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DECARBOXYLATION OF b -KETOACIDS. HYDROLYSIS OF A b -KETOESTER GIVES A b -KETOACID. Claisen Condensation. b -Ketoester. hydrolysis. - CO 2. Ketone. … then, the b -ketoacid decarboxylates in acid. b -Ketoacid. Decarboxylation of a b -Ketoacid. 6-ring. concerted step.

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Presentation Transcript
slide1

DECARBOXYLATION

OF b-KETOACIDS

slide2

HYDROLYSIS OF A b-KETOESTER GIVES A b-KETOACID

Claisen

Condensation

b-Ketoester

hydrolysis

- CO2

Ketone

… then, the b-ketoacid

decarboxylates in acid

b-Ketoacid

slide3

Decarboxylation of a b-Ketoacid

6-ring

concerted step

heat + acid

enol

tautomerization

Carbon dioxide is easily lost

since a nice 6-membered

ring transition state can be

formed for a concerted

process.

ketone

slide4

SYNTHESIS OF KETONES

FROM b-KETOESTERS

ALKYLATION / DECARBOXYLATION OF b-KETOESTERS

slide5

STEP ONE - MAKING THE b-KETOESTER

….. discussed earlier

a b-ketoester

Claisen

condensation

a different R group

could be used here

slide7

STEP THREE - HYDROYSIS AND DECARBOXYLATION

b-ketoester

b-ketoacid

this

part will be

the ketone

b-ketoacids decarboxylate

easily when heated in acid

(see next slide)

ketone

slide8

ALKYLATION CAN BE SKIPPED

an unbranched ketone

slide10

-

..

C

S

anion is stabilized

by empty 3d

orbitals on sulfur

Conversion of Aldehydes to Ketones

Dithiane Anions

a 1,3-dithiane

acidic H

H2SO4

aldehyde

-H2O

butyllithium

(strong base)

various

methods

..

H2/Ni

-

or

ketone

Hg(OAC)2

SN2

H2SO4

slide12

PATTERNS

Type of

Condensation

Reaction

b-hydroxy to C=O

ALDOL

3-hydroxyaldehyde or

(H)

3-hydroxyketone

(H)

-H2O

a,b-unsaturated C=O

ALDOL

2-propen-1-al or

(with loss of H2O)

2-propen-1-one

b-keto ester

CLAISEN

slide13

PATTERNS

CONTINUED

b-keto acid

DECARBOXYLATION

H3O+

heat

H3O+

R-X (alkylation)

+

CO2

from Claisen

ketone

slide16

Acyloin Condensation

Na

xylene

H2O

mechanism

not required

an acyloin

This is a good way to make large rings.

High dilution.

slide17

A Cyclic Acyloin Condensation

Na

xylene

Zn(Hg)

Conc HCl

cyclotetratriacontane

A VERY BIG RING !

slide18

PATTERN

a-hydroxyketone

ACYLOIN

an acyloin

slide20

Catenanes

Interlocked

rings.

slide21

HELL-VOLHARD-ZELINSKY

a-BROMOACIDS AND a-BROMOESTERS

slide22

Hell-Volhard-Zelinsky Reaction

1)

P, Br2

2)

H3O+

a-bromoacid

PBr3

H3O+

(workup step)

Acid halides have

a larger enol content

than ketones.

..

:

Double bonds react

with bromine!

Resonance increases the electron

density in the double bond. Enols

are more reactive than alkenes.

slide23

Synthesis of Aminoacids and a-Hydroxyacids

neutralization step not shown

+

phenylalanine

NaOH

lactic acid

slide24

REFORMATSKY REACTION

A COUSIN TO THE GRIGNARD REACTION

slide25

Reformatsky Reaction

Fischer

Esterification

or via the

acid chloride

a-bromoester

Zn, ether

1)

Reformatsky

reagent

2)

H3O+

Reacts like a

Grignard reagent!