Study on the accelerated Gutknecht self-cyclocondensation of amino-sugars under atmospheric pressure chemical ionization conditions

Article abstract of DOI:10.1039/c5ra22331h

An unexpected gas phase Gutknecht self-condensation of d-glucosamine hydrochloride to 2,5-deoxyfructosazine (2,5-DOF) in atmospheric pressure chemical ionization mass spectrometry (APCI-MS) was described. Mechanistic studies indicated that the thermospray conditions in APCI largely accelerate the irreversible Gutknecht self-cyclocondensation reaction of amino-sugars. Our observations provide a promising clue for a new borate-free synthetic method of 2,5-DOF by mimicking the APCI conditions.

Full text of DOI:10.1039/c5ra22331h

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Study on the accelerated Gutknecht self-
cyclocondensation of amino-sugars under
atmospheric pressure chemical ionization
conditions†
Cite this: RSC Adv., 2015, 5, 105079
Received 25th October 2015
Accepted 28th November 2015
Hao-Yang Wang, a Jun-Ting Zhang,a Shi-Hao Sun,b Shu-Sheng Zhang,a Fang Zhang,a
*
DOI: 10.1039/c5ra22331h
a
a
*
Hui Zhu and Yin-Long Guo
www.rsc.org/advances
An unexpected gas phase Gutknecht self-condensation of D-glucos- why APCI conditions are suitable for such reaction, but ESI not;
amine hydrochloride to 2,5-deoxyfructosazine (2,5-DOF) in atmo- (3) the APCI-based Gutknecht self-condensation is only an
spheric pressure chemical ionization mass spectrometry (APCI-MS) interesting ambient gas phase reaction or it also provides clues
was described. Mechanistic studies indicated that the thermospray for new synthetic methods of 2,5-DOF.
conditions in APCI largely accelerate the irreversible Gutknecht self-
As a major ionization method bridged the atmosphere
cyclocondensation reaction of amino-sugars. Our observations condition to vacuum environment of mass spectrometry, APCI
provide a promising clue for a new borate-free synthetic method of shares some common aspects with ESI. However, compared
2,5-DOF by mimicking the APCI conditions.
with ESI, APCI has its unique ionization mechanism initiated by
ion–molecule reactions at atmospheric pressure. The rst step
of APCI requires an effective thermal vaporization or desorption
to generate the free neutral gas phase molecules, which are
further ionized by a corona discharge to corresponding ionic
species.4 The most signicant character of APCI is generating
ions from the neutrals, but ESI prefers to directly release ionic
species from reaction solution by desolvation.4,5 The unique
characters of APCI make it an ideal method for studying certain
reaction mechanism,6,7 as well as monitoring reaction process.8
At the same time some useful APCI-based ambient gas-phase
reactions have been studied and reported.9–11 Therefore, in
some special cases, the ionic species from APCI have same m/z
values with their counterparts from ESI but their ionization
positions12 and even detailed structures, as well as their
congurations, would be different.
D-glucosamine is a prominent amino-sugar in the biochem-
ical synthesis of glycosylated proteins and lipids. And it is also
an important dietary supplement in treatment for osteoar-
thritis.13 Its three possible congurations might exist at certain
equilibrium in solution (Scheme 1). Nowadays, the advantages
of IM-MS (ion-mobility mass spectrometry) provide us a valu-
able chance to reveal the structural information of the shape
and size for ionic species14 and attract lots of attentions from
chemists.15 Besides elemental compositions obtained with high
resolution measurement and fragmentation patterns acquired
by CID (collision induced dissociation), the IM-Q-TOF MS (ion-
mobility quadrupole time-of-ight mass spectrometry) could
provide another dimensional information-dri time for the
characterization of the ionic species generated in ambient
reactions, even in the trace amount level. In another words, IM-
In 1879, H. Gutknecht rst reported the method for synthesis of
pyrazine by reduction of the a-oximino ketone.1 Since then
many variations of Gutknecht pyrazine synthesis were devel-
oped, but the basic principle of these methods was the self-
cyclocondensation of a-amino carbonyl compound to a dihy-
dropyrazine and ultimately formed the aromatic molecule pyr-
azine by further oxidation or dehydration.2,3 Just recently, we
found an unexpected Gutknecht self-cyclocondensation reac-
tion of D-glucosamine to 2,5-deoxyfructosazine (2,5-DOF)
showing the protonated signal at m/z 305 by consequent loss of
3H2O in atmospheric pressure chemical ionization mass spec-
trometry (APCI-MS) condition, whereas the electrospray mass
spectrometry (ESI-MS) analysis only showed the signals of M$H+
at m/z 180 and 2 M$H+ at m/z 359. Then, in order to fully
understand the scientic questions behind such interesting
APCI-MS experiments of amino-sugars, we need to answer the
following three important questions: (1) the specic reaction
mechanism of such Gutknecht self-cyclocondensation reaction
in APCI conditions and its difference with the typical borate-
catalyzed Gutknecht pyrazine synthesis in solution phase; (2)
aState Key Laboratory of Organmetallic Chemistry and National Center for Organic
Mass Spectrometry in Shanghai, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences, 345 Lingling Road, Shanghai, 200032, China. E-mail:
haoyangwang@sioc.ac.cn; ylguo@sioc.ac.cn
bZhengzhou Tobacco Research Institute, China National Tobacco Corporation, 2
Fengyang Road, Zhengzhou 450001, China
† Electronic supplementary information (ESI) available: Details of experimental
conditions and related data and spectra. See DOI: 10.1039/c5ra22331h
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Scheme 1 Three amino-sugars: D-glucosamine (1), D-mannosamine
(2), D-galactosamine (3) and the three possible configurations of D-
glucosamine: a-, b- and open chain forms.
Scheme 2 The Gutknecht self-condensation of amino-sugars (1–3)
to 2,5-deoxyfructosazine (4) or its isomer 5.
MS could work as a specic “chromatographic method” for fast
analysis ionic species based on their size and shape in extremely
short time scale (normally less than 60 ms). Therefore, IM-Q-
TOF MS would become a valuable tool for exploration of the
mechanisms for ion–molecule reactions at ambient condition.14
Just as the concept of “reactions on the y in ambient mass
spectrometry” presented by Cooks group recently,16 we want to
achieve “characterization of the reaction products and even
intermediates on the y in ambient mass spectrometry” with
the help of IM-MS.
First we studied the mass spectrometry behaviors of three
amino-sugar hydrochloride salts: D-glucosamine (1), D-man-
nosamine (2), D-galactosamine (3) by ESI-IM-Q-TOF MS. The
results of D-glucosamine hydrochloride showed major ions 1$H+
at m/z 180 and its dehydration signal at m/z 162, 1$Na+ at m/z
202, 2(1)$H+ at m/z 359 and 2(1)$Na+ at m/z 381 (Fig. 1a). The
similar results were obtained for compounds 2$HCl and 3$HCl
(Fig. S1†). However, to our big surprise, APCI-IM-Q-TOF-MS
analysis of the D-glucosamine hydrochloride salt (1$HCl)
showed no signals of 1$H+ at m/z 180 and 2(1)$H+ at m/z 359 at
all, but giving the signicantly large amount of ion at m/z 305
(Fig. 1b). The analogous results could also be obtained from the
APCI analysis of compounds 2$HCl and 3$HCl (Fig. S1 and
Table S1†). The ion at m/z 305 in APCI of 1$HCl was primarily
assigned as the protonated 2,5-deoxyfructosazine (2,5-DOF, 4)
formed by the Gutknecht self-cyclocondensation reaction
(Scheme 2). Compared with D-glucosamine 1, D-mannosamine 2
has the different 2-position-amino group conguration, but
their rest sugar chain structures remain the same. They should
give the same Gutknecht reaction product in APCI-condition.
The authentic 2,5-DOF (4) was synthesized and its APCI-IM-Q-
TOF-MS spectrum showed 4$H+ at m/z 305, which was quite
similar to APCI mass spectra of 1$HCl and 2$HCl (Fig. 1c and
S1†). The detailed MS/MS dissociation patterns of ions at m/z
305 from APCI analysis of 1$HCl and 2$HCl and the authentic
4$H+ at m/z 305 are nearly identical and they also have quite
similar dri time at about 21.50 ms (Fig. 2).
Fig. 1 (a) ESI-MS spectrum of D-glucosamine hydrochloride (1$HCl), (b) Fig. 2 APCI-MS/MS spectra and the drift time spectra of ion at m/z
APCI-MS spectrum of 1$HCl and (c) APCI-MS spectrum of 2,5-DOF (4). 305 from the APCI-analysis of: (a) 1$HCl; (b) 2$HCl; (c) the 2,5-DOF (4).
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Fig. 3 (a) APCI-MS/MS spectrum of ion at m/z 305 from APCI-IM-Q-
TOF-MS analysis of D-galactosamine hydrochloride (3$HCl) and; (b)
APCI-MS/MS spectrum of 5$H+ at m/z 305 from the APCI-IM-Q-TOF-
MS analysis of authentic 5 and its drift time spectrum. Both the MS/MS
fragmentation patterns and the drift time experiment results supported
the assignment of the ion at m/z 305 formed in APCI-MS spectrum of
the D-galactosamine hydrochloride (3$HCl) was the protonated 5.
Scheme 3 The proposed fragmentation patterns of ion 4$H+ at m/z
305 in MS/MS.
solution makes the purication taking time and energy, as well
as decreasing the total yields. It is proposed that the similar
point of the Gutknecht self-cyclocondensation reaction in APCI
and the solution phase reaction in presence of base and borate
relied on the liberation and activation of the hydrochloride salts
of amino-sugars to the highly reactive free neutral open chain
state. According to such hypothesis, the Gutknecht self-
cyclocondensation reaction tendency would be retarded or
even blocked when the ring-chain tautomerization to the active
open chain a-amino aldehyde structure was restrained by acyl-
ation of the 2-position-amino group and protection of the 1-
position hemiacetal-hydroxyl group. The following experiments
conrmed our hypothesis.
The N-acetyl-D-glucosamine (6) also gave rise to the ion at m/z
305 via Gutknecht self-condensation at same APCI-conditions.
Such results showed that the acylation of the amino group
could not completely stop the ring-chain tautomerization in gas
phase of the APCI condition (Fig. 4). The possible formation
process of ion at m/z 305 from N-acetyl-D-glucosamine at APCI
was depicted in Scheme 4. The dri time comparison and MS/
MS experiments of such ion by IM-Q-TOF conrmed it to be
the protonated 2,5-DOF (4). The 4-methylumbelliferyl N-acetyl-
b-D-glucosaminide (7) is the derivate of D-glucosaminide with
acylation of the 2-position-amino group and the protection of 1-
position hemiacetal-hydroxyl group (Fig. 4). Thus, its ring-chain
tautomerization process was completely blocked by such
chemical modications. Just as our expectation, the APCI-IM-Q-
TOF-MS spectrum of 7 showed no signal at m/z 305, but giving
the 7$H+ at m/z 380, the dehydration signal at m/z 362 and its
fragment ion at m/z 177 (Scheme 4). All the experimental results
showed that the most important two driving forces for the
interesting Gutknecht self-cyclocondensation reaction of
unprotected amino-sugar hydrochloride salts in APCI condition
were: (1) expulsion of HCl in thermospray process of APCI
conditions; (2) the ring-chain tautomerization to the highly
active neutral open chain a-amino aldehyde structure.
Compared with D-glucosamine 1, D-galactosamine 3 has the
different conguration at 4-position-hydroxyl group, therefore it
would give a different Gutknecht self-condensation reaction
product compound 5 (isomer of 2,5-DOF). Then the experi-
mental results from the same IM-Q-TOF MS conditions show
that the ion at m/z 305 from APCI analysis of 3$HCl has slightly
difference in the fragment ion distributions of MS/MS spectrum
to that of 4$H+ at m/z 305 and also has shorter dri time at 21.31
ms. The authentic compound 5 was synthesized and the IM-Q-
TOF MS experiments supported that the Gutknecht self-
condensation reaction product of 3$HCl was the compound 5
(Fig. 3, S2 and S3†). Therefore, with the help of IM-Q-TOF MS
experiments, the ionic specie at m/z 305 in the APCI analysis of
amino-sugars were conrmed to be the protonated Gutknecht
self-condensation products, but not the products or complexes
formed by random dehydrations. Meanwhile the obvious
difference in the detailed MS/MS dissociation patterns of 4$H+
and 5$H+ at m/z 305 also provides extra proof to differentiate
them. The ESI-IM-Q-TOF-MS experiments of compounds 4 and
5 showed that the dri time and the MS/MS dissociation
patterns of 4$Na+ (20.51 ms) and 5$Na+ (20.31 ms) at m/z 327
were also different (Fig. S3†). The possible fragmentation
patterns of 4$H+/5$H+ at m/z 305 and 4$Na+/5$Na+ at m/z 327
were proposed and depicted in Scheme 3 and Scheme S1 and
S2,† respectively.
It has been reported that D-glucosamine can be converted to
complicated products, involving the deoxyfructosazines as
major components, in aqueous condition.17–19 Addition of
borate or phenylboronate could effectively prevent the random
dehydrations as side reactions, because borate could chelate
with the diol moieties of D-glucosamine, giving 2,5-DOF as the
major product in base condition.17 From previous studies, the
open form of D-glucosamine has the highest affinity for borate
and in turn the borate or phenylboronate help the conguration
change of amino-sugar to the active open chain structure.20 But
the presence of borate or phenylboronate in the reaction
The amino-sugars in this research were obtained as their
hydrochloride salts. The protonation of amino-group in 2-
position makes them quite stable in both solid form and solu-
tion. At the same time, aldehyde group prefers to react with the
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Scheme 4 The chemical structures of the N-acylated amino-sugars 6
and 7, and their possible gas-phase transformation reactions at APCI
condition.
adding extra catalysts and reagents. ESI ionization of amino-
sugar hydrochloride salts more prefer to transfer the proton-
ated amino-sugars directly from solution to gas phase. For this
reason, ESI analysis of amino-sugar hydrochloride salts only
gave rise to M$H+ and 2 M$H+. Increasing the desolvation
Fig. 4 (a) ESI-IM-Q-TOF-MS spectrum of N-acetyl-b-D-glucosami-
nide (6), showing 6$H+ at m/z 222, 6$Na+ at m/z 242, 2(6)$H+ at m/z
443 and 2(6)$Na+ at m/z 465; (b) APCI-IM-Q-TOF-MS spectrum of N-
acetyl-b-D-glucosaminide (6), also showing ion at m/z 305; (c) ESI-IM-
Q-TOF-MS spectrum of 4-methylumbelliferyl N-acetyl-b-D-gluco-
saminide (7), showing 7$Na+ at m/z 402; (d) APCI-IM-Q-TOF-MS
spectrum of N-acetyl-b-D-glucosaminide (7), showing 7$H+ at m/z
380 and the ion at m/z 362 by loss of water and 4-methylumbelliferyl
cation at m/z 177.
ꢀ
temperature of ESI ion source to 350 C only led to the gener-
ation of dehydration fragment ions of [M$H À H2O]+ at m/z 162
and [2 M$H À H2O]+ at m/z 341 of glucosamine but not helped to
generate the ionic species of 2,5-DOF [M + H]+ at m/z 305
(Fig. S4†). This study enriched our knowledge of the difference
in the ionization mechanism between ESI and APCI. In tradi-
tional opinions, APCI was more suitable for less polar organic
compounds and its applications for polar carbohydrate
compounds were limited by the possible decomposition or
fragmentation reactions that happened in APCI. However, the
unique capacity of APCI for activation of amino-sugars might
have its own alternative potentials for developing new on-line
APCI-based derivatization method for analysis of small molec-
ular sugars and amino-sugars. Such idea provides chemists
more choices in ionization methods when they study small
molecular carbohydrate compounds by mass spectrometry.
Meanwhile, the IM-MS method was applied to provide the dri-
time as an elegant proof for the occurrence of ambient Gut-
knecht self-condensation reaction of amino-sugars at APCI and
such capacity of IM-MS is also useful to probe the reaction
mechanism for some other ambient organic reactions.
5-position intramolecular OH-group to form hemiacetal.
Although the equilibrium of the ring-chain tautomerization of
these salts might give rise to their open chain form, the
protonation of the a-amino group entirely restrained the rst
bimolecular imine formation step of Gutknecht pyrazine
synthesis. Thus, the release of the neutral a-amino aldehyde
open chain structure by expulsion of HCl is the most critical
requirement for such Gutknecht self-cyclocondensation reac-
tion of amino-sugars hydrochloride. The unique thermal
ꢀ
vaporization step (desolvation temperature at 350 C) in APCI
not only helped the amino-sugar hydrochloride salts to get rid
of HCl giving their neutral state but also could activate the
neutral state to the highly active a-amino aldehyde open chain
structure. On the other hand, the thermalization condition of
APCI facilities the dehydration steps in forming the imine and
dihydropyrazine intermediates. All these factors in APCI largely
accelerate the irreversible Gutknecht self-cyclocondensation
reaction of amino-sugars and make the reaction nished in
the amazing millisecond timescale in APCI ionization process.
Thus, APCI-IM-MS provides all the suitable conditions for
Gutknecht pyrazine synthesis and detection of 2,5-DOF directly
and immediately from amino-sugar hydrochloride salts without
In 2012, Cooks group developed the ESI-based synthetic
methods and reported that the ambient reaction condition
could signicantly increase the reaction rate of some conden-
sation reactions.21 Herein, our experimental results showed that
APCI conditions could also signicantly increase the Gutknecht
condensation reaction of amino-sugars even in absence of
borate or phenylboronate. We even could collect small amount
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Science Foundation of China (21532005, 21475145 and
21472228) and Youth Innovation Promotion Association CAS
(2013171) and Opening Projects of Key Laboratory of Tobacco
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