398
G. Oliviero et al. / Tetrahedron Letters 48 (2007) 397–400
O2N
mic acid (90% yield). The reaction of the MMTCl poly-
O
O
styrene resin (1.3 mequiv/g) with 2 or 3 in anhydrous
pyridine at room temperature and in the presence of
4-(N,N-dimethylamino)pyridine (DMAP) afforded
support 4 or 5, respectively, in almost quantitative yield.
The structure and the loading of supports 4 and 5 were
confirmed by NMR analysis and quantitative UV exper-
iments, on the released inosine derivatives 2 and 3 ob-
tained by treating the support with 2% TFA in DCM
(8 min, rt). Supports 4 and 5 were then reacted with sev-
eral N-nucleophiles (R2–NH2, Table 1, entries a–e) to
give supports 6a–e and 7a–e, respectively. In a typical
reaction, 100 mg (0.13 mmol) of support 4 (or 5), swol-
len in DMF, was left in contact with the R2–NH2 nucleo-
phile (5.0 mmol) in 1.5 mL of DMF under shaking
(8 h, at 50 °C). After washings with DMF and MeOH,
the support was dried under reduced pressure and the
reaction yield was evaluated detaching the nucleoside
material from a weighted amount of resin. The reaction
of 4 or 5 with ethylenediamine (Table 1, entry f) fur-
nished, as expected,14 supports 12 and 13 bearing the
20-30-isopropylidene-AICAR and the AICAR, respec-
tively, in almost quantitative yields. The structure and
the loading of supports 6, 7, 12, and 13 were ascertained
by analyzing with HPLC,à 1H NMR (Table 1) and MS§
the corresponding detached N-1 alkyl inosine deriva-
tives 8a–e, 9a–e as well as AICAR 14 and its derivative
15. The purity and the yield of the above detached
nucleosides, confirmed by HPLC analysis and quantita-
tive UV experiments, resulted to be always over 90–95%,
(Table 1). Starting from 20 mg of solid support (4 or 5),
and considering an average molecular weight of 300 g/
mol, 6–7 mg of each nucleoside derivative could be
obtained in 90–98% purity, thanks to the almost quanti-
tative reaction yields.
HN
N
N
N
O2N
N
N
N
N
O
O
i
HO
HO
O
O
R1O
OR1
2 : R1-R1 = Acetonide
3 : R1 = H
1
ii
Scheme 1. Reagents and conditions: (i) DNCB (2.2 equiv), K2CO3
(2.0 equiv) 2 h, 80 °C; (ii) HCOOH/H2O (6:4, v/v), 4 h, rt.
50-DMT-nucleosides by the classical 30-succinyl linkage9–11
or by the 30-acyloxyaryl phosphate linker,12 have been
successful exploited to prepare very large nucleic-acid-
bases (NABTM) libraries of 50-phosphoramidate nucleo-
side derivatives and nucleic acid fragments to be tested
in their antiviral activity.
In an effort to enlarge the nucleoside chemical reactivity
on the solid phase and consequently the number of
accessible structurally diverse analogues, we report here
the synthesis and exploitation of the new nucleoside
functionalized supports 4 and 5 which bind the N-1-di-
nitrophenyl-inosine derivatives 2 or 3 through the 50-O-
trityl function. These supports have been employed in
the synthesis of the N-1 substituted inosine 8a–e, 9a–e,
the related 20,30-seconucleoside derivatives 11a–e and
the 5-aminoimidazole-4-carboxamide riboside (AICAR)
derivatives 14 and 15. The here proposed solid phase
strategy is based on our previous studies on the C-2
reactivity of the N-1-dinitrophenyl-20-deoxyinosine
toward N-nucleophiles13,14 that allowed to obtain N-1
substituted inosine and AICAR derivatives. In particu-
lar, according to the reported reaction mechanism, when
a strong electron-withdrawing group (such as the 2,4-
dinitrophenyl, nitro,15 or arylsulfonyl16 group) is
attached to the N-1 atom of the hypoxanthine ring,
the C-2 carbon becomes electrophilic enough to react
with amino nucleophiles (R2–NH2). This leads to the
opening of the six membered ring through the cleavage
of the (N-1)–(C-2) bond. The successive fast ring re-clo-
sure, favoured by the loss of the 2,4-dinitroaniline as the
leaving group, furnished the N-1 alkyl inosine deriva-
tives. It is to be noted that as a consequence of the
purine rearrangement, the endocyclic N-1 atom is
substituted by the nitrogen atom of the nucleophilic
reactant. This purine reactivity has also been used by
others to introduce modified purine bases into oligo-
nucleotides.17 Thus, to obtain a small library of N-1 alkyl
inosine derivatives, we bound the 1-(2,4-dinitrophenyl)-
20,30-O-isopropylideneinosine 2, or the corresponding
unprotected inosine derivative 3 (Scheme 1 and 2), to
the commercially available polystyrenemonomethoxy-
trityl chloride (MMTCl) resin by 50-O-trityl ether linkage.
Inosine derivative 2 was synthesized by reaction of the
commercially available 20,30-O-isopropylidene inosine 1
with 2,4-dinitrochlorobenzene (DNCB) essentially as
previously described.18 The 20-30 deprotected inosine
derivative 3 was obtained treating 2 with aqueous for-
The second goal of this work was to combine the set of
the N-1 alkylated inosine derivatives (from supports 7)
with a ribose modification. As an example we have
examined the well known 20,30-oxidative cleavage of
the ribose moiety generally carried out by reaction with
metaperiodate followed by reduction of the di-aldehyde
derivative which leads to 20,30-seconucleosides.19 In a
typical reaction, supports 7a–e (100 mg, 0.13 mmol)
were left in contact with a solution of NaIO4 (1.3 mmol)
in DMF/H2O (1.5 mL, 1:1, v/v) and shaken for 12 h at
60 °C. The resulting support, after washings with DMF
and EtOH, was treated with NaBH4 (2.6 mmol) in
Product 3, as a 1:1 mixture of atropisomers at N-1 phenyl bond; 1H
NMR (400 MHz, CD3OD): d 9.05 (s, 1H, H-3 DNP); 8.77 (dd, 1H,
H-5 DNP); 8.49, 8.49, 8.48, 8.40 (s’s, H-2 and H-8); 8.02 (dd, 1H, H-6
DNP); 6.10 and 6.12 (d’s, 0.5 H each, H-10); 4.69 and 4.64 (dd’s, 0.5 H
each, H-20); 4.35 (m, 1H, H-30); 4.15 (m, 1H, H-40); 3.88 and 3.77 (m,
1H each, H-50).
à HPLC analyses: RP18 analytic column eluted with a linear gradient
of CH3CN in 0.1 M TEAB (pH 7.0, from 0% to 60% in 60 min, flow
1.0 mL/min).
§ ESI MS data m/z (calcd): 9a 325.2 (M+H+) (324.1); 9b 313.2 (M+H+)
(312.1); 9c 327.1 (M+H+) (326.1); 9d 355.1 (M+H+) (354.1); 9e 343.2
(M+H+) (342.1); 11a 349.1 (M+Na+) (326.2); 11b 337.2 (M+Na+)
(314.1); 11c 351.2 (M+Na+) (328.1); 11d 379.2 (M+Na+) 356.2; 11e
367.2 (M+Na+) 344.1; 15 259.0 (M+H+) (258.1).