Synthesis of a pseudodisaccharide α-C-glycosidically linked to an 8-alkylated guanine

Article abstract of DOI:10.3390/molecules18043906

The synthesis of stable guanofosfocin analogues has attracted considerable attention in the past 15 years. Several guanofosfocin analogues mimicking the three constitutional elements of mannose, ribose, and guanine were designed and synthesized. Interest in ether-linked pseudodisaccharides and 8-alkylated guanines is increasing, due to their potential applications in life science. In this article, a novel guanofosfocin analogue 6, an ether-linked pseudodisaccharide connected α-C-glycosidically to an 8-alkylated guanine, was synthesized in a 10-longest linear step sequence from known diol 13, resulting in an overall yield of 26%. The key steps involve the ring-opening of cyclic sulfate 8 by alkoxide generated from 7 and a reductive cyclization of 4-N-acyl-2,4-diamino-5- nitrosopyrimidine 19 to form compound 6.

Full text of DOI:10.3390/molecules18043906

Molecules 2013, 18, 3906-3916; doi:10.3390/molecules18043906
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis of a Pseudodisaccharide α-C-Glycosidically Linked to
an 8-Alkylated Guanine
Mu-Hua Huang 1,2,*, Jan Duchek 2 and Andrea Vasella 2
1
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
2
Laboratorium für Organische Chemie, Departement Chemie und Angewandte Biowissenschaften,
ETH Zürich, HCI, Zürich CH-8093, Switzerland
* Author to whom correspondence should be addressed; E-Mail: mhhuang@bit.edu.cn;
Tel./Fax: +86-10-6891-1608.
Received: 25 February 2013; in revised form: 21 March 2013 / Accepted: 21 March 2013 /
Published: 2 April 2013
Abstract: The synthesis of stable guanofosfocin analogues has attracted considerable
attention in the past 15 years. Several guanofosfocin analogues mimicking the three
constitutional elements of mannose, ribose, and guanine were designed and synthesized.
Interest in ether-linked pseudodisaccharides and 8-alkylated guanines is increasing, due to
their potential applications in life science. In this article, a novel guanofosfocin analogue 6,
an ether-linked pseudodisaccharide connected α-C-glycosidically to an 8-alkylated guanine,
was synthesized in a 10-longest linear step sequence from known diol 13, resulting in an
overall yield of 26%. The key steps involve the ring-opening of cyclic sulfate 8 by
alkoxide generated from 7 and a reductive cyclization of 4-N-acyl-2,4-diamino-5-
nitrosopyrimidine 19 to form compound 6.
Keywords: guanofosfocin; 8-alkylated guanine; ether-linked pseudodisaccharide;
analogues; α-C-glycoside; nitrosopyrimidine
1. Introduction
Stable carbohydrate mimics are used widely to study the biological functions of oligo- and
polysaccharides [1–3]. Well-known examples include C-glycosides [4], carbasugars [5] and
thiooligosaccharides [6].

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Guanofosfocin B (1, Figure 1) is one of the three guanofosfocins which were isolated in 1996 by
Nippon Roche [7] from the fermentation broth of Streptmyces sp. AB 2570 and Trichoderma sp. FD
5372. Guanofosfocins are of interest as strong inhibitors of chitin synthases (IC50: 1–10 nM). Detailed
investigations of their biological activity were, however, hampered by their rapid decomposition. The
instability, not surprising considering the two activated acetal moieties found in 1, was addressed by
several groups by synthesizing stable open chain analogues of 1 while potentially retaining the
promising biological properties. Sugimura et al. designed several analogues such as 2 (Figure 1) that
maintained the alkoxy substituent attached to the C(8) of guanine [8–10]. Later on, they replaced the
mannosyl moiety by a carba-mannosyl unit, and synthesized analogue 3 (Figure 1) [11]. Vasella et al.
aimed at C-mannosides, replacing the anomeric oxygen by a methylene group, and prepared analogues
4 [12] and 5 [13] (Figure 1). We also considered the guanofosfocin analogue 6 of interest, in analogy
to other, stable ether-linked pseudo-disaccharides [14–16] and report a synthesis of this ether-linked
pseudo-disaccharide containing an 8-alkylated guanine.
Figure 1. Compounds prepared on the way to stable analogues of guanofosfocins.
OBn
OBn
OBn
OH
OH
O
OH
O
NHTr
N
NHTr
N
BnO
TBSO
BnO
BnO
O
N
N
N
N
N
O
HO
O
O
N
N
O
NH
NH2
O
O
N
TBDPSO
N
O P
TrO
HO
O
O
O
O
P
OH
OH
O
O
OH
O
OH
O
Guanofosfocin B (1)
2
3
Isolated in 1996
by Sugimura et al. in 1998
Sugimura et al. in 2007
OBn
OBn
BnO
OBn
OBn
OBn
O
O
OBn
OBn
N
O
BnO
HO
OBn
BnO
BnO
N
N
N
O
N
N
O
O
N
N
O
O
N
H
N
N
NH2
HO
N
H
H
AcO
O
NH2
N
O
O
O
O
BnO
OBn
O
O
O
O
4
5
6
by Vasella et al. in 2004
by Vasella et al. in 2011
This work
2. Results and Discussion
The structure of 6 is characterized by an ether-linked pseudo-disaccharide with the α-C-mannopyranosyl
unit linked to C(8) of a guanine via a methylene group. Accordingly, we had to incorporate a
methylene group between the guanyl and mannosyl moieties and install the ether bond between the
secondary C(3)-OH group of mannose and the C(6)-OH group of an allofuranose. Retrosynthetically
(Scheme 1), the ether bond could be formed by ring-opening cyclic sulfate 8 by the alkoxy anion
corresponding to alcohol 7 [17], and the 8-substituted guanine could be formed by regioselective
4-N-acylation of 2,4-diamino-5-nitrosopyrimidine 9, followed by reductive cyclization, using a procedure
developed by Vasella et al. [18].

Molecules 2013, 18
Scheme 1. Retrosynthesis of pseudodisaccharide-guanine hybrid 6.
3908
BnO
OBn
O
Cyclization
BnO
O
OBn
O
BnO
HO
Etherification
BnO
OBn
O
O
O
S
N
N
7
O
OBn
N
O
N
N
N
NH2
O
AcO
H
O
O
O
BnO
H2N
O
NH2
N
BnO
O
9
8
6
Our synthesis started with the preparation of the protected α-allofuranose-diol 13, following known
procedures as outlined in Scheme 2 [19–22]. The secondary alcohol 7 was synthesized from
commercially available methyl α-D-mannoside in 36% overall yield using the three-step sequence reported
by Vasella et al. [13]. Epoxide formation from vicinal diol 13 under Mitsunobu conditions [23] yielded
66% of 14. With epoxide 14 in hand, we carried out the oxirane ring-opening by the secondary
alkoxide of 7 to get the pseudo-disaccharide 15 in 39% yield, besides 49% of recovered epoxide 14.
Scheme 2. Synthesis of ether-linked pseudodisaccharide 15.
O
O
O
O
O
O
O
OH
O
ref 20
76%
ref 19
ref 21
92%
ref 22
96%
O
D-glucose
O
O
O
O
OH
O
OBn
O
10
11
12
OBn
OBn
HO
HO
OBn
O
OBn
O
O
BnO
a)
O
b)
O
O
BnO
HO
O
O
HO
O
O
OBn
7
O
OBn
O
OBn
O
ref 13
13
14
15
Reagents and conditions: (a) Ph3P, DIAD, toluene, 66%; (b) 7, NaH, DMF, 39%.
To improve the yield of the etherification, we prepared the cyclic sulfate 8 (Scheme 3). It was
obtained in 69% from diol 13 by treatment with thionyl chloride and subsequent oxidation with NaIO4
in the presence of catalytic Ru(II)Cl2·xH2O [24]. The cyclic sulfate 8 was a colorless solid that
darkened upon storage, even in the refrigerator (0–5 °C) and so was used immediately. The reaction
between the cyclic sulfate 8 and the oxyanion derived from alcohol 7 in HMPA/THF took place
smoothly to furnish the ether-linked pseudo-disaccharide 15 in a yield of 86% upon acidic aqueous
work-up. The polar solvent proved crucial for the high yield. Alcohol 15 was acetylated and converted
to the carboxylic acid 18 in a yield of 88% by dihydroxylation of 16 by OsO4/NMO, cleavage of the
resulting diol by NaIO4, and oxidation of the resulting aldehyde by NaH2PO4/NaClO2. Treatment of
acid 18 with oxalyl chloride in the presence of catalytic DMF furnished the acid chloride. Though the

Molecules 2013, 18
3909
reaction was carried out for 1 h, the conversion was completed within 5 min, as observed by monitoring
the reaction mixture by IR spectroscopy. The acid chloride was stable enough to allow routine
1
13
characterization (IR, H-NMR and C-NMR). It reacted with 6-(benzyloxy)-5-nitrosopyrimidine-2,4-
diamine (9) to afford amide 19, accompanied by a color change from purple to blue-green. As
purification of amide 19 by chromatography led to yields below 50%, presumably caused by a strong
absorption of amide 19 on silica gel, the crude amide 19 was treated directly with triphenylphosphine
in xylene under reflux to furnish guanine 6 in an overall yield of 50% from 18 (Scheme 3). The
reductive cyclisation was accompanied by a color change from blue-green to brown.
Scheme 3. Synthesis of pseudodisaccharide-guanine hybrid 6 from cyclic sulfate 8.
OBn
OBn
OBn
O
OBn
O
O
O
BnO
S
O
BnO
O
O
O
O
a)
b)
c)
e),f)
R
13
O
AcO
O
RO
O
O
O
O
OBn O
OBn
O
OBn
8
15 R = H
17 R = CHO
d)
g)
16 R = Ac
18 R = COOH
OBn
OBn
OBn
OBn
O
OBn
N
O
O
OBn
O
N
OBn
N
BnO
BnO
N
O
O
O
j)
N
N
h), i)
N
N
NH2
N
H
N
NH2
H
AcO
AcO
H2N
N
NH2
O
O
9
O
O
O
OBn
O
OBn
Ref 12
19
6
Reagents and conditions: (a) SOCl2, Pyr., CH2Cl2; (b) RuCl2.xH2O, NaIO4, 69% for 2 steps; (c) 7, NaH,
HMPA/THF, 86%; (d) Ac2O, DMAP, Et3N; (e) OsO4, NMO; (f) NaIO4; (g) NaH2PO4, NaClO2, 88% for
4 steps; (h) (COCl)2, DMF(cat.), CH2Cl2; (i) 9, Pyr., THF; (j) Ph3P, xylene, reflux; 50% for 3 steps.
With guanine 6 in hand, we performed a few scouting reactions to test its macrocyclization
reactions via direct intra-molecular N-glycosylation. The first results showed that acetonide in 6 was
not a good glycosyl donor for N-glycosylation. We then hydrolysed 6 to its corresponding vicinal diol.
The macrocyclization is under investigation and the results will be reported in due course.
3. Experimental
General
Commercially available reagents were used without further purification. Water-free solvents were
dried: THF was distilled from Na/benzophenone; toluene from Na; CH2Cl2, MeCN, MeOH, pyridine,
and triethylamine from CaH2; acetone, and chloroform were dried over 4 Å molecular sieves.
Technical solvents were distilled: AcOEt, CH2Cl2 from K2CO3; Et2O from FeSO4·7 H2O; cyclohexane,
hexane, MeOH, and toluene without any other additive. The reactions were carried out in oven-dried
glassware, under an N2 or Ar atmosphere, unless stated otherwise. Qualitative TLC: precoated silica-gel

Molecules 2013, 18
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plates (Merck silica gel 60 F254); detection by heating with ‘mostain’ (400 mL of 10% H2SO4 soln., 20 g
of (NH4)6Mo7O24·4H2O, 0.4 g of Ce(SO4)2·4 H2O), or by UV. FCC (flash column chromatography):
silica gel Fluka 60 (0.04–0.063 mm) or Merck silica gel 60 (0.063–0.200 mm) under slightly elevated
pressure (0.1–0.4 bar). Melting points were measured on a Büchi B-540 melting point apparatus using
open glass capillaries and are uncorrected. Optical rotations were measured with a PerkinElmer digital
polarimeter: 1-dm cell at 25 °C, 589 nm, concentration (c) in g/100 mL. Infrared spectroscopy (IR)
were recorded on a Perkin Elmer Spectrum RX-I FT-IR: ca. 2% soln. in CHCl3; absorptions in cm–1.
NMR-spectra were recorded on Bruker magnetic resonance spectrometer (1H at 300 MHz, 13C at 75 MHz):
chemical shifts in ppm relative to a residual undeuterated solvent peak. MS spectra were recorded on
an IONSPEC Ultima ESI-FT-ICR spectrometer at 4.7 T.
3-O-Benzyl-1,2-O-isopropylidene-α-D-allofuranose, 5,6-cyclic sulfate (8). A solution of thionyl
chloride (0.11 mL, 1.5 mmol) in CH2Cl2 (0.85 mL) was added dropwise to an ice-cooled solution of
diol 13 (230 mg, 0.74 mmol) in CH2Cl2 (5 mL) and pyridine (0.24 mL, 3 mmol). The mixture was
stirred for 5 min, when TLC revealed the disappearance of starting material. The mixture was diluted
with CH2Cl2 and washed with water. The combined aqueous layers were extracted with CH2Cl2. The
combined organic layers were dried (Na2SO4) and concentrated. The residue was dissolved in
CH2Cl2/MeCN/H2O (2/2/3), to which was added NaIO4 (320 mg, 1.5 mmol) followed by
Ru(II)Cl2·xH2O (10 mg). After 10 min, the mixture was diluted with CH2Cl2, the organic layer was
separated, the water layer was extracted with CH2Cl2. The combined organic layers were dried
(Na2SO4) and concentrated in vacuo. The residue was purified by Flash Column Chromatography
(FCC, EtOAc/cyclohexane, 1/3→1/1) to afford the cyclic sulfate 8 as a white solid (191 mg, 69%).
1
m.p. 120–122 °C (dec.) (EtOAc/hexane). H-NMR (CDCl3, 300 MHz): (ppm) 7.40–7.37 (m, 5 H),
5.74 (d, 1 H, J = 3.3 Hz), 5.17 (dt, 1 H, J = 2.4, 7.2 Hz), 4.79 (dd, 1 H, J = 7.8, 9.0 Hz), 4.75 (d, 1 H,
J = 12.0 Hz), 4.62 (d, 1 H, J = 12.0 Hz), 4.60 (d, 1 H, J= 6.9 Hz), 4.57 (dd, 1 H, J = 1.2, 5.4 Hz), 4.23
13
(dd, 1 H, J = 2.4, 9.0 Hz), 3.99 (dd, 1 H, J = 4.5, 9.0 Hz), 1.59 (s, 3 H), 1.36 (s, 3 H). C-NMR
(CDCl3, 75 MHz): (ppm) 136.51, 128.53, 128.36, 113.53, 103.95, 79.99, 77.13, 77.05, 76.41, 72.59,
67.86, 26.90, 26.46. HR-MS(ESI), m/z 395.07695 [M+Na]+ (C16H20NaO8S+, required 395.07711).
5,6-Anhydro-3-O-benzyl1,2-O-isopropylidene-α-D-allofuranose (14). To a solution of diol 13 (305 mg,
0.98 mmol) and triphenylphosphine (310 mg, 1.18 mmol) in dry toluene (6 mL) was added diisopropyl
azodicarboxylate (0.25 mL, 94% pure, 1.18 mmol) dropwise at room temperature. The mixture was
stirred under reflux overnight. The solvent was removed in vacuo, the residue was purified by FCC
(EtOAc/cyclohexane, 1/3) to give epoxide 14 as a colorless oil (189 mg, 66%). IR (Film, cm−1): 3021
(s), 2934 (w), 2870 (w), 1751 (w), 1455 (w), 1384 (w), 1375 (w), 1315 (w), 1254 (w), 1163 (w), 1131
1
(m), 1102 (s), 1022 (s). H-NMR (CDCl3, 300 MHz): (ppm) 7.40–7.29 (m, 5 H), 5.75 (d, 1 H,
J = 3.9 Hz), 4.75 (d, 1 H, J = 11.4 Hz), 4.59–4.55 (m, 2 H), 4.21 (dd, 1 H, J = 3.0, 9.0 Hz), 3.66 (dd, 1
H, J = 4.2, 8.7 Hz), 3.19 (dd, 1 H, J = 3.0, 7.2 Hz), 2.81–2.73 (m, 2 H), 1.59 (s, 3 H), 1.36 (s, 3 H).
13C-NMR (CDCl3, 75 MHz): (ppm) 137.26, 128.57, 128.20, 128.16, 113.12, 104.21, 77.73, 77.57,
77.54, 72.02, 50.70, 44.45, 26.87, 26.60. [α]2D5 +85.92° (c 1.05 in CHCl3).

Molecules 2013, 18
3911
2,6-Anhydro-1,3,5-tri-O-benzyl-4-O-(3-O-benzyl-1,2-O-isopropylidene-6-deoxy-α-D-allofuranos-6-yl)-
7,8,9-trideoxy-D-glycero-D-mannonon-8-enitol (15). To a mixture of NaH (60% in oil, 93 mg, 2.32 mmol)
in HMPA (3 mL) and THF (1 mL) was added alcohol 7 (1.10 g, 2.32 mmol) in THF (15 mL). The
resulting slightly yellow solution was stirred at room temperature for 20 min, and then treated with
cyclic sulfate 8 (720 mg, 1.93 mmol) in THF (5 mL). The resulting mixture was stirred at room
temperature overnight when TLC revealed the disappearance of the cyclic sulfate. The reaction
mixture was treated with H2SO4/THF/H2O (1/100/0.3, 3 mL), stirred for 1 h, treated with saturated
aqueous NaHCO3 solution, and extracted with EtOAc. The combined organic layers were dried
(MgSO4), concentrated, and the residue purified by FCC (EtOAc/cyclohexane, 1/2→1/1) to afford 15
as a colorless oil (1.28 g, 86%). IR (Film, cm−1): 3475 (w), 3067 (w), 3032 (w), 3009 (m), 2928 (m),
2872 (w), 1732 (w), 1672 (w), 1496 (w), 1454 (w), 1374 (w), 1313 (w), 1224 (m), 1094 (s), 1027 (s).
1H-NMR (CDCl3, 300 MHz): (ppm) 7.34–7.23 (m, 20 H), 5.84–5.70 (m, 1 H), 5.72 (d, 1 H, J = 3.3 Hz),
5.08–5.01 (m, 2 H), 5.72–4.48 (m, 9 H), 4.05–3.91 (m, 4 H), 3.86–3.48 (m, 8 H), 2.88 (d, 1 H, J = 2.4 Hz),
2.38–2.25 (m, 2 H), 1.57 (s, 3 H), 1.35 (s, 3 H). 13C-NMR (CDCl3, 75 MHz): (ppm) 138.22, 137.94,
137.90, 137.40, 134.16, 128.29, 128.19, 127.94, 127.87, 127.64, 127.39, 117.17, 112.86, 104.03,
78.66, 78.13, 77.71, 77.60, 75.14, 74.94, 73.86, 73.55, 73.29, 72.28, 72.13, 71.81, 71.10, 70.29, 68.96,
34.61, 26.93, 26.67. ([α]2D5 +45.45° (c 1.00 in CHCl3). HR-MS (ESI), m/z 789.36095 [M+Na]+
+
(C46H54NaO10 , required 789.36092).
2,6-Anhydro-1,3,5-tri-O-benzyl-4-O-(5-O-acetyl-3-O-benzyl-1,2-O-isopropylidene-6-deoxy-α-D-allofuranos-
6-yl)-7,8,9-trideoxy-D-glycero-D-mannonon-8-enitol (16). Triethylamine (0.32 mL, 2.29 mmol) and
DMAP (61 mg, 0.50 mmol) in dry CH2Cl2 (2 mL), were added dropwise at 0 °C to a solution of
alcohol 15 (0.88 g, 1.15 mmol) and acetic anhydride (0.21 mL, 2.29 mmol) in dry CH2Cl2 (3 mL). The
resulting mixture was stirred at room temperature overnight. The reaction was quenched by adding
water (1 mL), and the solvent was removed in vacuo. A solution of the residue in EtOAc (20 mL) was
washed with water and 0.1 N HCl. The aqueous layer was extracted with EtOAc, the combined organic
layers were dried (MgSO4) and concentrated to give the acetate 16 as a slightly yellow oil (0.94 g,
100%). IR (cm−1, CHCl3): 3067 (w), 3032 (w), 3010 (m), 2928 (w), 1738 (s), 1642 (w), 1496 (w),
1
1374 (m), 1374 (m), 1307 (w), 1240 (s), 1093 (s), 1027 (s). H-NMR (CDCl3, 300 MHz): (ppm)
7.34–7.26 (m, 20 H), 5.85–5.73 (m, 1 H), 5.64 (d, 1 H, J = 3.7 Hz), 5.31 (ddd, 1 H, J = 4.8, 3.0, 6.6 Hz),
5.11–5.04 (m, 2 H), 4.73 (d, 1 H, J = 11.3 Hz), 4.67 (d, 1 H, J = 11.2 Hz), 4.61–4.40 (m, 7 H), 4.12
(dd, 1 H, J = 5.3, 8.6 Hz), 4.05–3.98 (m, 1 H), 3.90 (dd, 1 H, J = 4.3, 8.9 Hz), 3.83–3.62 (m, 8 H),
13
2.41–2.05 (m, 2 H), 1.85 (s, 3 H), 1.53 (s, 3 H), 1.31 (s, 3 H). C-NMR (CDCl3, 75 MHz): (ppm)
170.27 (CO), 138.57, 138.45, 138.37, 137.50, 128.40, 128.38, 128.36, 128.29, 128.14, 127.99, 127.83,
127.74, 127.63, 127.58, 127.43, 117.27, 113.08, 104.17, 79.81, 78.99, 77.20, 76.46, 75.53, 75.22,
73.93, 73.57, 73.97, 73.57, 73.28, 72.84, 72.21, 71.92, 71.75, 69.28, 68.92, 34.43, 26.80, 26.60, 21.00.
[α]2D5 +43.73° (c 1.65 in CHCl3). HR-MS (ESI), m/z, 831.37633 [M+Na]+ (C48H56NaO11 , requires
+
831.37148).
2-[3-O-(5-O-Acetyl-3-O-benzyl-1,2-O-isopropylidene-6-deoxy-α-D-allofuranos-6-yl)-2,4,6-tri-O-benzyl-
α-D-mannopyranosyl]acetaldehyde (17). To a mixture of alkene 16 (0.850 g, 1.05 mmol) and
N-methylmorpholine N-oxide (NMO) (213 mg, 1.58 mmol) in acetone (6 mL) and water (2 mL) was

Molecules 2013, 18
3912
added osmium tetroxide (0.2 w% in water, 2.6 mL, 0.021 mmol, 0.02 eq.) at 0 °C. The resulting
mixture was stirred for 45 h when TLC revealed the disappearance of the alkene 16. Upon addition of
sodium sulfite (1.12 g) the yellow suspension turned into a slightly yellow two-layered solution. The
upper organic layer was separated, the aqueous layer was extracted with EtOAc, the combined organic
layers were dried (MgSO4) and concentrated to give the crude diol (802 mg, 91%) as a mixture of two
1
epimers in a ratio of ca. 0.75:1 (based on the integrals in the H-NMR spectrum). IR (cm−1, CHCl3):
3473 (w), 3031 (vw), 2931 (w), 2871 (w), 1737 (m), 1496 (w), 1454 (m), 1372 (m), 1236 (s), 1164
(w), 1090 (s), 1070 (s), 1024 (s). 1H-NMR (CDCl3, 300 MHz): (ppm) 7.32–7.24 (m, 32.3 H), 5.63 (d,
0.6 H, J = 3.0 Hz), 5.60 (d, 0.8 H, J = 3.3 Hz), 5.29–5.23 (m, 1.5 H), 5.67–4.39 (m, 13.9 H), 4.17–4.06
(m, 4.0 H), 3.95–3.78 (m, 7.2 H), 3.72–3.42 (m, 13.9 H), 4.17–4.06 (m, 4.0 H), 3.95–3.78 (m, 7.2 H),
3.72–3.42 (m, 13.9 H), 2.04 (d, 1.2 H, J = 0.9 Hz), 1.84–1.83 (m, 5.1 H), 1.78–1.51 (m, 2.9 H), 1.52
13
(s, 5.3 H), 1.31 (br s, 5.2 H). C-NMR (CDCl3, 75 MHz): (ppm) 170.29, 138.17, 138.10, 138.07,
138.00, 137.98, 137.49, 137.46, 128.46, 128.42, 128.16, 128.14, 128.02, 127.94, 127.90, 127.83,
127.80, 127.74, 127.70, 127.66, 133.11, 133.09, 104.15, 79.84, 77.20, 76.79, 76.67, 76.40, 76.31,
75.25, 73.61, 73.45, 73.42, 73.18, 71.93, 71.80, 69.47, 69.39, 66.38, 32.81, 32.69, 26.82, 26.60, 21.01.
+
HR-MS(ESI), m/z, 865.37635 [M+Na]+ (C48H58NaO13 , required 865.37696).
To the diol (630 mg, 0.747 mmol) in MeOH (3 mL) and H2O (5 mL) was added sodium periodate
(190 mg, 0.897 mmol) in H2O (3 mL) at 0 °C, the mixture was stirred for 60 min, and then extracted
with EtOAc. The combined organic layers were dried (Mg2SO4) and concentrated to give the aldehyde
17 as a slightly yellow oil which was pure according to the NMR spectrum. IR (cm−1, CHCl3): 3030
(vw), 2933 (w), 2870 (w), 1739 (m), 1726 (m), 1496 (w), 1454 (w), 1371 (m), 1307 (m), 1234 (s),
1091 (s), 1071 (s), 1025 (s). 1H-NMR (CDCl3, 300 MHz): (ppm) 9.68 (t, 1 H, J = 2.3 Hz), 7.36–7.21
(m, 20 H), 5.63 (d, 1 H, J = 3.7 Hz), 5.28 (ddd, 1 H, J = 4.6, 3.4, 6.4 Hz), 4.74–4.42 (m, 10 H), 4.13
(dd, 1 H, J = 5.2, 8.9 Hz), 3.96–3.61 (m, 9 H), 2.62 (dd, 1 H, J = 1.1, 2.0 Hz), 2.60 (t, 1 H, J = 2.3 Hz),
1.87 (s, 3 H), 1.55 (s, 3 H), 1.34 (s, 3 H). 13C-NMR (CDCl3, 75 MHz): (ppm) 200.50, 170.21,
138.40, 138.07, 137.92, 137.79, 137.54, 129.09, 128.50, 128.48, 128.44, 128.38, 128.11, 128.08,
128.03, 127.90, 127.87, 127.82, 127.79, 127.57, 125.36, 113.11, 104.17, 79.80, 77.21, 76.52, 76.43,
76.00, 74.91, 74.43, 73.24, 72.85, 72.18, 71.80, 71.49, 69.65, 68.36, 66.57, 45.32, 26.53, 26.62, 21.52,
21.00. [α]2D5 +56.27° (c 2.00 in CHCl3). HR-MS(ESI), m/z, 833.35133 [M+Na]+ (C47H54NaO12 ,
+
required 833.35075).
2-[3-O-(5-O-Acetyl-3-O-benzyl-1,2-O-isopropylidene-6-deoxy-α-D-allofuranos-6-yl)-2,4,6-tri-O-benzyl-
α-D-mannopyranosyl]acetic acid (18). The crude aldehyde 17 in acetonitrile (6 mL) was treated with
NaH2PO4 (34 mg, 0.25 mmol) in H2O (2 mL) and H2O2 (30%, 0.12 mL, 1.12 mmol), respectively, at
0 °C, followed by addition NaClO2 (0.17 g, 80%, 1.49 mmol) in H2O (2 mL) at 0 °C. The resulting
mixture was stirred overnight, brought to pH 2.0 with 1N HCl, and extracted with EtOAc. The
combined organic layers were dried (MgSO4) and concentrated to give acid 18 as a colorless gum
(0.60 g, 88% from 16). IR(cm−1, CHCl3): 3062 (w), 3030 (w), 2933 (w), 2872 (w), 1739 (s), 1714 (m),
1
1496 (w), 1454 (w), 1372 (m), 1306 (w), 1236 (s), 1162 (m), 1095 (s), 1026 (s). H-NMR (CDCl3,
300 MHz): (ppm) 7.33–7.25 (m, 20 H), 5.61 (d, 1 H, J = 3.6 Hz), 5.23 (dd, 1 H, J = 5.3, 10.2 Hz),
4.68–4.36 (m, 10 H), 4.34–4.30 (m, 1 H), 4.10 (dd, 1 H, J = 5.3, 8.9 Hz), 3.97 (br. s, 1 H), 3.89–3.78
(m, 3 H), 3.73 (d, 1 H , J = 2.2 Hz), 3.64–3.56 (m, 3 H), 2.70 (dd, 1 H, J = 4.3, 15.7 Hz), 2.56 (dd,

Molecules 2013, 18
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1 H, J = 8.6, 15.9 Hz), 1.84 (s, 3 H), 1.52 (s, 3 H), 1.31 (s, 3 H). 13C-NMR (CDCl3, 75 MHz): (ppm)
175.43, 170.35, 138.31, 138.06, 137.88, 137.50, 128.47, 128.43, 128.36, 128.17, 128.04, 127.97,
127.89, 127.86, 127.81, 127.56, 133.13, 104.17, 79.80, 77.20, 76.86, 76.45, 75.66, 74.84, 74.47, 73.33,
73.05, 72.24, 71.84, 71.50, 69.45, 68.40, 68.16, 36.25, 26.98, 26.81, 26.61, 20.98. [α]2D5 +50.97° (c 4.52
+
in CHCl3). HR-MS(ESI), m/z, 849.34625 [M+Na]+ (C47H54NaO13 , requires 849.34621), 871.32809
+
[M−H+Na2]+ (C47H534Na2O13 , requires 871.32816).
2-[3-O-(5-O-Acetyl-3-O-benzyl-1,2-O-isopropylidene-6-deoxy-α-D-allofuranos-6-yl)-2,4,6-tri-O-benzyl-
α-D-mannopyranosyl]-N-[2-amino-6-(benzyloxy)-5-nitrosopyrimidin-4-yl]acetamide (19). To a solution
of acid 18 (0.470 g, 0.57 mmol) in CH2Cl2 (4 mL) was added oxalyl chloride (0.15 mL, 1.7 mmol)
followed by a drop of DMF at 0 °C. The resulting mixture was stirred for 1 h and concentrated in
vacuo. The crude acid chloride was pure according to its NMR spectra and used directly for next step.
IR (cm−1, CHCl3): 3031 (vw), 2932 (vw), 2868 (vw), 1798 (m), 1740 (s), 1496 (w), 1454 (m), 1371
(m), 1234 (s), 1091 (s), 1069 (s), 1023 (s). 1H-NMR (CDCl3, 300 MHz): (ppm) 7.36–7.24 (m, 20 H,),
5.61 (d, 1 H, J = 3.7 Hz), 5.23 (dd, 1 H, J = 6.0, 10.0 Hz), 4.95 (d, 1 H, J = 11.0 Hz), 4.73–4.31 (m, 9 H),
4.10 (dd, 1 H, J = 5.4, 8.9 Hz), 3.97 (t, 1 H, J = 5.8 Hz), 3.86 (dd, 1 H, J = 4.3, 8.8 Hz), 3.81–3.67 (m,
4 H), 3.64–3.57 (m, 3 H), 3.21–3.14 (m, 1 H), 2.99–2.90 (m, 1 H), 1.84 (s, 3 H), 1.52 (s, 3 H), 1.31 (s,
13
3 H). C-NMR (CDCl3, 75 MHz): (ppm) 171.27, 170.22, 138.39, 137.93, 137.54, 137.48, 128.55,
128.48, 128.34, 128.16, 128.02, 127.99, 127.84, 127.77, 127.52, 113.12, 104.13, 79.89, 77.32, 77.23,
76.30, 75.93, 75.25, 74.78, 74.64, 73.34, 72.56, 72.23, 71.74, 71.21, 69.72, 68.25, 67.23, 49.09, 26.82,
26.61, 20.95, 13.97. [α]2D5 +43.68° (c 2.35 in CHCl3).
To a solution of 2,4-diamino-5-nitrosopyrimidine 9 (0.21 g, 0.86 mmol) and pyridine (0.07 mL) in
dry THF (12 mL) was added the above acid chloride (0.47 g, 0.57 mmol) in THF (7 mL) at 0 °C
dropwise. Once the acid chloride was added, the deep-blue solution turned green. After stirring for 1 h,
the purple solid was filtered off, and the filtrate was concentrated in vacuo to afford the amide 19 (600 mg).
IR (cm−1, CHCl3): 3328 (w), 3228 (w), 3031 (w), 2933 (w), 2870 (w), 1739 (m), 1631 (m), 1598 (s),
1535 (s), 1497 (m), 1454 (s), 1371 (m), 1347 (s), 1255 (s), 1209 (s), 1091 (s), 1072 (s), 1026 (s).
1H-NMR (CDCl3, 300 MHz): (ppm) 12.38 (br. s, 1 H, exchange with D2O), 7.51–7.23 (m, 20 H), 7.17
(br. s, 1 H, exchange with D2O), 6.03 (br. s, 1 H, exchange with D2O), 5.65 (d, 1 H, J= 3.7 Hz), 5.63
(s, 2 H), 5.27 (dd, 1 H, J = 5.8, 10.0 Hz), 4.70 (d, 1 H, J = 11.0 Hz), 4.64 (d, 1 H, J = 11.6 Hz), 4.63
(d, 1 H, J = 11.8 Hz), 4.57–4.43 (m, 6 H), 4.14 (dd, 1 H, J = 5.5, 8.9 Hz), 4.06 (br. s, 1 H), 3.88 (dd, 1 H,
J = 4.3, 8.9 Hz), 3.83–3.65 (m, 7 H), 2.97 (dd, 1 H, J = 5.3, 14.7 Hz), 2.89 (dd, 1 H, J = 7.6, 14.9 Hz),
1.83 (s, 3 H), 1.51 (s, 3 H), 1.32 (s, 3 H). 13C-NMR (CDCl3, 75 MHz): (ppm) 171.45, 170.35,
163.78, 138.91, 138.31, 138.04, 137.86, 137.46, 135.49, 128.64, 128.46, 128.44, 128.32, 128.26,
128.20, 128.06, 127.95, 127.80, 127.72, 127.51, 113.12, 104.15, 79.92, 77.33, 77.24, 76.20, 76.05,
74.74, 74.12, 73.13, 72.25, 71.91, 71.62, 69.40, 68.72, 68.62, 41.33, 26.72, 26.61, 20.97. [α]2D5 +81.92°
+
(c 0.053 in CHCl3). HR-MS (ESI), m/z, 1076.4270 [M+Na]+ (C58H63N5NaO14 , required 1076.4264).
8-[(3-O-((5-O-Acetyl-3-O-benzyl-1,2-O-isopropylidene-6-deoxy-α-D-allofuranos-6-yl)-2,4,6-tri-O-benzyl-
α-D-mannopyranosyl)methyl]-O6-benzylguanine (6). A mixture of the amide 19 (600 mg, 0.57 mmol)
and triphenyl phosphine (449 mg, 1.71 mmol) in xylene was stirred at 140 °C overnight and then
cooled to room temperature. The mixture was passed through silica gel (EtOAc/cyclohexane 1/1→1/0)

Molecules 2013, 18
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to give guanine 6 as a yellowish oil (291 mg, 50% from acid 18). IR (cm−1, CHCl3): 3528 (w), 3420
(w), 3067 (w), 3010 (m), 2928 (m), 1738 (m), 1625 (s), 1591 (s), 1496 (w), 1455 (m), 1374 (m), 1325
1
(m), 1227 (s), 1146 (m), 1090 (s). H-NMR (CDCl3, 300 MHz): (ppm) 11.65 (br s, 1 H, NH,
exchange with D2O), 7.50–7.20 (m, 25 H), 5.57 (d, 1 H, J = 3.7 Hz), 5.53 (s, 2 H), 5.18 (dd, 1 H,
J = 5.0, 10.7 Hz), 4.80 (br. s, 2 H, exchange with D2O), 4.71–4.35 (m, 10 H), 4.21 (t, 1 H, J = 8.1 Hz),
4.08 (dd, 2 H, J = 5.0, 8.9 Hz), 3.86–3.70 (m, 3 H), 3.61 (dd, 1 H, J = 2.6, 7.7 Hz), 3.58–3.53 (m,
2 H), 3.47 (dd, 1 H, J= 4.0, 10.2 Hz), 3.30 (d, 1 H, J = 14.0 Hz), 2.97 (dd, 1 H, J = 10.8, 16.0 Hz),
1.84 (s, 3 H), 1.49 (s, 3 H), 1.30 (s, 3 H). 13C-NMR (CDCl3, 75 MHz): (ppm) 170.14, 160.10,
158.72, 155.66, 148.61, 137.74, 137.65, 137.42, 136.79, 132.20, 132.07, 128.61, 128.56, 128.49,
128.46, 128.40, 128.35, 128.32, 128.22, 128.04, 127.98, 127.92, 127.82, 127.69, 114.66, 13.06,
104.16, 79.68, 77.26, 77.11, 76.47, 75.95, 75.27, 74.37 (CH2), 73.14, 72.70, 72.15, 71.79, 69.86,
67.79, 26.95, 26.78, 26.55, 20.98. [α]2D5 +31.76° (c 1.41 in CHCl3). HR-MS (ESI), m/z, 1044.4378
[M+Na]+ (C58H63N5NaO12 , required 1044.4365).
+
4. Conclusions
In summary, a new linear analogue of guanofosfocin 6 was synthesized in 10 steps from the known
sugar diol 13 in 26% overall yield. Key steps were the ring-opening of cyclic sulfate 8 by a sugar
sec-alkoxide and the reductive cyclization of 4-N-acyl-2,4-diamino-5-nitrosopyrimidine 19. The
robustness of the above etherification and of the reductive cyclization will find more applications in
synthetic organic chemistry. The structure of 6 was characterized as an ether-linked pseudodisaccharide
α-C-glycosidically linked to an 8-alkylated guanine. This compound, together with previously
synthesized analogues of guanofosfocins, allows expansion of the relevant research in this field of
medicinal chemistry and chemical biology.
Acknowledgments
We thank ETH Zürich and The National Natural Science Foundation of China (No. 21202008) for
generous support.
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