Synthesis of the transfer-RNA nucleoside queuosine by using a chiral allyl azide intermediate

Article abstract of DOI:10.1002/anie.200604579

Chilled out: Chiral allyl azides are rarely used in natural product synthesis because of their tendency to undergo a [3.3] sigmatropic rearrangement (see scheme, top). In allylic cyclopentenyl azides, this rearrangement can be suppressed at just 0°C, enabling a short convergent synthesis of the hypermodified transfer-RNA nucleoside queuosine. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200604579

Angewandte
Chemie
DOI: 10.1002/anie.200604579
Nucleosides
Synthesis of the Transfer-RNA Nucleoside Queuosine by Using a
Chiral Allyl Azide Intermediate**
Florian Klepper, Eva-Maria Jahn, Volker Hickmann, and Thomas Carell*
Transfer RNAs (tRNAs) are key molecules needed for the
decoding of the genetic information at the ribosome.[1] The
tRNAs contain the four canonical RNA bases plus a large set
of modified nucleotides whose function in the tRNA charging
and decoding process is largely unknown.[2] Queuosine (Q;
1)[3] and its galactosylated, mannosylated, or amino acid
modified derivatives[4] (Scheme 1) are important hypermodi-
bases play during the translational process and to study their
largely unknown biosynthesis,[6] efficient synthetic procedures
for the modified bases and ultimately of tRNA containing
such modified bases are a prerequisite.
Herein we describe an efficient stereoselective synthesis
of the hypermodified tRNA nucleoside queuosine[7] using a
reductive amination of 7-deazaguanosine-aldehyde 3 with the
cyclopentenylamine 2,[8] which is prepared from an allyl azide
precursor (Scheme 1).
The short synthesis of the allyl amine 2 from the allyl
azide precursor 4, however, requires that the Mitsunobu
reaction (Scheme 2, step e)[9] proceeds with a defined regio-
(SN2vs. S N2’) and stereochemistry (inversion of configura-
tion). In addition, the [3.3] sigmatropic rearrangement, which
allyl azides such as 4 are prone to undergo, has to be
controlled.[10] Indeed, owing to the efficient [3.3] sigmatropic
rearrangement of allylic azides and the reported lack of
regiocontrol of Mitsunobu aminations of allylic alcohols,[19]
such a synthetic approach looks, at first, rather unattractive.
The cyclopentenyl building block 2 was synthesized
starting from d-(À)-ribose 5. Protection of the 2’ and 3’
hydroxy groups as an acetonide, conversion into the mono-
Scheme 1. Chemical structures of queuosine (1), its derivatives, and
key synthetic intermediates. Bz=benzoyl.
fied RNA bases present in the anticodon stem loop of various
tRNAs. It is currently hypothesized that these base deriva-
tives are needed to fine-regulate the translational process.[5]
To enable investigation of the role that these hypermodified
[*] F. Klepper, E.-M. Jahn, V. Hickmann, Prof. Dr. T. Carell
Department of Chemistry and Biochemistry
Ludwig-Maximilians-Universität
Butenandtstrasse 5–13, 81377 München (Germany)
Fax: (+49)89-2180-77756
E-mail: thomas.carell@cup.uni-muenchen.de
[**] We thank the Deutsche Forschungsgemeinschaft (DFG), the
Volkswagen Foundation (Priority Program: Conformational Control
of Biomolecular Function), the EU Marie Curie Training Mobility
Program (CLUSTOXDNA), and the Fonds der Chemischen Indus-
trie for generous financial support of this research.
Scheme 2. Synthesis of the cyclopentenylamine 2: a)2,2-dimethoxypro-
pane, MeOH, HClO4, acetone, 94%; b)PCC (4.0 equiv), benzene,
reflux, 46%; c)(MeO) 2P(O)CH3, nBuLi, THF, 08C, 56%; d)NaBH 4,
CeCl3, MeOH, 08C, 95%; e)DIAD, PPh 3, HN3 (1.3m in toluene), THF,
08C; f)PPh 3, THF, 08C, 72% (65% from 11)in two steps; g)toluoyl
chloride, NEt3, CH2Cl2, 60%. PCC=pyridinium chlorochromate, DIA-
D=diisopropylazodicarboxylate.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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methylacetal 6, and subsequent oxidation with excess pyridi-
suppressed, which allows chiral allyl azides to be utilized as
convenient synthetic intermediates. In addition, the ability to
suppress the racemization demonstrates that the Mitsunobu
reaction proceeds, in our case, unexpectedly under strict
control of the regiochemistry:[9] we observe exclusively an SN2
and not anti-SN2’-attack of the azide on the allylic alcohol.
The unusual selectivity of the Mitsunobu amination is
further demonstrated by the observation that the use of the
diastereomeric allylic alcohol 11[14] provides the same allyl
azide 4 and, after Staudinger reduction, the allylic amine 2.
This result is only explainable if, in this case, we assume an
interesting, and for the Mitsonobu amination untypical, syn-
SN2’ reactivity.[15,16,19]
For the preparation of 7-formyl-7-deazaguanosine as the
second key building block needed for the synthesis of
queuosine (1), a second short synthesis was developed
(Scheme 3). Condensation of chloroacetaldehyde (12) and
2,4-diamino-6-hydroxypyrimidine furnished 7-deaza-guanine
(13). Chlorination of the 5-position with phosphoryl chloride,
protection of the exocyclic amino function with pivaloyl
chloride, and subsequent iodination with NIS gave the
deazapurine derivative 14. This compound was coupled to
protected d-(À)-ribose under Vorbrüggen conditions to give
the iodonucleoside 15 in good yields.[17]
À
nium chlorochromate gave, under C C bond cleavage lactone
7. An intramolecular Wittig reaction with dimethylmethyl-
phosphonate and nBuLi furnished pentenone 8.[11] Stereo-
specific reduction of 8 with sodium borohydride allowed the
preparation of the key allyl alcohol intermediate 9 in just four
steps with an overall yield of 23%. Subsequent Mitsunobu
reaction and then a Staudinger reduction at room temper-
ature gave exclusively 2. To analyze the optical purity of the
allyl amine 2, we converted it into the toluoyl protected
derivative 10. Analysis of 10 by chiral HPLC, showed to our
disappointment that compound 2 was obtained with an
unsatisfactory ee value of < 80%. The reason could be
either a partial anti-SN2’-type reaction, as a result of the
lack of regiocontrol, or racemization that occurs because of
the allyl azide rearrangement.[12] To analyze the allyl azide
rearrangement pathway in more detail, we heated the allyl
azide 4 to about 608C. Indeed, within a few hours the optical
rotation dropped to zero, showing that the allyl azide [3.3]
sigmatropic rearrangement is indeed a problem at room
temperature.
To discover how to control the unwanted allyl azide
rearrangement we measured the disappearing optical rotation
of a solution of 4 in pure ethanol at various temperatures
(Figure 1). From this plot it is evident that the allyl azide
equilibration, which is indeed quite fast at room temperature,
can be efficiently suppressed at just 08C. After checking that
the racemization rate in THF and EtOH was identical, we
carried out the Mitsunobu reaction 9 to 4 at 08C. A
Staudinger reduction, also performed at 08C, provided
compound 2, which was again converted into the derivative
10 for analysis by chiral HPLC. This time, the key allyl amine
intermediate 2 was indeed obtained with an excellent ee value
of 95% in 72% yield over both steps.
After dechlorination of 15 with DABCO and triethyl-
amine, the aldehyde group was introduced by a palladium-
In contrast to recently published procedures, in which the
fast allyl azide equilibrium was exploited for synthesis,[10a,13]
we show herein that the rearrangement can also be efficiently
Scheme 3. Synthesis of queuosine (1): a) 1) NaOMe, H2O, DMF;
2)2,4-diamino-6-hydroxypyrimidine, NaOAc, 86%; b)POCl 3, 1208C,
78%; c)PivCl, pyridine, 64%; d)NIS, THF, 81%; e)BSA, TMSTf, 1- O-
acetyl-2,3,5-tri-O-benzoyl-d-ribofuranose, MeCN, 60%; f)DABCO, NEt 3,
CsOAc, DMF, 63%; g)[Pd 2(dba)3] PPh3, CO, Bu3SnH, 73%; h) 2,
benzene, then NaBH4, EtOH, 85%; i)NaOH, MeOH; j)HCl (2 n),
45%. Piv=pivaloyl, NIS=N-iodosuccinimide, BSA=bis(trimethylsilyl)-
acetamide, TMSTf=trimethylsilyltriflate, DABCO=1,4-diazabicyclo-
[2.2.2]octane.
Figure 1. [3.3] sigmatropic rearrangement of 4 at T=0, 35, 45, 55, and
658C in EtOH.
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hypermodified tRNA nucleoside queuosine (1), which is
found in the anticodon stem loop of tRNAs. We show that the
synthesis is efficiently performed using the allyl azide
intermediate 4. The [3.3] sigmatropic rearrangement of the
allyl azide intermediate, which causes racemization of this key
intermediate, could be efficiently suppressed at just 08C. This
result shows that chiral allyl azides can be used as highly
valuable intermediates in natural-product synthesis. The
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analysis of its function during the decoding process.
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Received: November 9, 2006
Published online: February 20, 2007
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Keywords: allyl compounds · azides · Mitsunobu reaction ·
nucleosides · rearrangement
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