Synthesis of the four 1-(1-deoxy-d-pentitol-1-yl)thymines and conformational properties of the acyclic sugar chain

Article abstract of DOI:10.1016/j.carres.2006.12.003

Acetylated d-pentose diethyl dithioacetals were coupled by way of 1-bromo-1-ethylthio derivatives with 2,4-bis(trimethylsilyl)thymine to afford diastereomeric pairs of acyclic-sugar nucleoside analogues bearing a thymin-1-yl and an ethylthio group at C-1. Free-radical desulfurization by the action of tributylstannane removed the ethylthio group to afford the corresponding acetylated 1-(1-deoxy-d-pentitol-1-yl)thymines and subsequently the free title compounds in the arabino, lyxo, ribo, and xylo series. Conformations of the intermediates and products were studied in detail and the final products were evaluated for their potential as agents active against plant viruses and rice blast fungus.

Full text of DOI:10.1016/j.carres.2006.12.003

Carbohydrate Research 342 (2007) 806–818
Synthesis of the four 1-(1-deoxy-D-pentitol-1-yl)thymines
and conformational properties of the acyclic sugar chain
Scott M. Vejcik and Derek Horton*
Department of Chemistry, American University, 4400 Massachusetts Avenue, NW, Washington, DC 20016, USA
Received 23 October 2006; received in revised form 28 November 2006; accepted 5 December 2006
Available online 12 December 2006
Abstract—Acetylated D-pentose diethyl dithioacetals were coupled by way of 1-bromo-1-ethylthio derivatives with 2,4-bis(trimeth-
ylsilyl)thymine to afford diastereomeric pairs of acyclic-sugar nucleoside analogues bearing a thymin-1-yl and an ethylthio group at
C-1. Free-radical desulfurization by the action of tributylstannane removed the ethylthio group to afford the corresponding acetyl-
ated 1-(1-deoxy-D-pentitol-1-yl)thymines and subsequently the free title compounds in the arabino, lyxo, ribo, and xylo series.
Conformations of the intermediates and products were studied in detail and the final products were evaluated for their potential
as agents active against plant viruses and rice blast fungus.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Nucleoside analogue; Acyclic sugar; Free-radical desulfurization; NMR spectroscopy; Chain conformation; Antifungal agent; Antiviral
agent
1
. Introduction
N-1 the four D-pentoses in acyclic form, initially linking
an acyclic 1-bromo-1-ethylthio acetylated derivative of
the sugar to the heterocycle to generate a diastereomeric
pair of 1-ethylthio-1-(thymine-1-yl) derivatives, which
were individually characterized, and then removing the
ethylthio group by a free-radical procedure, ultimately
to yield the title compounds.
Nucleoside analogues in which the sugar component is
in the open-chain form have been of sustained synthetic
1
–4
interest in this laboratory, as such products should in
principle be inert to nuclease enzymes, could modify the
solubility properties and transport behavior of the
attached bases in biological systems, and might act as
pro-drugs susceptible to phosphorylation by kinases,
especially if the conformational disposition of the sugar
chain would present a favorable orientation simulating a
natural nucleoside. Several examples have demonstrated
2. Results and discussion
2.1. Synthesis
3
,5
biological activity, including in vivo activity against
the murine L-1210 tumor, and cytostatic activity against
Streptococcus faecalis and the K-12 strain of Escherichia
coli. The clinically useful antiviral drug 9-(2-hydroxyeth-
The starting compounds were the peracetylated diethyl
dithioacetals (1–4) of D-arabinose, D-lyxose, D-ribose,
and D-xylose, respectively. Treatment of these compounds
with one molar equivalent of bromine in cold diethyl
6
,7
oxymethyl)guanine (Acyclovir, Zovirax) is a guanine
derivative bearing a truncated, acyclic chain in place
of the b-D-ribofuranosyl group in the natural nucleoside
guanosine. The objective of the present work was to pre-
pare a series of thymine derivatives having attached at
8
ether, following the precedent of Gauthier, Weygand
9
1–4
and associates, and our earlier work converted each
dithioacetal into a product of lower TLC mobility
through replacement of one ethylthio group by bromine
and affording, after removal of volatile materials, the
corresponding 1-bromo-1-ethylthio derivatives 5–8,
respectively, as syrups that were unstable on storage
*
Corresponding author. Tel.: +1 202 885 1767; fax: +1 202 885 1095;
e-mail: carbchm@american.edu
0
008-6215/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2006.12.003

S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
807
0
and were used directly in the next step. The fusion cou-
as a levorotatory oil identified as the (1 S) isomer 13.
In similar fashion, the D-lyxo bromide 6 afforded in
48% yield a 1:1.2 diastereomeric mixture that was
resolved chromatographically to afford 19% of the
faster-migrating isomer as a levorotatory foam identified
1
0,11
pling procedure of Nishimura and co-workers
was
employed. Each of the bromo derivatives was mixed
1
2
with 1 equiv of 2,4-bis(trimethylsilyl)thymine
and
heated in a sealed tube to 140 °C, and the fused mixture
was maintained at this temperature for 45 min, resulting
in nucleophilic attack by N-1 on C-1 of the sugar deriv-
ative and elimination of bromotrimethylsilane. The cou-
pled products, still having a trimethylsilyloxy group at
C-4 of the pyrimidine, were subjected to an aqueous
methanolic isolation procedure to remove the remaining
0
as the (1 S) diastereomer 14 and 23% of a dextrorota-
tory oil identified as the (1 R) diastereomer 10.
Likewise, the D-ribo bromide 7 gave 82% of a 1:3.1
diastereomeric mixture that afforded 8% of the crystal-
line, minor, dextrorotatory (1 R) isomer 11 and 23% of
the syrupy, levorotatory major isomer, the (1 S) product
15. Finally, the D-xylo bromide 8 (which was particu-
0
0
0
0
trimethylsilyl group and afford a C-1 diastereomeric
pair of nucleoside derivatives in each of the four series.
larly unstable and was used without delay) gave 49%
O
O
Me
Me
HN
OSiMe3
Me
HN
1
.
Br
Br2
SEt
CH
N
CH(SEt)2
O
O
N
N
Me3SiO
N
(
CHOAc)3
(CHOAc)3
EtSCH
HCSEt
2
. MeOH, H O
CH OAc
2
2
CH OAc
+
2
(CHOAc)3
(
CHOAc)3
1
D-arabino
D-lyxo
5 D-arabino
6 D-lyxo
7 D-ribo
CH OAc
CH2OAc
2
2
3
4
D-ribo
9 D-arabino
10 D-lyxo
13 D-arabino
14 D-lyxo
15 D-ribo
D-xylo
8 D-xylo
1
1 D-ribo
12 D-xylo
16 D-xylo
1-(R) Series
1-(S) Series
Bu SnH, AIBN
3
O
N
O
Me
HN
Me
HN
O
O
NaOMe, MeOH
N
CH2
CH2
(
CHOAc)3
(
CHOH)3
CH OAc
2
CH OH
2
1
7 D-arabino
8 D-lyxo
9 D-ribo
0 D-xylo
2
1 D-arabino
2 D-lyxo
3 D-ribo
4 D-xylo
1
2
1
2
2
2
From the D-arabino bromide 5, there was obtained in
1% yield a 3.7:1 mixture of diastereomers that were
of a 2.2:1 mixture of diastereomers, which on chromato-
graphic resolution yielded a faster-migrating, levorota-
tory component, obtained crystalline in 22% yield
4
separated by column chromatography on silica gel to
yield 21% of the faster-migrating, crystalline major dia-
stereomer as a strongly dextrorotatory product identi-
0
and identified as the (1 S) isomer 16, along with 7% of
0
a dextrorotatory product identified as the (1 R) isomer
0
fied as the (1 R) isomer 9, and the minor isomer (5%)
12.

808
S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
All of the products 9–16 were chromatographically
Earlier efforts to remove the ethylthio group from
acyclic-sugar nucleosides similar to compounds 9–16
by a variety of reagents have in all instances led to cleav-
homogeneous and gave satisfactory elemental analyses,
most commonly as the hemihydrated form. Their elec-
tron-impact mass spectra were all essentially identical,
showing small molecular-ion (m/z 488.4) and M+1
peaks, along with significant peaks arising through C-
–N-1 cleavage at m/z 127 for the protonated base
1
5
age of the sugar–base linkage, but a free-radical proce-
dure has subsequently been shown to be successful in
1
6
application with acyclic-sugar purine nucleosides. In
the present work it was found that the individual epi-
mers 9 or 13, on treatment with 4–6 equiv of tributyl-
stannane in the presence of azobisisobutanonitrile
(AIBN) in boiling toluene for 8–12 h, led to clean removal
of the ethylthio group to afford the acetylated 1-N-(1-
deoxy-D-arabinitol-1-yl)thymine 17. The same proce-
dure conducted with the mixed pairs of epimers in the
arabino (9 þ 13), lyxo (10 þ 14), ribo (11 þ 15), and xylo
(12 þ 16) series afforded the corresponding acetylated
alditol-1-yl nucleosides 17–20 in 93, 82, 93, and 92%
yields, respectively. The electron-impact mass spectra
of these products all showed small molecular-ion peaks
0
1
and m/z 363 for the acyclic sugar cation. Loss of the ethyl-
thio group generates a major ion at m/z 427, which by
loss of ketene gives a cation at m/z 385. Other fragments
attributable to the sugar chain were also observed in the
mass spectra of the starting dithioacetals 1–4. Complete
1
3
details of the mass spectra are recorded elsewhere.
1
13
Tables 1 and 3 record H and C NMR data, respec-
tively, and proton–proton spin-coupling data are
recorded in Table 4. The H-1 resonances predictably
0
fall at lower field (ꢀ6 ppm) than the remaining proton
0
signals of the chain, and likewise the C-1 resonances
0
near 60 ppm lie at lower field than the remaining car-
bon signals. Most importantly, the NMR data provide
confirmation that the coupling reaction leads to attach-
ment of the sugar chain to N-1 and not to N-3 of the
pyrimidine. Had the attachment been at N-3, the reso-
nance of the proton at N-1 would have shown splitting
through coupling to the vicinal H-6, and concurrently
the H-6 resonance would be split by the proton at N-
at m/z 428 and fragments m/z 126 and 127 from C-1 –N-
1 cleavage, along with additional fragments listed else-
1
3
1
where. The H NMR spectra of 17–20 (Table 1)
0
showed for the methylene protons at C-1 the antici-
pated upfield shift (ꢀ2 ppm) and changed multiplicity
0
with respect to H-1 of the 1 -ethylthio precursors 9–16,
along with a comparable upfield shift (ꢀ10 ppm) of
1
3
0
the corresponding C resonances for C-1 (Table 3).
Conventional Zempl e´ n catalytic deacetylation of the
tetraacetates 17–20 by methanolic sodium methoxide
afforded in high yield the corresponding free tetrols
21–24 as high-melting crystalline solids. The arabino
(21), lyxo (22), and xylo (24) derivatives were hemihy-
drates and the ribo derivative (23) was a nonhydrated,
1
and the distal allylic methyl group at C-5. In fact,
however, the NH proton signal is observed as a singlet,
broadened by the quadrupolar effect of N-3, and not as
a broadened doublet. The lack of splitting of this signal
is a clear evidence of attachment of the sugar chain to
N-1 of the heterocycle. Furthermore, the H-6 resonance
is observed as a narrow (ꢀ1 Hz) doublet (and not as an
ABX splitting pattern) coupled to the methyl group at
C-5.
1
3
as indicated by microanalytical data. The C reso-
0
nances for C-1 (attachment to nitrogen) in 21–24 were
0
predictably less deshielded than those of C-5 (attach-
Additional evidence for the chain attachment at N-1 is
provided by UV spectral data. Thymine derivatives
ment to oxygen) (Table 3).
The chirality at C-1 in compounds 9–16 was assigned
on the basis of the Generalized Heterocycle Rule,
0
1
4
17
alkylated at N-1 show maximal absorption at lower
wavelength than those alkylated at N-3; thus 1,5-di-
methyluracil near neutral pH shows kmax at 272 nm,
whereas 3,5-dimethyluracil shows maximal absorbance
at 264.5 nm. The nucleoside derivatives 9–16 exhibit
maximal absorbance in the 272–276 nm range, further
supportive of the N-1 substitution assignment.
which attributes a dominant rotatory contribution to
0
the atom (C-1 in this instance) having attached to it
both a heterocycle and a chalcogen. By this rationale,
0
the C-1 epimers having strongly positive specific rota-
0
tions at the sodium D line (9–12) are the (1 R) isomers,
and those exhibiting corresponding levorotation (13–16)
1
Table 1. H NMR Chemical shifts (400 MHz, CHCl
3
4
; ppm relative to Me Si) for compounds 9–16
0
0
0
0
0
0
H-5b
Compound H-1
H-2
H-3
H-4
H-5a
H-6
CH
3
N–H
SCH
2
CH
2
CH
3
OAc
9
0
1
2
3
4
5
6
5.96d 5.42dd 5.25dd 5.12m 4.24dd 4.03dd 7.36d 1.99d 8.93br s 2.52q
6.05d 5.55dd 5.03dd 5.27m 4.23dd 3.71dd 7.37d 1.93d 8.97br s 2.48q
6.17d 5.56dd 5.32dd 5.31m 4.10dd 4.26dd 7.35d 1.89d 9.10br s 2.51q
1.25t
1.19t
1.23t
1.24t
1.23t
1.19t
1.24t
1.24t
2.03, 2.08, 2.13, 2.14
1.94, 2.02, 2.07, 2.08
1.97, 1.99, 2.04, 2.10
2.05, 2.06, 2.08, 2.13
1.97, 2.04, 2.05, 2.19
1.94, 2.02, 2.07, 2.08
2.05, 2.06, 2.08, 2.13
2.02, 2.03, 2.05, 2.17
1
1
1
1
1
1
1
a
a
5.98d 5.25dd 5.50dd 5.29m 4.06dd 4.27dd 7.37
1.94
9.04br s 2.54q
5.88d 5.63dd 5.27dd 5.07m 4.05dd 4.25dd 7.33d 1.92d 9.21br s 2.52q
6.05d 5.55dd 5.02dd 5.27m 4.23dd 3.71dd 7.37d 1.93d 8.97br s 2.48q
5.98d 5.25dd 5.50dd 5.29m 4.06dd 4.27dd 7.37a 1.94
9.04br s 2.54q
5.72d 5.48dd 5.36dd 5.44m 4.28dd 3.98dd 7.53a 1.92a 8.98br s 2.53q
a
Denotes an incompletely resolved signal.

S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
809
0
are the (1 S) isomers. Although this criterion is not
spectroscopy was used to determine vicinal and geminal
proton–proton coupling constants for all nucleoside
analogues prepared in this study and the data are re-
corded in Table 4, chloroform-d being used as a solvent
totally unambiguous, the closely related 1-fluoro ana-
logue of compound 9 (whose stereochemistry at C-1
has been definitively assigned on the basis of an X-ray
crystallographic study) has a specific rotation of
0
3
for all except tetrols 21–24, for which D O was the sol-
2
+
132° at the sodium D line. This value is practically
vent. Conformational attributions were made, following
1
9–21
identical to that (+135°) for compound 9, lending strong
support to the assignments made here for 9, and by
extension, for the complete series 9–16. Comparable
correlations between crystallographic and polarimetric
earlier precedent
on the concept of a time-averaged
dynamic equilibrium between all possible rotameric
states, with a large (ꢀ9 Hz) vicinal coupling indicating
principally anti disposition of vicinal protons, a small
(ꢀ2 Hz) coupling being indicative of gauche disposition,
1
8
data have been made in related examples.
1
Table 2. H NMR Chemical shifts (400 MHz, CHCl
compounds 21–24
3
4
; ppm relative to Me Si) for compounds 17–20 and (400 MHz, D
2
O; ppm relative to DSS) for
0
0
0
0
0
0
0
H-5b
Compound H-1 a
H-1 b
H-2
H-3
H-4
H-5a
H-6
CH
3
N–H
OAc
17
18
19
20
21
22
23
24
3.32dd
3.51dd
3.61d
4.13d
5.08m
5.24m
5.20m
5.30m
3.75m
3.55m
3.67m
3.59m
5.36dd
5.27dd
5.26dd
4.12dd
3.52dd
3.39dd
3.57dd
5.50m
5.36m
5.27m
5.41m
4.18m
3.78m
3.90m
3.88m
4.09dd
3.94dd
4.09dd
3.97dd
3.65dd
3.52dd
3.51dd
4.18dd
4.23dd
4.26dd
4.31dd
3.85dd
3.82dd
3.58dd
6.89d 1.83d
8.96br s 1.95, 1.99, 2.01, 2.12
9.03br s 1.98, 2.02, 2.06, 2.15
a
a
4.13dd
4.15dd
3.51dd
3.98dd
4.21dd
4.02dd
6.88
1.87
6.94d 1.82d 10.00br s 1.96, 1.99, 2.06, 2.09
a
a
2.89d
6.91
1.85
9.61br s 2.01, 2.02, 2.09, 2.10
3.81dd
3.52dd
3.64dd
7.48s
7.35s
7.31s
7.32s
1.87s
1.74s
1.73s
1.73s
a
a
a
a
a
a
a
a
a
a
a
a
a
a
3.59m
3.88m
3.59m
3.59m
3.59m
a
Denotes an incompletely resolved signal.
1
3
Table 3. C NMR Chemical shifts (75 MHz, CHCl
compounds 21–24
3
; ppm relative to Me
4
Si) for compounds 9–20 and (75 MHz, D
2
O; ppm relative to DSS) for
OCH
0
0
0
0
0
C-5
Compound C-1
C-2
C-3
C-4
C-2 C-4 C-5
C-6 6-CH CH C@O
3
CH
25.21 14.16 170.01, 170.14, 20.54, 20.78
70.74
27.27 16.06 171.05, 171.91, 22.39, 22.52,
72.19 22.57, 22.61
25.63 14.86 169.54, 169.89, 20.91, 21.10,
70.32, 171.03 21.32
25.58 14.22 169.59, 169.89, 20.35, 20.60
70.20, 170.62
25.39 14.28 169.95, 170.74 20.17, 20.54,
0.66
25.41 14.71 169.58, 170.43, 20.93, 21.07,
72.32 21.20
25.94 13.22 171.77, 171.79 19.56, 19.65,
9.81
25.51 14.10 169.65, 169.95, 20.29, 20.41,
70.62, 170.86 20.66
171.08 21.48
169.93, 170.48, 21.01, 21.06,
70.56, 170.88 21.10, 21.14
170.03, 170.22, 20.98, 21.04,
70.46, 170.95 21.19, 21.36
170.06, 170.18, 20.64, 20.77
70.49, 170.79
2
3
3
9
0
1
2
3
4
5
6
59.51 70.02 67.65 68.32 61.77 151.07
55.31 71.43 69.41 69.32 69.63 151.40
58.30 72.31 69.61 69.50 62.15 151.61
61.82 70.44 68.86 69.59 61.82 151.31
58.18 69.53 68.32 68.56 61.70 151.38
62.63 70.82 68.77 68.02 62.72 151.40
60.51 74.87 68.63 68.42 59.29 146.33
60.00 71.84 68.14 69.84 61.57 150.77
163.15 112.88 135.41 12.58
165.43 113.18 137.97 14.46
163.85 112.22 137.05 13.11
163.80 111.55 137.05 12.40
163.70 111.49 136.93 12.34
165.43 113.18 138.56 13.43
164.48 126.04 137.08 11.61
163.51 111.30 135.89 12.46
1
1
1
1
1
1
1
1
1
1
1
2
1
1
1
1
7
8
50.13 n.o.
68.89 69.56 62.46 151.83
165.06 111.82 141.02 12.98
164.41 111.25 140.68 12.68
1
48.79 70.14 68.54 68.89 62.03 151.63
47.99 69.93 69.35 68.84 62.04 151.34
48.88 76.69 69.09 69.64 61.87 151.06
41.43 54.50 57.09 65.68 62.46 155.23
1
1
2
9
0
164.91 110.91 140.88 12.55
164.36 111.05 140.37 12.21
169.87 113.06 146.67 17.70
1
1
21
22
23
24
n.o.
38.51 50.61 62.25 72.66 69.21 143.93
n.o. 51.78 62.69 69.64 69.33 152.70q 167.33 110.57 140.37 11.57
50.00 61.22 70.05 68.45 150.91
165.46 108.59 142.55
9.66
165.31 110.07 138.05 11.12
n.o. Not observed.
2
.2. Conformational studies
and intermediate coupling values being consistent with
substantial population of more than one rotameric state.
The extended planar zigzag (P) conformation favored
for a linear alkane chain on the basis of maximal
High-field NMR spectroscopy in conjunction with selec-
tive proton decoupling and two-dimensional correlation

810
S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
separation of large groups along each carbon–carbon
bond is subject to perturbation in polysubstituted
thy that in each epimer the rotameric preference along
0
0
the C-1 –C-2 bond is the one that avoids bringing a
bulky group (heterocycle or SEt) in 1,3-parallel disposi-
1
9–21
systems,
notably by unfavorable parallel 1,3-inter-
0
actions between bulky substituents as well as by polar
effects and by solvent interactions. Polysubstituted
systems may thus favor nonextended (sickle, gauche,
G) conformations or conformational mixtures, accord-
ing to the substitution mode. Particular G conforma-
tions derived by 120° rotation of the P form about a
tion with the acetoxy group at C-3 .
OAc SEt
H
H
H
H
H
OAc T
H
H
H
H
H
5
4
3
2
1
5
4
3
2
1
AcO
SEt AcO
T
AcO
H
OAc
AcO
H
OAc
2
1
particular bond are designated by the lower-numbered
atom and the sign of rotation when viewed from the
9a (1'R)-D-arabino
13a (1'S)-D-arabino
+
remote atom; thus 3G denotes the conformation
derived from the P conformation by 120° clockwise
rotation of the remote atom along the C-3–C-4 bond.
This behavior of these two epimers having the arabino
configuration, showing a strong preference for the P
3 2
Table 4. Proton–proton NMR spin-coupling data (400 MHz, CDCl ; Hz) for compounds 9–20 and (400 MHz, D O; Hz) for compounds 21–24
Compound
J1
0
0
J
0
0
J
0
0
J
0
0
J
0
0
J
0
0
J
0
0
J
0
0
J
0
0
1a ;1b
J
Me,6
;2
1a ;2
1b ;2
2 ;3
3 ;4
4 ;5a
4 ;5b
5a ;5b
9
8.4
2.5
9.1
7.8
8.8
4.0
8.3
4.9
3.0
9.8
2.5
3.9
2.9
9.3
2.8
6.8
2.3
7.1
3.9
4.2
8.4
2.0
9.3
5.9
8.8
2.2
9.1
3.9
9.1
3.3
8.8
2.9
2.7
5.3
5.4
5.4
2.9
4.5
7.1
4.4
4.3
6.8
5.7
5.7
5.7
7.6
2.4
3.9
5.9
7.6
3.3
6.8
2.5
5.3
3.0
3.7
12.3
11.6
12.2
11.7
12.7
11.8
11.9
11.7
12.7
11.6
12.3
12.0
1.2
0.9
1.3
a
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
1.0
1.0
a
a
9.8
8.6
8.9
3.0
2.7
2.8
2.1
8.8
14.4
14.6
14.5
14.4
9.1
a
4.0
a
1.1
a
8.8
a
6.2
a
9.1
a
11.7
a
14.1
a
7.1
a
2.6
a
8.8
a
2.8
a
7.1
a
5.9
a
12.1
a
14.3
a
a
a
Incompletely resolved signal.
2
1
2
.2.1. 2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-1-thio-1-
0
conformation, parallels the behavior observed in a
wide range of other compounds possessing a tetrasubsti-
tuted D-arabinitol chain.
(
(
thymin-1-yl)-aldehydo-D-pentose aldehydrols—(1 R)- and
0
1 S)-D-arabino epimers (9 and 13). The spectra of both
epimers were amenable to first-order analysis and
3
,5
0
0
proton assignments paralleled those made earlier for
peracetylated 1-S-alkyl-1-(pyrimidin-1-yl)-1-thiopentitols.
The H-1 resonance for the dextrorotatory epimer 9
2.2.2. (1 R)- and (1 S)-D-lyxo epimers (10 and 14).
Analysis of the spectra of both epimers was essentially
first order, but minor differences in the J
were evident. The H-1 signal for the (1 R) epimer 10
0
0
couplings
1
;2
0
0
was a wide doublet (J1
0
0
8.4 Hz) at d 5.96, and that
;2
for the levorotatory epimer 13 observed at d 5.88 was
likewise a wide doublet (J 8.8 Hz), indicating that
was a narrow doublet (J1
0
0
2.5 Hz) consistent with the
;2
0
0
0
0
;2
gauche disposition of H-1 and H-2 , whereas that for
the (1 S) epimer 14 showed a J
cating a major but not exclusive contribution of the H-
1 –H-2 gauche rotamer. For both epimers, the J
1
0
0
0
H-1 and H-2 are essentially antiparallel in both epi-
mers. The J2 3 values for 9 and 13 are 3.0 and 2.9 Hz,
respectively, indicating an essentially gauche disposition
0
0
value of 4.0 Hz, indi-
1
;2
0
0
0
0
0
0
and
2
;3
0
0
between H-2 and H-3 , while the respective J
0
0
values
J
0
0
values were similar (9.8 and 2.0 Hz, respectively, for
3
;4
3 ;4
are 8.4 and 8.8 Hz, consistent with the essentially anti-
10 and 9.3 and 2.2 Hz, respectively, for 14), indicating
the extended planar (P) orientation of the carbon back-
bone for each epimer. The rotameric state along the C-
0
0
parallel disposition of H-3 and H-4 . These data indi-
cate that each epimer strongly favors a fully extended,
planar zigzag conformation having all backbone carbon
0
0
1 –C-2 bond in each epimer is consistent with avoidance
0
0
atoms in the same plane. The (1 R) epimer has the SEt
of that rotamer that would place a large C-1 substituent
group also in that plane, as indicated in structure 9a,
whereas in the (1 S) epimer the heterocycle occupies
(heterocycle or SEt) in 1,3-parallel orientation with the
acetoxy group at C-3 , leading to the favored conforma-
0
0
the extended orientation, as shown in 13a. It is notewor-
tions depicted in 10a and 14a.

S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
811
0
ꢁ
H
H
AcO
4
H
H
SEt
T
H
H
AcO
4
H
H
T
shown as 12a, and that for the (1 S) epimer the G
2
5
3
2
1
5
3
2
1
conformer shown as 16a. Again, the rotameric popula-
AcO
AcO
SEt
0
0
tions about the C-1 –C-2 bond favor the orientation
that minimizes steric interactions of the heterocycle
and SEt groups.
AcO
H
H
OAc
AcO
H
H
OAc
1
0a (1'R)-D-lyxo
14a (1'S)-D-lyxo
0
0
2
.2.3. (1 R)- and (1 S)-D-ribo epimers (11 and 15). For
0
0
AcO
H
H
SEt
T
H
H
AcO
4
H
the (1 R) epimer 11 the H-1 signal is a wide doublet
AcO
OAc
with J1
0
0
9.1 Hz, indicating antiperiplanar disposition
4
3
2
1
5
3
2
;2
AcO
0
0
0
0
0
of H-1 and H-2 , and the disposition of H-3 and H-4
is likewise antiperiplanar (J3
and H-3 are gauche-disposed (J2
lar parameters are observed for the (1 S) epimer 15, with
1
H
H
AcO
5 CH OAc
AcO
H
H
H
EtS
SEt
16a (1'S)-D-xylo
0
0
9.3 Hz), whereas H-2
;4
2
0
T
0
0
2.5 Hz). Very simi-
0
;3
12a (1'R )-D-xylo
J1
0
0
8.3 Hz, J
0
0
2.8 Hz, and J
0
0
9.1 Hz, indicating
;2
2 ;3
3 ;4
essentially similar favored conformations for both epi-
2
.2.5. 2,3,4,5-Tetra-O-acetyl-1-deoxy-1-(thymin-1-yl)-D-
0
0
mers, with rotation from about the C-2 –C-3 bond to
alleviate the 1,3-parallel interaction between the acetoxy
pentitols (17–20). Removal of the ethylthio group from
the precursors 9–16 led to loss of the downfield H-1 res-
onance near 6 ppm (Table 1) and the appearance some
2
an ABX system for the methylene group at C-1 (Table
2
downfield of the C-1 methylene resonances, is attribut-
able to the C-5 methylene protons, whose chemical shifts
are little changed from their positions in the ethylthio
precursors. The C-5 methylene protons remain subject
to the same level of deshielding by the adjacent oxygen
substituent, whereas the less electronegative nitrogen
atom of the thymine substituent causes the C-1 methyl-
ene protons to resonate at somewhat higher field. Based
on these assignments, the proton–proton coupling
constants could be extracted (Table 4), and the confor-
mational behavior for compounds 17–20 determined.
0
0
0
groups at C-2 and C-4 that would have been present in
0
the P conformation. This brings C-1 out of the plane of
ppm further upfield of a two-proton AB portion of
the rest of the backbone chain. Again, as in the two pre-
ceding examples, the favored rotameric state about the
0
). A second AB portion of an ABX system, somewhat
0
0
C-1 –C-2 bond is the one that does not generate a
1
0
0
,3-parallel interaction between the 3 -OAc group and
0
either the heterocycle or the SEt group, and the favored
ꢁ
conformations are thus the G conformers depicted as
0
2
1
1a and 15a.
H
H
H
H
OAc
H
H
H
H
OAc
0
OAc
OAc
5
4
3
2
5
4
3
2
AcO
AcO
1
H
1
H
AcO
H
T
AcO
H
EtS
SEt
T
1
1a (1'R)-D-ribo
15a (1'S)-D-ribo
0
0
2
.2.4. (1 R)- and (1 S)-D-xylo epimers (12 and 16). The
2
.2.6. D-arabino Isomer (17). The observed values for
vicinal proton–proton couplings for both epimers devi-
ate significantly from those values diagnostic of
preponderantly antiperiplanar or gauche, and are
indicative of conformational mixtures with substan-
tial contributions from more than one conformer. The
value of 7.8 Hz for the (1 R) epimer 12 is consis-
tent with a major contribution from the antiperiplanar
orientation of H-1 and H-2 , but the J
.9 Hz for the (1 S) epimer 16 indicates comparably
J2 (2.3 Hz) and J (9.1 Hz) give clear support for
the extended planar zigzag (P) conformation for 17,
and the respective small (2.7 Hz) and large (9.8 Hz) cou-
plings of the C-1 methylene protons with H-2 are consis-
tent with the thymine N-1 atom lying in the same plane as
the carbon atoms of the chain, and the fully extended
conformation 17a depicted is clearly the favored form.
0
0
0
0
;3
3 ;4
2
2
0
0
0
J1
0
0
;2
0
0
0
0
coupling of
1
;2
0
4
H
H
H
OAc H
H
weighted contributions from antiperiplanar and gauche
5
4
3
2
1
AcO
T
dispositions of these protons. Likewise the respective
AcO
H
H
OAc
0
J2
0
0
and J
0
0
values are 3.9 and 5.9 Hz for the (1 R)
;3
3 ;4
0
epimer 12, and 6.8 and 3.9 Hz for the (1 S) epimer
1
7a D-arabino
1
6, again demonstrating conformational mixing with
no clear single, principal conformation for either epi-
mer. Avoidance of the 1,3-parallel interaction of the
2.2.7. D-lyxo Isomer (18). The conformationally diag-
nostic spin couplings for compound 18 show, respec-
0
0
acetoxy groups at C-2 and C-4 that would have been
present in the P conformation again would seem to be
the driving force in establishing the most favored dispo-
sition of these epimers, and from the possible contribu-
tors to the equilibrium population, a major contributor
tively, large (7.1 Hz) and small (3.3 Hz) values for J2
0
0
;3
and J
0
0
and the magnetically nonequivalent protons
3
0
;4
at C-1 show large (8.6 Hz) and small (2.8 Hz) couplings
with H-2 , leading to assignment of the P conformation
0
depicted in 18a, as the most favored, but not exclusive,
0
+
for the (1 R) epimer appears to be the G conformer
form; the observed J
0
0
and J
0
0
values are not at the
3
2 ;3
3 ;4

812
S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
limits expected for exclusive antiperiplanar and gauche
dispositions, respectively.
amounts of praseodymium chloride, an effective shift
2
1
reagent, to the D O solution.
2
For the D-arabino tetrol 21, the small J
0
0
coupling
2
;3
0
0
H
H
H
H
H
supports the gauche disposition of H-2 and H-3 , while
AcO
4
the value (8.8 Hz) of J
0
0
indicates antiperiplanar rela-
5
3
2
1
3 ;4
AcO
T
0
0
tionship between H-3 and H-4 , and the P conforma-
tion depicted (21a) can be assigned as the major
AcO
H
H
OAc
18a D-lyxo
conformer. For D-ribo tetrol 23, the J2
0
;3
0
and J
3 ;4
0
0
cou-
plings are, respectively, 8.8 and 2.8 Hz, and indicate
þ
2
.2.8. D-ribo Isomer (19). The large (8.8 Hz) value of
the
3
G conformer depicted (23a) as the major contrib-
0
0
utor to the conformational equilibrium, and a mixture
J3 is consistent with essentially exclusive antiperipla-
;4
0
0
0
0
0
0
0
value of
;3
of rotamers along C-4 –C-5 that favors O-5 oriented
nar orientation of H-3 and H-4 , but the J
.9 Hz is indicative of conformational instability with
2
2
2
0
antiperiplanar to C-3 .
3
an appreciable contribution of both the P conformer
having H-2 and H-3 antiperiplanar, and a larger con-
H
H
H
H
OH
H
H
H
OH H
H
0
0
ꢁ
5
HO
4
3
2
1
4
3
2
1
tribution of the G gauche form illustrated (19a), aris-
2
T
T
HO
0
0
H
ing through rotation about the C-2 –C-3 bond to
alleviate the 1,3-interaction between acetoxy groups at
C-2 and C-4 . As with the preceding two examples,
the large (8.9 Hz) and small (2.1 Hz) between the meth-
ylene protons at C-1 and H-2 establish that H-2 is anti-
periplanar to one of the C-1 hydrogen atoms.
OH
HO
H
H
HO
H
HO
5
H
0
0
21a D-arabino
23a D-ribo
0
0
2.3. Molecular modeling studies
H
H
H
OAc
An attempt was made to match the conformational
properties determined here from NMR data for the
tetraacetates 16–20 and the free tetrols 21–14 with
minimum-energy conformations predicted by molecular
modeling. The calculations were performed using
OAc
H
5
4
3
2
AcO
1
H
H
AcO
H
T
1
9a D-ribo
Ò
Hyperchem Release 5 for Windows, and were initiated
from the planar zigzag (P) conformation. In the simula-
tions for the tetrols the AMBER force field was employed
using the Polak–Ribiere minimization algorithm, and
simulations for the tetraacetates required use of the
2
.2.9. D-xylo Isomer (20). The key diagnostic couplings,
J2 and J , are, respectively, 4.2 and 2.9 Hz, and the
latter indicates that H-3 and H-4 are preponderantly
gauche, but the former is indicative of conformational
instability with rotation about the C-2 –C-3 bond to alle-
viate the 1,3-parallel interaction between the acetoxy
groups at C-2 and C-4 in the P conformation, and gen-
0
0
0
0
;3
3 ;4
0
0
+
MM force field. Full details are presented in Ref. 13.
0
0
The results overall showed no consistent agreement
with the conformations demonstrated by NMR experi-
mental studies, not even in the most qualitative sense.
It appears that the software used was not capable of
finding the global energy minima through sequential
rotation of intramolecular bonds, and presumably traps
the computed energy into false local minima. It may be
possible with use of more robust modeling software to
find reliably the true global energy minima, but the
results here emphasize the need for caution in using
popular software as a tool for predicting conformational
properties in the absence of direct experimental data.
0
0
ꢁ
erating the G conformer illustrated as 20a as a signifi-
cant contributor to the conformational equilibrium.
2
H
H AcO
H
OAc
5
4
3
2
AcO
1
H
AcO
H
H
H
T
20a D-xylo
2
.2.10. 1-Deoxy-1-(thymin-1-yl)-D-pentitols (21–24).
Removal of the ester substituents from 16 to 20 caused
significant compression of the signals of the backbone
protons into a narrow range, and selective decoupling
together with COSY experiments was required to assign
individual resonances and determine vicinal coupling
constants. This proved possible for D-arabino and D-ribo
tetrols 21 and 23, but extensive signal overlap in the
spectra of D-lyxo and D-xylo tetrols 22 and 24 precluded
specific assignments, even with the addition of small
2.4. Biological studies
The four acyclic-sugar nucleosides 21–24 were tested for
activity against typical plant viruses and a fungus. At
application levels of 100 ppm, all four compounds were
toxic to cucumber and tobacco plants, while rice plants
tolerated the compounds at levels of 100 ppm. The four
compounds were evaluated at levels of 10, 25, and

S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
813
1
13
5
0 ppm for their ability to combat infection by the
for H spectra and 75 MHz for C spectra. Chemical
tobacco mosaic virus in tobacco plants, and by the
Cucumber Green Mottle Mosaic Virus in cucumber
plants, and they showed no activity.
shifts are referenced to that of Me Si (0.0 ppm) for solu-
4
tions in CDCl (9–20) and with sodium 4,4-dimethyl-4-
3
silapentanesulfonate (DSS) for solutions in D O
2
Tested against the rice blast fungus Pyriculria oryxae,
the four compounds demonstrated biological activity
that varied according to their stereochemical configura-
tion, with the D-ribo derivative 23 showing the greatest
activity. At the 10 ppm application level, the D-arabino
and D-lyxo derivatives 21 and 22 showed no activity,
while the D-xylo derivative 24 actually enhanced the
development of the fungal symptoms (PV ¼ 127),
whereas the D-ribo derivative 23 showed significant inhi-
bition of the pathogen (PV ¼ 71). At the 50 ppm appli-
cation level, compounds 21 and 23 showed some
inhibitory activity, but at the 100 ppm level the D-ribo
compound 23 actually enhanced the fungal activity.
Details are recorded in Table 5.
(21–24). Assignments of signals were made by first-order
analysis of spectra and the assignments were supported
by two-dimensional correlation spectroscopy (COSY)
and homonuclear decoupling experiments (40–60 dB),
using commercial software supplied with the spectro-
meter (XWIN NMR Acquisition and XWIN NMR
Processing). Elemental analyses were performed by
Atlantic Microlab, Inc. All reactions were monitored
by silica gel (Whatman Al SIL G/UV, 250 lm thickness)
thin-layer chromatography (TLC) and detection was
accomplished by either UV absorption and/or charring
2 4
effected with 5% H SO in EtOH. Preparative chroma-
tography was performed using Whatman PKGF silica
gel 60A (1000 lm thickness). In all cases, column chro-
matography was performed using Kieselgel 60 (70–230
mesh ASTM).
a
Table 5. Protective values (PV, %) of compounds 21, 22, 23, and 24
and a control at three concentrations versus development of Pyriculria
oryzae symptoms in potted rice plants
12
3.2. Preparation of 2,4-bis(trimethylsilyl)thymine
Compound
Concentration (ppm)
50
1
0
100
To a magnetically stirred suspension of thymine (12.6 g,
.100 mol) in 300 mL of dry toluene was added 23.0 g
(2.1 equiv) of chlorotrimethylsilane. The flask was fitted
with an addition funnel, which was used to add 30.0 mL
0
2
2
2
2
1
2
3
4
103
105
71
127
100
82
123
84
101
100
162
118
203
105
100
of Et N (2.5 equiv) in 30 mL of toluene and the mixture
3
Control
was stirred overnight. The mixture was filtered and the
filtrate washed with three 25-mL portions of toluene.
The solvent was then removed by simple distillation
and the product isolated by vacuum distillation
a
PV ¼ average number of lesions in treated leaves ꢂ 100/average
number of lesions in nontreated leaves. Full details are recorded in
Ref. 13.
(124 °C/13 mmHg). The purified product crystallized
3. Experimental
upon being kept for several hours and was sufficiently
pure for subsequent condensation reactions; yield
21.2 g (78.4%).
3
.1. General methods
Infrared spectra were recorded with a Shimadzu FTIR-
300 Fourier-transform infrared spectrophotometer,
and UV spectra with a Hewlett–Packard Diode Array
Spectrophotometer Model 8452A. Optical rotations
3.3. Preparation of 2,3,4,5-tetra-O-acetyl-1-bromo-1-
deoxy-1-S-ethyl-1-thio-aldehydo-D-arabinose
aldehydrol (5)
8
2
(
sodium D line) were measured with a Perkin–Elmer
2,3,4,5-Tetra-O-acetyl-D-arabinose
diethyl
dithio-
2
3
Model 141 polarimeter. Electron-impact mass spectra
were obtained with a Shimadzu GCMS QP5050A gas
chromatograph–mass spectrometer at an ionization
potential of 70 eV and an accelerating potential of
acetal (1, 2.15 g, 5.06 mmol) was dissolved in 25 mL
of anhydrous diethyl ether in a sealed flask fitted with
a thermometer, a septum, and a magnetic stirring device.
To the solution was added 0.27 mL (1.05 equiv) of Br2,
at such a rate that the temperature remained below
10 °C. The reaction was monitored by TLC (3:1 tolu-
ene–MeOH) for 40 min at which time TLC showed the
absence of starting material and the appearance of a sin-
gle, more-slowly moving spot detectable by H SO char-
ring. The volatile materials were evaporated off using a
vacuum source and the residue was washed twice with
diethyl ether to yield 5 as a pale-yellow syrup; R 0.44
(3:1 toluene–MeOH). The product was not stable on
storage and was used without delay in the next step.
1
.50 kV. The column employed for all GLC analyses
was a 30 mꢂ0.25 mm Supelco fused-silica capillary
TM
(
PTE -5, 0.25 lm film). The carrier gas was set to flow
at 50.00 mL/min with a split ratio of 30.00. The sample
was applied through the injector (180 °C) and to the col-
umn at an initial temperature of 60.0 °C. The column
temperature was then elevated at 10 °C/min until the
final oven temperature of 280.0 °C was reached. Nuclear
magnetic resonance (NMR) spectra were obtained using
a Bruker Avance DRX400 NMR instrument at 400 MHz
2
4
f

814
S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
3
.4. (1R,S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-1-
thio-1-(thymin-1-yl)-aldehydo-D-arabinose aldehydrols
9 and 13)
NMR chemical shifts, and proton–proton coupling con-
stants are given in Tables 1, 3, and 4, respectively.
(
3.5. 1-(2,3,4,5-Tetra-O-acetyl-1-deoxy-D-arabinitol-1-yl)-
thymine (17)
To 4.72 g (10.1 mmol) of freshly prepared 2,3,4,5-tetra-
O-acetyl-1-bromo-1-deoxy-1-S-ethyl-1-thio-1-aldehydo-
D-arabinose aldehydrol (5) in a pressure reaction tube
To a 100-mL round-bottom flask fitted with a magnetic
stirring device was added 0.36 g (0.74 mmol) of com-
pounds 9 þ 13 (R,S mixture), followed by 25 mL of
freshly distilled toluene. To the solution was added
(
Ace Glass, Inc.) was added 2.74 g (1.0 equiv) of 2,4-
bis(trimethylsilyl)thymine in 25 mL of dry toluene. The
tube was sealed and the mixture heated in an oil bath
for 8 h at 140 °C. The darkened residue was then
removed and allowed to cool to room temperature.
The pressure was then carefully released and the mixture
triturated with 15 mL of 4:1 MeOH–water and then
0.85 g (4.0 equiv, 0.80 mL) of Bu SnH and 0.02 g of
3
azobisisobutanonitrile (AIBN). The flask was flushed
with dry N and maintained under N . The mixture
2
2
was heated to boiling (110 °C) under reflux for 8 h, at
which time TLC showed the absence of starting material
and the appearance of a single, more-slowly moving
CHCl (50 mL). The mixture was filtered and the filtrate
3
washed with distilled water, and dried (MgSO ). The
4
solution was decolorized with activated charcoal to yield
a pale-yellow solution. TLC (2:1 EtOAc–hexane)
showed two major products that were detectable by
both UV absorbance and H SO charring. Evaporation
2 4
spot detectable with both UV absorbance and H SO
charring. The flask was allowed to cool and the solvent
removed under vacuum. The clear syrup was dissolved
in 25 mL of N,N-dimethylformamide (DMF) and the
solution washed three times with 5 mL portions of
n-hexane. The clear syrup was applied to a column
(2.0 ꢂ 40 cm) of silica gel that was eluted with 2:1
EtOAc–hexane as eluent. Fractions containing product
17 were combined to yield 0.30 g (94%) of 12 as a color-
2
4
of the solvent yielded 4.40 g of yellow syrup; crude yield
9%. Analysis by GLC–MS indicated two products with
identical mass spectra and retention times of 24.1 and
4.6 min, interpreted as the diastereomeric products 9
8
2
and 13 in the ratio of 3.7:1.0. Column chromatographic
purification on silica gel afforded 3.20 g of the mixed
diastereomers 11a and 11b, as a white foam; yield 65%.
2
5
less syrup; ½aꢃ +45.9 (CHCl ); R 0.17 (2:1 EtOAc–hex-
D
3
f
ane); k
272 nm; EIMS: m/z 428 [M]. Anal. Calcd for
max
C H N O : C, 50.47; H, 5.66; N, 6.54. Found: C,
1
8
24
2
10
1
13
3
.4.1. (1R)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-1-
50.18; H, 5.80; N, 6.31. H NMR and C NMR chem-
ical shifts, and proton–proton coupling constants are
given in Tables 2–4, respectively.
thio-1-(thymin-1-yl)-aldehydo-D-arabinose aldehydrol (9).
The aforementioned foam (3.20 g) was dissolved in a
minimal amount of EtOAc and applied to a column
(
3.0 cmꢂ60 cm) of silica gel (200 g). The column was
3.6. 1-(1-Deoxy-D-arabinitol-1-yl)thymine (21)
eluted with 2:1 EtOAc–hexane, and concentration of
the fractions having R 0.48 afforded 9 as a white foam
To a magnetically stirred solution of 17 (2.10 g,
4.90 mmol) in 100 mL of anhydrous MeOH was added
a catalytic amount (ꢀ0.006 g) of freshly crushed Na.
The flask was immediately sealed and the contents were
stirred overnight. The basic reaction mixture was then
neutralized using cation-exchange resin (Amberlite IR-
f
that crystallized from pure EtOH; yield 2.07 g (42%);
25
mp 126.0 °C, ½aꢃ +135 (CHCl ); R 0.48 (2:1 EtOAc–
D
3
f
hexane); EIMS: m/z 488 [M]. Anal. Calcd for
C
20
H
28
N
2
O
10Sꢄ0.5H O: C, 48.27; H, 5.89; N, 5.63; S,
2
1
6
.44. Found: C, 48.48; H, 5.71; N, 5.81; S, 6.48. H
1
3
+
NMR and C NMR chemical shifts, and proton–pro-
ton coupling constants are given in Tables 1, 3, and 4,
respectively.
120, H form) and the solvent removed under dimin-
ished pressure. Addition of pure EtOH yielded 1.10 g
(86%) of 21 as small white crystals; mp 230–234 °C
2
D
5
(
dec), ½aꢃ +38.1 (H O); R 0.53 (1:2 toluene–MeOH).
2
f
3
1
.4.2. (1S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-
-thio-1-(thymin-1-yl)-aldehydo-D-arabinose aldehydrol
Anal. Calcd for C10
6.38; N, 10.40. Found: C, 44.79; H, 6.08; N, 10.23. H
NMR and C NMR chemical shifts, and proton–pro-
ton coupling constants are given in Tables 2–4,
respectively.
H
16
N
2
O
6
ꢄ0.5H O: C, 44.60; H,
2
1
1
3
(
13). The fractions from the previously described col-
umn containing the slower-moving product (R ) 0.40;
f
2
:1 EtOAc–hexane) were combined and the solvent was
removed to yield 0.64 g of a clear syrup, which failed to
crystallize. Further purification on a column (2.0ꢂ
3.7. Preparation of 2,3,4,5-tetra-O-acetyl-1-bromo-1-
deoxy-1-S-ethyl-1-thio-aldehydo-D-lyxose
aldehydrol (6)
4
0 cm) of silica gel yielded 0.54 g (11%) of 13 as a colorless
25
2
syrup; ½aꢃ ꢁ26.7 (CHCl ); R 0.40 (2:1 EtOAc–hexane);
D
3
f
EIMS: m/z 488 [M]. Anal. Calcd for C20
H
28
N
2
O
10Sꢄ
13
2
3
0
4
.5H O: C, 48.27; H, 5.89; N, 5.63; S, 6.44. Found: C,
8.54; H, 5.65; N, 5.78; S, 6.18. H NMR and
2,3,4,5-Tetra-O-acetyl-D-lyxose diethyl dithioacetal (2,
2
1
C
2.15 g, 5.06 mmol) was treated with Br and the reaction
2

S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
815
processed as described for compound 5 to yield 6 as a
scribed for 17 yielded 1.50 g (81.5%) of 18 as a color-
2
5
pale-yellow syrup; R 0.44 (3:1 toluene–MeOH), which
less syrup; ½aꢃ +60.3 (CHCl ); R 0.18 (2:1 EtOAc–
f
D
3
f
was used without delay in the next step.
hexane); k
276 nm (loge 3.87); EIMS: m/z 428
max
[
M]. Anal. Calcd for C H N O ꢄ0.5H O: C, 49.43;
1
8
24
2
10
2
3
.8. (1R,S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-1-
thio-1-(thymin-1-yl)-aldehydo-D-lyxose aldehydrol
10 and 14)
H, 5.77; N, 6.41. Found: C, 49.68; H, 5.81; N, 6.47.
1
13
H NMR and C NMR chemical shifts, and proton–
(
proton coupling constants are given in Tables 2–4,
respectively.
Compound 6 (4.72 g, 10.1 mmol) and 2.74 g (1.0 equiv)
of 2,4-bis(trimethylsilyl)thymine in 20 mL of dry toluene
were allowed to react and the products isolated as
described for compounds 9 and 13, to yield 3.60 g (73%)
of yellow syrup containing two major products detect-
able by both UV absorbance and H SO charring. Puri-
fication on a column of silica gel afforded 2.71 g of the
diastereomers 10 and 14, as a white foam; yield 54.7%.
Analysis by GLC–MS indicated two products with iden-
tical mass spectra and retention times of 24.4 and
3.10. 1-(1-Deoxy-D-lyxitol-1-yl)thymine (22)
Catalytic deesterification of 18 (1.40 g, 3.27 mmol) as
described for compound 21 yielded 0.79 g (93%) of
22 as small white crystals; mp 220.1–222.3 °C (dec),
2
4
2
D
5
½aꢃ +45.8 (H
O); R
0.49 (1:2 toluene–MeOH). Anal.
ꢄ0.5H O: C, 44.60; H, 6.38;
2
f
Calcd for C10
N, 10.40. Found: C, 44.57; H, 6.44; N, 10.36.
NMR and C NMR chemical shifts, and proton–
proton coupling constants are given in Tables 2–4,
respectively.
H
N
O
6
16
2
2
1
H
1
3
2
4.7 min, interpreted as the diastereomeric products 10
and 14 in the ratio of 1.0:1.2.
3
1
.8.1. (1S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-
-thio-1-(thymin-1-yl)-aldehydo-D-lyxose aldehydrol (10).
3.11. Preparation of 2,3,4,5-tetra-O-acetyl-1-bromo-1-
2
deoxy-1-S-ethyl-1-thio-aldehydo-D-ribose aldehydrol (7)
The aforementioned foam (2.71 g) was dissolved in a
minimal amount of EtOAc and applied to a column
2
3
2,3,4,5-Tetra-O-acetyl-D-ribose diethyl dithioacetal (3,
2.15 g, 5.06 mmol) was treated with Br and the reaction
processed as described for compound 5 to yield 7 as a
pale-yellow syrup; R 0.34 (3:1 toluene–MeOH), which
(
4.0 cmꢂ60 cm) of silica gel (200 g). The column was
2
eluted with 2:1 EtOAc–hexane to separate the products
from the reaction mixture. Concentration of the fractions
f
having R 0.44 afforded 10 as a white foam; yield 1.15 g
was used without delay in the next step.
f
2
5
(
23%); ½aꢃ ꢁ59.0 (CHCl ); R 0.44 (2:1 EtOAc–hexane);
20 28 2 10
D
3
f
EIMS: m/z 488 [M]. Anal. Calcd for C H N O Sꢄ
3.12. (1R,S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-1-
thio-1-(thymin-1-yl)-aldehydo-D-ribose aldehydrol
(11 and 15)
0
4
.5H O: C, 48.27; H, 5.89; N, 5.63; S, 6.44. Found: C,
2
1
13
8.32; H, 5.71; N, 5.54; S, 6.36. H NMR and
C
NMR chemical shifts, and proton–proton coupling
constants are given in Tables 1, 3, and 4, respectively.
Compound 7 (4.72 g, 10.1 mmol) and 2.74 g (1.0 equiv)
of 2,4-bis(trimethylsilyl)thymine (9) in 20 mL of dry tol-
uene were allowed to react and the products isolated as
described for compounds 9 and 13. Evaporation of the
solvent yielded 3.75 g of yellow syrup containing two
major products detectable by both UV absorbance and
3
1
.8.2. (1R)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-
-thio-1-(thymin-1-yl)-aldehydo-D-lyxose aldehydrol (14).
The fractions from the previously described column con-
taining the slower-moving product (R ) 0.40; 2:1 EtOAc–
f
hexane) were combined and the solvent removed to yield
2 4
H SO charring. Purification on a column of silica gel
1
.48 g of a clear syrup, which failed to crystallize. Further
afforded 3.12 g of the diastereomers 11 and 15, as a
white foam; yield 63%. Analysis by GC–MS indicated
two products with identical mass spectra and retention
times of 24.0 and 24.3, interpreted as the diastereomeric
products 11 and 15 in the ratio of 1.0:3.1.
purification on a column (2.0ꢂ40 cm) of silica gel yielded
25
1
.38 g (28%) of 14 as a colorless syrup; ½aꢃ +72.0
D
(
CHCl ); R 0.40 (2:1 EtOAc–hexane); EIMS: m/z 488
3 f
[
5
M]. Anal. Calcd for C H N O Sꢄ1H O: C, 47.41; H,
20
28
2
10
2
.98; N, 5.53; S, 6.33. Found: C, 47.54; H, 5.72; N, 5.54;
1
13
S, 6.27. H NMR and C NMR chemical shifts, and pro-
ton–proton coupling constants are given in Tables 1, 3,
and 4, respectively.
3.12.1. (1R)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-
1-thio-1-(thymin-1-yl)-aldehydo-D-ribose aldehydrol (11).
The aforementioned foam (3.12 g) was dissolved in a
minimal amount of EtOAc and applied to a column
(4.0 cm ꢂ 60 cm) of silica gel (200 g). The column was
eluted with 2:1 EtOAc–hexane to separate the products
from the reaction mixture. Concentration of the fractions
3.9. 1-(2,3,4,5-Tetra-O-acetyl-1-deoxy-D-lyxitol-1-yl)-
thymine (18)
Treatment of compounds 10 þ 14 (R,S mixture,
having R 0.48 afforded 11 as a white foam that crystal-
lized from pure EtOH; yield 0.75 g (15%); mp 126.0 °C,
f
2
.10 g, 4.30 mmol) with Bu SnH and AIBN as de-
3

816
S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
2
D
5
½
aꢃ +45.6 (CHCl ); R 0.48 (2:1 EtOAc–hexane); EIMS:
3.16. (1R,S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-1-
thio-1-(thymin-1-yl)-aldehydo-D-xylose aldehydrol
(12 and 16)
3
f
m/z 488 [M]. Anal. Calcd for C20
8.27; H, 5.89; N, 5.63; S, 6.44. Found: C, 48.11; H,
.89; N, 5.55; S, 6.12. H NMR and C NMR chemical
H
28
N
2
O
10Sꢄ0.5H O: C,
2
4
5
1
13
shifts, and proton–proton coupling constants are given
in Tables 1, 3, and 4, respectively.
Compound 8 (4.72 g, 10.1 mmol) and 2.74 g (1.0 equiv)
of 2,4-bis(trimethylsilyl)thymine in 20 mL of dry toluene
were allowed to react and the products isolated as de-
scribed for compounds 9 and 13 to yield 4.22 g (86%)
of yellow syrup containing two major products detect-
3
1
.12.2. (1S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-
-thio-1-(thymin-1-yl)-aldehydo-D-ribose aldehydrol (15).
2 4
able by both UV absorbance and H SO charring. Puri-
The fractions from the previously described column
fication on a column of silica gel afforded 3.22 g of the
diastereomers 12 and 16, as a white foam; yield 65%.
GLC–MS analysis indicated two products with identical
mass spectra and retention times of 24.3 and 24.8, inter-
preted as the diastereomeric products 12 and 16 in the
ratio of 2.2:1.0.
containing the slower-moving product (R 0.40; 2:1
f
EtOAc–hexane) were combined and the solvent was
removed to yield 2.20 g of a clear syrup, which failed
to crystallize. Further purification on a column
(
2.0 ꢂ 40 cm) of silica gel yielded 2.14 g (43.4%) of 15
2
D
5
as a colorless syrup; ½aꢃ ꢁ11.0 (CHCl ); R 0.46 (2:1
3
f
EtOAc–hexane); EIMS: m/z 488 [M]. Anal. Calcd for
C
20
H
28
N
2
O
10Sꢄ 0.5H O: C, 48.27; H, 5.89; N, 5.63; S,
3.16.1.
(1S)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-
2
1
6
.44. Found: C, 48.45; H, 5.90; N, 5.49; S, 6.16. H
1-thio-1-(thymin-1-yl)-aldehydo-D-xylose aldehydrol (16).
The aforementioned foam (3.22 g) was dissolved in a
minimal amount of EtOAc and applied to a column
(3.5 ꢂ 60 cm) of silica gel (200 g). The column was eluted
with 2:1 EtOAc–hexane to separate the products from
the reaction mixture. Concentration of the fractions
1
3
NMR and C NMR chemical shifts, and proton–pro-
ton coupling constants are given in Tables 1, 3, and 4,
respectively.
3.13. 1-(2,3,4,5-Tetra-O-acetyl-1-deoxy-D-ribitol-1-yl)-
thymine (19)
having R 0.50 afforded 16 as a white foam that crystal-
f
lized from pure EtOH; yield 2.07 g (42%); mp 124.0–
2
D
5
1
25.0 °C, ½aꢃ ꢁ116 (CHCl ); R 0.50 (2:1 EtOAc–
Treatment of compounds 11 þ 15 (R,S mixture, 0.36 g,
3
f
hexane); EIMS: m/z 488 [M]. Anal. Calcd for
1
.0 mmol) with Bu SnH and AIBN as described for 17
3
2
5
C
20
H
28
N
2
O
10Sꢄ0.5H O: C, 48.27; H, 5.89; N, 5.63; S,
yielded 0.30 g (93%) of 19 as a colorless syrup; ½aꢃ
2
D
1
6
.44. Found: C, 48.34; H, 5.70; N, 5.58; S, 6.19. H
ꢁ
41.0 (CHCl ); R 0.17 (2:1 EtOAc–hexane); k
3 f max
1
3
NMR and C NMR chemical shifts, and proton–pro-
ton coupling constants are given in Tables 1, 3, and 4,
respectively.
2
74 nm; EIMS: m/z 428 [M]. Anal. Calcd for
10: C, 50.47; H, 5.66; N, 6.54. Found: C,
18 24 2
C H N O
1
13
5
0.28; H, 5.90; N, 6.20. H NMR and C NMR chemi-
cal shifts, and proton–proton coupling constants are
given in Tables 2–4, respectively.
3
1
.16.2. (1R)-2,3,4,5-Tetra-O-acetyl-1-deoxy-1-S-ethyl-
-thio-1-(thymin-1-yl)-aldehydo-D-xylose aldehydrol (12).
3
.14. 1-(1-Deoxy-D-ribitol-1-yl)thymine (23)
The fractions from the previously described column con-
taining the slower-moving product (R 0.43; 2:1 EtOAc–
f
Catalytic deesterification of 19 (1.70 g, 3.97 mmol) as
described for compound 21 yielded 1.0 g (97%) of 23
hexane) were combined and the solvent removed to yield
1.18 g of a clear syrup, which failed to crystallize. Fur-
ther purification on a column (2.0 ꢂ 40 cm) of silica
25
as a white solid; mp 202–205 °C (dec), ½aꢃ ꢁ38.6
D
2
D
5
(
H O); R 0.48 (1:2 toluene–MeOH). Anal. Calcd for
gel yielded 1.10 g (22%) of 12 as a colorless syrup; ½aꢃ
2
f
C H N O : C, 46.14; H, 6.21; N, 10.77. Found: C,
+43.4 (CHCl
m/z 488 [M]. Anal. Calcd for C20
C, 48.27; H, 5.89; N, 5.63; S, 6.44. Found: C, 48.27;
3
); R
f
0.43 (2:1 EtOAc–hexane); EIMS:
1
0
16
2
6
1
13
4
5.84; H, 6.34; N, 10.52. H NMR and C NMR chem-
H
28
N
2
O
10Sꢄ0.5H O:
2
ical shifts, and proton–proton coupling constants are
given in Tables 2–4, respectively.
1
13
H, 5.76; N, 5.69; S, 6.19. H NMR and C NMR chemi-
cal shifts, and proton–proton coupling constants are
given in Tables 1, 3, and 4, respectively.
3.15. Preparation of 2,3,4,5-tetra-O-acetyl-1-bromo-1-
deoxy-1-S-ethyl-1-thio-aldehydo-D-xylose aldehydrol (8)
2
3.17. 1-(2,3,4,5-Tetra-O-acetyl-1-deoxy-D-xylitol-1-yl)-
thymine (20)
2
3
2
,3,4,5-Tetra-O-acetyl-D-xylose diethyl dithioacetal (4,
2
.15 g, 5.06 mmol) was treated with Br and the reaction
2
processed as described for compound 5 to yield 8 as a
Treatment of compounds 12þ16 (R,S mixture, 3.20 g,
pale-yellow syrup; R 0.31 (3:1 toluene–MeOH), which
6.55 mmol) with Bu SnH and AIBN as described for
f
3
was used without delay in the following step.
17 yielded 2.60 g (92.5%) of 20 as a colorless syrup;

S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
817
2
D
5
½
aꢃ +45.9 (CHCl ); R 0.17 (2:1 EtOAc–hexane); k
pH 7) was added to the mixture to facilitate the homoge-
nization process. The mixture was filtered through gauze
to yield a green solution of the virus. To this solution was
added 2.0 g of carborundum abrasive (C600 mesh). Fi-
nally the solution was applied to the cucumber plant
test-subjects via the air-brush technique. The plants were
returned to the climate-controlled greenhouse and mon-
itored daily for signs of infection (leaf discolorations, leaf
distortions, leaf spots, and/or stem symptoms).
3
f
max
2
74 nm; EIMS: m/z 428 [M]. Anal. Calcd for
C H N O ꢄ1H O: C, 48.45; H, 5.89; N, 6.28. Found:
1
8
24
2
10
2
1
13
C, 48.62; H, 5.63; N, 6.21. H NMR and C NMR
chemical shifts, and proton–proton coupling constants
are given in Tables 2–4, respectively.
3
.18. 1-(1-Deoxy-D-xylitol-1-yl)thymine (24)
Catalytic deesterification of 20 (2.30 g, 5.37 mmol) as
described for compound 21 yielded 1.30 g (93%) of 24
3.21. Resistance of N. tabacum treated with 1-(1-deoxy-D-
pentitol-1-yl)thymines (21–24) toward tobacco mosaic
virus (TMV) infection
2
5
as a white powder; mp 117.0–120.2 °C (dec), ½aꢃ
D
ꢁ
37.4 (H O); R 0.64 (1:2 toluene–MeOH). Anal. Calcd
2 f
for C H N O ꢄ0.5H O: C, 44.60; H, 6.38; N, 10.40.
1
0
16
2
6
2
1
13
Found: C, 44.55; H, 6.37; N, 10.49. H NMR and
C
A solution of TMV was prepared for inoculation in the
following manner: a sample of infected tobacco plant
leaves (2.0 g) was pulverized using a mortar and pestle.
A phosphate-buffer solution (0.1 M, 200.0 mL, pH 7)
was added to the mixture to facilitate the homogenization
process. The mixture was filtered through gauze to yield a
green solution of the virus. To this solution was added
NMR chemical shifts, and proton–proton coupling con-
stants, are given in Tables 2–4, respectively.
3.19. Toxicological effects of 1-(1-deoxy-D-pentitol-1-yl)-
thymines (21–24) versus Cucumus sativus and
Nicotiana tabacum
2.0 g of carborundum abrasive (C600 mesh). Finally the
A 100-ppm aqueous solution of each separate 1-(1-
deoxy-D-pentitol-1-yl)thymine was applied to cucumber
solution was applied to the tobacco plant test-subjects
via the air-brush technique. The plants were returned to
the climate-controlled greenhouse and monitored daily
for signs of infection (leaf discolorations, leaf distortions,
leaf spots including mosaics, and/or stem symptoms).
(
C. sativus, 14 days following germination) and tobacco
plants (N. tabacum, 20 days following germination)
2
4
using the previously described air-brush technique.
Each test group comprised 4 cucumber and 3 tobacco
plants, and received 10.0 mL of test solution. A group
of 4 cucumber plants and 3 tobacco plants treated in
the same manner using distilled water instead of the 1-
3.22. Toxicological effects of 1-(1-deoxy-D-pentitol-1-yl)-
thymines (21–24) versus potted rice plants
(v. Koshihikari)
(
1-deoxy-D-arabinitol-1-yl)thymine solution was estab-
lished as the control group. Following the application,
the plants remained in the application hood for 1 h and
were then moved to a climate-controlled greenhouse
Aqueous solutions of each 1-(1-deoxy-D-pentitol-1-yl)-
thymine were prepared at 10, 50, and 100 ppm con-
centrations. Using the previously described air-brush
technique, 20 mL of each test solution was applied to
test groups consisting of four rice plants each. The tested
plants were all selected at the 3.5–4 leaf stage. An addi-
tional group of four potted rice plants treated in a
similar manner using distilled water instead of the 1-
(1-deoxy-D-pentitol-1-yl)thymine solution was established
as the control group. Following the application, the
plants remained in the application hood for 1 h and were
then moved to a climate-controlled greenhouse (25 °C)
for 24 h. Following the incubation period, the plants
were studied for signs of stress resulting from the chem-
ical treatment. As no significant differences were noted
in comparison with the control group, the procedure
of fungal infection was undertaken.
(
25 °C) for 24 h. After the incubation period, the poor
condition of the plants indicated that they could not
withstand the concentration of the applied test solution.
Appropriate dilutions were performed to achieve test
solution concentrations of 10.0, 25.0, and 50.0 ppm. To
test groups identical to those just described, 10.0 mL of
each diluted solution was applied and the plants were
incubated for 24 h in the greenhouse. Following the incu-
bation period, the plants were studied for signs of stress
resulting from the chemical treatment. As no significant
differences were noted in comparison with the control
groups, the procedure of viral infection was undertaken.
3
.20. Resistance of C. sativus treated with 1-(1-deoxy-D-
pentitol-1-yl)thymines (21–24) toward cucumber green
mottle mosaic virus (CGMMV) infection
3.23. Resistance of potted rice plants (v. Koshihikari)
treated with 1-(1-deoxy-D-pentitol-1-yl)thymines (21–24)
toward Pyriculria oryzae, Hoku 1 race 007 infection
A solution of CGMMV was prepared for inoculation in
the following manner: a sample of infected cucumber
plant leaves (1.0 g) was pulverized using a mortar and
pestle. A phosphate-buffer solution (0.1 M, 200.0 mL,
A solution of the rice blast fungus, P. oryzae, was grown
on oatmeal agar medium for 2 weeks at 28 °C in the

818
S. M. Vejcik, D. Horton / Carbohydrate Research 342 (2007) 806–818
absence of light. The plates were then irradiated with a
UV source to induce conidiation. Abundant conidia
were produced on the surface within 3 days. A suspen-
sion of the conidia was prepared by pouring a small
amount of distilled water onto the plate and rubbing
the media with a sterilized brush. The conidia suspen-
sion was filtered and the concentration adjusted to
4. Horton, D.; Kokrady, S. S. Carbohydr. Res. 1980, 80, 364–
74.
3
5
. Horton, D.; Baker, D. C.; Kokrady, S. S. Ann. NY Acad.
Sci. 1975, 225, 131–150.
6
. Elion, G. B.; Furman, P. A.; Fyfe, J. A.; de Miranda, P.;
Beauchamp, L.; Schaeffer, H. J. Proc. Natl. Acad. Sci.
U.S.A. 1977, 74, 5716–5720.
7
. Schaeffer, H. J.; Beauchamp, L.; de Miranda, P.; Elion, G.
B.; Bauer, D. J.; Collins, P. Nature 1978, 272, 583–585.
. Gauthier, B. Ann. Pharm. Fr. 1954, 12, 281–285.
. Weygand, F.; Ziemann, H.; Bestmann, H. J. Chem. Ber.
1958, 91, 2534–2537.
7
approximately 10 /mL using distilled water. Finally
8
9
the solution was applied to the previously treated potted
rice plants in a partially enclosed chamber using the air-
brush technique. The chamber was maintained at a high
level of humidity, a temperature of 24 °C, and com-
pletely isolated from exterior light sources. Following
the 24-h incubation period in the dark, the plants were
returned to the climate-controlled greenhouse and mon-
itored daily for signs of infection (leaf discolorations,
leaf distortions, leaf spots, and/or stem symptoms).
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Acknowledgments
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006, 341, 2211–2218.
S.M.V. thanks the US National Science Foundation for
support for a Summer Program in Japan and Dr. Isamu
Yamaguchi and his laboratory for the opportunity pro-
vided to perform the plant toxicological studies reported
here.
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