An improved synthesis of morpholino-glycoamino acids

Article abstract of DOI:10.1016/j.tetlet.2006.10.133

The current synthesis of hybrid morpholino-glycoamino acids through double reductive amination is characterized by modest yields and lengthy reaction times. We propose an optimized procedure that results in improved yields and the shortest reaction times reported so far.

Full text of DOI:10.1016/j.tetlet.2006.10.133

Tetrahedron Letters 47 (2006) 9203–9206
An improved synthesis of morpholino-glycoamino acids
Marko Anderluh*
Faculty of Pharmacy, University of Ljubljana, Asˇkercˇeva 7, 1000 Ljubljana, Slovenia
Received 3 October 2006; revised 20 October 2006; accepted 26 October 2006
Abstract—The current synthesis of hybrid morpholino-glycoamino acids through double reductive amination is characterized by
modest yields and lengthy reaction times. We propose an optimized procedure that results in improved yields and the shortest reac-
tion times reported so far.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Morpholines are used extensively in organic synthesis as
simple bases or as N-alkylating agents.1 In recent years,
morpholines have received considerable attention as
glycomimetics—surrogates for monosaccharide moieties.
For example, morpholine oligomers have been used as
nucleic acid ribose surrogates with favourable antisense
properties.2 A disaccharide mimetic with a galactose
moiety transformed to a hybrid morpholino-glycopep-
tide was found to be a better substrate for human milk
fucosyltransferase than the native substrate.3 Further-
more, morpholines have been used as glycomimetics in
the synthesis of paromomycin analogues4 and of N-
substituted 1,5-dideoxy-1,5-imino-D-arabinitol 2,3,4-tri-
phosphates from the parent a-trinositol.5
methodology is closely related to that reported by Acton
et al.6 Osborn and Clark later reported a modification of
this synthesis.7 A careful control of NaIO4-mediated
oxidation can lead to ring cleavage with or without
extrusion of the glycopyranose C-atom at position 4,
thus opening up the route to stereodefined morpholines
and [1,4]-oxepanes.7 A similar procedure for synthesiz-
ing morpholines, starting from modified glycofuranos-
ides was reported by Grotenbreg et al.8 We needed to
develop an efficient synthesis of orthogonally protected,
functionalized morpholino-glycoamino acids as versatile
and ‘drug-like’ surrogates for N-acetylmuramic acid
(Fig. 1). The latter is a constituent of bacterial cell walls
and has been incorporated in a series of potent phosph-
inate inhibitors of the MurD enzyme, which are promis-
ing new antibacterial agents.9
Hindsgaul and Du have developed a straightforward
methodology for the synthesis of hybrid morpholino-
glycoamino acids consisting of NaIO4-mediated oxida-
tion of glycopyranosides to dialdehydes, and subsequent
double reductive amination to form functionalized
morpholines with preserved stereochemistry.3 The
The protected and functionalized morpholino-glyco-
amino acid should allow specific attachment of various
substituents after deprotection. We prepared morpholine
5 based on the synthetic pathway of Hindsgaul and Du,3
protecting
group
protecting
group
O
O
HO
HO
HO
O
O
O
N
O
O
HOOC
HOOC
O
N
n
R
n
AcHN
R
protecting
group
OH
OH
designed morpholine
as surrogate
N-acetylmuramic acid
designed protected
morpholine
Figure 1.
Keywords: Morpholino-glycoamino acids; Glycomimetics; Reductive amination; Microwave-assisted synthesis.
*
Tel.: +386 1 47 69 639; fax: +386 142 58 031; e-mail: marko.anderluh@ffa.uni-lj.si
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.133

9204
M. Anderluh / Tetrahedron Letters 47 (2006) 9203–9206
1. AcCl, 0 °C, 30 min
NaIO4/H2O, MeOH,
0-25 °C, 3 h
HO
HO
HO
HO
O
O
O
O
2. D-glucose, 70 °C, 4 h
OH
67%
95%
OH
O
O
1
2
3
Ph Ph
Ph
O
β-AlaOBn x TsOH,
NaCNBH3/MeOH,
25 °C, 15 h
OH
(Ph)3CCl, Et3N/CH2Cl2
0 °C, 1 h
O
N
O
O
O
N
O
O
35%
94%
BnO
BnO
4
5
Scheme 1.
OH
NaIO4/H2O
0-25 °C, 3 h
HO
O
reductant, amino acid/MeOH
various conditions
HO
O
O
O
HO
HO
O
N
O
n
OH
OMe
92%
OMe
6
7
R''O
R'
OMe
8; R'=H, R''=tBu, n=0
9; R'=Me, R''=Bn, n=0
10; R'=H, R''=Bn, n=1
Scheme 2.
the synthesis of which is outlined in Scheme 1. The main
synthetic issues to be addressed were orthogonal protec-
tion of the functionalities and the need for a practical,
high-yielding synthesis. The main drawback of this syn-
thesis was the NaCNBH3-mediated double reductive
amination (Borch reduction) of dialdehyde 3 with a pro-
tected amino acid, which in our hands gave irreproduc-
ible and moderate yields, even after long reaction
times. Comparable moderate yields and long reaction
times (4–24 h) with similar reactions have been reported,
especially when using (protected) amino acids as the do-
nors of the amino moiety.3,4,7 Even careful analysis of the
reaction mixture did not enable Clark and Osborn to of-
fer any explanation for the low yields of the double
reductive amination step.7 This prompted us to examine
systematically a model reaction starting from a-D-meth-
ylglucoside 6 (Scheme 2), in an attempt to increase the
yields and reduce reaction times.
NaIO4 to 5 molar equiv. In our hands, 2 molar equiv
yielded the desired product, as demonstrated by TLC
and by the products formed in the next step.
In the first series of morpholines synthesized (Table 1),
double reductive amination reactions were carried out
with dialdehyde 7, protected amino acids and a large
excess of the reductant at room temperature.à In order
to circumvent the possible formation of open chain
diamines, we carried out reactions in diluted reaction
mixtures in carefully adjusted acidic media, with molar
equivalents of dialdehyde 7 increasing to 3. Further-
more, the protected amino acid and dialdehyde 7 were
allowed to react for 30 min before the addition of the
reductant, and without molecular sieves or other dehy-
drating agents—in contrast to some previously reported
procedures.7,8 The presence of water in the reaction mix-
ture allowed equilibrium to be achieved between amine–
imine and amine–aldehyde, which resulted in the desired
amine–morpholine with no trace of acyclic diamine by-
product.
The initial dialdehyde 7 was obtained by the oxidation
of a-D-methylglucoside 6 with excess NaIO4. Clark
and Osborn reported insufficient transformation of the
desired dialdehyde and further formation of oxaze-
panes.7 Therefore, they recommended an increase of
à Typical procedure for double reductive amination under normal
conditions: To a solution of 3-hydroxy-2-(1-methoxy-2-oxoeth-
oxy)propanal (dialdehyde 7) in methanol (10 mL), the protected
amino acid (in the form of a salt) was added and the acidity of the
solution adjusted with glacial acetic acid to pH 5 (wet pH-indicator
strip). After stirring for 30 min, the reductant (sodium cyanoboro-
hydride or sodium triacetoxyborohydride) was added and stirring was
continued overnight (15 h). The mixture was concentrated in vacuo,
and the residue dissolved in ethyl acetate (30 mL) and washed with a
saturated solution of sodium hydrogen carbonate (3 · 10 mL), water
(1 · 10 mL) and brine (1 · 10 mL). The organic layer was dried over
sodium sulphate, the solvent evaporated in vacuo and the residue
further purified by column chromatography (silica gel, dichlorometh-
ane/methanol = 20/1) to yield morpholino derivatives 8–10.
A solution of sodium periodate (2 equiv) in distilled water was added
dropwise to an ice-cooled water solution of a-D-methylglucoside 6
(1 equiv). The reaction mixture was stirred for another 4 h at room
temperature and concentrated in vacuo. The resulting white solid was
washed with tetrahydrofuran and subjected to ultrasound for 5 min
(3·). The organic fractions were collected, filtered through a
FluoroporeTM (PTFE) membrane filter (0.5 lm pore size) and the
solvent evaporated in vacuo. The residual colourless syrup was
treated with diethyl ether and the solvent evaporated (3 · 30 mL) to
yield a white amorphous solid. The crude product was immediately
used in the next step.

M. Anderluh / Tetrahedron Letters 47 (2006) 9203–9206
Table 1. Morpholino-glycopeptides 8–10 obtained according to Scheme 2
9205
Entry
Reductant
Dialdehyde (molar equiv)a
Amino acid
Yielda,c (%)
8a
8b
4.0 equiv NaCNBH3
4.0 equiv NaCNBH3
5.0 equiv NaCNBH3
4.0 equiv Na(AcO)3BH
4.0 equiv NaCNBH3
4.0 equiv NaCNBH3
5.0 equiv NaCNBH3
10 equiv NaCNBH3
4.0 equiv Na(AcO)3BH
4.0 equiv Na(AcO)3BH
5.0 equiv Na(AcO)3BH
4.0 equiv NaCNBH3
4.0 equiv NaCNBH3
5.0 equiv NaCNBH3
4.0 equiv Na(AcO)3BH
0.9
1.5
3.0
1.5
0.9
1.5
3.0
3.0
0.9
1.5
3.0
0.9
1.5
3.0
1.5
GlyOtBu · HCl
GlyOtBu · HCl
GlyOtBu · HCl
GlyOtBu · HCl
14
38
39
32
22
42
50
52
19
33
44
26
50
59
32
8c
8d
9a
9b
L-AlaOBn · TsOHb
L-AlaOBn · TsOH
L-AlaOBn · TsOH
L-AlaOBn · TsOH
L-AlaOBn · TsOH
L-AlaOBn · TsOH
L-AlaOBn · TsOH
b-AlaOBn · TsOH
b-AlaOBn · TsOH
b-AlaOBn · TsOH
b-AlaOBn · TsOH
9c
9d
9e
9f
9g
10a
10b
10c
10d
a Calculated relative to the amino acid.
b TsOH = p-toluenesulfonic acid.
c Refers to the yield of the purified product.
Table 2. Morpholino-glycopeptides 8–14 obtained as in Scheme 2 with microwave-assisted chemistry
Entry
Reductant
Dialdehydea (molar equiv)
Amino acid/amine
Yielda,b (%)
Time (min)
8e
9h
9i
GlyOtBu · HCl
L-AlaOBn · TsOH
L-AlaOBn · TsOH
L-AlaOBn · TsOH
b-AlaOBn · TsOH
EtOOC(CH2)3NH2 · HCl
BnNH2
40
37
41
41
56
57
64
88
73
10
5
10
15
10
10
10
10
10
9j
4.0 equiv NaCNBH3
1.5
10e
11
12
13
14
PhNH2
4-nBuC6H4NH2
a Calculated relative to the amino acid.
b Refers to the yield of the purified product.
The reports of procedures involving different starting
quantities of the dialdehyde and amino acid encouraged
us to explore reaction yields as a function of the quantity
of reactants.3,4,7,8 The results (Table 1) clearly demon-
strate that yields increased with increasing molar equiv-
alents of dialdehyde 7, and yields of up to 59% were
achieved in this way (8c, 9d, 10c)10, which is a reason-
able improvement over the yields of Clark and Osborn
(29–33%).7 When calculating yield relative to both the
dialdehyde (which is usually more laborious to obtain)
and the amino acid, an optimum reagent ratio of dialde-
hyde:amino acid = 1.5:1 was obtained. Raising the
quantity of NaCNBH3 to 10 molar equiv did not signif-
icantly influence the overall yield.
soned that conducting the reaction under microwave
irradiation should significantly shorten the reaction
times. The reactions were shown to be complete after
heating the reaction mixtures for only 10 min at
100 ꢁC under microwave irradiation (9h–j).§ The prod-
ucts were obtained in yields (Table 2) similar to those
for the reactions performed at room temperature (8b,
9b, 10b compared to 8e, 9i, 10e). Although reactions
were conducted in methanol under acidic conditions,
no transesterification products were identified and the
§ Typical procedure for microwave-assisted double reductive amination:
To a solution of dialdehyde 7 in methanol (3 mL) the amine donor
was added and the acidity of the solution adjusted with glacial acetic
acid to pH 5 (wet pH-indicator strip). Sodium cyanoborohydride was
added and the mixture heated in a dedicated microwave reactor
(CEMTMDiscover) at 100 ꢁC using a power of 20 W for the reported
time. The power was allowed to fluctuate as the temperature reached
the desired value. Afterwards, the reaction mixture was concentrated
in vacuo, and the residue dissolved in ethyl acetate (30 mL) and
washed with a saturated solution of sodium hydrogen carbonate
(3 · 10 mL), water (1 · 10 mL) and brine (1 · 10 mL). The organic
layer was dried over sodium sulphate, the solvent evaporated in vacuo
and the residue further purified by column chromatography (silica
gel, dichloromethane/methanol = 20/1) to yield morpholino deriva-
tives 8–14.
A further reason for the earlier moderate yields could be
the competitive reduction of dialdehyde 7 or the inter-
mediate amine–aldehyde, with sodium cyanoboro-
hydride, to give the resulting alcohol. The use of the
milder reductant, sodium triacetoxyborohydride, should
therefore diminish the extent of this side reaction. This,
however, failed to produce any improvement in yield
(8d,9e–g,10d compared to 8b,9a–c,10b).
As long reaction times of up to 24 h have been reported
for a complete double reductive amination,7,8,11 we rea-

9206
M. Anderluh / Tetrahedron Letters 47 (2006) 9203–9206
7. Clark, S. M.; Osborn, H. M. I. Tetrahedron: Asymmetry
2004, 15, 3643–3652.
8. Grotenbreg, G. M.; Christina, A. E.; Buizert, A. E. M.;
van der Marel, G. A.; Overkleeft, H. S.; Overhand, M. J.
Org. Chem. 2004, 69, 8331–8339.
9. (a) Tanner, M. E.; Vaganay, S.; van Heijenoort, J.; Blanot,
D. J. Org. Chem. 1996, 61, 1756–1760; (b) Gegnas, L. D.;
Waddell, S. T.; Chabin, R. M.; Reddy, S.; Wong, K. K.
Bioorg. Med. Chem. Lett. 1998, 8, 1643–1648; (c) El
Zoeiby, A.; Sanschagrin, F.; Levesque, R. C. Mol.
Microbiol. 2003, 47, 1–12.
10. Representative examples: Compound 8: colourless oil. IR
(ATR-IR, thin film, cmꢀ1): 3432, 2922, 2840, 1723, 1447,
1373, 1217, 1155, 1038, 898, 832, 739. 1H NMR
(300 MHz, CDCl3): d (ppm) = 1.47 (s, 9H, t-Bu), 1.93
isolated morpholines were single isomers, as shown by
NMR. In order to test the general applicability of our
method, other alkyl and aryl amines were used (Table
2, compounds 11–14). Surprisingly, the highest yields
were obtained with aromatic amines.
In conclusion, we have reported an improved synthetic
route to hybrid morpholino-glycoamino acids through
double reductive amination. The microwave-assisted
procedure results in a reasonable improvement in the
reaction yield and the shortest reaction times reported
so far. The optimized procedure offers a feasible route
for multi-gram synthesis of orthogonally protected func-
tionalized morpholino-glycoamino acids, the synthesis
of which will be reported in due course.
(t, 1H, –OH, J = 5.4 Hz), 2.55 (t, 1H, H-5ax, J5ax,6
=
J5ax,5eq = 10.8 Hz), 2.68 (dd, 1H, H-3ax, J3ax,2 = 2.7 Hz,
J3ax,3eq = 11.4 Hz), 2.80 (m, 1H, H-5eq), 2.94 (m, 1H,
H-3eq), 3.17 (d, 1H, NCHH0COOtBu, JH,H = 17.0 Hz),
0
3.23 (d, 1H, NCHH0COOtBu, JH,H = 17.0 Hz), 3.43 (s,
Acknowledgements
0
3H, CH3O), 3.66 (m, 2H, CH2OH), 4.13 (m, 1H, H-6),
4.74 (br s, 1H, H-2). 13C NMR (75 MHz, CDCl3): d
(ppm) = 28.52 ((CH3)3C), 53.41 (CH3O), 55.12 (C-5),
55.29 (NCH2COOtBu), 59.67 (C-3), 64.22 (CHCH2OH),
69.25 (C-6), 81.86 ((CH3)3C), 97.37 (C-2), 169.94 (CO).
MS (EI) m/z: 261 (M)+. HRMS Calcd for C12H23NO5 m/
z: 261.158030 (M)+. Found 261.157623. Compound 9:
colourless oil. IR (ATR-IR, thin film, cmꢀ1): 3420, 2922,
2821, 1735, 1451, 1370, 1256, 1158, 1124, 1046, 984, 886,
805, 727, 688. 1H NMR (300 MHz, CDCl3): d (ppm) =
1.35 (d, 3H, CH3CH, J = 7.2 Hz), 2.08 (br s, 1H, –OH),
2.49 (t, 1H, H-5ax, J5ax,6 = J5ax,5eq = 10.8 Hz), 2.65 (dd,
1H, H-3ax, J3ax,2 = 2.7 Hz, J3ax,2 = 11.7 Hz), 2.75 (m, 1H,
H-5eq), 2.92 (br d, 1H, H-3eq, J3eq,2 = 11.7 Hz), 3.41 (s,
3H, CH3O), 3.41 (q, 1H, CH3CH, J = 7.2 Hz), 3.55 (dd,
1H, CHCHH0OH, J1 = 3.8 Hz, J2 = 11.7 Hz), 3.64 (dd,
1H, CHCHH0OH, J1 = 5.6 Hz, J2 = 11.7 Hz), 4.06 (m,
1H, H-6), 4.71 (br s, 1H, H-2), 5.12 (s, 2H, PhCH2O), 7.36
(m, 5H, Ph). 13C NMR (75 MHz, CDCl3): d (ppm) =
15.15 (CH3CH), 50.23 (C-5), 52.48 (CH3O), 55.44 (C-3),
62.55 (CHCH3), 64.29 (CHCH2OH), 66.54 (CH2Ph),
69.50 (C-6), 97.60 (C-2), 128.72, 128.97, 136.17 (C–Ar),
172.84 (CO). MS(EI) m/z: 309 (M)+. HRMS Calcd for
C16H23NO5 m/z: 309.158560 (M)+. Found 309.157623.
11. Kolar, C.; Kneissl, G.; Kno¨dler, U.; Dehmel, K. Carbo-
hydr. Res. 1991, 209, 89–100.
This work was supported by the European Union FP6
Integrated Project EUR-INTAFAR (Project No.
LSHM-CT-2004-512138) under the thematic priority
Life Sciences, Genomics and Biotechnology for Health,
and the Ministry of Education, Science and Sport of the
Republic of Slovenia. The author wishes to thank Dr.
Roger Pain for the critical reading of the manuscript.
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