Stereoselective cyclopropanation in the synthesis of 3'-deoxy-3'-C-hydroxymethyl-2',3'-methylene-uridine

Article abstract of DOI:10.1021/ol502383c

The synthesis of the novel 2',3'-cyclopropane nucleoside 3'-deoxy-3'-C-hydroxymethyl-2',3'-methylene-uridine is described. Stereoselective construction of the cyclopropane ring was achieved via Simmons-Smith cyclopropanation of a benzyl protected silyl enol ether, which was itself derived from 1,2-O-isopropylidene-α-D-xylofuranose.

Full text of DOI:10.1021/ol502383c

Letter
pubs.acs.org/OrgLett
Stereoselective Cyclopropanation in the Synthesis of 3′-Deoxy-
3′‑C‑hydroxymethyl-2′,3′-methylene-uridine
Zofia Komsta,*,‡ Benjamin A. Mayes,*,† Adel Moussa,† Montserrat Shelbourne,‡ Alistair Stewart,†
Andrew J. Tyrrell,‡ Laura L. Wallis,‡ Alexander C. Weymouth-Wilson,‡ and Alexander Yurek-George‡
†Idenix Pharmaceuticals, 320 Bent Street, Cambridge, Massachusetts 02141, United States
‡Dextra, Science and Technology Centre, Earley Gate, Whiteknights Road, Reading, RG6 6BZ, U.K.
S
* Supporting Information
ABSTRACT: The synthesis of the novel 2′,3′-cyclopropane nucleoside 3′-deoxy-3′-C-hydroxymethyl-2′,3′-methylene-uridine is
described. Stereoselective construction of the cyclopropane ring was achieved via Simmons−Smith cyclopropanation of a benzyl
protected silyl enol ether, which was itself derived from 1,2-O-isopropylidene-α-D-xylofuranose.
Scheme 1. 2-Ketofuranoside 9 Synthesis
3′-Deoxy-3′-C-hydroxymethyl-2′,3′-methylene-uridine 1 was
required as part of an investigation into the antihepatitis C
virus (HCV) activity of novel nucleoside scaffolds (Figure 1).
Although the introduction of four direct-acting antivirals1
(DAAs) over the past three years has significantly improved
patient treatment options, with a prevalence of 2.8% of the
world’s population,2 HCV remains a serious global disease
concern. Clinical validation of nucleoside inhibitors targeting the
HCV NS5B polymerase has been demonstrated with multiple
compounds in development over the past decade.3 In 2013, the
2′-C-methyl-uridine prodrug, sofosbuvir,1d achieved regulatory
approval, and thus further exploration of this important class of
DAAs continues.
Figure 1. Target nucleoside 1 combining 3′-C-hydroxymethyl 2 and
2′,3′-dideoxy-2′,3′-cyclopropane 3 structural features.
and the synthesis, characterization, and stereochemical con-
firmation of this novel ribonucleoside system are described
herein.
Synthesis of 1 from the commercially available acetonide
protected D-xylose derivative 4 started with the chemistry
reported by Suhara et al.10 to form the 1,2,5-protected 3-C-
methylene xylofuranoside 6 (Scheme 1). Pivaloyl protection of
the primary hydroxyl proceeded selectively, and the oxidation to
Incorporation of a 3′-C-hydroxymethyl group 2 has
historically led to a multitude of diverse sugar-modified
nucleoside systems displaying a broad range of antiviral (hepatitis
B virus,4 human immunodeficiency virus,5 varicella zoster virus6),
antimicrobial (tuberculosis7), and anticancer (leukemia8)
properties. In contrast, there are relatively few reports
concerning nucleosides bearing a 2′,3′-cyclopropane modifica-
tion 3, beyond the original 2′,3′-dideoxy analogs.9 Combination
of these structural elements to form 3′-deoxy-3′-C-hydroxy-
methyl-2′,3′-methylene-uridine 1 as a potential anti-HCV agent
Received: August 11, 2014
© XXXX American Chemical Society
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Organic Letters
Letter
5 was achieved using pyridinium dichromate in a similar yield to
the reported pyridinium chlorochromate oxidation. Non-
chromium based oxidation protocols, such as the Swern
oxidation or use of the Dess-Martin periodinane, were evaluated
but either were less selective or resulted in ketone hydrate
formation. Following the Wittig reaction to introduce the
exomethylene moiety in 6, hydroboration was performed using
9-BBN.10,11 This was achieved in high yield and excellent
stereoselectivity with only the 3R-isomer observed, and 7 was
isolated following treatment with H2O2 and NaOH. Phase
transfer conditions12 were employed to form the dibenzyl
protected intermediate 8 directly from compound 7. Treatment
with 4.0 M HCl in dioxane and BnOH effected acetonide
removal, and Fischer glycosylation to give the benzyl glycoside
and subsequent oxidation with Dess-Martin periodinane was
performed in good yield to give the protected 3-C-
hydroxymethyl 2-ketofuranoside 9.
Figure 2. Confirmation of stereochemistry by 2D NOESY NMR
spectroscopy (performed on a 400 MHz spectrometer; NOE
correlations reported from extracted 1D data sets).
Several examples of the cyclopropanation of pyranose
derivatives have been reported in the literature.13 Open chain
cyclopropanations followed by cyclization to give carbohydrate
ring systems are also known.14 However, the formation of
cyclopropane rings on carbohydrate substrates in their cyclic
furanose form is less well described. An example of Simmons−
Smith cyclopropanation on a 4,5-dihydrofuran was found to be
highly stereospecific.15 A high yielding Simmons−Smith type
cyclopropanation at the 3,4-position of an L-xylose derived
substrate has been described.16 However, to date, there has been
no report of a Simmons−Smith type cyclopropanation at the 2,3-
position of a carbohydrate furanose ring. 2,3-Methylene
pentofuranose derivatives have been synthesized, although
using alternative methods, such as ring contraction, conjugate
addition to a phenylselenoid derivative, a displacement approach
for gem-dimethylcyclopropanes, and via 1,3-dipolar cyclo-
additions of diazomethane to furanones.9a,17 2-Ketofuranoside
9 was transformed into silyl enol ether 10 using LDA and TBSCl
(Scheme 2). Cyclopropanation was subsequently performed
confirming their close spatial relationship on the β-face of the
furanose ring.
Interestingly, this selectivity was not readily reproduced and
subsequent cyclopropanations resulted in a mixture of 2R,3R and
2S,3S isomers, prompting investigation of the reaction
conditions and input materials. The purity of the silyl enol
ether substrate 10 was found to be critical to the stereochemical
outcome of the reaction. The presence of residual TBSOH due to
either its incomplete purging on chromatography or by gradual
decomposition of 10 on storage (as observed by proton NMR
spectroscopy) was determined to be the influencing factor.
Cyclopropanations performed on freshly purified 10 gave
exclusively (R,R)-11 in 60% yield, whereas a sample of 10
containing 34% w/w (2.2 mol equiv) of TBSOH gave a 1:2.9
mixture of (R,R)-11 and (S,S)-11 diastereomers with only a 22%
combined yield.
One of the major factors influencing the stereochemical
outcome of the cyclopropanation reaction is chelation of the zinc
carbenoid species.19 Under normal conditions it is proposed that
the zinc carbenoid species chelates to the oxygen atom at the
sugar C5-position directing cyclopropanation onto the same (β)
face of the ring to generate the (R,R)-11 isomer. If, however, the
cyclopropanation is performed in the presence of TBSOH,
binding of this impurity to the zinc carbenoid species may
compete with substrate chelation resulting in the loss of
stereocontrol. Cyclopropanation may then occur preferentially
on the less hindered α face to generate (S,S)-11 as the major
product.
Scheme 2. Stereoselective Cyclopropanation
Formation of the desired protected 3-C-hydroxymethyl 2,3-
methylene furanoside (R,R)-11 was achieved in 49% yield from
ketofuanose 9 (2 steps). After removal of the silyl group, the
tertiary hydroxyl was reprotected as acetate ester (R,R)-12
(Scheme 3). A 2D-NOESY NMR experiment performed on
(R,R)-12 further confirmed the required stereochemistry at C2
and C3 while also indicating the β anomeric configuration
(Figure 2). Benzyl group removal by hydrogenation and
subsequent triacetylation provided nucleosidation substrate
(R,R)-13 without issue. The (S,S)-13 isomer could not be
prepared since the debenzylated intermediate following hydro-
genation was unstable. Analogous cyclopropanated carbohy-
drates in their furanose form have been found to behave similarly,
and these results will be published in due course.
using the Simmons−Smith type conditions described by Gerber
and Vogel18 in which the substrate 10 was added to a premixed
solution of ZnEt2 and chloroiodomethane in DCE cooled to −30
°C, followed by warming to ambient temperature.
The cyclopropanation reaction was initially found to exhibit
remarkably high diastereoselectivity, with a greater than 8:1 ratio
of (R,R)-11 to (S,S)-11. The desired 2R,3R stereochemistry in 11
was investigated via 2D-NOESY NMR spectroscopy (Figure 2).
An NOE correlation was observed between one of the methylene
protons of the cyclopropane ring and the methylene H5 protons
Nucleosidation of (R,R)-13 was performed using silylated
uracil and TMS triflate in acetonitrile under Vorbruggen
̈
conditions.20 Protected uridine 14 was isolated in excellent
yield as the β-anomer only. Acetate deprotection of 14 was,
B
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Organic Letters
Letter
2014). (d) http://www.fda.gov/downloads/AdvisoryCommittees/
C o m m i t t e e s M e e t i n g M a t e r i a l s / D r u g s /
AntiviralDrugsAdvisoryCommittee/UCM371877.pdf (28 July 2014).
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Hepatol. 2013, 57, 1333.
Scheme 3. Synthesis of Nucleoside 12
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however, challenging due to gradual decomposition under
various basic deprotection conditions (NaOMe, Mg(OMe)2,
NH3/MeOH). Enzymatic hydrolysis using a range of lipases21
was ineffective with 14 remaining largely unreacted. Depro-
tection with n-BuNH2 in methanol provided the most suitable
reaction profile. Final purification was achieved by reversed phase
chromatography in 25% yield to give the target nucleoside 1.
3′-Deoxy-3′-C-hydroxymethyl-2′,3′-methylene-uridine 1 was
evaluated in a whole cell-based HCV replicon assay and was not
found to possess inhibitory activity (EC50 >100 μM) or
cytotoxicity (CC50 >100 μM).
In conclusion, this report is the first example of a stereo-
selective Simmons−Smith style cyclopropanation performed at
the 2,3-positions of a carbohydrate furanose ring system. This
key transformation facilitated the 16-step synthesis of the novel
nucleoside 3′-deoxy-3′-C-hydroxymethyl-2′,3′-methylene-uri-
dine which was evaluated for anti-HCV activity in vitro. The
synthesis and antiviral evaluation of further structurally diverse
2′,3′-cyclopropane nucleosides is underway and will be reported
in due course.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: zofia.komsta@dextrauk.com.
*E-mail: mayes.ben@idenix.com.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Mrs L. Morris is gratefully acknowledged for performing sample
analysis. In vitro biology assays were performed at Idenix by C.
Chapron, M. La Colla, M. Seifer, and I. Serra.
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