Pyrrolidine N-alkylphosphonates and related nucleotide analogues: synthesis and stereochemistry

Article abstract of DOI:10.1016/j.tet.2009.02.071

N-Phosphonoalkyl-trans-3,4-dihydroxypyrrolidine derivatives were synthesized and exploited as synthons for the preparation of hydroxypyrrolidine nucleoside phosphonic acids, the 3′-deoxynucleoside 5′-phosphate analogues. Simultaneously, an alternative route, the N-phosphoalkylation of the preformed pyrrolidine nucleosides employing Mannich- and Michael-type reactions, was investigated to obtain desired nucleotide analogues. In contrast to the latter approach, the former resulted in the formation of two diastereoisomers very likely due to the existence of two possible S_N2 transition states during a nucleophilic displacement. The stereochemistry of the prepared nucleotide analogues was studied by NMR spectroscopy.

Full text of DOI:10.1016/j.tet.2009.02.071

Tetrahedron 65 (2009) 3673–3681
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Pyrrolidine N-alkylphosphonates and related nucleotide analogues:
synthesis and stereochemistry
*
*
ˇ
´
´
Dominik Rejman , Radek Pohl, Petr Kocalka, Milena Masojıdkova, Ivan Rosenberg
´
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, 166 10 Prague 6, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
N-Phosphonoalkyl-trans-3,4-dihydroxypyrrolidine derivatives were synthesized and exploited as syn-
thons for the preparation of hydroxypyrrolidine nucleoside phosphonic acids, the 30-deoxynucleoside 50-
phosphate analogues. Simultaneously, an alternative route, the N-phosphoalkylation of the preformed
pyrrolidine nucleosides employing Mannich- and Michael-type reactions, was investigated to obtain
desired nucleotide analogues. In contrast to the latter approach, the former resulted in the formation of
two diastereoisomers very likely due to the existence of two possible SN2 transition states during
a nucleophilic displacement. The stereochemistry of the prepared nucleotide analogues was studied by
NMR spectroscopy.
Received 4 November 2008
Received in revised form 5 February 2009
Accepted 26 February 2009
Available online 5 March 2009
Keywords:
Tartaric acid
Pyrrolidine derivatives
Nucleoside analogues
Nucleotide analogues
Phosphonate
Ó 2009 Elsevier Ltd. All rights reserved.
Nucleobase
Mannich
Michael
1. Introduction
phosphate moiety in nucleotides seems to be the most successful
approach in contributing to the pool of potential antivirals. For
Over the past few decades, enormous effort has been devoted to
the synthesis and biochemical and biological evaluation of ana-
logues of natural nucleosides and nucleotides due to their useful-
instance, several types of structurally interesting aza-sugar nucle-
oside phosphonates, such as the pyrrolidine 4,5,6 isoxazolidine 57
and 6,8 and aziridine 79 ring-containing compounds have been
reported but none of them, except for analogues 6 (n¼1), which
exerted significant inhibition of reverse transcriptase comparable
to AZT in efficiency as well as low cytotoxicity, showed any re-
markable antiviral or antimicrobial activities.
Also nitrogen-containing heterocycles bearing phosphonate
moiety were reported as biologically active species. Thus, a tetra-
zole phosphonic acid10 was shown to inhibit multiplication of HSV
II, and imidazole, benzimidazole, and benzotriazole phosphonic
acids have been evaluated as plant growth factors.11,12 Besides these
compounds, many others, such as the triazole,13 aziridine,14 azeti-
dine,15 pyrazole,16 and pyrrolidine17–23 phosphonates were repor-
ted and biologically evaluated. We have recently described
prolinol-based nucleoside N-methylphosphonates24 as a new class
of nucleotide analogues with a conformationally flexible N-methyl-
phosphonate moiety. In this paper, we report on similar, flexible-
type, 3-hydroxypyrrolidine-based nucleoside N-alkylphosphonates
10–13 (Fig. 2) mimicking 30-deoxynucleoside 50-phosphates. For
the synthesis of these compounds, we considered two routes A and
B (Scheme 1) employing preformed pyrrolidine phosphonate syn-
thons and the recently published25 pyrrolidine nucleosides 8 and 9,
respectively.
ness as tools in investigations aimed at
a more thorough
understanding of metabolic processes. In many cases, the form of
a triphosphate is the one that effectively starts the active role of
such antimetabolites in the in vivo processes affecting biosynthesis
of DNA and RNA. Based on this knowledge, alternatives to the
natural phosphates were sought, and the enzymatically and
chemically stable phosphonate moiety was introduced1 to improve
the in vivo stability of parent compounds and hence their biological
properties.
Systematic biological investigation of the structurally diverse
nucleoside phosphonic acids has led to potent antiviral drugs based
on both the acyclic phosphonate nucleotides 12,3 and the cyclic
structures 2 and 3 (Fig. 1).4 Specifically, acyclic compounds 1 were
found to inhibit the replication of DNA viruses and retroviruses,
whereas cyclic compounds 2 and 3 exhibited favorable antiviral
profiles against HIV strains. The modification of the sugar-
* Corresponding authors. Tel.: þ420 220 183 381.
E-mail addresses: rejman@uochb.cas.cz (D. Rejman), ivan@uochb.cas.cz
(I. Rosenberg).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.02.071

3674
D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
O
HO P
OH
O
O
O
HO P
OH
B
N
B
B
O
B
X
O
HO P
X
HO P
O
O
OH
OH
R
1
2
3
4
B= A, Hx, C, T, U
B= A, C
R= H, CH3,CH2OH, CH2F
B= A, T
X= O, CH2
B= A, T
X= O, CH2
B
B
O
EtO
P
O
P
HN
HN
B
T
HO
HO
O
O
EtO
B
N
n
N
EtO P
OH
OH
OEt
N
O
H3C
9a B= A
9b B= T
8a B= A
8b B= T
6
7
5
H (CH2OH)
B=C,T,A,G,U5-subst
B=C,T,A,G
Figure 1.
chloride at reflux according to the described procedure.26 Cyclic
anhydride 15 was then opened with diisopropyl amino-
methylphosphonate27 (23a) or 2-aminoethylphosphonate28 (23b),
and the intermediates 16a and 16b were cyclized by heating with
acetic anhydride. The formed O-acetylated intermediates were
deacetylated to 17a and 17b with 1 M HCl in methanol (Scheme 2).
Reduction of these compounds with borane, generated in situ from
sodium borohydride and iodine in THF,29,30 afforded pyrrolidine
derivatives 18a and 18b after treatment of the reaction mixture
with hydrochloric acid to decompose the observed pyrrolidine–BH3
complex.
Compounds 18a and 18b were dimethoxytritylated in pyridine
and subsequently mesylated in DCM in the presence of DMAP. The
obtained mesyl derivatives 20a and 20b were subjected to the
nucleosidation reaction with adenine providing a mixture of two
diastereoisomers (21a/21c, trans/cis w1:1; 21b/21d, trans/cis
w2:1; determined by 1H NMR spectroscopy) in a moderate yield.
Removal of the dimethoxytrityl protecting groups gave the phos-
phono diesters 22a/24a and 22b/26 as diastereomeric mixtures.
The epimerization at the C3 atom in 20a and 20b during nu-
cleophilic displacement of the mesyl group could be explained by
the participation of the pyrrolidine nitrogen free electron pair. Two
hypothetical chiral SN2 transition state intermediates A and B
(Fig. 3) can be attacked on the C3 atom by adenine either from the
cis or trans position with respect to the dimethoxytrityloxy sub-
stituent, providing a mixture of diastereoisomers 21a/21c and 21b/
A
A
N
B
B
O
HO P
HO
O
HO P
HO
O
HO P
O
HO P
N
N
N
OH
OH
OH
OH
OH
OH
10a B= A
10b B= T
11a B= A
12
13
11b B= T
Figure 2.
O
O
B
B
OH
RO
RO
Y
RO
RO
Y
route B
route A
P
N
P
N
HN
8,9
18
OH
10-13,22 OH
OH
Y = -CH2-, -(CH2)2-
Scheme 1.
2. Results and discussion
2.1. Chemistry
The synthetic procedure started with L-tartaric acid (14), which
was transformed into anhydride 15 on treatment with acetyl
O
nP
O
nP
O
nP
NH
OiPr
OiPr
OiPr
OiPr
COOH
OH
O
O
O
OiPr
O
ii
i
OiPr
OH
iii, iv
v
N
N
O
O
HO
O
AcO
OAc
COOH
O
n
OiPr
HO
OH
HO
OH
AcO
OAc
P
OiPr
23a n= 1
NH2
17a n= 1
17b n= 2
16a n= 1
16b n= 2
18a n= 1; 44 % over 3 steps
18b n= 2; 28 % over 3 steps
15
14
23b n= 2
vi
O
nP
O
nP
O
nP
O
nP
O
O
nP
OiPr
OiPr
OiPr
OiPr
OiPr
OiPr
OiPr
OiPr
P
n
OiPr
OiPr
+
OiPr
+
OiPr
N
N
N
N
N
ix
vii
N
viii
DMTrO
DMTrO
OH
DMTrO
A
HO
A
DMTrO
A
OMs
HO
A
20a n= 1; 33 % over 2 steps
24a n= 1; 8 %
21c n= 1
21d n= 2
19a n= 1
22a n= 1; 8 %
21a n= 1
21b n= 2
26 n= 2; 8 %
20b n= 2; 64 % over 2 steps
19b n= 2
22b n= 2; 16 %
yields over 2 steps
Scheme 2. Synthesis of pyrrolidine N-alkylphosphonate derivatives. (i) AcCl, reflux; (ii) 23a or 23b, DCM; (iii) Ac2O, 90 ꢀC; (iv) 1 M HCl in MeOH; (v) NaBH4/I2/THF, aq HCl; (vi)
DMTrCl/pyridine; (vii) MsCl/DMAP/DCM; (viii) Adenine/Cs2CO3/DMF; (ix) 80% aq AcOH.

D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
3675
deuterated quaternary pyrrolidine nitrogen, at the high pD it is
a salt of phosphonic acid with fast flip-flop movement of the
phosphonoalkyl substituent. A special situation occurs at pD 6–8
when phosphonates 10–13 exist as zwitterions and the phospho-
noalkyl group occupies an exo (trans) arrangement relative to the
nucleobase as follows from stereospecific assignment of protons
using H,H-ROESY (Fig. 5).
A
B
Adenine
4
3
O
P
OiPr
3 +
OR
+
O
P
OiPr
Adenine
iPrO
N
OR
N
1-2
iPrO
4
1-2
Figure 3. Possible explanation of C3 epimerization during nucleophilic displacement.
Nothing is known about the conformation of the pyrrolidine
ring in the series of novel nucleotide analogues 10–13. The
conformation analysis using vicinal 3J(H,H) coupling constants of
pyrrolidine ring protons confronted with a molecular modeling
approach is underway and will be published elsewhere.
21d. Similar epimerization was observed during nucleosidation of
1-N-benzyl-3-dimethoxytrityloxy-4-mesyloxyyrrolidines.
The observed epimerization provides the possibility to obtain
two separable diastereoisomers in one reaction but we did not
attempt it in this case.
3. Conclusion
Two synthetic routes for the preparation of pyrrolidine
nucleoside phosphonates exploiting either nucleosidation of an
N-phosphonoalkylpyrrolidine synthon with a nucleobase, or
N-phosphonoalkylation of preformed pyrrolidine nucleoside have
been evaluated. In contrast to the latter approach, the former pro-
vided a mixture of separable epimers. This experimental arrange-
ment seems to be advantageous in cases when two
diastereoisomeric pyrrolidine nucleoside N-alkylphosphonates are
required, starting from one diastereoisomerically pure N-phospho-
noalkylpyrrolidine derivative. All synthetic steps in the preparation
of N-phosphono-alkylpyrrolidine derivatives are straightforward
giving good yields of the desired compounds. We can conclude that
the described synthetic strategies provide, in several simple steps,
new pyrrolidine N-alkylphosphonates as well as the appropriate
nucleotide analogues whose antiviral, anticancer, and antimicrobial
properties are currently underway. NMR-based stereochemical
studies of pyrrolidine nucleotide analogues revealed an exo (trans)
configuration of the N-phosphonoalkyl moiety regarding to the
pyrrolidine-nucleobase residue. Acquired knowledge will be eval-
uated at the synthesis of modified oligonucleotides containing
pyrrolidine nucleotide units in selected positions.
In order to obtain the desired phosphonates 10–13 in diaster-
eomerically pure forms, we performed N-phosphonylation re-
actions at the level of pyrrolidine nucleosides 8a,b and 9a,b. These
compounds easily underwent the Mannich-type reaction with
formaldehyde and diisopropyl phosphite31 to form phosphonates
24a,b and 25a,b (Schemes 3 and 4).
Pyrrolidine nucleosides 8a and 9a were subjected to the Michael
addition on diisopropyl vinylphosphonate32 giving a good yield of
diisopropyl phosphonates 26 and 27. The phosphoester groups of
24a,b and 25a,b were removed by treatment with bromo-
trimethylsilane in acetonitrile to yield the desired phosphonic acids
10a, 10b, 11a, 11b, 12, and 13.
2.2. NMR analysis
1H, 13C, and 31P NMR spectra of target nucleotide analogues 10–
13 were measured in D2O and all resonances were unambiguously
assigned based on H,H-COSY, H,H-ROESY, H,C-HSQC, and H,C-
HMBC experiments.
We have measured NMR spectra of 10–13 at different pD (using
D2O solutions of DCl and NaOD) and found them to be pD de-
pendent with similar behavior as proline. An example of 1H NMR
pD dependence for T-derivative is shown in Figure 4. Whereas at
low pD the compound 11b exists as a free phosphonic acid with
4. Experimental
4.1. General
Unless otherwise stated, all solvents used were anhydrous.
Final products were lyophilized from water, and dried over
phosphorus pentoxide at rt and 13 Pa. TLC was performed on
silica gel pre-coated aluminum plates UV 254 (Fluka), and the
compounds were detected by UV light (254 nm), by heating
(detection of dimethoxytrityl group; orange color), by spraying
with a 1% solution of ninhydrin (to visualize amines), and by
spraying with a 1% solution of 4-(4-nitrobenzyl)pyridine in eth-
anol followed by heating and treating with gaseous ammonia (to
expose the blue color of mono- and diesters of phosphonic acids).
Preparative column chromatography was carried out on silica gel
O
P
O
P
OiPr
OiPr
OiPr
OiPr
H
N
ii
i
iii
N
iii
N
10a; 62 %
10b; 38 %
12; 43 %
HO
B
HO
B
HO
B
24a B= A; 87 %
26 B= A; 69 %
8a B= A
8b B= T
24b B= T; 65 %
Scheme 3. Synthesis of cis-nucleotide analogues from pyrrolidine nucleosides. (i)
HP(O)(OiPr)2, formaldehyde; (ii) diisopropyl vinylphosphonate, CH3OH; (iii) Me3SiBr,
DMF (MeCN).
(40–60
mm; Fluka) neutralized with triethylamine (1 ml/100 g),
and the elution was performed at the flow rate of 40 ml/min. The
solvent systems used for TLC and preparative chromatography
were (v/v) toluene–ethyl acetate 1:1 (T) and chloroform–ethanol
9:1 (C1). Mass spectra were recorded on a ZAB-EQ (VG Analytical)
instrument, using FAB (ionization with Xe, accelerating voltage
8 kV, glycerol and thioglycerol were used as matrices) or on LTQ
Orbitrap XL (Thermo Fisher Scientific) instrument for ESI. NMR
spectra were measured on a Bruker Avance 600 (1H at 600.1 MHz,
13C at 150.9 MHz), Bruker Avance 500, Varian Unity 500 (1H at
500.0 MHz, 13C at 125.7 MHz, 31P at 202.3 MHz), and Bruker
Avance 400 (1H at 400.0 MHz, 13C at 100.6 MHz, 31P at 162.0 MHz)
spectrometers in CDCl3 (referenced to the TMS signal and to the
solvent signald77.0 ppm for 13C), DMSO-d6 (referenced to the
O
P
O
P
OiPr
OiPr
OiPr
OiPr
H
N
N
N
iii
ii
i
iii
11a; 58 %
11b; 67 %
13; 52 %
HO
B
HO
B
HO
B
9a B= A
9b B= T
25a B= A; 78 %
25b B= T; 54 %
27 B= A; 65 %
Scheme 4. Synthesis of trans-nucleotide analogues from pyrrolidine nucleosides. (i)
HP(O)(OiPr)2, formaldehyde; (ii) diisopropyl vinylphosphonate, CH3OH; (iii) Me3SiBr,
DMF (MeCN).

3676
D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
O
N
O
O
N
ND
ND
ND
O
N
O
O
- D+
+ D+
- D+
+ D+
D
N
D
N
OD
P
O
P
O
P
DO
O
DO
O
O
O
N
OD
OD
OD
pD = 12.2
pD = 10.5
pD = 8.3
pD = 6.5
pD = 4.1
4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 ppm
Figure 4. 1H NMR spectra of 11b measured at different pD values.
solvent signald2.5 ppm for 1H and 39.7 ppm for 13C), or D2O
(referenced to the 1,4-dioxane signal as an internal stand-
ardd3.75 ppm for 1H and 69.3 ppm for 13C) as indicated. 31P{1H}
NMR spectra were referenced to the signal of H3PO4 as an internal
standard in 2 mm coaxial capillary tube.
4.1.1. (3S,4R)-(4-(Adenin-9-yl)-3-hydroxypyrrolidin-1-
yl)methylphosphonic acid (10a)
TMSBr (0.88 ml, 6.7 mmol) was added to a solution of 24a
(0.53 g, 1.3 mmol) in DMF (15 ml), and the mixture was stirred at rt
overnight. On evaporation of the reaction mixture in vacuo, the
Figure 5. Part of H,H-ROESY spectrum of 11b at pD w7 showing spatial arrangement in zwitterionic form.

D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
3677
residue was dissolved in 0.5 M TEAB (4 ml) and the volatiles
4.1.4. (3R,4R)-(3-Hydroxy-4-(thymin-1-yl)-pyrrolidin-1-
evaporated again. The title compound was obtained by chroma-
tography on DEAE-Sephadex A25 using a linear gradient of 0.2 M
TEAB in water, converted to the sodium salt by passing through
small column of Dowex 50 in sodium form, and lyophilized from
water giving 62% yield (296 mg, 0.8 mmol) of a white amorphous
solid.
yl)methylphosphonic acid (11b)
The title compound was prepared, using the procedure reported
for compound 10a, from 25b (0.15 g, 0.39 mmol) and TMSBr
(0.25 ml, 0.39 mmol) in DMF (10 ml) in a 67% yield (90 mg,
0.26 mmol) as a white amorphous solid.
20
Mp 266.4 ꢀC (dec). [
a
]
ꢁ16.5 (c- 0.121, H2O). HRMS (FABþ) for
D
Mp >350 ꢀC. HRMS (ESþ) for C10H15N6O4PNa (MþHþNa)þ: calcd
C10H15N3O6P (MꢁH)þ: calcd 304.0699, found 304.0689. nmax (KBr)
3491 (w, br), 3183 (m, vbr), 1693 (vs), 1656 (s, sh), 1513 (w), 1470
(w), 1444 (w, br), 1384 (w), 1269 (w), 1071 (s, vbr), 909 (w, br), 768
(w). 1H NMR (500 MHz, D2O): 1.89 (d, 3H, J¼1.3, CH3-5), 3.13 and
3.18 (2ꢂdd, 2ꢂ1H, Jgem¼14.1, JH,P¼11.4, CH2P), 3.35 (dd, 1H,
Jgem¼12.0, J2b,3¼5.8, H-2b-pyrr), 3.82 (dd, 1H, Jgem¼12.7, J5b,4¼5.4,
H-5b-pyrr), 3.88 (dd, 1H, Jgem¼12.7, J5a,4¼9.1, H-5a-pyrr), 4.00 (dd,
1H, Jgem¼12.0, J2a,3¼6.9, H-2a-pyrr), 4.64 (ddd, 1H, J4,5¼9.1, 5.4,
J4,3¼4.0, H-4-pyrr), 4.82 (ddd, 1H, J3,2¼6.9, 5.8, J3,4¼4.0, H-3), 7.52
(q, 1H, J¼1.3, H-6). 13C NMR (125.7 MHz, D2O): 14.06 (CH3-5), 56.41
(d, JC,P¼129, CH2P), 59.49 (d, JC,P¼5, CH2-5-pyrr), 63.71 (d, JC,P¼5,
CH2-2-pyrr), 69.16 (CH-4-pyrr), 75.31 (CH-3-pyrr), 113.94 (C-5),
145.93 (CH-6), 154.91 (C-2), 169.50 (C-4).
20
337.0790, found 337.0793. [
a
]
þ26.0 (c- 0.123, H2O). nmax (KBr)
D
3329 (s), 3163 (vs, br), 1657 (vs), 1606 (s), 1573 (m), 1485 (m), 1422
(m), 1337, 1315 (m), 1216 (m), 1073 (s, br, sh), 909 (w, br), 795 (m),
644 (m). 1H NMR (500 MHz, D2O): 3.24 (dd, 1H, Jgem¼14.1, JH,P¼11.1,
CHaHbP), 3.29 (dd, 1H, Jgem¼14.1, JH,P¼11.6, CHaHbP), 3.76 (dd, 1H,
Jgem¼12.8, J2b,3¼5.8, H-2b-pyrr), 3.80 (dd, 1H, Jgem¼12.8, J2a,3¼3.7,
H-2a-pyrr), 3.94 (dd, 1H, Jgem¼12.8, J5b,4¼7.9, H-5b-pyrr), 4.22 (dd,
1H, Jgem¼12.8, J5a,4¼5.0, H-5a-pyrr), 4.88 (ddd, 1H, J3,4¼6.2, J3,2¼5.8,
3.7, H-3-pyrr), 5.44 (ddd, 1H, J4,5¼7.9, 5.0, J4,3¼6.2, H-4-pyrr), 8.23
(s, 1H, H-8), 8.26 (s, 1H, H-2). 13C NMR (125.7 MHz, D2O): 56.87 (d,
JC,P¼128, CH2P), 59.12 (CH-4-pyrr), 59.63 (d, JC,P¼6, CH2-5-pyrr),
63.92 (d, JC,P¼4, CH2-2-pyrr), 71.56 (CH-3-pyrr), 121.07 (C-5), 145.06
(CH-8), 151.81 (C-4), 155.36 (CH-2), 158.39 (C-6). 31P NMR
(162 MHz, D2O): 7.92.
4.1.5. (3S,4R)-2-(4-(Adenin-9-yl)-3-hydroxypyrrolidin-1-
yl)ethylphosphonic acid (12)
The title compound was prepared as described for 10a from 26
(0.904 g, 2.19 mmol) and TMSBr (1.45 ml, 11 mmol) in DMF (20 ml)
in a 43% yield (350 mg, 0.94 mmol) as a white amorphous solid.
4.1.2. (3S,4R)-(3-Hydroxy-4-(thymin-1-yl)-pyrrolidin-1-
yl)methylphosphonic acid (10b)
Mp >350 ꢀC. HRMS (ESþ) for C11H17N6NaO4P (MþHþNa)þ: calcd
The title compound was prepared via the same procedure as
that employed for compound 10a, using 24b (0.12 g, 0.31 mmol),
TMSBr (0.2 ml, 1.53 mmol), and DMF (5 ml), in a 38% yield (41 mg,
0.12 mmol) as a white amorphous solid.
20
351.0947, found 351.0957. [
a
]
þ31.3 (c- 0.115, H2O). nmax (KBr)
D
3450 (m, br, sh), 3329 (s), 3264 (s), 3176 (s, br), 3120 (s, br, sh), 1649
(vs), 1604 (s), 1576 (m), 1478 (m), 1418 (m), 1333 (m), 1306 (m), 1220
(m), 1066 (s), 901 (m, br), 798 (m), 647 (m). 1H NMR (500 MHz,
D2O): 1.95 (m, 2H, CH2P), 3.44 (m, 2H, CH2N), 3.56 (dd, 1H,
Jgem¼12.4, J2b,3¼2.4, H-2b-pyrr), 3.72 (dd, 1H, Jgem¼12.4, J2a,3¼4.9,
H-2a-pyrr), 3.89 (dd, 1H, Jgem¼11.9, J5b,4¼8.8, H-5b-pyrr), 4.01 (dd,
1H, Jgem¼11.9, J5a,4¼8.2, H-5a-pyrr), 4.80 (td, 1H, J3,4¼4.9, J3,2¼4.9,
2.4, H-3-pyrr), 5.45 (ddd, 1H, J4,5¼8.8, 8.2, J4,3¼4.9, H-4-pyrr), 8.29
(s, 1H, H-2), 8.19 (s, 1H, H-8). 13C NMR (125.7 MHz, D2O): 28.62 (d,
JC,P¼125, CH2P), 56.78 (CH2N), 56.322 (CH2-5-pyrr), 58.14 (CH-4-
pyrr), 62.15 (CH2-2-pyrr), 71.37 (CH-3-pyrr), 121.48 (C-5), 144.46
(CH-8), 151.98 (C-4), 155.33 (CH-2), 158.31 (C-6). 31P NMR
(162 MHz, D2O): 17.88.
Mp 289.2 ꢀC. HRMS (FABþ) for C10H15N3O6P (MþH)þ: calcd
20
304.0699, found 304.0689. [
a
]
þ38.1 (c- 0.162, H2O). nmax (KBr)
D
3595 (m, br, sh), 3525 (m, br, sh), 3320 (s, br, sh), 3268 (s, br), 1402
(m), 1705 (vs), 1669 (vs), 1511 (w, sh), 1485 (m), 1436 (m, br), 1381
(w, sh), 1292 (s), 1145 (s), 1073 (s), 1025 (m), 983 (m), 766 (m). 1H
NMR (500 MHz, D2O): 1.89 (d, 3H, JCH3,6¼1.1, CH3-5), 3.19 (d, 2H,
JH,P¼11.4, CH2P), 3.62 (dd, 1H, Jgem¼12.3, J2b,3¼5.4, H-2b-pyrr), 3.75
(dd, 1H, Jgem¼12.3, J2a,3¼2.0, H-2a-pyrr), 3.75 (dd, 1H, Jgem¼12.7,
J5a,4¼9.3, H-5a-pyrr), 4.00 (dd, 1H, Jgem¼12.7, J5a,4¼6.5, H-5a-pyrr),
4.70 (ddd, 1H, J3,4¼6.5, J3,2¼5.4 2.0, H-3-pyrr), 5.17 (dt, 1H, J4,5¼9.3,
6.5, J4,3¼6.5, H-4), 7.63 (q, 1H, J6,CH3¼1.1, H-6). 13C NMR (125.7 MHz,
D2O): 14.18 (CH3-5), 56.65 (d, JC,P¼130, CH2P), 58.46 (d, JC,P¼5, CH2-
5-pyrr), 60.36 (CH-4-pyrr), 64.86 (d, JC,P¼5, CH2-2-pyrr), 71.27 (CH-
3-pyrr), 112.99 (C-5), 144.53 (CH-6), 155.47 (C-2), 169.37 (C-4). 31P
NMR (162 MHz, D2O): 9.16.
4.1.6. (3R,4R)-2-(4-(Adenin-9-yl)-3-hydroxypyrrolidin-1-
yl)ethylphosphonic acid (13)
The titled compound was prepared according to the procedure
for 10a from 27 (0.144 g, 0,35 mmol) and TMSBr (0.23 ml,
1.75 mmol) in DMF (5 ml) in 52% yield (68 mg, 0.18 mmol) as
a white amorphous solid. þ
4.1.3. (3R,4R)-(4-(Adenin-9-yl)-3-hydroxypyrrolidin-1-
yl)methylphosphonic acid (11a)
Mp >350 ꢀC. HRMS (ES ) for C11H17N6NaO4P (MþHþNa)þ: calcd
20
The title compound was prepared via the same procedure as
that employed for compound 10a from 25a (0.5 g, 1.26 mmol),
TMSBr (0.83 ml, 6.28 mmol), and DMF (15 ml) in a 58% yield
(261 mg, 0.73 mmol) as a þwhite amorphous solid.
351.0947, found 351.0950. [
a]
ꢁ6.8 (c- 0.088, H2O). nmax (KBr)
D
3465 (w, br, sh), 3319 (s), 3260 (m), 3173 (s, br), 3115 (s, br, sh), 1647
(vs), 1602 (s), 1575 (m), 1476 (m), 1416 (m), 1331 (m), 1301 (m), 1219
(m), 1076 (vs, br), 900 (w, br), 798 (m), 649 (m). 1H NMR (500 MHz,
D2O): 1.90 (m, 2H, CH2P), 3.29 (m, 3H, H-2b-pyrr and CH2N), 3.67
(dd, 1H, Jgem¼11.9, J5b,4¼6.6, H-5b-pyrr), 3.75 (dd, 1H, Jgem¼11.6,
J2a,3¼6.5, H-2a-pyrr), 3.86 (dd, 1H, Jgem¼11.9, J5a,4¼8.4, H-5a-pyrr),
4.90 (ddd, 1H, J3,2¼6.5, 4.9, J3,4¼4.7, H-3-pyrr), 5.14 (ddd, 1H,
J4,5¼8.4, 6.6, J4,3¼4.7, H-4-pyrr), 8.14 (s, 1H, H-2), 8.23 (s, 1H, H-8).
13C NMR (125.7 MHz, D2O): 29.05 (d, JC,P¼126, CH2P), 55.19 (CH2N),
58.10 (CH2-5-pyrr), 61.32 (CH2-2-pyrr), 63.76 (CH-4-pyrr), 76.71
(CH-3-pyrr), 121.48 (C-5), 143.56 (CH-8), 151.69 (C-4), 155.28 (CH-
2), 158.31 (C-6). 31P NMR (162 MHz, D2O): 18.57.
Mp >350 ꢀC. HRMS (ES ) for C10H15N6O4PNa (MþHþNa)þ: calcd
20
337.0790, found 337.0788. [
a
]
ꢁ7.0 (c- 0.116, H2O). nmax (KBr) 3329
D
(s), 3163 (vs, br), 1657 (vs), 1606 (s), 1573 (m), 1485 (m), 1422 (m),
1337, 1315 (m), 1216 (m), 1073 (s, br, sh), 909 (w, br), 795 (m), 644
(m). 1H NMR (500 MHz, D2O): 3.26 (d, 2H, JH,P¼11.4, CH2P), 3.40 (dd,
1H, Jgem¼12.5, J2b,3¼5.1, H-2b-pyrr), 4.01 (dd, 1H, Jgem¼12.8,
J5b,4¼7.1, H-5b-pyrr), 4.04 (dd, 1H, Jgem¼12.8, J5a,4¼4.4, H-5a-pyrr),
4.16 (dd, 1H, Jgem¼12.5, J2a,3¼6.5, H-2a-pyrr), 4.76 (ddd, 1H, J3,2¼6.5,
5.1, J3,4¼3.1, H-3-pyrr), 5.17 (ddd,1H, J4,5¼7.1, 4.4, J4,3¼3.1, H-4-pyrr),
8.23 (s, 1H, H-8), 8.25 (s, 1H, H-2). 13C NMR (125.7 MHz, D2O): 56.89
(d, JC,P¼130, CH2P), 60.50 (d, JC,P¼6, CH2-5-pyrr), 63.63 (d, JC,P¼5,
CH2-2-pyrr), 64.28 (CH-4-pyrr), 77.27 (CH-3-pyrr), 121.55 (C-5),
144.20 (CH-8), 151.10 (C-4), 155.36 (CH-2), 158.39 (C-6). 31P NMR
(162 MHz, D2O): 8.57.
4.1.7. Diisopropyl ((3R,4R)-3,4-dihydroxy-2,5-dioxopyrrolidin-1-
yl)methylphosphonate (17a)
Diisopropyl aminomethylphosphonate (34.73 g, 178.13 mmol)
in DCM (300 ml) was added to a solution of 2,3-O-diacetyl-L-

3678
D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
tartaric anhydride (38.5 g, 178.13 mmol) in DCM (500 ml) over
2 h at rt. The mixture was stirred for 2 h more at rt, and then
DCM was evaporated. The obtained white foam was dissolved in
acetic anhydride (350 ml), and the mixture was stirred at 90 ꢀC
overnight. Then, acetic anhydride was evaporated. The diacetyl
derivative was purified by flash column chromatography on silica
gel using a linear gradient of ethyl acetate in toluene. After
evaporation, the product was dissolved in 1 M methanolic HCl
(600 ml) and stirred at rt for 8 h. The mixture was concentrated
in vacuo at a temperature below 40 ꢀC, and co-evaporated with
toluene (2ꢂ100 ml) and DCM (2ꢂ100 ml). The title compound
was obtained by purification on silica gel using a linear gradient
of ethanol in chloroform in 87% yield (47.87 g, 154.8 mmol) as
a yellowish oil.
1376 (m), 1255 (m, sh), 1232 (s), 1216 (m, sh), 1179 (m), 1143 (m),
1106 (m), 1074 (m), 1013 (vs), 990 (vs), 890 (m). 1H NMR (500 MHz,
DMSO-d6): 1.23 (d, 12H, Jvic¼6.2, (CH3)2CH), 2.48 (dd, 2H, Jgem¼9.6,
Jvic¼4.5, H-2b,5b-pyrr), 2.73 and 2.79 (2ꢂdd, 2H, Jgem¼15.0,
JH,P¼11.2, CH2P), 2.89 (dd, 2H, Jgem¼9.6, Jvic¼5.8, H-2a,5a-pyrr), 3.81
(m, 2H, H-3,4-pyrr), 4.57 (m, 2H, CH(CH3)2), 4.83 (d, 2H, Jvic¼4.4,
OH). 13C NMR (125.7 MHz, DMSO-d6): 23.96 and 24.08 (d, JC,P¼4,
(CH3)2CH), 52.41 (d, JC,P¼163, CH2P), 62.57 (d, JC,P¼10, CH2-2,5-
pyrr), 69.77 and 69.79 (d, JC,P¼7, CH(CH3)2), 77.54 (CH-3,4-pyrr). 31
NMR (202.3 MHz, DMSO-d6): 23.25.
P
4.1.10. Diisopropyl 2-((3S,4S)-3,4-dihydroxypyrrolidin-1-
yl)ethylphosphonate (18b)
The reduction of this compound 17b with diborane/THF com-
plex generated in situ was performed according to the procedure
described for compound 18a. Starting from diisopropyl ((3R,4R)-
3,4-dihydroxy-2,5-dioxopyrrolidin-1-yl)methylphosphonate (17b)
(5.27 g, 16.3 mmol), sodium borohydride (4 g, 106 mmol), and io-
dine (10.34 g, 40.75 mmol) in THF (200 ml), we obtained 13b in 30%
yield as a yellowish oil (1.44 g, 4.87 mmol).
HRMS (FABþ) for C11H21NO7P (MþH)þ: calcd 310.1056, found
310.1049. 1H NMR (500 MHz, DMSO-d6): 1.23 and 1.24 (2ꢂd, 2ꢂ6H,
Jvic¼6.2, (CH3)2CH), 3.75 (d, 2H, JH,P¼12.0, CH2P), 4.32 (br d, 2H,
Jvic¼4.9, H-3,4-pyrr), 4.57 (m, 2H, CH(CH3)2), 6.41 and 6.42 (2ꢂd,
2ꢂ1H, Jvic¼4.9, OH). 13C NMR (125.7 MHz, DMSO-d6): 23.86, 24.04,
24.07, and 24.10 (d, JC,P¼4, (CH3)2CH), 34.81 (d, JC,P¼157, CH2P),
71.36 and 71.38 (d, JC,P¼6, CH(CH3)2), 74.55 (CH-3,4-pyrr), 173.78
(C-2,5-pyrr). 31P NMR (202.3 MHz, DMSO-d6): 17.54.
HRMS (FABþ) for C12H27NO5P (MþH)þ: calcd 296.1627, found
296.1617. nmax (KBr) 3390 (m, br), 2980 (m), 1468 (m), 1386 (m),
1377 (m), 1255 (m, sh), 1220 (s), 1179 (m), 1143 (m), 1106 (s), 1077
(m, sh), 1011 (vs, sh), 990 (vs), 891 (w). 1H NMR (500 MHz, DMSO-
d6): 1.23 (d, 12H, Jvic¼6.2, (CH3)2CH), 1.85 (ddd, 2H, JH,P¼16.3,
Jvic¼8.9, 7.6, CH2P), 2.39 (dd, 2H, Jgem¼9.9, Jvic¼3.8, H-2b,5b-pyrr),
2.59 (m, 2H, CH2N), 2.82 (dd, 2H, Jgem¼9.9, Jvic¼5.7, H-2a,5a-pyrr),
3.84 (br m, 2H, H-3,4-pyrr), 4.55 (m, 2H, CH(CH3)2), 4.94 (br s, 2H,
OH). 13C NMR (125.7 MHz, DMSO-d6): 23.98 and 24.00 (d, JC,P¼4,
(CH3)2CH), 25.46 (d, JC,P¼138, CH2P), 49.73 (CH2N), 60.27 (CH2-2,5-
pyrr), 69.56 (d, JC,P¼6, CH(CH3)2), 77.28 (CH-3,4-pyrr). 31P NMR
(202.3 MHz, DMSO-d6): 28.15.
4.1.8. Diisopropyl ((3R,4R)-3,4-dihydroxy-2,5-dioxopyrrolidin-1-yl)-
ethylphosphonate (17b)
A
solution of diisopropyl 2-aminoethylphosphonate (4.6 g,
22 mmol) in DCM (40 ml) was added to a solution of 2,3-di-O-acetyl-
-tartaric anhydride (4.76 g, 22 mmol) in DCM (60 ml) over 2 h at rt.
L
The mixture was stirred for an additional hour at rt. The DCM was
evaporated and the resulting white foam was dissolved in acetic
anhydride (150 ml). The mixture was stirred at 90 ꢀC for 7 h. The
excess of acetic anhydride was evaporated and the mixture was pu-
rified on silica gel using a linear gradient of ethyl acetate in toluene.
The obtained product was dissolved in 1 M HCl in methanol (200 ml)
and stirred at rt for 8 h. The mixture was concentrated in vacuo at
4.1.11. Diisopropyl ((3S,4S)-3-dimethoxytrityloxy-4-hydroxy-
pyrrolidin-1-yl)methylphosphonate (19a)
a
temperature below 40 ꢀC, and co-evaporated with toluene
A solution of dimethoxytrityl chloride (8.47 g, 25 mmol) in DCM
(50 ml) was added dropwise to the solution of 18a (5.98 g,
21.26 mmol) and DMAP (3 g, 35 mmol) in DCM (150 ml). The reaction
mixture was left aside at rt overnight. The product was obtained by
purification on silica gel using a linear gradient of ethyl acetate in
toluene in a 40% yield in the form of a yellowish oil (5.0 g, 8.57 mmol).
HRMS (ESIþ) for C32H43NO7P (MþH)þ: calcd 584.2772, found
584.2771. 1H NMR (500 MHz, DMSO-d6): 1.21, 1.22, 1.23, and 1.24
(4ꢂd, 4ꢂ3H, Jvic¼6.2, (CH3)2CH), 2.12 (br dd,1H, Jgem¼10.0, J5b,4¼3.6,
H-5b-pyrr), 2.25 (br m, 1H, H-5a-pyrr), 2.57 (br m, 1H, H-2b-pyrr),
2.70 (br m, 2H, CH2P), 2.98 (br m, 1H, H-2a-pyrr), 3.75 (s, 6H, CH3O-
DMTr), 3.85 (br m, 1H, H-4-pyrr), 4.04 (br m, 1H, H-3-pyrr), 4.55 (m,
2H, CH(CH3)2), 6.87 (m, 4H, H-m-C6H4-DMTr), 7.22 (m, 1H, H-p-
C6H5-DMTr), 7.29 (m, 6H, H-o-C6H4-DMTr and H-m-C6H5-DMTr),
7.43 (m, 2H, H-o-C6H5-DMTr). 13C NMR (125.7 MHz, DMSO-d6):
23.83 and 23.94 (d, JC,P¼4, (CH3)2CH), 51.95 (d, JC,P¼162, CH2P), 55.15
(CH3O-DMTr), 60.67 (d, JC,P¼10, CH2-5-pyrr), 61.82 (d, JC,P¼9, CH2-2-
pyrr), 69.79 (d, JC,P¼7, CH(CH3)2), 76.45 (CH-3), 80.21 (CH-4), 86.08
(C-DMTr), 113.25 and 113.27 (CH-m-C6H4-DMTr), 126.73 (CH-p-
C6H5-DMTr),127.82 (CH-o-C6H5-DMTr),128.08 (CH-m-C6H5-DMTr),
130.01 and 130.10 (CH-o-C6H4-DMTr), 136.47 and 136.76 (C-i-C6H4-
DMTr), 145.83 (C-i-C6H5-DMTr), 158.24 (C-p-C6H4-DMTr).
(2ꢂ100 ml) and DCM (2ꢂ100 ml) to provide diisopropyl ((3R,4R)-3,4-
dihydroxy-2,5-dioxopyrrolidin-1-yl)methylphosphonate (17b) in a
95% yield (8.5 g, 21 mmol).
HRMS (ESIþ) for C12H23NO7P (MþH)þ: calcd 324.1207, found
324.1208. 1H NMR (500 MHz, DMSO-d6): 1.238, 1.240, 1.244, and
1.246 (4ꢂd, 4ꢂ3H, Jvic¼6.2, (CH3)2CH), 1.91–2.03 (m, 2H, CH2P),
3.42–3.58 (m, 2H, CH2N), 4.30 (s, 2H, H-3,4-pyrr), 4.54 and 4.55
(2ꢂdh, 2ꢂ1H, JH,P¼8.0, Jvic¼6.2, CH(CH3)2). 13C NMR (125.7 MHz,
DMSO-d6): 23.87, 23.89, 23.93, and 23.94 (d, JC,P¼4, (CH3)2CH),
24.42 (d, JC,P¼140, CH2P), 32.40 (CH2N), 69.96 and 70.01 (d, JC,P¼6,
CH(CH3)2), 74.48 (CH-3,4-pyrr), 174.45 (C-2,5-pyrr). 31P NMR
(202.3 MHz, DMSO-d6): 25.54.
4.1.9. Diisopropyl ((3S,4S)-3,4-dihydroxypyrrolidin-1-
yl)methylphosphonate (18a)
A solution of iodine (43.2 g, 170 mmol) in THF (500 ml) was
added dropwise under argon atmosphere to an ice bath cooled
suspension of sodium borohydride (18.2 g, 480 mmol) in the so-
lution of diisopropyl ((3R,4R)-3,4-dihydroxy-2,5-dioxopyrrolidin-
1-yl)methylphosphonate (21 g, 68 mmol) 17a in THF (800 ml) for
2.5 h. The reaction mixture was stirred overnight at rt, then cooled
to 0 ꢀC, and aqueous 3 M HCl (100 ml) was added carefully to de-
stroy an excess of sodium borohydride (gas evolving). The mixture
was diluted with water (1000 ml) and applied onto a Dowex 50
(Hþ) (2000 ml) column. The column was washed with water
(3000 ml) and the product was eluted with 3% aqueous ammonia.
After evaporation of solvents, 18a was obtained in 50% yield (9.6 g,
34 mmol) as a yellowish oil.
4.1.12. Diisopropyl ((3S,4S)-4-dimethoxytrityloxy-3-mesyloxy-
pyrrolidin-1-yl)methylphosphonate (20a)
Mesyl chloride (3.23 ml, 43 mmol) was added to the solution of
19a (5 g, 8.57 mmol) and DMAP (5.3 g, 43 mmol) in DCM (50 ml) at
0 ꢀC, and the reaction mixture was stirred at rt for 1 h. The reaction
was quenched with water (1 ml), diluted with chloroform (100 ml),
and washed with saturated solution of sodium hydrogen carbonate.
The organic phase was dried with sodium sulfate and evaporated.
HRMS (FABþ) for C11H25NO5P (MþH)þ: calcd 282.1470, found
282.1473. nmax (KBr) 3375 (m, br), 2980 (m), 1469 (m), 1386 (m),

D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
3679
The title product was obtained in an 82% yield (4.6 g, 7 mmol) by
purification on silica gel using a linear gradient of ethyl acetate in
toluene in the form of a yellowish oil.
chloroform in a 26% yield as a thick yellowish oil (0.311 g,
0.44 mmol).
HRMS (FABþ) for C38H48N6O6P (MþH)þ: calcd 715.3373, found
715.3351. 1H NMR (500 MHz, DMSO-d6): 1.17–1.22 (m, 24H,
(CH3)2CH-cisþtrans), 1.72 (m, 4H, CH2P-cisþtrans), 1.84 (dd, 1H,
Jgem¼10.0, J2b,3¼4.5, H-2b-pyrr-trans), 1.86 (dd, 1H, Jgem¼10.5,
J2b,3¼7.2, H-2b-pyrr-cis), 2.13 (dd, 1H, Jgem¼10.0, J2a,3¼7.0, H-2a-
pyrr-trans), 2.16 (dd, 1H, Jgem¼10.0, J2a,3¼7.0, H-2a-pyrr-cis), 2.40
(m, 4H, CH2N-cisþtrans), 2.68 (dd, 1H, Jgem¼9.5, J5b,4¼6.2, H-5b-
pyrr-trans), 2.72 (dd, 1H, Jgem¼10.0, J5b,4¼5.4, H-5b-pyrr-cis), 2.93
(dd, 1H, Jgem¼10.0, J5a,4¼3.6, H-5a-pyrr-cis), 2.96 (dd, 1H, Jgem¼9.5,
J5a,4¼7.2, H-5a-pyrr-trans), 4.34 (br dt, 1H, J3,5¼7.0, 4.5, J3,4¼4.5, H-
3-pyrr-trans), 4.36 (ddd, 1H, J3,4¼7.6, J3,5¼7.2, 5.0, H-3-pyrr-cis), 4.51
(m, 4H, CH(CH3)2-cisþtrans), 5.05 (ddd, 1H, J4,3¼7.6, J4,5¼5.4, 3.6, H-
4-pyrr-cis), 5.09 (ddd, 1H, J4,5¼7.2, 6.2, J4,3¼4.5, H-4-pyrr-trans),
6.58 and 6.71 (2ꢂm, 2ꢂ4H, H-m-C6H4-DMTr-trans), 6.72 and 6.73
(2ꢂm, 2ꢂ4H, H-m-C6H4-DMTr-cis), 6.95 (m, 2H, H-p-C6H5-DMTr-
cisþtrans), 7.02–7.17 (m, 12H, H-o-C6H4-DMTr-cisþtrans and H-m-
C6H5-DMTr-cisþtrans), 7.22 (br s, 2H, NH2-cis), 7.24 (br s, 2H,
NH2-trans), 7.45 (m, 2H, H-o-C6H5-DMTr), 8.145 and 8.148 (2ꢂs,
2ꢂ1H, H-2,8-trans), 8.17 and 8.46 (2ꢂs, 2ꢂ1H, H-2,8-cis).
HRMS (FABꢁ) for C33H43NO9PS (MꢁH)ꢁ: calcd 660.2396, found
660.2446. 1H NMR (500 MHz, DMSO-d6): 1.15, 1.16, 1.17, and 1.18
(4ꢂd, 4ꢂ3H, Jvic¼6.2, (CH3)2CH), 1.82 (dd, 1H, Jgem¼10.0, J5b,4¼5.2,
H-5b-pyrr), 2.07 (dd, 1H, Jgem¼10.0, J5a,4¼6.6, H-5a-pyrr), 2.64 (d,
2H, JH,P¼11.5, CH2P), 2.93 (dd, 1H, Jgem¼11.2, J2b,3¼5.1, H-2b-pyrr),
2.95 (dd, 1H, Jgem¼11.2, J2a,3¼3.9, H-2a-pyrr), 3.20 (s, 3H, CH3S),
3.737 and 3.740 (2ꢂs, 2ꢂ3H, CH3O-DMTr), 3.98 (ddd, 1H, J4,5¼6.6,
5.2, J4,3¼2.6, H-4-pyrr), 4.48 (dh, 2H, JH,P¼7.8, Jvic¼6.2, CH(CH3)2),
5.09 (ddd, 1H, J3,2¼5.1, 3.9, J3,4¼2.6, H-3-pyrr), 6.89 (m, 4H, H-m-
C6H4-DMTr), 7.23 (m, 1H, H-p-C6H5-DMTr), 7.26 and 7.28 (2ꢂm,
2ꢂ2H, H-o-C6H4-DMTr), 7.31 (m, 2H, H-m-C6H5-DMTr), 7.42 (m, 2H,
H-o-C6H5-DMTr).
4.1.13. Diisopropyl 2-((3S,4S)-4-dimethoxytrityloxy-3-
mesyloxypyrrolidin-1-yl)ethylphosphonate (20b)
Dimethoxytrityl chloride (1.7 g, 5.0 mmol) was added to the
solution of 18b (1.44 g, 4.9 mmol) in pyridine (20 ml). The reaction
mixture was stirred at rt overnight and quenched by the addition of
methanol (3 ml). The solvents were evaporated and compound 19b
was obtained by chromatography on silica gel using a linear gra-
dient of ethanol in chloroform in a 64% yield (1.7 g, 2.8 mmol). It
was characterized only by MS (HRMS (FABþ) for C33H45NO7P
(MþH)þ: calcd 598.2934, found 598.2921), and used directly for the
further step.
Mesyl chloride (1.1 ml, 14.0 mmol) was added to the solution of
19b (1.7 g, 2.8 mmol) and DMAP (1.7 g, 14.0 mmol) in DCM (40 ml)
at 0 ꢀC, and the reaction mixture was stirred at rt for 2 h. The re-
action was quenched with water (1 ml), diluted with chloroform
(100 ml), and washed with saturated solution of sodium hydrogen
carbonate. The organic phase was dried with sodium sulfate and
evaporated. The desired product 20b was obtained by purification
on silica gel using a linear gradient of ethanol in chloroform in a 60%
yield as a yellowish oil (1.134 g, 1.7 mmol).
4.1.15. Diisopropyl (3S,4S)-(4-(adenin-9-yl)-3-hydroxypyrrolidin-
1-yl)methylphosphonate (22a) and diisopropyl (3S,4R)-(4-(adenin-
9-yl)-3-hydroxypyrrolidin-1-yl)methylphosphonate (24a)
Dimethoxytrityl derivatives 21a and 21c were prepared from
20a (4.6 g, 7 mmol), adenine (1.1 g, 8 mmol), and cesium carbonate
(2.6 g, 8 mmol) in DMF (40 ml) using the same procedure as de-
scribed for compound 21b and 21d. The mixture of products 21a
and 21c obtained by purification on silica gel using a linear gradient
of H3 in ethyl acetate was dissolved in 80% aqueous acetic acid
(30 ml) and the solution was stirred at rt for 3 h. Acetic acid was
removed in vacuo, and the crude residue was co-evaporated with
water (2ꢂ20 ml), ethanol (2ꢂ20 ml), and toluene (2ꢂ20 ml). A
mixture of products 22a and 24a (in the ratio 1:1) was obtained by
purification on silica gel using a linear gradient of H3 in ethyl
acetate in a 17% overall yield as a yellowish thick oil (0.48 g,
1.21 mmol).
HRMS (FABþ) for C34H47NO9PS (MþH)þ: calcd 676.2709, found
676.2627. 1H NMR (500 MHz, DMSO-d6): 1.175, 1.18, and 1.19 (3ꢂd,
12H, Jvic¼6.2, (CH3)2CH), 1.54 (dd, 2H, Jgem¼10.0, J5b,4¼5.3, H-5b-
pyrr), 1.62 (m, 2H, CH2P), 1.76 (dd, 1H, Jgem¼10.0, J5a,4¼6.4, H-5a-
pyrr), 2.33 (q, 2H, JH,P¼Jvic¼8.2, CH2N), 2.73 (dd, 1H, Jgem¼11.0,
J2b,3¼3.3, H-2b-pyrr), 2.78 (dd, 1H, Jgem¼11.0, J2a,3¼6.0, H-2a-pyrr),
3.22 (s, 3H, CH3S), 3.74 (s, 6H, CH3O-DMTr), 3.96 (ddd, 1H, J4,5¼6.4,
5.3, J4,3¼2.8, H-4-pyrr), 4.50 (m, 2H, CH(CH3)2), 5.12 (ddd, 1H,
J3,2¼6.0, 3.3, J3,4¼2.8, H-3-pyrr), 6.89 (m, 4H, H-m-C6H4-DMTr), 7.25
(m, 1H, H-p-C6H5-DMTr), 7.30 (m, 4H, H-o-C6H4-DMTr), 7.31 (m, 2H,
H-m-C6H5-DMTr), 7.45 (m, 2H, H-o-C6H5-DMTr). 13C NMR
(125.7 MHz, DMSO-d6): 23.88 and 23.93 (d, JC,P¼4, (CH3)2CH), 25.08
(d, JC,P¼139, CH2P), 38.07 (CH3S), 48.58 (CH2N), 56.78 and 57.94 (CH2-
2,5-pyrr), 55.26 (CH3O-DMTr), 69.44 (d, JC,P¼6, CH(CH3)2), 77.99
(CH-3), 85.37 (CH-4), 86.69 (C-DMTr), 113.49 and 113.53 (CH-m-
C6H4-DMTr), 127.04 (CH-p-C6H5-DMTr), 127.97 (CH-o-C6H5-DMTr),
128.10 (CH-m-C6H5-DMTr), 130.04 and 130.13 (CH-o-C6H4-DMTr),
135.77 and 136.15 (C-i-C6H4-DMTr), 145.24 (C-i-C6H5-DMTr), 158.51
and 158.52 (C-p-C6H4-DMTr).
HRMS (FABþ) for C16H28N6O4P (MþH)þ: calcd 399.1910, found
399.1935. For 1H, 13C and 31P NMR see compound 24a (cis-isomer)
and 25a (trans-isomer).
4.1.16. Diisopropyl (3S,4S)-2-(4-(adenin-9-yl)-3-hydroxypyrrolidin-
1-yl)ethylphosphonate (22b) and diisopropyl (3S,4R)-2-(4-
(adenin-9-yl)-3-hydroxypyrrolidin-1-yl)ethylphosphonate (26)
Compound 21b (0.311 g, 0.435 mmol) was dissolved in 80%
aqueous acetic acid (30 ml) and the whole was stirred at rt for 3 h.
The acetic acid was removed in vacuo, and the crude product was co-
evaporated with water (2ꢂ20 ml), ethanol (2ꢂ20 ml), and toluene
(2ꢂ20 ml). A mixture of products 22b and 26 (2:1) was obtained in
a 99% yield (0.177 g, 0.43 mmol) by purification on silica gel using
a linear gradient of H3 in ethyl acetate as a thick yellowish oil.
HRMS (FABþ) for C17H30N6O4P (MþH)þ: calcd 413.2066, found
413.2084. For 1H, 13C and 31P NMR, see compound 26 (cis-isomer)
and 27 (trans-isomer).
4.1.14. Diisopropyl (3S,4S)-2-(4-(adenin-9-yl)-3-dimethoxy-
trityloxypyrrolidin-1-yl)ethylphosphonate (21b) and diisopropyl
(3S,4R)-2-(4-(adenin-9-yl)-3-dimethoxytrityloxypyrrolidin-1-
yl)ethylphosphonate (21d)
The mixture of 20b (1.134 g, 1.7 mmol) and adenine (0.27 g,
2 mmol) was co-evaporated with DMF (2ꢂ20 ml) and redissolved
in DMF (20 ml). Cesium carbonate (0.55 g, 1.7 mmol) was added
and the reaction mixture was stirred at 120 ꢀC under a drying tube
for two days. A mixture of products 21b and 21d (2:1) was obtained
by purification on silica gel using a linear gradient of ethanol in
4.1.17. Diisopropyl (3S,4R)-(4-(adenin-9-yl)-3-hydroxypyrrolidin-
1-yl)methylphosphonate (24a)
Aqueous formaldehyde (14.5 M, 1 ml, 14.5 mmol) was added to
the suspension of 8a (0.34 g, 1.537 mmol) in diisopropyl phosphite
(3 ml). The mixture was homogenized using an ultrasonic bath and
stirred at 60 ꢀC for 3 h. Aqueous sulfuric acid (0.02 M, 40 ml) was
added and the reaction mixture was stirred at rt for two days. The
solutionwas applied ontothe column of Dowex 50 (40 ml). The resin
was washed with 50% aqueous ethanol (200 ml), and the crude
product was eluted with 3% ammonia in an ethanol–water (1:1)

3680
D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
mixture. Pure compound 24a was obtained by column chromatog-
raphy on silica gel using a linear gradient of ethanol in chloroform in
87% yield (0.53 g, 1.332 mmol) as a thick yellowish oil.
CH(CH3)2), 75.06 (CH-3-pyrr),118.87 (C-5),139.47 (CH-8),149.52 (C-
4),152.54 (CH-2),156.19 (C-6). 31P NMR (162 MHz, DMSO-d6): 22.84.
HRMS (FABþ) for C16H28N6O4P (MþH)þ: calcd 399.1910, found
399.1905. nmax (KBr) 3423 (vs, br), 3200 (m, br), 2979 (m), 1641 (s),
1600 (s), 1573 (m, sh), 1469 (m), 1415 (m), 1387 (m), 1376 (m), 1331
(m),1299 (m),1245 (m, br, sh),1222 (m, br),1179 (m),1143 (w),1079
(m),1062 (m),1012 (s, sh), 990 (vs), 889 (w), 799 (w), 763 (w, br), 647
(w), 553 (m).1H NMR (400 MHz, DMSO-d6): 1.26 and 1.27 (2ꢂd,12H,
Jvic¼6.2, (CH3)2CH), 2.85 (dd, 1H, Jgem¼10.2, J2b,3¼3.7, H-2b-pyrr),
2.91 (dd, 1H, Jgem¼15.0, JH,P¼11.3, CHaHb-P), 2.97 (dd, 1H, Jgem¼15.0,
JH,P¼11.6, CHaHb-P), 3.00 (dd, 1H, Jgem¼10.2, J2a,3¼6.9, H-2a-pyrr),
3.03 (dd, 1H, Jgem¼9.9, J5b,4¼6.5, H-5b-pyrr), 3.15 (dd, 1H, Jgem¼9.9,
J5a,4¼5.1, H-5a-pyrr), 4.40 (m, 1H, J3,2¼6.9, 3.7, J3,4¼6.5, J3,OH¼5.0, H-
3-pyrr), 4.61 and 4.62 (2ꢂdh, 2ꢂ1H, JH,P¼7.8, Jvic¼6.2, CH(CH3)2),
5.01 (td, 1H, J4,3¼6.5, J4,5¼6.5, 5.1, H-4-pyrr), 5.13 (d, 1H, JOH,3¼5.0,
OH), 7.16 (br s, 2H, NH2), 8.12 (s, 1H, H-2), 8.16 (s, 1H, H-8). 13C NMR
(100.6 MHz, DMSO-d6): 24.01 and 24.09 (d, JC,P¼4, (CH3)2CH), 51.14
(d, JC,P¼163, CH2P), 55.05 (CH-4-pyrr), 58.87 (d, JC,P¼11, CH2-5-pyrr),
62.61 (d, JC,P¼10, CH2-2-pyrr), 68.86 (CH-3-pyrr), 69.95 (d, JC,P¼6,
CH(CH3)2), 118.20 (C-5), 141.03 (CH-8), 150.19 (C-4), 152.29 (CH-2),
156.02 (C-6). 31P NMR (162 MHz, DMSO-d6): 22.49.
4.1.20. Diisopropyl (3R,4R)-(3-hydroxy-4-(thymin-1-yl)-pyrrolidin-
1-yl)methylphosphonate (25b)
The titled compound was prepared as described for 24a from 9b
(0.15 g, 0.71 mmol),14.5 M aqueous formaldehyde (0.5 ml, 7.25 mmol),
and diisopropyl phosphite (2 ml) in 54% yield (0.15 g, 0.385 mmol) as
a thick yellowish oil.
HRMS (ESIþ) for C16H28N3NaO6P (MþNa)þ: calcd 412.1608,
found 412.1611. 1H NMR (500 MHz, DMSO-d6): 1.21 and 1.22 (2ꢂd,
12H, Jvic¼6.2, (CH3)2CH), 1.74 (d, 3H, J¼1.2, CH3), 2.34 (dd, 1H,
Jgem¼9.7, J2b,3¼6.0, H-2b-pyrr), 2.79 (dd, 1H, Jgem¼15.2, JH,P¼10.6,
CHaHb-P), 2.85 (d, 2H, J5,4¼5.5, H-5-pyrr), 2.90 (dd, 1H, Jgem¼15.2,
JH,P¼12.6, CHaHb-P), 3.26 (dd, 1H, Jgem¼9.7, J2a,3¼6.9, H-2a-pyrr),
4.12 (br m, 1H, J3,2¼6.9, 6.0, J3,OH¼5.1, J3,4¼3.5, H-3-pyrr), 4.56 (dh,
2H, JH,P¼7.9, Jvic¼6.2, CH(CH3)2), 4.60 (td, 1H, J4,5¼5.5, J4,3¼3.5, H-4-
pyrr), 5.66 (d, 1H, JOH,3¼5.1, OH), 7.51 (q, 1H, J¼1.2, H-6), 11.27 (br s,
1H, NH). 13C NMR (125.7 MHz, DMSO-d6): 12.20 (CH3), 23.86, 23.87,
and 23.92 (d, JC,P¼4, (CH3)2CH), 50.95 (d, JC,P¼163, CH2P), 58.34 (d,
JC,P¼10, CH2-5-pyrr), 62.27 (CH-4-pyrr), 62.59 (d, JC,P¼11, CH2-2-
pyrr), 69.88 and 69.90 (d, JC,P¼7, CH(CH3)2), 74.95 (CH-3-pyrr),
109.19 (C-5), 138.19 (CH-6), 150.98 (C-2), 163.79 (C-4). 31P NMR
(202.3 MHz, DMSO-d6): 23.42.
4.1.18. Diisopropyl (3S,4R)-(3-hydroxy-4-(thymin-1-yl)-pyrrolidin-
1-yl)methylphosphonate (24b)
The title compound was prepared as described for 24a from 8b
(0.1 g, 0.47 mmol), 14.5 M aqueous formaldehyde (0.5 ml,
7.25 mmol), and diisopropyl phosphite (2 ml) in 65% yield (0.12 g,
0.31 mmol) as a thick yellowish oil.
4.1.21. Diisopropyl (3S,4R)-2-(4-(adenin-9-yl)-3-hydroxy-
pyrrolidin-1-yl)ethylphosphonate (26)
Diisopropyl vinylphosphonate (1.4 g, 7 mmol) was added to
a suspension of compound 8a (0.7 g, 3.18 mmol) in methanol
(30 ml), and the mixture was treated at reflux for five days. The so-
lutionwas applied onto a column of Dowex 50 (40 ml). The resinwas
washed with 50% aqueous ethanol (200 ml), and the crude product
was obtained byelutionwith 3% ammonia in an ethanol–water (1:1)
mixture. Pure compound 26 was obtained by column chromatog-
raphy on silica gel using a linear gradient of ethanol in chloroform,
providing a 69% yield (0.904 g, 2.192 mmol) of a thick yellowish oil.
HRMS (FABþ) for C17H30N6O4P (MþH)þ: calcd 413.2066, found
413.2084. nmax (KBr) 3400 (m, br, sh), 3328 (m, br), 3265 (m, br, sh),
3176 (m, br), 2978 (m), 1662 (s), 1645 (s), 1599 (s), 1573 (m), 1511
(w), 1474 (m), 1416 (m), 1385 (m), 1374 (m), 1334 (m), 1299 (m),
1249 (s, br), 1224 (m, br), 1179 (m), 1143 (m), 1064 (m), 1009 (s, br),
983 (vs, br), 888 (w), 799 (w), 793 (w), 774 (w, br), 647 (w), 529 (m,
br). 1H NMR (600 MHz, DMSO-d6): 1.24 and 1.25 (2ꢂd,12H, Jvic¼6.2,
(CH3)2CH), 1.94 (m, 2H, CH2P), 2.64 (dd, 1H, Jgem¼10.1, J2b,3¼3.7,
H-2b-pyrr), 2.68 (m, 2H, CH2N), 2.95 (dd, 1H, Jgem¼10.1, J2a,3¼6.8, H-
2a-pyrr), 2.96 (dd, 1H, Jgem¼9.6, J5b,4¼6.8, H-5b-pyrr), 2.99 (dd, 1H,
Jgem¼9.6, J5a,4¼6.3, H-5a-pyrr), 4.36 (m, 1H, J3,2¼6.8, 3.7, J3,4¼6.7,
J3,OH¼5.1, H-3-pyrr), 4.57 and 4.58 (2ꢂdh, 2ꢂ1H, JH,P¼8.0, Jvic¼6.2,
CH(CH3)2), 4.99 (ddd, 1H, J4,5¼6.8, 6.3, J4,3¼6.7, H-4-pyrr), 5.10 (d,
1H, JOH,3¼5.1, OH), 7.15 (br s, 2H, NH2), 8.12 (s, 1H, H-2), 8.19 (s, 1H,
H-8). 13C NMR (100.6 MHz, DMSO-d6): 24.00 (d, JC,P¼4, (CH3)2CH),
25.65 (d, JC,P¼138, CH2P), 8.99 (CH2N), 54.95 (CH-4-pyrr), 56.58
(CH2-5-pyrr), 60.66 (CH2-2-pyrr), 68.65 (CH-3-pyrr), 69.40 (d,
JC,P¼6, CH(CH3)2), 118.27 (C-5), 141.33 (CH-8), 150.14 (C-4), 152.23
(CH-2), 155.99 (C-6). 31P NMR (162 MHz, DMSO-d6): 29.13.
HRMS (ESIþ) for C16H28N3NaO6P (MþNa)þ: calcd 412.1608,
found 412.1600. nmax (KBr) 3397 (m, vbr), 3190 (m, br), 2980 (m),
1694 (vs, br), 1664 (s, br, sh), 1517 (w, sh), 1472 (m), 1389 (m), 1375
(m), 1280 (s), 1233 (m), 1179 (m), 1143 (m), 1069 (m), 1012 (s, sh),
988 (vs), 888 (m), 764 (w). 1H NMR (400 MHz, DMSO-d6): 1.26 and
1.27 (2ꢂd, 12H, Jvic¼6.2, (CH3)2CH), 1.74 (d, 3H, J¼1.2, CH3), 2.85 (dd,
1H, Jgem¼10.0, J2b,3¼5.9, H-2b-pyrr), 2.65 (dd, 1H, Jgem¼10.8,
J5b,4¼8.0, H-5b-pyrr), 2.76 (dd, 1H, Jgem¼15.0, JH,P¼10.7, CHaHb-P),
2.86 (dd, 1H, Jgem¼10.0, J2a,3¼2.2, H-2a-pyrr), 2.89 (dd, 1H,
Jgem¼15.0, JH,P¼12.8, CHaHb-P), 3.05 (dd, 1H, Jgem¼10.8, J5a,4¼3.4, H-
5a-pyrr), 4.27 (m, 1H, J3,4¼7.6, J3,2¼5.9, 2.2, J3,OH¼4.4, H-3-pyrr),
4.61 (m, 2H, CH(CH3)2), 4.92 (ddd, 1H, J4,5¼8.0, 3.4, J4,3¼7.6, H-4-
pyrr), 5.16 (d, 1H, JOH,3¼4.4, OH), 7.58 (q, 1H, J¼1.2, H-6), 11.15 (br s,
1H, NH). 13C NMR (100.6 MHz, DMSO-d6): 12.49 (CH3), 23.98, 24.01,
and 24.05 (d, JC,P¼4, (CH3)2CH), 50.88 (d, JC,P¼165, CH2P), 55.14 (CH-
4-pyrr), 58.04 (d, JC,P¼10, CH2-5-pyrr), 63.58 (d, JC,P¼12, CH2-2-
pyrr), 68.53 (CH-3-pyrr), 69.94 and 69.96 (d, JC,P¼7, CH(CH3)2),
106.86 (C-5), 140.79 (CH-6), 151.54 (C-2), 164.13 (C-4).
4.1.19. Diisopropyl (3R,4R)-(4-(adenin-9-yl)-3-hydroxypyrrolidin-
1-yl)methylphosphonate (25a)
The title compound was prepared as described for 24a from 9a
(0.9 g, 4.09 mmol), 14.5 M aqueous formaldehyde (1.4 ml,
20 mmol), and diisopropyl phosphite (20 ml) in 78% yield (1.27 g,
3.18 mmol) as a thick yellowish oil.
HRMS (FABþ) for C16H28N6O4P (MþH)þ: calcd 399.1910, found
399.1907. 1H NMR (400 MHz, DMSO-d6): 1.24 and 1.25 (2ꢂd, 12H,
Jvic¼6.2, (CH3)2CH), 2.56 (dd, 1H, Jgem¼10.0, J2b,3¼4.7, H-2b-pyrr),
2.89 (dd, 1H, Jgem¼15.0, JH,P¼10.7, CHaHb-P), 2.98 (dd, 1H, Jgem¼15.0,
JH,P¼12.4, CHaHb-P), 3.09 (d, 2H, J5,4¼5.8, H-5-pyrr), 3.35 (dd,
1H, Jgem¼10.0, J2a,3¼6.8, H-2a-pyrr), 4.40 (br m, 1H, J3,2¼6.8, 4.7,
J3,OH¼4.8, J3,4¼3.3, H-3-pyrr), 4.60 and 4.61 (2ꢂdh, 2ꢂ1H, JH,P¼7.8,
Jvic¼6.2, CH(CH3)2), 4.70 (td,1H, J4,5¼5.8, J4,3¼3.3, H-4-pyrr), 5.54 (d,
1H, JOH,3¼4.8, OH), 7.23 (br s, 2H, NH2), 8.14 (s,1H, H-8), 8.18 (s,1H, H-
2). 13C NMR (100.6 MHz, DMSO-d6): 24.00 and 24.08 (d, JC,P¼4,
(CH3)2CH), 50.94 (d, JC,P¼163, CH2P), 58.79 (d, JC,P¼10, CH2-5-pyrr),
61.67 (CH-4-pyrr), 62.69 (d, JC,P¼10, CH2-2-pyrr), 69.99 (d, JC,P¼6,
4.1.22. Diisopropyl (3R,4R)-2-(4-(adenin-9-yl)-3-hydroxy-
pyrrolidin-1-yl)ethylphosphonate (27)
The title compound was prepared from 9a (0.12 g, 0.54 mmol)
and diisopropyl vinylphosphonate (0.2 g, 1 mmol) in methanol
(10 ml) via the same procedure described for 26, in a 65% yield
(0.144 g, 0.35 mmol) as a thick yellowish oil.
HRMS (FABþ) for C17H30N6O4P (MþH)þ: calcd 413.2066, found
413.2074. 1H NMR (400 MHz, DMSO-d6): 1.234, 1.240, 12.42, and
1.243 (4ꢂd, 4ꢂ3H, Jvic¼6.2, (CH3)2CH), 1.92 (m, 2H, CH2P), 2.43 (dd,
1H, Jgem¼9.8, J2b,3¼4.6, H-2b-pyrr), 2.66 (m 2H, CH2N), 2.90 (dd, 1H,

D. Rejman et al. / Tetrahedron 65 (2009) 3673–3681
3681
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Jgem¼9.6, J5b,4¼5.1, H-5b-pyrr), 3.00 (dd, 1H, Jgem¼9.6, J5a,4¼7.1, H-
5a-pyrr), 3.18 (dd, 1H, Jgem¼9.8, J2a,3¼7.0, H-2a-pyrr), 4.41 (br m, 1H,
J3,5¼7.0, 4.6, J3,OH¼5.0, J3,4¼3.7, H-3-pyrr), 4.57 and 4.58 (2ꢂdh,
2ꢂ1H, JH,P¼7.8, Jvic¼6.2, CH(CH3)2), 4.72 (td, 1H, J4,5¼7.1, 5.1, J4,3¼3.7,
H-4-pyrr), 5.52 (d, 1H, JOH,3¼5.0, OH), 7.21 (br s, 2H, NH2), 8.13 (s,
1H, H-8), 8.24 (s, 1H, H-2). 13C NMR (100.6 MHz, DMSO-d6): 23.97
and 23.99 (d, JC,P¼4, (CH3)2CH), 25.60 (d, JC,P¼139, CH2P), 48.95
(CH2N), 57.17 (CH2-5-pyrr), 60.58 (CH2-2-pyrr), 61.60 (CH-4-pyrr),
69.41 (d, JC,P¼6, CH(CH3)2), 75.26 (CH-3-pyrr), 118.88 (C-5), 139.77
(CH-8), 149.56 (C-4), 152.48 (CH-2), 156.18 (C-6). 31P NMR
(162 MHz, DMSO-d6): 28.46.
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Support by grants #2B06065 (Ministry of Education, Youth and
Sports of the Czech Republic), #NR/9138-3 (Ministry of Health of
the Czech Republic), and MSM0021620835 (MEYS, CR), and Re-
search Centres LC06077 and LC06061 (MEYS, CR), and project
KAN200520801 (Acad. Sci., CR) under the Institute research project
Z40550506 is gratefully acknowledged. The authors are indebted to
the staff of the IOCB’s Department of Organic Analysis for mea-
surements of HRMS and IR spectra, and Prof. E. De Clercq (Rega
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