Synthesis of novel l-N-MCd4T as a potent anti-HIV agent

Article abstract of DOI:10.1039/b612537a

l-N-MCd4T (1) has been 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, RCM reaction and Mitsunobu reaction as key steps and wa

Full text of DOI:10.1039/b612537a

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Synthesis of novel L-N-MCd4T as a potent anti-HIV agent†
Ah-Young Park,a Hyung Ryong Moon,*a Kyung Ran Kim,a Moon Woo Chunb and Lak Shin Jeong*c
Received 30th August 2006, Accepted 21st September 2006
First published as an Advance Article on the web 11th October 2006
DOI: 10.1039/b612537a
L-N-MCd4T (1) has been 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, RCM reaction and Mitsunobu
reaction as key steps and was found to be a very potent anti-
HIV-1 (EC50 = 6.76 lg mL−1) agent without cytotoxicity up
to 100 lg mL−1, indicating that the anti-HIV-1 activity found
is similar to that of ddI (EC50 = 4.95 lg mL−1), which is used
clinically for the treatment of AIDS patients.
Human immunodeficiency virus (HIV) is a pathogenic retrovirus
and the causative agent of AIDS.1 In the search for an effective
chemotherapeutic agent of HIV infections, initial attempts were
focused on the development of 2ꢀ,3ꢀ-dideoxynucleosides (ddNs).2
Among them, AZT, ddI, ddC and d4T3 have been clinically used
for the treatment of AIDS patients. Peculiarly, d4T has a double
bond at its pseudosugar ring, which renders the pseudosugar
ring to be nearly planar and highly rigid. 3TC4 has been also
used as anti-HIV and anti-HBV agents. Because it does not have
hydroxyl substituents at both the 2ꢀ- and 3ꢀ-position, 3TC might be
considered as a member of ddNs family. Therefore, ddN analogs
still seem to be the most promising candidates for AIDS treatment,
although most of them have the property of easy cleavage of their
glycosidic bond, compared with normal nucleosides under acidic
conditions.
Carbocyclic nucleosides5 have been synthesized mainly with
the purpose of overcoming cleavage of the glycosidic bond,
which readily occurs in conventional nucleosides by chemical or
enzymatic means.5a,6 On the other hand, conformationally rigid
methanocarba (MC) nucleosides built on a bicyclo[3.1.0]hexane
template have a south (S, 3E)7 or north (N, 2E)8 conformation as in
normal nucleosides. These rigid MC nucleosides have been synthe-
sized to study the conformational preferences of various enzymes
involved in nucleoside and nucleotide metabolism. Recently, D-N-
MCd4T,9 bearing a locked North conformation and a 2ꢀ,3ꢀ-double
bond in a single structure has been synthesized and its anti-HIV
activity was compared with that of d4T (Fig. 1). Although D-N-
MCd4T was somewhat less potent than d4T, it showed significant
antiviral activity against HIV-1 and HIV-2 with less cytotoxicity
and was stable under conditions that would cleave d4T. On the
Fig. 1 The rationale for the design of the desired nucleoside, L-N-MCd4T
(1).
other hand, a number of L-nucleosides such as 3TC,4 FTC,10 L-
d4FC11 and L-5-FddC12 have been also reported to show highly
potent anti-HIV activity.
As a part of our ongoing efforts to search for novel antiviral
agents with better potency and less cytotoxicity, it was of interest
to synthesize L-N-MCd4T (1), an enantiomer of D-N-MCd4T as a
potent anti-HIV agent on the basis of these findings. Here, we wish
to report a new synthetic approach to conformationally locked
L-N-MCd4T (1) using cyclopropanation via tandem alkylation,
ring-closing metathesis (RCM) and Mitsunobu reaction as the
key steps from (R)-epichlorohydrin and its anti-HIV activity.
A new strategy for the synthesis of L-N-MCd4T (1) was
highly required because the method used for the synthesis of D-
N-MCd4T,9 the enantiomer of L-N-MCd4T (1) was somewhat
lengthy.
First, synthesis of a glycosyl donor, bicyclo[3.1.0]hexenol tem-
plate (2S)-10 for the synthesis of L-N-MCd4T (1) is described in
Scheme 1. Compound 2 was synthesized in two steps according to
the procedure reported by Tsuji and his co-workers.13,14 Reaction
of (R)-epichlorohydrin with diethyl malonate and sodium metal
gave cyclopropane-fused lactone 2 via tandem alkylation followed
by lactonization. The lactone moiety of 2 was more susceptible to
hydrolysis than the ester, maybe due to the structural constraint
caused by a fused cyclopropane ring. Treatment of 2 with 1 equiv.
of NaOH in ethanol afforded monocarboxylate sodium salt 2a,
the ester of which was chemoselectively reduced by NaBH4 under
reflux and recyclized back to lactone 3 under acidic conditions.
Protection of hydroxyl group of 3 with tert-butyldiphenylsilyl
aLaboratory of Medicinal Chemistry, College of Pharmacy and Research
Institute for Drug Development, Pusan National University, Busan, 609-
735, Korea. E-mail: mhr108@pusan.ac.kr; Fax: +82 51 513 6754; Tel: +82
51 510 2815
bCollege of Pharmacy, Seoul National University, Seoul, 151-742, Korea
cLaboratory of Medicinal Chemistry, College of Pharmacy, Ewha Womans
University, Seoul, 120-750, Korea
† Electronic supplementary information (ESI) available: 1H and 13C NMR
spectra for 8, 9, (2S)-10 and (2R)-10. See DOI: 10.1039/b612537a
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from a chiral bicyclo[3.1.0]hexane template,16 indicating that the
compound derived from 9 has the desired (2S)-stereochemistry.
(2S)-10 was ready for the coupling of nucleobases to yield various
nucleosides.
Synthesis of L-N-MCd4T (1) from bicyclo[3.1.0]hexenol (2S)-
10 via a coupling reaction with N3-benzoylthymine is described
in Scheme 2. Condensation of (2S)-10 with N3-benzoylthymine
under Mitsunobu conditions afforded N-alkylated product 11
in 52% yield. The desired final product, L-N-MCd4T (1) was
obtained after removal of benzoyl and TBDPS groups by suc-
cessively treating with ammonium hydroxide in methanol and
tetrabutylammonium fluoride (TBAF) in THF.
Scheme 2 Reagents and conditions: (a) N3-benzoylthymine, DEAD, PPh3,
THF, 0 ◦C, 6 h; (b) 28% NH4OH–MeOH (1 : 10), rt, 7 h; (c) TBAF, THF,
rt, 1 h.
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, 85% for (2S)-10, 84% for (2R)-10.
L-N-MCd4T (1) exactly matched with all spectral properties
of the enantio-counterpart, D-N-MCd4T prepared from chiral
bicyclo[3.1.0]hexane template except the sign of optical rotation
value.9
Antiviral activity of the synthesized L-N-MCd4T (1) was
measured against a HIV (human immunodeficiency virus) type
1 and 2. In MT4 (HTLV-1-infected human T lymphocyte) cells,
chloride produced silyl ether 4, which was reduced with Dibal-H
at −78 ◦C to give the corresponding lactol 5 in 97% yield. Wittig
reaction of lactol 5 with methyltriphenylphosphonium bromide in
the presence of potassium tert-butoxide afforded hydroxy olefin
6 in 88% yield after quenching with aqueous NH4Cl solution.
However, when after the Wittig reaction the reaction mixture
was extracted with EtOAc and water without adding aqueous
ammonium chloride solution, a new by-product appeared at a
higher Rf value than that of the desired compound 6 in a 10–
L-N-MCd4T (1) exhibited significant anti-HIV-1 activity (EC50
=
6.76 lg mL−1), which was about 1.3-fold less potent than ddI
being clinically used, without cytotoxicity up to 100 lg mL−1.
Interestingly, L-N-MCd4T (1) was inactive against HIV-2 whereas
ddI showed moderate activity (EC50 = 16.00 lg mL−1).
In conclusion, we have accomplished the synthesis of novel L-
N-MCd4T (1), an enantiomer of D-N-MCd4T as a potent anti-
HIV agent employing a novel synthetic strategy starting from (R)-
epichlorohydrin. Tandem alkylation, lactonization, chemoselec-
tive reduction of ester in the presence of lactone functional group,
Grignard reaction, RCM reaction, and Mitsunobu reaction were
the key reactions to achieve the synthesis of L-N-MCd4T (1). L-N-
MCd4T (1) was found to show potent anti-HIV-1 activity similar
to that of ddI, which is a clinically useful anti-AIDS drug, in MT-
4 cells without appreciable cytotoxicity up to 100 lg mL−1. This
observation shows that L-type bicyclo[3.1.0]hexene (2S)-10 might
be regarded as a new template for the development of anti-HIV
agents.
1
20% yield. It turned out to be acetylated compound 6a by H,
13C and COSY NMR experiments. 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 afford allylic alcohols 8‡ (48%) and 9§ (39%), easily
separated by normal silica gel column chromatography. The
stereochemistry of 8 and 9 couldn’t be determined until the
bicyclo[3.1.0]hexenol template was formed via the RCM reaction.
RCM reaction15 of diene 9 and 8 with a second generation Grubbs
catalyst produced the bicyclo[3.1.0]hex-3-en-2-ol templates (2S)-
10¶ and (2R)-10ꢁ, respectively. The stereochemistry of these was
confirmed by comparing their 1H NMR spectra with that of the au-
thentic enantio-counterpart, an enantiomer of (2S)-10, prepared
This work was supported for two years by Pusan National
University Research Grant (to H. R. Moon).
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Notes and references
‡ (−)-(1R)-1-[(1R,2S)-2-(tert-Butyl-diphenyl-silanyloxymethyl)-2-vinyl-
cyclopropyl]prop-2-en-1-ol (8). [a]2D5 −22.8 (c 1.06, in CHCl3) (Found: C,
76.51; H, 8.39. Calc. for C25H32O2Si: C, 76.48; H, 8.22%); dH (200 MHz;
=
CDCl3) 7.69–7.35 (10 H, m, 2 × Ph), 6.06–5.87 (2 H, m, 2 × -CH CH2),
=
5.37–5.09 (4 H, m, 2 × -CH CH2), 3.75 (1 H, d, J 10.4, TBDPSCHa),
3.75–3.66 (1 H, m, CHOH), 3.50 (1 H, d, J 10.4, TBDPSCHb), 1.25–1.13
(1 H, m, methine H), 1.07 (9 H, s, t-Bu), 0.89 (1 H, s, methylene Ha), 0.85
(1 H, s, methylene Hb); dC (50 MHz; CDCl3) 140.5, 137.3, 136.0, 134.0,
130.0, 130.0, 128.0, 116.3, 114.5, 73.5, 68.9, 30.1, 29.2, 27.2, 19.7, 13.6;
m/z (ESI) 415 (83, M + Na)+.
§ (−)-(1S)-1-[(1R,2S)-2-(tert-Butyl-diphenyl-silanyloxymethyl)-2-vinyl-
cyclopropyl]prop-2-en-1-ol (9). [a]2D5 −55.9 (c 1.84, in CHCl3) (Found: C,
76.20; H, 8.58. Calc. for C25H32O2Si: C, 76.48; H, 8.22%); dH (200 MHz;
=
CDCl3) 7.71–7.36 (10 H, m, 2 × Ph), 6.13–5.90 (2 H, m, 2 × -CH CH2),
=
5.33–5.09 (4 H, m, 2 × -CH CH2), 3.76 (1 H, d, J 10.4, TBDPSCHa),
3.67–3.65 (1 H, m, CHOH), 3.58 (1 H, d, J 10.4, TBDPSCHb), 1.23–1.11
(1 H, m, methine H), 1.09 (9 H, s, t-Bu), 0.86 (1 H, dd, J 5.2, 8.8, methylene
Ha), 0.72 (1 H, t, J 5.2, methylene Hb); dC (50 MHz; CDCl3) 140.1, 137.3,
136.1, 136.1, 134.0, 134.0, 130.1, 130.0, 128.0, 128.0, 116.6, 114.7, 73.5,
69.0, 30.4, 30.2, 27.3, 19.7, 14.2; m/z (ESI) 415 (60, M + Na)+.
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131.0, 129.8, 127.8, 78.0, 64.9, 38.8, 27.0, 22.9, 20.3, 19.4; m/z (EI) 346
(7, M − H2O)+.
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C, 76.03; H, 7.80. Calc. for C23H28O2Si: C, 75.78; H, 7.74%); dH (500 MHz;
CDCl3) 7.70–7.39 (10 H, m, 2 × Ph), 6.21 (1 H, d, J 5.5, vinylic H), 5.58–
5.57 (1 H, br d, J 5.5, vinylic H), 4.39 (1 H, d, J 1.5, 2-H), 3.97 (1 H, d, J
11.0, TBDPSOCHa), 3.74 (1 H, d, J 11.5, TBDPSOCHb), 1.64 (1 H, dd,
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130.1, 129.8, 127.8, 127.8, 77.6, 65.1, 37.2, 30.1, 27.0, 25.5, 19.5; m/z (EI)
346 (12, M − H2O)+.
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This journal is
The Royal Society of Chemistry 2006
Org. Biomol. Chem., 2006, 4, 4065–4067 | 4067
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