Page 315 - Advanced Organic Chemistry Part B - Reactions & Synthesis

of these substances. Several methods exist for forming the nucleoside bonds. 287
Application of 2-chloro-3-ethylbenzoxazolium chloride to this reaction was
PROBLEMS
investigated using 2,3,4,6-tetra-O-acetyl- -D-glucopyranose. Good yields were
observed and the reaction was stereospecific for the -nucleoside. Suggest a
mechanism to explain the retention of configuration.

C 3 H 5
+ CH 3
AcOCH 2 H N N
AcO O + N CH 3 Cl AcOCH 2 O
OH O AcO N
AcO CH 3
AcO N AcO
60°C, 10 h AcO
CH 3
H
3.19. A route to -glycosides involves treatment of a 2,3,4,6-tetra-O-benzyl- -D-
glucopyranosyl bromide with an alcohol, tetraethylammonium bromide, and
diisopropylethylamine in CH Cl . Explain the stereoselectivity of this reaction.
2
2

R′OH
ROCH 2 + – ROCH 2
4
RO O Et N Br RO O
RO EtN(i-Pr) , RO
2
OR Br CH 2 Cl 2 OR OR′
R = CH Ph
2

3.20. Write mechanisms for formation of 2-pyridylthio esters by the following
methods:

(a)
O
H +
RCO 2 SS + PPh 3 RC S N + Ph P O
3
N N
(b)
O +
RCO H + + R′ N + CO + R′ NH Cl
3
2
2
N SCCl 3 RC S N
O
3.21. The ionophoric antibiotic nonactin is a 32-membered macrocycle that contains
two units of (−)-nonactic acid and two units of (+)-nonactic acid in an alternating
sequence.

a. Assuming that you have access to both (+)- and (−)-nonactic acid, devise a
strategy and protecting group sequence that could provide the natural macro-
molecule in high stereochemical purity.