with N-methyl-N-(2-bromoethyl) thymidyl phosphoramidate
12 (Scheme 3). Interestingly, compound 12 reacted under
Scheme 4a
Scheme 3a
a Reagents and conditions: (a) MeNH2, rt, 1.5 h; (b) lithium
aluminum hydride, THF, rt, 2 h min; (c) Na2SO4‚10H2O; (d) HCl(g),
CH2Cl2, rt; (e) SOCl2, CH2Cl2, 0 °C to rt, 4 h; (f) benzyl alcohol,
a Reagents and conditions: (a) â-D-glucose 1-phosphate, TBAC,
pyridine, rt.
i
iPr2NEt, CH2Cl2, -78 °C, 15 min; (g) 17, Pr2NEt, CH2Cl2, -78
to -60 °C, 20 min; (h) thymidine, pyridine, -45 °C (titration); (i)
t-BuOOH, -45 ° to 0 °C, 30 min.
these conditions to form two major products: the desired
diphosphate TDP-Glc and five-membered ring product 14
(Scheme 3). Nucleophilic attack by the phosphate moiety at
the carbon of the aziridinium ion was not observed. Kinetic
analysis of the reaction leading to the formation of cyclized
product 14 suggests that this product arises from intramo-
lecular O-alkylation following formation of the aziridinium
ion (13).5 Formation of product 14 represents an undesirable
pathway and necessitates a larger excess of phosphoramidate
starting material to ensure complete conversion of the sugar
phosphate to product; thus, the halobutyl phosphoramidate
was used for the preparation of model sugar nucleoside
diphosphate TDP-Glc and ultimately TDP-Rha.
The synthesis of N-methyl-N-(2-chlorobutyl) thymidyl
phosphoramidate 19 is shown in Scheme 4. N-methyl-N-(2-
chlorobutyl)amine hydrochloride used for the preparation of
19 has previously been synthesized according to Kuznetsov
et al.;6 however, purification of the intermediate amino
alcohol is tedious and yields are low. An alternative synthesis
of halobutylamine hydrochloride 17 is reported here (Scheme
4) and employs a modified procedure of Beaucage et al.7
Briefly, reaction of γ-butyrolactone with excess condensed
methylamine proceeds quantitatively to amide 15 in <1 h.
Lithium aluminum hydride reduction of amide 15 followed
by reaction of the resulting amino alcohol 16 with thionyl
chloride3 provides the desired halobutylamine hydrochloride
17 in 65% overall yield.
Phosphoramidate 19 was synthesized according to the
procedure reported for the preparation of structurally similar
haloethyl phosphoramidates (Scheme 4).8 Briefly, this method
involves the in situ generation of reactive phosphorus III
intermediate 18. Phosphorylation of the nucleoside by
titration with the phosphorus III intermediate followed by
oxidation with tert-butyl hydroperoxide proceeds in 70-80%
overall yield.
The preparation of TDP-Glc was subsequently achieved
on a > 20 µmol scale using excess N-methyl-N-(2-chlo-
robutyl) thymidyl phosphoramidate 19 (Scheme 5). Phos-
phoramidate 19 was activated by hydrogenolysis in THF over
1.5 h. â-D-Glucose 1-phosphate and TBAC were then
dissolved in dry pyridine and added to the hydrogenolysis
reaction mixture. THF was removed under reduced pressure,
and the mixture was stirred at room temperature for 1 h.
Typically, >90% conversion of â-D-glucose 1-phosphate to
TDP-Glc was observed by 31P NMR. Self-condensation
product and small amounts of hydrolysis products in addition
to unreacted â-D-glucose 1-phosphate were also observed
by 31P NMR. Conversion of the crude product mixture to
the ammonium salts was accomplished on DOWEX 50W-
(5) Manuscript in preparation.
(6) Kuznetsov, S. G.; Ioffe, D. V. J. Gen. Chem. USSR (Eng. Transl.)
1961, 31, 2133.
(7) Wilk, A.; Grajkowski, A.; Phillips, L. R.; Beaucage, S. L. J. Org.
Chem. 1999, 64, 7515-7522.
(8) Meyers, C. L. F.; Borch, R. F. J. Med. Chem. 2000, 43, 4313.
Org. Lett., Vol. 3, No. 23, 2001
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