Stereoselective synthesis of N1-6-methyluridine and related 2-substituted analogues

Article abstract of DOI:10.3987/COM-06-S(K)3

Coupling reaction of 2-substituted-6-methyl-4(3H)-pyrimidinones (1a,b) with 1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose (ABR) afforded the corresponding N¹-2,6-disubstituted nucleosides (2a,b) without any trace of the N³-isomer.

Full text of DOI:10.3987/COM-06-S(K)3

HETEROCYCLES, Vol. 72, 2007
79
HETEROCYCLES, Vol. 72, 2007, pp. 79 - 83. © The Japan Institute of Heterocyclic Chemistry
Received, 2nd August, 2006, Accepted, 23rd August, 2006, Published online, 25th August, 2006. COM-06-S(K)3
STEREOSELECTIVE SYNTHESIS OF N1-6-METHYLURIDINE AND
RELATED 2-SUBSTITUTED ANALOGUES
Marco Radi, Elena Petricci, Federico Corelli, and Maurizio Botta*
Dipartimento Farmaco Chimico Tecnologico, Universita’ di Siena, via Aldo Moro,
53100 Siena, Italy. *Corresponding author: botta@unisi.it
Abstract – Coupling reaction of 2-substituted-6-methyl-4(3H)-pyrimidinones
(1a,b) with 1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose (ABR) afforded the
corresponding N1-2,6-disubstituted nucleosides (2a,b) without any trace of the
N3-isomer. The 2-methylthio-6-methyl derivative (2b) has then proved to be a key
intermediate for the synthesis of N1-6-methyluridine (4) and the corresponding
N1-2-amino-6-metyhl derivative (5), suggesting the possibility to obtain different
2,6-disubstituted derivatives by means of methylthio nucleophilic substitution. A
successful solid phase application was also shown.
To date, the main efforts in the discovery of novel antiviral and anticancer nucleosides were generally
devoted to the functionalization of the sugar moiety while base modified nucleoside have received so far
little attention. In this context, base-modified pyrimidine nucleosides have generally been substituted at
C-5 position (i.e. BVDU, CEDU)1 probably because of the easiness of functionalization at this site (i.e.
electrophilic substitution) which could be related to the highest single coefficient in the HOMO of C-5.
While the methods for the preparation of 5-substituted pyrimidine nucleosides are well established, there
is a need for yet a easier access to 6-derivatized analogues which, being constrained into a non natural
syn-conformation, are of interest as potential antimetabolites. Uridine derivatives bearing a 6-vinyl
substituent as well as a 6-fluoromethyl substituent have in fact shown to posses interesting antitumor
activity.2 However, according to the results published so far from Vorbrüggen et al.,3 very subtle steric
as well as energetic factors seem to drive the formation of N1- and N3- nucleosides by condensation of
6-substituted nucleobases with the suitable sugar. As a result, a complex mixture of N1- and N3-isomers,
which requires tedious purification, is always obtained.
In order to drive the coupling reaction between 6-substituted pyrimidines and the activated sugar derivative
to the exclusive formation of the N1-nucleoside, few different strategies have been developed so far: the

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most common method requires a preliminary protection of the amide moiety of the pyrimidine base to give
the corresponding N3-benzyl derivative, subsequent silylation, condensation with the activated sugar,
removal of the benzyl group and final deprotection of the sugar moiety. Otherwise, the formation of the
N3-nucleoside during the condensation reaction is prevented by transforming the 6-substituted pyrimidines
into a 4,6-disubstituted derivatives so that the NH moiety of the nucleobase is no more available for the
coupling with the sugar.2b The continued interest of our research group in the synthesis of unnatural
nucleosides has prompted us to investigate the synthesis of 6-substituted nucleoside in order to find out a
new method for maximal yields or exclusive formation of the N1-isomer.4 Since steric factors seem to be of
primary importance in the formation of either N1- or N3-nucleosides in the coupling reaction between
silylated 6-methyluridine and the activated sugar, initially we decided to study the effects of a reduced
bulkiness around the N1 atom on the outcome of the coupling reaction. Accordingly, a methoxy group was
introduced in the C-2 position of 6-methyluracil. The O-methyl group is chemically very similar to
O-trimethylsilyl group of the silylated nucleobase but its smaller size could influence the outcome of the
coupling reaction.
O
N
O
N
OTMS
H3CX
N
CH3
BzO
ii
i
HN
H3CX
N
O
CH3
H3CX
N
CH3
OBz OBz
1a: X = O
1b: X = S
2a: X = O
2b: X = S
Scheme 1. Reagents and conditions: i. HMDS, (NH4)2SO4; ii. ABR, TMSOTf, MeCN
Following the Vorbrüggen’s procedure,5 2-methoxy-6-methyl-4(3H)-pyrimidinone (1a) was reacted with
hexamethyldisilazane (HMDS) in the presence of catalytic ammonium sulfate (Scheme 1) to give the
corresponding 4-trimethylsilyl derivative. The latter intermediate was then reacted with
1-O-acetyl-2,3,5-tri-O-benzoyl-β-D-ribofuranose (ABR) in the presence of trimethylsilyl triflate
(TMSOTf) as a catalyst to give compound (2a) as the only product in 61% yield which configuration was
established through 1D NOE difference spectrometry (NOEDS) NMR experiments: irradiation of the
anomeric proton (H1') in (2a) gave a significative NOE effect both on C6 methyl group and H4',
characteristic for a β-N3-nucleoside. The same stereoselectivity in the outcome of the coupling reaction was
observed using the 2-methylthio-6-methyl-4(3H)-pyrimidinone (1b) even if nucleoside (2b) was obtained

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81
with lower yields (49%) probably due to the bigger size of the sulfur atom. The configuration of (2b) was
established through the irradiation of the anomeric proton in a 1D NOE experiment.
In order to get the desired N1-6-methyluridine (4), compound (2b) was reacted with 3 equivalents of
m-chloroperbenzoic acid (m-CPBA) in dichloromethane followed by aqueous workup obtaining compound
(3) in 72% yield (Scheme 2). Final deprotection with methanolic ammonia afforded N1-6-methyluridine (4)
which chemico-physical properties and NMR data were identical with that of an authentic sample.6 The
methylthio group in C-2 played therefore a key role for the stereoselectivity of the coupling reaction and
was moreover an interesting site for further functionalization: reacting in fact compound (2b) directly with
methanolic ammonia at room temperature for 48 hours, methylthio displacement and sugar deprotection
took place at the same time giving the 2-amino-4-methyluridine (5) in 68% yield.
O
N
O
N
HN
N
i
O
CH3
H3CS
CH3
RO
BzO
O
O
OR OR
OBz OBz
2b
3 R = Bz
4 R = H
ii
iii
O
N
H2N
N
CH3
HO
O
OH OH
5
Scheme 2. Reagents and conditions: i. m-CPBA (3 eq.), CH2Cl2, r.t.; ii. MeOH/NH3, r.t.; iii. MeOH/NH3,
r.t.
Our interest turned then into the development of additional 2,6-disubstituted nucleosides since, to the best
of our knowledge, they represent a new class of unnatural nucleosides which may display interesting
biological activity. A further increase in the steric hindrance of the C2 side chain of the nucleobase
(compound 6) did not afford however the expected coupling product (7a) following the Vorbrüggen’s
procedure (Scheme 3). Nevertheless, a modified coupling procedure allowed to obtain the desired
compound: one pot silylation with TMSOTf and TMSI catalyzed coupling reaction with ABR let us to

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obtain (7a) in a 54% yield (using 1 eq. of TMSI) while (7b) was obtained in a 30% yield using an excess of
TMSI (2 eq.) (Scheme 2).
O
N
O
N
i, ii
RO
BzO
S
N
CH3
HN
BnO
iii, iv
O
S
CH3
6
OBz OBz
7a R = Bn
7b R = H
Scheme 3. Reagents and conditions: i. HMDS, (NH4)2SO4; ii. ABR, TMSOTf, CH3CN; iii. Collidine,
TMSOTf, CH2Cl2; iv. ABR, TMSI (1 eq. for 7a and 2 eq for 7b).
As a further extension of our previous work on the solid phase synthesis of pyrimidine nucleosides,4e the
latter modified coupling procedure was also proved to be an interesting approach for the contemporary
coupling with the sugar and releasing of the 2,6-disubstituted nucleoside (7b) from the solid support
(Scheme 3) since our previously developed Oxone® cleavage methodology was not effective on solid
supported 6-substituted nucleosides. The application of the modified Vorbrüggen’s procedure using 3
equivalents of TMSI gave, after 72h, the expected nucleoside (7b) in a good 42% yield.
O
O
N
N
i., ii.
HN
HO
BzO
S
N
CH3
O
S
CH3
O
8
OBz OBz
7b
Scheme 4. Reagents and conditions: i. Collidine, TMSOTf, CH2Cl2; ii. ABR, TMSI (2 eq).
In conclusion, the present work has initially pointed out the interesting role played by the reduced steric
hindrance on the C-2 position of 6-methyluracil derivatives on the outcome of the coupling reaction. The
stereoselective formation of nucleosides (2a,b) was thus obtained. Subsequent reaction of (2b) with
m-CPBA and final deprotection, afforded N1-6-methyluridine as single isomer after only three steps
starting from (1b). It was moreover observed how, reacting compound (2b) with methanolic ammonia for
long time, it was possible to obtain both sugar deprotection and methylthio displacement achieving in this
way compound (5). Finally, a modified Vorbrüggen’s procedure was successfully applied to the

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83
solid-supported nucleobase (8) obtaining the contemporary coupling with the sugar and releasing of the
2,6-disubstituted nucleoside (7b) from the solid support. Both solution and solid phase approaches could
therefore be conveniently employed to obtain further 2,6-disubstituted nucleosides in order to study the
antimetabolite properties of this new class of compounds.
ACKNOWLEDGEMENTS
Support from the European TRIoH Consortium (LSHB-2003-503480) is gratefully acknowledged.
REFERENCES
1. (a) E. De Clercq, J. Descamps, P. De Somer, P. J. Barr, A. S. Jones, and R. T. Walker, Proc. Natl.
Acad. Sci. U.S.A., 1979, 76, 2947. (b) H. Griengl, M. Bodenteich, W. Hayden, E. Wanek, W.
Streicher, P. Stutz, H. Bachmayer, I. Ghazzouli, and B. Rosenwirth, J. Med. Chem., 1985, 28, 1679.
2. (a) S. Megati, R. Sodum, G. M. Otter, R. S. Klein, and B. A. Otter, Bioorg. Med. Chem. Lett., 1994,
4, 469. (b) K. Felczak, A. K. Drabikowska, J. A. Vilpo, T. Kulikowski, and D. Shugar, J. Med.
Chem., 1996, 39, 1720.
3. U. Niedballa and H. Vorbrüggen, J. Org. Chem., 1974, 39, 3660.
4. (a) M. Radi, C. Mugnaini, E. Petricci, F. Corelli, and M. Botta, Tetrahedron Lett., 2005, 46, 4361.
(b) E. Petricci, M. Radi, F. Corelli, and M. Botta, Tetrahedron Lett., 2003, 44, 9181. (c) C. Mugnaini,
M. Botta, M. Coletta, F. Corelli, F. Focher, S. Marini, M. L. Renzulli, and A. Verri, Bioorg. Med.
Chem., 2003, 11, 357. (d) L. Paolini, E. Petricci, F. Corelli, and M. Botta, Synthesis, 2003, 2003,
1039. (e) E. Petricci, M. L. Renzulli, M. Radi, F. Corelli, and M. Botta, Tetrahedron Lett., 2002, 43,
9667. (f) R. Saladino, C. Crestini, A. T. Palamara, M. C. Danti, F. Manetti, F. Corelli, E. Garaci, and
M. Botta, J. Med. Chem., 2001, 44, 4554.
5. H. Vorbrüggen, K. Krolikiewicz, and B. Bennua, Chem. Ber., 1981, 114, 1234.
6. (a) M. P. Schweizer, J. T. Witkowski, and R. K. Robins, J. Am. Chem. Soc., 1971, 93, 277. (b) M. W.
Winkley and R. K. Robins, J. Org. Chem., 1968, 33, 2822.