Nucleic Acids in Chemistry and Biology

from one base to another by a process known as transglycosylation.28–30This procedure makes use of the
fact that nucleoside formation described in the sections above, in the presence of Lewis acids, is a
reversible process. The reaction is particularly effective for transferring sugars from pyrimidines (which
are -deficient heterocycles) to the more basic purines (-excessive heterocycles). Some examples31,32are
shown in Figure 3.9.

Nucleosides and Nucleotides 83

BzO O OAc

BzO OBz

N

NN

OTMS

TMSO

+ BzO O

BzO OBz

N

N

N

OTMS

O

HO O

HO OH

N

N

NH

O

O

(i)

(ii), (iv)

(iii), (iv)

HO O

HO OH

N

N

N

N

O

93%

57%

HO O

HO OH

N

NH

N

N

O

NH 2

N

N N

N

OTMS

AcN

BzO O OAc

BzO OBz

(v), (iv)

66%

TMS TMS

N

N N

N

OTMS

TMSO

TMS

+

+

BzO O OAc

BzO OBz

HO O

HO OH

N

NH

N

N

O

O

(v), (iv)

49%

BzO O OAc

BzO OBz

N

N

NHTMS

TMSS

+ HO O

HO OH

N

N

NH 2

S

(i), (ii), (iv)

pTolO O

pTolO

N

N

OTMS

OTMS

+

HO O

HO

N

NH

O

O

(vi), (iv)

Br

Cl

Br

72%

95%

HO O

H 2 N OH

N

N

N

N

NH 2

N

N N

N

N

BzO O OBz

CF 3 CONH OBz

(i), (iv) 70%

TMS

+

Bz TMS

H

Figure 3.8 Examples of the silyl base method of nucleoside synthesis. Reagents: (i) SnCl 4 in ClCH 2 CH 2 Cl, 20°C;
(ii) aq. NaHCO 3 ; (iii) pyrrolidine; (iv) NH 3 , MeOH; (v) TMSOTf, ClCH 2 CH 2 Cl, reflux; and (vi) CHCl 3 , 20°C