Asymmetric synthesis and biological activity of l-bicyclocarba-d4T

Article abstract of DOI:10.1080/15257770701615599

Novel l-bicyclocarba-d4T (1), an enantiomer of d-N-MCd4T has been enantiopurely synthesized as a potent anti-HIV agent starting from (R)-epichlorohydrin using tandem alkylation, chemoselective reduction of ester in the presence of lactone functional group

Full text of DOI:10.1080/15257770701615599

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Asymmetric Synthesis and Biological
Activity of l-Bicyclocarba-d4T
Hyung Ryong Moon a , Ah-Young Park a , Kyung Ran Kim a , Moon Woo
Chun b & Lak Shin Jeong c
a College of Pharmacy and Research Institute for Drug Development ,
Pusan National University , Busan, Korea
b College of Pharmacy , Seoul National University , Seoul, Korea
c College of Pharmacy , Ewha Womans University , Seoul, Korea
Published online: 05 Dec 2007.
To cite this article: Hyung Ryong Moon , Ah-Young Park , Kyung Ran Kim , Moon Woo Chun & Lak
Shin Jeong (2007) Asymmetric Synthesis and Biological Activity of l-Bicyclocarba-d4T, Nucleosides,
Nucleotides and Nucleic Acids, 26:10-12, 1653-1657, DOI: 10.1080/15257770701615599
To link to this article: http://dx.doi.org/10.1080/15257770701615599
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Nucleosides, Nucleotides, and Nucleic Acids, 26:1653–1657, 2007
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1525-7770 print / 1532-2335 online
DOI: 10.1080/15257770701615599
ASYMMETRIC SYNTHESIS AND BIOLOGICAL ACTIVITY
OF L-BICYCLOCARBA-d4T
Hyung Ryong Moon, Ah-Young Park, and Kyung Ran Kim
College of
2
Pharmacy and Research Institute for Drug Development, Pusan National University,
Busan, Korea
Moon Woo Chun
Lak Shin Jeong
College of Pharmacy, Seoul National University, Seoul, Korea
College of Pharmacy, Ewha Womans University, Seoul, Korea
2
2
Novel L-bicyclocarba-d4T (1), an enantiomer of D-N -MCd4T has been enantiopurely syn-
2
thesized as a potent anti-HIV agent starting from (R)-epichlorohydrin using tandem alkylation,
chemoselective reduction of ester in the presence of lactone functional group, Grignard reaction,
RCM reaction, and Mitsunobu reaction as key steps. L-N-MCd4T (1) was found to be very potent
anti-HIV-1 (EC50 = 6.76 µg/mL) agent with no cytotoxicity.
Keywords Anti-HIV agent; novel L-bicyclocarba-d4T; tandem alkylation; (R)-
epichlorohydrin
INTRODUCTION
HIV (human immunodeficiency virus) is a pathogenic retrovirus and
the causative agent of AIDS.[1] Since AZT has been known as a potent
anti-HIV agent, initial attempts were focused on the development of 2ꢁ,3ꢁ-
dideoxynucleosides (ddNs).[2] Among them, ddI, ddC, and d4T[3] have
been clinically used for the treatment of AIDS patients. Particulary, d4T
has a double bond at its pseudosugar ring, which renders the pseudosugar
ring to be nearly planar. 3TC[4] also has been used as anti-HIV and anti-
HBV agents and might be considered as a member of ddNs family be-
cause it does not have hydroxyl substituents at both 2ꢁ- and 3ꢁ-position.
Therefore, ddN analogs still seem to be the most promising candidate
for the AIDS drug, although most of them undergo an easy glycosidic
This work was supported by the Korea Research Foundation Grant funded by the Korean
Government (KRF-2005-202-E00211).
Address correspondence to Hyung Ryong Moon, College of Pharmacy and Research Institute for
Drug Development, Pusan National University, San 30 Jangjeon-dong, Geumjeong-gu, Busan 609-735,
Korea. E-mail: mhr108@pusan.ac.kr
1653

1654
H. R. Moon et al.
FIGURE 1 The rationale for the design of the desired nucleoside, L-N-MCd4T (1).
bond cleavage, comparing to normal nucleosides under acidic conditions.
Carbocyclic nucleosides[5] have been synthesized mainly with the purpose
of overcoming cleavage of the glycosidic bond.[5,6] On the other hand,
conformationally rigid methanocarba (MC) nucleosides built on a bicy-
clo[3.1.0]hexane template have a south (S, 3E)[7] or north (N, 2E)[8,9] con-
formation as in normal nucleosides. Marquez and co-workers have synthe-
sized these rigid MC nucleosides to study the conformational preferences
of various enzymes involved in nucleoside and nucleotide metabolism. D-N-
MCd4T,[10] bearing a locked North conformation and a 2ꢁ,3ꢁ-double bond
in a single structure has been recently synthesized (Figure 1). Although D-N-
MCd4T was somewhat less potent than d4T, it still showed significant antivi-
ral activity against HIV-1 and -2 with a less cytotoxicity and was more stable
under acidic conditions. On the other hand, a number of L-nucleosides such
as 3TC,[4] FTC, L-d4FC, and L-5-FddC have been also reported to show highly
potent anti-HIV activity.
On the basis of these findings, it was interesting to synthesize L-N-MCd4T
(1, L-bicyclocarba-d4T), an enantiomer of D-N-MCd4T as a potent anti-HIV
agent. Here, we report a new synthetic methodology of L-N-MCd4T (1) using
cyclopropanation via tandem alkylation and ring-closing metathesis (RCM)
as key steps from (R)-epichlorohydrin and its anti-HIV activity.
CHEMISTRY
Synthesis of a glycosyl donor, bicyclo[3.1.0]hexenol template 10 for the
synthesis of L-N-MCd4T (1) is described in Scheme 1. Compound 2 was syn-
thesized starting from (R)-epichlorohydrin according to the procedure re-
ported by Tsuji and his coworkers.[11,12] The reaction was accomplished by
tandem alkylation followed by lactonization in 62% yield. The lactone moi-
ety of 2 was more susceptible to hydrolysis than the ester, probably due to
ring strain derived from fused cyclopropane ring. Therefore, it is possible
to chemoselectively reduce the ester in the presence of the lactone group.
Treatment of 2 with 1 equiv. of NaOH in ethanol afforded monocarboxy-
late sodium salt 2a and the ester was reduced by NaBH4 under reflux and
recyclized back to lactone 3 under acidic conditions. Protection of hydroxyl
group of 3 with tert-butyldiphenylsilyl chloride produced silyl ether 4, which

L-Bicyclocarba-d4T
1655
SCHEME 1 Reagents and conditions: (a) (EtO2C)2CH2, Na, EtOH, 80◦C, 20 h; (b) i) 1 eq. NaOH,
EtOH, rt, 16 h; ii) NaBH4, reflux, 3 h, then 2 M HCl, rt, 18 h; (c) TBDPSCl, imidazole, CH2Cl2,
rt, overnight; (d) Dibal-H, CH2Cl2, −78◦C, 30 min; (e) CH3PPh3Br, t- BuOK, THF, rt, 3 h; (f) oxalyl
chloride, DMSO, CH2Cl2, −78◦C, 1 h, then Et3N, rt, 1 h; (g) vinylmagnesium bromide, THF, −78◦C,
1 h; (h) 2nd generation Grubbs catalyst, CH2Cl2, rt, 1.5 h.
was reduced with Dibal-H to give the corresponding lactol 5. Wittig reaction
of lactol 5 with methylidenephosphorane afforded hydroxy olefin 6 in 88%
yield after quenched with aqueous NH4C1 solution. Swern oxidation of 6
with oxalyl chloride and DMSO at −78◦C smoothly gave aldehyde 7, which
was subjected to a Grignard reaction with vinylmagnesium bromide to af-
ford allylic alcohols 8 (48%) and 9 (39%) as an easily separable diastereoiso-
meric mixture. The stereochemistry of 8 and 9 could be determined after a
RCM reaction. RCM reaction[13,14] of diene 9 and 8 with second generation
Grubbs catalyst produced the bicyclo[3.1.0]hex-3-en-2-ol template 10 and its
(2R)-diastereomer, respectively. The stereochemistry of 10 and its diastere-
1
omer was confirmed by comparing their H NMR spectra with that of the
authentic enantio-counterpart, an enantiomer of 10, prepared from chiral
bicyclo[3.1.0]hexane template,[15] indicating compound 10 has the desired
(2S)-stereochemistry.
Synthesis of L-N-MCd4T (1) from glycosyl donor, bicyclo[3.1.0]hexenol
10 via a coupling reaction with N 3-benzoylthymine is described in Scheme 2.
Condensation of 10 with N 3-benzoylthymine under Mitsunobu conditions
afforded N 1-alkylated product 11 in 52% yield. L-N-MCd4T (1) was obtained
after removal of benzoyl and TBDPS groups by successively treating with am-
monium hydroxide in methanol and tetrabutylammonium fluoride (TBAF)
in THF. L-N-MCd4T (1) exactly matched with all spectral properties of the
authentic D-N-MCd4T except the sign of optical rotation value.[10]
SCHEME 2 Reagents and conditions: (a) N 3-benzoylthymine, DEAD, PPh3, THF, 0◦C, 6 h; (b) 28%
NH4OH/MeOH (1/10), rt, 7 h; (c) TBAF, THF, rt, 1 h.

1656
BIOLOGICAL RESULTS
H. R. Moon et al.
Anti-HIV type 1 and 2 activities of L-N-MCd4T (1) were measured in
cell culture assays with HIV-1IIIb and HIV-2ROD infected MT-4 cells. MTT
assay was performed for its IC50 and EC50. In MT4 (HTLV-1-infected hu-
man T lymphocyte) cells, L-N-MCd4T (1) showed potent anti-HIV-1 activity
(EC50 = 6.76 µg/mL), which was about 1.3-fold less potent than ddI being
clinically used for the treatment of AIDS patient, without cytotoxicity up to
100 µg/mL. However, L-N-MCd4T (1) was inactive against HIV-2 whereas
ddI showed moderate activity (EC50 = 16.00 µg/mL).
CONCLUSION
We have accomplished the enantiopure synthesis of novel L-N-MCd4T
(1) as a potent anti-HIV agent employing a novel synthetic strategy start-
ing from (R)-epichlorohydrin. For the synthesis of L-N-MCd4T (1), tandem
alkylation, lactonization, chemoselective reduction of ester in the presence
of lactone functional group, Grignard reaction and RCM reaction were used
as the key reactions. L-N-MCd4T (1) was found to exhibit very potent anti-
HIV-1 activity in MT-4 cells with no cytotoxicity. This observation indicates
that L-type bicyclo[3.1.0]hexene 10 might be regarded as one of novel tem-
plates for the development of anti-HIV agents.
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