Synthesis of site-specific, deuterium-substituted α-l-Rhap-(12)-α-L-Rhap-OMe

Article abstract of DOI:10.1016/S0008-6215(98)00254-7

The disaccharide α-l-Rhap-(12)-α-l-(2-²H)Rhap-OMe has been synthesized. The site-specific deuteration was performed at C-2 by reduction of a suitably protected β-linked 2-ulose derivative. In addition, α-l-Rhap-(12)-α-l-(2-²H)Rhap-O(²H₃)Me was also synthesized. Copyright (C) 1998 Elsevier Science Ltd.

Full text of DOI:10.1016/S0008-6215(98)00254-7

Carbohydrate Research 312 (1998) 233±237
Note
Synthesis of site-speci®c, deuterium-substituted
ꢀ-l-Rhap-(1!2)-ꢀ-l-Rhap-OMe
Peter Soderman, Stefan Oscarson, Goran Widmalm *
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm,
Sweden
Received 1 October 1997; accepted 12 September 1998
Abstract
The disaccharide ꢀ-l-Rhap-(1!2)-ꢀ-l-(2-2H)Rhap-OMe has been synthesized. The site-speci®c
deuteration was performed at C-2 by reduction of a suitably protected ꢁ-linked 2-ulose derivative.
In addition, ꢀ-l-Rhap-(1!2)-ꢀ-l-(2-2H)Rhap-O(2H3)Me was also synthesized. # 1998 Elsevier
Science Ltd. All rights reserved
Keywords: Carbohydrates; Rhamnose; Deuterium; NMR spectroscopy
The predominant technique used for conforma-
tional studies of carbohydrates in solution is NMR
spectroscopy. It is versatile and can give informa-
tion on proton±proton internuclear distances by
the employment of NOESY experiments. Informa-
tion on dihedral angles can be obtained from mea-
surement of coupling constants, which are used
both in homo- and heteronuclear cases. Studies of
nuclear spin relaxation rates by investigation of
spin±lattice and spin±spin relaxation times can give
information on dynamics of the molecule. For
carbohydrates, intramolecular hydrogen bonding
can occur for certain geometries. These conformers
may be stabilized by a hydrogen bond and the
conformational equilibrium shifted as a result.
Hydrogen bonding can for hydroxyl or amide
protons be studied by NMR spectroscopy through
In some cases, the problem under investigation
cannot be fully resolved due to the inability of the
technique to di€erentiate between various cases.
One such problem has been observed in the
conformational analysis of ꢀ-l-Rhap-(1!2)-ꢀ-l-
Rhap-OMe [1], where two possible conforma-
tional states could not be di€erentiated due to a
`three spin e€ect' between protons, which ren-
dered the NOEs for the two conformational states
essentially identical under the experimental con-
ditions employed. By simulation of the NOE
build-up curves from both conformers, it became
apparent that, if the H-2 proton at the glycosidic
linkage could be eliminated, then di€erentiation
between conformers should be possible. This
could be performed by a MINSY NMR experi-
ment or by substitution of H-2 by a deuterium
atom. Here, we report the synthesis of site-spe-
ci®c, deuterated analogues of ꢀ-l-Rhap-(1!2)-ꢀ-
l-Rhap-OMe.
1
the investigation of H/2H exchange rates, chemi-
cal shift displacement as a function of temperature
or by 1H/2H chemical shift isotope e€ects.
Site-speci®c deuteration of the title compound 1
* Corresponding author.
[2] was targeted to H-2 (1-d) and to H-2 and the
0008-6215/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved
PII S0008-6215(98)00254-7

234
P. Soderman et al./Carbohydrate Research 312 (1998) 233±237
È
O-methyl group (1-d4). In order to obtain a good
yield of product with the manno-con®guration by
stereoselective reduction of a 2-ulose derivative
with sodium borodeuteride, it is necessary that the
anomeric center should have the ꢁ-con®guration.
A key intermediate in the synthesis was therefore
methyl 3,4-di-O-benzyl-6-deoxy-ꢁ-l-arabino-hexo-
pyranosid-2-ulose.
by hydrolysis of the prop-1-enyl glycoside with
HgCl2±HgO in aqueous acetone [6,7], gave crys-
talline 4-O-benzyl-2,3-O-isopropylidene-ꢀ-l-rham-
nopyranose (3) in 70% yield. Preparation of
(2H3)methyl 4-O-benzyl-2,3-O-isopropylidene-ꢁ-l-
rhamnopyranoside (4-d3) was accomplished under
conditions described by Haines et al. [8], in which
the alcohol was methylated using sodium hydride
and (2H3)methyl iodide in tetrahydrofuran to give
4-d3 in 91% yield.
Acidic hydrolysis of the isopropylidene group
gave (2H3)methyl 4-O-benzyl-ꢁ-l-rhamnopyrano-
side (5-d3) (92%). (2H3)Methyl 3,4-di-O-benzyl-ꢁ-
l-rhamnopyranoside (6-d3) was ®rst prepared by
the original method of Handa et al. [9] which gave,
after activation by dibutyltin oxide and subsequent
regioselective benzylation of the diol in N,N-dime-
thylformamide and benzyl bromide, only a low
yield (<40%) of the desired regioisomer. However,
when the activated stannylene intermediate was
benzylated using neat benzyl bromide, the yield
was raised to 92%.
Swern oxidation (dimethyl sulfoxide/oxalyl
chloride/triethylamine) [10] of 6-d3 a€orded the 2-
ulo intermediate, which was not isolated. Sub-
sequent reduction with sodium borodeuteride in
methanol-d4 gave the isotopically enriched
(2H3)methyl
3,4-di-O-benzyl-ꢁ-l-(2-2H)rhamno-
pyranoside (7-d4) in 77% yield. The extent of deu-
teration was >98% as calculated by integration of
1
the remaining H-2 signal in the H NMR spec-
trum. Anomerization of 7-d4 with non deuterated
or deuterated methanolic hydrogen chloride gave
the glycosyl acceptors methyl 3,4-di-O-benzyl-ꢀ-l-
(2-2H)rhamnopyranoside (8-d) and (2H3)methyl
3,4-di-O-benzyl-ꢀ-l-(2-2H)rhamnopyranoside (8-d4)
in 80 and 82%, respectively.
Glycosylation of compounds 8-d and 8-d4 with
2,3,4-tri-O-benzoyl-ꢀ-l-rhamnopyranosyl bromide
[11] using silver tri¯ate as a promoter [12,13]
a€orded the disaccharides 9-d and 9-d4 in 81 and
82%, respectively. Hydrogenolysis of 9-d and 9-d4
over Pd±C and subsequent removal of the benzoate
protecting groups with methanolic sodium meth-
oxide gave after gel ®ltration the target molecules
1-d and 1-d4 in 87 and 89%, respectively. The
overall yield from 2 was ꢀ26% for the two target
This intermediate was synthesized from the
starting material, allyl 4-O-benzyl-2,3-O-iso-
propylidene-ꢀ-l-rhamnopyranoside (2), obtained
in a three-step synthesis from l-rhamnose accord-
ing to the procedure of Gigg et al. [3]. Isomeriza-
tion of the allyl group using the Wilkinson catalyst
[tris(triphenylphosphine)rhodium(I) chloride] and
ethyldiisopropylamine in ethanol [4,5], followed
1
compounds. The H NMR spectra of 1, 1-d and
1-d4 are given in Fig. 1. We are presently investi-
gating the three-dimensional structure of the title
compound making use of these site-speci®c
deuterated analogues.

P. Soderman et al./Carbohydrate Research 312 (1998) 233±237
È
235
1. Experimental
General methods.ÐConcentrations were per-
formed under reduced pressure at temperatures
<40 ꢁC (bath). Optical rotations were measured at
ꢁ
22 C for solutions in CHCl3 or water with a Per-
kin±Elmer 241ꢁ polarimeter. NMR spectra were
recorded at 30 C for solutions in CDCl3, Me2SO-
d6 or D2O using JEOL GSX-270 or Varian Inova
600 spectrometers. Chemical shifts are reported in
ppm relative to internal SiMe4 (ꢂH 0.00), internal
sodium 4,4-dimethyl-4-sila-2,2,3,3-(2H4)pentanoate
(ꢂH 0.00), residual Me2SO-d5 (ꢂH 2.57) or CDCl3
(ꢂC 77.17). Electrospray-Ionization mass spectro-
metry (ESIMS) was performed in the negative-ion
mode on a VG Quattro triple quadrupole mass
spectrometer (Micromass) with 1:1 MeOH±water
as the mobile phase. High-resolution fast atom
bombardment mass spectrometry (HR-FABMS)
was performed in the positive-ion mode at a reso-
lution of 10 000 on a JEOL SX-102 using glycerol
or m-nitrobenzylalcohol as a matrix. Selected
intermediates 4, 5, 6 and 8 were also prepared non-
deuterated (data not shown).
4-O-Benzyl-2,3-O-isopropylidene-a-l-rhamnopyr-
anose (3).ÐA mixture of 2 [3] (4 g, 12 mmol),
ethyldiisopropylamine (1.55 g, 12 mmol) and tris-
(triphenylphosphine)rhodium(I) chloride (0.11 g,
0.12 mmol) in EtOH (50 mL) was boiled under
re¯ux for 3 h. The mixture was concentrated, and
the residue was dissolved in 10:1 acetone±water
(25 mL) containing HgO (3.37 g, 16 mmol). A
solution of HgCl2 (3.57 g, 13 mmol) in 10:1 acet-
one±water (25 mL) was added dropwise to the sus-
pension. When TLC (5:1 toluene±EtOAc) showed
the reaction to be complete, the mixture was con-
centrated, and the residue was dissolved in Et2O,
washed with a saturated KI solution, dried, and
concentrated. Silica gel column chromatography
(7:1 toluene±EtOAc) gave 3 (2.46 g, 70%); [ꢀ]d
+21ꢁ (c 1.0, CHCl3); 1H NMR (Me2SO-d6): ꢂ 1.22
(H-6), 1.35 and 1.47 (CMe2), 3.18 (H-4), 3.81 (H-
5), 4.09 (H-2), 4.21 (H-3), 4.64 and 4.84 (CH2Ph),
5.17 (H-1); 13C NMR (CDCl3): ꢂ 18.1 (C-6), 26.4
and 28.0 (CCH3), 65.1, 73.1, 76.3, 78.2, 80.8, 92.1
(C-1), 109.4, 127.8, 128.2, 128.4, 138.2. ESIMS:
[M H]± m/z 293.3; HRFABMS: [M+Na]+ m/z
calcd for C16H22O5Na 317.1365; found 317.1351.
(2H3)Methyl 4-O-benzyl-b-l-rhamnopyranoside
(5-d3).ÐMeI-d3 (1.39 mL, 21.8 mmol) was added
to a mixture of 3 (2.26 g, 7.7 mmol) and NaH
(0.52 g, 21.8 mmol) in THF (50 mL), and the
Fig. 1. 1H NMR spectra of ꢀ-l-Rhap-(1!2)-ꢀ-l-Rhap-OMe
(A), ꢀ-l-Rhap-(1!2)-ꢀ-l-(2-2H)Rhap-OMe (B) and ꢀ-l-Rhap-
(1!2)-ꢀ-l-(2-2H)Rhap-O(2H3)Me (C).

236
P. Soderman et al./Carbohydrate Research 312 (1998) 233±237
È
ꢁ
mixture was stirred at room temperature. When
TLC (7:1 toluene±EtOAc) showed the reaction to
be complete, MeOH (2 mL) was added and the
mixture was concentrated. The residue was dis-
solved in CH2Cl2 and washed with water, dried,
concentrated and applied to a silica gel column
(10:1 toluene±EtOAc) to give (2H3)methyl 4-O-
benzyl-2,3-O-isopropylidene-ꢁ-l-rhamnopyrano-
side (4-d3) (2.18 g, 91%); 13C NMR (CDCl3): ꢂ 18.3
(C-6), 26.3 and 27.8 (CCH3), 56.5 (OMe-d3, t),
70.5, 72.7, 74.5, 80.0, 81.0, 99.5 (C-1), 110.4, 127.6,
127.8, 128.2, 138.1.
Compound 4-d3 (2.0 g, 6.4 mmol) was dissolved
in CHCl3 (10 mL) and aq 90% CF3COOH (10 mL)
was added. The mixture was stirred for 2 h at
room temperature, then washed with saturated
NaHCO3, dried, concentrated, and puri®ed by
silica gel column chromatography (1:1 toluene±
EtOAc) to give 5-d3 as a white solid (1.61 g, 92%);
[ꢀ]d +48ꢁ (c 1.0, CHCl3); 13C NMR (CDCl3): ꢂ
17.9 (C-6), 56.0 (OMe-d3, t), 71.1, 71.2, 74.2, 75.0,
81.4, 100.6 (C-1), 127.8, 128.0, 128.4, 138.4. HR-
FABMS: [M+Na]+ m/z calcd for C14H17D3O5Na
294.1397; found 294.1426.
(2H3)Methyl 3,4-di-O-benzyl-b-l-rhamnopyrano-
side (6-d3).ÐA suspension of compound 5-d3
(1.53 g, 5.6 mmol) and dibutyltin oxide (1.54 g,
6.2 mmol) in anhydrous MeOH (50 mL) was boiled
under re¯ux for 3 h. The solvent was evaporated,
and the resulting syrup (crude) was dried in a
vacuum line and thereafter dissolved in benzyl
bromide (10 mL, 8.4 mmol). The mixture was
heated at 90 ꢁC for 30 min, then diluted with
toluene and put onto a silica gel column and eluted
with toluene followed by 2:1 toluene±EtOAc to
give after concentration 6-d3 as a solid (1.87 g,
92%); [ꢀ]d +36ꢁ (c 1.0, CHCl3); 1H NMR
(CDCl3): ꢂ 1.37 (H-6), 3.33 (H-5), 3.53 (H-3,4),
4.09 (H-2), 4.29 (H-1), 4.65, 4.68, 4.76, 4.94
(CH2Ph); 13C NMR (CDCl3): ꢂ 18.0 (C-6), 68.5,
71.6, 76.7, 79.8, 81.5, 100.6 (C-1), 127.9±128.6,
138.0, 138.5. HRFABMS: [M+Na]+ m/z calcd for
C21H23D3O5Na 384.1866; found 384.1875.
added and the solution was allowed to reach 0 C.
Water was added and the two phases were sepa-
rated, the organic phase dried (Na2SO4) and con-
centrated. The residue was immediately dissolved
in MeOH-d4 (40 mL) and NaBD4 (0.185 g,
4.4 mmol) was added. After 15 min the mixture was
concentrated. Silica gel column chromatography
(4:1 toluene±EtOAc) gave (2H3)methyl 3,4-di-O-
benzyl-ꢁ-l-(2-2H)rhamnopyranoside (7-d4) (1.12 g,
77%); Rf (3:1 toluene±EtOAc): 0.19; [ꢀ]d +40ꢁ (c
1
1.0, CHCl3); H NMR (CDCl3): ꢂ 1.37 (H-6), 3.33
(H-5) 3.53 (H-3,4), 4.29 (H-1, s), 4.65, 4.68, 4.76,
4.94 (CH2Ph).
Compound 7-d4 (0.500 g, 1.38 mmol) was dis-
solved in MeOH-d4 and acidi®ed (0.5 mL 2 M
acetyl chloride in MeOH-d4). The mixture was
boiled under re¯ux for 1 h, then it was con-
centrated. The residue was puri®ed on a silica gel
column (4:1 toluene±EtOAc) to give 8-d4 (0.410 g,
82%); Rf (3:1 toluene±EtOAc): 0.37; 13C NMR
(CDCl3): ꢂ 18.0 (C-6), 67.3, 68.2 (C-2, t), 72.1 and
75.4 (CH2Ph), 80.1, 100.1 (C-1), 127.8-128.6, 138.1,
138.5. ESIMS: [M H]± m/z 361.1. HRFABMS:
[M+Na]+ m/z calcd for C21H22D4O5Na 385.1929;
found 385.1922.
Methyl 3,4-di-O-benzyl-a-l-(2-2H)rhamnopyr-
anoside (8-d).ÐThis compound was prepared as
described for 8-d4, but using MeOH, starting from
7-d4 (0.500 g, 1.38 mmol) to give 8-d (0.396 g, 80%);
[ꢀ]d 40ꢁ (c 1.0, CHCl3); 13C NMR (CDCl3): ꢂ
54.9 (OMe). ESIMS: [M H]± m/z 358.2. HR-
FABMS: [M+Na]+ m/z calcd for C21H25DO5Na
382.1741; found 382.1781.
(2H3)Methyl (2,3,4-tri-O-benzoyl-a-l-rhamnopyr-
anosyl)-(1!2)-3,4-di-O-benzyl-a-l-(2-2H)rhamno-
pyranoside (9-d4).ÐA mixture of 8-d4 (0.350 g,
0.97mmol), 2,3,4-tri-O-benzoyl-ꢀ-l-rhamnopyrano-
syl bromide (0.679 g, 1.26 mmol), collidine (59 ꢃL,
0.49 mmol) and 4A molecular sieves (0.5 g) in
CH2Cl2 (20 mL) was stirred for 15 ꢁmin. Then, the
temperature was lowered to 40 C and AgOTf
(0.325 g, 1.26mmol) was added. When TLC (10:1
toluene±EtOAc) showed a complete reaction, pyr-
idine (50 ꢃL) was added, and the mixture was ®ltered
through Celite and concentrated. Silica gel chroma-
tography of the residual syrup (toluene, 20:1 toluene±
EtOAc) gave 9-d4 (0.650 g, 82%); [ꢀ]d +96ꢁ (c 1.0,
CHCl3); 13C NMR (CDCl3): ꢂ 17.9 and 18.1 (C-6,60),
67.3, 68.2, 70.1, 70.8, 72.2, 72.7, 75.7, 79.9, 80.3,
99.6 and 100.0 (C-1,10), 127.6±133.2, 138.5, 138.8,
165.4, 165.6, 166.0. HRFABMS: [M+Na]+ m/z
calcd for C48H44D4O12Na 843.3295, found 843.3306.
(2H3)Methyl 3,4-di-O-benzyl-a-l-(2-2H)rhamno-
pyranoside (8-d4).ÐDimethyl sulfoxide (0.63 mL,
8.8 mmol) in CH2Cl2 (3 mL) was added to a
cooled solution ( 60 ꢁC) of oxalyl chloride
(0.38 mL, 4.4 mmol) in CH2Cl2 (15 mL), and the
mixture was stirred for 5 min. Thereafter com-
pound 6-d3 (1.45 g, 4.0 mmol) in CH2Cl2 (10 mL)
was added dropwise during 15 min. After stirring
ꢁ
for 30 min at 60 C, Et3N (2.8 mL, 20 mmol) was

P. Soderman et al./Carbohydrate Research 312 (1998) 233±237
È
237
Methyl (2,3,4-tri-O-benzoyl-a-l-rhamnopyrano-
Acknowledgements
syl)-(1!2)-3,4-di-O-benzyl-a-l-(2-2H)rhamnopyr-
anoside (9-d).ÐThis compound was prepared as
described for 9-d4 starting from 8-d (0.320 g,
This work was supported by a grant from the
Swedish Natural Science Research Council. We
thank Professor P.-E. Jansson for ESIMS and Ms
P. Se€ers for HRFABMS.
1
0.9 mmol) to give 9-d (0.600 g, 81%). H NMR
(CDCl3): ꢂ 1.34 and 1.39 (H-6,60), 3.36 (OMe).
HRFABMS: [M+Na]+ m/z calcd for C48H47
DO12Na 840.3106; found 840.3134.
References
(2H3)Methyl a-l-rhamnopyranosyl-(1!2)-a-l-
(2-2H)rhamnopyranoside (1-d4).ÐCompound 9-d4
(0.300 g, 0.366 mmol) was dissolved in a mixture of
1:1 EtOAc±EtOH (20 mL) and hydrogenolysed
over 10% Pd±C (catalytic amount) for 16 h. Then,
the mixture was ®ltered through Celite and con-
centrated. The residue was dissolved in 0.1 M
NaOMe/MeOH (25 mL). When TLC (12:3:3:1
EtOAc±HOAc±MeOH±H2O) showed that the
debenzoylation was complete, the mixture was
passed through a column of Dowex-50 (H+) and
concentrated to dryness. Gel ®ltration through a
Bio-Gel P-2 column eluted with water and freeze-
drying gave 107 mg of 1-d4 (89%). NMR data and
optical rotation are in agreement with those
previously published [2]. HRFABMS: [M+Na]+
m/z calcd for C13H20D4O9Na 351.1569; found
351.1554.
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(1977) C13±C16.
Methyl a-l-rhamnopyranosyl-(1!2)-a-l-(2-2H)
rhamnopyranoside (1-d).ÐThis compound was
prepared as described for 1-d4 starting from 9-d
(0.300 g, 0.367 mmol), to give 1-d (104 mg, 87%).
HRFABMS: [M+Na]+ m/z calcd for C13H23
DO9Na 348.1381, found 348.1374.