Organic Chemistry

Section 20.8 Oxidative Cleavage of Alkenes 865

PROBLEM 27 SOLVED

Describe how the following compound could be prepared, using the given starting materi-

als (perform a retrosynthetic analysis to help you arrive at your answer):

SOLUTION Retrosynthetic analysis shows that the target molecule can be disconnected

to give a five-carbon positively charged fragment and a four-carbon negatively charged

fragment (Section 18.12). Pentanal and a butyl magnesium bromide are the synthetic

equivalents for the two fragments. The five-carbon starting material can be converted to the

required four-carbon compound by ozonolysis.

Now the synthesis of pentanal and butyl magnesium bromide from the given starting

material can be written in the forward direction, including the reagents necessary for

the transformations.

OH

?

Table 20.1 Summary of the Methods Used to Oxidize an Alkene

O

CH 3 CCH 3

O

+ CH 3 CH

CH 3

CH 3 C CHCH 3

1. O3, – 78 °C
   2. Zn, H 2 O

CH 3

CH 3 C

O

CHCH 3

RCOOH

O

O

CH 3 CCH 3

O

+ CH 3 COH

1. O3, – 78 °C
   2. H 2 O 2
   O

CH 3 CCH 3

O

+ CH 3 COH

KMnO 4

H+

HIO 4

O

CH 3 CCH 3

O

+ CH 3 CO−

KMnO 4 , HO−

∆

CH 3

OH

CH 3 C

OH

CHCH 3

O

CH 3 CCH 3

O

+ CH 3 CH

KMnO 4 , HO−,^ H 2 O^

cold

HIO 4

CH 3

OH

CH 3 C

OH

CHCH 3

O

CH 3 CCH 3

O

+ CH 3 CH

1. OsO 4


2. H 2 O 2

OH OH

+ −

OH O

Br OH

O

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