, 99.64%); [α]28.1 = −75.67◦ (c 1.4, CHCl3);
D
1
Rf 0.37 (20% EtOAc: hexanes); H NMR (CDCl3, 500 MHz) δ 1.13 (d, Jβ,γ
=
6.5 Hz, 3H), 3.23 (brs, OH), 3.80 (s, 3H, CH3O), 3.81 (s, 3H, CH3O), 3.97 (d,
Jβ,α = 3.5 Hz, α-H), 4.32 (m, β-H), 5.13 (d, J = 12.0 Hz, CH2Ph), 6.84 (d, J = 9.0
Hz, 2H, ar), 6.85 (d, J = 8.5 Hz, 2H, ar), 6.97 (d, J = 8.5 Hz, 2H, ar), 7.29–7.35
(m, 5H, ar), 7.61 (d, J = 9.0 Hz, 2H, ar); 13C NMR (CDCl3, 125 MHz) δ 20.6
(γ-CH3), 55.2 (CH3O), 55.3 (CH3O’), 66.0 (β-CH2), 70.3 (α-CH), 66.5 (CH2Ph),
113.3 (CH, ar), 113.8 (CH, ar), 128.1 (2CH, ar), 128.4 (CH, ar), 128.4 (2CH, ar),
129.0 (2CH, ar), 130.5 (2CH, ar), 132.1 (C, q), 135.7 (C, q), 159.6 (C, q), 161.7
(C, q), 170.9 ( COO ), and 172.1 (C N). ESIMS C26H27NO5, m/z [M + H]+
434.1.
(S)-Benzyl-2,2-bis(4-methoxyphenyl)oxazolidine-4-carboxylate
(4b)
1H NMR (CDCl3, 500 MHz) δ 1.29 (d, Jβ,γ = 6.5 Hz, 3H), 3.23 (brs, NH),
3.60 (d, Jβ,α = 8.0 Hz, α-H), 3.74 (s, 3H, CH3O), 3.76 (s, 3H, CH3O), 4.09 (m,
β-H), 5.16 (d, J = 12.0 Hz, 2H, CH2Ph), 6.78 (d, J = 8.5 Hz, 2H, ar), 6.80 (d, J =
9.0 Hz, 2H, ar), 7.29–7.35 (m, 5H, ar), 7.38 (d, J = 8.5 Hz, 2H, ar), 7.52 (d, J =
8.5 Hz, 2H, ar); 13C NMR (CDCl3, 125 MHz) δ 20.3 (γ-CH3), 55.1 (CH3O), 55.2
(CH3O’), 67.0 (CH2Ph), 67.1 (α-CH), 77.3 (β-CH2), 99.7 (C, q), 113.2 (CH, ar),
113.4 (CH, ar), 127.1 (CH, ar), 127.6 (CH, ar), 128.2 (CH, ar), 128.6 (CH, ar),
135.2 (C, q), 136.5 (C, q), 137.2 (C, q), 158.8 (C, q), 159.0 (C, q), and 171.1
( COO ).
ACKNOWLEDGEMENTS
We thank the Office of Naval Research (N00014-05-1-0807 and N00014-02-1-
0471), the National Science Foundation (CHE-607917), and the National In-
stitutes of Health (NINDS-NS-052727) for Support. We also thank Dr. Gary
Nichol in the Department of Chemistry and Biochemistry, The University of
Arizona, for the x-ray data and analysis.

C.M. Keyari & R. Polt
204
REFERENCES
1. Stick, R.V. Carbohydrates: The Sweet Molecules of Life, Academic Press, Harcourt
Place, London, 2001, 113–178.
¨
2. Koenigs, W.; Knorr, E. Uber einige Derivate des Traubenzuckers und der Galac-
tose. Ber. Dtsch. Chem. Ges. 1901, 34, 957–981.
3. (a) Helfrich, K.; Weis, K. Zur Synthese von Glucosiden und von nicht-
reduzierenden Disacchariden. Chem. Ber. 1956, 89, 314–321 (b) Paulsen, H. Advances
in selective chemical syntheses of complex oligosaccharides. Angew. Chem. Int. Ed.
Engl. 1982, 21, 155–224.
4. Mitchell, S.A.; Pratt, M.R.; Hruby, V.J.; Polt, R. Solid-phase synthesis of O-linked
glycopeptide analogues of enkephalin. J. Org. Chem. 2001, 6, 2327–2342.
5. (a) Micheel, F.; Klemer, A.; Flitsch, R. The reaction mechanism of the glycoside
formation from α- and β-1-fluoro derivatives of D-glucose and D-mannose. Chem. Ber.
1958, 91, 663–667 (b) Micheel, F.; Hallermann, G. A further method for chemical syn-
thesis of polysaccharides. Tetrahedron Lett. 1962, 3, 19–20.
6. Takeuchi, K.; Mukaiyama, T. Trityl tetrakis(pentafluorophenyl)borate catalyzed
stereoselective glycosylation using glycopyranosyl fluoride as a glycosyl donor. Chem.
Lett. 1998, 555–556.
7. Nicolaou, K.C.; Dolle, R.E.; Papahatjis, D.P.; Randall, J.L. Practical synthesis of
oligosaccharides. Partial synthesis of avermectin B1a. J. Am. Chem. Soc. 1984, 106,
4189–4192.
8. Thiem, J.; Wiesner, M. Preparations and reactions of acylated and partially acy-
lated glycosyl fluorides. Carbohydr. Res. 1993, 249, 197–205.
9. (a) Brauns, D.H. Optical rotation and atomic dimension. VIII. Halogeno-hepta-
acetyl derivatives of melibiose and maltose. The structures of bioses and cellulose. J.
Am. Chem. Soc. 1929, 51, 1820–1831 (b) Gervay, J. Glycosyl iodides in organic syn-
thesis. In Organic Synthesis: Theory and Applications, Hudlicky, T., Ed., JAI Press,
Greenwich, CT, 1998, Vol. 4, 121–153.
10. Schmidt, R.R.; Michel, J. Facile synthesis of α- and β-O-glycosyl imidates; prepa-
ration of glycosides and disaccharides. Angew. Chem. Int. Ed. 1980, 19, 731–732.
11. Norberg, T. Glycosylation properties and reactivity of thioglycosides, sulfoxides
and other S-glycosides: current scope and future prospects. In Modern Methods in Car-
bohydrate Synthesis, Khan, S.H.; O’Neill, R.A., eds, Harwood Academic, Netherlands,
1996, 82–106.
12. Fu¨gedi, P.; Garegg, P.J. A novel promoter for the efficient construction of 1,2-trans
linkages in glycoside synthesis, using thioglycosides as glycosyl donors. Carbohydr. Res.
1986, 149, C9–C13.
13. Kahne, D.; Walker, S.; Cheng, S.; Engen, D.V. Glycosylation of unreactive sub-
strates. J. Am. Chem. Soc. 1989, 111, 6881–6882.
14. Fraser-Reid, B.; Merritt, J.R.; Handlon, A.; Andrews, C.W. The chemistry of N-
pentenyl glycosides: synthetic, theoretical, and mechanistic ramifications. Pure Appl.
Chem. 1993, 65, 779–786.
15. Salvador, L.A.; Elofsson, M.; Kilberg, J. Preparation of building blocks for gly-
copeptide synthesis by glycosylation of Fmoc amino acids having unprotected carboxyl
groups. Tetrahedron 1995, 51, 5643–5656.
16. Seibel, J.; Hillringhaus, L.; Moraru, R. Microwave-assisted glycosylation for the
synthesis of glycopeptides. Carbohydr. Res. 2005, 340, 507–511.

Serine and threonine Schiff base esters
205
17. Szabo´, L.; Li, Y.; Polt, R. O-glycopeptides: a simple β-stereoselective glycosidation
of serine and threonine via a favorable hydrogen bonding pattern. Tetrahedron Lett.
1991, 32, 585–588.
18. Polt, R.; Szabo´, L.; Treiberg, J.; Li, Y.; Hruby, V.J. General methods for alpha- or
beta-O-Ser/Thr glycosides and glycopeptides. Solid-phase synthesis of O-glycosyl cyclic
enkephalin analogs. J. Am. Chem. Soc. 1992, 114, 10249–10258.
19. Polt, R.; Porreca, F.; Szabo´, L.Z.; Bilsky, E.J.; Davis, P.; Abbruscato, T.J.; Davis,
T.P.; Harvath, R.; Yamamura, H.I.; Hruby, V.J. Glycopeptide enkephalin analogues pro-
duce analgesia in mice: evidence for penetration of the blood-brain barrier. Proc. Natl.
Acad. Sci. U.S.A. 1994, 91, 7114–7118.
20. Wijayaratne, T.; Collins, N.; Li, Y.; Bruck, M.A.; Polt, R. β-Alkoxy Schiff base-
oxazolidine tautomerism: solid-state structure of N-diphenylmethylene-L-threonine
methyl ester. Acta Cryst. 1993, B49, 316–320.
21. Dejaegher, Y.; Mangelinckx, S.; Kimpe, N.D. Synthesis of substituted benzhydry-
lamines. Synlett 2002, 113–115.
22. Cipera, J.D.; Nicholls, R.V.V. Preparation of benzyl esters of amino acids. Chem.
Industry 1955, 16–17.
23. Holden, K.G.; Mattson, M.N.; Cha, H.K.; Rapoport, H. Synthesis of chiral pilo-
carpine analogues via a C-8 ketone intermediate. J. Org. Chem. 2002, 67, 5913–5918.
24. O’Donnell, M.J.; Polt, R.L. A mild and efficient route to Schiff base derivatives of
amino acids. J. Org. Chem. 1982, 47, 2663–2666.
25. Pe´tursson, S.; Baldwin, J.E. Synthesis of δ-(L-α-aminoadipoyl)-L-cysteinyl-L-(O-
methyl)-D-allothreonine a substrate for isopenicillin-N synthase and its O-methyl-D-
threonine epimer. Tetrahedron 1998, 54, 6001–6010.
26. Gutmann, H.R.; Chang, S.F. DL- and L-Threonine p-toluenesulfonate benzyl ester.
J. Org. Chem. 1962, 27, 2248–2250.
27. (a) Wolfrom, M.L.; Thompson, A. β-Maltose monohydrate. In Methods in Carbohy-
drate Chemistry, Whistler, R.I.; Wolfrom, M.L., eds.; Academic Press, New York, 1962,
Vol. 1, 334 (b) Wolfrom, M.L.; Thompson, A. In Methods in Carbohydrate Synthesis,
Whistler, R.I.; Wolfrom, M.L., eds.; Academic Press, New York, 1963; Vol. 2, 211–215.
28. Katsuraya, K.; Ikushima, N.; Takahashi, N.; Shoji, T.; Nakashima, H.;
Yamamoto, N.; Yoshida,T.; Uryu, T. Synthesis of sulfated alkyl malto- and laminara-
oligosaccharides with potent inhibitory effects on AIDS virus infection. Carbohydr. Res.
1994, 260, 51–61.
29. Column: CHIRALCEL OD (Daicel Chemical Industries Ltd.), mobile phase:
hexane/2-propanol (70/30, v/v), flow rate 0.75 mL/min, detection: UV (254 nm), tem-
perature: rt.
30. (a) Fu¨lo¨p, F.; Pihlaja, K.; Neuvonen, K.; Berna´th, G. Ring-chain tautomerism in
1,3-oxazines. J. Org. Chem. 1987, 52, 3821–3825 (b) Fu¨lo¨p, F.; Pihlaja, K.; Neuvonen,
K.; Berna´th, G.; Argay, G.; Ka´lma´n, A. Ring-chain tautomerism in oxazolidines. J. Org.
Chem. 1993, 58, 1967–1969.
31. (a) Paukstelis, J.V.; Hammaker, R.M. The effect of hydrogen bonding on the ring-
chain tautomerism of oxazolidines. Tetrahedron Lett. 1968, 3557–3560 (b) Alva As-
tudillo, M.E.; Chokotko, N.C.J.; Jarvis, T.C.; Johnson, C.D.; Lewis, C.C.; McDonnell, P.D.
Hydroxy Schiff base-oxazolidine tautomerism: apparent breakdown of Baldwin’s rules.
Tetrahedron 1985, 41, 5919–5928 (c) Valters, R.E.; Flitsch, W. Intramolecular reversible
addition reactions to the C N group. In Ring-Chain Tautomerism, Katritzky, A.R., Ed.;
Plenum Press, New York City, 1985, 169–209.

C.M. Keyari & R. Polt
206
32. Darabantu, M.; Ple´, G.; Mager, S.; Cotora, E.; Gaina, L.; Costas, L.; Mates, A.
Synthesis and stereochemistry of some heterocyclic saturated compounds based on l-
p-nitrophenylserinol skeleton (I). Ring-chain tautomerism of some schiff bases of l-p-
nitrophenylserinol. Tetrahedron 1997, 53, 1873–1890.
33. Baldwin, J.E. Rules for ring closure. J. Chem.Soc. Chem. Commun. 1976, 734–736.
34. (a) Beesley, R.M.; Ingold, C.K.; Thorpe, J.F. Formation and stability of spiro-
compounds. I. Spiro-compounds from cyclohexano. J. Chem. Soc. 1915, 107, 1080–1106
(b) Ingold, C.K. The conditions underlying the formation of unsaturated and of cyclic
compounds from halogenated open-chain derivatives. I. Products derived from α-
halogenated glutaric acids. J. Chem. Soc. 1921, 119, 305–329 (c) Ingold, C.K.; Sako,
S.; Thorpe, J.F. Influence of substituents on the formation and stability of heterocyclic
compounds. I. Hydantoins. J. Chem. Soc. 1922, 1177–1198.
35. Smith, S.W.; Newman, M.S. Gem-dialkyl effect. III. Kinetic and equilibrium stud-
ies of steroid cyclic ketal formation and hydrolysis. J. Am. Chem. Soc. 1968, 88,
1253–1257.
36. Allinger, N.L.; Zalkow, V. Conformational analysis. IX. The Gem-Dimethyl effect.
J. Org. Chem. 1960, 25, 701–704.
37. (a) Paukstelis, J.V.; Lambing, L.L. Ring-chain tautomerism of oxazolidines. Tetra-
hedron Lett. 1970, 299–302 (b) McDonagh, A.F.; Smith, H.E. Ring-chain tautomerism of
derivatives of o-hydroxybenzylamine with aldehydes and ketones. J. Org. Chem. 1968,
33, 1–8.
38. Bock, K.; Pedersen, C. A study of 13CH coupling constants in hexopyranoses.
J. Chem. Soc., Perkin Trans 2, 1974, 293–297; Bock, K.; Pedersen, C. A study of 13CH
coupling constants in pentopyranoses and some of their derivatives. Acta Chem. Scand.
1975, B29, 258–264.
39. Box, V.G.S. Some consequences of lone pair interactions in the chemistry of
monosaccharides. Heterocycles 1982, 19, 1939–1966.
40. Hudson, C.S.; Dale, J.K. A comparison of the optical rotatory powers of the alpha
and beta forms of certain acetylated derivatives of glucose. J. Am. Chem. Soc. 1915, 37,
1264–1270.
41. Lemieux, R.U. The mercaptolysis of glucose and galactose pentaacetates. Can. J.
Chem. 1951, 29, 1079–1091.
42. Lemieux, R.U.; Brice, C. The mechanisms of glucose pentaacetate anomerization
and levoglucosan formation. Can. J. Chem. 1952, 30, 295–310.
43. Boons, G-J. In Organic Synthesis with Carbohydrates, Boons, G-J.; Hale, K.J., eds.;
Blackwell Science, Inc., Oxford, 2000, 103–154.
44. (a) Pickard, P.L.; Tolbert, T.L. Diphenylketimine. In Organic Syntheses, Baum-
garten, H.E., Ed.; Wiley, New York, 1973, Vol. V, 520–522 (b) Pickard, P.L.; Tolbert, T.L.
An improved method of ketimine synthesis. J. Org. Chem. 1961, 26, 4886–4888.
45. (a) Williams, H.B.; Koenig, P.E.; Huddleston, G.; Couvillon, T.; Castille, W. Fur-
ther studies of Schoenberg’s reagent and sulfur. J. Org. Chem. 1963, 28, 463–464
(b) Thoman, C.J.; Hunsberger, I.M. Chemistry of N-nitrosoketimines. J. Org. Chem.
1968, 33, 2852–2857.
46. Starrat, A.N.; Brown, B.E. Synthesis of proctolin, a pharmacologically active pen-
tapeptide in insects. Can. J. Chem. 1977, 55, 4238–4242.
47. Maclaren, J.A. Some amino acid esters—an improved preparative method. Aust.
J. Chem. 1978, 31, 1865–1868.