Novel preparation of a 2′-O-acetyl-1′-O-(4-methoxybenzyl)-L- biopterin derivative, a versatile precursor for a selective synthesis of L-biopterin glycosides

Article abstract of DOI:10.1055/s-2006-949620

L-Rhamnose was converted, over a 13-step-sequence, into 2′-O-acetyl-N²-(N,N-dimethylaminomethylene)-1′-O-(4- methoxybenzyl)-3-[2-(4-nitrophenyl)ethyl]-L-biopterin, an appropriately protected precursor of 1′-O- and 2′-O-monoglycosyl-L-biopterin.

Full text of DOI:10.1055/s-2006-949620

LETTER
2075
Novel Preparation of a 2¢-O-Acetyl-1¢-O-(4-methoxybenzyl)-L-biopterin
Derivative, a Versatile Precursor for a Selective Synthesis of L-Biopterin
Glycosides
Preparationof
a
a
2
¢
-
O
-Acet
d
yl-1
¢
-
O
-(4-m
a
ethoxybenz
s
yl)-
L
-biop
h
terin
D
eriva
t
i
ive Hanaya,* Hiroki Toyota, Hiroshi Yamamoto
Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700-8530, Japan
Fax +81(86)2517853; E-mail: hanaya@cc.okayama-u.ac.jp
Received 12 May 2006
Efficient preparation of various types of glycosides of
biopterin and related pterins by glycosylation has not been
achieved so far, despite considerable interest from the
viewpoint of their physiological function and biological
Abstract: L-Rhamnose was converted, over a 13-step-sequence,
into 2¢-O-acetyl-N2-(N,N-dimethylaminomethylene)-1¢-O-(4-meth-
oxybenzyl)-3-[2-(4-nitrophenyl)ethyl]-L-biopterin, an appropriate-
ly protected precursor of 1¢-O- and 2¢-O-monoglycosyl-L-biopterin.
Thus, the first selective synthesis of these L-biopterin glycosides activities as well as the structural proof of hitherto
was accomplished by treatment of the precursor with either DDQ or
reported natural products. Although we reported the
synthesis of 2¢-O-(D-glucopyranosyl)-L-biopterins,10 gly-
sodium methoxide, then with tetra-O-benzoyl-a-D-glucopyranosyl
bromide in the presence of silver triflate and tetramethylurea, fol-
cosylation of N2-(N,N-dimethylaminomethylene)-3-[2-(4-
lowed by removal of the remaining protecting groups.
nitrophenyl)ethyl]-L-biopterin (4) was not obtained with
Key words: L-biopterin glycosides, limipterin, glycosylations,
high selectivity: e.g., treatment of 4 with tetra-O-benzoyl-
pteridine, protecting groups
a-D-glucopyranosyl bromide11 (3 mol equiv) in the pres-
ence of tin(IV) chloride afforded 2¢-O-(b-D-gluco-
pyranosyl)-L-biopterin (5a, 41% yield), together with the
Various pterin derivatives have been isolated from many
1¢-O-glycosyl isomer 5b (15%) and the 1¢,2¢-di-O-glyco-
syl derivative 5c (14%; Scheme 1).
living organisms, including microorganisms, algae, in-
sects, fish, amphibians, and mammals.1 Among the pterin
derivatives, L-biopterin (1) is the most abundant of the
naturally occurring pterins found in human urine2 and
1) (Me3Si)2NH, (NH4)2SO4
CH2Cl2, reflux
O
N
OH
NPE
N
N
N
exhibits enzyme cofactor activity in hydroxylation of aro-
matic amino acids as the form of its tetrahydro derivative.3
Meanwhile, pterin glycosides having various kinds of
sugars attached to the side-chain at C-6 of the pteridine
ring were found to be produced by some prokaryotes. For
example, 2¢-O-(a-D-glucopyranosyl)-L-biopterin (2) was
isolated from cyanobacterium, Anacystis nidulans,4 Syn-
echococcus sp. PCC 7942,5 and Spirulina (Arthrospira)
platensis,6 whereas limipterin [2¢-O-(2-acetamido-2-
deoxy-b-D-glucopyranosyl)-L-biopterin] (3) was isolated
from a green sulfur photosynthetic bacterium Chlorobium
limicola f. thiosulfatophilum NCIB 8327 (Figure 1).7 Be-
sides these glycosides of L-biopterin, some glycosides of
other pterin derivatives have also been found in nature8
and some of them have remained obscure as for the
anomeric structure of the sugar moiety and the position of
the pterin moiety where the sugar attaches.9
N
BzO
O
2)
SnCl4,
CH2Cl2,
r.t., 18 h
OH
N
OBz
Br
BzO
4
BzO
O
N
OR1
NPE
N
N
N
N
OR2
N
BzO
5a R1 = H, R2 = Glc (41%)
5b R1 = Glc, R2 = H (15%)
5c R1 = R2 = Glc (14%)
O
Glc =
OBz
BzO
OBz
Scheme 1
This result prompted us to undertake an effective prepara-
tion of 1¢-O- and 2¢-O-monoprotected L-biopterin deriva-
tives as the potential key precursors to achieve the
selective 2¢-O- and 1¢-O-monoglycosylation. Although
synthetic procedures for biopterin itself12 and protection
of the pyrimidine ring moiety13 were well documented,
preparation of biopterin derivatives whose one hydroxy
group of the side-chain diols is protected has not been
reported yet, to the best of our knowledge. Taking into
consideration the available combination of protecting
groups employed for the synthetic pathways, we have
chosen p-methoxybenzyl (PMB) group for protection of
1¢-hydroxy group, since its cleavage can be effectively
performed under specific conditions14 that cause no
disruption of the rest of the molecule. We now describe
HO
O
OH
O
3
2'
2a R =
OH
N
N
6
3'
1'
HN
HO
OR
HO
7
2
H2N
N
HO
O
1 R = H (
L
-biopterin)
3 R =
OH
HO
NHAc
Figure 1
SYNLETT 2006, No. 13, pp 2075–2078
1
7
.0
8
.2
0
0
6
Advanced online publication: 09.08.2006
DOI: 10.1055/s-2006-949620; Art ID: U05406ST
© Georg Thieme Verlag Stuttgart · New York

2076
T. Hanaya et al.
LETTER
herein the preparation of a novel versatile 2¢-O-acetyl-1¢- 2,5,6-triamino-4-hydroxypyrimidine sulfate was carried
O-PMB-L-biopterin derivative and the first selective out in aqueous sodium bicarbonate solution to give an in-
glycosylation of L-biopterin.
separable mixture of the 6-substituted pterin 13a and its 7-
substituted isomer 13b.19 These products were separated
and characterized after having been converted into the ful-
ly-protected derivatives 14a,b by the following three
steps. Namely, treatment of 13a,b with N,N-dimethylfor-
mamide dimethyl acetal in DMF, the following acetyla-
tion of a hydroxy group afforded 2¢-O-acetyl-N2-(N,N-
dimethylaminomethylene)-1¢-O-PMB derivatives, whose
N-3 position was then protected with 2-(4-nitrophe-
nyl)ethyl (NPE) group by Mitsunobu reaction with NPE
alcohol in the presence of triphenylphoshine and diethyl
azodicarboxylate (DEAD) to provide 14a and 14b. These
products were separated by column chromatography over
silica gel into the desired 6-substituted pterin (L-biopter-
in) derivative 14a (53% overall yield from 12) and the 7-
substituted (L-primapterin) derivative 14b (15%).
Since no example of 3-O-protected 5-deoxy-L-arabinoses
has been reported so far, we designed a synthetic route for
the key intermediate 5-deoxy-3-O-PMB-L-arabinose (11)
by starting from L-rhamnose (Scheme 2). Thus, glyco-
sidation of L-rhamnose with allyl alcohol and the sub-
sequent acetalation with 2,2-dimethoxypropane provided
allyl 2,3-O-isopropylidene-a-L-rhamnopyranoside (6,15
80%) together with the corresponding b-anomer (8%).
Treatment of 6 with p-methoxybenzyl chloride and sodi-
um hydride in DMF gave the 4-O-PMB derivative 7. Con-
version of the allyl glycoside 7 into the 1-propenyl
glycoside with potassium tert-butoxide in DMSO, fol-
lowed by hydrolysis in 70% acetic acid,16 afforded 4-O-
PMB-L-rhamnopyranose (8).
The cleavage of C-1 of 8 was achieved by application of
the Hough and Taylor’s procedures17 with a slight modi-
fication. Namely, treatment of 8 with ethanethiol in the
presence of tosylic acid in acetic acid gave the dithioacetal
9, which was then oxidized with m-chloroperbenzoic acid
(MCPBA) to the corresponding sulfone 10. Degradation
of 10 with dilute aqueous ammonia afforded 5-deoxy-3-
O-PMB-L-arbinofuranose (11).
The structural assignment of 14a and 14b was achieved
primarily on the basis of their 13C NMR spectral data.20
The signals of C-6 and C-7 of 6-alkylkylpteridines gener-
ally appear at a similar field, whereas C-7 signals of 7-
alkyl derivatives shifts to a lower field (ca. 20 ppm) from
those of C-6.21 Therefore, the close values of 14a (C-6:
d = 150.71 ppm, C-7: d = 149.88 ppm) and the distant
values of 14b (C-6: d = 140.92 ppm, C-7: d = 159.98 ppm)
indicate the 6-substituted pterin for the former and the 7-
substituted pterin for the latter.
The selective oxidation for the 2-hydroxy group of 11
with cupric acetate18 provided the L-erythro-pentos-2-ul-
ose derivative 12. The pteridine ring formation of 12 with
1) allyl alcohol
HCl, dioxane
reflux, 2 h
1) t-BuOK
DMSO
100 °C, 20 min
HO
O
PMBO
O
O
CH3
PMBO
HO
O
CH3
O
CH3
O
CH3
PMBCl
OH
OH
2) Me2C(OMe)2
acetone, TsOH
r.t., 10 h
NaH, Bu4NI
DMF, r.t., 2 h
2) 70% AcOH aq
35 °C, 24 h
O
O
O
O
HO OH
HO OH
80%
95%
75%
L-rhamnose
6
7
8
EtS
SEt
EtO2S
SO2Et
OH
OH
OH
OH
O
OPMB
Cu(OAc)2
NH3 aq
EtSH
MCPBA
OH
PMBO
HO
PMBO
HO
TsOH, AcOH
10 °C, 40 min
dioxane
r.t., 1 h
r.t., 10 h
82%
MeOH–H2O
reflux, 1.5 h
42%
H3C
OH
CH3
CH3
10
73%
77%
9
11
O
OPMB
O
OPMB
N
N
NPE
1) Me2NCH(OMe)2
DMF, r.t., 3 h
2) Ac2O, Py
HN
N
N
O
OH
OAc
N
NH2
NH2
HN
H2N
H2N
N
O
N
N
N
N
O
O
OPMB
N
r.t., 12 h
OH
13a
14a (53% from 12)
+
+
H3C
3) NPEOH, PPh3
DEAD, THF
r.t., 12 h
NaHCO3
O
MeOH–H2O
reflux, 3.5 h
NPE
N
N
N
N
OAc
HN
OH
12
4) separation
N
N
N
H2N
N
OPMB
OPMB
13b
14b (15%)
Scheme 2
Synlett 2006, No. 13, 2075–2078 © Thieme Stuttgart · New York

LETTER
Preparation of a 2¢-O-Acetyl-1¢-O-(4-methoxybenzyl)-L-biopterin Derivative
2077
O
N
OPMB
OH
O
OPMB
OAc
O
OH
NPE
N
N
N
NPE
N
N
N
NPE
N
N
N
NaOMe
DDQ
N
N
N
OAc
CH2Cl2, H2O
r.t., 30 min
MeOH
r.t., 3 h
N
N
N
N
N
99%
92%
15
14a
16
1) NaOMe, MeOH
rt, 1 h
BzO
BzO
O
N
OR1
O
N
OH
O
OBz
NPE
N
N
2) NH3 aq, MeOH
r.t., 12 h
N
N
HN
H2N
Br
OR2
BzO
OR3
BzO
BzO
15
N
N
N
O
3) DBU, DMF
r.t., 12 h
R2
=
=
CF3SO3Ag, TMU
CH2Cl2, r.t., 3 h
OBz
17 R1 = PMB
5a R1 = H
2b
OBz
DDQ, CH2Cl2
rt, 2 h, 83%
89% (3 steps)
75%
HO
1) NaOMe, MeOH
r.t., 1 h
BzO
O
O
R3
O
OR2
O
N
OR3
OH
OBz
2) NH3 aq, MeOH
r.t., 12 h
NPE
N
HO
N
N
N
BzO
Br
N
HN
H2N
OH
BzO
16
OAc
N
OH
N
N
CF3SO3Ag, TMU
CH2Cl2, r.t., 3 h
3) DBU, DMF
r.t., 12 h
AcO
AcO
O
18
19
R4
=
=
88% (3 steps)
73%
OAc
NPhth
AcO
AcO
Br
O
N
OR1
O
OH
O
OAc
1) MeNH2, MeOH
r.t., 12 h
2) Ac2O, Py, r.t., 12 h
NPE
N
N
N
N
3
HO
N
HN
H2N
O
OR4
N
R5
NPhth
OR5
OH
15
N
N
HO
3) NH3 aq, MeOH
r.t., 12 h
CF3SO3Ag, TMU
CH2Cl2, r.t., 2.5 h
NHAc
20 R1 = PMB
21 R1 = H
4) DBU, DMF, r.t., 12 h
86% (4 steps)
DDQ, CH2Cl2
r.t., 2 h, 87%
77%
Scheme 3
Deprotection of 2¢-O-acetyl-1¢-O-PMB-L-biopterin deriv- performed by use of DDQ without harming glycosyl link-
ative 14a was then carried out under mutually exclusive age, affording 5a and 21, respectively.
conditions to give the mono-O-protected compounds
(Scheme 3). Namely, methanolysis of 14a in the presence
of sodium methoxide provided the 1¢-O-PMB derivative
Removal of the protecting groups of 5a and 18 was carried
out by the successive treatment with sodium methoxide
(to cleave all acyl groups), aqueous ammonia (to cleave
15, while treatment of 14a with 2,3-dichloro-5,6-dicyano-
the N,N-dimethylaminomethylene group), and then DBU
1,4-benzoquinone (DDQ) afforded the 2¢-O-acetyl com-
(to cleave the NPE group) to give 2¢-O-(b-D-glucopyrano-
pound 16. The partially protected pterins 15 and 16 are
syl)-L-biopterin (2b) and its 1¢-O-glycosyl analogue 19 in
important precursors for the 2¢- and 1¢-O-monoglycosyl
ca. 90% (overall yield from 5a and 18), respectively.
derivatives, respectively. In addition, these compounds
Similarly, removal of the phthaloyl group and the N,N-
dimethylaminomethylene group of 21 with methylamine,
followed by the action of acetic anhydride, afforded the
have such an advantage as to be sufficiently soluble in
dichloromethane, although pterin derivatives having the
side chain diols are sparingly soluble in nonpolar aprotic
fully-acetylated derivative, which was then treated with
solvents.
aqueous ammonia and then with DBU to give limipterin
Efficient glycosylation of 15 was exemplified by the con- (3) in 86% (overall yield from 21).
densation with tetra-O-benzoyl-a-D-glucopyranosyl bro-
In summary, we have developed a novel effective way for
mide in the presence of silver triflate22 and
selective preparation of L-biopterin glycosides via 2¢-O-
tetramethylurea (TMU) in dichloromethane at room tem-
perature for three hours, affording 2¢-O-(2,3,4,6-tetra-O-
acetyl-1¢-O-PMB-L-biopterin derivative 14a. The 1¢-O-
PMB derivatives 15 and the 2¢-O-acetyl compounds 16
benzoyl-b-D-glucopyranosyl)-L-biopterin derivative (17)
derived from 14a are regarded as highly useful precursors
as a sole product in 75% yield. The similar treatment of 16
respectively for the 2¢- and 1¢-O-glycosyl-L-biopterin de-
rivatives having various types of sugar moiety.
afforded 1¢-O-glycosyl analogue 18 in 73% yield. More-
over, glycosylation of 15 with 1,3,4,6-tetra-O-acetyl-2-
deoxy-2-phthalimindo-b-D-glucopyranosyl
bromide23
Acknowledgment
provided 2¢-O-(1,3,4,6-tetra-O-acetyl-2-deoxy-2-phthali-
mindo-b-D-glucopyranosyl)-L-biopterin derivative 20 in
77% yield.24 Cleavage of PMB group of 17 and 20 was
We are grateful to the SC-NMR Laboratory of Okayama University
for the NMR measurements.
Synlett 2006, No. 13, 2075–2078 © Thieme Stuttgart · New York

2078
T. Hanaya et al.
LETTER
(14) (a) Oikawa, Y.; Yoshioka, T.; Yonemitsu, O. Tetrahedron
Lett. 1982, 23, 885. (b) Oikawa, Y.; Tanaka, T.; Horita, K.;
Yonemitsu, O. Tetrahedron Lett. 1984, 25, 5397.
(c) Tanaka, T.; Oikaya, Y.; Hamada, T.; Yonemitsu, O.
Tetrahedron Lett. 1986, 27, 3651.
(15) Addition of 2,2-dimethoxypropane gave a higher yield of 6
(80%) than that of reported method by use of only acetone
(68%): Gigg, R.; Payne, S.; Conant, R. J. Carbohydr. Res.
1983, 2, 207.
(16) When acidic hydrolysis of methyl 2,3-O-isopropylidene-4-
O-PMB-a-L-rhamnopyranoside was attempted to obtain 4-
O-PMB-L-rhamnose(8), removal of the PMB group
preferentially took place rather than hydrolysis of methyl
glycoside. Therefore we employed 1-propenyl glycoside,
which is cleavable under weaker acidic conditions.
(17) Hough, L.; Taylor, T. J. J. Chem. Soc. 1955, 3544.
(18) Weinstock, J. US 3505329, 1970; Chem. Abstr. 1970, 72,
132787h.
(19) A similar condensation of non-protected 5-deoxy-L-
erythro-pentos-2-ulose with the same pyrimidine derivative
has been reported to provide an 8:2 mixture of 6- and 7-
substituted pterins in a relatively low yield (37%; ref. 18).
(20) Selected NMR data for 14a: 1H (600 MHz, CDCl3): d = 1.23
(3 H, d, J2¢,3¢ = 6.6 Hz, H-3¢), 4.77 (1 H, d, J1¢,2¢ = 4.4 Hz, H-
1¢), 5.36 (1 H, qd, H-2¢), 8.96 (1 H, s, H-7). 13C (151 MHz,
CDCl3): d = 15.92 (C-3¢), 71.76 (C-2¢), 82.21 (C-1¢), 128.36
(C-4a), 149.88 (C-7), 150.71 (C-6), 153.63 (C-8a), 157.55
(C-2), 161.83 (C-4).
References and Notes
(1) (a) Pfleiderer, W. Angew. Chem., Int. Ed. Engl. 1964, 3,
114. (b) Rembold, H.; Gyure, W. L. Angew. Chem., Int. Ed.
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(2) (a) Patterson, E. L.; Broquist, H. P.; Albrecht, A. M.; von
Saltza, M. H.; Stokstad, E. L. R. J. Am. Chem. Soc. 1955, 77,
3167. (b) Patterson, E. L.; von Saltza, M. H.; Stokstad, E. L.
R. J. Am. Chem. Soc. 1956, 78, 5871.
(3) (a) Kaufman, S.; Fisher, D. B. In Molecular Mechanisms of
Oxygen Activation; Hayaishi, O., Ed.; Academic Press: New
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Folates and Pterins, Vol. 2; Blakley, R.; Benkovic, S. J.,
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(4) Forrest, H. S.; Van Baalen, C.; Myers, J. Arch. Biochem.
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M.; Nishihara, H.; Sekine, M.; Inada, Y. Mar. Biotechnol.
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Selected NMR data for 14b: 1H (600 MHz, CDCl3): d = 1.22
(3 H, d, J2¢,3¢ = 6.6 Hz, H-3¢), 4.66 (1 H, d, J1¢,2¢ = 4.2 Hz, H-
1¢), 5.37 (1 H, qd, H-2¢), 8.78 (1 H, s, H-6). 13C (151 MHz,
CDCl3): d = 15.61 (C-3¢), 71.94 (C-2¢), 82.26 (C-1¢), 129.29
(C-4a), 159.98 (C-7), 140.92 (C-6), 153.17 (C-8a), 157.88
(C-2), 161.83 (C-4).
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(22) Use of SnCl4 as an activator resulted in the formation of diol
4 by cleavage of PMB group instead of glycosylation.
(23) Farkas, J.; Ledvina, M.; Brokes, J.; Jezek, J.; Zajicek, J.;
Zaoral, M. Carbohydr. Res. 1987, 163, 63.
(24) General Procedure for Glycosylation of 15.
To a solution of 15 (56 mg, 0.10 mmol), glycosyl bromide
(0.30 mmol) and TMU (0.012 mL, 0.10 mmol) in dry
CH2Cl2 (1.0 mL) was added silver triflate (56 mg, 0.22
mmol). The mixture was stirred at r.t. for 3 h, diluted with
CHCl3, and filtered through Celite®. The filtrate was washed
with aq NaHCO3, dried (MgSO4), and evaporated in vacuo.
The residue was purified by column chromatography to give
the 2¢-O-glucopyranosyl-L-biopterin derivative.
(11) Ness, R. K.; Fletcher, H. G. Jr.; Hudson, C. S. J. Am. Chem.
Soc. 1950, 72, 2200.
(12) (a) Patterson, E. L.; Milstrey, R.; Stockstad, E. L. R. J. Am.
Chem. Soc. 1956, 78, 5868. (b) Viscontini, M.; Provenzale,
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E. C.; Jacobi, P. A. J. Am. Chem. Soc. 1976, 98, 2301.
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Chem. 1984, 1815. (e) Mori, K.; Kikuchi, H. Liebigs Ann.
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Mogi, K.; Wasada, H. Synthesis 1992, 303.
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Pfleiderer, W. Helv. Chim. Acta 2003, 86, 1.
Synlett 2006, No. 13, 2075–2078 © Thieme Stuttgart · New York