Synthesis of the phosphoramidite derivatives of 2′-deoxy-2′-C- α-methylcytidine and 2′deoxy-2′-C- αhydroxymethylcytidine: Analogues for chemical dissection of RNA's 2′-hydroxyl group

Article abstract of DOI:10.1021/jo0495337

Oligonucleotides containing 2′-C-αmethyl and 2′-C-α-hydroxymethyl modifications enable strategies for delineation of the distinctive role fulfilled by the 2′-hydroxyl group in RNA structure and function. Synthetic routes to the phosphoramidite derivatives

Full text of DOI:10.1021/jo0495337

Syn th esis of th e P h osp h or a m id ite Der iva tives of
2′-Deoxy-2′-C-r-m eth ylcytid in e a n d
2′-Deoxy-2′-C-r-h yd r oxym eth ylcytid in e: An a logu es for Ch em ica l
Dissection of RNA’s 2′-Hyd r oxyl Gr ou p
Nan-Sheng Li and J oseph A. Piccirilli*
Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biology and Department of
Chemistry, University of Chicago, 5841 South Maryland Avenue, MC 1028, Chicago, Illinois 60637
jpicciri@midway.uchicago.edu
Received March 22, 2004
Oligonucleotides containing 2′-C-R-methyl and 2′-C-R-hydroxymethyl modifications enable strategies
for delineation of the distinctive role fulfilled by the 2′-hydroxyl group in RNA structure and function.
Synthetic routes to the phosphoramidite derivatives of 2′-deoxy-2′-C-R-methylcytidine (14%, 15 steps)
and 2′-deoxy-2′-C-R-hydroxymethylcytidine (19%, 10 steps) from methyl 3,5-di-O-(4-chlorobenzyl)-
R-D-ribofuranoside are developed.
CHART 1. P ossible Mech a n ism s by Wh ich RNA’s
2′-Hyd r oxyl Gr ou p Con tr ibu tes to Str u ctu r e a n d
Biologica l F u n ction
In tr od u ction
The 2′-hydroxyl group plays a key role in RNA biology,
mediating hydrogen bonds, coordinating to metal ions,
and providing a scaffold for interactions with water.1 The
locations of residues bearing important 2′-hydroxyl groups
therefore provide fundamental clues about RNA structure
and function. Deoxynucleotide substitution reliably iden-
tifies functionally important 2′-hydroxyl groups within
RNA but provides no information about the underlying
chemical mechanism by which they fulfill their functional
role.2,3 Concomitant with the loss of hydrogen bonding
capacity at the 2′-position, deoxynucleotides introduce a
steric void (a hydrogen atom occupies less volume than
a hydroxyl group), change the conformational preferences
of the nucleoside, and alter hydration.3
To decouple these effects, we designed three new
nucleoside analogues, 2′-â-CH3, 2′-R-CH3, and 2′-R-CH2-
OH nucleosides (Chart 1).3 The 2′-â-CH3 modification, in
which a methyl group replaces the 2′-hydrogen atom on
the â-face of the ribose, maintains the 3′-endo sugar
conformation even without the 2′-hydroxyl group.4 This
modification provides a way to evaluate the effect of
removing a 2′-hydroxyl group from an RNA residue
without changing the inherent sugar pucker preference.
The 2′-R-CH3 modification, in which a methyl group
replaces the 2′-OH, provides a means to evaluate the
effect of eliminating hydrogen bonding capacity without
a concomitant loss in spacing-filling capacity. The 2′-R-
hydroxymethyl modification provides a strategy to de-
termine whether a 2′-hydroxyl group interacts with a
water molecule, as the HOCH2- may mimic the hydra-
tion of the 2′-hydroxyl group. Additionally, the HOCH2-
analogue offers a means to test whether a 2′-hydroxyl
group imparts its functional contribution through space
or through the ribose σ-framework via an inductive effect.
These nucleoside analogues (Chart 1), when used in
combination with other analogues,3 allow attribution of
the deleterious effects observed upon 2′-deoxynucleotide
to hydrogen bond removal, cavity creation, sugar pucker,
inductive effects, or the hydration of the 2′-hydroxyl
group and thereby permit a new level of understanding
of the role that 2′-hydroxyl groups play in RNA structure
and function. In addition, oligonucleotides containing
these analogues merit investigation as therapeutic agents
as they could engender an increased nuclease resistance
compared to standard RNA. We reported previously the
synthesis of the phosphoramidite derivative of 2′-deoxy-
(1) (a) Auffinger, P.; Westhof, E. J . Mol. Biol. 1997, 274, 54 and
references therein. (b) Simons, R. W.; Grunberg-Manago, M. RNA
Structure and Function; Cold Spring Harbor Laboratory Press: Cold
Spring Harbor, NY, 1998.
(2) (a) Gesteland, R. F.; Cech, T. R.; Atkins, J . F. The RNA World,
2nd ed.; Cold Spring Harbor Laboratory Press: Cold Spring Harbor,
NY, 1999. (b) Eckstein, F.; Lilley, D. M. J . Catalytic RNA; Springer-
Verlag: Berlin, Germany, 1997.
(3) Gordon, P. M.; Fong, R.; Deb, S.; Li, N.-S.; Schwans, J . P.; Ye,
J .-D.; Piccirilli, J . A. Chem. Biol. 2004, 11, 237 and references therein.
(4) Li, N.-S.; Piccirilli, J . A. J . Org. Chem. 2003, 68, 6799 and
references therein.
10.1021/jo0495337 CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/12/2004
J . Org. Chem. 2004, 69, 4751-4759
4751

Li and Piccirilli
SCHEME 1a
a
Key: (i) Dess-Martin periodinane, CH2Cl2, 0 °C to rt, overnight; (ii) Ph3P+CH3Br-, s-BuLi, THF, -78 °C to rt, ovenight; (iii) (1)
9-BBN, rt, 20 h, (2) NaBO3, H2O, rt, 3 h; (iv) Ac2O, pyridine, DMAP, CH2Cl2, 0 °C to rt, 2 h; (v) N4-benzoyl bis(trimethylsilyl)cytosine,
SnCl4, CH3CN, rt, overnight, 60 °C, 30 min; (vi) bis(trimethylsilyl)uracil, SnCl4, CH3CN, rt, overnight.
2′-C-â-methylcytidine.4 Here, we report the synthesis of
the phosphoramidite derivatives of 2′-deoxy-2′-C-R-meth-
ylcytidine and 2′-deoxy-2′-C-R-hydroxymethylcytidine.
We chose modified cytidine as the target because of its
application to our mechanistic studies of the group II
intron ribozyme.3,5
the â-nucleoside. In adopting Schmit’s approach for our
synthesis of 2′-deoxy-2′-C-R-hydroxymethylcytidine, we
explored alternative reagents and conditions to improve
yields. We protected the glycosylating agent 5 as the
4-chlorobenzyl ether rather than as Schmit’s 2,4-dichlo-
robenzyl ether10 (Scheme 1). Methyl 3,5-di-O-(4-chloro-
benzyl)-R-D-ribofuranoside (1) was prepared according to
the method of Martin et al.9 and converted to methyl 2-R-
acetoxymethyl-3,5-di-O-(4-chlorobenzyl)-2-deoxy-R-D-ribo-
furanoside (5) essentially via the same sequence of
reactions used by Schmit to prepare the dichlorobenzyl
derivative. The major difference was that our procedures
allow more efficient access to a 2′-C-R-hydroxymethyl
ribofuranosylating agent than reported previously (48%
vs 31%8). Glycosylation of persilylated N4-benzoylcy-
tosine11 with 5 in the presence of tin(IV) chloride gave
the cytidine derivative (6) in 78% yield with 70:30 â/R
selectivity. Under similar conditions, 5 glycosylated bis-
(trimethylsilyl)uracil to give the uridine derivative (7) in
greater yield (89%) with greater selectivity (â/R 96:4)
(Scheme 1).
Conversion of 6 to a phosphoramidite suitable for solid-
phase oligonucleotide synthesis requires removal of the
benzyl groups. Attempts to debenzylate 6b (the â-isomer
of 6) directly, using hydrogen in the presence of Pd/C or
Pd(OH)2/C, resulted in deamination of cytosine nucleo-
base.12 To render the nucleobase less susceptible to
reduction, we removed the benzoyl group from 6b by
treatment with ammonia in methanol to give 2′-hy-
droxymethylcytidine derivative (8)13 in 83% yield (Scheme
2). The 2′-hydroxymethyl group was then protected as
the 2′-TBDMS ether (9) in 93% yield using TBDMS
Resu lts a n d Discu ssion
There exists no reported synthesis of the phosphor-
amidite derivative of 2′-deoxy-2′-C-R-hydroxymethylcy-
tidine. In 1998, Cosstick and O’Neil reported the syn-
thesis of the phosphoramidite derivative of 2′-deoxy-2′-
C-R-hydroxymethyluridine from 2′-allyl-2′-deoxy-3′,5′-
(TIPDS)uridine.6 They obtained the core nucleoside via
an ene reaction between the allyl nucleoside and 4-meth-
yl-1,2,4-triazoline-3,5-dione followed by ozonolysis. Suke-
da et al.7 obtained 2′-deoxy-2′-C-R-hydroxylmehtylcyti-
dine by conversion of 2′-vinyl-2′-deoxy-3′-(TBDMS)-5′-
(MMTr)uridine to the corresponding cytidine derivative
followed by deprotection and direct ozonolysis. Despite
their elegance, we made no attempt to use these ap-
proaches to prepare the phosphoramidite derivative of
2′-deoxy-2′-C-R-hydroxymethylcytidine. We favored ap-
proaches based on glycosylation of a 2-R-C-hydroxy-
methyl sugar derivative so as to allow access ultimately
to derivatives of A, G, C, and U in the most convergent
manner.
In 1994, Schmit reported the synthesis of the phos-
phoramidite derivative of 2′-deoxy-2′-C-R-hydroxymeth-
ylthymidine (5% overall yield) via stereoselective gly-
cosylation with methyl 2-R-acetoxymethyl-3,5-di-O-(di-
chlorobenzyl)-2-deoxy-R-D-ribofuranoside.8 This approach
appeared attractive for accessing our target molecules
because the starting material, methyl 3,5-di-O-(dichlo-
robenzyl)-R-D-ribofuranoside, was prepared easily from
D-ribose in three steps9 and because the subsequent
glycosylation reaction proceeded stereoselectively to give
(10) We used the less expensive 4-chlorobenzyl chloride [Aldrich
(03-04): $55.00/500 g] rather than dichlorobenzyl chloride [Aldrich
(03-04): $59.90/100 g] as reported in ref 8.
(11) (a) Tang, X.-Q.; Liao, X.-M.; Piccirilli, J . A. J . Org. Chem. 1999,
64, 747. (b) Li, N.-S.; Tang, X.-Q.; Piccirilli, J . A. Org. Lett. 2001, 3,
1025.
(12) Benzoylamide was formed during catalytic hydrogen debenzyla-
tion.
(5) Gordon, P. M.; Sontheimer, E. J .; Piccirilli, J . A. Biochemistry
2000, 39, 12939.
(6) Pavey, J . B. J .; Lawrence, A. J .; Potter, A. J .; Cosstick, R.; O’Neil,
I. A. Tetrahedron Lett. 1988, 39, 6967.
(7) Sukeda, M.; Shuto, S.; Sugimoto, I.; Ichikawa, S.; Matsuda, A.
J . Org. Chem. 2000, 65, 8988.
(8) Schmit, C. Synlett 1994, 238.
(13) NOESY experiments of compound 8 indirectly confirmed the
1′-â- and 2′-R-configuration of compound 6b. We observed a stronger
NOE between 2′-H (δ: 2.47) and 6-H (δ: 7.68) than between 2′-H (δ:
2.47) and 1′-H (δ: 6.18); a stronger NOE between 1′-H (δ: 6.18) and
2′-CH2 (δ: 3.89) than between 1′-H (δ: 6.18) and 2′-H (δ: 2.47); a
stronger NOE between 2′-H (δ: 2.47) and 3′-H (δ: 4.12) than between
1′-H (δ: 6.18) and 2′-H (δ: 2.47). These results suggest that the 2′-,
3′-, and 6-hydrogen atoms reside on one side of the ribose ring, and
the 1′-H and 2′-CH2 reside on the other side of the ribose ring.
(9) Martin, O. R.; Kurz, K. G.; Rao, S. P. J . Org. Chem. 1987, 52,
2922.
4752 J . Org. Chem., Vol. 69, No. 14, 2004

Analogues for Chemical Dissection of RNA’s 2′-Hydroxyl Group
SCHEME 2a
a
Key: (i) NH3, CH3OH, 0-4 °C, 2 days; (ii) TBDMSOTf, pyridine, rt, 24 h; (iii) (1) H2, Pd/C 10%, CH3OH, rt, 7 h, (2) BzCl, DMAP,
Et3N, DMF, rt, overnight; (iv) 2 N NaOH, pyridine, CH3OH, -20 to 0 °C, 1-2 h.
SCHEME 3a
triflate. Debenzylation of 9 with hydrogen in the presence
of 10% Pd/C followed by acylation with benzoyl chloride
gave perbenzoylated hydroxymethylcytidine (10) in 57%
yield. Reduction of the nucleobase still occurred to some
extent, accounting for the relatively low yield. However,
the loss of the TBDMS group proved even more prob-
lematic, as we could not differentially protect the primary
hydroxyl groups following selective hydrolysis of the
benzoyl esters. No further attempt was made to convert
11 to the phosphoramidite derivative of 2′-C-R-hydroxy-
methylcytidine. Before developing an alternative syn-
thesis (see below), we turned our attention to the
phosphoramidite derivative of 2′-C-R-methylcytidine,
envisioning the hydroxymethyluridine derivative (7) as
a possible precursor.
Schmit reported the synthesis of the phosphoramidite
derivatives of 2′-deoxy-2′-C-R-methylcytidine and 2′-
deoxy-2′-C-R-methylthymidine from the glycosylating
agent 3-O-benzoyl-5-O-TBDMS-2-deoxy-2-R-methylribose
(26% and 21% overall yield, respectively).8 The glycos-
ylation reactions in these syntheses gave product with
good â/R selectivity (6.7:1-9:1), but the preparation of
the glycosylating agent required a special chiral organo-
a
Key: (i) 2 N NaOH, MeOH, rt, 1 h; (ii) PhOCSCl, DMAP,
CH3CN, rt, 2 h; (iii) n-Bu3SnH/AIBN, toluene, reflux, 2 h.
iodine/triphenylphosphine16 gave the corresponding 2′-
R-iodomethyluridine (14) in quantitative yield. Hydrogen
in the presence of Pd/C and Pd(OH)2/C reduced the benzyl
ether and the iodomethyl group to give 2′-deoxy-2′-C-R-
methyluridine (15)17,18 in 82% yield over the two steps.
To obtain the corresponding cytidine derivative, we
protected the 3′- and 5′-hydroxyl groups as TBDMS
titanium reagent. No experimental details for the syn-
thesis and use of this reagent have been published,
however.14
As an alternative approach to access 2′-deoxy-2′-R-
methylpyrimidine nucleosides, we explored strategies to
deoxygenate the hydroxymethyluridine derivative (7)
(Schemes 3 and 4). The acetyl group was removed from
7 to give the corresponding 2′-deoxy-2′-R-hydroxymethyl-
uridine derivative (12) in 98% yield (Scheme 3). Deoxy-
genation via radical chemistry15 gave the corresponding
2′-C-R-methyluridine (13) in only 39% yield (Scheme 3).
Iodination followed by hydrogenation proved to be sig-
nificantly more efficient (Scheme 4). Reaction of 12 with
(16) Corey, E. J .; Nagata, R. Tetrahedron Lett. 1987, 28, 5391.
(17) The 1′-â- and 2′-R-configuration of 2′-deoxy-2′-C-R-methyluri-
dine (15) was confirmed by the NOESY experiments. We observed
strong NOEs between the 2′-H (δ: 2.39) and 6-H (δ: 7.78); between
the 5′-H (δ: 3.70) and 6-H (δ: 7.78); a stronger NOE between 1′-H (δ:
5.88) and 2′-CH3 (δ: 1.00) than between 1′-H (δ: 5.88) and 2′-H (δ:
2.39); we observed no NOE between 2′-CH3 (δ: 1.00) and 6-H (δ: 7.78).
These results suggest that the 2′-, 6-, and 5′- hydrogen atoms reside
on one side of the ribose ring and that the 1′-H and 2′-CH2 reside on
other side of the ribose ring.
(14) Duthaler, R. O.; Hafner, A.; Alsters, P. L.; Rothe-Streit, P.; Rihs,
G. Pure Appl. Chem. 1992, 64, 1897.
(15) Robins, M. J .; Wilson, J . S.; Hansske, F. J . Am. Chem. Soc.
1983, 105, 4059.
(18) Cicero et al. described the synthesis of 2′-deoxy-2′-C-â-meth-
yluridine and obtained 15 as a minor product. The yield was not
reported. See: Cicero, D. O.; Neuner, P. J . S.; Franzese, O.; D’Onofrio,
C.; Iribarren, A. M. Bioorg. Med. Chem. Lett. 1994, 4, 861.
J . Org. Chem, Vol. 69, No. 14, 2004 4753

Li and Piccirilli
SCHEME 4a
a
Key: (i) I2, PPh3, imidazole, C6H6/CH3CN (4:1), 0 °C to rt, 4.5 h; (ii) H2, Pd/C 10%, Et3N, CH3CO2Et, rt, 24 h; (iii) H2, Pd(OH)2/C 20%,
CH3OH/CH3CO2Et (1:1), rt, 24 h; (iv) TBDMSCl, DMF, imidazole, rt, ovrnight; 60 °C, 4 h; (v) (1) 2,4,6-triisopropylbenzenesulfonyl chloride,
DMAP, Et3N, CH3CN, rt, 24 h, (2) NH4OH, rt, 3 h; (vi) TBAF, THF, rt, overnight, reflux, 2 h; (vii) BzCl, DMAP, CH2Cl2, rt, overnight;
(viii) Et3N-3HF, Et3N, THF, 60 °C, overnight; (ix) DMTrCl, pyridine, DMAP, rt, overnight; (x) (i-Pr)2NP(Cl)OCH2CH2CN, i-Pr2NEt,
1-methylimidazole, CH2Cl2, 0 °C to rt, 2 h.
ethers (16, 80% yield) and transformed the uracil het-
erocycle to cytosine via a two step reaction sequence
(2,4,6-triisopropylbenzenesulfonyl chloride and ammo-
nia)19 giving 17 in 87% yield. The parent nucleoside was
obtained by TBAF desilylation to give 2′-deoxy-2′-C-R-
methylcytidine (18)20 in 66% yield. Continuing with the
synthesis, we converted 17 efficiently to a suitably
protected phosphoramidite 22 using consecutive benzo-
ylation, desilylation, 5′-dimethoxytritylation, and phos-
phitylation reactions (Scheme 4).
Returning to the hydroxymethylcytidine analogue, we
developed a direct and efficient route by removing the
benzyl groups from the glycosylating agent 5 before
installation of the nucleobase, thereby avoiding cytosine
side reactions (Scheme 5). Debenzylation of 5 with
hydrogen catalyzed by Pd(OH)2/C in the presence of
triethylamine gave methyl 2-acetoxymethyl-2-deoxy-R-
D-ribofuranoside (23) in 92% yield. Reprotection of the
hydroxyl groups with TBDMSCl gave the new glycos-
ylating agent, methyl 2-R-acetoxymethyl-3,5-di-O-(TB-
DMS)-D-ribofuranoside (24), in 97% yield as a mixture
of anomers (R/â ratio ∼ 85:15 based on 1H NMR).21
Glycosylation of persilylated N4-acetylcytosine in the
presence of tin(IV) chloride gave the cytidine nucleoside
25 in 63% yield with 9:1 â/R selectivity. Glycosylation of
bis(trimethylsilyl)uracil under similar conditions gave the
uridine nucleoside 26 in 57% yield with 94:6 â/R selectiv-
ity. Cytidine analogue 25 was converted efficiently to
phosphoramidite 29 via consecutive desilylation, 5′-
dimethyoxytritylation,22 and 3′-phosphitylation reac-
tions.23
Con clu sion
We modified the approach of Schmit8 to develop an
effective synthesis of methyl 2-R-acetoxymethyl-3, 5-di-
(21) The â-isomer of 24 might be formed by partial anomerization
of the R-isomer under the TBDMS protection conditions.
(22) The 1′-â- and 2′-R-configuration of tritylnucleoside 28 was
confirmed by the NOESY experiments. We observed stronger NOEs
between 2′-H (δ: 2.63) and 6-H (δ: 8.15) than between 2′-H (δ: 2.63)
and 1′-H (δ: 6.33); NOE between 5′-H (δ: 3.48) and 6-H (δ: 8.15). We
observed no NOE between 2′-CH2 (δ: 4.50, 4.40) and 6-H (δ: 8.15).
These results suggest that 2′-, 6-, and 5′- hydrogen atoms reside on
one side of the ribose ring.
(19) Iino, T.; Yoshimura, Y.; Matsuda, A. Tetrahedron 1994, 50,
10397.
(20) Matsuda et al. obtained 18 as a minor product in a lengthy
synthesis starting from uridine (12 steps, 6% overall yield). See: (a)
Matsuda, A.; Takenuki, K.; Sasaki, T.; Ueda, T. J . Med. Chem. 1991,
34, 234. (b) Matsuda, A.; Itoh, H.; Takenuki, K.; Sasaki, T.; Ueda, T.
Chem. Pharm. Bull. 1988, 36, 945.
(23) A phosphoramidte derivative of 2′-deoxy-2′-C-R-hydroxymeth-
ylcytidine might be accessed by ozonolysis of 2′,5′-O-(MMTr)-2′-deoxy-
2′-C-R-vinyl-3′-O-(TBDMS)cytidine, prepared by the approach of Suke-
da et al.7 This possible synthetic route would involve more than 10
steps starting from 2,2′-O-anhydrouridine.
4754 J . Org. Chem., Vol. 69, No. 14, 2004

Analogues for Chemical Dissection of RNA’s 2′-Hydroxyl Group
SCHEME 5a
a
Key: (i) H2, Pd(OH)2/C 20%, CH3CO2Et, Et3N, rt, overnight; (ii) TBDMSCl, imidazole, DMF, rt, overnight; (iii) N4-acetylbis(trimeth-
ylsilyl)cytosine or bis(trimethylsilyl)uracil, SnCl4, CH3CN, rt, 14 h; (iv) Et3N-3HF, Et3N, THF, 60 °C, 14 h; (v) DMTrCl, pyridine, DMAP,
rt, overnight; (vi) (i-Pr)2NP(Cl)OCH2CH2CN, i-Pr2NEt,1-methylimidazole, CH2Cl2, 0 °C to rt, 1 h.
Meth yl 3,5-Di-O-(4-ch lor oben zyl)-2-m eth ylen e-r-D-r i-
bofu r a n osid e (3). Under an argon atmosphere, s-BuLi in
cyclohexane (38.5 mL, 1.3 M, 50.0 mmol) was added to a
suspension of triphenylmethylphosphonium bromide (19.65 g,
55.0 mmol) in THF (100 mL) at -78 °C. After the reaction
mixture was stirred at -78 °C for 1 h, a solution of 2 (10.28 g,
25.0 mmol) in THF (50 mL) was transferred slowly into the
reaction mixture. The reaction mixture was allowed to warm
to room temperature and was stirred overnight. The solid was
filtered away and rinsed with ether. The filtrate was washed
sequentially with saturated ammonium chloride and brine and
dried over anhydrous MgSO4. After solvent was removed, the
residue was purified by silica gel chromatography, eluting with
hexane/ethyl acetate (v/v ) 4:1) to give 3 (6.40 g, 63% yield):
1H NMR (CDCl3/TMS) δ 7.30-7.20 (m, 8H), 5.45 (s, 1H), 5.39
(s, 1H), 5.24 (s, 1H), 4.59 (d, 1H, J ) 12.2 Hz), 4.54-4.45 (m,
3H), 4.27 (m, 2H), 3.56 (m, 2H), 3.43 (s, 3H); 13C NMR (CDCl3)
δ 146.6, 136.4, 136.3, 133.31, 133.27, 128.93, 128.86, 128.4,
114.3, 104.1, 80.9, 78.5, 72.5, 69.8, 69.7, 54.8; HRMS calcd for
O-(4-chlorobenzyl)-R-D-ribofuranoside (5). This glycos-
ylating agent allows efficient access to protected hy-
droxymethyluridine and, following conversion to methyl
2-R-acetoxymethyl-3,5-di-O-(TBDMS)-R-D-ribofurano-
side (24), allows direct and efficient access to hydroxy-
methylcytidine and the corresponding phosphoramidite
(29, Scheme 5). We further demonstrated that hy-
droxymethyluridine offers robust and convergent syn-
thetic entry into the 2′-deoxy-2′-methylnucleoside mani-
fold via an iodination/hydrogenation sequence. 2′-C-R-
Hydroxymethylnucleosides and 2′-C-R-methylnucleosides
serve as valuable analogues with which to explore the
role of RNA’s 2′-hydroxyl group.3
Exp er im en ta l Section
Meth yl 3,5-Di-O-(4-ch lor oben zyl)-2-k eto-r-D-r ibofu r a n -
osid e (2). Under argon, methyl 3,5-di-O-(4-chlorobenzyl)-R-
D-ribofuranoside (1) (5.762 g, 14.0 mmol) was added to a
solution of Dess-Martin periodinane (8.91 g, 21.0 mmol) in
dry dichloromethane (50 mL) at 0 °C. The reaction mixture
was stirred overnight at room temperature. The solvent was
removed, and the residue was triturated with diethyl ether
(100 mL). Following filtration through a pad of silica gel and
anhydrous magnesium sulfate (w/w ) 1:1), the ether solution
was washed with saturated aqueous sodium thiosulfate and
saturated aqueous NaHCO3 and dried over anhydrous mag-
nesium sulfate. The solvent was removed, and the residue was
dried under vacuum to give 2 as a colorless oil (5.283 g, 92%
yield): 1H NMR (CDCl3/TMS) δ 7.32-7.17 (m, 8H), 4.91 (d,
1H, J ) 11.6 Hz), 4.83 (s, 1H), 4.63 (d, 1H, J ) 11.6 Hz), 4.54
(d, 1H, J ) 12.0 Hz), 4.45 (d, 1H, J ) 12.0 Hz), 4.30 (m, 1H),
4.09 (d, 1H, J ) 8.4 Hz), 3.80 (dd, 1H, J ) 11.2, 2.4 Hz), 3.64
(dd, 1H, J ) 11.2, 3.6 Hz), 3.47 (s, 3H); 13C NMR (CDCl3) δ
207.6, 135.9, 135.4, 133.8, 133.5, 129.4, 129.2, 128.9, 128.5,
C
21H26Cl2NO4 [MNH4+] 426.1239, found 426.1251.
Meth yl 3,5-Di-O-(4-ch lor oben zyl)-2-d eoxy-2-r-h yd r oxy-
m eth yl-r-D-r ibofu r a n osid e (4). Under argon, the 9-BBN
dimer (3.05 g, 12.5 mmol) was added to the solution of 3 (7.524
g, 18.4 mmol) in THF (50 mL) at room temperature. After the
reaction mixture was stirred at room temperature for 20 h,
oxidizing agent sodium perborate tetrahydrate (11.5 g, 75.0
mmol) and water (25 mL) were added and the mixture was
stirred at room temperature for an additional 3 h. The organic
layer was separated, and the aqueous was extracted with ethyl
acetate (3 × 30 mL). The organic layers were combined and
dried over MgSO4. The solvent was removed, and the product
was purified by silica gel chromatography, eluting with hex-
ane/ethyl acetate (v/v ) 6:4) to give 4 (6.60 g, 84% yield): 1H
NMR (CDCl3/TMS) δ 7.31-7.15 (m, 8H), 5.02 (d, 1H, J ) 5.0
Hz), 4.55-4.40 (m, 4H), 4.26 (m, 1H), 3.95-3.85 (m, 3H), 3.44-
3.35 (m, 2H), 3.40 (s, 3H), 2.43 (m, 1H); 13C NMR (CDCl3) δ
136.3, 136.0, 133.2, 129.0, 128.8, 128.3, 105.1, 82.7, 78.6, 72.4,
98.6, 77.6, 75.3, 72.7, 71.6, 68.1, 55.8; HRMS calcd for C18H20
-
Cl2NO5 [MNH4+] 428.1032, found 428.1031.
J . Org. Chem, Vol. 69, No. 14, 2004 4755

Li and Piccirilli
71.0, 70.2, 57.3, 55.3, 48.9; HRMS calcd for C21H24Cl2NaO5
[MNa+] 449.0898, found 449.0912.
fully by addition of 10% aqueous NaHCO3 (75 mL) and stirred
for 15 min. The mixture was extracted with methylene
chloride, and the organic phase was washed with brine and
dried over MgSO4. After evaporation of solvent, the product
was purified by silica gel chromatography, eluting with hex-
ane/ethyl acetate (v/v ) 4:6) to give compound 7 (2.19 g, 89%
yield). The â/R selectivity is 96:4 based on the 1H NMR spectra
of 7. â-anomer: 1H NMR (CDCl3/TMS) δ 9.39 (br s, 1H), 7.65
(d, 1H, J ) 8.0 Hz), 7.37-7.30 (m, 4H), 7.21 (m, 4H), 6.23 (d,
1H, J ) 8.0 Hz), 5.56 (d, 1H, J ) 8.0 Hz), 4.55-4.40 (m, 5H),
4.26-4.20 (m, 2H), 4.13 (m, 1H), 3.74 (m, 1H), 3.54 (m, 1H),
2.65 (m, 1H), 1.97 (s, 3H); 13C NMR (CDCl3) δ 170.6, 163.1,
150.3, 139.9, 135.43, 135.39, 134.1, 133.9, 129.2, 129.0, 128.8,
128.7, 102.6, 87.3, 82.5, 79.1, 72.9, 71.0, 70.3, 60.1, 47.9, 20.7;
HRMS calcd for C26H27Cl2N2O7 [MH+] 549.1195, found 549.1194.
3′,5′-Di-O-(4-ch lor oben zyl)-2′-deoxy-2′-R-h ydr oxym eth yl-
â-cytid in e (8). Compound 6b (0.844 g, 1.29 mmol) in methanol
(50 mL) was saturated with ammonia gas at 0 °C for 30 min.
The mixture was kept in the refrigerator (4 °C) for 2 days.
The solvent was removed, the residue was isolated by silica
gel chromatography, eluting with 10% methanol in chloroform
to give 8 (0.543 g, 83% yield): 1H NMR (CDCl3/TMS) δ 7.97
(br s, 1H), 7.68 (d, 1H, J ) 7.4 Hz, 6-H), 7.32-7.15 (m, 8H),
6.48 (br s, 1H), 6.18 (d, 1H, J ) 6.4 Hz, 1′-H), 5.69 (d, 1H, J )
7.4 Hz, 5-H), 4.55-4.40 (m, 4H), 4.18 (m, 1H), 4.12 (dd, 1H,
J ) 3.6, 6.0 Hz, 3′-H), 3.89 (d, 2H, J ) 6.4 Hz, 2′-CH2), 3.68
(m, 1H, 5′-H), 3.52 (m, 1H, 5′-H), 2.47 (m, 1H, 2′-H); 13C NMR
(CDCl3) δ 165.7, 156.6, 140.4, 135.9, 133.6, 133.5, 129.1, 128.9,
128.6, 128.5, 95.6, 89.0, 82.3, 79.3, 72.6, 71.1, 69.8, 58.7, 51.1.
2′-R-(ter t-Bu tyldim eth ylsiloxym eth yl)-3′,5′-di-O-(4-ch lo-
r oben zyl)-2′-d eoxy-â-cytid in e (9). TBDMSOTf (0.53 g, 0.46
mL, 2.0 mmol) was added to a solution of 8 (0.478 g, 0.94 mmol)
in dry pyridine (10 mL). The reaction mixture was stirred at
room temperature for 24 h. The solvent was removed, and the
residue was diluted with chloroform. The solution was washed
with brine and dried over anhydrous magnesium sulfate. After
solvent was removed, the residue was purified by silica gel
chromatography, eluting with 8% methanol in chloroform to
give 9 (0.544 g, 93% yield): 1H NMR (CDCl3/TMS) δ 8.40 (br
s, 1H), 7.68 (d, 1H, J ) 8.0 Hz), 7.34-7.19 (m, 8H), 6.70 (br s,
1H), 6.18 (d, 1H, J ) 8.0 Hz), 5.76 (d, 1H, J ) 8.0 Hz), 4.55-
4.40 (m, 4H), 4.23 (m, 1H), 4.13 (m, 1H), 4.02 (m, 1H), 3.69
(m, 2H), 3.50 (m, 1H), 2.46 (m, 1H), 0.87 (s, 9H), -0.01 (s, 3H),
-0.02 (s, 3H); 13C NMR (CDCl3) δ 165.5, 156.0, 140.5, 136.3,
135.9, 133.4, 133.1, 128.9, 128.53, 128.47, 128.3, 95.3, 86.8,
82.5, 79.0, 72.4, 71.0, 70.2, 58.0, 51.3, 25.6, 17.8, -5.68, -5.73.
2′-R-(Ben zoxylm et h yl)-3′,5′-d i-O-b en zoyl-2′-d eoxy-N4-
ben zoyl-â-cytid in e (10). The mixture of compound 9 (0.544
g, 0.88 mmol) and Pd/C 10% (0.10 g) in methanol (10 mL) was
stirred under hydrogen atmosphere (hydrogen balloon) at room
temperature for 7 h. The catalyst was filtered off and rinsed
with methanol. The filtrate was concentrated, and the residue
was dried under vacuum for 1 h. The crude hydrogenation
product was then dissolved into anhydrous DMF (15 mL), and
DMAP (122 mg, 1.0 mmol), triethylamine (1.4 mL, 10.0 mmol)
and benzoyl chloride (0.703 g, 0.58 mL, 5.0 mmol) were added.
The reaction mixture was stirred at room temperature over-
night. TLC showed the reaction was complete. The solvent was
removed, and the residue was extracted with ether and washed
with 1 N HCl, saturated NaHCO3, and brine. The organic layer
was dried over MgSO4. After the solvent was removed, the
residue was purified by silica gel chromatography, eluting with
hexane/ethyl acetate (v/v ) 1:1) to give compound 10 (0.335
g, 57% yield): 1H NMR (CDCl3/TMS) δ 8.15-7.95 (m, 8H), 7.87
(d, 2H, J ) 8.0 Hz), 7.60-7.20 (m, 13H), 6.64 (d, 1H, J ) 8.0
Hz), 5.82 (dd, 1H, J ) 2.4, 6.0 Hz), 4.87 (m, 1H), 4.80-4.73
(m, 3H), 4.67 (m, 1H), 3.23 (m, 1H); 13C NMR (CDCl3) δ 170.8,
166.0, 165.8, 165.6, 162.7, 154.6, 143.6, 133.8, 133.6, 133.1,
133.0, 129.9, 129.7, 129.45, 129.40, 129.1, 129.0, 128.6, 128.5,
128.2, 128.1, 128.0, 126.9, 97.6, 88.0, 82.5, 74.2, 64.0, 60.3, 47.9.
Meth yl 2-r-Acetoxym eth yl-3,5-d i-O-(4-ch lor oben zyl)-
2-d eoxy-r-D-r ibofu r a n osid e (5). To a mixture of 4 (4.246 g,
9.94 mmol) in dry CH2Cl2 (50 mL), pyridine (7.92 mL, 98
mmol), and DMAP (1.22 g, 10.0 mmol) at 0 °C was added acetyl
anhydride (2.76 mL, 29.45 mmol). After the reaction mixture
was stirred at room temperature for 2 h, the mixture was
diluted with CH2Cl2 (100 mL) and washed with water (20 mL),
1 N HCl (100 mL), saturated NaHCO3 (20 mL), and brine (20
mL). The organic layer was dried over MgSO4. The solvent
was removed, and the residue was purified by silica gel
chromatography, eluting with hexane/ethyl acetate (v/v ) 4:1)
to give 5 (4.557 g, 98% yield): 1H NMR (CDCl3/TMS) δ 7.35-
7.15 (m, 8H), 4.99 (d, 1H, J ) 4.8 Hz), 4.60-4.25 (m, 7H), 3.88
(dd, 1H, J ) 2.0, 7.2 Hz), 3.41 (s, 3H), 3.45-3.30 (m, 2H), 2.44
(m, 1H), 2.02 (s, 3H); 13C NMR (CDCl3) δ 170.8, 136.4, 136.2,
133.5, 133.4, 129.2, 128.9, 128.5, 128.4, 105.0, 83.3, 78.3, 72.7,
71.1, 70.4, 59.3, 55.6, 46.4, 20.9; HRMS calcd for C23H30Cl2-
NO6 [MNH4+] 486.1450, found 446.1448.
2′-r-Acetoxym eth yl-3′,5′-di-O-(4-ch lor oben zyl)-2′-deoxy-
N4-ben zoylcytid in e (6). Persilylated N4-benzoylcytosine was
prepared by the reaction of N4-benzoylcytosine (1.18 g, 5.5
mmol) with TMS2NH (30 mL) and (NH4)2SO4 (15 mg) at reflux
under an argon atmosphere. After 1 h, the reaction mixture
became clear. The excess TMS2NH was evaporated under
vacuum, and the residue was dried under vacuum for 1 h.
Under argon, the persilylated base was dissolved into aceto-
nitrile (40 mL), and the solution was transferred into the flask
containing compound 5 (1.245 g, 2.65 mmol). SnCl4 (0.65 mL,
5.5 mmol) was added in one portion with vigorous stirring and
exclusion of moisture. The homogeneous pale yellow solution
was stirred at room temperature overnight and heated to 60
°C for 30 min. After it was cooled, the reaction mixture was
quenched carefully by addition of 10% aqueous NaHCO3 (40
mL) and stirred for 15 min. The mixture was extracted with
methylene chloride, and the organic phase was washed with
brine. The organic layer was dried over MgSO4. After evapora-
tion of solvent, 1H NMR showed that the crude product
contained both the â- and R-anomers (â/R ) 70:30). The
product was purified by silica gel chromatography, eluting with
hexane/ethyl acetate (v/v ) 1:1) to give â-anomer 6b (0.948 g,
55% yield, Rf ) 0.51 in ethyl acetate) and R-anomer 6a (0.406
g, 23% yield, Rf ) 0.38 in ethyl acetate). 6b: 1H NMR (CDCl3/
TMS) δ 9.40 (br s, 1H), 8.24 (d, 1H, J ) 7.6 Hz), 7.93 (d, 2H,
J ) 8.0 Hz), 7.50-7.15 (m, 12H), 6.26 (d, 1H, J ) 6.0 Hz),
4.65-4.40 (m, 6H), 4.35-4.25 (m, 2H), 4.19 (m, 1H), 3.80 (m,
1H), 3.57 (m, 1H), 2.77 (m, 1H), 1.98 (s, 3H); 13C NMR (CDCl3)
δ 170.5, 166.8, 162.1, 154.5, 144.4, 135.44, 135.41, 133.8, 133.6,
132.9, 129.0, 128.9, 128.5, 128.3, 128.2, 128.0, 127.6, 96.7, 88.4,
82.7, 77.7, 72.6, 71.1, 69.2, 60.1, 47.9, 20.7. 6a : 1H NMR
(CDCl3/TMS) δ 8.89 (br s, 1H), 8.02 (d, 1H, J ) 7.6 Hz), 7.90
(d, 2H, J ) 7.2 Hz), 7.60 (m, 1H), 7.53-7.15 (m, 11H), 6.55 (d,
1H, J ) 7.2 Hz), 4.65-4.40 (m, 5H), 4.21 (m, 1H), 4.14-4.09
(m, 2H), 3.50 (m, 2H), 3.22 (m, 1H), 1.95 (s, 3H); 13C NMR
(CDCl3) δ 170.3, 166.8, 162.1, 155.0, 145.7, 136.1, 135.8, 134.0,
133.7, 133.1, 129.5, 129.2, 129.0, 128.7, 128.6, 128.5, 127.5,
96.1, 87.5, 84.0, 78.8, 72.7, 71.3, 70.1, 58.3, 45.4, 20.7.
2′-R-Acetoxym eth yl-3′,5′-di-O-(4-ch lor oben zyl)-2′-deoxy-
â-u r id in e (7): Bis(trimethylsilyl)uracil was prepared under
an argon atmosphere by the reaction of uracil (1.26 g, 11.2
mmol) with refluxing TMS2NH (35 mL) in the presence of
(NH4)2SO4 (15 mg). After the reaction mixture became clear,
the excess TMS2NH was evaporated under vacuum, and the
residue was dried under vacuum for 1 h. Under argon, the
persilylated base was dissolved into acetonitrile (50 mL), and
the solution was transferred into the flask containing com-
pound 5 (2.10 g, 4.48 mmol). SnCl4 (1.31 mL, 11.2 mmol) was
added in one portion with vigorous stirring and exclusion of
moisture. The homogeneous pale yellow solution was stirred
at room temperature overnight. TLC showed that no starting
material remained. The reaction mixture was quenched care-
2′-Deoxy-2′-R-(h yd r oxym et h yl)-N4-b en zoyl-â-cyt id in e
(11). To the solution of compound 10 (300 mg, 0.446 mmol) in
4756 J . Org. Chem., Vol. 69, No. 14, 2004

Analogues for Chemical Dissection of RNA’s 2′-Hydroxyl Group
pyridine (5 mL) and methanol (1 mL) at -20 °C was added 2
N NaOH (0.67 mL, 1.34 mmol). The mixture was stirred below
0 °C until TLC showed the reaction was complete (1-2 h). The
reaction mixture was neutralized to pH ) 7 with DOWEX
50WX 8-200 resin. The resin was filtered off and washed with
a mixture of H2O/pyridine (v/v ) 4:1, 25 mL). The filtrate was
evaporated to dryness, and the residue was purified by silica
gel chromatography, eluting with 10% methanol in chloroform
to give compound 11 (81 mg, 50% yield): 1H NMR (CD3OD/
CDCl3/TMS) δ 8.54 (d, 1H, J ) 7.5 Hz), 7.99 (d, 2H, J ) 7.2
Hz), 7.70-7.60 (m, 2H), 7.60-7.50 (m, 2H), 6.18 (d, 1H, J )
6.6 Hz), 4.49 (dd, 1H, J ) 3.9, 6.4 Hz), 4.12 (m, 1H), 4.00 (m,
1H), 3.95-3.85 (m, 2H), 3.82 (m, 1H), 2.53 (m, 1H); 13C NMR
(CD3OD/CDCl3) δ 168.4, 163.7, 157.4, 146.1, 133.6, 132.2,
129.3, 128.5, 98.5, 89.8, 88.1, 72.1, 61.9, 58.8, 53.0.
3′,5′-Di-O-(4-ch lor oben zyl)-2′-deoxy-2′-R-h ydr oxym eth yl-
â-u r id in e (12). NaOH (2 N, 1.0 mL, 2.0 mmol) was added to
the solution of 2′-R-acetoxymethyl-3′,5′-di-O-(4-chlorobenzyl)-
2′-deoxy-â-uridine (659 mg, 1.2 mmol) in methanol (10 mL).
The mixture was stirred at room temperature for 1 h. The
solvent was removed, and the residue was dissolved into
methylene chloride (20 mL), washed with brine, and dried over
magnesium sulfate. After the solvent was removed, the residue
was purified by silica gel chromatography, eluting with ethyl
acetate to give 12 as a white foam (0.597 g, 98% yield): 1H
NMR (CDCl3/TMS) δ 9.77 (br s, 1H), 7.68 (d, 1H, J ) 8.1 Hz),
7.35-7.29 (m, 4H), 7.20 (m, 4H), 6.21 (d, 1H, J ) 7.4 Hz), 5.52
(d, 1H, J ) 8.1 Hz), 4.55-4.42 (m, 4H), 4.24 (m, 1H), 4.19 (dd,
1H, J ) 6.4, 2.8 Hz), 3.93 (m, 2H), 3.73 (m, 1H), 3.53 (dd, 1H,
J ) 10.4, 2.5 Hz), 3.03 (br s, 1H), 2.56 (m, 1H); 13C NMR
(CDCl3) δ 163.5, 150.9, 140.1, 135.63, 135.59, 134.0, 133.8,
129.2, 129.0, 128.8, 128.7, 102.5, 88.1, 82.8, 79.7, 72.9 71.3,
70.1, 58.7, 50.3; HRMS calcd for C24H25Cl2N2O6 [MH+] 507.1090,
found 507.1071.
3′,5′-Di-O-(4-ch lor oben zyl)-2′-d eoxy-2′-R-m eth yl-â-u r i-
d in e (13). To the solution of 12 (157 mg, 0.31 mmol) in dry
acetonitrile (20 mL) was added DMAP (122 mg, 1.0 mmol)
followed by the addition of phenyl chlorothionoformate (69 µL,
0.5 mmol). The mixture was stirred at room temperature for
2 h, but TLC showed that the reaction was not yet complete.
Additional phenyl chlorothionoformate (17 µL, 0.12 mmol) was
added, and the reaction mixture was stirred at room temper-
ature for 3 h. The solvent was removed, and the residue was
dissolved into ethyl acetate (50 mL), washed with 0.1 N HCl
(10 mL), water (10 mL), and brine (10 mL), and dried over
magnesium sulfate. After the solvent was removed, the residue
was dried under vacuum overnight. To the vacuum-dried crude
thioformate ester were added toluene (10 mL), n-Bu3SnH (133
µL, 0.5 mmol), and AIBN (10 mg) under an argon atmosphere.
After the reaction mixture was stirred at reflux for 2 h, the
solvent was removed, and the residue was purified by silica
gel chromatography, eluting with hexane/ethyl acetate (v/v )
1:1) to give 13 (59 mg, 39% yield): 1H NMR (CDCl3/TMS) δ
9.17 (br s, 1H), 7.68 (d, 1H, J ) 8.0 Hz), 7.36-7.19 (m, 8H),
5.98 (d, 1H, J ) 7.6 Hz), 5.53 (d, 1H, J ) 8.1 Hz), 4.55-4.43
(m, 4H), 4.22 (m, 1H), 3.98 (dd, 1H, J ) 6.0, 2.8 Hz), 3.74 (dd,
1H, J ) 10.4, 3.2 Hz), 3.56 (dd, 1H, J ) 10.4, 2.4 Hz), 2.34 (m,
1H), 1.13 (d, 3H, J ) 6.8 Hz); 13C NMR (CDCl3) δ 163.0, 150.5,
140.0, 136.0, 135.7, 134.1, 133.7, 129.2, 128.8, 128.7, 102.4,
90.1, 82.4, 80.2, 73.0, 71.0, 70.4, 43.0, 9.4.
3′,5′-Di-O-(4-ch lor oben zyl)-2′-d eoxy-2′-R-iod om eth yl-â-
u r id in e (14). Iodine (1.27 g, 5.0 mmol) was added to the
mixture of 3′,5′-di-O-(4-chlorobenzyl)-2′-deoxy-2′-R-hydroxy-
methyl-â-uridine (12) (483 mg, 0.95 mmol), PPh3 (1.31 g, 5.0
mmol), and imidazole (0.34 g, 5.0 mmol) in dry benzene/CH3-
CN (v/v ) 4:1, 25 mL) at 0 °C. After the mixture was stirred
at room temperature for 4.5 h, TLC showed that the reaction
was complete. The reaction mixture was diluted with ether,
and the solid was filtered off. The filtrate was washed with
saturated Na2S2O3 and brine and dried over magnesium
sulfate. The solvent was removed, and the residue was purified
by silica gel chromatography, eluting with hexane/ethyl acetate
(v/v ) 1:1) to give compound 14 (0.586 g, 100% yield): 1H NMR
(CDCl3/TMS) δ 9.44 (br s, 1H), 7.61 (d, 1H, J ) 8.1 Hz), 7.38-
7.23 (m, 8H), 6.07 (d, 1H, J ) 9.1 Hz), 5.60 (d, 1H, J ) 8.1
Hz), 4.60-4.48 (m, 4H), 4.29 (m, 1H), 4.11 (m, 1H), 3.73 (dd,
1H, J ) 10.3, 2.8 Hz), 3.49 (dd, 1H, J ) 8.5, 1.8 Hz), 3.39 (m,
1H), 3.10 (dd, 1H, J ) 9.9, 4.9 Hz), 2.71 (m, 1H); 13C NMR
(CDCl3) δ 163.0, 150.7, 139.5, 135.4, 134.1, 133.9, 129.4, 129.2,
128.9, 128.6, 103.1, 86.8, 82.0, 81.8, 72.8 71.7, 70.7, 52.5, 29.6;
HRMS calcd for
617.0100.
C
24H24Cl2IN2O5 [MH+] 617.0107, found
2′-Deoxy-2′-R-m eth yl-â-u r id in e (15). To the solution of
3′,5′-di-O-(4-chlorobenzyl)-2′-deoxy-2′-R-iodomethyl-â-uridine
(14) (458 mg, 0.74 mmol) in ethyl acetate (15 mL) were added
10% Pd/C (78 mg, 0.074 mmol) and triethylamine (0.52 mL,
3.7 mmol). The mixture was stirred at room temperature under
hydrogen atmosphere (hydrogen balloon) for 24 h. TLC showed
that no starting material (14) remained and compound (13)
had formed. The catalyst was filtered off and rinsed with ethyl
acetate. The filtrate was washed sequentially with 1 N HCl
(5 mL), water (5 mL), saturated NaHCO3, and brine and dried
over magnesium sulfate. After solvent was removed, the
residue was dissolved into a mixed solvent of methanol (10
mL) and ethyl acetate (10 mL). Pd(OH)2/C 20% (208 mg, 0.3
mmol) was added. The reaction mixture was stirred under
hydrogen atmosphere for 24 h. TLC showed that the reaction
was complete. The catalyst was filtered off and rinsed with
methanol. The filtrate was concentrated, and the residue was
purified by silica gel chromatography, eluting with 7.5%
methanol in ethyl acetate to give compound 1518(147 mg, 82%
yield): 1H NMR (D2O) δ 7.78 (d, 1H, J ) 8.1 Hz, 6-H), 5.88 (d,
1H, J ) 8.9 Hz, 1′-H), 5.85 (d, 1H, J ) 8.1 Hz, 5-H), 4.21 (dd,
1H, J ) 5.7, 2.1 Hz, 3′-H), 4.03 (m, 1H, 4′-H), 3.75 (dd, 1H,
J ) 12.5, 4.1 Hz, 5′-H), 3.70 (dd, 1H, J ) 12.5, 5.1 Hz, 5′-H),
2.39 (m, 1H, 2′-H), 1.00 (d, 3H, J ) 7.0 Hz, 2′-CH3); 13C NMR
(D2O) δ 166.0, 151.9, 141.6, 102.6, 89.4, 86.3, 72.9, 61.6, 42.3,
7.7; HRMS calcd for
243.0981.
C
10H15N2O5 [MH+] 243.0981, found
3′,5′-Di-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxy-2′-R-m eth -
yl-â-u r id in e (16). To the solution of 2′-deoxy-2′-R-methyl-â-
uridine (15) (100 mg, 0.41 mmol) in dry DMF (10 mL) were
added imidazole (338 mg, 4.96 mmol) and TBDMSCl (374 mg,
2.48 mmol). The reaction mixture was stirred at room tem-
perature overnight. TLC showed that the reaction was not yet
complete. The reaction mixture was then heated to 60 °C for
2 h. The solvent was removed, and the residue was extracted
with CH2Cl2, washed with saturated NaHCO3 and brine, and
dried over magnesium sulfate. After solvent was removed, the
residue was purified by silica gel chromatography, eluting with
hexane/ethyl acetate (v/v ) 3:1) to give compound 16 (154 mg,
80% yield): 1H NMR (CDCl3/TMS) δ 9.72 (br s, 1H), 7.83 (d,
1H, J ) 8.1 Hz), 5.96 (d, 1H, J ) 7.7 Hz), 5.69 (dd, 1H, J )
1.9, 8.1 Hz), 4.19 (dd, 1H, J ) 5.6, 2.1 Hz), 3.95 (m, 1H), 3.84
(dd, 1H, J ) 11.4, 2.8 Hz), 3.72 (dd, 1H, J ) 11.4, 1.9 Hz),
2.16 (m, 1H), 1.04 (d, 3H, J ) 6.9 Hz), 0.89 (s, 9H), 0.88 (s,
9H), 0.083 (s, 3H), 0.081 (s, 3H), 0.054 (s, 3H), 0.044 (s, 3H);
13C NMR (CDCl3) δ 163.6, 150.8, 140.2, 102.4, 89.7, 87.2, 74.1,
63.2, 44.7, 25.8, 25.6, 18.3, 18.0, 9.4, -4.8, -4.9, -5.6, -5.7;
HRMS calcd for
471.27108.
C
22H43N2O5Si2 [MH+] 471.27106, found
3′,5′-Di-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxy-2′-R-m eth -
yl-â-cytid in e (17). Triethylamine (0.14 mL, 1.0 mmol) was
added to a mixture of 16 (148 mg, 0.315 mmol), 2,4,6-
triisopropylbenzenesulfonyl chloride (303 mg, 1.0 mmol), and
DMAP (122 mg, 1.0 mmol) in CH3CN (10 mL). After the
mixture was stirred at room temperature for 24 h, concen-
trated ammonium hydroxide (28%, 15 mL) was added, and the
reaction mixture was stirred at room temperature for 3 h. The
organic solvent was removed, and the product was extracted
with chloroform, washed with brine, and dried over magne-
sium sulfate. After solvent was removed, the residue was
purified by silica gel chromatography, eluting with 5% metha-
nol in chloroform to give compound 17 (128 mg, 87% yield):
J . Org. Chem, Vol. 69, No. 14, 2004 4757

Li and Piccirilli
1H NMR (CDCl3/TMS) δ 9.72 (br s, 1H), 7.83 (d, 1H, J ) 8.0
Hz), 6.01 (d, 1H, J ) 8.0 Hz), 5.72 (d, 1H, J ) 8.0 Hz), 4.21
(dd, 1H, J ) 8.0, 4.0 Hz), 3.92 (m, 1H), 3.86 (dd, 1H, J ) 12.0,
4.0 Hz), 3.73 (dd, 1H, J ) 12.0, 4.0 Hz), 2.15 (m, 1H), 1.07 (d,
3H, J ) 8.0 Hz), 0.94 (s, 9H), 0.91 (s, 9H), 0.09 (s, 6H), 0.04 (s,
6H); 13C NMR (CDCl3) δ 165.7, 156.3, 141.0, 94.6, 90.3, 86.2,
73.3, 62.8, 45.0, 25.9, 25.7, 18.3, 18.0, 9.9, -4.8, -5.5, -5.6;
HRMS calcd for C22H44N3O4Si2 [MH+] 470.2870, found 470.2872.
2′-Deoxy-2′-R-m eth yl-â-cytid in e (18). To the solution of
17 (57 mg, 0.12 mmol) in THF (5 mL) was added TBAF (1.0
M solution in THF, 0.35 mL, 0.35 mmol). The mixture was
stirred at room temperature overnight, but TLC showed that
the reaction was not yet complete. The reaction mixture was
then heated to reflux for 2 h. After the solvent was removed,
the residue was purified by silica gel chromatography, eluting
with 20% methanol in chloroform to give compound 1820 (19
mg, 66% yield): 1H NMR (CD3OD/TMS) δ 7.96 (d, 1H, J ) 7.5
Hz), 6.00 (d, 1H, J ) 8.2 Hz), 5.91 (d, 1H, J ) 7.5 Hz), 4.19
(dd, 1H, J ) 5.7, 2.5 Hz), 3.96 (m, 1H), 3.77-3.73 (m, 2H),
2.28 (m, 1H), 1.07 (d, 3H, J ) 7.0 Hz); 13C NMR (CDCl3) δ
167.4, 158.7, 142.9, 96.3, 91.7, 88.1, 74.4, 63.3, 45.5, 9.4.
(dd, 1H, J ) 10.9, 3.2 Hz), 2.44 (m, 1H), 1.26 (d, 3H, J ) 8.0
Hz); 13C NMR (CDCl3) δ 162.6, 162.1, 158.6, 144.9, 144.2,
135.6, 135.4, 132.9, 130.0, 129.9, 129.1, 128.8, 128.1, 128.0,
127.7, 127.5, 127.0, 113.2, 96.8, 91.4, 86.9, 84.5, 71.7, 62.7, 55.2,
45.3, 10.0; HRMS calcd for C38H38N3O7 [MH+] 648.2710, found
648.2730.
5′-O-(Dim eth oxytr ityl)-2′-deoxy-2′-r-m eth yl-N4-ben zoyl-
â-cytid in e 3′-N,N-Diisop r op yl(cya n oeth yl)p h osp h or a m i-
d ite (22). N,N-Diisopropylethylamine (69 µL, 0.4 mmol),
2-cyanoethyl N,N-diisopropylchlorophosphoramidite (67 µL,
0.3 mmol), and 1-methylimidazole (8.0 µL, 0.1 mmol) were
quickly added to a solution of compound 21 (80 mg, 0.124
mmol) in dry dichloromethane (10 mL) at 0 °C under argon.
The reaction mixture was then warmed to room temperature
and stirred until TLC indicated that the reaction was complete
(2 h). The reaction mixture was cooled to 0 °C, quenched with
methanol (0.5 mL), and stirred for an additional 5 min. After
the solvent was removed, the residue was purified by silica
gel chromatography, eluting with 4% acetone in dichloro-
methane with 0.2% triethylamine to give the corresponding
phosphoramidite (22)8 (105 mg, 100% yield): 31P NMR (CDCl3)
δ 153.8, 151.8; MS (m/z) 848.3 [MH+], 303 (DMtr, 100).
3′,5′-Di-O-(ter t-bu tyld im eth ylsilyl)-2′-d eoxy-2′-R-m eth -
yl-N4-ben zoyl-â-cytid in e (19). A mixture of 17 (100 mg,
0.213 mmol), DMAP (52 mg, 0.426 mmol), and benzoyl chloride
(45 mg, 0.32 mmol) in dichloromethane (10 mL) was stirred
at room temperature overnight. TLC showed that the reaction
was complete. Methanol (1.0 mL) was added into the mixture.
The solvent was removed, and the residue was purified by
silica gel chromatography, eluting with hexane/ethyl acetate
(v/v ) 7:3) to give compound 19 (119 mg, 97% yield): 1H NMR
(CDCl3/TMS) δ 8.88 (br s, 1H), 8.35 (d, 1H, J ) 8.0 Hz), 7.88
(d, 2H, J ) 8.0 Hz), 7.55 (t, 1H, J ) 8.0 Hz), 7.47-7.39 (m,
3H), 5.99 (d, 1H, J ) 4.0 Hz), 4.23 (m, 1H), 3.96-3.91 (m, 2H),
3.74 (dd, 1H, J ) 12.0, 4.0 Hz), 2.22 (m, 1H), 1.12 (d, 3H, J )
8.0 Hz), 0.91 (s, 9H), 0.86 (s, 9H), 0.09 (s, 3H), 0.07 (s, 3H),
0.02 (s, 6H); 13C NMR (CDCl3) δ 161.9, 158.0, 144.7, 133.2,
133.0, 128.9, 127.6, 96.6, 91.2, 86.4, 72.5, 62.4, 45.7, 25.9, 25.6,
18.4, 18.0, 10.2, -4.6, -4.8, -5.5; HRMS calcd for C29H48N3O5-
Si2 [MH+] 574.3133, found 574.3138.
2′-Deoxy-2′-R-m eth yl-N4-ben zoyl-â-cytid in e (20). To the
solution of 19 (117 mg, 0.204 mmol) in THF (10 mL) was added
triethylamine trihydrofluoride (258 mg, 0.26 mL, 1.6 mmol).
After the mixture was stirred at room temperature for 40 h,
TLC showed that the reaction was not yet complete. Trieth-
ylamine (28 µL, 0.204 mmol) was then added into the mixture,
and the reaction was continued at 60 °C overnight. The solvent
was removed, and the residue was purified by silica gel
chromatography, eluting with 2% methanol in ethyl acetate
to give compound 208 (53 mg, 75% yield): 1H NMR (CD3OD/
TMS) δ 8.52 (d, 1H, J ) 8.0 Hz), 7.96 (d, 2H, J ) 8.0 Hz),
7.64-7.51 (m, 4H), 6.02 (d, 1H, J ) 8.0 Hz), 4.25 (m, 1H), 4.04
(m, 1H), 3.86 (dd, 1H, J ) 8.0, 4.0 Hz), 3.78 (dd, 1H, J ) 8.0,
4.0 Hz), 2.37 (m, 1H), 1.14 (d, 3H, J ) 8.0 Hz); 13C NMR (CD3-
OD) δ 164.6, 158.3, 146.7, 134.6, 134.1, 129.8, 129.1, 98.7, 92.7,
88.3, 73.7, 62.8, 46.5, 9.8; HRMS calcd for C17H20N3O5 [MH+]
346.1403, found 346.1387.
5′-O-(Dim eth oxytr ityl)-2′-deoxy-2′-R-m eth yl-N4-ben zoyl-
â-cytid in e (21). Compound 20 (53 mg, 0.154 mmol) was
coevaporated with pyridine (2 mL) and dissolved into pyridine
(5 mL) again. DMTrCl (171 mg, 0.51 mmol) and DMAP (42
mg, 0.34 mmol) were added to the pyridine solution of 20 under
argon. After the mixture was stirred at room temperature
overnight, methanol (1.0 mL) was added to the reaction
mixture. The solvent was removed, and the residue was
purified by silica gel chromatography, eluting with 3% metha-
nol in chloroform to give compound 218 (80 mg, 80% yield).
Some starting material (20) (11 mg, 20%) was also recovered
by silica gel chromatography, eluting with 8% methanol in
chloroform. 21: 1H NMR (CDCl3/TMS) δ 9.10 (br s, 1H), 8.33
(d, 1H, J ) 7.4 Hz), 7.81 (d, 2H, J ) 7.6 Hz), 7.60-7.10 (m,
12H), 6.85 (m, 5H), 6.05 (d, 1H, J ) 5.0 Hz), 4.44 (m, 1H),
4.19 (m, 1H), 3.79 (s, 6H), 3.53 (dd, 1H, J ) 10.9, 2.7 Hz), 3.48
Met h yl 2-Acet oxym et h yl-2-d eoxy-R-D-r ibofu r a n osid e
(23). A mixture of compound 5 (2.103 g, 4.48 mmol), 20% Pd-
(OH)2/C (754 mg, 1.07 mmol), and triethylamine (0.93 mL, 6.7
mmol) in ethyl acetate (30 mL) was stirred under a hydrogen
atmosphere (hydrogen balloon) at room temperature overnight.
TLC showed that the reaction was complete. The catalyst was
filtered off and rinsed with ethyl acetate. The filtrate was
concentrated, and the residue was isolated by silica gel
chromatography, eluting with ethyl acetate to give compound
23 (0.906 g, 92% yield): 1H NMR (CDCl3/TMS) δ 5.04 (d, 1H,
J ) 4.2 Hz), 4.41 (dd, 1H, J ) 11.5, 7.9 Hz), 4.27 (dd, 1H, J )
11.5, 6.4 Hz), 4.19-4.12 (m, 2H), 3.73-3.66 (m, 2H), 3.41 (s,
3H), 2.37 (m, 1H), 2.08 (s, 3H); 13C NMR (CDCl3) δ 171.3, 105.0,
87.8, 72.4, 62.8, 59.2, 55.1, 47.5, 20.8; HRMS calcd for C9H16O6-
Na [MNa+] 243.0845, found 243.0848.
Meth yl 2-Acetoxym eth yl-3,5-d i-O-(ter t-bu tyld im eth yl-
silyl)-2-d eoxy-R-D-r ibofu r a n osid e (24). A mixture of com-
pound 23 (220 mg, 1.0 mmol), TBDMSCl (904 mg, 6.0 mmol),
and imidazole (816 mg, 12 mmol) in DMF (30 mL) was stirred
at room temperature overnight. The solvent was removed, and
the residue was dissolved into CH2Cl2, washed with saturated
NaHCO3 and brine, and dried over MgSO4. The solvent was
removed, and the residue was isolated by 5% ethyl acetate in
pentane to give compound 24 (436 mg, 97% yield), a mixture
of anomers R/â ∼85:15 from 1H NMR spectra. R-Anomer: 1H
NMR (CDCl3/TMS) δ 4.93 (d, 1H, J ) 4.0 Hz), 4.25 (m, 3H),
4.00 (m, 1H), 3.66 (m, 1H), 3.55 (m, 1H), 3.31 (s, 3H), 2.34 (m,
1H), 2.01 (s, 3H), 0.86 (s, 9H), 0.85 (s, 9H), 0.05-0.02 (m, 12H);
13C NMR (CDCl3) δ 170.9, 105.1, 87.6, 72.2, 63.6, 59.8, 55.0,
47.2, 25.9, 25.7, 21.0, 18.3, 17.9, -4.4, -5.2, -5.3, -5.5; HRMS
calcd for C21H44O6Si2Na [MNa+] 471.2574, found 471.2583.
2′-Acetoxym eth yl-3′,5′-d i-O-(ter t-bu tyld im eth ylsilyl)-2′-
d eoxy-N4-a cetyl-â-cytid in e (25). Persilylated N4-acetylcy-
tosine was prepared by the reaction of N4-acetylcytosine (0.80
g, 5.2 mmol) with TMS2NH (20 mL) in the presence of (NH4)2-
SO4 (10 mg), as described for the synthesis of 6. Under argon,
the persilylated base was dissolved in acetonitrile (50 mL), and
the solution was transferred into the flask containing com-
pound 24 (938 mg, 2.09 mmol). SnCl4 (0.61 mL, 5.2 mmol) was
added in one portion with vigorous stirring and exclusion of
moisture. The homogeneous pale yellow solution was stirred
at room temperature overnight. TLC showed that no starting
material remained. The reaction was quenched carefully by
addition of 10% aqueous NaHCO3 (50 mL) and stirred for 15
min. The mixture was extracted with methylene chloride, and
the organic phase was washed with brine and dried over
MgSO4. The solvent was removed, and the residue was purified
by silica gel chromatography, eluting with hexane/ethyl acetate
(v/v ) 3:7) to give compound 25 (748 mg, 63% yield). The â/R
selectivity is 9:1 based on the 1H NMR spectra of 25. â-Ano-
4758 J . Org. Chem., Vol. 69, No. 14, 2004

Analogues for Chemical Dissection of RNA’s 2′-Hydroxyl Group
mer: 1H NMR (CDCl3/TMS) δ 10.45 (br s, 1H), 8.22 (d, 1H,
J ) 7.6 Hz), 7.38 (d, 1H, J ) 7.6 Hz), 6.22 (d, 1H, J ) 7.1 Hz),
4.40-4.20 (m, 3H), 4.02 (m, 1H), 3.88 (m, 1H), 3.75 (m, 1H),
2.48 (m, 1H), 2.26 (s, 3H), 1.93 (s, 3H), 0.89 (s, 9H), 0.86 (s,
9H), 0.10 (s, 3H), 0.08 (s, 3H), 0.04 (s, 6H); 13C NMR (CDCl3)
δ 171.4, 170.3, 163.0, 155.1, 144.5, 97.1, 87.6, 87.5, 72.2, 62.8,
59.8, 50.6, 25.8, 25.6, 24.8, 20.7, 18.3, 17.9, -4.8, -5.1, -5.5,
-5.6; HRMS calcd for C26H47N3O7Si2Na [MNa+] 592.2850,
found 592.2828.
2′-r-Acetoxym eth yl-5′-O-(d im eth oxytr ityl)-2′-d eoxy-N4-
a cetyl-â-cytid in e (28). Compound 27 (176 mg, 0.516 mmol)
was coevaporated with pyridine (5 mL) and dissolved into
pyridine (10 mL) again. To the solution of 27 in pyridine were
added DMTrCl (372 mg, 1.1 mmol) and DMAP (126 mg, 1.03
mmol). After the mixture was stirred at room temperature
overnight, methanol (5.0 mL) was added into the reaction
mixture. The solvent was removed, and the residue was
purified by silica gel chromatography, eluting with 3% metha-
nol in chloroform to give compound 28 (263 mg, 79% yield):
1H NMR (CDCl3/TMS) δ 9.83 (br s, 1H), 8.15 (d, 1H, J ) 7.5
Hz, 6-H), 7.40 (d, 2H, J ) 7.7 Hz), 7.30 (d, 6H, J ) 8.3 Hz),
7.24 (m, 1H), 7.21 (d, 1H, J ) 7.5 Hz, 5-H), 6.85 (d, 4H, J )
8.6 Hz), 6.33 (d, 1H, J ) 6.6 Hz, 1′-H), 4.65 (m, 1H, 3′-H), 4.50
(m, 1H, 2′-CH2), 4.45 (br s, 1H), 4.40 (m, 1H, 2′-CH2), 4.25 (m,
1H, 4′-H), 3.78 (s, 6H, MeO), 3.48 (m, 2H, 5′-H), 2.63 (m, 1H,
2′-H), 2.20 (s, 3H), 2.01 (s, 3H); 13C NMR (CDCl3) δ 171.5,
170.8, 162.6, 158.6, 155.6, 144.6, 144.2, 135.3, 135.2, 130.0,
128.0, 127.9, 127.0, 113.3, 97.0, 87.7, 86.9, 85.7, 71.4, 63.1, 60.4,
55.2, 50.9, 24.7, 20.8; HRMS calcd for C35H38N3O9 [MH+]
644.2608, found 644.2609.
2′-R-Acetoxym eth yl-5′-O-(d im eth oxytr ityl)-2′-d eoxy-N4-
a cet yl-â-cyt id in e 3′-N,N-Diisop r op yl(cya n oet h yl)p h os-
p h or a m id ite (29). To a solution of compound 28 (53 mg, 0.082
mmol) in dry dichloromethane (10 mL) at 0 °C under argon
were added N,N-diisopropylethylamine (50 µL, 0.29 mmol),
2-cyanoethyl N,N-diisopropylchlorophosphoramidite (45 µL,
0.20 mmol), and 1-methylimidazole (4.0 µL, 0.05 mmol). The
reaction mixture was stirred at room temperature until all
starting material was consumed (1 h). The reaction mixture
was cooled to 0 °C, quenched with methanol (0.5 mL), and
stirred for 5 min. After the solvent was removed, the residue
was purified by silica gel chromatography, eluting with 4%
acetone in dichloromethane with 0.5% triethylamine to give
the corresponding phosphoramidite (29) (65 mg, 94% yield):
31P NMR (CDCl3) δ: 151.14, 151.07. MS (m/z) 842.3 [M - 1,
100], 843.3, 844.3.
2′-Acetoxym eth yl-3′,5′-d i-O-(ter t-bu tyld im eth ylsilyl)-2′-
d eoxy-â-u r id in e (26). Bis(trimethylsilyl)uracil was prepared
by the reaction of uracil (0.168 g, 1.5 mmol) with TMS2NH (5
mL) in the presence of (NH4)2SO4 (5 mg) as described for the
synthesis of 7. Under argon, bis(trimethylsilyl)uracil was
dissolved into acetonitrile (10 mL), and the solution was
transferred into the flask containing compound 24 (188 mg,
0.42 mmol). SnCl4 (0.18 mL, 1.5 mmol) was added in one-
portion with vigorous stirring and exclusion of moisture. The
homogeneous pale yellow solution was stirred at room tem-
perature overnight (14 h). TLC showed that no starting
material remained. The reaction was quenched carefully by
addition of 10% aqueous NaHCO3 (10 mL) and stirred for 15
min. The mixture was extracted with methylene chloride, and
the organic phase was washed with brine and dried over
MgSO4. The solvent was removed, and the residue was purified
by silica gel chromatography, eluting with hexane/ethyl acetate
(v/v ) 6:4) to give compound 26 (126 mg, 57% yield). The â/R
selectivity is 94:6 based on the 1H NMR spectra of 26.
â-Anomer: 1H NMR (CDCl3/TMS) δ 9.65 (br s, 1H), 7.79 (d,
1H, J ) 8.0 Hz), 6.17 (d, 1H, J ) 8.0 Hz), 5.69 (d, 1H, J ) 8.0
Hz), 4.35 (m, 1H), 4.28 (m, 1H), 4.18 (m, 1H), 3.98 (m, 1H),
3.83 (m, 1H), 3.72 (dd, 1H, J ) 12.0, 4.0 Hz), 2.50 (m, 1H),
1.94 (s, 3H), 0.89 (s, 9H), 0.87 (s, 9H), 0.10 (s, 3H), 0.08 (s,
3H), 0.06 (s, 3H), 0.05 (s, 3H); 13C NMR (CDCl3) δ 170.4, 163.4,
150.4, 139.9, 102.6, 87.6, 86.5, 73.1, 63.3, 59.9, 50.0, 25.7, 25.6,
20.6, 18.2, 17.9, -4.8, -5.2, -5.6, -5.8; HRMS calcd for
C
24H45N2O7Si2 [MH+] 529.2765, found 529.2765.
2′-Acetoxym eth yl-2′-deoxy-N4-acetyl-â-cytidin e (27). Tri-
ethylamine trihydrofluoride (670 mg, 0.68 mL, 4.16 mmol) and
triethylamine (72 µL, 0.52 mmol) were added to the solution
of 25 (294 mg, 0.52 mmol) in dry THF (15 mL). The mixture
was heated to 60 °C and stirred for 14 h. TLC showed that
the reaction was complete. The reaction mixture was diluted
with pyridine (2 mL). Water (50 mL) and CH2Cl2 (20 mL) were
added, and the aqueous layer was collected. After evaporation
of water, the residue was purified by silica gel chromatography,
eluting with 10% methanol in chloroform to give compound
27 (168 mg, 95% yield): 1H NMR (CD3OD/TMS) δ 8.17 (d, 1H,
J ) 7.6 Hz), 7.27 (d, 2H, J ) 7.6 Hz), 6.14 (d, 1H, J ) 7.6 Hz),
4.40-4.34 (m, 2H), 4.22 (dd, 1H, J ) 11.4, 7.5 Hz), 4.09 (m,
1H), 3.78 (dd, 1H, J ) 12.4, 3.5 Hz), 3.73 (dd, 1H, J ) 12.4,
4.7 Hz), 2.68 (m, 1H), 2.14 (s, 3H), 1.87 (s, 3H); 13C NMR (CD3-
OD) δ 174.0, 173.8, 162.7, 157.0, 145.6, 98.5, 88.6, 86.9, 71.0,
61.2, 61.0, 47.8, 23.9, 20.0.
Ack n ow led gm en t. N.S.L. is a Research Specialist
and J .A.P. is an Associate Investigator of the Howard
Hughes Medical Institute. We thank S. R. Das, J . Ye,
J . Hougland, and R. Fong for helpful discussions and
critical comments on the manuscript.
Su p p or t in g In for m a t ion Ava ila b le: 1H NMR and 13C
NMR spectra of compounds 2-21 and 23-28. 1H NMR and
31P NMR spectra of compounds 22 and 29. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O0495337
J . Org. Chem, Vol. 69, No. 14, 2004 4759