maximize the purity of the unstable compound 10 and to
use it crude for the subsequent reaction. Oxidative coupling
of 10 and the 3′-aminonucleoside 1, carried out in the
presence of trimethylsilyl chloride (TMS-Cl) and a 5-fold
molar excess of iodine, resulted in the formation of the
desired diastereomers 4 and 5 in a 1/1 molar ratio. To prevent
protonation of the aminonucleoside under the reaction
conditions, resulting in an inhibition of the nucleophilic
substitution at phosphorus, we used an excess of triethyl-
amine in addition to pyridine as solvent. Reaction products
were isolated by aqueous workup and separated by column
or HPLC silica gel chromatography in a chloroform/methanol
solvent system (Table 1). The yield of the oxidative
amidation process was in the range 50-60%.
phosphorus of the diastereomers 4 and 5 have not been
assigned, except for compounds 4aa and 5aa, for which
absolute configurations have already been assigned.5 By
analogy with these compounds, the fast migrating dimer 4
(FAST) should be the [RP] diastereomer and slow migrating
dimer 5 (SLOW) should be the [SP] diastereomer.
All the dinucleoside diastereomers 4 and 5 were depro-
tected under basic conditions (for removal of the acetyl
protecting groups) or in the presence of fluoride anion14 (for
removal of the tert-butyldimethylsilyl group) in yields of
>80%. The dinucleoside methanephosphonamidate units 11
(originating from diastereomers 4, FAST) and 12 (originating
from diastereoisomer 5, SLOW) (Table 1)15 with a free 3′-
OH group were subjected to phosphitylation with chloro-2-
cyanoethyl-N,N-diisopropylphosphoramidite and then suc-
cessfully introduced into long-sequence chimeric oligonucleo-
tides by automated solid-phase synthesis.5,16 The physico-
chemical properties of these oligomers are presently under
investigation.
By comparison with Routes A and B, the third approach
seems to be the most attractive with respect to the overall
yield and the consumption of aminonucleoside 1, which is
obtained by a rather time-consuming chemical synthesis.4
Moreover, despite being more laborious than Routes A and
B, it yields exclusively a mixture of diastereomers 4 and 5,
uncontaminated with the side product 6. We tried to shorten
this approach by direct synthesis of the H-phosphonate
derivative 10 by means of a reaction of dichloromethylphos-
phine with nucleoside 2 and with water. The reaction resulted
in the formation of the desired compound 10, but all our
efforts of purification by column chromatography failed.
The structures of the dinucleoside methanephosphon-
amidates 4, 5, and 6 were confirmed by NMR, MS, and CD
Acknowledgment. The authors wish to thank Prof.
Wojciech J. Stec for permanent interest in this work, helpful
discussions, and critical evaluation of the manuscript. The
results presented here were obtained within the project funded
by the State Committee for Scientific Research (KBN, Grant
3T09A 03917 for B.N.).
Supporting Information Available: Detailed synthetic
procedures A, B, and C as well spectral characterization of
the products. This material is available free of charge via
1
analysis. Selected spectral data (31P and H NMR and MS)
are given in Table 1. The absolute configurations at
OL0259084
(9) Schweitzer, M.; Engels, J. W. J. Biomol. Struct. Dyn. 1999, 16, 1177.
(10) Kers, I.; Stawinski, J.; Kraszewski, A. Tetrahedron Lett. 1998, 39,
1219.
(11) Kers, I.; Bollmark, M.; Kraszewski, A.; Stawinski, J. Phosphorus,
Sulfur Silicon 1999, 144-146, 637.
(12) Agrawal, S.; Goodchild, J. Tetrahedron Lett. 1987, 28, 3539.
(13) Gryaznov, S. M.; Letsinger, R. L. Nucleic Acids Res. 1992, 20, 3403.
(14) Ogilvie, K. K.; Schifman, A. L.; Penney, Ch. L. Can. J. Chem.
1979, 57, 2230.
(15) Detailed synthetic procedures A, B, and C as well spectral
characterization of the products are available in the Supporting Information.
(16) Caruthers, M. H. Science 1985, 230, 281.
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Org. Lett., Vol. 4, No. 10, 2002