J. Am. Chem. Soc. 1997, 119, 3637-3638
3637
Scheme 1a
Elimination of Chlorine (Radical) or Tosylate
(Anion) from C2′ of Nucleoside C3′ Free Radicals
as Model Reactions Postulated To Occur at the
Active Site of Ribonucleotide Reductases1
Morris J. Robins,* Zhiqiang Guo, and Stanislaw F. Wnuk
Department of Chemistry and Biochemistry
Brigham Young UniVersity
ProVo, Utah 84602-5700
ReceiVed January 21, 1997
Ribonucleotide reductases (RNRs) catalyze the conversion
of ribonucleoside 5′-di- or -triphosphates to 2′-deoxynucleotides
that are required for DNA biosynthesis.2 The ribonucleoside
diphosphate reductase (RDPR) from Escherichia coli (EC
1.17.4.1) has two nonidentical subunits (R1 and R2) whose
structures have been determined by X-ray crystallography.3 The
R1 subunit contains allosteric control sites and five cysteine
residues that participate in catalytic turnover and/or as redox
dithiol/disulfide pairs. The R2 subunit contains a diiron chelate
and a tyrosine-centered free radical that is responsible for
generation of a proximate thiyl radical4 on R1 via coupled
electron and proton transfer reactions. The thiyl radical has
been proposed to initiate nucleotide reduction by abstraction
of H3′ from the substrate ribonucleotide.2c Water (O2′) is then
lost from C2′ of the resulting C3′ radical.2a,5 Interaction of a
carboxylate group (glutamate) with OH3′ has recently been
invoked to assist with the heterolytic release of water.2c
Abstraction of H3′ from 2′-chloro-2′-deoxynucleoside 5′-
diphosphates to generate C3′ radicals was proposed2a,c to initiate
reactions leading to inactivation of RDPR.6 Spontaneous loss
of chloride and transfer of the OH3′ proton to glutamate would
give 2′-deoxy-3′-ketonucleotide intermediates without involve-
ment of a cysteine pair on R1.2a,c Successive â-eliminations
(H2′/base and H4′/pyrophosphate) would give the Michael
acceptor 2-methylene-3(2H)-furanone, which could effect co-
valent inactivation of the enzyme.7
We recently demonstrated a mechanistic alternative for
potential generation of the Michael acceptor that involved loss
of a radical, rather than an anionic, species from C2′ of model
2′-substituted nucleosides.8,9 Thus, treatment of 2′-(azido,
bromo, chloro, iodo, or methylthio)nucleoside 3′-thionocarbon-
ates with tributylstannane/AIBN resulted in loss of the 2′-
substituents, as presumed radicals, to give 2′,3′-didehydro-2′,3′-
dideoxy derivatives upon generation of C3′ radicals (without
O3′); whereas 3′-thionocarbonates with 2′-fluoro or 2′-O-(mesyl
or tosyl) substituents underwent radical-mediated hydrogen
transfer to C3′ to give the 3′-deoxy-2′-[fluoro or O-(mesyl or
a (a) (i) (Bu3Sn)2O/CHCl3/∆; (ii) Br2. (b) TBDMSCl/pyridine. (c)
(i) TsNHNH2/MeOH; (ii) NaBH4/MeOH/∆. (d) (i) CrO3/pyridine/Ac2O;
(ii) NaBH4/EtOH. (e) TBAF/THF. (f) BzCl/pyridine. (g) (i) TFA/H2O;
(ii) Ac2O/pyridine. (h) (i) Adenine/SnCl4/CH3CN; (ii) NH3/MeOH. (i)
(i) Bu2SnO/MeOH; (ii) TsCl/Et3N. (j) (i) HCl/MeOH; (ii) Me2CO/
Me2C(OMe)2/∆. (k) HNO3/Ac2O/-60 °C. (l) (i) Amberlite IR-120 (H+)/
MeOH; (ii) Ac2O/DMAP.
tosyl)] products.8 We also demonstrated that 6′-oxy radicals
(e.g., 20, produced10 from the 6′-O-nitro derivative 19) generated
OH3′-containing C3′ radicals that underwent chlorine loss and
â-elimination (H/base) to provide the first model simulation of
the initiation/elimination cascade that occurs during mechanism-
based inactivation of RNRs with 2′-substituted nucleotides.9 We
now describe synthesis of 6′-O-nitro-2′-O-tosylhomoadenosine
(13) and its treatment with Bu3SnD/AIBN. Generation10 of the
6′-oxy radical, relay abstraction of H3′ (by [1,5]-hydrogen shift
via a six-membered transition state11) to produce the C3′ radical,
loss of tosylate, and elimination (H/base) gave partially deu-
terated 2(R)-(2-hydroxyethyl)-3-(2H)-furanone (18).
Regioselective oxidation of 1,2-O-isopropylidene-R-D-glu-
cofuranose (1) gave the 5-ulose 212,13 (91%, Scheme 1).
Silylation (O6) and deoxygenation (C5, via its tosylhydrazone14)
of 3 gave the 5-deoxy sugar 4 (∼60% from 1). Oxidation (C3)
of 4 and stereoselective reduction15 gave 5 which was desilylated
to give ribohexofuranose 6 (77%). Homoadenosine16 (7) was
obtained by benzoylation (O3 and O6) of 6, acetal hydrolysis,
acetylation, coupling17 of the anomeric acetates (adenine, SnCl4),
and deacylation. However, glycosyl cleavage occurred upon
attempted nitration18 of derivatives of 7.
Methanolysis19 of 5 and one-pot treatment with acetone gave
9, which was nitrated18 to give 10. Acetal hydrolysis [Amberlite
(H+)] and acetylation gave 11 (81%) which was coupled
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