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The aim was to check whether DNA and RNA-nucleoside- CHE-1004570 and by the NASA Astrobiology: Exobiology: Exo-
phosphoramidites could be produced under microwave conditions biology and Evolutionary Biology Program (Grant NNX07AK18G).
given the concerns for their sensitivity5 as ‘fragile’ nucleoside- We thank the Sharpless lab for the use of the microwave
phosphoramidites. The microwave-mediated phosphitylation reac- synthesizer.
tion was tested on suitably protected 20-deoxyadenosine 18 using
reagent 5 and the corresponding phosphitylated derivative 26
was isolated in 75% yield (Table S2, entry 1, ESI†). Since there
is a preference for the 2-cyanoethyl-N,N,N0,N0-tetraisopropyl
Notes and references
‡ TOM-derivatives were prepared based on protecting group com-
patibility considerations (originating from the difficulties encountered
phosphorodiamidite 6 as a phosphitylating reagent owing to
its stability against hydrolysis when compared to 5,1,2 we also
investigated microwave-assisted-phosphitylations of commercially
available DNA and RNA substrates employing 5 with Hu¨nig’s base
and 6 with 5-ethylthiotetrazole or pyridinium hydrochloride as
activators (Scheme 2C). Good to excellent yields (70–90%) of DNA
phosphoramidites 25–28 were obtained (Table S2, entries 1–6,
ESI†). In the case of RNA phosphoramidites 29, 30, and 32, the
yields were lower (44–62%) and in the case of 32, substantial
amounts of H-phosphonate by-products were formed indicating
that further optimizations are needed.8 However, the stability
of the phosphoramidites formed under microwave reaction
conditions is noteworthy.
Phosphitylation of sterically hindered nucleosides (which
was problematic under standard conditions) with commercially
available reagents was rendered efficient by microwave-irradiation.
The resultant phosphoramidites were formed within short time
spans (15–20 min) and found to be stable under the reaction
conditions. This suggests that phosphitylation reactions need not
be restricted to mild conditions and could have more flexibility
with respect to reaction parameters (e.g. higher temperature). This
microwave-assisted phosphitylation reaction has been shown to be
general in its nature for nucleosides, with some optimizations
needed in the RNA series. It is proposed that the microwave-
assisted phosphitylation procedure outlined here could become a
useful tool with the potential to accommodate a wide variety of
substrates.
in oligonucleotide synthesis using the phosphoramidites with the 40-O-
benzoyl group).
§ Consistent with this reasoning the corresponding xylulo-derived
nucleosides – where the 30-OH group is axial (but have the same protecting
groups at C-40 and C-20) – were phosphitylated under the standard reaction
conditions with phosphoramidite yields ranging from 78 to 90%.6
¶ This is inferred from the X-ray structure of the ribulo-adenine nucleo-
side that places the N(3) of the purine ring in close proximity to the
30-OH group.6
8 The lower (unoptimized) yields may be reflective of the difference in
the TOM vs. TBDMS protecting groups.
1 (a) R. Gukathasan, M. Massoudipour, I. Gupta, A. Chowdhury, S. Pulst,
S. Ratnam, Y. Sanghvi and S. A. Laneman, J. Organomet. Chem., 2005,
690, 2603–2607; (b) Y. S. Sanghvi, Z. Guo, H. M. Pfundheller and
A. Converso, Org. Process Res. Dev., 2000, 4, 175–181.
2 E.g. see C. Xie, M. A. Staszak, J. T. Quatroche, C. D. Sturgill, V. V.
Khau and M. Martinelli, Org. Process Res. Dev., 2005, 9, 730–737.
3 E.g. see N. Venkatesan, S. J. Kim and B. H. Kim, Curr. Med. Chem.,
2003, 10, 1973–1991.
4 A. J. A. Cobb, Org. Biomol. Chem., 2007, 5, 3260–3275.
5 M. H. Caruthers and S. L. Becauge, US Pat. 4,668,777, 1987.
6 M. Stoop, G. Meher, P. Karri and R. Krishnamurthy, Chem. – Eur. J.,
2013, 45, 15336–15345.
7 A. Eschenmoser, Angew. Chem., Int. Ed., 2011, 50, 12412–12472.
8 P. Brady, E. M. Morris, O. S. Fenton and B. R. Sculimbrene, Tetra-
hedron Lett., 2009, 50, 975–978.
9 (a) E. J. Amigues, C. Hardacre, G. Keane, M. E. Migaud, S. E. Norman
and W. R. Pitner, Green Chem., 2009, 11, 1391–1396; (b) C. Hardacre,
H. Huang, S. L. James, M. E. Migaud, S. E. Norman and W. R. Pitner,
Chem. Commun., 2011, 47, 5846–5848.
10 For application of MW in nucleic acid chemistry see: (a) B. C.
Bookser and N. B. Raffaele, J. Org. Chem., 2007, 73, 173–179;
(b) D.-A. Catana, M. Maturano, C. Payrastre, P. Lavedan, N. Tarrat
and J.-M. Escudier, Eur. J. Org. Chem., 2011, 6857–6863; (c) M. Meng,
C. Ahlborn, M. Bauer, O. Plietzsch, S. A. Soomro, A. Singh, T. Muller,
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This work was supported by the NSF and the NASA Astro-
biology Program under the NSF Centre for Chemical Evolution,
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7463--7465 | 7465