A facile preparation of uronates via selective oxidation with TEMPO/KBr/Ca(OCl)2 under aqueous conditions

Article abstract of DOI:10.1016/j.carres.2004.02.001

Addition of solid Ca(OCl)₂ as the terminal oxidant in the TEMPO-mediated selective oxidation has the benefit of easier operation. A variety of partially protected saccharide derivatives (1a-l) have been successfully converted into the corresponding uronate derivatives, including disaccharide building blocks for GAG fragments and precursors to saponins. The beneficial effect of Aliquat 336 was also disclosed in the oxidation of certain substrates.

Full text of DOI:10.1016/j.carres.2004.02.001

Carbohydrate
RESEARCH
Carbohydrate Research 339 (2004) 1219–1223
Note
A facile preparation of uronates via selective oxidation with
TEMPO/KBr/Ca(OCl)2 under aqueous conditions
Feng Lin,a Wenjie Peng,b Wen Xu,a Xiuwen Hanb and Biao Yua,*
aState Key Laboratory of Bio-organic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, Shanghai 200032, China
bState Key Laboratory of Catalyst, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Received 31 October 2003; accepted 3 February 2004
Abstract—Addition of solid Ca(OCl)2 as the terminal oxidant in the TEMPO-mediated selective oxidation has the benefit of easier
operation. A variety of partially protected saccharide derivatives (1a–l) have been successfully converted into the corresponding
uronate derivatives, including disaccharide building blocks for GAG fragments and precursors to saponins. The beneficial effect of
Aliquat 336 was also disclosed in the oxidation of certain substrates.
Ó 2004 Elsevier Ltd. All rights reserved.
Keywords: Uronate; Oxidation; TEMPO; Ca(OCl)2; Saponin
Repeating disaccharides composed of a uronic acid and
a 2-amino sugar constitute glycosaminoglycans (GAGs),
that is, hyaluronan, chondroitin, dermatan, heparin,
and heparan sulfate, which are ubiquitous in extracell-
uar matrix of animals and play essential roles in various
cellular processes, including cell growth and cell–cell
interactions.1 Uronates also occur as a important
structural units in a number of bacterial capsular poly-
saccharides and plant glycosides that show a variety of
biological activities.2;3 Furthermore, glucuronates are
frequently the final form of a drug or xenobiotic elimi-
nated from the body, performing a detoxification role.4
A key step in the synthesis of uronates usually employs
the oxidation of the primary hydroxyl groups of the
sugar moieties to carboxylic acids.4;5 A selective oxida-
tion in the presence of secondary hydroxyl groups would
greatly simplify the protecting group manipulations.6
This idea has been realized mostly resorting to the
oxidation protocol developed by Anelli and co-workers
in 1987 employing a TEMPO (2,2,6,6-tetramethyl-
piperidine-1-oxyl) (cat)–KBr (cat)–NaOCl system under
aqueous conditions.6–8 Here we applied a modified
protocol with Ca(OCl)2 as the terminal oxidant for the
selective oxidation of the substrates (1a–l) with a variety
of aqueous solubilities.
In the TEMPO–Brꢀ–NaOCl oxidation system, the
primary hydroxyl groups are oxidized to aldehydes by
the oxoammonium salt of TEMPO generated in situ via
oxidation with ClOꢀ and/or BrOꢀ. The hypohalogenous
species then oxidize the resulting aldehydes to carboxylic
acids.7;8 The latter oxidation takes place in the aqueous
phase; thus phase-transfer catalysts are required for
hydrophobic substrates. The pH is critically important
to the oxidation, which determines the interconversion
of the oxidation species in the system. In addition, oxi-
dation of a hydroxymethyl group to an aldehyde
involves release of a Hþ so as to drop the pH sharply.
Therefore, the reaction medium is usually buffered with
NaHCO3 at pH 8.6–9.5. In some cases, higher pH was
carefully controlled to execute the oxidation via adding
dilute NaOH solution during the reaction.6c;d;h;j;k NaOCl
is commercially available in dilute aqueous solution that
deteriorates rather rapidly; thus the solution needs
titration before use to define the quantity of oxidant
present.9a Alternatively, Ca(OCl)2 is a stable and inex-
pensive solid hypochlorite oxidant.9b Its use as an
* Corresponding author. Tel.: +86-21-6416-3300; fax: +86-21-6416-
6128; e-mail: byu@mail.sioc.ac.cn
0008-6215/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2004.02.001

1220
F. Lin et al. / Carbohydrate Research 339 (2004) 1219–1223
OH
OMe
O
1) TEMPO/KBr/Ca(ClO)
2
O
2) methyl ester formation
O
O
O
(HO)n
R
(HO)n
1a-l
2a-l
R
R
R
O
O
O
O
HO
HO
BzO
HO
HO
OBz
O
HO
BzO
AcHN
O
BzO
AcHN
O
OBn
1d, 2d
OBn
1c, 2c
OBz
1a, 2a
1b, 2b
MeO
O
O
OAc
O
OBn
O
BzO
O
HO
BnO
O
R
O
O
O
BnO
HO
BnO
HO
BzO
R
O
O
N3
N3
N3
BnO
OMe
BzO
OBn
R
O
1f, 2f
1e, 2e
1g, 2g
O
O
O
O
O
R
OH
R
O
O
HO
BzO
HO
BzO
O
HO
O
OH
OH
O
OBz
1h, 2h
OBz
R
1j, 2j
1i, 2i
O
O
R
R
O
O
O
O
HO
BzO
HO
O
O
O
O
OBz
O
OBz
O
OBz
O
O
O
OBz
O
BzO
BzO
BzO
OBz
OBz
OBz
OBz
OBz
OBz
BzO
OBz
BzO
BzO
BzO
1l, 2l
1k, 2k
OBz
BzO
1a-1l: R = CH2OH, 2a-2k: R = COOMe, 2l: R = COOH
Scheme 1.
alternative to NaOCl in the TEMPO-mediated oxida-
tion has been demonstrated by Torii and co-workers.10
However, only two papers have so far reported the
oxidation of two monosaccharide 4,6-diols with the
TEMPO–KBr–Ca(OCl)2 system.11 In addition, these
authors dissolved Ca(OCl)2 in an aqueous NaHCO3
solution before use, and in fact a NaOCl solution was
prepared.9b;c Addition of aqueous NaOCl solution
dilutes the reaction system so as to cause pH change and
the precipitation of certain substrates in the biphasic
reaction medium. We found the addition of Ca(OCl)2
solid to be more convenient and controllable. In fact,
substrates 1a–l were reliably oxidized into the corre-
sponding uronates (2a–l) in satisfactory yields under the
present modified operation conditions (Scheme 1, Table
1).
and 1l, affording the desired uronates, after methylation
(except for 2l), in satisfactory 71–89% yields (entries 1, 2,
4–6, 8, 11, 12). For the water-soluble GluNAc 3,4,6-triol
1c, the oxidation was carried out in an aqueous solution
free of the organic solvent, giving methyl uronate 2c in
85% yield after methylation with MeI (Na2CO3, DMF,
50 °C) (entry 3). Glucosyl oleanolic ester 1j is insoluble in
either water or CH2Cl2; therefore 1:4 H2O–t-BuOH was
used as the reaction medium. The oxidation was com-
pleted with 4 equiv of Ca(OCl)2 at 10 °C, affording after
methylation the desired uronate 2j in a satisfactory 65%
yield, where the three secondary OHs on the glucose
The 1H NMR signal changes from the starting pyranosides to the
resulting methyl uronates are very diagnostic: the H-5 signal moves
downfield from a multiplet to a doublet (J ¼ ꢁ9.6 Hz, for glucuro-
nates), the two H-6 signals disappear, and the nascent COOMe
singlet appears downfield (3.74–3.88 ppm).
The present typical procedure is nicely applicable to
the partially protected monosaccharides 1a,b, and 1d,
disaccharides 1e and 1f, and the saponin derivatives 1h,k,

F. Lin et al. / Carbohydrate Research 339 (2004) 1219–1223
1221
(C8H17)3NþClꢀ) was employed in place of the previous
phase-transfer catalyst, Bu4NþBrꢀ, oxidation of 1i pro-
vided 2i in 51% yield (entry 9). The Aliquat 336 is capable
of forming inverted micelles13 that might provide
accommodations to the lipophilic steroidal moiety so as
to prevent the double bond from attack by the haloge-
nous species. The beneficial effect of using Aliquat 336
instead of Bu4NþBrꢀ was also found when oxidizing
disaccharide 1g, where the yield of uronate 2g increased
from 54% to 93% (entry 7).
In summary, the use of solid Ca(OCl)2 in place of
NaOCl in the TEMPO-mediated oxidation has the
benefit of easier operation. The present modified oper-
ation has been reliably applied to a variety of substrates
to prepare uronates, where disaccharides 2e–g are
important building blocks for the synthesis of heparin
and heparan sulfate fragments, and 2h–l are advanced
precursors to the synthesis of glucuronic acid-containing
saponins.14 This is the first application of the TEMPO-
mediated selective oxidation to the saponin substrates. It
shall be also valuable to disclose the beneficial effect of
Aliquat 336 in the oxidation of certain substrates.
Table 1. Selective oxidation to prepare uronates with TEMPO–KBr–
Ca(OCl)2 under aqueous conditions
Entry
Substrate
Conditionsa;b
Product
Yield (%)
1
1a
1b
1c
1d
1e
1f
A
2a
2b
2c
2d
2e
2f
83
70
2
A
3
Bb
Ab
A
85
71
4
5
73
83
6
A
7
1g
1h
1i
A; C
A
2g
2h
2i
54; 93
78
8
9
A; C
Db
A
0; 51
65
72
10
11
12
1j
2j
1k
1l
2k
2l
Ac
89
aConditions: (A) TEMPO (0.02equiv), KBr (0.2equiv), Ca(OCl)
2
(2equiv), Bu 4NþBrꢀ (0.2equiv), 1:1 CH 2Cl2–aq NaHCO3, 0 °C,
45 min; then CH2N2, THF, rt. See the typical procedure for detailed
experiment. (B) Free of organic solvent (CH2Cl2). (C) Aliquat 336
(CH3(C8H17)3NþClꢀ, 0.2equiv) was used in place of Bu 4NþBrꢀ. (D)
The solvent was changed to 1:4 H2O–t-BuOH; 4 equiv of Ca(OCl)2
was required to complete the oxidation.
bSubsequent methylation was carried out with MeI in DMF in the
presence of Na2CO3 (1 equiv) at 50 °C for 3 h.
cMethylation was not carried out.
moiety and the secondary 3-OH of the aglycone re-
mained intact (entry 10). Although, subjection of the
steroidal glycosides 1h to the present typical oxidation
procedure gave, after methylation, the expected uronate
2h in good yield (78%) (entry 8), oxidation of 1i, which
bears an additional 5,6-double bond on the steroid, un-
der the similar conditions failed to provide the desired
uronate (2i). In the major products isolated the 5,6-
double bond was absent as indicated by 1H NMR
spectroscopy.12 It is noted that the double bonds in 1b
and 1j–l remained intact during oxidation under similar
conditions. Interestingly, when Aliquat 336 (CH3-
1. Experimental
1.1. General methods
See Ref. 15.
A typical procedure is as follows: To a solution of the
alcohol 1f (0.6 mmol) in CH2Cl2 (1.5 mL) containing
TEMPO (2mg, 0.02equiv) was added a solution of satd
aq NaHCO3 (1.5 mL, its pH was adjusted to 9.5 with satd
aq Na2CO3) containing KBr (11 mg, 0.2equiv), and
Bu4NþBrꢀ (14 mg, 0.2equiv). The mixture was cooled to
0 °C. Under vigorous stirring, Ca(OCl)2 was added
Table 2. 1H NMR and MS data for compounds 2a–l
Compound 1H NMR (300 MHz, CDCl3): d
ESIMS (m=z)
2a
a-Isomer: 6.82(d, 1H, J 3.9 Hz, H-1), 4.58 (d, 1H, J 10.2Hz, H-5), 3.86 (s, 3H, OMe);
b-isomer: 6.21 (d, 1H, J 7.5 Hz, H-1), 3.83 (s, 3H, OMe).
543.4 [MþNaþ]
2b
5.89 (dd, 1H, J 9.6 Hz, H-3), 5.86 (m, 1H, OCH2CH@CH2), 5.37 (d, 1H, J 3.6 Hz, H-1), 479.1303 [MþNaþ] (calcd 479.1313)
5.32(m, 1H, OCH 2CH@CH2), 5.27 (m, 1H, OCH2CH@CH2), 5.17 (d, 1H, J 10.5 Hz,
H-2), 4.45 (d, 1H, J 9.6 Hz, H-5), 4.30 (m, 1H, OCH2CH@CH2), 4.04 (m, 2H), 3.88 (s,
3H, OMe).
2c
2d
((CD3)2CO): 7.40–7.29 (m, 5H, Ph), 4.95 (d, 1H, J 3.6 Hz, H-1), 4.75 and 4.52(AB, 2H,
12.3 Hz), 4.13 (d, 1H, J 9.3 Hz, H-5), 4.00 (m, 1H, H-2), 3.74 (s, 3H, OMe), 3.76–3.66 (m,
2H), 1.91 (s, 3H, Ac).
7.42–7.27 (5H, Ph), 5.59 (d, 1H, J 9.6 Hz, NH), 5.06 (s, 1H, OH), 4.89 (d, 1H, J 3.9 Hz, 448.1558 [MþNaþ] (calcd 448.1578)
H-1), 4.75 and 4.46 (AB, 2H, J 11.4 Hz), 4.39 (m, 1H, H-3), 4.22 (d, 1H, J 9.6 Hz, H-5),
4.13 (m, 1H, H-2), 3.88 (m, 1H), 3.85 (s, 3H, OMe), 3.75 (s, 3H, OMe), 3.40 (t, 1H, J
9.3 Hz, H-4), 1.96 (s, 3H, Ac), 1.38 (d, 3H, J 6.0 Hz, Me).
J
362.1221 [MþNaþ] (calcd 362.1210)
2e
5.49 (br s, 1H, H-1), 5.29 (dd, 1H, J 1.5 Hz, H-3), 4.99 (AB, 1H, J 10.8 Hz), 4.87 (AB, 2H, 622.2 [MþNaþ]
J 11.4 Hz), 4.77 (AB, 1H, J 10.8 Hz), 4.64 (d, 1H, J 7.5 Hz, H-10), 4.57 (d, 1H, J 5.1 Hz,
H-5), 4.02(dd, 1H, J 7.5, 0.9 Hz, H-4), 3.88 (dd, 1H, J 8.0 Hz), 3.84 (d, 1H, J 9.6 Hz,
H-50), 3.79 (s, 3H, OMe), 3.76 (m, 1H), 3.65 (br s, 1H), 3.55 (m, 2H), 3.20 (br, 1H, H-2),
2.12 (s, 3H, OAc).
(continued on next page)

1222
F. Lin et al. / Carbohydrate Research 339 (2004) 1219–1223
Table 2 (continued)
Compound 1H NMR (300 MHz, CDCl3): d
ESIMS (m=z)
2f
2g
2h
2i
5.52(s, 1H, H-1), 5.36 (s, 1H, H-1 0), 5.31 (m, 1H, H-20), 4.90 (s, 1H, H-50), 4.88 (AB, 1H, J 684.2[M þNaþ]
12.0 Hz), 4.75 (d, 1H, J 5.5 Hz, H-5), 4.68 (AB, 1H, J 12.0 Hz), 4.57 (d, 1H, J 7.0 Hz,
CH2Ph), 4.13 (d, 1H, J 7.0 Hz), 4.05 (d, 1H, J 7.2Hz, H-4), 3.92(br, 1H), 3.82 (m, 1H),
3.79 (s, 3H, OMe), 3.77 (m, 1H), 3.68 (m, 1H), 3.26 (d, 1H, J 3 Hz, H-2).
8.10–7.25 (m, 20H, Ph), 5.37 (s, 1H, H-10), 5.21 (br, 1H, H-20), 4.97 (d, 1H, J 2.1 Hz,
H-50), 4.76 (AB, 2H, J 11.4 Hz), 4.81–4.67 (m, 4H), 4.48 (AB, 1H, J 12.0 Hz), 4.02 (d, 1H,
J 7.8 Hz), 4.13–3.96 (m, 2H), 3.92–3.86 (m, 2H), 3.48 (s, 3H, OMe), 3.45 (s, 3H, OMe),
3.44 (m, 1H), 2.66 (d, 1H, J 9.0 Hz, OH).
820.3 [MþNaþ]
7.97–7.37 (m, 10H, Ph), 5.51 (dd, 1H, J 9.5 Hz, H-30), 5.39 (dd, 1H, J 8.6 Hz, H-20), 4.86 832.5 [MþNHþ4 ]
(d, 1H, J 7.8 Hz, H-10), 4.39 (m, 1H, H-16), 4.19 (dd, 1H, J 9.3 Hz, H-40), 4.07 (d, 1H, J
9.6 Hz, H-50), 3.86 (s, 3H, OMe), 3.62(m, 1H, H-3), 3.47–3.30 (m, 2H), 0.74 (d, 3H,
J
6.9 Hz), 0.78 (d, 3H, J 6.0 Hz), 0.72, 0.68 (s each, 2 ꢂ Me).
8.01–7.35 (m, 10H, Ph), 5.53 (dd, 1H, J 10.2Hz, H-3 0), 5.41 (dd, 1H, J 9.6 Hz, H-20), 5.26 835.4 [MþNaþ]
(d, 1H, J 4.8 Hz, H-6), 4.87 (d, 1H, J 5.4 Hz, H-10), 4.40 (m, 1H, H-16), 4.19 (m, 1H), 4.08
(d, 1H, J 9.9 Hz, H-50), 3.86 (s, 3H, OMe), 3.59–3.34 (m, 3H), 0.98 (d, 3H, J 6.6 Hz, Me),
0.92(s, 3H, Me), 0.79 (d, 3H, J 6.6 Hz, Me), 0.76 (s, 3H, Me).
2j
(CD3OD): 5.43 (d, 1H, J 7.7 Hz, H-10), 5.26 (t, 1H, J 3.6 Hz, H-12), 3.91 (d, 1H, J 9.6 Hz, 669.4 [MþNaþ]
H-50), 3.77 (s, 3H, OMe), 3.57 (t, 1H, J 9.1 Hz), 3.45 (t, 1H, J 8.4 Hz), 3.39 (t, 1H, J
9.1 Hz), 3.15 (m, 1H, H-3), 2.88 (m, 1H, H-18), 1.28, 1.16, 0.98, 0.95, 0.92, 0.78, 0.77
(7 ꢂ s, 7 ꢂ Me).
2k
5.97 (t, 1H, J 9.6 Hz), 5.94 (d, 1H, J 8.5 Hz), 5.90 (t, 1H, J 9.8 Hz), 5.78–5.69 (m, 2H), 2403.3 [MþNaþ]
5.58 (t, 2H, J 8.4 Hz), 5.52–5.43 (m, 2H), 5.38 (d, 1H, J 3.6 Hz), 5.27 (s, 1H), 4.68 (d, 1H,
J 7.7 Hz), 4.60 (d, 1H, J 8.0 Hz), 4.56 (dd, 1H, J 12.4, 2.8 Hz), 4.45 (dd, 1H, J 12.2,
4.8 Hz), 4.42–4.30 (m, 2H), 4.28–4.16 (m, 2H), 4.03 (dd, 1H, J 11.0, 7.7 Hz), 3.88–3.75 (m,
2H), 3.78 (s, 3H), 3.70 (d, 1H, J 9.6 Hz), 3.58 (t, 1H, J 8.6 Hz), 3.53 (s, 1H), 3.02(dd, 1H,
J 15.7, 4.2Hz), 2.78 (d, 1H, J 9.0 Hz), 2.62 (m, 1H), 2.33 (t, 1H, J 6.9 Hz), 1.16, 1.02, 0.98,
0.95, 0.86, 0.82, 0.42 (7 ꢂ s, 7 ꢂ Me).
2l
See Ref. 14.
2. (a) Lindberg, B.; Kenne, L. In The Polysaccharides;
Academic: New York, 1985; Vol. 2, p 287; (b) Lindberg,
B. Adv. Carbohydr. Chem. Biochem. 1990, 48, 279.
3. Hostettmann, K.; Marston, A. Saponins; Cambridge, UK:
Cambridge University Press, 1995.
4. For a comprehensive review on glucuronide synthesis, see:
Stachulski, A. V.; Jenkins, A. N. Nat. Prod. Rep. 1998,
173–186.
slowly in small portions. After 45 min at 0 °C, the reac-
tion was quenched with Na2S2O3 (100 mg). After addi-
tion of water (1.5 mL) and CH2Cl2 (4 mL), aq HCl (6 N)
was added to adjust the final pH to pH 3. Then the
organic phase was separated, and the remaining aq phase
was extracted with CH2Cl2. The combined organic phase
was washed with brine, dried with Na2SO4, and then
concentrated in vacuo. In order to facilitate the purifi-
cation and characterization, the corresponding methyl
uronate was prepared via treating the crude uronate with
CH2N2 in THF at rt. Chromatography on a silica gel
column provided the desired 2f (329 mg, 83%). See Table
2for characterization data for compounds 2a–l.
5. Relevant Ref. cited in: Yu, B.; Zhu, X.; Hui, Y. Org. Lett.
2000, 2, 2539–2541.
6. For selective oxidation of saccharides to uronates with
TEMPO–Brꢀ–NaOCl, see, for example: (a) Davis, N. J.;
Flitsch, S. L. Tetrahedron Lett. 1993, 34, 1181–1184; (b)
Davis, N. J.; Flitsch, S. L. J. Chem. Soc., Perkin Trans. 1
1994, 359–368; (c) de Nooy, A. E. J.; Besemer, A. C.; van
Bekkum, H. Carbohydr. Res. 1995, 269, 89–98; (d) Heeres,
A.; van Doren, H. A.; Gotlieb, K. F.; Bleeker, I. P.
Carbohydr. Res. 1997, 299, 221–227; (e) Battistelli, C. L.;
De Castro, C.; Iadonisi, A.; Lanzetta, R.; Mangoni, L.;
Parrilli, M. J. Carbohydr. Chem. 1999, 18, 69–86; (f)
Garegg, P. J.; Oscarson, S.; Tedebark, U. J. Carbohydr.
Chem. 1998, 17, 587–594; (g) Yeung, B. K. S.; Hill, D. C.;
Janicka, M.; Petillo, P. A. Org. Lett. 2000, 2, 1279–1282;
(h) Kraus, T.; Budesinsky, M.; Zavada, J. Eur. J. Org.
Chem. 2000, 3133–3137; (i) Haller, M.; Boons, G.-J.
J. Chem. Soc., Perkin Trans. 1 2001, 814–822; (j) Thabu-
ret, J.-F.; Merbouh, N.; Ibert, M.; Marsais, F.; Queguiner,
G. Carbohydr. Res. 2001, 330, 21–29; (k) Soderman, P.;
Widmalm, G. Eur. J. Org. Chem. 2001, 3453–3456; (l)
Lefeber, D. J.; Arevalo, E. A.; Kamerling, J. P.; Vliegent-
hart, J. F. G. Can. J. Chem. 2002, 80, 76–81; (m) Bouktaib,
M.; Atmani, A.; Rolando, C. Tetrahedron Lett. 2002, 43,
6263–6266; (n) Brochette-Lemoine, S.; Joannard, D.;
Descotes, G.; Bouchu, A.; Queneau, Y. J. Mol. Catalysis
Acknowledgements
This work was supported by the National Natural Sci-
ence Foundation of China (29925203) and the Com-
mittee of Science and Technology of Shanghai
(02QMA1401).
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