Organic Reactions Volume 4 - Sciencemadness Dot Org

284 ORGANIC REACTIONS
even traces of the isomer results. Both benzoins XI and XII on treatment
with benzoyl chloride give the same monobenzoate. Presumably
an equilibrium mixture of isomers is produced first; benzoylation then
occurs readily with only one of the isomers. 88
The diketone corresponding to XI or XII, when treated with zinc
and acetic acid for five minutes, yields a mixture of the isomeric benzoins;
but on heating for ten hours only the desoxy compound XIII is formed,
and no further reduction takes place.
More Stable to Less Stable
More stable benzoins can be converted into the less stable isomers by
indirect means through derivatives which have the appropriate isomeric
structures and which in turn can be transformed readily into the desired
benzoins. 4-Methoxybenzoin upon reduction gives the desoxy compound
related to the isomeric benzoin (82% yield); the desoxybenzoin
can be brominated to the bromo ketone (84% yield). This on treatment
with sodium ethoxide followed by acidification is converted (74%) into
the isomer of the original benzoin. Isolation of the bromo ketone is not
necessary. 65 ' 71 ' 72 ' 81
C6H6CHOHCOC6H4OCH3-4^-%
More stable HC1 Br
C6H6COCH2C6H4OCH3-4 > C6H6COCHBrC6H4OCH3~4
NaOC2H5
HCl
CeH5COCHOHO6H4OOH8^ < C6H6C (OC2H6)2CHC6H4OCH3-4
Leas stable
ONa
Another procedure for the conversion of the more stable isomeric
benzoin into the less stable one is illustrated also with 4-methoxybenzoin.
18 ' u The more stable benzoin is converted into the oxime in 50-99%
yield. This is reduced catalytically to the corresponding amine in
C6HBCHOHCOC6H4OCHr4 > C6H5CHOHCC6H4OCH3-4
More stable
NOH
C6HBCOCHC6H4OCHr4 ^ - C6HBCHOHCHC6H4OCH8-4
I I
NH2
HNO2
C6H6COCHOHC6H4OCHjHt
Less stable
NH2