Anomeric stereospecific synthesis of 2′-C-methyl β-nucleosides; the Holy reaction of cyanamide with 2-C-methyl-d-arabinose

Article abstract of DOI:10.1016/j.tetlet.2007.04.105

The sequential reactions of 2-C-methyl-d-arabinose with cyanamide and methyl propiolate afford an anhydronucleoside, which may be opened under acid conditions with inversion at C2′, to give 2′-C-methyl uridine; ring opening with sodium hydroxide gave 2′-C

Full text of DOI:10.1016/j.tetlet.2007.04.105

Tetrahedron Letters 48 (2007) 4441–4444
Anomeric stereospecific synthesis of 20-C-methyl b-nucleosides;
the Holy reaction of cyanamide with 2-C-methyl-D-arabinose
Sarah F. Jenkinson,a Nigel A. Jones,a Adel Moussa,b Alistair J. Stewart,b
Thomas Heinzc and George W. J. Fleeta,*
aChemistry Research Laboratory, Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, UK
bIdenix Pharmaceuticals Inc., 60 Hampshire Street,Cambridge, MA 02139, USA
cChemical and Analytical Development, Novartis Pharma AG, Postfach, CH-4002 Basel, Switzerland
Received 1 March 2007; accepted 12 April 2007
Available online 4 May 2007
Abstract—The sequential reactions of 2-C-methyl-D-arabinose with cyanamide and methyl propiolate afford an anhydronucleoside,
which may be opened under acid conditions with inversion at C20, to give 20-C-methyl uridine; ring opening with sodium hydroxide
gave 20-C-methyl arabino-uridine with retention of configuration at C20. This gives complete stereospecific control to yield only
b-nucleosides.
Ó 2007 Elsevier Ltd. All rights reserved.
Although nucleosides with methyl branches on the sugar
moiety have been known for a long time,1 a number of
20-C-methyl nucleoside analogues have been identified
as promising therapeutics against hepatitis C virus
NS5B RNA-dependent RNA polymerase.2 In particular
Valopicitabine (NM283), a valine ester of 20-C-methyl
cytidine, is in phase IIb clinical trials for the treatment
of hepatitis C.3,4
Previous strategies for the synthesis of 20-C-methyl
nucleosides, illustrated by approaches to 20-C-methyl
uridine 1 (Scheme 1) have been (A) the coupling of a
O
NH
CHO
HO
H
H
Me
OH
OH
O
O
O
HOH2C
HO
HOH2C
HO
HOH2C
HO
N
O
CO2Me
N
N
NH2CN
O
Me
OH
N
NH2
O
O
Me
Me
CH2OH
1
2
3
4
(B)
(A)
O
O
NH
NH
O
O
O
HOH2C
N
O
HOH2C
HO
N
O
XOH2C
XO
OX
Me
OX
OH
Me
HO
OH
7
5
6
Scheme 1.
*
Corresponding author. E-mail: george.fleet@chem.ox.ac.uk
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.04.105

4442
S. F. Jenkinson et al. / Tetrahedron Letters 48 (2007) 4441–4444
suitable protected activated derivative of a branched
sugar 5 [with problems of anomeric specificity]3,5 and
(B) the introduction of a methyl group to a b-nucleoside
6 [which requires a number of protections and func-
tional group manipulations].6,7 The Holy reaction of
cyanamide with arabinose8 and other sugars9 allows
the specific generation of pyrimidine b-nucleoside ana-
logues;10,11 the reaction of cyanamide with sugar precur-
sors has been suggested as a prebiotic route to nucleic
acids.12 This Letter presents a new strategy for the syn-
thesis of 20-C-methyl uridine 1; reaction of 2-C-methyl
arabinose 4 with cyanamide afforded oxazoline 3, which
upon treatment with methyl propiolate gave the 20-C-
methyl anhydronucleoside 2. The tricyclic system 2 can
be opened with the hydroxide ion to give the arabino-
branched nucleoside 7 whereas benzoylation followed
by acid treatment predominantly gives the target 20-C-
methyl uridine 1.
Dowex resin (50W X8 (H+)), to the same lactol 4 in 49%
yield over two steps. The unprotected branched arabi-
nose 4 is a complex mixture of furanose and pyranose
forms; full details of the NMR of 2-C-methyl arabinose
4 will be discussed elsewhere. Although at present the
availability of the branched sugar 2-C-methyl-D-arabi-
nose is limited to chemical syntheses such as this,
advances in biotechnology and the production of rare
monosaccharides by Izumoring18 and other green tech-
niques suggest that a wide range of such rare and novel
monosaccharides could be readily accessible.
2-C-Methyl arabinose 4 with cyanamide and aqueous
ammonia underwent a Holy reaction to form oxazoline
319 in 62% yield; subsequent reaction of 3 with methyl
propiolate in the presence of DBU gave the 20-C-
methyl-anhydrouridine 220 (Scheme 3). There were
various ambiguities about the structure of 2, such as
the sugar moiety could be in a pyranose or furanose
form; the structure of the anhydronucleoside 2 was
confirmed by X-ray crystallographic analysis.21
The synthesis of 2-C-methyl arabinose 4 is shown in
Scheme 2. The Kiliani reaction of cyanide on ketohex-
oses13,14 provides easy access to branched carbohydrate
chirons. Reaction of methyl magnesium bromide with
the protected D-erythronolactone 8 afforded the pro-
tected 1-deoxy ribulose 9 in 99% yield. The 1-deoxyketo-
pentose 9 with sodium cyanide underwent a highly
diastereoselective Kiliani ascension15 to give the pro-
tected arabinono-1,5-lactone 1016 together with the
unprotected arabinono-1,4-lactone 1117 and a small
amount of the epimeric ribono-lactone 12. The com-
bined yield of 10 and 11 was 60%; as the acetonide in
10 can easily be hydrolysed to give 11, the relative pro-
portions of 10 and 11, generated in the Kiliani reaction,
depend on the precise reaction conditions. The protected
lactone 10 was easily isolated by column chromatogra-
phy, but the two lactones, 11 and 12, were not easy to
separate—but serendipitously 11 was found to crystal-
lise out of the isomeric lactone mixture.
Treatment of anhydronucleoside 2 with sodium hydr-
oxide in methanol gave the branched arabino-nucleoside
722,7 in quantitative yield. Nucleophilic addition of
hydroxide ion at C2 of the pyrimidine ring of 2 afforded
adduct 14, which fragmented to 7 without cleavage of
the sugar C20–O bond, thus retaining the configuration
at C-20.
Conversion of the anhydronucleoside 2 to uridine 1
required inversion of configuration at C-2 of the sugar;
SN1 displacement would proceed via a tertiary carbocat-
ion at C-2 stabilised by the methyl branch. Numerous
conditions were investigated and benzoyl protection of
the anhydronucleoside allowed a satisfactory procedure.
Esterification of 2 with benzoyl chloride in pyridine gave
20
the dibenzoate 15 [mp 221–223 °C; ½aꢀD ꢁ55.5 (c 0.96 in
CH3CN)] in 98% yield. Treatment of 15 with para-tolu-
enesulfonic acid in acetonitrile gave a quantitative yield
of the inseparable dibenzoates 16 and 17. Deprotection
of the benzoyl protecting groups in 16 and 17 by sodium
methoxide in methanol gave the ribo-1 and the arabino-7
branched nucleosides in a ratio of 11/1 (calculated from
integration in the 1H NMR spectrum). The epimers were
not easy to separate by chromatography so the mixture
was treated with acetone in the presence of anhydrous
Both 10 and 11 can be used to access the desired lactol 4
(Scheme 2). Protection of 11 with triethylsilyl groups was
required prior to reduction with DIBALH to give lactol
13 in 97% yield. This was subsequently deprotected using
Dowex resin (50W X8 (H+)) to give 4 in a quantitative
yield. Alternatively, the 3,4-O-isopropylidene protected
c-lactone 10 was converted in two steps, reduction to
the lactol using DIBALH followed by deprotection with
O
O
O
O
O
O
O
HOH2C
HO
OH
Me
HOH2C
HO
O
OH
Me
O
Me
(i)
OH
(ii)
O
Me
OH
O
O
O
O
O
10
12
11
(iv)
9
(iii)
8
CHO
O
O
HOH2C
HO
O
OH
OH
Et3SiOH2C
O
HO
H
Me
OH
OH
OH
OH
Me
(v)
Me
OSiEt3
Et3SiO
HO
H
Me
HO
CH2OH
13
4
Scheme 2. Reagents and conditions: (i) MeMgBr, THF, ꢁ78 °C; (ii) NaCN, H2O; then H+; (iii) isoBu2AlH, CH2Cl2 ꢁ78 °C; then Dowex (H+), H2O;
(iv) Et3SiCl, imidazole, DMF; (v) isoBu2AlH, CH2Cl2 ꢁ78 °C; then Dowex (H+), dioxane–H2O.

S. F. Jenkinson et al. / Tetrahedron Letters 48 (2007) 4441–4444
4443
CHO
Me
O
O
O
HOH2C
HO
HOH2C
HOH2C
N
N
HO
H
N
(ii)
(iii)
(i)
O
O-
7
OH
OH
N
N
OH
NH2
O
O
O
Me
HO Me
HO Me
H
CH2OH
4
3
2
(iv)
14
O
O
NH
NH
O
O
O
BzOH2C
BzOH2C
BzO
N
O
BzOH2C
BzO
N
O
(v)
N
+
O
OH
Me
Me
OH
N
O
BzO Me
15
16
17
Bz = PhCO
(vi)
O
O
O
O
NH
NH
NH
NH
(vii)
O
(viii)
O
O
O
HOH2C
HO
N
O
HOH2C
HO
N
O
HOH2C
N
O
HOH2C
HO
N
O
+
OH
Me
Me
OH
Me
Me
OH
O
O
1
18
1
7
Scheme 3. Reagents and conditions: (i) NH2CN, concd NH4OH, 80 °C; (ii) methyl propiolate, 50% aq EtOH, DBU, reflux; (iii) NaOH, MeOH; (iv)
BzCl, Pyr; (v) p-toluenesulfonic acid, MeCN, reflux; (vi) NaOMe, MeOH; (vii) CuSO4, concd H2SO4 acetone; (viii) Dowex (H+), H2O.
O
N
O
O
NH
NH
NH
O
BzOH2C
N
O
O
O
OH+
BzOH2C
O
BzOH2C
N
O
BzOH2C
N
O
N
16
O
Me
O
O
Me
Me
neighbouring
group participation
O
Me
O
O
O
O
Ph
HO
Ph
Ph
O
Ph
H2O
19
20
22
21
16 + 17
Scheme 4.
copper sulfate and sulfuric acid to form the easily puri-
ing of branched 2-C-methyl anhydronucleosides, ensur-
ing that the stereochemistry of the anomeric position is
completely defined. The likelihood of rare monosaccha-
rides, such as 2-C-methyl arabinose 4, being readily
available in the near future from advances in biotechnol-
ogy means that such antiviral nucleosides may be made
by green chemistry.
20
fied acetonide 18 [oil, ½aꢀD +32.2 (c 1.23 in CH3CN)] in
74% over three steps. Removal of the acetonide in 18 by
treatment with Dowex resin (H+) in water gave the pure
20-C-methyl uridine 123,24 in 95% yield.
The excellent diastereoselectivity in the acid catalysed
ring opening of the dibenzoyl anhydronucleoside 15
may be rationalised (Scheme 4). Initial protonation
would give the cation 19 which might fragment to the
tertiary carbocation 20 in which the C–O bond at C2
had been broken. The tertiary cation 20, stabilised by
the methyl substituent could be attacked to give either
of the epimers 16 and 17. Alternatively the benzoyl
group in 20 may act as a neighbouring group to form
21, which would undergo capture by water to give 22
and would then ring open only to dibenzoate 16.
Acknowledgements
Funding of post-doctoral fellowships (S.F.J. and
N.A.J.) by Idenix Pharmaceuticals Inc and Novartis
Pharma AG is gratefully acknowledged.
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20
19. Oxazoline 3: yellow oil, ½aꢀD ꢁ52.6 (c 1.255 in MeOH);
mmax (thin film): 3349 (s, br, OH), 1668 (s, NC@N), 1601
(m); dH (D2O, 400 MHz): 1.38 (3H, s, Me), 3.55–3.59 (1H,
dd, H5, J 6.2, 12.3), 3.68–3.72 (1H, dd, H50, J 3.8, 12.3),
3.75–4.00 (1H, m, H4), 4.08 (1H, d, H3, J 6.3), 5.30 (1H, s,
H1); dc (D2O, 100 MHz): 16.6 (Me), 61.4 (C5), 76.5 (C3),
82.7 (C4), 95.2 (C2), 102.1 (Cl), 164.9 (NC@N).
20
20. Anhydro-20-C-methyl undine 2: mp 240–243 °C; ½aꢀD
ꢁ52.0 (c 0.23 in MeOH); mmax (thin film): 3332 (m, br,
OH), 1651 (s, NC@O), 1522 (m, NC@O), 1479 (s); dH
(MeOD, 400 MHz): 1.64 (3H, s, Me0), 3.51–3.56 (1H, dd,
H50, J 4.6, 12.0), 3.59–3.63 (1H, dd, H500, J 3.7, 12.0),
4.06–4.09 (1H, a-dd, H40, J 4.6, 8.4), 4.34 (1H, d, H30, J
4.8), 5.90 (1H, s, H10), 6.06 (1H, d, H5, J 7.4), 7.79 (1H, d,
H6, J 7.4); dc (MeOD, 125 MHz): 17.1 (Me0), 61.9 (C50),
77.4 (C30), 89.4 (C40), 95.3 (C10), 97.6 (C20), 109.6 (C5),
138.7 (C6), 161.4 (NC@N), 175.4 (C@O).
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22. 20-C-Methyl arabino-uridine 7: mp 132–135 °C [lit.7 mp
20
137–138 °C]; ½aꢀD +79.7 (c 1.04 in MeOH); mmax (thin
film): 3418 (s, br, OH/NH), 1653 (s, HNC@O), 1448 (s);
dH (D2O, 400 MHz): 1.24 (3H, s, Me0), 3.71–3.76 (1H, m,
H50), 3.77–3.84 (1H, m, H500), 3.85 (2H, m, H30, H40), 5.74
(1H, d, H5, J 8.2), 5.86 (1H, s, H10), 7.75 (1H, d, H6, J
8.2); dc (D2O, 100 MHz): 18.8 (Me0), 61.3 (C50), 77.0 (C30
or C40), 79.4 (C20), 83.7 (C40 or C30), 89.0 (C10), 101.4
(C5), 143.6 (C6), 152.0 (C2), 166.5 (C4).
23. 20-C-Methyl uridine 1: mp 110–112 °C [lit.24 mp 118–
119 °C (softening at 101 °C)]; ½aꢀ2D0 +75.0 (c 1.25 in MeOH)
20
[lit.24 ½aꢀD +82 (c 0.7 in water)]; mmax (thin film): 3390 (s,
br, OH), 1694 (s, br, HNC@O); dH (MeOD, 400 MHz):
1.16 (3H, s, Me0), 3.77–3.81 (1H, dd, H50, J 2.5, 12.5), 3.85
(1H, d, H30, J 9.2), 3.91–3.95 (1H, ddd, H40, J 2.2, 2.4,
9.2), 3.97–4.00 (1H, dd, H500, J 2.1, 12.5), 5.70 (1H, d, H5,
J 8.1), 5.96 (1H, s, H10), 8.16 (1H, d, H6, J 8.2); dH (D2O,
400 MHz): 1.16 (3H, s, Me0), 3.79–3.83 (1H, dd, H50, J
4.2, 13.5), 3.86 (1H, d, H30, J 9.2), 3.97–4.01 (2H, m, H40,
H500), 5.84 (1H, d, H5, J 8.1), 5.95 (1H, s, H10), 7.89 (1H,
d, H6, J 8.2); dc (MeOD, 100 MHz): 20.2 (Me0), 60.4
(C50), 73.3 (C30), 80.1 (C20), 83.8 (C40), 93.1 (C10), 102.3
(C5), 142.5 (C6), 152.5 (C2), 166.1 (C4); dc (D2O,
100 MHz): 19.3 (Me0), 59.8 (C50), 72.6 (C30), 79.4 (C20),
82.0 (C40), 91.9 (C10), 102.5 (C5), 141.8 (C6), 151.9 (C2),
166.4 (C4).
24. Beigelman, L. N.; Ermolinsky, B. S.; Gurskaya, G. V.;
Tsapkina, E. N.; Karpeisky, M. Y.; Mikhailov, S. N.
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