Section 19.16 Intramolecular Condensation and Addition Reactions 815to completion by carrying out the reaction with enough base to remove a proton from
the of the ester product. When the reaction is over, acid is added to
reprotonate the condensation product.
PROBLEM 28Write the mechanism for the base-catalyzed formation of a cyclic ester from a
1,7-diester.Intramolecular Aldol Additions
Because a 1,4-diketone has two different sets of two different intramol-
ecular addition products can potentially form—one with a five-membered ring, the
other with a three-membered ring. The greater stability of five- and six-membered
rings causes them to be formed preferentially (Section 2.11). In fact, the five-
membered ring product is the only product formed from the intramolecular aldol ad-
dition of a 1,4-diketone.
The intramolecular aldol addition of a 1,6-diketone potentially can lead to either a
seven- or a five-membered ring product. Again, the more stable product—the one with
the five-membered ring—is the only product of the reaction.
1,5-Diketones and 1,7-diketones undergo intramolecular aldol additions to form
six-membered ring products.
a-hydrogens,b-ketoa-carbon b-ketoCH 2 CH 2 COCH 3CH 2 COCH 3CH 2O O OO
OCH 3 O−mechanism for the Dieckmann condensation
CH 2 CHCOCH 3CH 2 COCH 3CH 2OOCH+ 3 OH+ CH 3 O−COCH 3 COCH 3OOCH 3−−CH 3 CCH 2 CH 2 CCH 2C CH 3OOCH 3 CCH 2 CH 2 CCH 3
2,5-hexanedione
a 1,4-diketoneOOO
H 2 OH 2 O CH
3O H 2 O
−O
CH 3O
− HOHO−HO−CH 3 CCH 2 CHCCH 3OOCH 3−O H 2 O C CH 3O− CH 3HOnot formedCH 3 CCH 2 CH 2 CH 2 CH 2 CCH 3
2,7-octanedione
a 1,6-diketone
OOH 2 OH 2 OHO−HO−CH 3 CCH 2 CH 2 CH 2 CHCCH 3OOCH 3H 2 O−O−CCH 3OCH 3CCH 3OCH 3 CCH 2 CH 2 CH 2 CH 2 CCH 2OOHOH 2 O
−OCH 3O−OCH 3OHnot formed