Organic Chemistry

PROBLEM 34 SOLVED

a. What carbonyl compound and what phosphonium ylide are required for the synthesis of

the following alkenes?

b. What alkyl halide is required to prepare each of the phosphonium ylides?

SOLUTION TO 34a (1) The atoms on either side of the double bond can come from

the carbonyl compound, so there are two pairs of compounds that can be used.

SOLUTION TO 34b (1) The alkyl halide required depends on which phosphonium

ylide is used; it would be either 1-bromobutane or 2-bromopropane.

The primary alkyl halide would be more reactive in the reaction required to make the

ylide, so the best method would be to use acetone and the ylide obtained from 1-bromobutane.

SN 2

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

Br

or

CHCH 2 CH 3 CH CH 2

CH 3 CH 2 CH 2 CH CCH 3 (C 6 H 5 ) 2 C“CHCH 3

CH 3

CH 3 CCH 3 + CHCH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH CCH 3

CH 3

(C 6 H 5 ) 3 P(Cor 6 H 5 ) 3 P

O

+

O

764 CHAPTER 18 Carbonyl Compounds II

is used in the food industry to color margarine.

Many people take -carotene as a dietary supplement because

there is some evidence that high levels of -carotene are associat-

ed with a low incidence of cancer. More recent evidence, howev-

er, suggests that -carotene taken in pill form does not have the

cancer-preventing effects of -carotene obtained from vegetables.b

b

b

b

b-Carotene

O

H

P(C 6 H 5 ) 3

H

+

vitamin A aldehyde

-carotene

-CAROTENE

is found in yellow-orange fruits and

vegetables such as apricots, mangoes, carrots, and

sweet potatoes. The synthesis of -carotene from vitamin A is

an important example of the use of the Wittig reaction in indus-

try. Notice that the ylide is a stabilized ylide and the product has

the Econfiguration at the reaction site.

b

b-Carotene

B