Synthesis of lactosamine from lactulose: Scalable approach for the Heyns rearrangement

Article abstract of DOI:10.1016/j.tetlet.2016.04.119

A scalable approach for the preparation of lactosamine hydrochloride from lactulose is described. The reported procedure is based on the preparation of a new dibenzylamino derivative of lactosamine hydrochloride. Lactosamine hydrochloride was prepared in a two-pot reaction sequence from commercially available lactulose.

Full text of DOI:10.1016/j.tetlet.2016.04.119

Tetrahedron Letters 57 (2016) 2595–2597
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of lactosamine from lactulose: scalable approach
for the Heyns rearrangement
⇑
⇑
Károly Ágoston , Gyula Dékány, István Bajza, Markus Hederos
Glycom A/S, Diplomvej 373, 1, DK-2800 Kgs-Lyngby, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
A scalable approach for the preparation of lactosamine hydrochloride from lactulose is described. The
reported procedure is based on the preparation of a new dibenzylamino derivative of lactosamine
hydrochloride. Lactosamine hydrochloride was prepared in a two-pot reaction sequence from commer-
cially available lactulose.
Received 18 March 2016
Revised 22 April 2016
Accepted 27 April 2016
Available online 6 May 2016
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Heyns rearrangement
Scale up
Lactosamine
Introduction
The procedure was revisited in 1999 by Stütz and co-workers7
and later improved by the same group.8 The crucial improvement
Lactosamine is one of the most important building blocks of
biologically relevant oligosaccharides and is a basic structural ele-
ment of Lewis type, a-gal type, and human milk oligosaccharides.
in their procedure was the use of benzylamine for the rearrange-
ment which resulted in reduced formation of the manno epimer
(Scheme 1). As the first step of the reaction cascade a ketosyl amine
(2) was prepared and isolated. The isolated ketosyl amine was trea-
ted with acid under anhydrous condition affording the lactosamine
derivative (3).
Furthermore, it forms the backbone of several cell surface glycans
such as sialylated glycans or keratin sulfates.1 Due to the biological
importance of lactosamine several biological and chemical synthe-
ses have been developed for the preparation of lactosamine and its
derivatives. The enzymatic synthesis is generally based on the
transfer of a galactose unit onto glucosamine by glycosyl hydrolase
or transferase enzymes.2
The chemical synthesis of lactosamine was achieved by the gly-
cosylation of a suitably protected glucosamine acceptor with a
galactose donor,3 however, this is labor and time consuming.
Another possibility is to convert commonly available lactose into
lactosamine,4 but this reaction path is similarly tedious as the pre-
vious one. The shortest route starts from lactulose (1) employing
the Heyns rearrangement which was first reported in 1952 by Kurt
Heyns.5 With rearrangement, ketoses can be transformed into 2-
deoxy-2amino aldoses by reacting with an amine. The reaction
was further studied and different N-substituted glucosamine
derivatives were prepared from fructose and different amines by
both Carson and co-workers and by the Heyns group.6
The bottleneck of the reaction pathway is the isolation of the
ketosylamine (2). The excess of benzylamine has to be removed
from the reaction mixture and the only way to achieve this is the
precipitation of compound 2.9 To date the only solvent found to
be suitable for this precipitation is Et2O. Recently, a practical pro-
tocol for this reaction pathway has been published, but did not
manage to overcome the isolation problem of the ketosylamine.10
This fact makes the procedure non-scalable beyond lab scale. In
this manuscript we describe a scalable procedure for the prepara-
tion of lactosamine from lactulose via the Heyns rearrangement.
Due to the hydrolytic instability of glycosylamines it remains
difficult to isolate these compounds, and as a consequence we tried
to avoid this. It became clear upon deeper understanding of the
reaction that only trace acid was required for the rearrangement.
Furthermore, when the acid was added at the beginning of the
reaction, this led to a new product which had not been identified
before in the reaction mixture.
Treatment of lactulose with benzylamine in the presence of acid
led to the formation of compound 5 (Scheme 2) which was isolated
by crystallization from the crude reaction mixture making the
whole procedure scalable. The rearrangement could be catalyzed
with a wide range of Brønsted and Lewis acids.11 Furthermore,
⇑
Corresponding authors at present address: Carbosynth LTD, 8&9 Old Station
Business Park, RG20 6NE Compton, Berkshire, UK (K.Á.).
E-mail addresses: karoly.agoston@carbosynth.com (K. Ágoston), markus.
hederos@glycom.com (M. Hederos).
http://dx.doi.org/10.1016/j.tetlet.2016.04.119
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

2596
K. Ágoston et al. / Tetrahedron Letters 57 (2016) 2595–2597
HO
HO
OH
OH
OH
NHR
OH
HO
HO
O
O
NBn
OH
NHBn
+BnNH2
-H2O
HO
HO
HO
HO
O
O
OH
a
HO
O
O
OH
OH
OH
O
O
OH
OH
OH
HO
HO
OH
OH
I
III
1
2
HO
OH
HO
OH
b
II
Bronsted or Lewis acid
HO
O
HO
HO
OH
O
O
O
HO
HO
OH
NHR
NHBn
NHBn
O
OH
NHBn
HO
HO
OH
OH
HO
HO
OH
OH
OH
OH
3
HO
Scheme 1. The Heyns rearrangement. Reagents and conditions: (a) benzylamine,
0–40 °C, 48 h; (b) AcOH, MeOH, rt, 1 h, 38–45%.7
VI
OH
V
IV
+BnNH2
-H2O
the rearrangement can be performed using different disaccharides
as stating material and various substituted benzylamines as aro-
matic amines for the reaction.11 Palatinose, nigerose, turanose,
and maltulose were used as keto disaccharides for the rearrange-
ment and resulted in the formation of the corresponding 1,2-
dibenzylamino substituted disaccharides.11 A similar reaction has
been developed parallel to our work using fructose as a starting
material and ZnCl2 as a Lewis acid.12 The synthesized compounds
can be regarded as potential surfactants. Although the authors iso-
lated the products in very high yield, we were unable to repeat
this. In our case the yields were moderate and the mother liquor
of the crystals contained an anomeric mixture of compound 5
and traces of the C-2 epimer of compound 5.
NBn
NHBn
HO
HO
O
NHBn
HO
OH
OH
OH
NHBn
HO
VII
VIII
Scheme 3. Suggested intermediates for the reaction cascade.
ketosamine (III) proceeding through a Schiff base intermediate
(II). In the absence of an acid catalyst the reaction stops at this
stage and the ketosamine can be isolated.7–10 In the presence of
an acid catalyst the Schiff-base intermediate (II) enters to the
Heyns reaction cascade. The reaction proceeds through an enol
intermediate (IV) followed by the formation of aldosamine inter-
mediates V and VI which are in equilibrium (VI could be isolated
if needed) but in the presence of benzylamine intermediate V
reacts further to form VII and VIII as a final stage of the reaction
path. Although all steps are reversible due to the excess of benzy-
lamine used, the reaction cascade is pushed toward the dibenzy-
lamino derivatives. In most cases compound VIII can be isolated
as a crystalline compound.
In conclusion a scalable route for the preparation of lactosamine
hydrochloride is presented. The reaction starts from commercially
available lactulose and applies the Heyns rearrangement. A new
1,2-dibenzylamino substituted lactosamine was isolated as a crys-
talline intermediate allowing the possibility of avoiding ether type
solvents for the whole process. The procedure has been performed
on a hundred-kilogram scale under industrial conditions.
Compound 5 was treated with dilute acid to hydrolyze the
anomeric benzylamino residue affording compound 3 (Scheme 2).
Compound 3 could be isolated if required, but more practically Pd/
C was added to the reaction mixture and stirred under H2 to afford
lactosamine hydrochloride (4). A high isolated yield of compound 4
was achieved by using recrystallized starting material for the
reaction.
The reaction proceeds via well-known intermediates
(Scheme 3).7,10 Ketosugar (I) reacts with benzlyamine to form
HO
HO
OH
OH
NHBn
OH
O
O
HO
HO
HO
HO
OH
O
O
a
OH
O
O
1
2
HO
OH
HO
OH
c
b
HO
O
HO
HO
HO
HO
O
O
O
O
NHBn
OH
HO
O
d
Acknowledgment
HO
5
NHBn
f
HO
3
NHBn
HO
OH
HO
OH
Authors thank to Glycom A/S for support.
e
Supplementary data
HO
O
HO
HO
O
O
Supplementary data (experimental description, NMR spectra,
HPLC chromatogram) associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.04.
119.
OH
HO
NH2 HCl
HO
OH
4
Scheme 2. Preparation of lactosamine (4) from lactulose (1) via the bisbenzyl
derivative. Reagents and conditions: (a) BnNH2, 0–40 °C, 48 h; (b) AcOH, MeOH, rt,
2 h, 65–70%7 over two steps; (c) BnNH2, BnNH3Cl, 40 °C, 96 h, 25%; (d) HCl, H2O,
MeOH, rt, 30 min, 80%; (e) Pd(OH)2/C, H2, HCl, H2O, MeOH, rt, 2 h 38–45% from
compound 1;7 (f) HCl, H2O, MeOH, then Pd/C, H2, H2O, MeOH, rt, 24 h, 95%.
References and notes
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Laboratory Press: Cold Spring Harbor, NY, 2009.

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2597
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