A short, concise synthesis of queuine

Article abstract of DOI:10.1016/j.tetlet.2010.06.008

A short, concise synthesis of queuine was accomplished in a 36% overall yield through a convergent scheme utilizing a reductive amination as the penultimate step. The synthesis demonstrates the utility of silylation to facilitate reactions of various pyrrolo[2,3-d]pyrimidine intermediates, and offers the possibility of easily accessing related pyrrolo[2,3-d]pyrimidines as well as making additional analogues of queuine.

Full text of DOI:10.1016/j.tetlet.2010.06.008

Tetrahedron Letters 51 (2010) 4163–4165
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A short, concise synthesis of queuine
*
Allen F. Brooks, George A. Garcia, H. D. Hollis Showalter
Department of Medicinal Chemistry, University of Michigan, Ann Arbor, MI 48109, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 February 2010
Revised 1 June 2010
Accepted 2 June 2010
Available online 8 June 2010
A short, concise synthesis of queuine was accomplished in a 36% overall yield through a convergent
scheme utilizing a reductive amination as the penultimate step. The synthesis demonstrates the utility
of silylation to facilitate reactions of various pyrrolo[2,3-d]pyrimidine intermediates, and offers the pos-
sibility of easily accessing related pyrrolo[2,3-d]pyrimidines as well as making additional analogues of
queuine.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Pyrrolo[2,3-d]pyrimidine
Queuine
Mitsunobu azididation
Reductive amination
RNA transglycosylase
Queuine (1; Fig. 1) is one of the approximately one hundred
modified nucleotides found in RNA.1 It was first discovered in bo-
vine amniotic fluid and later determined to be conserved through-
out the eukaryl and eubacterial kingdoms with very few
exceptions.2,3 Queuine is unusual in that, unlike the majority of
modified nucleotides that result from modifications of the geneti-
cally encoded nucleotides, it is incorporated into RNA by transgly-
cosylation. Queuine modification differs depending on the type of
organism under consideration.2 Eukaryae incorporate queuine into
RNA, whereas eubacteria incorporate preQ1 (2; Fig. 1), which then
undergoes modification to yield queuine. Queuine has long been
known to occur in the wobble position of four tRNAs: aspartic acid,
asparagine, histidine, and tyrosine.4,5 However, recent in vitro
studies of the Escherichia coli transglycosylase have demonstrated
the potential for queuine to exist in other RNA species.6–8 To more
fully probe the occurrence of queuine and to utilize it in mechanis-
tic studies of eukaryl transglycosylases requires an efficient,
straightforward synthesis.
Grubb9c employs a different disconnection of the molecule, pro-
ceeding via a key Mitsunobu reaction to introduce the cyclopente-
nylamine side chain and a subsequent cyclocondensation reaction
to build up the core heterocyclic moiety. While relatively straight-
forward and efficient, it requires 11 linear steps. In this Letter, we
report on an exceptionally short and high yielding synthesis of
queuine from readily available precursors.
Our synthesis is shown in Scheme 1 and utilizes a strategy
wherein easily accessed side chain amine and core heterocycle
moieties are condensed via a reductive amination step followed
by a global deprotection step. The most direct route to the desired
heterocyclic moiety of queuine was determined to be through
preQ0 (3),10 which was difficult to enter into further reaction due
to its poor organic solubility. A solution to this was to draw from
the procedure of Barnett and Kobierski11 to first silylate 3 with ex-
cess N,O-bis(trimethylsilyl)acetamide (BSA) and after removing
volatiles in vacuo, treat the residual oil with trityl chloride in pyr-
idine to provide tritylated preQ0 (4). The site of tritylation, while
not important for subsequent reactions, was determined by 1H
NMR to be on the N-2 position.
There are three previous reports on the synthesis of queuine.9
That of Kondo et al.9a proceeds via a Schiff base between a pro-
tected pyrrolopyrimidine aldehyde and protected queuine side
chain amine in a total of 19 steps. The synthesis of Akimoto et
al.9b is much shorter and proceeds via a Mannich reaction to incor-
porate the protected side chain of queuine regioselectively to the
C-5 position of a pyrrolopyrimidine precursor. Its main drawback
is the requirement of several equivalents of the side chain in a
key exchange reaction. The more recent synthesis of Barnett and
H
O
N
O
NH2
HN
H2N
HN
H2N
HO
N
H
N
H
OH
N
N
Queuine
PreQ1
2
( )
1
( )
* Corresponding author. Tel.: +1 734 764 5504; fax: +1 734 647 8430.
E-mail addresses: showalh@umich.edu, hdhshow@gmail.com (H.D. Hollis Sho-
walter).
Figure 1. Structures of queuine and preQ1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.06.008

4164
A. F. Brooks et al. / Tetrahedron Letters 51 (2010) 4163–4165
O
N
O
N
O
CN
CHO
CN
1. BSA, DMF
1. HMDS
HN
H2N
HN
RHN
aq
HN
2. TrtCl, pyridine
N
2. DIBAL-H
CH2Cl2
N
N
H
TrtHN
N
64%
H
H
PreQ0
(3)
Trt = C(Ph)3
5: R = Trt
HOAc 6: R = H
4
65 %
1. HMDS 2. DIBAL-H, CH2Cl2
45%
1. PPh3
(PhO)2P(O)N3
DIAD, THF
O
OMe
HO
H2N
HO
3 steps
O
O
O
O
O
O
21%
(ref 13)
2. PPh3, THF
then H2O
69 %
7
8
9
H
N
O
N
Na2SO4, MeOH, 6h
1.25 M HCl
HN
TrtHN
. HCl
1
5
9
O
N
in MeOH
87 %
then NaBH4
99%
O
H
10
Scheme 1. Convergent synthesis of queuine (1) via reductive amination of pyrrolo[2,3-d]pyrimidine (5) and amino alcohol (9).
Several trials were attempted to transform nitrile 4 to desired
aldehyde 5 with DIBAL-H before a reproducible method with an
acceptable yield was found. A key breakthrough was to utilize sily-
lation of the heterocycle to impart sufficient solubility such that
reduction could be conducted at a lower temperature. Hence, pro-
tected nitrile 4 was silylated with HMDS in refluxing toluene, and
upon removal of volatiles the remaining glass-like solid was dis-
solved in dichloromethane and cooled to À78 °C. DIBAL-H was
then added to complete the transformation to 5 after mild acidic
workup. Acid hydrolysis of 5 then provided highly insoluble alde-
hyde 6.12 We found that this could also be obtained directly from
unprotected nitrile 3 under similar DIBAL-H reduction conditions
described above, albeit in modest yield. The chemistry shown in
Scheme 1 completed a two-pot sequence in an overall 42% yield
to key heterocyclic aldehyde 5 for subsequent entry into a reduc-
tive amination reaction.
With the aldehyde in hand, our attention focused on synthesiz-
ing the amine side chain of queuine. A recent synthesis by Klepper
et al.13 was deemed to be the best available approach and was uti-
lized in our synthesis with some modifications. Our decision to fol-
low this route was based on its length, overall yield, and the fact
that it addresses a [3.3] sigmatropic rearrangement of an allylic
azide intermediate that leads to racemization, which likely occurs
in earlier routes employing similar intermediates. Klepper et al.
were able to suppress this by conducting the Mitsunobu reaction
and subsequent Staudinger reduction in the same pot at a reduced
temperature.
reduction. The four-pot route from 7 to 9 proceeded in 14.5% over-
all yield and was highly reproducible.
Before completing the synthesis, the chiral purity of amine 9
needed to be verified due to possible racemization of the azide
intermediate derived from 8. This was accomplished by generating
the Mosher amide14 under standard amidation conditions. Subse-
quent 19F NMR demonstrated that only one enantiomer was pres-
ent. This was confirmed by comparing the spectrum of the Mosher
amide of 9 to that of racemic 9, synthesized via an alternate route
that failed to suppress the [3.3] sigmatropic rearrangement of the
allylic azide (see Supplementary data).
With key reaction partners aldehyde 5 and amine 9 in hand, the
synthesis of queuine was completed. As shown in Scheme 1, this
was done under standard reductive amination conditions to pro-
vide penultimate adduct 10 in near quantitative yield. This was en-
tered directly into a global deprotection with methanolic HCl to
provide queuine (1) which precipitated out of solution as the
monohydrochloride salt in 87% yield.
In conclusion, we have developed a concise synthesis of queu-
ine, which is considerably shorter than previously reported synthe-
ses. Our synthesis proceeds in an overall 36% yield in a four-pot
linear sequence from starting pyrimidine 3. Our route demon-
strates the utility of silylation to facilitate reactions of various pyr-
rolo[2,3-d]pyrimidine intermediates, which otherwise would be
difficult to conduct reliably. Furthermore, our synthesis, especially
in the facile generation of aldehydes 5 and 6, offers the possibility
of easily accessing related pyrrolo[2,3-d]pyrimidines as well as
making further analogues of queuine. Future publications will re-
port on the synthesis of radiolabeled queuine, wherein NaBT4 is
utilized in the reductive amination step, and the application of this
compound to study the prevalence of queuine as well as to conduct
kinetic studies of the eukaryl transglycosylase.
As shown in Scheme 1, the synthesis of the cyclopentenylamine
side chain begins with commercially available 7. This contains the
requisite chirality for the two alcohol functions in queuine. The
reactions from 7 to alcohol 8 were conducted similarly as previ-
ously described,13 although minor adjustments were made in sev-
eral steps (see Supplementary data). For the key final step (8–9),
we modified the procedure of Klepper et al. to avoid the use of haz-
ardous hydrazoic acid in the Mitsunobu reaction. Utilizing diphen-
ylphosphoryl azide (DPPA) instead, 9 was procured in good yield
with careful control of temperature followed by in situ Staudinger
Acknowledgments
This work was supported by the University of Michigan College
of Pharmacy Vahlteich and Upjohn Research funds (GAG, HDHS),

A. F. Brooks et al. / Tetrahedron Letters 51 (2010) 4163–4165
4165
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5. Durand, J. M.; Okada, N.; Tobe, T.; Watarai, M.; Fukudu, I.; Suzuki, T.; Nakata,
N.; Komatsu, K.; Yoshikawa, M.; Sasakawa, C. J. Bacteriol. 1994, 176, 4627.
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8. Curnow, A. W.; Garcia, G. A. Biochimie 1994, 76, 1183.
Supplementary data
9. (a) Kondo, T.; Ohgi, T.; Goto, T. Chem. Lett. 1983, 419; (b) Akimoto, H.; Imamiya,
E.; Hitaka, T.; Nomura, H. J. Chem. Soc., Perkin Trans. 1. 1988, 1637; (c) Barnett,
C. J.; Grubb, L. M. Tetrahedron 2000, 56, 9221.
10. Migawa, M. T.; Hinkley, J. M.; Hoops, G. C.; Townsend, L. Synth. Commun. 1996,
26, 3317. the compound is also commercially available..
11. Barnett, C. J.; Kobierski, M. E. J. Heterocycl. Chem. 1994, 31, 1181.
12. Hoops, G. C.; Townsend, L. B.; Garcia, G. A. Biochemistry 1995, 34, 15381.
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2325.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.06.008.
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