Organic Chemistry

710 CHAPTER 17 Carbonyl Compounds I

17.19 Designing a Synthesis IV:

The Synthesis of Cyclic Compounds

Most of the reactions we have studied are intermolecular reactions: The two reacting

groups are in different molecules. Cyclic compounds are formed from intramolecu-

lar reactions: The two reacting groups are in the same molecule. We have seen that

intramolecular reactions are particularly favorable if the reaction forms a compound

with a five- or a six-membered ring (Section 11.11).

In designing the synthesis of a cyclic compound, we must examine the target

molecule to determine what kinds of reactive groups will be necessary for a suc-

cessful synthesis. For example, we know that an ester is formed from the acid-cat-

alyzed reaction of a carboxylic acid with an alcohol. Therefore, a cyclic ester

(lactone) can be prepared from a reactant that has both a carboxylic acid group and

an alcohol group in the same molecule. The size of the lactone ring will be deter-

mined by the number of carbon atoms between the carboxylic acid group and the

alcohol group.

A compound with a ketone group attached to a benzene ring can be prepared

using a Friedel–Crafts acylation reaction (Section 15.13). Therefore, a cyclic

ketone will result if a Lewis acid is added to a compound that contains both

a benzene ring and an acyl chloride group separated by the appropriate number of

carbon atoms.

A cyclic ether can be prepared by an intramolecular Williamson ether synthesis

(Section 11.9).

CH 3

CH CH^3

Cl 2 CH 2 CH 2 CCH 3

CH 3

Na

O

−O

ClCH 2 CH 2 CH 2 CCH 3

CH 3

OH

1. AlCl 3


2. H 2 O


3. AlCl 3


4. H 2 O

CH 2 CH 2 CCl

CH 2 CH 2 CH 2 CCl

O

O

O

O

(AlCl 3 )

HCl

HCl

HOCH 2 CH 2 CH 2 CH 2 COH

O

O

O

O

O

HOCH 2 CH 2 CH 2 COH

O

three intervening carbon atoms

four intervening carbon atoms