A chiral template-driven synthesis of a 3′-deoxy-3′-18F-fluorothymidine precursor

Article abstract of DOI:10.1016/j.ejmech.2017.10.047

An asymmetric synthesis of a 3′-deoxy-3′-¹⁸F-fluorothymidine (¹⁸F-FLT) precursor has been developed wherein the deoxysugar moiety was synthesized using a novel Ga-mediated allylation of (R)-2,3-cyclohexylideneglyceraldehyde as the key step. The synthesis deviates significantly from the previous syntheses of the ¹⁸F-FLT precursors wherein the expensive starting material, thymidine was used.

Full text of DOI:10.1016/j.ejmech.2017.10.047

Accepted Manuscript
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A chiral template-driven synthesis of a 3′-deoxy-3′- F-fluorothymidine precursor
Papiya Dey, K.S. Ajish Kumar, Anubha Sharma, Subrata Chattopadhyay
PII:
S0223-5234(17)30847-4
10.1016/j.ejmech.2017.10.047
EJMECH 9839
DOI:
Reference:
To appear in: European Journal of Medicinal Chemistry
Received Date: 26 May 2017
Revised Date: 12 October 2017
Accepted Date: 15 October 2017
Please cite this article as: P. Dey, K.S. Ajish Kumar, A. Sharma, S. Chattopadhyay, A chiral template-
18
driven synthesis of a 3′-deoxy-3′- F-fluorothymidine precursor, European Journal of Medicinal
Chemistry (2017), doi: 10.1016/j.ejmech.2017.10.047.
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ACCEPTED MANUSCRIPT
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A chiral template-driven synthesis of a 3ˈ-deoxy-3ˈ-18F-fluorothymidine precursor
Papiya Dey,1,2 K. S. Ajish Kumar,1 Anubha Sharma,1,2 and Subrata Chattopadhyay*,1,2
1Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai 400 085, India.
2Homi Bhabha National Institute, Training School Complex, Anushakti Nagar,
Mumbai -400094, India.
Fax: +91 22 25505151; e-mail: schatt@barc.gov.in
ABSTRACT– An asymmetric synthesis of a 3ˈ-deoxy-3ˈ-18F-fluorothymidine (18F-FLT)
precursor has been developed wherein the deoxysugar moiety was synthesized using a novel Ga-
mediated allylation of (R)-2,3-cyclohexylideneglyceraldehyde as the key step. The synthesis
deviates significantly from the previous syntheses of the 18F-FLT precursors wherein the
expensive starting material, thymidine was used.
Keywords: asymmetric synthesis; chiral template; FLT; PET imaging agent

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Highlights
•
•
Formulation of an efficient synthesis of an 18F-FLT precursor.
Application of an inexpensive natural chiral pool material for the synthesis.

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1. Introduction
Molecular imaging tools have gained importance for tumor detection and assessment of
its prognosis as well as development of therapeutic strategies. To this end, positron emission
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tomography (PET) with F-fluorodeoxyglucose (18F-FDG) is being extensively used in clinical
practice for diagnosis, staging and restaging of a wide variety of tumor types, and in the
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evaluation of the treatment regime [1,2]. However, F-FDG may not be tumor-specific, since
glucose is metabolized in all types of cells. Besides, low glucose consumption by some
18
carcinomas, higher uptake of F-FDG in brain and during inflammation, and its high urinary
excretion rate are some of the limitations of 18F-FDG [3]. It may not also make distinction
between inflammation, infection and tumors [4].
Instead, the thymidine analog, 3ˈ-deoxy-3ˈ-18F-fluorothymidine (18F-FLT, 1) has been
proposed as an alternative to 18F-FDG for in vivo imaging of proliferating tissues and malignant
tumors [5,6]. The hypothesis is based on the premises that FLT is a substrate of thymidine kinase
1 (TK1), and TK1 activity closely correlates with cellular proliferation and DNA synthesis [7].
Thus, the cellular concentration of FLT is proportional to TK1 activity, and therefore, to cellular
proliferation. FLT being a substrate for only TK1, but not for mitochondrial TK2, it is a more
specific tracer for assessing cellular proliferation. Moreover, due to the presence of the fluorine
atom at the 3ˈ-position in the sugar, 1 may function as a terminator of the growing DNA chain,
ensuring its insignificant accumulation in DNA. But it will be retained in the cells due to the TK-
1 (an S-phase specific enzyme)-catalyzed phosphorylation to 3-fluorothymidine monophosphate
(FLT-MP) [8,9]. The intracellularly trapped FLT-MP is not significantly degraded in vivo, as it
is not a substrate for thymidine phosphorylase. Taken together, this would result in retention of
FLT in the cells without being incorporated into DNA.

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Considerable interest in FLT as an imaging agent has been developed over the past
decade. Synthesis of 1 can be achieved by the nucleophilic substitution at C-3ˊ of a suitable
thymidine precursor such as 1a with the F- ion followed by deprotection and HPLC purification.
However, the seemingly simple synthesis is fraught with several key issues such as good
radiochemical yield and ease of automation in the process. The low radiochemical yields of
many of the existing methods have been a major obstacle in the routine use of 1 as a PET
imaging agent. Therefore, a convenient and concise synthetic scheme from a suitable precursor is
desirable. A comparative analysis of various precursors and their radiochemical yields is
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reported in literature [10]. The initial syntheses of F-FLT starting from an anhydrothymidine
derivative were short, but the radiochemical yields were low [11-13]. Subsequently another
synthesis used a precursor having a nosyl (Ns) group at O-3ˊ,4,4ˊ-dimethoxytrityl (DMTr) group
at O-5ˊ and a 2,4-dimethoxybenzyl protection at N-3 [14]. However, this required seven long
chemical steps for precursor synthesis followed by three additional steps for 18F-labeling.
Moreover, the final deprotection step involved use of ceric ammonium nitrate, resulting in
precipitation of ceric salt and causing problems in the automated synthesis. Compared to these,
the precursor 1a with a nosyl (Ns) leaving group for fluorination and an N-Boc-protecting group
on the pyrimidine ring appears better suited for the synthesis of 1. The radiolabeling of 1a is fast,
and the protocol provided the advantage of one step hydrolysis of the Boc group in a
homogeneous solution [15,16]. In our own Radiation Medicine Centre also, automated
radiosynthesis of 1 from 1a has been carried out [17]. The chemical structures of 1 and 1a are
shown in Figure 1.

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The low half-life (110 min) of F necessitates that compound 1 is synthesized from its
precursor, just prior to its use. Earlier, syntheses of the precursors were accomplished starting
from the commercially available, but expensive, thymidine. Hence, the primary aim of the
present work was to develop an alternate synthesis of 1a using readily available, inexpensive
materials/reagents and operationally simple reactions. To this end, we used (R)-2,3-
cyclohexylideneglyceraldehyde (2) as the starting material to formulate an efficient and
economic synthesis of 1a, which is presented in this paper.
2. Results and discussion
It was planned to construct the precursor 1a from the homoallylic alcohol 3 that can be
accessed by a Barbier-type allylation of the aldehyde 2 with allyl bromide. Previously we have
observed that the diastereoselectivity in the metal-mediated reaction between 2 and allylic
bromides depends on the choice of the metal-solvent combinations [18-22]. More recently, we
have shown that the inexpensive room temperature ionic liquid (RTIL), [bmim][Br] can act both
as a solvent and a metal activator in the Ga-mediated allylation/crotylation of various aldehydes
even at room temperature and offers excellent diastereoselectivity with 2 as the substrate [23,24].
Currently, the RTILs have emerged as good and reusable solvents for a wide range of organic
transformations [25,26]. All these prompted us to explore the Ga-[bmim][Br] combination for a
green allylation protocol of the aldehyde 2. The reaction between 2 and allyl bromide was very

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fast, and could be carried out with almost stoichiometric amounts of the reagents to furnish the
anti-alcohol 3 as the major product in 90% yield and 95:5 anti:syn diastereoselectivity. The pure
anti-alcohol 3 was easily isolated by column chromatography. We are routinely using this
protocol to synthesize 3 in about 8-10 g-scale with excellent reproducibility and convenience of
product isolation. The RTIL was reused three times without any significant change in the
reaction outcome. However, about 10-15% fresh [bmim][Br] was added on each repetition to
compensate for the loss of the solvent during recovery. The diastereoselectivity of the Zn-
mediated reaction between 2 and allyl bromide in H2O-THF was similar to that of the present
protocol. However, it requires a much longer time, a large excess of Zn and allyl bromide (each
3.0 equivalents) and use of NH4Cl as the metal-activator.
With alcohol 3 in hand, we proceeded for the synthesis. For this, the alcohol 3 was
directly esterified with benzoic acid under the Mitsunobu conditions to obtain 4 (83% yield) in a
single step (Scheme 1.). Mitsunobu inversion of secondary alcohols is an important synthetic
transformation in organic synthesis, and is usually accomplished using para-nitrobenzoic acid
(PNBA) [27-29]. We also carried out the conventional protocol to convert the alcohol 3 to the
benzoate 4. This involved reaction of 3 with PNBA, alkaline hydrolysis of the para-
nitrobenzoate and benzoylation of the resultant alcohol with BzCN/Et3N/CH2Cl2. This provided
the benzoate 4 in 76% overall yield from 3 in three steps, each of which required isolation and
purification of the respective products. Thus, besides a better yield, the direct esterification route
was advantageous in terms of isolation of the product, less use of reagents/solvents and simpler
work-up protocol. Earlier, coupling of benzoic acids with phenols under the Mitsunobu
conditions has been reported [30]. However, our literature search did not reveal any example of
using benzoic acid for inverting stereochemistry of secondary alcohols. In a recent report,

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reaction between benzoic acid and silibinin in THF led to esterification at the primary carbinol,
but also led to dehydration at the secondary carbinol function [31]. In comparison, our results are
much better. Treatment of 4 with aqueous trifluoroacetic acid (TFA) in CH2Cl2 furnished the diol
5. Its benzoylation with BzCN proceeded chemo-selectively at the primary carbinol site to
furnish the dibenzoate 6. Reductive ozonolysis (O3/Ph3P) of 6 followed by treatment with para-
toluene sulphonic acid (p-TsOH)/MeOH directly afforded the furanose 7 as a mixture of
anomers, which could not be separated by column chromatography. The required introduction of
the thymine moiety at the anomeric centre of 7 was achieved in presence of N,O-bis-
(trimethylsilyl)trifluoroacetamide (BSTFA) and trimethylsilyl trifluoromethanesulphonate
(TMSOTf) [32,33] to obtain 8 (67%) yield.
To improve the yield of the process for the required β-anomer of 8, we adopted a
different protocol. Thus, compound 6 was converted to the acetate 9 by reductive ozonolysis
followed by acetylation. In this case, the furanose acetate 9 was obtained in a slightly improved
(72%) yield, but as a mixture of chromatographically inseparable anomers. However,
introduction of the thymine moiety at the anomeric centre of 9 by the Vorbrüggen’s coupling
[34] proceeded with a much improved overall yield (84%) to furnish 8 as a mixture of β:α
anomers. Next, the NH group of the pyrimidine was protected with di-tert-butyldicarbonate
(Boc2O) and 4-dimethylaminopyridine (DMAP) to obtain compound 10. Unlike a previous
report [16], the Boc protection was achieved smoothly in only 2 h. Debenzoylation of 10 with
K2CO3/MeOH furnished the corresponding diol anomers 11a and 11b. Gratifyingly the α- and β-
anomers were separated at this stage by column chromatography and were obtained in 16:84
ratio (based on isolated yields of the individual isomers). In parallel experiments, we also used
the compound 8, obtained from the furanoside 7, and followed the same sequence of reaction as

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above to obtain 11a and 11b in a much less (3:7) diastereomeric ratio of the chromatographically
separable α:β anomers. The primary and secondary hydroxyl groups of 11b were sequentially
protected with DMTrCl and 4-nitrobenzenesulphonyl chloride (NsCl) respectively to obtain the
desired precursor 1a (Scheme 1.). The reaction with DMTrCl proceeded chemoselectively
without furnishing any secondary carbinol protected or diprotected products. Compound 1a was
characterized from its NMR spectra that matched very well with the reported values [16].

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OH
O
O
O
ii
iii
86%
i
RO
O
O
O
90%
83%
CHO
OBz
5 R = H
6 R = Bz
OBz
4
OH
3
2
iv
77%
O
H
O
N
O
N
OMe
O
v
vi
BzO
BzO
BzO
69%
67%
BzO
8
7
O
R
O
N
O
N
OAc
viii
x
81%
O
vii
72%
BzO
6
BzO
BzO
ix
84%
BzO
9
8 R = H
10 R = Boc
72%
O
O
N
Boc
Boc
O
N
N
HO
N
N
O
O
xi,xii
58%
N
O
O
+
HO
O
O
HO
DMTrO
Boc
from 11b
HO
NsO
1a
11a
11b
i) Ga/allyl bromide/[bmim][Br]/25 oC/4 h, ii) Benzoic acid/Ph3P/DIAD/THF/reflux/4 h, iii) Aqueous TFA/
CH2Cl2/0 oC/6 h, iv) BzCN/Et3N/CH2Cl2/0-25 oC/3 h, v) O3/Ph3P/CH2Cl2/-78-25 oC/18 h; p-TsOH/MeOH/25
oC/5 h, vi) Thymine/BSTFA/MeCN/25 oC/3 h; TMSOTf/-30-25 oC/7 d, vii) O3/Ph3P/CH2Cl2/-78-25 oC/18 h;
Ac2O/ pyridine/DMAP/CH2Cl2/0 oC/3 h, viii) Thymine/BSTFA/TMSOTf/MeCN/reflux/4 h, ix) Boc2O/DMAP/25
oC/2 h, x) K2CO3/MeOH/25 oC/5 h, xi) DMTrCl/pyridine/25 oC/12 h, xii) NsCl/pyridine/25 oC/22 h.
Scheme 1.
3. Conclusions
The primary aim of a target-driven synthesis is to accomplish the mission in minimum
steps with good yields. Hence, we adopted a practical strategy using several known, but efficient
reactions. To this end, we have been able to develop a 10-step efficient syntheses of the 18F-FLT

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precursor 1a in 8.5% overall yield, starting from the inexpensive aldehyde (R)-2, without using
expensive thymidine as the starting material. At this stage, we have not calculated the cost
benefit of the present synthesis vis-à-vis that of the reported methods. Compound 8 of our route,
is equivalent to the respective key intermediates of the previous syntheses of 1a or its equivalent
[14-16]. We synthesized compound 8 via operationally simple reactions, using inexpensive
chemicals/reagents. Thus, the cost difference between our synthesis vis-à-vis the thymidine-
based previous routs may be predominantly dictated by the relative prices of thymidine and
thymine. Presently, thymine is ~85-90% less expensive than thymidine. Thus, it is expected that
our synthesis will be competitive to the existing syntheses in terms of cost, while offering ease of
availability of the chemicals/reagents from the local market.
4. Experimental Section
4.1 General chemistry
The chemicals (Fluka and Lancaster) were used as received. Other reagents were of AR
grade. All anhydrous reactions were carried out under Ar atmosphere, using freshly dried
solvents. The organic extracts were dried over anhydrous Na2SO4. The IR spectra as thin films
1
13
were scanned with a Bruker tensor II spectrometer. H and C NMR spectra were recorded in
CDCl3 (unless otherwise mentioned) with a Bruker AC-200 instrument. or 500 MHz Varian
NMR spectrometer. Optical rotations were recorded on a JASCO DIP-360 digital polarimeter.
Elemental analyses were recorded on Elementar Vario micro cube.
4.2 (2R,3S)-1,2-Cyclohexylidenedioxy-5-hexen-3-ol (3)
A mixture of Ga metal (1.4 g, 20.0 mmol) and allyl bromide (2.1 mL, 24.8 mmol) in
[bmim][Br] (25 mL) was stirred at room temperature for 0.5 h, followed by addition of 2 (3.4 g,
20.0 mmol). The reaction mixture was stirred at room temperature till completion of the reaction

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(cf. TLC, 4 h), extracted with Et2O (3 × 15 mL), the ether extract was concentrated in vacuo and
the residue was purified by column chromatography (silica gel, 0–15% EtOAc/hexane) to obtain
24
25
3 (3.82 g, 90%). colorless oil; [α]D +10.5 (c 1.24, CHCl3) (lit. [18] [α]D +10.2 (c 1.41,
CHCl3)); IR: 3453, 1642 cm-1; 1H NMR (200 MHz, CDCl3): δ 1.39-1.43 (m, 2H), 1.57-1.82 (m,
8H), 2.10 (broad s, 1H), 2.14-2.38 (m, 2H), 3.73-3.81 (m, 1H), 3.89-4.02 (m, 3H), 5.09-5.17 (m,
13
2H), 5.72-5.93 (m, 1H); C NMR (50 MHz, CDCl3): δ 23.7, 23.9, 25.1, 34.7, 36.2, 37.6, 64.8,
70.5, 77.7, 109.6, 118.1, 134.0. Anal. Calcd. for C12H20O3: C, 67.89; H, 9.50%. C, 67.79; H
9.42%.
4.3 (4R,5R)-4-Benzoyloxy-5,6-cyclohexylidenedioxy-1-hexene (4)
o
To a cooled (0 C) and stirred solution of 3 (3.0 g, 14.15 mmol), PPh3 (5.56 g, 21.23
mmol), PhCO2H (2.07 g, 16.98 mmol) in THF (50 mL), DIAD (4.2 mL, 21.23 mmol) was added
dropwise. The mixture was brought to room temperature and subsequently refluxed for 4 h, and
concentrated in vacuo. The residue was diluted with EtOAc (50 mL), the organic extract was
successively washed with H2O (2 × 20 mL) and brine (1 × 5 mL), dried and concentrated in
vacuo. The residue was purified by column chromatography (silica gel, 0-5% EtOAc/hexane) to
26
afford pure 4 (3.71 g, 83%). colorless oil; [α]D +15.2 (c 1.06, CHCl3); IR: 1722, 1641, 1463
1
cm-1; H NMR (500 MHz, CDCl3): δ 1.37-1.40 (m, 2H), 1.54-1.72 (m, 8H), 2.48-2.57 (m, 2H),
3.80 (dd, J = 8.5, 6.5 Hz, 1H), 4.04 (dd, J = 8.5, 6.5 Hz, 1H), 4.31-4.34 (m, 1H), 5.06-5.08 (m,
1H), 5.12-5.16 (m, 1H), 5.22-5.26 (m, 1H), 5.79-5.86 (m, 1H), 7.42-7.45 (m, 2H), 7.54-7.57 (m,
1H), 8.06 (d, J = 8.0 Hz, 2H); 13C NMR (125 MHz, CDCl3): δ 23.8, 23.9, 25.1, 34.9, 35.4, 35.9,
65.2, 73.0, 75.7, 110.1, 118.2, 128.3, 129.7, 130.2, 132.9, 133.1, 166.1. Anal.Calcd. for
C19H24O4: C, 72.13; H, 7.65%. Found: C, 72.11; H, 7.90%.
4.4 (2R,3R)-3-Benzoyloxyhex-5-ene-1,2-diol (5)

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A mixture of 4 (3.25 g, 10.28 mmol) and aqueous 80% TFA (5 mL) in CH2Cl2 (15 mL)
o
was stirred at 0 C till completion of the reaction (cf. TLC, ~6 h). The reaction mixture was
neutralized with aqueous saturated NaHCO3, extracted with CHCl3 (3 × 20 mL), the organic
extract was successively washed with H2O (3 × 10 mL) and brine (1 × 5 mL), and dried. Solvent
removal followed by column chromatography (silica gel, 0-5% MeOH/CHCl3) of the residue
gave pure 5 (2.09 g, 86%). colorless thick oil; [α]D24 + 8.0 (c 1.04, CHCl3); IR: 3423, 1718, 1642
cm-1; 1H NMR (200 MHz, CDCl3): δ 2.47 (broad s, 2H), 2.57 (t, J = 7.0 Hz, 2H), 3.63 (d, J = 5.6
Hz, 2H), 3.87-4.11 (m, 1H), 5.05-5.14 (m, 2H), 5.19-5.30 (m, 1H), 5.71-5.88 (m, 1H), 7.39-7.46
13
(m, 2H), 7.53-7.60 (m, 1H), 8.00-8.09 (m, 2H); C NMR (50 MHz, CDCl3): δ 34.9, 63.2, 72.5,
73.8, 118.1, 128.2, 129.5, 129.6, 133.0, 166.5. Anal. Calcd. for C13H16O4: C, 66.09; H, 6.83%.
Found: C, 66.49; H, 6.81%.
4.5 (2R,3R)-1,3-Dibenzoyloxyhex-5-en-2-ol (6)
To a cooled (0 oC) and stirred solution of 5 (2.05 g, 8.69 mmol) and Et3N (1.33 mL, 9.56
mmol) in CH2Cl2 (30 mL) was added BzCN (1.19 g, 9.12 mmol). The mixture was brought to
room temperature and stirred till consumption of the starting material (cf. TLC, ~3 h). The
reaction mixture was treated with ice-cold H2O (30 mL). The organic layer was separated, the
aqueous portion extracted with CHCl3 (2 × 25 mL), the combined organic extracts washed with
H2O (2 × 10 mL) and brine (1 × 5 mL), and dried. Solvent removal followed by column
chromatography (silica gel, 0-10% EtOAc/hexane) of the residue furnished 6 (2.28 g, 77%).
1
colorless oil; IR: 3455, 1725, 1642, 1463 cm-1; [α]D26 -1.6 (c 0.8, CHCl3); H NMR (200 MHz,
CDCl3): δ 2.03 (s, 1H), 2.59-2.68 (m, 2H), 4.09-4.23 (m, 1H), 4.44-4.47 (m, 2H), 5.08-5.23 (m,
2H), 5.31-5.39 (m, 1H), 5.78-5.86 (m, 1H), 7.35-7.57 (m, 6H), 7.96-8.09 (m, 4H); 13C NMR (50

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MHz, CDCl3): δ 34.8, 65.9, 70.0, 73.7, 118.5, 128.2, 129.4, 129.5, 129.6, 129.7, 132.9, 133.0,
166.0, 166.6. Anal. Calcd. for C20H20O5: C, 70.57; H, 5.92%. Found : C, 70.56; H, 5.90%.
4.6 Methyl 3,5-O-dibenzoyl-2-deoxy-α,β-D-lyxofuranoside (7)
O3 was bubbled through a cooled (-78 oC) and stirred solution of 6 (2.2 g, 6.47 mmol) in
CH2Cl2 (100 mL) until a blue color persisted. After 15 min, excess O3 was removed by flushing
with N2, PPh3 (2.03 g, 7.73 mmol) was added into the mixture and stirring continued for 18 h at
o
25 C. The mixture was concentrated in vacuo, the residue taken in hexane (30 mL) and
concentrated to obtain a residue that was purified by column chromatography (silica gel, 0-5%
MeOH/CHCl3) to obtain the intermediate furanose (1.79 g, 81%).
A solution of the above furanose (1.76 g, 5.15 mmol) and p-TsOH (89 mg, 0.515 mmol)
o
in MeOH (50 mL) was stirred at 25 C for 5 h. The mixture was neutralized with aqueous 10%
NaHCO3, concentrated in vacuo, and the residue was purified by column chromatography (silica
gel, 0-25% EtOAc/hexane) to afford 7 (1.56 g, 85%). colorless oil; IR: 1792, 1722, 1150 cm-1.
1
[α]D24+ 11.7 (c 1.03, CHCl3); H NMR (200 MHz, CDCl3): δ 2.29-2.54 (m, 2H), 3.41 (s, 3H),
4.54-4.64 (m, 3H), 5.24-5.28 (m, 1H), 5.72-5.79 (m, 1H), 7.37-7.58 (m, 6H), 8.01 (d, J = 8.2 Hz,
4H); 13C NMR (50 MHz, CDCl3): δ 39.5, 55.1, 64.1, 72.6, 74.8, 78.6, 104.6, 128.2, 128.3, 129.6,
129.7, 132.9, 133.1, 165.8, 166.2. Anal.Calcd. for C20H20O6: C, 67.41; H, 5.66%. Found : C,
67.72; H, 5.92%.
4.7 1-[3,5-O-Dibenzoyl-2-Deoxy-α,β-D-lyxofuranosyl]-thymine (8)
To a stirred suspension of 7 (1.51 g, 4.24 mmol) and thymine (1.60 g, 12.72 mmol) in
anhydrous CH3CN (30 mL) under Ar, BSTFA (5.2 mL, 21.20 mmol) was added dropwise at 25
o
oC. The mixture was stirred for 3 h, cooled to -30 C and treated with TMSOTf (3.1 mL, 17.0
mmol). After stirring for 7 d at 25 oC, the mixture was diluted with CH2Cl2 (30 mL). The organic

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layer was successively washed with aqueous saturated NaHCO3 (1 × 10 mL), H2O (2 × 20 mL)
and brine (1 × 5 mL), and dried. After removal of solvent in vacuo, the residue was purified by
column chromatography (silica gel, 0-5% MeOH/CHCl3) to obtain 8 (1.28 g, 67%) as an
22
inseparable mixture of α- and β-anomers. colorless oil; [α]D +34.9 (c 0.66, CHCl3); IR: 3584,
1
1716, 1695 cm-1; H NMR (200 MHz, CDCl3): δ 1.82 and 1.95 (two s, 3H), 2.28-2.38, 2.58-
2.82, 2.88-3.02 (three m, 2H), 4.50-4.61, 4.62-4.68, 4.70-4.85 and 4.88-4.95 (four m, 3H), 5.77-
5.94 (two m, 1H), 6.25-6.36 (m, 1H), 7.12 and 7.26 (two s, 1H), 7.38-7.65 (two m, 6H), 7.95-
13
8.05 (m, 4H), 8.54 (broad s, 1H); C NMR (50 MHz, CDCl3): δ 12.4, 12.5, 38.9, 39.6, 61.9,
62.7, 72.6, 73.9, 80.2, 80.5, 84.4, 87.3, 110.9, 111.3, 128.4, 128.5, 128.7, 128.8, 129.0, 129.2,
129.4, 129.6, 129.7, 133.2, 133.3, 133.6, 133.9, 135.1, 135.8, 150.4, 150.5, 163.9, 164.0, 165.1,
165.4, 166.1. Anal. Calcd. for C24H22N2O7: C, 63.99; H, 4.92; N, 6.22%. Found : C, 63.84; H,
4.59; N, 6.56%.
4.8 3,5-O-Dibenzoyl-2-deoxy-α,β-D-lyxofuranosyl-1-acetate (9)
O3 was bubbled through a cooled (-78 oC) and stirred solution of 6 (1.5 g, 4.41 mmol) in
CH2Cl2 (100 mL) until a blue color persisted. After 15 min, excess O3 was removed by flushing
with N2, PPh3 (1.39 g, 5.29 mmol) was added into the mixture and stirring continued for 18 h at
o
25 C. The mixture was concentrated in vacuo, the residue taken in hexane (30 mL) and
concentrated to obtain a residue that was purified by column chromatography (silica gel, 0-5%
MeOH/CHCl3) to obtain the intermediate furanose (1.24 g, 82%) as a colorless oil.
o
To a cooled (0 C) and stirred solution of the above furanose (1.02 g, 2.98 mmol),
pyridine (0.3 mL, 3.58 mmol) and DMAP (catalytic) in CH2Cl2 (20 mL), Ac2O (0.3 mL, 3.58
mmol) was added. After stirring for 3 h, the mixture was neutralized with aqueous 2N HCl,
diluted with CHCl3 (20 mL), washed with H2O (3 × 20 mL) and brine (1 × 10 mL), and dried.

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Solvent removal followed by column chromatography (silica gel, 0-20% EtOAc/hexane) of the
28
residue gave 9 (1.0 g, 88%) as an anomeric mixture. white solid; [α]D + 4.7 (c 1.37, CHCl3);
1
IR: 1728, 1715 cm-1; H NMR (500 MHz, CDCl3): δ 2.05 and 2.09 (two s, 3H), 2.40-2.43 and
2.51-2.62 (two m, 2H), 4.59-4.67 (m, 2H), 4.69-4.72 (m, 1H), 5.81-5.86 and 5.91-5.93 (two m,
1H), 6.43-6.44 and 6.53-6.55 (two m, 1H), 7.39-7.47 (m, 4H), 7.53-7.58 (m, 2H), 7.98-8.05 (m,
13
4H); C NMR (125 MHz, CDCl3): δ 21.2, 21.3, 39.3, 39.7, 62.4, 62.8, 72.1, 73.1, 78.5, 97.2,
97.6, 128.3, 128.5, 129.7, 129.9, 133.1, 133.5, 165.7, 166.1, 170.1, 170.2. Anal. Calcd. for
C21H20O7: C, 65.62; H, 5.24%. Found: C, 65.38; H, 5.44%.
4.9 1-[3,5-O-Dibenzoyl-2-Deoxy-α,β-D-lyxofuranosyl]-thymine (8) from compound 9
To a stirred solution of 9 (800 mg, 2.08 mmol) and thymine (394 mg, 3.12 mmol) in
MeCN (20 mL), BSTFA (2.0 mL, 8.20 mmol) was added dropwise. The reaction mixture was
o
refluxed for 1 h, and cooled to 0 C, treated with TMSOTf (0.6 mL, 3.33 mmol) and
subsequently refluxed for 4 h. The reaction was quenched with cold aqueous saturated NaHCO3
(10 mL) and extracted with CHCl3 (2 × 10 mL). The combined organic extracts were washed
with aqueous saturated NaHCO3 (1 × 5 mL) and brine (1 × 5 mL) and dried. The solvent was
removed in vacuo and the residue was subjected to column chromatography (silica gel, 0-5%
25
1
MeOH/CHCl3) to obtain 8 (788 mg, 84%). colorless oil; [α]D +14.2 (c 1.05, CHCl3); The H
and 13C NMR spectra of the sample was similar as that of the sample obtained from 7.
4.10 3-N-Boc-1-[3,5-O-dibenzoyl-2-deoxy-α,β-D-lyxofuranosyl]thymine (10)
To a solution of 8 (1.23 g, 2.73 mmol) in THF (50 mL) was added DMAP (0.670 g, 5.46
mmol) followed by Boc2O (1.19 g, 5.46 mmol). The mixture was stirred for 2 h at 25 oC, diluted
with EtOAc (20 mL), the organic extract washed with H2O (2 × 20 mL) and brine (1 × 5 mL),
and dried. Solvent removal followed by column chromatography (silica gel, 0-35%

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EtOAc/hexane) of the residue gave pure 10 as an anomeric mixture (1.08 g, 72%). colorless oil;
25
[α]D +27.9 (c 1.06, CHCl3). IR: 1782, 1720, 1667, 1601 cm-1; 1H NMR (500 MHz, CDCl3): δ
1.60 and 1.61 (two s, 9H), 1.83 and 1.96 (two s, 3H), 2.34-2.38, 2.63-2.69, 2.75-2.80 and 2.92-
2.97 (four m, 2H), 4.53-4.56, 4.58-4.67, 4.72-4.80 and 4.90-4.93 (four m, 3H), 5.79-5.81 and
5.90-5.92 (two m, 1H), 6.25-6.27 and 6.30-6.32 (two m, 1H), 7.14 and 7.27 (two s, 1H), 7.39-
13
7.47 (m, 4H), 7.55-7.63 (m, 2H), 7.95-8.02 (m, 4H); C NMR (125 MHz, CDCl3): δ 12.5, 12.6,
27.4, 39.1, 39.7, 61.9, 62.6, 72.5, 73.7, 80.3, 80.7, 84.7, 86.8, 86.9, 87.7, 110.5, 111.0, 128.4,
128.5, 128.6, 128.7, 128.9, 129.0, 129.4, 129.5, 129.7, 133.3, 133.4, 133.7, 133.9, 134.4, 135.0,
147.7, 148.3, 148.4, 161.1, 161.2, 165.1, 165.4, 166.1. Anal. Calcd. for C29H30N2O9: C, 63.27; H,
5.49; N, 5.09%. Found : C, 63.04; H, 5.45; N, 5.36%.
4.11 3-N-Boc-1-[2-deoxy-β-D-lyxofuranosyl]-thymine (11a and 11b)
A mixture of 10 (1.02 g, 1.85 mmol) and K2CO3 (639 mg, 4.63 mmol) in MeOH (25 mL)
was stirred for 5 h. It was concentrated in vacuo, the residue diluted with H2O (10 mL), and the
aqueous layer extracted with EtOAc (3 × 10 mL). The combined organic extracts were washed
with brine (1 × 5 mL), dried and concentrated in vacuo to obtain the crude product, which was
purified by column chromatography (silica gel, 0-5% MeOH/CHCl3) to obtain 11a and 11b (514
mg, 81%, 16:84).
11a: colorless oil; [α]D24 -11.0 (c 0.49, MeOH); IR: 3456 cm-1; 1H NMR (200 MHz, CD3OD): δ
1.19 (s, 3H), 1.48 (s, 6H), 1.82 (d, J = 6.2 Hz, 3H), 2.12-2.42 (m, 2H), 3.71 (d, J = 5.4 Hz, 2H),
13
4.22-4.29 (m, 1H), 4.39-4.41 (m, 1H), 6.10-6.20 (m, 1H), 7.41 and 7.46 (two s, 1H) ; C NMR
(50 MHz, CD3OD): δ 12.6, 27.8, 30.8, 42.6, 62.1, 72.7, 86.5, 88.0, 88.7, 111.3, 138.1, 149.5,
150.0, 163.4. Anal. Calcd. For C15H22N2O7: C, 52.63; H, 6.48; N, 8.18%. Found : C, 52.61; H,
6.47; N, 8.20%.

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23
11b: colorless oil; [α]D +2.9 (c 0.51, MeOH); IR: 3455 cm-1; 1H NMR (200 MHz, CD3OD): δ
1.20 (s, 3H), 1.49 (s, 6H), 1.81 (s, 3H), 1.93-2.07 (m, 1H), 2.49-2.63 (m, 1H), 3.81-3.90 (m, 3H),
13
4.28-4.32 (m, 1H), 6.03-6.07 (m, 1H), 7.81-7.90 (m, 1H); C NMR (50 MHz, CD3OD): δ 12.7,
27.8, 30.8, 42.5, 61.7, 70.9, 86.6, 86.9, 87.9, 110.5, 139.1, 149.5, 150.2, 163.4. Anal. Calcd. for
C15H22N2O7: C, 52.63; H, 6.48; N, 8.18%. Found : C, 52.96; H, 6.10; N 8.54%.
4.12 3-N-Boc-1-[3-O-Nosyl-5-O-(4,4ˊ-dimethoxytrityl)-2-deoxy-β-D-lyxofuranosyl]thymine (1a)
To a stirred solution of 11b (400 mg, 1.17 mmol) in dry pyridine (7 mL) under Ar was
o
added DMTrCl (474 mg, 1.40 mmol). After stirring for 12 h at 25 C, the reaction mixture was
poured into ice-cold water (100 mL), stirred for 0.5 h, and extracted with EtOAc (3 × 30 mL).
The combined organic extracts were washed with brine (1 × 10 mL), dried and concentrated in
vacuo to obtain the crude product, which was purified by column chromatography (silica gel, 0-
35% EtOAc/hexane) to obtain the corresponding 5ˊ-O-DMTr product (610 mg, 81%) as a
colorless syrup.
To a cooled (0 oC) and stirred solution of the above 5ˊ-O-DMTr compound (410 mg, 0.64
mmol) in pyridine (7 mL) under Ar was added NsCl (284 mg, 1.28 mmol). After stirring for 1 h,
the reaction mixture was brought to room temperature and stirred further for 21 h. It was diluted
with EtOAc (20 mL), filtered, the filtrate washed with ice-cold H2O (2 × 20 mL) and brine (1 × 5
mL), dried, and concentrated in vacuo. The residue was purified by column chromatography
o
(silica gel, 0-25% EtOAc/hexane) to obtain pure 1a (386 mg, 72%). yellow solid; m.p.: 105 C
1
(lit. [16] mp: 106 °C); H NMR (500 MHz, CD3CN) δ 1.58 (s, 9H), 1.68 (s, 3H), 2.36 (dd, J =
16.0, 1.6 Hz, 1H), 2.80 (ddd, J =16.0, 7.6, 5.1 Hz, 1H), 3.16 (dd, J = 10.5, 4.5 Hz, 1H), 3.40 (dd,
J = 10.5, 7.0 Hz, 1H), 3.80 (s, 6H), 4.31-4.33 (m, 1H), 5.30 (t, J = 4.0 Hz, 1H), 6.11 (dd, J =8.0,
2.5 Hz, 1H), 6.89 (d, J = 8.0 Hz, 4H), 7.20 (d, J = 1.0 Hz, 1H), 7.24-7.38 (m, 7H), 7.41-7.44 (m,

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13
2H), 7.95 (d, J = 9.0 Hz, 2H), 8.29 (d, J = 9.0 Hz, 2H); C NMR (125 MHz, CD3CN) δ 12.8,
27.7, 40.0, 56.0, 62.7, 82.2, 82.9, 86.0, 87.4, 87.6, 110.5, 114.2, 118.3, 125.9, 128.0, 129.0,
130.1, 131.0, 136.0, 136.6, 142.3, 145.8, 149.3, 149.4, 152.1, 159.9, 162.1. Anal. Calcd for
C42H43N3O13S: C, 60.79; H, 5.22; N, 5.06, S, 3.86%. Found: C, 61.02; H, 5.50; N, 5.23, S,
3.53%.

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Graphical abstract
A chiral template-driven synthesis of a 3ˈ-deoxy-3ˈ-18F-fluorothymidine precursor