Efficient synthesis of deazaguanosine-derived tRNA nucleosides PreQ 0, PreQ1, and archaeosine using the Turbo-Grignard method

Article abstract of DOI:10.1002/ejoc.201000987

The natural products PreQ₁, PreQ₀, and archaeosine are deazaguanosine-derived natural products of immense biological importance. All three are critical components of tRNAs and, as such, are involved in decoding genetic information. W

Full text of DOI:10.1002/ejoc.201000987

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000987
Efficient Synthesis of Deazaguanosine-Derived tRNA Nucleosides PreQ0,
PreQ1, and Archaeosine Using the Turbo-Grignard Method
Tobias Brückl,[a] Ines Thoma,[a] Andreas J. Wagner,[a] Paul Knochel,[a] and
Thomas Carell*[a]
Keywords: Grignard reaction / Nucleosides / tRNA / Natural products
The natural products PreQ1, PreQ0, and archaeosine are de-
azaguanosine-derived natural products of immense bio-
logical importance. All three are critical components of
tRNAs and, as such, are involved in decoding genetic infor-
mation. We were able to consolidate and shorten the synthe-
ses of all three compounds by using a novel Turbo-Grignard-
based approach. The reported iodine/magnesium exchange
followed by a Grignard reaction with the deazaguanosine
skeleton showed a high tolerance towards important func-
tional groups such as esters and amides.
tral biosynthetic precursors of queuosine and archaeosine,
respectively.[2,15] They are incorporated into tRNA by
tRNA guanine transglycosylases (TGTs),[11] one of which
has been identified as a target for the treatment of shigello-
sis.[16] In addition, deazaguanosine-derived nucleosides are
of significant interest from a medicinal chemistry point of
view, as they have antibacterial, antifungal, antiviral, and
anticancer activity.[17]
Introduction
Naturally occurring RNAs like messenger RNA
(mRNA), ribosomal RNA (rRNA), and transfer RNA
(tRNA) contain modified nucleosides, which structurally
deviate from the canonical ones.[1] The found changes range
from simple methylations of the base or of the C2Ј–OH of
the sugar to hypermodifications featuring large side chains
and additional rings. The most complex hypermodifications
are probably those derived from deazaguanosines. These
bases not only contain an additional side chain but also
have an altered ring system, in which the N7 of guanosine
is replaced by a carbon atom.
In archaea, the deazaguanosine nucleoside archaeosine
(G+) occurs in the 15-position of nearly all tRNAs,[2] where
it is suspected to stabilize their structural integrity (Fig-
ure 1).[3] Under physiological pH it is positively charged
and placed in a cleft with high negative electrostatic poten-
tial.[2] Queuosine (Q) occurs in the wobble position of the
anticodon in tRNAHis, tRNAAsn, tRNAAsp, and tRNATyr
of both eukaryotes and prokaryotes (Figure 1).[4] It is in-
volved in numerous key processes such as translational fi-
delity,[5] development,[6] proliferation,[7] and bacterial viru-
lence.[8] There is a multitude of excellent reviews discussing
the individual aspects of queuosine biosynthesis[2,9,10,11] and
functions.[9,12] Furthermore, the influence of the Q base on
the activity of proto-oncogenes[13] and correlations between
a decreased Q-content and malignancy as well as tumor
differentiation implies a participation of Q-deficiency in the
neoplastic processes.[14] PreQ1 and PreQ0 (Figure 1) are cen-
Figure 1. The hypermodified deazanucleosides PreQ0, archaeosine,
PreQ1, and queuosine are all available from iodonucleoside 1. The
synthesis of queuosine has been presented before.[18]
Results and Discussion
The high biological importance of deazaguanosines
prompted us to establish a chemo- and regioselective ap-
proach for the functionalization of deazaguanosines in the
critical 7-position starting from the common key intermedi-
ate 1 (Figure 1). The method was intended to tolerate a
broad range of functionalities such as protecting groups,
[a] Center for Integrative Protein Science (CiPSM), Department of
Chemistry, Ludwig Maximilians University, Munich
Butenandtstrasse 5-13, 81377 Munich, Germany
Fax: +49-89-2180 77756
E-mail: thomas.carell@cup.uni-muenchen.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000987.
Eur. J. Org. Chem. 2010, 6517–6519
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. Brückl, I. Thoma, A. J. Wagner, P. Knochel, T. Carell
SHORT COMMUNICATION
both at the sugar moiety and the heterocycle. For this study, –78 °C was first deprotonated with MeMgCl (2 equiv.) and
we chose the naturally occurring nucleosides PreQ0, PreQ1, then converted into Grignard compound 7 by addition of
and archaeosine.
iPrMgCl·LiCl. This procedure leads to almost quantitative
The synthesis of nucleoside 1 was reported in preceding formation of the corresponding metallated species 7. This
publications.[18,19] For the initial step of the synthesis, which was shown by treatment of intermediate 7 with water, which
requires preparation of nucleobase 2,[20] we found that con- caused complete deiodination of the starting material. The
densation of pyrimidinone 3 with bromoacetaldehyde, in most likely structure of compound 7 is shown in Scheme 2.
situ generated from bromoacetaldehyde diethyl acetal 4 is Interestingly, the iodine/magnesium exchange reaction takes
superior to the procedure published before (Scheme 1). place despite the presence of two negatively charged posi-
Subsequent steps towards nucleoside 1 were performed as tions in the starting material after deprotonation with
published[18] yielding critical intermediate 1 in six steps and MeMgCl. Grignard reagent 7 was subsequently trapped by
10% overall yield.
addition of TosCN. The corresponding nitrile nucleoside 8
was isolated by using a standard extraction procedure fol-
lowed by column chromatography.
Scheme 1. Synthesis of the natural deazaguanosine derivative
PreQ1. Reagents and conditions: (a) HCl, NaOAc, water, 90 °C and
80 °C, 4 h, 74%; (b) Pd2(dba)3, triphenylphosphane, CO, tributyltin
hydride, toluene, 50 °C, 2 h, 73%; (c) hydroxylamine hydrochloride,
NaHCO3, H2O, EtOH, –20 °C, 12 h, 65%; (d) NiCl2·H2O, NaBH4,
MeOH, 0 °C, 1 h, then NaOH in MeOH (0.5 m), room temp., 12 h,
42%.
Starting from nucleoside 1 we recently reported a for-
mylation strategy that led to the hypermodified nucleoside
queuosine in nine steps.[18] This approach was also used for
the preparation of PreQ1 (Scheme 1). After palladium-cata-
lyzed formylation of nucleoside 1 with CO and Bu3SnH,
the resulting aldehyde 5 was converted into the correspond-
ing oxime 6. Reduction of the oxime moiety was initially
attempted with NaBH4, which however proved to be too
unreactive. Functional groups that are inert to treatment
with NaBH4 have been shown to gain susceptibility to re-
Scheme 2. Synthesis of the deazaguanosine-containing tRNA bases
PreQ0 and archaeosine. Reagents and conditions: (a) MeMgCl,
iPrMgCl·LiCl, toluene, –65 °C, 6 h; (b) TosCN, 65 °C to room
temp., 18 h; (c) NH3 in H2O (28%), 60 °C, 16 h, 78%; (d) HCl (g),
MeOH, then NH3 in MeOH (7 m), room temp., 19 h, 40%. Com-
plexed LiCl in intermediate 7 has been omitted for the sake of
clarity.
The established iodine/magnesium exchange reaction has
duction after addition of NiCl2·H2O.[21] Application of the great advantage of tolerating various functional groups
these conditions indeed allowed clean reduction of the ox- including ester and amide moieties present during the con-
ime to furnish the benzoylated and pivaloylated PreQ1. The version. Furthermore, the reactive C8-H position, which
crude product was deprotected with 0.5 m NaOH in MeOH has been used for C–H activation before,[29] is not attacked
to yield PreQ1 in excellent purity after HPLC. In summary, under the conditions presented here. This high tolerance
formylation and subsequent reduction of nucleoside 1 al- towards functional groups is strictly limited to the Turbo-
lows preparation of PreQ1 in only nine straightforward Grignard reagent. Experiments with iPrMgCl alone re-
steps and in 1.9% overall yield. This is significantly shorter sulted in decomposition of nucleoside 1, which showed that
and higher yielding than previous routes.[22,23]
For the synthesis of C7-substituted deazaguanosine de- mation.[30]
the presence of LiCl is essential for enabling the transfor-
rivatives such as PreQ0 and archaeosine, we expanded the
Starting from nitrile nucleoside 8 we prepared PreQ0, the
scope of the novel and versatile reagent iPrMgCl·LiCl biosynthetic precursor of archaeosine, by simple deprotec-
(Turbo-Grignard)[24] from uracil[25,26] and adenosines[26,27] tion with NH3 in water. PreQ0 is available by this route in
to a further class of nucleosides: the deazaguanosines only eight steps and 4.7% overall yield.[22,31]
(Scheme 2). Using the Turbo-Grignard enables mild condi-
Finally, nitrile nucleoside 8 was converted into archaeo-
tions at low temperature compared to a reaction using sine by using Pinner conditions. This approach not only
CuCN.[28] To this end, iodo compound 1 in toluene at converted the nitrile efficiently into the amidinium moiety
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Efficient Synthesis of Deazaguanosine-Derived tRNA Nucleosides
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To the best of our knowledge this report describes the
first Grignard and iodine/magnesium exchange reaction
performed with deazapurines. Application of the Turbo-
Grignard reagent allowed us to perform functionalization
of the C7 position in the presence of benzoyl and pivaloyl
protecting groups. This facilitated the synthesis of the de-
azaguanosine-derived tRNA nucleosides archaeosine,
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course of this study, introduction of the versatile nitrile
moiety through the presented Turbo-Grignard reaction has
been established. Expansion of the scope of this reaction to
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might turn the here-established reaction into a general tool
for the manipulation of deazaguanosines.
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Acknowledgments
We thank the Deutsche Forschungsgemeinschaft (grants CA275/8-
4 and SFB749) as well as the Volkswagen Foundation for financial
support.
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Received: July 13, 2010
Published Online: October 22, 2010
Eur. J. Org. Chem. 2010, 6517–6519
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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