2444-103111

2444-103111 - 1 Preparation of esters One of the most...

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Preparation of esters 1 • One of the most efFcient methods to prepare esters is the reaction of an alcohol with an acid chloride. • 2-Methyl-2-propanol ( 59 ; tert- butyl alcohol) dissolved in diethyl ether, in the presence of dimethylaniline ( 61 ) as an amine base, is heated to re±ux and then treated with acetyl chloride ( 60 ) to give a 62% yield of 1,1-dimethylethyl ethanoate ( tert- butyl acetate, 62 ) HO NMe 2 59 62 O O CH 3 Cl O CH 3 60 Cl O CH 3 O H Cl O CH 3 O – Cl 63 61 64
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Preparation of esters 2 Acid anhydrides also react with alcohols, in the presence of amine bases, to give esters. OH O O CH 3 59 62 O O O reflux + CH 3 CO 2 H
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Preparation of esters 3 Acid catalyzed reaction of an alcohol with an acid. 21 Me O OH Me OH O H Bu OH Me O OH H Me OH OBu OH Me O OH H Me O OBu H Me O– H OBu 70 Me O OBu Me OH OBu O H H + H + + BuOH + H + 65 66 67 68 69 + BuOH - H + - H 2 O - H + H 3 C O OH OH 21 70 H 3 C O O cat. H 2 SO 4 , reflux 69%
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Transesterification 4 Transesterifcation is a very specifc term that means exchanging the alcohol unit oF one ester with a diFFerent alcohol unit to make a new ester. C 3 H 7 O OMe C 3 H 7 OH O MeO H Et C 3 H 7 O OMe H C 3 H 7 OH OEt O C 3 H 7 O OMe H C 3 H 7 O OEt H C 3 H 7 O–H OEt 76 C 3 H 7 O OEt C 3 H 7 OH OEt O H Me 29 + H + +EtOH 75 74 73 72 + H + 71 +EtOH Me - H + - MeOH - H +
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DCC Coupling 5 • A carbodiimide is a unique chemical entity that is effectively a "di-imine”, and it has the generic structure shown for 81 . • Carbodiimides serve as "dehydration agents" for various OH-bearing compounds, including carboxylic acids. • One specialized and readily available carbodiimide called dicyclohexylcarbodiimide (usually abbreviated DCC) will be used to illustrate this transformation, and it has structure 78 . When 3-methylbutanoic acid ( 77 ; isobutyric acid) is stirred with tert- butanol in the presence of a catalytic amount of 4-pyrrolidinopyridine ( 79 ), for example, tert- butyl 3-methylbutanoate ( 80 ) is isolated in 65% yield. In addition to the ester, one molar equivalent of dicyclohexyl urea (see 87 ) is also isolated. Urea is 82 . N C N O OH N N 77 79 OH , O O 80 78 cat. H 2 N O NH 2 N C N R R 82 81
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DCC Coupling 6 C O N H N H O O CHMe 2 CHMe 2 80 N C N H O O Me 2 HC N C N H O O O H CHMe 2 Me 2 HC H O CHMe 2 N C N N C N H O O H CHMe 2 77 O O Me 2 HC + 87 85 86 78 84 83 ! + ! + In effect the DCC unit "activates" the acyl carbon (derived from butanoic acid) to attack by the weak nucleophile, ethanol.
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Lactones 7 O O O O O O 88 89 90 • Cyclic esters in which the acyl carbon and the "alcohol" oxygen are constituent members of a ring, as in 88 - 90 , are called lactones . • The common names for structurally simple lactones arise by using the sufFx for the open chain carboxylic acid precursor and the word lactone. • Compound
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2444-103111 - 1 Preparation of esters One of the most...

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