Organic Chemistry

Section 11.12 Designing a Synthesis II: Approaching the Problem 431

Now the reaction sequence can be written in the forward direction, indicating the

reagents needed to carry out each reaction. A bulky base is used in the elimination

reaction in order to maximize the amount of elimination product.

Example 4.How could the following cyclic ether be prepared from the given starting

material?

In order to obtain a cyclic ether, the two groups required for ether synthesis (the alkyl

halide and the alcohol) must be in the same molecule. To obtain a five-membered ring,

the carbons bearing the two groups must be separated by two additional carbons.

Addition of water to the given starting material will give the required bifunctional

compound.

PROBLEM 27

How could you have prepared the target molecule in the preceding synthesis using

4-penten-1-ol as the starting material? Which synthesis would give you a higher yield

of the target molecule?

H+

H 2 O

NaH

BrCH 2 CH 2 CH 2 CH CH 2 BrCH 2 CH 2 CH 2 CHCH 3

OH

O CH^3

target molecule

synthesis

BrCH 2 CH 2 CH 2 CHCH 3 BrCH 2 CH 2 CH 2 CH CH 2

OH

O

target molecule

CH 3

retrosynthetic analysis

BrCH 2 CH 2 CH 2 CH CH 2

O CH^3

?

Tutorial:

Retrosynthetic analysis

CH 3 CH 2 CCH 3 CH 3 CH 2 CCHCH 3 CH 2 CHCH 2 Br CH 3 CH 2 CH CH 2 CH 3 CH 2 CH 2 CH 2 Br

Br

O

retrosynthetic analysis

target molecule

CH 3 CH 2 C

CH 3 CH 2 CCH 3

O

CH 3 CH 2 CH 2 CH 2 Br CH 3 CH 2 CH CH 2 CH 3 CH 2 CHCH 2 Br CH

Br

target molecule

Br 2

CH 2 Cl 2

H 2 O

H 2 SO 4

HgSO 4

NH 2

high

concentration

tert-BuO−

tert-BuOH

−

synthesis

BRUI11-400_436r3 26-03-2003 10:20 AM Page 431