Dolbier HW Solutions 349 - NaBH 4 or LiAlH 4 The...

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( q ) Cyanohydrin formation takes place. ( r ) Cyclopentanone is not oxidized readily with chromic acid. 17.23 ( a ) The fi rst step in analyzing this problem is to write the structure of the starting ketone in stereo- chemical detail. Reduction of the ketone introduces a new stereogenic center, which may have either the R or the S con fi guration; the con fi guration of the original stereogenic center is unaffected. In prac- tice the 2 R ,3 S diastereomer is observed to form in greater amounts than the 2 S ,3 S (ratio 2.5:1 for LiAlH 4 reduction). ( b ) Reduction of the ketone can yield either cis - or trans -4- tert -butylcyclohexanol. It has been observed that the major product obtained on reduction with either lithium alu- minum hydride or sodium borohydride is the trans alcohol (trans/cis 9:1). ( c ) The two reduction products are the exo and endo alcohols. The major product is observed to be the endo alcohol (endo/exo 9:1) for reduction with
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Unformatted text preview: NaBH 4 or LiAlH 4 . The stereoselectivity observed in this reaction is due to decreased steric hindrance to attack of the hydride reagent from the exo face of the molecule, giving rise to the endo alcohol. H OH endo-Bicyclo[2.2.1]-heptan-2-ol OH H exo-Bicyclo[2.2.1]-heptan-2-ol O Bicyclo[2.2.1]-heptan-2-one 1 1 4-tert-Butylcyclo-hexanone H (CH 3 ) 3 C O trans-4-tert-Butylcyclohexanol H (CH 3 ) 3 C OH H cis-4-tert-Butylcyclohexanol H (CH 3 ) 3 C H OH 1 metal hydride reduction ( S )-3-Phenyl-2-butanone O H C 6 H 5 (2 R ,3 S )-3-Phenyl-2-butanol HO H H C 6 H 5 (2 S ,3 S )-3-Phenyl-2-butanol H OH H C 6 H 5 O Cyclopentanone Cyclopentanone cyanohydrin CN HO NaCN H 2 SO 4 ALDEHYDES AND KETONES: NUCLEOPHILIC ADDITION TO THE CARBONYL GROUP 443...
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