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lecture 16.3 - Organic Chemistry William H Brown...

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16- 16-1 Organic Organic Chemistry Chemistry William H. Brown William H. Brown Christopher S. Foote Christopher S. Foote Brent L. Iverson Brent L. Iverson
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16- 16-2 Aldehydes Aldehydes & & Ketones Ketones Chapter 16 Chapter 16
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16- 16-3 Acetals as Protecting Grps Acetals as Protecting Grps Suppose you wish to bring about a Grignard reaction between these compounds 5-Hydroxy-5-phenylpentanal (racemic) 4-Bromobutanal Benzaldehyde ?? + H O H O H OH O Br
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16- 16-4 Acetals as Protecting Groups Acetals as Protecting Groups a Grignard reagent prepared from 4- bromobutanal will self-destruct! first protect the -CHO group as an acetal then do the Grignard reaction hydrolysis (not shown) gives the target molecule H O Br A cyclic acetal + H + H 2 O HO OH + Br O O Br O O 1. Mg, ether 2. C 6 H 5 CHO O O O - MgBr + A chiral magnesium  alkoxide (produced as a racemic mixture)
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16- 16-5 Acetals as Protecting Groups Acetals as Protecting Groups Tetrahydropyranyl (THP) protecting group the THP group is an acetal and, therefore, stable to neutral and basic solutions, and to most oxidizing and reducting agents it is removed by acid-catalyzed hydrolysis RCH 2 OH + O O RCH 2 O Dihydropyran A tetrahydropyranyl ether H + THP group
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16- 16-6 Addition of Nitrogen Nucleophiles Addition of Nitrogen Nucleophiles Ammonia, 1° aliphatic amines, and 1° aromatic
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