SolutionsChiral auxiliaries
Exercise 1
Which cleavage conditions of the Evans auxiliary would you use to convert the aldol in the scheme into (a) the acid, (b) the methyl ester and (c) the 1,3-diol? Why is LiOOH, rather than LiOH alone, used to obtain the acid?
(a) LiOH and H2O2 in THF/water; (b) sodium or magnesium methoxide in methanol; (c) LiBH4. LiOH preferentially attacks the less hindered ring carbonyl (the carbamate) and destroys the auxiliary; the hydroperoxide anion, more nucleophilic and less basic, selectively attacks the exocyclic carbonyl and releases the acid and the intact auxiliary.
Exercise 2
If the enantiomeric oxazolidinone, (R)-4-benzyl, is used in the reaction of the scheme, which product is obtained?
The enantiomer of the aldol: the syn aldol with (2R,3S) configuration and, after hydrolysis, (2R,3S)-3-hydroxy-2,4-dimethylpentanoic acid. The whole system is the mirror image of the original.