Synthesis of pyrrolidinone PNA: A novel conformationally restricted PNA analogue

Article abstract of DOI:10.1021/jo001000k

To preorganize PNA for duplex formation, a new cyclic pyrrolidinone PNA analogue has been designed. In this analogue the aminoethylglycine backbone and the methylenecarbonyl linker are connected, introducing two chiral centers compared to PNA. The four stereoisomers of the adenine analogue were synthesized, and the hybridization properties of PNA decamers containing one analogue were measured against complementary DNA, RNA, and PNA strands. The (3S,5R) isomer was shown to have the highest affinity toward RNA, and to recognize RNA and PNA better than DNA. The (3S,5R) isomer was used to prepare a fully modified decamer which bound to rU₁₀ with only a small decrease in T_m (ΔT_m/mod = 1°C) relative to aminoethylglycine PNA.

Full text of DOI:10.1021/jo001000k

J . Org. Chem. 2001, 66, 707-712
707
Syn th esis of P yr r olid in on e P NA: A Novel Con for m a tion a lly
Restr icted P NA An a logu e
Ask Pu¨schl,†,‡ Thomas Boesen,‡ Guido Zuccarello,‡ Otto Dahl,‡ Stefan Pitsch,§ and
Peter E. Nielsen*,†
Center for Biomolecular Recognition, Department for Biochemistry and Genetics, Biochemistry
Laboratory B, The Panum Institute, Blegdamsvej 3C, DK-2200, Copenhagen Denmark, Department of
Chemistry, The H. C. Ørsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100,
Copenhagen Denmark, and Laboratorium fu¨r Organische Chemie, ETH-Zentrum, Zu¨rich Switzerland
pen@imbg.ku.dk
Received J uly 3, 2000
To preorganize PNA for duplex formation, a new cyclic pyrrolidinone PNA analogue has been
designed. In this analogue the aminoethylglycine backbone and the methylenecarbonyl linker are
connected, introducing two chiral centers compared to PNA. The four stereoisomers of the adenine
analogue were synthesized, and the hybridization properties of PNA decamers containing one
analogue were measured against complementary DNA, RNA, and PNA strands. The (3S,5R) isomer
was shown to have the highest affinity toward RNA, and to recognize RNA and PNA better than
DNA. The (3S,5R ) isomer was used to prepare a fully modified decamer which bound to rU10 with
only a small decrease in Tm (∆Tm/mod ) 1 °C) relative to aminoethylglycine PNA.
In tr od u ction
the carboxyl end of PNA.5a-d Rotation around the meth-
ylene carbonyl linker in single stranded PNA can be
prevented by connecting the linker to the backbone
through a methylene bridge in a cyclic structure. A
somewhat similar approach using olefinic PNA analogues
has recently been published,6a-c where the central func-
tionality was replaced by a configurationally stable
carbon-carbon double bond to give E-OPA and Z-OPA
(Figure 1). Incorporation of the OPA monomers (B ) T)
once in the middle of a PNA sequence showed that the E
isomer, which has a configuration similar to the PNA
rotamer preferred in duplexes, gave a higher affinity to
antiparallel complementary DNA than the Z isomer.
However, both OPA isomers resulted in a substantial
decrease in affinity of the modified PNA toward DNA
(∆Tm ) -6.5 and -14.2 °C for a decamer PNA containing
one central E-, respectively, Z-OPA unit). No data were
given for the hybridization to RNA. Recently D’Costa et
al. published the synthesis of “aminoethylprolyl PNAs”,
in which the glycyl component in the PNA backbone
has been substituted by a prolyl unit, to which a thymine
base is attached at position 4.7 Most interestingly these
chiral and cationic PNA analogues showed increased
binding strength toward DNA, but further data is needed
to fully evaluate the properties of these interesting
analogues.
Peptide nucleic acid (PNA) is an acyclic mimic of
natural nucleic acids.1 Only the nucleobases are retained,
linked together by an achiral, uncharged pseudopeptide
backbone. PNA is an excellent structural mimic of DNA
and RNA; it binds strongly and with high specificity to
complementary oligonucleotides. The properties of PNA
and PNA analogues have recently been reviewed.2a,b
PNA/DNA or PNA/RNA duplex formation is accompanied
by a decrease in entropy. This entropy loss could be
reduced by using a more rigid PNA analogue. This has
been attempted by using conformationally constrained
chiral backbones.3a,b We have now designed a new
constrained structure, pyr-PNA, in which the aminoet-
hylglycine backbone and the methylenecarbonyl linker
are connected (Figure 1). NMR studies of single stranded
PNA oligomers clearly show that a complex mixture (up
to 2n) of isomers are present.4 Thus PNA hybridization
may be disfavored by a slow rotamer equilibrium of the
linker-backbone amide. The aim was to shift the equi-
librium between single strands and duplexes toward
duplex formation by preorganizing PNA in the duplex
strand conformation and thereby reducing the entropy
loss upon duplex formation. Examination of PNA/DNA,
PNA/RNA, and PNA/PNA duplexes as well as PNA2/DNA
triplexes, reveals that the linker-carbonyl points toward
Here, we report the synthesis and hybridization prop-
erties toward DNA, RNA, and PNA of the first “pyrroli-
† The Panum Institute.
‡ University of Copenhagen.
§ ETH-Zentrum.
(1) Nielsen, P. E.; Egholm, M.; Berg, R. H.; Buchardt, O. Science
1991, 254, 1497.
(2) (a) Nielsen, P. E.; Haaima, G. Chemical Soc. Rev. 1997, 73. (b)
Uhlmann, E.; Peyman, A.; Breipohl, G.; Will, D. W. Angew. Chem.,
Int. Ed. 1998, 37, 2796.
(5) (a) Eriksson, M.; Nielsen, P. E. Nat. Struct. Biol. 1996, 3, 410.
(b) Brown, S. C.; Thomson, S. A.; Veal, J . M.; Davis, D. G. Science 1994,
265, 777. (c) Rasmussen, H.; Kastrup, J . S.; Nielsen, J . N.; Nielsen, J .
M.; Nielsen, P. E. Nat. Struct. Biol. 1997, 4, 98. (d) Betts, L.; J osey, J .
A.; Veal, J . M.; J ordan, S. R. Science 1995, 270, 1838.
(3) (a) Lagriffoule, P.; Wittung, P.; Eriksson, M.; J ensen, K. K.;
Norde´n, B.; Buchardt, O.; Nielsen, P. E. Chem. Eur. J . 1997, 3, 912.
(b) Gangamani, B. P.; Kumar, V. A.; Ganesh, K. N. Tetrahedron 1999,
55, 177.
(4) Leijon, M.; Gra¨slund, A.; Nielsen, P. E.; Buchardt, O.; Norde´n,
B.; Kristensen, S. M.; Eriksson, M. Biochemistry 1994, 33, 9820.
(6) (a) Cantin, M.; Schu¨tz, R.; Leumann, C. J . Tetrahedron Lett.
1997, 38, 4211. (b) Roberts, C. D.; Schu¨tz, R.; Leumann, C. J . Synlett.
1999, 6, 819. (c) Schu¨tz, R.; Cantin, M.; Roberts, C.; Greiner, B.;
Uhlmann, E.; Leumann, C. Angew. Chem., Int. Ed. 2000, 39, 1250.
(7) D’Costa, M.; Kumar, V. A.; Ganesh, K. N. Org. Lett. 1999, 1,
1513.
10.1021/jo001000k CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/10/2001

708 J . Org. Chem., Vol. 66, No. 3, 2001
Pu¨schl et al.
F igu r e 1. Structures of DNA, PNA, and PNA analogues.
F igu r e 2. Synthesis of the (3R,5R) (10) and (3S,5R) (13) pyr-PNA A monomers. (a) HMDS, BuLi, THF; (b) MoOPH; (c) Bom-
chloride, DIEA, CH2Cl2; (d) TFA: CH2Cl2 (1:1) 0 °C; (e) NaH, BrCH2CO2CH3, THF; (f) (C2H5)3N‚3HF, THF; (g) MsCl, pyridine; (h)
NaN3, DMF, 80 °C; (i) H2, 10% Pd/C, Boc2O, AcOEt; (j) H2, 10% Pd(OH)2/C, MeOH; (k) adenine, PPh3, DEAD, dioxane; (l)
N-benzyloxycarbonyl-N′-methylimidazolium triflate, CH2Cl2; (m) LiOH, THF, then HCl; (n) PhCO2H, PPh3, DEAD, dioxane; (o)
NaOMe, MeOH.
dinone PNA” analogues, the adenine pyrrolidinone ana-
logues; 3S,5S pyr-PNA, 3R,5S pyr-PNA, 3R,5R pyr-PNA,
and 3S,5R pyr-PNA (Figure 1, B ) adenin-9-yl).8 Com-
pared to PNA, two chiral centers are introduced making
the syntheses of pyr-PNA more challenging since stereo-
selective synthesis is required.
Resu lts a n d Discu ssion
Syn th esis of th e (3R,5R) a n d (3S,5R) Mon om er s.
The adenine monomers were prepared as outlined in
Figure 2. Compound 1 was synthesized from D-pyro-
glutamic acid in five steps and 49% yield.9 The stereo-
selective hydroxylation of 1 has been reported in the
literature.10 The procedure uses oxodiperoxymolybde-
num-pyridine-hexamethylphosphoric triamide (MoOPH)
(8) Part of this work has been presented at a conference: Pu¨schl,
A.; Boesen, T.; Zuccarello, G.; Dahl, O.; Nielsen, P. E. Peptide Nucleic
Acids with a Constrained Cyclic Backbone. Presented at the Sixth
International Symposium of Solid-Phase Synthesis & Combinatorial
Libraries, Aug 31, 1999; Poster 33.
(9) Ackermann, J .; Matthes, M.; Tamm, C. Helv. Chim. Acta 1990,
73, 122.

Synthesis of Pyrrolidinone PNA
J . Org. Chem., Vol. 66, No. 3, 2001 709
Ta ble 1. Meltin g Tem p er a tu r es (Tm Va lu es)a
entry
sequence
alone
DNA complement
RNA complement
PNA complement
1
2
3
4
5
6
5′-d(AGT-GTA-CTA-C)
<20
37 (13%)
49.0
37.0 (11%)
55.5
39.5 (11%)
38.0 (9%)b
41.0 (13%)
52.0 (11%)
49.5
67.0
58.0 (21%)
65.0 (20%)
60.0
H-AGT-GTA-CTA-C-Lys-NH2
H-AGT-GTASS-CTA-C-Lys-NH2
H-AGT-GTARS-CTA-C-Lys-NH2
H-AGT-GTARR-CTA-C-Lys-NH2
H-AGT-GTASR-CTA-C-Lys-NH2
44
50 (8%)
37 (10%)
46 (7%)
40 (5%)
ntdc
39.0b
45 (9%)b
43.0 (9%)
64.0 (23%)
a
Tm ) melting temperature (measured in medium salt buffer: 100 mM NaCl, 10 mM phosphate, 0.1 mM EDTA, pH ) 7.0). Heating
b
rate: 1 K/min. UV absorbance measured at 254 nm. Hyperchromicity in parentheses. UV-titration (J ob-plot) failed to detect any binding.
ntd ) no transition detected.
Ta ble 2. Mism a tch a ga in st RNAa
entry
sequence
RNA-complement
MM-Ab
42.0
37.5 (9%)
MM-Cb
42.5
39.5 (11%)
MM-Gb
42.0
41.0 (11%)
7
8
H-AGT-GTA-CTA-C-Lys-NH2
H-AGT-GTASR-CTA-C-Lys-NH2
55.5
52.0 (11%)
a
Tm ) melting temperature (measured in medium salt buffer: 100 mM NaCl, 10 mM phosphate, 0.1 mM EDTA, pH ) 7.0). Heating
b
rate: 1 K/min. UV absorbance measured at 254 nm. Hyperchromicity in parentheses. MM-X ) The T in the complement is substituted
with X.
Ta ble 3. Meltin g Tem p er a tu r es (Tm Va lu es)a
as the oxidizing agent, and it is reported to proceed in
entry
sequence
alone
rU10
ent-rU10
58% yield with total stereoselectivity. In our hands, using
the same conditions, the hydroxylation gave, on a 21
mmol scale, a 95:5 mixture of 3S- and 3R-2. Protection
of the hydroxy group as its benzyloxymethyl (Bom)11
ether was followed by removal of the Boc group. The Bom
protective group was not completely stable to neat TFA,
so milder conditions for removal of Boc (TFA/CH2Cl2 (1:
1), 0 °C, 8 min) were used. At this point the pure 3S-
isomer (3) could be obtained by recrystallization from
AcOEt/hexane. The lactam nitrogen was alkylated12 and
the silyl protecting group was removed with Et3N‚3HF
in THF to produce 5 in nearly quantitative yield. This
was converted to the azide 7 via mesylate 6. Using the
mesyl derivative was found to give higher yield than
using the corresponding tosyl derivative (see Supporting
Information). The azide was hydrogenated to the amine
which was Boc protected in a one-pot procedure using
10% Pd/C as catalyst.13 The Bom protecting group was
almost completely resistant to hydrogenation under these
conditions. It was, however, removed by hydrogenation
using Pearlman’s catalyst to give 8. Under Mitsunobu
conditions, the configuration in the 3-position was in-
verted to yield the diasteromer 11. The hydroxy groups
in 8 and 11 were substituted with adenine under Mit-
sunobu conditions. 13C NMR of these intermediates
proved that the correct N-9 isomers had formed. The
exocyclic amine in the nucleobase was Z-protected using
Rapoport’s reagent.14 Finally, the methyl esters 9 and 12
were hydrolyzed to yield the fully protected monomers
10 and 13. After purification by chromatography (MeOH:
HOAc:CH2Cl2 10:5:85) the monomers were evaporated
several times from toluene and dried under a high
vacuum for one week to remove the HOAc completely
before oligomerization.15
9
10
11
H-eg1-(A)10-Lys-NH2
44
44
44
35
26
nd
nd
nd
24
H-(ASR
H-(ARS
)
)
10-Lys-NH2
10-Lys-NH2
a
Tm ) melting temperature (measured in medium salt buffer:
100 mM NaCl, 10 mM phosphate, 0.1 mM EDTA, pH ) 7.0).
Heating rate: 1 K/min. UV absorbance measured at 254 nm.
nd ) not determined.
synthesized from L-pyroglutamic acid in a similar way
as the (3R,5R) and (3S,5R) monomers except that the
tosyl analogue of 6 was used and TBAF was used instead
of Et3N‚3HF to remove TBDPS. Full experimental condi-
tions are given in the Supporting Information.
Th er m a l Sta bility. Each of the four novel monomers
was incorporated once in the middle of a decameric PNA
(Table 1, entries 3-6) and the thermal stability of the
complexes of the decamers with a complementary oligo-
nucleotide was compared with those formed by an
unmodified PNA oligomer (Table 1, entries 1-2). The
decamer incorporating the (3S,5R) analogue (Table 1,
entry 6) binds stronger to complementary RNA compared
to the other three decamers. In fact it binds only slightly
less efficiently than unmodified PNA (∆Tm ) 3.5 °C).
Furthermore the (3S,5R) PNA appears to discriminate
more between binding to RNA versus DNA (∆Tm ) 9 °C).
The remaining three stereoisomers (Table 1, entries 3-5)
hybridize with lower stability to complementary RNA,
although binding to the more flexible PNA was less
impaired. These results are in accordance with models
which indicated that the (3S,5R ) analogue best ap-
proximates the conformation of PNA in a PNA:RNA5b
double helix.
The (3S,5R) analogue was also shown to bind with high
sequence specificity to RNA (Table 2). To further evaluate
the affinity of the (3S,5R ) analogue toward RNA, a fully
modified adenine decamer was synthesized and the
binding affinity toward oligo U-RNA was evaluated
(Table 3 entries 9-10). The affinity of this analogue
toward RNA is only slightly decreased (∆Tm /mod ) 1
°C) compared to unmodified PNA. This was confirmed
by synthesizing a fully modified decamer of the (3R,5S )
analogue and measure the binding affinity (entry 11)
toward ent-RNA (also known as mirror-image RNA).16a,b
Virtually the same Tm was found in this case.
Syn th esis of th e (3R,5S) a n d (3S,5S) Mon om er s.
The (3R,5S) and the (3S,5S) adenine monomers were
(10) Shin, C.-G.; Nakamura, Y.; Yamada, Y.; Yonezawa, Y.; Ume-
mura, K.; Yoshimura, J . Bull. Chem. Soc. J pn. 1995, 68, 3151.
(11) Connor, D. S.; Klein, G. W.; Taylor, G. N.; Boeckman, R. K.,
J r.; Medwid, J . B. Organic Syntheses; Wiley: New York, 1973; Collect.
Vol. VI, p 101.
(12) Simanadan, T. Synth. Commun. 1996, 1827.
(13) Saito, S.; Nakajima, H.; Inaba, M.; Moriwake, T. Tetrahedron
Lett. 1989, 30, 837.
(14) Watkins, B. E.; Rapoport, H. J . Org. Chem. 1982, 47, 4471.
(15) Koch, T.; Hansen, H. F.; Andersen, P.; Larsen, T.; Batz, H. G.;
Otteson, K.; Ørum, H. J . Pept. Res. 1997, 49, 80.

710 J . Org. Chem., Vol. 66, No. 3, 2001
Pu¨schl et al.
the aqueous phase was extracted with AcOEt (3 ×). The
organic phases were combined, washed with brine, dried (Na2-
SO4), and evaporated in vacuo to give 13.43 g of a crude
product which was purified by chromatography (a stepwise
gradient of AcOEt in hexane from 2:3 to 2:1). Yield: 4.12 g of
2 as a white solid (42%). The product was contaminated with
5% of the 3R,5R-isomer as judged by 1H NMR. mp 145-147
°C. Rf ) 0.15 (AcOEt/heptane 2:3). 1H NMR (400 MHz, DMSO-
d6): (chemical shifts for the minor isomer are given in
parentheses) δ 7.61-7.37 (m, 10 H), 5.80 (d, J ) 7.5, 1H), 4.52
(m, 1H), 4.12 (m, 1H), 3.84 (dd, J ) 10.5, 3.6, 1H), 3.64 (dd, J
) 10.5, 2.0, 1H), 2.39 (m, 1H), 1.96 (m, 1H), 1.35 (1.32) (s, 9H),
0.95 (0.98) (s, 9H). Anal. Calcd for C26H35NO5Si: C, 66.49; H,
7.51; N, 2.98. Found: C, 66.27; H, 7.61; N, 2.97.
Surprisingly, the binding stoichiometry was found by
titration (J ob-plot) to be 2:1 (entry 10 versus rU10)
indicating a PNA2-RNA triplex structure. Most likely
the two PNA strands bind to each other through reverse
Hoogsteen hydrogen bonding. Titration of the binding of
the unmodified PNA to complementary rU10 showed the
expected 1:1 binding mode.
Con clu sion
Four new conformationally restricted PNA adenine
monomers have been synthesized. The restriction, a five-
membered ring, forces the carbonyl group of the linker
to point toward the COOH-terminus of the backbone, as
is usually the case when aminoethylglycine PNA oligo-
mers are bound to complementary DNA, RNA, or PNA.
The novel monomers were built into PNA oligomers
once by standard PNA synthesis, and the melting tem-
peratures of the modified oligomers toward complemen-
tary DNA, RNA, and PNA strands were measured. The
(3S,5R ) isomer was shown to have the highest affinity
toward RNA and to recognize RNA (and PNA) better than
DNA. The (3S,5R ) isomer was used to prepare a fully
modified decamer which bound to rU10 with only a small
decrease in Tm (∆Tm/mod ) 1 °C). The slightly decreased
Tm compared to unmodified PNA indicates that the five-
membered ring may not be the optimal modification to
restrict PNA, and work is in progress to examine other
ring structures.
(3S,5R)-3-Ben zyloxym eth oxy-5-[(ter t-bu tyld ip h en ylsi-
lyloxy)m eth yl]-2-p yr r olid in on e (3). 2 (12.59 g, 26.8 mmol)
was dried by coevaporation from CH3CN/CH2Cl2 (1:1) and then
redissolved in CH2Cl2 (60 mL). Bom-chloride (11.2 mL, 80.4
mmol) and then DIEA (14.0 mL, 80.4 mmol) were added at 0
°C. The solution was stirred at room temperature overnight.
More CH2Cl2 was added, and the solution was extracted with
half saturated aqueous NH4Cl (2 ×). The organic phase was
washed with brine, dried (Na2SO4), and evaporated in vacuo.
Purification by chromatography (a stepwise gradient of AcOEt
in heptane from 1:4 to 2:1) afforded the BOM-protected
intermediate as a clear oil. Yield: 14.9 g (94%). Rf: 0.55
1
(AcOEt/hexane 2:3). H NMR (300 MHz, DMSO-d6): δ 7.59-
7.28 (m, 15 H), 4.96 (d, J ) 6.5, 1H), 4.86-4.79 (m, 2H), 4.60
(s, 2H) 4.21-4.18 (m, 1H), 3.85 (d, J ) 7.6, 1H), 3.66 (d, J )
10.6, 1H), 2.45-2.38 (m, 1H), 2.18-2.07 (m, 1H), 1.36 (s, 9H),
0.92 (s, 9H). This purified intermediate (14.9 g, 25.2 mmol)
was dissolved in CH2Cl2 (25 mL). TFA (25 mL) was added
dropwise at 0 °C during 1 min, and the solution was stirred
for 8 min. The solution was transferred to a 1 L Erlenmeyer
flask and quenched by addition of saturated aqueous NaHCO3.
The aqueous phase was extracted with CH2Cl2 and AcOEt (2
×). The combined organic phases were dried (Na2SO4) and
evaporated in vacuo. Purification by chromatography (AcOEt/
heptane 2:1) afforded 9.27 g of a white solid. Recrystallization
from a mixture of AcOEt (35 mL) and hexane (80 mL) afforded
Exp er im en ta l Section
Gen er a l. Reagents, except benzyl chloromethyl ether,11
MoOPH,17 and N-benzyloxycarbonyl-N′-methylimidazolium
triflate,18 which were prepared according to literature proce-
dures, were purchased from Sigma-Aldrich and used without
purification. Solvents were HPLC-grade from LAB-SCAN.
Acetonitrile, N,N′-dimethylformamide, benzene, toluene, di-
oxane, methylene chloride, and pyridine were dried over 4 Å
molecular sieves. Tetrahydrofuran was dried by distillation
from sodium/ benzophenone. TLC was run on Merck 5554
silica 60 aluminum sheets. Column chromatography was
performed as flash chromatography on Merck 9385 silica gel
60 (0.040-0.063 mm). Reactions were carried out under
nitrogen except in the case of hydrogenations. FAB-MS were
recorded in the positive ion mode. Elemental analyses were
performed at the Microanalytical Laboratory, Department of
Chemistry, University of Copenhagen. NMR spectra were
obtained on a 300 or 400 MHz spectrometer. δ-Values are in
ppm relative to DMSO-d6 (2.50 for proton and 39.5 for carbon)
or CDCl3 (7.29 for proton and 76.9 for carbon) or CD3OD (3.35
for proton and 49.1 for carbon).
(3S,5R)-N-ter t-Bu toxyca r bon yl-5-[(ter t-bu tyld ip h en yl-
silyloxy)m eth yl]-3-h ydr oxy-2-pyr r olidin on e (2). BuLi (42.1
mL, 63.2 mmol, 1.5 M in hexane) was added dropwise to a
solution of hexamethyldisilazane (13.3 mL, 63.2 mmol) in THF
(50 mL) at -78 °C. The solution was stirred at -78 °C for 30
min. A solution of 1 (9.55 g, 21.05 mmol) in THF (50 mL) was
added over a period of 5 min. Stirring was continued for
another 30 min at -78 °C, and then the reaction mixture was
heated to -40 °C during the next 20 min, before MoOPH (18.3
g, 42.1 mmol) was added in two portions. The green solution
was stirred between -40 °C and -30 °C for 45 min, and the
reaction was then quenched by the addition of half saturated
aqueous NH4Cl (150 mL). The THF was evaporated off, and
1
3 as white needles. Yield: 7.31 g (59%). mp 108-109 °C. H
NMR (400 MHz, DMSO-d6): δ 8.02 (s, 1H), 7.62-7.59 (m, 4H),
7.49-7.39 (m, 6H), 7.35-7.27 (m, 5H), 4.98 (d, J ) 6.8, 1H),
4.79 (d, J ) 6.6, 1H), 4.59 (s, 2H), 4.45 (t, J ) 8.2, 1H), 3.66-
3.63 (m, 1H), 3.55 (s, 2H), 2.30-2.24 (m, 1H), 2.07-2.01 (m,
1H), 0.97 (s, 9H). 13C NMR (100.6 MHz, DMSO-d6): δ 174.6,
138.1, 135.2, 135.1, 132.8, 132.5, 130.0, 129.9, 128.2, 128.0,
127.9, 127.7, 127.4, 93.6, 72.3, 68.9, 66.4, 51.6, 31.6, 26.6, 18.8.
Anal. Calcd for C29H35NO4Si: C, 71.13; H, 7.20; N, 2.86.
Found: C, 71.20; H, 7.17; N, 2.93. [R]22D -57.8 (c 1.45, MeOH).
(3S,5R)-3-Ben zyloxym eth oxy-5-[(ter t-bu tyld ip h en ylsi-
lyloxy)m eth yl]-N-[m eth oxyca r bon ylm eth yl]-2-p yr r olid i-
n on e (4). 3 (4.06 g, 8.29 mmol) was dissolved in THF (40 mL).
NaH (60% suspension in mineral oil, 0.663 g, 16.6 mmol) and
then methyl bromoacetate (1.57 mL, 16.6 mmol) were added
at 0 °C, and the reaction mixture was warmed to room
temperature and stirred overnight. More NaH (60% suspen-
sion in mineral oil, 0.663 g, 16.6 mmol) and methyl bromoac-
etate (1.57 mL, 16.6 mmol) were added, and the reaction
mixture stirred for 2 h at room temperature. The reaction was
quenched by the slow addition of half saturated aqueous NH4-
Cl (125 mL), and the solution was extracted with AcOEt (2
×). The combined organic phases were washed with brine,
dried (Na2SO4), and evaporated in vacuo to give the crude
product as an oil (6.2 g) which was purified by chromatography
(AcOEt/heptane 2:3). Yield: 4.65 g (100%) of 4 as a clear oil.
Rf: 0.52 (AcOEt/heptane 2:1). 1H NMR (300 MHz, DMSO-d6):
δ 7.60-7.29 (m, 15 H), 4.98 (d, J ) 7.0, 1H), 4.80 (d, J ) 6.5,
1H), 4.60 (s, 2H), 4.17 (d, J ) 17.6, 2H), 4.04-3.63 (m, 3H),
3.62 (s, 3H), 2.28 (m, 1H), 1.99 (m, 1H), 0.95 (s, 9H). Anal.
Calcd for C32H39NO6Si: C, 68.42; H, 7.00; N, 2.49. Found: C,
67.85; H, 7.24; N, 2.50.
(16) (a) Ashley, G. W. J . Am. Chem. Soc. 1992, 114, 9731. (b) Pitsch,
S. Helv. Chim. Acta 1997, 80, 2286.
(17) Vedejs, E.; Larsen, S. Organic Syntheses; Wiley: New York,
1990; Collect. Vol. VII, p 277.
(18) Haaima, G.; Hansen, H. F.; Christensen, L.; Dahl, O.; Nielsen,
P. E. Nucleic Acids Res. 1997, 25, 4639.
(3S,5R)-3-Ben zyloxym eth oxy-5-h ydr oxym eth yl-N-m eth -
oxyca r bon ylm eth yl-2-p yr r olid in on e (5). 4 (6.57 g, 11.7
mmol) was dried by coevaporation from CH3CN/CH2Cl2 (1:1)

Synthesis of Pyrrolidinone PNA
J . Org. Chem., Vol. 66, No. 3, 2001 711
and then redissolved in THF (60 mL). Et3N‚3HF (3.7 mL, 23
mmol)) was slowly added, and the reaction was stirred at 50
°C for 2 h after which another portion of Et3N‚3HF (3.7 mL,
23 mmol) was added. The reaction was stirred at 50 °C for an
additional 8 h. The solvent was evaporated off and the crude
product purified by chromatography (12% MeOH in CH2Cl2).
Yield: 3.73 g (99%) of 5 as a clear oil. 1H NMR (300 MHz,
DMSO-d6): δ 7.36-7.29 (m, 5H), 4.96 (d, J ) 6.5, 1H), 4.85 (t,
J ) 4.8, 1H), 4.79 (d, J ) 6.5, 1H), 4.60 (s, 2H), 4.44 (t, J )
8.1, 1H), 4.19 (d, J ) 17.3, 1H), 3.93 (d, J ) 17.3, 1H), 3.65 (s,
3H), 3.55 (m, 1H), 3.37 (m, 1H), 2.24 (m, 1H), 2.00 (m, 1H).
13C NMR (100.6 MHz, DMSO-d6): δ 172.7, 169.3, 138.0, 128.3,
127.8, 127.5, 93.7, 72.5, 69.0, 61.4, 56.8, 52.0, 42.3, 30.4. Anal.
added, and the mixture was hydrogenated overnight at 1 atm
and room temperature and then passed through Celite. The
solution was evaporated, and the crude product (2.75 g) was
purified by chromatography (CH2Cl2/MeOH 9:1). Yield: 2.23
1
g of 8 as a white solid (94%), Rf: 0.16 (AcOEt). H NMR (300
MHz, DMSO-d6): δ 6.91 (t, J ) 6.0, 1H), 5.58 (d, J ) 5.1, 1H),
4.19 (m, 1H), 4.13 (d, J ) 18.0, 1H) 3.87 (d, J ) 17.9, 1H),
3.65 (s, 3H), 3.62 (m, 1H), 3.11 (m, 2H), 2.22 (m, 1H), 1.83 (m,
1H), 1.36 (s, 9H). 13C NMR (75.5 MHz, CDCl3): δ 174.6, 169.5,
155.9, 78.0, 67.2, 55.1, 51.9, 42.2, 41.0, 32.3, 28.2. Anal. Calcd
for C13H22N2O6‚0.25 H2O: C, 50.89; H, 7.39; N, 9.13. Found:
C, 50.63; H, 7.37; N, 9.00.
(3R,5R)-3-[N6-(Ben zyloxyca r bon yl)a d en in -9-yl]-5-ter t-
bu toxycar bon ylam in om eth yl-N-m eth oxycar bon ylm eth yl-
2-p yr r olid in on e (9). 8 (302 mg, 1.00 mmol) was dried by
coevaporation from dry CH3CN (5 mL) and then redissolved
in dry dioxane (20 mL). PPh3 (0.565 g, 2.50 mmol) was added
followed by adenine (675 mg, 5.00 mmol). To this suspension
was slowly (during 30 min) added DEAD (0.32 mL, 2.0 mmol)
at room temperature and the suspension stirred at room
temperature overnight. The solvent was evaporated off, and
the residue was purified twice by chromatography (first
column: AcOEt then 10% MeOH/CH2Cl2. Second column: 15%
MeOH/ CH2Cl2). Yield: 175 mg white solid (42%). 1H NMR
(300 MHz, CD3OD): δ 8.23 (s, 1H), 8.11 (s, 1H), 5.46 (t, J )
9.9, 1H), 4.45 (d, J ) 18.0, 1H), 4.09 (d, J ) 18.0, 1H), 4.08
(m, 1H), 3.76 (s, 3H), 3.53 (d, J ) 15, 1H), 3.35 (d, J ) 15,
1H), 2.86 (m, 1H), 2.38 (m, 1H), 1.42 (s, 9H, Boc). 13C NMR
(75.5 MHz, CD3OD): δ 172.9, 170.5, 158.6, 157.4, 153.9, 150.5,
142.3, 120.3, 80.7, 56.4, 56.3, 53.1, 43.7, 42.2, 29.6, 28.8. This
intermediate (155 mg, 0.37 mmol) was dissolved in dry CH2-
Cl2 (2.5 mL), and N-benzyloxycarbonyl-N′-methylimidazolium
triflate (542 mg, 1.48 mmol) was added. The reaction was
stirred overnight, after which more N-benzyloxycarbonyl-N′-
methylimidazolium triflate (125 mg, 0.23 mmol) was added,
and the reaction was stirred 3 h. Half-saturated aqueous
NaHCO3 (25 mL) and CH2Cl2 were added, and the aqueous
phase was extracted with CH2Cl2 and AcOEt. The combined
organic phases were dried (MgSO4) and evaporated in vacuo.
The crude product was purified by chromatography (AcOEt
then 15% MeOH/AcOEt). Yield: 132 mg (64%) of 9 as a white
Calcd for
Found: C, 58.44; H, 6.39; N, 4.20.
C
16H21NO6‚1/4H2O: C, 58.63; H, 6.63; N, 4.28.
(3S,5R)-3-Ben zyloxym eth oxy-N-m eth oxyca r bon ylm e-
th yl-5-[m eth ylsu lfon yloxym eth yl]-2-p yr r olid in on e (6). 5
(376 mg, 1.16 mmol) was dried by evaporation from CH3CN/
CH2Cl2 (3:5) and then redissolved in CH2Cl2 (6 mL). Et3N (242
µL, 1.74 mmol) and methanesulfonyl chloride (99 µL, 1.28
mmol) were added at 0 °C. The reaction was stirred at 0 °C
for 45 min after which more methanesulfonyl chloride (99 µL,
1.28 mmol) was added, and the reaction was stirred at 0 °C
for 90 min, when TLC (MeOH:CH2Cl2 4:96) showed the
reaction to be complete. The reaction mixture was quenched
by addition of half saturated aqueous NaHCO3 (25 mL) and
CH2Cl2. The aqueous phase was extracted with CH2Cl2 (2 ×).
The combined organic phases were dried over MgSO4 and
evaporated in vacuo to give 520 mg of crude product which
was purified by chromatography (a stepwise gradient of 2-4%
MeOH in CH2Cl2). Yield: 346 mg (86%) of 6 as a clear oil. Rf:
0.46 (MeOH:CH2Cl2 4:96). 1H NMR (300 MHz, CDCl3): δ 7.38-
7.28 (m, 5 H), 5.10 (d, J ) 7.2, 1H), 4.89 (d, J ) 6.9, 1H), 4.69
(s, 2H), 4.52 (t, J ) 7.8, 1H), 4.35-4.00 (m, 5H), 3.76 (s, 3H),
3.03 (s, 3H), 2.29 (m, 2H). 13C NMR (75.5 MHz, CDCl3): δ
173.1, 168.7, 137.4, 128.3, 127.8, 127.6, 94.2, 71.6, 69.9, 68.8,
55.9, 52.3, 43.1, 37.5, 30.1. FABMS m/z 402.1 (M + H). Anal.
Calcd for C17H23NO8S: C, 50.87; H, 5.77; N, 3.49. Found: C,
50.43; H, 5.94; N, 3.44.
(3S,5R)-5-Azid om et h yl-3-b en zyloxym et h oxy-N-m et h -
oxyca r bon ylm eth yl-2-p yr r olid in on e (7). 6 (3.80 g, 9.47
mmol) was dissolved in DMF (50 mL), and NaN3 was (3.08 g,
47.4 mmol) added. The solution was stirred at 80 °C overnight.
The solvent was evaporated off and the resulting oil parti-
tioned between half saturated aqueous NaHCO3 and AcOEt.
The aqueous phase was extracted with AcOEt (2 ×) and CH2-
Cl2. The combined organic phases were dried (MgSO4) and
evaporated in vacuo. The crude product was purified by
chromatography (4% MeOH in CH2Cl2). Yield: 3.14 g (98%)
of 7 as an oil. 1H NMR (300 MHz, DMSO-d6): δ 7.36-7.28
(m, 5H), 4.97 (d, J ) 6.6, 1H), 4.81 (d, J ) 6.7, 1H), 4.61 (s,
2H), 4.50 (t, J ) 8.5, 1H), 4.17 (d, J ) 17.6, 1H), 4.03 (d, J )
17.6, 1H), 3.80 (m, 1H), 3.73-3.68 (m, 1H), 3.67 (s, 3H), 3.53-
3.49 (m, 1H), 2.27-2.21 (m, 1H), 2.10-2.02 (m, 1H). 13C NMR
(75.5 MHz, DMSO-d6): δ 172.6, 169.0, 137.9, 128.3, 127.8,
127.5, 93.8, 71.9, 69.0, 54.5, 52.2, 52.0, 42.3, 30.9. Anal. Calcd
for C16H20N4O5‚1/4H2O: C, 54.47; H, 5.87; N, 15.88. Found: C,
54.65; H, 5.84; N, 16.00.
1
solid. Total yield 24%. H NMR (300 MHz, CDCl3): δ 8.66 (s,
1H), 7.96 (s, 1H), 7.33-7.22 (m, 5H), 6.56 (br s, 1H), 5.19 (s,
2H), 5.08 (m, 1H), 4.44 (d, J ) 17.9, 1H), 4.07 (m, 1H), 3.82
(d, J ) 17.9, 1H), 3.66 (s, 3H), 3.60 (d, J ) 14.7, 1H), 3.24 (d,
J ) 13.5, 1H), 2.70 (m, 1H), 2.44 (m, 1H), 1.35 (s, 9H). 13C
NMR (75.5 MHz, CDCl3): δ 169.4, 168.4, 156.1, 152.2, 151.0,
150.2, 149.6, 143.2, 135.2, 128.3, 128.1, 122.2, 79.5, 67.4, 55.0,
53.4, 52.2, 42.0, 40.7, 28.1, 27.2. FABHRMS m/z 554.2364 (M
+ H, C26H32N7O7 requires 554.2363).
(3R,5R)-3-[N6-(Ben zyloxyca r bon yl)a d en in -9-yl]-5-ter t-
bu toxyca r bon yla m in om eth yl-N-ca r boxym eth yl-2-p yr r o-
lid in on e (10). 9 (120 mg, 0.217 mmol) was dissolved in THF
(2.2 mL), and 1 M aqueous LiOH (0.54 mL, 0.54 mmol) was
added at 0 °C. After 20 min, H2O (4 mL) was added and the
THF was evaporated off. 10 was precipitated by addition of 4
M HCl (0.25 mL) at 0 °C. The crude product was purified by
chromatography (CH2Cl2:MeOH:HOAc 80:15:5). Fractions con-
taining 10 were pooled, and the solvent was removed in vacuo.
HOAc was completely removed by coevaporation from MeOH/
toluene (three times) and MeOH/CH3CN. The resulting white
solid was dried under a high vacuum for one week. Yield: 68
mg (58%) of 10 as a white solid, Rf: 0.24 (CHCl3/EtOH/AcOH
80:15:5). 1H NMR (400 MHz, DMSO-d6): δ 10.65 (s, 1H), 8.59
(s, 1H), 8.42 (s, 1H) 7.47-7.32 (m, 5H), 7.12 (m, 1H), 5.49 (t,
J ) 9.9, 1H), 5.22 (s, 2H), 4.24 (d, J ) 17.8, 1H), 3.96 (m, 1H),
3.95 (d, J ) 17.6, 1H), 3.30 (s, 2H), 2.72 (m, 1H), 2.34 (m, 1H),
1.37 (s, 9H). 13C NMR (100.6 MHz, DMSO-d6): δ 172.0, 170.3,
169.8, 156.0, 152.1, 151.6, 151.4, 149.7, 143.8, 136.4, 128.4,
128.0, 127.9, 123.6, 78.1, 66.3, 54.7, 54.4, 42.7, 41.1, 28.2, 21.1.
FABHRMS m/z 540.2227 (M + H, C25H30N7O7 requires
(3S,5R)-5-ter t-Bu toxycar bon ylam in om eth yl-3-h ydr oxy-
N-m eth oxyca r bon ylm eth yl-2-p yr r olid in on e (8). 10% Pd/C
(0.61 g) was added to a stirred solution of 7 (3.10 g, 8.90 mmol)
and Boc2O (3.90 g, 17.8 mmol) in AcOEt (90 mL) at 0 °C. The
mixture was hydrogenated at 1 atm for 90 min at room
temperature and then passed through Celite. The solvent was
evaporated off, and the crude product (5.9 g) was purified by
chromatography (AcOEt). Yield: 3.37 g (89%) of the BOM-
protected intermediate. 1H NMR (300 MHz, DMSO-d6):δ 7.35-
7.28 (m, 5H), 6.95 (t, J ) 5.7, 1H), 4.97 (d, J ) 6.9, 1H), 4.78
(d, J ) 6.6, 1H), 4.59 (m, 2H), 4.39 (t, J ) 8.4, 1H), 4.20 (d, J
) 18.0, 1H), 4.02 (m, 2H), 3.88 (d, J ) 18.0, 1H), 3.71 (m, 1H),
3.77 (s, 3H), 2.38-2.31 (m, 1H), 2.03-1.95 (m, 1H), 1.34 (s,
9H). 13C NMR (75.5 MHz, DMSO-d6):δ 172.4, 169.3, 156.0,
137.9, 128.2, 127.7, 127.5, 93.5, 78.1, 71.9, 69.0, 55.5, 52.0, 42.1,
40.9, 30.6, 28.1. This purified intermediate (3.32 g, 7.84 mmol)
was dissolved in MeOH (112 mL), 10% Pd(OH)2/C (0.66 g) was
540.2207). [R]22 +29.4 (c 0.333, MeOH).
D
(3R,5R)-5-ter t-Bu toxycar bon ylam in om eth yl-3-h ydr oxy-
N-m eth oxyca r bon ylm eth yl-2-p yr r olid in on e (11). 8 (2.19
g, 7.24 mmol) was dried by evaporation from CH3CN (15 mL)

712 J . Org. Chem., Vol. 66, No. 3, 2001
Pu¨schl et al.
and then redissolved in THF (25 mL). A solution of PPh3 (5.72
g, 21.72 mmol) in THF (25 mL) and a solution of benzoic acid
(4.44 g, 36.2 mmol) in toluene (70 mL) was successively added
at 0 °C, and the mixture stirred for 2 min, before DEAD (5.71
mL, 36.2 mmol) was added dropwise. The clear yellow solution
was allowed to warm to room temperature and stirred over-
night. AcOEt was added, and the mixture was extracted with
0.5 M aq citric acid, brine, saturated aqueous NaHCO3, and
brine. The organic phase was dried (MgSO4) and evaporated.
The crude product was purified by chromatography (AcOEt/
hexane 4:1, then pure AcOEt). Yield: 2.92 g (99%) of the
intermediate as a white foam. 1H NMR (300 MHz, DMSO-d6):
δ 8.00-7.52 (m, 5H), 6.97 (br s, 1H), 5.51 (t, J ) 7.8, 1H), 4.31
(d, J ) 17.7, 1H), 4.05 (d, J ) 18.3, 1H), 3.78 (br s, 1H), 3.68
(s, 3H), 3.23 (m, 2H), 2.66 (m, 1H), 1.88 (m, 1H), 1.34 (s, 9H).
13C NMR (75.5 MHz, DMSO-d6): δ 170.4, 169.0, 164.9, 156.0,
133.7, 129.3, 129.1, 128.8, 78.0, 70.5, 54.7, 52.1, 42.3, 40.8, 28.6,
28.1. This intermediate (2.88 g, 7.09 mmol) was dissolved in
MeOH (50 mL) and cooled to 0 °C. NaOMe in methanol (1.05
M, 13.5 mL, 14.18 mmol) was added dropwise, and the solution
was stirred at 0 °C for 30 min. The reaction was quenched by
addition of half-saturated aqueous NH4Cl (100 mL). The
aqueous phase was extracted with AcOEt (4 ×) and CH2Cl2.
The organic phases were combined, dried over Na2SO4 and
evaporated in vacuo. Chromatography (AcOEt, then CH2Cl2/
MeOH 9:1) afforded 11 as a white foam. Yield: 1.88 g (88%).
aqueous NaHCO3 (40 mL) and CH2Cl2 were added, and the
aqueous phase was extracted with CH2Cl2 and AcOEt. The
combined organic phases were dried (MgSO4) and evaporated
in vacuo. The crude product was purified by chromatography
(AcOEt then 10% MeOH/AcOEt). Yield: 507 mg (70%) of 12
1
as a white solid. Total yield 49%. H NMR (400 MHz, DMSO-
d6): δ 10.68 (s, 1H), 8.60 (s, 1H), 8.38 (s, 1H), 7.45-7.36 (m,
5H), 7.05 (br s, 1 H), 5.53 (t, 1H), 5.19 (s, 2H), 4.23 (d, J )
17.8, 1H), 4.10 (d, J ) 17.8, 1H), 3.98 (m, 1H), 3.68 (s, 3H),
3.24 (m, 2H), 2.75 (m, 1H), 2.58 (m, 1H), 1.40 (s, 9H). 13C NMR
(100.6 MHz, DMSO-d6): δ 169.8, 169.1, 156.1, 152.1, 152.0,
151.6, 149.7, 143.2, 136.4, 128.4, 128.0, 127.9, 123.4, 78.3, 66.3,
56.1, 53.6, 52.2, 43.2, 41.3, 29.2, 28.2. FABMS m/z 554.2 (M +
H). Anal. Calcd for C26H31N7O7‚1.5 H2O: C, 53.81; H, 5.92; N,
16.90. Found: C, 54.01; H, 5.52; N, 16.43.
(3S,5R)-3-[N6-(Ben zyloxyca r bon yl)a d en in -9-yl]-5-ter t-
bu toxyca r bon yla m in om eth yl-N-ca r boxym eth yl-2-p yr r o-
lid in on e (13). LiOH (1M, 2.1 mL, 2.1 mmol) was slowly added
to a stirred solution of 12 (467 mg, 0.84 mmol) in THF (9 mL)
at 0 °C, and the reaction was stirred at 0 °C for 20 min. H2O
(20 mL) was added, and the THF was evaporated off. The
product was precipitated by the slow addition of 4 M HCl (0.6
mL) at 0 °C. The crude product was purified by chromatog-
raphy (CH2Cl2:MeOH:HOAc 85:10:5). Fractions containing 13
were pooled, and the solvent was removed in vacuo. HOAc was
completely removed by coevaporation from MeOH/toluene
(three times) and MeOH/CH3CN. The resulting white solid was
dried in high vacuum for one week. Yield: 152 mg (33%). Rf:
0.55 (CHCl3/EtOH/AcOH 80:15:5). 1H NMR (300 MHz, DMSO-
d6): δ 10.68 (s, 1H), 8.59 (s, 1H), 8.42 (s, 1H), 7.44-7.29 (m,
5H), 7.11 (m, 1H), 5.49 (t, J ) 9.0, 1H), 5.20 (s, 2H), 3.97 (m,
2H), 3.28-3.22 (m, 2H), 2.76-2.60 (m, 2H), 1.40 (s, 9H). 13C
NMR (100.6 MHz, DMSO-d6): δ 170.5, 169.6, 156.1, 152.2,
152.0, 151.6, 149.7, 143.2, 136.4, 128.4, 128.0, 127.9, 123.3,
78.3, 66.3, 56.5, 53.7, 44.3, 41.2, 29.5, 28.3. FABHRMS m/z
540.2212 (M + H, C25H30N7O7 requires 540.2207). Anal. Calcd
for C25H29N7O7‚2H2O: C, 52.17; H, 5.78; N, 17.03. Found: C,
1
Total yield: 87%. H NMR (300 MHz, DMSO-d6): δ 6.90 (t, J
) 5.7, 1H), 5.66 (d, J ) 5.1, 1H), 4.19 (d, J ) 17.7, 1H), 4.11
(m, 1H), 3.94 (d, J ) 18.0, 1H), 3.64 (s, 3H), 3.55 (m, 1H), 3.17
(m, 2H), 2.31 (m, 1H), 1.51 (m, 1H), 1.38 (s, 9H). 13C NMR
(75.5 MHz, DMSO-d6): δ 175.4, 169.3, 155.9, 78.0, 68.1, 53.5,
51.9, 41.9, 41.3, 31.8, 28.2. Anal. Calcd for C13H22N2O6‚0.25
H2O: C, 50.88; H, 7.41; N, 9.13. Found: C, 50.87; H, 7.46; N,
8.93.
(3S,5R)-3-[N6-(Ben zyloxyca r bon yl)a d en in -9-yl]-5-ter t-
bu toxycar bon ylam in om eth yl-N-m eth oxycar bon ylm eth yl-
2-p yr r olid in on e (12). 11 (604 mg, 2.0 mmol) was dried by
coevaporation from dry CH3CN and then redissolved in dry
dioxane (40 mL). PPh3 (1.13 g, 5.0 mmol) was added followed
by adenine (1.35 g, 10.0 mmol). To this suspension was slowly
(during 30 min) added DEAD (0.63 mL, 4.0 mmol) at room
temperature, and the suspension stirred at room temperature
overnight. The solvent was evaporated off and the residue
purified by chromatography (AcOEt then 10% MeOH/CH2Cl2).
52.60; H, 5.33; N, 16.71. [R]22 -11.7 (c 0.3, MeOH).
D
Ackn owledgm en t. Annette W. Jørgensen and Jolan-
ta Ludvigsen are thanked for their expert technical
assistance. Dr Henrik Frydenlund Hansen (Pantheco
A/S) is thanked for providing the UV-melting curves
regarding Table 3. The Danish Natural Science Re-
search Council and The Danish Technical Research
Council are acknowledged for financial support.
1
Yield: 587 mg (70%). H NMR (400 MHz, DMSO-d6): δ 8.13
(s, 1H), 8.04 (s, 1H), 7.27 (s, 2H), 7.07 (t, J ) 5.7, 1H), 5.43 (t,
J ) 5.43, 1H), 4.23 (d, J ) 17.8, 1H), 4.11 (d, J ) 17.8, 1H),
3.94 (m, 1H), 3.70 (s, 3H), 3.26 (m, 2H), 2.66 (m, 1H), 2.56 (m,
1H). 13C NMR (100.6 MHz, DMSO-d6): δ 170.2, 169.1, 156.1,
152.5, 149.6, 139.6, 118.6, 78.3, 56.0, 53.1, 52.2, 43.2, 41.3, 29.6,
28.2. The intermediate (547 mg, 1.31 mmol) was dissolved in
dry CH2Cl2 (9 mL) and N-benzyloxycarbonyl-N′-methylimida-
zolium triflate (1.92 g, 5.24 mmol) was added. The reaction
was stirred at room temperature overnight. Half-saturated
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures for the synthesis of the (3R,5S) and (3S,5S) monomers.
Copies of 1H and 13C NMR spectra of compound 10. UV-melting
curves and UV-titration curves. This material is available free
of charge via the Internet at http://pubs.acs.org.
J O001000K