Efficient regeneration of the 5-acetamido group of a neuraminic acid aryl glycoside from the O-nethyl acetimidate function under acidic conditions

Article abstract of DOI:10.1007/s11172-010-0030-6

Treatment of protected N-acetylneuraminic acid 2-formyl-4-nitrophenyl glycoside with (diethyl-amino)sulfur trifluoride (DAST) and methanol resulted, apart from the known transformation of the formyl group into a difluoromethyl one, in an undocumented form

Full text of DOI:10.1007/s11172-010-0030-6

Russian Chemical Bulletin, International Edition, Vol. 58, No. 2, pp. 442—445, February, 2009
442
Efficient regeneration of the 5ꢀacetamido group of a neuraminic acid aryl
glycoside from the Oꢀmethyl acetimidate function under acidic conditions*
A. V. Orlova, A. I. Zinin, and L. O. Kononov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 137 6148. Eꢀmail: kononov@ioc.ac.ru
Treatment of protected Nꢀacetylneuraminic acid 2ꢀformylꢀ4ꢀnitrophenyl glycoside with (diꢀ
ethylꢀamino)sulfur trifluoride (DAST) and methanol resulted, apart from the known transforꢀ
mation of the formyl group into a difluoromethyl one, in an undocumented formation of 5ꢀ(Oꢀ
methyl acetimidate) of Neu5Ac. This imidate was converted into the target 5ꢀacetamido
derivative under the action of aqueous TFA. The yield of the target product (94%) was twice as
high as that reported earlier.
Key words: DAST, Neu5Ac, Nꢀacetylneuraminic acid, aryl glycoside, acetimidate.
corresponding signal in the spectrum of compound 2 (δC 49.3).
In addition, the 13C NMR spectrum of compound 3 conꢀ
Fluorination with (diethylamino)sulfur trifluoride
(DAST)1 is widely used in carbohydrate chemistry for
replacement of hydroxy groups and carbonyl O atoms
by fluorine atoms.1—6 For instance, the formyl group in
methyl Nꢀacetylneuraminate aryl glycoside 1 has been
transformed under the action of DAST into a difluoroꢀ
methyl group to give product 2 in 47% yield (Scheme 1).7
While reproducing this transformation, we discovered
an unknown side process. In addition, the reaction of
aldehyde 1 with DAST was found to occur at a much
lower rate than reported earlier,7 giving a mixture of
products 2 (see Ref. 7) and 3 in comparable amounts.
Product 3 was isolated by column chromatography in
58% yield. The 1H and 13C NMR spectra of compound 3
show signals unusual for Nꢀacetylneuramic acid (Neu5Ac)
derivatives. The signals for the methyl group CH3—C—N(5)
are shifted upfield compared to analogous signals in
the spectra of compound 2: δH 1.93/δC 23.1 (2) versus
Scheme 1
δH 1.81/δC 15.3 (3). Two signals (rather than one) for the
MeO groups appear in both the 1H (δH 3.62 and 3.69) and
13C NMR spectra (δC 52.8 and 53.4). It should also be
noted that the 1H NMR spectrum of compound 3 exhibits
an unusual highꢀfield triplet for the H(5) atom (δH 3.29),
while the corresponding signal in the spectrum of the
target product 2 is more complex and appears at δH 4.10.
This suggests the absence of a proton at N(5) (indeed, the
spectrum of compound 3 contains no signal for NH) and
the absence of the electronꢀwithdrawing Nꢀacetyl group.
The signal for C(5) in the 13C NMR spectrum of comꢀ
pound 3 is shifted downfield (δC 57.7) compared to the
* On the occasion of the 75th anniversary of the foundation
of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
Reagents and conditions: a. 1) DAST, CH2Cl2; 2) MeOH;
b. TFA, THF—H2O (4 : 1).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 433—436, February, 2009.
1066ꢀ5285/09/5802ꢀ0442 © 2009 Springer Science+Business Media, Inc.

Neu5Ac Oꢀmethyl acetimidate
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 2, February, 2009
443
Scheme 2
tains a signal for the C=N group, which is shifted upfield
(δC 165.0) compared to the signals for the amide and ester
C=O groups (δC 167—171). All these spectral features, as
well as the literature data for related Oꢀalkyl acetimidate
derivatives of Neu5Ac8,9 and glucosamine,10 suggest that
product 3 is a Neu5Ac derivative with the 5ꢀ(Oꢀmethyl
acetimidate) function instead of the 5ꢀacetamido group.
Usually, Oꢀalkyl acetimidates like compound 3 are
synthesized from acetamide derivatives under the action
of various alkylating reagents (MeOTf, [Et3O]+[BF4]–,
etc.)8,10 or through the formation of imidoyl triflates (with
Tf2O9) followed by their reactions with alcohols. It should
be emphasized that none of these reagents were employed
in our case.
Since Oꢀmethyl acetimidate 3 contains an additional
methoxy group, its formation is obviously due to the deꢀ
composition of excess DAST with methanol. Apparently,
amide 1 reacts with DAST to give imidoyl fluoride 4
(Scheme 2), which is transformed under the action of
MeOH into imidate 3 (reactions of imidoyl fluorides with
methanol are known11). The formation of similar imidoyl
fluorides from the acetamido group has been detected
by mass spectrometry and 19F NMR spectroscopy in
attempted highꢀtemperature fluorination of the oxo group
in Cꢀglycoside derivatives of Neu5Ac with [bis(2ꢀmethoxyꢀ
ethyl)amino]sulfur trifluoride (DeoxoꢀFluor), a more
potent fluorinating agent than DAST.12 At the same time,
we found no literature data on similar transformations in
the presence of DAST.
(47%). Apparently, Lu et al.7 faced a similar problem but
1
did not optimize the fluorination step. The H, 13C, and
19F NMR spectra of compound 2 agree with the literature
data.7
The proposed approach to the recovery of the 5ꢀacetaꢀ
mide function with aqueous TFA differs substantially from
the action of acids on other acetimidates of amino sugꢀ
ars.8—10,13 The most striking result was obtained when
imidate 6 (synthesized from Neu5Ac benzyl glycoside 5)
was treated with anhydrous TFA in MeOH9 (Scheme 3).
In this case, cleavage of the C=N bond smoothly gave the
corresponding amine 7 in high yield (95%).9 In contrast,
hydrolysis of imidate 3 with TFA in aqueous THF reꢀ
sulted in cleavage of the C—O bond and the formation
of amide 2 only (see Scheme 1). Moreover, treatment of
Oꢀmethyl acetimidate 9 (obtained in 65% yield from
Neu5Ac 4ꢀnitrophenyl glycoside 8 (see Ref. 14) under the
action of MeOTf in the presence of 2,6ꢀdi(tertꢀbutyl)ꢀ
4ꢀmethylpyridine (DTBMP)) with aqueous TFA under
similar conditions gave a mixture of products, from which
the expected acetamide 8 could not be isolated (see
Scheme 3).
Hydrolysis of Oꢀalkyl imidates is believed to be conꢀ
trolled stereoelectronically (e.g., see Ref. 15). In this
context, the hydrolysis pathway is mainly determined by
the structure of a tetrahedral intermediate, which in turn
depends on the configuration and conformation of the
starting imidate. It is not unlikely that the different conꢀ
figurations (Z or E with respect to the double C=N bond)
and conformations (syn or anti) of imidates 3, 6 and 9
It is known that the Oꢀalkyl acetimidate function is
unstable under acidic conditions and can be hydrolyzed
to the amino group. This property is widely used for
Nꢀdeacetylation of acetamido sugars (e.g., see Refs 9, 10,
13). The resulting amine can be acetylated in situ to
recover the acetamide function. For instance, such a
singleꢀstep transformation has been reported for a gluꢀ
cosamine derivative in the presence of 23% AcOH in
Ac2O at 55 °C (see Ref. 10). Note, however, that direct
recovery of the acetamide function from a 5ꢀ(Oꢀmethyl
acetimidate) derivative of Neu5Ac as a component of a
hexasaccharide has been carried out under other condiꢀ
tions:8 Bu4NI/Amberlyst H+/acetone or H2O2/AcOH/
CH2Cl2.
We found that the use of aqueous trifluoroacetic acid
is very efficient for recovery of the acetamido group from
Oꢀmethyl acetimidate 3. When a solution of acetimidate 3
in THF—H2O (4 : 1) was treated with a dilute aqueous
solution of trifluoroacetic acid (TFA) for 1 h, the starting
compound 3 was completely converted into acetamide 2
(TLC data). Therefore, acetamide 2 and acetimidate 3
obtained in the fluorination of aldehyde 1 need not be
separated; their mixture can be treated in situ with aqueꢀ
ous TFA to give acetamide 2 as the sole product. It was
isolated by column chromatography in a yield (94%) that
is twice as high as that reported earlier7 for compound 2

444
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 2, February, 2009
Orlova et al.
Scheme 3
Reagents and conditions: a. 1) Tf2O, 2,6ꢀdi(tertꢀbutyl)pyridine, MeCN (Ref. 9: 50% yield), 2) EtOH (Ref. 9: 95% yield);
b. 1) TFA, MeOH, 2) Amberlite IRAꢀ420 (Cl–) (Ref. 9); c. MeOTf, DTBMP, CH2Cl2; d. TFA, THF—H2O (4 : 1).
1H), signals of CDCl3 (δ 77.0; 13C{1H}), and signals of CFCl3
obtained in different ways might be decisive for the aforeꢀ
mentioned difference in the acid hydrolysis results. To
gain insight into the factors determining conformation of
both the imidate itself and a tetrahedral intermediate proꢀ
duced by a nucleophilic attack on the imidate, further
investigations are required. At the moment, one can only
state that the acetamido group can be regenerated by acid
treatment of an Oꢀalkyl acetimidate of Neu5Ac, though
this approach is not of general character.
In summary, when reproducing the reported transforꢀ
mation of the formyl group of a Neu5Ac aryl glycoside
into a difluoromethyl group under the action of DAST,
we discovered an unknown formation of Oꢀmethyl acetꢀ
imidate of Neu5Ac and optimized the conditions for the
regeneration of the amide function in the difluoromethyl
derivative. The yield of the target product was nearly quanꢀ
titative and twice as high as that reported earlier.
C
(δF 0.0; 19F; external reference). The signals in the 13C{1H} NMR
spectra were assigned using the APT (JMODXH) experiment.
Methyl [(2ꢀdifluoromethylꢀ4ꢀnitrophenyl) 5ꢀacetamidoꢀ
4,7,8,9ꢀtetraꢀOꢀacetylꢀ3,5ꢀdideoxyꢀDꢀglyceroꢀαꢀDꢀgalactoꢀnonꢀ
2ꢀulopyranosid]onate (2). A. (Diethylamino)sulfur trifluoride
(168 μL, 221.4 mg, 1.374 mmol) was added to a cooled (ice—
water bath) solution of aldehyde 1 (220 mg, 0.343 mmol) (see
Ref. 7) in anhydrous CH2Cl2 (5 mL). The mixture was stirred
at ~20 °C for 20 h and another portion of DAST (168 μL,
221.4 mg, 1.374 mmol) was added. Then two more portions of
DAST (200 μL, 263.6 mg, 1.636 mmol, each) were added after
5 and 18 h. After 1.5 h, MeOH (5 mL) was added and volatiles
were removed on a rotary evaporator. The residue was dissolved
in THF—H2O (4 : 1) (7 mL) and 1% aqueous TFA (4.5 mL) was
added. After 1 h, volatiles were removed on a rotary evaporator.
The residue was subjected to column chromatography with
gradient elution from toluene to toluene—acetone (1 : 1). The
yield of amide 2 was 214.6 mg (94%), Rf 0.54 (toluene—acetone,
1 : 1). 1H NMR (CDCl3), δ: 1.93 (s, 3 H, CH3CON); 2.04, 2.05,
2.10, 2.18 (all s, 3 H each, CH3CO); 2.28—2.36 (m ~ “br.t”,
1 H, Hax(3)); 2.79 (dd, 1 H, Heq(3), J3ax,3eq = 13.1 Hz,
J3eq,4 = 4.7 Hz); 3.66 (s, OCH3); 4.06—4.14 (m, 2 H, H(5),
H(9a)); 4.23 (dd, 1 H, H(9b), J9a,9b = 12.4 Hz, J8,9b = 2.2 Hz);
4.62 (br.d, 1 H, H(6), J5,6 = 10.8 Hz); 5.03 (ddd, 1 H, H(4),
J4,5 = 10.9 Hz, J3eq,4 = 4.7 Hz, J3ax,4 = 10.9 Hz); 5.32—5.40 (m,
2 H, H(7), H(8)); 5.46 (d, 1 H, NH, J5,NH = 9.8 Hz); 6.89
(t, 1 H, CHF2, JH,F = 55.0 Hz); 7.42 (d, 1 H, Harom(6),
J = 9.2 Hz); 8.32 (dd, 1 H, Harom(5), J = 9.2 Hz, J = 2.2 Hz);
8.45 (d, 1 H, Harom(3), J = 2.2 Hz). 13C NMR (CDCl3), δ: 20.6,
20.7, 20.8, 20.9 (AcO); 23.1 (AcN); 38.5 (C(3)); 49.3 (C(5));
53.5 (OCH3); 62.2 (C(9)); 67.0, 68.0, 68.1, 73.7 (C(4), C(6),
Experimental
Solvents for chromatography and extraction were distilled
and purified according to standard procedures. Thinꢀlayer chroꢀ
matography was carried out on DCꢀAlufolien Kieselgel 60 F254
plates (Merck). The eluents for TLC are specified for each parꢀ
ticular experiment (the ratios are expressed as v/v). Spots were
visualized under UV light (254 nm) or by heating the plates at
>150 °C, immersing in 85% H3PO4—96% EtOH (1 : 10), and
heating again. Preparative separation was carried out by column
chromatography on silica gel 60 (40—63 μm, Merck).
1H and 13C NMR spectra were recorded on Bruker ACꢀ200
(200.13 and 50.32 MHz, respectively) and Bruker Avance 600
instruments (600.0 and 150.9 MHz, respectively). 19F NMR spectra
were recorded on a Bruker ACꢀ200 instrument (188.31 MHz). Chemiꢀ
1
C(7), C(8)); 100.1 (C(2)); 110.1 (t, CHF2, JC,F = 238.7 Hz);
118.5, 122.5 (t, JC,F = 6.0 Hz); 125.3, 127.8, 143.3, 156.5 (t,
JC,F = 5.3 Hz) (Carom); 167.5 (C(1)); 169.9, 170.0, 170.3, 170.5,
170.7 (C=O). 19F NMR (CDCl3), δ: –116.1 (d, CHF2, J = 55.5 Hz).
cal shifts are referenced to residual signals of CHCl3 (δ 7.27;
H

Neu5Ac Oꢀmethyl acetimidate
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 2, February, 2009
445
B. A 1% aqueous solution of TFA (0.5 mL) was added to a
solution of imidate 3 (see below) in THF—H2O (4 : 1) (1 mL).
The reaction mixture was kept at ~20 °C. According to TLC
data (Rf 0.80 (3) and 0.54 (2), toluene—acetone (1 : 1)), the
reaction mixture after 1 h contained only compound 2.
H(9)); 4.61 (d, 1 H, H(6), J5,6 = 9.7 Hz); 4.87 (ddd, 1 H, H(4),
J3ax,4 = 11.4 Hz, J4,5 = 9.7 Hz); 5.30—5.55 (m, 2 H, H(7),
H(8)); 7.23 (d, 2 H, Harom, J = 9.0 Hz); 8.22 (d, 2 H, Harom
,
J = 9.0 Hz). 13C NMR (CDCl3), δ: 15.3 (CH3C(OMe)=N);
20.7, 20.9, 21.0, 21.4 (CH3CO); 37.4 (C(3)); 52.9, 53.2 (2 OCH3);
57.9 (C(5)); 62.0 (C(9)); 67.9, 68.2, 70.6, 75.5 (C(4), C(6),
C(7), C(8)); 99.8 (C(2)); 116.2, 118.7, 125.6, 146.4 (Carom);
164.8 (CH3OC=N); 167.4 (C(1)); 167.8, 169.2, 169.5 (C=O).
Methyl [(2ꢀdifluoromethylꢀ4ꢀnitrophenyl) 4,7,8,9ꢀtetraꢀ
Oꢀacetylꢀ5ꢀ(1ꢀmethoxyethylideneamino)ꢀ3,5ꢀdideoxyꢀDꢀglyceroꢀ
αꢀDꢀgalactoꢀnonꢀ2ꢀulopyranosid]onate (3). (Diethylamino)sulfur
trifluoride (76 μL, 100.6 mg, 0.624 mmol) was added to a cooled
(ice—water bath) solution of aldehyde 1 (100 mg, 0.156 mmol)
(see Ref. 7) in anhydrous CH2Cl2 (1 mL). The mixture was
stirred at ~20 °C for 18 h and another portion of DAST (76 μL,
100.6 mg, 0.624 mmol) was added. Then two more portions of
DAST (200 μL, 263.6 mg, 1.636 mmol, each) were added after
6 and 20 h. After 18 h, MeOH (5 mL) was added and volatiles
were removed on a rotary evaporator. Product 3 was isolated
from the residue by column chromatography with gradient eluꢀ
tion from toluene to toluene—acetone (8 : 2). The yield was
60.8 mg (58%), Rf 0.80 (toluene—acetone, 1 : 1). 1H NMR
(CDCl3), δ: 1.81 (s, 3 H, CH3C(OMe)=N); 2.02, 2.05, 2.06,
2.21 (all s, 3 H each, CH3CO); 2.186 (dd, 1 H, Hax(3)); 2.87
(dd, 1 H, Heq(3), J3ax,3eq = 13.0 Hz, J3eq,4 = 4.8 Hz); 3.29 (t,
1 H, H(5)); 3.62 (s, 3 H, CH3OC=N); 3.69 (s, 3 H, OCH3);
4.14 (dd, 1 H, H(9a), J9a,9b = 12.5 Hz, J8,9a = 5.3 Hz); 4.24 (dd,
1 H, H(9b), J8,9b = 2.6 Hz); 4.56 (dd, 1 H, H(6), J5,6 = 10.0 Hz,
J6,7 = 1.5 Hz); 4.84 (ddd, 1 H, H(4), J3ax,4 = 11.4 Hz, J4,5 = 9.4 Hz);
5.33 (dd, 1 H, H(7), J7,8 = 9.2 Hz); 5.40 (ddd, 1 H, H(8));
We are grateful to Prof. Z. Guo (USA) for fruitful
discussion of the possible mechanism of imidate formaꢀ
tion and to V. I. Bragin (State Research Center of the
Russian Federation Federal State Unitary Enterprise
“GNIIKhTEOS”) for recording NMR spectra on a Bruker
AVANCE 600 instrument.
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 08ꢀ03ꢀ00839)
and the Council on Grants at the President of the Russian
Federation (State Support Program for Young Ph.D. Sciꢀ
entists, Grant MKꢀ3244.2008.3).
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2
6.91 (t, 1 H, CHF2, JH,F = 55.0 Hz); 7.50 (d, 1 H, Harom(6),
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Methyl [(4ꢀnitrophenyl) 4,7,8,9ꢀtetraꢀOꢀacetylꢀ5ꢀ(1ꢀmethoxyꢀ
ethylideneamino)ꢀ3,5ꢀdideoxyꢀDꢀglyceroꢀαꢀDꢀgalactoꢀnonꢀ
2ꢀulopyranosid]onate (9). Methyl triflate (30 μL, 51.6 mg,
0.344 mmol) was added under argon to a cooled (–40 °C) soluꢀ
tion of nitrophenyl glycoside 8 (92 mg, 0.152 mmol) (see
Ref. 14) and DTBMP (132.8 mg, 0.647 mmol) in anhydrous
CH2Cl2 (2 mL). The reaction mixture was stirred at ~20 °C for
3 h. Additional amount of MeOTf (30 μL, 51.6 mg, 0.344 mmol)
and DTBMP (132.8 mg, 0.647 mmol) were added. The reaction
mixture was stirred for 20 h and concentrated on a rotary evapoꢀ
rator. The residue was dissolved in CH2Cl2; the solution was
washed with 1 M H2SO4 (50 mL), a saturated solution of
NaHCO3 (2×50 mL), and brine (50 mL), filtered through a
cotton wool plug, and concentrated on a rotary evaporator.
Imidate 9 was isolated from the residue by column chromatoꢀ
graphy with gradient elution from toluene to toluene—acetone
(1 : 1). The yield was 62.2 mg (65%), Rf 0.82 (toluene—acetone,
1 : 1). 1H NMR (CDCl3), δ: 1.84 (s, 3 H, CH3C(OMe)=N);
2.04, 2.08, 2.11, 2.24 (all s, 3 H each, CH3CO); 2.10—2.28
(m, 1 H, Hax(3)); 2.88 (dd, 1 H, Heq(3), J3ax,3eq = 13.0 Hz,
J3eq,4 = 4.8 Hz); 3.27 (t, 1 H, H(5), J = 9.7 Hz); 3.65 (s, 3 H,
CH3OC=N); 3.71 (s, 3 H, OCH3); 4.12—4.33 (m, 2 H,
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Received October 31, 2008;
in revised form January 23, 2009