Nucleic Acids in Chemistry and Biology

(MSNT). A mixed phosphoryl–sulfonyl anhydride is produced initially, in which the methyl groups of the
mesitylene ring provide steric hindrance to reaction at the sulfur atom and ensure condensation at the
phosphorus atom. Subsequent complex reactions, which may also involve condensed phosphate intermedi-
ates, lead to the triester product (Figure 3.54). The final conversion of the triester into the desired diester
uses one of the specific cleavages described earlier (Section 3.2.2).

3.2.3.4 Syntheses via Phosphite Triesters.88,91,94 The P(III) triester route to oligonucleotides ini-

tially introduced by Letsinger (Figures 3.53 and 3.55) was designed to exploit the intrinsically greater
reactivity shown by PCl 3 as compared to POCl 3 to achieve faster coupling steps. A major breakthrough
was achieved by Caruthers,95,96who established the value of alkyl phosphoramidites(X i-Pr 2 N) as stable
3 -derivatives of nucleosides, which nonetheless react rapidly and efficiently with nucleoside 5-hydroxyl
groups in the presence of azole catalysts. The resulting product is an unstable phosphite triester, which
must be oxidised immediately to give the stable phosphate triester in a process that can be cycled up to 100
times on a solid-phase support. Removal of the phosphate-protecting group affords the phosphate diester
(Figure 3.55). Racemization that occurs during reactions carried out with purified Rp and Sp diastereoiso-
mers have confirmed the intermediacy of a phosphorotetrazolideduring formation of the phosphite triester
(Figure 3.56).^97

3.2.3.5 Syntheses via H-phosphonate Diesters.^92 The H-phosphonate monoesters of protected

3 -nucleosides are readily prepared using PCl 3 and excess imidazole followed by mild hydrolysis. The

Nucleosides and Nucleotides 109

HO

HO

P

O

i-Pr 2 N

X

NC P

O

i-Pr 2 N

OR

NC

P

O

OR'

OR

O P NC

R'O OR

NC

O

PCl 3 P

O

Cl

Cl

NC

P

O

R'O OR

O

R = 3'

5'

R' =

(i) (ii) (iii)

(iv)

(vi) (v)

Figure 3.55 Synthesis of phosphate triesters and diesters via phosphite triesters. Reagents: (i) 2-cyanoethanol,
pyridine, ether78°C; (ii) i-Pr 2 NH, ether,20°C: XCl, 2eq. or Xi-Pr 2 N 4 eq.; (iii) XCl:
ROH, i-Pr 2 EtN,THF, or Xi-Pr 2 N: ROH, i-Pr 2 EtNH 2 tetrazolidein CH 2 Cl 2 or THF; (iv) tetrazole
in CH 3 CN and ROH; (v) I 2 in pyridine/H 2 O/THF; and (vi) aq NH 4 OH

O R'

H

P

O

i-Pr 2 N

OR

NC

P

O

i-Pr 2 N

OR

NC

H

N

N

N

N

P

O

N

OR

NC

N

NN

P

O

OR'

OR

NC

N

N

N

N

H

P

O OR'

RO

NC

O oxidation

phosphate triester [P(V)] phosphite triester [P(III)]

Figure 3.56 Synthesis of phosphate triesters using phosphoramidite reagents