Enantioselective total synthesis of the potent Anti-HIV nucleoside EFdA

Article abstract of DOI:10.1021/ol202116k

A concise enantioselective total synthesis of 4′-ethynyl-2-fluoro- 2′-deoxyadenosine (EFdA), an extremely potent anti-HIV agent, has been accomplished from (R)-glyceraldehyde acetonide in 18% overall yield by a 12-step sequence involving a highly diastere

Full text of DOI:10.1021/ol202116k

ORGANIC
LETTERS
2011
Vol. 13, No. 19
5264–5266
Enantioselective Total Synthesis of the
Potent Anti-HIV Nucleoside EFdA
Masayuki Kageyama,† Tomohiro Nagasawa,† Mayumi Yoshida,† Hiroshi Ohrui,‡ and
Shigefumi Kuwahara*,†
Laboratory of Applied Bioorganic Chemistry, Graduate School of Agricultural Science,
Tohoku University, Tsutsumidori-Amamiyamachi, Aoba-ku, Sendai 981-8555,
Japan, and Yokohama College of Pharmacy, 601 Matano-cho, Totsuka-ku,
Yokohama 245-0066, Japan
skuwahar@biochem.tohoku.ac.jp
Received August 4, 2011
ABSTRACT
A concise enantioselective total synthesis of 40-ethynyl-2-fluoro-20-deoxyadenosine (EFdA), an extremely potent anti-HIV agent, has been
accomplished from (R)-glyceraldehyde acetonide in 18% overall yield by a 12-step sequence involving a highly diastereoselective ethynylation of
an R-alkoxy ketone intermediate.
EFdA [40-ethynyl-2-fluoro-20-deoxyadenosine (1), also
abbreviated as 40Ed2FA] is a nucleoside analog designed
by Ohrui and co-workers as a nucleoside reverse tran-
scriptase inhibitor (NRTI) for the treatment of human
immunodeficiency virus (HIV) infection (Figure 1).1 The
20-deoxyadenosine analog 1, modified at the 2- and 40-
positions of the parent natural nucleoside, forms a stark
structural contrast to eight existing NRTIs clinically ap-
proved for AIDS treatment in its retention of the 30-
hydroxyl group; any of the currently prescribed drugs
such as zidovudine (AZT) and stavudine lack the 30-
hydroxyl function requisite for DNA chain elongation
by viral reverse transcriptases and thereby serve as
chain elongation terminators.2 The modifications at the
two positions (2 and 40) of 20-deoxyadenosine while retain-
ing its 30-hydroxyl endowed 1 with promising properties as
an anti-HIV drug as follows: (1) exceptionally potent
inhibitory activity against HIV-1 replication [e.g., EC50
(HIV-1NL4ꢀ3), 50 pM in phytohemagglutinin-activated
peripheralbloodmononuclearcells, which isseveral orders
of magnitude better than those of any currently prescribed
NRTIs such as AZT (22 nM) and tenofovir (3300 nM)];3
(2) excellent in vitro selectivity indices (SI = CC50/EC50)
(e.g., 200,000 for HIV-1NL4ꢀ3, 134,000 for HIV-1IIIB);3ꢀ5
(3) no acute toxicity in ICR mice at a dose of 100 mg/kg;1a,6
(4) retention of efficacy even against a wide spectrum of
† Tohoku University.
‡ Yokohama College of Pharmacy.
(3) Michailidis, E.; Marchand, B.; Kodama, E. N.; Singh, K.;
Matsuoka, M.; Kirby, K. A.; Ryan, E. M.; Sawani, A. M.; Nagy, E.;
Ashida, N.; Mitsuya, H.; Parniak, M. A.; Sarafianos, S. G. J. Biol.
Chem. 2009, 284, 35681–35691.
(4) Kawamoto, A.; Kodama, E.; Sarafianos, S. G.; Sakagami, Y.;
Kohgo, S.; Kitano, K.; Ashida, N.; Iwai, Y.; Hayakawa, H.; Nakata, H.;
Mitsuya, H.; Arnold, E.; Matsuoka, M. Int. J. Biochem. Cell Biol. 2008,
40, 2410–2420.
(5) Nakata, H.; Amano, M.; Koh, Y.; Kodama, E.; Yang, G.; Bailey,
C. M.; Kohgo, S.; Hayakawa, H.; Matsuoka, M.; Anderson, K. S.;
Cheng, Y.-C.; Mitsuya, H. Antimicrob. Agents Chemother. 2007, 51,
2701–2708.
(1) (a) Kohgo, S.; Ohrui, H.; Kodama, E.; Matsuoka, M.; Mitsuya, H.
40-C-Substituted-2-halo-adenosine derivative. Can. Pat. CA 2502109,
2005. (b) Kohgo, S.; Yamada, K.; Kitano, K.; Iwai, Y.; Sakata, S.; Ashida,
N.; Hayakawa, H.; Nameki, D.; Kodama, E.; Matsuoka, M.; Mitsuya, H.;
Ohrui, H. Nucleosides Nucleotides Nucleic Acids 2004, 23, 671–690.
(c) Ohrui, H. Chem. Rec. 2006, 6, 133–143. (d) Ohrui, H.; Hayakawa,
H.; Kohgo, S.; Matsuoka, M.; Kodama, E.; Mitsuya, H. J. Synth. Org.
Chem. Jpn. 2006, 64, 716–723. (e) Ohrui, H.; Kohgo, S.; Hayakawa, H.;
Kodama, E.; Matsuoka, M.; Nakata, T.; Mitsuya, H. Nucleosides
Nucleotides Nucleic Acids 2007, 26, 1543–1546. (f) Ohrui, H. Proc.
Jpn. Acad., Ser. B 2011, 87, 53–65.
(2) Nikolenko, G. N.; Kotelkin, A. T.; Oreshkova, S. F.; Ilyichev,
A. A. Mol. Biol. 2011, 45, 93–109. (b) Sharma, B. Neurobehav. HIV Med.
2011, 3, 27–40.
(6) Hattori, S.; Ide, K.; Nakata, H.; Harada, H.; Suzu, S.; Ashida, N.;
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r
10.1021/ol202116k
Published on Web 09/02/2011
2011 American Chemical Society

drug-resistant HIV-1 variants including multidrug-resis-
tant clinical isolates;4ꢀ6 and (5) a longer intracellular
half-life value (t1/2, 17.2 h) of its active form (EFdA-50-
triphosphate) than that of AZT-50-triphosphate (2.8 h),
which may enable a once-daily or twice-daily regimen.5
From these favorable pharmacological profiles, EFdA (1)
is increasingly coming under the spotlight as a potential
therapeutic agent for AIDS.
by diastereoselective addition of an acetylide anion to
alkoxy ketone 5, which in turn could be readily prepared
from (R)-glyceraldehyde acetonide 6 via diastereoselective
allylation and subsequentfunctionalgroup manipulations.
Scheme 2. Synthesis of EEdA (1)
Figure 1. Structure of the anti-HIV nucleoside EFdA (1).
Despite the remarkable physiological properties of 1
reported one after another, there has been only one way
to supply 1, the synthesis by Kohgo and co-workers which
required a considerably lengthy 18-step sequence from the
expensive starting material 2-amino-20-deoxyadenosine,
resulting in a modest overall yield of 2.5%.1a,b The limited
availability of 1 as well as its striking antiviral potency and
low toxicity in mice prompted our synthetic efforts to
supply a sufficient amount of 1 to further promote its
biological and clinical research. We describe herein a new
efficient 12-step synthesis of 1 in 18% overall yield from
(R)-glyceraldehyde acetonide.
Scheme 1. Retrosynthetic Analysis of EFdA (1)
Taking into account the diastereoselective ethynylation
step in the retrosynthetic plan (5 f 4, Scheme 1), we chose
MOM-protected acetal 7 as the starting point,7 a known
compound readily obtainable from 6 in 83% overall yield
through two steps consisting of diastereoselective allyla-
tion of 6 with allylzinc bromide and MOM protection
of the resulting secondary alcohol (Scheme 2).8 Deprotec-
tion of the acetonide group of 7 under acidic conditions
followed by selective TBDPS protection of the primary
hydroxyl group of the resulting diol 8 afforded 9.
Our retrosynthetic analysis of 1 is shown Scheme 1. We
planned toobtain the targetmolecule1viaN-glycosylation
of activated cyclic hemiacetal 2 with 2-fluoroadenine 3.
The glycosyl donor 2 would be prepared through oxidative
cleavage of the terminal double bond of 4 followed by
spontaneous hemiacetal ring formation. The stereochem-
istry at the 40-position of 4 was considered to be installable
(7) For examples of diastereoselective addition of acetylide anions to
MOM-protected R-hydroxy ketones, see: (a) Maddaford, A.; Wainwright,
P.; Glen, R.; Fisher, R.; Dragovich, P. S.; Gonzalez, J.; Kung, P.-P.;
Middleton, D. S.; Pryde, D. C.; Stephenson, P. S.; Sutton, S. C. Synthesis
2007, 1378–1384. (b) Druais, V.; Hall, M. J.; Corsi, C.; Wendeborn, S. V.;
Meyer, C.; Cossy, J. Tetrahedron 2010, 66, 6358–6375.
Org. Lett., Vol. 13, No. 19, 2011
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Oxidation of the alcohol 9 with the DessꢀMartin period-
inane gave ketone 10, which was then subjected to the
addition of the acetylide anion prepared by treating
(trimethylsilyl)acetylene with EtMgBr. Expectedly, this
reaction proceeded highly diastereoselectively, providing
12 as a single stereoisomer in 91% overall yield from 9 after
removal of the TMS protecting group of the resulting
adduct 11 followed by chromatographic purification. De-
protection of the MOM group of 12 was effected efficiently
with ZnBr2 and 1-dodecanethiol in CH2Cl2 to give diol 13
in nearly quantitative yield,9 the enantiomeric excess of
which was determined to be >95% by 1H NMR analyses
of the corresponding (R)- and (S)-30-mono-MTPA esters.
Ozonolysis of the terminal double bond of 13accompanied
by spontaneous hemiacetal ring formation afforded 14 as
an anomeric mixture (R/β = 2.8:1 as judged by 1H NMR
analyses including NOE experiments).10 After acetylation
of the anomeric and 30-hydroxyl groups to form 15 (R/β =
1:1.4),10 the resulting anomeric mixture was subjected to
the silyl-HilbertꢀJohnson reaction using 2-fluoroadenine
(3) as a nucleophilic base to afford 16 in an isolated yield of
46% along with the corresponding R-anomer (25%).11
Finally, desilylation of 16 with NH4F and subsequent
one-pot hydrolysis of the acetate function gave the target
molecule EFdA (1) as a white microcrystalline solid,12 the
1H NMR of which was identical with that of an authentic
sample of EFdA.
In conclusion, the enantioselective total synthesis of
EFdA (1) was accomplished in 18% overall yield from the
commercially available protected aldehyde 6 via 12 steps (or
22% overall yield from the known compound 7 through 10
steps) using the chelation-controlled diastereoseletive ethy-
nylation of R-alkoxy ketone 10 as the key transformation.
Attempts to improve the stereoselectivity of the N-glycosy-
lation step as well as to convert the undesired R-anomer of
16 into the corresponding β-anomer are now underway.
(8) Reddy, B. V. S.; Reddy, B. P.; Pandurangam, T.; Yadav, J. S.
Tetrahedron Lett. 2011, 52, 2306–2308. (b) Nagaiah, K.; Sreenu, D.;
Purnima, K. V.; Rao, R. S.; Yadav, J. S. Synthesis 2009, 1386–1392. (c)
Einhorn, C.; Luche, J.-L. J. Organomet. Chem. 1987, 322, 177–183.
(9) For the use of ZnBr2/n-BuSH and ZnBr2/n-PrSH to deprotect
MOM ethers, see: (a) Sohn, J.-H.; Waizumi, N.; Zhong, H. M.; Rawal,
V. H. J. Am. Chem. Soc. 2005, 127, 7290–7291. (b) Han, J. H.; Kwon,
Y. E.; Sohn, J.-H.; Ryu, D. H. Tetrahedron 2010, 66, 1673–1677.
(10) For diagnostic NOE correlations in the anomers of 14 and 15,
see the Supporting Information.
Acknowledgment. This work was financially supported,
in part, by a Grant-in-Aid for Scientific Research (B) from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan (No. 17380070).
Supporting Information Available. Experimental pro-
cedures, characterization data, and NMR spectra for new
compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
(11) (a) Wright, G E.; Dudycz, L. W. J. Med. Chem. 1984, 27, 175–
€
181. (b) Vorbruggen, H.; Ruh-Pohlenz, C. Org. React. 2000, 55, 1–630.
1H NMR and TLC analyses of the crude reaction product suggested the
formation of small amounts of by-products derived from deprotection
the 50-O-TBDPS group of 15, while no N7-glycosyl isomer of 16 was
detected in this reaction.
(12) (a) Zhang, W.; Robins, M. J. Tetrahedron Lett. 1992, 33, 1177–
1180. (b) Kaburagi, Y.; Kishi, Y. Org. Lett. 2007, 9, 723–826.
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