Ch_322b_17.05

Ch_322b_17.05 - Chapter 17.5 lecture note

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The Crossed Aldol Condensation An aldol reaction using two different carbonyl compounds is a crossed aldol condensation . Because of the possibility of four different condensation products , this reaction is not synthetically useful unless it is carefully designed. Directed Crossed Aldol Condensations A simple designed synthesis includes one carbonyl compound with no ! -H, which eliminates two of the possible products. An example is the crossed aldol condensation of benzaldehyde and acetaldehyde . To minimize self-condensation of acetaldehyde, the enolizable carbonyl is slowly added to a mixture of benzaldehyde and dilute HO - . CH O = benzaldehyde (no enolizable H) + CH 3 CH O dil. HO - CHCH 2 CH O OH a directed aldol product fast (-H 2 O) CH=CHCH O cinnamaldehyde (3-phenyl-2-propen-1-one)
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Additional examples of directed crossed aldol reactions using benzaldehyde: CH O = + HO - benzaldehyde (no enolizable H) slowly add CH 3 CCH 3 O (-H 2 O) CH= CHCCH 3 O 4-phenyl-3-buten-2-one slowly add C 6 H 5 CCH 3 O (-H 2 O) CH= CHC O 1,3-diphenyl-2-propenone
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Ch_322b_17.05 - Chapter 17.5 lecture note

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